Mitogen activated protein kinase-dependent activation of c-Jun and c-Fos is required for neuronal differentiation but not for growth and stress response in PC12 cells

MAPK‐dependent activation of AP‐1 protein c‐Jun is involved in PC12 cell differentiation and apoptosis. However, the role of other AP‐1 proteins and their connection to MAPKs during growth, differentiation and apoptosis has remained elusive. Here we studied the activation of AP‐1 proteins in response to ERK, JNK, and p38 signaling upon NGF, EGF and anisomycin exposures. All treatments caused different kinetics and strength of MAPK and AP‐1 activities. NGF induced persistent ERK and AP‐1 activities, whereas upon EGF and anisomycin exposures, their activities were only weakly and transiently induced. The sustained AP‐1 activity was associated with concomitant c‐Fos and c‐Jun expression and phoshorylation, which were JNK and ERK dependent. While inhibition of the ERK, JNK, and p38 activities partially prevented AP‐1 activity and suppressed differentiation, none of them was required for anisomycin‐induced apoptosis. The importance of c‐Fos and c‐Jun as mediators of differentiation was demonstrated by the findings that the corresponding siRNAs suppressed NGF‐induced neurite outgrowth. However, the capacity of c‐Fos to promote differentiation required cooperation with Jun proteins. In contrast, Fra‐2 expression was not required for the differentiation response. Together, the results show that sustained c‐Jun and c‐Fos activities mediate MAPK signaling and are essential for differentiation of PC12 cells. J. Cell. Physiol. 210: 538–548, 2007. © 2006 Wiley‐Liss, Inc.