HIF1A knockdown abolished the effect of microRNA-18a on cell metastatic properties. (A) Expression fold changes of hypoxia-inducible factor α (HIF1A) and hypoxia-responsive genes in MDA-MB-231 cells expressing microRNA-18a (miR-18a) inhibitor and/or HIF1A short-hairpin RNA (shRNA). The mRNA levels were measured by quantitative PCR after cells were exposed to hypoxia (2% O2 for 6 hours) and normalized to ACTB. Data are presented as mean ± SD (n = 3). (B) HIF1A protein expression levels in nuclear extracts from untreated or hypoxia-treated (2% O2 for 6 hours) cells were detected by immunoblotting assay. TATA-binding protein (TBP) was used as a loading control. (C) Effect of miR-18a inhibitor and HIF1A shRNA on viability of cells cultured in suspension for 6 days. (D) Effect of miR-18a inhibitor and HIF1A shRNA on cell invasion. The results are presented as mean ± SD (n = 4). *P < 0.05 by Student’s t-test. n, the number of experimental repeats.