Targeting RIP3 inhibits osteoarthritis development by restoring anabolic-catabolic balance in the bone-cartilage unit

Background:Osteoarthritis(OA)is a debilitating joint disorder characterized by pro-gressive cartilage degeneration.During OA,subchondral bone undergoes micro-structural and molecular changes that precede cartilage degradation.However,spe-cific 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.Methods:RIP3-mediated pathological and metabolic alterations in chondrocytes,os-teoblasts,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.Results:RIP3 was upregulated in the cartilage and subchondral bone of OA patients and post-traumatic OA mouse model.RIP3 overexpression not only inhibited extra-cellular 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 degen-eration 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 re-sponses,senescence,and osteoclast differentiation.Clofibrate,a hypolipidemic drug,was identified as a novel RIP3 inhibitor that reverses ECM catabolism in OA.Conclusions: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.