Hypoxia-inducible factor-1α promotes upregulation of extracellular matrix in articular chondrocytes

Cartilage homeostasis is regulated by multiple mechanisms including growth factors, cytokines, oxygen supply and mechanical force. Hypoxia is a stimulus for articular cartilage development and regeneration. These processes are regulated by the transcription factor hypoxia-inducible factors-1α (HIF-1α). Under normoxia, HIF-1α is hydroxylated by prolyl hydroxylase, ubiquinated and then degradated by proteosoma. Hypoxia inhibits the activity of the HIF-targeting prolyl hydroxylase enzymes, therefore HIF-1α traslocates into the nucleus and dimerizes with HIF-1β to induce transcription of target genes1. Recent study reported that HIF-1a is able to promotes the synthesis of relevant extracellular matrix (ECM) components2. In this study we investigate the effect of hypoxia on gene expression of extracellular matrix components, such as collagen type II, aggrecan, biglycan and decorin, in primary articular chondrocytes. Cells cultured under normoxia were treated with deferoxamine mesylate, which inhibits HIF-1α hydroxilation, to simulate hypoxia, and transfected with siRNAs targeting HIF-1α. Treatment of cells with deferoxamine mesylate increased the levels of HIF-1αprotein. The levels of collagen type II, aggrecan, biglycan and decorin mRNAs and proteins assessed by real time RT-PCR and Western blotting, respectively, were significantly higher under simulated hypoxia than under normoxia. The specific siRNA knocked down mRNA and protein levels of ECM gene under hypoxia. These data seem to confirm that hypoxia, by activating HIF-1α, promotes the anabolic activity of chondrocytes that increase ECM production. Particularly our goal has been shown that also biglycan and decorin were upregulated under simulate hypoxia, since these SLRP are reported to be key molecules in modulating the physiological processes, such as cartilage regeneration. The results suggest that HIF-1α has an important role in cartilage metabolism and modulation of HIF pathway could be a useful strategy to promote articular cartilage repair through regulating chondrocyte ECM synthesis.