Growth hormone (GH)-stimulated insulin-like growth factor I gene expression is mediated by a tyrosine phosphorylation pathway depending on C-terminal region of human GH receptor in human GH receptor-expressing Ba/F3 cells.

The signaling pathway of GH-stimulated IGF-I gene expression is still unclear, although it has been reported that the Janus kinase (JAK)-signal transducers and activators of transcription (STAT)5b pathway plays an important role in liver IGF-I expression. In this study, the GH-dependent IGF-I gene expression and its intracellular signaling mechanism have been examined in mouse pro-B, Ba/F3 cells stably expressing human GH receptor (Ba/F3-hGHR). The IGF-I gene expression was stimulated by human GH (0.01-10 nm) in a dose-dependent fashion in Ba/F3-hGHR cells. The specific inhibitors for JAK2 remarkably suppressed the GH-induced IGF-I gene expression, but MAPK or phosphatidylinositol 3 kinase-specific inhibitors failed to block the GH stimulation of the IGF-I gene expression. However, genistein, a nonspecific tyrosine kinase inhibitor that does not inhibit JAK2 and STAT5 phosphorylation, significantly suppressed the GH-induced IGF-I gene expression. Additionally, a Ba/F3-hGHR mutant that contained the truncated C-terminal hGHR up to D351 showed no IGF-I gene expression in response to human GH. The D351 form normally has the GH-induced JAK/STAT5 tyrosine phosphorylation. These results suggest that the JAK-STAT5 pathway and the novel tyrosine phosphorylation pathway, dependent on signaling from the C-terminal region of hGHR, might be involved in the GH-stimulated IGF-I gene expression in Ba/F3 cells.