Unique long-acting antiglucocorticoid in whole and broken cell systems

The biological properties of cortisol 21-mesylate (CM), an alkylating derivative of cortisol, were investigated in a line of rat hepatoma tissue culture (HTC) cells. CM appears to bind to glucocoticoid receptors in cell-free extracts because CM inhibits the specific binding of [3H]dexamethasone. However, in whole cells CM not only fails to induce the enzyme tyrosine aminotransferase (TyrATase) but also inhibits the induction of TyrATase by dexamethasone. Thus CM is an antiglucocorticoid. This is not caused by cell death, because CM is relatively nontoxic up to concentrations of 10 microM. The concentration of CM needed for half maximal inhibition of TyrATase induction is an order of magnitude lower than that predicted from the apparent cell-free affinity of CM for the glucocorticoid receptors of HTC cells, which suggests that the cell-free binding data does not reflect an equilibrium situation. In fact, the reactive alpha-keto mesylate group was intentionally incorporated into cortisol in hopes of obtaining a steroid capable of undergoing irreversible reactions. When HTC cells were preincubated with either CM or the reversible antiglucocorticoid progesterone and then washed to remove free steroid, only the CM-treated cells failed to show subsequent induction of TyrATase by dexamethasone. Furthermore, preincubation of HTC-cell cytosol with CM blocked approximately 75% of the subsequent exchange binding of [3H]dexamethasone to glucocorticoid receptor sites. Thus, the actions of CM in whole and broken cells either require an exceptionally long time for reversal or are not reversible. Together, these results indicate that CM is a unique antagonist and could be an irreversible antiglucocorticoid in vitro.