227 CALCIUM CHANNEL BLOCKER MODULATES ANDROGEN RECEPTOR-MEDIATED GENE EXPRESSION AND INDUCES CYTOTOXICITY IN PROSTATE CANCER CELLS

INTRODUCTION AND OBJECTIVES: Calcium (Ca ), a ubiquitous secondary messenger, is involved in several cellular processes such as apoptosis, proliferation and carcinogenesis. It has also been shown that Ca signaling is involved in androgen-induced PSA secretion. Recently published epidemiologic data demonstrated that patients on calcium channel blockers had significantly lower incidences of prostate cancer. Therefore, we examined the role of various voltagegated calcium channels in androgen signaling and prostate cancer development. METHODS: Multiple prostate cancer cell lines including LNCaP, LAPC-4, and C4-2 (all androgen receptor positive) were treated with calcium channel blockers nifedipine, mibefradil, and tetrandine. The medications were added to the cell culture media at a dose range of 5-50 micromolar for various time points between 2 and 24 hours. Cell proliferaion was measured using an MTS assay. Reverse transcriptase polymerase chain reaction (RT-PCR) was used to determine expression of several Ca channel genes in different cell lines. Knockdown of Ca channel expression using an siRNA was also utilized to determine effects on cell lines. RESULTS: Blocking L-type channels with nifedipine and tetrandrine significantly suppressed cell proliferation and androgen receptormediated gene expression in prostate cancer cells. Higher doses of tetrandrine also induced a dramatic cytotoxic effect. In contrast, a T-type specific channel blocker mibefradil induced androgen receptor transactivation without androgen addition and also drastically enhanced androgen receptor transactivation in a reporter gene assay. RT-PCR demonstrated that only L-type CACNA1D (cav1.3) and T-type CACNA1H (cav3.2) alpha-subunit genes are highly expressed in androgen receptor-positive prostate cancer cell lines, while L-type CACNA1F (cav1.4) is only weakly expressed in androgen receptorpositive cell lines but highly expressed in androgen receptor-negative cell lines. Other subunits are not expressed in any prostate cancer cells. Furthermore, consistently, knocking down CACNA1D (cav1.3) expression by the siRNA approach abrogated AR transactivation while knocking down CACNA1H (cav3.2) expression enhanced AR transactivation. CONCLUSIONS: These results suggest that the L-type calcium channel subunit (cav1.3) has the potential as a therapeutic target for prostate cancer intervention, although further clinical testing is warranted.