title: GREM1, FRZB and DKK1 mRNA levels correlate with osteoarthritis and are regulated by osteoarthritis-associated factors.
abstract: BACKGROUND
  Osteoarthritis is, at least in a subset of patients, associated with hypertrophic differentiation of articular chondrocytes. Recently, we identified the bone morphogenetic protein (BMP) and wingless-type MMTV integration site (WNT) signaling antagonists Gremlin 1 (GREM1), frizzled-related protein (FRZB) and dickkopf 1 homolog (Xenopus laevis) (DKK1) as articular cartilage's natural brakes of hypertrophic differentiation. In this study, we investigated whether factors implicated in osteoarthritis or regulation of chondrocyte hypertrophy influence GREM1, FRZB and DKK1 expression levels.
  
  
  METHODS
  GREM1, FRZB and DKK1 mRNA levels were studied in articular cartilage from healthy preadolescents and healthy adults as well as in preserved and degrading osteoarthritic cartilage from the same osteoarthritic joint by quantitative PCR. Subsequently, we exposed human articular chondrocytes to WNT, BMP, IL-1β, Indian hedgehog, parathyroid hormone-related peptide, mechanical loading, different medium tonicities or distinct oxygen levels and investigated GREM1, FRZB and DKK1 expression levels using a time-course analysis.
  
  
  RESULTS
  GREM1, FRZB and DKK1 mRNA expression were strongly decreased in osteoarthritis. Moreover, this downregulation is stronger in degrading cartilage compared with macroscopically preserved cartilage from the same osteoarthritic joint. WNT, BMP, IL-1β signaling and mechanical loading regulated GREM1, FRZB and DKK1 mRNA levels. Indian hedgehog, parathyroid hormone-related peptide and tonicity influenced the mRNA levels of at least one antagonist, while oxygen levels did not demonstrate any statistically significant effect. Interestingly, BMP and WNT signaling upregulated the expression of each other's antagonists.
  
  
  CONCLUSIONS
  Together, the current study demonstrates an inverse correlation between osteoarthritis and GREM1, FRZB and DKK1 gene expression in cartilage and provides insight into the underlying transcriptional regulation. Furthermore, we show that BMP and WNT signaling are linked in a negative feedback loop, which might prove essential in articular cartilage homeostasis by balancing BMP and WNT activity.
authors.forename: Jeroen C H
authors.initials: JC
authors.lastname: Leijten
authors.forename: Steffan D
authors.initials: SD
authors.lastname: Bos
authors.forename: Ellie B M
authors.initials: EB
authors.lastname: Landman
authors.forename: Nicole
authors.initials: N
authors.lastname: Georgi
authors.forename: Holger
authors.initials: H
authors.lastname: Jahr
authors.forename: Ingrid
authors.initials: I
authors.lastname: Meulenbelt
authors.forename: Janine N
authors.initials: JN
authors.lastname: Post
authors.forename: Clemens A
authors.initials: CA
authors.lastname: van Blitterswijk
authors.forename: Marcel
authors.initials: M
authors.lastname: Karperien
mesh_terms: D000293:Adolescent; D000368:Aged; D000369:Aged, 80 and over; D000818:Animals; D055396:Bone Morphogenetic Protein 2 / Q000235:genetics / Q000494:pharmacology; D002417:Cattle; D015687:Cell Hypoxia; D045744:Cell Line, Tumor; D002478:Cells, Cultured; D002648:Child; D019902:Chondrocytes / Q000187:drug effects / Q000378:metabolism; D015870:Gene Expression / Q000187:drug effects / Q000235:genetics*; D006023:Glycoproteins / Q000235:genetics*; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins / Q000235:genetics*; D053583:Interleukin-1beta / Q000494:pharmacology; D047908:Intracellular Signaling Peptides and Proteins; D008875:Middle Aged; D009994:Osmolar Concentration; D010003:Osteoarthritis / Q000235:genetics* / Q000378:metabolism / Q000473:pathology; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D011994:Recombinant Proteins / Q000494:pharmacology; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013314:Stress, Mechanical; D060509:Wnt3A Protein / Q000235:genetics / Q000494:pharmacology; D000099274:Frizzled-Related Proteins
chemical_list: D055396:Bone Morphogenetic Protein 2; D006023:Glycoproteins; D036341:Intercellular Signaling Peptides and Proteins; D053583:Interleukin-1beta; D047908:Intracellular Signaling Peptides and Proteins; D012333:RNA, Messenger; D011994:Recombinant Proteins; D060509:Wnt3A Protein; D000099274:Frizzled-Related Proteins; C501126:DKK1 protein, human; C486399:GREM1 protein, human