INACTIVATION OF P42 MAP KINASE BY PROTEIN PHOSPHATASE 2A AND A PROTEIN-TYROSINE-PHOSPHATASE, BUT NOT CL100, IN VARIOUS CELL-LINES

Background: Mitogen-activated protein (MAP) kinase is central to a signal transduction pathway that triggers cell proliferation or differentiation. Activation of the p42(mapk) isoform requires its phosphorylation at two residues, Thr 183 and Tyr 185, and this phosphorylation is catalysed by MAP kinase kinase (MAPKK). Relatively little is known, however, about the enzymes that dephosphorylate these residues, thereby inactivating the pathway. Recently, the CL100 phosphatase has been shown to inactivate p42(mapk) in vitro by dephosphorylating Thr 183 and Tyr 185 at similar rates. CL100, the product of an immediate early gene, is synthesized within one hour of stimulating cells with growth factors or exposure to oxidative stress or heat shock. Incubation of NIH 3T3 fibroblasts with cycloheximide prevents both synthesis of CL100 and inactivation of p42(mapk) after stimulation with serum. Results: Depleting cells of CL100 and preventing its induction using cycloheximide stopped the inactivation of p42(mapk) in Swiss 3T3 fibroblasts following stimulation with epidermal growth factor (EGF), but had no effect on the rapid inactivation of p42(mapk) in response to EGF in adipose (3T3-L1) or chromaffin (PC12) cells or in response to platelet-derived growth factor (PDGF) in endothelial (PAE) cells. Moreover, maximal induction of CL100 mRNA and a CL100-like activity did not trigger inactivation of p42(mapk), which was sustained at a high level after stimulation of PC12 cells with nerve growth factor, PAE cells with serum, or Swiss 3T3 cells with PDGF. Dephosphorylation of Tyr 185 but not Thr 183 of p42(mapk) was suppressed by vanadate in EGF-stimulated PC12 cells; dephosphorylation of Thr 183, by contrast, was elicited by a vanadate-insensitive activity. Protein phosphatase-2A was the only vanadate-insensitive phosphatase acting on Thr 183 of p42(mapk) or on MAPKK to be detected in PC12 cell extracts. Phosphorylation of Thr183 also inhibited the dephosphorylation of Tyr 185 in vitro by the major vanadate-sensitive Tyr 185-specific phosphatase, explaining why dephosphorylation of Thr 183 is rate-limiting for p42(mapk) inactivation in PC12 cells after stimulation with EGF. Conclusions: The rapid inactivation of p42(mapk) initiated five minutes after stimulation of endothelial, adipose and chromaffin cells with growth factor is not catalysed by CL100, but rather by protein phosphatase 2A and by a protein tyrosine phosphatase distinct from CL100. Induction of CL100 is not accompanied by the inactivation of p42(mapk) in a number of situations.