Osteoporosis is a global bone disease characterized by reduced bone mineral density (BMD) and increased risk of fractures. The risk of developing osteoporosis increases with aging, especially after menopause in women. Discovering the signaling pathways that play a significant role in aging‐ and menopause‐induced osteoporosis should accelerate osteoporosis drug discovery. In this study, we found that bile acid membrane receptor Tgr5 knockout C57BL/6J mice had similar bone mass as wild‐type mice during early and middle‐age (before 4 months old) bone remodeling; however, Tgr5‐/‐ markedly decreased bone mass in aged (more than 7 months old) and ovariectomized (OVX) mice compared with wild‐type mice. Moreover, Tgr5 knockout strongly induced osteoclast differentiation but had no effect on osteoblast activity. Treatment with different TGR5 agonists consistently inhibited osteoclast differentiation. Importantly, our results showed that Tgr5 regulates osteoclastogenesis by the AMP‐activated protein kinase (AMPK) signaling pathway, which is a central metabolic pathway involved in the pathophysiology of aging and age‐related diseases. The bile acid nuclear receptor FXR is an established regulator of bone metabolism. We screened the derivatives of betulinic acid (BA), a known TGR5 agonist, to identify novel dual agonists of FXR and TGR5. The derivative SH‐479, a pentacyclic triterpene acid, could activate both TGR5 and FXR, with a better inhibitory effect on osteoclastogenesis compared with agonists solely activating FXR or TGR5 and additionally enhanced osteoblastogenesis. Furthermore, SH‐479 therapeutically abrogated bone loss in C57BL/6J mice through the bone remodeling pathways. Together, our results demonstrate that dual targeting the bile acid membrane receptor TGR5 and nuclear receptor FXR is a promising strategy for osteoporosis. © 2018 American Society for Bone and Mineral Research.

Inflammatory diseases decrease the extracellular environmental pH. However, whether proton-activated G protein-coupled receptors (GPCRs) can regulate the development of osteoarthritis (OA) is largely unknown. In this study, we report that proton-activated GPR4 is essential for OA development. We found a marked increase in expression of the proton-activated GPR4 in human and mouse OA cartilage. Lentivirus-mediated overexpression of GPR4 in mouse joints accelerated the development of OA, including promotion of articular cartilage damage, synovial hyperplasia, and osteophyte formation, while Gpr4 knockout effectively attenuated the development of posttraumatic and aging-associated OA in mice. We also found that inhibition of GPR4 with the antagonist NE52-QQ57 ameliorated OA progression in mice, promoted extracellular matrix (ECM) production, and protected cartilage from degradation in human articular cartilage explants. Moreover, GPR4 overexpression upregulated matrix-degrading enzymes’ expression and inflammation factors under pro-inflammatory and slightly acidic conditions. Mechanistically, GPR4 suppressed chondrocyte differentiation and upregulated cartilage homeostasis through NF-κB/MAPK signaling activation by regulating CXCR7/CXCL12 expression. Together, our results take the lead to illustrate that proton-activated GPCR acts as a key regulator for OA pathogenesis in vivo, and support that GPR4 could be a promising therapeutic target for OA treatment.

Articular cartilage repair and regeneration is an unmet clinical need because of the poor self-regeneration capacity of the tissue. In this study, we found that the expression of prostaglandin E receptor 4 (PTGER4 or EP4) was largely increased in the injured articular cartilage in both humans and mice. In microfracture (MF) surgery-induced cartilage defect (CD) and destabilization of the medial meniscus (DMM) surgery-induced CD mouse models, cartilage-specific deletion of EP4 remarkably promoted tissue regeneration by enhancing chondrogenesis and cartilage anabolism, and suppressing cartilage catabolism and hypertrophy. Importantly, knocking out EP4 in cartilage enhanced stable mature articular cartilage formation instead of fibrocartilage, and reduced joint pain. In addition, we identified a novel selective EP4 antagonist HL-43 for promoting chondrocyte differentiation and anabolism with low toxicity and desirable bioavailability. HL-43 enhanced cartilage anabolism, suppressed catabolism, prevented fibrocartilage formation, and reduced joint pain in multiple pre-clinical animal models including the MF surgery-induced CD rat model, the DMM surgery-induced CD mouse model, and an aging-induced CD mouse model. Furthermore, HL-43 promoted chondrocyte differentiation and extracellular matrix (ECM) generation, and inhibited matrix degradation in human articular cartilage explants. At the molecular level, we found that HL-43/EP4 regulated cartilage anabolism through the cAMP/PKA/CREB/Sox9 signaling. Together, our findings demonstrate that EP4 can act as a promising therapeutic target for cartilage regeneration and the novel EP4 antagonist HL-43 has the clinical potential to be used for cartilage repair and regeneration.