NATURE OF STEROID-RECEPTOR INTERACTIONS

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The objective of this project is to define the initial, intracellular events of steroid hormone action. These events include steroid binding to the intracellular receptor molecule, "activation" of the receptor-steroid complex to a DNA-binding and nuclear-binding species, and binding of the activated complex to those nuclear acceptor sites involved in the regulation of transcription of specific genes. One approach that has been used to examine these steps is to compare the properties of various steroids in different cell lines. Thus previous studies of the amount of induction of tyrosine aminotransferase (TAT) by several glucocorticoids in two rat hepatoma tissue culture lines (HTC and Fu5-5) revealed that the steroid concentration required for 50% of maximal TAT induction in HTC cells was about 6-fold higher than in Fu5-5 cells. The same difference is now seen in the induction of TAT enzyme and mRNA levels by the stable cAMP derivative, (8-(4- chlorophenylthio)cAMP). These data suggest that a common pre- translational event determines the different sensitivity of TAT induction by glucocorticoids and by cAMP in HTC and Fu5-5 cells. A second approach has been to examine the properties of the irreversible antiglucocorticoid (and affinity label) dexamethasone 21-mesylate (Dex-Mes) at a molecular level. Dex-Mes specifically reacts with the cysteines of proteins in basic aqueous solutions. Dex-Mes reaction with the glucocorticoid receptor occurs uniquely at Cys-656 in the steroid binding site. This identification of the first amino acid associated with a biological property of the glucocorticoid receptor should facilitate future structure-function studies.