The role of TRPV6 in breast carcinogenesis

TRPV6 is a calcium entry channel that is strongly expressed in breast adenocarcinoma tissue. In this PhD-study we further confirmed this observation by analysis of breast cancer tissues, which indicated that TRPV6 mRNA expression was upregulated compared to normal breast tissue. While TRPV6 is known to be expressed in the cancer tissue, its role as a calcium channel in breast carcinogenesis is poorly understood. Therefore, we investigated how TRPV6 affects viability, apoptosis, and calcium transport in breast cancer cell lines. TRPV6 expression can be regulated by estrogen, progesterone, 1, 25-vitamin D, and tamoxifen (a selective estrogen receptor modulator that is widely used in breast cancer therapy) in T47D cells. Knockdown of TRPV6 in this cell model leads to a significant decrease of cell viability. Moreover, the effect of tamoxifen on cell viability was enhanced when TRPV6 expression was silenced with siRNA. Interestingly, tamoxifen reduced expression of TRPV6 in T47D cells and is able to inhibit its calcium transport activity (IC50=7.5 μM) in Xenopus oocytes. We examined the effect of tamoxifen on TRPV6 function and intracellular calcium homeostasis in MCF-7 breast cancer cells transiently transfected with EYFP-C1-TRPV6 in detail. TRPV6 activity was measured with fluorescence microscopy using Fura-2. Tamoxifen decreased the transport rates of calcium and barium in transfected cells by 50%. This inhibitory effect was not blocked by the estrogen receptor antagonist, ICI 182, 720 and a similar inhibition effect was also observed in MDA-MB-231 estrogen receptor negative cells. The effect of tamoxifen was completely blocked by activation of protein kinase C. Inhibiting PKC with calphostin C decreased TRPV6 activity but did not alter the effect of tamoxifen. These findings illustrate how tamoxifen might be effective in estrogen receptor negative breast carcinomas and suggest that the mechanisms of tamoxifen and PKC inhibitors used in breast cancer therapy might involve TRPV6 mediated calcium entry. This PhD-study highlights a possible role of TRPV6 as a therapeutic target in breast cancer therapy.