Stimulatory effect of nobiletin, a citrus polymethoxy flavone, on catecholamine synthesis through Ser^sup 19^ and Ser^sup 40^ phosphorylation of tyrosine hydroxylase in cultured bovine adrenal medullary cells

Abstract

We previously reported the dual effects of nobiletin, a compound of polymethoxy flavones found in citrus fruits, on catecholamine secretion in cultured bovine adrenal medullary cells. Here, we report the effects of nobiletin on catecholamine synthesis in the cells. Nobiletin increased the synthesis of ^sup 14^C-catecholamines from [^sup 14^C]tyrosine in a time (20-30 min)- and concentration (1.0-100 [mu]M)-dependent manner. Nobiletin (10-100 [mu]M) also activated tyrosine hydroxylase activity. The stimulatory effect of nobiletin on ^sup 14^C-catecholamine synthesis was not observed when extracellular Ca^sup 2+^ was not present in the incubation medium. Protein kinase inhibitors including H-89, an inhibitor of cyclic AMP-dependent protein kinase, and KN-93, an inhibitor of Ca^sup 2+^/calmodulin-dependent protein kinase II, suppressed the stimulatory effects of nobiletin on catecholamine synthesis as well as tyrosine hydroxylase activity. Nobiletin also induced the phosphorylation of tyrosine hydroxylase at Ser^sup 19^ and Ser^sup 40^. Nobiletin (1.0-100 [mu]M) inhibited ^sup 14^C-catecholamine synthesis induced by acetylcholine. The present findings suggest that nobiletin, by itself, stimulates catecholamine synthesis through tyrosine hydroxylase phosphorylation at Ser^sup 19^ and Ser^sup 40^, whereas it inhibits catecholamine synthesis induced by acetylcholine in bovine adrenal medulla.