Abstract
Androgen treatment promotes up- or downregulation of androgen receptor (AR) mRNA in a tissue-specific manner. Since androgen responsiveness depends upon AR levels, autoregulation may control cellular sensitivity to androgen. Consistent with this idea, our laboratory has shown that androgen-mediated upregulation of AR mRNA and protein in a human osteosarcoma cell line (U2OS) correlated with enhanced androgen-regulated gene expression. Although androgen regulates transcription of the AR gone, the AR promoter and upstream region do not contain the requisite androgen response elements (AREs) for AR mRNA autoregulation. We previously showed that sequences present within the AR cDNA mediate AR mRNA autoregulation. Four AREs and a Myc site are involved in this AR mRNA upregulation and are present in a 350 bp fragment of the AR cDNA comprising exons D and E of the AR gene. Each ARE and the Myc site was required for maximal AR binding to and androgen regulation of the 350 bp region as determined by EMSA and reporter gene assay. Use of dominant negative myc or max mutants profoundly decreased androgen regulation of the 350 bp region but not of the MMTV-LTR. Within the AR gene, the AREs and Myc site are contained in a 6.5 kb fragment that, like the 350 bp AR cDNA region, was androgen regulated in a cell-specific manner. Androgen regulated the 350 bp and 6.5 kb regions in cell lines that upregulate AR mRNA after androgen treatment, but not in cells that undergo downregulation of AR mRNA. Thus, the requisite sequences for upregulation of the native AR gene are contained within these androgen responsive regions and a cell-specific factor(s) is likely to be involved in this process. Since the exonic autoregulatory elements are distributed over 6.5 kb of the AR gene and distal to the AR promoter, hormonal regulation of chromatin structure is likely to play an important role in transcriptional events occurring at this unique, intragenic responsive region. Unlike many other enhancers of androgen regulated genes, the 350 bp region and 6.5 kb genomic fragment are specifically regulated by AR, but not glucocorticoid receptor (GR). Use of functional chimeric receptors comprised of cassettes containing the NH-terminus, DNA binding, and ligand binding domains of AR and GR showed that the AR NH-terminus confers androgen specificity. Our findings suggest that cell-specific, androgen mediated upregulation of AR mRNA occurs via exonic AREs and a Myc site and influences hormonal responsiveness. The observation that a Myc family member may be involved in androgenic upregulation of AR mRNA suggests a potential link between AR and important regulators of cellular growth and differentiation.