Inhibition of PGE 2 /EP4 receptor signaling enhances oxaliplatin efficacy in resistant colon cancer cells through modulation of oxidative stress

Abstract

The platinum-based chemotherapeutic agent, oxaliplatin, is used to treat advanced colorectal cancer (CRC). Unfortunately, nearly all patients acquire resistance to oxaliplatin after long-term use, limiting its therapeutic efficacy. Since COX-2 and PGE 2 signaling can impact colon cancer cell proliferation and survival, we examined how this pathway was affected in an oxaliplatin resistant colon cancer cell line. PGE 2 levels were significantly elevated in oxaliplatin-resistant HT29 cells (OXR) compared to naïve parental HT29 cells (PAR). This increase was associated with elevated COX-2 (17.9-fold; P = 0.008) and reduced 15-hydroxyprostaglandin dehydrogenase (2.9-fold; P < 0.0001) expression. RNAi knockdown of microsomal prostaglandin E synthase-1, the rate-limiting enzyme in PGE 2 synthesis, sensitized OXR cells to oxaliplatin. Downstream effects of PGE 2 in OXR cells were also examined. Selective inhibition of the EP4 PGE 2 receptor by the small molecule inhibitor, L-161,982 enhanced oxaliplatin-induced apoptosis in OXR cells. L-161,982 also reduced expression of the colonic stem cell markers, CD133 and CD44, and inhibited tumor sphere formation. The accumulation of intracellular reactive oxygen species (ROS), a key component of oxaliplatin cytotoxicity, was significantly increased by EP4 inhibition (2.4 -fold; P < 0.0001). Overall, our findings uncover an important role for the COX-2/PGE 2 /EP4 signaling axis in oxaliplatin resistance via regulation of oxidative stress.