Subcutaneous Abdominal Adipose Tissue Subcompartments: Potential Role in Rosiglitazone Effects

Abdominal visceral tissue (VAT) and subcutaneous adipose tissue (SAT), comprised of superficial‐SAT (sSAT) and deep‐SAT (dSAT), are metabolically distinct. The antidiabetic agents thiazolidinediones (TZDs), in addition to their insulin‐sensitizing effects, redistribute SAT suggesting that TZD action involves adipose tissue depot‐specific regulation. We investigated the expression of proteins key to adipocyte metabolism on differentiated first passage (P1) preadipocytes treated with rosiglitazone, to establish a role for the diverse depots of abdominal adipose tissue in the insulin‐sensitizing effects of TZDs. Adipocytes and preadipocytes were isolated from sSAT, dSAT, and VAT samples obtained from eight normal subjects. Preadipocytes (P1) left untreated (U) or treated with a classic differentiation cocktail (DI) including rosiglitazone (DIR) for 9 days were evaluated for strata‐specific differences in differentiation including peroxisome proliferator‐activated receptor‐γ (PPAR‐γ) and lipoprotein lipase (LPL) expression, insulin sensitivity via adiponectin and glucose transport‐4 (GLUT4), glucocorticoid metabolism with 11β‐hydroxysteroid dehydrogenase type‐1 (11βHSD1), and alterations in the adipokine leptin. While depot‐specific differences were absent with the classic differentiation cocktail, with rosiglitazone sSAT had the most potent response followed by dSAT, whereas VAT was resistant to differentiation. With rosiglitazone, universal strata effects were observed for PPAR‐γ, LPL, and leptin, with VAT in all cases expressing significantly lower basal expression levels. Clear dSAT‐specific changes were observed with decreased intracellular GLUT4. Specific sSAT alterations included decreased 11βHSD1 whereas secreted adiponectin was potently upregulated in sSAT with respect to dSAT and VAT. Overall, the subcompartments of SAT, sSAT, and dSAT, appear to participate in the metabolic changes that arise with rosiglitazone administration.     

Impacts of Visceral Adipose Tissue and Subcutaneous Adipose Tissue on Metabolic Risk Factors in Middle‐aged Japanese

Regional fat distribution rather than overall fat volume has been considered to be important to understanding the link between obesity and metabolic disorders. We aimed to evaluate the independent associations of visceral adipose tissue (VAT) and subcutaneous adipose tissue (SAT) with metabolic risk factors in apparently healthy middle‐aged Japanese. Participants were 1,119 men and 854 women aged 38–60 years who were not taking medications for diabetes, hypertension, or dyslipidemia. VAT and SAT were measured by use of computed tomography (CT) scanning. VAT and SAT were significantly and positively correlated with each other in men (r = 0.531, P < 0.001) and women (r = 0.589, P < 0.001). In multiple regression analyses, either measure of abdominal adiposity (VAT or SAT) was positively associated with blood pressure, fasting plasma glucose, and log triglyceride (P < 0.001) and inversely with high‐density lipoprotein (HDL)‐cholesterol (P < 0.001). When VAT and SAT were simultaneously included in the model, the association of VAT with triglycerides was maintained (P < 0.001) but that of SAT was lost. The same was true for HDL‐cholesterol in women. For fasting plasma glucose, the association with VAT was strong (P < 0.001) and the borderline association with SAT was maintained (P = 0.060 in men and P = 0.020 in women). Both VAT and SAT were independently associated with blood pressure (P < 0.001). Further adjustment for anthropometric indices resulted in the independent association only with VAT for all risk factors. In conclusion, impacts of VAT and SAT differed among risk factors. VAT showed dominant impacts on triglyceride concentrations in both genders and on HDL‐cholesterol in women, while SAT also had an independent association with blood pressure.     

Pioglitazone Increases the Proportion of Small Cells in Human Abdominal Subcutaneous Adipose Tissue

Rodent and in vitro studies suggest that thiazolidinediones promote adipogenesis but there are few studies in humans to corroborate these findings. The purpose of this study was to determine whether pioglitazone stimulates adipogenesis in vivo and whether this process relates to improved insulin sensitivity. To test this hypothesis, 12 overweight/obese nondiabetic, insulin‐resistant individuals underwent biopsy of abdominal subcutaneous adipose tissue at baseline and after 12 weeks of pioglitazone treatment. Cell size distribution was determined via the Multisizer technique. Insulin sensitivity was quantified at baseline and postpioglitazone by the modified insulin suppression test. Regional fat depots were quantified by computed tomography (CT). Insulin resistance (steady‐state plasma insulin and glucose (SSPG)) decreased following pioglitazone (P < 0.001). There was an increase in the ratio of small‐to‐large cells (1.16 ± 0.44 vs. 1.52 ± 0.66, P = 0.03), as well as a 25% increase in the absolute number of small cells (P = 0.03). The distribution of large cell diameters widened (P = 0.009), but diameter did not increase in the case of small cells. The increase in proportion of small cells was associated with the degree to which insulin resistance improved (r = ?0.72, P = 0.012). Visceral abdominal fat decreased (P = 0.04), and subcutaneous abdominal (P = 0.03) and femoral fat (P = 0.004) increased significantly. Changes in fat volume were not associated with SSPG change. These findings demonstrate a clear effect of pioglitazone on human subcutaneous adipose cells, suggestive of adipogenesis in abdominal subcutaneous adipose tissue, as well as redistribution of fat from visceral to subcutaneous depots, highlighting a potential mechanism of action for thiazolidinediones. These findings support the hypothesis that defects in subcutaneous fat storage may underlie obesity‐associated insulin resistance.     

Abdominal Subcutaneous and Visceral Adipose Tissue and Insulin Resistance in the Framingham Heart Study

Insulin resistance is associated with central obesity and an increased risk of cardiovascular disease. Our objective is to examine the association between abdominal subcutaneous (SAT) and visceral adipose tissue (VAT) and insulin resistance, to determine which fat depot is a stronger correlate of insulin resistance, and to assess whether there was an interaction between SAT, VAT, and age, sex, or BMI. Participants without diabetes from the Framingham Heart Study (FHS), who underwent multidetector computed tomography to assess SAT and VAT (n = 3,093; 48% women; mean age 50.4 years; mean BMI 27.6 kg/m²), were evaluated. Insulin resistance was measured using the homeostasis model and defined as HOMA_IR ≥75th percentile. Logistic regression models, adjusted for age, sex, smoking, alcohol, menopausal status, and hormone replacement therapy use, were used to assess the association between fat measures and insulin resistance. The odds ratio (OR) for insulin resistance per standard deviation increase in SAT was 2.5 (95% confidence interval (CI): 2.2–2.7; P < 0.0001), whereas the OR for insulin resistance per standard deviation increase in VAT was 3.5 (95% CI: 3.1–3.9; P < 0.0001). Overall, VAT was a stronger correlate of insulin resistance than SAT (P < 0.0001 for SAT vs. VAT comparison). After adjustment for BMI, the OR of insulin resistance for VAT was 2.2 (95% CI: 1.9–2.5; P < 0.0001). We observed an interaction between VAT and BMI for insulin (P interaction = 0.0004), proinsulin (P interaction = 0.003), and HOMA_IR (P interaction = 0.003), where VAT had a stronger association in obese individuals. In conclusion, SAT and VAT are both correlates of insulin resistance; however, VAT is a stronger correlate of insulin resistance than SAT.     

Subdivision of the Subcutaneous Adipose Tissue Compartment and Lipid‐Lipoprotein Levels in Women

Objective: The aim of this study was to compare the relative importance of computed tomography‐measured abdominal fat compartment areas, including adipose tissue located posterior to the subcutaneous Fascia, in predicting plasma lipid‐lipoprotein alterations. Research Methods and Procedures: Areas of visceral as well as subcutaneous deep and superficial abdominal adipose tissue were measured by computed tomography in a sample of 66 healthy women, ages 37 to 60 years, for whom a detailed lipid‐lipoprotein profile was available. Results: Strong significant associations were observed between visceral adipose tissue area and most variables of the lipid‐lipoprotein profile (r = ?0.25, p < 0.05 to 0.62, p < 0.0001). Measures of hepatic lipoprotein synthesis such as very‐low‐density lipoprotein‐triglyceride and cholesterol content as well as total and very‐low‐density lipoprotein‐apolipoprotein B levels were also strongly associated with visceral adipose tissue area (r = 0.57, 0.57, 0.61, and 0.62, respectively, p < 0.0001). Significant associations were found between these variables and the deep subcutaneous adipose tissue area or DXA‐measured total body fat mass. However, the correlation coefficients were of lower magnitude compared to those with visceral adipose tissue area. Multivariate regression analyses demonstrated that visceral adipose tissue area was the strongest predictor of lipid‐lipoprotein profile variables (7% to 48% explained variance, 0.02 ≥ p ≤ 0.0001). Discussion: Although previous studies have generated controversial data as to which abdominal adipose tissue compartment was more closely associated with insulin resistance, our results suggest that visceral adipose tissue area is a stronger correlate of other obesity‐related outcomes such as lipid‐lipoprotein alterations.     

Characteristic Expression of Extracellular Matrix in Subcutaneous Adipose Tissue Development and Adipogenesis; Comparison with Visceral Adipose Tissue

Adipose tissue is a connective tissue specified for energy metabolism and endocrines, but functional differences between subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) have not been fully elucidated. To reveal the physiological role of SAT, we characterized in vivo tissue development and in vitro adipocyte differentiation. In a DNA microarray analysis of SAT and VAT in Wistar rats, functional annotation clusters of extracellular matrix (ECM)-related genes were found in SAT, and major ECM molecules expressed in adipose tissues were profiled. In a histological analysis and quantitative expression analysis, ECM expression patterns could be classified into two types: (i) a histogenesis-correlated type such as type IV and XV collagen, and laminin subunits, (ii) a high-SAT expression type such as type I, III, and V collagen and minor characteristic collagens. Type (i) was related to basal membrane and up-regulated in differentiated 3T3-L1 cells and in histogenesis at depot-specific timings. In contrast, type (ii) was related to fibrous forming and highly expressed in 3T3-L1 preadipocytes. Exceptionally, fibronectin was abundant in developed adipose tissue, although it was highly expressed in 3T3-L1 preadipocytes. The present study showed that adipose tissues site-specifically regulate molecular type and timing of ECM expression, and suggests that these characteristic ECM molecules provide a critical microenvironment, which may affect bioactivity of adipocyte itself and interacts with other tissues. It must be important to consider the depot-specific property for the treatment of obesity-related disorders, dermal dysfunction and for the tissue regeneration.     

Measurement Site and the Association Between Visceral and Abdominal Subcutaneous Adipose Tissue With Metabolic Risk in Women

The associations between visceral adipose tissue (VAT) and abdominal subcutaneous adipose tissue (ASAT) and metabolic risk may be influenced by measurement site. The aim of this study was to compare the strength of the associations between VAT and ASAT, as assessed by a cross‐sectional image (area) or total volume, and prevalent metabolic syndrome (MetS). We also examined the association between changes in abdominal AT area and volume with concomitant changes in metabolic risk. Abdominal AT volume and areas were derived using ～35 continuous computed tomography (CT) images from T10–T11 to L5–S1 in overweight or obese postmenopausal women before (n = 67) and after (n = 39) a 6‐month exercise intervention. At baseline, measurement site did not influence the inter‐relationship between ASAT area and total volume, and between ASAT and MetS. Conversely, VAT areas at L1–L2 and L2–L3 were stronger correlates of VAT volume at baseline (L1–L2 (r = 0.94), L2–L3 (r = 0.95), L4–L5 (r = 0.89)) and changes therein (L1–L2 (r = 0.77), L2–L3 (r = 0.75), L4–L5 (r = 0.55)) as compared to L4–L5, but were not significantly better predictors of MetS as compared to L4–L5 or the total volume (L2–L3: odds ratio (OR) = 2.68 (1.6–4.4), L1–L2: OR = 1.88 (1.2–3.0), L4–L5: OR = 2.56 (1.6–4.1), volume: OR = 2.07 (1.1–3.8)). Changes in VAT and ASAT were not associated with changes in MetS (P > 0.10). Although measurement site has an impact on the prediction of VAT volume, this does not translate into an improved prediction for the MetS. Thus, there is not enough evidence to support changing the current research practice of assessing VAT volume or at L4–L5 for the prediction of metabolic risk.     

Visceral and Subcutaneous Adipose Tissue Diacylglycerol Acyltransferase Activity in Humans

Diacylglycerol acyltransferase (DGAT) could be a rate limiting step in triglyceride (TG) synthesis as it is the final step in this pathway. As such, between depot differences in DGAT activity could influence regional fat storage. DGAT activity and in vitro rates of direct free fatty acid (FFA) storage were measured in abdominal subcutaneous and omental adipose tissue samples from 12 nonobese (BMI <30 kg/m²) and 23 obese men and women (BMI >30 kg/m²) undergoing elective surgery. DGAT activity was greater in omental than in abdominal subcutaneous adipose tissue from nonobese patients (2.0 ± 0.9 vs. 0.9 ± 0.3 pmol/min/mg lipid, respectively, P = 0.003), but not from obese patients (1.4 ± 0.6 vs. 1.7 ± 0.7 pmol/min/mg lipid, respectively, P = 0.10). DGAT activity per unit adipose weight was negatively correlated with adipocyte size (P < 0.01) and positively correlated with direct FFA storage in omental (P < 0.001) but not in abdominal subcutaneous fat. Tissue DGAT activity varies as a function of adipocyte size, but this relationship differs between visceral and abdominal subcutaneous fat in obese and nonobese humans. Our results are consistent with the hypothesis that interindividual variations in DGAT activity may be an important regulatory step in visceral adipose tissue FFA uptake/storage.     

Validation of Computed Tomographic Estimates of Intra‐abdominal and Subcutaneous Adipose Tissue in Rats and Mice

We characterized the accuracy, sensitivity, and reliability of computed tomographic (CT) estimates of intra‐abdominal (IA) and subcutaneous (S) adipose tissue (AT) in rats and mice using the Aloka rodent CT. Here, we present the first comparisons of CT estimates of the weights of AT samples ex vivo to balance weights of the same samples, of CT estimates of AT weights in vivo to the weights of resected whole‐body AT, and of CT estimates of the weights of pieces of AT inserted IA or S in vivo to the weights of the same pieces ex vivo. CT underestimated AT weight ex vivo by ～10%, and correction of the automated categorization of IAAT and SAT by Aloka software was required. After these adjustments, correlations (r) of CT estimates and balance weights of resected AT were ≥0.99 in rats and ≥0.92 in mice. CT was impressively sensitive: the 95% probability range of CT estimates of 10,000 mg AT inserts into rats was ±780 mg and of 500 mg inserts into mice, ±20 mg. Scans limited to the abdominal region correlated well (r > 0.90) with whole‐body scan measures of IAAT and SAT in rats and with IAAT, but not SAT (r < 0.80), in mice. Sums of IAAT and SAT correlated well with body weight in rats (r > 0.90), but not in mice (r < 0.80). Coefficients of variance (CVs) of duplicate scans were <5%. We conclude that CT is a valid tool for studies of AT weight in rats and mice, especially when rapid throughput or longitudinal measures are desired.     

Adipose Tissue Dysfunction Occurs Independently of Obesity in Adipocyte‐Specific Oncostatin Receptor Knockout Mice

Objective

This study examined the phenotypic effects of adipocyte‐specific oncostatin M receptor (OSMR) loss in chow‐fed mice.

Methods

Chow‐fed adipocyte‐specific OSMR knockout (FKO) mice and littermate OSMR^fl/fl controls were studied. Tissue weights, insulin sensitivity, adipokine production, and stromal cell immunophenotypes were assessed in epididymal fat (eWAT); serum adipokine production was also assessed. In vitro, adipocytes were treated with oncostatin M, and adipokine gene expression was assessed.

Results

Body weights, fasting blood glucose levels, and eWAT weights did not differ between genotypes. However, the eWAT of OSMR^FKO mice was modestly less responsive to insulin stimulation than that of OSMR^fl/fl mice. Notably, significant increases in adipokines, including C‐reactive protein, lipocalin 2, intercellular adhesion molecule‐1, and insulinlike growth factor binding protein 6, were observed in the eWAT of OSMR^FKO mice. In addition, significant increases in fetuin A and intercellular adhesion molecule‐1 were detected in OSMR^FKO serum. Flow cytometry revealed a significant increase in leukocyte number and modest, but not statistically significant, increases in B cells and T cells in the eWAT of OSMR^FKO mice.

Conclusions

The chow‐fed OSMR^FKO mice exhibited adipose tissue dysfunction and increased proinflammatory adipokine production. These results suggest that intact adipocyte oncostatin M–OSMR signaling is necessary for adipose tissue immune cell homeostasis.

     

Preadipocyte Number in Omental and Subcutaneous Adipose Tissue of Obese Individuals

Objective: To determine the variation in preadipocyte isolation procedure and to assess the number and function of preadipocytes from subcutaneous and omental adipose tissue of obese individuals. Research Methods and Procedures: The preadipocyte number per gram of adipose tissue in the abdominal‐subcutaneous and abdominal‐omental adipose stores of 27 obese subjects with a BMI of 44 ± 10 kg/m² and an age of 40 ± 9 years was determined. Results: The assessment of the preadipocyte number was found to be labor intensive and error prone. Our data indicated that the number of stromal vascular cells (SVCs), isolated from the adipose tissue by collagenase digestion, was dependent on the duration of collagenase treatment and the size and the origin of the biopsy. In addition, the fat accumulation and leptin production by differentiated SVCs were dependent on the number of adherent SVCs (aSVCs) in the culture plate and the presence of proteins derived from serum and peroxisome proliferator‐activated receptor ligands. Discussion: Using our standardized isolation and differentiation protocol, we found that the number of SVCs, aSVCs, leptin production, and fat accumulation still varied considerably among individuals. Interestingly, within individuals, the number of SVCs, aSVCs, and the leptin production by differentiating aSVCs from both the subcutaneous and the omental fat depots were associated, whereas fat accumulation was not. In obese to severely obese subjects, differences in BMI and age could not explain differences in SVCs, aSVCs, leptin production, and fat accumulation.     

Lipolysis in Adipocytes Isolated from Deep and Superficial Subcutaneous Adipose Tissue

Objective: Abdominal subcutaneous adipose tissue (SAT) occurs in two depots separated by a fascial plane: deep SAT and superficial SAT. In a recent study it was demonstrated that the amount of deep SAT has a much stronger relationship to insulin resistance than does superficial SAT. Because insulin resistance may be related to fatty acid release from adipose tissue, we hypothesized that the two SAT depots may have a different lipolytic activity. Research Methods and Procedures: To test this hypothesis, we obtained samples of deep and superficial SAT from patients undergoing elective abdominal surgery. The rate of lipolysis was determined in the collagenase‐digested adipocytes obtained from the two fat depots by measuring glycerol release in the presence and absence of isoproterenol. In addition, the relative concentration of hormone‐sensitive lipase was determined in both SAT depots by Western blot analysis. Results: Our results showed that the rate of isoproterenol‐stimulated lipolysis was ～20% higher in cells from deep SAT compared with those from superficial SAT, indicating that the deep SAT is more lipolytically active. The concentration of hormone‐sensitive lipase did not differ between the two adipose tissue depots. Discussion: These findings suggest that the higher lipolytic activity of deep SAT may account for its stronger association with insulin resistance. The mechanism seems to be independent of differences in hormone‐sensitive lipase concentration.     

Dietary Calcium Intake Is Associated With Less Gain in Intra‐Abdominal Adipose Tissue Over 1 Year

Calcium intake is reported to enhance weight loss with a preferential loss in trunk fat. Discrepant findings exist as to the effects of calcium intake on longitudinal changes in total fat mass and central fat deposition. Therefore, the purpose of this study was to determine associations between dietary calcium intake and 1‐year change in body composition and fat distribution, specifically intra‐abdominal adipose tissue (IAAT). A total of 119 healthy, premenopausal women were evaluated at baseline and 1 year later. Average dietary calcium was determined via 4‐day food records. Total fat was determined by dual‐energy X‐ray absorptiometry (DXA) and subcutaneous abdominal adipose tissue (SAAT) and IAAT by computed tomography. Over the study period, participants' reported daily calcium and energy intakes were 610.0 ± 229.9 mg and 1,623.1 ± 348.5 kcal, respectively. The mean change in weight, total fat, IAAT, and SAAT was 4.9 ± 4.4 kg, 5.3 ± 4.0 kg, 7.7 ± 19.5 cm², and 49.3 ± 81.1 cm², respectively. Average calcium intake was significantly, inversely associated with 1‐year change in IAAT (standardized β: ?0.23, P < 0.05) after adjusting for confounding variables. For every 100 mg/day of calcium consumed, gain in IAAT was reduced by 2.7 cm². No significant associations were observed for average calcium intake with change in weight, total fat, or SAAT. In conclusion, dietary calcium intake was significantly associated with less gain in IAAT over 1 year in premenopausal women. Further investigation is needed to verify these findings and determine the calcium intake needed to exert beneficial effects on fat distribution.     

A Combined Transcriptomics and Lipidomics Analysis of Subcutaneous,Epididymal and Mesenteric Adipose Tissue Reveals Marked Functional Differences

Depot-dependent differences in adipose tissue physiology may reflect specialized functions and local interactions between adipocytes and surrounding tissues. We combined time-resolved microarray analyses of mesenteric- (MWAT), subcutaneous- (SWAT) and epididymal adipose tissue (EWAT) during high-fat feeding of male transgenic ApoE3Leiden mice with histology, targeted lipidomics and biochemical analyses of metabolic pathways to identify differentially regulated processes and site-specific functions. EWAT was found to exhibit physiological zonation. De novo lipogenesis in fat proximal to epididymis was stably low, whereas de novo lipogenesis distal to epididymis and at other locations was down-regulated in response to high-fat diet. The contents of linoleic acid and α-linolenic acid in EWAT were increased compared to other depots. Expression of the androgen receptor (Ar) was higher in EWAT than in MWAT and SWAT. We suggest that Ar may mediate depot-dependent differences in de novo lipogenesis rate and propose that accumulation of linoleic acid and α-linolenic acid in EWAT is favored by testosterone-mediated inhibition of de novo lipogenesis and may promote further elongation and desaturation of these polyunsaturated fatty acids during spermatogenesis.