title: MicroRNA-33a regulates cholesterol synthesis and cholesterol efflux-related genes in osteoarthritic chondrocytes.
abstract: BACKGROUND
  Several studies have shown that osteoarthritis (OA) is strongly associated with metabolism-related disorders, highlighting OA as the fifth component of the metabolic syndrome (MetS). On the basis of our previous findings on dysregulation of cholesterol homeostasis in OA, we were prompted to investigate whether microRNA-33a (miR-33a), one of the master regulators of cholesterol and fatty acid metabolism, plays a key role in OA pathogenesis.
  
  
  METHODS
  Articular cartilage samples were obtained from 14 patients with primary OA undergoing total knee replacement surgery. Normal cartilage was obtained from nine individuals undergoing fracture repair surgery. Bioinformatics analysis was used to identify miR-33a target genes. miR-33a and sterol regulatory element-binding protein 2 (SREBP-2) expression levels were investigated using real-time PCR, and their expression was also assessed after treatment with transforming growth factor-β1 (TGF-β1) in cultured chondrocytes. Akt phosphorylation after treatment with both TGF-β1 and miR-33a inhibitor or TGF-β1 and miR-33a mimic was assessed by Western blot analysis. Furthermore, we evaluated the effect of miR-33a mimic and miR-33a inhibitor on Smad7, a negative regulator of TGF-β signaling, on cholesterol efflux-related genes, ATP-binding cassette transporter A1 (ABCA1), apolipoprotein A1 (ApoA1) and liver X receptors (LXRα and LXRβ), as well as on matrix metalloproteinase-13 (MMP-13), using real-time PCR.
  
  
  RESULTS
  We found that the expression of miR-33a and its host gene SREBP-2 was significantly elevated in OA chondrocytes compared with normal chondrocytes. Treatment of cultured chondrocytes with TGF-β1 resulted in increased expression of both miR-33a and SREBP-2, as well as in rapid induction of Akt phosphorylation, whereas TGF-β-induced Akt phosphorylation was enhanced by miR-33a and suppressed by inhibition of miR-33a, as a possible consequence of Smad7 regulation by miR-33a. Moreover, treatment of normal chondrocytes with miR-33a resulted in significantly reduced ABCA1 and ApoA1 mRNA expression levels and significantly elevated MMP-13 expression levels, promoting the OA phenotype, whereas miR-33a's suppressive effect was reversed using its inhibitor.
  
  
  CONCLUSIONS
  Our findings suggest, for the first time to our knowledge, that miR-33a regulates cholesterol synthesis through the TGF-β1/Akt/SREBP-2 pathway, as well as cholesterol efflux-related genes ABCA1 and ApoA1, in OA chondrocytes, pointing to its identification as a novel target for ameliorating the OA phenotype.
authors.affiliation: Department of Cytogenetics and Molecular Genetics, School of Medicine, University of Thessaly, Biopolis, 41110, Larissa, Greece. fotini.kostopoulou@gmail.com.|Center for Research and Technology Hellas (CERTH), 6th Km Charilaou-Thermi Road, PO Box 60361, GR 57001 Thermi, Thessaloniki, Greece. fotini.kostopoulou@gmail.com.
authors.forename: Fotini
authors.initials: F
authors.lastname: Kostopoulou
authors.affiliation: Center for Research and Technology Hellas (CERTH), 6th Km Charilaou-Thermi Road, PO Box 60361, GR 57001 Thermi, Thessaloniki, Greece. malizos@med.uth.gr.|Department of Orthopaedics, School of Medicine, University of Thessaly, Biopolis, 41110, Larissa, Greece. malizos@med.uth.gr.
authors.forename: Konstantinos N
authors.initials: KN
authors.lastname: Malizos
authors.affiliation: Department of Cytogenetics and Molecular Genetics, School of Medicine, University of Thessaly, Biopolis, 41110, Larissa, Greece. ioanna_papathanasiou@yahoo.gr.
authors.forename: Ioanna
authors.initials: I
authors.lastname: Papathanasiou
authors.affiliation: Department of Cytogenetics and Molecular Genetics, School of Medicine, University of Thessaly, Biopolis, 41110, Larissa, Greece. atsezou@med.uth.gr.|Center for Research and Technology Hellas (CERTH), 6th Km Charilaou-Thermi Road, PO Box 60361, GR 57001 Thermi, Thessaloniki, Greece. atsezou@med.uth.gr.|Department of Biology, School of Medicine, University of Thessaly, Biopolis, 41110, Larissa, Greece. atsezou@med.uth.gr.
authors.forename: Aspasia
authors.initials: A
authors.lastname: Tsezou
mesh_terms: D000368:Aged; D002478:Cells, Cultured; D002784:Cholesterol / Q000378:metabolism*; D019902:Chondrocytes / Q000378:metabolism* / Q000473:pathology; D004247:DNA / Q000235:genetics*; D005260:Female; D005786:Gene Expression Regulation; D006801:Humans; D008297:Male; D035683:MicroRNAs / Q000096:biosynthesis / Q000235:genetics*; D020370:Osteoarthritis, Knee / Q000235:genetics* / Q000378:metabolism / Q000473:pathology; D060888:Real-Time Polymerase Chain Reaction
chemical_list: C550416:MIRN33a microRNA, human; D035683:MicroRNAs; D004247:DNA; D002784:Cholesterol