ESSENTIAL ROLE OF CALCIUM IN THE REGULATION OF MAP KINASE PHOSPHATASE-1 EXPRESSION

Mitogen-activated protein (MAP) kinase phosphatase-l (MKP-1) is a dual -specificity protein phosphatase encoded by an immediate-early gene re sponsive to growth factors and stress. The MKP-1 protein selectively i nactivates MAP kinases in vitro by dephosphorylation of the regulatory Thr and Tyr residues. Little is known on the mechanisms that regulate MKP-1 gene expression. Here, we demonstrate that Ca2+ is both necessa ry and sufficient for the induction of MKP-1 gene expression. Treatmen t of Rat1 fibroblasts with the Ca2+ chelating agent BAPTA completely s uppressed serum-induced MKP-1 expression in a dose- and time-dependent manner. The inhibitory effect of BAPTA was observed at the level of t he protein and the mRNA. Importantly, Ca2+ chelation blocked the induc tion of MKP-1 expression in response to all stimuli tested and in diff erent cell types. Increasing the intracellular concentration of Ca2+ w ith the ionophore A23187 was sufficient to induce MKP-1 mRNA and prote in expression in rat fibroblasts. We also provide evidence that activa tion of MAP kinases is not an absolute requirement for induction of th e MKP-1 gene. Exposure of rat fibroblasts to A23187 induced MKP-1 expr ession without activating the JNK and p38 MAP kinase pathways. Also, i nhibition of the ERK pathway with the selective MEK inhibitor PD98059 did not interfere with serum-stimulated MKP-1 mRNA expression. These r esults will help define the regulatory mechanisms that govern MKP-1 ge ne transcription in target cells.