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 stimuli is severely inhibited by prolonged (16-20 hr) treatment with nanomolar dexamethasone (dex). It was shown earlier that this inhibition was accompanied 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 synthesis inhibitor, caused an almost complete reversion of the inhibitory effects of dex on cPLA(2) activation. These results indicate that the effects depend on new protein synthesis. This is consistent with other data, obtained 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 possibility 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 addressed. While the level of these MAP kinases and their phorbol myristate acetate (PMA)-induced activation were unaffected by dex, there was a partial inhibition of their zymosan-induced activation. However, this inhibition was not as pronounced as the dex-mediated inhibition of cPLA(2) activation. These data were confirmed by Western blot using antibodies against the phosphorylated forms of ERK, p38, and JNK. The results suggest that dex-mediated inhibition of PMA-induced cPLA(2) activation is exerted downstream of the MAP kinases, while the partial inhibition of the zymosan-induced activation may be explained by effects exerted more upstream. Thus, the MAP kinases investigated here do not appear to be main targets for the inhibitory effects of dex on cPLA(2) activation.