Estrogens

Estrogens are female sex steroid hormones derived from cholesterol. The most potent estrogen is 3,17b-estradiol (estradiol 1,3,5-estratriene-3,17b-diol). The 16a-hydroxylated derivative of estradiol, estriol, is a weak estrogen, whereas the 17b-oxidized compound, estrone, is hormonally inactive. Synthetic nonsteroid estrogens, such as diethylstilbestrol, have been introduced as therapeutic drugs in various endocrine diseases. Estrogen antagonists (eg, tamoxifen) have also found use in therapy, particularly of breast cancer.

 Estradiol is synthesized in the ovary, the placenta, the adrenal cortex, and the testis. In the theca cells of the ovarian follicles, cholesterol is metabolized to progesterone and to androstendione, which in the granulosa cells is subsequently transformed to estradiol by aromatization of the A ring by the aromatase complex. This process is stimulated by FSH (folitropin).

During fetal and early postembryonic development, estrogens are responsible for inducing female characteristics and for the formation of the female genitals (oviduct, uterus, vagina). In the adult, estrogens act on the development of the secondary female sexual characteristics—for example, on the growth and differentiation of the breast glands, on the appearance of pubic and axillar hair, and on fat deposition in the skin. Estrogens affect the central nervous system and can modify sexual behavior. They also act positively on bone mass. In collaboration with progesterone, estradiol regulates the menstrual cycle. Specifically, in the first half of the cycle, estradiol stimulates proliferation of the epithelial and stromal cells of the endometrium, in preparation for ovulation.

The molecular action of estrogens, like that of the other steroid hormones, is by modulation of gene transcription. The hormone first binds to the estrogen receptor, a member of the superfamily of nuclear receptors, which then homodimerizes and binds to specific nucleotide sequences, the estrogen response elements. This triggers activation of gene transcription. Among the genes regulated by estrogens are protooncogenes of endometrium cells encoding transcription factors involved in cell replication (c-fos, c-jun, c-myc) (1-3), those encoding the progesterone receptor, growth factors, and various structural and adhesion proteins (4). In the liver, estrogens stimulate the synthesis of steroid- and thyroxine-binding proteins. The molecular effects of estradiol on the synthesis of phosphoproteins in the chick oviduct, such as ovalbumin, have been studied in detail and were instrumental in developing concepts on the mode of action of steroid hormones (5).

References

1.D. S. Loose-Mitchell, C. Chiappetta, and G. M. Stancel (1988) Mol. Endocrinol. 2, 946–951. 

2. C. Chiappetta et al. (1992) J. Steroid Biochem. Mol. Biol. 41, 1134–123. 

3. S. M. Hyder et al (1992) J. Biol. Chem. 267, 18047–18054. 

4. R. Grummer et al. (1994) Biol. Reprod. 51, 1109–1116. 

5. M. Kalimi et al. (1976) J. Biol. Chem. 251, 516–523. 

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