Prostaglandin E2 Receptors Regulate The Progression Of Osteoarthritis By Modulating Subchondral Bone Remodeling

Osteoarthritis is a common senile disease.The clinical symptoms are mainly degeneration of articular cartilage,subchondral bone sclerosis,synovial inflammation,joint cysts,etc.The current treatment for osteoarthritis is mainly to repair articular cartilage and inhibit the degradation of articular cartilage.Recent studies have shown that subchondral bone plays an important role in the progress of osteoarthritis.Targeting subchondral bone to treat osteoarthritis can become a new treatment strategy.In the early stage of osteoarthritis,external pressure can cause damage to the subchondral bone.Osteoblasts and osteocytes at the injury site secrete RANKL and prostaglandin E2(PGE2)to recruit osteoclast precursor cells and initiate uncoupled bone remodeling.Osteoclasts will further absorb bone,secrete cytokines,promote blood vessels and nerves to invade the subchondral bone and calcified cartilage layer,intensify the uncoupled bone remodeling of subchondral bone,and promote the production of osteophytes.A large number of studies have shown that PGE2 signaling pathway plays an important role in diseases such as osteoporosis,osteoarthritis,tumors,nerve pain,kidney and cardiovascular diseases.The research of PGE2 signaling pathway in osteoarthritis mainly focuses on regulating the homeostasis of articular cartilage.Studies have shown that PGE2 can inhibit the synthesis of chondrocyte proteoglycans and promote the degradation of extracellular matrix by activating EP4.Studies have also shown that PGE2 can inhibit the catabolism of articular cartilage by activating EP2 and inhibit the expression of MMP-1,MMP-3,MMP-13 and ADAMTS5.However,some studies have shown that PGE2 can inhibit proteoglycan synthesis in chondrocytes by activating EP2.In terms of pain in osteoarthritis,studies have shown that PGE2 can activate EP4 in sensory nerves,thereby regulating the pain caused by osteoarthritis.PGE2 can also activate EP4 in dendritic cells to promote Th1 cell differentiation,proliferation,and IL-23 production to promote inflammation and produce pain.PGE2 also plays an important role in the regulation of subchondral bone homeostasis.Studies have shown that osteoblasts and osteocytes in the subchondral bone tissue of osteoarthritis can secrete a large amount of PGE2,destroy the homeostasis of the subchondral bone,and aggravate joint degeneration.Specifically knocking out cyclooxygenase-2(COX-2)in osteocytes or inhibiting the activity of COX-2 can inhibit the destruction of subchondral bone homeostasis and protect joints.However,exactly how PGE2 regulates subchondral bone homeostasis is still unclear.PGE2 can regulate cell biological functions through its four receptors(EP1,EP2,EP3,EP4).To this end,this topic starts with the 4 downstream receptors of PGE2,and explores how PGE2 regulates subchondral bone homeostasis.A large number of studies have shown that osteoclasts play an important role in the process of subchondral bone remodeling,and drugs used to inhibit osteoclasts(such as alendronate)have a certain therapeutic effect on patients with osteoarthritis.Therefore,this topic uses osteoclasts as the starting point to explore how PGE2 regulates subchondral bone homeostasis.Experimental results show that PGE2 can significantly promote the migration and differentiation of osteoclast precursor cells.During the differentiation of osteoclasts stimulated by PGE2,the expression of Ep2 and Ep4 increased.The expression of Ep1 did not change significantly,and the expression of Ep3 decreased.So we further researched EP2 and EP4.EP2 and EP4 specific small molecule antagonists and knocking out Ep2 and Ep4 in osteoclast precursor cells can inhibit the migration and differentiation of osteoclast precursor cells induced by PGE2.In the case of osteoarthritis,the expression of EP4 in the subchondral bone tissue of patients and mice increased,and the expression of EP2 did not change significantly.In vivo experiments,the systemic deletion of Ep2 could not inhibit the osteoarthritis induced by anterior cruciate ligament rupture(ACLT),and had no protective effect on the destruction of subchondral bone homeostasis.The specific knockout of Ep4 in osteoclast precursor cells can relieve osteoarthritis caused by ACLT,significantly inhibit osteoclast activity,reduce the number of subchondral bone osteoclast precursor cells,and inhibit subchondral bone uncoupled bone Remodeling and maintaining subchondral bone homeostasis.In addition,specifically knocking out Ep4 in osteoclast precursor cells can inhibit the secretion of Netrin-1 by osteoclasts,thereby inhibiting CGRP+ sensory nerves from invading subchondral bone,thereby inhibiting the production of pain.The specific knockout of Ep4 in osteoclast precursor cells can inhibit the secretion of PDGF-BB by osteoclast precursor cells,thereby inhibiting the generation of H-type blood vessels in subchondral bone.According to the previous research,we used the screening strategy of PGE2 to induce osteoclast differentiation to screen the EP4-specific small molecule antagonist HL-1-148.In the in vivo ACLT mouse osteoarthritis model,HL-1-148 can maintain subchondral bone homeostasis,inhibit osteoclast activity,and inhibit subchondral bone uncoupled bone remodeling.In naturally aging mice,HL-1-148 can also maintain subchondral bone homeostasis.In terms of pain control,HL-1-148 can inhibit the invasion of CGRP+ sensory nerves in the subchondral bone,thereby alleviating pain.In addition,HL-1-148 can also inhibit the production of H-type blood vessels in the subchondral bone and delay the development of osteoarthritis.In terms of mechanism,we found that PGE2 can promote the migration and differentiation of osteoclast precursor cells by activating Gas,and it does not depend on β-arrestin1 and β-arrestin2.PGE2 can activate the MAPK and NF-κB signaling pathways in osteoclast precursor cells.Inhibitors of MAPK signaling pathway can significantly inhibit the migration and differentiation of osteoclast precursor cells promoted by PGE2.Knocking out Ep4 on osteoclast precursor cells can inhibit the activation of MAPK signaling pathway and NF-κB signaling pathway by PGE2.In summary,in the case of osteoarthritis,PGE2 can activate EP4 in osteoclast precursor cells,promote the migration and differentiation of osteoclast precursor cells,promote uncoupled bone remodeling in subchondral bone,and destroy subchondral bone homeostasis.Targeting EP4 to develop specific small molecule antagonists can be a new strategy for the treatment of osteoarthritis by inhibiting subchondral bone uncoupled bone remodeling,laying a good foundation for osteoarthritis treatment.