Shp-1 mediates the antiproliferative activity of tissue inhibitor of metalloproteinase-2 in human microvascular endothelial cells

The tissue inhibitors of metalloproteinases (TIMPs) regulate matrix metalloproteinase activity required for cell migration/invasion associated with cancer progression and angiogenesis. TIMPs also modulate cell proliferation in vitro and angiogenesis in vivo independent of their matrix metalloproteinase inhibitory activity. Here, we show that TIMP-2 mediates G1 growth arrest in human endothelial cells through de novo synthesis of the cyclin-dependent kinase inhibitor p27Kip1. TIMP-2-mediated inhibition of Cdk4 and Cdk2 activity is associated with increased binding of p27Kip1 to these complexes in vivo. Protein-tyrosine phosphatase inhibitors or expression of a dominant negative Shp-1 mutant ablates TIMP-2 induction of p27Kip1. Finally, angiogenic responses to fibroblast growth factor-2 and vascular endothelial growth factor-A in "motheaten viable" Shp-1-deficient mice are resistant to TIMP-2 inhibition, demonstrating that Shp-1 is an important negative regulator of angiogenesis in vivo.     

Structural and functional uncoupling of the enzymatic and angiogenic inhibitory activities of tissue inhibitor of metalloproteinase-2 (TIMP-2): loop 6 is a novel angiogenesis inhibitor

Tissue inhibitors of metalloproteinases (TIMPs) regulate tumor growth, progression, and angiogenesis in a variety of experimental cancer models and in human malignancies. Results from numerous studies have revealed important differences between TIMP family members in their ability to inhibit angiogenic processes in vitro and angiogenesis in vivo despite their universal ability to inhibit matrix metalloproteinase (MMP) activity. To address these differences, a series of structure-function studies were conducted to identify and to characterize the anti-angiogenic domains of TIMP-2, the endogenous MMP inhibitor that uniquely inhibits capillary endothelial cell (EC) proliferation as well as angiogenesis in vivo. We demonstrate that the COOH-terminal domain of TIMP-2 (T2C) inhibits the proliferation of capillary EC at molar concentrations comparable with those previously reported for intact TIMP-2, while the NH2-terminal domain (T2N), which inhibits MMP activity, has no significant anti-proliferative effect. Interestingly, although both T2N and T2C inhibited embryonic angiogenesis, only T2C resulted in the potent inhibition of angiogenesis driven by the exogenous addition of angiogenic mitogen, suggesting that MMP inhibition alone may not be sufficient to inhibit the aggressive neovascularization characteristic of aberrant angiogenesis. We further mapped the anti-proliferative activity of T2C to a 24-amino acid peptide corresponding to Loop 6 of TIMP-2 and show that Loop 6 is a potent inhibitor of both embryonic and mitogen-stimulated angiogenesis in vivo. These findings demonstrate that TIMP-2 possesses two distinct types of anti-angiogenic activities which can be uncoupled from each other, the first represented by its MMP-dependent inhibitory activity which can inhibit only embryonic neovascularization and the second represented by an MMP-independent activity which inhibits both normal angiogenesis and mitogen-driven angiogenesis in vivo. In addition, we report, for the first time, the discovery of Loop 6 as a novel and potent inhibitor of angiogenesis.     

Tissue inhibitor of metalloproteinases-2 (TIMP-2) suppresses TKR-growth factor signaling independent of metalloproteinase inhibition

The tissue inhibitors of metalloproteinases (TIMPs) block matrix metalloproteinase (MMP)-mediated increases in cell proliferation, migration, and invasion that are associated with extracellular matrix (ECM) turnover. Here we demonstrate a direct role for TIMP-2 in regulating tyrosine kinase-type growth factor receptor activation. We show that TIMP-2 suppresses the mitogenic response to tyrosine kinase-type receptor growth factors in a fashion that is independent of MMP inhibition. The TIMP-2 suppression of mitogenesis is reversed by the adenylate cyclase inhibitor SQ22536, and implicates cAMP as the second messenger in these effects. TIMP-2 neither altered the release of transforming growth factor alpha from the cell surface, nor epidermal growth factor (EGF) binding to the cognate receptor, EGFR. TIMP-2 binds to the surface of A549 cells in a specific and saturable fashion (K(d) = 147 pm), that is not competed by the synthetic MMP inhibitor BB-94 and is independent of MT-1-MMP. TIMP-2 induces a decrease in phosphorylation of EGFR and a concomitant reduction in Grb-2 association. TIMP-2 prevents SH2-protein-tyrosine phosphatase-1 (SHP-1) dissociation from immunoprecipitable EGFR complex and a selective increase in total SHP-1 activity. These studies represent a new functional paradigm for TIMP-2 in which TIMP suppresses EGF-mediated mitogenic signaling by short-circuiting EGFR activation.     

Tissue inhibitor of metalloproteinase-2 (TIMP-2) regulates myogenesis and beta1 integrin expression in vitro

Myogenesis in vitro involves myoblast cell cycle arrest, migration, and fusion to form multinucleated myotubes. Extracellular matrix (ECM) integrity during these processes is maintained by the opposing actions of matrix metalloproteinase (MMP) proteases and their inhibitors, the tissue inhibitor of metalloproteinases (TIMPs). Here, we report that TIMP-2, MMP-2, and MT1-MMP are differentially expressed during mouse myoblast differentiation in vitro. A specific role for TIMP-2 in myogenesis is demonstrated by altered TIMP-2(-/-) myotube formation. When differentiated in horse serum-containing medium, TIMP-2(-/-) myotubes are larger than wild-type myotubes. In contrast, when serum-free medium is used, TIMP-2(-/-) myotubes are smaller than wild-type myotubes. Regardless of culture condition, myotube size is directly correlated with MMP activity and inversely correlated with beta1 integrin expression. Treatment with recombinant TIMP-2 rescues reduced TIMP-2(-/-) myotube size and induces increased MMP-9 activation and decreased beta1 integrin expression. Treatment with either MMP-2 or MMP-9 similarly rescues reduced myotube size, but has no effect on beta1 integrin expression. These data suggest a specific regulatory relationship between TIMP-2 and beta1 integrin during myogenesis. Elucidating the role of TIMP-2 in myogenesis in vitro may lead to new therapeutic options for the use of TIMP-2 in myopathies and muscular dystrophies in vivo.     

Tissue inhibitor of metalloproteinases-2 inhibits bFGF-induced human microvascular endothelial cell proliferation

Tissue inhibitor of metalloproteinase-2 (TIMP-2), a protease inhibitor that binds to the latent and active forms of 72 kDa type IV collagenase (gelatinase A), was found to inhibit the in vitro proliferation of human microvascular endothelial (HME) cells stimulated with bFGF and 5% serum. The maximal inhibitory effect of TIMP-2 on incorporation of ³H-thymidine was evident 24 hours after bFGF stimulation of these cells and ranged between 45 and 60%. The half-maximal effective concentration of TIMP-2 was 107 ± 12 nM (S.D.). In contrast, TIMP-1 was not found to slow the growth of HME cells. The inhibition of cell proliferation observed with TIMP-2 was not mimicked by addition to the culture medium of BB94, a general matrix metalloproteinase inhibitor, nor antibodies to the 72 kDa type IV collagenase. In addition to growth, two other cell functions associated with the angiogenic process were tested for sensitivity to TIMP-2. Cell adhesion to tissue culture platic was slightly stimulated by TIMP-2, and cell migration was inhibited with short-term exposure to TIMP-2, but neither process was affected by longer-term exposure. The ability of TIMP-2 to inhibit cultured endothelial cell proliferation independent of protease inhibitory activity suggests that TIMP-2 may have additional actions which may limit neovascularization associated with solid tumor growth and metastasis in vivo. © 1993 Wiley-Liss, Inc.     

Functional proteomics study reveals that N-Acetylglucosaminyltransferase V reinforces the invasive/metastatic potential of colon cancer through aberrant glycosylation on tissue inhibitor of metalloproteinase-1

N-Acetylglucosaminyltransferase-V (GnT-V) has been reported to be up-regulated in invasive/metastatic cancer cells, but a comprehensive understanding of how the transferase correlates with the invasive/metastatic potential is not currently available. Through a glycomics approach, we identified 30 proteins, including tissue inhibitor of metalloproteinase-1 (TIMP-1), as a target protein for GnT-V in human colon cancer cell WiDr. TIMP-1 was aberrantly glycosylated as characterized by the addition of beta1,6-N-acetylglucosamine, polylactosaminylation, and sialylation in GnT-V-overexpressing WiDr cells. Compared with normal TIMP-1, the aberrantly glycosylated TIMP-1 showed the weaker inhibition on both matrix metalloproteinase (MMP)-2 and MMP-9, and this aberrancy was closely associated with cancer cell invasion and metastasis in vivo as well as in vitro. Integrated data, both of TIMP-1 expression level and aberrant glycosylation, could provide important information to aid to improve the clinical outcome of colon cancer patients.     

Modulation of angiogenesis by tissue inhibitor of metalloproteinase-4

Despite the importance of MMP activity in the regulation of angiogenesis, relatively little is known about the role of TIMP-4, the most recently discovered endogenous MMP inhibitor, in modulating neovascularization. It has largely been assumed that all TIMPs are capable of inhibiting angiogenesis in vivo. However, it is now widely appreciated that TIMPs-1, -2, and -3 differ significantly in their ability to modulate angiogenic processes in vitro and angiogenesis in vivo. In order to study the effect of TIMP-4 in controlling angiogenesis, we have cloned and expressed TIMP-4 in a Pichia pastoris expression system, purified it to homogeneity, and tested its ability to regulate angiogenesis in vivo and in vitro. Our studies demonstrate that TIMP-4 is an inhibitor of capillary endothelial cell migration, but not of proliferation or of angiogenesis in vivo.     

A Cdc25A antagonizing K vitamin inhibits hepatocyte DNA synthesis in vitro and in vivo

Thioalkyl containing K vitamin analogs have been shown to be potent inhibitors of hepatoma cell growth and antagonizers of protein tyrosine phosphatase activity. We now show that they inhibit the activity of specific protein tyrosine phosphatases (PTP) in cell-free conditions in vitro, particularly the dual specificity phosphatase Cdc25A. Using primary cultures of adult rat hepatocytes that are in G0/G1 phase until stimulated into DNA synthesis by epidermal growth factor, we found that 2-(2-mercaptoethanol)-3-methyl-1,4-naphthoquinone or Compound 5 (Cpd 5) inhibited hepatocyte DNA synthesis and PTP activity in cell culture and in vivo after a two-thirds partial hepatectomy. We found a selective inhibition of Cdc25A activity in vitro, using both synthetic substrates and authentic cellular substrate, immunoprecipitated phospho-Cdk4. Intact Cpd 5-treated cells had decreased cellular Cdc25A activity and increased tyrosine phosphorylation of Cdk4, resulting in decreased phosphorylation of retinoblastoma (Rb). Loss of Cdk4 activity was confirmed using Cdk4 immunoprecipitates from either Cpd 5-treated or untreated cells and measuring its kinase activity using GST-Rb as target. We found a similar order of activity for inhibition of growth and Cdc25A activity using several thiol-containing analogs. Cdc25A inhibitors may thus be useful for defining biochemical pathways involving protein tyrosine phosphorylation that mediate cell growth inhibition.     

Exogenously added GPI-anchored tissue inhibitor of matrix metalloproteinase-1 (TIMP-1) displays enhanced and novel biological activities

The family of tissue inhibitors of metalloproteinases (TIMPs) exhibits diverse physiological/biological functions including the inhibition of active matrix metalloproteinases, regulation of proMMP activation, cell growth, and the modulation of angiogenesis. TIMP-1 is a secreted protein that can be detected on the cell surface through its interaction with surface proteins. The diverse biological functions of TIMP-1 are thought to lie, in part, in the kinetics of TIMP-1/MMP/surface protein interactions. Proteins anchored by glycoinositol phospholipids (GPIs), when purified and added to cells in vitro, are incorporated into their surface membranes. A GPI anchor was fused to TIMP-1 to generate a reagent that could be added directly to cell membranes and thus focus defined concentrations of TIMP-1 protein on any cell surface independent of protein-protein interaction. Unlike native TIMP-1, exogenously added GPI-anchored TIMP-1 protein effectively blocked release of MMP-2 and MMP-9 from osteosarcoma cells. TIMP-1-GPI was a more effective modulator of migration and proliferation than TIMP-1. While control hTIMP-1 protein did not significantly affect migration of primary microvascular endothelial cells at the concentrations tested, the GPI-anchored TIMP-1 protein showed a pronounced suppression of endothelial cell migration in response to bFGF. In addition, TIMP-1-GPI was more effective at inducing microvascular endothelial proliferation. In contrast, fibroblast proliferation was suppressed by the agent. Reagents based on this method should assist in the dissection of the protease cascades and activities involved in TIMP biology. Membrane-fixed TIMP-1 may represent a more effective version of the protein for use in therapeutic expression.     

Chrysin restores PDGF-induced inhibition on protein tyrosine phosphatase and reduces PDGF signaling in cultured VSMCs

Previous studies have shown that an increased intake of dietary flavonoids is associated with a decreased risk of cardiovascular diseases (CVDs). PDGF is a major mitogen for vascular smooth muscle cell (VSMC) and participates in the pathogenesis of many CVDs. The study investigated whether the flavone chrysin affected PDGF functions in VSMCs and neointma formation in rat artery. We found that chrysin concentration-dependently inhibited PDGF-induced proliferation and chemotaxis and reduced PDGF signaling in VSMCs. Chrysin attenuated H(2)O(2) signaling and PDGF-induced reactive oxygen species production and NADPH oxidase activation but did not interfere with PDGF binding to VSMCs. The further analyses revealed that chrysin relieved PDGF-induced inhibition on activity of protein tyrosine phosphatase (PTP) and reduced PDGF-induced oxidation of PTP cysteinyl active site. Moreover, it inhibited PDGF receptor autophosphorylation induced by low-dose vanadate (an inhibitor for PTP). The effect of chrysin, but not of the flavonoid (-)-epigallocatechin-3-gallate and antioxidant N-acetylcysteine, on PDGF signaling and PTP activity was reversed by depletion of intracellular glutathione (GSH), suggesting an involvement of chrysin on GSH/glutaredoxin system for PTP reactivation. Finally, to demonstrate the effectiveness of chrysin in vivo, we showed that oral administration of chrysin before and after angioplasty could reduce neointima formation in balloon-injured carotid artery in rats. In conclusion, we provide here evidence that chrysin can regulate intracellular PTP activity during PDGF signaling, inhibits PDGF-induced VSMC proliferation and chemotaxis, and reduces arterial intima hyperplasia in vivo.     

VCAM-1 activation of endothelial cell protein tyrosine phosphatase 1B

Lymphocytes migrate from the blood into tissue by binding to and migrating across endothelial cells. One of the endothelial cell adhesion molecules that mediate lymphocyte binding is VCAM-1. We have reported that binding to VCAM-1 activates endothelial cell NADPH oxidase for the generation of reactive oxygen species (ROS). The ROS oxidize and stimulate an increase in protein kinase C (PKC)alpha activity. Furthermore, these signals are required for VCAM-1-dependent lymphocyte migration. In this report, we identify a role for protein tyrosine phosphatase 1B (PTP1B) in the VCAM-1 signaling pathway. In primary cultures of endothelial cells and endothelial cell lines, Ab cross-linking of VCAM-1 stimulated an increase in serine phosphorylation of PTP1B, the active form of PTP1B. Ab cross-linking of VCAM-1 also increased activity of PTP1B. This activation of PTP1B was downstream of NADPH oxidase and PKCalpha in the VCAM-1 signaling pathway as determined with pharmacological inhibitors and antisense approaches. In addition, during VCAM-1 signaling, ROS did not oxidize endothelial cell PTP1B. Instead PTP1B was activated by serine phosphorylation. Importantly, inhibition of PTP1B activity blocked VCAM-1-dependent lymphocyte migration across endothelial cells. In summary, VCAM-1 activates endothelial cell NADPH oxidase to generate ROS, resulting in oxidative activation of PKCalpha and then serine phosphorylation of PTP1B. This PTP1B activity is necessary for VCAM-1-dependent transendothelial lymphocyte migration. These data show, for the first time, a function for PTP1B in VCAM-1-dependent lymphocyte migration.     

Sodium stibogluconate is a potent inhibitor of protein tyrosine phosphatases and augments cytokine responses in hemopoietic cell lines.

Using in vitro protein tyrosine phosphatase (PTPase) assays, we found that sodium stibogluconate, a drug used in treatment of leishmaniasis, is a potent inhibitor of PTPases Src homology PTPase1 (SHP-1), SHP-2, and PTP1B but not the dual-specificity phosphatase mitogen-activated protein kinase phosphatase 1. Sodium stibogluconate inhibited 99% of SHP-1 activity at 10 micrograms/ml, a therapeutic concentration of the drug for leishmaniasis. Similar degrees of inhibition of SHP-2 and PTP1B required 100 micrograms/ml sodium stibogluconate, demonstrating differential sensitivities of PTPases to the inhibitor. The drug appeared to target the SHP-1 domain because it showed similar in vitro inhibition of SHP-1 and a mutant protein containing the SHP-1 PTPase domain alone. Moreover, it forms a stable complex with the PTPase: in vitro inhibition of SHP-1 by the drug was not removed by a washing process effective in relieving the inhibition of SHP-1 by the reversible inhibitor suramin. The inhibition of cellular PTPases by the drug was suggested by its rapid induction of tyrosine phosphorylation of cellular proteins in Baf3 cells and its augmentation of IL-3-induced Janus family kinase 2/Stat5 tyrosine phosphorylation and proliferation of Baf3 cells. The augmentation of the opposite effects of GM-CSF and IFN-alpha on TF-1 cell growth by the drug indicated its broad activities in the signaling of various cytokines. These data represent the first evidence that sodium stibogluconate inhibits PTPases and augments cytokine responses. Our results provide novel insights into the pharmacological effects of the drug and suggest potential new therapeutic applications.     

Tyrosine phosphatase PTP1B modulates store-operated calcium influx

We have studied modulation of “store-operated calcium influx” by tyrosine phosphatases in the pancreatic acinar cell line AR42J and in HEK 293 cells. We show that inhibition of tyrosine phosphatases by bis-(N,N-dimethyl-hydroxamido) hydrooxovanadate (DMHV) leads to an increase in Ca²⁺ release-activated Ca²⁺ (CRAC) entry. This effect can be blocked in the presence of 2-aminoethyldiphenyl borate (2-APB). Furthermore, transfection of HEK 293 cells with the human wild-type tyrosine phosphatase PTP1B leads to inhibition of CRAC influx, whereas transfection with the substrate-trapping mutant of PTP1B (D181A) slightly increases Ca²⁺ influx. It also decreases enzymatic activity of PTP1B as compared to non-transfected cells. Our data suggest that CRAC influx is modulated by tyrosine phosphorylation and dephosphorylation which involves the tyrosine phosphatase PTP1B.     

Glycogen synthase kinase-3β regulates Tyr307 phosphorylation of protein phosphatase-2A via protein tyrosine phosphatase 1B but not Src

GSK-3β (glycogen synthase kinase-3β), a crucial tau kinase, negatively regulates PP2A (protein phosphatase 2A), the most active tau phosphatase that is suppressed in the brain in AD (Alzheimer's disease). However, the molecular mechanism is not understood. In the present study we found that activation of GSK-3β stimulates the inhibitory phosphorylation of PP2A at Tyr307 (pY307-PP2A), whereas inhibition of GSK-3β decreased the level of pY307-PP2A both in vitro and in vivo. GSK-3β is a serine/threonine kinase that can not phosphorylate tyrosine directly, therefore we measured PTP1B (protein tyrosine phosphatase 1B) and Src (a tyrosine kinase) activities. We found that GSK-3β can modulate both PTP1B and Src protein levels, but it only inhibits PTP1B activity, with no effect on Src. Furthermore, only knockdown of PTP1B but not Src by siRNA (small interfering RNA) eliminates the effects of GSK-3β on PP2A. GSK-3β phosphorylates PTP1B at serine residues, and activation of GSK-3β reduces the mRNA level of PTP1B. Additionally, we also observed that GSK-3 negatively regulates the protein and mRNA levels of PP2A, and knockdown of CREB (cAMP-response-element-binding protein) abolishes the increase in PP2A induced by GSK-3 inhibition. The results of the present study suggest that GSK-3β inhibits PP2A by increasing the inhibitory Tyr307 phosphorylation and decreasing the expression of PP2A, and the mechanism involves inhibition of PTP1B and CREB.     

Identification of protein-tyrosine phosphatase 1B as the major tyrosine phosphatase activity capable of dephosphorylating and activating c-Src in several human breast cancer cell lines

c-Src tyrosine kinase activity is elevated in several types of human cancer, and this has been attributed to elevated c-Src expression levels, increased c-Src specific activity, and activating mutations in c-Src. We have found a number of human breast cancer cell lines with elevated c-Src specific activity that also possess elevated phosphatase activity directed against the carboxyl-terminal negative regulatory domain of Src family kinases. To identify this phosphatase, cell extracts from MDA-MB-435S cells were chromatographed and the fractions were assayed for phosphatase activity. Four peaks of phosphatase activity directed against the nonspecific substrate poly(Glu/Tyr) were detected. One peak also dephosphorylated a peptide modeled against the c-Src carboxyl-terminal negative regulatory domain and intact human c-Src. Immunoblotting and immunodepletion experiments identified the phosphatase as protein-tyrosine phosphatase 1B (PTP1B). Examination of several human breast cancer cell lines with increased c-Src activity showed elevated levels of PTP1B protein relative to normal control breast cells. In vitro c-Src reactivation experiments confirmed the ability of PTP1B to dephosphorylate and activate c-Src. In vivo overexpression of PTP1B in 293 cells caused a 2-fold increase of endogenous c-Src kinase activity. Our findings indicate that PTP1B is the primary protein-tyrosine phosphatase capable of dephosphorylating c-Src in several human breast cancer cell lines and suggests a regulatory role for PTP1B in the control of c-Src kinase activity.     

TIMP-1 inhibits microvascular endothelial cell migration by MMP-dependent and MMP-independent mechanisms

It was reported over a decade ago that tissue inhibitor of metalloproteinases-1 (TIMP-1) suppresses angiogenesis in experimental models but the mechanism is still incompletely understood. This in vitro study focused on the molecular basis of TIMP-1-mediated inhibition of endothelial cell (EC) migration, a key step in the angiogenic process. Both recombinant human TIMP-1 and the synthetic MMP inhibitors, GM6001 and MMP-2-MMP-9 Inhibitor III, suppressed migration of human dermal microvascular endothelial cells (HDMVEC) in a dose-dependent fashion. The MMP-dependent inhibition of migration was associated with increased expression of the junctional adhesion proteins, VE-cadherin and PECAM-1, and VE-cadherin accumulation at cell-cell junctions. TIMP-1 also caused MMP-independent dephosphorylation of focal adhesion kinase (FAK) (pY397) and paxillin, which was associated with reduced number of F-actin stress fibers and focal adhesions. Moreover, TIMP-1 stimulated expression of PTEN that has been shown to reduce phosphorylation of FAK and inhibit cell migration. Our data suggest that TIMP-1 inhibits HDMVEC migration through MMP-dependent stimulation of VE-cadherin and MMP-independent stimulation of PTEN with subsequent dephosphorylation of FAK and cytoskeletal remodeling.     

Bromophenols as inhibitors of protein tyrosine phosphatase 1B with antidiabetic properties

A series of bromophenol derivatives were synthesized and evaluated as protein tyrosine phosphatase 1B (PTP1B) inhibitors in vitro and in vivo based on bromophenol 4e (IC(50)=2.42 μmol/L), which was isolated from red algae Rhodomela confervoides. The results showed that all of the synthesized compounds displayed weak to good PTP1B inhibition at tested concentration. Among them, highly brominated compound 4g exhibited promising inhibitory activity against PTP1B with IC(50) 0.68 μmol/L, which was approximately fourfold more potent than lead compound 4e. Further, compound 4g demonstrated high selectivity against other PTPs (TCPTP, LAR, SHP-1 and SHP-2). More importantly, in vivo antidiabetic activities investigations of compound 4g also demonstrated inspiring results.     

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.     

Mechanism of insulin sensitization by BMOV (bis maltolato oxo vanadium); unliganded vanadium (VO4) as the active component

Organovanadium compounds have been shown to be insulin sensitizers in vitro and in vivo. One potential biochemical mechanism for insulin sensitization by these compounds is that they inhibit protein tyrosine phosphatases (PTPs) that negatively regulate insulin receptor activation and signaling. In this study, bismaltolato oxovanadium (BMOV), a potent insulin sensitizer, was shown to be a reversible, competitive phosphatase inhibitor that inhibited phosphatase activity in cultured cells and enhanced insulin receptor activation in vivo. NMR and X-ray crystallographic studies of the interaction of BMOV with two different phosphatases, HCPTPA (human low molecular weight cytoplasmic protein tyrosine phosphatase) and PTP1B (protein tyrosine phosphatase 1B), demonstrated uncomplexed vanadium (VO(4)) in the active site. Taken together, these findings support phosphatase inhibition as a mechanism for insulin sensitization by BMOV and other organovanadium compounds and strongly suggest that uncomplexed vanadium is the active component of these compounds.