Structural and histochemical aspects of perirenal adipose tissue in fetal pigs: relationships between stromal-vascular characteristics and fat cell concentration and enzyme activity

Depot-specific mRNA expression programs in human adipocytes suggest physiological specialization via distinct developmental programs

Adipose tissue is distributed in diverse locations throughout the human body. Not much is known about the extent to which anatomically distinct adipose depots are functionally distinct, specialized organs, nor whether depot-specific characteristics result from intrinsic developmental programs, as opposed to reversible physiological responses to differences in tissue microenvironment. We used DNA microarrays to compare mRNA expression patterns of isolated human adipocytes and cultured adipose stem cells, before and after ex vivo adipocyte differentiation, from seven anatomically diverse adipose tissue depots. Adipocytes from different depots display distinct gene expression programs, which are most closely shared with anatomically related depots. mRNAs whose expression differs between anatomically diverse groups of depots (e.g., subcutaneous vs. internal) suggest important functional specializations. These depot-specific differences in gene expression were recapitulated when adipocyte progenitor cells from each site were differentiated ex vivo, suggesting that progenitor cells from specific anatomic sites are deterministically programmed to differentiate into depot-specific adipocytes. Many developmental transcription factors show striking depot-specific patterns of expression, suggesting that adipocytes in each anatomic depot are programmed during early development in concert with anatomically related tissues and organs. Our results support the hypothesis that adipocytes from different depots are functionally distinct and that their depot-specific specialization reflects distinct developmental programs.

Perirenal fat thickness as a superior obesity-related marker of subclinical carotid atherosclerosis in type 2 diabetes mellitus

Objective

Emerging evidence highlighted that perirenal adipose tissue might regulate the cardiovascular and metabolism system through several pathways. This study aimed to assess the association between perirenal fat thickness (PrFT) and subclinical carotid atherosclerosis (SCCA) in type 2 diabetes mellitus (T2DM).

Method

A total of 670 participants with complete data were included in this study. The trained reviewer collected demographic and anthropometric information. Laboratory assessments were determined by standard methods. PrFT and SCCA were evaluated by computed tomography and ultrasound. Binomial logistic regression analysis was conducted to assess the association between PrFT and SCCA. Receiver operating characteristic (ROC) curve analysis was conducted to evaluate the identifying value of PrFT for SCCA.

Results

Overall, the prevalence of SCCA was 61.8% in T2DM. PrFT was significantly increased in the SCCA group. Growing trends were observed in the prevalence of hypertension, carotid intima-media thickness (cIMT) > 1, plaque, and SCCA across the PrFT quartiles. Spearman correlation analysis revealed that PrFT was positively associated with cIMT (r = 0.401, p < 0.001). This correlation remained significant after adjustment for visceral fat area (VFA), subcutaneous fat area (SFA), and traditional metabolic risk factors (β = 0.184, p < 0.001). Meanwhile, PrFT was independently correlated with plaque, cIMT > 1 mm, and SCCA. The ORs (95% CI) were 1.072 (1.014-1.135), 1.319 (1.195-1.455), and 1.216 (1.119-1.322). Furthermore, PrFT remained correlated considerably with SCCA in subgroup analysis after stratification for age, sex, smoking, hypertension, and body mass index. From the ROC curve analysis, the AUCs (95% CI) of PrFT, VFA, and SFA identifying SCCA were 0.794 (0.760-0.828), 0.760 (0.724-0.796), and 0.697 (0.656-0.737), respectively. The AUC of PrFT was significantly higher than VFA (p = 0.028) and SFA (p < 0.001). The optimal cutoff values of PrFT were 14.0 mm, with a sensitivity of 66.7% and a specificity of 76.2%.

Conclusion

PrFT was independently associated with cIMT, plaque, cIMT > 1 mm, and SCCA as a superior obesity-related marker of SCCA in T2DM.

Clinical trial registration

Clinical Trials.Gov, identifier ChiCTR2100052032.

Importance of the Microenvironment and Mechanosensing in Adipose Tissue Biology

The expansion of adipose tissue is an adaptive mechanism that increases nutrient buffering capacity in response to an overall positive energy balance. Over the course of expansion, the adipose microenvironment undergoes continual remodeling to maintain its structural and functional integrity. However, in the long run, adipose tissue remodeling, typically characterized by adipocyte hypertrophy, immune cells infiltration, fibrosis and changes in vascular architecture, generates mechanical stress on adipose cells. This mechanical stimulus is then transduced into a biochemical signal that alters adipose function through mechanotransduction. In this review, we describe the physical changes occurring during adipose tissue remodeling, and how they regulate adipose cell physiology and promote obesity-associated dysfunction in adipose tissue.

Preadipocyte and adipose tissue differentiation in meat animals: influence of species and anatomical location

Early in porcine adipose tissue development, the stromal-vascular (SV) elements control and dictate the extent of adipogenesis in a depot-dependent manner. The vasculature and collagen matrix differentiate before overt adipocyte differentiation. In the fetal pig, subcutaneous (SQ) layer development is predictive of adipocyte development, as the outer, middle, and inner layers of dorsal SQ adipose tissue develop and maintain layered morphology throughout postnatal growth of SQ adipose tissue. Bovine and ovine fetuses contain brown adipose tissue but SQ white adipose tissue is poorly developed structurally. Fetal adipose tissue differentiation is associated with the precocious expression of several genes encoding secreted factors and key transcription factors like peroxisome proliferator activated receptor (PPAR)γ and CCAAT/-enhancer-binding protein. Identification of adipocyte-associated genes differentially expressed by age, depot, and species in vivo and in vitro has been achieved using single-gene analysis, microarrays, suppressive subtraction hybridization, and next-generation sequencing applications. Gene polymorphisms in PPARγ, cathepsins, and uncoupling protein 3 have been associated with back fat accumulation. Genome scans have mapped several quantitative trait loci (QTL) predictive of adipose tissue-deposition phenotypes in cattle and pigs.

Stromal Vascular Cells and Adipogenesis: Cells within Adipose Depots Regulate Adipogenesis

A collection of investigations indicate the importance of adipose tissue stromal/stem cells to vasculogenesis and angiogenesis during adipogenesis. Early in development the stromal-vascular (S-V) elements control and dictate the extent of adipogenesis. For instance, the vasculature and connective tissue collagen matrix develops before overt adipocyte differentiation. Definitive studies of human adipose tissue stem cells (ADSC) provided an understanding of stem cell identity and function. In this regard, a novel vascular stem cell theory proposes that ADSC are a mixed population of vascular stem cells (VSC) with differential potential proportional to the angiogenic potential of the vasculature. The differential potential of VSC can range considerably in a continuous fashion and can include vascular smooth cells, endothelial cells (EC) and adipocytes. These observations are consistent with fetal adipose tissue studies that show location-dependent angiogenic potential ranging from more to less in regards to a predominant presence of EC and developing arterioles before overt adipogenesis.

The Influence of Human Growth Hormone (GH) and Thyroxine (T4) on the Differentiation of Adipose Tissue in the Fetus

Late term fetuses from genetically obese dams have slightly larger fat cells, greater adipose tissue lipoprotein lipase (LPL) activities, elevated levels of thyroid hormones, and depressed growth hormone (GH) levels when compared to fetuses from lean dams. We have investigated the influence of thyroid hormone and GH status per se on these and other adipose tissue traits by chronically treating hypophysectomized (hypox) fetuses (day 70) between day 90 and 105 of gestation with either thyroxine (T4) or human GH. Treatment with T4 decreased body weights (P<.05), increased serum T4 levels (P<.05), and enhanced skin and hair development (P<.05). Quantitative analysis of sections of perirenal and subcutaneous adipose tissue indicated that T4 increased LPL activity (P<.05), slightly increased fat cell size, and more than doubled (P<.05) lipid accretion. A hypox induced deficit in fat cell cluster number in the outer layer of subcutaneous tissue was normalized by T4 (P<.05). Conversely, human GH (hGH) treatment had no influence on body weight, increased serum hGH levels, decreased fat cell size (P<.05) and LPL activity (P<.05) but had no influence on lipid accretion. Quantitative analysis of adipose tissue sections provided direct and indirect evidence of a "critical" or "sensitive" period between 90 and 105 days, since fetal hypox at day 70 severely impeded preadipocyte recruitment/replication during this period. Furthermore, T4 but not GH effectively normalized this hypox-induced deficiency in preadipocyte development. Therefore, T4 may have a major role in preadipocyte recruitment/replication during late fetal life.

The Influence of Hydrocortisone (HC) on Differentiation of Adipose Tissue is Dependent on Fetal Age

Elevated serum hydrocortisone (HC) levels are associated with larger fat cells and elevated levels of lipogenic and associated enzymes in late term pig fetuses from genetically obese dams. We have investigated the influence of HC status per se on these and other adipose tissue traits by chronically treating pig fetuses hypophysectomized (hypox) on day 70 with HC between either day 70 and 90 or 90 and 105 of gestation. Treatment with HC during both periods increased serum HC levels (P<.05) and increased fat cell size (P<.05) in the perirenal (PERI) and subcutaneous (SQ) depots, but failed to influence body weights, insulin-like growth factor-1 and insulin levels. Quantitative analysis of sections of PERI and SQ adipose tissue indicated that HC increased lipoprotein lipase (LPL), esterase and glucose-6-phosphate dehydrogenase (G6PDH) activities. The degree of esterase and G6PDH, but not LPL response to HC, was greater during the 90- to 105-day period than during the earlier period. HC significantly increased lipid accretion only in the SQ depot between 90 and 105 days. Overall, HC significantly augmented hypox-induced alterations in cellular and metabolic traits of developing adipose tissue. The general increase in fat cell size (21%) with moderate (SQ-105d) or no (PERI-90, 105d; SQ-90d) increase in lipid accretion indicates that HC either did not influence or decreased apparent fat cell number. Regardless, these data indicate that changes in serum HC per se may account for adipose tissue traits that characterize fetuses from genetically obese dams.

Fat depot-specific differences in angiogenic growth factor gene expression and its relation to adipocyte size in cattle

Adipocytes derived from different anatomical sites vary in the expression of adipocytokines and growth factor genes. Adipogenesis is tightly associated with angiogenesis, although the regional variation of angiogenic growth factor gene expression in adipose tissues remains unclear. In this experiment, we studied the fat depot-specific differences (subcutaneous, intramuscular, intermuscular, renal, and mesenteric) in the expression of angiogenic growth factor mRNA [vascular endothelial growth factor (VEGF), fibroblast growth factor-2 (FGF-2), fibroblast growth factor-10 (FGF-10), hepatocyte growth factor (HGF), and leptin], as well as the relationship between angiogenic growth factor mRNA level and adipocyte size in bovine adipose tissues. Intermuscular, renal, and mesenteric adipose tissues expressed significantly higher VEGF, FGF-2, and leptin mRNA levels than did subcutaneous and intramuscular adipose tissues. Mesenteric adipose tissue also expressed higher FGF-10 mRNA levels than did subcutaneous and intramuscular adipose tissues. There was no significant difference in the expression of HGF mRNA among adipose tissue depots. A significant correlation existed between adipocyte size and VEGF, FGF-2, FGF-10, and leptin mRNA levels. These results indicate that fat depot-specific difference in angiogenic growth factor gene expression results from the difference in adipocyte size.

Angiogenic growth factor synergism in a murine tissue engineering model of angiogenesis and adipogenesis

De novo tissue generation stimulated by three angiogenic growth factors administered in a factorial design was studied in an in vivo murine tissue engineering chamber. A silicone chamber was implanted around the epigastric pedicle and filled with Matrigel with 100 ng/ml of recombinant mouse vascular endothelial growth factor-120 (VEGF120), recombinant human basic fibroblastic growth factor (FGF-2), or recombinant rat platelet-derived growth factor-BB (PDGF-BB) added as single, double, or triple combinations. Angiogenesis, supporting tissue ingrowth, and adipogenesis were assessed at 2 and 6 weeks by immunohistochemistry and morphometry. At 2 weeks angiogenesis was synergistically enhanced by VEGF120 + FGF-2 (P = 0.019). FGF-2 (P = 0.008) and PDGF-BB (P = 0.01) significantly increased connective tissue/inflammatory cell infiltrate (macrophages, pericytes, and preadipocytes) in double and triple combinations compared with control. At 6 weeks sequential addition of growth factors increased the percent volume of adipose tissue (P < 0.0005, each main effect), with a synergistic increase in adipose tissue in combination treatments (P < 0.0005). Groups containing 300 ng/ml of single growth factors produced significantly less adipose tissue than the triple growth factor combination (P < 0.0005, VEGF120 and PDGF-BB; P < 0.001, FGF-2). In conclusion, angiogenic growth factor combinations increased early angiogenesis and cell infiltration resulting in synergistically increased adipose tissue growth at 6 weeks. Two way and higher level synergies are likely to be important in therapeutic applications of angiogenic growth factors.

Adipose tissue angiogenesis

A review of adipose tissue angiogenesis includes the morphological and cytochemical development of adipose tissue vasculature and the concept of primitive fat organs. Spatial and temporal relationships between fetal vascular and fat cell development are discussed, including depot- and genetic-dependent arteriolar differentiation. The relationship between connective tissue deposition and elaboration of adipose tissue vasculature is discussed with respect to regulating adipocyte development in a depot-dependent manner. In vitro studies indicated that depot-dependent vascular traits may be attributable to intrinsic growth characteristics of adipose tissue endothelial cells. These studies indicate that adipogenesis may be regulated by factors that drive angiogenesis. Fundamental aspects of angiogenesis, including basement membrane breakdown, vasculogenesis, angiogenic remodeling, vessel stabilization, and vascular permeability were reviewed. Critical angiogenic factors include vascular endothelial growth factor (VEGF), VEGF receptors, angiopoietins (Ang), ephrins, matrix metalloproteinases, and the plasminogen enzymatic system. Vascular endothelial growth factor is the most critical factor because it initiates the formation of immature vessels and disruption of a single VEGF allele leads to embryonic lethality in mice. Expression of VEGF is influenced by hypoxia, insulin, growth factors, and several cytokines. Angiogenic factors secreted and/or produced by adipocytes or preadipocytes are discussed. Vascular endothelial growth factor expression and secretion by adipocytes is regulated by insulin and hypoxia, and is associated with adipose tissue accretion. Vascular endothelial growth factor accounts for most of the angiogenic activity of adipose tissue. The proposed role of leptin as an adipogenic factor is reviewed with respect to efficacy on various aspects of angiogenesis relative to other angiogenic factors. The VEGF and leptin genes are both hypoxia inducible, but potential links between VEGF and leptin gene expression have not been examined. Finally, several studies including a study of mice treated with antiangiogenic factors indicate that adipose tissue accretion can be controlled through the vasculature per se.

Vascular and cellular development in fetal adipose tissue: lectin binding studies and immunocytochemistry for laminin and type IV collagen

A cytochemical study of vascular and cellular development in fetal adipose tissue was conducted utilizing 10 plant lectins (fluorescein isothiocyanate (FITC)-labeled), antibodies against laminin, types II and IV collagen, and a probe for actin. Throughout fetal development (50-110 days) blood vessels were stained by galactose binding lectins and stained for actin, type IV collagen, and laminin. Adipocyte reactivity for laminin was strong throughout development, whereas adipocyte staining for type IV collagen and several lectins increased from weak to moderate between 70 and 110 days of fetal life. In general, staining intensity for lectins was greater for blood vessels than for adipocytes at every age, and staining for lectins and type IV collagen was detected much earlier on blood vessels than on adipocytes. However, the ontogeny and intensity of laminin staining were similar for developing adipocytes and vasculature. Adipocyte staining by several lectins was dependent on location within the tissue, whereas blood vessel lectin staining was not location-dependent. Neuraminidase pretreatment abolished the variation in cellular lectin staining due to location (within the tissue) but did not alter age-related changes in cellular staining. This study indicates that the differentiation of the extracellular matrix of blood vessels and adipocytes is clearly distinct in regard to glycoconjugate composition and temporal pattern of glycoconjugate and type IV collagen deposition.