27P MiR-221/222 may enhance epithelial-mesenchymal transition and tamoxifen resistance by down-regulating GATA3

In recent years, studies have shown that GATA3 can promote the expression of ERα and inhibit tamoxifen (TAM) resistance, whereas miR-221/222 could promote epithelial-to-mesenchymal transition (EMT), which could result in TAM resistance. Moreover, GATA3 also inhibits EMT. However, the specific mechanism by which miR-221/222 interacts with GATA3 and induces TAM resistance remains unknown. Our study aims to investigate the regulatory mechanism of miR-221/222 on GATA3 expression and clarify its regulatory role in the process of TAM resistance. After the establishment of TAM-resistant MCF7 cell lines, we detected and compared the endogenous expression levels of miR-221/222, GATA3, ERα, E-cadherin and vimentin in MCF7 and MCF-TAMR strains, as well as in different breast cancer cell lines, by PCR and western blotting. After overexpressing miR-221/222 in MCF7 cell lines, we characterized the GATA3, ERα and E-cadherin protein levels, and evaluated the migration and invasion ability of MCF7 cells by comparing wound healing and transwell migration with or without a miR-221/222 inhibitor. In order to confirm the effect of miR221/222 and GATA 3 on TAM resistance, a Cell Counting Kit-8 (CCK8) assay was used to quantify TAM resistance in the MCF-TAMR cell lines. In the TAM-resistant and ERα-negative breast cancer cell lines, endogenous expression of miR-221/222 and vimentin were higher, whereas the expression of GATA3, ERα and E-cadherin was relatively low. After overexpressing miR-221/222, the protein levels of GATA3, ERα and E-cadherin decreased, indicating an inverse correlation between miR-221/222 and GATA3, ERα and E-cadherin. Wound healing and transwell experiments showed that miR221/222 promoted the migration and invasion of breast cancer cells, and CCK8 assay demonstrated that miR-221/222 can promote TAM resistance and that GATA 3 can reverse it. Overall, our results suggest that miR-221/222 may down-regulate GATA3 expression, thereby down-regulating ERα and promoting breast cancer EMT to ultimately promote tamoxifen resistance. This regulating network may be due to the targeting of miR-221/222 to the 3’ UTR of GATA3 mRNA.