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 Ca(2+) movement and cytotoxicity in human 
breast cancer cells is unknown. This study examined the relationship between 
1-induced Ca(2+) signaling and cytotoxicity in ZR-75-1 human breast cancer 
cells. The in vitro investigations reported herein produced the following 
results: (i) Compound 1 increased intracellular Ca(2+) concentration ([Ca(2+)]i) 
in a concentration-dependent manner. The signal was decreased by approximately 
50% by removal of extracellular Ca(2+). (ii) Compound 1-triggered [Ca(2+)]i 
increases were significantly suppressed by store-operated Ca(2+) channel 
blockers 2-aminoethoxydiphenyl borate (2-APB) and the PKC inhibitor GF109203X. 
(iii) In Ca(2+)-free medium, compound 1-induced [Ca(2+)]i increases were also 
inhibited by GF109203X. Furthermore, pretreatment with the endoplasmic reticulum 
Ca(2+) pump inhibitor thapsigargin (TG) or 2,5-ditert-butylhydroquinone (BHQ) 
abolished 1-induced [Ca(2+)]i increases. Inhibition of phospholipase C (PLC) 
with U73122 abolished 1-induced [Ca(2+)]i increases. (iv) Compound 1 (20-40 μM) 
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 Ca(2+) 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 [Ca(2+)]i increase by inducing PLC-dependent Ca(2+) 
release from the endoplasmic reticulum and Ca(2+) entry via PKC-dependent, 
2-APB-sensitive store-operated Ca(2+) channels. Moreover, baicalein (1) induced 
Ca(2+)-associated apoptosis involved ROS production in ZR-75-1 cells.