The current obesity epidemic poses a major public health issue since obesity predisposes towards several chronic diseases BMI and total adiposity are positively correlated with cardiometabolic disease risk at the population level However, body fat distribution and an impaired adipose tissue function, rather than total fat mass, better predict insulin resistance and related complications at the individual level Adipose tissue dysfunction is determined by an impaired adipose tissue expandability, adipocyte hypertrophy, altered lipid metabolism, and local inflammation Recent human studies suggest that adipose tissue oxygenation may be a key factor herein A subgroup of obese individuals - the 'metabolically healthy obese' (MHO) - have a better adipose tissue function, less ectopic fat storage, and are more insulin sensitive than obese metabolically unhealthy persons, emphasizing the central role of adipose tissue function in metabolic health However, controversy has surrounded the idea that metabolically healthy obesity may be considered really healthy since MHO individuals are at increased (cardio)metabolic disease risk and may have a lower quality of life than normal weight subjects due to other comorbidities Detailed metabolic phenotyping of obese persons will be invaluable in understanding the pathophysiology of metabolic disturbances, and is needed to identify high-risk individuals or subgroups, thereby paving the way for optimization of prevention and treatment strategies to combat cardiometabolic diseases

/pdf/the-metabolic-phenotype-in-obesity-fat-mass-body-fat-1f28h51lv5.pdf

The Metabolic Phenotype in Obesity: Fat Mass, Body Fat Distribution, and Adipose Tissue Function

The worldwide epidemic of obesity has brought considerable attention to research aimed at understanding the biology of adipocytes (fat cells) and the events occurring in adipose tissue (fat) and in the bodies of obese individuals. Accumulating evidence indicates that obesity causes chronic low-grade inflammation and that this contributes to systemic metabolic dysfunction that is associated with obesity-linked disorders. Adipose tissue functions as a key endocrine organ by releasing multiple bioactive substances, known as adipose-derived secreted factors or adipokines, that have pro-inflammatory or anti-inflammatory activities. Dysregulated production or secretion of these adipokines owing to adipose tissue dysfunction can contribute to the pathogenesis of obesity-linked complications. In this Review, we focus on the role of adipokines in inflammatory responses and discuss their potential as regulators of metabolic function.

/pdf/adipokines-in-inflammation-and-metabolic-disease-2i05swlyav.pdf

Adipokines in inflammation and metabolic disease

Components of the immune system are altered in obesity and type 2 diabetes (T2D), with the most apparent changes occurring in adipose tissue, the liver, pancreatic islets, the vasculature and circulating leukocytes. These immunological changes include altered levels of specific cytokines and chemokines, changes in the number and activation state of various leukocyte populations and increased apoptosis and tissue fibrosis. Together, these changes suggest that inflammation participates in the pathogenesis of T2D. Preliminary results from clinical trials with salicylates and interleukin-1 antagonists support this notion and have opened the door for immunomodulatory strategies for the treatment of T2D that simultaneously lower blood glucose levels and potentially reduce the severity and prevalence of the associated complications of this disease.

Type 2 diabetes as an inflammatory disease.

https://www.cell.com/cell-stem-cell/pdf/S1934-5909(10)00341-3.pdf

Oxygen in Stem Cell Biology: A Critical Component of the Stem Cell Niche

Obesity is a critical risk factor for the development of type 2 diabetes (T2D), and its prevalence is rising worldwide. White adipose tissue (WAT) has a crucial role in regulating systemic energy homeostasis. Adipose tissue expands by a combination of an increase in adipocyte size (hypertrophy) and number (hyperplasia). The recruitment and differentiation of adipose precursor cells in the subcutaneous adipose tissue (SAT), rather than merely inflating the cells, would be protective from the obesity-associated metabolic complications. In metabolically unhealthy obesity, the storage capacity of SAT, the largest WAT depot, is limited, and further caloric overload leads to the fat accumulation in ectopic tissues (e.g., liver, skeletal muscle, and heart) and in the visceral adipose depots, an event commonly defined as “lipotoxicity.” Excessive ectopic lipid accumulation leads to local inflammation and insulin resistance (IR). Indeed, overnutrition triggers uncontrolled inflammatory responses in WAT, leading to chronic low-grade inflammation, therefore fostering the progression of IR. This review summarizes the current knowledge on WAT dysfunction in obesity and its associated metabolic abnormalities, such as IR. A better understanding of the mechanisms regulating adipose tissue expansion in obesity is required for the development of future therapeutic approaches in obesity-associated metabolic complications.

Adipose Tissue Dysfunction as Determinant of Obesity-Associated Metabolic Complications

OBJECTIVE— Based on rodent studies, we examined the hypothesis that increased adipose tissue (AT) mass in obesity without an adequate support of vascularization might lead to hypoxia, macrophage infiltration, and inflammation.

RESEARCH DESIGN AND METHODS— Oxygen partial pressure (AT pO2) and AT temperature in abdominal AT (9 lean and 12 overweight/obese men and women) was measured by direct insertion of a polarographic Clark electrode. Body composition was measured by dual-energy X-ray absorptiometry, and insulin sensitivity was measured by hyperinsulinemic-euglycemic clamp. Abdominal subcutaneous tissue was used for staining, quantitative RT-PCR, and chemokine secretion assay.

RESULTS— AT pO2 was lower in overweight/obese subjects than lean subjects (47 ± 10.6 vs. 55 ± 9.1 mmHg); however, this level of pO2 did not activate the classic hypoxia targets (pyruvate dehydrogenase kinase and vascular endothelial growth factor [VEGF]). AT pO2 was negatively correlated with percent body fat ( R = −0.50, P < 0.05). Compared with lean subjects, overweight/obese subjects had 44% lower capillary density and 58% lower VEGF, suggesting AT rarefaction (capillary drop out). This might be due to lower peroxisome proliferator–activated receptor γ1 and higher collagen VI mRNA expression, which correlated with AT pO2 ( P < 0.05). Of clinical importance, AT pO2 negatively correlated with CD68 mRNA and macrophage inflammatory protein 1α secretion ( R = −0.58, R = −0.79, P < 0.05), suggesting that lower AT pO2 could drive AT inflammation in obesity.

CONCLUSIONS— Adipose tissue rarefaction might lie upstream of both low AT pO2 and inflammation in obesity. These results suggest novel approaches to treat the dysfunctional AT found in obesity.

https://diabetes.diabetesjournals.org/content/diabetes/58/3/718.full.pdf

Reduced Adipose Tissue Oxygenation in Human Obesity: Evidence for Rarefaction, Macrophage Chemotaxis, and Inflammation Without an Angiogenic Response

Relation of adipose tissue to metabolic flexibility

Objectives: Basic science studies show that the extracellular matrix of adipose tissue, mainly represented by collagen VI, is dysfunctional in obesity and contributes to the development of the metabolic syndrome. We hypothesized in humans that increased collagen VI α3-subunit (COL6A3) mRNA is associated with adipose tissue macrophage chemotaxis and inflammation and that weight gain is accompanied by changes in the expression of COL6A3. Research Design and Methods: Adipose tissue biopsies were obtained from a cross-sectional study (n = 109), an overfeeding study (n = 9), and a pioglitazone treatment study (n = 14). Adipose tissue gene expression was measured by quantitative RT-PCR, immunohistochemistry, and adipocyte sizing by fixation with osmium and Coulter counting. Body composition was measured by dual-energy x-ray absorptiometry and visceral adipose tissue by computed tomography. Patients with high or low COL6A3 mRNA were compared by one-way ANOVA. Results: In humans, immunohistochemistry revealed tha...

/pdf/adipose-tissue-collagen-vi-in-obesity-4y45x5x1qq.pdf

Adipose Tissue Collagen VI in Obesity

Objectives: We hypothesized that compared to obese, patients with type 2 diabetes have lower total adipocyte number with less small adipocytes. Research design and methods: Abdominal subcutaneous adipose tissue was obtained from lean and obese with or without type 2 diabetes matched for BMI. Adipocyte size was measured by osmium fixation and sizing/counting in a Coulter counter. Adipocyte size and number sub-distributions [small, medium, large and very-large] were determined. Results: Compared to obese, type 2 diabetes patients had greater mean adipocyte size and 67% bigger very-large adipocytes; total adipocyte number was lower, but the fraction of small adipocytes was increased by 27%. Conclusions: Total adipocyte cellularity is lower in type 2 diabetes vs. obese. We found no evidence for depletion of small adipocytes in patients with type 2 diabetes. This suggests the presence of a defect in early maturation of adipocytes in patients with type 2 diabetes.

http://care.diabetesjournals.org/content/diacare/32/5/900.full.pdf

Lower Total Adipocyte Number but No Evidence for Small Adipocyte Depletion in Patients With Type 2 Diabetes

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Papers

Reduced Adipose Tissue Oxygenation in Human Obesity: Evidence for Rarefaction, Macrophage Chemotaxis, and Inflammation Without an Angiogenic Response

Relation of adipose tissue to metabolic flexibility

Adipose Tissue Collagen VI in Obesity

Lower Total Adipocyte Number but No Evidence for Small Adipocyte Depletion in Patients With Type 2 Diabetes