Yourka D. Tchoukalova

Sex dimorphism and depot differences in adipose tissue function.

Obesity, characterized by excessive adiposity, is a risk factor for many metabolic pathologies, such as type 2 diabetes mellitus (T2DM). Numerous studies have shown that adipose tissue distribution may be a greater predictor of metabolic health. Upper-body fat (visceral and subcutaneous abdominal) is commonly associated with the unfavorable complications of obesity, while lower-body fat (gluteal-femoral) may be protective. Current research investigations are focused on analyzing the metabolic properties of adipose tissue, in order to better understand the mechanisms that regulate fat distribution in both men and women. This review will highlight the adipose tissue depot- and sex-dependent differences in white adipose tissue function, including adipogenesis, adipose tissue developmental patterning, the storage and release of fatty acids, and secretory function. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

Effect of 8 Weeks of Overfeeding on Ectopic Fat Deposition and Insulin Sensitivity: Testing the “Adipose Tissue Expandability” Hypothesis

OBJECTIVE The presence of large subcutaneous adipocytes in obesity has been proposed to be linked with insulin resistance and type 2 diabetes through the “adipose tissue expandability” hypothesis, which holds that large adipocytes have a limited capacity for expansion, forcing lipids to be stored in nonadipose ectopic depots (skeletal muscle, liver), where they interfere with insulin signaling. This hypothesis has, however, been largely formulated by cross-sectional findings and to date has not been prospectively demonstrated in the development of insulin resistance in humans. RESEARCH DESIGN AND METHODS Twenty-nine men (26.8 ± 5.4 years old; BMI 25.5 ± 2.3 kg/m2) were fed 40% more than their baseline requirement for 8 weeks. Before and after overfeeding, insulin sensitivity was determined using a two-step hyperinsulinemic-euglycemic clamp. Intrahepatic lipid (IHL) and intramyocellular lipid (IMCL) were measured by 1H-MRS and abdominal fat by MRI. Subcutaneous abdominal adipose and skeletal muscle tissues were collected to measure adipocyte size and markers of tissue inflammation. RESULTS Subjects gained 7.6 ± 2.1 kg (55% fat) and insulin sensitivity decreased 18% (P < 0.001) after overfeeding. IHL increased 46% from 1.5% to 2.2% (P = 0.002); however, IMCL did not change. There was no association between adipocyte size and ectopic lipid accumulation. Despite similar weight gain, subjects with smaller fat cells at baseline had a greater decrease in insulin sensitivity, which was linked with upregulated skeletal muscle tissue inflammation. CONCLUSIONS In experimental substantial weight gain, the presence of larger adipocytes did not promote ectopic lipid accumulation. In contrast, smaller fat cells were associated with a worsened metabolic response to overfeeding.

Differential Effect of Weight Loss on Adipocyte Size Subfractions in Patients With Type 2 Diabetes

The size of adipocytes influences their function suggesting a differential responsiveness to intervention. We hypothesized that weight loss in patients with type 2 diabetes mellitus (T2DM) predominantly decreases the size of large and very‐large adipocyte subfractions in parallel with beneficial changes in serum adipokines and improved insulin sensitivity. A total of 44 volunteers from the Look Action for Health in Diabetes trial, who lost weight after 1‐year of intense lifestyle intervention, were included. Insulin sensitivity (hyperinsulinemic–euglycemic clamp), size of subcutaneous abdominal adipocytes (osmium fixation), and selected serum adipokines were measured. A 13% weight loss was accompanied by 46% improvement in insulin sensitivity (increased glucose disposal rate from 5.9 ± 2.2 to 8.6 ± 2.7 mg/min/kg fat‐free mass, P < 0.05) in parallel with a 36% increase in plasma adiponectin concentration (6.1 ± 3.1 to 8.3 ± 3.9 µg/ml, P < 0.05], but no changes in the proinflammatory cytokines interleukin‐6 and tumor necrosis factor‐α. Change in adiponectin correlated with changes in glucose disposal rate (r = 0.34, P < 0.05). Mean adipocyte size decreased (0.84 ± 0.25 to 0.64 ± 0.23 µl, P < 0.05), mainly due to changes in the large adipocyte subfraction (size 0.75–0.44 µl, relative number 19–26%; P < 0.05). Our data suggest that change in the large adipocyte subfraction may contribute to the improvement in insulin sensitivity via an increase in serum adiponectin. Such a relationship, which does not imply cause and effect, could not be obtained by measuring only mean adipocyte size. These data provide support for the measures of adipocyte size distribution in concert with in vitro adipokine secretion and lipolysis in future studies.

Weight Gain Reveals Dramatic Increases in Skeletal Muscle Extracellular Matrix Remodeling

CONTEXT In animal models of obesity, chronic inflammation and dysregulated extracellular matrix remodeling in adipose tissue leads to insulin resistance. Whether similar pathophysiology occurs in humans is not clear. OBJECTIVE The aim of this study was to test whether 10% weight gain induced by overfeeding triggers inflammation and extracellular matrix remodeling (gene expression, protein, histology) in skeletal muscle and sc adipose tissue in humans. We also investigated whether such remodeling was associated with an impaired metabolic response (hyperinsulinemic-euglycemic clamp). DESIGN, SETTING, PARTICIPANTS, AND INTERVENTION Twenty-nine free-living males were fed 40% over their baseline energy requirements for 8 weeks. RESULTS Ten percent body weight gain prompted dramatic up-regulation of a repertoire of extracellular matrix remodeling genes in muscle and to a lesser degree in adipose tissue. The amount of extracellular matrix genes in the muscle were directly associated with the amount of lean tissue deposited during overfeeding. Despite weight gain and impaired insulin sensitivity, there was no change in local adipose tissue or systemic inflammation, but there was a slight increase in skeletal muscle inflammation. CONCLUSION We propose that skeletal muscle extracellular matrix remodeling is another feature of the pathogenic milieu associated with energy excess and obesity, which, if disrupted, may contribute to the development of metabolic dysfunction.

In Vivo Adipogenesis in Rats Measured by Cell Kinetics in Adipocytes and Plastic-Adherent Stroma-Vascular Cells in Response to High-Fat Diet and Thiazolidinedione

Impairment of adipogenesis contributes to the development of obesity-related insulin resistance. The current in vitro approaches for its assessment represent crude estimates of the adipogenic potential because of the disruption of the in vivo microenvironment. A novel assessment of in vivo adipogenesis using the incorporation of the stable isotope deuterium (2H) into the DNA of isolated adipocytes and stroma-vascular fraction from adipose tissue has been developed. In the current study, we have refined this technique by purifying the adipocytes via a negative immune selection and sorting the plastic adherent stroma-vascular (aSV) subfraction (using 3 h culture) that contains mostly adipocyte progenitor cells and ∼10% of small adipocytes. Using a 3-week 8% 2H2O ingestion with a high-fat diet (HFD) or HFD plus pioglitazone (HFD-P), we demonstrate that the fractions of new aSV cells (faSV) and immunopurified adipocytes (fAD) (the ratio of their 2H-enrichment of DNA to the maximal 2H-enrichment of DNA of bone marrow reference cells) recapitulate the known hyperplastic mechanism of weight gain with pioglitazone treatment. We conclude that faSV and fAD are reliable indices of in vivo adipogenesis. The proposed method represents a valuable tool for studying the effect of interventions (drugs, diets, and exercise) on in vivo adipogenesis.

Potential role of increased matrix metalloproteinase-2 (MMP2) transcription in impaired adipogenesis in type 2 diabetes mellitus

We measured gene expression of paracrine regulators involved in adipocyte differentiation within the stromovascular fraction of abdominal subcutaneous adipose tissue from obese individuals with (n=30) and without (n=18) type 2 diabetes mellitus (T2DM). Despite similar adiposity by design, subjects with T2DM had larger adipocytes (0.92+/-0.28 vs. 0.75+/-0.17 microl, p<0.05) than controls. Gene expression of the adipogenic marker aP2 was lower (0.35+/-0.16 vs. 0.58+/-0.27 arbitrary units, p<0.05) whereas the expression of matricellular peptidase, MMP2 was higher (1.65+/-0.17 vs. 1.27+/-0.21, p=0.02) in T2DM vs. controls. The gene expression levels between the aP2 and MMP2 were inversely correlated (r=-0.32, p=0.03). We conclude that early steps of adipogenesis may be impaired in T2DM independently of obesity due, in part, to an upregulation of the MMP2 transcription.

Fetal baboon sex-specific outcomes in adipocyte differentiation at 0.9 gestation in response to moderate maternal nutrient reduction

Objective:To investigate in vitro adipocyte differentiation in baboon fetuses in response to reduced maternal nutrition.Design:Cross-sectional comparison of adipocyte differentiation in normally grown fetuses and fetuses of pregnant baboons fed 70% of the control global diet from 30 days of pregnancy to term.Subjects:The subjects comprised control (CTR) fetuses (five female and five male) of mothers fed ad libitum and fetuses of mothers fed 70% of the global diet consumed by CTR (maternal nutrient reduction (MNR), five female and five male fetuses). The expression of genes/proteins involved in adipogenesis (PPARγ, FABP4 and adiponectin) and brown adipose tissue development (UCP1, TBX15 and COXIV) were determined in in vitro-differentiated stromal–vascular cultures from subcutaneous abdominal, subcutaneous femoral and omental adipose tissue depots. Adipocyte number per area (mm2) was determined histologically to assist in the evaluation of adipocyte size.Results:Maternal suboptimal nutrition suppressed growth of male but not female fetuses and led to adipocyte hypertrophy accompanied by increased markers of white- and, particularly, brown-type adipogenesis in male but not female fetuses.Conclusion:Adipose tissue responses to fetal nonhuman primate undernutrition are sexually dimorphic. While female fetuses adapt adequately, the male ones enhance pathways involved in white and brown adipose tissue development but are unable to compensate for a delayed development of adipose tissue associated with intrauterine growth restriction. These differences need to be considered when assessing developmental programming of adiposity in response to suboptimal maternal nutrition.

Potential effects of aerobic exercise on the expression of perilipin 3 in the adipose tissue of women with polycystic ovary syndrome: a pilot study.

OBJECTIVE Polycystic ovary syndrome (PCOS) is associated with reduced adipose tissue lipolysis that can be rescued by aerobic exercise. We aimed to identify differences in the gene expression of perilipins and associated targets in adipose tissue in women with PCOS before and after exercise. DESIGN AND METHODS We conducted a cross-sectional study in eight women with PCOS and eight women matched for BMI and age with normal cycles. Women with PCOS also completed a 16-week prospective aerobic exercise-training study. Abdominal subcutaneous adipose tissue biopsies were collected, and primary adipose-derived stromal/stem cell cultures were established from women with PCOS before 16 weeks of aerobic exercise training (n=5) and controls (n=5). Gene expression was measured using real-time PCR, in vitro lipolysis was measured using radiolabeled oleate, and perilipin 3 (PLIN3) protein content was measured by western blotting analysis. RESULTS The expression of PLIN1, PLIN3, and PLIN5, along with coatomers ARF1, ARFRP1, and βCOP was ∼ 80% lower in women with PCOS (all P<0.05). Following exercise training, PLIN3 was the only perilipin to increase significantly (P<0.05), along with coatomers ARF1, ARFRP1, βCOP, and SEC23A (all P<0.05). Furthermore, PLIN3 protein expression was undetectable in the cell cultures from women with PCOS vs controls. Following exercise training, in vitro adipose oleate oxidation, glycerol secretion, and PLIN3 protein expression were increased, along with reductions in triglyceride content and absence of large lipid droplet morphology. CONCLUSIONS These findings suggest that PLIN3 and coatomer GTPases are important regulators of lipolysis and triglyceride storage in the adipose tissue of women with PCOS.

Adipose Stem Cells and Adipogenesis

Adipocytes are highly specialized cells that form and store fat in adipose tissue and play a major role in energy homeostasis in vertebrate organisms. Obesity results from an energy surplus and is characterized by an increased storage of lipid and expansion of adipose tissue. Obesity modifies the endocrine and metabolic functions of adipocytes and is a risk factor for many other metabolic diseases, including type II diabetes, cardiovascular ischemic disease, atherosclerosis, and hypertension.

Implications of 2H-labeling of DNA protocol to measure in vivo cell turnover in adipose tissue

Adipose tissue expansion in obesity involves a series of cycles of adipocyte hyperplasia, hypertrophy and hypoplasia due to alterations in adipogenesis, adipocyte cellular metabolism and cell death, respectively. Increased frequency of these cycles may lead to deterioration of adipocyte function and viability, accelerated exhaustion of the adipocyte progenitor pool and extensive adipose tissue remodeling, all leading to impaired expandability of subcutaneous adipose tissue, ectopic lipid accumulation and insulin resistance. Understanding the mechanisms that contribute to adipocyte turnover is thus important. We have recently refined and published an existing method to assess in vivo adipogenesis using incorporation of the stable isotope deuterium into the DNA of isolated adipocytes and adipocyte progenitors from adipose tissue. In this commentary, we highlight further implications of this method to determine the rate of adipocyte hypertrophy and adipocyte death that will enhance our understanding of adipocyte cell turnover and cellular mechanisms that control regional adipose tissue growth.