INDUCTION OF C-FOS AND C-JUN MESSENGER-RNA EXPRESSION BY BASIC FIBROBLAST GROWTH-FACTOR IN CULTURED RAT MULLER CELLS

PURPOSE. Exogenous basic fibroblast growth factor (bFGF) induces bFGF gene expression in cultured rat Muller cells. To elucidate the mechani sm that links exogenous bFGF to transcriptional regulation of bFGF gen e expression in these cells, the authors examined mRNA expression of t he proto-oncogenes c-fos and c-jun in response to exogenous bFGF in cu ltured rat Muller cells. METHODS. Muller cells from 1- to 3-day-old Sp rague-Dawley rats were isolated and cultured in essential modified Eag le's medium + 10% fetal calf serum. Cultured cells were identified by immunocytochemical analysis using antibodies against vimentin, carboni c anhydrase C, and glutamine synthetase. Cells of passages 1 through 4 were treated with bFGF (0.01, 0.1, 1, 10, and 100 ng/ml), either the protein kinase C (PKC) inhibitors H-7 (30 mu M) and GF109203X (1 mu M) or the PKC activator phorbol 12-myristate 13-acetate (PMA; 1, 10, 100 , 500 nM), and either adenylate cyclase activator forskolin (5 mu M) o r adenylate cyclase inhibitor SQ22536 (100 mu M). Northern blot analys is was performed to determine the mRNA expression of c-fos, c-jun, and bFGF. RESULTS. Addition of bFGF to culture medium induced c-fos and c -jun mRNA expression in a dose- and time-dependent manner. Induction o f c-fos mRNA was observed as early as 10 minutes (9.6-fold) after expo sure to bFGF at a dose of 10 ng/ml. It reached a maximum of 17.4-fold by 30 minutes. A rapid decline of c-fos mRNA level was observed after 45 minutes of bFGF treatment. The temporal pattern of c-jun gene expre ssion was similar to that of c-fos, whereas a maximum induction of c-j un mRNA (8.2-fold) was seen after 45 minutes of treatment. Induction o f c-fos and c-jun gene expression started at a bFGF concentration of 0 .1 ng/ml. It reached peak levels of 15-fold for c-fos and 7.6-fold for c-jun mRNA at 10 ng/ml. A dose-dependent upregulation of c-fos and c- jun gene expression by the PKC activator PMA was also observed. A maxi mum induction was seen at 100 nM PMA. The induction of c-fos and c-jun gene expression by bFGF or by PMA was blocked by the PKC inhibitors H -7 (30 mu M) or GF109203X (1 mu M). SQ22536 (100 mu M), an adenylate c yclase inhibitor, did not inhibit bFGF-induced c-fos and c-jun gene ex pression, whereas forskolin (5 mu M), an adenylate cyclase activator, upregulated the expression. CONCLUSIONS. These results indicate that e xogenous bFGF induces c-fos and c-jun gene expression in cultured rat Muller cells through PKC activation. The proto-oncogenes c-fos and c-j un may play a role in the regulation of bFGF gene expression in respon se to exogenous bFGF in retinal Muller cells. These findings provide f urther insight into the roles of Muller cells and exogenous bFGF in pr otecting against photoreceptor degeneration.