Uncarboxylated Osteocalcin Enhances Glucose Uptake Ex Vivo in Insulin-Stimulated Mouse Oxidative But Not Glycolytic Muscle

Osteocalcin: A Multifaceted Bone-Derived Hormone

Together, loss- and gain-of-function experiments have identified the bone-derived secreted molecule osteocalcin as a hormone with a broad reach in rodents and primates. Following its binding to one of three receptors, osteocalcin exerts a profound influence on various aspects of energy metabolism as well as steroidogenesis, neurotransmitter biosynthesis and thereby male fertility, electrolyte homeostasis, cognition, the acute stress response, and exercise capacity. Although this review focuses mostly on the regulation of energy metabolism by osteocalcin, it also touches on its other functions. Lastly, it proposes what could be a common theme between the functions of osteocalcin and between these functions and the structural functions of bone.

Prior aerobic exercise mitigates the decrease in serum osteoglycin and lipocalin-2 following high-glucose mixed-nutrient meal ingestion in young men

Osteoglycin (OGN) and lipocalin-2 (LCN2) are hormones that can be secreted by bone and have been linked to glucose homeostasis in rodents. However, the endocrine role of these hormones in humans is contradictory and unclear. We examined the effects of exercise and meal ingestion on circulating serum OGN and LCN2 levels in eight healthy males {age: 28 [25, 30] years [median ± interquartile range (IQR)] and body mass index [BMI]: 24.3 [23.6, 25.5] kg/m²}. In a randomized crossover design, participants ingested a high-glucose (1.1 g glucose/kg body wt) mixed-nutrient meal (45% carbohydrate, 20% protein, and 35% fat) on a rest-control day and 3 and 24 h after aerobic cycling exercise (1 h at 70%-75% V̇o_2peak). Acute aerobic exercise increased serum LCN2 levels immediately after exercise (∼61%), which remained elevated 3-h postexercise (∼55%). In contrast, serum OGN remained similar to baseline levels throughout the 3-h postexercise recovery period. The ingestion of a high-glucose mixed-nutrient meal led to a decrease in serum OGN at 90-min (approximately -17%) and 120-min postprandial (approximately -44%), and a decrease in LCN2 at 120-min postprandial (approximately -26%). Compared with the control meal, prior exercise elevated serum OGN and LCN2 levels at 120-min postprandial when the meal was ingested 3-h (OGN: ∼74% and LCN2: ∼68%) and 24-h postexercise (OGN: ∼56% and LCN2: ∼16%). Acute exercise increases serum LCN2 and attenuates the postprandial decrease in OGN and LCN2 following high-glucose mixed-nutrient meal ingestion. The potential endocrine role of circulating OGN and LCN2 in humans warrants further investigation.NEW & NOTEWORTHY We provide novel evidence that OGN and LCN2 decrease 120 min after ingesting a high-glucose mixed-nutrient meal in healthy adults. Acute aerobic exercise increases circulating LCN2 for up to 3-h postexercise, whereas circulating OGN remains similar to baseline. Despite differing postexercise responses, postprandial LCN2 and OGN are elevated when the high-glucose meal is ingested 3-h and 24-h postexercise. Findings support that OGN and LCN2 are dynamically linked to energy homeostasis in humans.

Improvement of Lipotoxicity-Induced Islet β Cellular Insulin Secretion Disorder by Osteocalcin

BACKGROUND

Osteocalcin (OCN) has been proved to be closely related with the development of type 2 diabetes mellitus (T2DM). We aimed to study if OCN could improve the disorder of islet cell caused by lipotoxicity.

METHODS

Alizarin red staining was used to investigate the mineralization. Western blotting and ELISA methods were used to measure protein expression. Immunofluorescence staining was used to investigate the protein nuclear transfer.

RESULTS

High glucose and high fat inhibited the differentiation of osteoblast precursors. Overexpression of insulin receptor (InsR^OE) significantly promoted the Runx2 and OCN expression. The increase of insulin, Gprc6a, and Glut2 by osteoblast culture medium overexpressing insulin receptor was reversed by osteocalcin neutralizing antibody. Undercarboxylated osteocalcin (ucOC) suppressed the lipotoxic islet β-cell damage caused by palmitic acid. The FOXO1 from intranuclear to extranuclear was also significantly increased after ucOC treatment compared with the group PA. Knockdown of Gprc6a or suppression of PI3K/AKT signal pathway could reverse the upregulation of GPRC6A/PI3K/AKT/FoxO1/Pdx1 caused by ucOC.

CONCLUSION

OCN could activate the FOXO1 signaling pathway to regulate GLUT2 expression and improve the insulin secretion disorder caused by lipotoxicity.

Effects of ergosteroside combined risedronate on fracture healing and BMP-2, BMP-7 and VEGF expression in rats

Purpose

To evaluate the effect of ergosterol combined with risedronate on fracture healing.

Methods

Sixty male Sprague Dawley fracture model rats were assigned into group A (n=20), group B (n=20), and group C (n=20) at random. All rats were fed by gavage until their sacrifice as it follows: group A with ergosteroside and risedronate, group B with risedronate, and group C with saline solution. At weeks 2 and 4, 10 rats of each group were sacrificed. Healing effect and bone tissue changes in the fractures site were assessed by using hematoxylin and eosin stain histology. Enzyme-linked immunosorbent assay was used to detect the expression of serum bone morphogenetic protein-2 (BMP-2), bone morphogenetic protein-7 (BMP-7), and vascular endothelial growth factor (VEGF). Reverse transcriptase polymerase chain reaction was applied to detect the expression of osteoprotegerin (OPG) mRNA, osteocalcin (OCN) mRNA and core-binding factor subunit-?1 (CBF-?1) mRNA.

Results

In terms of serum BMP-2, BMP-7, and VEGF expression at weeks 2 and 4 after gavage, group A < group B < group C (P<0.05). At week 4 after gavage, serum VEGF expression in the three groups harbored positive relationship with serum BMP-2 and BMP-7 expression (P<0.05). Regarding serum OPG, OCN and CBF-?1 mRNA expression at weeks 2 and 4 after gavage, group A <group B <group C (P<0.05). Hematoxylin and eosin staining results showed that the recovery effect of trabecular bone and callus in the cases of group A was better than the other two groups after intragastric administration.

Conclusion

Ergosteroside combined risedronate can patently ameliorate the healing effect of fracture in rats.

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.

Decarboxylated osteocalcin, a possible drug for type 2 diabetes, triggers glucose uptake in MG63 cells

BACKGROUND

Uncarboxylated osteocalcin (GluOC) has been reported to improve glucose metabolism, prevent type 2 diabetes, and decrease the severity of obesity in mice with type 2 diabetes. GluOC can increase glucose uptake in a variety of cells. Glucose metabolism is the main source of energy for osteoblast proliferation and differentiation. We hypothesized that decarboxylated osteocalcin (dcOC), a kind of GluOC, can increase glucose uptake in MG63 cells (osteoblast-like osteosarcoma cells) and influence their proliferation and differentiation.

AIM

To investigate the effects of dcOC on glucose uptake in human osteoblast-like osteosarcoma cells and the possible signaling pathways involved.

METHODS

MG63 cells (human osteoblast-like osteosarcoma cells) were treated with dcOC (0, 0.3, 3, 10, or 30 ng/mL) for 1 and 72 h, and glucose uptake was measured by flow cytometry. The effect of dcOC on cell proliferation was measured with a CCK-8 assay, and alkaline phosphatase (ALP) enzyme activity was measured. PI3K was inhibited with LY294002, and hypoxia-inducible factor 1 alpha (HIF-1α) was silenced with siRNA. Then, GPRC6A (G protein-coupled receptor family C group 6 subtype A), total Akt, phosphorylated Akt, HIF-1α, and glucose transporter 1 (GLUT1) levels were measured by Western blot to elucidate the possible pathways by which dcOC modulates glucose uptake.

RESULTS

The glucose uptake of MG63 cells was significantly increased compared with that of the paired control cells after short-term (1 h) treatment with dcOC at different concentrations (0.3, 3, and 10 ng/mL groups, P < 0.01; 30 ng/mL group, P < 0.05). Glucose uptake of MG63 cells was significantly increased compared with that of the paired control cells after long-term (72 h) treatment with dcOC at different concentrations (0.3, 3, and 10 ng/mL groups, P < 0.01; 30 ng/mL group, P < 0.05). DcOC triggered Akt phosphorylation in a dose-dependent manner, and the most effective stimulatory concentration of dcOC for short-term (1 h) was 3 ng/mL (P < 0.01). LY294002 abolished the dcOC-mediated (1 h) promotion of Akt phosphorylation and glucose uptake without affecting GLUT1 protein expression. Long-term dcOC stimulation triggered Akt phosphorylation and increased the protein levels of HIF-1α, GLUT1, and Runx2 in a dose-dependent manner. Inhibition of HIF-1α with siRNA abolished the dcOC-mediated glucose uptake and substantially decreased GLUT1 protein expression. DcOC intervention promoted cell proliferation in a time- and dose-dependent manner as determined by the CCK-8 assay. Treatment with both 3 ng/mL and 10 ng/mL dcOC affected the ALP activity in MG63 cells after 72 h (P < 0.01).

CONCLUSION

Short- and long-term dcOC treatment can increase glucose uptake and affect proliferation and ALP activity in MG63 cells. This effect may occur through the PI3K/Akt, HIF-1α, and GLUT1 signaling factors.

Cardiac metallothionein overexpression rescues diabetic cardiomyopathy in Akt2-knockout mice

To efficiently prevent diabetic cardiomyopathy (DCM), we have explored and confirmed that metallothionein (MT) prevents DCM by attenuating oxidative stress, and increasing expression of proteins associated with glucose metabolism. To determine whether Akt2 expression is critical to MT prevention of DCM, mice with either global Akt2 gene deletion (Akt2-KO), or cardiomyocyte-specific overexpressing MT gene (MT-TG) or both combined (MT-TG/Akt2-KO) were used. Akt2-KO mice exhibited symptoms of DCM (cardiac remodelling and dysfunction), and reduced expression of glycogen and glucose metabolism-related proteins, despite an increase in total Akt (t-Akt) phosphorylation. Cardiac MT overexpression in MT-TG/Akt2-KO mice prevented DCM and restored glucose metabolism-related proteins expression and baseline t-Akt phosphorylation. Furthermore, phosphorylation of ERK1/2 increased in the heart of MT-TG/Akt2-KO mice, compared with Akt2-KO mice. As ERK1/2 has been implicated in the regulation of glucose transport and metabolism this increase could potentially underlie MT protective effect in MT-TG/Akt2-KO mice. Therefore, these results show that although our previous work has shown that MT preserving Akt2 activity is sufficient to prevent DCM, in the absence of Akt2 MT may stimulate alternative or downstream pathways protecting from DCM in a type 2 model of diabetes, and that this protection may be associated with the ERK activation pathway.

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.

The effect of exercise training on osteocalcin, adipocytokines, and insulin resistance: a systematic review and meta-analysis of randomized controlled trials

Recently, it has been reported that osteocalcin (OC), in particular its undercarboxylated (ucOC) form, is not only a bone remodeling marker but also an active hormone that intercedes glucose metabolism in humans. This study aimed to determine the impact of an exercise intervention on ucOC, adiponectin, leptin, and insulin resistance (measured by HOMA-IR). PubMed, CINAHL, Medline, Google Scholar, and Scopus databases and reference lists of included studies were searched. Twenty-two randomized controlled trials (RCTs) of exercise training impact in adults were included in the analysis. Results showed an overall significant increase in serum ucOC (MD: 0.15 ng/ml; 95% CI: 0.05 to 0.25) and adiponectin (MD: 2.83 mg/ml; 95% CI: 1.67 to 3.98), a significant decline in leptin (MD: - 4.89 pg/ml; 95% CI: - 6.94 to - 2.84), fasting glucose (MD: - 2.29 mg/dl; 95% CI: - 4.04 to - 0.54), fasting insulin (MD, - 8.90 μIU/ml; 95% CI: - 13.81 to - 3.98), and HOMA-IR (MD: - 1.96; 95% CI: - 3.11 to - 0.80). However, after removal of studies that had prescribed a balanced diet along with exercise intervention, total OC (TOC) levels also increased in the exercise group compared with the control group (MD: 0.36 ng/ml; 95% CI: 0.07 to 0.65). Our findings demonstrate that exercise-induced increases in ucOC are the probable cause of increased adiponectin. Additionally, increases in ucOC itself are probably due to changes in leptin levels and other factors, rather than its direct impact on bone and its osteoblastic activity. Further studies are required to clarify the mechanisms underlying the impact of exercise training on ucOC, adipocytokines, and insulin resistance.

Recombinant Antibodies with Unique Specificities Allow for Sensitive and Specific Detection of Uncarboxylated Osteocalcin in Human Circulation

Osteocalcin is a bone-specific protein which contains three glutamic acid residues (Glu) that undergo post-translational gamma-carboxylation. Uncarboxylated osteocalcin (ucOC) may participate in the regulation of glucose metabolism, thus measurement of ucOC could be useful in evaluating interactions between bone and glucose metabolism. We developed recombinant antibodies and immunoassay to specifically detect ucOC in human blood samples. ucOC-specific recombinant antibodies were selected from an antibody library by phage display. Four candidates were characterized, and one (Fab-AP13) was used to set up an immunoassay with a pre-existing MAb. Plasma ucOC levels were measured in subjects with normal fasting blood glucose (≤ 6 mmol/l, N = 46) or with hyperglycemia (≥ 7 mmol/l, N = 29). Further, we analyzed ucOC in age- and gender-matched patients with diagnosed type 2 diabetes (T2D, N = 49). Antibodies recognized ucOC without cross-reaction to carboxylated osteocalcin. Antibodies had unique binding sites at the carboxylation region, with Glu17 included in all epitopes. Immunoassay was set up and characterized. Immunoassay detected ucOC in serum and plasma, with on average 1.6-fold higher levels in plasma. ucOC concentrations were significantly lower in subjects with hyperglycemia (median 0.58 ng/ml, p = 0.008) or with T2D diagnosis (0.68 ng/ml, p = 0.015) than in subjects with normal blood glucose (1.01 ng/ml). ucOC negatively correlated with fasting plasma glucose in subjects without T2D (r = - 0.24, p = 0.035) but not in T2D patients (p = 0.41). Our immunoassay, based on the novel recombinant antibody, allows for specific and sensitive detection of ucOC in human circulation. Correlation between ucOC and plasma glucose suggests interactions between osteocalcin and glucose metabolism in humans.

Measurement of bioactive osteocalcin in humans using a novel immunoassay reveals association with glucose metabolism and β-cell function

Osteocalcin (OCN) is a bone-derived hormone involved in the regulation of glucose metabolism. In serum, OCN exists in carboxylated and uncarboxylated forms (ucOCN), and studies in rodents suggest that ucOCN is the bioactive form of this hormone. Whether this is also the case in humans is unclear, because a reliable assay to measure ucOCN is not available. Here, we established and validated a new immunoassay (ELISA) measuring human ucOCN and used it to determine the level of bioactive OCN in two cohorts of overweight or obese subjects, with or without type 2 diabetes (T2D). The ELISA could specifically detect ucOCN concentrations ranging from 0.037 to 1.8 ng/mL. In a first cohort of overweight or obese postmenopausal women without diabetes (n = 132), ucOCN correlated negatively with fasting glucose (r = -0.18, P = 0.042) and insulin resistance assessed by the homeostatic model assessment of insulin resistance (r = -0.18, P = 0.038) and positively with insulin sensitivity assessed by a hyperinsulinemic-euglycemic clamp (r = 0.18, P = 0.043) or insulin sensitivity index derived from an oral glucose tolerance test (r = 0.26, P = 0.003). In a second cohort of subjects with severe obesity (n = 16), ucOCN was found to be lower in subjects with T2D compared with those without T2D (2.76 ± 0.38 versus 4.52 ± 0.06 ng/mL, P = 0.009) and to negatively correlate with fasting glucose (r = -0.50, P = 0.046) and glycated hemoglobin (r = -0.57, P = 0.021). Moreover, the subjects with ucOCN levels below 3 ng/mL had a reduced insulin secretion rate during a hyperglycemic clamp (P = 0.03). In conclusion, ucOCN measured with this novel and specific assay is inversely associated with insulin resistance and β-cell dysfunction in humans.

Undercarboxylated Osteocalcin Improves Insulin-Stimulated Glucose Uptake in Muscles of Corticosterone-Treated Mice

Short-term administration of glucocorticoids (GCs) impairs muscle insulin sensitivity at least in part via the reduction of undercarboxylated osteocalcin (ucOC). However, whether ucOC treatment reverses the GC-induced muscle insulin resistance remains unclear. To test the hypothesis that ucOC directly ameliorates impaired insulin-stimulated glucose uptake (ISGU) induced by short-term GC administration in mice muscle and to identify the molecular mechanisms, mice were implanted with placebo or corticosterone (CS) slow-release pellets. Two days post-surgery, insulin-tolerance tests (ITTs) were performed. On day 3, serum was collected and extensor digitorum longus (EDL) and soleus muscles were isolated and treated ex vivo with vehicle, ucOC (30 ng/mL), insulin (60 µU/mL), or both. Circulating hormone levels, muscle glucose uptake, and muscle signaling proteins were assessed. CS administration reduced both serum osteocalcin and ucOC levels, whole-body insulin sensitivity, and muscle ISGU in EDL. Ex vivo ucOC treatment restored ISGU in CS-affected muscle, without increasing non-insulin-stimulated glucose uptake. In CS-affected EDL muscle, ucOC enhanced insulin action on phosphorylated (p-)protein kinase B (Akt)^Ser473 and the p-extracellular signal-regulated kinase isoform 2 (ERK2)^{Thr202/Tyr204} /total (t)ERK2 ratio, which correlated with ISGU. In CS-affected soleus muscle, ucOC enhanced insulin action on p-mammalian target of rapamycin (mTOR)^Ser2481 , the p-mTOR^Ser2481 /tmTOR ratio, p-Akt substrate of 160kD (AS160)^Thr642 , and p-protein kinase C (PKC) (pan)^Thr410 , which correlated with ISGU. Furthermore, p-PKC (pan)^Thr410 correlated with p-Akt^Ser473 and p-AS160^Thr642 . ucOC exerts direct insulin-sensitizing effects on CS-affected mouse muscle, likely through an enhancement in activity of key proteins involved in both insulin and ucOC signaling pathways. Furthermore, these effects are muscle type-dependent. © 2019 American Society for Bone and Mineral Research.

Activation of cancer-related and mitogen-activated protein kinase signaling pathways in human mature osteoblasts isolated from patients with type 2 diabetes

Diabetes mellitus is a disease of glucose metabolism, and it adversely affects bone metabolism and increases the risk of cancer development. Previously, we reported a method for the direct isolation of human mature osteoblasts and indicated that osteoblasts were associated with type 2 diabetes mellitus-related signaling pathways. In addition, a recent report suggested that osteoblasts are involved in glucose metabolism. Thus, we sought to examine the effects of diabetes on osteoblast signaling in vivo. We recruited eight patients with type 2 diabetes and eight non-diabetic individuals. We isolated human mature osteoblasts from the resected femoral heads during orthopaedic surgery and extracted their RNA. We compared the gene expression between the two groups by RNA microarray and pathway analyses. Microarray analysis showed significant differences in 885 of 19,463 genes between the two groups (p < 0.05), and pathway analysis revealed that pathways related to cancer and the mitogen-activated protein kinase signaling pathway were significantly activated in the diabetes group (p < 0.01). These preliminary findings suggest that diabetes affects intracellular signaling in human mature osteoblasts and that osteoblasts might not only play a key role in the regulation of bone and glucose metabolism, but might also be related to cancer metabolism. We plan to conduct further studies to examine signaling in diabetic osteoblasts and to further investigate the genes and pathways identified here.

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Compared microarray data from in vivo DM and healthy control osteoblasts

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MAPK and cancer-related signaling genes were enriched in DM osteoblasts.

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DM may increase cancer risk by activating cancer-related pathways in osteoblasts.

High maternal osteocalcin levels during pregnancy is associated with low birth weight infants: A nested case-control study in China

BACKGROUND

Low birth weight infants (LBW) are at risk of chronic diseases in later life due to the disorder of energy metabolism during pregnancy. Osteocalcin (OC) has been identified as a hormone that regulate energy metabolism. However, few studies have researched on the associations between maternal serum OC levels and low birth weight infants.

OBJECTIONS

To examine the associations between maternal serum OC concentrations and LBW.

METHODS

This was a nested case-control study involving a total of 230 pregnant women delivering LBW and 382 control pregnant women (matched for infant gender, gestational age at blood draw, region of Maternity and Child Healthcare Hospital and maternal age in 1: (1-2) ratio). One serum sample was collected from each pregnant woman at 5-35 weeks' gestation. Pregnant women were divided into 3 groups (1st, 2nd and 3rd trimester group). There were 60 and 142 and 28 pregnant women delivering LBW in the first, second and third trimester, respectively. Similarly, there were 101 and 233 and 48 controls in the first, second and third trimester, respectively. Maternal serum OC and 25(OH)D concentrations were categorized into low and high levels, the low level used as reference in analyses. Binary logistic regression model was used to compute odd radio (ORs) for LBW according to levels of maternal serum OC and 25(OH)D.

RESULTS

Compared with the subjects in low level in first trimester, LBW was two times as likely to occur among pregnancy women with high serum OC concentrations (OR = 2.04, 95%CI:1.05-3.96). After adjusted for confounding factors, a significant positive relationship still existed (adjusted ORs = 2.29, 95%CI: 1.11-4.72). In second trimester, women in high level of serum OC had nearly 1.6 times the risk of delivering LBW infants as those in the low level (OR = 1.55, 95%CI: 1.01-2.37). After adjusted for confounding factors, the ORs increased (ORs = 1.59, 95%CI:1.03-2.45). No significant associations were found between maternal serum OC levels and LBW in third trimester. In addition, there were no associations between maternal 25(OH)D concentrations and LBW during pregnancy.

CONCLUSION

High maternal serum OC levels in the first or the second trimester during pregnancy may be associated with the risk of LBW.

Undercarboxylated Osteocalcin: Experimental and Human Evidence for a Role in Glucose Homeostasis and Muscle Regulation of Insulin Sensitivity

Recent advances have indicated that osteocalcin, and in particular its undercarboxylated form (ucOC), is not only a nutritional biomarker reflective of vitamin K status and an indicator of bone health but also an active hormone that mediates glucose metabolism in experimental studies. This work has been supported by the putative identification of G protein-coupled receptor, class C, group 6, member A (GPRC6A) as a cell surface receptor for ucOC. Of note, ucOC has been associated with diabetes and with cardiovascular risk in epidemiological studies, consistent with a pathophysiological role for ucOC in vivo. Limitations of existing knowledge include uncertainty regarding the underlying mechanisms by which ucOC interacts with GPRC6A to modulate metabolic and cardiovascular outcomes, technical issues with commonly used assays for ucOC in serum, and a paucity of clinical trials to prove causation and illuminate the scope for novel health interventions. A key emerging area of research is the role of ucOC in relation to expression of GPRC6A in muscle, and whether exercise interventions may modulate metabolic outcomes favorably in part via ucOC. Further research is warranted to clarify potential direct and indirect roles for ucOC in human health and cardiometabolic diseases.