Growth hormone exerts antiapoptotic and proliferative effects through two different pathways involving nuclear factor-kappaB and phosphatidylinositol 3-kinase.

Dependence of murine pro-B Ba/F3 cells on interleukin-3 can be substituted by GH when cells are stably transfected with the GH receptor (GHR) complementary DNA. Recently, we demonstrated that Ba/F3 cells produce GH, which is responsible for the survival of cells expressing the GHR. This GH effect involves the activation of nuclear factor-kappaB (NF-kappaB). Here, we examined the signaling pathways mediating proliferation of growth factor-deprived Ba/F3 GHR cells. Exogenous GH stimulation of Ba/F3 GHR cells induced cyclins E and A and the cyclin-dependent kinase inhibitor p21(waf1/cip1) and repressed cyclin-dependent kinase inhibitor p27(kip1). The presence of the phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor Ly 294002 abolished proliferation induced by GH, arresting Ba/F3 GHR cells at the G(1)/S boundary, but did not promote apoptosis. Thus, the proliferative effect of GH is closely related to PI 3-kinase activation, whereas PI 3-kinase is not essential for GH-induced cell survival. Addition of Ly 294002 resulted in a moderate decrease in NF-kappaB activation by GH, suggesting a possible link between PI 3-kinase and NF-kappaB signaling by GH. Expression of c-myc was also induced by GH in Ba/F3 GHR cells, and inactivation of either PI 3-kinase or NF-kappaB reduced this induction. Overexpression of the dominant negative repressor mutant c-Myc-RX resulted in an inhibition of the GH proliferative effect, suggesting the involvement of c-myc in GH-induced proliferation. Taken together, these results suggest that the effects of GH on cell survival and proliferation are mediated through two different signaling pathways, NF-kappaB and PI 3-kinase, respectively; although cross-talk between them has not been excluded. NF-kappaB, which has been shown to be responsible for the antiapoptotic effect of GH, could also participate in GH-induced proliferation, as c-myc expression is promoted by PI 3-kinase, in an NF-kappaB-dependent and -independent manner.