A neuronal action of sirtuin 1 suppresses bone mass in young and aging mice.

The various functions of the skeleton are influenced by extracellular cues, hormones, and neurotransmitters. One type of neuronal regulation favors bone mass accrual by inhibiting sympathetic nervous system (SNS) activity. This observation raises questions about the transcriptional mechanisms regulating catecholamine synthesis. Using a combination of genetic and pharmacological studies, we found that the histone deacetylase sirtuin 1 (SIRT1) is a transcriptional modulator of the neuronal control of bone mass. Neuronal SIRT1 reduced bone mass by increasing SNS signaling. SIRT1 did so by increasing expression of monoamine oxidase A (MAO-A), a SIRT1 target that reduces brain serotonin levels by inducing its catabolism and by suppressing tryptophan hydroxylase 2 (Tph2) expression and serotonin synthesis in the brain stem. SIRT1 upregulated brain catecholamine synthesis indirectly through serotonin, but did not directly affect dopamine ß hydroxylase (Dbh) expression in the locus coeruleus. These results help us to understand skeletal changes associated with selective serotonin reuptake inhibitors (SSRIs) and may have implications for treating skeletal and metabolic diseases.

Lipocalin-2 is an anorexigenic signal in primates.

In the mouse, the osteoblast-derived hormone Lipocalin-2 (LCN2) suppresses food intake and acts as a satiety signal. We show here that meal challenges increase serum LCN2 levels in persons with normal or overweight, but not in individuals with obesity. Postprandial LCN2 serum levels correlate inversely with hunger sensation in challenged subjects. We further show through brain PET scans of monkeys injected with radiolabeled recombinant human LCN2 (rh-LCN2) and autoradiography in baboon, macaque, and human brain sections, that LCN2 crosses the blood-brain barrier and localizes to the hypothalamus in primates. In addition, daily treatment of lean monkeys with rh-LCN2 decreases food intake by 21%, without overt side effects. These studies demonstrate the biology of LCN2 as a satiety factor and indicator and anorexigenic signal in primates. Failure to stimulate postprandial LCN2 in individuals with obesity may contribute to metabolic dysregulation, suggesting that LCN2 may be a novel target for obesity treatment.

Obesity has reached epidemic proportions worldwide and affects more than 40% of adults in the United States. People with obesity have a greater likelihood of developing type 2 diabetes, cardiovascular disease or chronic kidney disease. Changes in diet and exercise can be difficult to follow and result in minimal weight loss that is rarely sustained overtime. In fact, in people with obesity, weight loss can lower the metabolism leading to increased weight gain. New drugs may help some individuals achieve 5 to 10% weight loss but have side effects that prevent long-term use. Previous studies in mice show that a hormone called Lipocalin-2 (LCN2) suppresses appetite. It also reduces body weight and improves sugar metabolism in the animals. But whether this hormone has the same effects in humans or other primates is unclear. If it does, LCN2 might be a potential obesity treatment. Now, Petropoulou et al. show that LCN2 suppressed appetite in humans and monkeys. In human studies, LCN2 levels increased after a meal in individuals with normal weight or overweight, but not in individuals with obesity. Higher levels of LCN2 in a person's blood were also associated with a feeling of reduced hunger. Using brain scans, Petropoulou et al. showed that LCN2 crossed the blood-brain barrier in monkeys and bound to the hypothalamus, the brain center regulating appetite and energy balance. LCN2 also bound to human and monkey hypothalamus tissue in laboratory experiments. When injected into monkeys, the hormone suppressed food intake and lowered body weight without toxic effects in short-term studies. The experiments lay the initial groundwork for testing whether LCN2 might be a useful treatment for obesity. More studies in animals will help scientists understand how LCN2 works, which patients might benefit, how it would be given to patients and for how long. Clinical trials would also be needed to verify whether it is an effective and safe treatment for obesity.

Lipocalin-2 counteracts metabolic dysregulation in obesity and diabetes.

Regulation of food intake is a recently identified endocrine function of bone that is mediated by Lipocalin-2 (LCN2). Osteoblast-secreted LCN2 suppresses appetite and decreases fat mass while improving glucose metabolism. We now show that serum LCN2 levels correlate with insulin levels and ß-cell function, indices of healthy glucose metabolism, in obese mice and obese, prediabetic women. However, LCN2 serum levels also correlate with body mass index and insulin resistance in the same individuals and are increased in obese mice. To dissect this apparent discrepancy, we modulated LCN2 levels in mice. Silencing Lcn2 expression worsens metabolic dysfunction in genetic and diet-induced obese mice. Conversely, increasing circulating LCN2 levels improves metabolic parameters and promotes ß-cell function in mouse models of ß-cell failure acting as a growth factor necessary for ß-cell adaptation to higher metabolic load. These results indicate that LCN2 up-regulation is a protective mechanism to counteract obesity-induced glucose intolerance by decreasing food intake and promoting adaptive ß-cell proliferation.

Bone: Another potential target to treat, prevent and predict diabetes.

Type 2 diabetes mellitus is now a worldwide health problem with increasing prevalence. Mounting efforts have been made to treat, prevent and predict this chronic disease. In recent years, increasing evidence from mice and clinical studies suggests that bone-derived molecules modulate glucose metabolism. This review aims to summarize our current understanding of the interplay between bone and glucose metabolism and to highlight potential new means of therapeutic intervention. The first molecule recognized as a link between bone and glucose metabolism is osteocalcin (OCN), which functions in its active form, that is, undercarboxylated OCN (ucOC). ucOC acts in promoting insulin expression and secretion, facilitating insulin sensitivity, and favouring glucose and fatty acid uptake and utilization. A second bone-derived molecule, lipocalin2, functions in suppressing appetite in mice through its action on the hypothalamus. Osteocytes, the most abundant cells in bone matrix, are suggested to act on the browning of white adipose tissue and energy expenditure through secretion of bone morphogenetic protein 7 and sclerostin. The involvement of bone resorption in glucose homeostasis has also been examined. However, there is evidence indicating the implication of the receptor activator of nuclear factor κ-B ligand, neuropeptide Y, and other known and unidentified bone-derived factors that function in glucose homeostasis. We summarize recent advances and the rationale for treating, preventing and predicting diabetes by skeleton intervention.

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 ß-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.

Corrigendum: MC4R-dependent suppression of appetite by bone-derived lipocalin 2.

This corrects the article DOI: 10.1038/nature21697.

MC4R-dependent suppression of appetite by bone-derived lipocalin 2.

Bone has recently emerged as a pleiotropic endocrine organ that secretes at least two hormones, FGF23 and osteocalcin, which regulate kidney function and glucose homeostasis, respectively. These findings have raised the question of whether other bone-derived hormones exist and what their potential functions are. Here we identify, through molecular and genetic analyses in mice, lipocalin 2 (LCN2) as an osteoblast-enriched, secreted protein. Loss- and gain-of-function experiments in mice demonstrate that osteoblast-derived LCN2 maintains glucose homeostasis by inducing insulin secretion and improves glucose tolerance and insulin sensitivity. In addition, osteoblast-derived LCN2 inhibits food intake. LCN2 crosses the blood-brain barrier, binds to the melanocortin 4 receptor (MC4R) in the paraventricular and ventromedial neurons of the hypothalamus and activates an MC4R-dependent anorexigenic (appetite-suppressing) pathway. These results identify LCN2 as a bone-derived hormone with metabolic regulatory effects, which suppresses appetite in a MC4R-dependent manner, and show that the control of appetite is an endocrine function of bone.

Leukaemogenesis induced by an activating ß-catenin mutation in osteoblasts.

Cells of the osteoblast lineage affect the homing and the number of long-term repopulating haematopoietic stem cells, haematopoietic stem cell mobilization and lineage determination and B cell lymphopoiesis. Osteoblasts were recently implicated in pre-leukaemic conditions in mice. However, a single genetic change in osteoblasts that can induce leukaemogenesis has not been shown. Here we show that an activating mutation of ß-catenin in mouse osteoblasts alters the differentiation potential of myeloid and lymphoid progenitors leading to development of acute myeloid leukaemia with common chromosomal aberrations and cell autonomous progression. Activated ß-catenin stimulates expression of the Notch ligand jagged 1 in osteoblasts. Subsequent activation of Notch signalling in haematopoietic stem cell progenitors induces the malignant changes. Genetic or pharmacological inhibition of Notch signalling ameliorates acute myeloid leukaemia and demonstrates the pathogenic role of the Notch pathway. In 38% of patients with myelodysplastic syndromes or acute myeloid leukaemia, increased ß-catenin signalling and nuclear accumulation was identified in osteoblasts and these patients showed increased Notch signalling in haematopoietic cells. These findings demonstrate that genetic alterations in osteoblasts can induce acute myeloid leukaemia, identify molecular signals leading to this transformation and suggest a potential novel pharmacotherapeutic approach to acute myeloid leukaemia.

FOXO1 orchestrates the bone-suppressing function of gut-derived serotonin.

Serotonin is a critical regulator of bone mass, fulfilling different functions depending on its site of synthesis. Brain-derived serotonin promotes osteoblast proliferation, whereas duodenal-derived serotonin suppresses it. To understand the molecular mechanisms of duodenal-derived serotonin action on osteoblasts, we explored its transcriptional mediation in mice. We found that the transcription factor FOXO1 is a crucial determinant of the effects of duodenum-derived serotonin on bone formation We identified two key FOXO1 complexes in osteoblasts, one with the transcription factor cAMP-responsive element-binding protein 1 (CREB) and another with activating transcription factor 4 (ATF4). Under normal levels of circulating serotonin, the proliferative activity of FOXO1 was promoted by a balance between its interaction with CREB and ATF4. However, high circulating serotonin levels prevented the association of FOXO1 with CREB, resulting in suppressed osteoblast proliferation. These observations identify FOXO1 as the molecular node of an intricate transcriptional machinery that confers the signal of duodenal-derived serotonin to inhibit bone formation.

FoxO1 protein cooperates with ATF4 protein in osteoblasts to control glucose homeostasis.

The Forkhead transcription factor FoxO1 inhibits through its expression in osteoblasts ß-cell proliferation, insulin secretion, and sensitivity. At least part of the FoxO1 metabolic functions result from its ability to suppress the activity of osteocalcin, an osteoblast-derived hormone favoring glucose metabolism and energy expenditure. In searching for mechanisms mediating the metabolic actions of FoxO1, we focused on ATF4, because this transcription factor also affects glucose metabolism through its expression in osteoblasts. We show here that FoxO1 co-localizes with ATF4 in the osteoblast nucleus, and physically interacts with and promotes the transcriptional activity of ATF4. Genetic experiments demonstrate that FoxO1 and ATF4 cooperate to increase glucose levels and decrease glucose tolerance. These effects result from a synergistic effect of the two transcription factors to suppress the activity of osteocalcin through up-regulating expression of the phosphatase catalyzing osteocalcin inactivation. As a result, insulin production by ß-cells and insulin signaling in the muscle, liver and white adipose tissue are compromised and fat weight increases by the FoxO1/ATF4 interaction. Taken together these observations demonstrate that FoxO1 and ATF4 cooperate in osteoblasts to regulate glucose homeostasis.

Genetic evidence points to an osteocalcin-independent influence of osteoblasts on energy metabolism.

The skeleton has been shown recently to regulate glucose metabolism through an osteoblast-specific hormone, osteocalcin, which favors ß-cell proliferation, insulin secretion, insulin sensitivity, and energy expenditure. An implication of this finding is that a decrease in osteoblast numbers would compromise glucose metabolism in an osteocalcin-dependent manner. To test this hypothesis, osteoblasts were inducibly ablated by cross-breeding transgenic mice expressing a tamoxifen-regulated Cre under the control of the osteocalcin promoter with mice in which an inactive form of the diphtheria toxin A chain was introduced into a ubiquitously expressed locus. Ablation of osteoblasts in adult mice profoundly affected glucose metabolism. In a manner similar to what is seen in the case of osteocalcin deficiency, a partial ablation of this cell population resulted in hypoinsulinemia, hyperglycemia, glucose intolerance, and decreased insulin sensitivity. However, and unlike what is seen in osteocalcin-deficient mice, osteoblast ablation also decreased gonadal fat and increased energy expenditure and the expression of resistin, an adipokine proposed to mediate insulin resistance. While administration of osteocalcin reversed (fully) the glucose intolerance and reinstated normal blood glucose and insulin levels, it only partially restored insulin sensitivity and did not affect the improved gonadal fat weight and energy expenditure in osteoblast-depleted mice. These observations not only strengthen the notion that osteoblasts are necessary for glucose homeostasis and energy expenditure but also suggest that in addition to osteocalcin, other osteoblast-derived hormones may contribute to the emerging function of the skeleton as a regulator of energy metabolism.

Opposite regulation of the human apolipoprotein M gene by hepatocyte nuclear factor 1 and Jun transcription factors.

HDL is a negative risk factor for atherosclerosis because of its multiple atheroprotective functions. Inflammation converts HDL particles from anti-atherogenic to pro-atherogenic, and this transformation is associated with changes in HDL structure and composition. Apolipoprotein M (apoM) has been recently shown to play a role in the maturation of HDL in plasma and to protect from atherosclerosis. ApoM gene is expressed primarily in the liver and kidney and is down-regulated by pro-inflammatory signals. We now show that the human apoM promoter harbors a dual specificity regulatory element in the proximal region that binds hepatocyte nuclear factor 1 (HNF-1) and members of the AP-1 family of pro-inflammatory transcription factors (c-Jun and JunB). Overexpression of c-Jun or JunB repressed both the basal and the HNF-1-mediated transactivation of the human apoM promoter. Treatment of HepG2 cells with potent inflammation-inducing phorbol esters or overexpression of PKCα was associated with a marked inhibition of apoM gene expression in a c-Jun/JunB-dependent manner. We provide evidence for a novel mechanism of inflammation-induced transcriptional repression that is based on the competition between HNF-1 and Jun proteins for binding to the same regulatory region. A similar mechanism accounts for the down-regulation of the liver-specific apolipoprotein A-II gene by Jun factors. Our studies provide novel insights on the mechanisms that control the expression of liver-specific apolipoprotein genes during inflammation and could affect the maturation and the functionality of HDL particles.

Regulation of human apolipoprotein m gene expression by orphan and ligand-dependent nuclear receptors.

Apolipoprotein M (apoM) plays an important role in the biogenesis and the metabolism of anti-atherogenic HDL particles in plasma and is expressed primarily in the liver and the kidney. We investigated the role of hormone nuclear receptors in apoM gene regulation in hepatic cells. Overexpression via adenovirus-mediated gene transfer and siRNA-mediated gene silencing established that hepatocyte nuclear factor 4 (HNF-4) is an important regulator of apoM gene transcription in hepatic cells. apoM promoter deletion analysis combined with DNA affinity precipitation and chromatin immunoprecipitation assays revealed that HNF-4 binds to a hormone-response element (HRE) in the proximal apoM promoter (nucleotides -33 to -21). Mutagenesis of this HRE decreased basal hepatic apoM promoter activity to 10% of control and abolished the HNF4-mediated transactivation of the apoM promoter. In addition to HNF-4, homodimers of retinoid X receptor and heterodimers of retinoid X receptor with receptors for retinoic acid, thyroid hormone, fibrates (peroxisome proliferator-activated receptor), and oxysterols (liver X receptor) were shown to bind with different affinities to the proximal HRE in vitro and in vivo. Ligands of these receptors strongly induced human apoM gene transcription and apoM promoter activity in HepG2 cells, whereas mutations in the proximal HRE abolished this induction. These findings provide novel insights into the role of apoM in the regulation of HDL by steroid hormones and into the development of novel HDL-based therapies for diseases such as diabetes, obesity, metabolic syndrome, and coronary artery disease that affect a large proportion of the population in Western countries.