Involvement of the prostaglandin E receptor EP2 in paeoniflorin-induced human hepatoma cell apoptosis

Abstract

Prostaglandin E2 (PGE2) has been shown to play an important role in tumor development and progression. PGE2 mediates its biological activity by binding any one of four prostanoid receptors (EP1 through EP4). The present study was designed to determine the role of the EP2 receptor during the proliferation and apoptosis of human HepG2 and SMMC-7721 hepatoma cell lines and the effect of paeoniflorin, a monoterpene glycoside. The proliferation of HepG2 and SMMC-7721 cells was determined by methyl thiazolyl tetrazolium after exposure to the selective EP2 receptor agonists butaprost and paeoniflorin. Apoptosis of HepG2 and SMMC-7721 cells was also quantified by flow cytometry with annexin V-fluorescein isothiocyanate and propidium iodide staining. The expression levels of Bcl-2 and Bax were quantified by western blotting and immunohistochemistry. The expression of the EP2 receptor and cysteine-aspartic acid protease (caspase)-3 was determined by western blotting. Butaprost significantly increased proliferation in HepG2 and SMMC-7721 cells. Paeoniflorin significantly inhibited the proliferation of HepG2 and SMMC-7721 cells stimulated by butaprost at multiple time points (24, 48, and 72 h). Paeoniflorin induced apoptosis in HepG2 and SMMC-7721 cells, which was quantified by annexin-V and propidium iodide staining. Our results indicate that the expression of the EP2 receptor and Bcl-2 was significantly increased, whereas that of Bax and cleaved caspase-3 was decreased in HepG2 and SMMC-7721 cells after stimulation by butaprost. Paeoniflorin significantly decreased the expression of the EP2 receptor and Bcl-2 and increased Bax and caspase-3 activation in HepG2 and SMMC-7721 cells on addition of butaprost. Our results show that the PGE2 receptor subtype EP2 may play a vital role in the survival of both HepG2 and SMMC-7721 cells. Paeoniflorin, which may be a promising agent in the treatment of liver cancer, induced apoptosis in hepatocellular carcinoma cells by downregulating EP2 expression and also increased the Bax-to-Bcl-2 ratio, thus upregulating the activation of caspase-3.