REGULATION OF BFGF GENE-EXPRESSION AND SUBCELLULAR-DISTRIBUTION OF BFGF PROTEIN IN ADRENAL-MEDULLARY CELLS

Basic fibroblast growth factor (bFGF), a potent mitogenic/neurotrophic factor, controls the development and plasticity of many types of neur al cells. In adrenal chromaffin cells, the appearance of bFGF protein coincided with the establishment of functional innervation, suggesting induction by trans-synaptic signals. In cultured bovine adrenal medul lary cells Western blot analysis revealed 18-, 23-, and 24-kD bFGF iso forms in the cytosolic and nuclear fractions. Stimulation of acetylcho line nicotinic receptors or hormonal angiotensin II receptors or the d irect stimulation of adenylate cyclase with forskolin or protein kinas e C (PKC) with PMA increased the content of all bFGF isoforms. Increas es in the levels of intracellular bFGF did not result in detectable pr esence of bFGF proteins in culture medium. Instead, bFGF proteins accu mulated in the cytoplasm or the nucleus depending on whether PKC or cA MP pathways were activated. The long-term nuclear forskolin-induced ac cumulation of bFGF was prevented by cycloheximide or by antisense bFGF oligonucleotide and was also accompanied by an increase in bFGF mRNA. We used luciferase reporter plasmids containing the human bFGF promot er to show that the induction of bFGF resulted from transcriptional ac tivation of the bFGF gene and was mediated by regulatory sequences loc ated upstream from its transcription start site. Stimulation of bFGF g ene expression by forskolin and PMA was synergistic and was mediated t hrough different promoter regions. The results suggest that stimulatio n by cAMP and PKC is mediated through novel cis elements. The regulati on of bFGF protein content also involves posttranscriptional mechanism s since changes in the levels of individual bFGF isoforms were differe nt depending on whether cells were treated with carbachol or angiotens in II, forskolin, or PMA. The present study indicates that bFGF is an intracrine cytoplasmic-nuclear factor, whose expression is regulated b y trans-synaptic and hormonal stimuli and which may act as a direct me diator of genomic responses to afferent stimulation.