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SirT1 Regulation of Aggrecan Expression in Human Chondrocytes

Final Report Summary - EUCUREOA (SirT1 Regulation of Aggrecan Expression in Human Chondrocytes)

Osteoarthritis (OA) is a degenerative joint disease of the articular cartilage (AC) characterized by accelerated extracellular matrix (ECM) degradation and severe pain-dysfunction. In a previous study, the NAD-dependent protein deacetylase, Sirt1, was shown to positively regulate cartilage-type ECM gene expression, namely aggrecan and collagen 2a1. Histological and biochemical evaluations of OA cartilage exhibit reduced Sirt1 protein levels as compared to normal cartilage samples. For collagen 2a1, Sirt1 forms complexes with several transcription factors (TF) and coactivators on the promoter sites of these genes, inducing their transcription and expression. Since both aggrecan and collagen 2a1 are co-expressed in healthy cartilage, it is hypothesized that both genes are regulated by Sirt1. We hence envision that reduced Sirt1 activity and protein levels are mechanistically linked with the impaired capacity of chondrocytes to express these genes, thereby perpetuating the pathogenesis of OA.

Numerous reports show that enhanced exposure to inflammatory cytokines is an integral part of OA progression. To this end, my group has additionally investigated the effect inflammatory mediators on Sirt1, and the attributes of such alterations in cartilage biology and homeostasis. Our observations established that the lysosomal cathepsin B cleaves Sirt1 in human chondrocytes treated with TNFalpha and/or IL1beta. We additionally show that this cleaved fragment (75kDa Sirt1; 75Sirt1) could prolong chondrocyte viability under pro-inflammtory conditions. These data were further validated in-vivo using a comparison between Sirt1 w.t. and haploinsufficient mice at young (1 month) and in older ages (9 months).

Expectedly, research obtained regarding Sirt1 in cartilage biology, will impact the generation of novel disease-modifying drugs to modulate Sirt1 activity and prevent age-related cartilage degeneration, as seen in OA.