The regulation of MAPKs in Y1 mouse adrenocortical tumor cells

The regulation of the MAPKs, Erk(1) and Erk(2), and the MAPK kinase, Mek, w ere examined in the Y1 mouse adrenocortical tumor cell line and in the prot ein kinase A-defective mutant, Kin-8. ACTH and basic fibroblast growth fact or each increased Mek phosphorylation and stimulated Mek activity in both c ell lines and also activated the Erks at concentrations that paralleled the ir effects on Mek. The specific Mek inhibitor, PD98059, blocked the activat ion of the Erks by ACTH and basic fibroblast growth factor, indicating that Mek is the up-stream activator of Erk. PD98059 did not block the phosphory lation of Mek, as might have been expected from previous studies; instead P D98059 inhibited the activity of the activated enzyme. In ACTH-stimulated, mutant Kin-8 cells, PD98059 paradoxically increased the amount of phosphory lated Mek, while preventing the activation of Erk. These results are interp reted as reflecting the loss of a protein kinase A-mediated inhibitory infl uence on Mek phosphorylation and activation.