Manju Sharma

Linking β-Catenin to Androgen-signaling Pathway

The androgen-signaling pathway is important for the growth and progression of prostate cancer cells. The growth-promoting effects of androgen on prostate cells are mediated mostly through the androgen receptor (AR). There is increasing evidence that transcription activation by AR is mediated through interaction with other cofactors. β-Catenin plays a critical role in embryonic development and tumorigenesis through its effects on E-cadherin-mediated cell adhesion and Wnt-dependent signal transduction. Here, we demonstrate that a specific protein-protein interaction occurs between β-catenin and AR. Unlike the steroid hormone receptor coactivator 1 (SRC1), β-catenin showed a strong interaction with AR but not with other steroid hormone receptors such as estrogen receptor α, progesterone receptor β, and glucocorticoid receptor. The ligand binding domain of AR and the NH2terminus combined with the first six armadillo repeats of β-catenin were shown to be necessary for the interaction. Through this specific interaction, β-catenin augments the ligand-dependent activity of AR in prostate cancer cells. Moreover, expression of E-cadherin in E-cadherin-negative prostate cancer cells results in redistribution of the cytoplasmic β-catenin to the cell membrane and reduction of AR-mediated transcription. These data suggest that loss of E-cadherin can elevate the cellular levels of β-catenin in prostate cancer cells, which may directly contribute to invasiveness and a more malignant tumor phenotype by augmenting AR activity during prostate cancer progression.

Androgen Receptor Interacts with a Novel MYST Protein, HBO1

The androgen receptor (AR), a member of the nuclear receptor superfamily, plays a central role in male sexual differentiation and prostate cell proliferation. Results of treating prostate cancer by androgen ablation indicate that signals mediated through AR are critical for the growth of these tumors. Like other nuclear receptors, AR exerts its transcriptional function by binding tocis-elements upstream of promoters and interacting with other transcriptional factors (e.g. activators, repressors and modulators). To determine the mechanism of AR-regulated transcription, we used the yeast two-hybrid system to identify AR-associated proteins. One of the proteins we identified is identical to the human origin recognition complex-interacting protein termed HBO1. A ligand-enhanced interaction between AR and HBO1 was further confirmed in vivo and in vitro. Immunofluorescence experiments showed that HBO1 is a nuclear protein, and Northern blot analysis revealed that it is ubiquitously expressed, with the highest levels present in human testis. HBO1 belongs to the MYST family, which is characterized by a highly conserved C2HC zinc finger and a putative histone acetyltransferase domain. Surprisingly, two yeast members of the MYST family, SAS2 and SAS3, have been shown to function as transcription silencers, despite the presence of the histone acetyltransferase domain. Using a GAL4 DNA-binding domain assay, we mapped a transcriptional repression domain within the N-terminal region of HBO1. Transient transfection experiments revealed that HBO1 specifically repressed AR-mediated transcription in both CV-1 and PC-3 cells. These results indicate that HBO1 is a new AR-interacting protein capable of modulating AR activity. It could play a significant role in regulating AR-dependent genes in normal and prostate cancer cells.

hZimp10 is an androgen receptor co-activator and forms a complex with SUMO-1 at replication foci

The androgen receptor (AR) plays a central role in male sexual development and in normal and malignant prostate cell growth and survival. It has been shown that transcriptional activation of AR is regulated through interaction with various co‐factors. Here we identify a novel PIAS‐like protein, hZimp10, as an AR‐interacting protein. The transactivation domain (TAD) of AR and the central region of hZimp10 were found to be responsible for the interaction. A strong intrinsic transactivation domain was identified in the C‐terminal, proline‐rich region of hZimp10. Endogenous AR and hZimp10 proteins were co‐stained in the nuclei of prostate epithelial cells from human tissue samples. In human prostate cancer cells, hZimp10 augmented the transcriptional activity of AR. Moreover, hZimp10 co‐localized with AR and SUMO‐1 at replication foci throughout S phase, and it was capable of enhancing sumoylation of AR in vivo. Studies using sumoylation deficient AR mutants suggested that the augmentation of AR activity by hZimp10 is dependent on the sumoylation of the receptor. Taken together, these data demonstrate that hZimp10 is a novel AR co‐regulator.