Hormone Depletion-Insensitivity of Prostate Cancer Cells Is Supported by the AR Without Binding to Classical Response Elements

Abstract

A need for androgen response elements (AREs) for androgen receptor (AR)-dependent growth of hormone depletion-insensitive prostate cancer is generally presumed. In such cells, androgen-independent activation by AR of certain genes has been attributed to selective increases in basal associations of AR with putative enhancers. We examined the importance of AR binding to DNA in prostate cancer cells in which proliferation in the absence of hormone was profoundly (∼ 90%) dependent on endogenous AR and where the receptor was not up-regulated or mutated but was predominantly nuclear. Here, ARE-mediated promoter activation and the binding of AR to a known ARE in the chromatin remained entirely androgen dependent, and the cells showed an androgen-responsive gene expression profile with an unaltered sensitivity to androgen dose. In the same cells, a different set of genes primarily enriched for cell division functions was activated by AR independently of hormone and significantly overlapped the signature gene overexpression profile of hormone ablation-insensitive clinical tumors. After knockdown of endogenous AR, hormone depletion-insensitive cell proliferation and AR apoprotein-dependent gene expression were rescued by an AR mutant that was unable to bind to ARE but that could transactivate through a well-established AR tethering protein. Hormone depletion-insensitive AR binding sites in the chromatin were functional, binding, and responding to both the wild-type and the mutant AR and lacked enrichment for canonical or noncanonical ARE half-sites. Therefore, a potentially diverse set of ARE-independent mechanisms of AR interactions with target genes must underlie truly hormone depletion-insensitive gene regulation and proliferation in prostate cancer.

The ETS Domain Transcription Factor ELK1 Directs a Critical Component of Growth Signaling by the Androgen Receptor in Prostate Cancer Cells

“…genes strikingly overlaps the signature gene overexpression profile of clinical castration-recurrent prostate tumors and is enriched for gene clusters primarily supporting mitotic cell division (9,20). Furthermore, we have demonstrated that in those cells the AR apoprotein can support growth through gene activation that occurs without the direct binding of AR to AREs and likely through tethered associations of the receptor with its target genes (9).…”

“…Furthermore, we have demonstrated that in those cells the AR apoprotein can support growth through gene activation that occurs without the direct binding of AR to AREs and likely through tethered associations of the receptor with its target genes (9). Our previous detailed studies of the interaction of AR with CCAAT/enhancer-binding protein ␣ (involved in terminal tissue differentiation) suggested that tethered associations of AR with DNA may not require hormone except in cell contexts in which androgen is needed for nuclear import of AR (21).…”

“…The growth could be driven by circulating androgen, postablation synthesis of intratumoral androgen, or by AR acting completely independently of androgen (5)(6)(7)(8)(9)(10). In all cases, the current clinical paradigm for adjuvant therapy is total and ubiquitous attenuation of AR signaling by androgen ablation and the use of AR ligands that antagonize, sequester, or deplete AR (11).…”

“…However, in prostate cancer cells that are adapted to grow in the absence of hormone, the AR apoprotein is localized in an active form in the nucleus. In such cells, the AR apoprotein activates a gene set that is distinct from genes that require androgen for activation in the same cells (9,20). This set of Tables 1-3 and Supplement 8.…”

“…However, in genome-wide chromatin binding studies using cell line models of advanced prostate cancer in the absence of hormone, putative tethered associations of AR with chromatin generally give relatively weak signals at best. This is presumably due to the poor efficiency of immunoprecipitation of AR in such complexes (9,20). Nevertheless, the association of AR with DNA-bound transcription factors has been reported by screening a synthetic cis-element array of transcription factor binding sites for AR recruitment from a nuclear extract of LNCaP cells (22).…”