The androgen receptor (AR) is a ligand-activated nuclear receptor that plays a critical role in normal prostate physiology, as well as in the development and progression of prostate cancer. In addition to the classical paradigm in which AR exerts its biological effects in the nucleus by orchestrating the expression of the androgen-regulated transcriptome, there is considerable evidence supporting a rapid, nongenomic activity mediated by membrane-associated AR. Although the genomic action of AR has been studied in depth, the molecular events governing AR transport to the plasma membrane and the downstream AR signaling cascades remain poorly understood. In this study, we report that AR membrane transport is microtubule-dependent. Disruption of the function of kinesin 5B (KIF5B), but not of kinesin C3 (KIFC3), interfered with AR membrane association and signaling. Co-immunoprecipitation and pulldown assays revealed that AR physically interacts with KIF5B and that androgen enhances this interaction. Furthermore, we show that heat shock protein 27 (HSP27) is activated by membrane-associated AR and that HSP27 plays an important role in mediating AR-mediated membrane-to-nuclear signal transduction. Together, these results indicate that AR membrane translocation is mediated by the microtubule cytoskeleton and the motor protein KIF5B. By activating HSP27, membrane-associated AR potentiates the transcriptional activity of nuclear AR. We conclude that disruption of AR membrane translocation may represent a potential strategy for targeting AR signaling therapeutically in prostate cancer.
Keywords: androgen receptor, microtubule, plasma membrane, kinesin, heat shock protein (HSP), HSP27, KIF5B, prostate cancer, steroid hormone receptor, type I nuclear receptor
The androgen receptor (AR),3 along with the estrogen receptor (ER), glucocorticoid receptor (GR), progesterone receptor (PR), and the mineralocorticoid receptor (MR), is a member of the Type I nuclear receptor subfamily (1, 2). AR plays an important role in the development and maintenance of the male sexual phenotype (3, 4). Similar to other steroid hormone receptors, AR is located in the cytoplasm in the absence of androgens, forming a complex with chaperones and co-chaperones such as heat shock proteins and immunophilins (5). Upon androgen binding, AR undergoes conformational changes and translocates to the nucleus, where it binds to the androgen response elements (AREs) as a homodimer; activates the expression of AR target genes; and induces cell proliferation, differentiation, and survival (6, 7). This mode of action is the classical AR signaling pathway, also known as the genomic action. In addition to the classical pathway, many observations suggest androgens, as well as some other steroid hormones, can affect cellular processes in a nongenomic fashion (8). Cinar et al. found that within minutes of androgen stimulation, AR is localized to the membrane lipid rafts microdomain, interacts with AKT, and activates AKT signaling (9). Pedram et al. showed palmitoylation of a conserved motif in the ligand-binding domain is critical for membrane localization of estrogen receptor, progesterone receptor, and AR (10). Additionally, studies have shown AR interacts with caveolin-1 (Cav-1), a major component of the caveolae membrane structure (11, 12). Down-regulation of Cav-1 decreases AR membrane localization (11, 12). Overall, studies on membrane-associated AR are limited and very little is understood regarding the mechanisms underlying AR translocation to the plasma membrane.
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