Tamoxifen Inhibits TRPV6 Activity via Estrogen Receptor–Independent Pathways in TRPV6-Expressing MCF-7 Breast Cancer Cells

Abstract

The epithelial calcium channel TRPV6 is upregulated in breast carcinoma compared with normal mammary gland tissue. The selective estrogen receptor modulator tamoxifen is widely used in breast cancer therapy. Previously, we showed that tamoxifen inhibits calcium uptake in TRPV6-transfected Xenopus oocytes. In this study, 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. TRPV6 activity was measured with fluorescence microscopy using Fura-2. The basal calcium level was higher in transfected cells compared with nontransfected cells in calcium-containing solution but not in nominally calcium-free buffer. Basal influxes of calcium and barium were also increased. In transfected cells, 10 μmol/L tamoxifen reduced the basal intracellular calcium concentration to the basal calcium level of nontransfected cells. 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. Similarly, a tamoxifen-induced inhibitory 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 protein kinase C 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 therapeutic effect of tamoxifen and protein kinase C inhibitors used in breast cancer therapy might involve TRPV6-mediated calcium entry. This study highlights a possible role of TRPV6 as therapeutic target in breast cancer therapy.

Tamoxifen Suppresses the Growth of Malignant Pleural Mesothelioma Cells

“…In common with our observations for MPM, tamoxifen induced cell death in malignant peripheral nerve sheath tumours at <5 μM and inhibited mitosis at <1 μM; these effects were independent of nuclear ER, but were mimicked by direct calmodulin inhibition (22). The interaction of calmodulin with other effector proteins is modulated by the availability of intracellular Ca 2+ , and the dysregulation of intracellular Ca 2+ homeostasis has also been attributed to ERindependent actions of tamoxifen on breast cancer cells (23).…”

Rosiglitazone Promotes AQP2 Plasma Membrane Expression In Renal Cells Via a Ca2+-Dependent/cAMP-Independent Mechanism

“…Unfortunately, there is no selective blocker for TRPV6-mediated calcium influx available [41,42], therefore we considered the knockdown approach to uncover the role of TRPV6 in mediating the Ca 2+ influx responsible of the AQP2 membrane accumulation. As shown in Fig.…”

Calcium Channel TRPV6 as a Potential Therapeutic Target in Estrogen Receptor–Negative Breast Cancer

“…Increased TRPV6 levels are a feature of T-47D breast cancer cells compared with MDA-MB-231 and MCF-7 breast cancer cells (35). TRPV6 expression is also elevated (2-to 15-fold) in 7 of 12 clinical breast tumors (7).…”