Esculetin, a natural coumarin compound, evokes Ca2+ movement and activation of Ca2+-associated mitochondrial apoptotic pathways that involved cell cycle arrest in ZR-75-1 human breast cancer cells

摘要

Esculetin (6,7-dihydroxycoumarin), a derivative of coumarin compound, is found in traditional medicinal herbs. It has been shown that esculetin triggers diverse cellular signal transduction pathways leading to regulation of physiology in different models. However, whether esculetin affects Ca2+ homeostasis in breast cancer cells has not been explored. This study examined the underlying mechanism of cytotoxicity induced by esculetin and established the relationship between Ca2+ signaling and cytotoxicity in human breast cancer cells. The results showed that esculetin induced concentration-dependent rises in the intracellular Ca2+ concentration ([Ca2+](i)) in ZR-75-1 (but not in MCF-7 and MDA-MB-231) human breast cancer cells. In ZR-75-1 cells, this Ca2+ signal response was reduced by removing extracellular Ca2+ and was inhibited by the store-operated Ca2+ channel blocker 2-aminoethoxydiphenyl borate (2-APB). In Ca2+-free medium, pre-treatment with the endoplasmic reticulum Ca2+ pump inhibitor thapsigargin (TG) abolished esculetin-induced [Ca2+](i) rises. Conversely, incubation with esculetin abolished TG-induced [Ca2+](i) rises. Esculetin induced cytotoxicity that involved apoptosis, as supported by the reduction of mitochondrial membrane potential and the release of cytochrome c and the proteolytic activation of caspase-9/caspase-3, which were partially reversed by pre-chelating cytosolic Ca2+ with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester (BAPTA-AM). Moreover, esculetin increased the percentage of cells in G2/M phase and regulated the expressions of p53, p21, CDK1, and cyclin B1. Together, in ZR-75-1 cells, esculetin induced [Ca2+](i) rises by releasing Ca2+ from the ER and causing Ca2+ influx through 2-APB-sensitive store-operated Ca2+ entry. Furthermore, esculetin activated Ca2+-associated mitochondrial apoptotic pathways that involved G2/M cell cycle arrest.