Background/Purpose: Osteoarthritis (OA) is a multifactorial disease and is characterized by focal degradation of articular cartilage. Several components of the inflammatory pathway, such as IL-6, are found to be highly expressed in OA joints. There is growing evidence to support the involvement of IL-6 in OA progression and severity. Harpagoside is a natural compound isolated principally from the secondary roots of Harpagophytum procumbens (Hp). In the present study we used an in vitro model of joint inflammation to study the therapeutic potential of harpagoside in OA.

Methods: Primary human OA chondrocytes were isolated from the non-affected cartilage obtained from OA patients who underwent total knee arthroplasty. Human OA chondrocytes were cultured and pre-treated with harpagoside (300 µg/ml) and then cultured with and without IL-1β (1-10 ng/ml). Chondrocyte viability was assayed using CytoTox 96® Non-Radioactive Cytotoxicity Assay (Promega). mRNA levels of 92 chemokines and chemokine-associated genes were measured by TaqMan human chemokine PCR array (Life Technologies). Secreted levels of IL-6 in the culture supernatants were assayed by ELISA and immunoblotting. Total protein levels and phosphorylation status of proteins were measured by immunoblotting using specific antibodies. mRNA levels of individual genes were quantified using the TaqMan assays (Life Technologies). Subcellular localization of c-FOS and IL-6 was studied using confocal microscopy (Olympus FV1000 microscope). Activation of transcription factor c-FOS/AP-1 was analyzed by ELISA (Active Motif). Data were analyzed and plotted using Origin 6.1 software and P<0.05 was considered significant.

Results: Harpagoside significantly modulated the IL-1β-induced expression of a number of chemokines in primary human OA chondrocytes. Several of the genes, including IL-6, that showed robust expression in response to IL-1β were markedly downregulated by treatment with harpagoside. Treatment with IL-1β markedly stimulated the mRNA expression as well as protein secretion of IL-6 in the culture supernatants. In contrast, OA chondrocytes pre-treated with harpagoside showed a significant reduction (p<0.05) in IL-1β-induced expression of IL-6. Importantly, harpagoside did not inhibit the IL-1b-induced activation of NF-κB and C/EBPb but specifically suppressed the IL-1β-triggered over-expression, phosphorylation and activity of c-FOS, one of the major components of AP-1. Further, we show here that harpagoside imparts its inhibitory effect on c-FOS activity and IL-6 production partially through suppression of IL-1β induced ROS production in human OA chondrocytes.

Conclusion: Taken together, our data showed that harpagoside markedly inhibited the IL-1β-induced expression and production of IL-6 in human OA chondrocytes. Importantly, a novel mechanism of IL-6 suppression by harpagoside which involves the inhibition of c-FOS expression was identified. These data provide strong evidence that harpagoside may have a chondroprotective effect and may be a potential therapeutic choice to prevent and/or to slow down the progression of OA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/low-molecular-weight-polyphenol-harpagoside-inhibits-il-1-induced-expression-of-il-6-by-blocking-the-expression-and-activity-of-c-fos-in-primary-human-osteoarthritis-chondrocytes/