Curcumin induces apoptosis by inhibiting sarco/endoplasmic reticulum Ca²⁺ ATPase activity in ovarian cancer cells

Abstract

Aberrant increase in the expression levels of sarco/endoplasmic reticulum calcium ATPase (SERCA), which regulates Ca²⁺ homeostasis, has been observed in ovarian cancers. In this study, we demonstrated that curcumin increases cytosolic Ca²⁺ concentration through inhibition of SERCA activity, causing apoptosis in ovarian cancer cells but not in normal cells, including peripheral blood mononuclear cells (PBMCs) and ovarian surface epithelial cells (OSE). Curcumin induced apoptosis in ovarian cancer cells in a concentration- and time-dependent manner. Cytosolic Ca²⁺ flux was evident after the curcumin treatment (15 µM). Treatment with Ca²⁺ chelator reduced curcumin-induced apoptosis, confirming the possible involvement of increased cytosolic Ca²⁺ concentration in this response. Basal mRNA and protein levels of SERCA2 were significantly higher in ovarian cancer cells than in OSE. SERCA activity was suppressed by curcumin, with no effect on protein expression. Forced expression of the SERCA2b gene in ovarian cancer cells prevented curcumin-induced cytosolic Ca²⁺ elevation and subsequent apoptosis, supporting an important role of SERCA in curcumin-induced apoptosis of ovarian cancer cells. Taken together, inhibition of SERCA activity by curcumin disrupts the Ca²⁺ homeostasis and thereby promotes apoptosis in ovarian cancer cells.

Introduction

Ovarian cancer is the fifth leading cause of gynecologic cancer-related death worldwide [1], [2]. Due to the late diagnosis and high recurrence rate (80%), ovarian cancer causes the highest mortality rate among all of the gynecologic malignancies [3], [4]. The recurrence of ovarian cancer is commonly followed by the resistance to conventional chemotherapy [5], [6], rendering its 5-year survival rate low with little change for decades [7], [8]. Thus, there is a pressing need to find the efficacious therapy for the ovarian cancer.

Calcium is a major signaling molecule involved in the regulation of numerous physiological processes, including cell proliferation and apoptosis. Elevated cytosolic Ca²⁺ concentration can promote cell survival and apoptosis, depending on the extent of the Ca²⁺ response, which in turn is controlled by various intracellular signaling mechanisms [9], [10]. One of the regulators of Ca²⁺ signaling is the sarco/endoplasmic reticulum calcium ATPase (SERCA). SERCA is a major Ca²⁺ pump which regulates cellular Ca²⁺ flux from the cytosol to the endoplasmic reticulum (ER) [11]. Various cancer cells exhibit dysregulated Ca²⁺ signaling [12] and overexpression of SERCA activates cell survival pathways through the effect on cellular Ca²⁺ peak and oscillations, in various cancers, including liposarcoma and colorectal cancer [11], [13], [14], [15]. SERCA2 mRNA is overexpressed in ovarian cancer tissues compared to normal ovarian surface epithelial cell (OSE; data accessible at NCBI GEO database, accession number is GDS3592) [16], suggesting the possibility of involvement of SERCA in ovarian cancer development. Thus SERCA may be a novel therapeutic target in ovarian cancer.

Curcumin, a yellow pigment from Curcuma longa, is well known for its anti-cancer activity in various types of malignancies including ovarian cancer [17], [18], [19], [20], [21] through its effect on cell cycle arrest and caspase-mediated apoptosis [22], [23]. However, the mechanism of anticancer action of curcumin is not fully understood. Several studies showed that curcumin inhibits SERCA activity in skeletal muscle cells, Cos7 cells, and cancer cells [24], [25]. In this study, we demonstrated that curcumin disrupts calcium homeostasis through inhibition of SERCA activity and induces apoptosis in ovarian cancer cells.