Opposite regulation of deoxyribonucleic acid synthesis and iodide uptake in rat thyroid cells by basic fibroblast growth factor: correlation with opposite regulation of c-fos and thyrotropin receptor gene expression.

Basic fibroblast growth factor (bFGF) increases DNA synthesis in rat FRTL-5 thyroid cells, as measured by increased incorporation of tritiated thymidine into DNA. We show that this action is associated with the ability of bFGF to increase cytosolic Ca2+ levels and transiently increase c-fos mRNA levels. Other agents that increase c-fos mRNA levels and DNA synthesis in FRTL-5 cells include TSH, insulin, insulin-like growth factor-I, phorbol esters, A23187, and alpha 1-adrenergic agents; the last two agents also act by increasing cytosolic Ca2+ levels. Despite its enhancement of DNA synthesis, however, bFGF decreases TSH-induced cAMP-mediated iodide uptake. This action appears to reflect two separate actions of bFGF. First, bFGF decreases TSH receptor mRNA levels and the ability of TSH to acutely increase cAMP levels in FRTL-5 cells. The ability of bFGF to negatively regulate TSH receptor mRNA levels is additive to and independent of the ability of TSH and its cAMP signal to negatively autoregulate TSH receptor mRNA levels. This is consistent with the effect of bFGF on cytosolic Ca2+ levels and the ability of increased cytosolic Ca2+ to decrease TSH receptor mRNA levels. Second, bFGF inhibits cAMP signal expression, as evidenced by its ability to inhibit (Bu)2cAMP-induced iodide uptake in FRTL-5 cells. Both effects are, presumably, associated with the ability of bFGF to counteract TSH/cAMP-induced increases in thyroid peroxidase mRNA levels, which we demonstrate. We suggest, therefore, that bFGF causes opposite effects on DNA synthesis and iodide uptake because of its effect on cytosolic Ca2+ levels and because increases in cytosolic Ca2+ can have opposite effects on gene transcription, particularly in the case of the TSH receptor and c-fos genes.