A potential role of Ras-mediated signal transduction for the enhancement of depolarization-induced Ca 2+ responses in hippocampal neurons by basic fibroblast growth factor

Abstract

Chronic treatment with basic fibroblast growth factor (bFGF) increases the expression of functional L-type voltage-dependent Ca 2+ channels (VDCCs) in fetal rat hippocampal neurons. We investigated the intracellular signaling mechanisms involved in this effect, using high K + depolarization-induced elevation of intracellular Ca 2+ concentrations as a measure. Genistein, a protein tyrosine kinase inhibitor, significantly attenuated the effect of bFGF. The effect of bFGF was also diminished by concurrent application of a Ras inactivator, N-acetyl- S-farnesyl- l-cysteine. In contrast, a phospholipase C inhibitor U73122, a phosphatidylinositol-3 kinase inhibitor wortmannin, Li + which inhibits inositol phospholipid turnover, or a protein kinase inhibitor calphostin C did not inhibit the effect of bFGF. Phorbol 12-myristate 13-acetate, a protein kinase C activator, did not mimic the effect of bFGF. On the other hand, an adenylyl cyclase activator forskolin and a cyclic AMP analog 8-Br-cyclic AMP markedly attenuated the effect of bFGF, which indicates the presence of a cyclic AMP-mediated negative regulatory mechanism, possibly the interference of Ras–Raf interaction. These results suggest that Ras-mediated signal transduction is required for the enhancement by bFGF of VDCC responses in hippocampal neurons.