Tonic activation of hypoxia-inducible factor 1alpha in avascular articular cartilage and implications for metabolic homeostasis.
OBJECTIVE
To determine whether oxygen-dependent activation patterns of hypoxia-inducible factor 1alpha (HIF-1alpha) observed in vascularized tissues are conserved within avascular and hypoxic articular cartilage and whether HIF-1alpha affects cartilage matrix synthesis.


METHODS
Explants of bovine articular cartilage and primary chondrocytes were exposed to normoxia (21% O2), hypoxia (2% O2), and simulated hypoxia (21% O2 plus CoCl2). Western blot and immunofluorescence analyses of HIF-1alpha were performed to determine HIF-1alpha activation patterns. To simulate cartilage loss from disease or injury, the top layers of cartilage were removed from osteochondral explants, and the residual cartilage was assessed for HIF-1alpha immunolocalization and proteoglycan synthesis.


RESULTS
We demonstrated continuous nuclear translocation of HIF-1alpha in deeper layers of intact articular cartilage. HIF-1alpha was not completely degraded in chondrocytes exposed to normoxia, but rather, colocalized to the Golgi complex, a finding not previously reported for any cell type. Following alteration of the oxygen gradient by removal of the top layers of cartilage, predominantly perinuclear HIF-1alpha was found in the deeper layers. Restoration of intranuclear HIF-1alpha to these areas was achieved by hypoxia and simulated hypoxia. Under conditions in which HIF-1alpha was inactivated, matrix synthetic activity was altered (P < 0.0001) compared with control cartilage.


CONCLUSIONS
These findings demonstrate that hypoxia-dependent activation of HIF-1alpha is highly conserved and that changes in oxygen tensions following cartilage loss from injury or disease alter cartilage metabolism in part by changing HIF-1alpha activity. The discovery of tonic activation of HIF-1alpha within intact articular cartilage underscores its potential importance to cartilage homeostasis.
Brucker
Peter U
PU
Cartilage Restoration Laboratory, Pittsburgh, Pennsylvania 15232, USA.
Izzo
Nicholas J
NJ
Chu
Constance R
CR
D000818:Animals; D002358:Cartilage, Articular / Q000098:blood supply / Q000166:cytology / Q000378:metabolism*; D002417:Cattle; D002467:Cell Nucleus / Q000378:metabolism; D002470:Cell Survival / Q000502:physiology; D002478:Cells, Cultured; D019902:Chondrocytes / Q000166:cytology / Q000378:metabolism*; D006056:Golgi Apparatus / Q000378:metabolism; D006706:Homeostasis / Q000502:physiology*; D000860:Hypoxia / Q000378:metabolism; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D010003:Osteoarthritis / Q000378:metabolism; D010100:Oxygen / Q000378:metabolism; D011509:Proteoglycans / Q000096:biosynthesis; D014157:Transcription Factors / Q000378:metabolism*
D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D011509:Proteoglycans; D014157:Transcription Factors; D010100:Oxygen