Insulin receptor protein-tyrosine phosphatases. Leukocyte common antigen-related phosphatase rapidly deactivates the insulin receptor kinase by preferential dephosphorylation of the receptor regulatory domain

Burdock miR8175 in diet improves insulin resistance induced by obesity in mice through food absorption

The incidence of type 2 diabetes mellitus (T2DM) induced by obesity is rapidly increasing. Although there are many synthetic drugs for treating T2DM, they have various side effects. Here, we report that miR8175, a plant miRNA from burdock root, has effective antidiabetic activity. After administration of burdock decoction or synthetic miR8175 by gavage, both burdock decoction and miR8175 can significantly improve the impaired glucose metabolism of diabetic mice induced by a high-fat diet (HFD). Our results demonstrate that burdock decoction and miR8175 enhance the insulin sensitivity of the hepatic insulin signaling pathway by targeting Ptprf and Ptp1b, which may be the reason for the improvement in metabolism. This study provides a theoretical basis for the main active component and molecular mechanism of burdock to improve insulin resistance. And the study also suggests that plant miRNA may be an indispensable nutrient for maintaining human health.

A type 2 diabetes disease module with a high collective influence for Cdk2 and PTPLAD1 is localized in endosomes

Despite the identification of many susceptibility genes our knowledge of the underlying mechanisms responsible for complex disease remains limited. Here, we identified a type 2 diabetes disease module in endosomes, and validate it for functional relevance on selected nodes. Using hepatic Golgi/endosomes fractions, we established a proteome of insulin receptor-containing endosomes that allowed the study of physical protein interaction networks on a type 2 diabetes background. The resulting collated network is formed by 313 nodes and 1147 edges with a topology organized around a few major hubs with Cdk2 displaying the highest collective influence. Overall, 88% of the nodes are associated with the type 2 diabetes genetic risk, including 101 new candidates. The Type 2 diabetes module is enriched with cytoskeleton and luminal acidification–dependent processes that are shared with secretion-related mechanisms. We identified new signaling pathways driven by Cdk2 and PTPLAD1 whose expression affects the association of the insulin receptor with TUBA, TUBB, the actin component ACTB and the endosomal sorting markers Rab5c and Rab11a. Therefore, the interactome of internalized insulin receptors reveals the presence of a type 2 diabetes disease module enriched in new layers of feedback loops required for insulin signaling, clearance and islet biology.

Unbiased identification of substrates of protein tyrosine phosphatase ptp-3 in C. elegans

The leukocyte antigen related (LAR) family of receptor-like protein tyrosine phosphatases has three members in humans - PTPRF, PTPRD and PTPRS - that have been implicated in diverse processes including embryonic development, inhibition of cell growth and axonal guidance. Mutations in the LAR family are associated with developmental defects such as cleft palate as well as various cancers including breast, neck, lung, colon and brain. Although this family of tyrosine phosphatases is important for many developmental processes, little is known of their substrates. This is partially due to functional redundancy within the LAR family, as deletion of a single gene in the LAR family does not have an appreciable phenotype, but a dual knockout is embryonically lethal in mouse models. To circumvent the inability to knockout multiple members of the LAR family in mouse models, we used a knockout of ptp-3, which is the only known ortholog of the LAR family in Caenorhabditis elegans and allows for the study of the LAR family at the organismal level. Using SILAC-based quantitative phosphoproteomics, we identified 255 putative substrates of ptp-3, which included four of the nine known annotated substrates of the LAR family. A motif analysis of the identified phosphopeptides allowed for the determination of sequences that appear to be preferentially dephosphorylated. Finally, we discovered that kinases were overrepresented in the list of identified putative substrates and tyrosine residues whose phosphorylation is known to increase kinase activity were dephosphorylated by ptp-3. These data are suggestive of ptp-3 as a potential negative regulator of several kinase families, such as the mitogen activated kinases (MAPKs), and multiple tyrosine kinases including FER, MET, and NTRK2.

Establishment of Leptin-Responsive Cell Lines from Adult Mouse Hypothalamus

Leptin resistance is considered to be the primary cause of obesity. However, the cause of leptin resistance remains incompletely understood, and there is currently no cure for the leptin-resistant state. In order to identify novel drug-target molecules that could overcome leptin resistance, it would be useful to develop in vitro assay systems for evaluating leptin resistance. In this study, we established immortalized adult mouse hypothalamus—derived cell lines, termed adult mouse hypothalamus (AMH) cells, by developing transgenic mice in which SV40 Tag was overexpressed in chromogranin A—positive cells in a tamoxifen-dependent manner. In order to obtain leptin-responsive clones, we selected clones based on the phosphorylation levels of STAT3 induced by leptin. The selected clones were fairly responsive to leptin in terms of STAT3, ERK, and Akt phosphorylation and induction of c-Fos mRNA induction. Pretreatment with leptin, insulin, and palmitate attenuated the c-Fos mRNA response to leptin, suggesting that certain aspects of leptin resistance might be reconstituted in this cellular model. These cell lines are useful tools for understanding the molecular nature of the signal disturbance in the leptin-resistant state and for identifying potential target molecules for drugs that relieve leptin resistance, although they have drawbacks including de-differentiated nature and lack of long-time stability.

The R3 receptor-like protein tyrosine phosphatase subfamily inhibits insulin signalling by dephosphorylating the insulin receptor at specific sites

The autophosphorylation of specific tyrosine residues occurs in the cytoplasmic region of the insulin receptor (IR) upon insulin binding, and this in turn initiates signal transduction. The R3 subfamily (Ptprb, Ptprh, Ptprj and Ptpro) of receptor-like protein tyrosine phosphatases (RPTPs) is characterized by an extracellular region with 6-17 fibronectin type III-like repeats and a cytoplasmic region with a single phosphatase domain. We herein identified the IR as a substrate for R3 RPTPs by using the substrate-trapping mutants of R3 RPTPs. The co-expression of R3 RPTPs with the IR in HEK293T cells suppressed insulin-induced tyrosine phosphorylation of the IR. In vitro assays using synthetic phosphopeptides revealed that R3 RPTPs preferentially dephosphorylated a particular phosphorylation site of the IR: Y960 in the juxtamembrane region and Y1146 in the activation loop. Among four R3 members, only Ptprj was co-expressed with the IR in major insulin target tissues, such as the skeletal muscle, liver and adipose tissue. Importantly, the activation of IR and Akt by insulin was enhanced, and glucose and insulin tolerance was improved in Ptprj-deficient mice. These results demonstrated Ptprj as a physiological enzyme that attenuates insulin signalling in vivo, and indicate that an inhibitor of Ptprj may be an insulin-sensitizing agent.

Inputs and outputs of insulin receptor

The insulin receptor (IR) is an important hub in insulin signaling and its activation is tightly regulated. Upon insulin stimulation, IR is activated through autophosphorylation, and consequently phosphorylates several insulin receptor substrate (IRS) proteins, including IRS1-6, Shc and Gab1. Certain adipokines have also been found to activate IR. On the contrary, PTP, Grb and SOCS proteins, which are responsible for the negative regulation of IR, are characterized as IR inhibitors. Additionally, many other proteins have been identified as IR substrates and participate in the insulin signaling pathway. To provide a more comprehensive understanding of the signals mediated through IR, we reviewed the upstream and downstream signal molecules of IR, summarized the positive and negative modulators of IR, and discussed the IR substrates and interacting adaptor proteins. We propose that the molecular events associated with IR should be integrated to obtain a better understanding of the insulin signaling pathway and diabetes.

Role of protein tyrosine phosphatases in the modulation of insulin signaling and their implication in the pathogenesis of obesity-linked insulin resistance

Insulin resistance is a major disorder that links obesity to type 2 diabetes mellitus (T2D). It involves defects in the insulin actions owing to a reduced ability of insulin to trigger key signaling pathways in major metabolic tissues. The pathogenesis of insulin resistance involves several inhibitory molecules that interfere with the tyrosine phosphorylation of the insulin receptor and its downstream effectors. Among those, growing interest has been developed toward the protein tyrosine phosphatases (PTPs), a large family of enzymes that can inactivate crucial signaling effectors in the insulin signaling cascade by dephosphorylating their tyrosine residues. Herein we briefly review the role of several PTPs that have been shown to be implicated in the regulation of insulin action, and then focus on the Src homology 2 (SH2) domain-containing SHP1 and SHP2 enzymes, since recent reports have indicated major roles for these PTPs in the control of insulin action and glucose metabolism. Finally, the therapeutic potential of targeting PTPs for combating insulin resistance and alleviating T2D will be discussed.

Variants of insulin-signaling inhibitor genes in type 2 diabetes and related metabolic abnormalities

Insulin resistance has a central role in the pathogenesis of several metabolic diseases, including type 2 diabetes, obesity, glucose intolerance, metabolic syndrome, atherosclerosis, and cardiovascular diseases. Insulin resistance and related traits are likely to be caused by abnormalities in the genes encoding for proteins involved in the composite network of insulin-signaling; in this review we have focused our attention on genetic variants of insulin-signaling inhibitor molecules. These proteins interfere with different steps in insulin-signaling: ENPP1/PC-1 and the phosphatases PTP1B and PTPRF/LAR inhibit the insulin receptor activation; INPPL1/SHIP-2 hydrolyzes PI3-kinase products, hampering the phosphoinositide-mediated downstream signaling; and TRIB3 binds the serine-threonine kinase Akt, reducing its phosphorylation levels. While several variants have been described over the years for all these genes, solid evidence of an association with type 2 diabetes and related diseases seems to exist only for rs1044498 of the ENPP1 gene and for rs2295490 of the TRIB3 gene. However, overall the data recapitulated in this Review article may supply useful elements to interpret the results of novel, more technically advanced genetic studies; indeed it is becoming increasingly evident that genetic information on metabolic diseases should be interpreted taking into account the complex biological pathways underlying their pathogenesis.

Involvement of protein tyrosine phosphatases and inflammation in hypothalamic insulin resistance associated with ageing: effect of caloric restriction

Aged Wistar rats present central insulin resistance associated with ageing. Several steps of the insulin signaling pathway have been described to be impaired in aged rats at hypothalamic level. In the present article we have explored possible alterations in protein tyrosine phosphatases (PTPs) involved in insulin receptor dephosphorylation, as well as pro-inflammatory pathways and serine kinases such as inhibitory kappa β kinase-nuclear factor kappa-B (IKKβ-NFκB), p38 mitogen-activated protein kinase (p38) and protein kinase C θ (PKCθ) that may also be involved in the decreased insulin signaling during ageing. We detected that ageing brings about a specific increase in insulin receptor tyrosine phosphatase activity and PTP1B serine phosphorylation. Increased association of PTP1B and leukocyte common antigen-related tyrosine protein phosphatase (LAR) with insulin receptor was also observed in hypothalamus from aged rats. Besides these mechanisms, increased activation of the IKKβ-NFκB pathway, p38 and PKCθ serine/threonine kinases were also detected. These data contribute to explain the hypothalamic insulin resistance associated with ageing. Caloric restriction ameliorates most of the effects of ageing on the above mentioned increases in PTPs and serine/threonine kinases activities and points to age-associated adiposity and inflammation as key factors in the development of age-associated insulin resistance.

T cell protein tyrosine phosphatase (TCPTP) deficiency in muscle does not alter insulin signalling and glucose homeostasis in mice

AIMS/HYPOTHESIS

Insulin activates insulin receptor protein tyrosine kinase and downstream phosphatidylinositol-3-kinase (PI3K)/Akt signalling in muscle to promote glucose uptake. The insulin receptor can serve as a substrate for the protein tyrosine phosphatase (PTP) 1B and T cell protein tyrosine phosphatase (TCPTP), which share a striking 74% sequence identity in their catalytic domains. PTP1B is a validated therapeutic target for the alleviation of insulin resistance in type 2 diabetes. PTP1B dephosphorylates the insulin receptor in liver and muscle to regulate glucose homeostasis, whereas TCPTP regulates insulin receptor signalling and gluconeogenesis in the liver. In this study we assessed for the first time the role of TCPTP in the regulation of insulin receptor signalling in muscle.

METHODS

We generated muscle-specific TCPTP-deficient (Mck-Cre;Ptpn2(lox/lox)) mice (Mck, also known as Ckm) and assessed the impact on glucose homeostasis and muscle insulin receptor signalling in chow-fed versus high-fat-fed mice.

RESULTS

Blood glucose and insulin levels, insulin and glucose tolerance, and insulin-induced muscle insulin receptor activation and downstream PI3K/Akt signalling remained unaltered in chow-fed Mck-Cre;Ptpn2(lox/lox) versus Ptpn2(lox/lox) mice. In addition, body weight, adiposity, energy expenditure, insulin sensitivity and glucose homeostasis were not altered in high-fat-fed Mck-Cre;Ptpn2(lox/lox) versus Ptpn2(lox/lox) mice.

CONCLUSIONS/INTERPRETATION

These results indicate that TCPTP deficiency in muscle has no effect on insulin signalling and glucose homeostasis, and does not prevent high-fat diet-induced insulin resistance. Thus, despite their high degree of sequence identity, PTP1B and TCPTP contribute differentially to insulin receptor regulation in muscle. Our results are consistent with the notion that these two highly related PTPs make distinct contributions to insulin receptor regulation in different tissues.

Identification of novel, less toxic PTP-LAR inhibitors using in silico strategies: pharmacophore modeling, SADMET-based virtual screening and docking

Human leukocyte antigen-related (PTP-LAR) is a receptor-like transmembrane phosphatase and a potential target for diabetes, obesity and cancer. In the present study, a sequence of in silico strategies (pharmacophore mapping, a 3D database searching, SADMET screening, and docking and toxicity studies) was performed to identify eight novel nontoxic PTP-LAR inhibitors. Twenty different pharmacophore hypotheses were generated using two methods; the best (hypothesis 2) consisted of three hydrogen-bond acceptor (A), one ring aromatic (R), and one hydrophobic aliphatic (Z) features. This hypothesis was used to screen molecules from several databases, such as Specs, IBS, MiniMaybridge, NCI, and an in-house PTP inhibitor database. In order to overcome the general bioavailability problem associated with phosphatases, the hits obtained were filtered by Lipinski's rule of five and SADMET properties and validated by molecular docking studies using the available crystal structure 1LAR. These docking studies suggested the ligand binding pattern and interactions required for LAR inhibition. The docking analysis also revealed that sulfonylurea derivatives with an isoquinoline or naphthalene scaffold represent potential LAR drugs. The screening protocol was further validated using ligand pharmacophore mapping studies, which showed that the abovementioned interactions are indeed crucial and that the screened molecules can be presumed to possess potent inhibitory activities.

Phosphoinositide 3-kinase signaling in the vertebrate retina

The phosphoinositide (PI) cycle, discovered over 50 years ago by Mabel and Lowell Hokin, describes a series of biochemical reactions that occur on the inner leaflet of the plasma membrane of cells in response to receptor activation by extracellular stimuli. Studies from our laboratory have shown that the retina and rod outer segments (ROSs) have active PI metabolism. Biochemical studies revealed that the ROSs contain the enzymes necessary for phosphorylation of phosphoinositides. We showed that light stimulates various components of the PI cycle in the vertebrate ROS, including diacylglycerol kinase, PI synthetase, phosphatidylinositol phosphate kinase, phospholipase C, and phosphoinositide 3-kinase (PI3K). This article describes recent studies on the PI3K-generated PI lipid second messengers in the control and regulation of PI-binding proteins in the vertebrate retina.

Enhanced retinal insulin receptor-activated neuroprotective survival signal in mice lacking the protein-tyrosine phosphatase-1B gene

Protein-tyrosine phosphatase 1B (PTP1B) has been implicated in the negative regulation of insulin signaling. We previously demonstrated that light-induced tyrosine phosphorylation of the retinal insulin receptor (IR) results in the activation of phosphoinositide 3-kinase/Akt survival pathway in rod photoreceptor cells. The molecular mechanism behind light-induced activation of IR is not known. We investigated the in vivo mechanism of IR activation and found that PTP1B activity in dark-adapted retinas was significantly higher than in light-adapted retinas. We made a novel finding in this study that the light-dependent regulation of PTP1B activity is signaled through photobleaching of rhodopsin. Conditional deletion of PTP1B in rod photoreceptors by the Cre-loxP system resulted in enhanced IR signaling. Further PTP1B activity negatively regulated the neuroprotective survival signaling in the retina. One of the challenging questions in the retina research is how mutations in human rhodopsin gene slowly disable and eventually disrupt photoreceptor functions. Our studies suggest that a defect in the photobleaching of rhodopsin and mutation in rhodopsin gene enhances the activity of PTP1B, and this activated activity could down-regulate the IR survival signaling. Our studies suggest that PTP1B antagonists could be potential therapeutic agents to treat stress-induced photoreceptor degenerations and provide further evidence that rhodopsin photoexcitation may trigger signaling events alternative to the classic phototransduction.

Regulation of Src homology 2-containing protein tyrosine phosphatase by advanced glycation end products: the role on atherosclerosis in diabetes

Advanced glycation end products (AGEs), among the most important causes of atherosclerosis in diabetes mellitus, stimulate the proliferation of smooth muscle cells (SMCs). Smooth muscle cells are central in the formation of atherosclerotic lesions, where they show both increased migration and accelerated proliferation. In investigating how AGEs stimulate SMC proliferation, we focused on protein tyrosine phosphatase, especially Src homology 2-containing protein tyrosine phosphatase (SHP2), which is considered important in regulating cell proliferation. Advanced glycation end products increased activity of SHP2 in the membrane fraction of rat aortic SMCs compared with control bovine serum albumin (P < .05). Upon characterizing the genomic and promoter structure of SHP2, we detected nuclear factor-kappaB (NF-kappaB) binding sites in the promoter area. Advanced glycation end product stimulation increased luciferase activity in cells transfected with SHP2 promoter region including NF-kappaB binding sites (P < .05) and increased SHP2 expression (P < .05). These data indicate that AGE stimulation appears to activate NF-kappaB. Activated NF-kappaB binds to sites on the SHP2 promoter, resulting in increased SHP2 expression, SHP2 activity, and, ultimately, SMC proliferation. It suggests that AGE stimulation induces SMC proliferation via SHP2, underscoring the importance of control of AGE for suppressing macroangiopathy in diabetes mellitus.

Monitoring phosphatase reactions of multiple phosphorylated substrates by reversed-phase HPLC

In an approach to gain insight into the sequence-dependent dephosphorylation of multiple phosphotyrosyl-containing peptides by the phosphatases SHP-1 and PTP1B, we applied a chromatographic technique for the analysis of the dephosphorylation products. Mono-, bi- and triphosphorylated reference peptides corresponding to positions 1999-2014 in the activation loop of the receptor tyrosine kinase Ros were first analyzed by reversed-phase HPLC and MALDI-TOF/TOF mass spectrometry. Then, the respective products from enzymatic treatment were investigated by HPLC and compared to the standard peptides. The results obtained in this study emphasize the advantage of monitoring phosphatase reactions for mono- and biphosphorylated peptides using the described procedure rather than spectrophotometric and fluorimetric methods that do not allow for a clear identification of the products formed.

Leukocyte antigen-related deficiency enhances insulin-like growth factor-1 signaling in vascular smooth muscle cells and promotes neointima formation in response to vascular injury

Increase in the expression of leukocyte antigen-related (LAR) protein causes insulin resistance, an important contributor to atherosclerosis. However, the function of LAR in atherosclerosis is not known. To address whether LAR is important in the response of vascular cells to atherogenic stimuli, we investigated cell proliferation, migration, and insulin-like growth factor-1 receptor (IGF-1R) signaling in wild-type and LAR(-/-) mouse vascular smooth muscle cells (VSMC) treated with IGF-1. Absence of LAR significantly enhanced proliferation and migration of VSMC compared with wild-type cells after IGF-1 treatment. U0126 and LY249002, specific inhibitors of MAPK/ERK kinase (MEK) and phosphoinositide 3-kinase, respectively, inhibited IGF-1-induced DNA synthesis and migration in both wild-type and LAR(-/-) VSMC. IGF-1 markedly enhanced IGF-1R phosphorylation in both wild-type and LAR(-/-) VSMC, but the phosphorylation was 90% higher in knock-out cells compared with wild-type cells. Absence of LAR enhanced phosphorylation of insulin receptor substrate-1 and insulin receptor substrate-1-associated phosphoinositide 3-kinase activity in VSMC treated with IGF-1. IGF-1-induced phosphorylation of ERK1/2 also increased significantly in LAR(-/-) VSMC compared with wild-type cells. Furthermore, LAR directly binds to IGF-1R in glutathione S-transferase-LAR pull-down and IGF-1R immunoprecipitation experiments and recombinant LAR dephosphorylates IGF-1R in vitro. Neointima formation in response to arterial injury and IGF-1R phosphorylation in neointima increased significantly in LAR(-/-) mice compared with wild-type mice. A significant decrease in body weight, fasting insulin, and IGF-1 levels were observed in LAR(-/-) mice compared with wild-type mice. Together, these data indicate that LAR regulates IGF-1R signaling in VSMC and dysregulation of this phosphatase may lead to VSMC hyperplasia.

Discovery of novel inhibitor of human leukocyte common antigen-related phosphatase

Human leukocyte common antigen-related phosphatase (LAR) may play a role in type 2 diabetes and cancer, and in the development of the nervous system, and it may be an attractive target for the treatment of diabetes and cancer. We identified eight hits from the random screening of LAR D1 with a high-throughput screening assay. Further validation of the eight hits showed that the meD insertion was associated with inhibition of LAR D1D2 and LAR D1Q. These data suggest that the inserted meD peptide influences the interaction of the enzyme and inhibitor, which is consistent with the kinetic catalysis constants of the substrate pNPP. Our data showed that Hit 1, the first published novel inhibitor of LAR, is a competitive inhibitor with a K(i) of 330 nM that displays obvious selectivity for LAR and mouse PTPsigma, but not for other protein tyrosine phosphatases.

Adipocyte-specific overexpression of FOXC2 prevents diet-induced increases in intramuscular fatty acyl CoA and insulin resistance

Insulin resistance plays a major role in the development of type 2 diabetes and may be causally associated with increased intracellular fat content. Transgenic mice with adipocyte-specific overexpression of FOXC2 (forkhead transcription factor) have been generated and shown to be protected against diet-induced obesity and glucose intolerance. To understand the underlying mechanism, we examined the effects of chronic high-fat feeding on tissue-specific insulin action and glucose metabolism in the FOXC2 transgenic (Tg) mice. Whole-body fat mass were significantly reduced in the FOXC2 Tg mice fed normal diet or high-fat diet compared with the wild-type mice. Diet-induced insulin resistance in skeletal muscle of the wild-type mice was associated with defects in insulin signaling and significant increases in intramuscular fatty acyl CoA levels. In contrast, FOXC2 Tg mice were completely protected from diet-induced insulin resistance and intramuscular accumulation of fatty acyl CoA. High-fat feeding also blunted insulin-mediated suppression of hepatic glucose production in the wild-type mice, whereas FOXC2 Tg mice were protected from diet-induced hepatic insulin resistance. These findings demonstrate an important role of adipocyte-expressed FOXC2 on whole-body glucose metabolism and further suggest FOXC2 as a novel therapeutic target for the treatment of insulin resistance and type 2 diabetes.

Coordinated regulation of insulin signaling by the protein tyrosine phosphatases PTP1B and TCPTP

The protein tyrosine phosphatase PTP1B is a negative regulator of insulin signaling and a therapeutic target for type 2 diabetes. Our previous studies have shown that the closely related tyrosine phosphatase TCPTP might also contribute to the regulation of insulin receptor (IR) signaling in vivo (S. Galic, M. Klingler-Hoffmann, M. T. Fodero-Tavoletti, M. A. Puryer, T. C. Meng, N. K. Tonks, and T. Tiganis, Mol. Cell. Biol. 23:2096-2108, 2003). Here we show that PTP1B and TCPTP function in a coordinated and temporally distinct manner to achieve an overall regulation of IR phosphorylation and signaling. Whereas insulin-induced phosphatidylinositol 3-kinase/Akt signaling was prolonged in both TCPTP-/- and PTP1B-/- immortalized mouse embryo fibroblasts (MEFs), mitogen-activated protein kinase ERK1/2 signaling was elevated only in PTP1B-null MEFs. By using phosphorylation-specific antibodies, we demonstrate that both IR beta-subunit Y1162/Y1163 and Y972 phosphorylation are elevated in PTP1B-/- MEFs, whereas Y972 phosphorylation was elevated and Y1162/Y1163 phosphorylation was sustained in TCPTP-/- MEFs, indicating that PTP1B and TCPTP differentially contribute to the regulation of IR phosphorylation and signaling. Consistent with this, suppression of TCPTP protein levels by RNA interference in PTP1B-/- MEFs resulted in no change in ERK1/2 signaling but caused prolonged Akt activation and Y1162/Y1163 phosphorylation. These results demonstrate that PTP1B and TCPTP are not redundant in insulin signaling and that they act to control both common as well as distinct insulin signaling pathways in the same cell.

Protein tyrosine phosphatases: their role in insulin action and potential as drug targets

Protein tyrosine phosphatases (PTPases) are the enzymes responsible for the selective dephosphorylation of tyrosine residues. PTPases function to regulate a wide array of biological responses mediated by growth factors and other stimuli by balancing the cellular level of phosphotyrosine in concert with their counterparts, protein tyrosine kinases. The important roles which PTPases play in regulating intracellular signalling and, ultimately, biological function along with the recent availability of information regarding their structural features has highlighted them as potential targets for pharmacological modulation. This is demonstrated by the increased level of activity directed towards the identification of novel small-molecule PTPase inhibitors. The rationale and potential utility of this drug discovery approach is discussed here, with particular emphasis on its application for the treatment of insulin resistance and Type 2 diabetes.

Transgenic overexpression of protein-tyrosine phosphatase 1B in muscle causes insulin resistance, but overexpression with leukocyte antigen-related phosphatase does not additively impair insulin action

Previous studies implicate protein-tyrosine phosphatase 1B (PTP1B) and leukocyte antigen-related phosphatase (LAR) as negative regulators of insulin signaling. The expression and/or activity of PTP1B and LAR are increased in muscle of insulin-resistant rodents and humans. Overexpression of LAR selectively in muscle of transgenic mice causes whole body insulin resistance. To determine whether overexpression of PTP1B also causes insulin resistance, we generated transgenic mice overexpressing human PTP1B selectively in muscle at levels similar to those observed in insulin-resistant humans. Insulin-stimulated insulin receptor (IR) tyrosyl phosphorylation and phosphatidylinositol 3'-kinase activity were impaired by 35% and 40-60% in muscle of PTP1B-overexpressing mice compared with controls. Insulin stimulation of protein kinase C (PKC)lambda/zeta activity, which is required for glucose transport, was impaired in muscle of PTP1B-overexpressing mice compared with controls, showing that PTP1B overexpression impairs activation of these PKC isoforms. Furthermore, hyperinsulinemic-euglycemic clamp studies revealed that whole body glucose disposal and muscle glucose uptake were decreased by 40-50% in PTP1B-overexpressing mice. Overexpression of PTP1B or LAR alone in muscle caused similar impairments in insulin action; however, compound overexpression achieved by crossing PTP1B- and LAR-overexpressing mice was not additive. Antibodies against specific IR phosphotyrosines indicated overlapping sites of action of PTP1B and LAR. Thus, overexpression of PTP1B in vivo impairs insulin sensitivity, suggesting that overexpression of PTP1B in muscle of obese humans and rodents may contribute to their insulin resistance. Lack of additive impairment of insulin signaling by PTP1B and LAR suggests that these PTPs have overlapping actions in causing insulin resistance in vivo.

Protein-tyrosine phosphatase activity in human adipocytes is strongly correlated with insulin-stimulated glucose uptake and is a target of insulin-induced oxidative inhibition

Protein-tyrosine phosphatases (PTPases), in particular PTP1B, have been shown to modulate insulin signal transduction in liver and skeletal muscle in animal models; however, their role in human adipose tissue remains unclear. The uptake of (14)C-D-glucose in response to 10 or 100 nmol/L insulin was measured in isolated subcutaneous adipocytes from subjects with a mean age of 44 years (range, 26 to 58) and mean body mass index (BMI) of 35.6 (range, 29.7 to 45.5). The endogenous activity of total PTPases and specifically of PTP1B in immunoprecipitates was measured in cell lysates under an inert atmosphere with and without added reducing agents. Using nonlinear regression analysis, higher BMI was significantly correlated with lower adipocyte glucose uptake (r = 0.73, P =.01) and with increased endogenous total PTPase activity (r = 0.64, P =.04). Correlation with waist circumference gave similar results. The endogenous total PTPase activity also strongly correlated with insulin-stimulated glucose uptake (R =.89, P <.0001); however, the activity of PTP1B was unrelated to the level of glucose uptake. Consistent with the insulin-stimulated oxidative inhibition of thiol-dependent PTPases reported for 3T3-L1 adipocytes and hepatoma cells, treatment of human adipocytes with 100 nmol/L insulin for 5 minutes lowered endogenous PTPase activity to 37% of control (P <.001), which was increased 25% by subsequent treatment with dithiothreitol in vitro. Cellular treatment with diphenyleneiodonium (DPI), an NADPH oxidase inhibitor that blocks the cellular generation of H(2)O(2) and reduces the insulin-induced reduction of cellular PTPase activity, also diminished insulin-stimulated glucose uptake by 82% (P =.001). These data suggest that total cellular PTPase activity, but not the activity of PTP1B, is higher in more obese subjects and is negatively associated with insulin-stimulated glucose transport. The insulin-stimulated oxidative inhibition of PTPases may also have an important permissive role in the transmission of the insulin signal to glucose transport in human adipocytes.

Overexpression of leucocyte common antigen (LAR) P-subunit in thyroid carcinomas

Protein tyrosine phosphatase (PTPase) dephosphorylation and protein tyrosine kinase (PTKs) phosphorylation of key signal transduction proteins may be regulated by extracellular signals, making PTPases important in the regulation of cell proliferation. Leucocyte common antigen (LAR), a receptor-like PTPase, consists of E-subunit, containing the cell adhesion molecule-like receptor region, and P-subunit specific for a short segment of the extracellular region, the transmembrane peptide, and two cytoplasmic PTPase domains. We produced a monoclonal antibody against the LAR P-subunit for immunohistochemical screening of LAR expression in normal and tumourous tissues. Gliomas and gastric, colorectal, lung, breast and prostate cancers showed weak and relatively infrequent expression. Intense and diffuse expression, however, was detected in 95% (227 out of 239) of thyroid carcinomas, but only 12% (22 out of 128) of adenomas and no cases of benign thyroid disease were immunopositive. In contrast to broad staining in carcinomas, LAR expression in thyroid adenomas was often found in small focal or locally invasive areas. Western blot analysis similarly detected LAR P-subunit protein in thyroid carcinomas, but not in normal tissues. We believe this to be the first demonstration of LAR overexpression in thyroid carcinoma and may help to elucidate the role of PTPases in the development of malignancy.

Proteasome inhibitors regulate tyrosine phosphorylation of IRS-1 and insulin signaling in adipocytes

Insulin rapidly stimulates the tyrosine kinase activity of its receptor, resulting in the phosphorylation of insulin receptor substrates (IRS), which in turn associates and activates PI 3-kinase, leading to an increase in glucose uptake. Phosphorylation of IRS proteins and activation of downstream kinases by insulin are transient and the mechanisms for the subsequent downregulation of their activity are largely unknown. We report here that the insulin-induced IRS-1 tyrosine phosphorylation and PI 3-kinase association to IRS-1 were strongly sustained by the proteasome inhibitors, MG132 and lactacystin. In contrast, no effect was detected on the insulin receptor and IRS-2 tyrosine phosphorylation. Interestingly, lactacystin also preserved PKB activation and insulin-induced glucose uptake. In contrast, calpeptin, a calpain inhibitor, was ineffective. Tyrosine phosphatase assays were also performed, showing that lactacystin was not functioning directly as a tyrosine phosphatase inhibitor "in vitro." In conclusion, proteasome inhibitors can regulate the tyrosine phosphorylation of IRS-1 and the downstream insulin signaling pathway, leading to glucose transport.

Role of SRC homology 2-containing tyrosine phosphatase 2 on proliferation of rat smooth muscle cells

OBJECTIVE

Src homology 2-containing phosphotyrosine phosphatase 2 (SHP2) is ubiquitously expressed and believed to function as part of a positive signaling pathway mediating growth factor-induced protein tyrosine phosphorylation. Proliferation of aortic vascular smooth muscle cells (SMCs) is an important contributor to atherosclerosis. We examined the effect of SHP2 expression on SMC proliferative activity.

METHODS AND RESULTS

SHP2 was abundant in cultured aortic SMCs, and SHP2 staining was markedly increased in the thickened aortic intima in rats with balloon-induced injury. We obtained several SMC clones by using geneticin screening. Endogenous SHP2 expression varied among individual clones. Significant positive relationships were observed between SHP2 expression and bromodeoxyuridine uptake in SMCs stimulated by FBS, platelet-derived growth factor, or insulin-like growth factor-1. In SMCs transiently transfected with SHP2, FBS stimulation significantly increased bromodeoxyuridine uptake beyond the uptake by control SMCs.

CONCLUSIONS

Increased SHP2 expression in SMCs may accelerate aortic atherosclerosis by increasing cell growth.

Use of an Anaerobic Chamber Environment for the Assay of Endogenous Cellular Protein-Tyrosine Phosphatase Activities

Protein-tyrosine phosphatases (PTPases) have a catalytic cysteine residue whose reduced state is integral to the reaction mechanism. Since exposure to air can artifactually oxidize this highly reactive thiol, PTPase assays have typically used potent reducing agents to reactivate the enzymes present; however, this approach does not allow for the measurement of the endogenous PTPase activity directly isolated from the in vivo cellular environment. Here we provide a method for using an anaerobic chamber to preserve the activity of the total PTPase complement in a tissue lysate or of an immunoprecipitated PTPase homolog to characterize their endogenous activation state. Comparison with a sample treated with biochemical reducing agents allows the determination of the activatable (reducible) fraction of the endogenous PTPase pool.

Hydrogen peroxide generated during cellular insulin stimulation is integral to activation of the distal insulin signaling cascade in 3T3-L1 adipocytes

In a variety of cell types, insulin stimulation elicits the rapid production of H(2)O(2), which causes the oxidative inhibition of protein-tyrosine phosphatases and enhances the tyrosine phosphorylation of proteins in the early insulin action cascade (Mahadev, K., Zilbering, A., Zhu, L., and Goldstein, B. J. (2001) J. Biol. Chem. 276, 21938-21942). In the present work, we explored the potential role of insulin-induced H(2)O(2) generation on downstream insulin signaling using diphenyleneiodonium (DPI), an inhibitor of cellular NADPH oxidase that blocks insulin-stimulated cellular H(2)O(2) production. DPI completely inhibited the activation of phosphatidylinositol (PI) 3'-kinase activity by insulin and reduced the insulin-induced activation of the serine kinase Akt by up to 49%; these activities were restored when H(2)O(2) was added back to cells that had been pretreated with DPI. Interestingly, the H(2)O(2)-induced activation of Akt was entirely mediated by upstream stimulation of PI 3'-kinase activity, since treatment of 3T3-L1 adipocytes with the PI 3'-kinase inhibitors wortmannin or LY294002 completely blocked the subsequent activation of Akt by exogenous H(2)O(2). Preventing oxidant generation with DPI also blocked insulin-stimulated glucose uptake and GLUT4 translocation to the plasma membrane, providing further evidence for an oxidant signal in the regulation of the distal insulin-signaling cascade. Finally, in contrast to the cellular mechanism of H(2)O(2) generation by other growth factors, such as platelet-derived growth factor, we also found that insulin-stimulated cellular production of H(2)O(2) may occur through a unique pathway, independent of cellular PI 3'-kinase activity. Overall, these data provide insight into the physiological role of insulin-dependent H(2)O(2) generation, which is not only involved in the regulation of tyrosine phosphorylation events in the early insulin signaling cascade but also has important effects on the regulation of downstream insulin signaling, involving the activation of PI 3'-kinase, Akt, and ultimately cellular glucose transport in response to insulin.

Insulin-stimulated hydrogen peroxide reversibly inhibits protein-tyrosine phosphatase 1b in vivo and enhances the early insulin action cascade

The insulin signaling pathway is activated by tyrosine phosphorylation of the insulin receptor and key post-receptor substrate proteins and balanced by the action of specific protein-tyrosine phosphatases (PTPases). PTPase activity, in turn, is highly regulated in vivo by oxidation/reduction reactions involving the cysteine thiol moiety required for catalysis. Here we show that insulin stimulation generates a burst of intracellular H(2)O(2) in insulin-sensitive hepatoma and adipose cells that is associated with reversible oxidative inhibition of up to 62% of overall cellular PTPase activity, as measured by a novel method using strictly anaerobic conditions. The specific activity of immunoprecipitated PTP1B, a PTPase homolog implicated in the regulation of insulin signaling, was also strongly inhibited by up to 88% following insulin stimulation. Catalase pretreatment abolished the insulin-stimulated production of H(2)O(2) as well as the inhibition of cellular PTPases, including PTP1B, and was associated with reduced insulin-stimulated tyrosine phosphorylation of its receptor and high M(r) insulin receptor substrate (IRS) proteins. These data provide compelling new evidence for a redox signal that enhances the early insulin-stimulated cascade of tyrosine phosphorylation by oxidative inactivation of PTP1B and possibly other tyrosine phosphatases.

Overexpression of the LAR (leukocyte antigen-related) protein-tyrosine phosphatase in muscle causes insulin resistance

Previous reports indicate that the expression and/or activity of the protein-tyrosine phosphatase (PTP) LAR are increased in insulin-responsive tissues of obese, insulin-resistant humans and rodents, but it is not known whether these alterations contribute to the pathogenesis of insulin resistance. To address this question, we generated transgenic mice that overexpress human LAR, specifically in muscle, to levels comparable to those reported in insulin-resistant humans. In LAR-transgenic mice, fasting plasma insulin was increased 2.5-fold compared with wild-type controls, whereas fasting glucose was normal. Whole-body glucose disposal and glucose uptake into muscle in vivo were reduced by 39-50%. Insulin injection resulted in normal tyrosyl phosphorylation of the insulin receptor and insulin receptor substrate 1 (IRS-1) in muscle of transgenic mice. However, phosphorylation of IRS-2 was reduced by 62%, PI3' kinase activity associated with phosphotyrosine, IRS-1, or IRS-2 was reduced by 34-57%, and association of p85alpha with both IRS proteins was reduced by 39-52%. Thus, overexpression of LAR in muscle causes whole-body insulin resistance, most likely due to dephosphorylation of specific regulatory phosphotyrosines on IRS proteins. Our data suggest that increased expression and/or activity of LAR or related PTPs in insulin target tissues of obese humans may contribute to the pathogenesis of insulin resistance.

Combinatorial control of the specificity of protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs), the enzymes that dephosphorylate tyrosyl phosphoproteins, were initially believed to be few in number and serve a 'housekeeping' role in signal transduction. Recent work indicates that this is totally incorrect. Instead, PTPs comprise a large superfamily whose members play critical roles in a wide variety of cellular processes. Moreover, PTPs exhibit exquisite substrate specificity in vivo. Recent evidence has led us to propose that members of the PTP family achieve selectivity through different combinations of specific targeting strategies and intrinsic catalytic domain specificity.

Distinct functions of the two protein tyrosine phosphatase domains of LAR (leukocyte common antigen-related) on tyrosine dephosphorylation of insulin receptor

Most receptor-like, transmembrane protein tyrosine phosphatases (PTPases), such as CD45 and the leukocyte common antigen-related (LAR) molecule, have two tandemly repeated PTPase domains in the cytoplasmic segment. The role of each PTPase domain in mediating PTPase activity remains unclear; however, it has been proposed that PTPase activity is associated with only the first of the two domains, PTPase domain 1, and the membrane-distal PTPase domain 2, which has no catalytic activity, would regulate substrate specificity. In this paper, we examine the function of each PTPase domain of LAR in vivo using a potential physiological substrate, namely insulin receptor, and LAR mutant proteins in which the conserved cysteine residue was changed to a serine residue in the active site of either or both PTPase domains. LAR associated with and preferentially dephosphorylated the insulin receptor that was tyrosine phosphorylated by insulin stimulation. Its association was mediated by PTPase domain 2, because the mutation of Cys-1813 to Ser in domain 2 resulted in weakening of the association. The Cys-1522 to Ser mutant protein, which is defective in the LAR PTPase domain 1 catalytic site, was tightly associated with tyrosine-phosphorylated insulin receptor, but failed to dephosphorylate it, indicating that LAR PTPase domain 1 is critical for dephosphorylation of tyrosine-phosphorylated insulin receptor. This hypothesis was further confirmed by using LAR mutants in which either PTPase domain 1 or domain 2 was deleted. Moreover, the association of the extracellular domains of both LAR and insulin receptor was supported by using the LAR mutant protein without the two PTPase domains. LAR was phosphorylated by insulin receptor tyrosine kinase and autodephosphorylated by the catalytic activity of the PTPase domain 1. These results indicate that each domain of LAR plays distinct functional roles through phosphorylation and dephosphorylation in vivo.

Decreased in situ insulin receptor dephosphorylation in hyperglycemia-induced insulin resistance in rat adipocytes

The regulation of insulin receptor (IR) tyrosine (tyr) phosphorylation is a key step in the control of insulin signaling. Augmented IR tyr dephosphorylation by protein tyrosine phosphatases (PTPs) may contribute to insulin resistance. To investigate this possibility in hyperglycemia-induced insulin resistance, primary cultured rat adipocytes were rendered insulin-resistant by chronic exposure (18 h) to 15 mmo/l glucose combined with 10(-7) mol/l insulin. Insulin-resistant adipocytes showed a decrease in insulin sensitivity and a maximum response of 2-deoxyglucose uptake, which was associated with a decrease in maximum insulin-stimulated IR tyr phosphorylation in situ. To assess tyr dephosphorylation, IRs of insulin-stimulated permeabilized adipocytes were labeled with [gamma-32P]ATP and chased for 2 min with unlabeled ATP in the presence of EDTA. In a nonradioactive protocol, insulin-stimulated adipocytes were permeabilized and exposed to EDTA and erbstatin for 2 min, and IRs were immunoblotted with anti-phosphotyrosine (pY) antibodies. Both methods showed a similar diminished extent of IR tyr dephosphorylation in resistant cells. Immunoblotting of four candidate IR-PTPs demonstrated no change in PTP1B or the SH2 domain containing phosphatase-2 (SHP-2), whereas a significant decrease in leukocyte antigen-related phosphatase (LAR) (51 +/- 3% of control) and an increase in PTP-alpha (165 +/- 16%) were found. Activity of immunoprecipitated PTPs toward a triple tyr phosphorylated IR peptide revealed a correlation with protein content for PTP1B, SHP-2, and LAR but a decrease in apparent specific activity of PTP-alpha. The data indicate that decreased IR tyr phosphorylation in hyperglycemia-induced insulin resistance is not due to enhanced dephosphorylation. The diminished IR tyr dephosphorylation observed in this model is associated with decreased LAR protein content and activity.

Increased energy expenditure, decreased adiposity, and tissue-specific insulin sensitivity in protein-tyrosine phosphatase 1B-deficient mice

Protein-tyrosine phosphatase 1B (PTP-1B) is a major protein-tyrosine phosphatase that has been implicated in the regulation of insulin action, as well as in other signal transduction pathways. To investigate the role of PTP-1B in vivo, we generated homozygotic PTP-1B-null mice by targeted gene disruption. PTP-1B-deficient mice have remarkably low adiposity and are protected from diet-induced obesity. Decreased adiposity is due to a marked reduction in fat cell mass without a decrease in adipocyte number. Leanness in PTP-1B-deficient mice is accompanied by increased basal metabolic rate and total energy expenditure, without marked alteration of uncoupling protein mRNA expression. In addition, insulin-stimulated whole-body glucose disposal is enhanced significantly in PTP-1B-deficient animals, as shown by hyperinsulinemic-euglycemic clamp studies. Remarkably, increased insulin sensitivity in PTP-1B-deficient mice is tissue specific, as insulin-stimulated glucose uptake is elevated in skeletal muscle, whereas adipose tissue is unaffected. Our results identify PTP-1B as a major regulator of energy balance, insulin sensitivity, and body fat stores in vivo.

Bradykinin enhances insulin receptor tyrosine kinase in 32D cells reconstituted with bradykinin and insulin signaling pathways

We have previously shown that bradykinin potentiated insulin-induced glucose uptake through GLUT4 translocation in canine adipocytes and skeletal muscles. The aim of this study was to determine the molecular mechanism of bradykinin enhancement of the insulin signal. For this purpose, 32D cells, which express a limited number of insulin receptors and lack endogenous bradykinin B2 receptor (BK2R) or insulin receptor substrate (IRS)-1 were transfected with BK2R cDNA and/or insulin receptor cDNA and/or IRS-1 cDNA, and analyzed. In 32D cells that expressed BK2R and insulin receptor (32D-BKR/IR), bradykinin alone had no effect on the phosphorylation of the insulin receptor, but it enhanced insulin-stimulated tyrosine phosphorylation of the insulin receptor. In 32D cells that expressed BK2R, insulin receptor and IRS-1 (32D-BKR/IR/IRS1), bradykinin also enhanced insulin-stimulated tyrosine phosphorylation of the insulin receptor and IRS-1. An increase in insulin-stimulated phosphorylation of IRS-1 by treatment with bradykinin in 32D-BKR/IR/IRS1 cell was associated with increased binding of 85 kD subunit of phosphatidylinositol 3 (PI 3)-kinase and increased IRS-1 associated PI 3-kinase activity. These effects of bradykinin were not observed in 32D cells which lack the expression of BK2R (32D-IR/IRS1) or insulin receptor (32D-BKR/IRS1). Furthermore, tyrosine phosphatase activity against insulin receptor beta-subunit in plasma membrane fraction of 32D-BKR/IR cells was significantly reduced by bradykinin, suggesting that the effect of bradykinin was in part mediated by inhibition of protein tyrosine phosphatase(s). Our results clearly demonstrated that bradykinin enhanced insulin-stimulated tyrosine kinase activity of the insulin receptor and downstream insulin signal cascade through the BK2R mediated signal pathway.

Phosphatase activity in rat adipocytes: effects of insulin and insulin resistance

Insulin regulates the activity of both protein kinases and phosphatases. Little is known concerning the subcellular effects of insulin on phosphatase activity and how it is affected by insulin resistance. The purpose of this study was to determine insulin-stimulated subcellular changes in phosphatase activity and how they are affected by insulin resistance. We used an in vitro fatty acid (palmitate) induced insulin resistance model, differential centrifugation to fractionate rat adipocytes, and a malachite green phosphatase assay using peptide substrates to measure enzyme activity. Overall, insulin alone had no effect on adipocyte tyrosine phosphatase activity; however, subcellularly, insulin increased plasma membrane adipocyte tyrosine phosphatase activity 78 +/- 26% (n = 4, P < 0.007), and decreased high-density microsome adipocyte tyrosine phosphatase activity 42 +/- 13% (n = 4, P < 0.005). Although insulin resistance induced specific changes in basal tyrosine phosphatase activity, insulin-stimulated changes were not significantly altered by insulin resistance. Insulin-stimulated overall serine/threonine phosphatase activity by 16 +/- 5% (n = 4, P < 0.005), which was blocked in insulin resistance. Subcellularly, insulin increased plasma membrane and crude nuclear fraction serine/threonine phosphatase activities by 59 +/- 19% (n = 4, P < 0. 005) and 21 +/- 7% (n = 4, P < 0.007), respectively. This increase in plasma membrane fractions was inhibited 23 +/- 7% (n = 4, P < 0. 05) by palmitate. Furthermore, insulin increased cytosolic protein phosphatase-1 (PP-1) activity 160 +/- 50% (n = 3, P < 0.015), and palmitate did not significantly reduce this activity. However, palmitate did reduce insulin-treated low-density microsome protein phosphatase-1 activity by 28 +/- 6% (n = 3, P < 0.04). Insulin completely inhibited protein phosphatase-2A activity in the cytosol and increased crude nuclear fraction protein phosphatase-2A activity 70 +/- 29% (n = 3, P < 0.038). Thus, the major effects of insulin on phosphatase activity in adipocytes are to increase plasma membrane tyrosine and serine/threonine phosphatase, crude nuclear fraction protein phosphatase-2A, and cytosolic protein phosphatase-1 activities, while inhibiting cytosolic protein phosphatase-2A. Insulin resistance was characterized by reduced insulin-stimulated serine/threonine phosphatase activity in the plasma membrane and low-density microsomes. Specific changes in phosphatase activity may be related to the development of insulin resistance.

Dynamics of protein-tyrosine phosphatases in rat adipocytes

Protein-tyrosine phosphatases (PTPases) play a key role in maintaining the steady-state tyrosine phosphorylation of the insulin receptor (IR) and its substrate proteins such as insulin receptor substrate 1 (IRS-1). However, the PTPase(s) that inactivate IR and IRS-1 under physiological conditions remain unidentified. Here, we analyze the subcellular distribution in rat adipocytes of several PTPases thought to be involved in the counterregulation of insulin signaling. We found that the transmembrane enzymes, protein-tyrosine phosphatase (PTP)-alpha and leukocyte common antigen-related (LAR), were detected predominantly in the plasma membrane and to a lesser extent in the heavy microsomes, a distribution similar to that of insulin receptor. PTP-1B and IRS-1 were present in light microsomes and cytosol, whereas SHPTP2/Syp was exclusively cytosolic. Insulin induced a redistribution of PTP-alpha from the plasma membrane to the heavy microsomes in a parallel fashion with the receptor. The distribution of PTP-1B in the light microsomes from resting adipocytes was similar to that of IRS-1 as determined by sucrose velocity gradient fractionation. Analysis of the catalytic activity of partially purified rat adipocyte PTP-alpha and LAR and recombinant PTP-1B showed that all three PTPases dephosphorylate IR. When a mix of IR/IRS-1 was used as a substrate, PTP-1B was particularly effective in dephosphorylating IRS-1. Considering that IR and IRS-1 can be dephosphorylated in internal membrane compartments from rat adipocytes (Kublaoui, B., Lee, J., and Pilch, P.F. (1995) J. Biol. Chem. 270, 59-65) and that PTP-alpha and PTP-1B are the respective PTPases in these fractions, we conclude that these PTPases are responsible for the counterregulation of insulin signaling there, whereas both LAR and PTP-alpha may act upon cell surface insulin receptors.

Tyrosine dephosphorylation and deactivation of insulin receptor substrate-1 by protein-tyrosine phosphatase 1B. Possible facilitation by the formation of a ternary complex with the Grb2 adaptor protein

Regulation of the steady-state tyrosine phosphorylation of the insulin receptor and its postreceptor substrates are essential determinants of insulin signal transduction. However, little is known regarding the molecular interactions that influence the balance of these processes, especially the phosphorylation state of postinsulin receptor substrates, such as insulin receptor substrate-1 (IRS-1). The specific activity of four candidate protein-tyrosine phosphatases (protein-tyrosine phosphatase 1B (PTP1B), SH2 domain-containing PTPase-2 (SHP-2), leukocyte common antigen-related (LAR), and leukocyte antigen-related phosphatase) (LRP) toward IRS-1 dephosphorylation was studied using recombinant proteins in vitro. PTP1B exhibited the highest specific activity (percentage dephosphorylated per microg per min), and the enzyme activities varied over a range of 5.5 x 10(3). When evaluated as a ratio of activity versus IRS-1 to that versus p-nitrophenyl phosphate, PTP1B remained significantly more active by 3.1-293-fold, respectively. Overlay blots with recombinant Src homology 2 domains of IRS-1 adaptor proteins showed that the loss of IRS-1 binding of Crk, GRB2, SHP-2, and the p85 subunit of phosphatidylinositol 3'-kinase paralleled the rate of overall IRS-1 dephosphorylation. Further studies revealed that the adaptor protein GRB2 strongly promoted the formation of a stable protein complex between tyrosine-phosphorylated IRS-1 and catalytically inactive PTP1B, increasing their co-immunoprecipitation from an equimolar solution by 13.5 +/- 3.3-fold (n = 7; p < 0.01). Inclusion of GRB2 in a reaction mixture of IRS-1 and active PTP1B also increased the overall rate of IRS-1 tyrosine dephosphorylation by 2.7-3.9-fold (p < 0.01). These results provide new insight into novel molecular interactions involving PTP1B and GRB2 that may influence the steady-state capacity of IRS-1 to function as a phosphotyrosine scaffold and possibly affect the balance of postreceptor insulin signaling.

Increased activity and expression of MAP kinase in HCC model rats induced by 3'-methyl-4-dimethylamino-azobenzene

BACKGROUND/AIMS

The ras-mitogen-activated protein kinase (MAPK) cascade plays an important role not only in the mitogenic signal transduction pathway but also in the development of cancer, and it is believed to be one of the important regulators in normal hepatocytes and hepatocellular carcinoma. The aim of this study was to determine the role of insulin receptor substrate-1 and the MAPK cascade in rats with hepatocellular carcinoma induced by 3'-methyl-4-dimethylamino-azobenzene (3'-MeDAB).

METHODS

Liver cancer was induced in rats by feeding 3'-MeDAB, and the changes in expression of IRS-1 and MAPK were analyzed in tumorous, non-tumorous and control liver.

RESULTS

Expression of insulin receptor substrate-1 (IRS-1) showed a 1.4-fold increase at protein level in the tumors (p<0.01), but the tyrosine phosphorylation of IRS-1 did not differ between the tumor and control liver. Expression of MAPK and its activity were elevated 4.5-7.5-fold (p<0.01) and 4.6-fold (p<0.01) in the tumor compared with control liver. In non-tumorous lesions from rats fed with 3'-MeDAB, expression of MAPK, but not IRS-1, increased significantly (p<0.01). Between tumorous and adjacent non-tumorous lesions, there was a significant difference in MAPK expression (p<0.05) and activities (p<0.05).

CONCLUSIONS

The increased expression of MAPK may play an important role in the progression or initiation of HCC in this rat model.

Dissociation of PTPase levels from their modulation of insulin receptor signal transduction

Protein tyrosine phosphatases have been implicated in the regulation of receptor tyrosine kinase signalling pathways, including that of the insulin receptor. Here, cell density-dependent changes in PTPase expression have been exploited to investigate the relationship between cellular PTPase levels and the insulin receptor signal transduction pathway. Increasing cell density (20%, 50%, and >90%) in the rat McA-RH7777 hepatoma cell line resulted in increased protein expression of the receptor-like PTPase LAR (14-fold), and the nonreceptor PTPases PTP1B (11-fold) and SHP2 (10-fold). Each of these PTPases has previously been implicated in regulating insulin receptor signal transduction. Despite these marked increases, maximum insulin receptor autophosphorylation as well as receptor expression actually increased 2-fold. MAP kinase also increased approximately 2-fold as a function of cell density and paralleled increases in expression levels. Neither sensitivity nor maximum responsiveness to insulin were decreased at increasing cell densities as assessed by activation of PI 3-kinase. Duration of response was also unimpaired. These results suggest that expression levels of relevant PTPases are not the primary determinant in their modulation of insulin receptor kinase activity. Restricted accessibility at the molecular level or involvement of accessory proteins may be more critical parameters.

Use of an antisense strategy to dissect the signaling role of protein-tyrosine phosphatase alpha

The protein-tyrosine phosphatase PTPalpha has been proposed to play an important role in controlling the dephosphorylation of a number of key signaling proteins and in regulating insulin signaling. To examine the potential cellular functions and physiological substrates of PTPalpha, a potent phosphorothioate oligonucleotide-based antisense strategy was developed that specifically depleted endogenous PTPalpha from 3T3-L1 adipocytes. The antisense probe, alphaAS1, achieved PTPalpha depletion levels normally of >/=85% and which varied up to levels where PTPalpha was not detected at all. Elimination of PTPalpha by 85% inhibited c-Src activity by 80%. Abolishing PTPalpha to levels undetected did not alter the tyrosine dephosphorylation of the insulin receptor or insulin receptor substrate proteins. Moreover, the ability of insulin to activate ERK2 or to stimulate DNA synthesis was not altered by alphaAS1. It is concluded that endogenous PTPalpha is a key regulator of c-Src activity in 3T3-L1 adipocytes and that PTPalpha is not required for the dephosphorylation of the insulin receptor or the insulin receptor substrate proteins or for the regulation of several downstream insulin signaling events in 3T3-L1 adipocytes. Finally, the development of the antisense probe, alphaAS1, provides an important molecular tool of general applicability for further dissecting the roles and precise targets of endogenous PTPalpha.

Protein tyrosine phosphatases as negative regulators of mitogenic signaling

The regulation of tyrosine phosphorylation represents a key mechanism governing cell proliferation. In fibroblasts, inputs from both growth factor and extracellular matrix receptors are required for cell division. Triggering such receptors induces a wave of tyrosine phosphorylation on key signaling molecules, culminating in the activation of cyclin-dependent kinases and cell cycle progression. In general, protein tyrosine kinases stimulate, while protein tyrosine phosphatases inhibit, such cell proliferation pathways. The role of protein tyrosine kinases in mitogenesis has been extensively studied, but the identity and targets of the protein tyrosine phosphatases that regulate cell growth are not well described. In this review, I will survey recent advances in the identification and regulation of protein tyrosine phosphatases that downregulate cell proliferation.

Marked impairment of protein tyrosine phosphatase 1B activity in adipose tissue of obese subjects with and without type 2 diabetes mellitus

Protein tyrosine phosphatases (PTPs) are required for the dephosphorylation of the insulin receptor (IR) and its initial cellular substrates, and it has recently been reported that PTP-1B may play a role in the pathogenesis of insulin resistance in obesity and type 2 diabetes mellitus (DM). We therefore determined the amount and activity of PTP-1B in abdominal adipose tissue obtained from lean nondiabetic subjects (lean control (LC)), obese nondiabetic subjects (obese control (OC)), and subjects with both type 2 DM (DM2) and obesity (obese diabetic (OD)). PTP-1B protein levels were 3-fold higher in OC than in LC (1444 +/- 195 U vs 500 +/- 146 U (mean +/- SEM), P < .015), while OD exhibited a 5.5-fold increase (2728 +/- 286 U, P < .01). PTP activity was assayed by measuring the dephosphorylating activity toward a phosphorus 32-labeled synthetic dodecapeptide. In contrast to the increased PTP-1B protein levels, PTP-1B activity per unit of PTP-1B protein was markedly reduced, by 71% and 88% in OC and OD, respectively. Non-PTP-1B tyrosine phosphatase activity was comparable in all three groups. Similar results were obtained when PTP-1B activity was measured against intact human IR. A significant correlation was found between body mass index (BMI) and PTP-1B level (r = 0.672, P < .02), whereas BMI and PTP-1B activity per unit of PTP-1B showed a strong inverse correlation (r = -0.801, P < .002). These data suggest that the insulin resistance of obesity and DM2 is characterized by the increased expression of a catalytically impaired PTP-1B in adipose tissue and that impaired PTP-1B activity may be pathogenic for insulin resistance in these conditions.

Inhibition of the transmembrane protein tyrosine phosphatase lar by 3S-peptide-I enhances insulin receptor phosphorylation in intact cells

3S-peptide-I, a tris-sulfotyrosyl dodecapeptide that corresponds to the major autophosphorylation domain within the insulin receptor beta-subunit, selectively enhances insulin signal transduction by specifically inhibiting dephosphorylation of the insulin receptor catalyzed by protein tyrosine phosphatases (PTPases). Because of the potential role of the transmembrane PTPase LAR in the regulation of insulin signaling, we assessed the effect of 3S-peptide-I on recombinant LAR PTPase activity and in McA-RH7777 rat hepatoma cells overexpressing full-length LAR protein (McA4B/LAR). 3S-peptide-I significantly reduced insulin receptor dephosphorylation by recombinant LAR (p < 0.001) while blocking dephosphorylation of the insulin receptor by approximately 72% in semi-permeabilized McA4B/LAR cells (p < 0.001). Increased LAR expression resulted in 40% reduction in ligand-mediated phosphorylation of the insulin receptor compared with null vector control (p < 0.001). However, treatment of intact McA4B/LAR cells with a fatty acid derivative of 3S-peptide-I (50 microM) led to an enhancement of insulin-stimulated receptor phosphorylation by 89% (p < 0.001). As a result, control and McA4B/LAR cells showed comparable steady-state levels of insulin receptor phosphorylation in the presence of insulin. These findings provide evidence that 3S-peptide-I may improve insulin responsiveness in intact cells by inhibiting LAR, an enzyme whose activity has been implicated in the pathogenesis of insulin resistance.

Development of vanadate sensitive human erythrocytes insulin receptor tyrosine phosphatase assay

The aim of this study was to investigate the effect of sodium orthovanadate on the alterations of human erythrocytes insulin receptor autophosphorylation. Human erythrocytes were incubated with insulin in a cell system and then lysed. The autophosphorylated insulin receptors were measured with the aid of a two-site immunofluorometric assay and using a monoclonal anti-insulin receptor antibody to label the insulin receptors and a monoclonal anti-phosphotyrosine antibody to assess tyrosine phosphorylation. When the erythrocytes were treated with insulin and then reincubated in insulin-free medium, vanadate completely inhibited insulin receptor dephosphorylation, although it had no effect on in vitro receptor autophosphorylation. Thus insulin receptor tyrosine phosphatase activity is postulated to be [% (autophosphorylated insulin receptors with vanadate - autophosphorylated insulin receptors without vanadate)/total insulin receptors] under overall steady conditions in a cell system. Using this assay, the insulin receptor tyrosine phosphatase activities of 25 control and 32 diabetic subjects were studied. There was no significant difference in insulin receptor tyrosine phosphatase activity between control subjects and diabetic subjects (0.173 +/- 0.062 vs 0.209 - +/- 0.057 autophosphorylated insulin receptors units/insulin receptors units). The assay used in this study requires only 0.6 ml of whole blood, and so should be a useful tool for detecting patients who are insulin-resistant due to abnormal insulin receptor tyrosine phosphatase activity.

Expression of protein-tyrosine phosphatases in the major insulin target tissues

Protein-tyrosine phosphatases (PTPs) are key regulators of the insulin receptor signal transduction pathway. We have performed a detailed analysis of PTP expression in the major human insulin target tissues or cells (liver, adipose tissue, skeletal muscle and endothelial cells). To obtain a representative picture, all tissues were analyzed by PCR using three different primer sets corresponding to conserved regions of known PTPs. A total of 24 different PTPs were identified. A multiprobe RNase protection assay was developed to obtain a semiquantitative measure of the expression levels of selected PTPs. Surprisingly, PTP-LAR, previously suggested to be a major regulator of the insulin receptor tyrosine kinase, was expressed in extremely low levels in skeletal muscle, whereas the related receptor-type PTP-sigma and PTP-alpha were expressed in relatively high levels in all four tissues. The low levels of LAR PTP mRNA in skeletal muscle were further confirmed by Northern blot analysis.