IGF-II, IGF-binding proteins and IGF receptors in pancreatic beta-cell lines

The IGFs are mitogenic agents which are closely linked to regulatory processes in carbohydrate metabolism. Because limited information is available on the occurrence of the IGF system in the pancreatic beta-cell milieu, we evaluated the presence of IGFs, IGF receptors, and IGF-binding proteins (IGFBPs) in the beta-cell lines beta TC3 and HIT T-15. Serum-free conditioned media (SFCM) from beta TC3 cells contained IGF-II at concentrations greater than 100 ng/ml. High (15 kDa) and low (7.5 kDa) molecular weight IGF-II were detected both by column chromatography followed by RIA and by immunoblotting. GH (10-1000 ng/ml) conditioning of beta TC3 cells stimulated IGF-II secretion in a dose-dependent manner. IGF-II mRNA was detected in beta TC3 cells using Northern blots, and also showed a GH-dependent relationship. IGF-II peptide was detected in SFCM from HIT cells, albeit at lower concentrations. To evaluate the presence of IGF receptors in beta-cell lines, affinity cross-linking studies were performed on beta TC3 cells, demonstrating type I IGF receptors which bound iodinated IGF-II with high affinity, iodinated IGF-I with lesser affinity, and had minimal appreciable binding to iodinated insulin. Type II IGF receptors were not detected. SFCM from beta TC3 and HIT cells was subjected to Western ligand blotting, which disclosed the presence of two major IGFBPs of 29 kDa and 24 kDa, characteristic of IGFBP-2 and IGFBP-4. The identity of the specific IGFBPs was confirmed by immunoprecipitation and Northern blotting. Varying the glucose concentration had no significant effect on the levels of IGFBPs, nor did preconditioning with GH, IGF-I, IGF-II, insulin, or glucagon. Levels of both IGFBPs in beta TC3 cell-conditioned media increased in the presence of dexamethasone at concentrations of 10(-6) M or greater. In summary, we present evidence that beta-cell lines comprise an environment for GH and IGF action. We speculate that IGFs, their receptors and binding proteins function as a complex interactive system which regulates beta-cell growth and function.