Insulin receptor signaling is augmented by antisense inhibition of the protein tyrosine phosphatase LAR

Exposure to per- and polyfluoroalkyl substances and high-throughput proteomics in Hispanic youth

BACKGROUND

Strong epidemiological evidence shows positive associations between exposure to per- and polyfluoroalkyl substances (PFAS) and adverse cardiometabolic outcomes (e.g., diabetes, hypertension, and dyslipidemia). However, the underlying cardiometabolic-relevant biological activities of PFAS in humans remain largely unclear.

AIM

We evaluated the associations of PFAS exposure with high-throughput proteomics in Hispanic youth.

MATERIAL AND METHODS

We included 312 overweight/obese adolescents from the Study of Latino Adolescents at Risk (SOLAR) between 2001 and 2012, along with 137 young adults from the Metabolic and Asthma Incidence Research (Meta-AIR) between 2014 and 2018. Plasma PFAS (i.e., PFOS, PFOA, PFHxS, PFHpS, PFNA) were quantified using liquid-chromatography high-resolution mass spectrometry. Plasma proteins (n = 334) were measured utilizing the proximity extension assay using an Olink Explore Cardiometabolic Panel I. We conducted linear regression with covariate adjustment to identify PFAS-associated proteins. Ingenuity Pathway Analysis, protein-protein interaction network analysis, and protein annotation were used to investigate alterations in biological functions and protein clusters.

RESULTS

Results after adjusting for multiple comparisons showed 13 significant PFAS-associated proteins in SOLAR and six in Meta-AIR, sharing similar functions in inflammation, immunity, and oxidative stress. In SOLAR, PFNA demonstrated significant positive associations with the largest number of proteins, including ACP5, CLEC1A, HMOX1, LRP11, MCAM, SPARCL1, and SSC5D. After considering the mixture effect of PFAS, only SSC5D remained significant. In Meta-AIR, PFAS mixtures showed positive associations with GDF15 and IL6. Exploratory analysis showed similar findings. Specifically, pathway analysis in SOLAR showed PFOA- and PFNA-associated activation of immune-related pathways, and PFNA-associated activation of inflammatory response. In Meta-AIR, PFHxS-associated activation of dendric cell maturation was found. Moreover, PFAS was associated with common protein clusters of immunoregulatory interactions and JAK-STAT signaling in both cohorts.

CONCLUSION

PFAS was associated with broad alterations of the proteomic profiles linked to pro-inflammation and immunoregulation. The biological functions of these proteins provide insight into potential molecular mechanisms of PFAS toxicity.

Vanadium Complexes with Thioanilide Derivatives of Amino Acids: Inhibition of Human Phosphatases and Specificity in Various Cell Models of Metabolic Disturbances

In the text, the synthesis and characteristics of the novel ONS-type vanadium (V) complexes with thioanilide derivatives of amino acids are described. They showed the inhibition of human protein tyrosine phosphatases (PTP1B, LAR, SHP1, and SHP2) in the submicromolar range, as well as the inhibition of non-tyrosine phosphatases (CDC25A and PPA2) similar to bis(maltolato)oxidovanadium(IV) (BMOV). The ONS complexes increased [14C]-deoxy-D-glucose transport into C2C12 myocytes, and one of them, VC070, also enhanced this transport in 3T3-L1 adipocytes. These complexes inhibited gluconeogenesis in hepatocytes HepG2, but none of them decreased lipid accumulation in the non-alcoholic fatty liver disease model using the same cells. Compared to the tested ONO-type vanadium complexes with 5-bromosalicylaldehyde and substituted benzhydrazides as Schiff base ligand components, the ONS complexes revealed stronger inhibition of protein tyrosine phosphatases, but the ONO complexes showed greater activity in the cell models in general. Moreover, the majority of the active complexes from both groups showed better effects than VOSO4 and BMOV. Complexes from both groups activated AKT and ERK signaling pathways in hepatocytes to a comparable extent. One of the ONO complexes, VC068, showed activity in all of the above models, including also glucose utilizatiand ONO Complexes are Inhibitors ofon in the myocytes and glucose transport in insulin-resistant hepatocytes. The discussion section explicates the results within the wider scope of the knowledge about vanadium complexes.

Effects of gestational exposure to bisphenol A on the hepatic transcriptome and lipidome of rat dams: Intergenerational comparison of effects in the offspring

Our previous studies demonstrated that prenatal bisphenol A (BPA) exposure affected the hepatic transcriptome and lipidome in rat offspring in a sex- and age-dependent manner. In this study, we investigated the effects of gestational exposure to BPA on the rat dams, after weaning period, and compared them with those of their offspring. Our results showed alterations in hepatic transcriptome related to insulin signaling, circadian rhythm, and infectious disease pathways in BPA-treated dams even 4 weeks after the exposure, whereas slight modifications on the lipid profile were found. Alterations in lipid and transcriptome profiles were more prominent in the prenatally BPA-exposed offspring at postnatal day (PND) 1 and 21 than those in the dams, suggesting that in utero exposure to BPA is more serious than exposure in the adulthood. Cryptochrome-1 (Cry1) and peroxisome proliferator-activated receptor delta (Ppard) were commonly altered in both dams and offspring. Nevertheless, the results of DIABLO (Data Integration Analysis for Biomarker discovery using Latent cOmponents), showed that multi-omics data successfully distinguished the exposed dams from the corresponding controls and their offspring with a high level of accuracy. The accuracy rates in BPA50 models (including control and 50 μg BPA/kg bw/day exposed groups) were smaller than those in BPA5000 models (control and 5000 μg BPA/kg bw/day exposed groups), suggesting dose-dependent severity in BPA effects. Palmitic acid and genes related to circadian rhythm, insulin responses, and lipid metabolism (e.g., 1-acylglycerol-3-phosphate O-acyltransferase 2 (Agpat2), B-cell CLL/lymphoma 10 (Bcl10), Cry1, Harvey rat sarcoma virus oncogene (Hras), and NLR family member X1 (Nlrx1)) were identified through DIABLO models as novel biomarkers of effects of BPA across two generations.

All Good Things Must End: Termination of Receptor Tyrosine Kinase Signal

Receptor tyrosine kinases (RTKs) are membrane receptors that regulate many fundamental cellular processes. A tight regulation of RTK signaling is fundamental for development and survival, and an altered signaling by RTKs can cause cancer. RTKs are localized at the plasma membrane (PM) and the major regulatory mechanism of signaling of RTKs is their endocytosis and degradation. In fact, RTKs at the cell surface bind ligands with their extracellular domain, become active, and are rapidly internalized where the temporal extent of signaling, attenuation, and downregulation are modulated. However, other mechanisms of signal attenuation and termination are known. Indeed, inhibition of RTKs’ activity may occur through the modulation of the phosphorylation state of RTKs and the interaction with specific proteins, whereas antagonist ligands can inhibit the biological responses mediated by the receptor. Another mechanism concerns the expression of endogenous inactive receptor variants that are deficient in RTK activity and take part to inactive heterodimers or hetero-oligomers. The downregulation of RTK signals is fundamental for several cellular functions and the homeostasis of the cell. Here, we will review the mechanisms of signal attenuation and termination of RTKs, focusing on FGFRs.

Role of Receptor Protein Tyrosine Phosphatases (RPTPs) in Insulin Signaling and Secretion

Changes in lifestyle in developed countries have triggered the prevalence of obesity and type 2 diabetes mellitus (T2DM) in the latest years. Consequently, these metabolic diseases associated to insulin resistance, and the morbidity associated with them, accounts for enormous costs for the health systems. The best way to face this problem is to identify potential therapeutic targets and/or early biomarkers to help in the treatment and in the early detection. In the insulin receptor signaling cascade, the activities of protein tyrosine kinases and phosphatases are coordinated, thus, protein tyrosine kinases amplify the insulin signaling response, whereas phosphatases are required for the regulation of the rate and duration of that response. The focus of this review is to summarize the impact of transmembrane receptor protein tyrosine phosphatase (RPTPs) in the insulin signaling cascade and secretion, and their implication in metabolic diseases such as obesity and T2DM.

Protein tyrosine phosphatases (PTPs) in diabetes: causes and therapeutic opportunities

Protein tyrosine phosphatases (PTPs) have an emerging paradigm for the development of antidiabetic drugs. Herein, we provide a comprehensive overview of the relevance of PTPs to type 2 diabetes (T2D) and the therapeutic opportunities thereof, while critically evaluating the potential challenges for PTP inhibitors to be next generation antidiabetics. This review briefly discusses the structure and function of PTPs. An account of importance and relevance of PTPs in various human diseases is presented with special attention to diabetes. The PTPs relevant to T2D have been targeted by small molecule inhibitors such as natural products and synthetic compounds as well as antisense nucleic acids. This review will give better understanding of the important concepts helpful in outlining the strategies for the development of new therapeutic agents with promising antidiabetic activities.

Lar maintains the homeostasis of the hematopoietic organ in Drosophila by regulating insulin signaling in the niche

Stem cell compartments in metazoa get regulated by systemic factors as well as local stem cell niche-derived factors. However, the mechanisms by which systemic signals integrate with local factors in maintaining tissue homeostasis remain unclear. Employing the Drosophila lymph gland, which harbors differentiated blood cells, and stem-like progenitor cells and their niche, we demonstrate how a systemic signal interacts and harmonizes with local factor/s to achieve cell type-specific tissue homeostasis. Our genetic analyses uncovered a novel function of Lar, a receptor protein tyrosine phosphatase. Niche-specific loss of Lar leads to upregulated insulin signaling, causing increased niche cell proliferation and ectopic progenitor differentiation. Insulin signaling assayed by PI3K activation is downregulated after the second instar larval stage, a time point that coincides with the appearance of Lar in the hematopoietic niche. We further demonstrate that Lar physically associates with InR and serves as a negative regulator for insulin signaling in the Drosophila larval hematopoietic niche. Whether Lar serves as a localized invariable negative regulator of systemic signals such as insulin in other stem cell niches remains to be explored.

In utero exposure to diesel exhaust particulates is associated with an altered cardiac transcriptional response to transverse aortic constriction and altered DNA methylation

In utero exposure to diesel exhaust air pollution has been associated with increased adult susceptibility to heart failure in mice, but the mechanisms by which this exposure promotes susceptibility to heart failure are poorly understood. To identify the potential transcriptional effects that mediate this susceptibility, we have performed RNA sequencing analysis on adult hearts from mice that were exposed to diesel exhaust in utero and that have subsequently undergone transverse aortic constriction. We identified 3 target genes, Mir133a-2, Ptprf, and Pamr1, which demonstrate dysregulation after exposure and aortic constriction. Examination of expression patterns in human heart tissues indicates a correlation between expression and heart failure. We subsequently assessed DNA methylation modifications at these candidate loci in neonatal cultured cardiac myocytes after in utero exposure to diesel exhaust and found that the promoter for Mir133a-2 is differentially methylated. These target genes in the heart are the first genes to be identified that likely play an important role in mediating adult sensitivity to heart failure. We have also shown a change in DNA methylation within cardiomyocytes as a result of in utero exposure to diesel exhaust.-Goodson, J. M., Weldy, C. S., MacDonald, J. W., Liu, Y., Bammler, T. K., Chien, W.-M., Chin, M. T. In utero exposure to diesel exhaust particulates is associated with an altered cardiac transcriptional response to transverse aortic constriction and altered DNA methylation.

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.

Reciprocal regulation of insulin and plasma 5'-AMP in glucose homeostasis in mice

A previous investigation has demonstrated that plasma 5'-AMP (pAMP) exacerbates and causes hyperglycemia in diabetic mice. However, the crosstalk between pAMP and insulin signaling to regulate glucose homeostasis has not been investigated in depth. In this study, we showed that the blood glucose level was more dependent on the ratio of insulin to pAMP than on the absolute level of these two factors. Administration of 5'-AMP significantly attenuated the insulin-stimulated insulin receptor (IR) autophosphorylation in the liver and muscle tissues, resulting in the inhibition of downstream AKT phosphorylation. A docking analysis indicated that adenosine was a potential inhibitor of IR tyrosine kinase. Moreover, the 5'-AMP treatment elevated the ATP level in the pancreas and in the isolated islets, stimulating insulin secretion and increasing the plasma level of insulin. The insulin administration decreased the 5'-AMP-induced hyper-adenosine level by the up-regulation of adenosine kinase activities. Our results indicate that blood glucose homeostasis is reciprocally regulated by pAMP and insulin.

Identification of receptor protein tyrosine phosphatases (RPTPs) as regulators of receptor tyrosine kinases (RTKs) using an RPTP siRNA-RTK substrate screen

Receptor tyrosine kinase (RTK) signaling exists in equilibrium between RTK tyrosyl phosphorylation and RTK tyrosyl dephosphorylation. Despite a detailed understanding of RTK tyrosyl phosphorylation, much less is known about RTK tyrosyl dephosphorylation. The receptor PTPs (RPTPs) are outstanding targets for the dephosphorylation of RTKs because of their mutual membrane proximity. In this chapter, we describe how to identify RPTPs that modulate the activity of RTKs using a siRNA screen and commercially available proteomic applications. The validation of putative RTKs as RPTP substrates using substrate-trapping approaches is detailed.

Functional genomics identified a novel protein tyrosine phosphatase receptor type F-mediated growth inhibition in hepatocarcinogenesis

It is unclear how proliferating cells elicit suppression on cell proliferation and how cancer cells evade this growth suppression. Using a loss-of-function screening of the human kinome and phosphatome to identify genes suppressing tumor initiation in human hepatocellular carcinoma (HCC), we identified 19 genes and characterized one of the top-scoring tumor suppressor candidates, protein tyrosine phosphatase receptor type F (PTPRF). We found that PTPRF was induced during cell proliferation by cell-cell contact. Ectopic expression of wild-type PTPRF, but not the phosphatase-inactive mutant, suppressed cell proliferation and colony formation in soft-agar assays. In contrast, PTPRF silencing led to cell hyperproliferation, enhanced tumor colony formation in soft agar, and increased xenograft tumor growth in nude mice. Mechanistically, PTPRF silencing showed aberrant ERK-dependent signaling including the phosphorylation/stabilization of v-myc avian myelocytomatosis viral oncogene homolog (MYC) through the direct activation of v-src avian sarcoma viral oncogene homolog (SRC) and suppression of PP2A. This PTPRF-mediated growth suppression during cell proliferation functioned independently of the Hippo-Yap pathway. Clinically, PTPRF was down-regulated in 42% HCC (37/89), 67% gastric cancer (27/40), and 100% colorectal cancer (40/40). PTPRF up-regulation was found in 24% HCC (21/89) and associated with better clinical outcomes.

CONCLUSION

A novel PTPRF-mediated growth suppression pathway was identified by way of a functional genomics screening in human hepatoma cells. Induction of PTPRF by cell-cell contact during cell proliferation quenched the activated ERK-dependent proliferation signaling to prevent cell hyperproliferation and tumor initiation. PTPRF down-regulation in HCC facilitated tumor development. Our findings shed light on how cancer cells can evade growth suppression and open a new avenue for future development of anticancer therapies.

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.

Receptor protein tyrosine phosphatase-receptor tyrosine kinase substrate screen identifies EphA2 as a target for LAR in cell migration

Receptor tyrosine kinases (RTKs) exist in equilibrium between tyrosyl-phosphorylated and dephosphorylated states. Despite a detailed understanding of how RTKs become tyrosyl phosphorylated, much less is known about RTK tyrosyl dephosphorylation. Receptor protein tyrosine phosphatases (RPTPs) can play essential roles in the dephosphorylation of RTKs. However, a complete understanding of the involvement of the RPTP subfamily in RTK tyrosyl dephosphorylation has not been established. In this study, we have employed a small interfering RNA (siRNA) screen to identify RPTPs in the human genome that serve as RTK phosphatases. We observed that each RPTP induced a unique fingerprint of tyrosyl phosphorylation among 42 RTKs. We identified EphA2 as a novel LAR substrate. LAR dephosphorylated EphA2 at phosphotyrosyl 930, uncoupling Nck1 from EphA2 and thereby attenuating EphA2-mediated cell migration. These results demonstrate that each RPTP exerts a unique regulatory fingerprint of RTK tyrosyl dephosphorylation and suggest a complex signaling interplay between RTKs and RPTPs. Furthermore, we observed that LAR modulates cell migration through EphA2 site-specific dephosphorylation.

Leukocyte antigen-related inhibition attenuates palmitate-induced insulin resistance in muscle cells

Palmitate has been shown to induce insulin resistance in skeletal muscle cells. The aim of this study was to investigate the role of the leukocyte common antigen-related (LAR) gene in palmitate-induced insulin resistance in C2C12 cells. A stable C2C12 cell line was generated using LAR short hairpin RNA. The levels of LAR protein and phosphorylation of insulin receptor substrate-1 (IRS1) and Akt were detected by western blot analysis. 2-Deoxyglucose uptake was measured in LAR knockdown and control cells using d-[2-(3)H]glucose. LAR protein level was decreased by 65% in the stable cell line compared with the control cells. Palmitate (0.5 mM) significantly induced LAR mRNA (65%) and protein levels (40%) in myotubes compared with untreated cells. Palmitate significantly reduced insulin-stimulated glucose uptake in both the control and LAR knockdown cells by 33 and 51% respectively. However, LAR depletion improved insulin-stimulated glucose uptake in myotubes treated with palmitate. Furthermore, the inhibition of LAR prevented palmitate-induced decreases in phosphorylation of IRS1(Tyr632) and Akt(Ser473) in C2C12 cells. In conclusion, these results reveal that palmitate induces LAR expression in C2C12 cells. We also provided evidence that the inhibition of LAR attenuates palmitate-induced insulin resistance in myotubes.

Topiramate treatment improves hypothalamic insulin and leptin signaling and action and reduces obesity in mice

Topiramate (TPM) treatment has been shown to reduce adiposity in humans and rodents. The reduction in adiposity is related to decreased food intake and increased energy expenditure. However, the molecular mechanisms through which TPM induces weight loss are contradictory and remain to be clarified. Whether TPM treatment alters hypothalamic insulin, or leptin signaling and action, is not well established. Thus, we investigate herein whether short-term TPM treatment alters energy balance by affecting insulin and leptin signaling, action, or neuropeptide expression in the hypothalamus of mice fed with a high-fat diet. As expected, short-term treatment with TPM diminished adiposity in obese mice mainly due to reduced food intake. TPM increased anorexigenic signaling by enhancing the leptin-induced leptin receptor/Janus kinase 2/signal transducer and activator of transcription 3 pathway and the insulin-induced insulin receptor substrate/Akt/forkhead box O1 pathway in parallel to reduced phosphatase protein expression in the hypothalamus of obese mice. These effects were independent of body weight. TPM also raised anorexigenic neuropeptides such as POMC, TRH, and CRH mRNA levels in obese mice. In addition, TPM increased the activation of the hypothalamic MAPK/ERK pathway induced by leptin, accompanied by an increase in peroxisome proliferator-activated receptor-coactivator α and uncoupling protein 1 protein levels in brown adipose tissue. Furthermore, TPM increased AMP-activated protein kinase and acetyl-coenzyme A carboxylase phosphorylation in peripheral tissues, which may help improve energy metabolism in these tissues. Together, these results provide novel insights into the molecular mechanisms through which TPM treatment reduces adiposity.

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.

Regulation of adipogenic differentiation by LAR tyrosine phosphatase in human mesenchymal stem cells and 3T3-L1 preadipocytes

Mesenchymal stem cells (MSCs) are multipotent adult stem cells that can differentiate into a variety of mesodermal-lineage cells. MSCs have significant potential in tissue engineering and therapeutic applications; however, the low differentiation and proliferation efficiencies of these cells in the laboratory are fundamental obstacles to their therapeutic use, mainly owing to the lack of information on the detailed signal-transduction mechanisms of differentiation into distinct lineages. With the aid of protein-tyrosine-phosphatase profiling studies, we show that the expression of leukocyte common antigen related (LAR) tyrosine phosphatase is significantly decreased during the early adipogenic stages of MSCs. Knockdown of endogenous LAR induced a dramatic increase in adipogenic differentiation, whereas its overexpression led to decreased adipogenic differentiation in both 3T3-L1 preadipocytes and MSCs. LAR reduces tyrosine phosphorylation of the insulin receptor, in turn leading to decreased phosphorylation of the adaptor protein IRS-1 and its downstream molecule Akt (also known as PKB). We propose that LAR functions as a negative regulator of adipogenesis. Furthermore, our data support the possibility that LAR controls the balance between osteoblast and adipocyte differentiation. Overall, our findings contribute to the clarification of the mechanisms underlying LAR activity in the differentiation of MSCs and suggest that LAR is a candidate target protein for the control of stem-cell differentiation.

A novel liver specific isoform of the rat LAR transcript is expressed as a truncated isoform encoded from a 5'UTR located within intron 11

BACKGROUND

The leukocyte common antigen related receptor (LAR) protein has been shown to modulate the signal transduction of a number of different growth factors, including insulin and insulin-like growth factor 1. Splice variants exhibit differing roles and are expressed according to tissue type and developmental stage.

RESULTS

Using 5'RACE, we identified a 5'UTR within intron 11 of the rat LAR gene. We demonstrated that this gives rise to a novel isoform of the LAR transcript encoded from the identified region within intron 11. By priming across the site from exon 11 to exon 15 we show that the novel 5'UTR is not represented in the full-length transcript and thus, it produces a truncated form of the LAR mRNA. We examined the tissue distribution of this novel isoform and found it to be exclusively expressed in liver. We additionally identified a liver specific 150 kDa band with western blotting which we propose may represent the protein product of the novel transcript. Luciferase assays showed the region immediately upstream of the 5'UTR to possesses considerable promoter activity and that this may be conferred by the presence of a number of putative binding sites for liver enriched transcription factors.

CONCLUSION

In summary, we describe a novel, liver specific, truncated isoform of the LAR transcript transcribed under the control of an intronic promoter, potentially representing a previously unidentified modulator of hepatic insulin signalling.

Leukocyte antigen-related protein tyrosine phosphatase negatively regulates hydrogen peroxide-induced vascular smooth muscle cell apoptosis

Reactive oxygen species (ROS) have been implicated in vascular smooth muscle cell (VSMC) apoptosis, a hallmark of advanced atherosclerotic lesions. Transient oxidation and inactivation of protein-tyrosine phosphatases play a critical role in cellular response to ROS production. However, the function of leukocyte antigen-related (LAR) protein-tyrosine phosphatase in ROS signaling is not known. To determine the expression of LAR in ROS-induced apoptosis, we investigated hydrogen peroxide-induced cell death and signaling in aortic VSMCs from wild-type and LAR(-/-) mice. Histone-associated DNA fragmentation and caspase-3/7 activity were significantly enhanced, mitochondrial membrane integrity was compromised, and cell viability was significantly decreased following H(2)O(2) treatment in LAR(-/-) VSMCs compared with wild-type cells. Stronger and sustained increase in autophosphorylation and activity of Fyn, an Src family tyrosine kinase, was observed in LAR(-/-) cells compared with wild-type cells following H(2)O(2) treatment. LAR binds to activated Fyn in H(2)O(2)-treated VSMCs, and recombinant LAR dephosphorylates phosphorylated-Fyn in vitro. In addition, LAR deficiency enhanced H(2)O(2)-induced phosphorylation of Janus kinase 2 (JAK2), signal transducer and activator of transcription 3 (STAT3), and p38 mitogen-activated protein kinase (MAPK). PP2, a Fyn-specific inhibitor, blocked JAK2, STAT3, and p38 MAPK activation and significantly attenuated apoptosis induced by H(2)O(2). AG490, a JAK2-specific inhibitor, significantly attenuated H(2)O(2)-induced apoptosis, and blocked H(2)O(2)-induced activation of STAT3, but not p38 MAPK in both wild-type and LAR(-/-) VSMCs. Attenuation of Fyn expression by short hairpin RNA significantly decreased H(2)O(2)-induced downstream signaling and apoptosis in VSMCs. Together, these data indicate that LAR regulates Fyn/JAK2/STAT3 and Fyn/p38 MAPK pathways involved in ROS-induced apoptosis.

Cell cycle regulation by oncogenic tyrosine kinases in myeloid neoplasias: from molecular redox mechanisms to health implications

Neoplastic expansion of myeloid cells is associated with specific genetic changes that lead to chronic activation of signaling pathways, as well as altered metabolism. It has become increasingly evident that transformation relies on the interdependency of both events. Among the various genetic changes, the oncogenic BCR-ABL tyrosine kinase in patients with Philadelphia chromosome positive chronic myeloid leukemia (CML) has been a focus of extensive research. Transformation by this oncogene is associated with elevated levels of intracellular reactive oxygen species (ROS). ROS have been implicated in processes that promote viability, cell growth, and regulation of other biological functions such as migration of cells or gene expression. Currently, the BCR-ABL inhibitor imatinib mesylate (Gleevec) is being used as a first-line therapy for the treatment of CML. However, BCR-ABL transformation is associated with genomic instability, and disease progression or resistance to imatinib can occur. Imatinib resistance is not known to cause or significantly alter signaling requirements in transformed cells. Elevated ROS are crucial for transformation, making them an ideal additional target for therapeutic intervention. The underlying mechanisms leading to elevated oxidative stress are reviewed, and signaling mechanisms that may serve as novel targeted approaches to overcome ROS-dependent cell growth are discussed.

Cytosolic protein tyrosine phosphatase-epsilon is a negative regulator of insulin signaling in skeletal muscle

Whereas positive regulatory events triggered by insulin binding to insulin receptor (IR) have been well documented, the mechanism by which the activated IR is returned to the basal status is not completely understood. Recently studies focused on the involvement of protein tyrosine phosphatases (PTPs) and how they might influence IR signaling. In this study, we examined the possibility that cytosolic PTPepsilon (cytPTPepsilon) is involved in IR signaling. Studies were performed on L6 skeletal muscle cells. cytPTPepsilon was overexpressed by using pBABE retroviral expression vectors. In addition, we inhibited cytPTPepsilon by RNA silencing. We found that insulin induced rapid association of cytPTPepsilon with IR. Interestingly, this association appeared to occur in the plasma membrane and on stimulation with insulin the two proteins internalized together. Moreover, it appeared that almost all internalized IR was associated with cytPTPepsilon. We found that knockdown of cytPTPepsilon by RNA silencing increased insulin-induced tyrosine phosphorylation of IR and IR substrate (IRS)-1 as well as phosphorylation of protein kinase B and glycogen synthase kinase-3 and insulin-induced stimulation of glucose uptake. Moreover, overexpression of wild-type cytPTPepsilon reduced insulin-induced tyrosine phosphorylation of IR, IRS-1, and phosphorylation of protein kinase B and glycogen synthase kinase-3 and insulin-induced stimulation of glucose uptake. Finally, insulin-induced tyrosine phosphorylation of IR and IRS-1 was greater in skeletal muscle from mice lacking the cytPTPepsilon gene than that from wild-type control animals. We conclude that cytPTPepsilon serves as another major candidate negative regulator of IR signaling in skeletal muscle.

Inhibition of the protein tyrosine phosphatase PTP1B: potential therapy for obesity, insulin resistance and type-2 diabetes mellitus

The global epidemic of obesity and type-2 diabetes mellitus (T2DM) has highlighted the need for new therapeutic approaches. The association of insulin resistance with these disorders and the knowledge that insulin receptor signaling is mediated by tyrosine (Tyr) phosphorylation have generated great interest in the regulation of the balance between Tyr phosphorylation and dephosphorylation. Several protein Tyr phosphatases (PTPs) have been implicated in the regulation of insulin action, with the most convincing data for PTP1B. Murine models targeting PTP1B, PTP1B(-/-)mice, demonstrate enhanced insulin sensitivity without the weight gain seen with other insulin sensitizers such as peroxisome proliferator-activated receptor gamma (PPARgamma) agonists, probably due to a second action of PTP1B as a negative regulator of leptin signaling. Despite intensive efforts and recent progress, a safe, selective and efficacious PTP1B inhibitor has yet to be identified.

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.

Insulin-dependent phosphorylation of DPP IV in liver. Evidence for a role of compartmentalized c-Src

Dipeptidyl peptidase IV (DPP IV, CD26, EC 3.4.14.5) serves as a model aimed at elucidating protein sorting signals. We identify here, by MS, several tyrosine-phosphorylated proteins in a rat liver Golgi/endosome (G/E) fraction including DPP IV. We show that a pool of DPP IV is tyrosine-phosphorylated. Maximal phosphorylation was observed after 2 min following intravenous insulin injection. DPP IV coimmunoprecipitated with the cellular tyrosine kinase Src (c-Src) with maximal association also observed after 2 min following insulin injection. DPP IV was found phosphorylated after incubation of nonsolubilized G/E membranes with [gamma-32P]ATP. The c-Src inhibitor PP2 inhibited DPP IV phosphorylation. Oriented proteolysis experiments indicate that a large pool of c-Src is protected in G/E fractions. Following injection of the protein-tyrosine phosphatase inhibitor bpV(phen), DPP IV levels markedly decreased by 40% both in plasma membrane and G/E fractions. In the fraction designated Lh, DPP IV levels decreased by 50% 15 min following insulin injection. Therefore, a pool of DPP IV is tyrosine-phosphorylated in an insulin-dependent manner. The results suggest the presence of a yet to be characterized signalling mechanism whereby DPP IV has access to c-Src-containing signalling platforms.

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.

Involvement of the small protein tyrosine phosphatases TC-PTP and PTP1B in signal transduction and diseases: from diabetes, obesity to cell cycle, and cancer

As in other fields of biomedical research, the use of gene-targeted mice by homologous recombination in embryonic stem cells has provided important findings on the function of several members of the protein tyrosine phosphatase (PTP) family. For instance, the phenotypic characterization of knockout mice has been critical in understanding the sites of action of the related PTPs protein tyrosine phosphatase 1B (PTP1B) and T-cell-PTP (TC-PTP). By their increased insulin sensitivity and insulin receptor hyperphosphorylation, PTP1B null mice demonstrated a clear function for this enzyme as a negative regulator of insulin signaling. As well, TC-PTP has also been recently involved in insulin signaling in vitro. Importantly, the high identity in their amino acid sequences suggests that they must be examined simultaneously as targets of drug development. Indeed, they possess different as well as overlapping substrates, which suggest complementary and overlapping roles of both TC-PTP and PTP1B. Here, we review the function of PTP1B and TC-PTP in diabetes, obesity, and processes related to cancer.

Oxidative enhancement of insulin receptor signaling: experimental findings and clinical implications

Signaling through the insulin receptor and several other receptor tyrosine kinases is subject to redox regulation. Prolonged exposure to hydrogen peroxide impairs the action of insulin, and may account to some extent for the decreased insulin responsiveness in hyperglycemic diabetic patients. However, insulin receptor kinase (IRK) autophosphorylation and/or kinase activity were found to be markedly enhanced by a more limited exposure to hydrogen peroxide or by an oxidative shift in the thiol/disulfide redox status. Oxidative enhancement of IRK function may be mediated by two different mechanisms with similar effects, i.e., by direct oxidative activation of IRK activity or by oxidative inactivation of a protein tyrosine phosphatase, which otherwise down-regulates IRK-mediated signaling. As both mechanisms enhance IRK activity in the absence of insulin, there is a strong possibility that the background IRK activity in the postabsorptive period may be abnormally increased in certain oxidative conditions and thereby disturb the metabolism of glucose and other energy substrates. This remains to be tested. In line with the oxidative enhancement of IRK activity, clinical studies have shown that treatment with a thiol-containing antioxidant increases the postabsorptive glucose and/or insulin concentrations (i.e., the HOMA-R index) at least under certain conditions. This effect may have therapeutic implications.

Role of insulin-induced reactive oxygen species in the insulin signaling pathway

Oxidants, including hydrogen peroxide (H2O2), have been recognized for years to mimic insulin action on glucose transport in adipose cells. Early studies also demonstrated the complementary finding that H2O2 was elaborated during treatment of cells with insulin, suggesting that cellular H2O2 generation was integral to insulin signaling. Recently, reactive oxygen species elicited by various hormones and growth factors have been shown to affect signal transduction pathways in various cell types. We recently reported that insulin-stimulated H2O2 modulates proximal and distal insulin signaling, at least in part through the oxidative inhibition of protein tyrosine phosphatases (PTPases) that negatively regulate the insulin action pathway. Nox4, a homologue in the family of NADPH oxidase catalytic subunits, was found to be prominently expressed in insulin-sensitive cells. By various molecular approaches, Nox4 was shown to mediate insulin-stimulated H2O2 generation and impact the insulin signaling cascade. Overexpression of Nox4 also significantly reversed the inhibition of insulin-stimulated receptor tyrosine phosphorylation by PTP1B, a widely expressed PTPase implicated in the negative regulation of insulin signaling, by inhibiting its catalytic activity. These recent studies have provided insight into Nox4 as a novel molecular link between insulin-stimulated reactive oxygen species and mechanisms involved in their modulation of insulin signal transduction.

Knock-down of LAR protein tyrosine phosphatase induces insulin resistance

To test the role of the leukocyte common antigen-related protein tyrosine phosphatase (LAR) as a regulator of insulin receptor (IR) signalling, an siRNA probe against LAR was developed. Knock-down of LAR induced post-receptor insulin resistance with the insulin-induced activation of PKB/Akt and MAP kinases markedly inhibited. The phosphorylation and dephosphorylation of the IR and insulin receptor substrate (IRS) proteins were unaffected by LAR knock-down. These results identify LAR as a crucial regulator of the sensitivity of two key insulin signalling pathways to insulin. Moreover, the siRNA probe provides a molecular tool of general applicability for further dissecting the precise targets and roles of LAR.

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.

Prolactin-modulated gene expression profiles in pancreatic islets from adult female rats

The effects of prolactin (PRL) on transcript profile expression in 24h cultured pancreatic adult rat islets were investigated by cDNA expression array analysis to identify possible candidate mRNA species that encode proteins involved in the maturation and growth of the endocrine pancreas. The expression of 54 out of 588 genes was altered by treatment with PRL. The differentially expressed transcripts identified were distributed in six main categories involved in cell proliferation and differentiation, namely, cell cycle regulation, signal transduction, transcription factors and coactivators, translational machinery, Ca(2+)-mediated exocytosis, and immuno-response. Treatment with PRL also reduced the expression of genes related to apoptosis. Several genes, whose expression was previously not known to be modulated by PRL were also identified including macrophage migration inhibitory factor and Ca(2+)/calmodulin-dependent protein kinase IV. These genes have recently been shown to play a crucial role in insulin secretion and insulin gene expression, respectively. Treatment with PRL also modified the expression of AKT2 and bone morphogenetic protein receptor 1A that control glucose homeostasis and directly affect the behavior of endocrine pancreas and/or the sensitivity of target tissues to insulin. In conclusion, PRL induces several patterns of gene expression in pancreatic islet cells. The analysis of these different patterns will be useful for understanding the complex mechanism of action of PRL in the maturation and differentiation of pancreatic islets.

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.

Cellular effects of small molecule PTP1B inhibitors on insulin signaling

Protein tyrosine phosphatase 1B (PTP1B) is implicated as a negative regulator of insulin receptor (IR) signaling and a potential drug target for the treatment of type 2 diabetes and other associated metabolic syndromes. To further define the role of PTP1B in insulin signaling and to test the hypothesis that blocking the activity of PTP1B would augment the action of insulin, we prepared several cell permeable, potent and selective, small molecule PTP1B inhibitors, and evaluated their biological effects in several insulin sensitive cell lines. Our data indicate that PTP1B inhibitors bind to and colocalize with PTP1B on the surface of the endoplasmic reticulum and PTP1B exerts its negative effect on insulin signaling upstream of phosphatidylinositol 3-kinase and MEK1. Treatment of cells with PTP1B inhibitors, both in the presence and in the absence of insulin, markedly enhances IRbeta and IRS-1 phosphorylation, Akt and ERK1/2 activation, Glut4 translocation, glucose uptake, and Elk1 transcriptional activation and cell proliferation. These results indicate that small molecule inhibitors targeted to PTP1B can act as both insulin mimetics and insulin sensitizers. Taken together, our findings combined with results from PTP1B knockout, antisense, and biochemical studies provide strong evidence that PTP1B negatively regulates insulin signaling and that small molecule PTP1B inhibitors have the ability to potentiate and augment the action of insulin.

Redox regulation of protein tyrosine phosphatases during receptor tyrosine kinase signal transduction

In addition to protein phosphorylation, redox-dependent post-translational modification of proteins is emerging as a key signaling system that has been conserved throughout evolution and that influences many aspects of cellular homeostasis. Both systems exemplify dynamic regulation of protein function by reversible modification, which, in turn, regulates many cellular processes such as cell proliferation, differentiation and apoptosis. In this article we focus on the interplay between phosphorylation- and redox-dependent signaling at the level of phosphotyrosine phosphatase-mediated regulation of receptor tyrosine kinases (RTKs). We propose that signal transduction by oxygen species through reversible phosphotyrosine phosphatase inhibition, represents a widespread and conserved component of the biochemical machinery that is triggered by RTKs.

A cellular model for myeloma cell growth and maturation based on an intraclonal CD45 hierarchy

Multiple myeloma (MM) is a plasma cell malignancy mainly characterized by the accumulation of malignant plasma cells within the bone marrow. This review shows that the biology of CD45 illuminates that of MM and, more specifically, provides a better delineation of a tumor cell 'hierarchy' of clinical interest. We show that in MM, as in normal plasma cell differentiation, there is an intraclonal CD45 hierarchy that is a gradient of CD45 expression on myeloma cells directly related to their proliferation rate and differentiation status. This CD45 hierarchy allows for the design of a cellular model for MM-cell growth and maturation in which CD45 bright myeloma cells represent the proliferating compartment and CD45 low myeloma cells the quiescent compartment. This model includes an aberrant phenotype that is annihilation rather than decline of CD45, annihilation reflecting the terminal phase of the disease and/or an aggressive presentation of MM. Data from the literature suggest that CD45 bright myeloma cells are targeted by interleukin (IL)-6, whereas CD45 negative myeloma cells with a high clonogenic capacity are targeted by insulin/insulin-like growth factor 1 (IGF-1). This model will be useful for both a better understanding of the basic biology of MM and a better stratification of and therapeutic approach to the patients. Finally, this model presents MM as a self-renewing plasma cell disease, although the first oncogenic events such as 14q32 translocations clearly occur earlier in a B cell.

Protein tyrosine phosphatases: the quest for negative regulators of insulin action

Type 2 diabetes is increasing at an alarming rate worldwide, and there has been a considerable effort in several laboratories to identify suitable targets for the design of drugs against the disease. To this end, the protein tyrosine phosphatases that attenuate insulin signaling by dephosphorylating the insulin receptor (IR) have been actively pursued. This is because inhibiting the phosphatases would be expected to prolong insulin signaling and thereby facilitate glucose uptake and, presumably, result in a lowering of blood glucose. Targeting the IR protein tyrosine phosphatase, therefore, has the potential to be a significant disease-modifying strategy. Several protein tyrosine phosphatases (PTPs) have been implicated in the dephosphorylation of the IR. These phosphatases include PTPalpha, LAR, CD45, PTPepsilon, SHP2, and PTP1B. In most cases, there is evidence for and against the involvement of the phosphatases in insulin signaling. The most convincing data, however, support a critical role for PTP1B in insulin action. PTP1B knockout mice are not only insulin sensitive but also maintain euglycemia (in the fed state), with one-half the level of insulin observed in wild-type littermates. Interestingly, these mice are also resistant to diet-induced obesity when fed a high-fat diet. The insulin-sensitive phenotype of the PTP1B knockout mouse is reproduced when the phosphatase is also knocked down with an antisense oligonucleotide in obese mice. Thus PTP1B appears to be a very attractive candidate for the design of drugs for type 2 diabetes and obesity.

Glucose regulates insulin mitogenic effect by modulating SHP-2 activation and localization in JAr cells

The glucose effect on cell growth has been investigated in the JAr human choriocarcinoma cells. When JAr cells were cultured in the presence of 6 mm glucose (LG), proliferation and thymidine incorporation were induced by serum, epidermal growth factor, and insulin-like growth factor 1 but not by insulin. In contrast, at 25 mm glucose (HG), proliferation and thymidine incorporation were stimulated by insulin, serum, epidermal growth factor, and insulin-like growth factor 1 to a comparable extent, whereas basal levels were 25% lower than those in LG. HG culturing also enhanced insulin-stimulated insulin receptor and insulin receptor substrate 1 (IRS1) tyrosine phosphorylations while decreasing basal phosphorylations. These actions of glucose were accompanied by an increase in cellular tyrosine phosphatase activity. The activity of SHP-2 in HG-treated JAr cells was 400% of that measured in LG-treated cells. SHP-2 co-precipitation with IRS1 was also increased in HG-treated cells. SHP-2 was mainly cytosolic in LG-treated cells. However, HG culturing largely redistributed SHP-2 to the internal membrane compartment, where tyrosine-phosphorylated IRS1 predominantly localizes. Further exposure to insulin rescued SHP-2 cytosolic localization, thereby preventing its interaction with IRS1. Antisense inhibition of SHP-2 reverted the effect of HG on basal and insulin-stimulated insulin receptor and IRS1 phosphorylation as well as that on thymidine incorporation. Thus, in JAr cells, glucose modulates insulin mitogenic action by modulating SHP-2 activity and intracellular localization.

Coordinated action of protein tyrosine phosphatases in insulin signal transduction

Insulin is the principal regulatory hormone involved in the tight regulation of fuel metabolism. In response to blood glucose levels, it is secreted by the beta cells of the pancreas and exerts its effects by binding to cell surface receptors that are present on virtually all cell types and tissues. In humans, perturbations in insulin function and/or secretion lead to diabetes mellitus, a severe disorder primarily characterized by an inability to maintain blood glucose homeostasis. Furthermore, it is estimated that 90-95% of diabetic patients exhibit resistance to insulin action. Thus an understanding of insulin signal transduction and insulin resistance at the molecular level is crucial to the understanding of the pathogenesis of this disease. The insulin receptor (IR) is a transmembrane tyrosine kinase that becomes activated upon ligand binding. Consequently, the receptor and its downstream substrates become tyrosine phosphorylated. This activates a series of intracellular signaling cascades which coordinately initiate the appropriate biological response. One important mechanism by which insulin signaling is regulated involves the protein tyrosine phosphatases (PTPs), which may either act on the IR itself and/or its substrates. Two well characterized examples include leuckocyte antigen related (LAR) and protein tyrosine phosphatase-1B (PTP-1B). The present review will discuss the current knowledge of these two and other potential PTPs involved in the insulin signaling pathway.

The linkage between beta1 integrin and the actin cytoskeleton is differentially regulated by tyrosine and serine/threonine phosphorylation of beta1 integrin in normal and cancerous human breast cells

BACKGROUND

Structural requirements for the beta1 integrin functions in cell adhesion, spreading and signaling have been well documented mainly for fibroblasts. In this study, we examined the reason for the reduced surface expression of beta1 integrin in human breast cancer MCF-7 cells compared to normal human breast epithelial (HBE) cells, both of which adhered to collagen type IV.

RESULTS

The beta1 integrin immunoprecipitates from either HBE or MCF-7 cells involved alpha-actinin while actin coprecipitated with beta1 integrin from HBE cells but not from MCF-7 cells. Immunoblotting using the anti-phosphotyrosine (PY) antibody indicated the phosphorylation of beta1 integrin at least at tyrosine in both cells. Dephosphorylation of beta1 integrin from HBE cells by protein tyrosine phosphatase (PTP), but not by protein serine/threonine phosphatase (PP), caused dissociation of actin from beta1 integrin, although dephosphorylation of it from MCF-7 cells by either PTP or PP caused association of the two proteins. In MCF-7 cells beta1 integrin coprecipitated doublet of proteins having the Ca2+/calmodulin-dependent protein kinase (CaMK) II activity that was susceptible to KN-62, a specific inhibitor of CaMKII.

CONCLUSION

The results suggest that beta1 integrin is tyrosine phosphorylated and links with actin via alpha-actinin in HBE cells but prevented from linking with actin in MCF-7 cells by phosphorylation at both tyrosine and serine/threonine of beta1 integrin which forms a complex with alpha-actinin and CaMKII. Thus the linkage formation of beta1 integrin with actin may be differentially regulated by its tyrosine and serine/threonine phosphorylation in normal HBE cells and breast cancer MCF-7 cells.