Expression and subcellular distribution of the beta-isoform of the human glucocorticoid receptor

Alternative splicing of the human glucocorticoid receptor (hGR) primary transcript produces two highly homologous protein isoforms, termed hGR alpha and hGRbeta, that differ at their carboxy-termini. In contrast to the well characterized hGR alpha isoform, which modulates gene expression in a hormone-dependent fashion, the biological significance of hGRbeta has only recently begun to emerge. We and others have shown that the hGRbeta messenger RNA transcript is widely expressed in human tissues and that the hGRbeta protein functions as a dominant negative inhibitor of hGR alpha in transfected cells. Unfortunately, these initial studies did not determine whether the hGRbeta protein was made in vivo. Such analyses are hindered because available anti-hGR antibodies cannot discriminate between the similarly sized hGR alpha and hGRbeta proteins. Therefore, to investigate the expression of the hGRbeta protein, we have produced an antipeptide, hGRbeta-specific antibody termed BShGR. This antibody was made against the unique 15-amino acid peptide at the carboxy-terminus of hGRbeta and recognizes both the native and denatured conformations of hGRbeta, but does not cross-react with hGR alpha. Using BShGR on Western blots and in immunoprecipitation experiments, we detected the hGRbeta protein in a variety of human cell lines and tissues. Immunocytochemistry was then performed with BShGR on HeLa S3 and CEM-C7 cells and on tissue sections prepared from lung, thymus, and liver to assess the cellular and subcellular distribution of hGRbeta. In all immunopositive cells, hGRbeta was found in the nucleus independent of glucocorticoid treatment. Within tissues, the hGRbeta protein was expressed most abundantly in the epithelial cells lining the terminal bronchiole of the lung, forming the outer layer of Hassall's corpuscle in the thymus, and lining the bile duct in the liver. As a potential in vivo inhibitor of hGR alpha activity, expression of hGRbeta may be an important factor regulating target cell responsiveness to glucocorticoids.