bFGF promotes photoreceptor cell survival in vitro by PKA-mediated inactivation of glycogen synthase kinase 3β and CREB-dependent Bcl-2 up-regulation

Although there is substantial evidence supporting the neuroprotective efficacy of basic fibroblast growth factor (bFGF) in the rodent retina there is no consensus to date as to the protective mechanism involved. We hypothesise that bFGF can assert its neuroprotective effects directly on mouse photoreceptors transduced via the activation of specific intracellular signalling pathways. In mouse photoreceptor-derived 661W cells, bFGF promoted a rapid inactivation of glycogen synthase kinase 3β (GSK3β) by phosphorylation at Ser9.The effects of bFGF on GSK3β were dependent on protein kinase A (PKA) activation, as inhibition of this pathway blocked inactivation. Furthermore, bFGF protection against oxidative stress was dependent on PKA inactivation of GSK3β as PKA inhibition attenuated bFGF-induced protection. Furthermore, transfection of cells with mutant dominant negative GSK3βS9A that cannot be phosphorylated on Ser9 also abrogated neuroprotection. Activation of the transcription factor cAMP-response element binding protein (CREB) and subsequent up-regulation of Bcl-2 in response to bFGF was also dependent on PKA as inhibition with H-89 attenuated increased pCREB levels and Bcl-2 expression. These results indicate that the protective efficacy of bFGF in mouse photoreceptors involves PKA-dependent inactivation of GSK3β and subsequent up-regulation of Bcl-2 via CREB activation.