Nitric oxide down-regulates MKP-3 mRNA levels - Involvement in endothelialcell protection from apoptosis

MAP kinase-dependent phosphorylation processes have been shown to interfere with the degradation of the antiapoptotic protein Bcl-2. The cytosolic MAP kinase phosphatase MAP kinase phosphatase-3 (MKP-3) induces apoptosis of e ndothelial cells in response to tumor necrosis factor cu (TNF alpha) via de phosphorylation of the MAP kinase ERK1/2, Leading to Bcl-2 proteolysis. Her e we report that the endothelial cell survival factor nitric oxide (NO) dow n-regulated MKP-3 by destabilization of MKP-3 mRNA. This effect of NO was p aralleled by a decrease in MKP-3 protein levels. Moreover, ERK1/2 was found to be protected against TNF alpha-induced dephosphorylation by coincubatio n of endothelial cells with the NO donor. Subsequently, both the decrease i n Bcl-2 protein levels and the mitochondrial release of cytochrome c in res ponse to TNF alpha were largely prevented by exogenous NO. In cells overexp ressing MKP-3, no differences in phosphatase activity in the presence or ab sence of NO were found, excluding potential posttranslational modifications of MKP-3 protein by NO. These data demonstrate that upstream of the S-nitr osylation of caspase-3, NO exerts additional antiapoptotic effects in endot helial cells, which rely on the down-regulation of MKP-3 mRNA.