title: Hypoxia represses microRNA biogenesis proteins in breast cancer cells.
abstract: BACKGROUND
  Cancers are commonly characterised by hypoxia and also by global reductions in the levels of mature microRNAs. We have examined the hypothesis that hypoxia might mediate this reduction through repressive effects on microRNA biogenesis proteins.
  
  
  METHODS
  Breast cancer cell lines were exposed to hypoxia and manipulations of hypoxia inducible factor (HIF) and HIF hydroxylase activity. The effects of hypoxia on the mRNA and protein levels of enzymes involved in microRNA biogenesis (Dicer, Drosha, TARPB2, DCGR8, XPO5) was determined by RT PCR and immunoblotting. The effect of hypoxia on microRNAs was determined with microarray studies, RT PCR and reporter assays.
  
  
  RESULTS
  In breast cancer lines there was significant reduction of Dicer mRNA and protein levels in cells exposed to hypoxia. This effect was independent of HIF but dependent on the HIF hydroxylase PHD2 and was partly mediated by feedback effects via microRNAs. Furthermore, several other proteins with critical roles in microRNA biogenesis (Drosha, TARBP2 and DCGR8) also showed significant and co-ordinated repression under hypoxic conditions. Despite these substantial alterations no, or modest, changes were observed in mature microRNA production.
  
  
  CONCLUSIONS
  These observations provide further and important interfaces between oxygen availability and gene expression and a potential mechanistic explanation for the reduced levels of microRNAs observed in some cancers. They provide further support for the existence of feedback mechanisms in the regulation of the microRNA biogenesis pathway and the relative stability of microRNAs.
authors: Veronika
authors: V
authors: Bandara
authors: Michael Z
authors: MZ
authors: Michael
authors: Renal Department, Flinders Medical Centre, Flinders University School of Medicine, Bedford Park, Adelaide, South Australia 5042, Australia. jonathan.gleadle@health.sa.gov.au.
authors: Jonathan M
authors: JM
authors: Gleadle
mesh_terms: D051792:Basic Helix-Loop-Helix Proteins / Q000378:metabolism; D001943:Breast Neoplasms / Q000378:metabolism / Q000473:pathology*; D015687:Cell Hypoxia; D045744:Cell Line, Tumor; D053487:DEAD-box RNA Helicases / Q000235:genetics / Q000378:metabolism; D005260:Female; D019073:HT29 Cells; D061307:Human Umbilical Vein Endothelial Cells; D006801:Humans; D061986:MCF-7 Cells; D035683:MicroRNAs / Q000378:metabolism*; D008969:Molecular Sequence Data; D043244:Ribonuclease III / Q000235:genetics / Q000378:metabolism
chemical_list: D051792:Basic Helix-Loop-Helix Proteins; D053487:DEAD-box RNA Helicases; D035683:MicroRNAs; D043244:Ribonuclease III; C408087:DICER1 protein, human