Insulin receptor substrate-4 signaling in quiescent rat hepatocytes and in regenerating rat liver

Is Insulin Receptor Substrate4 (IRS4) a Platform Involved in the Activation of Several Oncogenes?

The IRS (insulin receptor substrate) family of scaffold proteins includes insulin receptor substrate-4 (IRS4), which is expressed only in a few cell lines, including human kidney, brain, liver, and thymus and some cell lines. Its N-terminus carries a phosphotyrosine-binding (PTB) domain and a pleckstrin homology domain (PH), which distinguishes it as a member of this family. In this paper, we collected data about the molecular mechanisms that explain the relevance of IRS4 in the development of cancer and identify IRS4 differences that distinguish it from IRS1 and IRS2. Search engines and different databases, such as PubMed, UniProt, ENSEMBL and SCANSITE 4.0, were used. We used the name of the protein that it encodes "(IRS-4 or IRS4)", or the combination of these terms with the word "(cancer)" or "(human)", for searches. Terms related to specific tumor pathologies ("breast", "ovary", "colon", "lung", "lymphoma", etc.) were also used. Despite the lack of knowledge on IRS4, it has been reported that some cancers and benign tumors are characterized by high levels of IRS-4 expression. Specifically, the role of IRS-4 in different types of digestive tract neoplasms, gynecological tumors, lung cancers, melanomas, hematological tumors, and other less common types of cancers has been shown. IRS4 differs from IRS1 and IRS2 in that can activate several oncogenes that regulate the PI3K/Akt cascade, such as BRK and FER, which are characterized by tyrosine kinase-like activity without regulation via extracellular ligands. In addition, IRS4 can activate the CRKL oncogene, which is an adapter protein that regulates the MAP kinase cascade. Knowledge of the role played by IRS4 in cancers at the molecular level, specifically as a platform for oncogenes, may enable the identification and validation of new therapeutic targets.

Dysregulated Liver Metabolism and Polycystic Ovarian Syndrome

A significant fraction of couples around the world suffer from polycystic ovarian syndrome (PCOS), a disease defined by the characteristics of enhanced androgen synthesis in ovarian theca cells, hyperandrogenemia, and ovarian dysfunction in women. Most of the clinically observable symptoms and altered blood biomarker levels in the patients indicate metabolic dysregulation and adaptive changes as the key underlying mechanisms. Since the liver is the metabolic hub of the body and is involved in steroid-hormonal detoxification, pathological changes in the liver may contribute to female endocrine disruption, potentially through the liver-to-ovary axis. Of particular interest are hyperglycemic challenges and the consequent changes in liver-secretory protein(s) and insulin sensitivity affecting the maturation of ovarian follicles, potentially leading to female infertility. The purpose of this review is to provide insight into emerging metabolic mechanisms underlying PCOS as the primary culprit, which promote its incidence and aggravation. Additionally, this review aims to summarize medications and new potential therapeutic approaches for the disease.

Concerted regulation of non-alcoholic fatty liver disease progression by microRNAs in apolipoprotein E-deficient mice

The prevalence of non-alcoholic fatty liver disease (NAFLD) is constantly increasing, and altered expression of microRNAs (miRNAs) fosters the development and progression of many pathologies, including NAFLD. Therefore, we explored the role of new miRNAs involved in the molecular mechanisms that trigger NAFLD progression and evaluated them as biomarkers for diagnosis. As a NAFLD model, we used apolipoprotein E-deficient mice administered a high-fat diet for 8 or 18 weeks. We demonstrated that insulin resistance and decreased lipogenesis and autophagy observed after 18 weeks on the diet are related to a concerted regulation carried out by miR-26b-5p, miR-34a-5p, miR-149-5p and miR-375-3p. We also propose circulating let-7d-5p and miR-146b-5p as potential biomarkers of early stages of NAFLD. Finally, we confirmed that circulating miR-34a-5p and miR-375-3p are elevated in the late stages of NAFLD and that miR-27b-3p and miR-122-5p are increased with disease progression. Our results reveal a synergistic regulation of key processes in NAFLD development and progression by miRNAs. Further investigation is needed to unravel the roles of these miRNAs for developing new strategies for NAFLD treatment.

This article has an associated First Person interview with the joint first authors of the paper.

Summary:Apoe^−/− mice administered a high-fat diet represent a model of non-alcoholic fatty liver disease, revealing the synergistic regulation of key processes in disease progression by miRNAs and indicating some miRNAs as biomarkers for diagnosis.

Actinomycin D Arrests Cell Cycle of Hepatocellular Carcinoma Cell Lines and Induces p53-Dependent Cell Death: A Study of the Molecular Mechanism Involved in the Protective Effect of IRS-4

Actinomycin D (ActD) is an FDA-approved NCI oncology drug that specifically targets and downregulates stem cell transcription factors, which leads to a depletion of stem cells within the tumor bulk. Recently, our research group demonstrated the importance of IRS-4 in the development of liver cancer. In this study, we evaluated the protective effects of IRS-4 against ActD. For this study, three hepatocellular carcinoma cell lines (HepG2, Huh7, and Chang cells) were used to study the mechanism of actinomycin D. Most assays were carried out in the Hep G2 cell line, due to the high expression of stem cell biomarkers. We found that ActD caused HepG2 cell necroptosis characterized by DNA fragmentation, decreased mitochondrial membrane potential, cytochrome c depletion, and decreased the levels of reduced glutathione. However, we did not observe a clear increase in apoptosis markers such as annexin V presence, caspase 3 activation, or PARP fragmentation. ActD produced an activation of MAP kinases (ERK, p38, and JNK) and AKT. ActD-induced activation of AKT and MAP kinases produced an activation of the Rb-E2F cascade together with a blockage of cell cycle transitions, due to c-jun depletion. ActD led to the inhibition of pCdK1 and pH3 along with DNA fragmentation resulting in cell cycle arrest and the subsequent activation of p53-dependent cell death in the HepG2 cell line. Only JNK and AKT inhibitors were protective against the effects of ActD. N-Acetyl-L-cysteine also had a protective effect as it restored GSH levels. A likely mechanism for this is IRS-4 stimulating GCL-GSH and inhibiting the Brk-CHK1-p53 pathway. The assessment of the IRS-4 in cancer biopsies could be of interest to carry out a personalized treatment with ActD.

Possible Role of IRS-4 in the Origin of Multifocal Hepatocellular Carcinoma

Simple Summary

Hepatocellular carcinoma (HCC) is a potentially deadly liver cancer with a high prevalence worldwide. Despite the very efforts placed on this cancer, most cases are associated with poor prognosis and the understanding of the molecular mechanisms implicated in the development of HCC are arising as a potential therapeutic approach of this cancer. In this sense, we aimed to evaluate the established role of insulin receptor substrate 4 (IRS-4) in the tumorigenesis and progression of HCC. Thus, we leaded a histopathological study of this component, along with additional cancer biomarkers such as PCNA, Ki67, and pH3. In addition, in vitro models of different cell lines were used to describe the effects of IRS-4 overexpression/silencing. Finally, immunoblot analysis and transfection experiments were also conducted. Our research demonstrates that IRS-4 is involved in multiple tumoral effects such as proliferation, cell migration, and cell-collagen adhesion as well as the appearance of multifocal HCC.

Abstract

New evidence suggests that insulin receptor substrate 4 (IRS-4) may play an important role in the promotion of tumoral growth. In this investigation, we have evaluated the role of IRS-4 in a pilot study performed on patients with liver cancer. We used immunohistochemistry to examine IRS-4 expression in biopsies of tumoral tissue from a cohort of 31 patient suffering of hepatocellular carcinoma (HCC). We simultaneously analyzed the expression of the cancer biomarkers PCNA, Ki-67, and pH3 in the same tissue samples. The in vitro analysis was conducted by studying the behavior of HepG2 cells following IRS-4 overexpression/silencing. IRS-4 was expressed mainly in the nuclei of tumoral cells from HCC patients. In contrast, in healthy cells involved in portal triads, canaliculi, and parenchymal tissue, IRS-4 was observed in the cytosol and the membrane. Nuclear IRS-4 in the tumoral region was found in 69.9 ± 3.2%, whereas in the surrounding healthy hepatocytes, nuclear IRS-4 was rarely observed. The percentage of tumoral cells that exhibited nuclear PCNA and Ki-67 were 52.1 ± 7%, 6.1 ± 1.1% and 1.3 ± 0.2%, respectively. Furthermore, we observed a significant positive linear correlation between nuclear IRS-4 and PCNA (r = 0.989; p < 0.001). However, when we correlated the nuclear expression of IRS-4 and Ki-67, we observed a significant positive curvilinear correlation (r = 0.758; p < 0.010). This allowed us to define two populations, (IRS-4 + Ki-67 ≤ 69%) and (IRS-4 + Ki-67 > 70%). The population with lower levels of IRS-4 and Ki-67 had a higher risk of suffering from multifocal liver cancer (OR = 16.66; CI = 1.68–164.8 (95%); p < 0.05). Immunoblot analyses showed that IRS-4 in normal human liver biopsies was lower than in HepG2, Huh7, and Chang cells. Treatment of HepG2 with IGF-1 and EGF induced IRS-4 translocation to the nucleus. Regulation of IRS-4 levels via HepG2 transfection experiments revealed the protein’s role in proliferation, cell migration, and cell-collagen adhesion. Nuclear IRS-4 is increased in the tumoral region of HCC. IRS-4 and Ki-67 levels are significantly correlated with the presence of multifocal HCC. Moreover, upregulation of IRS-4 in HepG2 cells induced proliferation by a β-catenin/Rb/cyclin D mechanism, whereas downregulation of IRS-4 caused a loss in cellular polarity and in its adherence to collagen as well as a gain in migratory and invasive capacities, probably via an integrin α2 and focal adhesion cascade (FAK) mechanism.

Metastasis of Uveal Melanoma with Monosomy-3 Is Associated with a Less Glycogenetic Gene Expression Profile and the Dysregulation of Glycogen Storage

The prolonged storage of glucose as glycogen can promote the quiescence of tumor cells, whereas the accumulation of an aberrant form of glycogen without the primer protein glycogenin can induce the metabolic switch towards a glycolytic phenotype. Here, we analyzed the expression of n = 67 genes involved in glycogen metabolism on the uveal melanoma (UM) cohort of the Cancer Genome Atlas (TCGA) study and validated the differentially expressed genes in an independent cohort. We also evaluated the glycogen levels with regard to the prognostic factors via a differential periodic acid-Schiff (PAS) staining. UMs with monosomy-3 exhibited a less glycogenetic and more insulin-resistant gene expression profile, together with the reduction of glycogen levels, which were associated with the metastases. Expression of glycogenin-1 (Locus: 3q24) was lower in the monosomy-3 tumors, whereas the complementary isoform glycogenin-2 (Locus: Xp22.33) was upregulated in females. Remarkably, glycogen was more abundant in the monosomy-3 tumors of male versus female patients. We therefore provide the first evidence to the dysregulation of glycogen metabolism as a novel factor that may be aggravating the course of UM particularly in males.

Transcriptome analysis of Takifugu obscurus liver in response to acute retene exposure

Retene (1-methyl-7-isopropyl-phenanthrene, RET) is an alkyl polycyclic aromatic hydrocarbon (PAH) with environmental risk to aquatic animals. Takifugu obscurus is a migratory fish species with high economic and ecological value. To assess the toxic effects of RET on molecular metabolism, juvenile T. obscurus in this study were acutely exposed to 44.30 µg/L of RET for four days. The transcriptome profiles of livers were compared between RET treatment group and the control, and the results revealed that 1,897 genes were significantly differentially expressed (DEGs) after exposure to RET, which enriched 17 KEGG pathways. Among these, glycerolipid metabolism, glycerophospholipid metabolism, insulin signaling pathway, and FOXO signaling pathways were significantly activated. Further exploration indicated that RET exposure disrupted glucose metabolism, stimulated insulin metabolism, and activated cell proliferation genes. Overall, these findings help explain the molecular mechanisms underlying RET toxicity, and may offer evidence to support T. obscurus protection.

Liver-specific insulin receptor isoform A expression enhances hepatic glucose uptake and ameliorates liver steatosis in a mouse model of diet-induced obesity

Among the main complications associated with obesity are insulin resistance and altered glucose and lipid metabolism within the liver. It has previously been described that insulin receptor isoform A (IRA) favors glucose uptake and glycogen storage in hepatocytes compared with isoform B (IRB), improving glucose homeostasis in mice lacking liver insulin receptor. Thus, we hypothesized that IRA could also improve glucose and lipid metabolism in a mouse model of high-fat-diet-induced obesity. We addressed the role of insulin receptor isoforms in glucose and lipid metabolism in vivo. We expressed IRA or IRB specifically in the liver by using adeno-associated viruses (AAVs) in a mouse model of diet-induced insulin resistance and obesity. IRA, but not IRB, expression induced increased glucose uptake in the liver and muscle, improving insulin tolerance. Regarding lipid metabolism, we found that AAV-mediated IRA expression also ameliorated hepatic steatosis by decreasing the expression of Fasn, Pgc1a, Acaca and Dgat2 and increasing Scd-1 expression. Taken together, our results further unravel the role of insulin receptor isoforms in hepatic glucose and lipid metabolism in an insulin-resistant scenario. Our data strongly suggest that IRA is more efficient than IRB at favoring hepatic glucose uptake, improving insulin tolerance and ameliorating hepatic steatosis. Therefore, we conclude that a gene therapy approach for hepatic IRA expression could be a safe and promising tool for the regulation of hepatic glucose consumption and lipid metabolism, two key processes in the development of non-alcoholic fatty liver disease associated with obesity.

This article has an associated First Person interview with the first author of the paper.

Summary: Adeno-associated-virus-mediated gene therapy for insulin receptor isoform A expression in the liver improves glucose disposal and alleviates lipid accumulation in wild-type mice under a high-fat diet.

Overexpression of IRS-4 Correlates with Procaspase 3 Levels in Tumoural Tissue of Patients with Colorectal Cancer

We reported that insulin receptor substrate 4 (IRS-4) levels increased in tissue from colorectal cancer (CRC) patients and promoted retinoblastoma-cyclin-dependent kinase activation. The aim of the present study was to evaluate the effect of IRS-4 on IGF-1 receptor pathway and its impact on procaspase 3 and PARP expression in RKO and HepG2 cancer cell lines. The results obtained in vitro were compared with those obtained from biopsies of patients with CRC (n = 18), tubulovillous adenomas (TA) (n = 2) and in matched adjacent normal colorectal (MANC) tissue (n = 20). IRS-4 overexpression in cultured cells induced the overactivation of IGF-1/BRK/AKT/GSK-3/β-catenin/cyclin D1 pathways, which led to increased expression of procaspase 3 and PARP protein levels. Studies carried out on CRC and TA tissues revealed the overactivation of the IGF-1 receptor signalling pathway, as well as the overexpression of procaspase 3 and PARP in tumoural tissue with respect to MANC tissue. The upregulation of IRS-4 in tumoural samples correlated significantly with the increase in pIGF-1 receptor (Tyr 1165/1166) (r = 0.84; p < 0.0001), procaspase 3 (r = 0. 77; p < 0. 0005) and PARP (r = 0. 89; p < 0. 0005). Similarly, we observed an increase in the proteolysis of procaspase 3 in tumoural tissue with respect to MANC tissue, which correlated significantly with the degradation of PARP (r = 0.86; p < 0.0001), p53 (r = 0.84; p < 0.0001), and GSK-3 (r = 0.78; p < 0.0001). The stratification of patient samples using the TNM system revealed that procaspase 3 and caspase 3 increased gradually with T values, which suggests their involvement in the size and local invasion of primary tumours. Taken together, our findings suggest that IRS-4 overexpression promotes the activation of the IGF-1 receptor pathway, which leads to the increase in procaspase 3 levels in CRC.

Insulin receptor substrate-4 is overexpressed in colorectal cancer and promotes retinoblastoma-cyclin-dependent kinase activation

BACKGROUND

Insulin receptor substrate 4 (IRS-4) is an adaptor protein for which new evidence suggests plays a role in tumour promotion.

METHODS

We described nuclear IRS-4 in RKO colon cancer cell lines in biopsies of patients with colorectal cancer (CRC) (n = 20) and in matched adjacent normal colorectal (MANC) tissue (n = 20).

RESULTS

Treatment with physiological doses of IGF-1 promoted nuclear influx of IRS-4 from cellular cytosol in RKO cells. When exogenous IRS-4 was overexpressed in RKO cells, there was an increase in cyclin D1, cyclin E, E2F1, pRB Ser 809/811 and pRB Ser 705 levels compared with the empty vector-transfected cells. Some of these changes returned to control values after wortmannin treatment. Subcellular fractionation showed an overexpression of IRS-4 in the cytoplasm, membrane, and nuclei of tumour samples, whereas the levels of the protein were barely detectable in the three compartments of normal samples. Immunohistochemical studies showed positive nuclear IRS-4 staining in over 74% of the tumour cells. IRS-4 was strongly overexpressed in tumoural tissues from CRC patients compared to MANC tissues. The up-regulation of IRS-4 in CRC samples correlated significantly with the increase of several G1 checkpoint proteins including cyclin D1 (r = 0.6662), Rb (r = 0.7779), pRb Serine 809/811 (r = 0.6864), pRb serine 705 (r = 0.6261) and E2F1 (r = 0.8702).

CONCLUSIONS

Taken together, our findings suggest that IRS-4 promotes retinoblastoma-cyclin-dependent kinase activation and it may serve as a pharmacological target since its expression is very low in normal tissue, including colonic epithelium.

aPKCζ-dependent Repression of Yap is Necessary for Functional Restoration of Irradiated Salivary Glands with IGF-1

Xerostomia and salivary hypofunction often result as a consequence of radiation therapy for head and neck cancers, which are diagnosed in roughly 60,000 individuals every year in the U.S. Due to the lack of effective treatments for radiation-induced salivary hypofunction, stem cell-based therapies have been suggested to regenerate the irradiated salivary glands. Pharmacologically, restoration of salivary gland function has been accomplished in mice by administering IGF-1 shortly after radiation treatment, but it is not known if salivary stem and progenitor cells play a role. We show that radiation inactivates aPKCζ and promotes nuclear redistribution of Yap in a population of label-retaining cells in the acinar compartment of the parotid gland (PG)- which comprises a heterogeneous pool of salivary progenitors. Administration of IGF-1 post-radiation maintains activation of aPKCζ and partially rescues Yap's cellular localization in label retaining cells, while restoring salivary function. Finally, IGF-1 fails to restore saliva production in mice lacking aPKCζ, demonstrating the importance of the kinase as a potential therapeutic target.

Insulin receptor isoform A confers a higher proliferative capability to pancreatic beta cells enabling glucose availability and IGF-I signaling

The main compensatory response to insulin resistance is the pancreatic beta cell hyperplasia to account for increased insulin secretion. In fact, in a previous work we proposed a liver-pancreas endocrine axis with IGF-I (insulin-like growth factor type I) secreted by the liver acting on IRA insulin receptor in beta cells from iLIRKO mice (inducible Liver Insulin Receptor KnockOut) that showed a high IRA/IRB ratio. However, the role of insulin receptor isoforms in the IGF-I-induced beta cell proliferation as well as the underlying molecular mechanisms remain poorly understood. For this purpose, we have used four immortalized mouse beta cell lines: bearing IR (IRLoxP), lacking IR (IRKO), expressing exclusively IRA (IRA), or alternatively expressing IRB (IRB). Pancreatic beta cell proliferation studies showed that IRA cells are more sensitive than those expressing IRB to the mitogenic response induced by IGF-I, acting through the pathway IRA/IRS-1/2/αp85/Akt/mTORC1/p70S6K. More importantly, IRA beta cells, but not IRB, showed an increased glucose uptake as compared with IRLoxP cells, this effect being likely owing to an enhanced association between Glut-1 and Glut-2 with IRA. Overall, our results strongly suggest a prevalent role of IRA in glucose availability and IGF-I-induced beta cell proliferation mainly through mTORC1. These results could explain, at least partially, the role played by the liver-secreted IGF-I in the compensatory beta cell hyperplasia observed in response to severe hepatic insulin resistance in iLIRKO mice.

Irs2 and Irs4 synergize in non-LepRb neurons to control energy balance and glucose homeostasis

Insulin receptor substrates (Irs1, 2, 3 and Irs4) mediate the actions of insulin/IGF1 signaling. They have similar structure, but distinctly regulate development, growth, and metabolic homeostasis. Irs2 contributes to central metabolic sensing, partially by acting in leptin receptor (LepRb)-expressing neurons. Although Irs4 is largely restricted to the hypothalamus, its contribution to metabolic regulation is unclear because Irs4-null mice barely distinguishable from controls. We postulated that Irs2 and Irs4 synergize and complement each other in the brain. To examine this possibility, we investigated the metabolism of whole body Irs4^−/y mice that lacked Irs2 in the CNS (bIrs2^−/−·Irs4^−/y) or only in LepRb-neurons (Lepr^∆Irs2·Irs4^−/y). bIrs2^−/−·Irs4^−/y mice developed severe obesity and decreased energy expenditure, along with hyperglycemia and insulin resistance. Unexpectedly, the body weight and fed blood glucose levels of Lepr^∆Irs2·Irs4^−/y mice were not different from Lepr^∆Irs2 mice, suggesting that the functions of Irs2 and Irs4 converge upon neurons that are distinct from those expressing LepRb.

Effect of IRS4 levels on PI 3-kinase signalling

Insulin receptor substrate 1 (IRS1) and IRS2 are well-characterized adapter proteins that relay signals from receptor tyrosine kinases to downstream components of signalling pathways. In contrast, the function of IRS4 is not well understood. IRS4 overexpression has been associated with acute lymphoblastic leukaemia and subungual exostosis, while point mutations of IRS4 have been found in melanomas. Here, we show that while IRS4 expression is low in most cancer cell lines, IRS4 mRNA and protein levels are markedly elevated in certain cells including the NCI-H720, DMS114, HEK293T and HEK293AAV lines. Surprisingly, IRS4 expression was also strongly induced when HEK293 cells were infected with retroviral particles and selected under puromycin, making IRS4 expression a potential off-target effect of retroviral expression vectors. Cells with high expression of IRS4 displayed high phosphatidylinositol (3,4,5)-trisphosphate (PIP3) levels, as well as elevated Akt and p70 S6 kinase activities, even in the absence of growth factors. PI 3-kinase (PI3K) signalling in these cells depends on IRS4, even though these cells also express IRS1/2. Knockdown of IRS4 also inhibited cell proliferation in cells with high levels of IRS4. Together, these findings suggest IRS4 as a potential therapeutic target for cancers with high expression of this protein.

Novel binding partners and differentially regulated phosphorylation sites clarify Eps8 as a multi-functional adaptor

Eps8 is involved in both cell signalling and receptor trafficking. It is a known phosphorylation substrate for two proteins involved in the fibroblast growth factor receptor (FGFR) signalling pathway: the receptor itself and Src. Here we report a differential proteomic analysis of Eps8 aimed to identify specific FGFR and Src family kinase dependent phosphosites and co-associated phosphodependent binding partners. This study reveals a total of 22 Eps8 pTyr and pSer/Thr phosphorylation sites, including those that are dependent on Src family and FGFR kinase activity. Peptide affinity purification of proteins that bind to a selection of the pTyr phosphosites has identified a range of novel Eps8 binding partners including members of the intracellular vesicle trafficking machinery (clathrin and AP-2), proteins which have been shown to regulate activated receptor trafficking (NBR1 and Vav2), and proteins involved in receptor signalling (IRS4 and Shp2). Collectively this study significantly extends the understanding of Eps8 post-translational modification by regulated phosphorylation, identifies novel Eps8 binding partners implicated in receptor trafficking and signalling, and confirms the functions of Eps8 at the nexus of receptor signalling and vesicular trafficking.

A BRET assay for monitoring insulin receptor interactions and ligand pharmacology

The insulin receptor (IR) belongs to the receptor tyrosine kinase super family and plays an important role in glucose homeostasis. The receptor interacts with several large docking proteins that mediate signaling from the receptor, including the insulin receptor substrate (IRS) family and Src homology-2-containing proteins (Src). Here, we applied the bioluminescence resonance energy transfer 2 (BRET2) technique to study the IR signaling pathways. The interaction between the IR and the substrates IRS1, IRS4 and Shc was examined in response to ligands with different signaling properties. The association between IR and the interacting partners could successfully be monitored when co-expressing green fluorescent protein 2 (GFP2) tagged substrates with Renilla reniformis luciferase 8 (Rluc8) tagged IR. Through additional optimization steps, we developed a stable and flexible BRET2 assay for monitoring the interactions between the IR and its substrates. Furthermore, the insulin analogue X10 was characterized in the BRET2 assay and was found to be 10 times more potent with respect to IRS1, IRS4 and Shc recruitment compared to human insulin. This study demonstrates that the BRET2 technique can be applied to study IR signaling pathways, and that this assay can be used as a platform for screening and characterization of IR ligands.

RNAi-mediated silencing of insulin receptor substrate-4 enhances actinomycin D- and tumor necrosis factor-alpha-induced cell death in hepatocarcinoma cancer cell lines

Insulin receptor substrate-4 (IRS-4) transmits signals from the insulin-like growth factor receptor (IGF-IR) and the insulin receptor (IR) to the PI3K/AKT and the ERK1/2 pathways. IRS-4 expression increases dramatically after partial hepatectomy and plays an important role in HepG2 hepatoblastoma cell line proliferation/differentiation. In human hepatocarcinoma, IRS-4 overexpression has been associated with tumor development. Herein, we describe the mechanism whereby IRS-4 depletion induced by RNA interference (siRNA) sensitizes HepG2 cells to treatment with actinomycin D (Act D) and combined treatment with Act D plus tumor necrosis factor-alpha (TNF-alpha). Similar results have been obtained in HuH 7 and Chang cell lines. Act D therapy drove the cells to a mitochondrial-dependent apoptotic program involving cytochrome c release, caspase 3 activation, PARP fragmentation and DNA laddering. TNF-alpha amplifies the effect of Act D on HepG2 cell apoptosis increasing c-jun N-terminal kinase (JNK) activity, IkappaB-alpha proteolysis and glutathione depletion. IRS-4 depleted cells that were treated with Act D showed an increase in cytochrome c release and procaspase 3 and PARP proteolysis with respect to control cells. The mechanism involved in IRS-4 action is independent of Akt, IkappaB kinase and JNK. IRS-4 down regulation, however, decreased gamma-glutamylcysteine synthetase content and cell glutathione level in the presence of Act D plus TNF-alpha. These results suggest that IRS-4 protects HepG2 cells from oxidative stress induced by drug treatment.

Expression and function of the insulin receptor substrate proteins in cancer

The Insulin Receptor Substrate (IRS) proteins are cytoplasmic adaptor proteins that function as essential signaling intermediates downstream of activated cell surface receptors, many of which have been implicated in cancer. The IRS proteins do not contain any intrinsic kinase activity, but rather serve as scaffolds to organize signaling complexes and initiate intracellular signaling pathways. As common intermediates of multiple receptors that can influence tumor progression, the IRS proteins are positioned to play a pivotal role in regulating the response of tumor cells to many different microenvironmental stimuli. Limited studies on IRS expression in human tumors and studies on IRS function in human tumor cell lines and in mouse models have provided clues to the potential function of these adaptor proteins in human cancer. A general theme arises from these studies; IRS-1 and IRS-4 are most often associated with tumor growth and proliferation and IRS-2 is most often associated with tumor motility and invasion. In this review, we discuss the mechanisms by which IRS expression and function are regulated and how the IRS proteins contribute to tumor initiation and progression.

Beta-Cell hyperplasia induced by hepatic insulin resistance: role of a liver-pancreas endocrine axis through insulin receptor A isoform

OBJECTIVE

Type 2 diabetes results from a combination of insulin resistance and impaired insulin secretion. To directly address the effects of hepatic insulin resistance in adult animals, we developed an inducible liver-specific insulin receptor knockout mouse (iLIRKO).

RESEARCH DESIGN AND METHODS

Using this approach, we were able to induce variable insulin receptor (IR) deficiency in a tissue-specific manner (liver mosaicism).

RESULTS

iLIRKO mice presented progressive hepatic and extrahepatic insulin resistance without liver dysfunction. Initially, iLIRKO mice displayed hyperinsulinemia and increased beta-cell mass, the extent of which was proportional to the deletion of hepatic IR. Our studies of iLIRKO suggest a cause-and-effect relationship between progressive insulin resistance and the fold increase of plasma insulin levels and beta-cell mass. Ultimately, the beta-cells failed to secrete sufficient insulin, leading to uncontrolled diabetes. We observed that hepatic IGF-1 expression was enhanced in iLIRKO mice, resulting in an increase of circulating IGF-1. Concurrently, the IR-A isoform was upregulated in hyperplastic beta-cells of iLIRKO mice and IGF-1-induced proliferation was higher than in the controls. In mouse beta-cell lines, IR-A, but not IR-B, conferred a proliferative capacity in response to insulin or IGF-1, providing a potential explanation for the beta-cell hyperplasia induced by liver insulin resistance in iLIRKO mice.

CONCLUSIONS

Our studies of iLIRKO mice suggest a liver-pancreas endocrine axis in which IGF-1 functions as a liver-derived growth factor to promote compensatory pancreatic islet hyperplasia through IR-A.

Synergistic premalignant effects of chronic ethanol exposure and insulin receptor substrate-1 overexpression in liver

AIM

Insulin receptor substrate, type 1 (IRS-1) transmits growth and survival signals, and is overexpressed in more than 90% of hepatocellular carcinomas (HCCs). However, experimental overexpression of IRS-1 in the liver was found not to be sufficient to cause HCC. Since chronic alcohol abuse is a risk factor for HCC, we evaluated potential interactions between IRS-1 overexpression and chronic ethanol exposure by assessing premalignant alterations in gene expression.

METHODS

Wild-type (wt) or IRS-1 transgenic (Tg) mice, constitutively overexpressing the human (h) transgene in the liver, were pair-fed isocaloric liquid diets containing 0% or 24% ethanol for 8 weeks. The livers were used for histopathologic study and gene expression analysis, focusing on insulin, insulin-like growth factor (IGF) and wingless (WNT)-Frizzled (FZD) pathways, given their known roles in HCC.

RESULTS

In wt mice, chronic ethanol exposure caused hepatocellular microsteatosis with focal chronic inflammation, reduced expression of proliferating cell nuclear antigen (PCNA) and increased expression of IGF-I and IGF-I receptor. In hIRS-1 Tg mice, chronic ethanol exposure caused hepatic micro- and macrosteatosis, focal chronic inflammation, apoptosis and disordered lobular architecture. These effects of ethanol in hIRS-1 Tg mice were associated with significantly increased expression of IGF-II, insulin, IRS-4, aspartyl-asparaginyl beta hydroxylase (AAH), WNT-1 and FZD 7, as occurs in HCC.

CONCLUSION

In otherwise normal liver, chronic ethanol exposure mainly causes liver injury and inflammation with impaired DNA synthesis. In contrast, in the context of hIRS-1 overexpression, chronic ethanol exposure may serve as a cofactor in the pathogenesis of HCC by promoting expression of growth factors, receptors and signaling molecules known to be associated with hepatocellular transformation.

Insulin receptor substrate 4 couples the leptin receptor to multiple signaling pathways

Leptin is an adipokine that regulates food intake and energy expenditure by activating its hypothalamic leptin receptor (LR). Members of the insulin receptor substrate (IRS) family serve as adaptor proteins in the signaling pathways of several cytokines and hormones and a role for IRS2 in central leptin physiology is well established. Using mammalian protein-protein interaction trap (MAPPIT), a cytokine receptor-based two-hybrid method, in the N38 hypothalamic cell line, we here demonstrate that also IRS4 interacts with the LR. This recruitment is leptin dependent and requires phosphorylation of the Y1077 motif of the LR. Domain mapping of IRS4 revealed the critical role of the pleckstrin homology domain for full interaction. In line with its function as an adaptor protein, IRS4 interacted with the regulatory p85 subunit of the phosphatidylinositol 3-kinase, phospholipase Cgamma, and the suppressor of cytokine signaling (SOCS) family members SOCS2, SOCS6, and SOCS7 and thus can modulate LR signaling.

Human, but not rat, IRS1 targets to the plasma membrane in both human and rat adipocytes

Adipocytes are primary targets for insulin control of metabolism. The activated insulin receptor phosphorylates insulin receptor substrate-1 (IRS1), which acts as a docking protein for downstream signal mediators. In the absence of insulin stimulation, IRS1 in rat adipocytes is intracellular but in human adipocytes IRS1 is constitutively targeted to the plasma membrane. Stimulation of adipocytes with insulin increased the amount of IRS1 at the plasma membrane 2-fold in human adipocytes, but >10-fold in rat adipocytes, with the same final amount of IRS1 at the plasma membrane in cells from both species. Cross-transfection of rat adipocytes with human IRS1, or human adipocytes with rat IRS1, demonstrated that the species difference was due to the IRS1 protein and not the cellular milieus or posttranslational modifications. Chimeric IRS1, consisting of the conserved N-terminus of rat IRS1 with the variable C-terminal of human IRS1, did not target the plasma membrane, indicating that subtle sequence differences direct human IRS1 to the plasma membrane.

Role of insulin receptor substrate-4 in IGF-I-stimulated HEPG2 proliferation

BACKGROUNDS/AIMS

Insulin receptor substrate-4 (IRS-4) is a scaffold protein that mediates the actions of insulin-like growth factor-I (IGF-I). Its expression increases dramatically after partial hepatectomy (a liver regeneration model). Herein, we report IRS-4 expression in a human hepatoblastoma cell line (HepG2) and IGF-I-dependent IRS-4 tyrosine phosphorylation.

METHODS

The role of IRS-4 in HepG2 proliferation was established by RNA interference (siRNA). After 72h of transfection with IRS-4 siRNA, we observed a specific reduction in IRS-4 expression.

RESULTS

Depletion of IRS-4 levels decreased ERK phosphorylation, p70S6K phosphorylation and IGF-I-stimulated cell proliferation. Changes in ERK phosphorylation in IRS-4-depleted cells were independent of ras/raf/MEK1/2- and PI3K/Akt-cascades. IRS-4 down-regulation abolished IGF-I-, TPA- and IGF-I plus TPA-stimulated ERK and p70S6K activities. Our results suggest that PKC-epsilon mediates the effect of IRS-4 on ERK activity. Moreover, decreased IRS-4 levels diminished FBS- and IGF-I-stimulated HepG2 growth and cause stress fiber disruption in HepG2 cell line.

CONCLUSIONS

Collectively, our data suggest that IRS-4 plays an important role in HepG2 proliferation/differentiation and exerts its actions through ERK and p70S6K activation in a ras/raf/MEK1/2- and PI3Kinase/Akt-independent manner and in a PKC-dependent way.

Angiotensin II modulates tyr-phosphorylation of IRS-4, an insulin receptor substrate, in rat liver membranes

Angiotensin II (Ang II), a major regulator of blood pressure, is also involved in the control of cellular proliferation and hypertrophy and might exhibit additional actions in vivo by modulating the signaling of other hormones. As hypertension and Insulin (Ins) resistance often coexist and are risk factors for cardiovascular diseases, Ang II and Insulin signaling cross-talk may have an important role in hypertension development. The effect of Ins on protein tyrosine phosphorylation was assayed in rat liver membrane preparations, a rich source of Ins receptors. Following stimulation, Ins (10(-7) M) induced tyr-phosphorylation of different proteins. Insulin consistently induced tyr-phosphorylation of a 160 kDa protein (pp160) with maximum effect between 1 and 3 min. The pp160 protein was identified by anti-IRS-4 but not by anti-IRS-1 antibody. Pre-stimulation with Ang II (10(-7) M) diminishes tyr-phosphorylation level of pp160/IRS-4 in a dose-dependent manner. Okadaic acid, the PP1A and PP2A Ser/Thr phosphatase inhibitor, increases pp160 phosphorylation induced by Ins and prevents the inhibitory effect of Ang II pre-stimulation. Genistein, a tyrosine kinase inhibitor, diminishes tyr-phosphorylation level of IRS-4. PI3K inhibitors Wortmanin and LY294002, both increase tyr-phosphorylation of IRS-4, either in the presence of Ins alone or combined with Ang II. These results suggest that Ins and Ang II modulate IRS-4 tyr-phosphorylation in a PI3K-dependent manner. In summary, we showed that Ins induces tyr-phosphorylation of IRS-4, an effect modulated by Ang II. Assays performed in the presence of different inhibitors points toward a PI3K involvement in this signaling pathway.

Hepatocyte proliferation during liver regeneration is impaired in mice with liver-specific IGF-1R knockout

Recent evidence indicates that growth hormone (GH) is involved in liver regeneration. To test whether insulin-like growth factor I (IGF-I) mediates this effect, we studied liver regeneration induced by partial hepatectomy in liver-specific IGF type 1 receptor knockout (LIGFREKO) mice. The absence of IGF-1R caused a significant decrease in hepatocyte proliferation in males (-52%), but not in females, as assessed by Ki67 immunohistochemistry. Cyclin D1 and cyclin A protein levels in the livers of LIGFREKO males were only half those in controls, indicating that cyclin induction during liver regeneration is dependent on IGF-1R signaling. Analyzing the signaling cascade initiated by IGF-1R, we observed a lack of IRS-1 induction in LIGFREKO livers. In contrast, the induction of IRS-2 synthesis was similar in LIGFREKO and control groups, suggesting the existence of differential regulation of IRS synthesis during liver regeneration. Regenerating livers from LIGFREKO animals also showed significantly less activated ERKs than controls. Our findings demonstrate that IGF-1R makes a significant contribution to liver regeneration. Using the LIGFREKO model, we provide new evidence that IGF-1R/IRS-1/ERK signaling may be the intracellular pathway controlling the cell cycle via cyclin D1 and cyclin A in the regenerating liver.

Irs1 and Irs2 signaling is essential for hepatic glucose homeostasis and systemic growth

Insulin receptor substrates, including Irs1 and Irs2, integrate insulin and IGF receptor signals with heterologous pathways to coordinate growth and metabolism. Since Irs2 is thought to be especially important in hepatic nutrient homeostasis, we deleted Irs2 [corrected] from hepatocytes of WT mice (called LKO) or genetically insulin-resistant Irs1-/- mice (called LKO::Irs1-/-). Viable LKO::Irs1-/- mice were 70% smaller than WT or LKO mice, and 40% smaller than Irs1-/- mice. Hepatic insulin receptors were functional in all the mice, but insulin signaling via the Akt-FoxO1 pathway was reduced in Irs1-/- and LKO liver, and undetected in LKO::Irs1-/- liver; however, Gsk3beta phosphorylation (Ser9) and hepatic glycogen stores were nearly normal in all of the mice. LKO and Irs1-/- mice developed insulin resistance and glucose intolerance that never progressed to diabetes, whereas LKO::Irs1-/- mice developed hyperglycemia and hyperinsulinemia immediately after birth. Regardless, few hepatic genes changed expression significantly in Irs1-/- or LKO mice, whereas hundreds of genes changed in LKO::Irs1-/- mice--including elevated levels of Pck1, G6pc, Ppargc1, Pparg, and Igfbp1. Thus, signals delivered by Irs1 or Irs2 regulate hepatic gene expression that coordinates glucose homeostasis and systemic growth.

Interaction between Brk kinase and insulin receptor substrate-4

Breast tumor kinase (Brk) is a member of the Frk family of nonreceptor tyrosine kinases that is overexpressed in a high percentage of human breast tumors. The downstream substrates and effectors of Brk remain largely unidentified. In this study, we carried out immunoprecipitation and mass spectrometry experiments to identify new Brk binding partners. One interacting protein was insulin receptor substrate 4 (IRS-4), a member of the IRS family. We confirmed that Brk associates with IRS-4 in resting and insulin-like growth factor 1 (IGF-1)-stimulated HEK 293 cells. The SH3 and SH2 domains of Brk are both involved in the association. The tyrosine phosphorylation of Brk increases after stimulation with IGF-1, and in MCF-7 breast cancer cells we show that the presence of IRS-4 enhances this effect. Finally, we demonstrate that endogenous Brk and IRS-4 interact in A431 human epidermoid carcinoma cells.

Insulin stimulates Ca2+ uptake via PKC, cAMP, and p38 MAPK in mouse embryonic stem cells

Embryonic stem (ES) cells are provided as a powerful tool for developmental biology and have been shown to respond to insulin. However, little is known about the effect of insulin on [Ca2+]i regulation in the ES cells, although many cellular functions are tightly regulated by [Ca2+]i. Therefore, we examined the effect of insulin on Ca2+ uptake and its related signal pathways in the mouse ES cells. Mouse ES cells expressed alkaline phosphatase (AP), transcription factor Oct-4, and stage-specific embryonic antigen-1 (SSEA-1). Insulin increased the Ca2+ uptake in a time- and dose-dependent manner and the effect was blocked by L-type Ca2+ channel blockers, nifedifine and methoxyverapamil. Genistein or herbimycin A (tyrosine kinase inhibitors), wortmannin (PI-3K inhibitor), and staurosporine or bisindolylmaleimide I (PKC inhibitors) completely prevented insulin-induced increase of Ca2+ uptake. Wortmannin blocked insulin-induced PKC activation, but SQ 22536 (adenylate cyclase inhibitor) did not. Insulin also rapidly increased formation of inositol phosphates (IPs). We examined the involvement of MAPKs in mediating the effect of insulin on Ca2+ uptake. SB 203580 (p38 MAPK inhibitor) but not PD 98059 (p44/42 MAPKs inhibitor) blocked insulin-induced increase of Ca2+ uptake. Insulin significantly increased the phosphorylation of p38 MAPK but not p44/42 MAPKs. In addition, genistein, PKI, and bisindolylmaleimide I blocked the phosphorylation of p38 MAPK by insulin, suggesting a causal relationship. In conclusion, insulin partially stimulated Ca2+ uptake via PKC, cAMP, and p38 MAPK signaling pathways in mouse ES cells.

Protein phosphatase 4 interacts with and down-regulates insulin receptor substrate 4 following tumor necrosis factor-alpha stimulation

Protein phosphatase 4 (PP4; also named PPX or PPP4) is a PP2A-related protein serine/threonine phosphatase with important roles in a variety of cellular processes such as microtubule growth/organization, apoptosis, tumor necrosis factor (TNF)-alpha signaling, and activation of c-Jun N-terminal kinase and NF-kappaB. To further investigate the cellular functions of PP4, we isolated and identified PP4-interacting proteins using a proteomic approach. We found that insulin receptor substrate 4 (IRS-4) interacted with PP4 and that this interaction was enhanced following TNF-alpha stimulation. We also found that PP4, but not PP2A, down-regulated IRS-4 in a phosphatase activity-dependent manner. Pulse-chase analysis revealed that PP4 decreased the half-life of IRS-4 from 4 to 1 h. Moreover, we found that TNF-alpha stimulated a PP4-dependent degradation of IRS-4, as indicated by the blockage of the degradation by a potent PP4 inhibitor (okadaic acid) and a phosphatase-dead PP4 mutant (PP4-RL). Taken together, our studies indicate that IRS-4 is subject to regulation by TNF-alpha and that PP4 mediates TNF-alpha-induced degradation of IRS-4.