Androgen receptor (AR) signaling regulates the development and homeostasis of male reproductive organs, including the prostate. Deregulation of AR and AR coregulators, expression, or activity is involved in the initiation of prostate cancer and contributes to the transition of the disease to hormone-refractory stage. The ubiquitous ␤Arrestin proteins are now recognized as bona fide adapters and signal transducers with target effectors found in both the cytosol and nucleus. Here, we provide evidence that ␤Arrestin2 forms a complex with AR and acts as an AR corepressor in androgen-dependent prostate cancer cells. Accordingly, the forced overexpression of ␤Arrestin2 diminishes, and knockdown of ␤Arrestin2 expression with RNAi increases the androgen-induced prostate-specific antigen (PSA) gene expression. ␤Arrestin2 serves as an adapter, bringing into close proximity the Mdm2 E3 ligase and AR, thereby promoting AR ubiquitylation and degradation. Human prostate tissues evidence an inverse relationship between the expression of ␤Arrestin2 and AR activity: glands that express high levels of ␤Arrestin2 exhibit low expression of PSA, and those glands that express low levels of ␤Arrestin2 evidence elevated PSA levels. We conclude that ␤Arrestin2 acts as a corepressor of AR by serving as a scaffold for Mdm2 leading to the AR ubiquitylation and degradation.beta-arrestin ͉ ubiquitin ͉ Mdm2 ͉ testosterone ͉ androgen deprivation therapy P rostate cancer accounts for approximately one-third of all male cancer cases in the United States, and 186,320 cases were diagnosed in 2008 (1). The cancer often presents as an androgendependent (AD), hormone-sensitive disease that can be successfully managed with targeted therapies that aim to inhibit function of the androgen receptor (AR). Although these therapies are initially effective, a significant portion of the cancer patients develop advanced androgen-independent (AI), hormone-refractory disease (2). Not surprising then, effective management of prostate cancer remains elusive and an estimated 28,660 American lives were claimed by the disease in 2008 alone (1). The deregulation of expression and activity of AR, and AR-interacting partners, is thought to be involved in the progression of the prostate cancer to advanced disease (3, 4).The AR is a member of the nuclear hormone receptor superfamily of ligand-controlled transcription factors, and it regulates expression of multiple genes that are involved in the normal development and malignant transformation of the prostate (4-6). Recent evidence suggests that the AR also participates in the transition of the prostate cancer to AI disease (7). Indeed, approximately one-third of AI prostate carcinomas show amplification and overexpression of the wild-type AR, suggesting it adjusts to capture the low levels of circulating androgen (8, 9). In another one-third of AI cancer, the AR is mutated allowing it to become activated by other steroids or even anti-androgens (2, 10). In the remaining one-third of AI prostate cancers, no discernable AR m...

Background: G protein-coupled receptor kinases (GRKs) are important regulators of receptor function although little is known about their cellular localization. Results: GRK5 is localized in the centrosome during interphase and regulates normal cell cycle progression. Conclusion: GRKs appear to dynamically modulate regulation of the cell cycle. Significance: These findings reveal an additional mechanism by which plasma membrane signaling can regulate the cell cycle.

In addition to regulating receptor activity, non-visual arrestins function as scaffolds for numerous intracellular signaling cascades and as regulators of gene transcription. Here we report that the two non-visual arrestins, arrestin2 and arrestin3, localize to the centrosome, a key organelle involved in microtubule nucleation and bipolar mitotic spindle assembly. Both arrestins co-localized with the centrosomal marker ␥-tubulin during interphase and mitosis and were found in purified centrosome preparations. In vitro binding assays demonstrated that both arrestins directly interact with ␥-tubulin. Knockdown of either arrestin by RNA interference resulted in multinucleation, centrosome amplification, and mitotic defects, although only the loss of arrestin2 triggered aberrant microtubule nucleation. Importantly, overexpression of wild type arrestin rescued the multinucleation phenotype and restored normal centrosome number in arrestin siRNA-transfected cells. Moreover, overexpression of arrestin2 or -3 rescued the multinucleation defect observed in MDA-MB-231 breast cancer cells. Taken together, our data reveal that non-visual arrestins are novel centrosomal components and regulate normal centrosome function.

Background: G protein-coupled receptor kinases (GRKs) are important regulators of receptor signaling although little is known about their functions beyond their receptor modifying activities. Results: GRK5 binds and phosphorylates nucleophosmin. Conclusion: GRK5 and polo-like kinase 1 coordinately regulate nucleophosmin phosphorylation and cell sensitivity to inhibitor-induced apoptosis. Significance: GRKs play an important role in regulating normal cell functions such as cell cycle regulation and apoptosis.

GRK2 is localized to centrosomes and regulates EGF receptor–promoted separation of duplicated centrosomes. This pathway depends on Mst2 and Nek2A and involves GRK2-mediated phosphorylation and activation of Mst2. Thus GRK2 plays a central role in mitogen-promoted centrosome separation via its ability to phosphorylate Mst2.