SIGNALING FROM G-PROTEIN COUPLED RECEPTORS TO THE C-JUN PROMOTER INVOLVES THE MEF2 TRANSCRIPTION FACTOR - EVIDENCE FOR A NOVEL C-JUN AMINO-TERMINAL KINASE-INDEPENDENT PATHWAY

The c-Jun amino-terminal kinases (JNKs) are a subfamily of mitogen-act ivated protein kinases that phosphorylate c-Jun and ATF2, and it has b een postulated that phosphorylated c-Jun enhances its own expression t hrough AP-1 sites on the c-jun promoter. In this study, we asked wheth er signals activating JNK regulate the c-jun promoter, Using NIH 3T3 c ells expressing G protein-coupled mi acetylcholine receptors as an exp erimental model, we have recently shown that the cholinergic agonist c arbachol, but not platelet derived growth factor, potently elevates JN K activity, Consistent with these findings, carbachol, but not platele t-derived growth factor, increased the activity of a c-jun promoter-dr iven reporter gene (for chloramphenicol acetyltransferase), However, c oexpression of JNK kinase kinase (MEKK) effectively increased JNK acti vity, but resulted in surprisingly limited induction of the c- jun pro moter, This raised the possibility that pathway(s) distinct from JNK c ontrol the c-jun promoter, and prompted us to explore which of its reg ulatory elements participate in transcriptional control. We observed t hat deletion of the 3' AP-1 site diminished chloramphenicol acetyltran sferase activity in response to carbachol, but only to a limited exten t, In contrast, deletion of a MEF2 site dramatically reduced expressio n, and deletion of both the MEF2 and 3' AP-1 sites abolished induction , Furthermore, cotransfection with MEF2C and MEF2D cDNAs potently enha nced the activity of the c-jun promoter in response to carbachol, and stimulation of mi receptors, but not direct JNK activation, induced ex pression of a MEF2-responsive plasmid. Taken together, these data stro ngly suggest that MEF2 mediates c-jun promoter expression by G protein -coupled receptors through a yet to be identified pathway, distinct fr om that of JNK.