Basic fibroblast growth factor stimulates phosphorylation of CCAAT/enhancer-binding protein δ by activation of mitogen-activated protein kinase

Stimulation of cultured astrocytes with basic fibroblast growth factor (bFGF) increased phosphorylation of CCAAT/enhancer-binding protein (C/EBP)δ, but not C/EBPβ. The maximal phosphorylation was obtained 60 min after stimulation. PD098059, a specific inhibitor of mitogen-activated protein kinase kinase (MAP kinase kinase), inhibited phosphorylation of C/EBPδ. Immunohistochemistry showed that bFGF stimulation of cultured astrocytes induced the translocation of activated MAP kinase from the cytoplasm to the nucleus and surrounding regions. This translocation was inhibited by addition of PD098059. Dibutyryl cyclic AMP (dbcAMP) increased the total amount of C/EBPδ protein but had no effect on its phosphorylation state. KN93, a specific inhibitor of Ca²⁺/calmodulin-dependent protein kinase II, and calphostin C, a specific inhibitor of protein kinase C, did not inhibit phosphorylation of C/EBPδ following stimulation with bFGF. Gel mobility shift analysis using nuclear extracts from bFGF-treated cells showed increases in C/EBP site binding activities 60 min after stimulation with bFGF. These results suggest that bFGF stimulates phosphorylation of C/EBPδ through activation of MAP kinase, and is involved in stimulation of gene expression by phosphorylation of C/EBPδ.