Abstract Anthracyclines, such as epirubicin, activate the mitogen-activated protein kinase (MAPK) pathway in breast cancer (BC). Better predictors of tumor response are needed to guide de-intensification of anthracyclines when used in BC treatment. Here, we aimed to see if MAPK activation was responsible and targetable for epirubicin resistance, and to explore mechanisms and predictive markers for resistance. MAPK pathway inhibitors were used to ameliorate epirubicin resistance. Negative regulators of MAPK were screened to identify the essential gene cascades required for activation of the pathway. In vitro and in vivo approaches were applied to investigate epirubicin resistance. The regulatory miRNA was identified through bioinformatics-based screening and luciferase reporter assay. 114 estrogen receptor negative patients who received epirubicin monotherapy were included to evaluate predictive markers for tumor response. The MAPK pathway was activated in cells resistant to epirubicin; MAPK inhibition ameliorated epirubicin resistance. In triple-negative breast cancer (TNBC), progestin and adipoQ receptor 3 (PAQR3) were the most significantly decreased negative MAPK regulators. PAQR3 increased epirubicin sensitivity and suppressed MAPK activation in resistant cells. MiR-138-5p was increased in epirubicin resistant cells, and was shown to downregulate PAQR3, causing resistance in epirubicin resistant cells. PAQR3 was an independent factor in predicting response to epirubicin (OR=4.86, 95%CI=1.13-20.87, P=0.034), and possessed a high negative predictive value (NPV) (0.93; 95%CI=0.83-0.97). In addition, PAQR3 exhibited a better predictive value in patients older than 50 years (12.92 (95% CI, 1.43-116.78, P = 0.023). The combinative use of PAQR3 and topoisomerase II-α (TOP2A) led to an increased specificity (0.70; 95%CI=0.61-0.79), when compared with either PAQR3 or TOP2A alone. MAPK inhibition ameliorated epirubicin resistance. MiR-138-5p-induced PAQR3 reduction causes epirubicin resistance in TNBC via activation of MAPK cascades. PAQR3 is an independent and favorable predictor response to epirubicin in BC.

MAPK Pathway Inhibition as A Rational Therapeutic Strategy for MiR-138-5p/PAQR3 Dysregulation-mediated Epirubicin Resistance in Triple-negative Breast Cancer