Medicine plus 

Abstract: Osteoarthritis (OA) is a debilitating joint disorder characterized by progressive cartilage degeneration. During OA, subchondral bone undergoes microstructural and molecular changes that precede cartilage degradation. However, specific mechanisms underlying metabolic dysregulation of the bone-cartilage unit remain unclear. This study aims to investigate the role of receptor-interacting protein kinase-3 (RIP3) in OA progression, focusing on bone-cartilage metabolic homeostasis. RIP3-mediated pathological and metabolic alterations in chondrocytes, osteoblasts, and bone marrow-derived macrophages (BMMs) were evaluated. RIP3-mediated OA manifestations in cartilage and more importantly, subchondral bone were determined by intra-articular overexpression of RIP3 in rats. The protective effect of RIP3 deficiency on the bone-cartilage unit during OA was systematically investigated using Rip3 knockout mice. The CMap database was used to screen for compounds that abrogate RIP3-induced OA pathological changes. RIP3 was upregulated in the cartilage and subchondral bone of OA patients and post-traumatic OA mouse model. RIP3 overexpression not only inhibited extracellular matrix (ECM) anabolism in chondrocytes but also attenuated osteoblast differentiation, whereas RIP3 deficiency blunted receptor activator of NF-kappaB ligand-mediated osteoclastogenesis of BMMs. Intra-articular RIP3 overexpression induced the imbalance of SP7+ osteoblasts/tartrate-resistant acid phosphatase (TRAP)+ osteoclasts within the subchondral bone in addition to cartilage degeneration in rats, while Rip3 deletion significantly improved structural outcomes of the bone-cartilage unit, and achieved pain relief as well as functional improvement in surgery-induced and spontaneous OA mouse models. Mechanistically, RIP3 initiates OA by perturbing critical events, including cartilage metabolism, inflammatory responses, senescence, and osteoclast differentiation. Clofibrate, a hypolipidemic drug, was identified as a novel RIP3 inhibitor that reverses ECM catabolism in OA. RIP3 is an essential governor of whole joint metabolic homeostasis by regulating both cartilage metabolism and subchondral bone remodeling. Reconstruction of the bone-cartilage unit by targeting RIP3 might provide a two-birds-one-stone approach for the development of future OA therapies.