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Synthesis of Ribonucleic Acid during Early Estrogen Action

From the ¹ From the National Institute for Medical Research, Mill Hill, London, England N. W. 7

The data reported indicate that rat uterus nuclei possess the capacity to synthesize RNA in vivo and in vitro by mechanisms similar to those previously reported for rat liver nuclei.

Tritiated uridine administered to the intact adult rat is rapidly taken up by the uterus and incorporated into nuclear RNA. Maximum specific activity of the nuclear RNA is reached at 20 min following administration of the radioisotope and decreases thereafter, while that of cytoplasmic RNA increases to a maximum at 6 to 12 hours, which indicates a turnover of RNA from the nucleus to the cytoplasm.

RNA polymerase (EC 2.7.7.6) in intact nuclei isolated from the uterus was studied in the presence of Mg²⁺ ions at low ionic strength or in the presence of Mn²⁺ ions and 0.4 m ammonium sulfate. Both RNA polymerase reactions are inhibited by actinomycin D, DNase, or RNase. Analyses of base composition and nearest neighbor base frequencies indicate that the product of the Mg²⁺-activated RNA polymerase reaction is similar to ribosomal RNA, whereas that of the Mn²⁺-(NH₄)₂SO₄-activated reaction is more DNA-like. Sucrose density gradient sedimentation indicates extensive degradation of the RNA synthesized during incubation in both conditions.

During the 3-week period following ovariectomy of the adult rat, there is a progressive decline in the rate of uterine RNA synthesis. An initial decrease in the rate of synthesis of nuclear RNA rapidly labeled in vivo is followed sequentially by decreases in the activities of Mg²⁺-activated and Mn²⁺-(NH₄)₂SO₄-activated RNA polymerase measured in isolated nuclei. The concentration of whole tissue RNA or protein per mg of DNA in the organ decreases following ovariectomy, but the ratio of nuclear RNA or protein per mg of DNA appears to be unaltered. A single injection of 10 µg of estradiol-17ß to the adult rat ovariectomized 3 weeks previously stimulates uterine RNA synthesis. Nuclear RNA synthesis in vivo is accelerated prior to sequential stimulations of the two RNA polymerase activities. Concentrations of total RNA and protein are restored to normal levels as a result of hormone treatment. At 20 min following hormone treatment, the rate of synthesis of nuclear RNA rapidly labeled in vivo is accelerated 500 to 600%. This acceleration is accompanied by an increased uptake of ³H-uridine by the organ. Whether the increase in specific activity is a cause or a consequence of the increased uptake is unknown.

Ovariectomy or administration of estradiol-17ß influences neither the principal characteristics and kinetics of the RNA polymerase reactions nor the base composition and nearest neighbor base frequencies of their products.

The stimulatory effect of estrogen in vivo on RNA synthesis in uterine nuclei is inhibited 80 to 100% by prior treatment of the ovariectomized rat with actinomycin D or cycloheximide. Treatment or prior treatment of the immature rat with histamine, the antihistamine mepyramine, or the histamine releaser p-methoxy-n-methylphenethylamine ("48-80") has no effect on estrogen-stimulated RNA synthesis in uterine nuclei. No stimulation of RNA polymerase activity has yet been obtained by the incubation in vitro of uterine nuclei with estradiol-17ß.

It is concluded that an acceleration of the synthesis of all types of RNA, but particularly of that of ribosomal RNA and of ribosomes, is an essential feature of the early action of estrogen in the uterus.

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