c-Fos protein is a mediator in mitogenic response to ACTH.

Pulses (up to 2 h) of the adrenocorticotropic hormone (ACTH) rapidly activa te p42 and p44 MAPK (5 min), induce the c-Fos protein (1 h, 80% of cells) a nd stimulate entry of mouse Y-1 adrenocortical cells into the S phase of th e cell cycle. This set of sequential events is also triggered in Y-1 cells by bFGF, and reflects a mitogenic response to ACTH. We report here that 90 % inhibition of c-fos mRNA translation with a c-fos antisense oligodeoxynuc leotide completely blocks the entry of Y1 cells into S phase stimulated by pulses of ACTH. These results indicate that c-Fos protein is an intracellul ar mediator of the mitogenic response to ACTH.