ABSTRACT
The bone-muscle unit refers to the reciprocal regulation between bone and muscle by mechanical interaction and tissue communication via soluble factors. The receptor activator of NF-κB ligand (RANKL) stimulation induces mitochondrial biogenesis and increases the oxidative capacity in osteoclasts and adipocytes. RANKL may bind to the membrane bound receptor activator of NF-κB (RANK) or to osteoprotegerin (OPG), a decoy receptor that inhibits RANK-RANKL activation. RANK is highly expressed in skeletal muscle, but the contribution of RANKL to healthy skeletal muscle fiber remains elusive. Here we show that RANKL stimulation in C2C12-derived myotubes induced activation of mitochondrial biogenesis pathways as detected by RNA-seq and western blot. RANKL expanded the mitochondrial reticulum, as shown by mitochondrial DNA quantification and MitoTracker staining, and boosted the spare respiratory capacity. Using MEK and MAPK inhibitors, we found that RANKL signals via ERK and p38 to induce mitochondrial biogenesis. The soleus from OPG-/- and OPG+/- mice showed higher respiratory rates compared to C57BL6/J wild-type (WT) mice, which correlates with high serum RANKL levels. RANKL infusion using a mini-osmotic pump in WT mice increased the number of mitochondria, boosted the respiratory rate, increased succinate dehydrogenase (SDH) activity in skeletal muscle, and improved the fatigue resistance of gastrocnemius. Therefore, our findings reveal a new role of RANKL as an osteokine-like protein that impacts muscle fiber metabolism.
Bone modeling and remodeling are processes intricately related to bone health regulated by the RANKL system. The RANKL (receptor activator of NF-κB ligand) is a protein essential for bone resorption. RANKL activates RANK (receptor activator of NFκB) in the cell membrane of osteoclasts and can also bind to OPG (osteoprotegerin), which acts as a soluble decoy receptor. Therefore, the levels of RANKL and OPG determine the degree of osteoclast activation and bone resorption. Bone and muscle mechanically interact for movement as bone is a lever for skeletal muscle to exert force. They also communicate via soluble factors that reciprocally regulate their function. Skeletal muscle fibers express RANK, but the role of RANKL signaling in healthy myotubes was still unknown. Here, we propose that RANKL regulates muscle metabolism by inducing mitochondrial biogenesis. We show that RANKL increases mitochondrial area in myotubes and the expression of mitochondrial markers, boosting the spare respiratory capacity. In mice knockout for OPG, which shows high levels of RANKL and unopposed RANK-RANKL stimulation, we found higher respiratory rates than in the wild-type mice. We also infused a low dose of RANKL in wild-type mice, which is around ten times lower than the dose to induce osteoporosis, and found increased mitochondrial number and higher respiratory rates in soleus. In the gastrocnemius, we also observed increased phosphorylative respiration and improved resistance to fatigue compared to mice treated with the vehicle solution. Our findings indicate that RANKL regulates both bone and muscle under physiological conditions by inducing mitochondrial biogenesis and oxidative metabolism in skeletal muscle fibers.
ABSTRACT
High serum levels of osteoprotegerin (OPG) are found in patients with obesity, type 2 diabetes, sepsis, or septic shock and are associated with a high mortality rate in stroke. The primary known function of OPG is to bind to the receptor activator of NF-κB ligand (RANKL), and by doing so, it inhibits the binding between RANKL and its receptor (RANK). TLR4 signaling in macrophages involves TRAF6 recruitment and contributes to low-grade chronic inflammation in adipose tissue. LPS is a classical activator of the TLR4 pathway and induces the expression of inflammatory cytokines in macrophages. We have previously observed that in the presence of RANKL, there is no LPS-induced activation of TLR4 in macrophages. In this study, we investigated the crosstalk between RANK and TLR4 pathways in macrophages stimulated with both RANKL and LPS to unveil the role of OPG in inflammatory processes. We found that RANKL inhibits TLR4 activation by binding to RANK, promoting the binding between TRAF6 and RANK, lowering TLR4 activation and the expression of proinflammatory mediators. Furthermore, high OPG levels aggravate inflammation by inhibiting RANKL. Our findings elect RANKL as a candidate for drug development as a way to mitigate the impact of obesity-induced inflammation in patients.
