The role of androgens in prostate cancer is well established; yet not all facets of androgen signaling are well understood. Sex hormone-binding globulin (SHBG), which binds androgens in plasma, mediates a rapid androgen response in prostate cells through a specific membrane-based receptor (RSHBG) independent of the androgen receptor (AR). SHBG and its associated signaling pathways could provide potential therapeutic targets for prostate cancer. SHBG mRNA and protein is expressed by prostate cells, suggesting autocrine/paracrine properties. We hypothesize that SHBG regulates gene expression through RSHBG, and also acts as an intracellular buffer for androgens to modulate AR activation. The SHBG and p53 tumor suppressor genes colocalize on chromosome 17 to a region often deleted in tumors, implicating a role in prostate cancer for aberrant SHBG expression. In this study, we used microarray analysis to acertain the effects of SHBG on androgen signaling in the LNCaP prostate cell line. L5S2, a LNCaP-derived clonal cell line, reproducibly induces SHBG overexpression upon Ponasterone A (PonA) treatment. L5V4 is its noniducible isogenic sister vector control cell line. L5S2 and L5V4 cells were grown in androgen depleted medium, treated for 24hrs with PonA to induce SHBG or vehicle alone, then with DHT or vehicle alone for another 24 hours. Experiments were performed in triplicate. Cellular RNAs were isolated, labeled, and hybridized to Affymetrix Human Genome U133 Plus 2.0 Array chips. In response to SHBG overexpression and DHT, 1131 genes were significantly induced or repressed by conditions resulting in RSHBG activation and 187 DHT responsive genes were modulated by SHBG. Real-time PCR analysis was used to validate microarray results. Our results support a biologic role for RSHBG signaling in LNCaP cells. We also provide the first evidence that intracellular SHBG may regulate AR activity- decreased SHBG expression in prostate tumors may allow the AR to function under reduced intracellular androgen concentrations. Putative downstream targets of SHBG + DHT signaling include STEAP4, FKBP5, TIMP2, GPR89a, GPR30, and c-MYC, genes with important and provocative cellular functions with respect to androgen signaling and prostate cancer. If confirmed, we plan to identify intermediaries in SHBG-mediated androgen signaling, and address the functional role of dysregulated SHBG expression in prostate cancer.
99th AACR Annual Meeting-- Apr 12-16, 2008; San Diego, CA