STANOZOLOL AND DANAZOL, UNLIKE NATURAL ANDROGENS, INTERACT WITH THE LOW-AFFINITY GLUCOCORTICOID-BINDING SITES FROM MALE-RAT LIVER-MICROSOMES

Some 17 alpha-alkylated androgens used as anabolic agents, such as sta nozolol (ST) and danazol (DA), have specific effects on the liver that are not exerted by testosterone. This gives rise to the possibility t hat a steroid-binding protein, other than the androgen receptor, could modulate the intracellular actions of these agents. Male rat liver mi crosomes contain a homogeneous population of [H-3]dexamethasone ([H-3] DEX)-binding sites which we have denominated low affinity glucocortico id-binding sites (LAGS). Because glucocorticoids, progestagens, and th e synthetic estrogen ethynyl estradiol compete with [H-3] DEX for bind ing to the LAGS, we aimed to study the possible interactions between a ndrogens and the LAGS. To investigate whether several androgens had th e capability of interacting with the LAGS, we performed competition ex periments. The LAGS had no affinity for testosterone or methyltrienolo ne (R1881). However, some 17 alpha-alkylated androgens (DA (IC50, 116 nM) > ST much greater than fluoxymesterone > mestaline > methandriol m uch greater than methandrostenolone > methyltestosterone) were able to compete with [H-3]DEX binding to liver microsomes. ST and DA were pot ent inhibitors of [H-3]DEX binding to liver microsomes. They decreased both the affinity and the number of [H-3]DEX-binding sites, increased the dissociation rate of [H-3]DEX from the LAGS, and provoked a time- and dose-dependent inactivation of the [H-3]DEX-binding site. These r esults strongly suggest that ST and DA exert a negative allosteric mod ulation on [H-3]DEX binding to the LAGS. The in vivo administration of ST (but not other androgens) to male rats provoked a time- and dose-d ependent decrease in the LAGS level. Full recovery of the LAGS concent ration required at least 8 h and was blocked by protein synthesis inhi bitors. Such results suggest that ST irreversibly inactivates the [H-3 ]DEX-binding site in vivo as it does in vitro. Taken together, these o bservations are indicative of an irreversible interaction between some 17 alpha-alkylated androgens and the LAGS both in vitro and in vivo a nd suggest that ST may be an important pharmacological tool that can b e used in the elucidation of the molecular structure of the LAGS. Thes e results also mean that the LAGS are a steroid-binding entity able to distinguish between natural androgens and 17 alpha-alkylated testoste rone derivatives used as anabolic agents.