We have studied the phosphorylation state of the insulin receptor during receptor-mediated endocytosis in the well-differentiated rat hepatoma cell line Fao. Insulin induced the rapid internalization of surface-iodinated insulin receptors into a trypsin-resistant compartment, with a 3-fold increase in the internalization rate over that seen in the absence of insulin. Within 20 min of insulin stimulation, 30-35% of surface receptors were located inside the cell. This redistribution was half-maximal by 10.5 min. Similar results were obtained when the loss of surface receptors was measured by 125I-insulin binding. Tyrosyl phosphorylation of internalized insulin receptors was measured by immunoprecipitation with antiphosphotyrosine antibody. Immediately after insulin stimulation, 70-80% of internalized receptors were tyrosine phosphorylated. Internalized receptors persisted in a phosphorylated state after the dissociation of insulin but were dephosphorylated prior to their return to the plasma membrane. After 45-60 min of insulin stimulation, the tyrosine phosphorylation of the internal receptor pool decreased by 45%, whereas the phosphorylation of surface receptors was unchanged. These data suggest that insulin induces the internalization of phosphorylated insulin receptors into the cell and that the phosphorylation state of the internal receptor pool may be regulated by insulin.

The receptors for insulin and epidermal growth factor undergo tyrosine autophosphorylation in response to ligand stimulation, while pp60v-src is an unregulated tyrosine kinase. In this report we show that each of the kinases phosphorylates an exogenous peptide that corresponds to the insulin proreceptor sequence 1142-1153. When the kinases were pre-phosphorylated, saturable Michaelis-Menten kinetics were observed. However, when the kinases had not been pre-phosphorylated biphasic kinetics were observed; at progressively higher substrate concentrations (greater than Km) less substrate phosphorylation was seen. Furthermore, when the kinases had not been pre-phosphorylated kinase autophosphorylation was inhibited at high substrate concentrations. On this basis we postulated that the substrate inhibition of substrate phosphorylation resulted directly from substrate inhibition of kinase autophosphorylation. To test this we designed additional peptides to function specifically as inhibitors of the kinases. Each of the 3 tyrosine residues within the substrate sequence were replaced either by 4-methoxyphenylalanine or phenylalanine, residues structurally similar to tyrosine but unable to accept phosphoryl transfer. Both analogs inhibited insulin and epidermal growth factor receptor autophosphorylation, whereas only the Phe-substituted analog inhibited pp60v-src phosphorylation. These data suggest that autophosphorylation of tyrosine residues near the kinase active site is a generalized mechanism for tyrosine kinase activation and that activation can be selectively blocked by substrates and nonphosphorylatable analogs.

Insulin actions and receptors were studied in capillary endothelial cells cultured from diabetic BB rats and their nondiabetic colony mates. The endothelial cells from diabetic rats of 2 mo duration had persistent biological and biochemical defects in culture. Compared with normal rats, endothelial cells from diabetic rats grew 44% more slowly. Binding studies of insulin and insulin-like growth factor I (IGF-I) showed that cells from diabetic rats had 50% decrease of insulin receptor binding (nondiabetic: 4.6 +/- 0.7; diabetic: 2.6 +/- 0.4% per milligram protein, P less than 0.01), which was caused by a 50% decrease in the number of binding sites per milligram protein, whereas IGF-I binding was not changed. Insulin stimulation of 2-deoxy-glucose uptake and alpha-aminoisobutyric acid uptake were also severely impaired with a 80-90% decrease in maximal stimulation, in parallel with a 62% decrease in insulin-stimulated autophosphorylation (P less than 0.05). 125I-insulin cross-linking revealed an 140-kD alpha subunit of the insulin receptor similar to that in cells from nondiabetic rats, although bands at greater than 200 kD were also detected. The molecular weight of the insulin receptor beta subunit (by SDS-PAGE) was smaller in cells from diabetic than from normal rats (88-90 vs. 95 kD). Neuraminadase treatment of the partially purified insulin receptors decreased the molecular weight of the insulin receptors from nondiabetic rats to a greater degree than its diabetic counterpart. In contrast, Northern blot analysis of insulin receptor mRNAs using human cDNA probes revealed two species of 9.4 and 7.2 kb with no difference in mRNA abundance between cells from diabetic and nondiabetic rats. We conclude that the exposure of capillary endothelial cells to a diabetic milieu in vivo can cause specific and persistent changes in the insulin receptor and insulin action.

Insulin and insulin-like growth factor-I (IGF-I) effects on protein phosphorylation were investigated in intact skeletal muscle cells at different stages of differentiation. In undifferentiated L6 myoblasts, stimulation by either insulin or IGF-I, but not IGF-II, led to a 3- to 5-fold increase in phosphorylation of insulin and IGF receptor beta-subunits and the appearance of a 175,000 mol wt (Mr) phosphoprotein (pp175). These effects reached a maximum within 3 min, were maintained for 12 min, and then declined. Dose-response curves for pp175 phosphorylation in response to insulin (ED50 = 2 nM) and IGF-I (ED50 = 0.2 nM) were consistent with occupancy and stimulation of each receptor kinase by its specific hormone. The 175,000 Mr phosphoprotein was not precipitated by antireceptor antibodies, and the phosphoamino acid composition differed markedly from that of insulin and IGF-I receptors, with a 10-fold lower phosphotyrosine/phosphoserine ratio after insulin stimulation. In contrast to insulin and IGF-I receptors, pp175 was not extracted by the nonionic detergent Triton X-100, but required sodium dodecyl sulfate for solubilization. When experiments were carried out with L6 cells after differentiation into skeletal muscle myotubes, hormone-induced phosphorylation of pp175 was almost undetectable. We conclude that pp175 is a phosphoprotein distinct from insulin and IGF-I receptors that is involved in the early phosphorylation events that follow the activation of the insulin and IGF-I receptor kinases. Its disappearance after terminal differentiation of the L6 cells is consistent with a role in hormonal stimulation of cell proliferation.

Using the well differentiated rat hepatoma Fao we have studied the regulation of phosphoenolpyruvate carboxykinase (PEPCK) mRNA by insulin and glucose and compared these results to glucose production as estimated by glucose release into the medium. Fao cells possess an active gluconeogenic pathway and, when grown in glucose-free medium, release glucose for over 8 h. Addition of the cAMP analog, 8-(4-chlorophenyl-thio) cAMP (8-CTP-cAMP) or increasing the concentration of dihydroxyacetone and oxaloacetate results in an increase in glucose release which can be suppressed by insulin at concentrations between 1 and 100 nM. These effect of cAMP and insulin are associated with parallel changes in the level of mRNAPEPCK. Insulin treatment reduces mRNAPEPCK levels in these cells by 80%; this effect is transient reaching a maximum at 2-4 h. Addition of glucose to cells grown in glucose-free (G-) medium produces a decrease in mRNAPEPCK which is similar in magnitude and kinetics to that produced by insulin. Conversely, when cells grown in normal medium are placed in G- medium mRNAPEPCK levels triple over a period of 8 h, then return toward the basal value. Cells grown in G- medium or in G- medium plus 10nM insulin for 1 yr exhibit only slightly increased levels of mRNAPEPCK and respond to both 8-CTP-cAMP, and insulin, although the response to 8-CTP-cAMP is slightly blunted. These data indicate that glucose and insulin can play independent roles in regulation of PEPCK gene expression, and that these regulatory effects are usually transient.

We studied the structure of the insulin-receptor gene in normal individuals and in four unrelated patients with leprechaunism (Minn-1, Ark-1, Ark-2, Can-1) and four unrelated patients with the type A syndrome of insulin resistance, both disorders associated with genetic alterations in affinity, binding capacity, and kinase activity of the insulin receptor. Genomic cloning and Southern blot analysis indicate that the normal human insulin-receptor gene is greater than or equal to 150 kilobases long and consists of a minimum of 17 exons, 6 in the genomic region of the alpha-subunit and 11 in the region of the beta-subunit. Three of the patients, one with leprechaunism and two with type A syndrome, have decreases in insulin-receptor mRNA but on genomic blot analysis have no obvious abnormalities in the insulin-receptor gene. No distinctive pattern of restriction-fragment-length polymorphisms or evidence for major insertion or deletion mutations of the insulin-receptor gene was found in any of the patients. These data indicate that the insulin-receptor gene is greater than 35 times larger than coding regions and has a complex structure. Although leprechaunism and type A syndrome are most likely due to defects in the structure and expression of the insulin-receptor gene, they are likely to be associated with specific point mutations rather than major changes in gene structure.

Phosphotyrosine phosphatase (PTPase) activity in rat liver was measured using a phosphopeptide substrate containing sequence identity to the major site of insulin receptor autophosphorylation. PTPase activity was detected in both cytosolic and particulate fractions of rat liver and produced linear dephosphorylation over a 15-min time course. In rats made insulin-deficient diabetic by streptozotocin treatment (STZ), cytosolic PTPase activity increased to 180% of the control values after 2 d of diabetes and remained elevated at 30 d (P less than 0.02). Gel filtration on Sephadex-75 revealed a single peak of activity in the cytosol in both control and diabetic animals and confirmed the increased levels. In BB diabetic rats, another model of insulin deficiency, the PTPase activity in the cytosolic fraction was increased to approximately 230% of control values. PTPase activity in the particulate fraction of liver was also increased by 30 and 80% after 2 and 8 d of STZ diabetes, respectively. However, this increase was not sustained and after 30 d of STZ diabetes, PTPase activity associated with the particulate fraction in the BB diabetic rat was reduced to approximately 70% of the control levels. Treatment of STZ diabetic rats with subcutaneous insulin or vanadate in their drinking water for 3 d reduced tyrosine PTPase activity in the particulate, but not in the cytosolic fraction. This was associated with a change in blood glucose toward normal. These data indicate insulin deficient diabetes is accompanied by significant changes in hepatic PTPase activity. Since tyrosine phosphorylation plays a central role in the cellular action of insulin receptor, an increase in PTPase activity may be an important factor in the altered insulin action associated with these diabetic states.

The relation between insulin-stimulated autophosphorylation of the insulin receptor and internalization of the receptor was studied in Fao rat hepatoma cells. Treatment of Fao cells with 2,4-dinitrophenol for 45 min depleted cellular ATP by 80% and equally inhibited insulin-stimulated receptor autophosphorylation, as determined by immunoprecipitation of surface-iodinated or [32P]phosphate-labeled cells with anti-phosphotyrosine antibody. In contrast, internalization of the insulin receptor and internalization and degradation of 125I-labeled insulin by 2,4-dinitrophenol-treated cells were normal. These data show that autophosphorylation of the insulin receptor is not required for the receptor-mediated internalization of insulin in Fao cells and suggest that insulin receptor recycling is independent of autophosphorylation.

Leprechaunism is a genetic form of insulin resistance characterized by severe growth retardation and early death. To clarify the molecular basis of the insulin resistance, we investigated the insulin binding and kinase properties of the insulin receptor and the receptor gene in cultured skin fibroblasts of two patients (Ark-1 and Ark-2) with leprechaunism and in those of three of their parents. Specific insulin binding to fibroblasts was markedly reduced (less than 25% of control) in both patients with leprechaunism but was essentially normal in the parents. In contrast, insulin receptor autophosphorylation in 1% Triton X-100 cell lysates was reduced in both patients and parents. In Ark-1, the 70% reduction in autophosphorylation correlated with the decrease in binding, whereas in Ark-2 and in the three parents included in the study, autophosphorylation of the insulin receptor was reduced below the level accounted for by a change in receptor content. Analysis of the insulin receptor gene by hybridization with the receptor cDNA probes revealed no gross defect in either Ark-1 or Ark-2. Both parents of Ark-2 were heterozygous for a restriction fragment length polymorphism in the beta-subunit detected with Bam HI digestion (observed in 15% of controls). Ark-2 was homozygous for the more common allele of this polymorphism (observed in 84% of controls). Thus, we have biochemically characterized a new family of leprechaunism (Ark-2) and have found insulin receptor phosphorylation defects in their phenotypically normal parents.