Glucocorticoid resistance in a multiple myeloma cell line is regulated by a transcription elongation block in the glucocorticoid receptor gene (NR3C1).
Glucocorticoid (GC) effects are mediated by the glucocorticoid receptor (GR). Several studies have demonstrated that a lower number of receptors per cell were associated with poor GC response. The regulation of GR expression is complex; the levels of GR can be autologously regulated by its ligand and also by transcriptional, post-transcriptional and post-translational mechanisms. Using three human myeloma cell lines that parallel the development of GC resistance, this work describes the mechanism involved in the downregulation of GR expression. The decreased expression was neither due to mutations in the gene encoding GR, NR3C1, nor due to methylation of the promoters. A gradual decrease in NR3C1 transcripts was seen during the development of resistance, the level of expression of exon 1 to 2 RNA fragments remained the same in sensitive and resistant cell lines but a chromatin immunoprecipitation assay demonstrated that RNA polymerase II, detectable throughout exon 2 to 3 in the sensitive cells, was undetectable on exon 3 in the resistant variant, suggesting lower or no transcription at this site. These studies demonstrated that downregulation of NR3C1 mRNA in a resistant cell line involves a block to transcriptional elongation within intron B of NR3C1. This block may represent an important element in the regulation of GR expression.
Sánchez-Vega
Beatriz
B
Department of Experimental Therapeutics, University of Texas M. D. Anderson Cancer Center, Houston, TX 77030, USA.
Gandhi
Varsha
V
D001483:Base Sequence; D002460:Cell Line; D047369:Chromatin Immunoprecipitation / Q000379:methods; D004252:DNA Mutational Analysis / Q000379:methods; D015536:Down-Regulation; D019008:Drug Resistance, Neoplasm / Q000235:genetics*; D005938:Glucocorticoids / Q000627:therapeutic use; D006801:Humans; D007438:Introns; D008969:Molecular Sequence Data; D009101:Multiple Myeloma / Q000188:drug therapy / Q000235:genetics* / Q000378:metabolism; D009154:Mutation; D011401:Promoter Regions, Genetic; D011965:Receptors, Glucocorticoid / Q000235:genetics* / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction / Q000379:methods; D014158:Transcription, Genetic
D005938:Glucocorticoids; C497713:NR3C1 protein, human; D011965:Receptors, Glucocorticoid