Extracellular Ca2+-dependent enhancement of cytocidal potency of zoledronic acid in human oral cancer cells

Abstract

Direct antitumor effects of bisphosphonates (BPs) have been demonstrated in various cancer cells in vitro. However, the effective concentrations of BPs are typically much higher than their clinically relevant concentrations. Oral cancers frequently invade jawbone and may lead to the release of Ca2+ in primary lesions. We investigated the effects of the combined application of zoledronic acid (ZA) and Ca2+ on proliferation and apoptosis of oral cancer cells. Human oral cancer cells, breast cancer cells, and colon cancer cells were treated with ZA at a wide range of concentrations in different Ca2+ concentration environments. Under a standard Ca2+ concentration (0.6mM), micromolar concentrations of ZA were required to inhibit oral cancer cell proliferation. Increasing extracellular Ca2+ concentrations greatly enhanced the potency of the ZA cytocidal effect. The ability of Ca2+ to enhance the cytocidal effects of ZA was negated by the Ca2+-selective chelator EGTA. In contrast, the cytocidal effect of ZA was less pronounced in breast and colon cancer cells regardless of whether extracellular Ca2+ was elevated. In oral cancer cells incubated with 1.6mM Ca2+, ZA up-regulated mitochondrial Bax expression and increased mitochondrial Ca2+ uptake. This was associated with decreased mitochondrial membrane potential and increased release of cytochrome c. We suggest that ZA can specifically produce potent cytocidal activity in oral cancer cells in an extracellular Ca2+-dependent manner, implying that BPs may be useful for treatment of oral squamous cell carcinoma with jawbone invasion leading to the hypercalcemic state.