Combination of the C1429T polymorphism in the G-protein beta-3 subunit gene and the A1330V polymorphism in the low-density lipoprotein receptor-related protein 5 gene is a risk factor for hypercholesterolaemia

Regulation of Osteoblast Metabolism by Wnt Signaling

Wnt/β-catenin signaling plays a critical role in the achievement of peak bone mass, affecting the commitment of mesenchymal progenitors to the osteoblast lineage and the anabolic capacity of osteoblasts depositing bone matrix. Recent studies suggest that this evolutionarily-conserved, developmental pathway exerts its anabolic effects in part by coordinating osteoblast activity with intermediary metabolism. These findings are compatible with the cloning of the gene encoding the low-density lipoprotein related receptor-5 (LRP5) Wnt co-receptor from a diabetes-susceptibility locus and the now well-established linkage between Wnt signaling and metabolism. In this article, we provide an overview of the role of Wnt signaling in whole-body metabolism and review the literature regarding the impact of Wnt signaling on the osteoblast's utilization of three different energy sources: fatty acids, glucose, and glutamine. Special attention is devoted to the net effect of nutrient utilization and the mode of regulation by Wnt signaling. Mechanistic studies indicate that the utilization of each substrate is governed by a unique mechanism of control with β-catenin-dependent signaling regulating fatty acid β-oxidation, while glucose and glutamine utilization are β-catenin-independent and downstream of mammalian target of rapamycin complex 2 (mTORC2) and mammalian target of rapamycin complex 1 (mTORC1) activation, respectively. The emergence of these data has provided a new context for the mechanisms by which Wnt signaling influences bone development.

Lack of Lrp5 Signaling in Osteoblasts Sensitizes Male Mice to Diet-Induced Disturbances in Glucose Metabolism

Wnt signaling through the low-density lipoprotein-related receptor 5 (Lrp5) coreceptor regulates osteoblast maturation, matrix mineralization, and intermediary metabolism. In the mature osteoblast, signals downstream of Lrp5 are required for normal long-chain fatty acid β-oxidation. Mice rendered deficient for this coreceptor in osteoblasts and osteocytes accumulate body fat with elevated serum lipid levels but retain normal insulin sensitivity. In the present study, we challenged Lrp5-mutant mice with a high-fat diet (HFD) to determine whether they were more susceptible to diet-induced disturbances in glucose homeostasis. The HFD-fed Lrp5 mutant mice maintained a low bone mass phenotype with an increase in adipose tissue mass and hypertriglyceridemia and hypercholesterolemia. Examination of glucose metabolism revealed that Lrp5 deficiency in the osteoblast also resulted in hyperglycemia and hyperinsulinemia, with reductions in glucose tolerance, insulin sensitivity, and serum undercarboxylated osteocalcin. The results from in vivo genetic epistasis and in vitro studies suggest that this phenotype proceeds via the accumulation of diacylglycerol species and impaired insulin signaling in Lrp5-deficient osteoblasts. In turn, glucose uptake and osteocalcin production are diminished in mutant osteoblasts. Taken together, these data identify a link between Wnt-Lrp5 signaling and insulin signaling in the osteoblast that has the potential to influence energy balance and compound the detrimental effects of a HFD on whole-body metabolism.

Bone and the regulation of global energy balance

The skeleton, populated by large numbers of osteoblasts and long-lived osteocytes, requires a constant supply of energy-rich molecules to fuel the synthesis, deposition and mineralization of bone matrix during bone modelling and remodelling. When these energetic demands are not met, bone acquisition is suppressed. Recent findings suggest that key developmental signals emanating from Wnt low-density lipoprotein-related receptor 5 and hypoxia-inducible factor pathways impact osteoblast bioenergetics to accommodate the energy requirements for bone cells to fulfil their function. In vivo studies in several mutant mouse strains have confirmed a link between bone cells and global metabolism, ultimately leading to the identification of hormonal interactions between the skeleton and other tissues. The hormones insulin and leptin affect postnatal bone acquisition, whilst osteocalcin produced by the osteoblast in turn stimulates insulin secretion by the pancreas. These observations have prompted additional questions regarding the nature of the mechanisms of fuel sensing and processing in the osteoblast and their contribution to overall energy utilization and homeostasis. Answers to such questions should advance our understanding of metabolic diseases and may ultimately improve management of affected patients. In this review, we highlight recent studies in this field and offer a perspective on the evolutionary implications of bone as a metabolic endocrine organ.

A1330V polymorphism of low-density lipoprotein receptor-related protein 5 gene and self-reported incident fractures in Japanese female patients with rheumatoid arthritis

We attempted to determine whether the A1330V polymorphism of the low-density lipoprotein receptor-related protein 5 (LRP5) gene is associated with a risk of self-reported incident fractures and hypercholesterolemia in Japanese patients with rheumatoid arthritis (RA). DNA samples, laboratory data, and clinical data were obtained from 563 female RA patients who participated in the Institute of Rheumatology Rheumatoid Arthritis (IORRA) observational cohort study. A1330V genotyping was performed using a custom TaqMan assay. Multiple logistic regression analyses showed that any incident fracture was significantly associated with older age (P = 0.000000036), high Japanese Health Assessment Questionnaire (J-HAQ) score (P = 0.016), and high daily prednisolone dose (P = 0.031), but not with the A1330V polymorphism, while serum total cholesterol levels >or=220 mg/100 mL were independently correlated with baseline older age (P = 0.00011), low J-HAQ score (P = 0.0098), high body mass index (P = 0.024), 1330VV genotype (P = 0.027), and high daily prednisolone dose (P = 0.031). Our results suggest that this LPR5 polymorphism does not appear to be a clinically useful marker for the prediction of fracture risk in Japanese female RA patients, although it is associated with increased serum total cholesterol levels.