Mechanism of Action of Gonadotropin

Abstract The time course of chorionic gonadotropin stimulation of nuclear basic and acidic protein biosynthesis, phosphorylation, and acetylation as well as DNA biosynthesis in ovaries of immature rats has been investigated. After injection of radioactive precursor and chorionic gonadotropin into immature rats, ovarian histone fractions F1, F2a1, F2a2, F2b, and F3 were isolated by selective acid extraction. Acidic protein fractions AP1 and AP2 were extracted with 4 m CsCl at pH 11.6 and 14, and were separated from DNA by equilibrium density centrifugation. Histones, acidic proteins, and DNA were purified by Sephadex chromatography. Chorionic gonadotropin was demonstrated to accelerate the incorporation of [3H]lysine into nuclear acidic protein fraction AP1 and into histones F2a1, F2a2, and F3 within 10 min following a single injection of the hormone. Increased rates of phosphorylation of histones F1 and F2a1 and acidic proteins AP1 and AP2, as well as increased rates of acetylation of histone F2a1 and acidic protein AP2, were also observed within 10 min following gonadotropin injection. Stimulation of DNA, histones F1 and F2b and acidic protein AP2 biosynthesis lagged and was not observed until 30 to 180 min after hormone administration. A similarly delayed response to hormonal stimulation was observed when phosphorylation and acetylation of histones F2a2 and F3 were studied. Actinomycin D and cycloheximide administration abolished gonadotropin-stimulated biosynthesis but not phosphorylation and acetylation of histones and acidic proteins. This indicates that hormonally stimulated modification of nuclear proteins by acetylation and phosphorylation was not entirely dependent on protein synthesis. It is suggested that these results would be compatible with the idea that modification of ovarian nuclear protein metabolism is an early event in the action of chorionic gonadotropin.