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.
CONCLUSION: 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.