Estrogen receptors and androgen receptors in the mammalian liver

An estrogen receptor and an androgen receptor are present in the mammalian liver. In the liver of the rat, the estrogen receptor concentration increases markedly at puberty and this change correlates with enhanced estrogen stimulation of plasma renin substrate synthesis. High doses of estrogen are required for nuclear binding in liver when compared to doses for the uterus. The high dose requirement appears to be predominantly due to extensive metabolism in the hepatocyte of the estrogen to inactive derivatives. Furthermore, estradiol is much weaker than ethinyl estradiol for promoting nuclear binding in the liver. This is due to extremely rapid and extensive metabolism of estradiol. In human liver the concentration of estrogen receptor is low. An androgen receptor is present in high concentration in rabbit liver and is located predominantly in the nucleus after androgen administration. High concentrations of a putative androgen receptor are also present in human liver cytosol. Preliminary studies indicate that synthetic progestins can attach to the human liver androgen receptor. To date, a progesterone receptor has not been found in the mammalian liver. Thus, it appears that extensive steroid metabolism in liver preferentially diminishes sex steroid interaction with liver receptors and that androgen receptors may mediate progestin effects in liver. These observations provide a scientific basis for improved safety of oral contraceptives. Lowering the estrogen and progestin doses in oral contraceptives will decrease the major side-effects, which are liver mediated, and still maintain the desired effects at the hypothalamic-pituitary axis and uterus. Furthermore, it is likely that by selecting which estrogen, progestin or androgen is administered as well as by utilizing a parenteral route of administration that sex steroid effects on the liver could be minimized.