AR is necessary for AR nuclear export and is dominant over the NLS in the DNAbinding domain and hinge region in the absence of hormone. Our findings suggest that androgen can regulate NES AR and, subsequently, the NLS of the AR, providing a mechanism by which androgen regulates AR nuclear/ cytoplasmic shuttling. Estrogen receptor ␣ and mineralocorticoid receptor also contain functional NES, suggesting that this ligand-regulated NES is conserved among steroid receptors. Androgen receptor (AR)1 is a member of the steroid receptor superfamily that regulates gene expression in a ligand-dependent manner (1, 2). AR is necessary for male accessory organ development and is involved in prostate cancer progression. The human AR contains 918 amino acids and consists of the N-terminal transactivation domain (1-555), DNA-binding domain (DBD) (556 -623), hinge region (624 -665), and C-terminal ligand-binding domain (LBD) (666 -918) (3). The DBD, hinge region, and LBD of AR share a high degree of homology with other steroid receptors. Intracellular localization of AR and other steroid receptors such as glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) is ligand-dependent. The limit for passive diffusion across the nuclear pore complex has been suggested to fall within the range of 20 -40 kDa (4). Larger proteins such as AR, which is ϳ100 kDa in size, must be actively transported through the nuclear pore complex to enter or leave the nucleus. AR is localized to the cytoplasm in the absence of ligand and translocates into the nucleus in the presence of ligand (5-7). Likewise, nuclear AR can be exported to the cytoplasm upon ligand withdrawal (8). These observations indicate that AR contains both a nuclear localization signal (NLS) and nuclear export signal (NES) and that their activities are regulated by androgen either directly or indirectly. Because nuclear localization is necessary for steroid receptors to transactivate their target genes, the ligand-dependent nuclear import/export represents a critical mechanism by which steroids regulate the activity of their cognate receptors.The signal involved in the transport of steroid receptors from the cytoplasm to the nucleus has been studied extensively. Mutagenesis studies of the human AR have defined a bipartite NLS in the DBD and hinge region at amino acids 617-633 (9). This NLS is composed of two clusters of basic amino acids (underlined) separated by 10 amino acid residues: RKCY-EAGMTLGARKLKK. Similar NLS were found in other steroid receptors. In addition to the NLS in the DBD and hinge region, Picard and Yamamoto (10) demonstrated the existence of a ligand-dependent NLS present in the LBD of GR. When fused to ␤-galactosidase or to N-terminal fragments of the GR lacking the NLS, the LBD of GR conferred ligand-dependent nuclear localization to the fusion proteins. Similarly, a ligand-dependent NLS also exists in the LBD of AR, which is capable of inducing nuclear import in the absence of the NLS in the DBD and hinge region (11,12).Studies on sequences responsible...

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More than two dozen androgen-responsive genes were identified from the castrated rat ventral prostate on the basis of their induction by exogenous testosterone. One of the identified genes encodes a novel 886-amino-acid protein that was named prominin-like protein 2 (PROML2) because it shares 32% identity to prominin and prominin-like protein 1, a family of important plasma membrane proteins with five transmembrane domains. The rat PROML2 gene is expressed abundantly in the glandular epithelial cells of the rat ventral prostate. The PROML2 gene is expressed in the human prostate and human prostate cancer cell lines with the highest level in less aggressive LNCaP cells and low expression in highly aggressive PC3 and DU145 cells, suggesting a correlation between PROML2 down-regulation and aggressiveness of prostate cancer cells. Transient transfection of green fluorescent protein-tagged rat PROML2 expression vector into prostate cancer cell lines showed that PROML2 protein is localized to the nuclear envelope and perinuclear region and induces cell death in all of the transfected prostate cancer cells. Taken together, our results argue that PROML2 is a novel proapoptotic membrane protein and its down-regulation may associate with aggressive phenotype of prostate cancer cells.