CONDITIONAL EXPRESSION OF THE MITOGEN-ACTIVATED PROTEIN-KINASE (MAPK)PHOSPHATASE MKP-1 PREFERENTIALLY INHIBITS P38 MAPK AND STRESS-ACTIVATED PROTEIN-KINASE IN U937 CELLS

Phorbol ester tumor promoters, such as phorbol 12-myristate 13-acetate (PMA), are potent activators of extracellular signal-regulated kinase 2 (ERK2), stress-activated protein kinase (SAPK), and p38 mitogen-act ivated protein kinase (MAPK) in U937 human leukemic cells, These kinas es are regulated by the reversible dual phosphorylation of conserved t hreonine and tyrosine residues, The dual specificity protein phosphata se MAPK phosphatase-l (MKP-1) has been shown to dephosphorylate and in activate ERK2, SAPK, and p38 MAPK in transient transfection studies. H ere we demonstrate that PMA treatment induces MKP-1 protein expression in U937 cells, which is detectable within 30 min with maximal levels attained after 4 h, This time course coincides with the rapid inactiva tion of PMA-induced SAPK activity, but not ERK2 phosphorylation, which remains elevated for up to 6 h, To examine directly the role of MKP-1 in the regulation of these protein kinases in vivo, we established a U937 cell line that conditionally expresses MKP-1 from the human metal lothionein IIa promoter, Conditional expression of MKP-1 inhibited PMA -induced ERK2, SAPK, and p38 MAPK activity, By titrating the levels of MKP-1 expression from the human metallothionein IIa promoter, however , it was found that p38 MAPK and SAPK were much more sensitive to inhi bition by MKP-1 than ERK2, This differential substrate specificity of MKP-1 can be functionally extended to nuclear transcriptional events i n that PMA-induced c-Jun transcriptional activity was more sensitive t o inhibition by MKP-1 than either Elk-l or c-Myc. Conditional expressi on of MKP-1 also abolished the induction of endogenous MKP-1 protein e xpression in response to PMA treatment, This negative feedback regulat ory mechanism is likely due to MKP-1-mediated inhibition of ERK2, as s tudies utilizing the MEK1/2 inhibitor PD98059 suggest that ERK2 activa tion is required for PMA-induced MKP-1 expression, These findings sugg est that ERK2-mediated induction of MKP-1 may play an important role i n preferentially attenuating signaling through the p38 MAPK and SAPK s ignal transduction pathways.