The pharmacology and clinical uses of tamoxifen

The data discussed herein demonstrate the great variation in target-tissue response that can occur after administration of steroid hormones. The female sex steroids can exert regulatory effects on the synthesis, activity, and possibly even the degradation of tissue enzymes and structural proteins. Each response, nevertheless, appears to be dependent on the synthesis of nuclear RNA. In many instances, the steroid actually promotes a qualitative change in the base composition and sequence of the RNA synthesized by the target cell, implying a specific effect on gene transcription. Most important is our direct quantitative evidence that sex steroids cause a net increase in the intracellular amounts of specific mRNA molecules in target tissues. It thus appears that we are discovering a pattern of steroid hormone action which includes (Fig. 1): (i) uptake of the hormone by the target cell and binding to a specific cytoplasmic receptor protein; (ii) transport of the steroid-receptor complex to the nucleus; (iii) binding of this "active" complex to specific "acceptor" sites on the genome (chromatin DNA and acidic protein); (iv) activation of the transcriptional apparatus resulting in the appearance of new RNA species which includes specific mRNA's; (v) transport of the hormone-induced RNA to the cytoplasm resulting in synthesis of new proteins on cytoplasmic ribosomes; and (vi) the occurrence of the specific steroid-mediated "functional response" characteristic of that particular target tissue. To elucidate fully the mechanism of steroid hormone action we must study the biochemistry of the process by which information held by the steroid hormone-receptor complex is transferred to the nuclear transcription apparatus. If our assumptions are correct, we should ultimately be able to discover how this hormone-receptor complex exerts a specific regulatory effect on nuclear RNA metabolism. Such regulation might be achieved (i) by direct effects on chromatin template leading to increased gene transcription and thus RNA synthesis; (ii) by activation of the polymerase complex itself; (iii) by inhibition of RNA breakdown; or (iv) by intranuclear processing of large precursor molecules so that smaller biologically active sequences are produced, and (v) by transport of RNA from the nucleus to the cytoplasmic sites of cellular protein synthesis.

https://science.sciencemag.org/content/sci/183/4125/610.full.pdf

Female steroid hormones and target cell nuclei.

Investigation of the actions of the benzodiazepines has provided insights into the neurochemical mechanisms underlying anxiety, seizures, muscle relaxation, and sedation. Behavioral, electrophysical, pharmacological, and biochemical evidence indicates that the benzodiazepines exert their therapeutic effects by interacting with a high-affinity binding site (receptor) in the brain. The benzodiazepine receptor interacts with a receptor for gamma-aminobutyric acid, a major inhibitory neurotransmitter, and enhances its inhibitory effects. The benzodiazepine receptor may also interact with endogenous substances and several naturally occurring compounds, including the purines and nicotinamide, are candidates for this role. Both the purines and nicotinamide possess some benzodiazepine-like properties in vivo, although further work will be required to confirm their possible roles as endogenous benzodiazepines.

https://science.sciencemag.org/content/sci/207/4428/274.full.pdf

Receptors for the age of anxiety: pharmacology of the benzodiazepines

BONE differs from reproductive tissues in that many aspects of skeletal growth, turnover, and function occur in the absence of gonadal hormones. Nevertheless, sex steroids have an important impact on bone physiology: they participate in the sexual dimorphism of the skeleton, play a role in maintenance of mineral homeostasis during reproduction, and are essential to maintaining bone balance in adults. Insufficient levels of certain sex steroids predispose the human skeleton to bone loss and to osteoporotic fractures. Estrogen appears to be the most important sex steroid in preventing osteoporosis in women and is the subject of this review. Serious difficulties must be overcome to understand the action of estrogen on mineralized tissues. The skeleton is heterogeneous and contains many cell types. The cells that synthesize (osteoblasts), lie within (osteocytes), and degrade (osteoclasts) bone matrix are present in small numbers, are difficult to isolate as pure populations, and probably do not proliferate.

Skeletal Effects of Estrogen

Estrogen control of progesterone receptor in human breast cancer. Correlation with nuclear processing of estrogen receptor.

A method was developed for the determination of the number of specific oestradiol-binding sites in the nuclear fraction of oestrogen-sensitive tissues. The method is based on the exchange of [3H]oestradiol with non-labelled oestradiol that is bound to nuclear binding sites. The number of specific nuclear binding sites after the injection of 2.5μg of oestradiol, an amount sufficient to saturate all binding sites, is 1.6–1.7pmol per immature uterus. The number of sites occupied after an injection of physiological amounts of oestradiol (0.1μg) was 0.46pmol. The injection of oestradiol results in an increased number of nuclear binding sites in uterus and vagina, but has no effect on kidney or muscle. Injections of testosterone or progesterone failed to increase the number of uterine nuclear binding sites. This method permits an evaluation of the number of oestradiol-binding sites in the nuclear fraction of various tissues as a function of either endogenous oestradiol or non-labelled oestradiol administered by injection.

Oestrogen and nuclear binding sites. Determination of specific sites by [3H]oestradiol exchange

The relationship between nuclear receptor-estrogen binding and uterotrophic responses.

The quantitative relationship between the amount of receptor—estrogen complex in the nuclear fraction of rat uterine cells and the extent of early uterotropic responses was studied in the immature rat. The quantity of nuclear receptor—estrogen complex, R—E, is maximal 1 hr after estradiol treatment. A decrease in nuclear R—E between 1 and 6 hr following estradiol treatment was observed which was dependent on hormonal dosage. During this 5–hr period, 70–80% of the sites were lost from the nuclear fraction following pharmacologic doses (1.0–2.5 μg) while a 7–9% decrease was observed with physiological doses (0.05–0.1 μg). Maximal increases in uterine weight were obtained when levels of nuclear R—E represented only a fraction of the total quantity of receptor in the uterus. However, a high correlation was observed between water imbibition and the quantity of nuclear R—E present at 3 and 6 hr after estradiol injection. In addition, positive correlations were found between the quantity of nuclear R—E and the e...

Nuclear receptor-estrogen complex: relationship between concentration and early uterotrophic responses.

The effect of estradiol injection on the quantity of estrogen receptors in the nuclear fraction of the hypothalamus and the anterior pituitary was examined by the 3H-estradiol exchange assay. The accumulation of the receptor estradiol complex (RE) in the nuclear fraction of both tissues in the female rat reached a maximum one hour after an injection of 25 μg of estradiol. Between 1 and 12 hr after treatment, parallel declines in hypothalamic and hypophyseal nuclear RE were observed. The quantity of receptor in the pituitary is several-fold greater than the quantity in the hypothalamus. However, hypothalamic receptors are confined to the basal medial region, and the concentration in this region (1.1 fmole/μg DNA) is similar to the concentration in the pituitary (1.3 fmole/μg DNA). The increases in nuclear RE in both tissues are equally dependent on the quantity of estrogen administered. One hr after an injection of a physiological dose (0.5 μg) of estradiol the amount of receptor in the nuclei in both tiss...

Nuclear receptor estrogen complex: accumulation, retention and localization in the hypothalamus and pituitary.

A quantitative method was used to determine the concentration of receptor-estrogen complex in the nuclear fraction of rat uterine cells throughout the estrous cycle. The concentrations of nuclear receptor-estrogen complex were: metestrus, 0.22; diestrus, 0.75; proestrus, 1.29; and estrus, 0.31 picomoles per milligram of DNA. This cyclic fluctuation in the nuclear complex closely parallels the secretion of ovarian estrogen during the estrous cycle, an indication that the accumulation of receptor-estrogen complex by the nuclear fraction of uterine cells may be of physiological significance, and under the control of endogenous estrogen.

E. J. Peck

Papers

Oestrogen and nuclear binding sites. Determination of specific sites by [3H]oestradiol exchange

The relationship between nuclear receptor-estrogen binding and uterotrophic responses.

Nuclear receptor-estrogen complex: relationship between concentration and early uterotrophic responses.

Nuclear receptor estrogen complex: accumulation, retention and localization in the hypothalamus and pituitary.

Receptor-Estrogen Complex in the Nuclear Fraction of Rat Uterine Cells during the Estrous Cycle