Tamoxifen inhibits the proliferation of non­melanoma skin cancer cells by increasing intracellular calcium concentration.

Tamoxifen is an estrogen receptor (ER) antagonist used as first-line chemotherapy in breast cancer. Recent studies suggest that tamoxifen may be effective not only for ER­positive but also for ER­negative cancer cases. The aim of the present study was to investigate the antiproliferative effect of tamoxifen against human non­melanoma skin cancer cells. Tamoxifen inhibited the proliferation of the skin squamous cell carcinoma (SCC) cell lines A431, DJM­1 and HSC­1. A431 cells did not express ER­α or -ß, suggesting that tamoxifen may exert antiproliferative effects on skin SCC cells via a non­ER­mediated pathway. Tamoxifen increased the intracellular calcium concentration of skin SCC cells, and this increase in intracellular calcium concentration by calcium ionophore A23187 suppressed the proliferation of skin SCC cells. These data indicate that tamoxifen inhibited the proliferation of human skin SCC cells via increasing intracellular calcium concentration. Voltage-gated calcium channels and non­selective cation channels are involved in the increase in intracellular calcium concentration induced by tamoxifen. The broad-spectrum protein kinase C (PKC) inhibitor phloretin significantly attenuated the antiproliferative effect of tamoxifen on skin SCC cells. From these data, it may be concluded that tamoxifen inhibits the proliferation of skin SCC cells by induction of extracellular calcium influx via calcium channels in the plasma membrane and by subsequent activation of PKC.

Anti-proliferative effect of ridaifen-B on hepatoma cells.

Ridaifens (RIDs), a novel series of tamoxifen derivatives, exhibit a potent growth-inhibitory effect against numerous tumor cells regardless of the expression of estrogen receptors, and are thus promising candidates as novel anti-tumor drugs. RID-B is a first generation RIDs, and inhibits the proliferation of several tumor cell lines. However, the potentially growth inhibitory effect of RID-B against hepatoma cells, and the detailed mechanism underlying RID-B-mediated tumor cell death remain to be elucidated. The purpose of the current study was to evaluate the anti-proliferative effect of RID-B against hepatoma cells. The anti-proliferative effect of RID-B against human hepatoma Huh-7 cells was investigated by cell proliferation assay using WST-1 reagent, and caspase-3 activity was evaluated by using specific fluorescent substrate. In addition, DNA fragmentation in Huh-7 cells induced by RID-B was estimated by terminal deoxynucleotidyl transferase dUTP nick-end labelling assay, and binding of RID-B to double-stranded DNA was confirmed by mass spectrometry. RID-B (0.5, 1 and 2 µM) inhibited the growth of Huh-7 cells, seemingly dose-dependently, but did not inhibit the growth of normal primary rat hepatocytes in the same concentration range. Furthermore, the caspase-3 activity of Huh-7 cells was increased by RID-B (0.5 and 5 µM), and the anti-proliferative effect of RID-B (1 µM) on Huh-7 cells was partially suppressed by the addition of the caspase inhibitor, Z-VAD-FMK. Additionally, RID-B (10 µM) directly bound to double-stranded DNA, and the addition of DNA suppressed RID-B-mediated cell growth inhibition and DNA fragmentation in Huh-7 cells. From these data, it may be concluded that RID-B inhibited cell growth and induced apoptosis via activating caspase-3 and binding to DNA directly, leading to DNA fragmentation in hepatoma cells.