Long noncoding RNA CASC2 promotes paclitaxel resistance in breast cancer through regulation of miR-18a-5p/CDK19.

Breast cancer is one of the most prevalent cancers in women. Chemoresistance is a major obstacle for the treatment of breast cancer. We investigated the role of long noncoding RNA (lncRNA) cancer susceptibility candidate 2 (CASC2) in paclitaxel (PTX) resistance in breast cancer. CASC2 expression was increased in PTX-resistant clinical samples and cell lines. PTX induced CASC2 expression in a concentration-dependent manner. Downregulation of CASC2 increased PTX toxicity and decreased IC50 value, while upregulation of CASC2 decreased PTX toxicity and increased IC50 value in MCF-7/PTX and MDA-MB-231/PTX cells. Moreover, downregulation of CASC2 decreased tumor growth in xenograft mice implanted with MCF-7/PTX cells. miR-18a-5p possessed a putative binding site in 3'-UTR of CASC2 and cyclin-dependent kinase 19 (CDK19). In PTX-resistant breast cancer cells, miR-18a-5p expression was decreased. CASC2 and miR-18a-5p could negatively regulate the expression of each other. CDK19 expression could be negatively regulated by miR-18a-5p, but positively regulated by CASC2. miR-18a-5p mimics or downregulation of CDK19 decreased tumor growth in xenograft mice implanted with MCF-7/PTX cells. In summary, we identified that CASC2 activated PTX resistance in breast cancer through regulation of miR-18a-5p/CDK19. We highlight the importance of CASC2/miR-18a-5p/CDK19 axis in the chemoresistance of breast cancer and provide potential targets for the improving chemotherapy of breast cancer.