Insulin differentially regulates protein phosphotyrosine phosphatase activity in rat hepatoma cells

Rapid reversal of insulin-stimulated AS160 phosphorylation in rat skeletal muscle after insulin exposure

Increased phosphorylation of Akt substrate of 160 kDa (AS160) is essential to trigger the full increase in insulin-stimulated glucose transport in skeletal muscle. The primary aim of this study was to characterize the time course for reversal of insulin-stimulated AS160 phosphorylation in rat skeletal muscle after insulin removal. The time courses for reversal of insulin effects both upstream (Akt phosphorylation) and downstream (glucose uptake) of AS160 were also determined. Epitrochlearis muscles were incubated in vitro using three protocols which differed with regard to insulin exposure: no insulin (never exposed to insulin), transient insulin (30 min with 1.8 nmol/l insulin, then incubation without insulin for 10, 20 or 40 min), or sustained insulin (continuously incubated with 1.8 nmol/l insulin). After removal of muscles from insulin, Akt and AS160 phosphorylation reversed rapidly, each with a half-time of <10 min and essentially full reversal by 20 min. Glucose uptake reversed more slowly (half time between 10 and 20 min with essentially full reversal by 40 min). Removal of muscles from insulin resulted in a rapid reversal of the increase in AS160 phosphorylation which preceded the reversal of the increase in glucose uptake, consistent with AS160 phosphorylation being essential for maintenance of insulin-stimulated glucose uptake.

Common pathological processes in Alzheimer disease and type 2 diabetes: a review

Alzheimer disease (AD) and type 2 diabetes mellitus (T2DM) are conditions that affect a large number of people in the industrialized countries. Both conditions are on the increase, and finding novel treatments to cure or prevent them are a major aim in research. Somewhat surprisingly, AD and T2DM share several molecular processes that underlie the respective degenerative developments. This review describes and discusses several of these shared biochemical and physiological pathways. Disturbances in insulin signalling appears to be the main common impairment that affects cell growth and differentiation, cellular repair mechanisms, energy metabolism, and glucose utilization. Insulin not only regulates blood sugar levels but also acts as a growth factor on all cells including neurons in the CNS. Impairment of insulin signalling therefore not only affects blood glucose levels but also causes numerous degenerative processes. Other growth factor signalling systems such as insulin growth factors (IGFs) and transforming growth factors (TGFs) also are affected in both conditions. Also, the misfolding of proteins plays an important role in both diseases, as does the aggregation of amyloid peptides and of hyperphosphorylated proteins. Furthermore, more general physiological processes such as angiopathic and cytotoxic developments, the induction of apoptosis, or of non-apoptotic cell death via production of free radicals greatly influence the progression of AD and T2DM. The increase of detailed knowledge of these common physiological processes open up the opportunities for treatments that can prevent or reduce the onset of AD as well as T2DM.

Insulino-mimetic and anti-diabetic effects of vanadium compounds

Compounds of the trace element vanadium exert various insulin-like effects in in vitro and in vivo systems. These include their ability to improve glucose homeostasis and insulin resistance in animal models of Type 1 and Type 2 diabetes mellitus. In addition to animal studies, several reports have documented improvements in liver and muscle insulin sensitivity in a limited number of patients with Type 2 diabetes. These effects are, however, not as dramatic as those observed in animal experiments, probably because lower doses of vanadium were used and the duration of therapy was short in human studies as compared with animal work. The ability of these compounds to stimulate glucose uptake, glycogen and lipid synthesis in muscle, adipose and hepatic tissues and to inhibit gluconeogenesis, and the activities of the gluconeogenic enzymes: phosphoenol pyruvate carboxykinase and glucose-6-phosphatase in the liver and kidney as well as lipolysis in fat cells contributes as potential mechanisms to their anti-diabetic insulin-like effects. At the cellular level, vanadium activates several key elements of the insulin signal transduction pathway, such as the tyrosine phosphorylation of insulin receptor substrate-1, and extracellular signal-regulated kinase 1 and 2, phosphatidylinositol 3-kinase and protein kinase B activation. These pathways are believed to mediate the metabolic actions of insulin. Because protein tyrosine phosphatases (PTPases) are considered to be negative regulators of the insulin-signalling pathway, it is suggested that vanadium can enhance insulin signalling and action by virtue of its capacity to inhibit PTPase activity and increase tyrosine phosphorylation of substrate proteins. There are some concerns about the potential toxicity of available inorganic vanadium salts at higher doses and during long-term therapy. Therefore, new organo-vanadium compounds with higher potency and less toxicity need to be evaluated for their efficacy as potential treatment of human diabetes.

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

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

Metabolic effects of vanadyl sulfate in humans with non-insulin-dependent diabetes mellitus: in vivo and in vitro studies

To investigate the efficacy and mechanism of action of vanadium salts as oral hypoglycemic agents, 16 type 2 diabetic patients were studied before and after 6 weeks of vanadyl sulfate (VOSO4) treatment at three doses. Glucose metabolism during a euglycemic insulin clamp did not increase at 75 mg/d, but improved in 3 of 5 subjects receiving 150 mg VOSO4 and 4 of 8 subjects receiving 300 mg VOSO4. Basal hepatic glucose production (HGP) and suppression of HGP by insulin were unchanged at all doses. Fasting glucose and hemoglobin A1c (HbA1c) decreased significantly in the 150- and 300-mg VOSO4 groups. At the highest dose, total cholesterol decreased, associated with a decrease in high-density lipoprotein (HDL). There was no change in systolic, diastolic, or mean arterial blood pressure on 24-hour ambulatory monitors at any dose. There was no apparent correlation between the clinical response and peak serum level of vanadium. The 150- and 300-mg vanadyl doses caused some gastrointestinal intolerance but did not increase tissue oxidative stress as assessed by thiobarbituric acid-reactive substances (TBARS). In muscle obtained during clamp studies prior to vanadium therapy, insulin stimulated the tyrosine phosphorylation of the insulin receptor, insulin receptor substrate-1 (IRS-1), and Shc proteins by 2- to 3-fold, while phosphatidylinositol 3-kinase (PI 3-kinase) activity associated with IRS-1 increased 4.7-fold during insulin stimulation (P = .02). Following vanadium, there was a consistent trend for increased basal levels of insulin receptor, Shc, and IRS-1 protein tyrosine phosphorylation and IRS-1-associated PI 3-kinase, but no further increase with insulin. There was no discernible correlation between tyrosine phosphorylation patterns and glucose disposal responses to vanadyl. While glycogen synthase fractional activity increased 1.5-fold following insulin infusion, there was no change in basal or insulin-stimulated activity after vanadyl. There was no increase in the protein phosphatase activity of muscle homogenates to exogenous substrate after vanadyl. Vanadyl sulfate appears safe at these doses for 6 weeks, but at the tolerated doses, it does not dramatically improve insulin sensitivity or glycemic control. Vanadyl modifies proteins in human skeletal muscle involved in early insulin signaling, including basal insulin receptor and substrate tyrosine phosphorylation and activation of PI 3-kinase, and is not additive or synergistic with insulin at these steps. Vanadyl sulfate does not modify the action of insulin to stimulate glycogen synthesis. Since glucose utilization is improved in some patients, vanadyl must also act at other steps of insulin action.

Synergistic interaction of magnesium and vanadate on glucose metabolism in diabetic rats

The effect of vanadate (V) alone, magnesium (Mg) alone, and the combination of Mg plus V (MgV) on insulin-mediated glucose disposal and glucose tolerance was investigated in normal and streptozotocin-induced diabetic rats. MgV, magnesium sulfate (MgSO4) and sodium metavanadate (NaV) were added to the drinking water of normal or diabetic rats (approximately 300 g) for 3 weeks. After 3 weeks of V treatment (both MgV and NaV), diabetic rats demonstrated a normal meal tolerance test without any increase in the plasma insulin response. Rats also received a euglycemic insulin clamp (12 mU/kg x min for 120 minutes) with 3-3H-glucose infusion to quantify total body glucose disposal, glycolysis (3H2O production), and glycogen synthesis (total body glucose disposal minus glycolysis). Total glucose disposal was decreased in diabetic versus control rats (29 +/- 2 v 35 +/- 2 mg/kg x min, P < .01) and returned to levels greater than the nondiabetic control values after MgV (41 +/- 2, P < .01). Supersensitivity to insulin was not observed in diabetic rats treated with NaV (34 +/- 1). Glycogen synthesis was increased by both MgV and NaV treatment (23 +/- 21, P < .01 and 18 +/- 1, P < .05 v 14 +/- 2 mg/kg x min) in diabetic rats. A small increase in glycolysis was observed in MgSO4 and MgV rats (18 +/- 1 and 18 +/- 1 v 16 +/- 1, P < .05). NaV alone had no effect on glycolysis. Thus, Mg has a synergistic effect with V to increase muscle glycogen synthesis in diabetic rats. In normal rats, neither MgSO4 nor NaV had any effect on glucose utilization. However, MgV increased glucose disposal to rates that were significantly higher than the rate in untreated control rats (P < .05). Based on these results, MgV is superior to either V alone or Mg alone in improving insulin sensitivity and glycogen synthesis in diabetic rats.

Cell growth inhibition by a novel vitamin K is associated with induction of protein tyrosine phosphorylation

We have shown that a synthetic vitamin K analog, 2-(2-mercaptoethanol)-3-methyl-1,4-naphthoquinone or compound 5 (Cpd 5), potently inhibits cell growth and suggested that the analog exerts its effects mainly via sulfhydryl arylation rather than redox cycling. Since protein-tyrosine phosphatases (PTPases), which have pivotal roles in many cellular functions, have a critical cysteine in their active site, we have proposed PTPases as likely targets for Cpd 5. To test this hypothesis, we examined the effects of Cpd 5 on protein tyrosine phosphorylation of cellular proteins and on the activity of PTPases. We found that Cpd 5 rapidly induced protein tyrosine phosphorylation in a human hepatocellular carcinoma cell line (Hep3B) at growth inhibitory doses, and the effect was blocked by thiols but not by non-thiol antioxidants or tyrosine kinase inhibitors. Cpd 5 inhibited PTPase activity, which was also significantly antagonized by reduced glutathione. Furthermore, the well studied PTPase inhibitor orthovanadate also induced protein tyrosine phosphorylation and growth inhibition in Hep3B cells. These results suggest that inhibition of cellular PTPases by sulfhydryl arylation and subsequent perturbation of protein tyrosine phosphorylation may be involved in the mechanisms of Cpd 5-induced cell growth inhibition.

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

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

Alterations in skeletal muscle protein-tyrosine phosphatase activity and expression in insulin-resistant human obesity and diabetes

Obese human subjects have increased protein-tyrosine phosphatase (PTPase) activity in adipose tissue that can dephosphorylate and inactivate the insulin receptor kinase. To extend these findings to skeletal muscle, we measured PTPase activity in the skeletal muscle particulate fraction and cytosol from a series of lean controls, insulin-resistant obese (body mass index > 30) nondiabetic subjects, and obese individuals with non-insulin-dependent diabetes. PTPase activities in subcellular fractions from the nondiabetic obese subjects were increased to 140-170% of the level in lean controls (P < 0.05). In contrast, PTPase activity in both fractions from the obese subjects with non-insulin-dependent diabetes was significantly decreased to 39% of the level in controls (P < 0.05). By immunoblot analysis, leukocyte antigen related (LAR) and protein-tyrosine phosphatase 1B had the greatest increase (threefold) in the particulate fraction from obese, nondiabetic subjects, and immunodepletion of this fraction using an affinity-purified antibody directed at the cytoplasmic domain of leukocyte antigen related normalized the PTPase activity when compared to the activity from control subjects. These findings provide further support for negative regulation of insulin action by specific PTPases in the pathogenesis of insulin resistance in human obesity, while other regulatory mechanisms may be operative in the diabetic state.

cAMP counter-regulates insulin-mediated protein phosphatase-2A inactivation in rat skeletal muscle cells

In this study, we examined the mechanism of recently reported inactivation of protein phosphatase-2A (PP-2A) by insulin (Srinivasan, M., and Begum, N. (1994) J. Biol. Chem. 269, 12514-12520) and its counter-regulation by cAMP agonists. Exposure of L6 myotubes to insulin resulted in a rapid inhibition of PP-2A that was accompanied by a 3-fold increase in the phosphotyrosine content of the immunoprecipitated PP-2A catalytic subunit. Pretreatment with (Sp)-cAMP, a cAMP agonist, completely blocked insulin-mediated inhibition of PP-2A activity and decreased the tyrosine phosphorylation of PP-2A catalytic subunit to control levels. To understand the mechanism of counter-regulation of PP-2A by (Sp)-cAMP, cells were pretreated with sodium orthovanadate, an inhibitor of phosphotyrosine phosphatases. Vanadate prevented the effect of (Sp)-cAMP on PP-2A activity and increased the phosphorylation status of PP-2A catalytic subunit to the level observed with insulin. Wortmannin, a phosphatidylinositol 3-kinase inhibitor, and rapamycin, an inhibitor of 70-kDa S6 kinase activation, prevented insulin-mediated inactivation of PP-2A, suggesting that these pathways may participate in insulin-mediated phosphorylation and inactivation of PP-2A. These results show that insulin signaling results in a rapid inactivation of PP-2A by increased tyrosine phosphorylation and cAMP agonists counter-regulate insulin's effect on PP-2A by decreasing phosphorylation, presumably via an activated phosphatase.

Suppression of insulin receptor activation by overexpression of the protein-tyrosine phosphatase LAR in hepatoma cells

Protein-tyrosine phosphatases (PTPases) play an essential role in the regulation of reversible tyrosine phosphorylation of cellular proteins that mediate insulin action. In order to explore the potential role of the transmembrane PTPase (LAR) in insulin receptor signal transduction, we overexpressed the full-length LAR protein in McA-RH7777 rat hepatoma cells and found that modest increases in the abundance of LAR protein expression downregulated a number of insulin-stimulated cellular responses closely related to the activation of the receptor kinase. An increase in LAR protein of 2.4-fold over the level in control cells caused a 40% reduction in insulin receptor autophosphorylation in intact cells, without an alteration in insulin receptor mass or a change in the insulin-stimulated receptor kinase activity measured with partially purified receptors in vitro. In addition, insulin-stimulated tyrosine phosphorylation of the endogenous insulin receptor substrates IRS-1 and Shc were decreased to 57% and 73% of control, respectively, and IRS-1 associated phosphatidylinositol 3'-kinase activity was reduced to 47% of control of the cells overexpressing LAR. The present results, taken with our recent data demonstrating that reducing the abundance of LAR by expression of antisense mRNA enhances insulin receptor signal transduction (Kulas D. T., et al. J. Biol. Chem. 270:2435, 1995), supports the hypothesis that LAR acts as a physiological modulator of insulin action in insulin-sensitive hepatoma cells.

Altered basal and insulin-stimulated phosphotyrosine phosphatase (PTPase) activity in skeletal muscle from NIDDM patients compared with control subjects

To measure possible changes in basal and insulin-stimulated phosphotyrosine phosphatase (PTPase) activity in skeletal muscle from insulin-resistant individuals, soluble and particulate muscle fractions were prepared from biopsies taken before and after a 3-h hyperinsulinaemic euglycaemic clamp in eight non-insulin-dependent diabetic (NIDDM) patients and nine control subjects. We used a sensitive sandwich-immunofluorescence assay and the human insulin receptor as the substrate. PTPase activity was expressed as percentage of dephosphorylation of phosphotyrosyl-residues in immobilized insulin receptors per 2 h incubation time per 83 micrograms and 19 micrograms muscle fraction protein (soluble and particulate fraction, respectively). In the diabetic soluble muscle fractions, the basal PTPase activity was decreased compared with that of control subjects (11.5 +/- 5.5 vs 27.5 +/- 3.3, p < 0.04, mean +/- SEM). In the particulate muscle fractions from the control subjects, PTPase activity was increased after 3 h hyperinsulinaemia (20.0 +/- 3.2 vs 30.2 +/- 3.6, p < 0.03) and in the corresponding soluble fractions PTPase activity seemed decreased (27.5 +/- 3.3 vs 19.9 +/- 5.9, NS). No effect of insulin on PTPase activity was found in NIDDM patients (25.1 +/- 4.1 vs 27.2 +/- 5.2, 11.5 +/- 5.5 vs 15.1 +/- 4.5 [particulate and soluble fractions], NS). In conclusion, we found that the basal PTPase activity in soluble muscle fractions was decreased in NIDDM patients; furthermore, insulin stimulation was unable to increase PTPase activities in the particulate fractions, as opposed to the effect of insulin in control subjects.

In vivo and in vitro studies of vanadate in human and rodent diabetes mellitus

In vivo vanadate and vanadyl have been shown to mimic the action of insulin and to be effective treatment for animal models of both Type I and Type II diabetes. The molecular mechanism of action of the vanadium salts on insulin sensitivity remains uncertain, and several potential sites proposed for the insulin-like effects are reviewed. In human trials, insulin sensitivity improved in patients with NIDDM, as well as in some patients with IDDM after two weeks of treatment with sodium metavanadate. This increase in insulin sensitivity was primarily due to an increase in non-oxidative glucose disposal, whereas oxidative glucose disposal and both basal and insulin stimulated suppression of hepatic glucose output (HGP) were unchanged. Clinically, oral vanadate was associated with a small decrease in insulin requirements in IDDM subjects. Of additional benefit, there was a decrease in total cholesterol levels in both IDDM and NIDDM subjects. Furthermore, there was an increase in the basal activities of MAP and S6 kinases to levels similar to the insulin-stimulated levels in controls, but there was little or no further stimulation with insulin was seen. Further understanding of the mechanism of vanadium action may ultimately be useful in the design of drugs that improve glucose tolerance.

Erythrocyte insulin and insulin-like growth factor-I receptor tyrosine kinase activity in hypertension in pregnancy

We have shown that preeclampsia is associated with insulin resistance. In the present study, we examined young normal, preeclamptic (PE), and gestational hypertensive (GH) nulliparous African-American women at term to investigate cellular determinants of this resistance and insulin and insulin-like growth factor-I (IGF-I) binding to partially purified erythrocyte receptors and receptor tyrosine kinase activity (TKA). Blood pressure was significantly elevated in PE and GH subjects as compared with controls. Insulin binding was similar in number and affinity in the three groups (femtomoles per microgram). IGF-I binding was increased in PE subjects as compared with either normals or GH subjects (0.2 +/- 0.02, 0.15 +/- 0.01, and 0.14 +/- 0.02 fmol/microgram protein). Insulin receptor TKA was increased in PE subjects as compared with normals when assessed either per microgram protein or per femtomole insulin binding (P < .01). In contrast, IGF-I-potentiated TKA was elevated in PE subjects only when assessed per microgram protein (P < .03). Thus, the increased number of IGF-I receptors in erythrocytes of PE subjects yields a net increase in receptor tyrosine kinase. Also, there is an augmentation of insulin receptor TKA in PE subjects. Together, these two alterations may be a compensatory mechanism for the insulin resistance associated with hypertensive diseases of pregnancy.

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)

Purification, identification and subcellular distribution of three predominant protein-tyrosine phosphatase enzymes in skeletal muscle tissue

Protein-tyrosine phosphatases (PTPases) play a key role in the regulation of insulin action. In order to identify PTPases in skeletal muscle, the major site of insulin-mediated glucose disposal in vivo, we purified PTPases from rat muscle tissue fractions by a series of column chromatographic techniques. PTPase activities were assayed by measuring the dephosphorylation of a rat insulin receptor kinase domain, derivatized lysozyme and p-nitrophenylphosphate, and the enzymes were further characterized by immunoblotting. Of the total PTPase activity in muscle homogenates, 51-64% was localized to the solubilized particulate fraction, with the specific PTPase activity 3.3-fold and 5.6-fold higher in the particulate fraction towards RCM-lysozyme or the insulin receptor, respectively. The major peak (> 75%) of PTPase activity in the particulate fraction was purified further to 700-fold; 75% of this activity passed through a Blue-3GA column and revealed immunoreactivity for both LAR and SH-PTP2. PTPase activity retained on the Blue-3GA column contained PTPase1B. The major peak (> 70%) from muscle cytosol was further purified to 1500-fold. After the Blue-3GA step, immunoblotting revealed both SH-PTP2 and PTPase1B in the cytosol fraction, but LAR was absent from this fraction. LRP (RPTP-alpha) was not detected by blotting the PTPase activities from the purified particulate or cytosol fractions. Immunodepletion studies demonstrated that LAR, SH-PTP2 and PTPase1B were quantitatively major PTPase activities in the initial muscle homogenate, together accounting for over 70% of the total activity towards RCM-lysozyme. These studies provide insight into the relative abundance and subcellular distribution of specific PTPases in muscle tissue that are involved in the regulation of reversible tyrosine phosphorylation in this tissue.

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.

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.

Molecular determinants of insulin action

Insulin elicits an array of biologic responses. Insulin exerts a regulatory role in almost all cells of the body and is the primary hormone responsible for signaling the storage and utilization of basic nutrients. On the molecular level, the actions of insulin are initiated by binding of insulin to the insulin receptor. Interaction of the alpha and beta subunits of the receptor results in tyrosine kinase activity, which is integral to the initiation of cascades of phosphorylation/dephosphorylation reactions that mediate a large number of the actions of insulin. Insulin-receptor substrate 1 may be central to phosphorylation reactions through a role in serine and threonine kinase activity. Insulin action may also involve the generation of low-molecular-weight mediators capable of modulating intracellular enzymes. The regulation of glucose transport is a primary feature of the physiologic role of insulin and is performed by a family of glucose-transporter proteins with different characteristics. One mechanism by which insulin exerts its effect on glucose transport is the stimulation of the translocation of the glucose transporter to the plasma membrane. Degradation of insulin occurs through diverse mechanisms at numerous sites in the body. Reversal of the insulin signal at the cellular level may be accomplished by a class of enzymes termed phosphotyrosine phosphatases, which may play a role in certain pathophysiologic states. Important roles for insulin-receptor kinase, glucose transporters, insulin-receptor substrate 1, and various intracellular enzymes in the actions of insulin have been demonstrated; nonetheless, the formulation of potential therapeutic strategies directed at particular stages of the insulin action cascade will require further elucidation of its components.