Effects of dexamethasone on mitogen-activated protein kinases in mouse macrophages - Implications for the regulation of 85 kDa cytosolic phospholipase A(2)

In mouse macrophages, arachidonate mobilisation in response to several stim uli is severely inhibited by prolonged (16-20 hr) treatment with nanomolar dexamethasone (dex). It was shown earlier that this inhibition was accompan ied by a dual effect on cPLA(2); down-regulation of the enzyme protein and inhibition of its activation. We now report that cycloheximide, a protein s ynthesis inhibitor, caused an almost complete reversion of the inhibitory e ffects of dex on cPLA(2) activation. These results indicate that the effect s depend on new protein synthesis. This is consistent with other data, obta ined with a glucocorticoid receptor antagonist, indicating that the effects are mediated via the glucocorticoid receptor. Northern blot results showed pronounced down-regulation of cPLA(2) at the level of its mRNA. The possib ility that dex also targeted the level or activation of one or more of the three mitogen-activated protein kinases (MAP kinases), extracellular signal -regulated kinase (ERK), p38, or c-Jun N-terminal kinase (JNK) was also add ressed. While the level of these MAP kinases and their phorhol myristate ac etate (PMA)-induced activation were unaffected by dex, there was a partial inhibition of their zymosan-induced activation. However, this inhibition wa s not as pronounced as the dex-mediated inhibition of cPLA(2) activation Th ese data were confirmed by Western blot using antibodies against the phosph orylated forms of ERK, p38, and JNK. The results suggest that dex-mediated inhibition of PMA-induced cPLA(2) activation is exerted downstream of the M AP kinases, while the partial inhibition of the zymosan-induced activation may be explained by effects exerted more upstream. Thus, the MAP kinases in vestigated here do not appear to be main targets for the inhibitory effects of dex on cPLA(2) activation. BIOCHEM PHARMACOL 60;4:545-551, 2000. (C) 20 00 Elsevier Science Inc.