Metabolic Rewiring Controlled by C-Fos Governs Cartilage Integrity in Osteoarthritis
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Objectives: The activator protein-1 (AP-1) transcription factor component c-Fos regulates chondrocyte proliferation and differentiation, but its involvement in osteoarthritis (OA) has not been functionally assessed.
Methods: c-Fos expression was evaluated by immunohistochemistry on articular cartilage sections from patients with OA and mice subjected to the destabilisation of the medial meniscus (DMM) model of OA. Cartilage-specific c-Fos knockout (c-Fos) mice were generated by crossing to mice. Articular cartilage was evaluated by histology, immunohistochemistry, RNA sequencing (RNA-seq), quantitative reverse transcription PCR (qRT-PCR) and metabolic enzyme assays. The effect of dichloroacetic acid (DCA), an inhibitor of pyruvate dehydrogenase kinase (Pdk), was assessed in c-Fos mice subjected to DMM.
Results: FOS-positive chondrocytes were increased in human and murine OA cartilage during disease progression. Compared with c-Fos mice, c-Fos mice exhibited exacerbated DMM-induced cartilage destruction. Chondrocytes lacking c-Fos proliferate less, have shorter collagen fibres and reduced cartilage matrix. Comparative RNA-seq revealed a prominent anaerobic glycolysis gene expression signature. Consistently decreased pyruvate dehydrogenase (Pdh) and elevated lactate dehydrogenase (Ldh) enzymatic activities were measured , which are likely due to higher expression of hypoxia-inducible factor-1α, , and Pdk1 in chondrocytes. treatment of c-Fos mice with DCA restored Pdh/Ldh activity, chondrocyte proliferation, collagen biosynthesis and decreased cartilage damage after DMM, thereby reverting the deleterious effects of c-Fos inactivation.
Conclusions: c-Fos modulates cellular bioenergetics in chondrocytes by balancing pyruvate flux between anaerobic glycolysis and the tricarboxylic acid cycle in response to OA signals. We identify a novel metabolic adaptation of chondrocytes controlled by c-Fos-containing AP-1 dimers that could be therapeutically relevant.
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