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

IGF2BPs as novel m6A readers: Diverse roles in regulating cancer cell biological functions, hypoxia adaptation, metabolism, and immunosuppressive tumor microenvironment

m6A methylation is the most frequent modification of mRNA in eukaryotes and plays a crucial role in cancer progression by regulating biological functions. Insulin-like growth factor 2 mRNA-binding proteins (IGF2BP) are newly identified m6A 'readers'. They belong to a family of RNA-binding proteins, which bind to the m6A sites on different RNA sequences and stabilize them to promote cancer progression. In this review, we summarize the mechanisms by which different upstream factors regulate IGF2BP in cancer. The current literature analyzed here reveals that the IGF2BP family proteins promote cancer cell proliferation, survival, and chemoresistance, inhibit apoptosis, and are also associated with cancer glycolysis, angiogenesis, and the immune response in the tumor microenvironment. Therefore, with the discovery of their role as 'readers' of m6A and the characteristic re-expression of IGF2BPs in cancers, it is important to elucidate their mechanism of action in the immunosuppressive tumor microenvironment. We also describe in detail the regulatory and interaction network of the IGF2BP family in downstream target RNAs and discuss their potential clinical applications as diagnostic and prognostic markers, as well as recent advances in IGF2BP biology and associated therapeutic value.

Atractylodes macrocephala III suppresses EMT in cervical cancer by regulating IGF2BP3 through ETV5

Atractylodes macrocephala III (ATL III), with anti-inflammatory and antitumor effects, is the main compound of Atractylodes macrocephala. Whether ATL III has an effect on cervical cancer and the specific mechanism are still unclear. Here, we investigated the effects of ATL III on cervical cancer cells at different concentrations and found that ATL III downregulates insulin-like growth factor 2 mRNA-binding protein 3 (IGF2BP3), which was found to be highly expressed in cervical cancer tissue by RNA-Seq. In this study, we found that ATL III promotes apoptosis and regulates epithelial-mesenchymal transition (EMT) in cervical cancer cells (HeLa and SiHa cells) and that IGF2BP3 is a common target gene of ATL III in HeLa and SiHa cells. The expression level of IGF2BP3 in cervical cancer cells was proportional to their migration and invasion abilities. This was verified by transfection of cells with a small interfering RNA and an IGF2BP3 overexpression plasmid. After ATL III treatment, the migration and invasion abilities of cervical cancer cells were obviously reduced, but these effects were attenuated after overexpression of IGF2BP3. In addition, the transcription factor IGF2BP3 was predicted by the JASPAR system. After intersection with our sequencing results, we verified the promotional effect of ETV5 (ETS translocation variant 5) on IGF2BP3 and found that ALT III inhibited ETV5. In general, our research showed that ATL III inhibits the migration and invasion of cervical cancer cells by regulating IGF2BP3 through ETV5.

The mTORC2 signaling network: targets and cross-talks

The mechanistic target of rapamycin, mTOR, controls cell metabolism in response to growth signals and stress stimuli. The cellular functions of mTOR are mediated by two distinct protein complexes, mTOR complex 1 (mTORC1) and mTORC2. Rapamycin and its analogs are currently used in the clinic to treat a variety of diseases and have been instrumental in delineating the functions of its direct target, mTORC1. Despite the lack of a specific mTORC2 inhibitor, genetic studies that disrupt mTORC2 expression unravel the functions of this more elusive mTOR complex. Like mTORC1 which responds to growth signals, mTORC2 is also activated by anabolic signals but is additionally triggered by stress. mTORC2 mediates signals from growth factor receptors and G-protein coupled receptors. How stress conditions such as nutrient limitation modulate mTORC2 activation to allow metabolic reprogramming and ensure cell survival remains poorly understood. A variety of downstream effectors of mTORC2 have been identified but the most well-characterized mTORC2 substrates include Akt, PKC, and SGK, which are members of the AGC protein kinase family. Here, we review how mTORC2 is regulated by cellular stimuli including how compartmentalization and modulation of complex components affect mTORC2 signaling. We elaborate on how phosphorylation of its substrates, particularly the AGC kinases, mediates its diverse functions in growth, proliferation, survival, and differentiation. We discuss other signaling and metabolic components that cross-talk with mTORC2 and the cellular output of these signals. Lastly, we consider how to more effectively target the mTORC2 pathway to treat diseases that have deregulated mTOR signaling.

IGF2BP1/IMP1 Deletion Enhances a Facultative Stem Cell State via Regulation of MAP1LC3B

BACKGROUND & AIMS

The intestinal epithelium interfaces with a diverse milieu of luminal contents while maintaining robust digestive and barrier functions. Facultative intestinal stem cells are cells that survive tissue injury and divide to re-establish the epithelium. Prior studies have shown autophagic state as functional marker of facultative intestinal stem cells, but regulatory mechanisms are not known. The current study evaluated a post-transcriptional regulation of autophagy as an important factor for facultative stem cell state and tissue regeneration.

METHODS

We evaluated stem cell composition, autophagic vesicle content, organoid formation, and in vivo regeneration in mice with intestinal epithelial deletion of the RNA binding protein IGF2 messenger RNA binding protein 1 (IMP1). The contribution of autophagy to resulting in vitro and in vivo phenotypes was evaluated via genetic inactivation of Atg7. Molecular analyses of IMP1 modulation of autophagy at the protein and transcript localization levels were performed using IMP1 mutant studies and single-molecule fluorescent in situ hybridization.

RESULTS

Epithelial Imp1 deletion reduced leucine rich repeat containing G protein coupled receptor 5 cell frequency but enhanced both organoid formation efficiency and in vivo regeneration after irradiation. We confirmed prior studies showing increased autophagy with IMP1 deletion. Deletion of Atg7 reversed the enhanced regeneration observed with Imp1 deletion. IMP1 deletion or mutation of IMP1 phosphorylation sites enhanced expression of essential autophagy protein microtubule-associated protein 1 light chain 3β. Furthermore, immunofluorescence imaging coupled with single-molecule fluorescent in situ hybridization showed IMP1 colocalization with MAP1LC3B transcripts at homeostasis. Stress induction led to decreased colocalization.

CONCLUSIONS

Depletion of IMP1 enhances autophagy, which promotes intestinal regeneration via expansion of facultative intestinal stem cells.

RNA binding protein IGF2BP2 expression is induced by stress in the heart and mediates dilated cardiomyopathy

The IGF2BP family of RNA binding proteins consists of three paralogs that regulate intracellular RNA localization, RNA stability, and translational control. Although IGF2BP1 and 3 are oncofetal proteins, IGF2BP2 expression is maintained in many tissues, including the heart, into adulthood. IGF2BP2 is upregulated in cardiomyocytes during cardiac stress and remodeling and returns to normal levels in recovering hearts. We wondered whether IGF2BP2 might play an adaptive role during cardiac stress and recovery. Enhanced expression of an IGF2BP2 transgene in a conditional, inducible mouse line leads to dilated cardiomyopathy (DCM) and death within 3-4 weeks in newborn or adult hearts. Downregulation of the transgene after 2 weeks, however, rescues these mice, with complete recovery by 12 weeks. Hearts overexpressing IGF2BP2 downregulate sarcomeric and mitochondrial proteins and have fragmented mitochondria and elongated, thinner sarcomeres. IGF2BP2 is also upregulated in DCM or myocardial infarction patients. These results suggest that IGF2BP2 may be an attractive target for therapeutic intervention in cardiomyopathies.

In vivo CRISPR screen identifies LTN1 as a novel tumor suppressor ubiquitinating insulin-like growth factor 2 mRNA-binding protein 1 in hepatocellular carcinoma

BACKGROUND AND AIMS

Hepatocellular carcinoma (HCC) is a frequent and aggressive kind of cancer. Although E3 ligases play important roles in HCC development, several E3 ligases remain unknown.

APPROACH AND RESULTS

Through in vivo CRISPR knockout (KO) screens targeting related E3 ligase genes in HCC nude mice models, we discovered LTN1 as a novel tumor suppressor in HCC. Co-IP paired with 2D-LC-MS/MS and subsequent western blotting in HCC cells were used to identify the interactome of LTN1. Compared to matched normal tissues, the expression of LTN1 was decreased in human HCC tissues (ANT) (157/209). Clinically, patients with HCC who expressed low levels of LTN1 had a poor prognosis. Forced expression of LTN1 decreased cell growth in vitro and in vivo, whereas knockdown of LTN1 increased cell growth. Mechanistically, elevated LTN1 expression inhibited HCC cell growth by ubiquitinating and destabilizing the IGF2BP1 protein, which inhibited the c-Myc and IGF-1R signaling pathways. There was a negative correlation between the LTN1 protein expression and the IGF2BP1 protein expression in HCC tissues (R2=0.2799, P=0.0165).

CONCLUSIONS

LTN1 may be a crucial tumor suppressor for determining the prognosis and a possible therapeutic target since it inhibits the proliferation of HCC cells by ubiquitinating IGF2BP1.

Parkin regulates IGF2BP3 through ubiquitination in the tumourigenesis of cervical cancer

BACKGROUND

Insulin-like growth Factor 2 mRNA-binding protein 3 (IGF2BP3) is a highly conserved RNA-binding protein and plays a critical role in regulating posttranscriptional modifications.

METHODS

Immunoprecipitation was used to examine the interaction of Parkin and IGF2BP3. Mass spectrometry was performed to identify the ubiquitination sites of IGF2BP3. RNA-immunoprecipitation was conducted to examine the target genes of IGF2BP3. Xenograft mouse model was constructed to determine the tumorigenesis of IGF2BP3.

RESULTS

IGF2BP3 expression is negatively correlated with Parkin expression in human cervical cancer cells and tissues. Parkin directly interacts with IGF2BP3, and overexpression of Parkin causes the proteasomal degradation of IGF2BP3, while knockdown of PARK2 increases the protein levels of IGF2BP3. Mechanistically, in vivo and in vitro ubiquitination assays demonstrated that Parkin is able to ubiquitinate IGF2BP3. Moreover, the ubiquitination site of IGF2BP3 was identified at K213 in the first KH domain of IGF2BP3. IGF2BP3 mutation results in the loss of its oncogenic function as an m6A reader, resulting in the inactivation of the phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) signalling pathways. In addition, IGF2BP3 mutation results in the attenuation of Parkin-mediated mitophagy, indicating its inverse role in regulating Parkin. Consequently, the tumourigenesis of cervical cancer is also inhibited by IGF2BP3 mutation.

CONCLUSION

IGF2BP3 is ubiquitinated and regulated by the E3 ubiquitin ligase Parkin in human cervical cancer and ubiquitination modification plays an important role in modulating IGF2BP3 function. Thus, understanding the role of IGF2BP3 in tumourigenesis could provide new insights into cervical cancer therapy.

Oncofetal protein IGF2BPs in human cancer: functions, mechanisms and therapeutic potential

N⁶-methyladenosine (m⁶A) is the most prevalent and well-characterized internal chemical modification in eukaryotic RNA, influencing gene expression and phenotypic changes by controlling RNA fate. Insulin-like growth factor-2 mRNA-binding proteins (IGF2BPs) preferentially function as m⁶A effector proteins, promoting stability and translation of m⁶A-modified RNAs. IGF2BPs, particularly IGF2BP1 and IGF2BP3, are widely recognized as oncofetal proteins predominantly expressed in cancer rather than normal tissues, playing a critical role in tumor initiation and progression. Consequently, IGF2BPs hold potential for clinical applications and serve as a good choice for targeted treatment strategies. In this review, we discuss the functions and mechanisms of IGF2BPs as m⁶A readers and explore the therapeutic potential of targeting IGF2BPs in human cancer.

A combined computational and functional approach identifies IGF2BP2 as a driver of chemoresistance in a wide array of pre-clinical models of colorectal cancer

AIM

Chemoresistance is a major cause of treatment failure in colorectal cancer (CRC) therapy. In this study, the impact of the IGF2BP family of RNA-binding proteins on CRC chemoresistance was investigated using in silico, in vitro, and in vivo approaches.

METHODS

Gene expression data from a well-characterized cohort and publicly available cross-linking immunoprecipitation sequencing (CLIP-Seq) data were collected. Resistance to chemotherapeutics was assessed in patient-derived xenografts (PDXs) and patient-derived organoids (PDOs). Functional studies were performed in 2D and 3D cell culture models, including proliferation, spheroid growth, and mitochondrial respiration analyses.

RESULTS

We identified IGF2BP2 as the most abundant IGF2BP in primary and metastastatic CRC, correlating with tumor stage in patient samples and tumor growth in PDXs. IGF2BP2 expression in primary tumor tissue was significantly associated with resistance to selumetinib, gefitinib, and regorafenib in PDOs and to 5-fluorouracil and oxaliplatin in PDX in vivo. IGF2BP2 knockout (KO) HCT116 cells were more susceptible to regorafenib in 2D and to oxaliplatin, selumitinib, and nintedanib in 3D cell culture. Further, a bioinformatic analysis using CLIP data suggested stabilization of target transcripts in primary and metastatic tumors. Measurement of oxygen consumption rate (OCR) and extracellular acidification rate (ECAR) revealed a decreased basal OCR and an increase in glycolytic ATP production rate in IGF2BP2 KO. In addition, real-time reverse transcriptase polymerase chain reaction (qPCR) analysis confirmed decreased expression of genes of the respiratory chain complex I, complex IV, and the outer mitochondrial membrane in IGF2BP2 KO cells.

CONCLUSIONS

IGF2BP2 correlates with CRC tumor growth in vivo and promotes chemoresistance by altering mitochondrial respiratory chain metabolism. As a druggable target, IGF2BP2 could be used in future CRC therapy to overcome CRC chemoresistance.

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.

miR-193b-3p Promotes Proliferation of Goat Skeletal Muscle Satellite Cells through Activating IGF2BP1

As a well-known cancer-related miRNA, miR-193b-3p is enriched in skeletal muscle and dysregulated in muscle disease. However, the mechanism underpinning this has not been addressed so far. Here, we probed the impact of miR-193b-3p on myogenesis by mainly using goat tissues and skeletal muscle satellite cells (MuSCs), compared with mouse C2C12 myoblasts. miR-193b-3p is highly expressed in goat skeletal muscles, and ectopic miR-193b-3p promotes MuSCs proliferation and differentiation. Moreover, insulin-like growth factor-2 mRNA-binding protein 1 (IGF2BP1) is the most activated insulin signaling gene when there is overexpression of miR-193b-3p; the miRNA recognition element (MRE) within the IGF1BP1 3' untranslated region (UTR) is indispensable for its activation. Consistently, expression patterns and functions of IGF2BP1 were similar to those of miR-193b-3p in tissues and MuSCs. In comparison, ectopic miR-193b-3p failed to induce PAX7 expression and myoblast proliferation when there was IGF2BP1 knockdown. Furthermore, miR-193b-3p destabilized IGF2BP1 mRNA, but unexpectedly promoted levels of IGF2BP1 heteronuclear RNA (hnRNA), dramatically. Moreover, miR-193b-3p could induce its neighboring genes. However, miR-193b-3p inversely regulated IGF2BP1 and myoblast proliferation in the mouse C2C12 myoblast. These data unveil that goat miR-193b-3p promotes myoblast proliferation via activating IGF2BP1 by binding to its 3' UTR. Our novel findings highlight the positive regulation between miRNA and its target genes in muscle development, which further extends the repertoire of miRNA functions.

The role of insulin-like growth factor 2 mRNA binding proteins in female reproductive pathophysiology

Insulin-like growth factor 2 (IGF2) mRNA binding proteins (IMPs) family belongs to a highly conserved family of RNA-binding proteins (RBPs) and is responsible for regulating RNA processing including localization, translation and stability. Mammalian IMPs (IMP1-3) take part in development, metabolism and tumorigenesis, where they are believed to play a major role in cell growth, metabolism, migration and invasion. IMPs have been identified that are expressed in ovary, placenta and embryo. The up-to-date evidence suggest that IMPs are involved in folliculogenesis, oocyte maturation, embryogenesis, implantation, and placentation. The dysregulation of IMPs not only contributes to carcinogenesis but also disturbs the female reproduction, and may participate in the pathogenesis of reproductive diseases and obstetric syndromes, such as polycystic ovary syndrome (PCOS), pre-eclampsia (PE), gestational diabetes mellitus (GDM) and gynecological tumors. In this review, we summarize the role of IMPs in female reproductive pathophysiology, and hope to provide new insights into the identification of potential therapeutic targets.

mTOR substrate phosphorylation in growth control

The target of rapamycin (TOR), discovered 30 years ago, is a highly conserved serine/threonine protein kinase that plays a central role in regulating cell growth and metabolism. It is activated by nutrients, growth factors, and cellular energy. TOR forms two structurally and functionally distinct complexes, TORC1 and TORC2. TOR signaling activates cell growth, defined as an increase in biomass, by stimulating anabolic metabolism while inhibiting catabolic processes. With emphasis on mammalian TOR (mTOR), we comprehensively reviewed the literature and identified all reported direct substrates. In the context of recent structural information, we discuss how mTORC1 and mTORC2, despite having a common catalytic subunit, phosphorylate distinct substrates. We conclude that the two complexes recruit different substrates to phosphorylate a common, minimal motif.

IGF2BP3 promotes cell metastasis and is associated with poor patient survival in nasopharyngeal carcinoma

Metastasis contributes to treatment failure in nasopharyngeal carcinoma (NPC) patients. Our study aimed at elucidating the role of insulin-like growth factor 2 mRNA binding protein 3 (IGF2BP3) in NPC metastasis and the underlying mechanism involved. IGF2BP3 expression in NPC was determined by bioinformatics, quantitative polymerase chain reaction and immunohistochemistry analyses. The biological function of IGF2BP3 was investigated by using in vitro and in vivo studies. In this study, IGF2BP3 mRNA and protein levels were elevated in NPC tissues. In addition, IGF2BP3 exerted an oncogenic role by promoting epithelial-mesenchymal transition (EMT), thereby inducing NPC cell migration and invasion. Further studies revealed that IGF2BP3 regulated the expression of key regulators of EMT by activating AKT/mTOR signalling, thus stimulating NPC cell migration and invasion. Remarkably, targeting IGF2BP3 delayed NPC metastasis through attenuating p-AKT and vimentin expression and inducing E-cadherin expression in vivo. Moreover, IGF2BP3 protein levels positively correlated with distant metastasis after initial treatment. Importantly, IGF2BP3 expression served as an independent prognostic factor in predicting the overall survival and distant metastasis-free survival of NPC patients. This work identifies IGF2BP3 as a novel prognostic marker and a new target for NPC treatment.

The calcium signaling enzyme CD38 - a paradigm for membrane topology defining distinct protein functions

CD38 is a single-pass transmembrane enzyme catalyzing the synthesis of two nucleotide second messengers, cyclic ADP-ribose (cADPR) from NAD and nicotinic acid adenine dinucleotide phosphate (NAADP) from NADP. The former mediates the mobilization of the endoplasmic Ca²⁺-stores in response to a wide range of stimuli, while NAADP targets the endo-lysosomal stores. CD38 not only possesses multiple enzymatic activities, it also exists in two opposite membrane orientations. Type III CD38 has the catalytic domain facing the cytosol and is responsible for producing cellular cADPR. The type II CD38 has an opposite orientation and is serving as a surface receptor mediating extracellular functions such as cell adhesion and lymphocyte activation. Its ecto-NADase activity also contributes to the recycling of external NAD released by apoptosis. Endocytosis can deliver surface type II CD38 to endo-lysosomes, which acidic environment favors the production of NAADP. This article reviews the rationale and evidence that have led to CD38 as a paradigm for membrane topology defining distinct functions of proteins. Also described is the recent discovery of a hitherto unknown cADPR-synthesizing enzyme, SARM1, ushering in a new frontier in cADPR-mediated Ca²⁺-signaling.

Insulin action at a molecular level - 100 years of progress

The discovery of insulin 100 years ago and its application to the treatment of human disease in the years since have marked a major turning point in the history of medicine. The availability of purified insulin allowed for the establishment of its physiological role in the regulation of blood glucose and ketones, the determination of its amino acid sequence, and the solving of its structure. Over the last 50 years, the function of insulin has been applied into the discovery of the insulin receptor and its signaling cascade to reveal the role of impaired insulin signaling-or resistance-in the progression of type 2 diabetes. It has also become clear that insulin signaling can impact not only classical insulin-sensitive tissues, but all tissues of the body, and that in many of these tissues the insulin signaling cascade regulates unexpected physiological functions. Despite these remarkable advances, much remains to be learned about both insulin signaling and how to use this molecular knowledge to advance the treatment of type 2 diabetes and other insulin-resistant states.

Structures and target RNA preferences of the RNA-binding protein family of IGF2BPs: An overview

Insulin-like growth factor 2 mRNA-binding proteins (IMPs, IGF2BPs) act in mRNA transport and translational control but are oncofetal tumor marker proteins. The IMP protein family represents a number of bona fide multi-domain RNA-binding proteins with up to six RNA-binding domains, resulting in a high complexity of possible modes of interactions with target mRNAs. Their exact mechanism in stability control of oncogenic mRNAs is only partially understood. Our and other laboratories' recent work has significantly pushed the understanding of IMP protein specificities both toward RNA engagement and between each other from NMR and crystal structures serving the basis for systematic biochemical and functional investigations. We here summarize the known structural and biochemical information about IMP RNA-binding domains and their RNA preferences. The article also touches on the respective roles of RNA secondary and protein tertiary structures for specific RNA-protein complexes, including the limited knowledge about IMPs' protein-protein interactions, which are often RNA mediated.

mTORC2 deploys the mRNA binding protein IGF2BP1 to regulate c-MYC expression and promote cell survival

mTORC2 promotes cell survival by phosphorylating AKT and enhancing its activity. Inactivation of mTORC2 reduces viability through down-regulation of E2F1 caused by up-regulation of c-MYC. An additional target of mTORC2 is IGF2BP1, an oncofetal RNA binding protein expressed de novo in a wide array of malignancies. IGF2BP1 enhances c-MYC expression by protecting the coding region instability sequence (CRD) of its mRNA from endonucleolytic cleavage. Here we show that repression of mTORC2 signalling and prevention of Ser181 phosphorylation of IGF2BP1 enhanced translation and destabilization of the endogenous c-myc mRNA as well as the mRNA of reporter transcripts carrying the CRD sequence in frame. The consequent increase in c-MYC protein was accompanied by the emergence of an apoptotic c-MYC overexpressing population. On the other hand, preventing phosphorylation of IGF2BP1 on Tyr396 by Src kinase caused the accumulation of translationally silent transcripts through sequestration by IGF2BP1 into cytoplasmic granules. The apoptotic effect of mTORC2 signalling deprivation was augmented when preceded by inhibition of IGF2BP1 phosphorylation by the Src kinase in concert with further increase of c-MYC levels because of enhanced translation of the previously stored mRNA only in the presence of IGF2BP1. Furthermore, the combined administration of mTORC2 and Src inhibitors exhibited synergism in delaying xenograft growth in female NOD.CB17-Prkdc^scid/J mice. The above in vitro and in vivo findings may be applied for the induction of targeted apoptosis of cells expressing de novo the oncofetal protein IGF2BP1, a feature of aggressive malignancies resulting in a more focused anticancer therapeutic approach.

mTOR-autophagy promotes pulmonary senescence through IMP1 in chronic toxicity of methamphetamine

It is growingly concerned about methamphetamine (MA)-induced lung toxicity. IMP1 is identified as a key molecule for cell life processes, but the role of IMP1 in MA-induced senescence remains unclear. The purpose of this study was to investigate whether chronic exposure to MA can cause autophagy and senescence of the lungs, whether there are interactions between Mammalian target of rapamycin (mTOR) and IMP1 and whether IMP1 is involved in pulmonary senescence promoted by mTOR-autophagy. The rats were randomly divided into control group and MA group, following by H&E staining, immunohistochemistry staining and Western blot. The alveolar epithelial cells were proceeded by ß-galactosidase staining, cell cycle detection, transfection and co-immunoprecipitation. Long-term exposure to MA led to the thickening of alveolar septum and more compact lungs. MA promoted the conversion of LC3-I to LC3-II and inhibited the activation of mTOR to induce autophagy. Bioinformatics and co-immunoprecipitation results presented the interactions between IMP1 and mTOR. MA induced cell senescence by decreasing IMP1, up-regulating p21 and p53, arresting cell cycle and increasing SA-β-gal. Overexpression of IMP1 reduced p21 and SA-β-gal to inhibit the senescence of alveolar epithelial cells. These results demonstrated that mTOR-autophagy promotes pulmonary senescence through IMP1 in chronic toxicity of methamphetamine.

Regulation of mTORC2 Signaling

Mammalian target of rapamycin (mTOR), a serine/threonine protein kinase and a master regulator of cell growth and metabolism, forms two structurally and functionally distinct complexes, mTOR complex 1 (mTORC1) and mTORC2. While mTORC1 signaling is well characterized, mTORC2 is relatively poorly understood. mTORC2 appears to exist in functionally distinct pools, but few mTORC2 effectors/substrates have been identified. Here, we review recent advances in our understanding of mTORC2 signaling, with particular emphasis on factors that control mTORC2 activity.

The Diverse Functions of IMP2/IGF2BP2 in Metabolism

The human insulin-like growth factor 2 (IGF2) mRNA binding protein family (IMPs/IGF2BPs) is involved in a spectrum of biological processes, including development, tumorigenesis, and stemness. IMPs play a major role in post-transcriptional regulation of RNAs through the ribonucleoprotein complex (RNP). They have emerged as direct mammalian target of rapamycin (mTOR) substrates that coordinate nutrient stimulation and RNA life cycle control. IMP2 is a human type 2 diabetes (T2D) gene associated with impaired insulin secretion. Recently, using murine models, the substantial progress in understanding disease mechanisms has highlighted the significance of IMP2 in metabolism. This new knowledge may have the potential for therapeutic benefit.

miRNA-98-5p Targeting IGF2BP1 Induces Mesenchymal Stem Cell Apoptosis by Modulating PI3K/Akt and p53 in Immune Thrombocytopenia

Immune thrombocytopenia (ITP) is a common hematological autoimmune disease, in which defective mesenchymal stem cells (MSCs) are potentially involved. Our previous study suggested that MSCs in ITP patients displayed enhanced apoptosis. MicroRNAs (miRNAs) play important roles in ITP by affecting megakaryopoiesis, platelet production and immunoregulation, whereas the roles of miRNAs in ITP-MSCs remain unknown. In a previous study, we performed microarray analysis to obtain mRNA and miRNA profiles of ITP-MSCs. In the present study, we reanalyze the data and identify miR-98-5p as a candidate miRNA contributing to MSC deficiency in ITP. miR-98-5p acts through targeting insulin-like growth factor 2 mRNA-binding protein 1 (IGF2BP1), and the subsequent downregulation of insulin-like growth factor 2 (IGF-2) causes inhibition of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway, which is involved in the process of MSC deficiency. Furthermore, miR-98-5p upregulates p53 by inhibiting β-transducin repeat-containing protein (β-TrCP)-dependent p53 ubiquitination. Moreover, miR-98-5p overexpression impairs the therapeutic effect of MSCs in ITP mice. All-trans retinoic acid (ATRA) protects MSCs from apoptosis by downregulating miR-98-5p, thus providing a potential therapeutic approach for ITP. Our findings demonstrate that miR-98-5p is a critical regulator of ITP-MSCs, which will help us thoroughly understand the pathogenesis of ITP.

The Oncogenic Functions of Insulin-like Growth Factor 2 mRNA-Binding Protein 3 in Human Carcinomas

IGF2BP3 (also known as IMP3, KOC), a member of the insulin-like growth factor mRNA-binding protein family (IMPs), has been a research target in recent studies of promoting embryo development and exacerbating cancer. IGF2BP3 is ubiquitously expressed in early embryogenesis stages but limited in postembryonic stages, which is important in many physiological aspects such as stem cell renewal, morphological development and metabolism. A large number of studies show that IGF2BP3 interacts with many kinds of non-coding RNAs and proteins to promote cancer cell proliferation and metastasis and inhibit cancer cell apoptosis. As IGF2BP3 is highly expressed in advanced cancers and associated with poor overall survival rates of patients, it may be a potential molecular marker in cancer diagnosis for the detection of cancerous tissues and an indicator of cancer stages. Therefore, anti-IGF2BP3 drugs or monoclonal antibodies are expected as new therapeutic methods in cancer treatment. This review summarizes recent findings among IGF2BP3, RNA and proteins in cancer processes, with a focus on its cancer-promoting mechanisms and potential application as a new biomarker for cancer diagnosis and treatment.

Roles of insulin-like growth factor II in regulating female reproductive physiology

The number of growth factors involved in female fertility has been extensively studied, but reluctance to add essential growth factors in culture media has limited progress in optimizing embryonic growth and implantation outcomes, a situation that has ultimately led to reduced pregnancy outcomes. Insulin-like growth factor II (IGF-II) is the most intricately regulated of all known reproduction-related growth factors characterized to date, and is perhaps the predominant growth factor in human ovarian follicles. This review aims to concisely summarize what is known about the role of IGF-II in follicular development, oocyte maturation, embryonic development, implantation success, placentation, fetal growth, and in reducing placental cell apoptosis, as well as present strategies that use growth factors in culture systems to improve the developmental potential of oocytes and embryos in different species. Synthesizing the present knowledge about the physiological roles of IGF-II in follicular development, oocyte maturation, and early embryonic development should, on the one hand, deepen our overall understanding of the potential beneficial effects of growth factors in female reproduction and on the other hand support development (optimization) of improved outcomes for assisted reproductive technologies.

The IGF-II-Insulin Receptor Isoform-A Autocrine Signal in Cancer: Actionable Perspectives

Insulin receptor overexpression is a common event in human cancer. Its overexpression is associated with a relative increase in the expression of its isoform A (IR^A), a shorter variant lacking 11 aa in the extracellular domain, conferring high affinity for the binding of IGF-II along with added intracellular signaling specificity for this ligand. Since IGF-II is secreted by the vast majority of malignant solid cancers, where it establishes autocrine stimuli, the co-expression of IGF-II and IR^A in cancer provides specific advantages such as apoptosis escape, growth, and proliferation to those cancers bearing such a co-expression pattern. However, little is known about the exact role of this autocrine ligand–receptor system in sustaining cancer malignant features such as angiogenesis, invasion, and metastasis. The recent finding that the overexpression of angiogenic receptor kinase EphB4 along with VEGF-A is tightly dependent on the IGF-II/IR^A autocrine system independently of IGFIR provided new perspectives for all malignant IGF2omas (those aggressive solid cancers secreting IGF-II). The present review provides an updated view of the IGF system in cancer, focusing on the biology of the autocrine IGF-II/IR^A ligand–receptor axis and supporting its underscored role as a malignant-switch checkpoint target.

IGF2BP3 From Physiology to Cancer: Novel Discoveries, Unsolved Issues, and Future Perspectives

RNA network control is a key aspect of proper cellular homeostasis. In this context, RNA-binding proteins (RBPs) play a major role as regulators of the RNA life cycle due to their capability to bind to RNA sequences and precisely direct nuclear export, translation/degradation rates, and the intracellular localization of their target transcripts. Alterations in RBP expression or functions result in aberrant RNA translation and may drive the emergence and progression of several pathological conditions, including cancer. Among the RBPs, insulin-like growth factor 2 mRNA-binding protein 3 (IGF2BP3) is of particular interest in tumorigenesis and tumor progression. This review highlights the molecular mechanisms underlying the oncogenic functions of IGF2BP3, summarizes the therapeutic potential related to its inhibition and notes the fundamental issues that remain unanswered. To fully exploit IGF2BP3 for tumor diagnosis and therapy, it is crucial to dissect the mechanisms governing IGF2BP3 re-expression and to elucidate the complex interactions between IGF2BP3 and its target mRNAs as normal cells become tumor cells.

Resolving the topological enigma in Ca2+ signaling by cyclic ADP-ribose and NAADP

Cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) are two structurally distinct messengers that mobilize the endoplasmic and endolysosomal Ca2+ stores, respectively. Both are synthesized by the CD38 molecule (CD38), which has long been thought to be a type II membrane protein whose catalytic domain, intriguingly, faces to the outside of the cell. Accordingly, for more than 20 years, it has remained unresolved how CD38 can use cytosolic substrates such as NAD and NADP to produce messengers that target intracellular Ca2+ stores. The discovery of type III CD38, whose catalytic domain faces the cytosol, has now begun to clarify this topological conundrum. This article reviews the ideas and clues leading to the discovery of the type III CD38; highlights an innovative approach for uncovering its natural existence; and discusses the regulators of its activity, folding, and degradation. We also review the compartmentalization of cADPR and NAADP biogenesis. We further discuss the possible mechanisms that promote type III CD38 expression and appraise a proposal of a Ca2+-signaling mechanism based on substrate limitation and product translocation. The surprising finding of another enzyme that produces cADPR and NAADP, sterile α and TIR motif-containing 1 (SARM1), is described. SARM1 regulates axonal degeneration and has no sequence similarity with CD38 but can catalyze the same set of multireactions and has the same cytosolic orientation as the type III CD38. The intriguing finding that SARM1 is activated by nicotinamide mononucleotide to produce cADPR and NAADP suggests that it may function as a regulated Ca2+-signaling enzyme like CD38.

mTORC2 affects the maintenance of the muscle stem cell pool

Background

The mammalian target of rapamycin complex 2 (mTORC2), containing the essential protein rictor, regulates cellular metabolism and cytoskeletal organization by phosphorylating protein kinases, such as PKB/Akt, PKC, and SGK. Inactivation of mTORC2 signaling in adult skeletal muscle affects its metabolism, but not muscle morphology and function. However, the role of mTORC2 in adult muscle stem cells (MuSCs) has not been investigated.

Method

Using histological, biochemical, and molecular biological methods, we characterized the muscle phenotype of mice depleted for rictor in the Myf5-lineage (RImyfKO) and of mice depleted for rictor in skeletal muscle fibers (RImKO). The proliferative and myogenic potential of MuSCs was analyzed upon cardiotoxin-induced injury in vivo and in isolated myofibers in vitro.

Results

Skeletal muscle of young and 14-month-old RImyfKO mice appeared normal in composition and function. MuSCs from young RImyfKO mice exhibited a similar capacity to proliferate, differentiate, and fuse as controls. In contrast, the number of MuSCs was lower in young RImyfKO mice than in controls after two consecutive rounds of cardiotoxin-induced muscle regeneration. Similarly, the number of MuSCs in RImyfKO mice decreased with age, which correlated with a decline in the regenerative capacity of mutant muscle. Interestingly, reduction in the number of MuSCs was also observed in 14-month-old RImKO muscle.

Conclusions

Our study shows that mTORC2 signaling is dispensable for myofiber formation, but contributes to the homeostasis of MuSCs. Loss of mTORC2 does not affect their myogenic function, but impairs the replenishment of MuSCs after repeated injuries and their maintenance during aging. These results point to an important role of mTORC2 signaling in MuSC for muscle homeostasis.

mTOR signaling in Brown and Beige adipocytes: implications for thermogenesis and obesity

Brown and beige adipocytes are mainly responsible for nonshivering thermogenesis or heat production, despite the fact that they have distinguished features in distribution, developmental origin, and functional activation. As a nutrient sensor and critical regulator of energy metabolism, mechanistic target of rapamycin (mTOR) also plays an important role in the development and functional maintenance of adipocytes. While the recent studies support the notion that mTOR (mTORC1 and mTORC2) related signaling pathways are of great significance for thermogenesis and the development of brown and beige adipocytes, the exact roles of mTOR in heat production are controversial. The similarities and disparities in terms of thermogenesis might be ascribed to the use of different animal models and experimental systems, distinct features of brown and beige adipocytes, and the complexity of regulatory networks of mTORC1 and mTORC2 in energy metabolism.

IGF2BP1 Significantly Enhances Translation Efficiency of Duck Hepatitis A Virus Type 1 without Affecting Viral Replication

As a disease characterized by severe liver necrosis and hemorrhage, duck viral hepatitis (DVH) is mainly caused by duck hepatitis A virus (DHAV). The positive-strand RNA genome of DHAV type 1 (DHAV-1) contains an internal ribosome entry site (IRES) element within the 5′ untranslated region (UTR), structured sequence elements within the 3′ UTR, and a poly(A) tail at the 3′ terminus. In this study, we first examined that insulin-like growth factor-2 mRNA-binding protein-1 (IGF2BP1) specifically interacted with the DHAV-1 3′ UTR by RNA pull-down assay. The interaction between IGF2BP1 and DHAV-1 3′ UTR strongly enhanced IRES-mediated translation efficiency but failed to regulate DHAV-1 replication in a duck embryo epithelial (DEE) cell line. The viral propagation of DHAV-1 strongly enhanced IGF2BP1 expression level, and viral protein accumulation was identified as the key point to this increment. Collectively, our data demonstrated the positive role of IGF2BP1 in DHAV-1 viral proteins translation and provided data support for the replication mechanism of DHAV-1.

RNA-Binding Protein IGF2BP2/IMP2 is a Critical Maternal Activator in Early Zygotic Genome Activation

A number of genes involved in zygotic genome activation (ZGA) have been identified, but the RNA-binding maternal factors that are directly related to ZGA in mice remain unclear. The present study shows that maternal deletion of Igf  2bp2 (also commonly known as Imp2) in mouse embryos causes early embryonic developmental arrest in vitro at the 2-cell-stage. Transcriptomics and proteomics analyses of 2-cell-stage embryos in mice reveal that deletion of IMP2 downregulates the expression of Ccar1 and Rps14, both of which are required for early embryonic developmental competence. IGF2, a target of IMP2, when added in culture media, increases the proportion of wild-type embryos that develop successfully to the blastocyst stage: from 29% in untreated controls to 65% (50 × 10-9 m IGF2). Furthermore, in an experiment related to embryo transfer, foster mothers receiving IGF2-treated embryos deliver more pups per female than females who receive untreated control embryos. In clinically derived human oocytes, the addition of IGF2 to the culture media significantly enhances the proportion of embryos that develop successfully. Collectively, the findings demonstrate that IMP2 is essential for the regulation and activation of genes known to be involved in ZGA and reveal the potential embryonic development-related utility of IGF2 for animal biotechnology and for assisted reproduction in humans.

Targeting mTOR for cancer therapy

Mechanistic target of rapamycin (mTOR) is a protein kinase regulating cell growth, survival, metabolism, and immunity. mTOR is usually assembled into several complexes such as mTOR complex 1/2 (mTORC1/2). In cooperation with raptor, rictor, LST8, and mSin1, key components in mTORC1 or mTORC2, mTOR catalyzes the phosphorylation of multiple targets such as ribosomal protein S6 kinase β-1 (S6K1), eukaryotic translation initiation factor 4E binding protein 1 (4E-BP1), Akt, protein kinase C (PKC), and type-I insulin-like growth factor receptor (IGF-IR), thereby regulating protein synthesis, nutrients metabolism, growth factor signaling, cell growth, and migration. Activation of mTOR promotes tumor growth and metastasis. Many mTOR inhibitors have been developed to treat cancer. While some of the mTOR inhibitors have been approved to treat human cancer, more mTOR inhibitors are being evaluated in clinical trials. Here, we update recent advances in exploring mTOR signaling and the development of mTOR inhibitors for cancer therapy. In addition, we discuss the mechanisms underlying the resistance to mTOR inhibitors in cancer cells.

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.

VICKZ1 enhances tumor progression and metastasis in lung adenocarcinomas in mice

The VICKZ (Igf2bp) family of RNA binding proteins regulate RNA function at many levels, including intracellular RNA localization, RNA stability, and translational control. One or more of the three VICKZ paralogs are upregulated in many different types of cancers. Here, we show how VICKZ1 enhances, and dominant negative VICKZ1 inhibits, cell migration, growth in soft agar, and wound healing in a mouse lung adenocarcinoma cell line containing a constitutively active, mutant Kras. Similarly, modulation of VICKZ1 activity promotes or inhibits metastases upon implantation of these cells into syngeneic mice. To test these effects in a genetic model system, we generated a mouse with an inducible VICKZ1 transgene and found that isolated overexpression of VICKZ1 in the lungs had no noticeable effect on morphology. Although directed overexpression of mutant Kras in the lungs led to the formation of small adenomas, concurrent overexpression of VICKZ1 remarkably accelerated tumor growth and formation of pulmonary adenocarcinomas. VICKZ1-containing ribonucleoprotein complexes are highly enriched in Kras mRNA in lung adenocarcinoma cells, and Kras signaling is enhanced in these cells by overexpression of VICKZ1. Analysis of lung carcinoma patients reveals that elevated VICKZ1 expression correlates with lower overall survival; this reduction is dramatically enhanced in those patients bearing a mutant Kras gene. Our study reveals that RNA binding proteins of the VICKZ family can synergize with Kras to influence signaling and oncogenic activity.

IGF2BP1 promotes SRF-dependent transcription in cancer in a m6A- and miRNA-dependent manner

The oncofetal mRNA-binding protein IGF2BP1 and the transcriptional regulator SRF modulate gene expression in cancer. In cancer cells, we demonstrate that IGF2BP1 promotes the expression of SRF in a conserved and N6-methyladenosine (m6A)-dependent manner by impairing the miRNA-directed decay of the SRF mRNA. This results in enhanced SRF-dependent transcriptional activity and promotes tumor cell growth and invasion. At the post-transcriptional level, IGF2BP1 sustains the expression of various SRF-target genes. The majority of these SRF/IGF2BP1-enhanced genes, including PDLIM7 and FOXK1, show conserved upregulation with SRF and IGF2BP1 synthesis in cancer. PDLIM7 and FOXK1 promote tumor cell growth and were reported to enhance cell invasion. Consistently, 35 SRF/IGF2BP1-dependent genes showing conserved association with SRF and IGF2BP1 expression indicate a poor overall survival probability in ovarian, liver and lung cancer. In conclusion, these findings identify the SRF/IGF2BP1-, miRNome- and m6A-dependent control of gene expression as a conserved oncogenic driver network in cancer.

Integrin alpha V beta 3 targeted dendrimer-rapamycin conjugate reduces fibroblast-mediated prostate tumor progression and metastasis

Therapeutic strategies targeting both cancer cells and associated cells in the tumor microenvironment offer significant promise in cancer therapy. We previously reported that generation 5 (G5) dendrimers conjugated with cyclic-RGD peptides target cells expressing integrin alpha V beta 3. In this study, we report a novel dendrimer conjugate modified to deliver the mammalian target of rapamycin (mTOR) inhibitor, rapamycin. In vitro analyses demonstrated that this drug conjugate, G5-FI-RGD-rapamycin, binds to prostate cancer (PCa) cells and fibroblasts to inhibit mTOR signaling and VEGF expression. In addition, G5-FI-RGD-rapamycin inhibits mTOR signaling in cancer cells more efficiently under proinflammatory conditions compared to free rapamycin. In vivo studies established that G5-FI-RGD-rapamycin significantly inhibits fibroblast-mediated PCa progression and metastasis. Thus, our results suggest the potential of new rapamycin-conjugated multifunctional nanoparticles for PCa therapy.

Alternative mechanisms of translation initiation: An emerging dynamic regulator of the proteome in health and disease

Eukaryotic mRNAs were historically thought to rely exclusively on recognition and binding of their 5' cap by initiation factors to effect protein translation. While internal ribosome entry sites (IRESs) are well accepted as necessary for the cap-independent translation of many viral genomes, there is now recognition that eukaryotic mRNAs also undergo non-canonical modes of translation initiation. Recently, high-throughput assays have identified thousands of mammalian transcripts with translation initiation occurring at non-canonical start codons, upstream of and within protein coding regions. In addition to IRES-mediated events, regulatory mechanisms of translation initiation have been described involving alternate 5' cap recognition, mRNA sequence elements, and ribosome selection. These mechanisms ensure translation of specific mRNAs under conditions where cap-dependent translation is shut down and contribute to pathological states including cardiac hypertrophy and cancer. Such global and gene-specific dynamic regulation of translation presents us with an increasing number of novel therapeutic targets. While these newly discovered modes of translation initiation have been largely studied in isolation, it is likely that several act on the same mRNA and exquisite coordination is necessary to maintain 'normal' translation. In this short review, we summarize the current state of knowledge of these alternative mechanisms of eukaryotic protein translation, their contribution to normal and pathological cell biology, and the potential of targeting translation initiation therapeutically in human disease.

Ras Suppresses TXNIP Expression by Restricting Ribosome Translocation

Oncogenic Ras upregulates aerobic glycolysis to meet the bioenergetic and biosynthetic demands of rapidly growing cells. In contrast, thioredoxin-interacting protein (TXNIP) is a potent inhibitor of glucose uptake and is frequently downregulated in human cancers. Our laboratory previously discovered that Ras activation suppresses TXNIP transcription and translation. In this study, we developed a system to study how Ras affects TXNIP translation in the absence of transcriptional effects. We show that whereas Ras drives a global increase in protein translation, it suppresses TXNIP protein synthesis by reducing the rate at which ribosomes transit the coding region of TXNIP mRNA. To investigate the underlying mechanism(s), we randomized or optimized the codons in the TXNIP message without altering the TXNIP primary amino acid sequence. Translation from these mRNA variants was still repressed by Ras, implying that mRNA secondary structure, microRNAs (miRNAs), RNA binding proteins, or codon usage does not contribute to the blockade of TXNIP synthesis. Rather, we show that the N terminus of the growing TXNIP polypeptide is the target for Ras-dependent translational repression. Our work demonstrates how Ras suppresses TXNIP translation elongation in the face of a global upregulation of protein synthesis and provides new insight into Ras-dependent metabolic reprogramming.

mTORC2 modulates the amplitude and duration of GFAT1 Ser-243 phosphorylation to maintain flux through the hexosamine pathway during starvation

The mechanistic target of rapamycin (mTOR) controls metabolic pathways in response to nutrients. Recently, we have shown that mTOR complex 2 (mTORC2) modulates the hexosamine biosynthetic pathway (HBP) by promoting the expression of the key enzyme of the HBP, glutamine:fructose-6-phosphate aminotransferase 1 (GFAT1). Here, we found that GFAT1 Ser-243 phosphorylation is also modulated in an mTORC2-dependent manner. In response to glutamine limitation, active mTORC2 prolongs the duration of Ser-243 phosphorylation, albeit at lower amplitude. Blocking glycolysis using 2-deoxyglucose robustly enhances Ser-243 phosphorylation, correlating with heightened mTORC2 activation, increased AMPK activity, and O-GlcNAcylation. However, when 2-deoxyglucose is combined with glutamine deprivation, GFAT1 Ser-243 phosphorylation and mTORC2 activation remain elevated, whereas AMPK activation and O-GlcNAcylation diminish. Phosphorylation at Ser-243 promotes GFAT1 expression and production of GFAT1-generated metabolites including ample production of the HBP end-product, UDP-GlcNAc, despite nutrient starvation. Hence, we propose that the mTORC2-mediated increase in GFAT1 Ser-243 phosphorylation promotes flux through the HBP to maintain production of UDP-GlcNAc when nutrients are limiting. Our findings provide insights on how the HBP is reprogrammed via mTORC2 in nutrient-addicted cancer cells.

Who does TORC2 talk to?

The target of rapamycin (TOR) is a protein kinase that, by forming complexes with partner proteins, governs diverse cellular signalling networks to regulate a wide range of processes. TOR thus plays central roles in maintaining normal cellular functions and, when dysregulated, in diverse diseases. TOR forms two distinct types of multiprotein complexes (TOR complexes 1 and 2, TORC1 and TORC2). TORC1 and TORC2 differ in their composition, their control and their substrates, so that they play quite distinct roles in cellular physiology. Much effort has been focused on deciphering the detailed regulatory links within the TOR pathways and the structure and control of TOR complexes. In this review, we summarize recent advances in understanding mammalian (m) TORC2, its structure, its regulation, and its substrates, which link TORC2 signalling to the control of cell functions. It is now clear that TORC2 regulates several aspects of cell metabolism, including lipogenesis and glucose transport. It also regulates gene transcription, the cytoskeleton, and the activity of a subset of other protein kinases.

Rapamycin-independent IGF2 expression in Tsc2-null mouse embryo fibroblasts and human lymphangioleiomyomatosis cells

Lymphangioleiomyomatosis (LAM) is a rare, almost exclusively female lung disease linked to inactivating mutations in tuberous sclerosis complex 2 (TSC2), a tumor suppressor gene that controls cell metabolic state and growth via regulation of the mechanistic target of rapamycin (mTORC1) signaling. mTORC1 is frequently activated in human cancers and, although the mTORC1 inhibitor rapamycin has a cytostatic effect, it is, in general, unable to elicit a robust curative effect or tumor regression. Using RNA-Seq, we identified (1) Insulin-like Growth Factor (IGF2) as one of the genes with the highest fold-change difference between human TSC2-null and TSC2-expressing angiomyolipoma cells from a patient with LAM, and (2) the mouse IGF2 homolog Igf2, as a top-ranking gene according to fold change between Tsc2^-/- and Tsc2^+/+ mouse embryo fibroblasts (MEFs). We extended transcript-level findings to protein level, observing increased Igf2 protein expression and Igf2 secretion by Tsc2^-/- MEFs. Increased Igf2 expression was not due to epigenetic imprinting, but was partially mediated through the Stat3 pathway and was completely insensitive to rapamycin treatment. An siRNA-mediated decrease of Igf2 resulted in decreased Stat3 phosphorylation, suggesting presence of an autocrine Igf2/Stat3 amplification cycle in Tsc2^-/- MEFs. In human pulmonary LAM lesions and metastatic cell clusters, high levels of IGF2 were associated with mTORC1 activation. In addition, treatment of three primary IGF2-expressing LAM lung cell lines with rapamycin did not result in IGF2 level changes. Thus, targeting of IGF2 signaling may be of therapeutic value to LAM patients, particularly those who are unresponsive to rapamycin.

The Roles of Insulin-Like Growth Factor 2 mRNA-Binding Protein 2 in Cancer and Cancer Stem Cells

RNA-binding proteins (RBPs) mediate the localization, stability, and translation of the target transcripts and fine-tune the physiological functions of the proteins encoded. The insulin-like growth factor (IGF) 2 mRNA-binding protein (IGF2BP, IMP) family comprises three RBPs, IGF2BP1, IGF2BP2, and IGF2BP3, capable of associating with IGF2 and other transcripts and mediating their processing. IGF2BP2 represents the least understood member of this family of RBPs; however, it has been reported to participate in a wide range of physiological processes, such as embryonic development, neuronal differentiation, and metabolism. Its dysregulation is associated with insulin resistance, diabetes, and carcinogenesis and may potentially be a powerful biomarker and candidate target for relevant diseases. This review summarizes the structural features, regulation, and functions of IGF2BP2 and their association with cancer and cancer stem cells.

The Role of Mammalian Target of Rapamycin (mTOR) in Insulin Signaling

The mammalian target of rapamycin (mTOR) is a serine/threonine kinase that controls a wide spectrum of cellular processes, including cell growth, differentiation, and metabolism. mTOR forms two distinct multiprotein complexes known as mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), which are characterized by the presence of raptor and rictor, respectively. mTOR controls insulin signaling by regulating several downstream components such as growth factor receptor-bound protein 10 (Grb10), insulin receptor substrate (IRS-1), F-box/WD repeat-containing protein 8 (Fbw8), and insulin like growth factor 1 receptor/insulin receptor (IGF-IR/IR). In addition, mTORC1 and mTORC2 regulate each other through a feedback loop to control cell growth. This review outlines the current understanding of mTOR regulation in insulin signaling in the context of whole body metabolism.

Influenza virus differentially activates mTORC1 and mTORC2 signaling to maximize late stage replication

Influenza A virus usurps host signaling factors to regulate its replication. One example is mTOR, a cellular regulator of protein synthesis, growth and motility. While the role of mTORC1 in viral infection has been studied, the mechanisms that induce mTORC1 activation and the substrates regulated by mTORC1 during influenza virus infection have not been established. In addition, the role of mTORC2 during influenza virus infection remains unknown. Here we show that mTORC2 and PDPK1 differentially phosphorylate AKT upon influenza virus infection. PDPK1-mediated phoshorylation of AKT at a distinct site is required for mTORC1 activation by influenza virus. On the other hand, the viral NS1 protein promotes phosphorylation of AKT at a different site via mTORC2, which is an activity dispensable for mTORC1 stimulation but known to regulate apoptosis. Influenza virus HA protein and down-regulation of the mTORC1 inhibitor REDD1 by the virus M2 protein promote mTORC1 activity. Systematic phosphoproteomics analysis performed in cells lacking the mTORC2 component Rictor in the absence or presence of Torin, an inhibitor of both mTORC1 and mTORC2, revealed mTORC1-dependent substrates regulated during infection. Members of pathways that regulate mTORC1 or are regulated by mTORC1 were identified, including constituents of the translation machinery that once activated can promote translation. mTORC1 activation supports viral protein expression and replication. As mTORC1 activation is optimal midway through the virus life cycle, the observed effects on viral protein expression likely support the late stages of influenza virus replication when infected cells undergo significant stress.

Drug-resistant influenza viruses commonly arise due to frequent genetic changes and current antiviral drugs are not highly efficient. These underscore the need for new antiviral therapies effective against influenza viruses. Understanding how influenza virus uses cellular proteins for infection can potentially identify novel targets for pharmacological intervention. Influenza virus modulates cellular pathways to promote its replication and avoid immune restriction. Here we reveal the interplay between the cellular protein mTOR, which functions in two distinct protein complexes, and influenza virus infection. mTOR complex 1 (mTORC1) is activated during influenza virus infection through a cascade of specific modifications, or phosphorylation events, and by reducing the levels of another cellular protein termed REDD1, which is an mTORC1 inhibitor. Activation of mTORC1 results in additional phosphorylation events that together promote viral protein expression and replication. On the other hand, mTOR complex 2 (mTORC2) phosphorylates AKT at a specific site during infection, which is a process mediated by the viral NS1 protein that is known to regulate viral-mediated cell death. Since these effects occur midway through the virus life cycle in the infected cell, mTORC1 and mTORC2 activation are likely important to regulate the cellular environment in order to facilitate the late stages of viral infection.

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.

Some types of cancers develop when genes known as oncogenes or tumor promoters become faulty, and are present at abnormally high levels or inappropriately turned on. For example, cancer cells often have extra copies of the gene IMP2 and therefore produce too much the IMP2 protein. Previous research has shown that mice that lack the IMP2 protein develop fewer cancers and live longer, while patients whose cancers make too much IMP2 have a poorer prognosis.

In healthy cells, the IMP2 protein normally helps to make new gene products by stabilising certain newly produced RNA molecules – the precursors of proteins, and in some cases by promoting the translation of these RNAs into proteins. For example, IMP2 binds to the mRNA that encodes the protein IGF2, which is a protein that helps cells to grow and is commonly produced in large quantities by cancer cells. However, until now it was not clear whether IMP2 only acts by increasing the production of IGF2 or also contributes to cancer growth in other ways.

Using a range of human cancer cell lines, and healthy mouse cells, Dai et al. first confirmed that without IMP2, cancer cells made less IGF2 and grew less quickly. When IGF2 was added to the cells lacking IMP2, it only partially restored their ability to grow. Further experiments revealed that cells without IMP2 had increased levels of proteins that counteract the effects of IGF2. Usually, IMP2 binds and stabilizes the mRNA that encodes the oncogenic protein HMGA1, which is known to regulate the number of ‘anti-IGF2 proteins’. However, without IMP2, the HMGA1 levels drop, which causes an increase of the anti-IGF2 proteins.

This indicates that IMP2 promotes cancer cell growth both by enabling cells to produce more IGF2 and by suppressing inhibitors of IGF2 action. This suggests that cancer patients whose tumors have abnormally high levels of IMP2 may be especially sensitive to drugs that target and inhibit IGF2.

Express yourself: how PP2A-B55Pab1 helps TORC1 talk to TORC2

The control of cell fate, growth and proliferation in response to nitrogen availability is a tightly controlled process, with the two TOR complexes (TORC1 and TORC2) and their effectors playing a central role. PP2A-B55^Pab1 has recently been shown to be a key element in this response in fission yeast, where it regulates cell cycle progression and sexual differentiation. Importantly, a recent study from our group has shown that PP2A-B55^Pab1 acts as a mediator between the activities of the two TOR signaling modules, enabling a crosstalk that is required to engage in the differentiation program. In this review, we recapitulate the studies that have led to our current understanding of the interplay between TOR complexes. Moreover, we discuss several aspects of the response to nitrogen availability that still require further attention, and which will be important in the future to fully realize the implications of phosphatase activity in the context of TOR signaling.

ZBP1 phosphorylation at serine 181 regulates its dendritic transport and the development of dendritic trees of hippocampal neurons

Local protein synthesis occurs in axons and dendrites of neurons, enabling fast and spatially restricted responses to a dynamically changing extracellular environment. Prior to local translation, mRNA that is to be translated is packed into ribonucleoprotein particles (RNPs) where RNA binding proteins ensure mRNA silencing and provide a link to molecular motors. ZBP1 is a component of RNP transport particles and is known for its role in the local translation of β-actin mRNA. Its binding to mRNA is regulated by tyrosine 396 phosphorylation, and this particular modification was shown to be vital for axonal growth and dendritic branching. Recently, additional phosphorylation of ZBP1 at serine 181 (Ser181) was described in non-neuronal cells. In the present study, we found that ZBP1 is also phosphorylated at Ser181 in neurons in a mammalian/mechanistic target of rapamycin complex 2-, Src kinase-, and mRNA binding-dependent manner. Furthermore, Ser181 ZBP1 phosphorylation was essential for the proper dendritic branching of hippocampal neurons that were cultured in vitro and for the proper ZBP1 dendritic distribution and motility.

Heterogeneous nuclear ribonucleoprotein M associates with mTORC2 and regulates muscle differentiation

Mammalian target of rapamycin (mTOR) plays a range of crucial roles in cell survival, growth, proliferation, metabolism, and morphology. However, mTOR forms two distinct complexes, mTOR complex 1 and mTOR complex 2 (mTORC1 and mTORC2), via association with a series of different components; this allows the complexes to execute their wide range of functions. This study explores further the composition of the mTORC2 complex. Utilizing Rictor knock-out cells, immunoprecipitation and mass spectrometry, a novel Rictor associated protein, heterogeneous nuclear ribonucleoprotein M (hnRNP M), was identified. The association between hnRNP M and Rictor was verified using recombinant and endogenous protein and the binding site was found to be within aa 1~532 of hnRNP M. The presence of hnRNP M significantly affects phosphorylation of SGK1 S422, but not of Akt S473, PKCα S657 and PKCζ T560. Furthermore, hnRNP M also plays a critical role in muscle differentiation because knock-down of either hnRNP M or Rictor in C2C12 myoblasts reduced differentiation. This decrease is able to be rescued by overexpression SGK S422D in hnRNP M knockdown C2C12 myoblasts. Taken together, we have identified a novel Rictor/mTOR binding molecule, hnRNP M, that allows mTORC2 signaling to phosphorylate SGK1 thus regulating muscle differentiation.

Putative RNA-directed adaptive mutations in cancer evolution

Understanding the molecular mechanisms behind the capacity of cancer cells to adapt to the tumor microenvironment and to anticancer therapies is a major challenge. In this context, cancer is believed to be an evolutionary process where random mutations and the selection process shape the mutational pattern and phenotype of cancer cells. This article challenges the notion of randomness of some cancer-associated mutations by describing molecular mechanisms involving stress-mediated biogenesis of mRNA-derived small RNAs able to target and increase the local mutation rate of the genomic loci they originate from. It is proposed that the probability of some mutations at specific loci could be increased in a stress-specific and RNA-depending manner. This would increase the probability of generating mutations that could alleviate stress situations, such as those triggered by anticancer drugs. Such a mechanism is made possible because tumor- and anticancer drug-associated stress situations trigger both cellular reprogramming and inflammation, which leads cancer cells to express molecular tools allowing them to “attack” and mutate their own genome in an RNA-directed manner.

Enhanced CLIP Uncovers IMP Protein-RNA Targets in Human Pluripotent Stem Cells Important for Cell Adhesion and Survival

Human pluripotent stem cells (hPSCs) require precise control of post-transcriptional RNA networks to maintain proliferation and survival. Using enhanced UV crosslinking and immunoprecipitation (eCLIP), we identify RNA targets of the IMP/IGF2BP family of RNA-binding proteins in hPSCs. At the broad region and binding site levels, IMP1 and IMP2 show reproducible binding to a large and overlapping set of 3' UTR-enriched targets. RNA Bind-N-seq applied to recombinant full-length IMP1 and IMP2 reveals CA-rich motifs that are enriched in eCLIP-defined binding sites. We observe that IMP1 loss in hPSCs recapitulates IMP1 phenotypes, including a reduction in cell adhesion and increase in cell death. For cell adhesion, we find IMP1 maintains levels of integrin mRNA specifically regulating RNA stability of ITGB5 in hPSCs. Additionally, we show that IMP1 can be linked to hPSC survival via direct target BCL2. Thus, transcriptome-wide binding profiles identify hPSC targets modulating well-characterized IMP1 roles.