Prostate cancer depends on the androgen receptor (AR) for growth and survival even in the absence of androgen. In the classical models of gene activation by AR, ligand activated AR signals through binding to the androgen response elements (AREs) in the target gene promoter/enhancer. In the present study the role of AREs in the androgen-independent transcriptional signaling was investigated using LP50 cells, derived from parental LNCaP cells through extended passage in vitro. LP50 cells reflected the signature gene overexpression profile of advanced clinical prostate tumors. The growth of LP50 cells was profoundly dependent on nuclear localized AR but was independent of androgen. Nevertheless, in these cells AR was unable to bind to AREs in the absence of androgen. Gene expression profiling of LP50 cells showed that AR regulates two largely distinct gene expression programs, androgen-dependent and apo-AR dependent. Furthermore, a DNA binding domain mutant of AR which is unable to bind to ARE rescued androgen depletion insensitive proliferation and gene activation in LP50 cells depleted of endogenous wild type AR. Furthermore, ChIP-chip promoter tiling arrays revealed enrichment for AR in chromatin sites that are functional but lack ARE.

To identify candidate transcription factors that tether AR to target genes in the absence of androgen, cis-elements of transcription factors in the AR interactome data set and AR chip peaks were used. We found direct interaction between AR and Elk-1 in both the C81 and C4-2 LNCaP variants of androgen depletion insensitive experimental model systems representing clinical advanced prostate cancer. AR dependent promoter activity of an Elk-1 driven promoter reporter construct and physical association between AR and Elk-1 by coimmunoprecipitation suggested that AR acts as a co-activator of Elk-1. Elk-1 was shown to be necessary for the proliferation of C81 and C4-2 cells. Expression profile studies further showed AR-dependent activation of gene clusters enriched for cell division function by Elk-1. This AR dependent gene regulation by Elk-1 was insensitive to androgen antagonist.

The present study suggests that in advanced prostate cancer AR can support the progression of the tumor through ARE independent mechanisms by acting as a transcriptional co-activator for transcription factors such as Elk-1. This mechanism of action of AR is insensitive to hormone as well as antiandrogen. Hence, therapeutic strategies selectively targeting the interactions between AR and critical tethering proteins could be a novel approach for the management of advanced prostate cancer.