Effects of hypoxia on glucose transport in primary equine chondrocytes in vitro and evidence of reduced GLUT1 gene expression in pathologic cartilage in vivo

Abstract

Articular chondrocytes exist in an environment lacking in oxygen and nutrients due to the avascular nature of cartilage. The main source of metabolic energy is glucose, which is taken up by glucose transporters (GLUTs). In diseased joints, oxygen tensions and glucose availability alter as a result of inflammation and changes in vascularisation. Accordingly, in this study we examined the effects of hypoxia and the hypoxia mimetic cobalt chloride (CoCl(2)) on glucose transport in equine chondrocytes and compared expression of the hypoxia responsive GLUT1 gene in normal and diseased cartilage. Monolayers of equine chondrocytes were exposed to 20% O(2), 1% O(2), CoCl(2) (75 microM), or a combination of 1% O(2) and CoCl(2). Glucose uptake was measured using 2-deoxy-D-[2,6-(3)H] glucose. GLUT1 protein and mRNA expression were determined by FACS analysis and qPCR, respectively. GLUT1 mRNA expression in normal and diseased cartilage was analyzed using explants derived from normal, OA, and OCD cartilage. Chondrocytes under hypoxic conditions exhibited a significantly increased glucose uptake as well as upregulated GLUT1 protein expression. GLUT1 mRNA expression significantly increased in combined hypoxia-CoCl(2) treatment. Analysis of clinical samples indicated a significant reduction in GLUT1 mRNA in OA samples. In OCD samples GLUT1 expression also decreased but did not reach statistical significance. The increase in glucose uptake and GLUT1 expression under hypoxic conditions confirms that hypoxia alters the metabolic requirements of chondrocytes. The altered GLUT1 mRNA expression in diseased cartilage with significance in OA suggests that reduced GLUT1 may contribute to the failure of OA cartilage repair.