Steroid induction of low-affinity glucocorticoid binding sites in rat liver microsomes

Abstract

Rat liver contains two glucocorticoid binding sites: the high-affinity or glucocorticoid receptor (GR) and the low-affinity glucocorticoid binding sites, or LAGS. The Kd of LAGS predicts that they can be half-saturated by plasma corticosteroids in some physiological circumstances and, therefore, that they can play relevant roles in the rat liver. [3H]dexamethasone was used as a ligand in exchange assays, to study the relative abundance of GR and LAGS in cell fractions of rat liver. GR were found in the cytosol, but not in the purified nuclei, the mitochondria, or the microsomes. LAGS were found in all the particulate fractions, being more abundant in the smooth-surfaced microsomes, but they were not found in the cytosol. The LAGS of microsomes and purified nuclei showed the same Kd and also the same broad range of steroid competition with [3H]dexamethasone (cortisol = progesterone > dexamethasone ≥ corticosterone > R5020 > DHEA > testosterone = estradiol). LAGS were found in liver, placenta and kidney, but not in other GR-containing organs. This suggests that the LAGS could be involved in physiological functions related to the metabolism of steroid hormones. The liver microsome LAGS were undetectable at rat birth, and became present in the 25-day-old rat. The level of LAGS then increased progressively, reaching its maximum level in the 2-3-month-old rats (10 pmol/mg protein), and declining afterwards to reach the adulthood level (5 pmol/mg protein) in 6-month-old rats. LAGS are mainly controlled by the corticoadrenal steroids, which is shown by their dramatic decrease after adrenalectomy, and especially after hypophysectomy. Many steroid hormones, like estradiol, testosterone, and corticosterone (but not progesterone) induce LAGS, estradiol being the most effective. A combination of T4 and corticosterone was more effective in inducing LAGS than when the two hormones were injected separately. It is possible to conclude that rat liver LAGS are mainly microsomal proteins, whose concentration is regulated by a multihormone system under pituitary control.