Nuclear receptors regulate lipid metabolism and oxidative stress markers in chondrocytes

Joint homeostasis failure can result in osteoarthritis (OA). Currently, there are no treatments to alter disease progression in OA, but targeting early changes in cellular behavior has great potential. Recent data show that nuclear receptors contribute to the pathogenesis of OA and could be viable therapeutic targets, but their molecular mechanisms in cartilage are incompletely understood. This study examines global changes in gene expression after treatment with agonists for four nuclear receptor implicated in OA (LXR, PPAR delta, PPAR gamma, and RXR). Murine articular chondrocytes were treated with agonists for LXR, PPAR delta, PPAR gamma, or RXR and underwent microarray, qPCR, and cellular lipid analyses to evaluate changes in gene expression and lipid profile. Immunohistochemistry was conducted to compare two differentially expressed targets (Txnip, Gsta4) in control and cartilage-specific PPAR delta knockout mice subjected to surgical destabilization of the medial meniscus (DMM). Nuclear receptor agonists induced different gene expression profiles with many responses affecting lipid metabolism. LXR activation downregulated gene expression of proteases involved in OA, whereas RXR agonism decreased expression of ECM components and increased expression of Mmp13. Functional assays indicate increases in cell triglyceride accumulation after PPAR gamma, LXR, and RXR agonism but a decrease after PPAR delta agonism. PPAR delta and RXR downregulate the antioxidant Gsta4, and PPAR delta upregulates Txnip. Wild-type, but not PPAR delta-deficient mice, display increased staining for Txnip after DMM. Collectively, these data demonstrate that nuclear receptor activation in chondrocytes primarily affects lipid metabolism. In the case of PPAR delta, this change might lead to increased oxidative stress, possibly contributing to OA-associated changes.