Human glucocorticoid receptor α transcript splice variants with exon 2 deletions:: Evidence for tissue- and cell type-specific functions

Alternative splicing of exon 9 in human glucocorticoid receptor (hGR) transcripts yields two native hGR transcripts and proteins, hGR alpha and hGR beta. We have now identified four novel hGRa transcripts that have various deletions of exon 2 sequences. Among these hGR alpha splice variants, three of them, 1A1/E2dist hGR alpha, 1A2/E2prox hGR alpha, and 1A3/E3 hGR alpha, arise from the hGR 1A promoter, while 1B/E3 hGR alpha 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 hGR alpha 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 hGR alpha, these four variants exhibit a cell type-specific activation of a reporter gene, and this is influenced by the hGR alpha 3' untranslated region in the hGR transcript. When equal amounts of the cDNAs for these GR alpha 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 hGR alpha and 1B/E3 hGR alpha transcripts are higher than for hGR beta transcripts, while 1A1/E2d1st hGR alpha and 1A2/E2prox hGE alpha transcript levels are comparable to the 1A1 hGR alpha and 1A2 hGR alpha (without the exon 2 deletions) transcript levels, respectively. Because these novel hGR, N-terrninal 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.