Phosphorylation and activation of protein tyrosine phosphatase (PTP) 1B by insulin receptor

We have previously reported a direct in vivo interaction between the activated insulin receptor and protein-tyrosine phosphatase-1B (PTP1B), which leads to an increase in PTP1B tyrosine phosphorylation. In order to determine if PTP1B is a substrate for the insulin receptor tyrosine kinase, the phosphorylation of the Cys 215 Ser, catalytically inactive mutant PTP1B (CS-PTP1B) was measured in the presence of partially purified and activated insulin receptor. In vitro, the insulin receptor tyrosine kinase catalyzed the tyrosine phosphorylation of PTP1B. 53% of the total cellular PTP1B became tyrosine phosphorylated in response to insulin in vivo. Tyrosine phosphorylation of PTP1B by the insulin receptor was absolutely dependent upon insulin-stimulated receptor autophosphorylation and required an intact kinase domain, containing insulin receptor tyrosines 1146, 1150 and 1151. Tyrosine phosphorylation of wild type PTP1B by the insulin receptor kinase increased phosphatase activity of the protein. Intermolecular transdephosphorylation was demonstrated both in vitro and in vivo, by dephosphorylation of phosphorylated CS-PTP1B by the active wild type enzyme either in a cell-free system or via expression of the wild type PTP1B into Hirc-M cell line, which constitutively overexpress the human insulin receptor and CS-PTP1B. These results suggest that PTP1B is a target protein for the insulin receptor tyrosine kinase and PTP1B can regulate its own phosphatase activity by maintaining the balance between its phosphorylated (the active form) and dephosphorylated (the inactive form) state.     

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.     

Interaction with Grb14 results in site-specific regulation of tyrosine phosphorylation of the insulin receptor

The dynamics of interaction of the insulin receptor (IR) with Grb14 was monitored, in real time, in living human embryonic kidney cells, using bioluminescence resonance energy transfer (BRET). We observed that insulin rapidly and dose-dependently stimulated this interaction. We also observed that insulin-induced BRET between the IR and protein tyrosine phosphatase 1B (PTP1B) was markedly reduced by Grb14, suggesting that Grb14 regulated this interaction in living cells. Using site-specific antibodies against phosphorylated tyrosines of the IR, we showed that Grb14 protected the three tyrosines of the kinase loop from dephosphorylation by PTP1B, while favouring dephosphorylation of tyrosine 972. This resulted in decreased IRS-1 binding to the IR and decreased activation of the extracellular signal-regulated kinase pathway. Increased Grb14 expression in human liver-derived HuH7 cells also seemed to specifically decrease the phosphorylation of Y972. Our work therefore suggests that Grb14 may regulate signalling through the IR by controlling its tyrosine dephosphorylation in a site-specific manner.     

Overexpression of protein-tyrosine phosphatase-1B in adipocytes inhibits insulin-stimulated phosphoinositide 3-kinase activity without altering glucose transport or Akt/Protein kinase B activation

Previous studies suggested that protein-tyrosine phosphatase 1B (PTP1B) antagonizes insulin action by catalyzing dephosphorylation of the insulin receptor (IR) and/or other key proteins in the insulin signaling pathway. In adipose tissue and muscle of obese humans and rodents, PTP1B expression is increased, which led to the hypothesis that PTP1B plays a role in the pathogenesis of insulin resistance. Consistent with this, mice in which the PTP1B gene was disrupted exhibit increased insulin sensitivity. To test whether increased expression of PTP1B in an insulin-sensitive cell type could contribute to insulin resistance, we overexpressed wild-type PTP1B in 3T3L1 adipocytes using adenovirus-mediated gene delivery. PTP1B expression was increased approximately 3-5-fold above endogenous levels at 16 h, approximately 14-fold at 40 h, and approximately 20-fold at 72 h post-transduction. Total protein-tyrosine phosphatase activity was increased by 50% at 16 h, 3-4-fold at 40 h, and 5-6-fold at 72 h post-transduction. Compared with control cells, cells expressing high levels of PTP1B showed a 50-60% decrease in maximally insulin-stimulated tyrosyl phosphorylation of IR and insulin receptor substrate-1 (IRS-1) and phosphoinositide 3-kinase (PI3K) activity associated with IRS-1 or with phosphotyrosine. Akt phosphorylation and activity were unchanged. Phosphorylation of p42 and p44 MAP kinase (MAPK) was reduced approximately 32%. Overexpression of PTP1B had no effect on basal, submaximally or maximally (100 nm) insulin-stimulated glucose transport or on the EC(50) for transport. Our results suggest that: 1) insulin stimulation of glucose transport in adipocytes requires 相似文献   

Phosphorylation of PTP1B at Ser(50) by Akt impairs its ability to dephosphorylate the insulin receptor

PTP1B is a protein tyrosine phosphatase that negatively regulates insulin sensitivity by dephosphorylating the insulin receptor. Akt is a ser/thr kinase effector of insulin signaling that phosphorylates substrates at the consensus motif RXRXXS/T. Interestingly, PTP1B contains this motif (RYRDVS(50)), and wild-type PTP1B (but not mutants with substitutions for Ser(50)) was significantly phosphorylated by Akt in vitro. To determine whether PTP1B is a substrate for Akt in intact cells, NIH-3T3(IR) cells transfected with either wild-type PTP1B or PTP1B-S50A were labeled with [(32)P]-orthophosphate. Insulin stimulation caused a significant increase in phosphorylation of wild-type PTP1B that could be blocked by pretreatment of cells with wortmannin or cotransfection of a dominant inhibitory Akt mutant. Similar results were observed with endogenous PTP1B in untransfected HepG2 cells. Cotransfection of constitutively active Akt caused robust phosphorylation of wild-type PTP1B both in the absence and presence of insulin. By contrast, PTP1B-S50A did not undergo phosphorylation in response to insulin. We tested the functional significance of phosphorylation at Ser(50) by evaluating insulin receptor autophosphorylation in transfected Cos-7 cells. Insulin treatment caused robust receptor autophosphorylation that could be substantially reduced by coexpression of wild-type PTP1B. Similar results were obtained with coexpression of PTP1B-S50A. However, under the same conditions, PTP1B-S50D had an impaired ability to dephosphorylate the insulin receptor. Moreover, cotransfection of constitutively active Akt significantly inhibited the ability of wild-type PTP1B, but not PTP1B-S50A, to dephosphorylate the insulin receptor. We conclude that PTP1B is a novel substrate for Akt and that phosphorylation of PTP1B by Akt at Ser(50) may negatively modulate its phosphatase activity creating a positive feedback mechanism for insulin signaling.     

Role and mechanism of homocysteine in affecting hepatic protein-tyrosine phosphatase 1B

BackgroundElevated homocysteine is epidemiologically related to insulin resistance. Protein-tyrosine phosphatase 1B (PTP1B) is a negative regulator of insulin signaling. However, the effect of homocysteine on PTP1B remains unclear.MethodsS-homocysteinylated PTP1B was identified by LC-ESI-MS/MS. The ability of thioredoxin system to recover active PTP1B from S-homocysteinylated PTP1B was confirmed by RNA interference. To address the mechanism for homocysteine to affect PTP1B activity, we performed 5-IAF insertion, activity assays, Western blotting, co-immunoprecipitation and glucose uptake experiments.ResultsThe thiol-containing form of homocysteine (HcySH) suppressed phosphorylation of insulin receptor-β subunit, but enhanced PTP1B activity. This phenomenon was partially related to the fact that HcySH promoted PTP1B expression. Although the disulfide-bonded form of homocysteine (HSSH) modified PTP1B to form an inactive S-homocysteinylated PTP1B, HcySH-induced increase in the activities of cellular thioredoxin and thioredoxin reductase, components of thioredoxin system, could recover active PTP1B from S-homocysteinylated PTP1B. Thioredoxin system transferred electrons from NADPH to S-homocysteinylated PTP1B, regenerating active PTP1B in vitro and in hepatocytes. The actions of HcySH were also related with decrease in hepatic glucose uptake.ConclusionsThe effect of HcySH/HSSH on PTP1B activity depends, at least partially, on the ratio of active PTP1B and S-homocysteinylated PTP1B. High HcySH-induced an increase in thioredoxin system activity is beneficial to de-S-homocysteinylation and is good for PTP1B activity.General significanceOur data provide a novel insight into post-translational regulation of PTP1B, and expand the biological functions of thioredoxin system.     

Liver-specific protein-tyrosine phosphatase 1B (PTP1B) re-expression alters glucose homeostasis of PTP1B-/-mice

Protein-tyrosine phosphatase 1B (PTP1B) is an important negative regulator of insulin and leptin signaling in vivo. Mice lacking PTP1B (PTP1B-/- mice) are hyper-responsive to insulin and leptin and resistant to diet-induced obesity. The tissue(s) that mediate these effects of global PTP1B deficiency remain controversial. We exploited the high degree of hepatotropism of adenoviruses to assess the role of PTP1B in the liver. Liver-specific re-expression of PTP1B in PTP1B-/- mice led to marked attenuation of their enhanced insulin sensitivity. This correlated with, and was probably caused by, decreased insulin-stimulated tyrosyl phosphorylation of the insulin receptor (IR) and IR substrate 2-associated phosphatidylinositide 3-kinase activity. Analysis using phospho-specific antibodies for the IR revealed preferential dephosphorylation of Tyr-1162/1163 compared with Tyr-972 by PTP1B in vivo. Our findings show that the liver is a major site of the peripheral action of PTP1B in regulating glucose homeostasis.     

Regulation of brown fat adipogenesis by protein tyrosine phosphatase 1B

Background

Protein-tyrosine phosphatase 1B (PTP1B) is a physiological regulator of insulin signaling and energy balance, but its role in brown fat adipogenesis requires additional investigation.

Methodology/Principal Findings

To precisely determine the role of PTP1B in adipogenesis, we established preadipocyte cell lines from wild type and PTP1B knockout (KO) mice. In addition, we reconstituted KO cells with wild type, substrate-trapping (D/A) and sumoylation-resistant (K/R) PTP1B mutants, then characterized differentiation and signaling in these cells. KO, D/A- and WT-reconstituted cells fully differentiated into mature adipocytes with KO and D/A cells exhibiting a trend for enhanced differentiation. In contrast, K/R cells exhibited marked attenuation in differentiation and lipid accumulation compared with WT cells. Expression of adipogenic markers PPARγ, C/EBPα, C/EBPδ, and PGC1α mirrored the differentiation pattern. In addition, the differentiation deficit in K/R cells could be reversed completely by the PPARγ activator troglitazone. PTP1B deficiency enhanced insulin receptor (IR) and insulin receptor substrate 1 (IRS1) tyrosyl phosphorylation, while K/R cells exhibited attenuated insulin-induced IR and IRS1 phosphorylation and glucose uptake compared with WT cells. In addition, substrate-trapping studies revealed that IRS1 is a substrate for PTP1B in brown adipocytes. Moreover, KO, D/A and K/R cells exhibited elevated AMPK and ACC phosphorylation compared with WT cells.

Conclusions

These data indicate that PTP1B is a modulator of brown fat adipogenesis and suggest that adipocyte differentiation requires regulated expression of PTP1B.     

Reduction of PTP1B induces differential expression of PI3-kinase (p85alpha) isoforms

Protein tyrosine phosphatase 1B (PTP1B) inhibition increases insulin sensitivity and normalizes blood glucose levels in animals. The molecular events associated with PTP1B inhibition that increase insulin sensitivity remain controversial. Insulin resistant, diabetic ob/ob mice, dosed with PTP1B antisense for 3 weeks exhibited a decrease in PTP1B protein levels and a change in the expression level of p85alpha isoforms in liver, characterized by a reduction in p85alpha and an upregulation of the p50alpha and p55alpha isoforms. Transfection of mouse hepatocytes with PTP1B antisense caused a downregulation PTP1B and p85alpha protein levels. Furthermore, transfection of mouse hepatocytes with PTP1B siRNA downregulated p85alpha protein expression and enhanced insulin-induced PKB phosphorylation. Treatment of mouse hepatocytes with p85alpha antisense oligonucleotide caused a reduction of p85alpha and an increase in p50alpha and p55alpha isoforms and enhanced insulin-stimulated PKB activation. These results demonstrate that PTP1B inhibition causes a direct differential regulation of p85alpha isoforms of PI3-kinase in liver and that reduction of p85alpha may be one mechanism by which PTP1B inhibition improves insulin sensitivity and glucose metabolism in insulin-resistant states.     

Insulin stimulates tyrosine phosphorylation and inactivation of protein-tyrosine phosphatase 1B in vivo

Protein-tyrosine phosphatase (PTP) 1B has been implicated in negative regulation of insulin action, although little is known of the ability of insulin to regulate PTP1B itself. The ability of insulin to regulate phosphorylation and activation of PTP1B was probed in vivo. Challenge with insulin in vivo provoked a transient, sharp increase in the phosphotyrosine content of PTP1B in fat and skeletal muscle that peaked within 15 min. Insulin stimulated a decline of 60--70% in PTP1B activity. In mouse adipocytes, the inhibition of PTP1B activity and increased tyrosine phosphorylation of the enzyme were blocked by the insulin receptor tyrosine kinase inhibitor AG1024. Phosphoserine content of PTP1B declined in response to insulin stimulation. Elevation of intracellular cyclic AMP provokes a sharp increase in PTP1B activity and leads to increased phosphorylation of serine residues and decreased tyrosine phosphorylation. Suppression of cyclic AMP levels or inhibition of protein kinase A leads to a sharp decline in PTP1B activity, a decrease in phosphoserine content, and an increase in PTP1B phosphotyrosine content. PTP1B appears to be a critical point for insulin and catecholamine counter-regulation.     

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.     

Impaired Integrin-mediated Adhesion and Signaling in Fibroblasts Expressing a Dominant-negative Mutant PTP1B

To investigate the role of nonreceptor protein tyrosine phosphatase 1B (PTP1B) in β1-integrin– mediated adhesion and signaling, we transfected mouse L cells with normal and catalytically inactive forms of the phosphatase. Parental cells and cells expressing the wild-type or mutant PTP1B were assayed for (a) adhesion, (b) spreading, (c) presence of focal adhesions and stress fibers, and (d) tyrosine phosphorylation. Parental cells and cells expressing wild-type PTP1B show similar morphology, are able to attach and spread on fibronectin, and form focal adhesions and stress fibers. In contrast, cells expressing the inactive PTP1B have a spindle-shaped morphology, reduced adhesion and spreading on fibronectin, and almost a complete absence of focal adhesions and stress fibers. Attachment to fibronectin induces tyrosine phosphorylation of focal adhesion kinase (FAK) and paxillin in parental cells and cells transfected with the wild-type PTP1B, while in cells transfected with the mutant PTP1B, such induction is not observed. Additionally, in cells expressing the mutant PTP1B, tyrosine phosphorylation of Src is enhanced and activity is reduced. Lysophosphatidic acid temporarily reverses the effects of the mutant PTP1B, suggesting the existence of a signaling pathway triggering focal adhesion assembly that bypasses the need for active PTP1B. PTP1B coimmunoprecipitates with β1-integrin from nonionic detergent extracts and colocalizes with vinculin and the ends of actin stress fibers in focal adhesions. Our data suggest that PTP1B is a critical regulatory component of integrin signaling pathways, which is essential for adhesion, spreading, and formation of focal adhesions.     

Protein Tyrosine Phosphatases as Targets of the Combined Insulinomimetic Effects of Zinc and Oxidants

Zinc ions have an insulin-like (insulinomimetic) effect. A particularly sensitive target of zinc ions is protein tyrosine phosphatase 1B (PTP 1B), a key regulator of the phosphorylation state of the insulin receptor. Modulation of insulin signaling by zinc chelating agents and the recognition of temporal and spatial fluctuations of zinc suggest a physiological role of zinc in insulin signal transduction. Tyrosine phosphatases seem to be regulated jointly by insulin-induced redox (hydrogen peroxide) signaling, which results in their oxidative inactivation, and by their zinc inhibition after oxidative zinc release from other proteins. In␣diabetes, the significant oxidative stress and associated changes in zinc metabolism modify the cell’s response and sensitivity to insulin. Zinc deficiency activates stress pathways and may result in a loss of tyrosine phosphatase control, thereby causing insulin resistance.     

Ursolic acid and its derivative inhibit protein tyrosine phosphatase 1B, enhancing insulin receptor phosphorylation and stimulating glucose uptake

Protein tyrosine phosphatase 1B (PTP1B) is a key element in the negative regulation of the insulin signaling pathway and may play an important role in diabetes and obesity. We identified ursolic acid, a natural pentacyclic triterpenoid that occurs widely in traditional Chinese medicinal herbs, as an inhibitor of PTP1B by screening an extract library of the traditional Chinese medicinal herbs used a diabetes clinic. By modifying urosolic acid, we designed and synthesized a derivative with a K(i) of 283 nM. As competitive inhibitors of PTP1B, ursolic acid and its derivative also inhibit T-cell protein tyrosine phosphatase and src homology phosphatase-2 but not leucocyte antigen-related phosphatase or protein tyrosine phosphatase alpha and epsilon, which are all possibly involved in the insulin pathway. The ursolic acid derivative enhanced insulin receptor phosphorylation in CHO/hIR cells and stimulate glucose uptake in L6 myotubes.     

Protein-tyrosine phosphatase-1B negatively regulates insulin signaling in l6 myocytes and Fao hepatoma cells

Insulin signaling is regulated by tyrosine phosphorylation of the signaling molecules, such as the insulin receptor and insulin receptor substrates (IRSs). Therefore, the balance between protein-tyrosine kinases and protein-tyrosine phosphatase activities is thought to be important in the modulation of insulin signaling in insulin-resistant states. We thus employed the adenovirus-mediated gene transfer technique, and we analyzed the effect of overexpression of a wild-type protein-tyrosine phosphatase-1B (PTP1B) on insulin signaling in both L6 myocytes and Fao cells. In both cells, PTP1B overexpression blocked insulin-stimulated tyrosine phosphorylation of the insulin receptor and IRS-1 by more than 70% and resulted in a significant inhibition of the association between IRS-1 and the p85 subunit of phosphatidylinositol 3-kinase and Akt phosphorylation as well as mitogen-activated protein kinase phosphorylation. Moreover, insulin-stimulated glycogen synthesis was also inhibited by PTP1B overexpression in both cells. These effects were specific for insulin signaling, because platelet-derived growth factor (PDGF)-stimulated PDGF receptor tyrosine phosphorylation and Akt phosphorylation were not inhibited by PTP1B overexpression. The present findings demonstrate that PTP1B negatively regulates insulin signaling in L6 and Fao cells, suggesting that PTP1B plays an important role in insulin resistance in muscle and liver.     

Modulation of cellular insulin signaling and PTP1B effects by lipid metabolites in skeletal muscle cells

Normal glucose regulation is achieved by having adequate insulin secretion and effective glucose uptake/disposal. Excess lipids in peripheral tissues — skeletal muscle, liver and adipose tissue — may attenuate insulin signaling through the protein kinase B (AKt) pathway and up-regulate protein tyrosine phosphatase 1B (PTP1B), a negative regulator of insulin signaling. We studied accumulation of lipid metabolites [triglycerides (TAGs), diglycerides (DAGs)] and ceramides in relation to insulin signaling and expression and phosphorylation of PTP1B by preincubating rat skeletal muscle cells (L6 myotubes) with three saturated and three unsaturated free fatty acids (FFAs) (200 μM). Cells were also evaluated in the presence of wortmannin, an inhibitor of phosphatidylinositol 3-kinases and thus AKt (0–100 nM). Unsaturated FFAs increased DAGs, TAGs and PTP1B expression significantly, but cells remained insulin sensitive as assessed by robust AKt and PTP1B phosphorylation at serine (Ser) 50, Ser 398 and tyrosine 152. Saturated palmitic and stearic acids increased ceramides, up-regulated PTP1B, and had AKt and PTP1B phosphorylation at Ser 50 impaired. We show a significant correlation between phosphorylation levels of AKt and of PTP1B at Ser 50 (R²=0.84, P<.05). The same was observed with increasing wortmannin dose (R²=0.73, P<.05). Only FFAs that increased ceramides caused impairment of AKt and PTP1B phosphorylation at Ser 50. PTP1B overexpression in the presence of excess lipids may not directly cause insulin resistance unless it is accompanied by decreased PTP1B phosphorylation. A clear relationship between PTP1B phosphorylation levels at Ser 50 and its negative effect on insulin signaling is shown.     

Modulation of protein tyrosine phosphatase 1B by erythropoietin in UT-7 cell line.

BACKGROUND/ AIMS: Since the reversible phosphorylation of tyrosyl residues is a critical event in cellular signaling pathways activated by erythropoietin (Epo), attention has been focused on protein tyrosine phosphatases (PTPs) and their coordinated action with protein tyrosine kinases. The prototypic member of the PTP family is PTP1B, a widely expressed non-receptor PTP located both in cytosol and intracellular membranes via its hydrophobic C-terminal targeting sequence. PTP1B has been implicated in the regulation of signaling pathways involving tyrosine phosphorylation induced by growth factors, cytokines, and hormones, such as the downregulation of erythropoietin and insulin receptors. However, little is known about which factor modulates the activity of this enzyme. METHODS: The effect of Epo on PTP1B expression was studied in the UT-7 Epo-dependent cell line. PTP1B expression was analyzed under different conditions by Real-Time PCR and Western blot, while PTP1B phosphatase activity was determined by a p-nitrophenylphosphate hydrolysis assay. RESULTS: Epo rapidly induced an increased expression of PTP1B which was associated with higher PTP1B tyrosine phosphorylation and phosphatase activity. The action of Epo on PTP1B induction involved Janus Kinase 2 (JAK2) and Phosphatidylinositol-3 kinase (PI3K). CONCLUSION: The results allow us to suggest for the first time that, besides modulating Epo/Epo receptor signaling, PTP1B undergoes feedback regulation by Epo.     

A bromopyrrole-containing diterpene alkaloid from the Okinawan marine sponge Agelas nakamurai activates the insulin pathway in Huh-7 human hepatoma cells by inhibiting protein tyrosine phosphatase 1B

Agelasine G (1), a known bromine-containing diterpene alkaloid, was isolated as a new type of protein tyrosine phosphatase (PTP) 1B inhibitor together with ageline B (2), an inactive debromo-derivative of 1, from the marine sponge Agelas nakamurai collected at Iriomote Island in Okinawa, Japan. Further biological evaluations revealed that compound 1 exhibited selective inhibitory activity against PTP1B over T-cell PTP and CD45 phosphatase. Compound 1 also enhanced the insulin-stimulated phosphorylation levels of Akt in Huh-7 cells more strongly than compound 2. The results obtained in this study suggest that compound 1 activates the insulin signaling pathway by inhibiting PTP1B activity.