Review Series
Abstract
Although obesity is typically associated with metabolic dysfunction and cardiometabolic diseases, some people with obesity are protected from many of the adverse metabolic effects of excess body fat and are considered “metabolically healthy.” However, there is no universally accepted definition of metabolically healthy obesity (MHO). Most studies define MHO as having either 0, 1, or 2 metabolic syndrome components, whereas many others define MHO using the homeostasis model assessment of insulin resistance (HOMA-IR). Therefore, numerous people reported as having MHO are not metabolically healthy, but simply have fewer metabolic abnormalities than those with metabolically unhealthy obesity (MUO). Nonetheless, a small subset of people with obesity have a normal HOMA-IR and no metabolic syndrome components. The mechanism(s) responsible for the divergent effects of obesity on metabolic health is not clear, but studies conducted in rodent models suggest that differences in adipose tissue biology in response to weight gain can cause or prevent systemic metabolic dysfunction. In this article, we review the definition, stability over time, and clinical outcomes of MHO, and discuss the potential factors that could explain differences in metabolic health in people with MHO and MUO — specifically, modifiable lifestyle factors and adipose tissue biology. Better understanding of the factors that distinguish people with MHO and MUO can produce new insights into mechanism(s) responsible for obesity-related metabolic dysfunction and disease.
Authors
Gordon I. Smith, Bettina Mittendorfer, Samuel Klein
Abstract
Obesity originates from an imbalance between caloric intake and energy expenditure that promotes adipose tissue expansion, which is necessary to buffer nutrient excess. Patients with higher visceral fat mass are at a higher risk of developing severe complications such as type 2 diabetes and cardiovascular and liver diseases. However, increased fat mass does not fully explain obesity’s propensity to promote metabolic diseases. With chronic obesity, adipose tissue undergoes major remodeling, which can ultimately result in unresolved chronic inflammation leading to fibrosis accumulation. These features drive local tissue damage and initiate and/or maintain multiorgan dysfunction. Here, we review the current understanding of adipose tissue remodeling with a focus on obesity-induced adipose tissue fibrosis and its relevance to clinical manifestations.
Authors
Geneviève Marcelin, Ana Letícia M. Silveira, Laís Bhering Martins, Adaliene V.M. Ferreira, Karine Clément
Abstract
Adipose tissue plays important roles in regulating whole-body energy metabolism through its storage function in white adipocytes and its dissipating function in brown and beige adipocytes. Adipose tissue also produces a variety of secreted factors called adipocytokines, including leptin and adiponectin. Furthermore, recent studies have suggested the important roles of extracellular vesicles of endosomal origin termed exosomes, which are secreted from adipocytes and other cells in adipose tissue and influence whole-body glucose and lipid metabolism. Adiponectin is known to be a pleiotropic organ-protective protein that is exclusively produced by adipocytes and decreased in obesity. Adiponectin accumulates in tissues such as heart, muscle, and vascular endothelium through binding with T-cadherin, a glycosylphosphatidylinositol-anchored (GPI-anchored) cadherin. Recently, adiponectin was found to enhance exosome biogenesis and secretion, leading to a decrease in cellular ceramides, excess of which is known to cause insulin resistance and cardiovascular disease phenotypes. These findings support the hypothesis that adipose tissue metabolism systemically regulates exosome production and whole-body metabolism through exosomes. This review focuses on intra-adipose and interorgan communication by exosomes, adiponectin-stimulated exosome production, and their dysregulation in metabolic diseases.
Authors
Shunbun Kita, Norikazu Maeda, Iichiro Shimomura
Abstract
In a society where physical activity is limited and food supply is abundant, metabolic diseases are becoming a serious epidemic. Metabolic syndrome (MetS) represents a cluster of metabolically related symptoms such as obesity, hypertension, dyslipidemia, and carbohydrate intolerance, and significantly increases type 2 diabetes mellitus risk. Insulin resistance and hyperinsulinemia are consistent characteristics of MetS, but which of these features is the initiating insult is still widely debated. Regardless, both of these conditions trigger adverse responses from the pancreatic β cell, which is responsible for producing, storing, and releasing insulin to maintain glucose homeostasis. The observation that the degree of β cell dysfunction correlates with the severity of MetS highlights the need to better understand β cell dysfunction in the development of MetS. This Review focuses on the current understanding from rodent and human studies of the progression of β cell responses during the development of MetS, as well as recent findings addressing the complexity of β cell identity and heterogeneity within the islet during disease progression. The differential responses observed in β cells together with the heterogeneity in disease phenotypes within the patient population emphasize the need to better understand the mechanisms behind β cell adaptation, identity, and dysfunction in MetS.
Authors
Laura I. Hudish, Jane E.B. Reusch, Lori Sussel
Abstract
The distribution of telomere length in humans is broad, but it has finite upper and lower boundaries. Growing evidence shows that there are disease processes that are caused by both short and long telomere length extremes. The genetic basis of these short and long telomere syndromes may be linked to mutations in the same genes, such as the telomerase reverse transcriptase (TERT), but through differential effects on telomere length. Short telomere syndromes have a predominant degenerative phenotype marked by organ failure that most commonly manifests as pulmonary fibrosis and are associated with a relatively low cancer incidence. In contrast, insights from studies of cancer-prone families as well as genome-wide association studies (GWAS) have identified both rare and common variants that lengthen telomeres as being strongly associated with cancer risk. We have hopothesized that these cancers represent a long telomere syndrome that is associated with a high penetrance of cutaneous melanoma and chronic lymphocytic leukemia. In this Review, we will synthesize the clinical and human genetic observations with data from mouse models to define the role of telomeres in cancer etiology and biology.
Authors
Emily J. McNally, Paz J. Luncsford, Mary Armanios
Abstract
The metabolic syndrome (MetS) encompasses medical conditions such as obesity, hyperglycemia, high blood pressure, and dyslipidemia that are major drivers for the ever-increasing prevalence of type 2 diabetes, cardiovascular diseases, and certain types of cancer. At the core of clinical strategies against the MetS is weight loss, induced by bariatric surgery, lifestyle changes based on calorie reduction and exercise, or pharmacology. This Review summarizes the past, current, and future efforts of targeting the MetS by pharmacological agents. Major emphasis is given to drugs that target the CNS as a key denominator for obesity and its comorbid sequelae.
Authors
Kerstin Stemmer, Timo D. Müller, Richard D. DiMarchi, Paul T. Pfluger, Matthias H. Tschöp
Abstract
Lipodystrophies are the result of a range of inherited and acquired causes, but all are characterized by perturbations in white adipose tissue function and, in many instances, its mass or distribution. Though patients are often nonobese, they typically manifest a severe form of the metabolic syndrome, highlighting the importance of white fat in the “safe” storage of surplus energy. Understanding the molecular pathophysiology of congenital lipodystrophies has yielded useful insights into the biology of adipocytes and informed therapeutic strategies. More recently, genome-wide association studies focused on insulin resistance have linked common variants to genes implicated in adipose biology and suggested that subtle forms of lipodystrophy contribute to cardiometabolic disease risk at a population level. These observations underpin the use of aligned treatment strategies in insulin-resistant obese and lipodystrophic patients, the major goal being to alleviate the energetic burden on adipose tissue.
Authors
Jake P. Mann, David B. Savage
Abstract
Skin and intestinal epithelial barriers play a pivotal role in protecting underlying tissues from harsh external environments. The protective role of these epithelia is, in part, dependent on a remarkable capacity to restore barrier function and tissue homeostasis after injury. In response to damage, epithelial wounds repair by a series of events that integrate epithelial responses with those of resident and infiltrating immune cells including neutrophils and monocytes/macrophages. Compromise of this complex interplay predisposes to development of chronic nonhealing wounds, contributing to morbidity and mortality of many diseases. Improved understanding of crosstalk between epithelial and immune cells during wound repair is necessary for development of better pro-resolving strategies to treat debilitating complications of disorders ranging from inflammatory bowel disease to diabetes. In this Review we focus on epithelial and innate immune cell interactions that mediate wound healing and restoration of tissue homeostasis in the skin and intestine.
Authors
Jennifer C. Brazil, Miguel Quiros, Asma Nusrat, Charles A. Parkos
Abstract
Immune cells are pivotal in the reaction to injury, whereupon, under ideal conditions, repair and resolution phases restore homeostasis following initial acute inflammation. Immune cell activation and reprogramming require transcriptional changes that can only be initiated if epigenetic alterations occur. Recently, accelerated deciphering of epigenetic mechanisms has extended knowledge of epigenetic regulation, including long-distance chromatin remodeling, DNA methylation, posttranslational histone modifications, and involvement of small and long noncoding RNAs. Epigenetic changes have been linked to aspects of immune cell development, activation, and differentiation. Furthermore, genome-wide epigenetic landscapes have been established for some immune cells, including tissue-resident macrophages, and blood-derived cells including T cells. The epigenetic mechanisms underlying developmental steps from hematopoietic stem cells to fully differentiated immune cells led to development of epigenetic technologies and insights into general rules of epigenetic regulation. Compared with more advanced research areas, epigenetic reprogramming of immune cells in injury remains in its infancy. While the early epigenetic mechanisms supporting activation of the immune response to injury have been studied, less is known about resolution and repair phases and cell type–specific changes. We review prominent recent findings concerning injury-mediated epigenetic reprogramming, particularly in stroke and myocardial infarction. Lastly, we illustrate how single-cell technologies will be crucial to understanding epigenetic reprogramming in the complex sequential processes following injury.
Authors
Katarzyna Placek, Joachim L. Schultze, Anna C. Aschenbrenner
Abstract
Acute organ injuries such as acute cerebrovascular accidents, myocardial infarction, acute kidney injury, acute lung injury, and others are among the leading causes of death worldwide. Dysregulated or insufficient organ repair mechanisms limit restoration of homeostasis and contribute to chronic organ failure. Studies reveal that both humans and mice harness potent non-stem cells that are capable of directly or indirectly promoting tissue repair. Specific populations of T lymphocytes have emerged as important reparative cells with context-specific actions. These T cells can resolve inflammation and secrete reparative cytokines and growth factors as well as interact with other immune and stromal cells to promote the complex and active process of tissue repair. This Review focuses on the major populations of T lymphocytes known to mediate tissue repair, their reparative mechanisms, and the diseases in which they have been implicated. Elucidating and harnessing the mechanisms that promote the reparative functions of these T cells could greatly improve organ dysfunction after acute injury.
Authors
Franco R. D’Alessio, Johanna T. Kurzhagen, Hamid Rabb
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Copyright © 2020 American Society for Clinical Investigation
ISSN: 0021-9738 (print), 1558-8238 (online)