Multidrug resistance-associated protein 3 confers resistance to chemoradiotherapy for rectal cancer by regulating reactive oxygen species and caspase-3-dependent apoptotic pathway

• Aberrant MRP3 indicated poor prognosis and pathological response to nCRT in rectal cancer.
• Down-regulation of MRP3 increased 5-fluorouracil or irradiation induced cell apoptosis.
• Inhibition of MRP3 decreased intracellular reactive oxygen species exporting.
• Inhibition of MRP3 activated caspase-3 dependent apoptotic pathway after irradiation.
• MRP3 seems as an attractive target in predicting chemo-radiosensitivity of rectal cancer.

This study aimed to clarify the role of multidrug resistance-associated protein 3 (MRP3) in resistance to neoadjuvant chemoradiotherapy and long-term prognosis of advanced rectal cancer. Immunohistochemistry was used to measure MRP3 expression in biopsy specimens of 144 stage II–III rectal cancer patients who received preoperative chemoradiotherapy. The effect of MRP3 expression on short-term pathological response and postoperative long-term prognosis were assessed using the Cox proportional hazards model. Short interfering RNAs targeting MRP3 were synthesized and used to transfect human colorectal carcinoma cell lines. The effect of MRP3 down-regulation on cell proliferation and apoptosis in response to 5-fluorouracil and/or irradiation were examined in vitro and in xenograft mouse models, respectively. The content of intracellular reactive oxygen species and the activity of caspase-3-dependent apoptotic pathway in response to irradiation were further evaluated. High expression (immunoreactive score > 6) of MRP3 significantly predicted poor pathological response to chemoradiotherapy (tumor regression grade ≤ 2 vs. ≥3, p = 0.002) in univariate analysis and unfavorable long-term prognosis (5-year overall survival: HR = 1.612, 95% CI, 1.094–2.375, p = 0.016; 5-year disease-free survival: HR = 1.513, 95% CI, 1.041–2.200, p = 0.030) in multivariate Cox analysis. MRP3 down-regulation significantly increased 5-fluorouracil or irradiation-induced cell apoptosis and attenuated tumor growth following irradiation in animal models. MRP3 inhibition significantly reduced intracellular reactive oxygen species exporting from cells following irradiation, and increased expression of cleaved poly ADP-ribose polymerase and caspase-3. Aberrant expression of MRP3 in rectal cancer confers chemo-radioresistance. MRP3 might be a predictive factor and an attractive target in treating advanced rectal cancer.