Mechanism of dexamethasone 21-mesylate antiglucocorticoid action: I. Receptor-antiglucocorticoid complexes do not competitively inhibit receptor-glucocorticoid complex activation of gene transcription in vivo.

The actions of dexamethasone 21-mesylate (DM) have been studied in two recently developed cultured murine cell lines containing approximately 200 copies of episomal minichromosome. This minichromosome contains the glucocorticoid regulatory element in the mouse mammary tumor virus long terminal repeat fused upstream of v-rasH sequences in a totally defined primary sequence environment. The levels of v-rasH mRNA were measured as an index of glucocorticoid regulated expression of this chimeric gene. In addition, expression of the endogenous single copy mouse metallothionein I (MT-I) gene was monitored simultaneously. DM was found to be an essentially pure antagonist of dexamethasone (dex)-stimulated expression of both the episomal chimeric gene and the endogenous MT-I gene. The covalent labeling efficiency by DM of glucocorticoid receptors in intact cells approached 100%, surpassing previously observed whole cell DM labeling efficiencies. These results strengthen the hypothesis that covalent complex formation is responsible for antiglucocorticoid action. The efficiency of whole cell nuclear binding of covalent receptor-DM complexes was found to be approximately 50% of that seen with receptor-dex complexes. Analyses of long terminal repeat initiated v-rasH mRNA and MT-I mRNA inductions by dex in cells previously exposed to a subsaturating concentration of DM indicated that receptor-DM complexes do not inhibit by a competitive mechanism the transcriptional activation of these glucocorticoid responsive genes by receptor-dex complexes. These results do not rule out the possibility, however, that covalent receptor-DM complexes may still bind to the biologically active nuclear sites. The implications of this result concerning the mechanism of DM irreversible antiglucocorticoid action are discussed.