Osteocalcin and Sex Hormone Binding Globulin Compete on a Specific Binding Site of GPRC6A

The dark sides of the GPCR tree - research progress on understudied GPCRs

A large portion of the human GPCRome is still in the dark and understudied, consisting even of entire subfamilies of GPCRs such as odorant receptors, class A and C orphans, adhesion GPCRs, Frizzleds and taste receptors. However, it is undeniable that these GPCRs bring an untapped therapeutic potential that should be explored further. Open questions on these GPCRs span diverse topics such as deorphanisation, the development of tool compounds and tools for studying these GPCRs, as well as understanding basic signalling mechanisms. This review gives an overview of the current state of knowledge for each of the diverse subfamilies of understudied receptors regarding their physiological relevance, molecular mechanisms, endogenous ligands and pharmacological tools. Furthermore, it identifies some of the largest knowledge gaps that should be addressed in the foreseeable future and lists some general strategies that might be helpful in this process.

'Toxic Masculinity': What Is Known about the Role of Androgen Receptors in Head and Neck Squamous Cell Carcinoma

Head and neck squamous cell carcinoma (HNSCC), the most prevalent cancer in the head and neck region, develops from the mucosal epithelium of the upper aerodigestive tract. Its development directly correlates with alcohol and/or tobacco consumption and infection with human papillomavirus. Interestingly, the relative risk for HNSCC is up to five times higher in males, so it is considered that the endocrine microenvironment is another risk factor. A gender-specific risk for HNSCC suggests either the existence of specific risk factors that affect only males or that females have defensive hormonal and metabolic features. In this review, we summarized the current knowledge about the role of both nuclear and membrane androgen receptors (nAR and mARs, respectively) in HNSCC. As expected, the significance of nAR is much better known; it was shown that increased nAR expression was observed in HNSCC, while treatment with dihydrotestosterone increased proliferation, migration, and invasion of HNSCC cells. For only three out of five currently known mARs-TRPM8, CaV1.2, and OXER1-it was shown either their increased expression in various types of HNSCC or that their increased activity enhanced the migration and invasion of HNSCC cells. The primary treatments for HNSCC are surgery and radiotherapy, but targeted immunotherapies are on the rise. On the other hand, given the evidence of elevated nAR expression in HNSCC, this receptor represents a potential target for antiandrogen therapy. Moreover, there is still plenty of room for further examination of mARs' role in HNSCC diagnosis, prognosis, and treatment.

Male subclinical hypogonadism and late-onset hypergonadotrophic hypogonadism: mechanisms, endothelial function, and interplay between reproductive hormones, undercarboxylated osteocalcin, and endothelial dysfunction

BACKGROUND

Pathogenesis and endothelial function in subclinical hypogonadism (SCH) remain unclear. Undercarboxylated osteocalcin (ucOC) participates in atherosclerosis and reproduction. We explored the underlying mechanisms and interplay of endothelial dysfunction, unOC and reproductive hormones in SCH and primary late-onset hypogonadism (LOH).

METHODS

In the SCH, LOH, and healthy eugonadal male groups, we measured serum unOC, calculated luteinizing hormone/testosterone (LH/T), LH.T product, and estradiol/T (E/T) as indicators of impaired Leydig cells, androgen sensitivity index (ASI), and aromatase activity, respectively (LH set-point regulators), and assessed flow-mediated dilation of the brachial artery (FMD%), carotid-intima media thickness (CIMT), and aortic stiffness (AS).

RESULTS

↑LH/T, ↑ASI, ↓aromatase activity, normal T, follicle-stimulating hormone (FSH) and sex hormone-binding globulin (SHBG) levels, ↑unOC, and enhanced atherosclerotic markers (↓FMD%, ↑CIMT, ↑AS) are characteristics of SCH. Testosterone was positively correlated with FMD% in SCH. The independent predictors were: SHBG and LH for FMD% and CIMT, respectively, and LH/T, ucOC, FSH, estradiol, and E/T ratio for AS in the LOH group; and LH for FMD% & AS and LH and LH/T for CIMT in all study subjects.

CONCLUSIONS

SCH is a distinct clinical entity characterized by impaired androgen sensitivity and aromatase activity, compensatory elevated unOC, endothelial dysfunction, and anti-atherogenic role of testosterone.

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.

The Interface of Nuclear and Membrane Steroid Signaling

Steroid hormones bind receptors in the cell nucleus and in the cell membrane. The most widely studied class of steroid hormone receptors are the nuclear receptors, named for their function as ligand-dependent transcription factors in the cell nucleus. Nuclear receptors, such as estrogen receptor alpha, can also be anchored to the plasma membrane, where they respond to steroids by activating signaling pathways independent of their function as transcription factors. Steroids can also bind integral membrane proteins, such as the G protein-coupled estrogen receptor. Membrane estrogen and progestin receptors have been cloned and characterized in vitro and influence the development and function of many organ systems. Membrane androgen receptors were cloned and characterized in vitro, but their function as androgen receptors in vivo is unresolved. We review the identity and function of membrane proteins that bind estrogens, progestins, and androgens. We discuss evidence that membrane glucocorticoid and mineralocorticoid receptors exist, and whether glucocorticoid and mineralocorticoid nuclear receptors act at the cell membrane. In many cases, integral membrane steroid receptors act independently of nuclear steroid receptors, even though they may share a ligand.

Effects of bovine cancellous bone powder/poly amino acid composites on cellular behaviors and osteogenic performances

Xenogeneic bone has good biological activity, but eliminating immunogenicity, while retaining osteogenic abilities, is a challenge. By combining xenogeneic bone with poly amino acid (PAA) that has an amide bond structure, a new type of composite conforming to bionics and low immunogenicity may be obtained. In this study, according to the principles of component bionics, three composites of delipidized cancellous bone powder (DCBP) and PAA were designed and obtained by anin situpolycondensation method, an extrusion molding (EM) method, and a solution-blend method. The three composites were all macroscopically uniform, non-cytotoxic, and demonstrated low immunogenicity by effective removal of residual antigens during preparation. Compared with PAA, mouse bone marrow mesenchymal stem cells (BMSCs) on the surfaces of three composites showed different cellular morphologies. The effects of different preparation methods and cellular morphology on cellular differentiation were confirmed by alkaline phosphatase activity, calcium nodule formation and the expression levels of osteogenic differentiation-related genes (bone morphogenetic protein 2, runt-related transcription factor 2, osteopontin and osteocalcin). Among these composites, DCBP/PAA EM showed best cell proliferation and osteogenic differentiationin vitro, and possessed greater bone formation than PAA in a rabbit femoral condyle study. This study may provide a new method for preparing bioactive bone repair materials with low immunogenicity and superior ability to stimulate differentiation of BMSCsin vitroand osteogenesisin vivo. DCBP/PAA EM might be a promising bone repair material for bone defect treatment.

Bone-Derived Modulators That Regulate Brain Function: Emerging Therapeutic Targets for Neurological Disorders

Bone has traditionally been regarded as a structural organ that supports and protects the various organs of the body. Recent studies suggest that bone also acts as an endocrine organ to regulate whole-body metabolism. Particularly, homeostasis of the bone is shown to be necessary for brain development and function. Abnormal bone metabolism is associated with the onset and progression of neurological disorders. Recently, multiple bone-derived modulators have been shown to participate in brain function and neurological disorders, including osteocalcin, lipocalin 2, and osteopontin, as have bone marrow-derived cells such as mesenchymal stem cells, hematopoietic stem cells, and microglia-like cells. This review summarizes current findings regarding the roles of these bone-derived modulators in the brain, and also follows their involvement in the pathogenesis of neurological disorders. The content of this review may aide in the development of promising therapeutic strategies for neurological disorders via targeting bone.

The half-life of the bone-derived hormone osteocalcin is regulated through O-glycosylation in mice, but not in humans

Osteocalcin (OCN) is an osteoblast-derived hormone with pleiotropic physiological functions. Like many peptide hormones, OCN is subjected to post-translational modifications (PTMs) which control its activity. Here, we uncover O-glycosylation as a novel PTM present on mouse OCN and occurring on a single serine (S8) independently of its carboxylation and endoproteolysis, two other PTMs regulating this hormone. We also show that O-glycosylation increases OCN half-life in plasma ex vivo and in the circulation in vivo. Remarkably, in human OCN (hOCN), the residue corresponding to S8 is a tyrosine (Y12), which is not O-glycosylated. Yet, the Y12S mutation is sufficient to O-glycosylate hOCN and to increase its half-life in plasma compared to wildtype hOCN. These findings reveal an important species difference in OCN regulation, which may explain why serum concentrations of OCN are higher in mouse than in human.

Bones provide support and protection for organs in the body. However, over the last 15 years researchers have discovered that bones also release chemicals known as hormones, which can travel to other parts of the body and cause an effect. The cells responsible for making bone, known as osteoblasts, produce a hormone called osteocalcin which communicates with a number of different organs, including the pancreas and brain.

When osteocalcin reaches the pancreas, it promotes the release of another hormone called insulin which helps regulate the levels of sugar in the blood. Osteocalcin also travels to other organs such as muscle, where it helps to degrade fats and sugars that can be converted into energy. It also has beneficial effects on the brain, and has been shown to aid memory and reduce depression.

Osteocalcin has largely been studied in mice where levels are five to ten times higher than in humans. But it is unclear why this difference exists or how it alters the role of osteocalcin in humans. To answer this question, Al Rifai et al. used a range of experimental techniques to compare the structure and activity of osteocalcin in mice and humans.

The experiments showed that mouse osteocalcin has a group of sugars attached to its protein structure, which prevent the hormone from being degraded by an enzyme in the blood. Human osteocalcin has a slightly different protein sequence and is therefore unable to bind to this sugar group. As a result, the osteocalcin molecules in humans are less stable and cannot last as long in the blood. Al Rifai et al. showed that when human osteocalcin was modified so the sugar group could attach, the hormone was able to stick around for much longer and reach higher levels when added to blood in the laboratory.

These findings show how osteocalcin differs between human and mice. Understanding this difference is important as the effects of osteocalcin mean this hormone can be used to treat diabetes and brain disorders. Furthermore, the results reveal how the stability of osteocalcin could be improved in humans, which could potentially enhance its therapeutic effect.

Humanized GPRC6AKGKY is a gain-of-function polymorphism in mice

GPRC6A is proposed to regulate energy metabolism in mice, but in humans a KGKY polymorphism in the third intracellular loop (ICL3) is proposed to result in intracellular retention and loss-of-function. To test physiological importance of this human polymorphism in vivo, we performed targeted genomic humanization of mice by using CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats-CRISPR associated protein 9) system to replace the RKLP sequence in the ICL3 of the GPRC6A mouse gene with the uniquely human KGKY sequence to create Gprc6a-KGKY-knockin mice. Knock-in of a human KGKY sequence resulted in a reduction in basal blood glucose levels and increased circulating serum insulin and FGF-21 concentrations. Gprc6a-KGKY-knockin mice demonstrated improved glucose tolerance, despite impaired insulin sensitivity and enhanced pyruvate-mediated gluconeogenesis. Liver transcriptome analysis of Gprc6a-KGKY-knockin mice identified alterations in glucose, glycogen and fat metabolism pathways. Thus, the uniquely human GPRC6A-KGKY variant appears to be a gain-of-function polymorphism that positively regulates energy metabolism in mice.

Role of GPRC6A in Regulating Hepatic Energy Metabolism in Mice

GPRC6A is a widely expressed G-protein coupled receptor that regulates energy metabolism. Global deletion of Gprc6a in mice is reported to result in a metabolic syndrome-like phenotype and conditional deletion of Gprc6a in pancreatic β-cell and skeletal muscle respectively impair insulin secretion and glucose uptake. In the current study, we explore the hepatic functions of GPRC6A by conditionally deleting Gprc6a in hepatocytes by cross breeding Alb-Cre and Gprc6aflox/flox mice to obtain Gprc6aLiver-cko mice. Gprc6aLiver-cko mice on a normal diet showed excessive hepatic fat accumulation and glycogen depletion. These mice also exhibit impaired glucose and pyruvate tolerance, but normal insulin sensitivity. Decreased circulating FGF-21 levels and FGF-21 message expression in the liver were found in Gprc6aLiver-cko mice. Hepatic transcriptome analysis identified alterations in multiple pathways regulating glucose, fat and glycogen metabolism in Gprc6aLiver-cko mice. Taken together, our studies suggest that GPRC6A directly regulates hepatic metabolism as well as regulates the production and release of FGF-21 to control systemic energy homeostasis. GPRC6A's unique regulation of β-cell, skeletal muscle and hepatic function may represent a new therapeutic target for treating disordered energy metabolism metabolic syndrome and type 2 diabetes.

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.

SHBG141-161 Domain-Peptide Stimulates GPRC6A-Mediated Response in Leydig and β-Langerhans cell lines

GPRC6A is acknowledged as a major regulator of energy metabolism and male fertility through the action of undercarboxylated osteocalcin (ucOCN), representing a possible therapeutic target. We recently showed that the sex hormone-binding globulin (SHBG) binds to GPRC6A through the likely involvement of the 141-161 domain. To confirm this model, here we investigated the possible binding and agonist activity of SHBG(141-161) domain-peptide (SHBG_141-161) on GPRC6A. The binding of SHBG_141-161 to GPRC6A and downstream dissociation from G_αi(GDP) protein was computationally modelled. SHBG_141-161 was obtained by solid-phase synthesis, characterized by circular dichroism (CD) and the receptor binding was assessed by displacement of ucOCN on HEK-293 cells transfected with GPRC6A gene. Agonist activity of SHBG_141-161 was assessed on Leydig MA-10 and Langerhans β-TC6 cell lines through the GPRC6A-mediated release of testosterone (T) and insulin. SHBG_141-161 was predicted to bind to GPRC6A and to reduce the affinity for G_αi(GDP) at computational level. Conformational properties and binding to GPRC6A of the synthetic SHBG_141-161 were confirmed by CD and displacement experiments. SHBG_141-161 stimulated cell secretion of T and insulin, with dose dependency from 10^-13 to 10^-11M for T release (respectively P = 0,041 10^-13M; P = 0,032 10^-12M; P = 0,008 10^-11M vs basal) and for 10^-12 to 10^-10M for insulin (respectively P = 0,041 10^-12M; P = 0,007 10^-11M; P = 0,047 10^-10M; P = 0,045 vs basal). Blockade with anti GPRC6A IgG abolished the response to SHBG_141-161, suggesting agonist specificity. SHBG_141-161 showed stimulating activity on GPRC6A, representing a template peptide with possible therapeutic use for metabolic and endocrine disorders.

Regulation of Atrial Fibrosis by the Bone

MR (mineralocorticoid receptor) antagonists have been demonstrated to provide beneficial effects on preventing atrial fibrosis. However, the underlying cellular and molecular mechanisms remain unclear. We aim to determine the role of osteoblast MR in atrial fibrosis and to explore the underlying mechanism. Using osteoblast MR knockout mouse in combination with mutant TGF (transforming growth factor)-β1 transgenic mouse, we demonstrated that MR deficiency in osteoblasts significantly attenuated atrial fibrosis. Mechanistically, MR directly regulated expression of OCN (osteocalcin) in osteoblasts. Both carboxylated and undercarboxylated OCNs (ucOC) were less secreted in osteoblast MR knockout mice. Mutant TGF-β1 transgenic mice supplemented with recombinant ucOC showed aggravated atrial fibrosis. In cultured atrial fibroblasts, ucOC treatment promoted proliferation and migration of atrial fibroblasts, whereas cotreatment with an antagonist for a GPRC6A (G-protein-coupled receptor, family C, group 6, member A) abolished these effects. Western blotting analysis revealed upregulation of PKA (protein kinase A) and CREB (cAMP-response element-binding protein) phosphorylation after ucOC treatment. Inhibition of PKA with its antagonist reduced ucOC-induced proliferation and migration of atrial fibroblasts. Finally, the impact of osteoblast MR deficiency on atrial fibrosis was abolished by ucOC administration in mutant TGF-β1 transgenic mice. Taken together, MR deficiency in osteoblasts attenuated atrial fibrosis by downregulation of OCN to promote proliferation and migration of atrial fibroblasts.

Androgen dihydrotestosterone (DHT) promotes the bladder cancer nuclear AR-negative cell invasion via a newly identified membrane androgen receptor (mAR-SLC39A9)-mediated Gαi protein/MAPK/MMP9 intracellular signaling

While androgens may function via nuclear androgen receptor (nAR) to increase bladder cancer (BCa) progression, the impact of androgens on muscle invasive BCa, which contains nearly 80% nAR-negative cells, remains unclear. To dissect the androgens potential impacts on these nAR-negative muscle invasive BCa, we first found that the androgens, dihydrotestosterone (DHT) might function via a novel membrane AR (mAR-SLC39A9) to increase nAR-negative BCa cell migration and invasion. Mechanism dissection revealed that DHT/mAR-SLC39A9 might function by altering G_αi protein-mediated MAPK/MMP9 intracellular signaling to increase nAR-negative BCa cell migration and invasion. Preclinical studies using multiple in vitro nAR-negative BCa cell lines and an in vivo mouse model all demonstrated that targeting this newly identified DHT/mAR-SLC39A9/G_αi/MAPK/MMP9 signaling with small molecules mAR-SLC39A9-shRNA or G_αi-shRNA, and not the classic antiandrogens including enzalutamide, bicalutamide, or hydroxyflutamide, could suppress nAR-negative BCa cell invasion. Results from human clinical samples surveys also indicated the positive correlation of this newly identified DHT/mAR signaling with BCa progression and prognosis. Together, these results suggest that androgens may not only function via the classic nAR to increase the nAR-positive BCa cell invasion, they may also function via this newly identified mAR-SLC39A9 to increase the nAR-negative/mAR-positive BCa cell invasion.

Membrane Androgen Receptors Unrelated to Nuclear Steroid Receptors

Rapid (nongenomic) membrane-initiated androgen actions have been described in nuclear androgen receptor-null cells. Four distinct proteins have been proposed as membrane androgen receptors (mARs) or sensors. Transient receptor potential melastatin 8 (TRPM8) is a calcium channel that acts as a pain receptor and mediates androgen- and menthol-induced increases in calcium levels and survival of prostate cancer cells. Testosterone (T) directly interacts with TRPM8, but extensive androgen receptor binding studies to confirm its role as an mAR are lacking. Oxoeicosanoid receptor 1 (OXER1) is highly expressed in prostate cancer tissues, and its major ligand, 5-oxoeicosatretraenoic acid (5-oxo-ETE), is a potent inducer of prostate cancer cell proliferation and survival. T competes for 5-oxo-ETE binding to OXER1 and antagonizes 5-oxo-ETE-mediated inhibition of cAMP production. However, OXER1 does not meet a traditional criterion for its designation as an mAR because T treatment alone does not alter cAMP signaling. GPRC6A is a class C G protein-coupled receptor activated by l-α-amino acids and is modulated by calcium. Although there has been controversy over the proposed role of T as a GPRC6A ligand, androgen induction of GPRC6A signaling has recently been confirmed by several researchers. ZIP9 belongs to the zinc transporter ZIP (SLC39A) family and displays specific T binding characteristic of an mAR. ZIP9 mediates androgen-dependent intracellular signaling and apoptosis of breast and prostate cancer cells through activation of G proteins. Androgen-signaling functions of ZIP9 have been confirmed in other cells, but the overall importance of ZIP9 in androgen physiology remains unclear. Here, the current status of these four proteins as mARs or sensors is critically reviewed.

Estrogen receptors and sex hormone binding globulin in neuronal cells and tissue

Estrogens exert a critical influence on neuronal tissues and cells. As demonstrated in many clinical studies, estrogens are neuroprotective to the extent that they improve prognosis for women with neurodegenerative diseases. Unfortunately, we still do not know exactly how these effects are mediated. Fifty years ago the first estrogen receptor was found, but since then many other new pathways of estrogen action have been identified. This review describes several of these pathways of estrogen effects and provides some conclusions and correlations about these as determined by recent studies with nerve growth factor differentiated rat pheochromocytoma cell line.

Three steroid-binding globulins, their localization in the brain and nose, and what they might be doing there

Steroid-binding globulins (SBGs) such as sex hormone binding globulin, corticosteroid binding globulin, and vitamin-D binding protein are receiving increasing notice as being actively involved in steroid actions. This paper reviews data of all three of these SBGs, focusing on their presence and possible activity in the brain and nose. We have found all three proteins in the brain in limbic areas such as the paraventricular (PVN) and supraoptic nuclei (SON) as well as other areas of the hypothalamus, hippocampus, and medial preoptic area. There is also evidence that all three are made in the PVN and SON, in conjunction with the neuropeptides oxytocin and vasopressin. The localization of these three SBGs is more variable within areas of the main olfactory area and the vomeronasal organ. However, all three are found in the mucus of these areas, suggesting that one of their functions is to sequester aerosol steroids, such as pheromones, and deliver them to sensory cells and then to deeper sensory areas. In this manuscript, we present multiple models of SBG action including: A) SBG binding to a membrane receptor, B) this SBG receptor being associated with a larger protein complex including cytoplasmic steroid receptors, C) when the SBGs binds to their SBG receptors, second messengers within the cells respond, D) after SBG binding to its receptor, it releases its associated steroid into the membrane's lipid bilayer, from which it gains access into the cell only when bound by an internal protein, E) the SBG, possibly with its bound SBG receptor, is internalized into the cell from which it can gain access to numerous organelles and possibly the cell's nucleus or F) associate with intracellular steroid receptors, G) SBGs produced in target cells are released from those cells upon specific stimulation, and H) according to the Free Steroid Hypothesis steroids released from the extracellular SBG passively diffuse across the plasma membrane of the cell. These models move the area of steroid endocrinology forward by providing important paths of steroid activity within many steroid target cells.

Androgen-dependent immune modulation in parasitic infection

Parasitic infections modulate the immune system of the host, resulting in either immune tolerance or the induction of pro-inflammatory defense mechanisms against the pathogen. In both cases, sex hormones are involved in the regulation of the immune response, as they are present in the systemic circulation and can act on a wide variety of cell types, including immune cells. Men and women have a different milieu of sex hormones, and these hormones play a role in determining immune responses to parasitic infections. Men, who have higher plasma levels of androgens than women, are generally more susceptible to parasitic infections. Many immune cells express the androgen receptor (AR), and the immunologic functions of these cells can be modulated by androgens. In this review, we will highlight the immune cell types that are sensitive to male steroid hormones and describe their roles during three parasitic diseases, amebiasis, leishmaniasis, and helminthiasis.

Overview of the Maturation Machinery of the H-Cluster of [FeFe]-Hydrogenases with a Focus on HydF

Hydrogen production in nature is performed by hydrogenases. Among them, [FeFe]-hydrogenases have a peculiar active site, named H-cluster, that is made of two parts, synthesized in different pathways. The cubane sub-cluster requires the normal iron-sulfur cluster maturation machinery. The [2Fe] sub-cluster instead requires a dedicated set of maturase proteins, HydE, HydF, and HydG that work to assemble the cluster and deliver it to the apo-hydrogenase. In particular, the delivery is performed by HydF. In this review, we will perform an overview of the latest knowledge on the maturation machinery of the H-cluster, focusing in particular on HydF.

Protective Effect of Sex Hormone-Binding Globulin against Metabolic Syndrome: In Vitro Evidence Showing Anti-Inflammatory and Lipolytic Effects on Adipocytes and Macrophages

Sex hormone-binding globulin (SHBG) is a serum protein released mainly by the liver, and a low serum level correlates with a risk for metabolic syndrome including diabetes, obesity, and cardiovascular events. However, the underlying molecular mechanism(s) linking SHBG and metabolic syndrome remains unknown. In this study, using adipocytes and macrophages, we focused on the in vitro effects of SHBG on inflammation as well as lipid metabolism. Incubation with 20 nM SHBG markedly suppressed lipopolysaccharide- (LPS-) induced inflammatory cytokines, such as MCP-1, TNFα, and IL-6 in adipocytes and macrophages, along with phosphorylations of JNK and ERK. Anti-inflammatory effects were also observed in 3T3-L1 adipocytes cocultured with LPS-stimulated macrophages. In addition, SHBG treatment for 18 hrs or longer significantly induced the lipid degradation of differentiated 3T3-L1 cells, with alterations in its corresponding gene and protein levels. Notably, these effects of SHBG were not altered by coaddition of large amounts of testosterone or estradiol. In conclusion, SHBG suppresses inflammation and lipid accumulation in macrophages and adipocytes, which might be among the mechanisms underlying the protective effect of SHBG, that is, its actions which reduce the incidence of metabolic syndrome.

Regulation of Energy Metabolism by Bone-Derived Hormones

Like many other organs, bone can act as an endocrine organ through the secretion of bone-specific hormones or "osteokines." At least two osteokines are implicated in the control of glucose and energy metabolism: osteocalcin (OCN) and lipocalin-2 (LCN2). OCN stimulates the production and secretion of insulin by the pancreatic β-cells, but also favors adaptation to exercise by stimulating glucose and fatty acid (FA) utilization by the muscle. Both of these OCN functions are mediated by the G-protein-coupled receptor GPRC6A. In contrast, LCN2 influences energy metabolism by activating appetite-suppressing signaling in the brain. This action of LCN2 occurs through its binding to the melanocortin 4 receptor (MC4R) in the paraventricular nucleus of the hypothalamus (PVN) and ventromedial neurons of the hypothalamus.

Androgen-Induced Immunosuppression

In addition to determining biological sex, sex hormones are known to influence health and disease via regulation of immune cell activities and modulation of target-organ susceptibility to immune-mediated damage. Systemic autoimmune disorders, such as systemic lupus erythematosus, rheumatoid arthritis, and multiple sclerosis are more prevalent in females, while cancer shows the opposite pattern. Sex hormones have been repeatedly suggested to play a part in these biases. In this review, we will discuss how androgens and the expression of functional androgen receptor affect immune cells and how this may dampen or alter immune response(s) and affect autoimmune disease incidences and progression.

Osteocalcin in the brain: from embryonic development to age-related decline in cognition

A remarkable, unexpected aspect of the bone-derived hormone osteocalcin is that it is necessary for both brain development and brain function in the mouse, as its absence results in a profound deficit in spatial learning and memory and an exacerbation of anxiety-like behaviour. The regulation of cognitive function by osteocalcin, together with the fact that its circulating levels decrease in midlife compared with adolescence in all species tested, raised the prospect that osteocalcin might be an anti-geronic hormone that could prevent age-related cognitive decline. As presented in this Review, recent data indicate that this is indeed the case and that osteocalcin is necessary for the anti-geronic activity recently ascribed to the plasma of young wild-type mice. The diversity and amplitude of the functions of osteocalcin in the brain, during development and postnatally, had long called for the identification of its receptor in the brain, which was also recently achieved. This Review presents our current understanding of the biology of osteocalcin in the brain, highlighting the bony vertebrate specificity of the regulation of cognitive function and pointing toward where therapeutic opportunities might exist.

Human sex hormone-binding globulin does not provide metabolic protection against diet-induced obesity and dysglycemia in mice

BACKGROUND

Sex hormone-binding globulin (SHBG) is the main transporter of sex hormones in most vertebrates. Low SHBG levels have been linked to increased risk for diabetes and metabolic syndrome. Polymorphisms of the SHBG gene linked to low SHBG protein levels also strongly predicted increased risk of type 2 diabetes, thus raising the possibility that SHBG may play a role in the pathogenesis of insulin resistance and diabetes.

AIM

To examine whether expression of human SHBG in mice may ameliorate the development of diabetes and metabolic syndrome in response to a high-fat diet (HFD).

METHODS

Transgene mice expressing a human SHBG transgene (SHBG+) (N = 10/11; males/females) and their wild type littermates (N = 12/8; males/females) were fed HFD for 4.5 months.

RESULTS

HFD induced comparable obesity in control and SHBG+ mice. Male transgenes had higher muscle mass after 2-3.5 months HFD (0.43 ± 0.028 (n = 4) vs 0.38 ± 0.053 g (n = 7), P = 0.05). Fasting blood glucose, as well as insulin or HOMA-IR, was not different in transgenic vs wild-type males after 4-5 months HFD. Female transgenes had higher fasting glucose (152 ± 29 (n = 7) vs 115 ± 27 mg/dL, P = 0.01 (n = 8)), but mean insulin and HOMA-IR were not different. Likewise, insulin tolerance test and intra-peritoneal glucose tolerance test (GTT) were not different. Finally, SHBG+ mice were not different from controls in terms of liver enzymes, serum triglyceride levels and blood pressure.

CONCLUSION

In mice with diet-induced obesity, human SHBG did not protect against development of obesity or dysglycemia.

Proprotein convertase furin regulates osteocalcin and bone endocrine function

Osteocalcin (OCN) is an osteoblast-derived hormone that increases energy expenditure, insulin sensitivity, insulin secretion, and glucose tolerance. The cDNA sequence of OCN predicts that, like many other peptide hormones, OCN is first synthesized as a prohormone (pro-OCN). The importance of pro-OCN maturation in regulating OCN and the identity of the endopeptidase responsible for pro-OCN cleavage in osteoblasts are still unknown. Here, we show that the proprotein convertase furin is responsible for pro-OCN maturation in vitro and in vivo. Using pharmacological and genetic experiments, we also determined that furin-mediated pro-OCN cleavage occurred independently of its γ-carboxylation, a posttranslational modification that is known to hamper OCN endocrine action. However, because pro-OCN is not efficiently decarboxylated and activated during bone resorption, inactivation of furin in osteoblasts in mice resulted in decreased circulating levels of undercarboxylated OCN, impaired glucose tolerance, and reduced energy expenditure. Furthermore, we show that Furin deletion in osteoblasts reduced appetite, a function not modulated by OCN, thus suggesting that osteoblasts may secrete additional hormones that regulate different aspects of energy metabolism. Accordingly, the metabolic defects of the mice lacking furin in osteoblasts became more apparent under pair-feeding conditions. These findings identify furin as an important regulator of bone endocrine function.

Osteocalcin-dependent regulation of glucose metabolism and fertility: Skeletal implications for the development of insulin resistance

The skeleton has recently emerged as a critical insulin target tissue that regulates whole body glucose metabolism and male reproductive function. While our understanding of these new regulatory axes remains in its infancy, the bone-specific protein, osteocalcin, has been shown to be centrally involved. Undercarboxylated osteocalcin acts as a secretagogue in a feed-forward loop to stimulate pancreatic β-cell proliferation and insulin secretion, improve insulin sensitivity, and promote testosterone production. Importantly, dysregulation of insulin signaling in bone causes a reduction in serum osteocalcin levels that is associated with elevated blood glucose and reduced serum insulin levels, suggesting that the skeleton may play a significant role in the development of diet-induced insulin resistance. Insulin signaling is negatively regulated by the mammalian target of rapamycin complex 1 (mTORC1) which becomes hyper-activated in response to nutrient overload. Loss- and gain-of function models suggest that mTORC1 function in bone is essential for normal skeletal development; however, the role of this complex in the regulation of glucose metabolism remains to be determined. This review highlights our current understanding of the role played by osteocalcin in the skeletal regulation of glucose metabolism and fertility. In particular, it examines data emerging from transgenic mouse models which have revealed a pancreas-bone-testis regulatory axis and discusses recent human studies which seek to corroborate findings from mouse models with clinical observations. Moreover, we review recent studies which suggest dysregulation of insulin signaling in bone leads to the development of insulin resistance and discuss the potential role of mTORC1 signaling in this process.

Multifaceted interaction of bone, muscle, lifestyle interventions and metabolic and cardiovascular disease: role of osteocalcin

Undercarboxylated osteocalcin (ucOC) may play a role in glucose homeostasis and cardiometabolic health. This review examines the epidemiological and interventional evidence associating osteocalcin (OC) and ucOC with metabolic risk and cardiovascular disease. The complexity in assessing such correlations, due to the observational nature of human studies, is discussed. Several studies have reported that higher levels of ucOC and OC are correlated with lower fat mass and HbA1c. In addition, improved measures of glycaemic control via pharmacological and non-pharmacological (e.g. exercise or diet) interventions are often associated with increased circulating levels of OC and/or ucOC. There is also a relationship between lower circulating OC and ucOC and increased measures of vascular calcification and cardiovascular disease. However, not all studies have reported such relationship, some with contradictory findings. Equivocal findings may arise because of the observational nature of the studies and the inability to directly assess the relationship between OC and ucOC on glycaemic control and cardiovascular health in humans. Studying OC and ucOC in humans is further complicated due to numerous confounding factors such as sex differences, menopausal status, vitamin K status, physical activity level, body mass index, insulin sensitivity (normal/insulin resistance/T2DM), tissue-specific effects and renal function among others. Current observational and indirect interventional evidence appears to support a relationship between ucOC with metabolic and cardiovascular disease. There is also emerging evidence to suggest a direct role of ucOC in human metabolism. Further mechanistic studies are required to (a) clarify causality, (b) explore mechanisms involved and

CRISPR/Cas9 targeting of GPRC6A suppresses prostate cancer tumorigenesis in a human xenograft model

Background

GPRC6A is implicated in the pathogenesis of prostate cancer, but its role remains uncertain because of a purported tolerant gene variant created by substitution of a K..Y polymorphism in the 3rd intracellular loop (IL) that evolved in the majority of humans and replaces the ancestral RKLP present in 40% of humans of African descent and all other species.

Methods

We determined whether the K..Y polymorphism is present in human-derived prostate cancer cell lines by sequencing the region of the 3rd IL and assessed the cellular localization of a “humanized” mouse GPRC6A containing the K..Y sequence by immunofluorescence. We assessed functions of GPRC6A in PC-3 cells expressing endogenous GPRC6A and in GPRC6A-deficient PC-3 cells created using CRISPR/Cas9 technology. The effect of GPRC6A on basal and ligand stimulated cell proliferation and migration was evaluated in vitro in wild-type and PC-3-deficient cell lines. The effect of editing GPRC6A on prostate cancer growth and progression in vivo was assessed in a Xenograft mouse model implanted with wild-type and PC-3 deficient cells and treated with the GPRC6A ligand osteocalcin.

Results

We found that all of the human prostate cancer cell lines tested endogenously express the “K..Y” polymorphism in the 3rd IL. Comparison of mouse wild-type GPRC6A with a “humanized” mouse GPRC6A construct created by replacing the “RKLP” with the “K..Y” sequence, found that both receptors were predominantly expressed on the cell surface. The transfected “humanized” GPRC6A receptor, however, preferentially activated mTOR compared to ERK signaling in HEK-293 cells. In contrast, in PC-3 cells expressing the endogenous GPRC6A with the “K..Y” polymorphism, the ligand osteocalcin stimulated ERK, AKT and mTOR phosphorylation, promoted cell proliferation and migration, and upregulated genes regulating testosterone biosynthesis. Targeting GPRC6A in PC-3 cells by CRISPR/Cas9 significantly blocked these responses in vitro. In addition, GPRC6A deficient PC-3 xenografts exhibited significantly less growth and were resistant to osteocalcin-induced prostate cancer progression compared to control PC-3 cells expressing GPRC6A.

Conclusions

Human GPRC6A is a functional osteocalcin and testosterone sensing receptor that promotes prostate cancer progression. GPRC6A may contribute to racial disparities in prostate cancer, and is a potential therapeutic target to develop antagonists to treat prostate cancer.

Electronic supplementary material

The online version of this article (doi:10.1186/s13046-017-0561-x) contains supplementary material, which is available to authorized users.

Osteocalcin, a bone-derived hormone with important andrological implications

Increasing evidence disclosed the existence of a novel multi-organ endocrine pathway, involving bone, pancreas and testis, of high penetrance in energy metabolism and male fertility. The main mediator of this axis is undercarboxylated osteocalcin (ucOC), a bone-derived protein-exerting systemic effects on tissues expressing the metabotropic receptor GPRC6A. The recognized effects of ucOC are the improvement of insulin secretion from the pancreas, the amelioration of systemic insulin sensitivity, in particular in skeletal muscle, and the stimulation of the global endocrine activity of the Leydig cell, including vitamin D 25-hydroxylation and testosterone production. The supporting evidence of this circuit in both animal and human models is here reviewed, with particular emphasis on the role of ucOC on testis function. The possible pharmacological modulation of this hormonal circuit for therapeutic aims is also discussed.

GPRC6A: Jack of all metabolism (or master of none)

Background

GPRC6A, a widely expressed G-protein coupled receptor, is proposed to be a master regulator of complex endocrine networks and metabolic processes. GPRC6A is activated by multiple ligands, including osteocalcin (Ocn), testosterone (T), basic amino acids, and various cations.

Scope of Review

We review the controversy surrounding GPRC6A functions. In mice, GPRC6A is proposed to integrate metabolic functions through the coordinated secretion of hormones, including insulin, GLP-1, T, and IL-6, and direct effects of this receptor to control glucose and fat metabolism in the liver, skeletal muscle, and fat. Loss-of-GPRC6A results in metabolic syndrome (MetS), and activation of GPRC6A stimulates proliferation of β-cells, increases peripheral insulin sensitivity, and protects against high fat diet (HFD) induced metabolic abnormalities in most mouse models. Bone, cardiovascular, immune, and skin functions of GPRC6A have also been identified in mice. Expression of GPRC6A is increased in prostate cancer (PCa) cells, and inhibition of GPRC6A attenuates PCa progression in mouse models. The function of GPRC6A in humans, however, is not clear. During evolution, a unique polymorphism of GPRC6A emerged mainly in humans of Asian and European decent that has been proposed to alter membrane trafficking and function. In contrast, the ancestral allele found in all other species is retained in 1%, 15%, and 40% of people of Asian, European and African descent, respectively, suggesting GPRC6A gene variants may contribute to the racial disparities in the risk of developing MetS and PCa.

Major Conclusions

If the regulatory functions of GPRC6A identified in mice translate to humans, and polymorphisms in GPRC6A are found to predict racial disparities in human diseases, GPRC6A may be a new gene target to predict, prevent, and treat MetS, PCa, and other disorders impacted by GPRC6A.

Three-dimensional printing of tricalcium silicate/mesoporous bioactive glass cement scaffolds for bone regeneration

Tricalcium silicate/mesoporous bioactive glass (C3S/MBG) cement scaffolds were successfully fabricated for the first time by 3D printing with a curing process, which combined the hydraulicity of C3S with the excellent biological property of MBG together.