The Mechanism of Ca2+ Movement in the Involvement of Baicalein-Induced Cytotoxicity in ZR-75-1 Human Breast Cancer Cells

Baicalein (5,6,7-trihydroxyflavone) (1) has been found to be active against a wide variety of cancer cells. However, the molecular mechanism underlying the effects of 1 on the induction of Ca2+ movement and cytotoxicity in human breast cancer cells is unknown. This study examined the relationship between 1-induced Ca2+ signaling and cytotoxicity in ZR-75-1 human breast cancer cells. The in vitro investigations repotted herein produced the following results: (i) Compound 1 increased intracellular Ca2+ concentration ([Ca2+](i)) in a concentration-dependent manner. The signal was decreased by approximately 50% by removal of extracellular Ca2+. (ii) Compound 1-triggered [Ca2+](i) increases were significantly suppressed by store-operated Ca2+ channel blockers 2-amino-ethoxydiphenyl borate (2-APB) and the PKC inhibitor GF109203X. (iii) In Ca2+-free medium, compound 1-induced [Ca2+](i) increases were also inhibited by GF109203X. Furthermore, pretreatment with the endoplasmic reticulum Ca2+ pump inhibitor thapsigargin (TG) or 2,5-ditert-butylhydroquinone (BHQ) abolished 1-induced [Ca2+](i) increases. Inhibition of phospholipase C (PLC) with U73122 abolished 1-induced [Ca2+](i) increases. (iv) Compound 1 (20-40 mu M) caused cytotoxicity, increased reactive oxygen species (ROS) production, and activated caspase-9/caspase-3. Furthermore, compound 1-induced apoptosis was significantly inhibited by prechelating cytosolic Ca2+ with BAPTA-AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester) or by decreasing ROS with the antioxidant NAC (N-acetylcysteine). Together, baicalein (1) induced a [Ca2+](i) increase by inducing PLC-dependent Ca2+ release from the endoplasmic reticulum and Ca2+ entry via PKC-dependent, 2-APB-sensitive store-operated Ca2+ channels. Moreover, baicalein (1) induced Ca2+-associated apoptosis Involved ROS production in ZR-75-1 cells.