Nuclear translocation of p90Rsk and phosphorylation of CREB is induced by ionomycin in a Ras-independent manner in PC12 cells.

In the present study we examined the possible role of p90Rsk in pathways leading to neuronal differentiation of PC12 cells induced by nerve growth factor (NGF) and the calcium ionophore ionomycin. PC12-M17 cells, expressing a dominant inhibitory Ras protein, do not undergo neuronal differentiation in response to NGF like wild-type PC12 cells, but exhibit neurite outgrowth when treated with NGF in combination with ionomycin. However, the blockade of Ras in these cells results in failure of activation of mitogen-activated protein kinase (MAPK)/extracellular signal regulation kinase (ERK) (MEK) and ERK activation as well, therefore kinases other than those of the ERK pathway might play a role in the induction of neuronal differentiation in this case. Here we show that p90Rsk translocates to the nucleus in response to ionomycin in both wild-type PC12 and PC12-M17 cells, and this spatial distribution is followed by increased phosphorylation of the cAMP response element binding protein (CREB). Since CREB is believed to be the transcription factor that can integrate Ca2+, growth factor and cAMP-induced signals, we suggest that p90Rsk may be one of the kinases which is able to replace ERKs under certain circumstances, thereby participating in Ras-independent neuronal differentiation induced by NGF plus ionomycin.