Biochemical and molecular characterization of the glucocorticoid receptor of lymphosarcoma p1798 variants

Three phenotypically distinct isolates from lymphosarcoma P1798 have been compared with respect to properties of the glucocorticoid receptor. Wild type P1798 cells express functional receptors and glucocorticoid treatment of such cells causes cy-tolysis in vivo. Wild type cells do not undergo cyto-lysis in culture. Rather, such cells exhibit reversible inhibition of proliferation in the presence of dexa-methasone. Two variant populations were selected from this background. One was selected for the ability to form tumors in mice receiving pharmacological doses of glucocorticoids. Cells from such tumors are resistant to the cytolytic effects of glucocorticoids in vivo, but are sensitive to the antiproliferative effects of the hormone in culture. Variants were also selected based upon their ability to proliferate in the presence of dexamethasone in culture. These variants were resistant to glucocorticoid-mediated cytolysis in vivo. Wild type P1798 cells express approximately 20,000 high affinity dexamethasone-binding sites per cell. Dexamethasone-mesylate labeling and im-munoblotting experiments indicate that hormone binding is due to a polypeptide of M_r 90-100 K. This polypeptide is encoded in an mRNA species that resolved as a single entity of approximately 7000 nucleotides. Variants selected for resistance to cytolysis in vivo are indistinguishable in any of these respects from wild type cells. The receptors are fully functional, as evidenced by their ability to precipitate growth arrest of dexamethasone-treated cultures. Variants selected for resistance in culture harbor a receptor mutation. They express fewer than 500 dexamethasone-binding sites per cell. Such variants contain neither detectable dexamethasone-mesy-late-binding protein nor any protein that is recognized by a receptor antibody. No receptor mRNA could be detected. The data indicate that P1798 cells are functionally haploid. If they contain a cryptic receptor allele, it does not give rise to mature receptor mRNA. Variants selected for resistance in culture have lost the ability to synthesize any receptor mRNA and are unambiguously of the r⁻ phenotype. On the other hand, selection for resistance to cytolysis in vivo is not associated with receptor mutation. The significance of this is discussed in reference to our understanding of the effects of glucocorticoids upon lymphoid cell proliferation.