Insulin activation of insulin receptor tyrosine kinase in intact rat adipocytes. An in vitro system to measure histone kinase activity of insulin receptors activated in vivo

Insulin stimulation of Akt/PKB phosphorylation in the placenta of preeclampsia patients

CONTEXT AND OBJECTIVE

Preeclampsia is a multi-systemic disease and one of the most frequent severe health problems during pregnancy. Binding of insulin triggers phosphorylation and activates cytoplasmic substrates such as phosphatidylinositol 3 kinase (PI3K). Phosphorylation of membrane phosphoinositide 2 (PIP2) to phosphoinositide 3 (PIP3) by PI3K starts Akt/PKB activation. Defects in phosphorylation of the insulin receptor and its substrates have an important role in insulin resistance. Studies have shown that insulin resistance is associated with preeclampsia and its pathophysiology. The aim here was to investigate insulin stimulation of the Akt/PKB pathway in the placenta, in normal and preeclampsia parturients.

DESIGN AND SETTING

Cross-sectional study in a tertiary public university hospital.

METHODS

Placentas were collected from 12 normal and 12 preeclampsia patients. These were stimulated and analyzed using Western blot to quantify the Akt/PKB phosphorylation.

RESULTS

The insulin stimulation was confirmed through comparing the stimulated group (1.14 ± 0.10) with the non-stimulated group (0.91 ± 0.08; P < 0.001). The phosphorylation of Akt/PKB did not differ between the placenta of the normal patients (1.26 ± 0.16) and those of the preeclampsia patients (1.01 ± 0.11; P = 0.237).

CONCLUSIONS

In vitro insulin stimulation of the human placenta has been well established. There was no difference in Akt/PKB phosphorylation, after stimulation with insulin, between placentas of normal and preeclampsia patients. Nevertheless, it cannot be ruled out that the Akt/PKB signaling pathway may have a role in the pathophysiology of preeclampsia, since the substrates of Akt/PKB still need to be investigated.

Mechanism of lipid induced insulin resistance: activated PKCε is a key regulator

Fatty acids (FAs) are known to impair insulin signaling in target cells. Accumulating evidences suggest that one of the major sites of FAs adverse effect is insulin receptor (IR). However, the underlying mechanism is yet unclear. An important clue was indicated in leptin receptor deficient (db/db) diabetic mice where increased circulatory FAs was coincided with phosphorylated PKCε and reduced IR expression. We report here that central to this mechanism is the phosphorylation of PKCε by FAs. Kinase dead mutant of PKCε did not augment FA induced IRβ downregulation indicating phosphorylation of PKCε is crucial for FA induced IRβ reduction. Investigation with insulin target cells showed that kinase independent phosphorylation of PKCε by FA occurred through palmitoylation. Mutation at cysteine 276 and 474 residues in PKCε suppressed this process indicating participation of these two residues in palmitoylation. Phosphorylation of PKCε endowed it the ability to migrate to the nuclear region of insulin target cells. It was intriguing to search about how translocation of phosphorylated PKCε occurred without having canonical nuclear localization signal (NLS). We found that F-actin recognized phospho-form of PKCε and chaperoned it to the nuclear region where it interact with HMGA1 and Sp1, the transcription regulator of IR and HMGA1 gene respectively and impaired HMGA1 function. This resulted in the attenuation of HMGA1 driven IR transcription that compromised insulin signaling and sensitivity.

Brazilin inhibits activities of protein kinase C and insulin receptor serine kinase in rat liver

Hypoglycemic action of brazilin was found to be based on the improvement of peripheral glucose utility, and this action might be correlated with the insulin action pathway. In the present study we investigated the effect of brazilin on the insulin receptor autophosphorylation, protein kinase C (PKC), protein phosphatase and insulin receptor serine kinase in order to confirm whether the hypoglycemic mechanism is concerned with insulin action pathway. Brazilin was found to inhibit PKC and insulin receptor serine kinase, which are involved in the regulation of insulin signal pathway. But any significant effect was not shown on insulin receptor tyrosine kinase activity, autophosphorylation and phosphatase activity. These findings suggest that brazilin might enhance insulin receptor function by decreasing serine phosphorylation, which might mediate hypoglycemic effect of brazilin.

Insulin stimulation of human adipocytes activates the kinase of only a fraction of the insulin receptors

The degree of insulin receptor kinase activation by in situ stimulation was studied in isolated human adipocytes. Although maximal in situ stimulation increased the kinase activity approximately 10-fold, this activity could again be doubled by subsequent activation in a cell-free system. To investigate how in situ stimulation resulted in incomplete activation, receptors binding or not binding to anti-phosphotyrosine antibody (alpha-PY) were studied separately. Even after maximal insulin stimulation of the cells, approximately 50% of the receptors did not bind to alpha-PY and had low kinase activity. In the cell-free system, however, these receptors reached activity levels similar to the other receptors, suggesting that they were intact and that factors in their cellular environment had prevented their activation. The activity of the alpha-PY-binding receptors could only be slightly increased in the cell-free system, suggesting that almost complete activation had been attained in situ. In situ stimulation with increasing insulin concentrations increased the number of activated receptors rather than their individual activity. We conclude that factors in the in situ environment prevent insulin activation of approximately 50% of the insulin receptors in human adipocytes and might therefore be important regulators of insulin signaling.

Dynamics of signaling during insulin-stimulated endocytosis of its receptor in adipocytes

Insulin causes rapid insulin receptor autophosphorylation, receptor endocytosis, and phosphorylation of its principle substrate (IRS-1). Using rat adipocytes, we studied the dynamics of receptor autophosphorylation, the kinase activity, and the IRS-1 phosphorylation state relative to the subcellular localization of these proteins. After 2 min of insulin exposure, the specific phosphotyrosine content of the insulin receptor in the internal membranes (IM) peaks at a level 5-6-fold higher than the plasma membrane (PM) receptor and then declines after 5-8 min to a level similar to the PM receptor. The exogenous kinase activity of these receptors exactly mirrored their phosphotyrosine content. The distribution of IRS-1 is 80% cytosolic, 20% IM-associated, and essentially undetectable in the PM. The phosphorylation state of IRS-1 in the IM parallels that of the insulin receptor, but cytosolic IRS-1 phosphorylation remains constant. Insulin-dependent GLUT4 translocation to the PM occurs after the peak of IRS-1 phosphorylation. The data are consistent with the hypothesis that insulin action may be mediated by receptor internalization and interaction with its substrate(s) associated with internal membranes. A small fraction of phosphorylated insulin receptors is sufficient for signal transduction. The dephosphorylation of the insulin receptor and IRS-1 in the IM appears to be a concerted process, possibly mediated by the same enzyme.

Molecular dynamics of insulin/IGF-I receptor transmembrane signaling

To examine the molecular basis of ligand-stimulated intramolecular beta-subunit autophosphorylation, hybrid receptors composed of wild-type and mutant insulin and insulin-like growth factor-1 (IGF-I) half-receptor precursors were characterized. Previous studies have demonstrated that assembly of the IGF-I wild-type half-receptor (alpha beta WT) with a kinase-defective half-receptor (alpha beta A/K) produced a substrate kinase-inactive holoreceptor in vitro [Treadway et al. (1991): Proc Natl Acad Sci USA 88:214-218]. To extend these studies, the vaccinia virus/bacteriophage T7 expression system was used to generate various hybrid receptor complexes in cultured cells. As was observed for hybrid receptors assembled in vitro, the wild-type/mutant hybrid receptors formed in situ were also incapable of phosphorylating several peptide substrates. However, ligand-stimulated beta-subunit autophosphorylation was still observed. To determine the molecular basis for this discrepancy, hybrid receptors were assembled from a truncated beta-subunit insulin half-receptor (alpha beta delta 43) and a kinase-defective half-receptor (alpha beta A/K). Under these conditions, insulin-stimulated autophosphorylation primarily occurred on the full-length kinase-inactive beta-subunit (alpha beta A/K) without significant labeling of the kinase-active truncated beta-subunit (alpha beta delta 43). A similar IGF-I hybrid receptor species was characterized, and the same pattern of autophosphorylation was observed in response to IGF-I. These data demonstrate that both insulin and IGF-I stimulate an intramolecular trans-autophosphorylation reaction between two adjacent beta-subunits within the holoreceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of benzyl succinate on insulin receptor function and insulin action in skeletal muscle: further evidence for a lack of spare high-affinity insulin receptors

Benzyl succinate inhibited insulin binding and tyrosine receptor kinase in a concentration-dependent manner in the partially purified insulin receptor preparation from rat skeletal muscle. Benzyl succinate lowered the apparent number of high-affinity insulin binding sites. We have made use of the inhibitory effect of benzyl succinate to investigate the possible presence of spare high-affinity insulin receptors in muscle. Benzyl succinate inhibited the effect of a supramaximal concentration of insulin on 3-O-methylglucose uptake, 2-(methylamino)isobutyric acid uptake and lactate production by the incubated muscle. Furthermore, the inhibitory effect of benzyl succinate on insulin binding in vitro closely correlated with its inhibitory effect on insulin action in vivo. These findings suggest the absence of spare high-affinity insulin receptors in skeletal muscle. In contrast to data obtained in skeletal muscle, benzyl succinate did not affect the maximally insulin-stimulated glucose transport, although it caused a marked decrease in insulin sensitivity in isolated rat adipocytes, for which the existence of spare insulin receptors is well documented.

Effect of oxidative stress on receptors and signal transmission

Reactive oxygen metabolites affect binding of ligands to membrane receptors and also coupling of receptors to G-proteins and effector enzymes. Peroxidation of membrane lipids may lead to a lowered receptor density and also will alter the viscosity of the plasma membrane, which affects receptor coupling. Reactive oxygen species may also interact with thiol/disulfide moieties on receptor proteins or on other factors in the receptor system, which is responsible for alterations in receptor binding or coupling. Moreover, lipid peroxidation is associated with the phospholipase A2 pathway, which might indirectly affect receptor function. Moreover, oxidative stress may lead to a disturbance in cellular Ca(2+)-homeostasis. This might be related to an effect on Ca(2+)-mobilizing receptors, but there is also evidence for a decreased Ca(2+)-sequestration by ATPases. In addition, peroxidation of membrane lipids increases membrane permeability to Ca2+. Finally, reactive oxygen species interfere with actions of nitric oxide, thus affecting another pharmacological messenger system.

An assessment of human insulin receptor phosphorylation and exogenous kinase activity following deletion of 69 residues from the carboxyl-terminus of the receptor beta-subunit

A mutant human insulin receptor with a carboxyl-terminal deletion of 69 amino acids (proreceptor residues 1287-1355) is expressed as a stable protein in transiently transfected COS cells. We find that in intact cells this mutant is phosphorylated in an insulin-dependent manner on core tyrosines 1158, 1163 and 1163. As expected, the carboxyl-terminal beta-subunit phosphorylation sites (serines 1305/6, tyrosines 1328/34 and threonine 1348) are absent from this mutant. However, the two major insulin-stimulated serine phosphopeptides remain. In intact cells, insulin stimulates exogenous substrate phosphorylation by the truncated receptor only approximately 1.9-fold (cf. approximately 9-fold for the wild-type receptor in these cells), a consequence of a approximately 4.8-fold elevation in basal insulin-independent kinase activity.

Insulin-like growth factor I-dependent tyrosine kinase activity in stromal cells of human endometrium in vitro

The study was undertaken to identify and characterize insulin-like growth factor I (IGF-I) receptors in human endometrial stromal cells in culture and to examine whether these receptors are modulated by estradiol (E2) and/or progesterone (P). We found that partially purified plasma membrane proteins from these cells contain specific high-affinity binding sites for IGF-I (10 fmol/micrograms protein). Chemical cross-linking with 125I-labeled IGF-I and autophosphorylation with [32P]ATP-labeled proteins of relative molecular weight 135,000 and 95,000 correspond to the known Mr values of the alpha- and the beta-subunits of IGF-I receptors. Receptor autophosphorylation and phosphorylation of the substrate poly(Glu,Na4Tyr1) was stimulated in vitro by IGF-I (half-maximally at 1 nM, maximally at 100 nM). After stimulation of intact cells with IGF-I (5 nM) and subsequent partial purification of receptors in the presence of phosphatase inhibitors, a 2.5- to 3.6-fold stimulation of the kinase activity toward poly(Glu,Na4Tyr1) was found. Preincubation of the cells for 16 h with E2, P, and E2 + P did not modify the IGF-I binding characteristics nor the effect of IGF-I (5 nM) on tyrosine kinase stimulation in intact cells. This suggests that, in isolated humans, endometrial cell modulation of IGF-I receptor function by estrogen and P does not occur.

Monoclonal antibody to the human insulin receptor, but not insulin, stimulates S6 kinase via human insulin receptors mutated at three major tyrosine autophosphorylation sites

Studies were carried out to examine the role of the major insulin receptor tyrosine autophosphorylation sites in stimulation of S6 kinase activity. For these studies, we employed HTC rat hepatoma cells transfected with and expressing human insulin receptors. In cells transfected with and expressing a large number of normal human insulin receptors (HTC-IR cells), the sensitivity of cells to insulin to stimulate S6 kinase was increased tenfold when compared to untransfected wild type HTC cells (HTC-WT cells). However, in cells transfected with and expressing a large number of mutated human insulin receptors where the tyrosines at three major autophosphorylation sites (1158, 1162, and 1163) were mutated to phenylalanines (HTC-F3 cells), there was no change in insulin sensitivity when compared to HTC-WT cells. We next studied the effect of a human-specific monoclonal antibody to the human insulin receptor, MA-5, on S6 kinase activation. In HTC-WT cells, MA-5 did not interact with endogenous rat insulin receptors and thus did not stimulate S6 kinase. In HTC-IR cells expressing normal human insulin receptors, MA-5 stimulated S6 kinase. Interestingly, MA-5, unlike insulin, was also able to stimulate S6 kinase in HTC-F3 cells expressing mutated receptors. In order to further understand the signaling mechanisms by MA-5 and insulin, two potential intermediate protein kinases were investigated. Neither insulin nor MA-5 appears to activate either microtubule-associated protein 2 (MAP-2) kinase or protein kinase C in these cells. These studies suggest therefore that: 1) insulin and MA-5 may signal S6 kinase activation by independent mechanisms that do not employ either MAP-2 kinase or protein kinase C; and 2) under certain circumstances, S6 kinase appears to be activated by mechanisms that are independent of insulin receptor tyrosine autophosphorylation.

Regulation of biological functions by an insulin receptor monoclonal antibody in insulin receptor beta-subunit mutants

We investigated the effects of MA-5, a human-specific monoclonal antibody to the insulin receptor alpha-subunit, on transmembrane signaling in cell lines transfected with and expressing both normal human insulin receptors and receptors mutated in their beta-subunit tyrosine kinase domains. In cell lines expressing normal human insulin receptors, MA-5 stimulated three biological functions: aminoisobutyric acid (AIB) uptake, thymidine incorporation, and S6 kinase activation. Under conditions where these biological functions were stimulated, there was no detectable stimulation of receptor tyrosine kinase. We then combined the use of this monoclonal antibody with cells expressing insulin receptors with mutations in the beta-subunit tyrosine kinase domain; two of ATP binding site mutants V1008 (Gly----Val) and M1030 (Lys----Met) and one triple-tyrosine autophosphorylation site mutant F3 (Tyr----Phe at 1158, 1162, and 1163). In cells expressing V1008 receptors, none of the three biological functions of insulin was stimulated. In cells expressing M1030 receptors, AIB uptake was stimulated to a small, but significant, extent whereas the other two functions were not. In cells expressing F3 receptors, AIB uptake and S6 kinase activation, but not thymidine incorporation, were fully stimulated. The data suggest, therefore, that (1) activation of insulin receptor tyrosine kinase may not be a prerequisite for signaling of all the actions of insulin and (2) there may be multiple signal transduction pathways to account for the biological actions of insulin.

Hepatic insulin and EGF receptor phosphorylation and dephosphorylation in fetal rats

Hepatic insulin receptor and epidermal growth factor (EGF) receptor phosphorylation and dephosphorylation were studied in normal and growth-retarded fetal rats. Insulin receptor autophosphorylation at a subsaturating ATP concentration (0.5 microM) increased by 10-fold from day 17 to 21 of gestation and decreased by 50% in term growth-retarded fetuses of fasted mothers. In vitro kinase activation at 0.5 mM ATP did not change with gestation or maternal fasting. EGF receptor autophosphorylation increased in parallel with receptor number with advancing gestation and did not change with maternal fasting. Protein tyrosine phosphatases (PTPases), which might attenuate receptor signaling in livers from growth-retarded fetuses, were measured using polybasic and polyacidic artificial substrates as well as the insulin receptor kinase domain. Fetal membrane PTPase activities were twofold higher than in the adult and declined with advancing gestation. However, activities were similar in normal and growth-retarded fetuses. We conclude that decreased hepatic growth in growth-retarded fetuses may involve decreased insulin receptor tyrosine kinase activation in vivo, as indicated by diminished receptor autophosphorylation at subsaturating ATP concentrations. Changes in EGF receptor kinase activity and PTPases could not be implicated based on our in vitro findings.

Control of insulin receptor autophosphorylation by polypeptide substrates: inhibition and stimulation by interaction with the catalytic subunit

Autophosphorylation of purified insulin receptor, in the absence of insulin, was stimulated by selected polypeptide substrates. In the presence of 1 microM insulin these peptides inhibited autophosphorylation. Stimulation was observed with reduced [S-(carboxamidomethyl)cysteinyl]lysozyme (RCAM-lysozyme) and three peptides generated by CNBr cleavage, V8 proteinase digestion and/or chemical modification. We also generated two peptide substrates from RCAM-lysozyme which did not stimulate receptor autophosphorylation and were very weak inhibitors. As a control peptide, the simple substrate angiotensin inhibited receptor autophosphorylation in the absence or presence of insulin. However, stimulatory peptide, but not insulin, significantly shifted the concentration dependence for inhibition by angiotensin. The stimulatory peptides also increased autophosphorylation of the cloned cytoplasmic domain of the kinase [R-BIRK; Villalba, M., Wente, S. R., Russell, D. S., Ahn, J., Reichelderfer, C. F., & Rosen, O. M. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 7848]. Therefore, stimulation occurs by interaction with the cytoplasmic process of the beta-subunit and not through interaction with the insulin binding alpha-subunit of the native receptor. Autophosphorylation was analyzed by mapping 32P-labeled tryptic phosphopeptides from the beta-subunit and from R-BIRK. Nearly identical phosphopeptide maps were found, comparing first, basal R-BIRK and basal native receptor, second, peptide- and insulin-stimulated native receptor, and third, peptide-stimulated R-BIRK and insulin-stimulated native receptor. Therefore, R-BIRK functions as a basal-state enzyme and can be stimulated in an insulin-like manner. On the basis of these observations, stimulation by insulin and by peptides yields similar functional results, but by apparently different mechanisms.

Insulin receptor activity and insulin sensitivity in mammary gland of lactating rats

The mammary gland is a tissue that is extremely sensitive to insulin during lactation; during weaning, the effect of insulin is rapidly abolished. The purpose of this study was to characterize the mammary gland insulin receptors and their kinase activity in lactating and weaned mammary gland. The apparent molecular weight of the alpha-subunit was slightly lower in the mammary gland than in liver and white adipose tissue (127,000 vs. 134,000), but the apparent molecular weight of the beta-subunit was similar in the three tissues (95,000). Insulin induced a 10-fold increase in beta-subunit autophosphorylation, and the half-maximal effect was achieved at 2 nM insulin. After 24 h of weaning, the number of insulin receptors was decreased by 30%, but the kinase activity of the beta-subunit was unchanged. During the euglycemic hyperinsulinemic clamp, insulin entirely activated pyruvate dehydrogenase in lactating rat mammary gland, whereas after 24 h of weaning it was unable to increase the proportion of the enzyme in the active form. These results suggest that the site of alteration in the action of insulin on the mammary gland during weaning is distal to the receptor.

Decreased autophosphorylation and kinase activity of insulin receptors in diabetic Chinese hamsters

To clarify the role of the insulin receptor in diabetes, the hepatic insulin receptor was investigated in the spontaneously diabetic Chinese hamsters, which are the animal models for insulin-deficient diabetes. Insulin binding in the diabetic animals increased mainly due to an increase in the number of receptors. It was also observed that both the autophosphorylation and kinase activity of the hepatic insulin receptor were decreased in the diabetic animals compared to the control animals. These changes in the hepatic insulin receptor may be caused by the diabetes itself. As the phosphorylated protein of 95 kDa was immunoprecipitated by the anti-insulin receptor antibody (B-10, human) in both diabetics and controls, it was supposed that the 95 kDa protein would be the beta-subunit of insulin receptors, as in other animals. These animals seem to be useful for examining insulin receptors in diabetes.

Evidence for the involvement of vicinal sulfhydryl groups in the insulin stimulation of intracellular glucose metabolism in Zajdela hepatoma cells

Phenylarsine oxide (PhAsO), a dithiol reagent that blocks insulin stimulation of glucose transport in 3T3 L1 cells, also altered insulin stimulation of intracellular glucose metabolism in Zajdela Hepatoma cultured cells. PhAsO (2 microM) similarly inhibited the insulin-induced glycogen and lipid syntheses without modifying the basal level of these processes, cell viability or the ATP content. Prior incubation of the cells with PhAsO did not prevent insulin binding to the cells, or activation of the receptor tyrosine kinase, while it minimally (16%) altered receptor internalization. These results indicate that cellular dithiols located at a post-receptor step are involved in the transduction of the insulin signal to intracellular glucose metabolism.

Intrinsic kinase activity of the insulin receptor

Since the identification of the insulin receptor by insulin-binding activity almost two decades ago, our understanding of the structure and function of the insulin receptor has progressed tremendously. The importance of the intrinsic tyrosine protein kinase activity of the insulin receptor is implied by the fact that the insulin receptor belongs to a family of receptor tyrosine kinases which play a role in growth control, by experiments demonstrating the intimate association of normal kinase activity and insulin action, and by evidence that the intrinsic kinase activity can be regulated under certain conditions. There are still some major gaps in our knowledge concerning the structure/function of the insulin receptor such as how activation of the intrinsic kinase activity of the receptor leads to altered cellular physiology. The kinase may phosphorylate endogenous substrates or autophosphorylation may simply alter beta subunit conformation so it can then interact with an effector system (i.e. a serine kinase) directly, or indirectly through a G-protein. The truth may lie somewhere between these two pathways.

Pervanadate [peroxide(s) of vanadate] mimics insulin action in rat adipocytes via activation of the insulin receptor tyrosine kinase

Both vanadate and hydrogen peroxide (H2O2) are known to have insulin-mimetic effects. We previously reported that the mixture of vanadate plus H2O2 results in the generation of a peroxide(s) of vanadate, which strongly enhances IGF-II binding to rat adipocytes (Kadota et al., 1987b). We now report that pervanadate mimics insulin in isolated rat adipocytes to (1) stimulate lipogenesis, (2) inhibit epinephrine-stimulated lipolysis, and (3) stimulate protein synthesis. The efficacy of pervanadate is comparable to that of insulin. However, it is 10(2)-10(3) times more potent than vanadate alone. Exposure of intact rat adipocytes to pervanadate was found to activate the WGA-purified insulin receptor tyrosine kinase assayed with the exogenous substrate poly(Glu80/Tyr20) in a dose-dependent manner to a maximum of 1464% of control at 10(-3) M compared with a maximum insulin effect of 1046% at 10(-6) M. In contrast, in vitro assayed autophosphorylation of the WGA-purified extract was increased 3-fold after exposure of intact cells to insulin but not significantly increased after pervanadate. Furthermore, high concentrations of pervanadate (10(-5) M) inhibited subsequent in vitro added insulin-stimulated autophosphorylation. In vitro addition of pervanadate to WGA-purified receptors could not stimulate autophosphorylation or exogenous tyrosine kinase activity and did not inhibit insulin-stimulated autophosphorylation. Labeling of intact adipocytes with [32P]orthophosphate followed by exposure to 10(-4) M pervanadate increased insulin receptor beta-subunit phosphorylation (7.9 +/- 3.0)-fold, while 10(-7) M insulin and 10(-4) vanadate increased labeling (5.3 +/- 1.8)- and (1.1 +/- 0.2)-fold, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

The insulin receptor: structure and function

Promising progress in understanding the molecular basis of insulin action has been achieved by demonstrating that the insulin receptor is an insulin-sensitive tyrosine kinase. Here we discuss the structure of this receptor kinase and compare it with receptors for related growth factors. We review the known modes to regulate the receptor kinase activity, either through its autophosphorylation (on tyrosine residues) or through its phosphorylation by other kinases (on serine and threonine residues). We discuss the role of the receptor kinase activity in hormone signal transduction in light of results indicating a reduced kinase activity in insulin-resistant states. Finally, studies to identify natural substrates for the insulin receptor kinase are presented. The possible physiological role of these phosphorylated substrates in mediating insulin action is evaluated.

Effects of hypercortisolemia and diabetes on skeletal muscle insulin receptor function in vitro and in vivo

Activation of skeletal muscle insulin receptor tyrosine kinase in vitro and in vivo was studied in two rat models of insulin resistance: insulinopenic diabetes and hypercortisolemia. In control rats, intravenous insulin administration resulted in dose-dependent in vivo activation of the muscle insulin receptor kinase towards histone H2b. Half-maximal and maximal activation were observed 5 min after injecting 0.1 and 0.5 U insulin/100 g, respectively. Diabetes (7 days) induced with streptozotocin did not affect insulin binding affinity of solubilized muscle receptors but depressed receptor kinase activation in vivo by 52 or 40% after intravenous insulin administration (0.1 or 2 U/100 g, respectively). Cortisone treatment (5 days) resulting in weight loss, hyperglycemia, and hyperinsulinemia did not affect the number, insulin binding affinity, or kinase activity of solubilized receptors activated with insulin in vitro or in vivo. It is concluded that impaired insulin receptor tyrosine kinase activation was demonstrated in vivo in rats with insulinopenic diabetes and that glucocorticoid-induced insulin resistance probably reflects postreceptor defect(s) in muscle.

Contrasting interactions between phorbol ester and insulin on the regulation of glycogen synthase activity and p33 mRNA accumulation in rat hepatoma cells

We have compared the effect of phorbol 12-myristate 13-acetate (PMA) with that of insulin on three targets of insulin action in H4IIEC3 (H4) rat hepatoma cells. These parameters are the phosphorylation state and tyrosine kinase activity of the insulin receptor, the activation state of glycogen synthase, and the accumulation of p33 mRNA. Under conditions where insulin treatment of H4 cells clearly activated receptor serine and tyrosine phosphorylation on the insulin receptor beta-subunit in situ, activated receptor tyrosine kinase activity in vitro, and activated glycogen synthase and p33 mRNA accumulation in situ, PMA alone did not influence the insulin receptor phosphorylation state or tyrosine kinase activity and did not affect glycogen synthase activity, but markedly increased p33 mRNA accumulation. When PMA was added in the presence of insulin, particularly if PMA was preincubated, the receptor phosphorylation state and the tyrosine kinase activity again were not affected, but insulin-activated glycogen synthase was significantly diminished or abolished. In contrast, increased p33 mRNA accumulation by PMA was additive with that of insulin. Thus, under conditions where no effect was observed on the insulin receptor phosphorylation state or the tyrosine kinase activity, PMA acted in an insulin-antagonistic manner on glycogen synthase and in an insulin-like manner on p33 mRNA accumulation, indicating that these actions of PMA are unrelated to early events in the pathway of the insulin action. Effects on glycogen synthase are most readily explained by an effect of protein kinase C-activated phosphorylation of glycogen synthase.

After insulin binds

Three recent advances pertinent to the mechanism of insulin action include (i) the discovery that the insulin receptor is an insulin-dependent protein tyrosine kinase, functionally related to certain growth factor receptors and oncogene-encoded proteins, (ii) the molecular cloning of the insulin proreceptor complementary DNA, and (iii) evidence that the protein tyrosine kinase activity of the receptor is essential for insulin action. Efforts are now focusing on the physiological substrates for the receptor kinase. Experience to date suggests that they will be rare proteins whose phosphorylation in intact cells may be transient. The advantages of attempting to dissect the initial biochemical pathway of insulin action include the wealth of information about the metabolic consequences of insulin action and the potential for genetic analysis in Drosophila and in man.

Specific inhibitors of tyrosine-specific protein kinase, synthetic 4-hydroxycinnamamide derivatives

Several newly synthesized 4-hydroxycinnamamide derivatives such as 3-(3',5'-di-isopropyl-4'-hydroxybenzylidene)-2-oxindol (ST 280), 3-(3',5'-di-methylthiomethyl-4'-hydroxybenzylidene)-2-oxindole (ST 458), alpha-cyano-3-ethoxy-4-hydroxy-5-phenylthiomethylcinnamamide (ST 638) and 3-(3'-ethoxy-4'-hydroxy-5'-phenylthiomethylbenzylidene)-2-pyrol idinone (ST 642) were found to inhibit tyrosine-specific protein kinase activity of the epidermal growth factor (EGF) receptor with IC50 values of 0.44 microM, 0.44 microM, 0.37 microM and 0.85 microM, respectively. None of them showed inhibitory effect on the enzyme activities of serine- and/or threonine-specific protein kinases such as cAMP-dependent protein kinase, Ca2+/phospholipid-dependent protein kinase C, casein kinase I and casein kinase II. In addition, none of them had effect on Na+/K+-ATPase or 5'-nucleotidase. The results suggest that the compound ST 280, ST 458, ST 638 and ST 642 are potent and specific inhibitors of tyrosine-specific protein kinase.