Chemotherapy resistance of tumor cells is a big challenge. Adaption to hypoxia is an essential cellular response that is controlled by the master oxygen-sensitive transcription factor HIF1 (hypoxia-inducible factor 1). The mechanism by which tumor cells acquire resistance to chemotherapy under hypoxic conditions is not fully understood. In this study, we found that hypoxia induces miR-424 expression and that miR-424 in turn suppresses the level of PDCD4 protein, a tumor suppressor that is involved in apoptosis, by targeting its 3′ untranslated region. Functionally, miR-424 overexpression decreases the sensitivity of cancer cells (HCT116 and A375) to doxorubicin (Dox) and etoposide. In contrast, the inhibition of miR-424 enhanced apoptosis and increased the sensitivity of cancer cells to Dox. In a xenograft tumor model, miR-424 overexpression promoted tumor growth following Dox treatment, suggesting that miR-424 promotes tumor cell resistance to Dox. Furthermore, miR-424 levels are inversely correlated with PDCD4 expression in clinical breast cancer samples. These results suggest that miR-424 is a potential molecular target for tumor therapy.

Aberrant protein fucosylation is associated with cancer malignancy. Fucosyltransferase IV (FUT4) is the key enzyme catalyzing the biosynthesis of α1,3-linkage fucosylated glycans carried by glycoproteins on the cell surface, such as the tumor-associated sugar antigen Lewis Y (LeY). An abnormal increase in the levels of FUT4 and LeY is observed in many cancers and correlated with cell proliferation and metastasis. Some microRNAs (miRNAs) are known to negatively regulate gene expression. FUT4 is an oncogenic glycogene, and thus it is important to identify the specific miRNA targeting FUT4. In current study, we first identified miR-200b as a specific miRNA that inhibited FUT4 expression. We found that miR-200b level was decreased, whereas that of FUT4 was increased in tissues and serum of breast cancer compared with that in the control by real-time PCR, western blotting and enzyme-linked immunosorbent assay. The alterations of miR-200b and FUT4 level were recovered after chemotherapy. The results also showed that miR-200b suppressed FUT4 expression and inhibited tumor growth and metastasis in MCF-7 and MDA-MB-231 breast cancer cells, as well as in the xenografted tumor tissues and metastatic lung tissues. miR-200b decreased the α1,3-fucosylation and LeY biosynthesis on epidermal growth factor receptor (EGFR), as well as inactivation of EGFR and downstream phosphoinositide-3 kinase/Akt signaling pathway. In conclusion, the study highlights that FUT4 could apply as a novel target for miR-200b that suppress the proliferation and metastasis of breast cancer cells by reducing α1,3-fucosylation and LeY biosynthesis of glycoproteins. miR-200b and FUT4 are potential diagnostic and therapeutic targets for breast cancer.

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