Different fat depots are distinct mini-organs:

"Is obesity linked to aging?": adipose tissue and the role of telomeres

Obesity is a condition in which excess or abnormal fat accumulation may present with adverse effects on health and decreased life expectancy. Increased body weight and adipose tissue accumulation amplifies the risk of developing various age-related diseases, such as cardiovascular disease, type 2 diabetes mellitus, musculoskeletal disorders, respiratory diseases and certain types of cancer. This imbalance in body composition and body weight is now recognized as a state of increased oxidative stress and inflammation for the organism. Increasing oxidative stress and inflammation affect telomeres. Telomeres are specialized DNA-protein structures found at the ends of eukaryotic chromosomes and serve as markers of biological aging rate. They also play a critical role in maintaining genomic integrity and are involved in age-related metabolic dysfunction. Erosion of telomeres is hazardous to healthy cells, as it is a known mechanism of premature cellular senescence and loss of longevity. The association of telomeres and oxidative stress is evident in cultured somatic cells in vitro, where oxidative stress enhances the process of erosion with each cycle of replication. Shorter telomeres have been associated with increasing body mass index, increased adiposity, and more recently with increasing waist to hip ratio and visceral excess fat accumulation. Furthermore, many of the metabolic imbalances of obesity (e.g. glycemic, lipidemic, etc.) give rise to organ dysfunction in a way that resembles the accelerated aging process. This article is a non-systematic review of the evidence linking obesity and accelerated aging processes as they are regulated by telomeres.

Fat tissue, aging, and cellular senescence

Fat tissue, frequently the largest organ in humans, is at the nexus of mechanisms involved in longevity and age-related metabolic dysfunction. Fat distribution and function change dramatically throughout life. Obesity is associated with accelerated onset of diseases common in old age, while fat ablation and certain mutations affecting fat increase life span. Fat cells turn over throughout the life span. Fat cell progenitors, preadipocytes, are abundant, closely related to macrophages, and dysdifferentiate in old age, switching into a pro-inflammatory, tissue-remodeling, senescent-like state. Other mesenchymal progenitors also can acquire a pro-inflammatory, adipocyte-like phenotype with aging. We propose a hypothetical model in which cellular stress and preadipocyte overutilization with aging induce cellular senescence, leading to impaired adipogenesis, failure to sequester lipotoxic fatty acids, inflammatory cytokine and chemokine generation, and innate and adaptive immune response activation. These pro-inflammatory processes may amplify each other and have systemic consequences. This model is consistent with recent concepts about cellular senescence as a stress-responsive, adaptive phenotype that develops through multiple stages, including major metabolic and secretory readjustments, which can spread from cell to cell and can occur at any point during life. Senescence could be an alternative cell fate that develops in response to injury or metabolic dysfunction and might occur in nondividing as well as dividing cells. Consistent with this, a senescent-like state can develop in preadipocytes and fat cells from young obese individuals. Senescent, pro-inflammatory cells in fat could have profound clinical consequences because of the large size of the fat organ and its central metabolic role.

Regional anatomic and age effects on cell function of human adipose-derived stem cells

Adipose tissue has been shown to contain adult mesenchymal stem cells that have therapeutic applications in regenerative medicine. There is evidence that the ability of adipose precursor cells to grow and differentiate varies among fat depots and changes with age. Defining these variations in cell function and molecular mechanisms of adipogenesis will facilitate the development of cell-based therapies. We compared cells harvested from 5 different subcutaneous (SC) adipose depots in 12 female patients classified into 3 age ranges (25-30, 40-45, and 55-60 years old). Capacity for differentiation of isolated adipose-derived stem cells (ASCs) with and without ciglitazone, a strong peroxisome proliferatoractivated receptors (PPAR)-gamma agonist, was assessed in vitro. ASCs were also characterized by lipolytic function, proliferation, and sensitivity to apoptosis. Additionally, PPAR-gamma-2 protein expression was determined. We observed a difference in the apoptotic susceptibility of ASCs from various SC depots, with the superficial abdominal depot (above Scarpas layer) significantly more resistant to apoptosis when compared with the 4 other depots. We have also demonstrated that a PPAR-gamma agonist aids in the induction of differentiation in cells from all depots and ages. Although sensitivity to apoptosis was linked to anatomic depot, differences in cell proliferation were related primarily to age. Stimulated free glycerol release has been shown to be highest in the arm depot. The arm depot has also consistently shown expression of PPAR-gamma-2 with and without a PPAR-gamma agonist. Younger patients have increased PPAR-gamma-2 expression in all depots, whereas the older patients have consistent elevated expression only in the arm and thigh depots. We have shown there is variability in function of ASCs that have been harvested from different SC depots. Additionally, we have shown age-related changes in function. These data will help select patients and cell harvest sites most suitable for tissue engineering therapies.

Abundance of two human preadipocyte subtypes with distinct capacities for replication, adipogenesis, and apoptosis varies among fat depots

Fat depots vary in function and size. The preadipocytes that fat cells develop from exhibit distinct regional characteristics that persist in culture. Human abdominal subcutaneous cultured preadipocytes undergo more extensive lipid accumulation, higher adipogenic transcription factor expression, and less TNF-alpha-induced apoptosis than omental preadipocytes. We found higher replicative potential in subcutaneous and mesenteric than in omental preadipocytes. In studies of colonies arising from single preadipocytes, two preadipocyte subtypes were found, one capable of more extensive replication, differentiation, and adipogenic transcription factor expression and less apoptosis in response to TNF-alpha than the other. The former was more abundant in subcutaneous and mesenteric than in omental preadipocyte populations, potentially contributing to regional variation in replication, differentiation, and apoptosis. Both subtypes were found in strains derived from single human preadipocytes stably expressing telomerase, confirming that both subtypes are of preadipocyte lineage. After subcloning of cells of either subtype, both subtypes were found, indicating that switching can occur between subtypes. Thus proportions of preadipocyte subtypes with distinct cell-dynamic properties vary among depots, potentially permitting tissue plasticity through subtype selection during development. Furthermore, mesenteric preadipocyte cell-dynamic characteristics are distinct from omental cells, indicating that visceral fat depots are not functionally uniform.

Thiazolidinediones block fatty acid release by inducing glyceroneogenesis in fat cells

Thiazolidinediones are used to treat type 2 diabetes mellitus because they decrease plasma glucose, insulin, triglyceride, and fatty acid levels. Thiazolidinediones are agonists for peroxisome proliferator-activated receptor gamma, a nuclear receptor that is highly expressed in fat tissue. We identify glyceroneogenesis as a target of thiazolidinediones in cultured adipocytes and fat tissues of Wistar rats. The activation of glyceroneogenesis by thiazolidinediones occurs mainly in visceral fat, the same fat depot that is specifically implicated in the progression of obesity to type 2 diabetes. The increase in glyceroneogenesis is a result of the induction of its key enzyme, phosphoenolpyruvate carboxykinase, whose gene expression is peroxisome proliferator-activated receptor gamma-dependent in adipocytes. The main role of this metabolic pathway is to allow the re-esterification of fatty acids via a futile cycle in adipocytes, thus lowering fatty acid release into the plasma. The importance of such a fatty acid re-esterification process in the control of lipid homeostasis is highlighted by the existence of a second thiazolidinedione-induced pathway involving glycerol kinase. We show that glyceroneogenesis accounts for at least 75% of the whole thiazolidinedione effect. Because elevated plasma fatty acids promote insulin resistance, these results suggest that the glyceroneogenesis-dependent fatty acid-lowering effect of thiazolidinediones could be an essential aspect of the antidiabetic action of these drugs.

Fat depot origin affects adipogenesis in primary cultured and cloned human preadipocytes

Fat distribution varies among individuals with similar body fat content. Innate differences in adipose cell characteristics may contribute because lipid accumulation and lipogenic enzyme activities vary among preadipocytes cultured from different fat depots. We determined expression of the adipogenic transcription factors peroxisome proliferator activated receptor-gamma (PPAR-gamma) and CCAAT/enhancer binding protein-alpha (C/EBP-alpha) and their targets in abdominal subcutaneous, mesenteric, and omental preadipocytes cultured in parallel from obese subjects. Subcutaneous preadipocytes, which had the highest lipid accumulation, glycerol-3-phosphate dehydrogenase (G3PD) activity, and adipocyte fatty acid binding protein (aP2) abundance, had highest PPAR-gamma and C/EBP-alpha expression. Levels were intermediate in mesenteric and lowest in omental preadipocytes. Overexpression of C/EBP-alpha in transfected omental preadipocytes enhanced differentiation. The proportion of differentiated cells in colonies derived from single subcutaneous preadipocytes was higher than in mesenteric or omental clones. Only cells that acquired lipid inclusions exhibited C/EBP-alpha upregulation, irrespective of depot origin. Thus regional variation in adipogenesis depends on differences at the level of transcription factor expression and is a trait conferred on daughter cells.