Progression of prostate cancer following castration is associated with increased androgen receptor (AR) expression and signaling despite AR blockade. Recent studies suggest that these activities are due to the generation of constitutively active AR splice variants, but the mechanisms by which these splice variants could mediate such effects are not fully understood. Here we have identified what we believe to be a novel human AR splice variant in which exons 5, 6, and 7 are deleted (AR v567es ) and demonstrated that this variant can contribute to cancer progression in human prostate cancer xenograft models in mice following castration. We determined that, in human prostate cancer cell lines, AR v567es functioned as a constitutively active receptor, increased expression of full-length AR (AR fl ), and enhanced the transcriptional activity of AR. In human xenografts, human prostate cancer cells transfected with AR v567es cDNA formed tumors that were resistant to castration. Furthermore, the ratio of AR v567es to AR fl expression within the xenografts positively correlated with resistance to castration. Importantly, we also detected AR v567es frequently in human prostate cancer metastases. In summary, these data indicate that constitutively active AR splice variants can contribute to the development of castration-resistant prostate cancers and may serve as biomarkers for patients who are likely to suffer from early recurrence and are candidates for therapies directly targeting the AR rather than ligand.

IntroductionThe C-type lectin-like stimulatory immune receptor NKG2D is expressed by all human NK and CD8 + T cells and by most γδ T cells (1-5). NKG2D-mediated immune activation can be triggered by interaction with its ligands (6-8): the family of stress-induced MHC class I chain-related molecules (MICs) MICA and MICB (5), and the UL16-binding protein (ULBP) family (9-11). In NK cells, the activation signal mediated by NKG2D was shown to dominate the inhibitory signal mediated by MHC class I binding to killer inhibitory receptors, leading to lysis of target cells that express NKG2D ligands (1-4). NKG2D-mediated activation signal also costimulates antigen-specific CD8 + T cell immunity and is necessary for activation of cytotoxic γδ T cells (5,(12)(13)(14)(15).According to current available data, MIC molecules are the best-characterized ligands and are the most frequently expressed ligands on epithelial tumors. MIC is induced on a broad range of epithelial tumor cells, such as melanoma, colon, breast, lung, ovary, renal, and hepatocellular carcinomas, but is absent from normal tissues (16-18). Cells expressing MIC on their surface are susceptible to NK and antigen-specific T cell immunity. Thus, surface expression of MIC on transformed cells is proposed to mark nascent tumors for immune surveillance (6-8). MIC pro-

Purpose:The type I insulin-like growth factor receptor (IGF-IR) and its ligands have been shown to play a critical role in prostate carcinoma development, growth, and metastasis. Targeting the IGF-IR may be a potential treatment for prostate cancer. A fully human monoclonal antibody, A12, specific to IGF-IR, has shown potent antitumor effects in breast, colon, and pancreatic cancers in vitro and in vivo. In this study, we tested the in vivo effects of A12 on androgen-dependent and androgen-independent prostate tumor growth. Experimental Design: Androgen-dependent LuCaP 35 and androgen-independent LuCaP 35V prostate tumors were implanted s.c. into intact and castrated severe combined immunodeficient mice, respectively.When tumor volume reached about150 to 200 mm 3 , A12 was injected at 40 mg/kg body weight thrice a week for up to 5 weeks. Results: We find that A12 significantly inhibits growth of androgen-dependent LuCaP 35 and androgen-independent LuCaP 35V prostate xenografts, however, by different mechanisms. In LuCaP 35 xenografts, A12 treatment induces tumor cell apoptosis or G 1 cycle arrest. In LuCaP 35V xenografts, A12 treatment induces tumor cell G 2 -M cycle arrest. Moreover, we find that blocking the function of IGF-IR down-regulates androgen-regulated gene expression in androgenindependent LuCaP 35V tumor cells. Conclusions: Our findings suggest that A12 is a therapeutic candidate for both androgendependent and androgen-independent prostate cancer. Our findings also suggest an IGFIR^dependent activity of the androgen receptor in androgen-independent prostate cancer cells.

The insulin-like growth factor type I receptor (IGF-IR) has been suggested to play an important role in prostate cancer progression and possibly in the progression to androgen-independent (AI) disease. The term AI may not be entirely correct, in that recent data suggest that expression of androgen receptor (AR) and androgen-regulated genes is the primary association with prostate cancer progression after hormone ablation. Therefore, signaling through other growth factors has been thought to play a role in AR-mediated prostate cancer progression to AI disease in the absence of androgen ligand. However, existing data on how IGF-IR signaling interacts with AR activation in prostate cancer are conflicting. In this Prospect article, we review some of the published data on the mechanisms of IGF-IR/AR interaction and present new evidence that IGF-IR signaling may modulate AR compartmentation and thus alter AR activity in prostate cancer cells. Inhibition of IGF-IR signaling can result in cytoplasmic AR retention and a significant change in androgen-regulated gene expression. Translocation of AR from the cytoplasm to the nucleus may be associated with IGF-induced dephosphorylation. Since fully humanized antibodies targeting the IGF-IR are now in clinical trials, the current review is intended to reveal the mechanisms of potential therapeutic effects of these antibodies on AI prostate cancers.