A possible role of osteocalcin in the regulation of insulin secretion: human in vivo evidence?

Anti-osteoporotic treatments in the era of non-alcoholic fatty liver disease: friend or foe

Over the last years non-alcoholic fatty liver disease (NAFLD) has grown into the most common chronic liver disease globally, affecting 17-38% of the general population and 50-75% of patients with obesity and/or type 2 diabetes mellitus (T2DM). NAFLD encompasses a spectrum of chronic liver diseases, ranging from simple steatosis (non-alcoholic fatty liver, NAFL) and non-alcoholic steatohepatitis (NASH; or metabolic dysfunction-associated steatohepatitis, MASH) to fibrosis and cirrhosis with liver failure or/and hepatocellular carcinoma. Due to its increasing prevalence and associated morbidity and mortality, the disease-related and broader socioeconomic burden of NAFLD is substantial. Of note, currently there is no globally approved pharmacotherapy for NAFLD. Similar to NAFLD, osteoporosis constitutes also a silent disease, until an osteoporotic fracture occurs, which poses a markedly significant disease and socioeconomic burden. Increasing emerging data have recently highlighted links between NAFLD and osteoporosis, linking the pathogenesis of NAFLD with the process of bone remodeling. However, clinical studies are still limited demonstrating this associative relationship, while more evidence is needed towards discovering potential causative links. Since these two chronic diseases frequently co-exist, there are data suggesting that anti-osteoporosis treatments may affect NAFLD progression by impacting on its pathogenetic mechanisms. In the present review, we present on overview of the current understanding of the liver-bone cross talk and summarize the experimental and clinical evidence correlating NAFLD and osteoporosis, focusing on the possible effects of anti-osteoporotic drugs on NAFLD.

Genomic loci mispositioning in Tmem120a knockout mice yields latent lipodystrophy

Little is known about how the observed fat-specific pattern of 3D-spatial genome organisation is established. Here we report that adipocyte-specific knockout of the gene encoding nuclear envelope transmembrane protein Tmem120a disrupts fat genome organisation, thus causing a lipodystrophy syndrome. Tmem120a deficiency broadly suppresses lipid metabolism pathway gene expression and induces myogenic gene expression by repositioning genes, enhancers and miRNA-encoding loci between the nuclear periphery and interior. Tmem120a-/- mice, particularly females, exhibit a lipodystrophy syndrome similar to human familial partial lipodystrophy FPLD2, with profound insulin resistance and metabolic defects that manifest upon exposure to an obesogenic diet. Interestingly, similar genome organisation defects occurred in cells from FPLD2 patients that harbour nuclear envelope protein encoding LMNA mutations. Our data indicate TMEM120A genome organisation functions affect many adipose functions and its loss may yield adiposity spectrum disorders, including a miRNA-based mechanism that could explain muscle hypertrophy in human lipodystrophy.

Isl1 Controls Patterning and Mineralization of Enamel in the Continuously Renewing Mouse Incisor

Rodents are characterized by continuously renewing incisors whose growth is fueled by epithelial and mesenchymal stem cells housed in the proximal compartments of the tooth. The epithelial stem cells reside in structures known as the labial (toward the lip) and lingual (toward the tongue) cervical loops (laCL and liCL, respectively). An important feature of the rodent incisor is that enamel, the outer, highly mineralized layer, is asymmetrically distributed, because it is normally generated by the laCL but not the liCL. Here, we show that epithelial-specific deletion of the transcription factor Islet1 (Isl1) is sufficient to drive formation of ectopic enamel by the liCL stem cells, and also that it leads to production of altered enamel on the labial surface. Molecular analyses of developing and adult incisors revealed that epithelial deletion of Isl1 affected multiple, major pathways: Bmp (bone morphogenetic protein), Hh (hedgehog), Fgf (fibroblast growth factor), and Notch signaling were upregulated and associated with liCL-generated ectopic enamel; on the labial side, upregulation of Bmp and Fgf signaling, and downregulation of Shh were associated with premature enamel formation. Transcriptome profiling studies identified a suite of differentially regulated genes in developing Isl1 mutant incisors. Our studies demonstrate that ISL1 plays a central role in proper patterning of stem cell-derived enamel in the incisor and indicate that this factor is an important upstream regulator of signaling pathways during tooth development and renewal. © 2017 American Society for Bone and Mineral Research.

Age and menopausal status affect osteoprotegerin and osteocalcin levels in women differently, irrespective of thyroid function

Osteoprotegerin (OPG) and osteocalcin (OC) are essential bone proteins. Recent studies have demonstrated that they are not secreted solely by bone cells; they play roles in the vascular function and energy metabolism, and they are influenced by multiple factors. The aim of the current study was to investigate the influence of menopause and age on OPG and OC in women with different thyroid-stimulating hormone (TSH) levels.

Insulin receptor isoforms are differently expressed during human osteoblastogenesis

The reciprocal influence and bidirectional cross-talk between bone and energy metabolism is a recent finding, since the discovery that the product of osteoblasts osteocalcin increases pancreatic β-cell proliferation, insulin secretion and sensitivity. Conversely, the anabolic effect of insulin is crucial for osteoblast function, as suggested by severe osteopenia and increased incidence of fracture in insulin-deficient diabetic patients. The Insulin Receptor (IR) tyrosine kinase, which is commonly expressed in the insulin-sensitive liver, muscle, and adipose tissues, is also found in animal and human bone. Here we show that in human bone two insulin receptor isoforms (IR-A and IR-B) are differently expressed. Mature human osteoblasts predominantly express IR-B, whereas IR-A is mainly expressed in osteoblast precursors, and IR-B/IR-A mRNA ratio significantly increases along the osteogenic differentiation of mesenchymal stromal precursors. Moreover, transfected osteoprogenitors overexpressing IR-A show an increased proliferation rate. In contrast, when transfected with and overexpressing IR-B, their proliferation rate is reduced, corresponding to a more differentiated phenotype. In conclusion, the fine regulation of the expression of different isoforms of IR during osteogenic differentiation confirms the important role played by IR in bone homeostasis, providing the basis for new perspectives on the various involvements of IR isoforms in bone pathophysiology.

p53 and MDM2 are involved in the regulation of osteocalcin gene expression

Osteocalcin (OC) is a major noncollagenous bone matrix protein and an osteoblast marker whose expression is limited to mature osteoblasts during the late differentiation stage. In previous studies we have shown osteosarcomas to lose p53 function with a corresponding loss of osteocalcin gene expression. Introduction of wild type p53 resulted in re expression of the osteocalcin gene. Using gel shift and chromatin immunoprecipitation assays, we have identified a putative p53 binding site within the rat OC promoter region and observed an increase in OC promoter activity when p53 accumulates using a CAT assay. The p53 inducible gene Mdm2 is a well-known downstream regulator of p53 levels. Our results showed a synergistic increase in the OC promoter activity when both p53 and MDM2 were transiently overexpressed. We further demonstrate that p53 is not degraded during overexpression of MDM2 protein. Increased OC expression was observed with concomitantly increased p53, VDR, and MDM2 levels in ROS17/2.8 cells during treatment with differentiation promoting (DP) media, but was significantly decreased when co-treated with DP media and the small molecule inhibitor of MDM2-p53 interaction, Nutlin-3. We have also observed a dramatic increase of the OC promoter activity in the presence of p53 and Mdm2 with inclusion of Cbfa-1 and p300 factors. Our results suggest that under some physiological conditions the oncoprotein MDM2 may cooperate with p53 to regulate the osteocalcin gene during osteoblastic differentiation.

Krüppel-like factor 4 regulates membranous and endochondral ossification

Krüppel-like factor 4 (KLF4/GKLF/EZF) is a zinc finger type of transcription factor highly expressed in the skin, intestine, testis, lung and bone. The role played by Klf4 has been studied extensively in normal epithelial development and maintenance; however, its role in bone cells is unknown. Previous reports showed that Klf4 is expressed in the developing flat bones but its expression diminishes postnatally. We now show that in the developing long bones, Klf4 is expressed in the perichondrium, trabecular osteoblasts and prehypertrophic chondrocytes. In contrast, osteoblasts lining at the surface of the bone collar showed extremely low levels of Klf4 expression. To investigate the possible roles played by Klf4 during skeletal development, we generated transgenic mice expressing Klf4 under mouse type I collagen regulatory sequence. Transgenic mice exhibited severe skeletal deformities and died soon after birth. Transgenic mice showed delayed formation of the calvarial bones; and over-expressing Klf4 in primary mouse calvarial osteoblasts in culture resulted in strong repression of mineralization indicating that this regulation of Klf4 is through an osteoblast-autonomous effect. Surprisingly, long bones of the transgenic mice exhibited delayed marrow cavity formation. Even at E18.5, the presumptive marrow space was occupied by cartilage anlage and invasion of the vascular endothelial cells and osteoclasts were seldom observed. Instead of entering the cartilage anlage, osteoclasts accumulated at the periosteum in the transgenic mice. Significantly, osteocalcin, which is known to chemotact osteoclasts, was up-regulated at the perichindrium as early as E14.5 in the mutants. In vitro studies showed that this induction of osteocalcin by Klf4 was regulated at its transcriptional level. Our results demonstrate that Klf4 regulates normal skeletal development through coordinating the differentiation and migration of osteoblasts, chondrocytes, vascular endothelial cells and osteoclasts.

The influence of inhaled fluticasone on bone metabolism and calciuria in asthmatic children

BACKGROUND

Inhaled corticosteroids have become the first-line medication for treatment of childhood asthma. Possible adverse effects, such as those on bone metabolism and urinary excretion of calcium are in the focus of interest. We investigated the influence of inhaled fluticasone on bone metabolism and renal excretion of calcium, sodium, and potassium in asthmatic children.

METHODS

Thirty asthmatic patients (mean age 12.24 ± 2.75 years) treated with 200-250 μg/day inhaled fluticasone were enrolled in the study. Prior to the initiation of therapy, as well as 12 weeks after following parameters were measured: serum sodium, potassium, calcium, phosphorus, creatinine, alkaline phosphatase, osteocalcin, intact parathyroid hormone levels, first-spot morning urine calcium/creatinine ratio, sodium/potassium ratio, and daily renal calcium excretion rate.

RESULTS

Serum electrolytes, alkaline phosphatase, parathyroid hormone, osteocalcin levels, as well as urinary calcium, sodium, and potassium excretion were within normal ranges. There was no statistical difference between values of those parameters prior and 12 weeks after initiation of the therapy.

CONCLUSIONS

Treatment with 200-250 μg/day inhaled fluticasone, in asthmatic children aged 9-16, during 12 weeks, did not affect serum osteocalcin level and renal excretion of calcium, sodium, and potassium.

Interplay between serum osteocalcin and insulin sensitivity in primary hyperparathyroidism

Osteocalcin (OC) has been proposed as a regulator of insulin sensitivity in both humans and other animals. Primary hyperparathyroidism (PHPT) is characterized by high OC levels and insulin resistance. The aim of this study was to evaluate whether in PHPT the link between OC levels and blood markers of insulin resistance was maintained. In a consecutive series of 219 adult PHPT patients, serum OC as well as fasting insulin and glucose levels were measured. Insulin sensitivity was estimated by homeostatic model assessment (HOMA2-S%). The same parameters were evaluated in a subgroup of 45 patients after parathyroidectomy (PTX). PHPT patients were characterized by markedly high OC levels. After subdividing them according to glucose tolerance, it was found that OC was similar in subjects with normal glucose tolerance (NGT) and impaired glucose tolerance (IGT), while diabetic subjects had lower serum OC than those with NGT (P < 0.02) or IGT (P < 0.04). OC was negatively associated with fasting glucose and positively associated with HOMA2-S%. OC independently predicted HOMA2-S% in a multivariate analysis. In the subgroup of surgically cured PHPT patients, OC levels significantly decreased after PTX, while HOMA2-S% did not change. Our findings indicate that in PHPT there is a positive relationship between OC and glucose metabolism, OC being one of the predictors of insulin sensitivity. However, data in surgically cured patients, showing OC normalization in spite of unchanged HOMA2-S%, suggest that OC does not likely play a major role in affecting insulin sensitivity in PHPT.

Effects of catalase gene (RS769217) polymorphism on energy homeostasis and bone status are gender specific

Az oxidatív stressz kóroki szerepe a csontállapot és a szénhidrát-anyagcsere romlásában ma már elfogadott. Vizsgálatunk során az egyik legismertebb antioxidáns enzim, a kataláz +22348C>T (RS769217) polimorfizmus hatását vizsgáltuk az inzulinérzékenységre, glükózfelhasználásra és a csontok denzitására. A glükózfelhasználás mérését (hyperinsulinaemiás klemp) és a genotipizálást 51 nő (24 egészséges, 27 glükózintoleráns: IFG, IGT és kezelést nem igénylő 2DM) és 90 férfi (64 egészséges és 26 glükózintoleráns) esetében végeztük el. Az allélfrekvenciákban a vizsgált dunántúli populációban, a nemek és csoportok között nem találtunk szignifikáns különbséget. A katalázgén-polimorfizmus anyagcsere- és csonthatása a nemek szerint különbözött. Nők esetében a T-allél megjelenése szignifikánsan jobb HOMA-IR indexet (CC: 2,95±1,8 vs. CT+TT: 2,06±0,9, p<0,05) és a TT-homozygoták esetében jobb teljestest-glükózfelhasználást eredményezett (M-1: CC: 9,43±4,4 vs. TT: 13,23±1,6 mg/kg/min, p<0,05), de a csontok denzitása nem különbözött. Férfiaknál a T-allél megjelenése alacsonyabb femurdenzitással (CC: 1,110±0,17 vs. CT+TT: 1,030±0,16, p<0,05 g/cm 2 ) és jobb HOMA-indexszel (CC: 2,42±2,3 vs. CT+TT: 1,50±0,2, p<0,05 ) társult, de javulást az izomszövet cukorfelhasználásában nem mértünk. A szervezet energia-háztartását és a csontanyagcserét összekapcsoló osteocalcin anyagcsere-kapcsolata nők esetében (r = +0,4424, p<0,05, n = 23) a T-allél megjelenésekor eltűnik. A többszörös korrelációs számítások szerint a leptin/adiponektin arány nők esetében a femur, férfiak esetében az L1-4 BMD-értékét befolyásolja, de ezek a kapcsolatok a T-allél megjelenésekor megszűntek. Eredményeink eltérnek a koreai nőkön mért adatoktól, és hangsúlyozzák a genetikai vizsgálatok különböző populációkon történő ismétlésének szükségét, és az anyagcsereadatok nemek szerinti értékelésének fontosságát.