In vivo effects of dexamethasone and cycloheximide on the phosphorylation of 110-kDa proteins and the protein kinase activities of rat liver nucleoli.

In vivo effects of dexamethasone and cycloheximide on the phosphorylation of nucleolar proteins and the activities of nucleolar protein kinases of rat livers were studied. Phosphorylation of nucleolar proteins was accomplished by incubation of isolated nucleoli with [gamma-32P]ATP at 37 degrees C for 10 min followed by electrophoretic separation and autoradiographic demonstration of phosphorylated proteins. Of several nucleolar phosphoproteins observed in the liver of adrenalectomized rats, the incorporation of 32P into nucleolar 110-kDa proteins was rapidly increased, reaching about 1.8-fold of the adrenalectomized level at 12 h after administration of dexamethasone. In addition, the activities of liver nucleolar protein kinases which were measured using casein or phosvitin as substrates were stimulated similarly by the administration of dexamethasone. It was found by DEAE-Sephadex column chromatography that nucleolar protein kinases were separated into NI and NII, the activities being increased about 35 and 80% by dexamethasone treatment (12 h), respectively, and that the enhancements were completely abolished by 1-h treatment of cycloheximide. The injection of cycloheximide also suppressed the hormone-induced enhancement of 32P incorporation into 110-kDa proteins but did not reduce the amount of the proteins at least until 2 h. Evidence was obtained indicating that the phosphorylation of the nucleolar 110-kDa proteins is accomplished primarily by protein kinase NII. These results suggest that continuous synthesis of protein(s) is necessary for glucocorticoid-induced enhancement of phosphorylation of 110-kDa proteins of liver nucleoli and that the nucleolar protein kinase NII is involved in the glucocorticoid-induced short-lived protein(s).