Background/Purpose:

Almost all cells contain a circadian clock. Cell clocks are key regulators of cell activity and cell differentiation. In osteoarthritis (OA), chondrocytes inappropriately undergo terminal differentiation leading to increased production of cartilage degrading enzymes. Recently, we found expression of two core circadian clock components, PER2 and BMAL1, is altered in chondrocytes in OA. In vitro and in vivo evidence indicates altered BMAL1 expression contributes to OA pathology. The purpose of this study was to determine potential causes of the altered PER2 and BMAL1 expression in OA, and to investigate the role of PER2 in regulating chondrocyte behaviour.

Methods: Chondrocytes were isolated from macroscopically normal (MN) and OA cartilage from 12 patients with OA. MN chondrocytes were treated with interleukin-1β (IL-1β, 10ng/ml), hydrogen peroxide (100µM) or basic calcium phosphate (BCP) crystals (50µg/ml) (stimuli previously shown to induce OA-associated changes in chondrocyte phenotype) for up to 48h. Clock gene expression was measured by RT-qPCR. RNAi was used to knockdown PER2 and chondrocyte phenotypic marker expression was assessed using RT-qPCR and ELISA.

Results: Consistent with previous findings, peak expression of BMAL1 was lower and peak expression of PER2 higher in OA compared to MN chondrocytes. In IL-1β, peroxide or BCP crystal-exposed MN chondrocytes, peak expression of BMAL1 was also lower, and PER2 levels remained elevated for longer, compared to untreated cells. IL-1β, peroxide and BCP-induced changes in disease-associated phenotypic markers (SOX9, ADAMTS5, MMP13) were partially mitigated by knockdown of PER2. PER2 knockdown in OA chondrocytes was associated with restoration of phenotypic marker expression to levels more similar to the non-diseased state. Notably, both RNA and protein levels of MMP13, the major enzyme implicated in the cartilage loss in OA, were reduced following PER2 knockdown in OA chondrocytes.

Conclusion: IL-1β, peroxide and BCP crystals induce similar changes in the expression of PER2 and BMAL1 as occur in OA. Increased PER2 expression contributes to the induction of OA-associated changes in chondrocyte phenotype following IL-1β, peroxide and BCP crystal treatment and contributes to the maintenance of the diseased phenotype in OA chondrocytes. Altered expression of PER2 and BMAL1 may be a central mechanism by which multiple different risk factors for OA induce disease-associated changes in chondrocyte activity.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/interleukin-1%ce%b2-oxidative-stress-and-basic-calcium-phosphate-crystals-induce-osteoarthritis-like-changes-in-chondrocyte-phenotype-by-altering-the-expression-of-period2-a-core-component-of-the-ch/