Growth hormone exerts antiapoptotic and proliferative effects through two different pathways involving nuclear factor-kappa B 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) compleme ntary DNA. Recently, we demonstrated that Ba/F3 cells produce GH, which is responsible for the survival of cells expressing the GHR. This GH effect in volves the activation of nuclear factor-kappaB (NF-kappaB). Here, we examin ed the signaling pathways mediating proliferation of growth factor-deprived Ba/F3 GHR cells. Exogenous GH stimulation of Ba/F3 GHR cells induced cycli ns E and A and the cyclin-dependent kinase inhibitor p21(waf1/cip1) and rep ressed cyclin-dependent kinase inhibitor p27(kip1). The presence of the pho sphatidylinositol 3-kinase (PI 3-kinase) inhibitor Ly 294002 abolished prol iferation 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 clos ely 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 in duced by GH in Ba/F3 GHR cells, and inactivation of either PI 3-kinase or N F-KB reduced this induction. Overexpression of the dominant negative repres sor mutant c-Myc-RX resulted in an inhibition of the GH proliferative effec t, suggesting the involvement of c-myc in GH-induced proliferation. Taken t ogether, these results suggest that the effects of GH on cell survival and proliferation are mediated through two different signaling pathways, NF-kap paB 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 a ntiapoptotic effect of GH, could also participate in GH-induced proliferati on, as c-nye expression is promoted by PI 3-kinase, in an NF-kappaB-depende nt and -independent manner.