Like all RTKs, Tyro-3, Axl, and Mer contain an extracellular domain, a transmembrane domain, and a conserved intracellular kinase domain. The TAM family is distinguished from other RTKs by a conserved sequence, KW (I/L)A(I/L)ES, within the kinase domain and adhesion molecule-like domains in the extracellular region (). More specifically, two immunoglobulin-like (Ig) domains and two fibronectin type III (FNIII) domains comprise nearly the entire ectodomain of each family member. These motifs are thought to be important in cell–cell contacts and mimic the structure of neural cell adhesion molecule (NCAM), which contains five Ig domains and two FNIII domains (
Yamagata et al., 2003). Among the RTKs, Tie (Tie1) and Tek (Tie2) are the only other receptors that contain both Ig and FNIII extra cellular domains. The FGF, VEGF, and PDGF growth factor receptor families contain Ig domains while the Ephrin and Insulin families contain FNIII domains. Although the TAM receptors share extracellular motifs with the above RTKs, the MET RTK family (com posed of Met and Ron) is most closely related to the TAM family on the basis of amino acid sequence of the kinase domain (
Robinson et al., 2000). The MET and TAM receptors activate common signaling molecules resulting in similar functions of the two RTK families (
Birchmeier et al., 2003;
Hafizi and Dahlback, 2006a). Thus, both the extracellular domain and the intracellular kinase domain are important determinants of the cellular processes regulated by specific RTKs.
The TAM receptor genes share similar genomic structure encoding transcripts which range in size from 3 to 5 kb (
Graham et al., 1994,
1995;
Mark et al., 1994;
O’Bryan et al., 1991). Within the TAM family, Tyro-3 and Axl appear to have the most similar genomic structure sharing the same number, 20, and size of exons (
Lewis et al., 1996b;
Lu et al., 1999;
Schulz et al., 1993). Mer is encoded by 19 exons (
Gal et al., 2000). Axl and Tyro-3 contain alternative splice sites, although the location and outcome of splicing are different. A splice variant of Mer has been suggested but not fully characterized (
Graham et al., 1995). Alter native splicing of Tyro-3 near the 5′ end results in three different splice variants containing either exon 2A, exon 2B, or exon 2C (
Biesecker et al., 1995;
Lewis et al., 1996b;
Lu et al., 1999). These exons encode a signal peptide, suggesting that the presence of these splice variants may impact posttranslational processing, localization, and/or function of Tyro-3. Two Axl variants have been observed resulting from alternative splicing of exon 10 (
Neubauer et al., 1994;
O’Bryan et al., 1991;
Schulz et al., 1993). This exon encodes part of the second FNIII domain just upstream from the transmembrane region (
Lu et al., 1999). It remains unknown whether the Tyro-3 and Axl variants are produced from a single transcript or from multiple promoters. However, analysis of Axl and Mer sequences upstream of their respective translation initiation sites revealed a GC-rich promoter region lacking traditional TATA or CAAT boxes (
Schulz et al., 1993;
Wong and Lee, 2002). Further analysis of the Mer promoter suggests that several transcription factors, including Sp1, Sp2, and E2F, may regulate promoter activity (
Wong and Lee, 2002).
In contrast to the striking similarity of genomic structure between Tyro-3 and Axl, Axl and Mer have the most similar tyrosine kinase domain amino acid sequence (
Graham et al., 1995;
Robinson et al., 2000). Overall, the protein sequences of the human TAMreceptors share 31–36%identical (52–57% similar) amino acids within the extracellular region. The intracellular domains share 54–59% sequence identity (72–75% similarity) with higher homology in the tyrosine kinase domain (
Graham et al., 1995). The fulllength Tyro-3, Axl, and Mer proteins contain 890, 894, and 999 amino acids, respectively. Although the predicted protein sizes are 97, 98, and 110 kDa for Tyro-3, Axl, and Mer, respectively, the actual molecular weights range from 100 to 140 kDa for Axl and Tyro-3 and 165–205 kDa for Mer due to posttranslational modifications, including glycosylation, phosphorylation, and ubiquitination (
Lu et al., 1999;
O’Bryan et al., 1991;
Sather et al., 2007;
Valverde, 2005). Such modifications are possible mediators of tissue-and cell type-specific variations in TAM receptor function (
Heiring et al., 2004;
Ling et al., 1996) (see Section II.D).
A. Cloning/Nomenclature
In addition to sequence and structural similarities, the TAM receptor kinases are unusual in that the entire family was discovered within a span of 3 years. In the early 1990s, each TAM receptor gene was cloned from multiple species by independent groups resulting in confusing nomenclature (). Axl was first detected in 1988 as an unidentified transforming gene in two patients with chronic myelogenous leukemia (CML) (
Liu et al., 1988). Three years later, two independent groups reported cloning of the human gene from patients with CML (
O’Bryan et al., 1991) and chronic myeloproliferative disorder (
Janssen et al., 1991). One group named the gene Axl from the Greek word for uncontrolled, anexelekto (
O’Bryan et al., 1991), and the other called the gene UFO indicating the unknown function of its protein product (
Janssen et al., 1991). Around the same time, a third group cloned the murine gene and named it Ark (adhesion-related kinase) (
Rescigno et al., 1991). In the same year, 13 novel PCR fragments comprising 50–60 amino acids of the conserved tyrosine kinase catalytic domain were isolated from rat brain and named Tyro-1 to -13 (
Lai and Lemke, 1991). Interestingly, the authors grouped Tyro-3, Tyro-7, and Tyro-12 into a novel subfamily based on the unique amino acid sequence found in their kinase domains. It would later be discovered that Tyro-7 is the same gene as Axl/UFO, Tyro-12 is the same gene as Mer, and Tyro-3 constituted the third member of the TAM family.
In 1992, a second member of the TAM family, v-ryk, was isolated from the chicken retrovirus RLP30 (
Jia et al., 1992). The cellular protooncogene, c-ryk, was later cloned from embryonic chicken brain and renamed c-eyk in order to avoid confusion with an unrelated tyrosine kinase also called ryk (
Jia and Hanafusa, 1994). Later that same year, our lab cloned the human gene from a B-lymphoblastoid λgt11 expression library and named it c-
mer because it was found in
monocytes as well as in
epithelial and
reproductive tissues (
Graham et al., 1994). We cloned murine c-mer the following year (
Graham et al., 1995). The human gene was cloned by a separate group and called Nyk for NCAM-related tyrosine kinase (
Ling and Kung, 1995). Mer was also named MerTK for Mer tyrosine kinase in a paper which mapped the human gene to chromosome 2q14.1 (
Weier et al., 1999).
In addition to the earlier mentioned PCR fragment isolated from rat (
Lai and Lemke, 1991), fragments of murine Tyro-3, called Etk-2 (
Biesecker et al., 1993), and human Tyro-3 (
Polvi et al., 1993) were cloned from mouse embryonic stem cells and human teratocarcinoma cell, bone marrow, and melanocyte cDNA libraries, respectively. In 1994, the murine and human genes were cloned by multiple labs. The murine gene was named Dtk (
Crosier et al., 1994), Brt (
Fujimoto and Yamamoto, 1994), Rse (
Mark et al., 1994), and Tyro-3 (
Lai et al., 1994) while the human gene was called Sky (
Ohashi et al., 1994), Tif (
Dai et al., 1994), or Rse (
Mark et al., 1994). Subsequent sequence analysis revealed that Dtk and Brt were alternative splice variants (
Lewis et al., 1996b). The chicken ortholog was cloned in 1996 but was given the name Rek because of limited amino acid sequence identity with the mouse and human genes (66% and 68%, respectively) (
Biscardi et al., 1996).
While many of these names were used initially in the literature, Tyro-3, Axl, and Mer (or MerTK) have become the most commonly published and will be used exclusively throughout the remainder of this review.
B. Expression Patterns
Although expression of TAM receptor mRNA has been observed in embryonic tissues (
Crosier et al., 1996;
Faust et al., 1992;
Graham et al., 1995;
Lai and Lemke, 1991), single, double, and even triple knockouts are viable without obvious signs of developmental defect at birth (
Lemke and Lu, 2003;
Lu and Lemke, 2001;
Lu et al., 1999). These data suggest that the TAM RTKs are largely nonessential for embryogenesis. Conversely, TAM adult knockout mice develop diverse phenotypes in a wide range of tissues revealing some of the most prominent cellular functions of TAM receptors (discussed in Section II.E).
In adult tissues, Tyro-3, Axl, and Mer exhibit widespread distribution with overlapping but unique expression profiles. Tyro-3 is most abundantly expressed in the nervous system, and is also found in ovary, testis, breast, lung, kidney, osteoclasts, and retina as well as a number of hematopoietic cell lines including monocytes/macrophages and platelets (
Angelillo-Scherrer et al., 2001;
Katagiri et al., 2001;
Lai et al., 1994;
Lu and Lemke, 2001;
Mark et al., 1994;
Prasad et al., 2006). Axl is expressed ubiquitously (
O’Bryan et al., 1991), with notable levels found in the hippocampus and cerebellum (
Bellosta et al., 1995) as well as monocytes/macrophages, platelets, endothelial cells, heart, skeletal muscle, liver, kidney, and testis (
Angelillo-Scherrer et al., 2001;
Graham et al., 1995;
Neubauer et al., 1994). Within the hematopoietic lineages, Mer is expressed in monocytes/macrophages, dendritic cells, NK cells, NKT cells, megakaryocytes, and platelets (
Angelillo-Scherrer et al., 2001;
Behrens et al., 2003;
Graham et al., 1994). High levels of Mer expression are also detected in ovary, prostate, testis, lung, retina, and kidney. Lower levels of Mer are found in heart, brain, and skeletal muscle (
Graham et al., 1994,
1995;
Prasad et al., 2006). Tyro-3, Axl, and Mer also display ectopic or overexpression in numerous cancers, including myeloid and lymphoblastic leukemias, melanoma, breast, lung, colon, liver, gastric, kidney, ovarian, uterine, and brain cancers (). However, the pattern differs for each family member, e.g. Mer is found in lymphoid leukemia while Axl is not (
Graham et al., 1994,
2006;
Neubauer et al., 1994).
| Table IITAM Receptor Expression in Human Cancers |
C. Ligands and Crystal Structures
The vitamin K-dependent protein Gas6 was first identified as a ligand for Axl in 1995 (
Stitt et al., 1995;
Varnum et al., 1995). The related vitamin K-dependent anticoagulation factor, Protein S, was described as a ligand for Tyro-3 (
Stitt et al., 1995). Although numerous subsequent studies confirmed that Gas6 binds to and activates all three members of the TAM receptor family, the validity of Protein S as a ligand for any of the TAM receptors became subject to extensive debate (
Chen et al., 1997;
Godowski et al., 1995;
Mark et al., 1996;
Nagata et al., 1996;
Ohashi et al., 1995). At the heart of the dispute was the issue of physiological relevance as the initial study used human Protein S to activate murine Tyro-3. Further studies were unable to demonstrate that Protein S could activate a TAM receptor of the same species, possibly due to the need for additional cofactor(s) or modification of the Protein S ligand. However, it was recently determined that purified recombinant murine Protein S does bind to and activate both endogenous murine Mer and heterologously expressed murine Tyro-3 (
Prasad et al., 2006). There is currently no evidence that Protein S activates Axl. A large number of additional studies have investigated the interspecies affinities of Gas6 and Protein S for TAM receptors (reviewed in
Hafizi and Dahlback, 2006b). Studies which evaluated the
Kd values for human Gas6 binding to each of the three human TAM receptors
in vitro suggest that Axl and Tyro-3 bind Gas6 with roughly equal affinity while Mer affinity for Gas6 is 3–10-fold lower (
Chen et al., 1997;
Fisher et al., 2005).
Gas6 and Protein S share 43% amino acid sequence identity and have the same domain structure with the exception of thrombin cleavage sites which are present in Protein S but not Gas6 (
Dahlback and Villoutreix, 2005;
Stenflo et al., 1987) (). The N-terminal domain contains glutamic acid residues which must be carboxylated in a vitamin K-dependent reaction before Gas6 and Protein S are biologically active (
Stenhoff et al., 2004). The γ-carboxyglutamic acid (Gla) domain is followed by four EGF-like repeats and two C-terminal globular laminin G-like (LG) domains. The Gla domain mediates Ca
2+-dependent binding to negatively charged membrane phospholipids exposed on the surface of apoptotic cells. The LG domains form a V-shaped structure stabilized by a calcium-binding site and mediate ligand–receptor interactions (
Mark et al., 1996;
Sasaki et al., 2002). Solution for the crystal structure of a Gas6 fragment containing the two LG domains revealed an unusual α-helix within LG2 located at the edge of the β-sandwich fold typical of all LG domains. In addition, five amino acids within LG2 constitute a patch of surface-exposed hydrophobic residues located near the crook of the “V” created by LG1 and LG2. These residues are also in close proximity to the stabilizing calcium-binding site. It has not been determined whether the calcium-binding site contributes to RTK binding. Mutagenesis studies and receptor activation assays suggested that the hydrophobic residues within LG2 comprise at least part of the Axl binding site (
Sasaki et al., 2002). However, LG2 alone does not bind to or activate Axl, and a later study by the same group determined that only LG1 of Gas6 binds Axl (
Sasaki et al., 2006). The authors suggest that the hydrophobic residues may still affect ligand/receptor binding indirectly. Direct binding between Axl and the LG1 domain of Gas6 was first demonstrated by
Fisher et al. (2005). An anti-Gas6 monoclonal antibody diminished Gas6 binding to Axl and the antibody binding epitope was mapped to residues 403–414 within the J–K loop of LG1. Notably, this region is located near the edge of the LG1 β-sandwich fold, distant from the hydrophobic patch within LG2.
The crystal structure of a Gas6/Axl complex finally revealed that the LG1 domain of Gas6 makes two separate contacts with the IG1 and IG2 domains of Axl (
Sasaki et al., 2006). Each contact is characterized by antiparallel alignment of edge β-strands such that continuous β-sheets span the molecular junction. Interestingly, no ligand/ligand or receptor/receptor contacts were reported in this minimal complex containing the LG domains of Gas6 and the Ig domains of Axl (). Additional experiments suggest that ligand-mediated TAM receptor dimerization occurs via a two-step mechanism whereby one molecule of Gas6 binds one receptor molecule with high affinity at the LG1/IG1 “major” contact. Lateral diffusion of these 1:1 ligand/receptor complexes results in dimerization of two 1:1 complexes via the LG1/IG2 “minor” contact. Thus, a 2:2 ligand/receptor complex is formed. Further evidence to support two Gas6/Axl binding sites was provided by receptor binding studies, which demonstrated that Gas6 can simultaneously bind Axl–Fc and a neutralizing Gas6 antibody (
Fisher et al., 2005). Receptor binding studies of an N-terminal fragment of Tyro-3 demonstrated that one site of Gas6/Tyro-3 receptor interaction is localized to the two Ig domains. Although the crystal structure of the Tyro-3 fragment and sequence alignment of the three TAM receptors predict the existence of a Gas6-binding site near the interface of the two Ig domains, no empirical evidence regarding the actual ligand binding site(s) was provided (
Heiring et al., 2004). Thus, additional studies are required to determine whether Tyro-3 and Mer bind Gas6 in the same manner as does Axl. Given that there is no current information describing Protein S as a ligand for Axl, it will be particularly interesting to see how Protein S interacts with Mer and Tyro-3.
Until recently, no structural information was available for the kinase domains of TAM receptors. The crystal structure of the catalytic domain of human Mer has been solved and may provide new insight into numerous aspects of TAM receptor biology, including mechanisms of receptor activation and interaction with downstream signaling molecules (
Walker et al., 2007).
D. Regulation of Receptor Kinase Activity
1. CONVENTIONAL ACTIVATION Typical activation of RTKs involves ligand binding to the extracellular domain (
Schlessinger, 2000). Ligand binding induces receptor dimerization and subsequent trans-autophosphorylation of tyrosine residues within the cytoplasmic domain (). The result of autophosphorylation is twofold: (1) increased catalytic efficiency leads to phosphorylation of other substrates and (2) tyrosine-phosphorylated RTKs and other proteins constitute docking sites that recruit signaling molecules containing SH2, PTB, or other phosphotyrosine-binding domains. This allows RTKs and other proteins to form macromolecular signaling complexes. For Mer, three tyrosine residues (Y-749, Y-753, and Y-754 in the human sequence) within the activation loop of the kinase domain have been identified as the primary sites of autophosphorylation (
Ling et al., 1996). Interestingly,
in vitro kinase assays utilizing peptides with two of the three tyrosines mutated to phenylalanine residues as substrates for WT Mer demonstrated that tyrosine 749 is the preferred site of autophosphorylation. Additional
in vitro kinase assays evaluated WT Mer versus mutant Mer phosphorylation of a synthetic peptide containing tyrosines 749, 753, and 754. Single mutations of tyrosines 749, 753, and 754 to phenylalanine reduced Mer kinase activity to 39%, 10%, and <6% of WT Mer, respectively, suggesting that all three residues are required for complete functional activity of the kinase (
Ling et al., 1996). These three tyrosines are conserved among the TAM receptors and correspond to residues 681, 685, and 686 in the human sequence on Tyro-3 and residues 698, 702, and 703 in the human sequence of Axl. Autophosphorylation of Tyro-3 and Axl have not been reported at these residues.
Three alternative tyrosine residues (Y-779, Y-821, and Y-866) within the C-terminal domain of Axl have been proposed as potential autophosphorylation sites (
Braunger et al., 1997). These three sites, and in particular Y-821, mediate interaction of Axl with a number of signaling molecules including phospholipase C (PLC), phosphatidyl inositol 3 kinase (PI3K), and Grb2 (
Braunger et al., 1997;
Fridell et al., 1996). All of the interactions identified were dependent on Axl tyrosine kinase activity; however, the studies do not provide clear evidence that tyrosine residues 779, 821, and 866 are indeed sites of autophosphorylation. The residue equivalent to Axl Y821 in Mer (Y-867/872 in the murine/human sequences) is also a probable site of interaction with multiple signaling molecules. Mutation of tyrosine 867/872 to phenylalanine did not reduce tyrosine phosphorylation of Mer, suggesting that this site does not regulate kinase activity efficiency (
Georgescu et al., 1999). Furthermore, Axl mutants lacking tyrosine 821 display normal ligand-induced tyrosine phosphorylation (
Fridell et al., 1996). Alternative to these tyrosines being sites of autophosphorylation, they may be phosphorylated by another kinase recruited by autophosphorylation at different residues. Src-family non-RTKs (SFKs) are potential candidates for this activity as they have been shown to interact with both Axl and Tyro-3 (
Braunger et al., 1997;
Toshima et al., 1995). The combination of site-directed mutagenesis and
in vitro kinase activity assays allows more definitive assignment of tyrosines 749, 753, and 754 as Mer autophosphorylation sites (
Ling et al., 1996). However, it remains possible that these and additional tyrosine or serine/threonine residues are phosphorylated by other kinases. It is also possible that a unique complement of residues becomes phosphorylated in response to specific stimuli within the cellular microenvironment. Expression of TAM receptors in certain cell types may also lead to distinct phosphorylation patterns. Future generation of phosphosite-specific antibodies will greatly aid our ability to address these types of questions.
2. ATYPICAL ACTIVATION In some cases, ligand-independent receptor dimerization and activation can occur (). For example, overexpression of Axl leads to cell aggregation via homophilic binding of the extracellular domains on neighboring cells (
Bellosta et al., 1995). Although cell aggregation correlated with increased tyrosine phosphorylation of Axl, activation of the kinase domain was not required for homophilic binding (
Bellosta et al., 1995). Because the specific residue(s) responsible for the observed increase in tyrosine phosphorylation remain unknown, it is possible that phosphorylation occurred at a site unrelated to receptor activation. Studies of Axl and Tyro-3 overexpression suggest that these receptors are also capable of ligand-independent dimerization and autophosphorylation (
Burchert et al., 1998;
Taylor et al., 1995a). Further evidence to support ligand-independent dimerization was provided by crystal structures of a Tyro-3 fragment containing the two N-terminal Ig domains (
Heiring et al., 2004). Importantly, a distinction must be made between dimerization of two receptors on the surface of one cell and homophilic binding of receptors on neighboring cells (i versus v in ) as exogenous expression of Tyro-3 in Sf9 cells (
Toshima et al., 1995) and basal expression of Axl in NIH3T3 cells (
Bellosta et al., 1995) are not sufficient to induce homophilic binding. Thus, it remains unknown whether this phenomenon occurs with any endogenous TAM receptor.
An increasingly common theme in cell signaling literature is cross-talk between receptor systems. Ligand-independent heterotypic receptor dimerization of Axl with interleukin-15 receptor alpha (IL-15Rα) has been reported in immortalized and primary fibroblasts (
Budagian et al., 2005b) (). Binding of IL-15 to IL-15Rα, not Axl, leads to Axl-mediated phosphorylation of IL-15Rα as well as Axl phosphorylation, although it is not known whether this is a direct action of the Axl kinase domain. Thus, IL-15 transactivates the Axl receptor and downstream signaling molecules, including PI3K, Akt, and ERK. Heterotypic dimerization of Axl with cytokine receptors seems to be specific to IL-15Rα as Axl does not coprecipitate IL-2, IL-4, IL-7, IL-9, or IL-21 receptor subunits, even in the presence of ligand (
Budagian et al., 2005b). To date, similar heterotypic receptor interactions have not been reported for Mer or Tyro-3.
Another unexplored possibility is an unusual heteromeric interaction among the three TAM receptors (). Homo-and heterodimerization have been reported for other RTK families such as EGFR family members. Recent studies suggest that Gas6-mediated phosphorylation/activation of one TAM receptor may require the presence of one or both of the other TAM receptors in some circumstances (
Angelillo-Scherrer et al., 2005;
Seitz et al., 2007). Interestingly, Western blotting studies suggest that relatively equal amounts of Axl total protein can be detected in whole cell lysates of platelets from WT and Tyro-3−/− mice. However, flow cytometry experiments demonstrated that surface expression of Axl is significantly reduced in Tyro-3−/− and Mer−/− mice (
Angelillo-Scherrer et al., 2005). Taken together, these data suggest that Axl may require the presence of Mer or Tyro-3 or both for functional surface delivery and stabilization within the plasma membrane.
3. MECHANISMS OF DEACTIVATION Cellular control of RTK signal attenuation is important as aberrant or continued receptor signaling can lead to numerous pathological states, including cancer. Cells have developed numerous methods for inactivation of RTKs, including antagonistic ligands, hetero-oligomerization with kinase inactive mutants, phosphorylation of inhibitory residues by other kinases, dephosphorylation of activating residues by phosphatases, and receptor endocytosis accompanied by ligand dissociation, receptor degradation, or both (
Schlessinger, 2000). Only a few of these pathways have been explored as possible mechanisms of TAM receptor regulation.
Many tyrosine kinases are negatively regulated by phosphorylation of an inhibitory residue. For example, phosphorylation of tyrosine 527 near the C-terminus of Src prevents activation of the kinase by promoting intramolecular binding to the SH2 domain, thus rendering the active site inaccessible. Interestingly, it has been postulated that tyrosine 866 on Axl, one of the same residues proposed as a site of autophosphorylation, might constitute an inhibitory phosphorylation site akin to C-terminal tyrosines found in SFKs and the EGFR (
Burchert et al., 1998). However, the same study concluded that the absence or mutation of this residue did not impact the ability of Axl-retroviruses to transform NIH3T3 cells. A second phosphorylation-mediated mechanism of receptor downregulation is receptor dephosphorylation by protein tyrosine phosphatases. The putative tyrosine phosphatase C1-TEN has been shown to bind Axl and overexpression of C1-TEN correlates with reduced cell survival, proliferation, and migration of 293 cells (
Hafizi et al., 2002,
2005b). Although neither enzymatic activity of C1-TEN nor direct dephosphorylation of Axl have been demonstrated, these results are consistent with C1-TEN-mediated Axl inactivation.
Soluble forms of Axl and Mer, produced by proteolytic cleavage and release of the ectodomain, can be detected in murine and human plasma (
Budagian et al., 2005a;
Costa et al., 1996;
O’Bryan et al., 1995;
Sather et al., 2007). Although a truncated form of Tyro-3 was found in the cytoplasm when expressed in 293 cells (
Taylor et al., 1995a), extracellular soluble Tyro-3 was not detected in human plasma (
Sather et al., 2007). Soluble Mer can also be produced by alternative splicing of the Mer transcript (our unpublished data). Although alternative splicing of Axl (
O’Bryan et al., 1991;
Schulz et al., 1993) and Tyro-3 (
Biesecker et al., 1995;
Lewis et al., 1996b) have been reported, the transcripts generated encode transmembrane proteins. Soluble TAM receptors bind to Gas6 and can act as a ligand sink and inhibit normal cellular functions of the full-length RTK (
Budagian et al., 2005a;
Sather et al., 2007). In the same regard, soluble TAM receptors may have therapeutic potential in pathological conditions, such as cancer, where TAM receptor activity is upregulated. This topic will be further explored in Section IV.
Evidence supporting endocytosis as a mechanism of TAM receptor downregulation was provided by a report which demonstrated that Gas6 stimulates interaction of Axl with the ubiquitin ligase c-Cbl and ubiquitination of Axl (
Valverde, 2005), a process that has been demonstrated with other RTKs such as the EGFR. Clearly the study of mechanisms which regulate TAM receptor function and turnover is an area that needs further investigation.
E. Cellular Functions
Stimulation of TAM receptors can produce diverse cellular functions depending on the ligand–receptor combination as well as the cell type and microenvironment. Initial studies of individual TAM receptors suggested that each kinase performs unique functions in specific cell types. However, as the number of publications investigating two or three TAM receptors in the same system increases, it is becoming evident that the TAM receptors can serve overlapping and possibly cooperative roles. While it is beyond the scope of this review to discuss every cell type which expresses TAM receptors, several cellular functions of TAM receptors are discussed here according to specific cell types.
1. MACROPHAGES/DENDRITIC CELLS TAM-receptor knockouts develop autoimmune diseases, including rheumatoid arthritis and lupus (
Cohen et al., 2002;
Lemke and Lu, 2003). Loss of Mer alone confers susceptibility to autoimmunity (
Scott et al., 2001). However, the phenotype is more pronounced in double knockouts and most severe in triple knockouts (
Lemke and Lu, 2003). These phenotypes likely result from accumulation of apoptotic cells and subsequent tissue necrosis combined with constitutive activation of the immune system. Studies of single, double, and triple mutants suggest that these defects are a result of TAM receptor loss from macrophages/dendritic cells (
Lu and Lemke, 2001).
a. Clearance of Apoptotic Cells Cell death via apoptosis is a necessary process for maintenance of normal cell number and health. Clearance of apoptotic cells plays an important role in many biological processes, including tissue development and homeostasis, lymphocyte maturation, and pathological responses such as inflammation. Progressive accumulation of apoptotic cells leads to tissue necrosis and release of intracellular contents into the local environment. Because it is more difficult for immune cells to locate and clear this cellular debris, necrosis leads to inflammation and, in some cases, activation of autoantibody production.
Although a number of different types of professional phagocytes can ingest infectious microorganisms and particles, clearance of apoptotic cells is primarily mediated by macrophages and, to a lesser degree, dendritic cells. Because the surface of apoptotic cells and the phagocytes which digest them are both negatively charged, proteins must mediate the processes of cell recognition and engulfment. Specifically, apoptotic cells express phosphatidylserine (PS) on their surface, which has been shown to bind directly to phagocytes via PS receptors or indirectly via binding to one of several soluble proteins, including the TAM receptor ligands Gas6 and Protein S (
Anderson et al., 2003;
Nakano et al., 1997). Macrophages express all three TAM receptors (
Graham et al., 1994;
Lu and Lemke, 2001;
Neubauer et al., 1994), suggesting a mechanism whereby TAM receptors and their ligands might mediate macrophage recognition of apoptotic cells.
Protein S binds to and stimulates phagocytosis of apoptotic cells (
Anderson et al., 2003). However, there is currently no empirical evidence which directly correlates Protein S-mediated phagocytosis with activation of a TAM receptor. Conversely,
in vitro studies demonstrated that Gas6 stimulates macrophage uptake of PS liposomes and uptake is blocked by the extracellular domain of Axl (
Ishimoto et al., 2000). Similarly, soluble Mer bound to the Fc domain of human immunoglobulin G (Mer–Fc) inhibits macrophage phagocytosis of apoptotic cells presumably by sequestering Mer ligand (
Sather et al., 2007). Several lines of evidence suggest that Mer is not required for binding to apoptotic cells but is essential for cell shape changes associated with engulfment of the apoptotic cell (
Cohen et al., 2002;
Guttridge et al., 2002;
Hu et al., 2004;
Scott et al., 2001;
Todt et al., 2004). The TAM ligands are proposed to mediate phagocytosis of apoptotic cells by bridging an interaction between PS-expressing cells and TAM receptor-expressing macrophages. Thus, the tyrosine kinase domains of TAM receptors, in particular Mer, likely activate downstream signaling events, including integrins such as αvβ5, which leads to cytoskeletal changes necessary for engulfment of apoptotic cells (
Wu et al., 2005).
It is likely that unique mechanisms mediate clearance of apoptotic cells depending on the type of phagocyte involved and the tissue microenvironment. Accordingly, a recent study by
Seitz et al. (2007) suggests that TAM receptor involvement in clearance of apoptotic cells varies according to cell and organ type. They found that Mer, and to a lesser degree Axl and Tyro-3, mediates macrophage clearance while dendritic cell clearance of apoptotic cells is largely mediated by Axl and Tyro-3. These findings are consistent with an earlier study which showed that dendritic cells from mice lacking Mer protein exhibit normal phagocytosis of apoptotic cells (
Behrens et al., 2003).
One of the most intensely studied examples of TAM receptor-mediated macrophage clearance of apoptotic cells is phagocytosis of photoreceptor outer segment membranes by retinal pigment epithelium (RPE) cells. The role of Mer in RPE phagocytosis was initially elucidated through the study of the Royal College of Surgeons (RCS) rat, a widely studied model of recessively inherited retinal degeneration and animal model for the human disease retinitis pigmentosa. Two groups independently discovered that the genetic basis for RPE dysfunction in the RCS rat was due to a deletion of the second exon of Mer leading to aberrant transcription with a frameshift and translation termination signal 20 codons after the AUG (
D’Cruz et al., 2000;
Nandrot et al., 2000). In a similar manner, transgenic mice (Mer
KD) containing a truncated form of the Mer gene lacking the kinase domain exhibit total loss of Mer protein expression and a retinal phenotype similar to that of the RCS rat (
Duncan et al., 2003). Subsequent work demonstrated that loss of function mutations in human Mer are present in a small subset of patients with severe and progressive retinitis pigmentosa (
Gal et al., 2000;
McHenry et al., 2004;
Thompson et al., 2002). It would be interesting to determine whether these patients exhibit other similarities to Mer knockout mice, such as predisposition to autoimmune disease. Recent studies have demonstrated that viral gene transfer of Mer into the RCS rat retina results in correction of the RPE phagocytosis defect and preservation of photoreceptors, suggesting the exciting possibility of gene therapy for retinitis pigmentosa patients with Mer mutations (
Tschernutter et al., 2005;
Vollrath et al., 2001).
b. Cytokine Secretion Cytokines are soluble proteins which mediate communication between cells of the immune system. Cytokines are released in response to extracellular stimuli, including microorganisms and antigens. A number of different cell types, including macrophages, secrete cytokines, and these soluble signaling molecules usually act over short distances. Cytokine levels indicate the status of the immune system and are subject to stringent regulation in order to avoid inappropriate immune responses. When cytokine levels are not held in check, constitutive activation of the immune system can occur resulting in development of autoimmunity. As mentioned previously, TAM receptor knockout mice develop autoimmune diseases likely due, at least in part, to abnormal regulation of cytokine release.
Mer
KD mice are more susceptible to lethal septic shock following lipopolysaccharide (LPS) challenge. LPS binds to surface receptors and activates nuclear factor (NF)-κB, which then initiates production of proinflammatory cytokines, including TNFα. Pretreatment with anti-TNFα antibody protects against LPS-induced death, suggesting that TNFα is a key upstream regulator of lethal septic shock. Following LPS treatment, Mer
KD mice have elevated NFκB and TNFα levels relative to wild-type controls (
Camenisch et al., 1999). In addition, a recent study demonstrated that Mer activation stimulates the PI3K/Akt pathway which negatively regulates NFκB activation, thus decreasing TNFα production in dendritic cells (
Sen et al., 2007). These data suggest that one of the normal functions of Mer in macrophages and dendritic cells is attenuation of proinflammatory cytokine responses following exposure to bacterial endotoxin. TAM receptors may also mediate other antiinflammatory macrophage responses. For example, interferon (IFN) α has been shown to upregulate expression of Axl and Gas6 in human macrophages resulting in reduced TNFα production (
Sharif et al., 2006). A role for TAM receptors in a broad spectrum of antiinflammatory responses is further supported by the observation of hyperactive macrophages in TAM receptor triple knockouts which produce higher levels of the proinflammatory cytokine IL-12 than do wild-type counterparts (
Lu and Lemke, 2001). TAM receptor regulation of the inflammatory response may be disrupted in various pathologies as microarray analysis of Mer kinase activation (via stimulation of FMS–Mer receptor chimera containing the extracellular domain of the M-CSF receptor and the transmembrane and cytoplasmic domains of Mer) in human prostate cancer cells indicated upregulation of proinflammatory cytokine genes, including IL-8, IL-11, and IL-24 (
Wu et al., 2004).
2. NATURAL KILLER CELLS NK cells are lymphocytes which do not express any of the antigen receptors characteristic of T- or B-cells. NKT cells exhibit characteristics similar to both NK and T cells. Expression of Mer in both NK and NKT cells was first reported by
Behrens et al. (2003), also demonstrating that the Mer tyrosine kinase domain is critical for normal cytokine release from NKT cells. A later study showed that NK cells also express Axl and Tyro-3 and all three TAM receptors are required for normal differentiation and functional maturation of NK cells (
Caraux et al., 2006).
3. PLATELETS The first evidence to suggest a role for TAM receptors in platelet function came from studies of Gas6 knockout mice. Gas6−/− mice were protected against thrombosis and exhibited defective platelet aggregation (
Angelillo-Scherrer et al., 2001). In the same study, RT-PCR analysis demonstrated that platelets express Tyro-3, Axl, and Mer. A follow-up study used single knockouts of Tyro-3, Axl, and Mer to demonstrate that all three receptors are required for normal platelet aggregation (
Angelillo-Scherrer et al., 2005). Loss of any one of the TAM receptors or application of soluble Axl protects against fatal thrombosis. These findings are supported by a study from our lab, which demonstrated that soluble Mer (Mer–Fc) reduces platelet aggregation
in vitro and protects against collagen/epinephrine-induced thrombosis
in vivo (
Sather et al., 2007). Furthermore, a recent study demonstrated that double and triple TAM receptor knockouts exhibit more severe impairment of platelet function than single knockouts (
Wang et al., 2007).
4. VASCULAR SMOOTH MUSCLE CELLS Some of the first studies which evaluated cellular function of TAM receptors were conducted in vascular smooth muscle cells (VSMCs). In these early studies, expression of Axl and Gas6 was increased following vascular injury (
Melaragno et al., 1998). In additional experiments, Gas6 stimulation induced migration of Axl-overexpressing VSMCs (
Fridell et al., 1998). Furthermore, Gas6 protects VSMCs from apoptosis induced by serum starvation in an Axl kinase-dependent manner (
Melaragno et al., 2004). These results suggest that TAM receptors may play a role in vascular diseases, such as atherosclerosis, which are characterized by accumulation of VSMCs. Indeed, Gas6 has been shown to stimulate scavenger receptor expression in normal VSMCs (
Murao et al., 1999). Scavenger receptors facilitate uptake of low-density lipoprotein (LDL) which may lead to transformation of the VSMCs into foam cells and development of atherosclerosis. In advanced atherosclerotic lesions, however, TAM receptors may help slow the progression of disease by mediating ingestion of apoptotic macrophages and attenuating the proinflammatory response (
Li et al., 2006).
5. OTHER Given their broad expression patterns, it is likely that TAM receptors perform important functions in numerous other cells types. For example, Tyro-3, Axl, Mer, and their mutual ligand Gas6 are all expressed in the central nervous system but their normal biological activity has not been widely studied in the brain (
Lai and Lemke, 1991;
Mark et al., 1994;
Prieto et al., 1999,
2000). One exception is an established line of evidence demonstrating a role for Axl in survival and migration of gonadotropin-releasing hormone (GnRH) neurons (
Allen et al., 1999,
2002;
Nielsen-Preiss et al., 2007). Similarly, Gas6 has been shown to reduce cell death of Tyro-3-expressing hippocampal neurons following serum starvation (
Funakoshi et al., 2002). Taken together, these studies suggest that TAM receptors may activate neurotrophic signaling pathways in specific regions of the central nervous system.
It also appears that the three TAM receptors act in concert to regulate spermatogenesis, as triple knockouts are infertile because of progressive degeneration of germ cells beginning one week prior to sexual maturity (
Lu et al., 1999). The mechanism of germ cell death remains unknown except that it likely involves reduced communication between the TAM receptor-expressing Sertoli cells which line the seminiferous tubules and the interstitial Leydig cells which express Gas6 and Protein S. TAM receptor regulation of GnRH neurons may also contribute to the infertility of these knockouts as impaired migration of GnRH neurons inhibits sexual maturation.
F. TAM Receptor Signaling Pathways
The first hint towards understanding TAM receptor signaling came from studies of FMS–Mer receptor chimera by
Ling and Kung in 1995. Around the same time, studies of EGF–Axl receptor chimera were published by an independent group (
Fridell et al., 1996). When the studies began, the ligand for TAM receptors was unknown, necessitating the use of receptor chimera composed of, in the latter report, the EGFR receptor ectodomain and transmembrane domain fused to the intracellular kinase domain of Axl. During the course of the studies, Gas6 was discovered as a ligand for Axl and Tyro-3 and additional work was conducted with the native Axl receptor. Two important findings came out of this seminal work. First, signaling pathway(s) downstream from the Mer and Axl kinase domains were determined to include PI3K, Ras, and ERK. Second, studies of the Axl receptor chimera compared to the native Axl RTK demonstrated that variation in the extracellular domain has a significant impact on downstream signaling.
In the 12 years since, an abundance of research has been conducted with the goal of outlining signaling pathways downstream of TAM receptors. Most of these experiments utilize Gas6 to stimulate TAM receptor function but discuss relevance to only one TAM receptor, usually Axl. It should be noted that Gas6 will also activate other TAM receptors endogenously expressed by the cells under investigation. For example, all three TAM receptors are expressed in platelets and are required for normal function of these cells (see Section II.E.3). The downstream signaling pathway whereby TAM receptors mediate platelet aggregation likely involves cross-talk with the integrin family of receptors as platelets from TAM receptor knockouts exhibit impaired spreading after adhesion to fibrinogen. Indeed, Gas6 stimulates phosphorylation of β
3 integrin, PI3K, and Akt in resting platelets from WT, but not TAM receptor knockout mice (
Angelillo-Scherrer et al., 2005). Importantly, the specific contributions of each TAM receptor to this signaling pathway have yet to be clarified.
To avoid uncertainty regarding which TAM receptor is responsible for the observed effects, some studies have continued to use the receptor chimera approach, fusing a TAM receptor intracellular kinase domain to an extracellular receptor kinase domain not normally expressed in the cells being studied. Although the use of chimeric receptors allows for determination of signaling pathways downstream from a single TAM receptor kinase, data from such experiments must be interpreted conservatively, given evidence provided by
Fridell et al. (1996), suggesting that the extracellular domain impacts downstream signaling. This issue along with inducible expression of TAM receptors in various cell types and unknown variables such as hetero-dimerization has made characterization of TAM receptor signaling pathways a complex task.
1. Mer SIGNALING Much of the evidence delineating Mer signaling pathways is provided by studies of chimeric receptors. This approach originated out of necessity as the ligand for Mer was unknown when many of the studies began. Three well-known signaling pathways, those downstream from PI3K/Akt, PLCγ, and MAPK/ERK (), were linked to Mer tyrosine kinase activation by early studies of chimeric Mer receptors expressed in NIH3T3 fibroblasts (
Ling and Kung, 1995). In this context, ligand stimulation of Mer kinase led to cellular transformation exemplified by increased proliferation and DNA synthesis. Additional experiments indicated that activation of the MAPK/ERK pathway correlated with activation of Raf and p90RSK kinases as well as phosphorylation of Shc and association of Grb2 with Mer (
Ling and Kung, 1995). Later studies identified Gas6 as a ligand for Mer and confirmed that ligand-dependent activation of endogenous Mer stimulates phosphorylation of ERK1/2 (
Chen et al., 1997). Phosphorylation and activation of PLCγ may occur through direct binding of one of its SH2 domains to endogenous phospho-Mer (
Todt et al., 2004). Similarly, there is evidence to suggest that PI3K may interact with Mer via an SH2 domain (
Sen et al., 2007). However, the coimmunopreciptiation experiments of the previous studies do not demonstrate direct binding and it is possible that association of PI3K and PLCγ with Mer is mediated by interaction of Mer tyrosine 872 with additional adapter proteins such as Grb2 (
Georgescu et al., 1999).
The ultimate downstream targets of the PI3K/Akt, PLCγ, and MAPK/ERK pathways may differ according to several factors, including cell type and the tissue microenvironment. In some cells, the PI3K/Akt and MAPK/ERK pathways may act in parallel. In leukemia cells, for example, ligand-dependent activation of an EGFR–Mer chimeric receptor stimulated phosphorylation of Akt, ERK1/2, and p38 MAPK resulting in reduced apoptosis without a change in proliferation (
Guttridge et al., 2002). The presence of multiple Mer signaling pathways which converge on the same prosurvival outcome gives these cells a strong advantage over noncancerous lymphocytes.
In other instances, the PI3K/Akt and MEK/Erk pathways may act in opposition. Similar to the study of leukemia cells discussed earlier, the PI3K/Akt and MAPK/ERK pathways were activated by ligand stimulation of an FMS–Mer chimeric receptor in prostate cancer cells. Additional experiments demonstrated that the Raf and p90RSK kinases act upstream and downstream, respectively, of MAPK/ERK, leading to transcriptional activation of IL-8 via c-Fos/c-Jun binding to the AP-1 promoter region (
Wu et al., 2004). Preincubation with a MEK inhibitor produced the expected result of decreased IL-8 production. However, preincubation with a PI3K inhibitor increased IL-8 production. The authors therefore speculated that the PI3K/Akt pathway may attenuate the effects of the MAPK/ERK pathway by phosphorylating and inhibiting Raf. In this case, activation of Mer may both stimulate and reduce proinflammatory cytokine production. It should be noted that other studies have suggested that Mer reduces production of proinflammatory cytokines in noncancerous cells (
Camenisch et al., 1999;
Sen et al., 2007). Ectopic expression of Mer in prostate cancer cells may therefore result in activation of altered downstream signaling pathways. The tonic strength of normal versus aberrant signaling may therefore determine the oncogenic potential of Mer activation and the ultimate phenotypic fate of the tissue.
Yet another possibility exists whereby activation of Mer stimulates a unique complement of signaling events under specific conditions, thus altering the downstream effect(s) of each individual pathway. For example, some studies of Mer signaling suggest that the PI3K/Akt pathway activates NFκB while others suggest that NFκB is inhibited by the PI3K/Akt pathway. Expression of a constitutively active CD8–Mer chimera in pro-B cells resulted in transcriptional activation of NFκB via PI3K/Akt (
Georgescu et al., 1999). Additional signaling pathways activated by CD8–Mer included p38/MAPK and MEK1. These cells were protected from apoptosis and became IL-3-independent. Conversely, pretreatment of dendritic cells with apoptotic cells prior to LPS exposure induces Mer-mediated stimulation of PI3K/Akt. Under these experimental conditions, the p38/MAPK, MEK1, and JNK signaling pathways were active but unaffected by Mer stimulation. The phenotypic result in this case was reduced production of the proinflammatory cytokine, TNFα, following exposure to LPS (
Sen et al., 2007). Additional experiments in the same study demonstrated that PI3K/Akt negatively regulates NFκB by inhibiting IKK activity and thus preventing degradation of IκB. As is observed with Axl-mediated survival (explained later), PI3K/Akt is classically thought to phosphorylate and activate IκB kinase (IKK), leading to phosphorylation and degradation of inhibitor of κB (IκB) releasing NFκB from the inhibitory complex. However, different isoforms of IKK have been discovered that are differentially phosphorylated by Akt (
Gustin et al., 2004). Thus, there are many factors that define the downstream effects of TAM signaling pathways, including the isoforms of numerous kinases involved and the concomitant activity of additional signaling pathways. Clearly, further investigation is needed to elucidate the myriad signaling pathways activated by Mer kinase.
In addition to the well-known pathways mediated by PI3K/Akt, PLCγ, and MAPK/ERK, some atypical signaling pathways have been proposed as a link between Mer and the actin cytoskeleton. Yeast two-hybrid experiments revealed Mer interactions with Grb2, SHC, and Vav1, the latter is a guanine nucleotide-exchange factor regulating Rac and cdc42 GDP to GTP exchange. Surprisingly, the Mer interaction with Vav1 involved the Vav1 SH2 domain but was constitutive and phosphotyrosine-independent (
Mahajan and Earp, 2003). Subsequent Mer activation induced both Vav1 tyrosine phosphorylation and release of Vav1 from Mer. GDP/GTP exchange on Rac1 and cdc42 followed. These small G proteins are commonly recognized as regulators of the actin cytoskeleton. The initial experiments cited earlier were conducted using an EGFR–Mer chimera expressed in 32D cells. Further study, however, demonstrated that Gas6 stimulation of endogenous Mer in human macrophages also results in Vav1 release and subsequent Rac1 and cdc42 GTP loading (
Mahajan and Earp, 2003). These data suggest a potential mechanism whereby activation of Mer may induce spatially focused regulation of the actin cytoskeleton, thus providing a model whereby Mer may mediate changes in cellular morphology necessary for phagocytosis of apoptotic cells bound at specific sites on the macrophage surface. Interestingly, the site of Vav1 interaction was mapped to amino acids 697–754 of Mer. This region contains the three putative Mer autophosphorylation sites (see Section II.D.1). As tyrosine phosphorylation of Vav1 was not sufficient for release from Mer, it is enticing to speculate that another SH2 domain-containing protein, perhaps with higher affinity for phosphorylated Mer, is required to release Vav1 and initiate cytoskeletal rearrangement. However, to our knowledge no other proteins have been suggested to interact with Mer in this region.
Another study suggests that Mer regulates the actin cytoskeleton via PLCγ2 and Src. Upon exposure of macrophages to apoptotic cells, PLCγ2 associates with Mer and becomes phosphorylated (
Todt et al., 2004). PLC can activate classical protein kinase Cs (PKCs) such as PKC βII, which is required for PS receptor-dependent phagocytosis in macrophages (
Todt et al., 2002). In addition, the Gas6–Mer system may also cooperate with the soluble bridging molecule milk fat globule-EGF factor 8 protein (MFG-E8) and its receptor αvβ5 integrin to stimulate the lamellipodia formation necessary for phagocytic engulfment of apoptotic cells. Studies utilizing constitutively active Mer chimera and kinase dead mutant Mer demonstrated that Mer stimulates Src-mediated phosphorylation of FAK and p130
CAS/CrkII/Dock180 complex activation of Rac1 in an αvβ5 integrin-dependent manner (
Wu et al., 2005). This pathway may also involve PLCγ2 as FAK association with αvβ5 integrin is dependent on PKC (
Lewis et al., 1996a).
Mer activation has also been linked to cell survival via atypical signaling pathways. Gas6 stimulation of a human prostate adenocarcinoma cell line resulted in phosphorylation of a 120-kDa protein that was identified as Cdc42-associated kinase (Ack1) by mass spectrometry (
Mahajan et al., 2005). Constitutive association of Mer and Ack1 could be detected by coimmunoprecipitation of the endogenous proteins. Experiments with constitutively active and kinase dead mutant constructs of Ack1 demonstrated that Ack1 is not a direct Mer substrate, but that Ack1 autophosphorylation (and presumably activation) is facilitated by ligand activation of cell surface Mer. Continued Ack1 kinase activity required the chaperone activity of heat shock protein 90β (Hsp90β). Additional mass spectrometry sequencing of constitutively active Ack immunoprecipitates identified the tumor suppressor Wwox as an Ack1-interacting protein. Further investigation suggests that Ack1 induces phosphorylation, ubiquitination, and degradation of Wwox. Downregulation of this proapoptotic tumor suppressor may be one mechanism by which Ack1 and perhaps Mer relay survival signals in cancer cells. Since the physiologic function of the high levels of Mer expressed in normal prostate is not known, it is difficult to assess the normal role of the Mer–Ack axis.
2. Axl SIGNALING Gas6/Axl signaling promotes the growth and survival of numerous cell types, including cardiac fibroblasts (
Stenhoff et al., 2004). These effects are likely mediated by Gas6/Axl-induced activation of the MAPK/ERK and PI3K signaling pathways (). Early studies utilized a chimeric EGFR/Axl receptor expressed in a leukemic cell line. These experiments demonstrated that ligand stimulation of the chimeric receptor leads to cell proliferation via activation of Grb2, Ras, Raf1, MEK-1, and ERK1/2 (
Fridell et al., 1996). Interestingly, Grb2 can be activated either by direct binding to tyrosine 821 on Axl or by association with Shc, which is phosphorylated upon ligand stimulation but does not associate with Axl. Later studies confirmed that the Ras/ERK pathway is essential for Gas6-induced mitogenesis of NIH3T3 cells (
Goruppi et al., 1999). Importantly, NIH3T3 cells also express Tyro-3 and therefore this mitogenic pathway may be activated by multiple TAM receptors. Although more than one study has suggested that weak or partial activation of the Ras/ERK pathway contributes to Axl-mediated survival (
Bellosta et al., 1997;
Fridell et al., 1996), more recent data indicate that Ras is dispensable for survival resulting from Gas6 stimulation of native TAM receptors in NIH3T3 cells (
Goruppi et al., 1999). However, the MAPK/ERK pathway may be important for Gas6/ TAM receptor-mediated survival in certain cell types, including GnRH neurons (
Allen et al., 1999).
While the MAPK/ERK pathway typically results in Axl-mediated proliferation, Axl binding to and activation of PI3K has been linked to multiple downstream pathways converging on increased cell survival. One pathway involves classical PI3K stimulation of Akt and S6K (
Goruppi et al., 1997). Gas6 also stimulates phosphorylation of Bad, a target of Akt commonly associated with prosurvival signaling (
Goruppi et al., 1999;
Lee et al., 2002). Other survival pathways downstream of Gas6–Axl signaling via PI3K/Akt include phosphorylation of NFκB, increased expression of antiapoptotic proteins such as Bcl-2 and Bcl-x
L, and inhibition of proapoptotic proteins such as caspase 3 (
Demarchi et al., 2001;
Hasanbasic et al., 2004). Transcriptional activation of Bcl-x
L occurs via the cannonical NFκB activation pathway whereby Akt phosphorylates and activates IKK, leading to phosphorylation and degradation of IκB releasing NFκB from the inhibitory complex (
Demarchi et al., 2001). NFκB then enters the nucleus where it binds to the promoter region of Bcl-x
L. Interestingly, this mechanism of NFκB regulation by Axl/PI3K/Akt differs from Mer activation of PI3K/Akt, which has been shown to inhibit IKK resulting in downregulation of NFκB-dependent transcription of TNFα (explained later). Another Gas6/Axl-induced survival pathway may involve PI3K activation of the small GTPases Rac and Rho as well as the downstream kinases Pak and JNK (
Goruppi et al., 1999). Many of these experiments were conducted in NIH3T3 cells which express both Axl and Tyro-3. However, Gas6 stimulation of fibroblasts from Axl−/− mice did not result in increased cell survival relative to Axl WT cells (
Bellosta et al., 1997). These results suggest that Axl is required for Gas6-mediated survival in some cell types. Additional studies suggest that Gas6/Axl receptor signaling activates PI3K-dependent survival pathways in numerous other cells types, including lens epithelial cells, vascular smooth muscle cells, GnRH neurons, and oligodendrocytes (
Allen et al., 1999;
Melaragno et al., 2004;
Shankar et al., 2003;
Valverde et al., 2004). Further study in oligodendrocytes from WT, Axl−/−, and Tyro-3−/− mice suggest that Axl is required for Gas6–PI3K–Akt-mediated survival (
Shankar et al., 2006).
In addition to the prototypic growth and survival pathways described earlier, Gas6/Axl signaling has also been linked to additional cellular functions such as neuronal cell migration and cytokine production. Studies of GnRH neurons suggest that Axl directs migration of these cells from the olfactory placode to the forebrain via a signaling pathway involving PI3K, Ras, Rac, p38 MAPK, MAPKAP kinase 2, and HSP25, which results in actin reorganization (
Allen et al., 2002;
Nielsen-Preiss et al., 2007). Interestingly, Axl is not expressed in postmigratory GnRH neurons (
Allen et al., 1999). With respect to cytokine production, IFNα-induced upregulation of Axl and Gas6 expression in human macrophages leads to increased Twist expression and reduced TNFα production (
Sharif et al., 2006). Twist is a basic helix loop helix protein that likely inhibits NFκB-mediated transcription of TNFα by binding to the E box region within the TNFα promoter. Given that macrophages also express Tyro-3 and Mer, these receptors may also regulate Twist expression. Consistent with this idea, Protein S (which has not been shown to activate Axl) stimulated Twist expression in the presence of IFNα.
A number of studies have suggested a physical association between Axl and various signaling molecules. For example, coimmunoprecipitation experiments demonstrated association of EGFR/Axl chimera and several coexpressed GST fusion proteins in 293 cells. In the same study, Far–Western analysis of mutant EGFR/Axl receptors as well as competition assays with phosphorylated Axl peptides revealed that tyrosine 821 of Axl mediates binding to PLCγ, p85α and p85β subunits of PI3K, Grb2, Src, and Lck (
Braunger et al., 1997). Axl tyrosine 866 also contributes to PLCγ binding while tyrosine 779 may constitute a nonessential, low affinity site of interaction with p85α and p85β. The interaction of Src and Lck likely involves additional contacts
in vivo as the Axl mutant receptor Y821F effectively coimmunoprecipitated both SFKs from 293 cells. Yeast two-hybrid experiments confirmed the interaction of Axl with PI3K and Grb2 while identifying four novel proteins which potentially interact with Axl: suppressor of cytokine signaling (SOCS)-1, Nck2, Ran-binding protein in microtubule organizing center (RanBPM), and C1-TEN (
Hafizi et al., 2002).
In many cases, such as the Grb2 and PI3K pathways described earlier, the signaling events downstream of these interactions have been subject to intense investigation. Conversely, Src-family kinase activity has been associated with Gas6-mediated mechanisms of proliferation and survival as well as neuronal migration, but the upstream and downstream components of these signaling pathway(s) have not been determined (
Goruppi et al., 1997;
Nielsen-Preiss et al., 2007). Many of the other Axl-interacting proteins have not been studied beyond their association with activated receptor. Nonetheless, there are reasonable hypotheses as to how some of these proteins may be involved in TAM receptor signaling. C1-TEN, for example, contains a tyrosine phosphatase motif. Thus Axl and other TAM receptors may be found in complex with both tyrosine kinases (SFKs) and phosphatases. Overexpression of C1-TEN in 293 cells has been shown to inhibit Akt signaling resulting in reduced cell survival, migration, and proliferation (
Hafizi et al., 2005b). These data are consistent with Axl inactivation mediated by the putative phosphatase C1-TEN. Furthermore, Axl signaling has been associated with attenuation of cytokine production (see Section II.E.1.b), including attenuation of proinflammatory cytokine production following exposure to LPS, a potential role for Axl SOCS-1 signaling as SOCS-1 is implicated in negative regulation of LPS-induced signaling (
Kinjyo et al., 2002;
Nakagawa et al., 2002).
3. Tyro-3 SIGNALING The Tyro-3 receptor is the least studied of the TAM receptors and the signaling pathways downstream of Tyro-3 activation are poorly understood. Nonetheless, a handful of studies provided clues as to the molecules which mediate Tyro-3 signaling (). Coimmunoprecipitation of Tyro-3 transiently expressed in COS cells revealed a potential interaction with a phosphorylated SFK (
Toshima et al., 1995). Because of cross-reactivity of the antibody used, it remains unknown which SFK(s) (Src, Yes, and/or Fyn) interact with Tyro-3. Importantly, all three of these SFKs are highly expressed in tissues of the central nervous system where they are likely to be found colocalized with Tyro-3. Yeast two-hybrid studies identified a number of proteins that potentially interact with Tyro-3, including RanBPM, protein phosphatase 1 (PP1), and the p85 β-subunit of PI3K (
Hafizi et al., 2005a;
Lan et al., 2000). Sequencing of the DNAs encoding the interacting proteins demonstrated that PI3K binds Tyro-3 via one of its SH2 domains and the interaction was confirmed
in vitro and
in vivo by GST pull-down assay and coimmunoprecipitation, respectively. Furthermore, ligand stimulation of an EGFR/Tyro-3 chimera induces phosphorylation of Tyro-3, PI3K, and Akt resulting in a transformed phenotype. A MAPK signaling pathway has also been linked to Tyro-3 activation as phosphorylation of ERK1/2 was increased by Gas6 stimulation of NIH3T3 cells which express endogenous Tyro-3 (
Chen et al., 1997). Phosphorylation of ERK1/2 was also upregulated by Gas6 stimulation of endogenous Tyro-3 in mouse osteoclasts, resulting in bone resorption (
Katagiri et al., 2001). Importantly, phosphorylation of Tyro-3 at specific residues has not been described. Clearly, further investigation is necessary to elucidate the signaling pathways downstream of Tyro-3 activation.
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