Role of GPRC6A in Regulating Hepatic Energy Metabolism in Mice

Osteocalcin: Beyond Bones

Apart from basic roles such as supporting the body, protecting internal organs, and storing calcium, the skeletal system also performs hormonal functions. In recent years, several reports have been published on proteins secreted by bones and their impact on the homeostasis of the entire body. These proteins include fibroblast growth factor 23, sclerostin, lipocalin 2, and osteocalcin. Osteocalcin, the most abundant non-collagenous protein in bone tissue, is routinely measured as a clinical marker for diagnosing bone metabolism disorders. Its molecule undergoes numerous transformations, with decarboxylation being the critical process. Decarboxylation occurs in the acidic environment typical of bone resorption, facilitating the release of the molecule into the bloodstream and enabling its hormonal action. Decarboxylated osteocalcin promotes insulin secretion and stimulates the proliferation of pancreatic islet β-cells. It also plays a role in reducing the accumulation of visceral fat and decreasing fat storage in the liver. Furthermore, decarboxylated osteocalcin levels are inversely correlated with fasting serum glucose levels, total body fat, visceral fat area, and body mass index. Apart from its role in energy metabolism, osteocalcin affects testosterone production and the synthesis of glucagon-like peptide-1. It is also actively involved in muscle-bone crosstalk and influences cognitive function.

Osteocalcin of maternal and embryonic origins synergize to establish homeostasis in offspring

Many physiological osteocalcin-regulated functions are affected in adult offspring of mothers experiencing unhealthy pregnancy. Furthermore, osteocalcin signaling during gestation influences cognition and adrenal steroidogenesis in adult mice. Together these observations suggest that osteocalcin may broadly function during pregnancy to determine organismal homeostasis in adult mammals. To test this hypothesis, we analyzed in unchallenged wildtype and Osteocalcin-deficient, newborn and adult mice of various genotypes and origin maintained on different genetic backgrounds, the functions of osteocalcin in the pancreas, liver and testes and their molecular underpinnings. This analysis revealed that providing mothers are Osteocalcin-deficient, Osteocalcin haploinsufficiency in embryos hampers insulin secretion, liver gluconeogenesis, glucose homeostasis, testes steroidogenesis in adult offspring; inhibits cell proliferation in developing pancreatic islets and testes; and disrupts distinct programs of gene expression in these organs and in the brain. This study indicates that osteocalcin exerts dominant functions in most organs it influences. Furthermore, through their synergistic regulation of multiple physiological functions, osteocalcin of maternal and embryonic origins contributes to the establishment and maintenance of organismal homeostasis in newborn and adult offspring.

Osteocalcin of maternal and embryonic origins synergize to establish homeostasis in offspring

Many physiological functions regulated by osteocalcin are affected in adult offspring of mothers experiencing an unhealthy pregnancy. Furthermore, osteocalcin signaling during gestation influences cognition and adrenal steroidogenesis in adult mice. Together these observations suggest that osteocalcin functions during pregnancy may be a broader determinant of organismal homeostasis in adult mammals than previously thought. To test this hypothesis, we analyzed in unchallenged wildtype and Osteocalcin -deficient, newborn, and adult mice of various genotypes and origin, and that were maintained on different genetic backgrounds, the functions of osteocalcin in the pancreas, liver and testes and their molecular underpinnings. This analysis revealed that providing mothers are themselves Osteocalcin -deficient, Osteocalcin haploinsufficiency in embryos hampers insulin secretion, liver gluconeogenesis, glucose homeostasis, testes steroidogenesis in adult offspring; inhibits cell proliferation in developing pancreatic islets and testes; and disrupts distinct programs of gene expression in these organs and in the brain. This study indicates that through their synergistic regulation of multiple physiological functions, osteocalcin ofmaternal and embryonic origins contributes to the establishment and maintenance of organismal homeostasis in newborn and adult offspring.

Untargeted Metabolomics Reveals the Function of GPRC6A in Amino Acid and Lipid Metabolism in Mice

GPRC6A is an amino acid sensor in the cytomembrane. Despite substantial evidence for the role of GPRC6A in metabolism, the specific effects and mechanism by which this gene acts on metabolic processes are still unresolved. In this study, serum biochemical parameters related to liver and kidney function and serum amino acid levels were determined in GPRC6A wild-type (WT) and knockout (KO) mice. An untargeted serum metabolomics analysis was also conducted for the first time, to the best of our knowledge, to decipher the function of GPRC6A in metabolic processes. GPRC6A was involved in lipid and amino acid metabolism, mainly by affecting liver function. A loss of GPRC6A function may perturb bile acid metabolism, thus leading to abnormal unsaturated fatty acid metabolism. GPRC6A KO may lead to excessive protein breakdown under starvation, and the loss of GPRC6A had a significant effect on phenylalanine metabolism-related pathways. Our metabolomics data provide a novel basis for further functional studies of GPRC6A.

GPRC6A Mediates Glucose and Amino Acid Homeostasis in Mice

GPRC6A, an important member of the G-protein-coupled receptor superfamily, has been widely studied in body health maintenance and related diseases. However, it is still controversial whether GPRC6A plays a vital role in glucose homeostasis, and the role of GPRC6A on amino acid homeostasis has not been reported. In this study, GPRC6A was knocked out in C57BL6 mice, and we found that GPRC6A plays an important role in the glucose metabolism, mainly affecting the glucose clearance capacity and gluconeogenesis in mice. GPRC6A plays an important role in maintaining amino acid homeostasis under dietary restrictions, and this may be realized by participating in the regulation of autophagy. Since a large amount of amino acid is lost from urine in aged GPRC6A^−/− mice, it is possible that GPRC6A regulates amino acid homeostasis by affecting the integrity of tissue structure. GPRC6A is involved in the regulation of mTORC1 activation but is not necessary for mTORC1 activation under sufficient nutritional supply. In the absence of exogenous amino acids, the loss of GPRC6A induces the GCN2 pathway activation and excessive autophagy of cells, leading to the overactivation of mTORC1, which may be detrimental to body health and cell survival. In summary, this study provides a theoretical and experimental basis for the metabolic process of GPRC6A in body growth and health.

Quercetin Attenuates Osteoporosis in Orchiectomy Mice by Regulating Glucose and Lipid Metabolism via the GPRC6A/AMPK/mTOR Signaling Pathway

Quercetin, a flavonoid found in natural medicines, has shown a role in disease prevention and health promotion. Moreover, because of its recently identified contribution in regulating bone homeostasis, quercetin may be considered a promising agent for improving bone health. This study aimed to elucidate the role of quercetin in androgen deprivation therapy-induced osteoporosis in mice. C57BL/6 mice were subjected to orchiectomy, followed by quercetin treatment (75 and 150 mg/kg/d) for 8 weeks. Bone microstructure was then assessed by micro-computed tomography, and a three-point bending test was used to evaluate the biomechanical parameters. Hematoxylin and eosin (H&E) staining was used to examine the shape of the distal femur, gastrocnemius muscle, and liver. The balance motion ability in mice was evaluated by gait analysis, and changes in the gastrocnemius muscle were observed via Oil red O and Masson's staining. ELISA and biochemical analyses were used to assess markers of the bone, glucose, and lipid metabolism. Western blotting analyses of glucose and lipid metabolism-related protein expression was performed, and expression of the GPCR6A/AMPK/mTOR signaling pathway-related proteins was also assessed. After 8 weeks of quercetin intervention, quercetin-treated mice showed increased bone mass, bone strength, and improved bone microstructure. Additionally, gait analysis, including stride length and frequency, were significantly increased, whereas a reduction of the stride length and gait symmetry was observed. H&E staining of the gastrocnemius muscle showed that the cross-sectional area of the myofibers had increased significantly, suggesting that quercetin improves balance, motion ability, and muscle mass. Bone metabolism improvement was defined by a reduction of serum levels of insulin, triglycerides, total cholesterol, and low-density lipoprotein, whereas levels of insulin-like growth factor-1 and high-density lipoprotein were increased after quercetin treatment. Expression of proteins involved in glucose uptake was increased, whereas that of proteins involved in lipid production was decreased. Moreover, the GPRC6A and the phospho-AMPK/AMPK expression ratio was elevated in the liver and tibia tissues. In contrast, the phospho-mTOR/mTOR ratio was reduced in the quercetin group. Our findings indicate that quercetin can reduce the osteoporosis induced by testosterone deficiency, and its beneficial effects might be associated with the regulation of glucose metabolism and inhibition of lipid metabolism via the GPCR6A/AMPK/mTOR signaling pathway.

Osteocalcin and the Physiology of Danger

Bone biology has long been driven by the question as to what molecules affect cell differentiation or the functions of bone. Exploring this issue has been an extraordinarily powerful way to improve our knowledge of bone development and physiology. More recently, a second question has emerged: does bone have other functions besides making bone? Addressing this conundrum revealed that the bone-derived hormone osteocalcin affects a surprisingly large number of organs and physiological processes, including acute stress response. This review will focus on this emerging aspect of bone biology taking osteocalcin as a case study and will show how classical and endocrine functions of bone help to define a new functional identity for this tissue.

Hepatic glycogenolysis is determined by maternal high-calorie diet via methylation of Pygl and it is modified by oteocalcin administration in mice

Objective

Accumulating evidence indicates that an adverse perinatal environment contributes to a higher risk of metabolic disorders in the later life of the offspring. However, the underlying molecular mechanisms remain largely unknown. Thus, we investigated the contribution of maternal high-calorie diet and osteocalcin to metabolic homeostasis in the offspring.

Methods

Eight-week-old C57Bl/6N female mice were mated with age-matched males and allocated randomly to three groups: a normal-diet (ND) or a high-fat, high-sucrose diet group, which was administered either saline (control) or GluOC (10 ng/g body mass) from the day of mating to that of delivery, and the dams were fed a ND after the delivery. Pups weaned at 24 days after birth were analyzed.

Results

A maternal high-fat, high-sucrose diet during pregnancy causes metabolic disorders in the liver of the offspring via hypermethylation of the Pygl gene, encoding glycogen phosphorylase L, which mediates hepatic glycogenolysis. The reduced expression of Pygl induced by the maternal diet causes the hepatic accumulation of glycogen and triglyceride in the offspring, which remains in adulthood. In addition, the administration of uncarboxylated osteocalcin during pregnancy upregulates Pygl expression via both direct CREBH and ATF4 and indirect epigenomic pathways, mitigating the maternal diet-induced obesity and abnormal glucose and lipid metabolism in adulthood.

Conclusions

We propose that maternal energy status is reflected in the hepatic glycogenolysis capacity of the offspring via epigenetic modification of Pygl and uncarboxylated osteocalcin regulates glycogenolysis.

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A high-calorie diet during pregnancy causes metabolic disorders in mouse offspring.

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These are mediated by low liver expression of Pygl encoding glycogen phosphorylase.

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Hypermethylation of the Pygl promoter in utero suppresses subsequent gene expression.

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Modification and phenotypic changes are prevented by GluOC administration during pregnancy.

Explaining Divergent Observations Regarding Osteocalcin/GPRC6A Endocrine Signaling

A new schema proposes that the bone-derived osteocalcin (Ocn) peptide hormone activates the G-protein-coupled receptor GPRC6A to directly regulate glucose and fat metabolism in liver, muscle, and fat, and to stimulate the release of metabolism-regulating hormones, including insulin, fibroblast growth factor 21, glucagon-like peptide 1, testosterone, and interleukin 6. Ocn/GPRC6A activation has also been implicated in cancer progression. GPRC6A is activated by cations, amino acids, and testosterone. The multiligand specificity, the regulation of energy metabolism in diverse tissues, and the coordinated release of metabolically active hormones make the GPRC6A endocrine networks unique. Recently, the significance of Ocn/GPRCA has been questioned. There is a lack of metabolic abnormalities in newly created genetically engineered Ocn- and Gprc6a-deficient mouse models. There are also paradoxical observations that GPRC6A may function as a tumor suppressor. In addition, discordant published studies have cast doubt on the function of the most prevalent uniquely human GPRC6A-KGKY polymorphism. Explanations for these divergent findings are elusive. We provide evidence that the metabolic susceptibility of genetically engineered Ocn- and Gprc6a-deficient mice is influenced by environmental challenges and genetic differences in mouse strains. In addition, the GPRC6A-KGKY polymorphism appears to be a gain-of-function variant. Finally, alternatively spliced isoforms of GPRC6A may alter ligand specificity and signaling that modulate oncogenic effects. Thus, genetic, post-translational and environmental factors likely account for the variable results regarding the functions of GPRC6A in animal models. Pending additional information, GPRC6A should remain a potential therapeutic target for regulating energy and fat metabolism, hormone production, and cancer progression.

Osteocalcin and vascular function: is there a cross-talk?

Background

The bone-derived protein osteocalcin (OC), in its undercarboxylated (ucOC) form, has a beneficial effect on energy metabolism and may be a future therapeutic target for metabolic diseases. Increasing evidence suggests a link between ucOC and cardiovascular disease (CVD) development; however, the exact relationship is conflicting and unclear.

Scope of review

The aim of this review was to summarise the current research examining the interaction between OC and vascular dysfunction, the initiating stage in the development of atherosclerosis and CVD.

Major conclusions

In humans, the association between OC and vascular function is inconsistent. Several studies report that total OC (tOC) is associated with adverse function or beneficial function, whereas others report that tOC and ucOC has no effect on vascular function. The conflicting data are likely due to several methodological inconsistencies, in particular the lack of studies reporting circulating ucOC levels. In animal models, the direct administration of ucOC to isolated blood vessels ex vivo produced minimal changes in endothelial function, but importantly, no adverse responses. Finally, in human endothelial and vascular smooth muscle cells, ucOC treatment did not influence classical markers of cellular function, including endothelin-1, vascular adhesion molecule-1 and monocyte chemoattractant protein-1 after exposure to high glucose and inflammatory conditions. The lack of adverse effects in ex vivo and in vitro studies suggests that ucOC may be targeted as a future therapeutic for metabolic diseases, without the risk of detrimental effects in the vasculature. However, further studies are needed to confirm these findings and to investigate whether there is a direct beneficial influence of ucOC.

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ucOC is implicated in the regulation of glucose homeostasis; but its role in the vasculature has been minimally reported.

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Studies which examine the association between ucOC and vascular function in humans often report inconsistent outcomes.

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In addition, ex vivo and in vitro studies have reported that ucOC likely does not directly regulate endothelial function.

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ucOC may be targeted as a therapeutic treatment for metabolic diseases without a risk of adverse effects in the vasculature.

G protein-coupled receptors as potential targets for nonalcoholic fatty liver disease treatment

Nonalcoholic fatty liver disease (NAFLD) is a broad-spectrum disease, ranging from simple hepatic steatosis to nonalcoholic steatohepatitis, which can progress to cirrhosis and liver cancer. Abnormal hepatic lipid accumulation is the major manifestation of this disease, and lipotoxicity promotes NAFLD progression. In addition, intermediate metabolites such as succinate can stimulate the activation of hepatic stellate cells to produce extracellular matrix proteins, resulting in progression of NAFLD to fibrosis and even cirrhosis. G protein-coupled receptors (GPCRs) have been shown to play essential roles in metabolic disorders, such as NAFLD and obesity, through their function as receptors for bile acids and free fatty acids. In addition, GPCRs link gut microbiota-mediated connections in a variety of diseases, such as intestinal diseases, hepatic steatosis, diabetes, and cardiovascular diseases. The latest findings show that gut microbiota-derived acetate contributes to liver lipogenesis by converting dietary fructose into hepatic acetyl-CoA and fatty acids. GPCR agonists, including peptides and natural products like docosahexaenoic acid, have been applied to investigate their role in liver diseases. Therapies such as probiotics and GPCR agonists may be applied to modulate GPCR function to ameliorate liver metabolism syndrome. This review summarizes the current findings regarding the role of GPCRs in the development and progression of NAFLD and describes some preclinical and clinical studies of GPCR-mediated treatment. Overall, understanding GPCR-mediated signaling in liver disease may provide new therapeutic options for NAFLD.

Adipocyte-specific GPRC6A ablation promotes diet-induced obesity by inhibiting lipolysis

The G protein–coupled receptor GPRC6A regulates various physiological processes in response to its interaction with multiple ligands, such as extracellular basic amino acids, divalent cations, testosterone, and the uncarboxylated form of osteocalcin (GluOC). Global ablation of GPRC6A increases the susceptibility of mice to diet-induced obesity and related metabolic disorders. However, given that GPRC6A is expressed in many tissues and responds to a variety of hormonal and nutritional signals, the cellular and molecular mechanisms underlying the development of metabolic disorders in conventional knockout mice have remained unclear. On the basis of our previous observation that long-term oral administration of GluOC markedly reduced adipocyte size and improved glucose tolerance in WT mice, we examined whether GPRC6A signaling in adipose tissue might be responsible for prevention of metabolic disorders. We thus generated adipocyte-specific GPRC6A knockout mice, and we found that these animals manifested increased adipose tissue weight, adipocyte hypertrophy, and adipose tissue inflammation when fed a high-fat and high-sucrose diet compared with control mice. These effects were associated with reduced lipolytic activity because of downregulation of lipolytic enzymes such as adipose triglyceride lipase and hormone-sensitive lipase in adipose tissue of the conditional knockout mice. Given that, among GPR6CA ligands tested, GluOC and ornithine increased the expression of adipose triglyceride lipase in cultured 3T3-L1 adipocytes in a manner dependent on GPRC6A, our results suggest that the constitutive activation of GPRC6A signaling in adipocytes by GluOC or ornithine plays a key role in adipose lipid handling and the prevention of obesity and related metabolic disorders.

The carboxylation status of osteocalcin has important consequences for its structure and dynamics

BACKGROUND

The carboxylation status of Osteocalcin (Ocn) not only influences formation and structure in bones but also has important endocrine functions affecting energy metabolism and expenditure. In this study, the role of γ-carboxylation of the glutamate residues in the structure-dynamics-function relationship in Ocn is investigated.

METHODS

Three forms of Ocn, differentially carboxylated at the Glu-17, 21 and 24 residues, along with a mutated form of Ocn carrying Glu/Ala mutations, are modeled and simulated using molecular dynamics (MD) simulation in the presence of calcium ions.

RESULTS

Characterization of the global conformational dynamics of Ocn, described in terms of the orientational variations within its 3-helical domain, highlights large structural variations in the non-carboxylated osteocalcin (nOcn). The bi-carboxylated Ocn (bOcn) and tri-carboxylated (tOcn) species, in contrast, display relatively rigid tertiary structures, with the dynamics of most regions strongly correlated. Radial distribution functions calculated for both bOcn and tOcn show long-range ordering of the calcium ion distribution around the carboxylated glutamate (γGlu) residues, likely playing an important role in promoting stability of these Ocns. Additionally, the same calcium ions are observed to coordinate with neighboring γGlu, better shielding their negative charges and in turn stabilizing these systems more than do the singly coordinating calcium ions observed in the case of nOcn. bOcn is also found to exhibit a more helical C-terminal structure, that has been shown to activate its cellular receptor GPRC6A, highlighting the allosteric role of Ocn carboxylation in modulating the stability and binding potential of the active C-terminal.

CONCLUSIONS

The carboxylation status of Ocn as well and its calcium coordination appear to have a direct influence on Ocn structure and dynamics, possibly leading to the known differences in Ocn biological function.

GENERAL SIGNIFICANCE

Modification of Ocn sequence or its carboxylation state may provide the blueprint for developing high-affinity peptides targeting its cellular receptor GPRC6A, with therapeutic potential for treatment of metabolic disorders.