The role of COX-2/PGE2 in gossypol-induced apoptosis of colorectal carcinoma cells

Our previous study showed that gossypol (GOS) exhibits potent cytotoxic effects via apoptosis induction against human colorectal carcinoma cells; however, the role of cyclooxygenase (COX)-2/prostaglandin (PG)E2 on GOS-induced apoptosis is still unknown. In the present study, 12-O-tetradecanoylphorbol-13-acetate (TPA) addition significantly inhibited GOS-induced apoptosis in human colorectal carcinoma HT-29 cells in accordance with inducing COX-2 protein/PGE2 production. TPA inhibition of GOS-induced apoptosis was blocked by adding protein kinase (PK)C inhibitors including staurosporine (ST), GF109203X (GF), and H7, characterized by the occurrence of cleaved caspase 3 proteins and a decrease in COX-2 protein/PGE2 production in HT-29 cells. The addition of COX activity inhibitors, including NS398 (NS), aspirin (AS), diclofenac (DI), and indomethacin (IN), suppressed TPA protection of GOS-induced apoptosis with decreased PGE2 production in HT-29 cells. Application of PGE2, but not it analogs PGD2, PGJ2, or PGF2α, protected HT-29 cells from GOS-induced DNA ladders, and the E-prostanoid (EP1) receptor agonist, 17PT-PGE2, mimicked the protection induced by PGE2, whereas the selective EP2 receptor agonist, butaprostol (BUT), the EP3 receptor agonist, sulprostol (SUL), and the EP4 receptor agonist, PGE1 alcohol (PGE1), showed no significant effects on GOS-induced apoptosis in HT-29 cells. PGE2's protection against GOS-induced apoptosis was reversed by adding the selective EP1 receptor antagonist, SC-19220. Furthermore, GOS had an effective apoptotic effect on COLO205 colorectal carcinoma cells which expressed undetectable level of endogenous COX-2 protein than HT-29 cells, and the decreased COX-2 protein level via COX-2 siRNA or addition of COX-2 activity inhibitor NS significantly elevated GOS-induced cell death in HT-29 cells. COLO205-T cells were established through sustained TPA incubation of COLO205 cells, and COLO205-T cells showed a lower sensitivity to GOS-induced cell death with increased COX-2 (not Bcl-2 and Mcl-1) protein than parental COLO-205 cells. A decrease in COX-2 protein expression in COLO205-T cells by COX-2 siRNA transfection or enhanced GOS-induced cell death according to MTT assay and DNA integrity assay. The notion of COX-2/PGE2 activation against GOS-induced apoptosis in colon carcinoma cells was demonstrated, and the combination of GOS and COX-2 inhibitors to treat colon carcinoma possesses clinical potential worthy of further investigation.