Glucocorticoid receptor isoforms generated by alternative splicing and alternative translation initiation
Alternative splicing of exon 9 in human glucocorticoid receptor (hGR) transcripts yields two native hGR transcripts and proteins, hGRalpha and hGRbeta. We have now identified four novel hGRalpha transcripts that have various deletions of exon 2 sequences. Among these hGRalpha splice variants, three of them, 1A1/E2dist hGRalpha, 1A2/E2prox hGRalpha, and 1A3/E3 hGRalpha, arise from the hGR 1A promoter, while 1B/E3 hGRalpha comes from the hGR 1B promoter. When fused to Flag and enhanced green fluorescent protein (EGFP) tags at the carboxy terminus, all transcript variants can be correctly translated in vitro and in vivo. The Flag-tagged hGRalpha protein variants can functionally bind to a glucocorticoid response element (GRE) and can mediate hormonal stimulation of a pGRE-luciferase reporter gene. Compared to the "classical", native hGRalpha, these four variants exhibit a cell type-specific activation of a reporter gene, and this is influenced by the hGRalpha 3' untranslated region in the hGR transcript. When equal amounts of the cDNAs for these GRalpha variant proteins are transfected into cells, they can exhibit lower or higher transcriptional activation compared to the classical GR. Furthermore, the EGFP-tagged proteins are nuclear localized, even in the absence of hormone. Using quantitative reverse transcription PCR, we found that these transcripts exist at a low level in CEM-C7 cells and IM-9 cells, although the concentrations of the 1A3/E3 hGRalpha and 1B/E3 hGRalpha transcripts are higher than for hGRbeta transcripts, while 1A1/E2dist hGRalphaand 1A2/E2prox hGRalpha transcript levels are comparable to the 1A1 hGRalpha and 1A2 hGRalpha (without the exon 2 deletions) transcript levels, respectively. Because these novel hGR, N-terminal deleted, protein variants have altered biological activity, their expression could potentially affect the hormone sensitivity or resistance in leukemia and be useful in diagnosing hormone-sensitive or -resistant disease.