Prolactin stimulates proliferation of the glucose-dependent beta-cell line INS-1 via different IRS-proteins

Prolactin is one of the most potent growth stimulating growth hormones of pancreatic beta cells. We investigated the role of prolactin on the proliferation of the beta-cell line INS-1. In particular, we investigated the involvement of intracellular signal transduction molecules in prolactin-dependent upregulation of INS-1 growth. The effect of prolactin on the growth of INS-1 cells was assessed in vitro under various feeding conditions. Cell proliferation was measured in the pancreatic beta-cell line INS-1 using 3H-thymidine incorporation. The activation of mitogenic signaling proteins was assessed by co-immunoprecipitation, immunoblot analysis and in proliferation assays using specific protein inhibitors. Prolactin (0.5-2 nM) increased INS-1 cell proliferation in the presence of 3-24 mM glucose up to 48 fold, having a maximum in the presence of physiological glucose concentrations (6 mM). Prolactin activated the JAK2/STAT5 pathway and phosphatidylinositol-3'-kinase (PI3'K) in the presence of all the glucose concentrations used (3-15 mM). At low glucose concentrations (3 mM), PI3'K activation occurred through IRS-2 phosphorylation whereas, in the presence of physiological glucose concentration IRS4 and at high glucose concentrations (15 mM), IRS-1 triggered a proliferative effect. PI3'K activation was essential for prolactin and glucose stimulated INS-1 cell proliferation. Co-stimulation with different growth factors (IGF-I, growth hormone) in addition to prolactin and glucose had no additive effects. These results define prolactin as an important hormone. mediating glucose-dependent pancreatic beta-cell proliferation primarily by the activation of PI3'K-dependent signaling pathways.