Different subcellular localization and phosphoinositides binding of insulin receptor substrate protein pleckstrin homology domains

Prospects for the Use of Intranasally Administered Insulin and Insulin-Like Growth Factor-1 in Cerebral Ischemia

Current approaches to the treatment of stroke have significant limitations, and neuroprotective therapy is ineffective. In view of this, searching for effective neuroprotectors and developing new neuroprotective strategies remain a pressing topic in research of cerebral ischemia. Insulin and insulin-like growth factor-1 (IGF-1) play a key role in the brain functioning by regulating the growth, differentiation, and survival of neurons, neuronal plasticity, food intake, peripheral metabolism, and endocrine functions. Insulin and IGF-1 produce multiple effects in the brain, including neuroprotective action in cerebral ischemia and stroke. Experiments in animals and cell cultures have shown that under hypoxic conditions, insulin and IGF-1 improve energy metabolism in neurons and glial cells, promote blood microcirculation in the brain, restore nerve cell functions and neurotransmission, and produce the anti-inflammatory and antiapoptotic effects on brain cells. The intranasal route of insulin and IGF-1 administration is of particular interest in the clinical practice, since it allows controlled delivery of these hormones directly to the brain, bypassing the blood-brain barrier. Intranasally administered insulin alleviated cognitive impairments in elderly people with neurodegenerative and metabolic disorders; intranasally administered insulin and IGF-1 promoted survival of animals with ischemic stroke. The review discusses the published data and results of our own studies on the mechanisms of neuroprotective action of intranasally administered insulin and IGF-1 in cerebral ischemia, as well as the prospects of using these hormones for normalization of CNS functions and reduction of neurodegenerative changes in this pathology.

Hot Spots for the Use of Intranasal Insulin: Cerebral Ischemia, Brain Injury, Diabetes Mellitus, Endocrine Disorders and Postoperative Delirium

A decrease in the activity of the insulin signaling system of the brain, due to both central insulin resistance and insulin deficiency, leads to neurodegeneration and impaired regulation of appetite, metabolism, endocrine functions. This is due to the neuroprotective properties of brain insulin and its leading role in maintaining glucose homeostasis in the brain, as well as in the regulation of the brain signaling network responsible for the functioning of the nervous, endocrine, and other systems. One of the approaches to restore the activity of the insulin system of the brain is the use of intranasally administered insulin (INI). Currently, INI is being considered as a promising drug to treat Alzheimer's disease and mild cognitive impairment. The clinical application of INI is being developed for the treatment of other neurodegenerative diseases and improve cognitive abilities in stress, overwork, and depression. At the same time, much attention has recently been paid to the prospects of using INI for the treatment of cerebral ischemia, traumatic brain injuries, and postoperative delirium (after anesthesia), as well as diabetes mellitus and its complications, including dysfunctions in the gonadal and thyroid axes. This review is devoted to the prospects and current trends in the use of INI for the treatment of these diseases, which, although differing in etiology and pathogenesis, are characterized by impaired insulin signaling in the brain.

Structural Consequences of IRS-2 nsSNPs and Implication for Insulin Receptor Substrate-2 Protein Stability

Single-Nucleotide Polymorphisms (SNPs) are common genetic variations implicated in human diseases. The non-synonymous SNPs (nsSNPs) affect the proteins' structures and their molecular interactions with other interacting proteins during the accomplishment of biochemical processes. This ultimately causes proteins functional perturbation and disease phenotypes. The Insulin receptor substrate-2 (IRS-2) protein promotes glucose absorption and participates in the biological regulation of glucose metabolism and energy production. Several IRS-2 SNPs are reported in association with type 2 diabetes and obesity in human populations. However, there are no comprehensive reports about the protein structural consequences of these nsSNPs. Keeping in view the pathophysiological consequences of the IRS-2 nsSNPs, we designed the current study to understand their possible structural impact on coding protein. The prioritized list of the deleterious IRS-2 nsSNPs was acquired from multiple bioinformatics resources, including VEP (SIFT, PolyPhen, and Condel), PROVEAN, SNPs&GO, PMut, and SNAP2. The protein structure stability assessment of these nsSNPs was performed by MuPro and I-Mutant-3.0 servers via structural modeling approaches. The atomic-level structural and molecular dynamics (MD) impact of these nsSNPs were examined using GROMACS 2019.2 software package. The analyses initially predicted 8 high-risk nsSNPs located in the highly conserved regions of IRS-2. The MD simulation analysis eventually prioritized the N232Y, R218C, and R104H nsSNPs that predicted to significantly compromise the structure stability and may affect the biological function of IRS-2. These nsSNPs are predicted as high-risk candidates for diabetes and obesity. The validation of protein structural impact of these shortlisted nsSNPs may provide biochemical insight into the IRS-2-mediated type-2 diabetes.

The P300 acetyltransferase inhibitor C646 promotes membrane translocation of insulin receptor protein substrate and interaction with the insulin receptor

Inhibition of P300 acetyltransferase activity by specific inhibitor C646 has been shown to improve insulin signaling. However, the underlying molecular mechanism of this improvement remains unclear. In this study, we analyzed P300 levels of obese patients and found that they were significantly increased in liver hepatocytes. In addition, large amounts of P300 appeared in the cytoplasm. Inhibition of P300 acetyltransferase activity by C646 drastically increased tyrosine phosphorylation of the insulin receptor protein substrates (IRS1/2) without affecting the tyrosine phosphorylation of the beta subunit of the insulin receptor (IRβ) in hepatocytes in the absence of insulin. Since IRS1/2 requires membrane translocation and binding to inositol compounds for normal functions, we also examined the role of acetylation on binding to phosphatidylinositol(4,5)P2, and found that IRS1/2 acetylation by P300 reduced this binding. In contrast, we show that inhibition of IRS1/2 acetylation by C646 facilitates IRS1/2 membrane translocation. Intriguingly, we demonstrate that C646 activates IRβ's tyrosine kinase activity and directly promotes IRβ interaction with IRS1/2, leading to the tyrosine phosphorylation of IRS1/2 and subsequent activation of insulin signaling even in the absence of insulin. In conclusion, these data reveal the unique effects of C646 in activating insulin signaling in patients with obesity and diabetes.

Phosphorylated and O-GlcNAc Modified IRS-1 (Ser1101) and -2 (Ser1149) Contribute to Human Diabetes Type II

BACKGROUND

The prevalence of the chronic metabolic disorder Type 2 diabetes mellitus (T2DM) is increasing steadily, and has even turned into an epidemic in some countries. T2DM results from defective responses to insulin and obesity is a major factor behind insulin resistance in T2DM. Insulin receptor substrate (IRS) proteins are adaptor proteins in the insulin receptor signalling pathway. The insulin signalling is controlled through tyrosine phosphorylation of IRS-1 and IRS-2, and dysregulation of IRS proteins signalling may lead to glucose intolerance and eventually insulin resistance.

OBJECTIVE

In this work, we suggest that both glycosylation (O-GlcNAc modification) and phosphorylation of IRS-1 and -2 are involved in the pathogenesis of T2DM.

METHODS

Phosphorylation and O-GlcNAc modifications (Ser1101 in IRS-1 and Ser1149 in IRS-2) proteins were determined experimentally by sandwich ELISA with specific antibodies and with bioinformatics tools.

RESULTS

When IRS-1 (on Ser1101) and IRS-2 (Ser1149) become glycosylated following an increase in UDP-GlcNAc pools, it may contribute to insulin resistance. Whereas when the same (IRS-1 on Ser1101 and IRS-2 on Ser1149) are phosphorylated, the insulin signalling is inhibited.

DISCUSSION

In this work OGlcNAc-modified proteins were specifically detected using O-Glc- NAc-specific antibodies, suggesting that elevated levels of O-GlcNAc-modified proteins are found, independently of their possible involvement in Advanced Glycation End products (AGEs).

CONCLUSION

This study suggests a mechanism, which is controlled by posttranslational modifications, and may contribute to the pathogenesis of type II diabetes.

Alterations of Gut Microbiota by Overnutrition Impact Gluconeogenic Gene Expression and Insulin Signaling

A high-fat, Western-style diet is an important predisposing factor for the onset of type 2 diabetes and obesity. It causes changes in gut microbial profile, reduction of microbial diversity, and the impairment of the intestinal barrier, leading to increased serum lipopolysaccharide (endotoxin) levels. Elevated lipopolysaccharide (LPS) induces acetyltransferase P300 both in the nucleus and cytoplasm of liver hepatocytes through the activation of the IRE1-XBP1 pathway in the endoplasmic reticulum stress. In the nucleus, induced P300 acetylates CRTC2 to increase CRTC2 abundance and drives Foxo1 gene expression, resulting in increased expression of the rate-limiting gluconeogenic gene G6pc and Pck1 and abnormal liver glucose production. Furthermore, abnormal cytoplasm-appearing P300 acetylates IRS1 and IRS2 to disrupt insulin signaling, leading to the prevention of nuclear exclusion and degradation of FOXO1 proteins to further exacerbate the expression of G6pc and Pck1 genes and liver glucose production. Inhibition of P300 acetyltransferase activity by chemical inhibitors improved insulin signaling and alleviated hyperglycemia in obese mice. Thus, P300 acetyltransferase activity appears to be a therapeutic target for the treatment of type 2 diabetes and obesity.

Signalling Properties of Inositol Polyphosphates

Several studies have identified specific signalling functions for inositol polyphosphates (IPs) in different cell types and have led to the accumulation of new information regarding their cellular roles as well as new insights into their cellular production. These studies have revealed that interaction of IPs with several proteins is critical for stabilization of protein complexes and for modulation of enzymatic activity. This has not only revealed their importance in regulation of several cellular processes but it has also highlighted the possibility of new pharmacological interventions in multiple diseases, including cancer. In this review, we describe some of the intracellular roles of IPs and we discuss the pharmacological opportunities that modulation of IPs levels can provide.

Inositol Polyphosphate-Based Compounds as Inhibitors of Phosphoinositide 3-Kinase-Dependent Signaling

Signaling pathways regulated by the phosphoinositide 3-kinase (PI3K) enzymes have a well-established role in cancer development and progression. Over the past 30 years, the therapeutic potential of targeting this pathway has been well recognized, and this has led to the development of a multitude of drugs, some of which have progressed into clinical trials, with few of them currently approved for use in specific cancer settings. While many inhibitors compete with ATP, hence preventing the catalytic activity of the kinases directly, a deep understanding of the mechanisms of PI3K-dependent activation of its downstream effectors led to the development of additional strategies to prevent the initiation of this signaling pathway. This review summarizes previously published studies that led to the identification of inositol polyphosphates as promising parent molecules to design novel inhibitors of PI3K-dependent signals. We focus our attention on the inhibition of protein-membrane interactions mediated by binding of pleckstrin homology domains and phosphoinositides that we proposed 20 years ago as a novel therapeutic strategy.

Fluorescent Biosensors for Multiplexed Imaging of Phosphoinositide Dynamics

Phosphoinositides constitute a critical family of lipids that regulate numerous cellular processes. Phosphatidylinositol 4,5-bisphosphate (PIP₂) is arguably the most important plasma membrane phosphoinositide and is involved in regulating diverse processes. It is also the precursor of phosphatidylinositol 3,4,5-trisphosphate (PIP₃), which is critical for growth factor signaling, as well as membrane polarization and dynamics. Studying these lipids remains challenging, because of their compartmentalized activities and location-dependent signaling profiles. Here, we introduce several new genetically encoded fluorescent biosensors, including FRET-based and dimerization-dependent fluorescent protein (ddFP)-based biosensors, that enable real-time monitoring of PIP₂ levels in live cells. In addition, we developed a red fluorescent biosensor for 3-phosphoinositides that can be co-imaged with the green PIP₂ indicator. Simultaneous visualization of the dynamics of PIP₂ and 3-phosphoinositides in the same cell shows that plasma membrane PIP₃ formation upon EGF stimulation is coupled to a decrease in the local pool of PIP₂.

Polyphosphoinositides in the nucleus: Roadmap of their effectors and mechanisms of interaction

Biomolecular interactions between proteins and polyphosphoinositides (PPIn) are essential in the regulation of the vast majority of cellular processes. Consequently, alteration of these interactions is implicated in the development of many diseases. PPIn are phosphorylated derivatives of phosphatidylinositol and consist of seven species with different phosphate combinations. PPIn signal by recruiting proteins via canonical domains or short polybasic motifs. Although their actions are predominantly documented on cytoplasmic membranes, six of the seven PPIn are present within the nucleus together with the PPIn kinases, phosphatases and phospholipases that regulate their turnover. Importantly, the contribution of nuclear PPIn in the regulation of nuclear processes has led to an increased recognition of their importance compared to their more accepted cytoplasmic roles. This review summarises our knowledge on the identification and functional characterisation of nuclear PPIn-effector proteins as well as their mode of interactions, which tend to favour polybasic motifs.

Modeling cell line-specific recruitment of signaling proteins to the insulin-like growth factor 1 receptor

Receptor tyrosine kinases (RTKs) typically contain multiple autophosphorylation sites in their cytoplasmic domains. Once activated, these autophosphorylation sites can recruit downstream signaling proteins containing Src homology 2 (SH2) and phosphotyrosine-binding (PTB) domains, which recognize phosphotyrosine-containing short linear motifs (SLiMs). These domains and SLiMs have polyspecific or promiscuous binding activities. Thus, multiple signaling proteins may compete for binding to a common SLiM and vice versa. To investigate the effects of competition on RTK signaling, we used a rule-based modeling approach to develop and analyze models for ligand-induced recruitment of SH2/PTB domain-containing proteins to autophosphorylation sites in the insulin-like growth factor 1 (IGF1) receptor (IGF1R). Models were parameterized using published datasets reporting protein copy numbers and site-specific binding affinities. Simulations were facilitated by a novel application of model restructuration, to reduce redundancy in rule-derived equations. We compare predictions obtained via numerical simulation of the model to those obtained through simple prediction methods, such as through an analytical approximation, or ranking by copy number and/or K_D value, and find that the simple methods are unable to recapitulate the predictions of numerical simulations. We created 45 cell line-specific models that demonstrate how early events in IGF1R signaling depend on the protein abundance profile of a cell. Simulations, facilitated by model restructuration, identified pairs of IGF1R binding partners that are recruited in anti-correlated and correlated fashions, despite no inclusion of cooperativity in our models. This work shows that the outcome of competition depends on the physicochemical parameters that characterize pairwise interactions, as well as network properties, including network connectivity and the relative abundances of competitors.

Cells rely on networks of interacting biomolecules to sense and respond to environmental perturbations and signals. However, it is unclear how information is processed to generate appropriate and specific responses to signals, especially given that these networks tend to share many components. For example, receptors that detect distinct ligands and regulate distinct cellular activities commonly interact with overlapping sets of downstream signaling proteins. Here, to investigate the downstream signaling of a well-studied receptor tyrosine kinase (RTK), the insulin-like growth factor 1 (IGF1) receptor (IGF1R), we formulated and analyzed 45 cell line-specific mathematical models, which account for recruitment of 18 different binding partners to six sites of receptor autophosphorylation in IGF1R. The models were parameterized using available protein copy number and site-specific affinity measurements, and restructured to allow for network generation. We find that recruitment is influenced by the protein abundance profile of a cell, with different patterns of recruitment in different cell lines. Furthermore, in a given cell line, we find that pairs of IGF1R binding partners may be recruited in a correlated or anti-correlated fashion. We demonstrate that the simulations of the model have greater predictive power than protein copy number and/or binding affinity data, and that even a simple analytical model cannot reproduce the predicted recruitment ranking obtained via simulations. These findings represent testable predictions and indicate that the outputs of IGF1R signaling depend on cell line-specific properties in addition to the properties that are intrinsic to the biomolecules involved.

Cellular phosphatase activity of C1-Ten/Tensin2 is controlled by Phosphatidylinositol-3,4,5-triphosphate binding through the C1-Ten/Tensin2 SH2 domain

Regulation of tyrosine phosphorylation on insulin receptor substrate-1 (IRS-1) is essential for insulin signaling. The protein tyrosine phosphatase (PTP) C1-Ten/Tensin2 has been implicated in the regulation of IRS-1, but the molecular basis of this dephosphorylation is not fully understood. Here, we demonstrate that the cellular phosphatase activity of C1-Ten/Tensin2 on IRS-1 is mediated by the binding of the C1-Ten/Tensin2 Src-homology 2 (SH2) domain to phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P₃). We show that the role of C1-Ten/Tensin2 is dependent on insulin-induced phosphoinositide 3-kinase activity. The C1-Ten/Tensin2 SH2 domain showed strong preference and high affinity for PtdIns(3,4,5)P₃. Using site-directed mutagenesis, we identified three basic residues in the C1-Ten/Tensin2 SH2 domain that were critical for PtdIns(3,4,5)P₃ binding but were not involved in phosphotyrosine binding and PTP activity. Using a PtdIns(3,4,5)P₃ binding-deficient mutant, we showed that the specific binding of the C1-Ten/Tensin2 SH2 domain to PtdIns(3,4,5)P₃ allowed C1-Ten/Tensin2 to function as a PTP in cells. Collectively, our findings suggest that the interaction between the C1-Ten/Tensin2 SH2 domain and PtdIns(3,4,5)P₃ produces a negative feedback loop of insulin signaling through IRS-1.

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.

Distinct signalling properties of insulin receptor substrate (IRS)-1 and IRS-2 in mediating insulin/IGF-1 action

Insulin/IGF-1 action is driven by a complex and highly integrated signalling network. Loss-of-function studies indicate that the major insulin/IGF-1 receptor substrate (IRS) proteins, IRS-1 and IRS-2, mediate different biological functions in vitro and in vivo, suggesting specific signalling properties despite their high degree of homology. To identify mechanisms contributing to the differential signalling properties of IRS-1 and IRS-2 in the mediation of insulin/IGF-1 action, we performed comprehensive mass spectrometry (MS)-based phosphoproteomic profiling of brown preadipocytes from wild type, IRS-1^-/- and IRS-2^-/- mice in the basal and IGF-1-stimulated states. We applied stable isotope labeling by amino acids in cell culture (SILAC) for the accurate quantitation of changes in protein phosphorylation. We found ~10% of the 6262 unique phosphorylation sites detected to be regulated by IGF-1. These regulated sites included previously reported substrates of the insulin/IGF-1 signalling pathway, as well as novel substrates including Nuclear Factor I X and Semaphorin-4B. In silico prediction suggests the protein kinase B (PKB), protein kinase C (PKC), and cyclin-dependent kinase (CDK) as the main mediators of these phosphorylation events. Importantly, we found preferential phosphorylation patterns depending on the presence of either IRS-1 or IRS-2, which was associated with specific sets of kinases involved in signal transduction downstream of these substrates such as PDHK1, MAPK3, and PKD1 for IRS-1, and PIN1 and PKC beta for IRS-2. Overall, by generating a comprehensive phosphoproteomic profile from brown preadipocyte cells in response to IGF-1 stimulation, we reveal both common and distinct insulin/IGF-1 signalling events mediated by specific IRS proteins.

Membrane and Protein Interactions of the Pleckstrin Homology Domain Superfamily

The human genome encodes about 285 proteins that contain at least one annotated pleckstrin homology (PH) domain. As the first phosphoinositide binding module domain to be discovered, the PH domain recruits diverse protein architectures to cellular membranes. PH domains constitute one of the largest protein superfamilies, and have diverged to regulate many different signaling proteins and modules such as Dbl homology (DH) and Tec homology (TH) domains. The ligands of approximately 70 PH domains have been validated by binding assays and complexed structures, allowing meaningful extrapolation across the entire superfamily. Here the Membrane Optimal Docking Area (MODA) program is used at a genome-wide level to identify all membrane docking PH structures and map their lipid-binding determinants. In addition to the linear sequence motifs which are employed for phosphoinositide recognition, the three dimensional structural features that allow peripheral membrane domains to approach and insert into the bilayer are pinpointed and can be predicted ab initio. The analysis shows that conserved structural surfaces distinguish which PH domains associate with membrane from those that do not. Moreover, the results indicate that lipid-binding PH domains can be classified into different functional subgroups based on the type of membrane insertion elements they project towards the bilayer.

Analysis of insulin receptor substrate signaling dynamics on microstructured surfaces

Insulin receptor substrates (IRS) are phosphorylated by activated insulin/insulin-like growth factor I receptor tyrosine kinases, with this comprising an initial key event for downstream signaling and bioactivities. Despite the structural similarities, increasing evidence shows that IRS family proteins have nonredundant functions. Although the specificity of insulin/insulin-like growth factor signaling and biological responses partly reflects which IRS proteins are dominantly phosphorylated by the receptors, the precise properties of the respective IRS interaction with the receptors remain elusive. In the present study, we utilized a technique that combines micropatterned surfaces and total internal reflection fluorescence microscopy for the quantitative analysis of the interaction between IRS proteins and insulin/insulin-like growth factor in living cells. Our experimental set-up enabled the measurement of equilibrium associations and interaction dynamics of these molecules with high specificity. We revealed that several domains of IRS including pleckstrin homology and phosphotyrosine binding domains critically determine the turnover rate of the receptors. Furthermore, we found significant differences among IRS proteins in the strength and kinetic stability of the interaction with the receptors, suggesting that these interaction properties could account for the diverse functions of IRS. In addition, our analyses using fluorescent recovery after photobleaching revealed that kinases such as c-Jun N-terminal kinase and IκB kinase β, which phosphorylate serine/threonine residues of IRS and contribute to insulin resistance, altered the interaction kinetics of IRS with insulin receptor. Collectively, our experimental set-up is a valuable system for quantitifying the physiological interaction of IRS with the receptors in insulin/insulin-like growth factor signaling.

Ankyrin-G palmitoylation and βII-spectrin binding to phosphoinositide lipids drive lateral membrane assembly

Ankyrin-G and βII-spectrin colocalize at sites of cell-cell contact in columnar epithelial cells and promote lateral membrane assembly. This study identifies two critical inputs from lipids that together provide a rationale for how ankyrin-G and βII-spectrin selectively localize to Madin-Darby canine kidney (MDCK) cell lateral membranes. We identify aspartate-histidine-histidine-cysteine 5/8 (DHHC5/8) as ankyrin-G palmitoyltransferases required for ankyrin-G lateral membrane localization and for assembly of lateral membranes. We also find that βII-spectrin functions as a coincidence detector that requires recognition of both ankyrin-G and phosphoinositide lipids for its lateral membrane localization. DHHC5/8 and βII-spectrin colocalize with ankyrin-G in micrometer-scale subdomains within the lateral membrane that are likely sites for palmitoylation of ankyrin-G. Loss of either DHHC5/8 or ankyrin-G-βII-spectrin interaction or βII-spectrin-phosphoinositide recognition through its pleckstrin homology domain all result in failure to build the lateral membrane. In summary, we identify a functional network connecting palmitoyltransferases DHHC5/8 with ankyrin-G, ankyrin-G with βII-spectrin, and βII-spectrin with phosphoinositides that is required for the columnar morphology of MDCK epithelial cells.

The AP-1 complex regulates intracellular localization of insulin receptor substrate 1, which is required for insulin-like growth factor I-dependent cell proliferation

The activation of the insulin/insulin-like growth factor I (IGF-I) receptor and the subsequent tyrosine phosphorylation of insulin receptor substrates (IRSs) are key initial events in a variety of insulin/IGF bioactivities, including mitogenesis. It has been reported that IRS-1 associates with intracellular membrane compartments, and this localization is believed to be important for insulin/IGF signal transduction. However, the molecular mechanisms underlying IRS-1 localization remain unclear. Here we show that in L6 myoblasts, IRS-1 associates with μ1A of the ubiquitously expressed AP-1 complex, which packages cargo proteins into clathrin-coated vesicles derived from intracellular membranes. While wild-type IRS-1 was predominantly localized to vesicular structures, IRS-1 mutants lacking three YXXΦ motifs responsible for binding to μ1A were mislocalized to the mannose-6-phosphate receptor-positive structures, suggesting that AP-1-dependent transport to peripheral vesicles is inhibited in these mutants. Furthermore, deletion of AP-1 binding sites in IRS-1 impaired IGF-I-induced cell proliferation, accompanied by reduced tyrosine phosphorylation of IRS-1 and its association with phosphoinositide (PI) 3-kinase. These data demonstrate the importance of AP-1-dependent localization of IRS-1 in mediating IGF-I-stimulated signaling and maximum mitogenic response.

Molecular basis of signaling specificity of insulin and IGF receptors: neglected corners and recent advances

Insulin and insulin-like growth factor (IGF) receptors utilize common phosphoinositide 3-kinase/Akt and Ras/extracellular signal-regulated kinase signaling pathways to mediate a broad spectrum of "metabolic" and "mitogenic" responses. Specificity of insulin and IGF action in vivo must in part reflect expression of receptors and responsive pathways in different tissues but it is widely assumed that it is also determined by the ligand binding and signaling mechanisms of the receptors. This review focuses on receptor-proximal events in insulin/IGF signaling and examines their contribution to specificity of downstream responses. Insulin and IGF receptors may differ subtly in the efficiency with which they recruit their major substrates (IRS-1 and IRS-2 and Shc) and this could influence effectiveness of signaling to "metabolic" and "mitogenic" responses. Other substrates (Grb2-associated binder, downstream of kinases, SH2Bs, Crk), scaffolds (RACK1, β-arrestins, cytohesins), and pathways (non-receptor tyrosine kinases, phosphoinositide kinases, reactive oxygen species) have been less widely studied. Some of these components appear to be specifically involved in "metabolic" or "mitogenic" signaling but it has not been shown that this reflects receptor-preferential interaction. Very few receptor-specific interactions have been characterized, and their roles in signaling are unclear. Signaling specificity might also be imparted by differences in intracellular trafficking or feedback regulation of receptors, but few studies have directly addressed this possibility. Although published data are not wholly conclusive, no evidence has yet emerged for signaling mechanisms that are specifically engaged by insulin receptors but not IGF receptors or vice versa, and there is only limited evidence for differential activation of signaling mechanisms that are common to both receptors. Cellular context, rather than intrinsic receptor activity, therefore appears to be the major determinant of whether responses to insulin and IGFs are perceived as "metabolic" or "mitogenic."

The CNK1 scaffold binds cytohesins and promotes insulin pathway signaling

Protein scaffolds play an important role in signal transduction, regulating the localization of signaling components and mediating key protein interactions. Here, we report that the major binding partners of the Connector Enhancer of KSR 1 (CNK1) scaffold are members of the cytohesin family of Arf guanine nucleotide exchange factors, and that the CNK1/cytohesin interaction is critical for activation of the PI3K/AKT cascade downstream from insulin and insulin-like growth factor 1 (IGF-1) receptors. We identified a domain located in the C-terminal region of CNK1 that interacts constitutively with the coiled-coil domain of the cytohesins, and found that CNK1 facilitates the membrane recruitment of cytohesin-2 following insulin stimulation. Moreover, through protein depletion and rescue experiments, we found that the CNK1/cytohesin interaction promotes signaling from plasma membrane-bound Arf GTPases to the phosphatidylinositol 4-phosphate 5-kinases (PIP5Ks) to generate a PIP(2)-rich microenvironment that is critical for the membrane recruitment of insulin receptor substrate 1 (IRS1) and signal transmission to the PI3K/AKT cascade. These findings identify CNK1 as a new positive regulator of insulin signaling.

Odorant-stimulated phosphoinositide signaling in mammalian olfactory receptor neurons

Recent evidence has revived interest in the idea that phosphoinositides (PIs) may play a role in signal transduction in mammalian olfactory receptor neurons (ORNs). To provide direct evidence that odorants indeed activate PI signaling in ORNs, we used adenoviral vectors carrying two different fluorescently tagged probes, the pleckstrin homology (PH) domains of phospholipase C delta 1 (PLC delta 1) and the general receptor of phosphoinositides (GRP1), to monitor PI activity in the dendritic knobs of ORNs in vivo. Odorants mobilized PI(4,5)P(2)/IP(3) and PI(3,4,5)P(3), the substrates and products of PLC and PI3K. We then measured odorant activation of PLC and PI3K in olfactory ciliary-enriched membranes in vitro using a phospholipid overlay assay and ELISAs. Odorants activated both PLC and PI3K in the olfactory cilia within 2s of odorant stimulation. Odorant-dependent activation of PLC and PI3K in the olfactory epithelium could be blocked by enzyme-specific inhibitors. Odorants activated PLC and PI3K with partially overlapping specificity. These results provide direct evidence that odorants indeed activate PI signaling in mammalian ORNs in a manner that is consistent with the idea that PI signaling plays a role in olfactory transduction.

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.

Identification of the JNK signaling pathway as a functional target of the tumor suppressor PTEN

Although most oncogenic phenotypes of PTEN loss are attributed to AKT activation, AKT alone is not sufficient to induce all of the biological activities associated with PTEN inactivation. We searched for additional PTEN-regulated pathways through gene set enrichment analysis (GSEA) and identified genes associated with JNK activation. PTEN null cells exhibit higher JNK activity, and genetic studies demonstrate that JNK functions parallel to and independently of AKT. Furthermore, PTEN deficiency sensitizes cells to JNK inhibition and negative feedback regulation of PI3K was impaired in PTEN null cells. Akt and JNK activation are highly correlated in human prostate cancer. These findings implicate JNK in PI3K-driven cancers and demonstrate the utility of GSEA to identify functional pathways using genetically defined systems.

Emerging roles of phosphatidylinositol 3-monophosphate as a dynamic lipid second messenger

The lipid products of phosphoinositide 3-kinase (PI3K) are involved in many cellular responses such as proliferation, migration and survival. Disregulation of PI3K-activated pathways is implicated in different disease including diabetes and cancer. Among the different products of PI3Ks, phosphatidylinositol-3,4,5-trisphosphate (PtdIns-3,4,5-P3) has a well established role in signal transduction whereas the monophosphate phosphatidylinositol-3-phosphate (PtdIns-3-P) has been considered for a long time just a cellular component confined in endosomal structures. Only recently several evidence have indicated that PtdIns-3-P can also act as a dynamic intracellular second messenger. The role of PtdIns-3-P as mediator of crucial intracellular signals is therefore just beginning to be appreciated. Here we review some of the latest evidence showing that pools of PtdIns-3-P can be generated upon cellular stimulation in compartments different from the "classical" endosomal region. We describe several proteins that can be targets in mediating signals deriving from such stimulated PtdIns-3-P pools. In addition we describe the potential mechanism of switching on and off such signals. Taken together all this evidence suggest a novel, key role for PtdIns-3-P in signal transduction.

PKCdelta-mediated IRS-1 Ser24 phosphorylation negatively regulates IRS-1 function

The IRS-1 PH and PTB domains are essential for insulin-stimulated IRS-1 Tyr phosphorylation and insulin signaling, while Ser/Thr phosphorylation of IRS-1 disrupts these signaling events. To investigate consensus PKC phosphorylation sites in the PH-PTB domains of human IRS-1, we changed Ser24, Ser58, and Thr191 to Ala (3A) or Glu (3E), to block or mimic phosphorylation, respectively. The 3A mutant abrogated the inhibitory effect of PKCdelta on insulin-stimulated IRS-1 Tyr phosphorylation, while reductions in insulin-stimulated IRS-1 Tyr phosphorylation, cellular proliferation, and Akt activation were observed with the 3E mutant. When single Glu mutants were tested, the Ser24 to Glu mutant had the greatest inhibitory effect on insulin-stimulated IRS-1 Tyr phosphorylation. PKCdelta-mediated IRS-1 Ser24 phosphorylation was confirmed in cells with PKCdelta catalytic domain mutants and by an RNAi method. Mechanistic studies revealed that IRS-1 with Ala and Glu point mutations at Ser24 impaired phosphatidylinositol-4,5-bisphosphate binding. In summary, our data are consistent with the hypothesis that Ser24 is a negative regulatory phosphorylation site in IRS-1.

Epidermal growth factor induces insulin receptor substrate-2 in breast cancer cells via c-Jun NH(2)-terminal kinase/activator protein-1 signaling to regulate cell migration

The epidermal growth factor (EGF) and insulin-like growth factor (IGF) signaling pathways are critically involved in cancer development and progression. However, how these two signals cross-talk with each other to regulate cancer cell growth is not clearly understood. In this study, we found that EGF remarkably induced expression of major IGF signaling components, insulin receptor substrate (IRS)-1 and IRS-2, an effect that could be blocked by EGF receptor (EGFR) tyrosine kinase inhibitors. Although both extracellular signal-regulated kinase and c-Jun NH(2)-terminal kinase (JNK) signaling pathways were involved in the EGF up-regulation of IRS-1, the IRS-2 induction by EGF was specifically mediated by JNK signaling. Consistent with this, EGF increased IRS-2 promoter activity, which was associated with recruitment of activator protein-1 (AP-1) transcription factors and was inhibited by blocking AP-1 activity. Moreover, EGF treatment enhanced IGF-I and integrin engagement-elicited tyrosine phosphorylation of IRS and their downstream signaling, such as binding to phosphatidylinositol 3'-kinase regulatory subunit p85. Finally, repressing the induction of IRS-2 levels abolished the EGF enhancement of cell motility, suggesting that increased IRS-2 is essential for the EGF regulation of breast cancer cell migration. Taken together, our results reveal a novel mechanism of cross-talk between the EGF and IGF signaling pathways, which could have implications in therapeutic applications of targeting EGFR in tumors. Because AP-1 activity is involved in breast cancer progression, our work may also suggest IRS-2 as a useful marker for aggressive breast cancer.

Regulation of insulin receptor substrate 1 pleckstrin homology domain by protein kinase C: role of serine 24 phosphorylation

Phosphorylation of insulin receptor substrate (IRS) proteins on serine residues is an important posttranslational modification that is linked to insulin resistance. Several phosphoserine sites on IRS1 have been identified; the majority are located proximal to the phosphotryosine-binding domain or near key receptor tyrosine kinase substrate- and/or Src-homology 2 domain-binding sites. Here we report on the characterization of a serine phosphorylation site in the N-terminal pleckstrin homology (PH) domain of IRS1. Bioinformatic tools identify serine 24 (Ser24) as a putative substrate site for the protein kinase C (PKC) family of serine kinases. We demonstrate that this site is indeed a bona fide substrate for conventional PKC. In vivo, IRS-1 is also phosphorylated on Ser24 after phorbol 12-myristate 13-acetate treatment of cells, and isoform-selective inhibitor studies suggest the involvement of PKCalpha. By comparing the pharmacological characteristics of phorbol 12-myristate 13-acetate-stimulated Ser24 phosphorylation with phosphorylation at two other sites previously linked to PKC activity (Ser307 and Ser612), we show that PKCalpha is likely to be directly involved in Ser24 phosphorylation, but indirectly involved in Ser307 and Ser612 phosphorylation. Using Ser24Asp IRS-1 mutants to mimic the phosphorylated residue, we demonstrate that the phosphorylation status of Ser24 does play an important role in regulating phosphoinositide binding to, and the intracellular localization of, the IRS1-PH domain, which can ultimately impinge on insulin-stimulated glucose uptake. Hence we provide evidence that IRS1-PH domain function is important for normal insulin signaling and is regulated by serine phosphorylation in a manner that could contribute to insulin resistance.

The insulin-like growth factor-I receptor as a target for cancer therapy

This review examines the rationale for targeting the insulin-like growth factor (IGF)-I receptor in the therapy of human tumours and their metastases. The rationale is based on two crucial findings: 1) in experimental animals, normal cells are only partially affected by the deletion of the IGF-I receptor, whereas tumour cells undergo apoptosis when the IGF-I receptor is downregulated; and 2) cells with a deleted IGF-I receptor are refractory to transformation by viral and cellular oncogenes. This review focuses on the mechanisms underlying the experimental findings, and discusses the possibility of extrapolating the results obtained in animals to the cure of human tumours.

Critical role for Kalirin in nerve growth factor signaling through TrkA

Kalirin is a multidomain guanine nucleotide exchange factor (GEF) that activates Rho proteins, inducing cytoskeletal rearrangement in neurons. Although much is known about the effects of Kalirin on Rho GTPases and neuronal morphology, little is known about the association of Kalirin with the receptor/signaling systems that affect neuronal morphology. Our experiments demonstrate that Kalirin binds to and colocalizes with the TrkA neurotrophin receptor in neurons. In PC12 cells, inhibition of Kalirin expression using antisense RNA decreased nerve growth factor (NGF)-induced TrkA autophosphorylation and process extension. Kalirin overexpression potentiated neurotrophin-stimulated TrkA autophosphorylation and neurite outgrowth in PC12 cells at a low concentration of NGF. Furthermore, elevated Kalirin expression resulted in catalytic activation of TrkA, as demonstrated by in vitro kinase assays and increased NGF-stimulated cellular activation of Rac, Mek, and CREB. Domain mapping demonstrated that the N-terminal Kalirin pleckstrin homology domain mediates the interaction with TrkA. The effects of Kalirin on TrkA provide a molecular basis for the requirement of Kalirin in process extension from PC12 cells and for previously observed effects on axonal extension and dendritic maintenance. The interaction of TrkA with the pleckstrin homology domain of Kalirin may be one example of a general mechanism whereby receptor/Rho GEF pairings play an important role in receptor tyrosine kinase activation and signal transduction.

Lipid-Membrane Affinity of Chimeric Metal-binding Green Fluorescent Protein

The Green Fluorescent Protein (GFP) is a useful marker to trace the expression of cellular proteins. However, little is known about changes in protein interaction properties after fusion to GFP. In this study, we present evidence for a binding affinity of chimeric cadmium-binding green fluorescent proteins to lipid membrane. This affinity has been observed in both cellular membranes and artificial lipid monolayers and bilayers. At the cellular level, the presence of Cd-binding peptide promoted the association of the chimeric GFP onto the lipid membrane, which declined the fluorescence emission of the engineered cells. Binding affinity to lipid membranes was further investigated using artificial lipid bilayers and monolayers. Small amounts of the chimeric GFP were found to incorporate into the lipid vesicles due to the high surface pressure of bilayer lipids. At low interfacial pressure of the lipid monolayer, incorporation of the chimeric Cd-binding GFP onto the lipid monolayer was revealed. From the measured lipid isotherms, we conclude that Cd-binding GFP mediates an increase in membrane fluidity and an expansion of the surface area of the lipid film. This evidence was strongly supported by epifluorescence microscopy, showing that the chimeric Cd-binding GFP preferentially binds to fluid-phase areas and defect parts of the lipid monolayer. All these findings demonstrate the hydrophobicity of the GFP constructs is mainly influenced by the fusion partner. Thus, the example of a metal-binding unit used here shines new light on the biophysical properties of GFP constructs.

Effects of growth hormone on insulin signal transduction in rat adipose tissue maintained in vitro

Growth hormone treatment (GH) decreases adipose tissue sensitivity to insulin. However, the exact molecular mechanism(s) involved remains unclear. In the present study, we have evaluated the chronic effects of GH on adipose tissue explants cultured in a defined media. The objective was to determine the effects of GH treatment for 24 and 48 hours on the early steps of the insulin signal transduction, including IRS-3. The 24-hour culture media contained no hormones or 100 ng/ml GH. The 48-hour culture media contained insulin and dexamethasone supplemented with or without 100 ng/ml of GH. Results demonstrated a reduction in the cellular concentration of IRS-1 by around 30% when adipose tissue was chronically treated with growth hormone for either 24 or 48 hours. IRS-3 protein levels were also decreased by 15% after the 24-hour treatment, and by 27% after culture with GH for 48 hours in the presence of insulin and dexamethasone. PI 3-kinase concentrations were also reduced by GH in both experiments by around 25%. At the end of the 24-hour culture with GH adipose explants were stimulated with insulin in a short-term incubation, after which phosphorylation and association of the IRSs with PI 3-kinase were evaluated. After the insulin stimulus, the association of PI 3-kinase with IRS-1 and IRS-3 were decreased in explants chronically cultured with GH by 44 and 28%, respectively. After this short-term insulin stimulus, the IRS-3 phosphorylation was also lowered in GH-treated explants. The results with chronic cultures of adipose presented here are consistent with similar changes in IRS-1 and IRS-2 concentration and phosphorylation observed for liver and muscle after long-term (3-5 days) in vivo treatment with GH. The data suggest that chronic GH treatment alters the early steps of the insulin signal transduction pathway, and may explain the changes in adipose tissue sensitivity to insulin.

Growth factor-specific regulation of insulin receptor substrate-1 expression in MCF-7 breast carcinoma cells: effects on the insulin-like growth factor signaling pathway

IGFs are potent mitogens that play a crucial role in cell proliferation and/or differentiation and tumorigenesis. Insulin receptor substrate-1 (IRS-1) is a key protein in the IGF signaling pathway in the estrogen-dependent MCF-7 breast carcinoma cell line. In this study, three growth factors [fibroblast growth factor (FGF), epidermal growth factor (EGF), and platelet-derived growth factor (PDGF)] were tested for their ability to modulate IRS-1 protein expression and the IGF-I signaling pathway. FGF and, to a lesser extent, EGF were found to increase IRS-1 protein, whereas PDGF had no effect. This indicates that growth factors can specifically modulate IRS-1 protein content. The increases provoked by EGF and FGF were dependent on the MAPK signaling pathway but independent of phosphatidylinositol 3-kinase (PI 3-kinase) signaling and required de novo protein synthesis. We noted that the kinetics of MAPK activation was continuous in response to FGF but transient in response to EGF. In addition, transfection of cells with a constitutively active form of MAPK kinase, which results in continuous MAPK activity, increased IRS-1 expression. Taken together, these results suggest that stimulation of IRS-1 expression was therefore stronger when MAPK activity was sustained. Pretreatment of cells with EGF, FGF, or PDGF for 24 h reduced IGF-I-induced tyrosine phosphorylation per molecule of IRS-1. However, IGF-I-induced PI 3-kinase activity was decreased by 24 h of pretreatment with EGF or PDGF but not with FGF. Our results therefore demonstrate that different growth factors are capable of specifically modulating the IGF-I signaling via IRS-1. They further suggest that the FGF-induced increase in IRS-1 counterbalances the inhibition of IRS-1 tyrosine phosphorylation to allow normal stimulation of IGF-I-induced PI 3-kinase activity.

Phosphatidylinositol 3-kinase interacts with the adaptor protein Dab1 in response to Reelin signaling and is required for normal cortical lamination

Reelin is a large secreted signaling protein that binds to two members of the low density lipoprotein receptor family, the apolipoprotein E receptor 2 and the very low density lipoprotein receptor, and regulates neuronal positioning during brain development. Reelin signaling requires activation of Src family kinases as well as tyrosine phosphorylation of the intracellular adaptor protein Disabled-1 (Dab1). This results in activation of phosphatidylinositol 3-kinase (PI3K), the serine/threonine kinase Akt, and the inhibition of glycogen synthase kinase 3beta, a protein that is implicated in the regulation of axonal transport. Here we demonstrate that PI3K activation by Reelin requires Src family kinase activity and depends on the Reelin-triggered interaction of Dab1 with the PI3K regulatory subunit p85alpha. Because the Dab1 phosphotyrosine binding domain can interact simultaneously with membrane lipids and with the intracellular domains of apolipoprotein E receptor 2 and very low density lipoprotein receptor, Dab1 is preferentially recruited to the neuronal plasma membrane, where it is phosphorylated. Efficient Dab1 phosphorylation and activation of the Reelin signaling cascade is impaired by cholesterol depletion of the plasma membrane. Using a neuronal migration assay, we also show that PI3K signaling is required for the formation of a normal cortical plate, a step that is dependent upon Reelin signaling.

Nuclear translocation of insulin receptor substrate-1 by the insulin receptor in mouse embryo fibroblasts

Translocation of the insulin receptor substrate-1 (IRS-1) to the nuclei has been reported to occur in cells stimulated by insulin-like growth factor-1 (IGF-I) or expressing certain viral and cellular oncogenes. We show here that insulin can also induce nuclear translocation of IRS-1 in mouse embryo fibroblasts (MEF), that do not express the type 1 insulin-like growth factor receptor (IGF-IR). Only the A isoform of the insulin receptor (IR) can induce IRS-1 nuclear translocation, which is significant when the receptor is over-expressed. At physiological receptor levels, translocation occurs only in a fraction of cells, and only at high concentrations of ligand.

Insulin-like growth factor I receptor signaling and nuclear translocation of insulin receptor substrates 1 and 2

The insulin receptor substrate 1 (IRS-1) can translocate to the nuclei and nucleoli of several types of cells. Nuclear translocation can be induced by an activated insulin-like growth factor 1 receptor (IGF-IR), and by certain oncogenes, such as the Simian virus 40 T antigen and v-src. We have asked whether IRS-2 could also translocate to the nuclei. In addition, we have studied the effects of functional mutations in the IGF-IR on nuclear translocation of IRS proteins. IRS-2 translocates to the nuclei of mouse embryo fibroblasts expressing the IGF-IR, but, at variance with IRS-1, does not translocate in cells expressing the Simian virus 40 T antigen. Mutations in the tyrosine kinase domain of the IGF-IR abrogate translocation of the IRS proteins. Other mutations in the IGF-IR, which do not interfere with its mitogenicity but inhibit its transforming capacity, result in a decrease in translocation, especially to the nucleoli. Nuclear IRS-1 and IRS-2 interact with the upstream binding factor, which is a key regulator of RNA polymerase I activity and, therefore, rRNA synthesis. In 32D cells, wild-type, but not mutant, IRS-1 causes a significant activation of the ribosomal DNA promoter. The interaction of nuclear IRS proteins with upstream binding factor 1 constitutes the first direct link of these proteins with the ribosomal DNA transcription machinery.

Reciprocal feedback regulation of insulin receptor and insulin receptor substrate tyrosine phosphorylation by phosphoinositide 3-kinase in primary adipocytes

Signalling by the insulin receptor substrate (IRS) proteins is critically dependent on the tyrosine phosphorylation of specific binding sites that recruit Src homology 2 (SH2)-domain-containing proteins, such as the p85 subunit of phosphoinositide 3-kinase (PI 3-kinase), the tyrosine phosphatase SHP-2 and the adapter protein Grb2. Here we show that stimulation by insulin of freshly isolated primary adipocytes resulted in the expected rapid tyrosine phosphorylation of the insulin receptor, IRS-1 and IRS-3. Inhibition of PI 3-kinase enhanced the insulin-stimulated phosphorylation of IRS-1 on (i) Tyr(612) and Tyr(941) (p85 binding sites), concomitant with an increased association of the p85 subunit of PI 3-kinase; (ii) Tyr(896) (a Grb2 binding site); and (iii) Tyr(1229) (an SHP-2 binding site), although little or no binding of SHP-2 to IRS-1 was detectable under any conditions. In contrast, inhibition of PI 3-kinase led to a decrease in insulin-stimulated p85 binding to IRS-3, but had no effect on SHP-2 binding. Furthermore, insulin-induced insulin receptor tyrosine phosphorylation, phosphorylation of Tyr(1158) and insulin receptor tyrosine kinase activity were all reduced by inhibition of PI 3-kinase at later time points (>or=20 min). The results demonstrate that, in primary adipocytes, PI 3-kinase feedback control of signalling by the insulin receptor and IRS proteins is multifaceted and reciprocal, illustrating the complexity of predicting the net flux of the insulin signal(s) through the IRS proteins.

Nuclear translocation of insulin receptor substrate-1 by oncogenes and Igf-I. Effect on ribosomal RNA synthesis

The insulin receptor substrate-1 (IRS-1) is one of the major substrates of both the insulin and IGF-I receptors and is generally localized in the cytosol/membrane fraction of the cell. We show here that a substantial fraction of IRS-1 is translocated to the nucleus in mouse embryo fibroblasts (MEF) expressing the simian virus 40 T antigen. Nuclear translocation of IRS-1 occurs also in MEF stimulated with IGF-I or in MEF expressing the oncogene v-src. Nuclear translocation of IRS-1 can be demonstrated by confocal microscopy, immunohistochemistry, or subcellular fractionation. An antibody to IRS-1 immunoprecipitates from nuclear fractions (but not from cytosolic fractions) the upstream binding factor, which is a key regulator of RNA polymerase I activity and ribosomal RNA (rRNA) synthesis. In agreement with this finding, in 32D murine hemopoietic cells, nuclear translocation of IRS-1 correlates with a markedly increased rRNA synthesis. Our experiments suggest that nuclear IRS-1 may play a specialized role in rRNA synthesis and/or processing.

The pleckstrin homology (PH) domain-interacting protein couples the insulin receptor substrate 1 PH domain to insulin signaling pathways leading to mitogenesis and GLUT4 translocation

Receptor-mediated tyrosine phosphorylation of the insulin receptor substrate 1 (IRS-1) is required for the propagation of many of insulin's biological effects. The amino-terminal pleckstrin homology (PH) domain of IRS-1 plays a pivotal role in promoting insulin receptor (IR)-IRS-1 protein interactions. We have recently reported the isolation of a PH domain-interacting protein, PHIP, which selectively binds to the IRS-1 PH domain and is stably associated with IRS-1 in mammalian cells. Here we demonstrate that overexpression of PHIP in fibroblasts enhances insulin-induced transcriptional responses in a mitogen-activated protein kinase-dependent manner. In contrast, a dominant-negative mutant of PHIP (DN-PHIP) was shown to specifically block transcriptional and mitogenic signals elicited by insulin and not serum. In order to examine whether PHIP/IRS-1 complexes participate in the signal transduction pathway linking the IR to GLUT4 traffic in muscle cells, L6 myoblasts stably expressing a myc-tagged GLUT4 construct (L6GLUT4myc) were transfected with either wild-type or dominant-interfering forms of PHIP. Whereas insulin-dependent GLUT4myc membrane translocation was not affected by overexpression of PHIP, DN-PHIP caused a nearly complete inhibition of GLUT4 translocation, in a manner identical to that observed with a dominant-negative mutant of the p85 subunit of phosphatidylinositol 3-kinase (Deltap85). Furthermore, DN-PHIP markedly inhibited insulin-stimulated actin cytoskeletal reorganization, a process required for the productive incorporation of GLUT4 vesicles at the cell surface in L6 cells. Our results are consistent with the hypothesis that PHIP represents a physiological protein ligand of the IRS-1 PH domain, which plays an important role in insulin receptor-mediated mitogenic and metabolic signal transduction.

Differential roles of insulin receptor substrates in the anti-apoptotic function of insulin-like growth factor-1 and insulin

Insulin-like growth factor-1 (IGF-1) and insulin are known to prevent apoptosis. The signaling network of IGF-1 and insulin occurs via multiple pathways involving different insulin receptor substrates (IRSs). To define their roles in the anti-apoptotic function of IGF-1 and insulin, we established brown pre-adipocyte cell lines from wild-type and IRS knockout (KO) animals. In response to 16 h of serum deprivation, IRS-1-deficient cells showed a significant decrease in response to IGF-1 protection from apoptosis, whereas no changes were observed in the IRS-2, IRS-3, or IRS-4 KO cells. Five hours after serum withdrawal, cells already began to undergo apoptosis. At this early time point, IGF-1 and insulin were able to protect both wild-type and IRS-1 KO cells from death by 85-90%. After a longer period of serum deprivation, the protective ability of insulin and IGF-1 was decreased, and this was especially reduced in the IRS-1 KO cells. Reconstitution of these cells with IRS-1, IRS-2, IRS-3, or IRS-1/IRS-2 chimeras restored the anti-apoptotic effects of IGF-1, whereas overexpression of IRS-4 had no effect at long time points and actually reduced the effect of IGF-1 at the short time point. The biochemical basis of the defect in anti-apoptosis was not dependent on phosphorylation of mitogen-activated protein kinase; whereas phosphoinositide 3-kinase activity was decreased by 30% in IRS-1 KO cells. Akt phosphorylation was slightly reduced in these cells. Phosphorylation of the transcription factors cAMP response element-binding protein and FKHR by IGF-1 and insulin was markedly reduced in IRS-1 KO cells. In addition, both IGF-1 and insulin prevented caspase-3 cleavage in the wild-type cells, and this effect was greatly reduced in the IRS-1-deficient cells. These findings suggest that the IRS proteins may play differential roles in the anti-apoptotic effects of IGF-1 and insulin in brown pre-adipocytes, with IRS-1 being predominant, possibly acting through caspase-3-, CREB-, and FKHR-dependent mechanisms.

Nuclear translocation of insulin receptor substrate-1 by the simian virus 40 T antigen and the activated type 1 insulin-like growth factor receptor

32D cells are a murine hemopoietic cell line that undergoes apoptosis upon withdrawal of interleukin-3 (IL-3) from the medium. 32D cells have low levels of the type 1 insulin-like growth factor (IGF-I) receptor and do not express insulin receptor substrate-1 (IRS-1) or IRS-2. Ectopic expression of IRS-1 delays apoptosis but cannot rescue 32D cells from IL-3 dependence. In 32D/IRS-1 cells, IRS-1 is detectable, as expected, in the cytosol/membrane compartment. The SV40 large T antigen is a nuclear protein that, by itself, also fails to protect 32D cells from apoptosis. Co-expression of IRS-1 with the SV40 T antigen in 32D cells results in nuclear translocation of IRS-1 and survival after IL-3 withdrawal. Expression of a human IGF-I receptor in 32D/IRS-1 cells also results in nuclear translocation of IRS-1 and IL-3 independence. The phosphotyrosine-binding domain, but not the pleckstrin domain, is necessary for IRS-1 nuclear translocation. Nuclear translocation of IRS-1 was confirmed in mouse embryo fibroblasts. These results suggest possible new roles for nuclear IRS-1 in IGF-I-mediated growth and anti-apoptotic signaling.

The intermolecular interaction between the PH domain and the C-terminal domain of Arabidopsis dynamin-like 6 determines lipid binding specificity

Dynamin and its related proteins are a group of mechanochemical proteins involved in the modulation of lipid membranes in various biological processes. Here we investigate the nature of membrane binding of the Arabidopsis dynamin-like 6 (ADL6) involved in vesicle trafficking from the trans-Golgi network to the central vacuole. Fractionation experiments by continuous sucrose gradients and gel filtration revealed that the majority of ADL6 is associated with membranes in vivo. Amino acid sequence analysis revealed that ADL6 has a putative pleckstrin homology (PH) domain. In vitro lipid binding assays demonstrated that ADL6 showed high affinity binding to phosphatidylinositol 3-phosphate (PtdIns-3-P) and that the PH domain was responsible for this interaction. However, the PH domain alone binds equally well to both PtdIns-3-P and phosphatidylinositol 4-phosphate (PtdIns-4-P). Interestingly, the high affinity binding of the PH domain to PtdIns-3-P was restored by a protein-protein interaction between the PH domain and the C-terminal region. In addition, deletion of the inserted regions within the PH domain results in high affinity binding of the PH domain to PtdIns-3-P. These results suggest that ADL6 binds specifically to PtdIns-3-P and that the lipid binding specificity is determined by the interaction between the PH domain and the C-terminal domain of ADL6.

ETV6-NTRK3 transformation requires insulin-like growth factor 1 receptor signaling and is associated with constitutive IRS-1 tyrosine phosphorylation

Congenital fibrosarcoma (CFS) and cellular mesoblastic nephroma (CMN) are pediatric spindle cell malignancies that share two specific cytogenetic abnormalities: trisomy of chromosome 11 and a t(12;15)(p13;q25) translocation. The t(12;15) rearrangement creates a transcriptionally active fusion gene that encodes a chimeric oncoprotein, ETV6-NTRK3 (EN). EN transforms NIH3T3 fibroblasts through constitutive activation of both the Ras-mitogen-activated protein kinase (MAPK) pathway and the phosphatidylinositol-3'kinase (PI3K)-Akt pathway. However, the role of trisomy 11 in CFS and CMN remains unknown. In this study we demonstrate elevated expression of the chromosome 11p15.5 insulin-like growth factor 2 gene (IGF2) in CFS and CMN tumors. Moreover, we present evidence that an intact IGF signaling axis is essential for in vitro EN-mediated transformation. EN only very weakly transformed so-called R-murine fibroblasts derived from mice with a targeted disruption of the IGF1 receptor gene (IGFRI), but transformation activity was fully restored in R- cells engineered to re-express IGFRI (R+ cells). We also observed that the major IGFRI substrate, insulin-receptor substrate-1 (IRS-1), was constitutively tyrosine phosphorylated and could be co-immunoprecipitated with EN in either R- or R+ cells expressing the EN oncoprotein. IRS-1 association with Grb2 and PI3K p85, which link IGFRI to the Ras-MAPK and PI3K-Akt pathways, respectively, was enhanced in both cell types in the presence of EN. However, activation of the Ras-MAPK and PI3K-Akt pathways was markedly attenuated in EN-expressing R- cells compared to EN-transformed R+ cells. This suggests that IRS-1 may be functioning as an adaptor in EN signal transduction, but that a link to EN transformation pathways requires the presence of IGFRI. Our findings indicate that an intact IGF signaling axis is essential for EN transformation, and are consistent with a role for trisomy 11 in augmenting this pathway in EN expressing tumors.

Insulin receptor substrate-3 functions as transcriptional activator in the nucleus

The family of insulin receptor substrates (IRSs) has been reported to play important roles for signal transduction of various hormones. Four members of the IRS family have been described. Each IRS is believed to have different functions; however, the distinct physiological roles of each IRS are unclear. This study was undertaken to determine the intracellular localization of IRS-3. IRS-3 was expressed in COS-7 cells as fusion with a green fluorescent protein (GFP), and subcellular localization of the chimera protein was analyzed by fluorescent microscopy. Surprisingly, GFP-IRS-3 was localized not only adjacent to the plasma membrane but also in the nucleus. We confirmed by immunostaining with anti-IRS-3 antibody that non-fused IRS-3 protein is also localized in the nucleus of COS-7 cells that were transfected with IRS-3 cDNA. In addition, we detected endogenous IRS-3 in the nucleus of isolated rat adipocytes. We then studied subcellular localization of deletion mutants and fragments of IRS-3 fused with GFP. We found that the region corresponding to amino acid residues 192-223 in the phosphotyrosine binding domain played an important role in nuclear localization. This region includes sequences that are unique to IRS-3. We then investigated intracellular localization of other IRSs fused with GFP. GFP-IRS-1, GFP-IRS-2, and GFP-IRS-4 were mainly localized in the cytosol or plasma membranes. Chimeric protein, Gal4 DNA binding domain fused with IRS-3 C-terminal region, increased transcription of the reporter gene containing Gal4 binding site in human embryonic kidney 293 cells. These results suggest that intracellular localization of IRS-3 is determined by a different mechanism from other IRS proteins, and that IRS-3 possesses a transcription-regulating activity.

Estradiol increases IRS-1 gene expression and insulin signaling in breast cancer cells

This study demonstrates how the potentiating effects of E2 on insulin signaling in ER-positive breast cancer cells are consequent to an enhanced IRS-1 expression [corrected]. It induces an increase of both PI-3K/AKT and ERK1/2 activities. A direct action of E2 in the regulating mouse IRS-1 gene is also investigated in both Chinese hamster ovary and MCF-7 cells that are transfected with mouse IRS-1 regulatory sequences. The authors have reported, for the first time, how E2 induction of IRS-1 mRNA was correlated with a direct positive regulatory role of E2 on the IRS-1 promoter. This effect seems to be not strictly related to the cell type.

Insulin receptor substrate-1 pleckstrin homology and phosphotyrosine-binding domains are both involved in plasma membrane targeting

The localization of insulin receptor substrate (IRS) molecules may be responsible for the differential biological activities of insulin and other peptides such as platelet-derived growth factor. The subcellular localization of IRS-1 is controversial, with some reports suggesting association with the cytoskeleton and other studies reporting membrane localization. In this study, we used immunofluorescence microscopy to define the localization of IRS-1. In the basal state, recombinant IRS-1 was localized predominantly in the cytoplasm. In response to insulin, recombinant IRS-1 translocated to the plasma membrane. We have also studied the localization of green fluorescent protein (GFP) fusion proteins. Unlike native IRS-1, a fusion protein containing GFP plus full-length IRS-1 appeared to localize in inclusion bodies. In contrast, when GFP was fused to the N terminus of IRS-1 (i.e. the pleckstrin homology and phosphotyrosine-binding domains), this fusion protein was targeted to the plasma membrane. Mutations of phosphoinositide-binding sites in both the pleckstrin homology and phosphotyrosine-binding domains significantly reduced the ability of Myc-tagged IRS-1 to translocate to the plasma membrane following insulin stimulation. However, these mutations did not cause a statistically significant impairment of tyrosine phosphorylation in response to insulin. This raises the possibility that IRS-1 tyrosine phosphorylation may occur prior to plasma membrane translocation.

Specificity in pleckstrin homology (PH) domain membrane targeting: a role for a phosphoinositide-protein co-operative mechanism

Pleckstrin homology (PH) domains are protein modules found in proteins involved in many cellular processes. The majority of PH domain-containing proteins require membrane association for their function. It has been shown that most PH domains interact directly with the cell membrane by binding to phosphoinositides with a broad range of specificity and affinity. While a highly specific binding of the PH domain to a phosphoinositide can be necessary and sufficient for the correct recruitment of the host protein to the membrane, a weaker and less specific interaction may be necessary but not sufficient, thus probably requiring alternative, co-operative mechanisms.

Crk family adaptors-signalling complex formation and biological roles

Crk family adaptors are widely expressed and mediate the timely formation of signal transduction protein complexes upon a variety of extracellular stimuli, including various growth and differentiation factors. Selective formation of multi-protein complexes by the Crk and Crk-like (CRKL) proteins depends on specific motifs recognized by their SH2 and SH3 domains. In the case of the first SH3 domains [SH3(1)] a P-x-x-P-x-K motif is crucial for highly selective binding, while the SH2 domains prefer motifs which conform to the consensus pY-x-x-P. Crk family proteins are involved in the relocalization and activation of several different effector proteins which include guanine nucleotide releasing proteins like C3G, protein kinases of the Abl- and GCK-families and small GTPases like Rap1 and Rac. Crk-type proteins have been found not only in vertebrates but also in flies and nematodes. Major insight into the function of Crk within organisms came from the genetic model organism C. elegans, where the Crk-homologue CED-2 regulates cell engulfment and phagocytosis. Other biological outcomes of the Crk-activated signal transduction cascades include the modulation of cell adhesion, cell migration and immune cell responses. Crk family adaptors also appear to play a role in mediating the action of human oncogenes like the leukaemia-inducing Bcr-Abl protein. This review summarizes some key findings and highlights recent insights and open questions.

PTEN expression causes feedback upregulation of insulin receptor substrate 2

PTEN is a tumor suppressor that antagonizes phosphatidylinositol-3 kinase (PI3K) by dephosphorylating the D3 position of phosphatidylinositol (3,4,5)-triphosphate (PtdIns-3,4,5-P3). Given the importance of PTEN in regulating PtdIns-3,4,5-P3 levels, we used Affymetrix GeneChip arrays to identify genes regulated by PTEN. PTEN expression rapidly reduced the activity of Akt, which was followed by a G(1) arrest and eventually apoptosis. The gene encoding insulin receptor substrate 2 (IRS-2), a mediator of insulin signaling, was found to be the most induced gene at all time points. A PI3K-specific inhibitor, LY294002, also upregulated IRS-2, providing evidence that it was the suppression of the PI3K pathway that was responsible for the message upregulation. In addition, PTEN, LY294002, and rapamycin, an inhibitor of mammalian target of rapamycin, caused a reduction in the molecular weight of IRS-2 and an increase in the association of IRS-2 with PI3K. Apparently, PTEN inhibits a negative regulator of IRS-2 to upregulate the IRS-2-PI3K interaction. These studies suggest that PtdIns-3,4,5-P3 levels regulate the specific activity and amount of IRS-2 available for insulin signaling.

Subcellular localization of insulin receptor substrate family proteins associated with phosphatidylinositol 3-kinase activity and alterations in lipolysis in primary mouse adipocytes from IRS-1 null mice

To clarify the roles of insulin receptor substrate (IRS) family proteins in phosphatidylinositol (PI) 3-kinase activation and insulin actions in adipocytes, we investigated the intracellular localization of IRS family proteins and PI 3-kinase activation in response to insulin by fractionation of mouse adipocytes from wild-type and IRS-1 null mice. In adipocytes from wild-type mice, tyrosine-phosphorylated IRS-1 and IRS-2, which were found to associate with PI 3-kinase in response to insulin, were detected in the plasma membrane (PM) and low-density microsome (LDM) fractions. By contrast, tyrosine-phosphorylated IRS-3 (pp60), which was found to associate with PI 3-kinase, was predominantly localized in the PM fraction. In adipocytes from IRS-1-null mice, insulin-stimulated PI 3-kinase activity in anti-phosphotyrosine (alphaPY) immunoprecipitates in the LDM fraction was almost exclusively mediated via IRS-2 and was reduced to 25%; however, insulin-stimulated PI 3-kinase activity in the PM fraction was primarily mediated via IRS-3 and was reduced to 60%. To determine the potential functional impact of the distinct subcellular localization of IRSs and associating PI 3-kinase activity on adipocyte-specific metabolic actions, we examined lipolysis in IRS-1 null mice. The level of isoproterenol-induced lipolysis was increased 5.1-fold in adipocytes from IRS-1 null mice as compared with wild-type mice. Moreover, hormone-sensitive lipase (HSL) protein was increased 4.3-fold in adipocytes from IRS-1-null mice compared with wild-type mice, and HSL mRNA expression was also increased. The antilipolytic effect of insulin in IRS-1 null adipocytes, however, was comparable to that in wild-type mice. Thus, discordance between these two insulin actions as well as the transcriptional and translational effect (HSL mRNA and protein regulation) and the PM effect (antilipolysis) of insulin may be explained by distinct roles of both PI 3-kinase activity associated with IRS-1/IRS-2 and PI 3-kinase activity associated with IRS-3 in insulin actions related to their subcellular localization.