Background: Intronic microRNAs (miRNAs) are considered to be transcribed using their host gene promoter. However, about one third of intronic miRNAs are predicted to have independent promoter elements. Materials and Methods: Human breast cancer cells were cultured under normoxia or hypoxia, and expression levels of intronic miR-106b-25 cluster miRNAs and their host gene minichromosome maintenance complex component 7 (MCM7) transcripts were analyzed by semi-quantitative polymerase chain reaction. The putative promoter element of miR-106b-25 cluster was analyzed by chromatin immunoprecipitation and luciferase assays. Results: Exposure to hypoxia reduced the expression of MCM7 mRNA and a primary transcript of miR-106b-25 cluster, but did not affect that of mature miRNAs. The putative promoter element of miR-106b-25 cluster was not bound by hypoxia-inducible factor 1-alpha (HIF1), and was not activated under hypoxia. Conclusion: Maintenance of miR-106b-25 cluster miRNA levels under hypoxia was not caused by the activation of an independent promoter element.
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