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Title: Promotion of chondrocyte-specific function by inhibition of HIF-targeting hydroxylases
Author: Thoms, Brendan Lawless
ISNI:       0000 0004 2709 8861
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2011
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Human articular cartilage is an avascular tissue, and therefore functions in a hypoxic environment. Cartilage cells, the chondrocytes, have adapted to this and actually use hypoxia to drive tissue-specific functions. Previous work in our laboratory has shown that human chondrocytes enhance cartilage matrix synthesis in response to hypoxia specifically through Hypoxia Inducible Factor 2alpha (HIF-2α) mediated upregulation of master regulator transcription factor SOX9, which in turn drives expression of the main cartilage-specific extracellular matrix genes. HIF-α isoforms are themselves regulated by specific prolyl hydroxylases (PHDs) which target them for proteosomal degradation. In fact, the PHDs are the true direct oxygen sensors since they require molecular oxygen as a co-substrate. Here we show that PHD2 is the dominant isoenzyme regulating HIF-2α (and HIF-1α) stability in human chondrocytes. Moreover, specific inhibition of PHD2 using RNAi-mediated depletion caused an upregulation of SOX9 and enhanced extracellular matrix protein production. Depletion of PHD2 resulted in greater HIF-2α levels, and therefore enhanced SOX9-induced matrix cartilage matrix production, even when performed in hypoxia (1% oxygen) implying that PHD2 inhibition offers a novel means to enhance cartilage repair. The need for HIF-specific hydroxylase inhibitors was highlighted as treatment with the oxoglutarate analogue dimethyloxalylglycine (which also inhibits the collagen prolyl hydroxylases) resulted in reduced secretion of type II collagen, a critical matrix component. Complimentary to these anabolic effects, hypoxia was found to mediate anti-catabolic effects, suppressing aggrecan degradation in human cartilage explants. In isolated chondrocytes hypoxia downregulates key matrix metalloproteinases (ADAMTS5 and MMP13) and upregulates metalloproteinase inhibitor (TIMP3). Unlike the anabolic effects (HIF-2α-dependent), these hypoxia induced anti-catabolic chondrocyte responses were predominantly HIF-1α dependent. Both anabolic and anti-catabolic hypoxia-induced responses were further enhanced by depletion of PHD2, whose inhibition thus offers a novel means to induce cartilage repair.
Supervisor: Murphy, Chris ; Saklatvala, Jeremy Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral