ATP-Citrate Lyase As A Novel Metabolic Target for Osteoarthritis

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Aging and obesity are the major risk factors for development of osteoarthritis (OA), the most common form of arthritis. Veterans are at high risk of developing OA, because over 12.4 millions veterans are age 65 or older, and nearly 80% of veterans are obese. For veterans and others, developing effective disease-modifying therapy for OA is a major unmet medical need. As OA progresses, failure of the synovial joint organ frequently develops, with degeneration of articular cartilage as a core disease feature. Chondrocytes, the sole cells in articular hyaline cartilage, are responsible for maintaining the homeostatic balance between extracellular matrix anabolism and catabolism. Dysfunction of chondrocytes in OA, amplified by local inflammatory processes, leads to an excess of chondrocyte catabolic activity, medicated by factors including nitric oxide (NO), matrix metalloproteinasess (MMPs) and aggrecanases. Cellular metabolism can intersect with certain epigenetic and transcription factor modifications to mediate cellular re-programming. ATP citrate lyase (ACLY) is a metabolic enzyme that converts citrate generated from mitochondria to acetyl-CoA in the cytosol and nucleus, which serves as an acetyl donor for de novo lipid synthesis and acetylation of proteins in the cytosol, and acetylation of histones and transcription factors in the nucleus, thereby modulating gene expression. Our preliminary studies reveal that human knee OA chondrocytes/cartilages have increased ACLY activity, associated with increased acetylation of histones. Pharmacologic inhibition of ACLY in OA chondrocytes increases anabolic and decreases inflammation-mediated catabolic activities through modulating acetylation of histones and transcription factors. Building on these findings in this translational project, we propose to test our central hypothesis that chondrocyte ACLY is a druggable metabolic target for OA in vivo. We will test hypotheses that (1) activation of Akt signaling by IL-1β and IGF-1, both of which are known to associate with aging and obesity and are upregulated in OA cartilage, drives increased ACLY activity in human chondrocytes; (2) Chondroprotection by limiting ACLY activity is mediated by increased chondrocyte autophagy; (3) Suppression of ACLY activity protects mice from OA development and progression in both obesity-induced OA via high-fat diet (HFD) and age-related spontaneous OA in vivo. Completion of these studies will provide new insights into how metabolic alterations modulated by ACLY influence cartilage tissue integrity, and may provide a novel approach by limiting ACLY activity to suppress or delay OA development and progression, particularly in those at increased risk due to aging and/or obesity.