INSULIN-LIKE GROWTH-FACTOR-I STIMULATES C-FOS AND C-JUN TRANSCRIPTIONIN PC12 CELLS

We analyzed the effects of insulin-like growth factor-I (IGF-I), a pol ypeptide growth factor which exerts mitogenic effects via specific mem brane receptors. The control of IGF-I on c-fos and c-jun transcription was studied in PC12 cells. Gel mobility shift assays with a labeled A P1 consensus binding sequence (TRE: TGACTCA) showed an increase in spe cific binding upon trIGF-I treatment. Gene transfer studies revealed t hat the increase in AP1 binding is functional since IGF-I stimulates t ranscription from a reporter gene containing the minimal TRE linked to the chloramphenicol acetyl transferase (CAT) reporter gene. To furthe r characterize the molecular mechanism by which IGF-I increases AP1 ac tivity, we analysed the transcription regulation of c-fos and c-jun us ing reporter genes containing the respective promoters or specific reg ulatory elements. Deletion studies with the c-jun promoter, showed tha t IGF-I stimulates c-jun transcription via a cis acting element(s) loc alized within the 132 base pairs prior to the transcription start site ; possibly the AP1 like element TGACATCA. Similar studies revealed tha t c-fos stimulation by IGF-I requires the presence of a regulatory seq uence spanning the dyad symmetry element (DSE) and the fos AP1-like se quence (FAP). Further experiments using specific elements linked to th e minimal unresponsive c-fos promoter, showed that the DSE is the main target for c-fos induction by IGF-I.