Elevated expression of hypoxia inducible factor-2α in terminally differentiating growth plate chondrocytes

Growth plate chondrocytes exist in a hypoxic environment where it is recognized that hypoxia-inducible factor-la (HIF-1 alpha) is essential for their survival. Its regulation of chondrocyte viability may be mediated by the increased expression of vascular endothelial growth factor (VEGF) and glycolytic enzymes. However, the full chondrocyte response to hypoxia and the molecular control of VEGF expression in relation to growth plate differentiation and vascularization remains poorly understood. Using Percoll density gradient centrifugation, chick chondrocytes were separated into populations of different maturational phenotype. A differential display analysis of the populations showed highly upregulated expression of HIF-2 alpha mRNA during chondrocyte differentiation. HIF-2 alpha is a homologue of the HIF-1a transcription factor, both of which play a role in the activation of a number of hypoxia responsive genes. HIF-1 alpha mRNA was also found to be expressed, although levels of expression were found to be similar in all of the chondrocyte fractions. The elevated expression of HIF-2 alpha during chondrocyte differentiation was accompanied by increased VEGF expression. Analysis of the murine chondrocyte cell line, ATDC5, which undergoes ordered maturation indicated that HIF-2 alpha, VEGF, placental growth factor, and glucose transporter-1 expression all increased in parallel with chondrocyte differentiation. This observation was supported by immunohistochemistry on sections of mouse bone which showed staining corresponding to the presence of HIF-2 alpha in hypertrophic growth plate chondrocytes. The presence of HIF-2 alpha was also observed in articular chondrocytes but was restricted to the Superficial tangential zone. HIF-2 alpha is, therefore, likely to be involved in the initiation of blood vessel formation and a metabolic shift in the growth plate, processes crucial for endochondral ossification.