Deficiency of Lipocalin-2 Promotes Proliferation and Differentiation of Osteoclast Precursors via Regulation of c-Fms Expression and Nuclear Factor-kappa B Activation

Astrocyte-secreted lipocalin-2 elicits bioenergetic failure-induced neuronal death that is causally related to high fatality in a mouse model of hepatic encephalopathy

Hepatic encephalopathy (HE) is a neurological complication arising from acute liver failure with poor prognosis and high mortality; the underlying cellular mechanisms are still wanting. We previously found that neuronal death caused by mitochondrial dysfunction in rostral ventrolateral medulla (RVLM), which leads to baroreflex dysregulation, is related to high fatality in an animal model of HE. Lipocalin-2 (Lcn2) is a secreted glycoprotein mainly released by astrocytes in the brain. We noted the presence of Lcn2 receptor (Lcn2R) in RVLM neurons and a parallel increase of Lcn2 gene in astrocytes purified from RVLM during experimental HE. Therefore, our guiding hypothesis is that Lcn2 secreted by reactive astrocytes in RVLM may underpin high fatality during HE by eliciting bioenergetic failure-induced neuronal death in this neural substrate. In this study, we first established the role of astrocyte-secreted Lcn2 in a liver toxin model of HE induced by azoxymethane (100 μg/g, ip) in C57BL/6 mice, followed by mechanistic studies in primary astrocyte and neuron cultures prepared from postnatal day 1 mouse pups. In animal study, immunoneutralization of Lcn2 reduced apoptotic cell death in RVLM, reversed defunct baroreflex-mediated vasomotor tone and prolonged survival during experimental HE. In our primary cell culture experiments, Lcn2 produced by cultured astrocytes and released into the astrocyte-conditioned medium significantly reduced cell viability of cultured neurons. Recombinant Lcn2 protein reduced cell viability, mitochondrial ATP (mitoATP) production, and pyruvate dehydrogenase (PDH) activity but enhanced the expression of pyruvate dehydrogenase kinase (PDK) 1, PDK3 and phospho-PDHA1 (inactive PDH) through MAPK/ERK pathway in cultured neurons, with all cellular actions reversed by Lcn2R knockdown. Our results suggest that astrocyte-secreted Lcn2 upregulates PDKs through MAPK/ERK pathway, which leads to reduced PDH activity and mitoATP production; the reinforced neuronal death in RVLM is causally related to baroreflex dysregulation that underlies high fatality associated with HE.

Role of lipocalin 2 in stroke

Stroke is the second leading cause of death worldwide; however, the treatment choices available to neurologists are limited in clinical practice. Lipocalin 2 (LCN2) is a secreted protein, belonging to the lipocalin superfamily, with multiple biological functions in mediating innate immune response, inflammatory response, iron-homeostasis, cell migration and differentiation, energy metabolism, and other processes in the body. LCN2 is expressed at low levels in the brain under normal physiological conditions, but its expression is significantly up-regulated in multiple acute stimulations and chronic pathologies. An up-regulation of LCN2 has been found in the blood/cerebrospinal fluid of patients with ischemic/hemorrhagic stroke, and could serve as a potential biomarker for the prediction of the severity of acute stroke. LCN2 activates reactive astrocytes and microglia, promotes neutrophil infiltration, amplifies post-stroke inflammation, promotes blood-brain barrier disruption, white matter injury, and neuronal death. Moreover, LCN2 is involved in brain injury induced by thrombin and erythrocyte lysates, as well as microvascular thrombosis after hemorrhage. In this paper, we review the role of LCN2 in the pathological processes of ischemic stroke; intracerebral hemorrhage; subarachnoid hemorrhage; and stroke-related brain diseases, such as vascular dementia and post-stroke depression, and their underlying mechanisms. We hope that this review will help elucidate the value of LCN2 as a therapeutic target in stroke.

Cellular and Molecular Mechanisms Associating Obesity to Bone Loss

Obesity is an alarming disease that favors the upset of other illnesses and enhances mortality. It is spreading fast worldwide may affect more than 1 billion people by 2030. The imbalance between excessive food ingestion and less energy expenditure leads to pathological adipose tissue expansion, characterized by increased production of proinflammatory mediators with harmful interferences in the whole organism. Bone tissue is one of those target tissues in obesity. Bone is a mineralized connective tissue that is constantly renewed to maintain its mechanical properties. Osteoblasts are responsible for extracellular matrix synthesis, while osteoclasts resorb damaged bone, and the osteocytes have a regulatory role in this process, releasing growth factors and other proteins. A balanced activity among these actors is necessary for healthy bone remodeling. In obesity, several mechanisms may trigger incorrect remodeling, increasing bone resorption to the detriment of bone formation rates. Thus, excessive weight gain may represent higher bone fragility and fracture risk. This review highlights recent insights on the central mechanisms related to obesity-associated abnormal bone. Publications from the last ten years have shown that the main molecular mechanisms associated with obesity and bone loss involve: proinflammatory adipokines and osteokines production, oxidative stress, non-coding RNA interference, insulin resistance, and changes in gut microbiota. The data collection unveils new targets for prevention and putative therapeutic tools against unbalancing bone metabolism during obesity.

Bone Cells Metabolic Changes Induced by Ageing

Bone is a living organ that exhibits active metabolic processes, presenting constant bone formation and resorption. The bone cells that maintain local homeostasis are osteoblasts, osteoclasts, osteocytes and bone marrow stem cells, their progenitor cells. Osteoblasts are the main cells that govern bone formation, osteoclasts are involved in bone resorption, and osteocytes, the most abundant bone cells, also participate in bone remodeling. All these cells have active metabolic activities, are interconnected and influence each other, having both autocrine and paracrine effects. Ageing is associated with multiple and complex bone metabolic changes, some of which are currently incompletely elucidated. Ageing causes important functional changes in bone metabolism, influencing all resident cells, including the mineralization process of the extracellular matrix. With advancing age, a decrease in bone mass, the appearance of specific changes in the local microarchitecture, a reduction in mineralized components and in load-bearing capacity, as well as the appearance of an abnormal response to different humoral molecules have been observed. The present review points out the most important data regarding the formation, activation, functioning, and interconnection of these bone cells, as well as data on the metabolic changes that occur due to ageing.

Lipocalin 2 serum levels correlate with age and bone turnover biomarkers in healthy subjects but not in postmenopausal osteoporotic women

Purpose

Lipocalin 2 (LCN2) is an adipokine involved in many physiological functions, including bone metabolism. We previously demonstrated its implication in mouse models of mechanical unloading-induced osteoporosis and in a cohort of bed rest volunteers. We therefore aimed at studying its involvement in postmenopausal osteoporosis.

Methods

We measured serum LCN2 and correlated its levels to Dickkopf WNT Signaling Pathway Inhibitor 1 (DKK1), Tartrate Resistant Acid Phosphatase 5B (TRAcP5B), sclerostin, urinary N-terminal telopeptide of type I collagen (NTX), serum C-terminal telopeptide of type I collagen (CTX), parathyroid hormone and vitamin K by ELISA performed in a cohort of younger (50–65 years) and older (66–90 years) osteoporotic women in comparison to healthy subjects. A cohort of male healthy and osteoarthritic patients was also included. Sobel mediation analysis was used to test indirect associations among age, LCN2 and DKK1 or NTX.

Results

LCN2 levels were unchanged in osteoporotic and in osteoarthritis patients when compared to healthy subjects and did not correlate with BMD. However, serum LCN2 correlated with age in healthy women (R = 0.44; P = 0.003) and men (R = 0.5; P = 0.001) and serum concentrations of DKK1 (R = 0.47; P = 0.003) and urinary NTX (R = 0.34; P = 0.04). Sobel mediation analysis showed that LCN2 mediates an indirect relationship between age and DKK1 (P = 0.02), but not with NTX, in healthy subjects.

Conclusions

Taken together, the results suggest a hitherto unknown association between LCN2, DKK1 and age in healthy individuals, but not in postmenopausal osteoporotic women.

Genome-Wide Association Identifies Risk Pathways for SAPHO Syndrome

SAPHO syndrome is a rare chronic inflammatory disease which is characterized by the comprehensive manifestations of bone, joint, and skin. However, little is known about the pathogenesis of SAPHO syndrome. A genome-wide association study (GWAS) of 49 patients and 121 control subjects have primarily focused on identification of common genetic variants associated with SAPHO, the data were analyzed by classical multiple logistic regression. Later, GWAS findings were further validated using whole exome sequencing (WES) in 16 patients and 15 controls to identify potentially functional pathways involved in SAPHO pathogenesis. In general, 40588 SNPs in genomic regions were associated with P < 0.05 after filter process, only 9 SNPs meet the expected cut-off P-value, however, none of them had association with SAPHO syndrome based on published literatures. And then, 15 pathways were found involved in SAPHO pathogenesis, of them, 6 pathways including osteoclast differentiation, bacterial invasion of epithelial cells, et al., had strong association with skin, osteoarticular manifestations of SAPHO or inflammatory reaction based published research. This study identified aberrant osteoclast differentiation and other pathways were involved in SAPHO syndrome. This finding may give insight into the understanding of pathogenic genes of SAPHO and provide the basis for SAPHO research and treatment.

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 β-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 β-cell function in mouse models of β-cell failure acting as a growth factor necessary for β-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 β-cell proliferation.

Lipoteichoic Acid Accelerates Bone Healing by Enhancing Osteoblast Differentiation and Inhibiting Osteoclast Activation in a Mouse Model of Femoral Defects

Lipoteichoic acid (LTA) is a cell wall component of Gram-positive bacteria. Limited data suggest that LTA is beneficial for bone regeneration in vitro. Thus, we used a mouse model of femoral defects to explore the effects of LTA on bone healing in vivo. Micro-computed tomography analysis and double-fluorochrome labeling were utilized to examine whether LTA can accelerate dynamic bone formation in vivo. The effects of LTA on osteoblastogenesis and osteoclastogenesis were also studied in vitro. LTA treatment induced prompt bone bridge formation, rapid endochondral ossification, and accelerated healing of fractures in mice with femoral bone defects. In vitro, LTA directly enhanced indicators of osteogenic factor-induced MC3T3-E1 cell differentiation, including alkaline phosphatase activity, calcium deposition and osteopontin expression. LTA also inhibited osteoclast activation induced by receptor activator of nuclear factor-kappa B ligand. We identified six molecules that may be associated with LTA-accelerated bone healing: monocyte chemoattractant protein 1, chemokine (C-X-C motif) ligand 1, cystatin C, growth/differentiation factor 15, endostatin and neutrophil gelatinase-associated lipocalin. Finally, double-fluorochrome, dynamic-labeling data indicated that LTA significantly enhanced bone-formation rates in vivo. In conclusion, our findings suggest that LTA has promising bone-regeneration properties.

Molecular Basis of Bone Aging

A decline in bone mass leading to an increased fracture risk is a common feature of age-related bone changes. The mechanisms underlying bone senescence are very complex and implicate systemic and local factors and are the result of the combination of several changes occurring at the cellular, tissue and structural levels; they include alterations of bone cell differentiation and activity, oxidative stress, genetic damage and the altered responses of bone cells to various biological signals and to mechanical loading. The molecular mechanisms responsible for these changes remain greatly unclear and many data derived from in vitro or animal studies appear to be conflicting and heterogeneous, probably due to the different experimental approaches; nevertheless, understanding the main physio-pathological processes that cause bone senescence is essential for the development of new potential therapeutic options for treating age-related bone loss. This article reviews the current knowledge concerning the molecular mechanisms underlying the pathogenesis of age-related bone changes.

Transcription factor 7-like 2 promotes osteogenic differentiation and boron-induced bone repair via lipocalin 2

Boron-containing mesoporous bioactive glass (B-MBG) scaffolds could be capable of promoting osteogenesis by activating Wnt/β-catenin signaling pathway during the process of bone defect repair. Despite this, more involving molecular controls are still largely unclear. In the present study, we identified that the downstream of Wnt/β-catenin signaling pathway named transcription factor 7-like 2 (TCF7L2) served as a key effector to promote boron-induced bone regeneration and osteogenesis through lipocalin 2 (LCN2). TCF7L2 was highly expressed in osteoblasts when treated with B-MBG scaffold extraction than MBG. LCN2, as a secreted bone factor, positively affected osteogenic differentiation of MC3T3-E1 and osteogenesis in vivo, which could be induced by TCF7L2. In addition, interference of TCF7L2 decreased the osteogenic differentiation of osteoblasts. Finally, we identified that rLCN2 could rescue the poor ability of osteogenic differentiation of MC3T3-E1 whose Tcf7l2 gene was knocked down by lentiviral transfection of shRNA. Our findings provide some new insights into the molecular controls of boron-associated bone regeneration and potential therapeutic targets for the treatment of bone defects.

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.

Hepatocyte-Derived Lipocalin 2 Is a Potential Serum Biomarker Reflecting Tumor Burden in Hepatoblastoma

Hepatoblastoma (HB) is the most common pediatric liver malignant tumor. Previously, we reported co-activation of β-catenin and Yes-associated protein-1 (YAP1) in 80% of HB. Hepatic co-expression of active β-catenin and YAP1 via sleeping beauty transposon/transposase and hydrodynamic tail vein injection led to HB development in mice. Here, we identify lipocalin 2 (Lcn2) as a target of β-catenin and YAP1 in HB and show that serum Lcn2 values positively correlated with tumor burden. Lcn2 was strongly expressed in HB tumor cells in our mouse model. A tissue array of 62 HB cases showed highest LCN2 expression in embryonal and lowest in fetal, blastemal, and small cell undifferentiated forms of HB. Knockdown of LCN2 in HB cells had no effect on cell proliferation but reduced NF-κB reporter activity. Next, liver-specific Lcn2 knockout (KO) mice were generated. No difference in tumor burden was observed between Lcn2 KO mice and wild-type littermate controls after sleeping beauty transposon/transposase and hydrodynamic tail vein injection delivery of active YAP1 and β-catenin, although Lcn2 KO mice with HB lacked any serum Lcn2 elevation, demonstrating that transformed hepatocytes are the source of serum Lcn2. More blastemal areas and inflammation were observed within HB in Lcn2 KO compared with wild-type tumors. In conclusion, Lcn2 expressed in hepatocytes appears to be dispensable for the pathogenesis of HB. However, transformed hepatocytes secrete serum Lcn2, making Lcn2 a valuable biomarker for HB.

A Complex Role for Lipocalin 2 in Bone Metabolism: Global Ablation in Mice Induces Osteopenia Caused by an Altered Energy Metabolism

Lipocalin 2 (Lcn2) is an adipokine that carries out a variety of functions in diverse organs. We investigated the bone phenotype and the energy metabolism of Lcn2 globally deleted mice (Lcn2^-/- ) at different ages. Lcn2^-/- mice were largely osteopenic, exhibiting lower trabecular bone volume, lesser trabecular number, and higher trabecular separation when compared to wild-type (WT) mice. Lcn2^-/- mice showed a lower osteoblast number and surface over bone surface, and subsequently a significantly lower bone formation rate, while osteoclast variables were unremarkable. Surprisingly, we found no difference in alkaline phosphatase (ALP) activity or in nodule mineralization in Lcn2^-/- calvaria osteoblast cultures, while less ALP-positive colonies were obtained from freshly isolated Lcn2^-/- bone marrow stromal cells, suggesting a nonautonomous osteoblast response to Lcn2 ablation. Given that Lcn2^-/- mice showed higher body weight and hyperphagia, we investigated whether their osteoblast impairment could be due to altered energy metabolism. Lcn2^-/- mice showed lower fasted glycemia and hyperinsulinemia. Consistently, glucose tolerance was significantly higher in Lcn2^-/- compared to WT mice, while insulin tolerance was similar. Lcn2^-/- mice also exhibited polyuria, glycosuria, proteinuria, and renal cortex vacuolization, suggesting a kidney contribution to their phenotype. Interestingly, the expression of the glucose transporter protein type 1, that conveys glucose into the osteoblasts and is essential for osteogenesis, was significantly lower in the Lcn2^-/- bone, possibly explaining the in vivo osteoblast impairment induced by the global Lcn2 ablation. Taken together, these results unveil an important role of Lcn2 in bone metabolism, highlighting a link with glucose metabolism that is more complex than expected from the current knowledge. © 2018 American Society for Bone and Mineral Research.

The relationship among serum lipocalin 2, bone turnover markers, and bone mineral density in outpatient women

PURPOSE

We aimed to investigate associations among serum levels of LCN2, bone resorption marker carboxy-terminal cross-linking telopeptide of type-1 collagen (CTx), bone formation marker osteocalcin (OCN), and bone mineral densities (BMDs) in ambulatory healthy women.

METHODS

This cross-sectional study analyzed 1012 previously enrolled outpatient Han Chinese women. BMDs of the lumbar spine and femoral neck were measured using dual energy X-ray absorptiometry. Serum levels of LCN2, CTx, OCN, and creatinine (Scr) were measured.

RESULTS

Circulating LCN2 was inversely correlated with BMDs at the lumbar spine and femoral neck (Spearman's r = -0.08, P = 0.010 and r = -0.14, P < 0.001; respectively). A significant positive correlation between LCN2 and CTx (r = 0.11, P < 0.001), OCN (r = 0.06, P = 0.047), age (r = 0.21, P < 0.001), and Scr (r = 0.24, P < 0.001) was also observed. After adjusting for age and Scr, the correlation among LCN2, BMDs and OCN disappeared, but LCN2 was still positively associated with CTx (r = 0.08, P = 0.010). The circulating concentration of LCN2 showed no significant difference between subjects with and without osteoporotic fractures (43.63 (35.29, 53.66) vs. 42.25 (34.43, 51.46) ng/ml, respectively, P = 0.111). Serum CTx concentrations rose with serum LCN2 increasing from the lowest to the highest quartile (P for trend = 0.005), even after adjusting for age and Scr (P for trend = 0.040). In multivariate regression analysis, LCN2 was one of the main determinants for changes in serum CTx (standard β = 0.061, P = 0.005).

CONCLUSIONS

In ambulatory healthy women, the relationships among serum LCN2 level, BMDs, and OCN were confounded by age and Scr. Although LCN2 was positively related with CTx, the correlation was very weak and may not be physiologically relevant.

Fexaramine Inhibits Receptor Activator of Nuclear Factor-κB Ligand-induced Osteoclast Formation via Nuclear Factor of Activated T Cells Signaling Pathways

Background

Osteoclasts are bone resorbing cells and are responsible for bone erosion in diseases as diverse as osteoporosis, periodontitis, and rheumatoid arthritis. Fexaramine has been developed as an agonist for the farnesoid X receptor (FXR). This study investigated the effects of fexaramine on receptor activator of nuclear factor (NF)-κB ligand (RANKL)-induced osteoclast formation and signaling pathways.

Methods

Osteoclasts were formed by culturing mouse bone marrow-derived macrophages (BMMs) with macrophage colony-stimulating factor (M-CSF) and RANKL. Bone resorption assays were performed using dentine slices. The mRNA expression level was analyzed by real-time polymerase chain reaction. Western blotting assays were conducted to detect the expression or activation level of proteins. Lipopolysaccharide-induced osteoclast formation was performed using a mouse calvarial model.

Results

Fexaramine inhibited RANKL-induced osteoclast formation, without cytotoxicity. Furthermore, fexaramine diminished the RANKL-stimulated bone resorption. Mechanistically, fexaramine blocked the RANKL-triggered p38, extracellular signal-regulated kinase, and glycogen synthase kinase 3β phosphorylation, resulting in suppressed expression of c-Fos and NF of activated T cells (NFATc1). Consistent with the in vitro anti-osteoclastogenic effect, fexaramine suppressed lipopolysaccharide-induced osteoclast formation in the calvarial model.

Conclusions

The present data suggest that fexaramine has an inhibitory effect on osteoclast differentiation and function, via downregulation of NFATc1 signaling pathways. Thus, fexaramine could be useful for the treatment of bone diseases associated with excessive bone resorption.

Novel bone metabolism-associated hormones: the importance of the pre-analytical phase for understanding their physiological roles

The endocrine function of bone is now a recognized feature of this tissue. Bone-derived hormones that modulate whole-body homeostasis, are being discovered as for the effects on bone of novel and classic hormones produced by other tissues become known. Often, however, the data regarding these last generation bone-derived or bone-targeting hormones do not give about a clear picture of their physiological roles or concentration ranges. A certain degree of uncertainty could stem from differences in the pre-analytical management of biological samples. The pre-analytical phase comprises a series of decisions and actions (i.e., choice of sample matrix, methods of collection, transportation, treatment and storage) preceding analysis. Errors arising in this phase will inevitably be carried over to the analytical phase where they can reduce the measurement accuracy, ultimately, leading discrepant results. While the pre-analytical phase is all important, in routine laboratory medicine, it is often not given due consideration in research and clinical trials. This is particularly true for novel molecules, such as the hormones regulating the endocrine function of bone. In this review we discuss the importance of the pre-analytical variables affecting the measurement of last generation bone-associated hormones and describe their, often debated and rarely clear physiological roles.