The Effect of Cortisol on Biosynthesis and Phosphorylation of Histones in Rat Liver

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Abstract
Administration of corticosteroid hormones to rats results in an increased activity of several enzymes in liver cells. The induced enzymes are related to the pathway of gluconeogenesis. The mechanism of specific gene activation by hormone action is still not well understood. Recent studies have shown that the steroid hormones combine with specific receptor protein(s) in cytosol of target cells (Jensen et al, 1968; Sherman et al, 1970). The hormone-protein(s) receptor penetrates the liver nucleus and interacts with specific “acceptor sites” of chromatin. These interactions may result in new sites of transcription, i.e. in specific gene activation. It was also demonstrated earlier that the restriction of template activity of chromatin may be due to the presence of the regulatory proteins associated with chromatin (histones) which mask, or repress the function of genome, i.e. play role in gene activation. Although studies on the rate of biosynthesis of histones have given little insight into their physiological role, there are established differences in the rates of biosynthesis of histones in hormone-treated and untreated animals. The rate of synthesis of various histone fractions was studied, following the hormone injection. Cortisol stimulated the synthesis of histones 20 min after hormone administration. The differences in elution pattern and pattern of radioactivity as revealed after column chromatography exist between control and hormone-treated animals. The arginine-rich histone fractions F3 and F2a showed the highest rate of [3H]lysine uptake. When the [3H]cortisol was injected into rats and histones were extracted from liver nuclei, the histone preparations were found to contain some radioactivity, suggesting the formation of his-tone-hormone complexes. The isolated nuclei incubated in the presence of radioactive Cortisol were extracted with 0.2 m HC1. Bound radioactivity was found also with histones. The cytosol protein(s)-cortisol complex formed in vivo, incubated with purified nuclei and in the presence of 32P has a significant effect on the phosphorylation of histones.

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