Basic fibroblast growth factor (bFGF) regulates tyrosine hydroxylase and proenkephalin mRNA levels in adrenal chromaffin cells

Abstract

bFGF is a neurotrophic protein expressed in various regions of the adult peripheral and central nervous system. The present study was undertaken to examine the role of bFGF in multihormonal, catecholaminergic and enkephalinergic cells of the adrenal medulla (AM). Western blot analysis revealed the presence of at least three bFGF isoforms (18, 22/23, and 24 kDa) in cultured bovine AM cells. Incubation of AM cells with the exogenous 18 kDa bFGF produced time-dependent increases in tyrosine hydroxylase (TH) and proenkephalin (PEK) mRNA, with maximal changes occurring at 12 h (TH) or 24 h (PEK) of bFGF exposure. Effects of bFGF on TH and PEK mRNA were non-additive with increases induced by exposure of AM cells to nicotine, the depolarizing agent veratridine, or the adenylate cyclase activator forskolin. These data indicate that bFGF effects may occur through intracellular pathways accessed during transsynaptic induction of TH and PEK genes. The increases in PEK mRNA induced by nicotine or bFGF were inhibited by the calcium antagonist TMB-8 also inhibited bFGF-induced increases in TH mRNA as well. However, treatment with TMB-8 increased basal levels of TH mRNA. The addition of bFGF increased endogenous levels of c- fos mRNA, c-Fos and c-Fos-related proteins, suggesting that bFGF may active TH and PEK gene expression through a calcium-AP1 transcriptional regulatory pathway. Immunohistochemical analysis revealed the presence of bFGF-immunoreactivity (bFGF-IR) in the cytoplasm and in the nucleus of AM cells. Incubation of cells with exogenous bFGF produced time-dependent increases of nuclear bFGF-IR. The intensity of nuclear bFGF-IR was also enhanced in cells treated with veratridine, indicating that in both cases nuclear translocation of bFGF occurs. The present study indicates that bFGF could serve as a paracrine or autocrine factor regulating the expression of TH and PEK genes in AM cells. It also suggests that nuclear translocation of bFGF in AM cells may play a role in transcriptional effects of bFGF in those cells.