The Regulation of Nuclear ErbB3 by the Androgen Receptor in Prostate Cancer Cells

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Introduction The receptor tyrosine kinase (RTK) ErbB3/HER3 increases in aggressive (castration-resistant) vs less aggressive (androgen-sensitive) prostate cancer. We detected nuclear ErbB3 in tumor tissue, which may predict biochemical recurrence of prostate cancer. Prostate tumors remain reliant on the androgen receptor (AR) so our goal was to investigate if AR altered the subcellular localization of nuclear ErbB3 and if these interactions might describe a mechanism for the involvement of nuclear ErbB3 in castration-resistance. Materials and Methods ErbB3 and AR localization were investigated in vitro (human prostate cancer cell lines, LNCaP, C4-2, CWR22Rv1, PC3 and PC3-wild-type AR). AR and ErbB3 were pharmacologically-manipulated by serum-depletion or ligand treatment with dihydrotestosterone (DHT) (AR ligand) or heregulin-1β (HRG) (ErbB3 ligand). ErbB3 and AR expression, activity and subcellular localization were analyzed using high-magnification microscopy/immunoblot/fractionation/ immunohistochemistry/reporter gene assays. Results ErbB3 was observed on the perinuclear membrane in human prostate tumor tissues and cell lines. AR and ErbB3 localization were altered by AR and ErbB3 ligands. This was pronounced in androgen-sensitive compared to castration-resistant cells. ErbB3 increased upon serum-depletion but nuclear ErbB3 decreased when DHT was added. Accordingly, cell viability was higher in cells co-cultured with HRG-1β and DHT but not HRG-1β alone. Castration-resistant cells did not display this additive effect. HRG selectively induced nuclear ErbB3 in castration-resistant cells. PC3 but not PC3-wt-AR (PC3 cells stably-transfected with wild-type AR) cells demonstrated increased full-length nuclear ErbB3. Conversely, PC3-wt-AR (but not PC3) cells displayed decreased cytoplasmic ErbB3 whose levels were more susceptible to HRG than DHT. The presence of nuclear ErbB3 modestly decreased cell viability and reporter gene readout in cells lacking AR protein but increased cell viability and reporter gene readouts in cells with active AR protein. Conclusions ErbB3 activation increases nuclear ErbB3 while AR inhibition increases cytoplasmic ErbB3 in prostate tumor cells. Nuclear ErbB3 function may depend upon AR, for we have previously shown that ErbB3 protein levels are regulated by the AR via the E3 ubiquitin ligase Nrdp1. The observed increases in nuclear and cytoplasmic ErbB3 protein may be a survival mechanism. Future studies will continue to examine the functional relevance of the nuclear ErbB3-AR interaction in prostate cancer.