IGF-I increases c-fos expression in FRTL5 rat thyroid cells by activating the c-fos promoter.

We studied the effect of IGF-1 on c-fos mRNA expression in FRTL5 rat thyroid cells. IFG-1, or calf serum, transiently increased c-fos mRNA levels in quiescent FRTL5 cells, with the latter being more potent. This increase was prevented by actinomycin D. FRTL5 cells were stably transfected with the c-fos promoter linked upstream to the reporter gene chloramphenicol acetyl transferase (CAT). Calf serum and IGF-1 increased c-fos promoter activity in a prolonged manner, and to an equal degree. Our data indicate that IGF-1, as well as additional factor(s) in serum, increase c-fos mRNA levels, at least in part, by activation of the c-fos promoter.