Human T2D-Associated Gene IMP2/IGF2BP2 Promotes the Commitment of Mesenchymal Stem Cells Into Adipogenic Lineage.

Excessive adiposity is the main cause of obesity and type two diabetes (T2D). Variants in human IMP2/IGF2BP2 gene are associated with increased risk of T2D. However, little is known about its role in adipogenesis and in insulin resistance. Here, we investigate the function of IMP2 during adipocyte development. Mice with Imp2 deletion in mesenchymal stem cells (MSC) are resistant to diet-induced obesity without glucose and insulin tolerance affected. Imp2 is essential for the early commitment of adipocyte-derived stem cells (ADSC) into preadipocytes, but the deletion of Imp2 in MSC is not required for the proliferation and terminal differentiation of committed preadipocytes. Mechanistically, Imp2 binds Wnt receptor Fzd8 mRNA and promotes its degradation by recruiting CCR4-NOT deadenylase complex in an mTOR-dependent manner. Our data demonstrate that Imp2 is required for maintaining white adipose tissue homeostasis through controlling mRNA stability in ADSC. However, the contribution of IMP2 to insulin resistance, a main risk of T2D, is not evident.

RNA m6A reader IMP2/IGF2BP2 promotes pancreatic ß-cell proliferation and insulin secretion by enhancing PDX1 expression.

BACKGROUND: Type 2 diabetes (T2D) is a common metabolic disease. Variants in human IGF2 mRNA binding protein 2 (IMP2/IGF2BP2) are associated with increased risk of T2D. IMP2 contributes to T2D susceptibility primarily through effects on insulin secretion. However, the underlying mechanism is not known. METHODS: To understand the role of IMP2 in insulin secretion and T2D pathophysiology, we generated Imp2 pancreatic ß-cell specific knockout mice (ßIMP2KO) by recombining the Imp2flox allele with Cre recombinase driven by the rat insulin 2 promoter. We further characterized metabolic phenotypes of ßIMP2KO mice and assessed their ß-cell functions. RESULTS: The deletion of IMP2 in pancreatic ß-cells leads to reduced compensatory ß-cell proliferation and function. Mechanically, IMP2 directly binds to Pdx1 mRNA and stimulates its translation in an m6A dependent manner. Moreover, IMP2 orchestrates IGF2-AKT-GSK3ß-PDX1 signaling to stable PDX1 polypeptides. In human EndoC-ßH1 cells, the over-expression of IMP2 is capable to enhance cell proliferation, PDX1 protein level and insulin secretion. CONCLUSION: Our work therefore reveals IMP2 as a critical regulator of pancreatic ß-cell proliferation and function; highlights the importance of posttranscriptional gene expression in T2D pathology.

A MicroRNA Linking Human Positive Selection and Metabolic Disorders.

Positive selection in Europeans at the 2q21.3 locus harboring the lactase gene has been attributed to selection for the ability of adults to digest milk to survive famine in ancient times. However, the 2q21.3 locus is also associated with obesity and type 2 diabetes in humans, raising the possibility that additional genetic elements in the locus may have contributed to evolutionary adaptation to famine by promoting energy storage, but which now confer susceptibility to metabolic diseases. We show here that the miR-128-1 microRNA, located at the center of the positively selected locus, represents a crucial metabolic regulator in mammals. Antisense targeting and genetic ablation of miR-128-1 in mouse metabolic disease models result in increased energy expenditure and amelioration of high-fat-diet-induced obesity and markedly improved glucose tolerance. A thrifty phenotype connected to miR-128-1-dependent energy storage may link ancient adaptation to famine and modern metabolic maladaptation associated with nutritional overabundance.

Liver-specific deletion of IGF2 mRNA binding protein-2/IMP2 reduces hepatic fatty acid oxidation and increases hepatic triglyceride accumulation.

Insulin-like growth factor 2 mRNA-binding proteins 1-3 (IGF2BP1-3, also known as IMP1-3) contribute to the regulation of RNAs in a transcriptome-specific context. Global deletion of the mRNA-binding protein insulin-like growth factor 2 mRNA-binding protein 2 (IGF2BP2 or IMP2) in mice causes resistance to obesity and fatty liver induced by a high-fat diet (HFD), whereas liver-specific IMP2 overexpression results in steatosis. To better understand the role of IMP2 in hepatic triglyceride metabolism, here we crossed mice expressing albumin-Cre with mice bearing a floxed Imp2 gene to generate hepatocyte-specific IMP2 knockout (LIMP2 KO) mice. Unexpectedly, the livers of LIMP2 KO mice fed an HFD accumulated more triglyceride. Although hepatocyte-specific IMP2 deletion did not alter lipogenic gene expression, it substantially decreased the levels of the IMP2 client mRNAs encoding carnitine palmitoyltransferase 1A (CPT1A) and peroxisome proliferator-activated receptor α (PPARα). This decrease was associated with their more rapid turnover and accompanied by significantly diminished rates of palmitate oxidation by isolated hepatocytes and liver mitochondria. HFD-fed control and LIMP2 KO mice maintained a similar glucose tolerance and insulin sensitivity up to 6 months; however, by 6 months, blood glucose and serum triglycerides in LIMP2 KO mice were modestly elevated but without evidence of liver damage. In conclusion, hepatocyte-specific IMP2 deficiency promotes modest diet-induced fatty liver by impairing fatty acid oxidation through increased degradation of the IMP2 client mRNAs PPARα and CPT1A This finding indicates that the previously observed marked protection against fatty liver conferred by global IMP2 deficiency in mice is entirely due to their reduced adiposity.

Pancreatic islet chromatin accessibility and conformation reveals distal enhancer networks of type 2 diabetes risk.

Genetic variants affecting pancreatic islet enhancers are central to T2D risk, but the gene targets of islet enhancer activity are largely unknown. We generate a high-resolution map of islet chromatin loops using Hi-C assays in three islet samples and use loops to annotate target genes of islet enhancers defined using ATAC-seq and published ChIP-seq data. We identify candidate target genes for thousands of islet enhancers, and find that enhancer looping is correlated with islet-specific gene expression. We fine-map T2D risk variants affecting islet enhancers, and find that candidate target genes of these variants defined using chromatin looping and eQTL mapping are enriched in protein transport and secretion pathways. At IGF2BP2, a fine-mapped T2D variant reduces islet enhancer activity and IGF2BP2 expression, and conditional inactivation of IGF2BP2 in mouse islets impairs glucose-stimulated insulin secretion. Our findings provide a resource for studying islet enhancer function and identifying genes involved in T2D risk.

IMP2 Increases Mouse Skeletal Muscle Mass and Voluntary Activity by Enhancing Autocrine Insulin-Like Growth Factor 2 Production and Optimizing Muscle Metabolism.

Insulin-like growth factor 2 (IGF2) mRNA binding protein 2 (IMP2) was selectively deleted from adult mouse muscle; two phenotypes were observed: decreased accrual of skeletal muscle mass after weaning and reduced wheel-running activity but normal forced treadmill performance. Reduced wheel running occurs when mice are fed a high-fat diet but is normalized when mice consume standard chow. The two phenotypes are due to altered output from different IMP2 client mRNAs. The reduced fiber size of IMP2-deficient muscle is attributable, in part, to diminished autocrine Igf2 production; basal tyrosine phosphorylation of the insulin and IGF1 receptors is diminished, and Akt1 activation is selectively reduced. Gsk3α is disinhibited, and S536-phosphorylated ε subunit of eukaryotic initiation factor 2B [eIF2Bε(S536)] is hyperphosphorylated. Protein synthesis is reduced despite unaltered mTOR complex 1 activity. The diet-dependent reduction in voluntary exercise is likely due to altered muscle metabolism, as contractile function is normal. IMP2-deficient muscle exhibits reduced fatty acid oxidation, due to a reduced abundance of mRNA of peroxisome proliferator-activated receptor α (PPARα), an IMP2 client, and PPARα protein. IMP2-deficient muscle fibers treated with a mitochondrial uncoupler to increase electron flux, as occurs with exercise, exhibit reduced oxygen consumption from fatty acids, with higher oxygen consumption from glucose. The greater dependence on muscle glucose metabolism during increased oxygen demand may promote central fatigue and thereby diminish voluntary activity.

Cryo-EM insight into the structure of MTOR complex 1 and its interactions with Rheb and substrates.

The mechanistic target of rapamycin (MTOR) is a giant protein kinase that, together with the accessory proteins Raptor and mLst8, forms a complex of over 1 MDa known as MTOR complex 1 (MTORC1). MTORC1, through its protein kinase activity, controls the accretion of cell mass through the regulation of gene transcription, mRNA translation, and protein turnover. MTORC1 is activated in an interdependent manner by insulin/growth factors and nutrients, especially amino acids, and is inhibited by stressors such as hypoxia and by the drug rapamycin. The action of insulin/growth factors converges on the small GTPase Rheb, which binds directly to the MTOR polypeptide in MTORC1 and, in its GTP-bound state, initiates kinase activation. Biochemical studies established that MTORC1 exists as a dimer of the MTOR/Raptor/mLst8 trimer, and progressive refinements in cryo-electron microscopy (cryo-EM) have enabled an increasingly clear picture of the architecture of MTORC1, culminating in a deep understanding of how MTORC1 interacts with and phosphorylates its best-known substrates-the eIF-4E binding protein/4E-BP, the p70 S6 kinase/S6K1B, and PRAS40/AKT1S1-and how this is inhibited by rapamycin. Most recently, Rheb-GTP has been shown to bind to MTORC1 in a cooperative manner at an allosteric site remote from the kinase domain that twists the latter into a catalytically competent configuration. Herein, we review the recent cryo-EM and associated biochemical studies of MTORC1 and seek to integrate these new results with the known physiology of MTORC1 regulation and signaling.

The Mst1 Kinase Is Required for Follicular B Cell Homing and B-1 B Cell Development.

The Mst1 and 2 cytosolic serine/threonine protein kinases are the mammalian orthologs of the Drosophila Hippo protein. Mst1 has been shown previously to participate in T-cell and B-cell trafficking and the migration of lymphocytes into secondary lymphoid organs in a cell intrinsic manner. We show here that the absence of Mst1 alone only modestly impacts B cell homing to lymph nodes. The absence of both Mst1 and 2 in hematopoietic cells results in relatively normal B cell development in the bone marrow and does not impact migration of immature B cells to the spleen. However, follicular B cells lacking both Mst1 and Mst2 mature in the splenic white pulp but are unable to recirculate to lymph nodes or to the bone marrow. These cells also cannot traffic efficiently to the splenic red pulp. The inability of late transitional and follicular B cells lacking Mst 1 and 2 to migrate to the red pulp explains their failure to differentiate into marginal zone B cell precursors and marginal zone B cells. Mst1 and Mst2 are therefore required for follicular B cells to acquire the ability to recirculate and also to migrate to the splenic red pulp in order to generate marginal zone B cells. In addition B-1 a B cell development is defective in the absence of Mst1.

IGF2 mRNA binding protein-2 is a tumor promoter that drives cancer proliferation through its client mRNAs IGF2 and HMGA1.

The gene encoding the Insulin-like Growth Factor 2 mRNA binding protein 2/IMP2 is amplified and overexpressed in many human cancers, accompanied by a poorer prognosis. Mice lacking IMP2 exhibit a longer lifespan and a reduced tumor burden at old age. Herein we show in a diverse array of human cancer cells that IMP2 overexpression stimulates and IMP2 elimination diminishes proliferation by 50-80%. In addition to its known ability to promote the abundance of Insulin-like Growth Factor 2/IGF2, we find that IMP2 strongly promotes IGF action, by binding and stabilizing the mRNA encoding the DNA binding protein HMGA1, a known oncogene. HMGA1 suppresses the abundance of IGF binding protein 2/IGFBP2 and Grb14, inhibitors of IGF action. IMP2 stabilization of HMGA1 mRNA plus IMP2 stimulated IGF2 production synergistically drive cancer cell proliferation and account for IMP2's tumor promoting action. IMP2's ability to promote proliferation and IGF action requires IMP2 phosphorylation by mTOR.

MST1/MST2 Protein Kinases: Regulation and Physiologic Roles.

The MST1 and MST2 protein kinases comprise the GCK-II subfamily of protein kinases. In addition to their amino-terminal kinase catalytic domain, related to that of the Saccharomyces cerevisiae protein kinase Ste20, their most characteristic feature is the presence near the carboxy terminus of a unique helical structure called a SARAH domain; this segment allows MST1/MST2 to homodimerize and to heterodimerize with the other polypeptides that contain SARAH domains, the noncatalytic polypeptides RASSF1-6 and Sav1/WW45. Early studies emphasized the potent ability of MST1/MST2 to induce apoptosis upon being overexpressed, as well as the conversion of the endogenous MST1/MST2 polypeptides to constitutively active, caspase-cleaved catalytic fragments during apoptosis initiated by any stimulus. Later, the cleaved, constitutively active form of MST1 was identified in nonapoptotic, quiescent adult hepatocytes as well as in cells undergoing terminal differentiation, where its presence is necessary to maintain those cellular states. The physiologic regulation of full length MST1/MST2 is controlled by the availability of its noncatalytic SARAH domain partners. Interaction with Sav1/WW45 recruits MST1/MST2 into a tumor suppressor pathway, wherein it phosphorylates and activates the Sav1-bound protein kinases Lats1/Lats2, potent inhibitors of the Yap1 and TAZ oncogenic transcriptional regulators. A constitutive interaction with the Rap1-GTP binding protein RASSF5B (Nore1B/RAPL) in T cells recruits MST1 (especially) and MST2 as an effector of Rap1's control of T cell adhesion and migration, a program crucial to immune surveillance and response; loss of function mutation in human MST1 results in profound immunodeficiency. MST1 and MST2 are also regulated by other protein kinases, positively by TAO1 and negatively by Par1, SIK2/3, Akt, and cRaf1. The growing list of candidate MST1/MST2 substrates suggests that the full range of MST1/MST2's physiologic programs and contributions to pathophysiology remains to be elucidated.

Kinases Mst1 and Mst2 positively regulate phagocytic induction of reactive oxygen species and bactericidal activity.

Mitochondria need to be juxtaposed to phagosomes for the synergistic production of ample reactive oxygen species (ROS) in phagocytes to kill pathogens. However, how phagosomes transmit signals to recruit mitochondria has remained unclear. Here we found that the kinases Mst1 and Mst2 functioned to control ROS production by regulating mitochondrial trafficking and mitochondrion-phagosome juxtaposition. Mst1 and Mst2 activated the GTPase Rac to promote Toll-like receptor (TLR)-triggered assembly of the TRAF6-ECSIT complex that is required for the recruitment of mitochondria to phagosomes. Inactive forms of Rac, including the human Rac2(D57N) mutant, disrupted the TRAF6-ECSIT complex by sequestering TRAF6 and substantially diminished ROS production and enhanced susceptibility to bacterial infection. Our findings demonstrate that the TLR-Mst1-Mst2-Rac signaling axis is critical for effective phagosome-mitochondrion function and bactericidal activity.

IGF2BP2/IMP2-Deficient mice resist obesity through enhanced translation of Ucp1 mRNA and Other mRNAs encoding mitochondrial proteins.

Although variants in the IGF2BP2/IMP2 gene confer risk for type 2 diabetes, IMP2, an RNA binding protein, is not known to regulate metabolism. Imp2(-/-) mice gain less lean mass after weaning and have increased lifespan. Imp2(-/-) mice are highly resistant to diet-induced obesity and fatty liver and display superior glucose tolerance and insulin sensitivity, increased energy expenditure, and better defense of core temperature on cold exposure. Imp2(-/-) brown fat and Imp2(-/-) brown adipocytes differentiated in vitro contain more UCP1 polypeptide than Imp2(+/+) despite similar levels of Ucp1 mRNA; the Imp2(-/-)adipocytes also exhibit greater uncoupled oxygen consumption. IMP2 binds the mRNAs encoding Ucp1 and other mitochondrial components, and most exhibit increased translational efficiency in the absence of IMP2. In vitro IMP2 inhibits translation of mRNAs bearing the Ucp1 untranslated segments. Thus IMP2 limits longevity and regulates nutrient and energy metabolism in the mouse by controlling the translation of its client mRNAs.

A genome-wide siRNA screen in mammalian cells for regulators of S6 phosphorylation.

mTOR complex1, the major regulator of mRNA translation in all eukaryotic cells, is strongly activated in most cancers. We performed a genome-wide RNAi screen in a human cancer cell line, seeking genes that regulate S6 phosphorylation, readout of mTORC1 activity. Applying a stringent selection, we retrieved nearly 600 genes wherein at least two RNAis gave significant reduction in S6-P. This cohort contains known regulators of mTOR complex 1 and is significantly enriched in genes whose depletion affects the proliferation/viability of the large set of cancer cell lines in the Achilles database in a manner paralleling that caused by mTOR depletion. We next examined the effect of RNAi pools directed at 534 of these gene products on S6-P in TSC1 null mouse embryo fibroblasts. 76 RNAis reduced S6 phosphorylation significantly in 2 or 3 replicates. Surprisingly, among this cohort of genes the only elements previously associated with the maintenance of mTORC1 activity are two subunits of the vacuolar ATPase and the CUL4 subunit DDB1. RNAi against a second set of 84 targets reduced S6-P in only one of three replicates. However, an indication that this group also bears attention is the presence of rpS6KB1 itself, Rac1 and MAP4K3, a protein kinase that supports amino acid signaling to rpS6KB1. The finding that S6 phosphorylation requires a previously unidentified, functionally diverse cohort of genes that participate in fundamental cellular processes such as mRNA translation, RNA processing, DNA repair and metabolism suggests the operation of feedback pathways in the regulation of mTORC1 operating through novel mechanisms.

YAP Inhibition Restores Hepatocyte Differentiation in Advanced HCC, Leading to Tumor Regression.

Defective Hippo/YAP signaling in the liver results in tissue overgrowth and development of hepatocellular carcinoma (HCC). Here, we uncover mechanisms of YAP-mediated hepatocyte reprogramming and HCC pathogenesis. YAP functions as a rheostat in maintaining metabolic specialization, differentiation, and quiescence within the hepatocyte compartment. Increased or decreased YAP activity reprograms subsets of hepatocytes to different fates associated with deregulation of the HNF4A, CTNNB1, and E2F transcriptional programs that control hepatocyte quiescence and differentiation. Importantly, treatment with small interfering RNA-lipid nanoparticles (siRNA-LNPs) targeting YAP restores hepatocyte differentiation and causes pronounced tumor regression in a genetically engineered mouse HCC model. Furthermore, YAP targets are enriched in an aggressive human HCC subtype characterized by a proliferative signature and absence of CTNNB1 mutations. Thus, our work reveals Hippo signaling as a key regulator of the positional identity of hepatocytes, supports targeting of YAP using siRNA-LNPs as a paradigm of differentiation-based therapy, and identifies an HCC subtype that is potentially responsive to this approach.

Amino acids activate mammalian target of rapamycin (mTOR) complex 1 without changing Rag GTPase guanyl nucleotide charging.

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 Ras(Ser-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 (32)P-labeled whole cells, or just from the pool associated with the lysosomal membrane, exhibit constitutive [(32)P]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.

The Ets transcription factor GABP is a component of the hippo pathway essential for growth and antioxidant defense.

The transcriptional coactivator Yes-associated protein (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 glutathione depletion inhibits GABP transcriptional activity and depletes YAP. In contrast, activating YAP by deleting Mst1/Mst2 strongly protects against acetaminophen-induced liver injury. Similar to its effects on YAP, 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.

G protein-coupled receptors engage the mammalian Hippo pathway through F-actin: F-Actin, assembled in response to Galpha12/13 induced RhoA-GTP, promotes dephosphorylation and activation of the YAP oncogene.

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.

Hippo signaling regulates differentiation and maintenance in the exocrine pancreas.

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. METHODS: We used a Cre/LoxP system to create mice with pancreas-specific disruptions in Mst1 and Mst2 (Pdx1-Cre;Mst1(-/-);Mst2(fl/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. RESULTS: Contrary to expectations, the pancreatic mass of Pdx1-Cre;Mst1(-/-);Mst2(fl/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(-/-);Mst2(fl/fl) mice recapitulated cellular and molecular changes observed during chemical-induced pancreatitis in mice. CONCLUSIONS: The mammalian Hippo kinases, and YAP, maintain postnatal pancreatic acinar differentiation in mice.

mTOR complex 2 phosphorylates IMP1 cotranslationally to promote IGF2 production and the proliferation of mouse embryonic fibroblasts.

Lack of IGF2 in mice results in diminished embryonic growth due to diminished cell proliferation. Here we show that mouse embryonic fibroblasts lacking the RNA-binding protein IMP1 (IGF2 mRNA-binding protein 1) have defective splicing and translation of IGF2 mRNAs, markedly reduced IGF2 polypeptide production, and diminished proliferation. The proliferation of the IMP1-null fibroblasts can be restored to wild-type levels by IGF2 in vitro or by re-expression of IMP1, which corrects the defects in IGF2 RNA splicing and translation. The ability of IMP1 to correct these defects is dependent on IMP1 phosphorylation at Ser181, which is catalyzed cotranslationally by mTOR complex 2 (mTORC2). Phosphorylation strongly enhances IMP1 binding to the IGF2-leader 3 5' untranslated region, which is absolutely required to enable IGF2-leader 3 mRNA translational initiation by internal ribosomal entry. These findings uncover a new mechanism by which mTOR regulates organismal growth by promoting IGF2 production in the mouse embryo through mTORC2-catalyzed cotranslational IMP1/IMP3 phosphorylation. Inasmuch as TORC2 is activated by association with ribosomes, the present results indicate that mTORC2-catalyzed cotranslational protein phosphorylation is a core function of this complex.

Protein kinases of the Hippo pathway: regulation and substrates.

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 Saccharomyces 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 orthologs 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.