Glucocorticoid regulation of phenylethanolamine N-methyltransferase in vivo

In vivo, supraphysiological doses of glucocorticoids are required to restore adrenal medullary phenylethanolamine N-methyltransferase (PNMT, E.C. 2.1.1.28) activity after hypophysectomy. However, in vitro, phenylethanolamine N-methyltransferase gene expression appears normally glucocorticoid-responsive. To explore this paradox, rats were given dexamethasone or the type II-specific glucocorticoid RU28362 (1-1000 micrograms/day), and adrenal phenylethanolamine N-methyltransferase activity and mRNA levels were determined. At low doses (1-30 micrograms/day), neither steroid altered mRNA whereas at higher doses (100-1000 micrograms/day), mRNA rose 10- to 20-fold, with dexamethasone approximately 3 times as potent as RU28362. In contrast, enzyme activity fell with low doses of either steroid, consistent with suppression of ACTH and endogenous steroidogenesis. At higher doses of RU28362, enzyme activity remained low and unchanged despite increased mRNA expression, whereas higher doses of dexamethasone progressively restored the enzyme to normal. These findings suggest 1) that glucocorticoid regulation of phenylethanolamine N-methyltransferase activity occurs largely independent of gene expression; 2) that glucocorticoid effects on enzyme activity are primarily indirect, probably through cosubstrate regulation and/or enzyme stabilization; and 3) that these effects are not mediated via a classical (type II) glucocorticoid receptor mechanism, given the high doses of dexamethasone and corticosterone required and the inability of RU28362 to mimic the effects of these less selective steroids.