Factors influencing the glucocorticoid-induced increase of ribonucleic acid polymerase activity of rat liver nuclei

Abstract

Liver nuclei from adrenalectomized rats receiving cortisone have a nuclear RNA polymerase activity higher than controls. The induced and control polymerase activity present the same general characteristics. However, ammonium sulfate added in vitro enhances RNA polymerase activity of controls more than that of treated nuclei and obliterates the hormonal response. The stimulating effect of cortisone on RNA polymerase activity is prevented in vivo by puromycin and actinomycin D. Cortisone abolishes in vivo the toxic effects of histones; however, neither cortisone nor cortisol reverses in vitro the inhibitory action of these compounds on RNA polymerase activity. Irradiation with X-rays (600 r) results in a reduced response of RNA polymerase to cortisone. Liver RNA polymerase activity is very low at birth and reaches the adult values after about 15 days of extrauterine life. No response to cortisone or cortisol is observed in vivo before the 13th day of life, whereas other enzymic activities, such as tyrosine transaminase and alanine transaminase, respond much earlier. Therefore it cannot be stated at present that the increase of liver enzyme activities induced by glucocorticoids is mediated by RNA polymerase, at least in newborn rats. Administration of cortisone has no appreciable effect on the hydrolysis of pulse-labeled nuclear RNA, but it reduces the hydrolysis of microsomal RNA. The possibility is prospected that this effect together with the increased RNA polymerase activity may contribute to increase the amount of liver RNA, which is a typical effect of glucocorticoid treatment.