Diet-induced obesity suppresses cortical bone accrual by a neuropeptide Y-dependent mechanism

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.

Energy Balance and Bone Health: a Nutrient Availability Perspective

PURPOSE OF REVIEW

Obesity is highly prevalent and is associated with bone fragility and fracture. The changing nutrient availability to bone in obesity is an important facet of bone health. The goal of this article is to summarize current knowledge on the effects of carbohydrate and dietary fat availability on bone, particularly in the context of other tissues.

RECENT FINDINGS

The skeleton is a primary site for fatty acid and glucose uptake. The trafficking of carbohydrates and fats into tissues changes with weight loss and periods of weight gain. Exercise acutely influences nutrient uptake into bone and may affect nutrient partitioning to bone. Bone cells secrete hormones that signal to the brain and other tissues information about its energetic state, which may alter whole-body nutrient trafficking. There is a critical need for studies to address the changes that metabolic perturbations have on nutrient availability in bone.

Obesity and Bone: A Complex Relationship

There is a large literature on the relationship between obesity and bone. What we can conclude from this review is that the increase in body weight causes an increase in BMD, both for a mechanical effect and for the greater amount of estrogens present in the adipose tissue. Nevertheless, despite an apparent strengthening of the bone witnessed by the increased BMD, the risk of fracture is higher. The greater risk of fracture in the obese subject is due to various factors, which are carefully analyzed by the Authors. These factors can be divided into metabolic factors and increased risk of falls. Fractures have an atypical distribution in the obese, with a lower incidence of typical osteoporotic fractures, such as those of hip, spine and wrist, and an increase in fractures of the ankle, upper leg, and humerus. In children, the distribution is different, but it is not the same in obese and normal-weight children. Specifically, the fractures of the lower limb are much more frequent in obese children. Sarcopenic obesity plays an important role. The authors also review the available literature regarding the effects of high-fat diet, weight loss and bariatric surgery.

Endocrine role of bone in the regulation of energy metabolism

Bone mainly functions as a supportive framework for the whole body and is the major regulator of calcium homeostasis and hematopoietic function. Recently, an increasing number of studies have characterized the significance of bone as an endocrine organ, suggesting that bone-derived factors regulate local bone metabolism and metabolic functions. In addition, these factors can regulate global energy homeostasis by altering insulin sensitivity, feeding behavior, and adipocyte commitment. These findings may provide a new pathological mechanism for related metabolic diseases or be used in the diagnosis, treatment, and prevention of metabolic diseases such as osteoporosis, obesity, and diabetes mellitus. In this review, we summarize the regulatory effect of bone and bone-derived factors on energy metabolism and discuss directions for future research.

Changes in bone mass associated with obesity and weight loss in humans: Applicability of animal models

The implications of obesity and weight loss for human bone health are not well understood. Although the bone changes associated with weight loss are similar in humans and rodents, that is not the case for obesity. In humans, obesity is generally associated with increased bone mass, an outcome which is exacerbated by advanced age and menopause. In rodents, by contrast, bone mass decreases in proportion to severity and duration of obesity, and is influenced by sex, age and mechanical load. Despite these discrepancies, rodents are frequently used to model the situation in humans. In this review, we summarise the existing knowledge of the effects of obesity and weight loss on bone mass in humans and rodents, focusing on the translatability of findings from animal models. We then describe how animal models should be used to broaden the understanding of the relationship between obesity, weight loss, and skeletal health in humans. Specifically, we highlight the aspects of study design that should be considered to optimise translatability of the rodent models of obesity and weight loss. Notably, the sex, age, and nutritional status of the animals should ideally match those of interest in humans. With these caveats in mind, and depending on the research question asked, our review underscores that animal models can provide valuable information for obesity and weight-management research.

Neuropeptide Y Regulation of Energy Partitioning and Bone Mass During Cold Exposure

The maintenance of whole body energy homeostasis is critical to survival and mechanisms exist whereby an organism can adapt to its environment and the stresses placed upon it. Environmental temperature and thermogenesis are key components known to affect energy balance. However, little is known about how these processes are balanced against the overall energy balance. We show that even mild cold exposure has a significant effect on energy expenditure and UCP-1 levels which increase by 43% and 400%, respectively, when wild-type (WT) mice at thermoneutral (29 °C) were compared to mice at room temperature (22 °C) conditions. Interestingly, bone mass was lower in cold-stressed WT mice with significant reductions in femoral bone mineral content (- 19%) and bone volume (- 13%). Importantly, these cold-induced skeletal changes were absent in mice lacking NPY, one of the main controllers of energy homeostasis, highlighting the critical role of NPY in this process. However, energy expenditure was significantly greater in cold-exposed NPY null mice, indicating that suppression of non-thermogenic tissues, like bone, contributes to the adaptive responses to cold exposure. Altogether, this work identifies NPY as being crucial in coordinating energy and bone homeostasis where it suppresses energy expenditure, UCP-1 levels and lowers bone mass under conditions of cold exposure.

Skeletal phenotype of the neuropeptide Y knockout mouse

Neuropeptide Y (NPY) is involved in multiple processes such as behavior, energy and bone metabolism. Previous studies have relied on global NPY depletion to examine its effects on bone. However, this approach is unable to distinguish the central or local source of NPY influencing bone. Our aim was to identify which cells within the skeleton express Npy and establish a model that will enable us to differentiate effects of NPY derived from different cell types. We have generated the NPY floxed (NPY^flox) mice using CRISPR technology. By crossing the NPY^flox mice with Hypoxanthine Phosphoribosyltransferase 1 (Hprt)-cre to generate a global knockout, we were able to validate and confirm loss of Npy transcript and protein in our global NPYKO. Global deletion of NPY results in a smaller femoral cortical cross-sectional area (-12%) and reduced bone strength (-18%) in male mice. In vitro, NPY-deficient bone marrow stromal cells (BMSCs) showed increase in osteogenic differentiation detected by increases in alkaline phosphatase staining and bone sialoprotein and osteocalcin expression. Despite both sexes presenting with increased adiposity, female mice had no alterations in bone mass, suggesting that NPY may have sex-specific effects on bone. In this study we identified Npy expression in the skeleton and examined the effect of global NPY depletion to bone mass. The differential impact of NPY deletion in cortical and cancellous compartments along with differences in phenotypes between in vitro and in vivo, highlights the complex nature of NPY signaling, indicative of distinct sources that can be dissected in the future using this NPY^flox model.