Induction of basic fibroblast growth factor mRNA by basic fibroblast growth factor in Muller cells

Purpose. To investigate the induction of basic fibroblast growth factor (bFGF) gene expression in cultured rat Muller cells by bFGF and to study the mechanism of induction. Methods. Muller cells from 1- to 3-day-old Sprague- Dawley rats were isolated and cultured with Dulbecco's modified Eagle's medium with 10% fetal calf serum. Cultured cells were identified by immunocytochemistry using antibodies against vimentin, carbonic anhydrase II, and glutamine synthetase. Cells of passages 1 through 4 were treated in bFGF, the protein kinase C (PKC) inhibitor, H-7; calphostin C, or the PKC activator, PMA; and protein kinase A (PKA) inhibitor, H-89; as well as the adenylate cyclase activator, forskolin; or the adenylate cyclase inhibitor, SQ22536. Northern blot analysis was performed to determine the mRNA expression of bFGF, ciliary neurotrophic factor (CNTF) and brain-derived neurotrophic factor (BDNF). Results. Addition of bFGF to culture medium induced bFGF gene expression in a dose- and time-dependent manner. Induction of bFGF mRNA started at a bFGF concentration of 0.1 ng/ml. The bFGF mRNA level was elevated by 2-fold at 1 ng/ml of bFGF, 2.8-fold at 5 ng/ml, and reached a peak of 4-fold at 10 ng/ml and 3.7-fold at 50 ng/ml. At 10 ng/ml of bFGF, induction of bFGF mRNA was observed as early as 2 hours (2-fold) after treatment. The bFGF mRNA level continued to increase to 3.7-fold by 4 hours, and reached a maximum of 4.4-fold by 8 hours. A slow decline of the bFGF mRNA level was observed after 8 hours of bFGF treatment (3.5-fold by 12 hours, and 3-fold by 24 hours). This induction of bFGF gene expression was blocked by PKC inhibitors H-7 (30 μM) and calphostin C (1 μM). The PKC activator PMA (0.1 μM) also upregulated bFGF gene expression, but the effects of bFGF and PMA were not additive. An adenylate cyclase inhibitor, SQ22536 (100 μM), did not inhibit bFGF-induced bFGF gene expression. Although forskolin (5 μM), an adenylate cyclase activator, also upregulated the level of bFGF mRNA, the effects of forskolin and bFGF were additive. In addition, no inhibitory effect on bFGF-induced expression of bFGF mRNA was found using H-89 (1 μM). Exogenous bFGF did not later the mRNA levels of CNTF and BDNF. Conclusions. These results indicate that bFGF induces bFGF gene expression in cultured rat Muller cells through PKC activation. The authors' findings raise the possibility that Muller cells in vivo also respond to available bFGF (for example, that released from the endogenous reservoirs in the case of injury) or to exogenous bFGF by producing more bFGF, which could in turn promote photoreceptor survival.