Signal transduction in G0/G1-arrested mouse Y1 adrenocortical cells stimulated by ACTH and FGF2.

In G0/G1 cell cycle arrested mouse Y1 adrenocortical tumor cells ACTH39, a weak mitogen and strong anti-mitogenic agent, blocks FGF2 mitogenic activity at G1 phase, keeping untouched ERK-MAPK activation and c-Fos protein induction. Here we report two anti-mitogenic mechanisms initiated in ACTH receptors and mediated by cAMP/PKA: a) post-transcriptional down regulation of c-Myc protein; b) dephosphorylation of AKT/PKB. In Y-1 cells the activity of the Mad/Max/Myc network of transcription factors seems to be regulated by c-Myc levels. FGF2 induces c-myc gene and stabilizes c-Myc protein by a process dependent on ERK-MAPK (PD98059 sensitive), but not on PI3K (Wortmannin resistant). ACTH39, on the other hand, causes rapid decrease in c-Myc levels induced by FGF2 in wild type Y1 cells, but not in PKA-deficient Y1 clones. The ACTH inhibition of DNA synthesis stimulated by FGF2 is reversed by transient transfection and induction of the MycER chimera (fusion of c-Myc and estrogen-receptor), suggesting that c-Myc down regulation is an efficient anti-mitogenic mechanism activated by ACTH. Y1 cells display high constitutive levels of AKT/PKB, that is dependent on elevated Ras x GTP. FGF2 up regulates Ras x GTP, PI3K and AKT/PKB. ACTH antagonizes this mitogenic effect of FGF2, promoting rapid dephosphorylation of AKT/PKB.