Growth hormone- and prolactin-induced proliferation of insulinoma cells, INS-1 depends on activation of STAT5 (signal transducer and activator of transcription 5)

GH and PRL stimulate proliferation and insulin production of pancreatic bet a -cells. Whereas GH- and PRL-regulated transcription of the insulin gene i n insulinoma cells has been shown to depend on STAT5 (signal transducer and activator of transcription 5), the signaling pathways involved in GH/PRL-i nduced p-cell replication are unknown. The roles of various signaling pathw ays in human GH (hGH)-induced DNA synthesis were studied by analysis of the effect of specific inhibitors in both the insulin-producing cell line, INS -1, and in primary beta -cells. The mitogen-activated protein kinase kinase (MEK)-inhibitor, PD98059, as well as the mitogen-activated protein kinase p38 (MAPKp38) inhibitor, SB203580, partially inhibited hGH-induced prolifer ation in INS-1 cells but had no significant effect in primary beta -cells. Staurosporine, a protein kinase C (PKC) and protein kinase A (PKA) inhibito r, blocked both basal and hGH-induced proliferation in INS-1 cells, but had no inhibitory effect in primary beta -cells. Wortmannin, a phosphatidytino sitol 3-kinase (P13K) inhibitor, inhibited hGH-induced proliferation neithe r in INS-1 cells nor in primary beta -cells, whereas the tyrosine kinase in hibitor, genistein, completely inhibited hGHinduced proliferation in both p rimary beta -cells and INS-1 cells. To analyze the possible role of STAT5 i n hGH-induced proliferation, a dominant negative STAT5 mutant, STAT5 Delta 49, was expressed in INS-1 cells under the control of a doxycycline-inducib le promoter by stable transfection. Two clones were found to exhibit dose-d ependent, doxycycline-inducible expression of STAT5 Delta 749 and suppressi on of hGH-stimulated transcriptional activation of a STAT5-regulated PRL re ceptor (PRLR) promoter-reporter construct. Furthermore, induction of STAT5 Delta 749 expression completely inhibited hGH-induced DNA synthesis. Analys is of endogenous gene expression revealed a doxycycline-dependent inhibitio n of hGH-stimulated PRLR and cyclin D2 mRNA levels. Our results suggest tha t GH/PRL-induced beta -cell proliferation is dependent on the Janus Kinase2 (JAK2)/STAT5 signaling pathway but not the MAPK, PI3K, and PKC signaling p athways. Furthermore, the cell cycle regulator cyclin D2 may be a crucial t arget gene for STAT5 in this process.