title: Hypoxia promotes redifferentiation and suppresses markers of hypertrophy and degeneration in both healthy and osteoarthritic chondrocytes.
abstract: BACKGROUND
  Hypoxia is considered to be a positive influence on the healthy chondrocyte phenotype and cartilage matrix formation. However, hypoxia-inducible factors (HIFs) have been implicated in the pathogenesis of osteoarthritis (OA). Thus, we assessed whether healthy and OA chondrocytes have distinct responses to oxygen, particularly with regard to hypertrophy and degradation during redifferentiation.
  
  
  METHODS
  Monolayer-expanded healthy and OA chondrocytes were redifferentiated for 14 days in pellet cultures under standard (20% oxygen) or hypoxic (2% oxygen) conditions. Cartilage matrix gene expression, matrix quality and quantity, degradative enzyme expression and HIF expression were measured.
  
  
  RESULTS
  In hypoxia, both healthy and OA chondrocytes had higher human collagen type II, α1 gene (COL2A1), and aggrecan (ACAN) expression and sulfated glycosaminoglycan (sGAG) accumulation, concomitant with lower human collagen type X, α1 gene (COL10A1), and human collagen type I, α1 gene (COL1A1), expression and collagen I extracellular accumulation. OA chondrocytes had significantly lower sGAGs/DNA than healthy chondrocytes, but only in high oxygen conditions. Hypoxia also caused significantly greater sGAG retention and hyaluronic acid synthase 2 (HAS2) expression by OA chondrocytes. Both healthy and OA chondrocytes had significantly lower expression of matrix metalloproteinases (MMPs) MMP1, MMP2, MMP3 and MMP13 in hypoxia and less active MMP2 enzyme, consistent with lower MMP14 expression. However, aggrecanase (ADAMTS4 and ADAMTS5) expression was significantly lowered by hypoxia only in healthy cells, and COL10A1 and MMP13 remained significantly higher in OA chondrocytes than in healthy chondrocytes in hypoxic conditions. HIF-1α and HIF-2α had similar expression profiles in healthy and OA cells, increasing to maximal levels early in hypoxia and decreasing over time.
  
  
  CONCLUSIONS
  Hypoxic culture of human chondrocytes has long been acknowledged to result in increased matrix accumulation, but still little is known of its effects on catabolism. We show herein that the increased expression of matrix proteins, combined with decreased expression of numerous degradative enzymes by hypoxia, minimizes but does not abolish differences between redifferentiated healthy and OA chondrocytes. Hypoxia-induced HIF expression is associated with hypertrophic marker and degradative enzyme downregulation and increased measures of redifferentiation in both healthy and OA chondrocytes. Therefore, though HIFs may be involved in the pathogenesis of OA, conditions that promote HIF expression in vitro promote matrix accumulation and decrease degradation and hypertrophy, even in cells from OA joints.
authors: Brandon D
authors: BD
authors: Markway
authors: Holly
authors: H
authors: Cho
authors: Brian
authors: B
authors: Johnstone
mesh_terms: D015153:Blotting, Western; D002358:Cartilage, Articular / Q000378:metabolism; D002454:Cell Differentiation / Q000502:physiology; D015687:Cell Hypoxia / Q000502:physiology*; D002478:Cells, Cultured; D019902:Chondrocytes / Q000166:cytology* / Q000378:metabolism*; D006801:Humans; D006984:Hypertrophy / Q000378:metabolism / Q000473:pathology; D007150:Immunohistochemistry; D010003:Osteoarthritis / Q000378:metabolism* / Q000473:pathology*; D060888:Real-Time Polymerase Chain Reaction
references: J Biol Chem. 1999 Jul 30;274(31):21893-9
references: 10419509
references: Arthritis Res Ther. 2009;11(5):R133
references: 19723327
references: Tissue Eng Part A. 2012 May;18(9-10):920-33
references: 22097912
references: Nat Med. 2010 Jun;16(6):678-86
references: 20495570
references: Cell Res. 2010 Sep;20(9):977-9
references: 20625379
references: Int Orthop. 2005 Feb;29(1):6-9
references: 15611874
references: Nat Med. 2011 Jan;17(1):25-6; author reply 27-9
references: 21217667
references: Inflamm Res. 2006 Feb;55(2):72-7
references: 16612567
references: J Cell Physiol. 2004 Jun;199(3):451-9
references: 15095292
references: Ann Rheum Dis. 2009 Jun;68(6):1051-8
references: 18662930
references: Int J Cancer. 2005 Aug 20;116(2):218-25
references: 15800931
references: J Biol Chem. 1999 Nov 19;274(47):33709-13
references: 10559262
references: J Orthop Res. 2011 Feb;29(2):258-64
references: 20706991
references: Arthritis Rheum. 2009 May;60(5):1406-15
references: 19404942
references: Cell. 1982 Aug;30(1):215-24
references: 7127471
references: J Cell Biochem. 2001 Mar 26;81(2):368-77
references: 11241676
references: Clin Sci (Lond). 2013 Jul 1;125(2):99-108
references: 23406266
references: BMC Res Notes. 2009 Dec 10;2:243
references: 20003239
references: Biochem J. 2001 Feb 15;354(Pt 1):17-24
references: 11171074
references: J Orthop Res. 2003 May;21(3):451-7
references: 12706017
references: Arthritis Res Ther. 2005;7(3):R560-8
references: 15899043
references: Clin Orthop Relat Res. 2004 Jan;(418):246-52
references: 15043126
references: J Proteome Res. 2010 Jun 4;9(6):3035-45
references: 20387907
references: Nat Med. 2010 Jun;16(6):641-4
references: 20526316
references: Oncogene. 2001 Sep 13;20(41):5779-88
references: 11593383
references: J Orthop Res. 2012 Aug;30(8):1244-8
references: 22247019
references: Arthritis Res Ther. 2010;12(2):R34
references: 20193091
references: J Biol Chem. 2013 Apr 5;288(14):10061-10072
references: 23417678
references: Nat Med. 2010 Jun;16(6):687-93
references: 20495569
references: Arthritis Rheum. 2013 May;65(5):1302-12
references: 23334958
references: Arthritis Rheum. 2007 Oct;56(10):3297-306
references: 17907154
references: J Biol Chem. 1999 Nov 12;274(46):32631-7
references: 10551817
references: Arthritis Res Ther. 2005;7(4):R904-14
references: 15987493
references: Arthritis Rheum. 2009 Oct;60(10):3038-48
references: 19790048