Introduction

Deregulated androgen receptor (AR) signaling due to either mutations or altered expression of the AR and its cofactors (activators or suppressors) has been identified as critical in prostate cancer development and progression. AR regulated oncogenic activation of Ets Related Gene (ERG) represents one of the most common and validated prostate cancer driver gene. In our recent studies using prostate specific ERG transgenic mouse prostate glands, we a observed novel morphological phenotypes of endoplasmic reticulum (ER) stress. Since AR was the critical regulator of ERG expression through TMPRSS2 promoter in human prostate cancer, the present study was aimed towards understanding the post-translational interactions between ERG and AR in ER stress and subsequent cell survival mechanisms in mouse and cell culture models. Understanding such mechanistic insights will potentially have major therapeutic implications.

Methods

Histological phenotype in the mouse prostate glands were examined by light and electron microscopy. Cell culture models of LNCaP, HEK293 and COS7 cells were utilized to examine the AR aggregations, Co-IP and Proximal Ligation Assay in the presence and absence of ERG. Various domain deletions of AR were utilized to identify specific AR domain interactions with ERG and its contribution in AR aggregation. Luminal cell surface markers on the isolated mouse prostate glands and spontaneously immortalized mouse prostate epithelial cells from ERG transgenic mouse (MoE1) were analyzed by FACS analysis.

Results

Co-expression of ERG and AR in LNCaP and COS-7 cells showed significant aggregation of AR in filter assays. Co-IP experiments and PLA assays in VCaP, LNCaP and HEK 293 cell revealed that ERG physically interacts with AR. Epithelial cells of ERG-Tg mouse prostates showed ~70% increase in CD49f (low) and Sca-1 (med) population with increased sphere formation capability and resistance to radiation induced cell death. Both epithelial cells grown into spheres and established MoE1 cells displayed increased CD49f (low) and significant increase in the EpCAM negative population.

Conclusions

Overall, our experiments demonstrate the mechanistic link that the physical interactions between ERG and AR initiate the ER stress in prostate epithelium through AR misfolding/aggregation. Our observation of ERG induced AR aggregation is one of the initial events that lead to ER stress to cell survival indicate a critical function for ERG in the etiology of prostate cancer initiation and progression.

Funding

This research in part was supported by the National Cancer Institute R01CA162383 (S. S.) and HU0001-10-2-0002 funds.