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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Semin Cell Dev Biol. Author manuscript; available in PMC Nov 27, 2012.
Published in final edited form as:
PMCID: PMC3507455
NIHMSID: NIHMS324878
14-3-3 Proteins As Signaling Integration Points for Cell Cycle Control and Apoptosis
Alexandra K. Gardinoa and Michael B. Yaffea*
aDepartments of Biology and Biological Engineering, David H. Koch Institute for Integrative Cancer Research, Massachusetts Institute for Technology, Cambridge, MA, 02139, USA
Alexandra K. Gardino: gardino/at/mit.edu; Michael B. Yaffe: myaffe/at/mit.edu
*Corresponding author – Michael B. Yaffe, 77 Massachusetts Ave., Bldg 76–353, Cambridge, MA 021329, Ph: 617-452-2103, Fax: 617-452-4978, myaffe/at/mit.edu
14-3-3 proteins play critical roles in the regulation of cell fate through phospho-dependent binding to a large number of intracellular proteins that are targeted by various classes of protein kinases. 14-3-3 proteins play particularly important roles in coordinating progression of cells through the cell cycle, regulating their response to DNA damage, and influencing life-death decisions following internal injury or external cytokine-mediated cues. This review focuses on 14-3-3-dependent pathways that control cell cycle arrest and recovery, and the influence of 14-3-3 on the apoptotic machinery at multiple levels of regulation. Recognition of 14-3-3 proteins as signaling integrators that connect protein kinase signaling pathways to resulting cellular phenotypes, and their exquisite control through feedforward and feedback loops, identifies new drug targets for human disease, and highlights the emerging importance of using systems-based approaches to understand signal transduction events at the network biology level.
Keywords: 14-3-3, DNA damage, cell cycle checkpoint, apoptosis, signal transduction, mitosis
14-3-3 proteins bind to their phospho-Serine and phospho-Threonine-containing ligands to regulate a wide range of cellular phenomena. Many 14-3-3-binding proteins contain sequences that match one of two general consensus motifs: RSx[pS/pT]xP and Rxxx[pS/pT]xP, which are recognized by all 14-3-3 isotypes [1]. The Pro in the pS/pT+2 position, while preferred, is not absolutely required [13], particularly if the motif is found at the C–terminus [4]. Two of the most well established roles for 14-3-3 proteins are in the control of cell cycle progression and the regulation of apoptosis, and these remain areas of active ongoing research. In this article we briefly summarize the current status of research in these areas, focusing on a selection of the most recent papers that illustrate new concepts or expand on previously established models.
2.1 The G2/M Checkpoint
Early studies implicated 14-3-3 as a critical integration point for many of the protein kinases and phosphatases that control the transition from G2 into M phase (Figure 1A). Normal mitotic entry from G2 is triggered by a rapidly enhanced activation of Cdk1/cyclin B (c.f. [5] and references therein). Cdk1 is activated by constitutive phosphorylation of a threonine residue (T161) within the “T-loop” by CAK (Cdc2-activating kinase) but maintained in an inhibited state by phosphorylation of a tyrosine residue (Y15) within the ATP-binding “P-loop” by the tyrosine kinase Wee-1 [5]. In addition, higher eukaryotes also have a membrane-bound kinase, Myt1, that phosphorylates both Y15 and the preceding threonine, T14, to ensure Cdk1 inhibition [5]. Removal of these inhibitory P-loop phosphorylations on Cdk1 to promote entry into mitosis from G2 is achieved by the Cdc25 family of dual-specificity protein phosphatases [6, 7]. Mammalian cells express three different Cdc25 proteins (A, B, and C), all of which have been implicated in G2/M progression. At the G2/M boundary, Cdc25B appears to be the trigger phosphatase that initially activates Cdk1/cyclin B, which in turn phosphorylates Cdc25C to create a binding site for a second mitotic kinase, Plk1 [8, 9], which in turn further phosphorylates Cdc25C [10]. This cascade of sequential phosphorylation events results in an autoactivation loop that subsequently drives entry into mitosis [7, 1113]. While Cdc25A is predominantly recognized for its involvement in earlier phases of cell cycle progression (see below), it has been shown to also play a role at the G2/M transition [14, 15] and is the only Cdc25 family member that has an embryonic lethal phenotype in knockout mice [16]. The relative importance of Cdc25A in G2/M progression in normal cells probably depends on the status of Cdc25B and C, which may be influenced by the prior history of the cells.
Figure 1
Figure 1
The role of 14-3-3 in G2/M checkpoint activation, maintenance, and release
Like Cdk1/cyclin B, the Cdc25 phosphatases are themselves regulated through activating and inhibitory phosphorylations. For Cdc25B and C, activating phosphorylations are generated by Cdk1 and Plk1 [10, 1719] through the positive feedback loop described above, although data in Xenopus extracts suggests that MAPKs (particularly Erk1/2) may play a major role [20]. Whether this is also true in mammalian cells is less clear. Inhibitory phosphorylation events during G2 probably arise from the detection of ongoing endogenous DNA damage from normal replication events during the preceding S-phase, or exogenous DNA damage that the cell has encountered during late S and G2, and are mediated by three DNA damage-responsive kinases, Chk1, Chk2, and MK2 [21, 22] (see below). The main mechanism by which these inhibitory phosphorylations block Cdc25B and C function is through the phosphorylation-dependent binding to 14-3-3. Phosphorylation of a single binding site on Cdc25B (S323 in human), or Cdc25C (S216 in human) [23, 24] is necessary and sufficient for 14-3-3 binding, although there are some indications that additional binding sites may also contribute to their regulation [25, 26] in ways that are somewhat mechanistically unclear.
There are likely to be multiple mechanisms through which 14-3-3 proteins inhibit Cdc25B/C function. First, 14-3-3 binding to Xenopus Cdc25C causes a direct two-fold reduction in its phosphatase activity when assayed in vitro using Myt1-phosphorylated 32P-labelled Cdk1 as a substrate [23]. A similar direct inhibition of Cdc25B catalytic activity is likely to occur based on pull-down data indicating that 14-3-3 binding to Cdc25B blocks Cdc25B binding to CyclinB/Cdk1 [25]. Second, 14-3-3 binding leads to the cytoplasmic sequestration of Cdc25B/C [2730], likely blocking access to its nuclear substrate Cdk1/cyclin B [31], required for mitotic entry. The mechanism by which 14-3-3 proteins regulate the subcellular localization of their bound targets involves the selective exposure or shielding of specific sequence elements. This has been most clearly experimentally elucidated for Cdc25C, although the identical mechanism almost certainly functions for Cdc25B. In between the N-terminal regulatory domain and the C-terminal catalytic domain of Cdc25C lies an intrinsic nuclear localization sequence (NLS) and a nuclear export sequence (NES) [28, 29]. These subcellular localization signals allow Cdc25C to shuttle between the nuclear and cytoplasmic compartments. The major 14-3-3 binding site in human and Xenopus Cdc25C (S216 and S287, respectively) is located right next to the NLS [28, 32], and in Xenopus, 14-3-3 binding to Cdc25C has been shown to block the binding of Cdc25C to the importin-α nuclear import receptor [29]. Thus, 14-3-3-binding causes defective nuclear import in the face of unaffected nuclear export [33], relocalizing most of Cdc25C to the cytoplasm.
Although all of the 14-3-3 isoforms except 14-3-3 σ can bind to Cdc25 isoforms in vitro [34, 35], it appears, on the basis of co-IP experiments using tagged overexpressed proteins that the ε and γ isoforms of 14-3-3 bind strongest to Cdc25C in cells [34], while yeast 2-hybrid experiments suggest that 14-3-3 β, η, τ, and ζ bind to Cdc25B [36]. Whether this marked 14-3-3 isoform exclusivity for the mitotic ‘initiator’ Cdc25B phosphatase versus the mitotic ‘sustainer’ Cdc25C phosphatase is biologically important remains unknown.
In addition to inhibiting the Cdc25 phosphatases that activate Cdk1, 14-3-3 proteins also bind to Wee1, the protein kinase that inhibits Cdk1. In this case, however, 14-3-3 binding serves to distribute Wee1 uniformly throughout the nucleus, and appears to increase the catalytic activity of both Xenopus and human Wee1 at least 5-fold [37, 38]. These two 14-3-3 dependent mechanisms of Wee1 activation work synergistically to enhance the phosphorylation and inactivation of nuclear Cdk1, and together with the 14-3-3-mediated nuclear exclusion and catalytic reduction of Cdc25B and C activity, acts as a ‘belt and suspenders’ mechanism to initiate a strong G2 checkpoint arrest (Figure 1A). Interference with either 14-3-3:Cdc25 or 14-3-3:Wee1 binding results in inappropriate mitotic entry [23, 24, 34, 38].
In contrast to the selectivity of Cdc25B/C for binding to 14-3-3 isoforms other than σ, it appears that Wee1 binds directly to14-3-3σ [38, 39], likely in addition to other 14-3-3 proteins. 14-3-3 σ, a unique isoform that appears to have first arisen in birds, appears to play an especially important role in G2/M checkpoint maintenance. Hermeking, Chan, Vogelstein and colleagues made the important finding that 14-3-3σ was potently up-regulated by p53 in colorectal carcinoma cells after ionizing radiation, where it functioned in parallel with p21 to ensure that DNA damaged cells maintained a G2 arrested state [4043]. Cells lacking 14-3-3 σ could initiate, but were unable to maintain a G2 arrest after irradiation [40], while cells lacking both 14-3-3σ and p21 showed both accelerated cell death and enhanced sensitivity to doxorubicin-induced DNA damage and 5-fluorouracil treatment [41]. Possible targets of 14-3-3σ may be Cyclin/Cdk1 and Cdk2 complexes themselves [40, 44], although neither of these targets was detected in an unbiased proteomic screen for 14-3-3σ ligands [39]. Importantly, 14-3-3σ likely functions, at least in part, through enhancing the activity of p53 as part of a coherent feed-forward loop ([45]; see below), as well as through enhancing the stability of the Cdk inhibitor protein p27Kip1 by blocking its AKT-mediated ubiquitination and degradation [46]. 14-3-3 regulation of p27Kip1 is more complex, however, since 14-3-3 bound p27Kip1 becomes re-localized in the cytoplasm, where it is presumably nonfunctional in suppressing Cdk activity [47]. This potentially allows tumor cells with high levels of PI 3-kinase/AKT activity to bypass G1 arrest (see below), while maintaining a sufficient total concentration of intracellular p27 to arrest the cell if DNA damage occurs subsequently.
14-3-3-binding sites on Cdc25, Wee-1, and a variety of other checkpoint modulators are generated through the action of DNA damage-responsive basophilic kinases, particularly the kinases Chk1, Chk2, and MK2 [22, 26, 4852]. It is thought that Chk1 predominantly responds to single strand breaks, lesions created during replication fork collapse, and bulky DNA adducts induced by UV irradiation, while Chk2 is thought to be the major kinase responding to double strand breaks, although these kinases appear to participate to varying extents in all three DNA damage responses [53]. Intriguingly, in fission yeast Chk1 itself binds to 14-3-3, and this binding appears to be important for phosphorylating and activating Chk1, and targeting it to the nucleus after DNA damage [54]. Recent data have now revealed that a third DNA damage kinase, the p38MAPK-activated kinase MK2, is critical for prolonged G2 arrest in tumor cells that are defective in p53 signaling [48, 49, 55]. The major direct targets of MK2 appear to be the Cdc25B and C phosphatases, which MK2 directly phosphorylates on their 14-3-3-binding motifs [48, 49, 56], and whose persistent 14-3-3-binding and nuclear exclusion requires persistent MK2 kinase activity. Importantly, the mechanism responsible for prolonged MK2 activity after DNA damage involves another example of a coherent feed-forward loop, in this case involving MK2-mediated phosphorylation of RNA binding proteins, which stabilize the production of the cell cycle regulator Gadd45α, which in turn binds to p38MAPK to maintain prolonged MK2 activity [55]. The fact that this pathway is critical for preventing inappropriate checkpoint release and mitotic catastrophe in p53-defective cells suggests that it may be a useful therapeutic target to preferentially kill p53-defective tumor cells [49].
2.2 G2/M Checkpoint Release
Re-entry into the mitosis from G2 upon completion of DNA repair appears to involve Cdk1 and Plk1-dependent inactivation of the four upstream kinase pathways responsible for 14-3-3-dependent inactivation of CyclinB/Cdk1 itself - the ATM-Chk2 pathway, the ATR-Chk1 pathway, the p38MAPK-MK2 pathway (all of which target Cdc25 for binding to 14-3-3), and the Wee1 pathway (Figure 1B). Inactivation of the ATR-Chk1 pathway occurs through Plk1 mediated phosphorylation of Claspin, an adaptor protein that facilitates ATR-dependent Chk1 activation. Plk1 phosphorylation releases Claspin from damaged chromatin [57], and targets it for ubiquitin-mediated degradation [5860]. Inactivation of the ATM-Chk2 pathway involves the Cdk1 and Plk1 dependent inactivation of the adaptor protein 53BP1 (that facilitates sustained Chk2 activation by ATM as well as direct inactivation of Chk2 itself in both mammalian cells and yeast [61, 62]. Inactivation of the p38MAPK-MK2 pathway is less well understood, but likely involves the dephosphorylation and inactivation of p38MAPK by the phosphatase Wip1 [63, 64], which also dephosphorylates the DNA damage marker histone γH2AX along with other ATM/ATR and DNA-PK targets [65, 66]. Inactivation of the Wee1 pathway involves the Cdk1 and Plk1 phosphorylation of Wee1, which targets it for ubiquitin-mediated proteolysis [67, 68]. These mechanisms function together to block upstream signals that target Cdc25B/C and Wee1 for 14-3-3 binding, thus shutting off the G2/M checkpoint regulated by 14-3-3-mediated Cdc25 inactivation and Wee1 activation, and allowing cells to re-enter the cell cycle.
The process of checkpoint release is still under active investigation. This is particularly important since it appears that G2/M checkpoint bypass may be an early event that triggers the progression from carcinoma in situ to full blown cancer [69]. In addition, the G2/M checkpoint appears to be an error-prone checkpoint, since both normal cells and tumor cells are released from G2 into M-phase despite the presence of a finite number of unrepaired DNA breaks [70], resulting in genomic instability. Taken together, these findings suggest that modulation of 14-3-3 function to enhance G2 arrest may be a viable mechanism to limit tumor cell proliferation and progression.
An important observation made by Kornbluth and colleagues was that 14-3-3 binding to Xenopus Cdc25 phosphorylated on S287 protected this mitotic phosphatase from premature dephosphorylation and activation [71]. In addition, protein phosphatase 1 was required to dephosphorylate Xenopus Cdc25 at S287 (the 14-3-3 binding site corresponding to S216 in human Cdc25C) [72]. Interestingly, the removal of 14-3-3 from Cdc25 preceded S287 dephosphorylation, implying that a phosphatase-independent pathway is responsible for the initial 14-3-3 removal from Cdc25, followed only later by dephosphorylation and loss of the 14-3-3 binding site on Cdc25. These authors postulated that the accumulation of phosphorylated intermediate filament proteins during mitosis may compete with Cdc25C for 14-3-3 binding, with a shift in the equilibrium away from 14-3-3:Cdc25C as the abundance of phosphorylated forms of intermediate filament proteins accumulate in mitosis. These findings agree well with a prior report from Tzivion et al. [73] that treatment of COS-7 cells with the phosphatase inhibitor Calyculin-A resulted in marked accumulation of 14-3-3:phosphovimentin complexes, displacing many of the other 14-3-3-bound ligands, and a series of strong papers from Omary and colleagues that showed binding of phosphorylated epithelial keratins K8/18 to 14-3-3 is increased during mitosis, where it may contribute to mitotic progression, at least in hepatocytes during liver regeneration [74, 75]. These studies raise the possibility that intermediate filament phosphorylation during mitosis may function to facilitate the release of specific ligands from 14-3-3 whose function is important during cell division by competitive binding, although additional mechanisms are likely to be operative during the initial stages of checkpoint release when mitotic kinase activity may not be high enough to produce an abundance of phosphokeratins and phosphovimentin. Importantly, the 14-3-3 strongest binding motif on Cdc25B and C corresponds to the sequence RSPSMP, where the bolded highlighted Ser is the critical 14-3-3-binding phosphorylation site. Once cells have entered mitosis, Cdk activity results in phosphorylation of the preceding Ser residue (RSPSMP), which renders these Cdc25 molecules incapable of binding to 14-3-3 proteins [76, 77], and thereby ensuring that mitotic progression is strictly unidirectional.
The fact that Cdk1 is intimately involved in nearly all of the G2 checkpoint release mechanisms discussed above shows that, like checkpoint establishment, checkpoint release also involves a feed-forward loop when viewed from a systems biology perspective.
2.3 Mitotic roles of 14-3-3 proteins
In addition to from blocking mitotic entry, 14-3-3 proteins appear to contribute substantially to enhancing progression once cells have entered mitosis through stimulating the process of cytokinesis. Parker and colleagues showed that 14-3-3 binding to protein kinase C epsilon (PKCε) during mitosis activates it in a lipid-independent manner, locking PKCε in an open active conformation near the actomyosin ring. The resulting PKCε activity is crucial during telophase to down-regulate RhoA activity at the midbody, and allow cells to complete abscission. Loss of the 14-3-3 binding sites on PKCε, loss of PKCε expression, expression of a catalytically dead form of PKCε or expression of a dominant negative form of 14-3-3 results in cytokinesis delay or failure and the production of multinucleated cells [78]. The cancer relevance of this phenomenon is unclear, since PKCε is generally thought to be a potent oncogene and its expression is upregulated in certain tumors and tumor-derived cell lines [79].
Our laboratory showed that 14-3-3σ plays an important role in cytokinesis by facilitating the mitotic switch from cap-dependent to cap-independent translation. Notably, 14-3-3σ is a tumor suppressor protein that is frequently lost in tumors of epithelial origin [80], inhibiting the ability of such cells to efficiently complete cytokinesis. This ultimately leads to the production of tetraploid cells that may serve as early precursors of cancer [81]. In normal epithelial cells, completion of normal cytokinesis occurs as a consequence of binding of the translation initiation factor eIF4B to 14-3-3σ during mitosis, impairing eIF4B function, and culminating in a switch from cap-dependent to cap-independent (IRES mediated) mRNA translation during mitosis. During mitosis, a variant form of Cdk11, termed p58-PITSLRE, is then translated in a cap-independent manner [82]. This cap-independent form is specifically required to complete the final stages of cell division [83, 84]. In the absence of 14-3-3σ, only the cap-dependent interphase form of Cdk11, p110-PITSLRE is produced, rendering 14-3-3σ-defective cells unable to sever the cell-cell attachment at the terminal midbody stage (abscission) of cell division [81]. These findings are particularly significant given the finding that the expression of 14-3-3σ is suppressed in normal breast tissue adjacent to the sites tumors arose (see below). In this case, loss of 14-3-3 probably contributes to ‘field cancerization’ by facilitating the development of pre-cancerous tetraploid lesions that subsequently undergo genomic instability during the tumorigenic process.
2.4 G1/S Checkpoint control
The remaining Cdc25 phosphatase that has not been discussed at length, Cdc25A, is best appreciated for its role in regulating the G1/S transition by controlling CyclinE and CyclinA/Cdk2 activity. Cdc25A has been generally accepted as being primarily regulated in response to DNA damage by Chk1-dependent phosphorylation and β-TrCP-mediated ubiquitination and proteosomal degradation [8587]. Other kinases however, including CK1, GSK-3β, and Plk3 have also been implicated in this process [8890]. In addition to its role in the G1/S transition, Cdc25A also appears to play a role in facilitating the G2/M transition by activating CyclinB/Cdk1 [91]. Piwnica-Worms and colleagues showed that Cdc25A has two 14-3-3-binding sites, and that mutants of Cdc25A that are defective in 14-3-3 binding have an enhanced ability to activate Cyclin B/Cdk1 [92]. One of the two 14-3-3-binding sites sits near the C-terminal region of Cdc25A, immediately adjacent to a putative Cyclin B docking site, suggesting that 14-3-3 binding disrupts the interaction of Cdc25A with its substrate. These data suggest that 14-3-3-binding to Cdc25A may be important for short-term control of Cyclin/Cdk complexes involved in both G1/S and G2/M progression, while ubiquitin-mediated destruction of Cdc25A may be a mechanism of long term inhibition of Cyclin/Cdk activation in response to larger amounts of genotoxic stress. Unfortunately, the structural basis for 14-3-3-mediated effects on Cdc25 phosphatases remains unclear, largely because the 14-3-3-binding sites are primarily localized in relatively unstructured regions of Cdc25 between the N-terminal regulatory domain and the C-terminal phosphates domain.
An additional role for 14-3-3 proteins in the G1/S transition has emerged from a recent finding that 14-3-3τ seems to facilitate the MDM2-dependent but ubiquitin-independent degradation of the Cdk inhibitor p21 to facilitate S-phase entry [93] (Figure 1B). Thus 14-3-3 proteins appear to have roles in both suppressing and enhancing the G1/S transition, likely as a function of which client proteins are bound, depending on the relative levels of activity of different 14-3-3 motif-generating kinases.
3.1 14-3-3 binding to Bcl-2 family members
An important role for 14-3-3 proteins in apoptosis emerged from a series of studies examining the interaction of 14-3-3 proteins with BH3 domain-containing proteins, particularly BAD and BAX. Early work identified BAD as a BH3 domain-only protein that was the 14-3-3 binding target of an IL-3 dependent survival mechanism in the hematopoetic cell line FL5.12 [94]. BAD binding to 14-3-3 proteins was found to involve the AKT-dependent phosphorylation of two sites on BAD [95], although additional kinases also may be involved [96]. The mechanism by which 14-3-3 facilitates the inactivation of BAD involves both transient sequestration of BAD to prevent it from binding to and inactivating the pro-survival protein Bcl-XL, along with holding BAD in a conformation that enhances the phosphorylation of an additional residue in its BH3 domain to result in a more permanent state of inactivation [97].
14-3-3 proteins were later shown to bind to, and functionally inactivate the critical pro-apoptotic effector BAX, in a phospho-independent manner [98]. The binding involves sequences at both the N- and C-terminus of BAX, and precludes BAX from transiting to the mitochondria, where it would otherwise interact with BAK to induce permeabilization of the mitochondrial outer membrane and trigger the release of cytochrome C and SMAC/DIABLO to activate caspases to kill the cell [99]. More recent work has now shown that this 14-3-3:BAX interaction is controlled by the stress MAP kinase JNK, which directly phosphorylates 14-3-3 on Ser-184 (14-3-3ζ notation) near the end of helix α7 to block BAX binding [100].
3.2 14-3-3-mediated control of signaling components and transcription factors involved in the apoptotic response
JNK, which triggers the release of BAX from 14-3-3, as well as p38MAPK, are activated, in part by the upstream MAP kinase kinase kinase ASK1 (apoptosis stimulating kinase 1) [101], which is itself negatively regulated by 14-3-3 binding [102]. ASK1 is activated downstream of the TNFα receptor and FAS. TNFα receptor-mediated apoptosis is negatively regulated by the zinc finger protein A20 [103], and just like ASK1, A20 is also a client protein whose function is repressed by binding to 14-3-3 [104]. Signaling from TNF receptor family members to BAX involves formation of a complex between the receptors and the proteins MOAP-1 and RASSF1A, a process that is inhibited by 14-3-3 binding to RASSF1A [105, 106]. Finally, in response to growth factor deprivation, and potentially other pro-apoptotic stimuli, members of the FOXO family of transcription factors drive the expression of a pro-apoptotic gene expression program [107]. Not surprisingly, then, a number of FOXO transcription factors have been shown to be under the control of 14-3-3 proteins, where their binding is regulated by AKT-dependent phosphorylation [108111]. Intriguingly, both AKT-dependent phosphorylation of FOXO proteins and 14-3-3 binding appears to occur in the nucleus [110], resulting in both an inability of FOXO proteins to bind to DNA [112114] and facilitating their export out of the nucleus through a combination of masking NLS sequences and exposing NES sequences [110]. Thus, 14-3-3 proteins control the induction of apoptosis at multiple levels, from effector BH3 domain-containing proteins to upstream activators and transcription factors. In good agreement with this, Masters and Fu showed that blocking of the 14-3-3 binding cleft with a peptide inhibitor (see below) enhanced the apoptotic response of multiple cell lines in culture [115] while Muslin and colleagues showed that transgenic expression of a dominant negative form of 14-3-3 could increase the apoptotic response of cardiomyocytes in the hearts of mice to cardiac pressure overload. The observation that direct phosphorylation of 14-3-3 by JNK disrupted binding of 14-3-3 to BAX and Forkhead transcription factors (and likely other 14-3-3 targets such as A20 and ASK1) [116], again illustrates the importance of feedforward and feedback loops in 14-3-3 mediated control of cell fate, since once a threshold of JNK activity has been exceeded, one would expect additional release of ASK1 and RASSF1A to enhance further JNK activity, BAX release and activation, and transcription of FOXO-driven apoptotic genes, while release of A20 would limit subsequent upstream input into this pathway from the TNF receptor, and allow additional pro- and anti-apoptotic pathways to be integrated into the overall response.
3.3 Therapeutic targeting 14-3-3:liagnd binding by drugs
The idea that global interference of 14-3-3 proteins with their target ligands could be used therapeutically was first explored by Fu and colleagues through the use of a non-phosphorylated 20 amino acid binding peptide, R18, that was isolated from a phage display library by virtue of its strong binding to 14-3-3 [117]. Difopein, a high affinity 14-3-3 antagonist, was then constructed by linking two R18 peptides together with a linker, thereby blocking both binding sites within a functional 14-3-3 dimer. Treatment of several cancer cell lines with Difopein resulted in enhanced apoptosis both alone, and when administered in combination with cisplatin [115], although the death was additive and not synergistic. An alternative approach taken by our laboratory involved the use of photo-activatable caged phosphopeptides to allow timed inhibition of 14-3-3 proteins, and this was shown to enhance UV-induced cell death [118]. Such an approach might have utility in various form of photodynamic therapy, such as that used for treatment of mycosis fungoides. However, a general caveat of both of these approaches is their non-isoform specificity, and the potential for significant toxicity in normal non-cancerous tissue. The recent surge of interest in RNAi-mediated therapeutics might be an ideal strategy for targeting specific members of protein families, such as particular 14-3-3 isoforms whose up-regulation may directly contribute to the oncogenic behavior of tumor cells and their resistance to anti-cancer agents.
Recently, the first non-peptidic 14-3-3 small molecule inhibitor was reported [119], and shown to enhance the apoptotic cell death of chronic myelogenous leukemia cells [120]. An alternative approach to killing cancer cells might be to block their proliferation by stabilizing the interaction of cell cycle regulators such as Wee1 and Cdc25 with 14-3-3 proteins. Interestingly, two small plant toxins, fusicoccin A and cotylenin A, bind to 14-3-3 and stabilize the interaction between 14-3-3 proteins and a subset of client proteins [121, 122], raising the possibility that this approach might be adapted for use in mammalian cells [123].
By functioning as master regulators of protein kinase signaling cascades and effector proteins involved in cell cycle control and apoptosis, 14-3-3 proteins function as signal transduction ‘integrators’ that connect signals to phenotypes. The diversity of 14-3-3 isoforms and their interactions reveals an abundance of potential drug targets that could be used to therapeutically treat diseases caused by aberrant cell proliferation such as cancer, as well as those diseases resulting from excessive inflammation and cell death, including degenerative diseases associated with aging. Examination of the signaling pathways in which 14-3-3 proteins participate reveals the striking importance of feedforward and feedback loops in the fine control of 14-3-3 regulated signaling networks.
Highlights
  • Checkpoint kinases use 14-3-3 proteins to establish and maintain a G2 cell cycle checkpoint by targeting Cdc25 and Wee1.
  • Cdk1 and Plk1 inactivate checkpoint kinase pathways targeting Cdc25B and Wee1 binding to 14-3-3 to drive mitotic entry.
  • 14-3-3 proteins prevent apoptotis by binding to BH3-containing proteins, FOXO transcription factors and apoptotic kinases.
  • JNK directly phosphorylates 14-3-3 proteins to block their anti-apoptotic effects and drive cell death.
Figure 2
Figure 2
Roles for 14-3-3 proteins at multiple points in the apoptotic signaling network
Acknowledgements
We apologize to many of our colleagues whose important work was not cited due to space limitations in the text. We gratefully acknowledge past and present members of the Yaffe laboratory for the insights and assistance, notably Drs. H. Christian Reinhardt and Erik W. Wilker. This work was supported by a grant from the American Cancer Society #PF-06-286-01-CCG to A.K.G., and NIH grants GM-60594, GM-68762, ES-015339, and CA-112967 to M.B.Y.
Footnotes
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1. Yaffe MB, Rittinger K, Volinia S, Caron PR, Aitken A, Leffers H, et al. The structural basis for 14-3-3:phosphopeptide binding specificity. Cell. 1997;91:961–71. [PubMed]
2. Johnson C, Crowther S, Stafford MJ, Campbell DG, Toth R, MacKintosh C. Bioinformatic and experimental survey of 14-3-3-binding sites. Biochem J. 427:69–78. [PMC free article] [PubMed]
3. Rittinger K, Budman J, Xu J, Volinia S, Cantley LC, Smerdon SJ, et al. Structural analysis of 14-3-3 phosphopeptide complexes identifies a dual role for the nuclear export signal of 14-3-3 in ligand binding. Mol Cell. 1999;4:153–66. [PubMed]
4. Coblitz B, Shikano S, Wu M, Gabelli SB, Cockrell LM, Spieker M, et al. C-terminal recognition by 14-3-3 proteins for surface expression of membrane receptors. J Biol Chem. 2005;280:36263–72. [PubMed]
5. Pines J. Four-dimensional control of the cell cycle. Nat Cell Biol. 1999;1:E73–9. [PubMed]
6. Boutros R, Lobjois V, Ducommun B. CDC25 phosphatases in cancer cells: key players? Good targets? Nat Rev Cancer. 2007;7:495–507. [PubMed]
7. Donzelli M, Draetta GF. Regulating mammalian checkpoints through Cdc25 inactivation. EMBO Rep. 2003;4:671–7. [PubMed]
8. Elia AE, Rellos P, Haire LF, Chao JW, Ivins FJ, Hoepker K, et al. The molecular basis for phosphodependent substrate targeting and regulation of Plks by the Polo-box domain. Cell. 2003;115:83–95. [PubMed]
9. Karaiskou A, Cayla X, Haccard O, Jessus C, Ozon R. MPF amplification in Xenopus oocyte extracts depends on a two-step activation of cdc25 phosphatase. Exp Cell Res. 1998;244:491–500. [PubMed]
10. Toyoshima-Morimoto F, Taniguchi E, Nishida E. Plk1 promotes nuclear translocation of human Cdc25C during prophase. EMBO Rep. 2002;3:341–8. [PubMed]
11. Gabrielli BG, De Souza CP, Tonks ID, Clark JM, Hayward NK, Ellem KA. Cytoplasmic accumulation of cdc25B phosphatase in mitosis triggers centrosomal microtubule nucleation in HeLa cells. J Cell Sci. 1996;109(Pt 5):1081–93. [PubMed]
12. Karlsson C, Katich S, Hagting A, Hoffmann I, Pines J. Cdc25B and Cdc25C differ markedly in their properties as initiators of mitosis. J Cell Biol. 1999;146:573–84. [PMC free article] [PubMed]
13. Nishijima H, Nishitani H, Seki T, Nishimoto T. A dual-specificity phosphatase Cdc25B is an unstable protein and triggers p34(cdc2)/cyclin B activation in hamster BHK21 cells arrested with hydroxyurea. J Cell Biol. 1997;138:1105–16. [PMC free article] [PubMed]
14. Donzelli M, Squatrito M, Ganoth D, Hershko A, Pagano M, Draetta GF. Dual mode of degradation of Cdc25 A phosphatase. EMBO J. 2002;21:4875–84. [PubMed]
15. Molinari M, Mercurio C, Dominguez J, Goubin F, Draetta GF. Human Cdc25 A inactivation in response to S phase inhibition and its role in preventing premature mitosis. EMBO Rep. 2000;1:71–9. [PubMed]
16. Lee G, White LS, Hurov KE, Stappenbeck TS, Piwnica-Worms H. Response of small intestinal epithelial cells to acute disruption of cell division through CDC25 deletion. Proc Natl Acad Sci U S A. 2009;106:4701–6. [PubMed]
17. Elia AE, Cantley LC, Yaffe MB. Proteomic screen finds pSer/pThr-binding domain localizing Plk1 to mitotic substrates. Science. 2003;299:1228–31. [PubMed]
18. Hoffmann I, Clarke PR, Marcote MJ, Karsenti E, Draetta G. Phosphorylation and activation of human cdc25-C by cdc2- -cyclin B and its involvement in the self- amplification of MPF at mitosis. EMBO J. 1993;12:53–63. [PubMed]
19. Izumi T, Maller JL. Elimination of cdc2 phosphorylation sites in the cdc25 phosphatase blocks initiation of M–phase. Mol Biol Cell. 1993;4:1337–50. [PMC free article] [PubMed]
20. Wang R, He G, Nelman-Gonzalez M, Ashorn CL, Gallick GE, Stukenberg PT, et al. Regulation of Cdc25C by ERK-MAP kinases during the G2/M transition. Cell. 2007;128:1119–32. [PubMed]
21. Bartek J, Lukas J. Chk1 and Chk2 kinases in checkpoint control and cancer. Cancer Cell. 2003;3:421–9. [PubMed]
22. Reinhardt HC, Yaffe MB. Kinases that control the cell cycle in response to DNA damage: Chk1, Chk2, and MK2. Curr Opin Cell Biol. 2009;21:245–55. [PMC free article] [PubMed]
23. Kumagai A, Yakowec PS, Dunphy WG. 14-3-3 proteins act as negative regulators of the mitotic inducer Cdc25 in Xenopus egg extracts. Mol Biol Cell. 1998;9:345–54. [PMC free article] [PubMed]
24. Peng CY, Graves PR, Thoma RS, Wu Z, Shaw AS, Piwnica-Worms H. Mitotic and G2 checkpoint control: regulation of 14-3-3 protein binding by phosphorylation of Cdc25C on serine-216. Science. 1997;277:1501–5. [PubMed]
25. Giles N, Forrest A, Gabrielli B. 14-3-3 acts as an intramolecular bridge to regulate cdc25B localization and activity. J Biol Chem. 2003;278:28580–7. [PubMed]
26. Zeng Y, Forbes KC, Wu Z, Moreno S, Piwnica-Worms H, Enoch T. Replication checkpoint requires phosphorylation of the phosphatase Cdc25 by Cds1 or Chk1. Nature. 1998;395:507–10. [PubMed]
27. Davezac N, Baldin V, Gabrielli B, Forrest A, Theis-Febvre N, Yashida M, et al. Regulation of CDC25B phosphatases subcellular localization. Oncogene. 2000;19:2179–85. [PubMed]
28. Graves PR, Lovly CM, Uy GL, Piwnica-Worms H. Localization of human Cdc25C is regulated both by nuclear export and 14-3-3 protein binding. Oncogene. 2001;20:1839–51. [PubMed]
29. Kumagai A, Dunphy WG. Binding of 14-3-3 proteins and nuclear export control the intracellular localization of the mitotic inducer Cdc25. Genes Dev. 1999;13:1067–72. [PubMed]
30. Lopez-Girona A, Furnari B, Mondesert O, Russell P. Nuclear localization of Cdc25 is regulated by DNA damage and a 14-3-3 protein. Nature. 1999;397:172–5. [PubMed]
31. Forrest A, Gabrielli B. Cdc25B activity is regulated by 14-3-3. Oncogene. 2001;20:4393–401. [PubMed]
32. Ogg S, Gabrielli B, Piwnica-Worms H. Purification of a serine kinase that associates with and phosphorylates human Cdc25C on serine 216. J Biol Chem. 1994;269:30461–9. [PubMed]
33. Yang J, Winkler K, Yoshida M, Kornbluth S. Maintenance of G2 arrest in the Xenopus oocyte: a role for 14-3-3-mediated inhibition of Cdc25 nuclear import. EMBO J. 1999;18:2174–83. [PubMed]
34. Dalal SN, Yaffe MB, DeCaprio JA. 14-3-3 family members act coordinately to regulate mitotic progression. Cell Cycle. 2004;3:672–7. [PubMed]
35. Wilker EW, Grant RA, Artim SC, Yaffe MB. A structural basis for 14-3-3sigma functional specificity. J Biol Chem. 2005;280:18891–8. [PubMed]
36. Mils V, Baldin V, Goubin F, Pinta I, Papin C, Waye M, et al. Specific interaction between 14-3-3 isoforms and the human CDC25B phosphatase. Oncogene. 2000;19:1257–65. [PubMed]
37. Lee J, Kumagai A, Dunphy WG. Positive regulation of Wee1 by Chk1 and 14-3-3 proteins. Mol Biol Cell. 2001;12:551–63. [PMC free article] [PubMed]
38. Rothblum-Oviatt CJ, Ryan CE, Piwnica-Worms H. 14-3-3 binding regulates catalytic activity of human Wee1 kinase. Cell Growth Differ. 2001;12:581–9. [PubMed]
39. Benzinger A, Popowicz GM, Joy JK, Majumdar S, Holak TA, Hermeking H. The crystal structure of the non-liganded 14-3-3sigma protein: insights into determinants of isoform specific ligand binding and dimerization. Cell Res. 2005;15:219–27. [PubMed]
40. Chan TA, Hermeking H, Lengauer C, Kinzler KW, Vogelstein B. 14-3-3Sigma is required to prevent mitotic catastrophe after DNA damage. Nature. 1999;401:616–20. [PubMed]
41. Chan TA, Hwang PM, Hermeking H, Kinzler KW, Vogelstein B. Cooperative effects of genes controlling the G(2)/M checkpoint. Genes Dev. 2000;14:1584–8. [PubMed]
42. Hermeking H. The 14-3-3 cancer connection. Nat Rev Cancer. 2003;3:931–43. [PubMed]
43. Hermeking H, Lengauer C, Polyak K, He TC, Zhang L, Thiagalingam S, et al. 14-3-3 sigma is a p53-regulated inhibitor of G2/M progression. Mol Cell. 1997;1:3–11. [PubMed]
44. Laronga C, Yang HY, Neal C, Lee MH. Association of the cyclin-dependent kinases and 14-3-3 sigma negatively regulates cell cycle progression. J Biol Chem. 2000;275:23106–12. [PubMed]
45. Lee MH, Lozano G. Regulation of the p53-MDM2 pathway by 14-3-3 sigma and other proteins. Semin Cancer Biol. 2006;16:225–34. [PubMed]
46. Yang H, Zhang Y, Zhao R, Wen YY, Fournier K, Wu HB, et al. Negative cell cycle regulator 14-3-3sigma stabilizes p27 Kip1 by inhibiting the activity of PKB/Akt. Oncogene. 2006;25:4585–94. [PubMed]
47. Fujita N, Sato S, Katayama K, Tsuruo T. Akt-dependent phosphorylation of p27Kip1 promotes binding to 14-3-3 and cytoplasmic localization. J Biol Chem. 2002;277:28706–13. [PubMed]
48. Manke IA, Nguyen A, Lim D, Stewart MQ, Elia AE, Yaffe MB. MAPKAP kinase-2 is a cell cycle checkpoint kinase that regulates the G2/M transition and S phase progression in response to UV irradiation. Mol Cell. 2005;17:37–48. [PubMed]
49. Reinhardt HC, Aslanian AS, Lees JA, Yaffe MB. p53-deficient cells rely on ATM-and ATR-mediated checkpoint signaling through the p38MAPK/MK2 pathway for survival after DNA damage. Cancer Cell. 2007;11:175–89. [PMC free article] [PubMed]
50. Walworth NC. DNA damage: Chk1 and Cdc25, more than meets the eye. Curr Opin Genet Dev. 2001;11:78–82. [PubMed]
51. Wang JY. Cellular responses to DNA damage. Curr Opin Cell Biol. 1998;10:240–7. [PubMed]
52. Zhou BB, Elledge SJ. The DNA damage response: putting checkpoints in perspective. Nature. 2000;408:433–9. [PubMed]
53. Jazayeri A, Falck J, Lukas C, Bartek J, Smith GC, Lukas J, et al. ATM- and cell cycle-dependent regulation of ATR in response to DNA double-strand breaks. Nat Cell Biol. 2006;8:37–45. [PubMed]
54. Dunaway S, Liu HY, Walworth NC. Interaction of 14-3-3 protein with Chk1 affects localization and checkpoint function. J Cell Sci. 2005;118:39–50. [PubMed]
55. Reinhardt HC, Hasskamp P, Schmedding I, Morandell S, van Vugt MA, Wang X, et al. DNA damage activates a spatially distinct late cytoplasmic cell-cycle checkpoint network controlled by MK2- mediated RNA stabilization. Mol Cell. 2010;40:34–49. [PMC free article] [PubMed]
56. Bouche J, Froment C, Dozier C, Esmenjaud-Mailhat C, Lemaire M, Monsarrat B, et al. NanoLC-MS/MS Analysis Provides New Insights into the Phosphorylation Pattern of Cdc25B in Vivo: Full Overlap with Sites of Phosphorylation by Chk1 and Cdk1/cycB Kinases in Vitro. Journal of Proteome Reserach. 2008;7:1264–73. [PubMed]
57. Yoo HY, Kumagai A, Shevchenko A, Dunphy WG. Adaptation of a DNA replication checkpoint response depends upon inactivation of Claspin by the Pololike kinase. Cell. 2004;117:575–88. [PubMed]
58. Mailand N, Bekker-Jensen S, Bartek J, Lukas J. Destruction of Claspin by SCFbetaTrCP restrains Chk1 activation and facilitates recovery from genotoxic stress. Mol Cell. 2006;23:307–18. [PubMed]
59. Mamely I, van Vugt MA, Smits VA, Semple JI, Lemmens B, Perrakis A, et al. Pololike kinase-1 controls proteasome-dependent degradation of Claspin during checkpoint recovery. Curr Biol. 2006;16:1950–5. [PubMed]
60. Peschiaroli A, Dorrello NV, Guardavaccaro D, Venere M, Halazonetis T, Sherman NE, et al. SCFbetaTrCP-mediated degradation of Claspin regulates recovery from the DNA replication checkpoint response. Mol Cell. 2006;23:319–29. [PubMed]
61. van Vugt MA, Gardino AK, Linding R, Ostheimer GJ, Reinhardt HC, Ong SE, et al. A mitotic phosphorylation feedback network connects Cdk1, Plk1, 53BP1, and Chk2 to inactivate the G(2)/M DNA damage checkpoint. PLoS Biol. 2009;8:e1000287. [PMC free article] [PubMed]
62. Vidanes GM, Sweeney FD, Galicia S, Cheung S, Doyle JP, Durocher D, et al. CDC5 inhibits the hyperphosphorylation of the checkpoint kinase Rad53, leading to checkpoint adaptation. PLoS Biol. 2010;8:e1000286. [PMC free article] [PubMed]
63. Takekawa M, Adachi M, Nakahata A, Nakayama I, Itoh F, Tsukuda H, et al. p53-inducible wip1 phosphatase mediates a negative feedback regulation of p38 MAPK-p53 signaling in response to UV radiation. EMBO J. 2000;19:6517–26. [PubMed]
64. Yamaguchi H, Minopoli G, Demidov ON, Chatterjee DK, Anderson CW, Durell SR, et al. Substrate specificity of the human protein phosphatase 2Cdelta, Wip1. Biochemistry. 2005;44:5285–94. [PubMed]
65. Cha H, Lowe JM, Li H, Lee JS, Belova GI, Bulavin DV, et al. Wip1 directly dephosphorylates gamma-H2AX and attenuates the DNA damage response. Cancer Res. 2010;70:4112–22. [PMC free article] [PubMed]
66. Macurek L, Lindqvist A, Lim D, Lampson MA, Klompmaker R, Freire R, et al. Polo-like kinase-1 is activated by aurora A to promote checkpoint recovery. Nature. 2008;455:119–23. [PubMed]
67. Asano S, Park JE, Sakchaisri K, Yu LR, Song S, Supavilai P, et al. Concerted mechanism of Swe1/Wee1 regulation by multiple kinases in budding yeast. EMBO J. 2005;24:2194–204. [PubMed]
68. Watanabe N, Arai H, Nishihara Y, Taniguchi M, Hunter T, Osada H. M-phase kinases induce phospho-dependent ubiquitination of somatic Wee1 by SCFbeta-TrCP. Proc Natl Acad Sci U S A. 2004;101:4419–24. [PubMed]
69. Bartkova J, Horejsi Z, Koed K, Kramer A, Tort F, Zieger K, et al. DNA damage response as a candidate anti-cancer barrier in early human tumorigenesis. Nature. 2005;434:864–70. [PubMed]
70. Krempler A, Deckbar D, Jeggo PA, Lobrich M. An imperfect G2M checkpoint contributes to chromosome instability following irradiation of S and G2 phase cells. Cell Cycle. 2007;6:1682–6. [PubMed]
71. Margolis SS, Walsh S, Weiser DC, Yoshida M, Shenolikar S, Kornbluth S. PP1 control of M phase entry exerted through 14-3-3-regulated Cdc25 dephosphorylation. EMBO J. 2003;22:5734–45. [PubMed]
72. Margolis SS, Perry JA, Weitzel DH, Freel CD, Yoshida M, Haystead TA, et al. A role for PP1 in the Cdc2/Cyclin B–mediated positive feedback activation of Cdc25. Mol Biol Cell. 2006;17:1779–89. [PMC free article] [PubMed]
73. Tzivion G, Luo ZJ, Avruch J. Calyculin A–induced vimentin phosphorylation sequesters 14-3-3 and displaces other 14-3-3 partners in vivo. J Biol Chem. 2000;275:29772–8. [PubMed]
74. Ku NO, Michie S, Resurreccion EZ, Broome RL, Omary MB. Keratin binding to 14-3-3 proteins modulates keratin filaments and hepatocyte mitotic progression. Proc Natl Acad Sci U S A. 2002;99:4373–8. [PubMed]
75. Liao J, Omary MB. 14-3-3 proteins associate with phosphorylated simple epithelial keratins during cell cycle progression and act as a solubility cofactor. J Cell Biol. 1996;133:345–57. [PMC free article] [PubMed]
76. Astuti P, Gabrielli B. Phosphorylation of Cdc25B3 Ser169 regulates 14-3-3 binding to Ser151 and Cdc25B activity. Cell Cycle. 2011;10:1960–7. [PubMed]
77. Bulavin DV, Higashimoto Y, Demidenko ZN, Meek S, Graves P, Phillips C, et al. Dual phosphorylation controls Cdc25 phosphatases and mitotic entry. Nat Cell Biol. 2003;5:545–51. [PubMed]
78. Saurin AT, Durgan J, Cameron AJ, Faisal A, Marber MSs, Parker PJ. The regulated assembly of a PKCepsilon complex controls the completion of cytokinesis. Nat Cell Biol. 2008;10:891–901. [PubMed]
79. Gorin MA, Pan Q, Protein kinase C. epsilon: an oncogene and emerging tumor biomarker. Mol Cancer. 2009;8:9. [PMC free article] [PubMed]
80. Lodygin D, Hermeking H. Epigenetic silencing of 14-3-3sigma in cancer. Semin Cancer Biol. 2006;16:214–24. [PubMed]
81. Wilker EW, van Vugt MA, Artim SA, Huang PH, Petersen CP, Reinhardt HC, et al. 14-3-3sigma controls mitotic translation to facilitate cytokinesis. Nature. 2007;446:329–32. [PubMed]
82. Cornelis S, Bruynooghe Y, Denecker G, Van Huffel S, Tinton S, Beyaert R. Identification and characterization of a novel cell cycle-regulated internal ribosome entry site. Mol Cell. 2000;5:597–605. [PubMed]
83. Hu D, Valentine M, Kidd VJ, Lahti JM. CDK11(p58) is required for the maintenance of sister chromatid cohesion. J Cell Sci. 2007;120:2424–34. [PubMed]
84. Petretti C, Savoian M, Montembault E, Glover DM, Prigent C, Giet R. The PITSLRE/CDK11p58 protein kinase promotes centrosome maturation and bipolar spindle formation. EMBO Rep. 2006;7:418–24. [PubMed]
85. Busino L, Donzelli M, Chiesa M, Guardavaccaro D, Ganoth D, Dorrello NV, et al. Degradation of Cdc25A by beta-TrCP during S phase and in response to DNA damage. Nature. 2003;426:87–91. [PubMed]
86. Jin J, Shirogane T, Xu L, Nalepa G, Qin J, Elledge SJ, et al. SCFbeta-TRCP links Chk1 signaling to degradation of the Cdc25A protein phosphatase. Genes Dev. 2003;17:3062–74. [PubMed]
87. Mailand N, Falck J, Lukas C, Syljuasen RG, Welcker M, Bartek J, et al. Rapid destruction of human Cdc25A in response to DNA damage. Science. 2000;288:1425–9. [PubMed]
88. Honaker Y, Piwnica-Worms H. Casein kinase 1 functions as both penultimate and ultimate kinase in regulating Cdc25A destruction. Oncogene. 2010;29:3324–34. [PMC free article] [PubMed]
89. Jin J, Ang XL, Ye X, Livingstone M, Harper JW. Differential roles for checkpoint kinases in DNA damage-dependent degradation of the Cdc25A protein phosphatase. J Biol Chem. 2008;283:19322–8. [PubMed]
90. Kang T, Wei Y, Honaker Y, Yamaguchi H, Appella E, Hung MC, et al. GSK-3 beta targets Cdc25A for ubiquitin-mediated proteolysis, and GSK-3 beta inactivation correlates with Cdc25A overproduction in human cancers. Cancer Cell. 2008;13:36–47. [PMC free article] [PubMed]
91. Mailand N, Podtelejnikov AV, Groth A, Mann M, Bartek J, Lukas J. Regulation of G(2)/M events by Cdc25A through phosphorylation-dependent modulation of its stability. EMBO J. 2002;21:5911–20. [PubMed]
92. Chen MS, Ryan CE, Piwnica-Worms H. Chk1 kinase negatively regulates mitotic function of Cdc25A phosphatase through 14-3-3 binding. Mol Cell Biol. 2003;23:7488–97. [PMC free article] [PubMed]
93. Wang B, Liu K, Lin HY, Bellam N, Ling S, Lin WC. 14-3-3Tau regulates ubiquitin-independent proteasomal degradation of p21, a novel mechanism of p21 downregulation in breast cancer. Mol Cell Biol. 2010;30:1508–27. [PMC free article] [PubMed]
94. Zha J, Harada H, Yang E, Jockel J, Korsmeyer SJ. Serine phosphorylation of death agonist BAD in response to survival factor results in binding to 14-3-3 not BCL-X(L) Cell. 1996;87:619–28. [PubMed]
95. Datta SR, Dudek H, Tao X, Masters S, Fu H, Gotoh Y, et al. Akt phosphorylation of BAD couples survival signals to the cell-intrinsic death machinery. Cell. 1997;91:231–41. [PubMed]
96. Tan Y, Ruan H, Demeter MR, Comb MJ. p90(RSK) blocks bad-mediated cell death via a protein kinase C-dependent pathway. J Biol Chem. 1999;274:34859–67. [PubMed]
97. Datta SR, Katsov A, Hu L, Petros A, Fesik SW, Yaffe MB, et al. 14-3-3 proteins and survival kinases cooperate to inactivate BAD by BH3 domain phosphorylation. Mol Cell. 2000;6:41–51. [PubMed]
98. Nomura M, Shimizu S, Sugiyama T, Narita M, Ito T, Matsuda H, et al. 14-3-3 Interacts directly with and negatively regulates pro-apoptotic Bax. J Biol Chem. 2003;278:2058–65. [PubMed]
99. Youle RJ, Strasser A. The BCL-2 protein family: opposing activities that mediate cell death. Nat Rev Mol Cell Biol. 2008;9:47–59. [PubMed]
100. Tsuruta F, Sunayama J, Mori Y, Hattori S, Shimizu S, Tsujimoto Y, et al. JNK promotes Bax translocation to mitochondria through phosphorylation of 14-3-3 proteins. EMBO J. 2004;23:1889–99. [PubMed]
101. Tobiume K, Matsuzawa A, Takahashi T, Nishitoh H, Morita K, Takeda K, et al. ASK1 is required for sustained activations of JNK/p38 MAP kinases and apoptosis. EMBO Rep. 2001;2:222–8. [PubMed]
102. Zhang L, Chen J, Fu H. Suppression of apoptosis signal-regulating kinase 1-induced cell death by 14-3-3 proteins. Proc Natl Acad Sci U S A. 1999;96:8511–5. [PubMed]
103. He KL, Ting AT. A20 inhibits tumor necrosis factor (TNF) alpha-induced apoptosis by disrupting recruitment of TRADD and RIP to the TNF receptor 1 complex in Jurkat T cells. Mol Cell Biol. 2002;22:6034–45. [PMC free article] [PubMed]
104. Vincenz C, Dixit VM. 14-3-3 proteins associate with A20 in an isoform-specific manner and function both as chaperone and adapter molecules. J Biol Chem. 1996;271:20029–34. [PubMed]
105. Baksh S, Tommasi S, Fenton S, Yu VC, Martins LM, Pfeifer GP, et al. The tumor suppressor RASSF1A and MAP-1 link death receptor signaling to Bax conformational change and cell death. Mol Cell. 2005;18:637–50. [PubMed]
106. Ghazaleh HA, Chow RS, Choo SL, Pham D, Olesen JD, Wong RX, et al. 14-3-3 mediated regulation of the tumor suppressor protein, RASSF1A. Apoptosis. 2010;15:117–27. [PubMed]
107. Fu Z, Tindall DJ. FOXOs, cancer and regulation of apoptosis. Oncogene. 2008;27:2312–9. [PMC free article] [PubMed]
108. Brownawell AM, Kops GJ, Macara IG, Burgering BM. Inhibition of nuclear import by protein kinase B (Akt) regulates the subcellular distribution and activity of the forkhead transcription factor AFX. Mol Cell Biol. 2001;21:3534–46. [PMC free article] [PubMed]
109. Brunet A, Bonni A, Zigmond MJ, Lin MZ, Juo P, Hu LS, et al. Akt promotes cell survival by phosphorylating and inhibiting a forkhead transcription factor. Cell. 1999;96:857–68. [PubMed]
110. Brunet A, Kanai F, Stehn J, Xu J, Sarbassova D, Frangioni JV, et al. 14-3-3 transits to the nucleus and participates in dynamic nucleocytoplasmic transport. J Cell Biol. 2002;156:817–28. [PMC free article] [PubMed]
111. Rena G, Prescott AR, Guo S, Cohen P, Unterman TG. Roles of the forkhead in rhabdomyosarcoma (FKHR) phosphorylation sites in regulating 14-3-3 binding, transactivation and nuclear targetting. Biochem J. 2001;354:605–12. [PubMed]
112. Dobson M, Ramakrishnan G, Ma S, Kaplun L, Balan V, Fridman R, et al. Bimodal regulation of FoxO3 by AKT and 14-3-3. Biochim Biophys Acta. 2011;1813:1453–64. [PMC free article] [PubMed]
113. Obsil T, Ghirlando R, Anderson DE, Hickman AB, Dyda F. Two 14-3-3 binding motifs are required for stable association of Forkhead transcription factor FOXO4 with 14-3-3 proteins and inhibition of DNA binding. Biochemistry. 2003;42:15264–72. [PubMed]
114. Silhan J, Vacha P, Strnadova P, Vecer J, Herman P, Sulc M, et al. 14-3-3 protein masks the DNA binding interface of forkhead transcription factor FOXO4. J Biol Chem. 2009;284:19349–60. [PubMed]
115. Masters SC, Fu H. 14-3-3 proteins mediate an essential anti-apoptotic signal. J Biol Chem. 2001;276:45193–200. [PubMed]
116. Sunayama J, Tsuruta F, Masuyama N, Gotoh Y. JNK antagonizes Akt-mediated survival signals by phosphorylating 14-3-3. J Cell Biol. 2005;170:295–304. [PMC free article] [PubMed]
117. Wang B, Yang H, Liu YC, Jelinek T, Zhang L, Ruoslahti E, et al. Isolation of high-affinity peptide antagonists of 14-3-3 proteins by phage display. Biochemistry. 1999;38:12499–504. [PubMed]
118. Nguyen A, Rothman DM, Stehn J, Imperiali B, Yaffe MB. Caged phosphopeptides reveal a temporal role for 14-3-3 in G1 arrest and S-phase checkpoint function. Nat Biotechnol. 2004;22:993–1000. [PubMed]
119. Corradi V, Mancini M, Manetti F, Petta S, Santucci MA, Botta M. Identification of the first non-peptidic small molecule inhibitor of the c-Abl/14-3-3 protein- protein interactions able to drive sensitive and Imatinib-resistant leukemia cells to apoptosis. Bioorg Med Chem Lett. 20:6133–7. [PubMed]
120. Mancini M, Corradi V, Petta S, Barbieri E, Manetti F, Botta M, et al. A new nonpeptidic inhibitor of 14-3-3 induces apoptotic cell death in chronic myeloid leukemia sensitive or resistant to imatinib. J Pharmacol Exp Ther. 336:596–604. [PubMed]
121. Ottmann C, Weyand M, Sassa T, Inoue T, Kato N, Wittinghofer A, et al. A structural rationale for selective stabilization of anti-tumor interactions of 14-3-3 proteins by cotylenin A. J Mol Biol. 2009;386:913–9. [PubMed]
122. Wurtele M, Jelich-Ottmann C, Wittinghofer A, Oecking C. Structural view of a fungal toxin acting on a 14-3-3 regulatory complex. Embo J. 2003;22:987–94. [PubMed]
123. Rose R, Erdmann S, Bovens S, Wolf A, Rose M, Hennig S, et al. Identification and structure of small-molecule stabilizers of 14-3-3 protein-protein interactions. Angew Chem Int Ed Engl. 2010;49:4129–32. [PubMed]