Insulin receptor tyrosine kinase activity in colon carcinoma

Colon carcinoma is the most common tumor of the gastrointestinal tract. According to some investigators, insulin, epidermal growth factor (EGF) and insulin-like growth factor I (IGF-I) man be involved in the neoplastic proliferation. Insulin-binding and receptor tyrosine kinase activity were investigated in colon carcinomas and in normal colons. The insulin receptor concentration, as shown by binding assays, was 17.4 +/- 4.3 fmol/micrograms in normal colon and 29.69 +/- 9.4 fmol/micrograms in colon carcinoma. Nevertheless, the insulin affinity of the receptor was similar in both groups (Kd identical to 1 nM). Both normal and neoplastic colon showed phosphorylation of the insulin receptor. The electrophoretic migration of the beta-subunit of the insulin receptors purified from colon carcinomas was similar to that of normal colon and both tissues demonstrated an insulin-dependent autophosphorylation. The receptor tyrosine kinase activity was measured by the incorporation of [gamma 32P]ATP into the beta-subunit. The basal and the insulin-stimulated tyrosine kinase activities were significantly higher in colon carcinomas compared to normal colon tissues (2.2 and 1.6 times, respectively). Understanding the metabolism of neoplastic cells may contribute to the development of prevention strategies as well as new therapies. It is now necessary to study other steps of the insulin signal transduction pathway, such as insulin receptor substrate 1 phosphorylation.

Characterization of receptors for insulin-like growth factor type I on cultured human endometrial stromal cells: downregulation by progesterone

Because insulin-like growth factor type I (IGF-I) is reputed to be involved in the endometrial decidualization, we analyzed the expression of IGF-I receptors in an in vitro system of human endometrial stromal cells. Competitive binding studies of both intact stromal cells and membrane preparation indicated the presence of specific components with high affinity for binding IGF-I. Half-maximum displacement was obtained with 2.3 nmol/l native IGF-I, whereas insulin was unable to achieve half-maximum displacement even at higher concentrations. This IGF-I binding component was found to be a saturable protein in respect of the radioligand [125I]IGF-I, with a dissociation constant of 0.16 nmol/l. Affinity cross-linking studies revealed a labelled band of approximate relative molecular mass 135000, corresponding to the known alpha-subunit of IGF-I receptor. This band was significantly inhibited dose-dependently by the IGF-I receptor monoclonal antibody alpha-IR3 or native IGF-I, suggesting that the IGF-I binding component in the membrane of stromal cells has the identity of the alpha-subunit of IGF-I receptor. Cell proliferation in vitro was stimulated by progesterone. Furthermore, progesterone downregulated the [125I]IGF-I binding activity by downregulation of the IGF-I membrane receptor of human endometrial stromal cells. These data show that the IGF-I receptor is a functionally integral component of the stromal cell membrane structure, and its expression might be directly modulated by progesterone and, therefore, might play an important role in the preparation of the stroma for successful embryo implantation.

Tumor necrosis factor-alpha- and hyperglycemia-induced insulin resistance. Evidence for different mechanisms and different effects on insulin signaling

Inhibition of insulin receptor signaling by high glucose levels and by TNF-alpha was recently observed in different cell systems. The aim of the present study was to characterize the mechanism of TNF-alpha-induced insulin receptor inhibition and to compare the consequences of TNF-alpha- and hyperglycemia-induced insulin receptor inhibition for signal transduction downstream from the IR. TNF-alpha (0.5-10 nM) and high glucose (25 mM) showed similar rapid kinetics of inhibition (5-10 min, > 50%) of insulin receptor autophosphorylation in NIH3T3 cells overexpressing the human insulin receptor. TNF-alpha effects were completely prevented by the phosphotyrosine phosphatase (PTPase) inhibitors orthovanadate (40 microM) and phenylarsenoxide (35 microM), but they were unaffected by the protein kinase C (PKC) inhibitor H7 (0.1 mM), the phosphatidylinositol-3 kinase inhibitor wortmannin (5 microM), and the thiazolidindione troglitazone (CS045) (2 microgram/ml). In contrast, glucose effects were prevented by PKC inhibitors and CS045 but unaffected by PTPase inhibitors and wortmannin. To assess effects on downstream signaling, tyrosine phosphorylation of the following substrate proteins of the insulin receptor was determined: insulin receptor substrate-1, the coupling protein Shc, focal adhesion kinase (FAK125), and unidentified proteins of 130 kD, 60 kD. Hyperglycemia (25 mM glucose) and TNF-alpha showed analogous (> 50% inhibition) effects on tyrosine phosphorylation of insulin receptor substrate-1, Shc, p60, and p44, whereas opposite effects were observed for tyrosine phosphorylation of FAK125, which is dephosphorylated after insulin stimulation. Whereas TNF-alpha did not prevent insulin-induced dephosphorylation of FAK125, 25 mM glucose blocked this insulin effect completely. In summary, the data suggest that TNF-alpha and high glucose modulate insulin receptor-signaling through different mechanisms: (a) TNF-alpha modulates insulin receptor signals by PTPase activation, whereas glucose acts through activation of PKC. (b) Differences in modulation of the insulin receptor signaling cascade are found with TNF-alpha and high glucose: Hyperglycemia-induced insulin receptor inhibition blocks both insulin receptor-dependent tyrosine phosphorylation and dephosphorylation of insulin receptor substrate proteins. In contrast, TNF-alpha blocks only substrate phosphorylation, and it does not block insulin-induced substrate dephosphorylation. The different effects on FAK125 regulation allow the speculation that long-term cell effects related to FAK125 activity might develop in a different way in hyperglycemia- and TNF-alpha-dependent insulin resistance.

Insulin- and insulin-like growth-factor-I receptor tyrosine-kinase activities in human renal carcinoma

We studied expression and functional characteristics of the insulin- and insulin-like-growth-factor-I (IGF-I) receptors in human renal carcinoma. Ligand-binding properties and tyrosine-kinase activity of both receptors, as well as the expression of the 2 isoforms of the human insulin receptor (HIR-A and -B) were analyzed in renal carcinoma and normal adjacent kidney tissue of 8 adult patients. Partially purified insulin- and IGF-I receptors from normal and renal cell carcinoma tissue possessed identical affinities for their ligands. Renal cell carcinoma, however, contained 3- to 4-fold more specific insulin-binding sites and 2-fold more IGF-I binding sites than adjacent normal kidney tissue. In addition, we determined the relative content of insulin/IGF-I receptor hybrids in both tissues. Renal cell carcinoma and adjacent normal tissue revealed similar amounts of insulin/IGF-I receptor hybrids, i.e., 44 +/- 8.2% of tracer IGF-I binding in normal tissue and 46 +/- 12.0% in renal cell carcinoma. When equal amounts of insulin- and IGF-I receptor protein were studied, we found significantly increased receptor autophosphorylation and elevated substrate phosphorylation in carcinoma tissue. To assess whether the differences in insulin-receptor tyrosine-kinase activity were caused by an altered pattern of insulin receptor isoform expression, we determined mRNA levels for HIR-A and -B. The 2 insulin receptor isoforms were, however, expressed in highly variable ratios in both normal and tumor tissue. Our experiments show that renal carcinoma expresses an elevated amount of insulin- and IGF-I receptor protein with increased specific autophosphorylation and tyrosine-kinase activity each. The increase of insulin-receptor tyrosine-kinase activity in renal carcinoma cannot be explained by an altered expression pattern of insulin receptor isoforms.

Mechanism of insulin receptor kinase inhibition in non-insulin-dependent diabetes mellitus patients. Phosphorylation of serine 1327 or threonine 1348 is unaltered

The tyrosine kinase activity of insulin receptor isolated from the skeletal muscle of NIDDM patients has previously been found to be decreased compared with the activity of receptor from nondiabetic subjects but the mechanism underlying this defect is unknown. Phosphorylation of receptor serine/threonine residues has been proposed to exert an inhibitory influence on receptor tyrosine kinase activity and Ser 1327 and Thr 1348 have been identified as specific sites of phosphorylation in the insulin receptor COOH terminal domain. To address the potential negative regulatory role of phosphorylation of these residues in vivo, we assessed the extent of phosphorylation of each site in insulin receptor isolated from the skeletal muscle of 12 NIDDM patients and 13 nondiabetic, control subjects. Phosphorylation of Ser 1327 and Thr 1348 was determined using antibodies that specifically recognize insulin receptor phosphorylated at these sites. In addition, a phosphotyrosine-specific antibody was used to monitor receptor tyrosine phosphorylation. The extent of insulin-induced tyrosine autophosphorylation was decreased in receptor isolated from diabetic versus nondiabetic muscle, thus confirming earlier reports. In contrast, there was no significant difference in the extent of phosphorylation of either Ser 1327 or Thr 1348 in receptor isolated from diabetic or nondiabetic muscle as assessed by immunoprecipitation (Ser 1327: 5.6 +/- 1.6% diabetics vs. 4.7 +/- 2.0% control; Thr 1348: 3.8 +/- 1.0% diabetics vs. 3.2 +/- 1.2% control). Moreover, within each group there was no correlation between the level of tyrosine kinase activity and the extent of serine/threonine phosphorylation. It is concluded that the stoichiometry of serine/threonine phosphorylation of insulin receptor in vivo is low, and that increased phosphorylation of Ser 1327 or Thr 1348 is not responsible for the decreased insulin receptor tyrosine kinase activity observed in the skeletal muscle of NIDDM patients.