Energy homeostasis in the bone

Lactate induces oxidative phosphorylation in osteoblasts via Gpr81-Stat3 signaling

Lactate has long been regarded as an end product of glycolysis and a metabolic "waste product" under hypoxic conditions, but recent studies have revealed that lactate plays a central role in energy metabolism reprogramming and intercellular communication. However, it remains unknown whether lactate promotes osteogenic differentiation through metabolic reprogramming. Here, we showed that lactate significantly increased the cellular ATP content, activated succinate dehydrogenase activity, and enhanced oxygen consumption rate in pre-osteoblast MC3T3-E1 cells. Moreover, lactate treatment increased oxidative phosphorylation (OXPHOS) in parathyroid hormone (PTH)-treated MC3T3-E1 cells. Microarray and RNA-sequencing analysis revealed that Stat3 signaling was enriched in MC3T3-E1 cells treated with lactate or co-treated with lactate and PTH. Immunoblotting verification analysis further showed that lactate activated the Jak2-Stat3-Y705 and Akt-Stat3-S727 signaling. Inhibition of Jak2-Stat3-Y705 signaling by AG490 interrupted lactate-induced osteoblast differentiation. Inhibition of Gpr81 by 3-OBA or decrease in Gpr81 expression by Gpr81 siRNA, but not the interruption of MCT1 by AZD3965, led to the inhibition of the Gpr81-Jak2-Stat3-Y705 and Gpr81-Akt-Stat3-S727 signaling, and OXPHOS and cell differentiation of MC3T3-E1 cells were also inhibited. Furthermore, we demonstrated that the Gpr81 subunit Gβγ plays a central role in lactate-Gpr81 signaling. Lastly, osteoblast Gpr81-deficient mice showed lower bone formation. Thus, these findings propose a novel signaling mechanism by which lactate regulates cell differentiation as well as OXPHOS through the activation of Stat3 signaling by Gpr81.

ED-71 ameliorates OVX-induced osteoporosis by regulating calcium homeostasis and SIRT1-mediated mitochondrial function, alleviating osteoblast senescence and suppressing osteoclastogenesis

Osteoporosis arising from estrogen deficiency is characterized by oxidative stress and cellular senescence accompanied by calcium loss and disrupted bone metabolism. The paracrine interaction between osteoblasts and osteoclasts, along with the ratio of receptor activator of nuclear factor-κB ligand (RANKL) to osteoprotegerin (OPG), play a pivotal role in maintaining bone homeostasis. Eldecalcitol (ED-71), a novel active form of vitamin D, can reduce the ratio of RANKL to OPG in osteoblasts. In this study, an ovariectomized (OVX) rat model was established in vivo, and a cell model was constructed in vitro using H₂O₂ to explore the specific mechanism by which ED-71 improved the release of RANKL/OPG in senescent osteoblasts. Mitochondrial dysfunction and calcium imbalance were identified as significant factors. Under oxidative stress conditions, ED-71 alleviated endoplasmic reticulum (ER) stress by decreasing the ratio of phosphorylated protein kinase R-like ER kinase (P-PERK/PERK), and augmented the expression levels of sarcoplasmic reticulum/endoplasmic reticulum calcium ATPase 2 (SCERA2) thereby promoting calcium uptake by the ER, enhancing ER calcium influx, and effectively ameliorating calcium homeostasis between the ER and mitochondria. Consequently, it mitigates mitochondrial calcium overload and associated dysfunction. In contrast, ED-71 increased the expression of silent information regulator 1 (SIRT1) and phosphorylated AMP-activated protein kinase (P-AMPK). This alleviates mitochondrial dysfunction and promotes adenosine triphosphate (ATP). The combined effects of these two factors synergistically contribute to the improvement in osteoblast senescence.

FSH exacerbates bone loss by promoting osteoclast energy metabolism through the CREB-MDH2-NAD+ axis

AIMS

Osteoclast energy metabolism is a promising target for treating diseases characterized by high osteoclast activity, such as osteoporosis. However, the regulatory factors involved in osteoclast bioenergetic processes are still in the early stages of being fully understood. This study reveals the effects of follicle-stimulating hormone (FSH) on osteoclast energy metabolism.

METHODS

The Lyz2-Cre-Flox model selectively deletes FSH receptor (FSHR) from osteoclast precursor cells to generate Fshrf/f; Lyz2-Cre (Fshrf/f; Cre) mice. Bone quality was assessed using micro-computed tomography, histomorphometric analysis, and dual-fluorescence labeling. The in vitro assays measured oxygen consumption rate, extracellular acidification rate, pyruvate content, and mitochondrial membrane potential to determine metabolic flux. RNA-seq, LC-MS, dual-luciferase reporter assays, and chromatin immunoprecipitation (ChIP) assays were used to elucidate the underlying mechanisms.

RESULTS

FSHR deficiency in osteoclasts protected bone from resorption under normal and ovariectomized conditions. FSHR-deficient osteoclasts have reduced nicotinamide adenine dinucleotide (NAD+) levels, impairing osteoclast activity and energy metabolism. Mechanistically, FSH influenced NAD+ levels via the CREB/MDH2 axis. Treatment with FSH monoclonal antibodies rescued bone loss in OVX mice and reduced bone marrow NAD+ levels.

CONCLUSIONS

Targeting FSH may be a promising metabolic modulation strategy for treating osteoporosis and other diseases associated with high osteoclast activity.

Exosomal non-coding RNAs in the regulation of bone metabolism homeostasis: Molecular mechanism and therapeutic potential

Bone metabolism is a dynamic balance between bone formation and absorption regulated by osteoblasts/osteoclasts. Bone metabolic disorders can lead to metabolic bone disease. Osteoporosis (OP), osteoarthritis (OA) and femoral head necrosis (ONFH) are common metabolic bone diseases. At present, the treatment of metabolic bone disease is still mainly to relieve pain and improve joint function. However, surgical treatment does not apply to the vast majority of high-risk groups, including postmenopausal women, patients with diabetes, cirrhosis, etc. Exosomes (Exos) are nanoscale membrane vesicles that are released by almost all cells. Exos are rich in a variety of bioactive substances, such as non-coding RNAs, nucleic acids, proteins and lipids. In view of the structure of Exos, it can protect the biologically active molecules can be smoothly delivered to the target cells and involved in the regulation of cell function. In this review, we focus on the regulation mechanism and function of bone homeostasis mediated by exosomal ncRNAs (Exos-ncRNAs), including macrophage polarization, autophagy, angiogenesis, signal transduction and competing endogenous RNA (ceRNA). We summarized the therapeutic strategies and potential drugs of Exos-ncRNAs in metabolic bone disease. Moreover, we discussed the shortcomings and potential research directions of Exos as carrier to deliver ncRNAs to play a role.

Functional Amino Acids in the Regulation of Bone and Its Diseases

Bone as a vigorous tissue is constantly undergoing bone remodeling. The homeostasis of bone remodeling requires combined efforts of multifarious bone cells. Amino acids (AA), known as essential components of life support, are closely related to the regulation of bone homeostasis. In recent years, the concept of functional amino acids (FAAs) has been proposed, which is defined as AA that regulate key metabolic pathways to improve health, survival, growth, development, lactation, and reproduction of organisms, to highlight their outstanding contributions in the body. In the hope of exploring new therapeutic strategies, this review focus on summarizing recent progress in the vital role of FAAs in bone homeostasis maintaining and potential implications of FAAs in bone-related diseases, and discussing related mechanisms. The results showed that FAAs are closely related to bone metabolism and therapeutic strategy targeting FAAs metabolism is one of the future trends for bone disorders, while the explorations about possible impact of FAAs-based diets are still limited.

Bone: A Neglected Endocrine Organ?

Bone has traditionally been viewed in the context of its structural contribution to the human body. Foremost providing necessary support for mobility, its roles in supporting calcium homeostasis and blood cell production are often afterthoughts. Recent research has further shed light on the ever-multifaceted role of bone and its importance not only for structure, but also as a complex endocrine organ producing hormones responsible for the autoregulation of bone metabolism. Osteocalcin is one of the most important substances produced in bone tissue. Osteocalcin in circulation increases insulin secretion and sensitivity, lowers blood glucose, and decreases visceral adipose tissue. In males, it has also been shown to enhance testosterone production by the testes. Neuropeptide Y is produced by various cell types including osteocytes and osteoblasts, and there is evidence suggesting that peripheral NPY is important for regulation of bone formation. Hormonal disorders are often associated with abnormal levels of bone turnover markers. These include commonly used bone formation markers (bone alkaline phosphatase, osteocalcin, and procollagen I N-propeptide) and commonly used resorption markers (serum C-telopeptides of type I collagen, urinary N-telopeptides of type I collagen, and tartrate-resistant acid phosphatase type 5b). Bone, however, is not exclusively comprised of osseous tissue. Bone marrow adipose tissue, an endocrine organ often compared to visceral adipose tissue, is found between trabecula in the bone cortex. It secretes a diverse range of hormones, lipid species, cytokines, and other factors to exert diverse local and systemic effects.