Abstract LB-263: miR-181a downregulation allows simultaneous overexpression of ABCG2 and induction of autophagy to mediate multidrug resistance in colon cancer
Authors: To Kenneth KW 
Published: August 2019

Abstract
Aim: The study investigated the cross-talk between overexpression of multidrug resistance (MDR) transporter ABCG2 and induction of autophagy in mediating drug resistance in colon cancer. We also studied the simultaneous inhibition of ABCG2 and autophagy as a novel strategy to circumvent MDR. Background: Colon cancer is a leading cause of cancer-related death worldwide. MDR, which is usually associated with overexpression of ATP-binding cassette efflux transporters including ABCG2, is a major unresolved obstacle to successful cancer chemotherapy in colon cancer. Recent investigations have shown that autophagy is also involved in the development of MDR. Autophagy is an adaptive and survival mechanism in response to stress and starvation in order to maintain cellular homeostasis. Interestingly, ABCG2 overexpression was found to enhance autophagy to promote cell survival in response to various stress. Co-inhibition of ABCG2 and autophagy might exert a synergistic cancer killing effect to help eradicate MDR cancer cells. Our team has recently reported the upregulation of ABCG2 in tumor specimens, relative to adjacent normal colon epithelial cells, obtained from colon cancer patients unresponsive to 5-fluorouracil-based chemotherapy. Interestingly, this is coincident with a remarkable downregulation of miR-181a, which is also known to target and repress a major autophagy regulator ATG5. Therefore, we hypothesize that miR-181a may form a mechanistic link between ABCG2 overexpression and autophagy induction to mediate MDR.
Method: Regulation of ABCG2 and ATG5 in MDR colon cancer cells after ectopic overexpression or genetic silencing of miR-181a was investigated. ABCG2 and ATG5 expression was measured by qPCR and Western blot analysis. Functional consequence of ABCG2 regulation was evaluated by measuring efflux and anticancer activity of ABCG2 substrate anticancer drugs. Autophagic flux was monitored by LC3 turnover and selective autophagy substrate p62 degradation assay.
Result: In ABCG2-overexpressing MDR colon cancer cell lines, miR-181a expression was found to be remarkably reduced in the resistant sublines relative to the parental cells. Ectopic overexpression of miR-181a was found to downregulate both ABCG2 and ATG5, which also lead to reduced drug efflux, inhibition of autophagic flux and enhanced anticancer activity of both ABCG2 substrate (SN-38) and non-substrate (oxaliplatin) anticancer drugs. We screened a compound library and identified autophinib as a compound capable of inhibiting both ABCG2 and autophagy. At non-toxic concentration, autophinib was found to potentiate anticancer activity in MDR colon cancer cells.
Conclusion: Our study revealed the causal and complementary relationship between ABCG2 overexpression and autophagy induction in the development of MDR in colon cancer. miR-181a may represent a new molecular target for MDR circumvention. Co-inhibition of ABCG2 and autophagy was found to circumvent drug resistance in MDR cells.
Note: This abstract was not presented at the meeting.
Citation Format: Kenneth KW To. miR-181a downregulation allows simultaneous overexpression of ABCG2 and induction of autophagy to mediate multidrug resistance in colon cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-263.