Background/Purpose: Osteoarthritis (OA) is the most common degenerative joint disease worldwide, traditionally classified as non-inflammatory. Recently, attention has been drawn to the importance of synovitis in the pathogenesis of OA. Fibroblast-like synoviocytes (FLS), which maintain structural and dynamic integrity of the joint under physiological conditions, have been proposed to be key factors in joint destruction during OA. Of these, pathogenic THY1- FLS of the synovial lining are assumed to destroy bone and cartilage, whereas THY1+ FLS of the sublining drive synovitis. The latter are classified as invasive proliferative cells characterized by a dominant glucose metabolism. In order to demonstrate inflammation-induced metabolic changes in FLS, we chose mesenchymal stromal cells (MSCs) as a phenotypically indistinguishable, non-activated fibroblast-like control. To identify novel targets for the diagnosis and treatment of OA, we compared human FLS isolated from tibial plateau samples collected during knee arthroplasty for OA with bone marrow-derived MSCs from patients with OA at the transcriptomic, proteomic, and metabolic levels.

Methods: We characterized FLS and MSCs for their multipotency, surface marker pattern, proliferation rate, expression of metabolic-related markers, and mitochondrial function using flow cytometry, immunofluorescence and Seahorse^TM technology. Using qPCR and mass spectrometry, we analyzed selected gene and protein expression patterns.

Results: We observed a comparable phenotype of FLS and non-inflamed MSCs with respect to the minimal criteria defining the MSC phenotype. Mapping the distribution of subsets within expanded FLS, we observed >90% THY1+ FLS proliferating faster than non-inflamed MSCs. Global proteome comparison of FLS with MSCs revealed 592 differentially expressed proteins. In detail, we observed no differences between the two cell types with respect to the expression of classical fibroblast markers. When mitochondrial function was analyzed, FLS showed significantly lower basal respiration and ATP production rates but higher spare respiratory capacity and number of mitochondria compared with MSCs. In addition, we identified the pyruvate dehydrogenase kinase (PDK) 3 to be highly expressed in proliferative FLS compared with MSCs. Inhibition of PDKs by dichloroacetate (DCA) significantly increased basal respiration and ATP production rates in FLS but not in MSCs. Finally, DCA significantly reduced the proliferation of FLS compared with untreated FLS controls.

Conclusion: Our data suggest that, although the classical fibroblast markers do not distinguish between non-inflammatory MSCs and THY1+ FLS, the latter have significantly higher expression of PDK isoforms. PDK isoforms are known to inhibit the entry of pyruvate into the tricarboxylic acid cycle, thereby limiting the mitochondrial ATP production, and may play a critical role in the expansion of FLS during OA pathogenesis. Therefore, it is likely, that reprogramming FLS metabolism from glycolysis to mitochondrial respiration by inhibiting PDK isoforms might be a potential new approach to treat OA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/metabolic-reprogramming-inhibiting-osteoarthritis-induced-expression-of-the-pyruvate-dehydrogenase-kinase-preserves-mitochondrial-respiration/