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.