Recurrence rates following radiotherapy for prostate cancer in the post-operative adjuvant or salvage setting remain substantial. Previous work from our institution demonstrated that published prostate bed CTV guidelines frequently do not cover the pre-operative MRI defined prostate. Inadequate target delineation may contribute to the high recurrence rates, but increasing target volumes may increase dose to organs at risk.
We propose guidelines for delineating post-prostatectomy target volumes based upon an individual’s co-registered pre-operative MRI. MRI-based CTVs and PTVs were compared to those created using the RTOG guidelines in 30 patients. Contours were analysed in terms of absolute volume, intersection volume (Jaccard Index) and the ability to meet the RADICALS and QUANTEC rectal and bladder constraints (tomotherapy IMRT plans with PTV coverage of V98% ≥98%).
CTV MRI was a mean of 18.6% larger than CTV RTOG: CTV MRI mean 138 cc (range 72.3 - 222.2 cc), CTV RTOG mean 116.3 cc (range 62.1 - 176.6 cc), (p < 0.0001). The difference in mean PTV was only 4.6%: PTV MRI mean 386.9 cc (range 254.4 – 551.2), PTV RTOG mean 370 cc (range 232.3 - 501.6) (p = 0.05). The mean Jaccard Index representing intersection volume between CTVs was 0.72 and 0.84 for PTVs. Both criteria had a similar ability to meet rectal and bladder constraints. Rectal DVH: 77% of CTV RTOG cases passed all RADICALS criteria and 37% all QUANTEC criteria; versus 73% and 40% for CTV MRI (p = 1.0 for both). Bladder DVH; 47% of CTV RTOG cases passed all RADICALS criteria and 67% all QUANTEC criteria, versus 57% and 60% for CTV MRI (p = 0.61for RADICALS, p = 0.79 for QUANTEC). CTV MRI spares more of the lower anterior bladder wall than CTV RTOG but increases coverage of the superior lateral bladder walls.
CTV contours based upon the patient’s co-registered pre-operative MRI in the post-prostatectomy setting may improve coverage of the individual’s prostate bed without substantially increasing the PTV size or dose to bladder/ rectum compared to RTOG CTV guidelines. Further evaluation of whether the use of pre-operative MRI improves local control rates is warranted.
Prostate cancer; Post-prostatectomy; Adjuvant; Salvage radiotherapy; MRI; Consensus guidelines; CTV
The Hippo pathway, a cascade of protein kinases that inhibits the oncogenic transcriptional coactivators YAP and TAZ, was discovered in Drosophila as a major determinant of organ size in development. Known modes of regulation involve surface proteins that mediate cell-cell contact or determine epithelial cell polarity which, in a tissue specific manner, use intracellular complexes containing FERM domain and actin-binding proteins to modulate the kinase activities or directly sequester YAP. Unexpectedly, recent work demonstrates that GPCRs, especially those signaling through Galpha12/13 such as the Protease Activated Receptor PAR1, cause potent YAP dephosphorylation and activation. This response requires active RhoA GTPase and increased assembly of filamentous (F-)actin. Morever, cell architectures that promote F-actin assembly per se also activate YAP by kinase-dependent and independent mechanisms. These findings unveil the ability of GPCRs to activate the YAP oncogene through a newly recognized signaling function of the actin cytoskeleton, likely to be especially important for normal and cancerous stem cells.
Nek6 and Nercc1/Nek9 belong to the NIMA family of protein kinases. Nercc1 is activated in mitosis whereupon it binds, phosphorylates and activates Nek6. Interference with Nek6 or Nercc1 in mammalian cells causes prometaphase/metaphase arrest, and depletion of XNercc from Xenopus egg extracts prevents normal spindle assembly. Herein we show that Nek6 is constitutively associated with Eg5, a kinesin necessary for spindle bipolarity. Nek6 phosphorylates Eg5 at several sites in vitro, and one of these sites, Ser1033, is phosphorylated in vivo during mitosis. While Cdk1 phosphorylates nearly all Eg5 during mitosis at Thr926, Nek6 phosphorylates ~3% of Eg5, primarily at the spindle poles. Eg5 depletion arrests cells with a monopolar spindle; this can be rescued by Eg5 wildtype but not by Eg5(Thr926Ala). Eg5(Ser1033Ala) rescues half as well as wildtype whereas an Eg5(Ser1033Asp) mutant is nearly as effective. Thus Nek6 phosphorylates a subset of Eg5 polypeptides during mitosis at a conserved site, whose phosphorylation is critical for the mitotic function of Eg5.
BACKGROUND & AIMS
The Hippo signaling pathway is a context-dependent regulator of cell proliferation, differentiation, and apoptosis in species ranging from Drosophila to humans. In this study, we investigated the role of the core Hippo kinases—Mst1 and Mst2—in pancreatic development and homeostasis.
We used a Cre/LoxP system to create mice with pancreas-specific disruptions in Mst1 and Mst2 (Pdx1-Cre;Mst1−/−;Mst2fl/fl mice), the mammalian orthologs of Drosophila Hippo. We used a transgenic approach to overexpress Yap, the downstream mediator of Hippo signaling, in the developing pancreas of mice.
Contrary to expectations, the pancreatic mass of Pdx1-Cre;Mst1−/−;Mst2fl/fl mice was reduced compared with wild-type mice, largely because of postnatal de-differentiation of acinar cells into duct-like cells. Development of this phenotype coincided with postnatal reactivation of YAP expression. Ectopic expression of YAP during the secondary transition (a stage at which YAP is normally absent) blocked differentiation of the endocrine and exocrine compartments, whereas loss of a single Yap allele reduced acinar de-differentiation. The phenotype of Pdx1-Cre;Mst1−/−;Mst2fl/fl mice recapitulated cellular and molecular changes observed during chemical-induced pancreatitis in mice.
The mammalian Hippo kinases, and YAP, maintain postnatal pancreatic acinar differentiation in mice.
mouse model; signal transduction; Hpo; Wts
The transcriptional co-activator YAP plays an important role in organ size control and tumorigenesis. However, how Yap gene expression is regulated remains unknown. This study shows that the Ets family member GABP binds to the Yap promoter and activates YAP transcription. The depletion of GABP downregulates YAP, resulting in a G1/S cell cycle block and increased cell death, both of which are substantially rescued by reconstituting YAP. GABP can be inactivated by oxidative mechanisms, and acetaminophen-induced GSH depletion inhibits GABP transcriptional activity and depletes YAP. In contrast, activating YAP by deleting Mst1/Mst2 strongly protects acetaminophen-induced liver injury. Similar to its effects on YAP, the Hippo signaling inhibits GABP transcriptional activity through several mechanisms. In human liver cancers, enhanced YAP expression is correlated with increased nuclear expression of GABP. Therefore, we conclude that GABP is an activator of Yap gene expression and a potential therapeutic target for cancers driven by YAP.
GABP; Hippo pathway; YAP; Glutathione depletion; Liver injury; liver cancer
Photoreceptor cell death is the definitive cause of vision loss in retinal detachment (RD). Mammalian STE20-like kinase (MST) is a master regulator of both cell death and proliferation and a critical factor in development and tumorigenesis. However, to date the role of MST in neurodegeneration has not been fully explored. Utilizing MST1−/− and MST2−/− mice we identified MST2, but not MST1, as a regulator of photoreceptor cell death in a mouse model of RD. MST2−/− mice demonstrated significantly decreased photoreceptor cell death and outer nuclear layer (ONL) thinning after RD. Additionally, caspase-3 activation was attenuated in MST2−/− mice compared to control mice after RD. The transcription of p53 upregulated modulator of apoptosis (PUMA) and Fas was also reduced in MST2−/− mice post-RD. Retinas of MST2−/− mice displayed suppressed nuclear relocalization of phosphorylated YAP after RD. Consistent with the reduction of photoreceptor cell death, MST2−/− mice showed decreased levels of proinflammatory cytokines such as monocyte chemoattractant protein 1 and interleukin 6 as well as attenuated inflammatory CD11b cell infiltration during the early phase of RD. These results identify MST2, not MST1, as a critical regulator of caspase-mediated photoreceptor cell death in the detached retina and indicate its potential as a future neuroprotection target.
retinal detachment photoreceptor cell; apoptosis; neurodegeneration; mammalian STE20-like kinase
Background: Signaling by mTOR complex 1 requires its amino acid-stimulated binding to Rag GTPase heterodimers.
Results: mTOR complex 1-Rag binding in vitro is independent of Rag guanyl nucleotide charging, and withdrawal of amino acids from cells does not alter Rag GTP charging.
Conclusion: Amino acids promote Rag binding to mTOR complex 1 without changing Rag GTP charging.
Significance: Amino acids promote Rag-mTORC1 binding by undefined mechanisms.
Activation of mammalian target of rapamycin complex 1 (mTORC1) by amino acids is mediated in part by the Rag GTPases, which bind the raptor subunit of mTORC1 in an amino acid-stimulated manner and promote mTORC1 interaction with Rheb-GTP, the immediate activator. Here we examine whether the ability of amino acids to regulate mTORC1 binding to Rag and mTORC1 activation is due to the regulation of Rag guanyl nucleotide charging. Rag heterodimers in vitro exhibit a very rapid, spontaneous exchange of guanyl nucleotides and an inability to hydrolyze GTP. Mutation of the Rag P-loop corresponding to RasSer-17 abolishes guanyl nucleotide binding. Such a mutation in RagA or RagB inhibits, whereas in RagC or RagD it enhances, Rag heterodimer binding to mTORC1. The binding of wild-type and mutant Rag heterodimers to mTORC1 in vitro parallels that seen with transient expression, but binding to mTORC1 in vitro is entirely independent of Rag guanyl nucleotide charging. HeLa cells stably overexpressing wild-type or P-loop mutant RagC exhibit unaltered amino acid regulation of mTORC1. Despite amino acid-independent raptor binding to Rag, mTORC1 is inhibited by amino acid withdrawal as in parental cells. Rag heterodimers extracted from 32P-labeled whole cells, or just from the pool associated with the lysosomal membrane, exhibit constitutive [32P]GTP charging that is unaltered by amino acid withdrawal. Thus, amino acids promote mTORC1 activation without altering Rag GTP charging. Raptor binding to Rag, although necessary, is not sufficient for mTORC1 activation. Additional amino acid-dependent steps couple Rag-mTORC1 to Rheb-GTP.
Cell Signaling; GTPase; Insulin; Lysosomes; mTOR Complex (mTORC); Rag GTPase; Amino Acids; Raptor
The “Hippo” signaling pathway has emerged as a major regulator of cell proliferation and survival in metazoans. The pathway, as delineated by genetic and biochemical studies in Drosophila, consists of a kinase cascade regulated by cell-cell contact and cell polarity that inhibits the transcriptional coactivator Yorkie and its proliferative, anti-differentiation, antiapoptotic transcriptional program. The core pathway components are the GC kinase Hippo, which phosphorylates the noncatalytic polypeptide Mats/Mob1 and, with the assistance of the scaffold protein Salvador, phosphorylates the ndr-family kinase Lats. In turn phospho-Lats, after binding to phospho-Mats, autoactivates and phosphorylates Yorkie, resulting in its nuclear exit. Hippo also uses the scaffold protein Furry and a different Mob protein to control another ndr-like kinase, the morphogenetic regulator Tricornered. Architecturally homologous kinase cascades consisting of a GC kinase, a Mob protein, a scaffolding polypeptide and an ndr-like kinase are well described in yeast; in S. cerevisiae e.g., the MEN pathway promotes mitotic exit whereas the RAM network, using a different GC kinase, Mob protein, scaffold and ndr-like kinase, regulates cell polarity and morphogenesis. In mammals, the Hippo orthologues Mst1 and Mst2 utilize the Salvador ortholog WW45/Sav1 and other scaffolds to regulate the kinases Lats1/Lats2 and ndr1/ndr2. As in Drosophila, murine Mst1/Mst2, in a redundant manner, negatively regulate the Yorkie ortholog YAP in the epithelial cells of the liver and gut; loss of both Mst1 and Mst2 results in hyperproliferation and tumorigenesis that can be largely negated by reduction or elimination of YAP. Despite this conservation, considerable diversification in pathway composition and regulation is already evident; in skin e.g., YAP phosphorylation is independent of Mst1Mst2 and Lats1Lats2. Moreover, in lymphoid cells, Mst1/Mst2, under the control of the Rap1 GTPase and independent of YAP, promotes integrin clustering, actin remodeling and motility while restraining the proliferation of naïve T cells. This review will summarize current knowledge of the structure and regulation of the kinases Hippo/Mst1&2, their noncatalytic binding partners, Salvador and the Rassf polypeptides, and their major substrates Warts/Lats1&2, Trc/ndr1&2, Mats/Mob1 and FOXO.
Protein kinase; Hippo; Mst1/2; Mob1; Lats1/2; ndr1/2
The transcriptional co-activator YAP is an evolutionarily conserved regulator of organ size and progenitor cell proliferation. YAP is overexpressed at high frequency in many common human cancers and can directly drive cancer development in mouse models. YAP abundance and nuclear localization are negatively regulated by the Hippo kinase cascade, which, in epithelia, is activated by physiological cell-cell contact. recent work in intestinal epithelium has established that YAP is constitutively inhibited by the Hippo pathway and entirely dispensable for normal development and homeostasis. YAP serves only in a standby capacity; should cell-cell contact be abrogated, as after intestinal damage, the loss of Hippo input permits increased YAP abundance and nuclear residence. in turn, YAP cooperates with β-catenin to transactivate genes that promote stem cell expansion for epithelial repair. This interplay between overexpressed YAP and β-catenin also drives proliferation of colon cancer cells. The dispensability of YAP in normal intestine makes YAP's expression or outputs attractive targets for cancer therapy.
YAP; Hippo; intestinal stem cell; β-catenin; colon cancer; liver cancer
Mammalian target of rapamycin (mTOR) is a giant protein kinase that controls cell proliferation, growth, and metabolism. mTOR is regulated by nutrient availability, by mitogens, and by stress, and operates through two independently regulated hetero-oligomeric complexes. We have attempted to identify the cellular components necessary to maintain the activity of mTOR complex 1 (mTORC1), the amino acid-dependent, rapamycin-inhibitable complex, using a whole genome approach involving RNAi-induced depletion of cellular polypeptides. We have used a pancreatic ductal adenocarcinoma (PDAC) cell line, Mia-PaCa for this screen; as with many pancreatic cancers, these cells exhibit constitutive activation of mTORC1. PDAC is the most common form of pancreatic cancer and the 5-year survival rate remains 3–5% despite current nonspecific and targeted therapies. Although rapamycin-related mTOR inhibitors have yet to demonstrate encouraging clinical responses, it is now evident that this class of compounds is capable of only partial mTORC1 inhibition. Identifying previously unappreciated proteins needed for maintenance of mTORC1 activity may provide new targets and lead to the development of beneficial therapies for pancreatic cancer.
rpS6; Phosphorylation; mTOR; Genome wide; RNAi; Screen; Immunofluorescence; Pancreatic cancer
In mice lacking both Mst1 and Mst2 in the lymphoid compartment, thymocyte development is normal, but single-positive thymocytes exhibit excessive apoptosis and greatly diminished thymic egress, accompanied by loss of chemokine activation of RhoA and Rac1.
The Mst1 kinase is an important regulator of murine T cell adhesion, migration, proliferation, and apoptosis. In this study, we analyze mice lacking both Mst1 and Mst2 in hematopoietic cells. Compared with wild-type mice, these double knockout (DKO) mice exhibit a severe reduction in the number of mature T cells in the circulation and in secondary lymphoid organs (SLOs). CD4+CD8− and CD4−CD8+ single-positive (SP) thymocytes in DKO mice resemble mature T cells of wild-type mice but undergo excessive apoptosis, and their egress from the thymus is reduced by >90%. Even when placed directly in the circulation, DKO SP thymocytes failed to enter SLOs. In SP thymocytes, deficiency of Mst1 and Mst2 abolished sphingosine-1 phosphate– and CCL21-induced Mob1 phosphorylation, Rac1 and RhoA GTP charging, and subsequent cell migration. When phosphorylated by Mst1 or Mst2, Mob1 binds and activates the Rac1 guanyl nucleotide exchanger Dock8, which is abundant in the thymus. Thus, the Mst1 and Mst2 kinases control Rho GTPase activation and the migratory responses of SP thymocytes.
No targeted immunotherapies reverse type 1 diabetes in humans. However, in a rodent model of type 1 diabetes, Bacillus Calmette-Guerin (BCG) reverses disease by restoring insulin secretion. Specifically, it stimulates innate immunity by inducing the host to produce tumor necrosis factor (TNF), which, in turn, kills disease-causing autoimmune cells and restores pancreatic beta-cell function through regeneration.
Translating these findings to humans, we administered BCG, a generic vaccine, in a proof-of-principle, double-blind, placebo-controlled trial of adults with long-term type 1 diabetes (mean: 15.3 years) at one clinical center in North America. Six subjects were randomly assigned to BCG or placebo and compared to self, healthy paired controls (n = 6) or reference subjects with (n = 57) or without (n = 16) type 1 diabetes, depending upon the outcome measure. We monitored weekly blood samples for 20 weeks for insulin-autoreactive T cells, regulatory T cells (Tregs), glutamic acid decarboxylase (GAD) and other autoantibodies, and C-peptide, a marker of insulin secretion. BCG-treated patients and one placebo-treated patient who, after enrollment, unexpectedly developed acute Epstein-Barr virus infection, a known TNF inducer, exclusively showed increases in dead insulin-autoreactive T cells and induction of Tregs. C-peptide levels (pmol/L) significantly rose transiently in two BCG-treated subjects (means: 3.49 pmol/L [95% CI 2.95–3.8], 2.57 [95% CI 1.65–3.49]) and the EBV-infected subject (3.16 [95% CI 2.54–3.69]) vs.1.65 [95% CI 1.55–3.2] in reference diabetic subjects. BCG-treated subjects each had more than 50% of their C-peptide values above the 95th percentile of the reference subjects. The EBV-infected subject had 18% of C-peptide values above this level.
We conclude that BCG treatment or EBV infection transiently modified the autoimmunity that underlies type 1 diabetes by stimulating the host innate immune response. This suggests that BCG or other stimulators of host innate immunity may have value in the treatment of long-term diabetes.
During development and regeneration, proliferation of tissue-specific stem cells is tightly controlled to produce organs of a predetermined size. The molecular determinants of this process remain poorly understood. Here, we investigate the function of Yap1, the transcriptional effector of the Hippo signaling pathway, in skin biology. Using gain- and loss-of-function studies we show that Yap1 is a critical modulator of epidermal stem cell proliferation and tissue expansion. Yap1 mediates this effect through interaction with TEAD transcription factors. Additionally, our studies reveal that α-catenin, a molecule previously implicated in tumor suppression and cell density sensing in the skin, is an upstream negative regulator of Yap1. α-catenin controls Yap1 activity and phosphorylation by modulating its interaction with 14-3-3 and the PP2A phosphatase. Together, these data identify Yap1 as a determinant of the proliferative capacity of epidermal stem cells and as an important effector of a ‘crowd control’ molecular circuitry in mammalian skin.
The mechanisms controlling mammalian organ size have long been a source of fascination for biologists. These controls are needed to both ensure the integrity of the body plan and to restrict inappropriate proliferation that could lead to cancer. Regulation of liver size is of particular interest inasmuch as this organ maintains the capacity for regeneration throughout life, and is able to regain precisely its original mass after partial surgical resection. Recent studies using genetically engineered mouse strains have shed new light on this problem; the Hippo signalling pathway, first elucidated as a regulator of organ size in Drosophila, has been identified as dominant determinant of liver growth. Defects in this pathway in mouse liver lead to sustained liver overgrowth and the eventual development of both major types of liver cancer, hepatocellular carcinoma and cholangiocarcinoma. In this review, we discuss the role of Hippo signalling in liver biology and the contribution of this pathway to liver cancer in humans.
liver cancer; hepatocellular carcinoma; cholangiocarcinoma; oval cells; Hippo; Rassf polypeptides; tumour suppressor pathway
The small GTPase Rheb is a positive upstream regulator of the target of rapamycin (TOR) complex 1 in mammalian cells and can bind directly to TOR complex 1. To identify the regions of the Rheb surface most critical for signaling to TOR complex 1, we created a set of 26 mutants wherein clusters of 1–5 putative solvent-exposed residues were changed to alanine, ultimately changing 65 residues distributed over the entire Rheb surface. The signaling function of these mutants was assessed by their ability, in comparison to wild type Rheb, to restore the phosphorylation of S6K1(Thr389) when expressed transiently in amino acid-deprived 293T cells. The major finding is that two mutants situated in the Rheb switch 2 segment, Y67A/I69A and I76A/D77A, exhibit a near total loss of function, whereas extensive replacement of the switch 1 segment and other surface residues with alanines causes relatively little disturbance of Rheb rescue of S6K1 from amino acid withdrawal. This is surprising in view of the minimal impact of guanyl nucleotide on Rheb switch 2 configuration. The loss of function Rheb switch 2 mutants are well expressed and exhibit partial agonist function in amino acid-replete cells. They are unimpaired in their ability to bind GTP or mammalian (m)TOR in vivo or in vitro, and the mTOR polypeptides retrieved with these inactive Rheb mutants exhibit kinase activity in vitro comparable with mTOR bound to wild type Rheb. We conclude that Rheb signaling to mTOR in vivo requires a Rheb switch 2-dependent interaction with an element other than the three known polypeptide components of TOR complex 1.
Nore1A was originally identified as a potential Ras effector, and Nore1B is an alternatively spliced isoform. Both share a Ras/Rap association domain (RA domain) but only Nore1A contains sequence motifs that predict SH3 domain binding and diacylglycerol/phorbol ester binding in the amino-terminal region. Here we report that Carma1 binds to Nore1A and Nore1B through the RA domain and that Carma1 interacts with active Ras in the presence of Nore1B. RNA interference against Nore1B attenuates NF-κB activation induced by T cell receptor (TCR) ligation, but not NF-κB activation induced by TNFα or lipoteichoic acid. In addition, Nore1B is also required for KiRas GV12-mediated ERK1 activation and Elk1 reporter activity in T cells. We also provide evidence that knockdown of Nore1B also impairs polarized redistribution of Ras at the B cell-T cell immune interface. Together, these findings suggest that endogenous Nore1B recruits active Ras to the APC-T cell interface and mediates the interaction between Ras and Carma1.
T-lymphocytes; NF-κB; Ras proteins; MAGUK proteins; Lymphocyte activation
We developed a pTOM construct that can express two proteins in a cell. Using pTOM ensures that two polypeptide-encoding nucleotide sequences are simultaneously transfected into the same cell. The ability to simultaneously express two separate polypeptide-encoding nucleotide sequences from the same vector in the same cell allows the user to determine the relationship between the two proteins. Another advantage is that one of the proteins can be used as a transfection marker/selector. The vector contains multiple cloning sites for insertions of polypeptide-encoding nucleotide sequences. Positive clones can be easily selected when performing cloning using ampicillin. Overall, this vector provides a convenient way to express dual proteins in a single mammalian cell.
The proline-rich Akt substrate of 40 kilodaltons (PRAS40) was identified as a raptor-binding protein that is phosphorylated directly by mammalian target of rapamycin (mTOR) complex 1 (mTORC1) but not mTORC2 in vitro, predominantly at PRAS40 (Ser183). The binding of S6K1 and 4E-BP1 to raptor requires a TOR signaling (TOS) motif, which contains an essential Phe followed by four alternating acidic and small hydrophobic amino acids. PRAS40 binding to raptor was severely inhibited by mutation of PRAS40 (Phe129 to Ala). Immediately carboxyl-terminal to Phe129 are two small hydrophobic amino acid followed by two acidic residues. PRAS40 binding to raptor was also abolished by mutation of the major mTORC1 phosphorylation site, Ser183, to Asp. PRAS40 (Ser183) was phosphorylated in intact cells; this phosphorylation was inhibited by rapamycin, by 2-deoxyglucose, and by overexpression of the tuberous sclerosis complex heterodimer. PRAS40 (Ser183) phosphorylation was also inhibited reversibly by withdrawal of all or of only the branched chain amino acids; this inhibition was reversed by overexpression of the Rheb GTPase. Overex-pressed PRAS40 suppressed the phosphorylation of S6K1 and 4E-BP1 at their rapamycin-sensitive phosphorylation sites, and reciprocally, overexpression of S6K1 or 4E-BP1 suppressed phosphorylation of PRAS40 (Ser183) and its binding to raptor. RNA interference-induced depletion of PRAS40 enhanced the amino acid-stimulated phosphorylation of both S6K1 and 4E-BP1. These results establish PRAS40 as a physiological mTORC1 substrate that contains a variant TOS motif. Moreover, they indicate that the ability of raptor to bind endogenous substrates is limiting for the activity of mTORC1 in vivo and is therefore a potential locus of regulation.
Hippo-Lats-Yorkie signaling regulates tissue overgrowth and tumorigenesis in Drosophila. We show that the Mst1 and Mst2 protein kinases, the mammalian Hippo orthologs, are cleaved and constitutively activated in the mouse liver. Combined Mst1/2 deficiency in the liver results in loss of inhibitory Ser127 phosphorylation of the Yorkie ortholog, Yap1, massive overgrowth, and hepatocellular carcinoma (HCC). Reexpression of Mst1 in HCC-derived cell lines promotes Yap1 Ser127 phosphorylation and inactivation, and abrogates their tumorigenicity. Notably, Mst1/2 inactivates Yap1 in liver through an intermediary kinase distinct from Lats1/2. Approximately 30% of human HCCs show low Yap1(Ser127) phosphorylation and a majority exhibit loss of cleaved, activated Mst1. Mst1/2 inhibition of Yap1 is an important pathway for tumor suppression in liver relevant to human HCC.
The pathways that regulate quiescence and tumor suppression in the liver have not been fully elucidated. We show that the Mst1 and Mst2 kinases are tumor suppressors and regulators of liver size in adults and that negative regulation of the transcriptional coactivator, Yap1, is central to Mst1/2 tumor suppressor function. Loss of both Mst1 and Mst2 is sufficient to initiate hepatocyte proliferation, resulting in dramatic liver overgrowth, resistance to pro-apoptotic stimuli, and the development of HCC. Mst1 and Mst2 promote phosphorylation of Yap1 and thereby suppress its oncogenic activity. Mst1/2 regulation of Yap1 is tissue-specific and, in the liver, involves an Mst1/2-regulated Yap1 kinase distinct from Lats1/2. Significantly, the Mst-Yap1 pathway is disrupted in a substantial fraction of human HCCs.
Mst1; Mst2; hepatocellular carcinoma; tumor suppressor; Yap1; hippo
The stimulatory effect of insulin on protein synthesis is due to its ability to activate various translation factors. We now show that insulin can increase protein synthesis capacity also by translational activation of TOP mRNAs encoding various components of the translation machinery. This translational activation involves the tuberous sclerosis complex (TSC), as the knockout of TSC1 or TSC2 rescues TOP mRNAs from translational repression in mitotically arrested cells. Similar results were obtained upon overexpression of Rheb, an immediate TSC1-TSC2 target. The role of mTOR, a downstream effector of Rheb, in translational control of TOP mRNAs has been extensively studied, albeit with conflicting results. Even though rapamycin fully blocks mTOR complex 1 (mTORC1) kinase activity, the response of TOP mRNAs to this drug varies from complete resistance to high sensitivity. Here we show that mTOR knockdown blunts the translation efficiency of TOP mRNAs in insulin-treated cells, thus unequivocally establishing a role for mTOR in this mode of regulation. However, knockout of the raptor or rictor gene has only a slight effect on the translation efficiency of these mRNAs, implying that mTOR exerts its effect on TOP mRNAs through a novel pathway with a minor, if any, contribution of the canonical mTOR complexes mTORC1 and mTORC2. This conclusion is further supported by the observation that raptor knockout renders the translation of TOP mRNAs rapamycin hypersensitive.
Mammalian sterile 20-like kinase 1 (Mst1) is a ubiquitously expressed serine/threonine kinase belonging to the family of Sterile 20-like kinases. MST1 has been inferred to play important roles in apoptosis and in the inhibition of proliferation in mammalian cells. Here, we describe the genetic characterization of Mst1-deficient mice produced by two distinct gene-trap insertions. Animals generated from clone RRT293 exhibit transmission ratio distortion favoring the mutated allele and is amplified with each generation. Inexplicably, while the mutated allele is favored for transmission, its homozygosity is embryonic lethal. By contrast, animals generated from the second Mst1 gene-trap clone, AJ0315, do not show any gross abnormalities. We find that the discrepancy in phenotype is most likely attributable to a second insertion in the RRT293 clone. Thus, a mutation in Mst1 alone does not affect survival. Our results set the stage for identification of the lethal second-site mutation that is paradoxically favored for transmission.
Mst1; Sterile 20-like kinases; transmission ratio distortion; gene-trap insertion
Insulin signaling through phosphatidylinositol 3-kinase (PI 3-kinase) activates the protein kinase Akt through phosphorylation of its threonine 308 and serine 473 residues by the PDK1 protein kinase and the Rictor-mammalian target of rapamycin complex (mTORC2), respectively. Remarkably, we show here that the Rictor protein is also present in cultured adipocytes in complexes containing Myo1c, a molecular motor that promotes cortical actin remodeling. Interestingly, the Rictor-Myo1c complex is biochemically distinct from the previously reported mTORC2 and can be immunoprecipitated independently of mTORC2. Furthermore, while RNA interference-directed silencing of Rictor results in the expected attenuation of Akt phosphorylation at serine 473, depletion of Myo1c is without effect. In contrast, loss of either Rictor or Myo1c inhibits phosphorylation of the actin filament regulatory protein paxillin at tyrosine 118. Furthermore, Myo1c-induced membrane ruffling of 3T3-L1 adipocytes is also compromised following Rictor knockdown. Interestingly, neither the mTORC2 inhibitor rapamycin nor the PI 3-kinase inhibitor wortmannin affects paxillin tyrosine 118 phosphorylation. Taken together, our findings suggest that the Rictor-Myo1c complex is distinct from mTORC2 and that Myo1c, in conjunction with Rictor, participates in cortical actin remodeling events.
The MAP kinases, discovered approximately twenty years ago, together with their immediate upstream regulators, are among the most highly studied signal transduction molecules. This body of work has shaped many aspects of our present views of signal transduction by protein kinases. The effort expended in this area reflects the extensive participation of these regulatory modules in the control of cell fate decisions, i.e., proliferation, differentiation and death, across all eukaryotic phylla and in all tissues of metazoans. The discovery of these kinases is reviewed, followed by a discussion of some of the features of this signaling module that account for its broad impact on cell function and its enormous interest to many investigators.