Site-specific dephosphorylation and deactivation of the human insulin receptor tyrosine kinase by particulate and soluble phosphotyrosyl protein phosphatases

A new highly efficient substrate-trapping mutant of protein tyrosine phosphatase 1B (PTP1B) reveals full autoactivation of the insulin receptor precursor

PTP1B is a protein tyrosine-phosphatase located on the cytosolic side of the endoplasmic reticulum that plays an important role in the regulation of the insulin receptor (IR). Replacement of the conserved Asp-181 by alanine is known to convert PTP1B into a substrate-trapping protein that binds to but cannot dephosphorylate its substrates. In this work, we have studied the effect of an additional mutation (Y46F) on the substrate-trapping efficiency of PTP1B-D181A. We observed that this mutation converts PTP1B-D181A into a highly efficient substrate-trapping mutant, resulting in much higher recovery of tyrosine-phosphorylated proteins coimmunoprecipitated with PTP1B. Bioluminescence resonance energy transfer (BRET) experiments were also performed to compare the dynamics of interaction of the IR with these mutants. Basal BRET, which mainly reflects the interaction of PTP1B with the IR precursor during its biosynthesis in the endoplasmic reticulum, was markedly increased with the PTP1B-D181A-Y46F mutant. In contrast, insulin-induced BRET was markedly reduced with PTP1B-D181A-Y46F. I(125) insulin binding experiments indicated that PTP1B-D181-Y46F reduced the expression of IR at the plasma membrane. Reduced expression at the cell surface was associated with higher amounts of the uncleaved IR precursor in the cell. Moreover, we observed that substantial amounts of the uncleaved IR precursor reached the Tris-phosphorylated, fully activated form in an insulin independent fashion. These results support the notion that PTP1B plays a crucial role in the control of the activity of the IR precursor during its biosynthesis. In addition, this new substrate-trapping mutant may be a valuable tool for the identification of new PTP1B substrates.

The use of bioluminescence resonance energy transfer for the study of therapeutic targets: application to tyrosine kinase receptors

During recent years, the bioluminescence resonance energy transfer (BRET) methodology has emerged as a powerful technique for the study of protein-protein interactions. This review focuses on recent work demonstrating the power of BRET for the study of tyrosine kinase receptors, using insulin and IGF-1 receptors as models. The authors show that BRET can be used to monitor ligand-induced conformational changes within homodimeric insulin and IGF-1 receptors, as well as heterodimeric insulin/IGF-1 hybrid receptors. BRET can also be used to study, in real time and in living cells, the interaction of tyrosine kinase receptors with cellular partners negatively or positively involved in the regulation of intracellular signalling (protein tyrosine phosphatases, molecular adaptors). In addition, BRET can be used to develop high-throughput screening assays for the search of molecules with therapeutic interest and could, therefore, constitute a valuable tool for laboratories involved in drug discovery.

Looking for an insulin pill? Use the BRET methodology!

Insulin exerts its biological effects through a plasma membrane receptor that possesses a tyrosine-kinase activity. This tyrosine-kinase activity depends on the autophosphorylation of the receptor on tyrosine residues and on its dephosphorylation by protein tyrosine-phosphatases. The discovery of pharmacological agents that specifically stimulate the autophosphorylation of the insulin receptor or inhibit its dephosphorylation will be of great importance for the treatment of insulin resistant or insulin deficient patients. Bioluminescence Resonance Energy Transfer (BRET) has developed in recent years as a new technique to study protein-protein interactions. In the BRET technique, one partner is fused to Renilla luciferase, whereas the other partner is fused to a fluorescent protein (e.g. YFP, Yellow Fluorescent Protein). The luciferase is excited by addition of its substrate, coelenterazine. If the two partners interact, resonance energy transfer occurs between the luciferase and the YFP, and a fluorescent signal, emitted by the YFP, can be detected. Our work indicates that this methodology could be an important tool for the search of molecules that activate insulin receptor autophosphorylation or that inhibit its dephosphorylation. Indeed, we first showed that the activation of the insulin receptor by different ligands can be monitored using a chimeric receptor with one B-subunit fused to Renilla luciferase and the other B-subunit fused to YFP. The conformational changes induced by different ligands could be detected as an energy transfer (BRET signal) between the luciferase and the YFP, that reflects the activation state of the receptor. This methodology allows for rapid analysis of the effects of agonists on insulin receptor activity and may therefore be used in high-throughput screening for the discovery of molecules with insulin-like properties. More recently, we demonstrated that the BRET methodology could also be used to monitor the interaction of the insulin receptor with protein tyrosine-phosphatase 1B, one of the main tyrosine-phosphatase that controls its activity. HEK cells were co-transfected with the insulin receptor fused to Renilla luciferase and a substrate-trapping mutant of PTP1B (PTP1B-D181A) fused to YFP. Insulin-induced BRET signal could be followed in real time for more than 30 min. Therefore, this methodology can also be used in high-throughput screening for the search of molecules that will specifically disrupt the interaction between the insulin receptor and PTP1B.

Molecular basis for the dephosphorylation of the activation segment of the insulin receptor by protein tyrosine phosphatase 1B

The protein tyrosine phosphatase PTP1B is responsible for negatively regulating insulin signaling by dephosphorylating the phosphotyrosine residues of the insulin receptor kinase (IRK) activation segment. Here, by integrating crystallographic, kinetic, and PTP1B peptide binding studies, we define the molecular specificity of this reaction. Extensive interactions are formed between PTP1B and the IRK sequence encompassing the tandem pTyr residues at 1162 and 1163 such that pTyr-1162 is selected at the catalytic site and pTyr-1163 is located within an adjacent pTyr recognition site. This selectivity is attributed to the 70-fold greater affinity for tandem pTyr-containing peptides relative to mono-pTyr peptides and predicts a hierarchical dephosphorylation process. Many elements of the PTP1B-IRK interaction are unique to PTP1B, indicating that it may be feasible to generate specific, small molecule inhibitors of this interaction to treat diabetes and obesity.

Vanadate enhances but does not normalize glucose transport and insulin receptor phosphorylation in skeletal muscle from obese women with gestational diabetes mellitus

OBJECTIVE

We compared the insulin-mimetic effects of vanadate, a protein-tyrosine phosphatase inhibitor, with the effects of insulin on skeletal muscle glucose transport and insulin receptor and insulin receptor substrate 1 phosphorylation to test the hypothesis that protein-tyrosine phosphatases participate in pregnancy-induced insulin resistance.

STUDY DESIGN

Skeletal muscle fiber strips were obtained from the rectus abdominis during cesarean delivery in 7 patients with gestational diabetes mellitus, 11 pregnant women with normal glucose tolerance (pregnant control group), and 11 nonpregnant women undergoing elective surgery (nonpregnant control group). Muscle tissues were incubated in vitro for 15 to 60 minutes with or without maximal insulin (100 nmol/L) or sodium vanadate (6 micromol/L). Insulin receptor and insulin receptor substrate 1 tyrosine phosphorylation were measured, as was 2-deoxyglucose transport. The levels of protein-tyrosine phosphatase 1B were measured by Western blot analysis.

RESULTS

Vanadate stimulated maximal 2-deoxyglucose transport more than did insulin alone in all samples (P<.05), but the value was still less in muscle tissues from pregnant control subjects and patients with gestational diabetes mellitus (P<.05). In muscle tissues from pregnant control subjects vanadate increased tyrosine phosphorylation of the insulin receptor and insulin receptor substrate 1 to levels similar to those in muscle tissues from nonpregnant control subjects. In patients with gestational diabetes mellitus vanadate increased insulin receptor and insulin receptor substrate 1 tyrosine phosphorylation, but these values remained less than in muscle tissues from nonpregnant control subjects (P<.05). Protein-tyrosine phosphatase 1B levels were not significantly different in skeletal muscles from each group.

CONCLUSION

Vanadate did not restore normal glucose transport activity during pregnancy complicated by gestational diabetes mellitus, which indicates that decreased glucose uptake is probably not caused by impaired tyrosine phosphorylation events alone. Increased serine kinase activity and impaired glucose transporter 4 translocation probably contribute to insulin signaling abnormalities associated with pregnancy, especially in patients with gestational diabetes mellitus.

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

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

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

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

Regulation of the insulin signalling pathway by cellular protein-tyrosine phosphatases

Protein-tyrosine phosphatases (PTPases) have been implicated in the physiological regulation of the insulin signalling pathway. In cellular and molecular studies, the transmembrane, receptor-type PTPase LAR and the intracellular, non-receptor enzyme PTP1B have been shown to have a direct impact on insulin action in intact cell models. Since insulin signalling can be enhanced by reducing the abundance or activity of specific PTPases, pharmaceutical agents directed at blocking the interaction between individual PTPases and the insulin receptor may have potential clinical relevance to the treatment of insulin-resistant states such as obesity and Type II diabetes mellitus.

Autophosphorylation of activation loop tyrosines regulates signaling by the TRK nerve growth factor receptor

Many receptor tyrosine kinases possess an "activation loop" containing three similarly placed tyrosine autophosphorylation sites. To examine their roles in the TRK NGF receptor, these residues (Tyr-670, Tyr-674, and Tyr-675) were mutated singly and in all combinations to phenylalanine and stably expressed in Trk-deficient PC12nnr5 cells. All mutant receptors showed significantly diminished nerve growth factor (NGF)-stimulated autophosphorylation, indicating impaired catalytic activity. NGF-induced neurite outgrowth exhibited dose-responsive behavior when transfectants were compared by relative receptor expression and exhibited a functional hierarchy: wild type > Y670F >/= Y674F >> Y675F >/= YY670/674FF = YY670/675FF >> YY674/675FF > YYY670/674/675FFF. NGF-induced tyrosine phosphorylation of Shc, ERKs, and SNT and immediate early gene inductions generally paralleled neurogenic potential. However, activation of phosphatidylinositol 3'-kinase and tyrosine phosphorylation of phospholipase Cgamma-1 was essentially abolished. The latter effect appears due to selective inability of the mutated TRKs to autophosphorylate the tyrosine residue (Tyr-785) required for binding phospholipase Cgamma-1 and indicates that the "activation loop" tyrosines participate in NGF-dependent changes in receptor conformation. Our findings stress the importance that expression levels play in assessing the consequences of receptor mutations and that all three activation loop tyrosines have roles regulating both overall and specific NGF-mediated signaling through TRK.

Phosphotyrosine specifies the phosphorylation by protein kinase CK2 of a peptide reproducing the activation loop of the insulin receptor protein tyrosine kinase

Protein kinase CK2 is a ubiquitous Ser/Thr-specific protein kinase responsible for the phosphorylation of many proteins implicated in signal transduction. It phosphorylates both threonyl and seryl residue(s) of the insulin receptor beta-subunit. In this study, a series of peptides, reproducing all the threonyl sites of the intracellular domain of the insulin receptor that display the consensus sequence for CK2, has been synthesized and used as substrate for purified rat liver CK2. The only peptide readily phosphorylated is the one reproducing the activation loop of the insulin receptor (EIYET1160DYYA), including three tyrosines (Y1158, Y1162 and Y1163) whose phosphorylation through an intermolecular autocatalytic process promotes the activation of the receptor kinase. The phosphorylation efficiency of T1160 is increased almost 20-fold if these three tyrosines are previously phosphorylated. By using variably phosphorylated peptides, the tyrosine mainly responsible for such a hierarchical phosphorylation process has been identified as Y1163. It can be concluded, from these data, that T1160 situated in the activation loop of the insulin receptor, represents an excellent target for CK2, its phosphorylation being triggered by the previous autophosphorylation of the three tyrosyl residues surrounding it, with special reference to Y1163. These data are consistent with the implication of CK2 in the regulation of the activation process of the insulin receptor protein tyrosine kinase.

A peptide-based protein-tyrosine phosphatase inhibitor specifically enhances insulin receptor function in intact cells

3S-peptide-I is a synthetic tris-sulfotyrosyl dodecapeptide corresponding to the major site of insulin receptor autophosphorylation that potently inhibits dephosphorylation of the insulin receptor in a cell-free system and in digitonin-permeabilized Chinese hamster ovary (CHO) cells overexpressing the human insulin receptors (CHO/HIRc cells) (Liotta, A. S., Kole, H. K., Fales, H. M., Roth, J., and Bernier, M. (1994) J. Biol. Chem. 269, 22996-23001). In the present study, we found that 3S-peptide-I was not capable of inhibiting dephosphorylation of the epidermal growth factor (EGF) receptors in digitonin-permeabilized CHO cells that overexpress human EGF receptors (CHO/EGF-R cells). Moreover, the addition of a N-stearyl derivative of 3S-peptide-I to intact CHO/HIRc cells caused a concentration-dependent increase in insulin-stimulated phosphorylation of the insulin receptor, with a maximum effect (approximately 2.7-fold) at 50 microM. In contrast, ligand-stimulated EGF receptor phosphorylation in CHO/EGF-R cells was not affected by the presence of stearyl 3S-peptide-I. Furthermore, treatment of CHO/HIRc cells with this N-stearyl peptide led to a significant enhancement of the insulin-induced association of phosphatidylinositol (PI) 3-kinase activity with insulin receptor substrate 1 and the activation of mitogen-activated protein kinase. However, stearyl 3S-peptide-I had no effect on the EGF-stimulated activation of PI-3-kinase and mitogen-activated protein kinase in CHO/EGF-R cells. These data indicate that this tris-sulfotyrosyl dodecapeptide selectively enhances insulin signal transduction by specifically inhibiting dephosphorylation of the insulin receptor in intact cells.

Selective down-regulation of the insulin receptor signal by protein-tyrosine phosphatases alpha and epsilon

Binding of insulin to its receptor (IR) causes rapid autophosphorylation with concomitant activation of its tyrosine kinase which transmits the signal by phosphorylating cellular substrates. The IR activity is controlled by protein-tyrosine phosphatases, but those directly involved in regulating the insulin receptor and its signaling pathways have not yet been identified. Using baby hamster kidney cells overexpressing the IR and a novel insulin-based selection principle, we established stable cell lines with functionally coupled expression of the IR and protein-tyrosine phosphatases. The two closely related protein-tyrosine phosphatases alpha and epsilon were identified as negative regulators of IR tyrosine kinase.

Insulin resistance is associated with abnormal dephosphorylation of a synthetic phosphopeptide corresponding to the major autophosphorylation sites of the insulin receptor

Insulin resistance in the ob/ob mouse model is associated with a reduction in insulin-induced protein-tyrosine phosphorylation in tissues such as liver. To ascertain whether this decrease in phosphorylation may be due to increased phosphatase activity, protein-tyrosine phosphatase (PTPase) activity was determined in particulate and soluble fractions from livers of 5- to 23-week-old ob/ob mice and age-matched lean littermates. PTPase activity was measured using a synthetic phosphopeptide, TRDIY(P)ETDY(P)Y(P)RK, as the substrate, corresponding to residues 1142 to 1153 of the insulin receptor and containing the major autophosphorylation sites of the regulatory domain. The ob/ob mice were hyperinsulinemic across all age groups, but only the youngest mice (aged 5 to 7 weeks) were hyperglycemic. Most PTPase activity was present in the liver particulate fraction and was 19% to 114% greater in ob/ob mice as compared with controls. PTPase activity in the liver soluble fraction was 26% less than control values in the youngest ob/ob mice (5 to 7 weeks), but increased with age and was 41% and 131% above control values at 21 to 23 and 25 to 27 weeks of age, respectively. Oral administration of the PTPase inhibitor sodium orthovanadate (0.6 mg/mL in drinking water for 2 weeks) to young ob/ob mice caused a significant reduction in the elevated particulate PTPase activity, with concomitant decreases in plasma insulin and plasma glucose. Assessment of PTPase activity with a monophosphate form of the same synthetic peptide, TRDIY(P)ETDYYRK, showed lower PTPase activities as compared with the triphosphate form and no significant differences between ob/ob and control preparations.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose-induced stimulation of human insulin-receptor mRNA and tyrosine kinase activity in cultured cells

The effects of high glucose on insulin-receptor tyrosine kinase activity and gene expression were investigated in 3T3-HIR cells. Cells incubated for 48 h in the presence of 25 mM glucose showed a 5-fold increase in the amount of insulin receptors per cell, receptor autophosphorylation and phosphorylation of the exogenous substrate poly(Glu/Tyr) compared with cells grown in the absence of glucose but in the presence of 25 mM fructose. These effects were associated with a 4-fold stimulation in steady-state levels of insulin-receptor mRNA. Significant cellular glucose utilization and lactate production were observed in the presence of high glucose in the culture medium, indicating a functional glycolytic pathway in glucose-treated cells, but not in cells treated with fructose. Such a differential response to hexoses favours the hypothesis of a carbohydrate regulation via a glycolytic intermediate. This was further supported by a similar glucose-induced increase in mRNA levels of the enzyme glyceraldehyde-3-phosphate dehydrogenase. To test the hypothesis that the stimulatory effect of glucose on amount of insulin receptors and phosphorylation state could result from post-transcriptional modifications, cells exposed to glucose were incubated with actinomycin D, a potent inhibitor of gene transcription. In cells challenged with high glucose plus inhibitor, insulin-receptor mRNA half-life was increased from 1 to 3 h, indicating that posttranscriptional mechanisms are involved in these processes of glucose regulation. Inhibition of protein synthesis by cycloheximide induced an overexpression of insulin-receptor mRNA levels in the presence of glucose, suggesting that labile repressor protein(s) could be implicated in the effects of glucose. We conclude that (1) long-term culture with high glucose increases the amount of insulin receptors and their tyrosine kinase activity and (2) the glucose-induced increase in insulin-receptor mRNA levels can be accounted for, at least in part, by posttranscriptional events.

Inhibitors of the insulin receptor tyrosine kinase

Insulin is a polypeptide hormone consisting of 51 amino acids. Insulin promotes a variety of anabolic enzymatic pathways and inhibits many catabolic enzymatic pathways involved in energy storage, as well as in synthesis of structural tissue proteins. In addition, insulin serves as a growth factor, modulating mitogenesis, growth and differentiation. Insulin mediates all of its effects by initially binding and activating its specific cell-surface receptor. Conformational changes induced by insulin binding lead to activation of intrinsic receptor tyrosine kinase. Thus, the study of tyrosine kinase inhibitors, whether synthetically produced or purified from microorganisms or humans, has led to elucidation of molecular details of physiological insulin signaling.

Dynamic regulation of intact and C-terminal truncated insulin receptor phosphorylation in permeabilized cells

Using digitonin-permeabilized Chinese hamster ovary (CHO) cells that were transfected with intact human insulin receptors (CHO/HIRc cells), we examined insulin receptor phosphorylation and dephosphorylation using pulse-chase techniques. Insulin activated receptor autophosphorylation on tyrosyl residues to a level severalfold over basal, reaching maximal levels after 2, 5, and 10 min of stimulation at 34, 18, and 6 degrees C, respectively. Phosphopeptide analysis revealed that the triply phosphorylated form of the 1146-kinase domain of the insulin receptor was the major species, which is characteristic of the fully active tyrosine kinase function. The dephosphorylation reaction was time- and temperature-dependent with t1/2 values of 0.67 and 2 min at 18 and 6 degrees C, respectively. Vanadate completely inhibited dephosphorylation. Under similar permeabilization conditions when compared with CHO/HIRc cells, CHO/delta CT cells (CHO cells overexpressing a mutated form of the receptor with a 43 amino acid deletion at the C-terminus) stimulated with insulin exhibited larger increases in receptor autophosphorylation levels and in tyrosine kinase activity toward a synthetic peptide substrate; the rate of CHO/delta CT receptor dephosphorylation was not reduced. There was near-complete absence of insulin receptor substrate 1 (IRS-1) in the cell ghosts after permeabilization. We therefore examined the pattern of tyrosine phosphorylation and dephosphorylation of residual cellular proteins in permeabilized CHO/HIRc cells by Western blot analysis. In addition to the 95-kDa receptor beta-subunit, we detected the phosphorylation of two glycoproteins which included the commonly found 120-kDa protein and a novel 195-kDa protein whose dephosphorylation rate is slower than that of receptor beta-subunit.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of an hepatic low-M(r) membrane-associated protein-tyrosine phosphatase

Protein-tyrosine phosphatases (PTPases), active against autophosphorylated insulin and epidermal growth factor (EGF) receptors in rat liver, are predominantly membrane associated. Fasting of rats for 48 h decreased hepatic particulate PTPase activity by 15.0-26.9%. This reduction in particulate PTPase activity was due to a rather specific decrease in activity of > 85% of a single species of PTPase, termed PTPase I. Disappearance of PTPase I activity from the particulate fraction was not accounted for by its translocation to the cytosol. PTPase I displayed the highest activity against autophosphorylated insulin and EGF receptors, relative to activity against a 32P-labelled peptide substrate, of three PTPases resolved from the liver particulate fraction. The M(r) value of PTPase I, as determined by gel filtration on a Superose 12 column was approx. 42,000, indicating that PTPase I belongs to the low-M(r) class of PTPases. An antibody raised against PTPase 1B, the prototype of this class of PTPases, did not react with PTPase I in Western blots. The potential importance of the novel change in activity of PTPase I in the regulation of insulin-receptor signal transduction is discussed.

Activation of signal transduction in platelets by the tyrosine phosphatase inhibitor pervanadate (vanadyl hydroperoxide)

The protein tyrosine phosphatase (PTPase) inhibitor pervanadate (vanadyl hydroperoxide) stimulated protein tyrosine phosphorylation 29-fold more than did thrombin in intact and saponin-permeabilized platelets. Increased tyrosine phosphorylation preceded, or was coincident with, a fall in PtdIns(4,5)P2 levels, production of PtdIns(3,4)P2 and phosphatidic acid, mobilization of intracellular Ca2+, stimulation of protein kinase C-dependent protein phosphorylation, secretion of dense and alpha-granules, increased actin polymerization, shape change and aggregation which required fibrinogen and was mediated by increased surface expression of GPIIb-IIIa. The tyrosine kinase inhibitor RG 50864 totally prevented induction of tyrosine phosphorylation by pervanadate, as well as all other responses measured; in contrast, the inactive structural analogue, tyrphostin #1, had no effect. Dense-granule secretion induced by pervanadate required protein kinase C activity; however, aggregation and alpha-granule secretion were independent of protein kinase C. In saponin-permeabilized platelets pervanadate and thrombin stimulated phospholipase C activity by GTP-independent and GTP-dependent mechanisms respectively. We conclude that PTPases are important regulators of signal transduction in platelets.

Dephosphorylation of human insulin-like growth factor I (IGF-I) receptors by membrane-associated tyrosine phosphatases

The insulin-like growth factor-I (IGF-I) receptor exhibits structural and functional similarities to the insulin receptor. Although the regulation of the insulin-receptor tyrosine kinase has been extensively investigated, the mechanisms involved in phosphorylation/dephosphorylation of the IGF-I receptor have received only little attention. To obtain a better understanding of the mode of IGF-I action, we have investigated the effects of protein phosphotyrosine phosphatases (PTPases) on the phosphorylation status of the IGF-I receptor. The dephosphorylation of the human IGF-I receptor by membrane-associated tyrosine phosphatases was studied by an immuno-enzymic assay based on the recognition of phosphotyrosine residues by anti-phosphotyrosine antibodies. Using intact IGF-I receptors as substrates, we show that they could be completely dephosphorylated by different cellular PTPases. Three pieces of evidence indicate that receptor dephosphorylation takes place on phosphotyrosine, i.e. the inhibition profile of phosphatase activity by zinc and vanadate, its absolute requirement for thiol compounds and the diminution of [32P]phosphotyrosine labelling of the beta subunit assessed by SDS/PAGE and phosphoamino acid analysis. Tyrosine kinase activity and autophosphorylation of the IGF-I receptor were decreased in a dose-dependent manner by PTPases, indicating that partial dephosphorylation of the receptor was associated with a decrease in its intrinsic activity. The sensitivity of the activated human IGF-I receptor to dephosphorylation on tyrosine leads to the speculation that IGF-I receptor activity might be regulated by mechanisms such as those described for the insulin receptor. Further investigation of the pathways of IGF-I receptor dephosphorylation will contribute to define the role(s) of PTPases in the overall mechanism of IGF-I signalling.

Insulin receptor and epidermal growth factor receptor dephosphorylation by three major rat liver protein-tyrosine phosphatases expressed in a recombinant bacterial system

Protein-tyrosine phosphatases (PTPases) play an essential role in the regulation of signal transduction mediated by reversible protein-tyrosine phosphorylation. In order to characterize individual rat hepatic PTPases that might have specificity for autophosphorylated receptor tyrosine kinases, we isolated cDNA segments encoding three PTPases (PTPase 1B, LAR and LRP) that are expressed in insulin-sensitive liver and skeletal muscle tissue, and evaluated their catalytic activity in vitro. The intrinsic PTPase activities of the full-length PTPase 1B protein and the cytoplasmic domains of LAR and LRP were studied by expression of recombinant cDNA constructs in the inducible bacterial vector pKK233-2 using extracts of a host strain of Escherichia coli that lacks endogenous PTPase activity. Each of the cloned cDNAs dephosphorylated a cognate phosphopeptide derived from the regulatory region of the insulin receptor. Despite having only 30-39% sequence identity in their catalytic domains, LAR and PTPase 1B had similar relative activities between the peptide substrate and intact insulin receptors, and also displayed similar initial rates of simultaneous dephosphorylation of insulin and epidermal growth factor (EGF) receptors. In contrast, LRP exhibited a higher rate of dephosphorylation of both intact receptors relative to the peptide substrate, and also dephosphorylated EGF receptors more rapidly than insulin receptors. These studies indicate that three PTPases with markedly divergent structures have the catalytic potential to dephosphorylate both insulin and EGF receptors in intact cells and that redundant PTPase activity may occur in vivo. For these PTPases to have specific physiological actions in intact cells, they must be influenced by steric effects of the additional protein segments of the native transmembrane enzymes, cellular compartmentalization and/or interactions with regulatory proteins.

Protein-tyrosine phosphatases and the regulation of insulin action

Protein-tyrosine phosphatases (PTPases) play an important role in the regulation of insulin action by dephosphorylating the active (autophosphorylated) form of the insulin receptor and attenuating its tyrosine kinase activity. PTPases can also modulate post-receptor signalling by catalyzing the dephosphorylation of cellular substrates of the insulin receptor kinase. Dramatic advances have recently been made in our understanding of PTPases as an extensive family of transmembrane and intracellular proteins that are involved in a number of pathways of cellular signal transduction. Identification of the PTPase(s) which act on various components of the insulin action cascade will not only enhance our understanding of insulin signalling but will also clarify the potential involvement of PTPases in the pathophysiology of insulin-resistant disease states. This brief review provides a summary of reversible tyrosine phosphorylation events in insulin action and available data on candidate PTPases in liver and skeletal muscle that may be involved in the regulation of insulin action.

Dephosphorylation of autophosphorylated insulin and epidermal-growth-factor receptors by two major subtypes of protein-tyrosine-phosphatase from human placenta

The identity of protein-tyrosine-phosphatases (PTPases) active against autophosphorylated insulin receptor was probed by using an insulin-receptor-related peptide phosphorylated on tyrosine (peptide 1142-1153). Two major peaks of PTPase activity were resolved from the particulate (Triton X-100-soluble) fraction of human placenta by chromatography on DEAE-cellulose. The two peaks were purified 1300-2300-fold; other peaks of PTPase activity (greater than 15%) were not detected. Properties of the PTPases indicated that they corresponded to subtypes 1A and 1B. Both subtypes appeared capable of catalysing dephosphorylation of all autophosphorylation sites in three domains of the insulin receptor, with no appreciable difference in the pattern of dephosphorylation detected by two-dimensional tryptic-peptide mapping. The tyrosine-1150 domain of the insulin receptor in triply phosphorylated form was found to be highly sensitive to the action of both PTPases, and was dephosphorylated at least 4 times faster than the doubly and singly phosphorylated forms of the tyrosine-1150 domain or phosphorylation sites in other domains by either PTPase. This is significant, as the level of the triphosphotyrosine-1150 species has been shown to correlate well with the capacity of the insulin-receptor tyrosine kinase to phosphorylate other proteins. Both subtypes also dephosphorylated autophosphorylated epidermal-growth-factor (EGF) receptor by greater than 95%. Placental particulate (and cytosolic) PTPase activity against either receptor distributed approximately 2:1 between subtypes 1A and 1B as assayed in the presence of EDTA. In summary, PTPases within two major subtypes have been identified as phosphotyrosyl-insulin and -EGF-receptor phosphatases in vitro. The PTPases identified exhibit high affinities for substrates and high activities in cells, which is commensurate with the PTPases being important in vivo in controlling or reversing autophosphorylation-induced regulatory or signalling events.