Aging and regional differences in fat cell progenitors - a mini-review

Macrophage-induced adipose tissue dysfunction and the preadipocyte: should I stay (and differentiate) or should I go?

Adipose tissue can be regarded as a multidepot organ responsible for metabolic homeostasis by managing sophisticated energy transactions as well as by producing bioactive molecules that regulate insulin sensitivity and immune and vascular responses. Chronic nutrient excess expands adipose tissue, and concomitant variations in its cellular and matrix remodeling can affect the extent of the metabolic dysfunction that is associated with obesity. Preadipocytes, also termed adipose progenitor cells, play a pivotal role in determining whether a dysfunctional hypertrophic state arises as opposed to a hyperplastic process in which mature adipocytes remain relatively responsive. Obesity is associated with infiltration of macrophages, and these immune cells have been shown to communicate with preadipocytes to influence how they differentiate, survive, and proliferate. Understanding macrophage-preadipocyte interactions and their effect on adipose remodeling mechanisms may identify potential therapeutic molecular targets to improve adipose tissue function, even in the face of obesity.

Human adipose tissue stem cells: relevance in the pathophysiology of obesity and metabolic diseases and therapeutic applications

Stem cells are unique cells exhibiting self-renewing properties and the potential to differentiate into multiple specialised cell types. Totipotent or pluripotent stem cells are generally abundant in embryonic or fetal tissues, but the use of discarded embryos as sources of these cells raises challenging ethical problems. Adult stem cells can also differentiate into a wide variety of cell types. In particular, adult adipose tissue contains a pool of abundant and accessible multipotent stem cells, designated as adipose-derived stem cells (ASCs), that are able to replicate as undifferentiated cells, to develop as mature adipocytes and to differentiate into multiple other cell types along the mesenchymal lineage, including chondrocytes, myocytes and osteocytes, and also into cells of endodermal and neuroectodermal origin, including beta-cells and neurons, respectively. An impairment in the differentiation potential and biological functions of ASCs may contribute to the development of obesity and related comorbidities. In this review, we summarise different aspects of the ASCs with special reference to the isolation and characterisation of these cell populations, their relation to the biochemical features of the adipose tissue depot of origin and to the metabolic characteristics of the donor subject and discuss some prospective therapeutic applications.

Prevalence and risk factors of non-alcoholic fatty liver disease in the elderly: results from the Rotterdam study

BACKGROUND & AIMS

The prevalence of non-alcoholic fatty liver disease (NAFLD) appears to increase with age. However, limited data are available concerning the prevalence of NAFLD in the elderly. Our aim was to determine the prevalence and risk factors of NAFLD in an elderly population.

METHODS

This study was based on participants in the population-based Rotterdam Study. Each participant was interviewed and had a clinical examination at the research center, including a fasting blood collection, liver ultrasonography, and anthropometric assessment. Ordinal and logistic regression analysis was used to assess associations between covariables and (severity of) NAFLD.

RESULTS

Data from 2811 participants (mean age 76.4 ± 6.0 years) were analyzed. The prevalence of NAFLD was 35.1%. The prevalence of NAFLD decreased with advancing age (p<0.001). In logistic regression analysis, age (OR 0.97; 95% CI 0.95-0.99; p<0.001), total physical activity level (OR 0.98, 95% CI 0.96-0.99; p=0.005), pack years of smoking (OR 1.01, 95% CI 1.00-1.01; p=0.02), waist circumference >88 cm for women and > 102 cm for men (OR 4.89; CI 4.00-5.96; p<0.001), fasting glucose ≥ 100 mg/dl or drug treatment for elevated blood glucose (OR 2.11, 95% CI 1.72-2.59; p<0.001), blood pressure ≥ 130/85 mmHg or drug treatment for elevated blood pressure (OR 1.80, 95% CI 1.08-3.01; p=0.03), and triglycerides ≥ 150 mg/dl or treatment with serum lipid reducing agents (OR 1.56, 95% CI 1.28-1.91; p<0.001) were associated with NAFLD.

CONCLUSIONS

NAFLD is common in the elderly, although the prevalence decreases with advancing age. Further studies are warranted exploring potential factors contributing to this apparent positive selection effect in the elderly.

Fatty acid-induced production of plasminogen activator inhibitor-1 by adipose macrophages is greater in middle-aged versus younger adult participants

BACKGROUND

Human aging is associated with heightened risk of diabetes and cardiovascular disease. Increased fat mass may contribute to age-related diseases by harboring inflammatory macrophages that produce metabolically important proteins such as plasminogen activator inhibitor-1 (PAI-1). Elevated PAI-1 concentrations have been implicated in the pathogenesis of such aging-related conditions as insulin resistance, obesity, and atherosclerosis. We have previously reported that increased plasma free fatty acid (FFA) concentrations augment both circulating PAI-1 concentrations and PAI-1 production by adipose tissue macrophages (ATMs).

METHODS

Because increasing age is associated with increased infiltration and reactivity of adipose macrophages, we performed euglycemic-hyperinsulinemic clamp studies and adipose tissue biopsies with and without elevated FFA concentrations in 31 nondiabetic participants stratified by age, to determine whether middle-aged individuals manifest heightened insulin resistance and PAI-1 production by ATMs in response to elevated nutrient signals relative to their young adult peers.

RESULTS

We observed that elevating FFA concentrations under euglycemic-hyperinsulinemic clamp conditions induced the same degree of insulin resistance in both middle-aged and younger body mass index-matched adults, whereas systemic PAI-1 concentrations were significantly increased in the middle-aged group. Likewise, elevated FFA and insulin concentrations induced larger increases in PAI-1 gene expression in the whole fat and ATMs of middle-aged compared with younger adult participants.

CONCLUSIONS

These studies reveal a heightened adipose inflammatory response to increased FFA and insulin availability in middle-aged individuals relative to younger adults, suggesting that increased susceptibility to the effects of fatty acid excess may contribute to the pathogenesis of age-related diseases.

The role of CUGBP1 in age-dependent changes of liver functions

Aging liver is characterized by alterations of liver biology and by a reduction of many functions which are important for the maintenance of body homeostasis. The main dysfunctions include appearance of enlarged hepatocytes, impaired liver regeneration after partial hepatectomy (PH), development of hepatic steatosis, reduction of secretion of proteins and alterations in the hepatic sinusoid. RNA binding proteins are involved in the regulation of gene expression in all tissues including regulation of biological processes in the liver. This review is focused on the role of a conserved, multi-functional RNA-binding protein, CUGBP1, in the development of aging phenotype in the liver. CUGBP1 has been identified as a protein which binds to RNA CUG repeats expanded in Myotonic Dystrophy type 1 (DM1). CUGBP1 is highly expressed in the liver and regulates translation of proteins which are critical for maintenance of liver functions. In livers of young mice, CUGBP1 forms complexes with eukaryotic translation initiation factor eIF2 and supports translation of C/EBPβ and HDAC1 proteins, which are involved in liver growth, differentiation and liver cancer. Aging changes several signaling pathways which lead to the elevation of the CUGBP1-eIF2α complex and to an increase of translation of C/EBPβ and HDAC1. These proteins form multi-protein complexes with additional transcription factors and with chromatin remodeling proteins causing epigenetic alterations of gene expression in livers of old mice. It appears that CUGBP1-mediated translational elevation of HDAC1 is one of the key events in the epigenetic changes in livers of old mice, leading to the development of age-associated dysfunctions of the liver. This review will also discuss a possible role of CUGBP1 in liver dysfunction in patients affected with DM1.

Structural and biochemical characteristics of various white adipose tissue depots

It is now widely accepted that white adipose tissue (WAT) is not merely a fuel storage organ, but also a key component of metabolic homoeostatic mechanisms. Apart from its major role in lipid and glucose metabolism, adipose tissue is also involved in a wide array of other biological processes. The hormones and adipokines, as well as other biologically active agents released from fat cells, affect many physiological and pathological processes. WAT is neither uniform nor inflexible because it undergoes constant remodelling, adapting the size and number of adipocytes to changes in nutrients' availability and hormonal milieu. Fat depots from different areas of the body display distinct structural and functional properties and have disparate roles in pathology. The two major types of WAT are visceral fat, localized within the abdominal cavity and mediastinum, and subcutaneous fat in the hypodermis. Visceral obesity correlates with increased risk of insulin resistance and cardiovascular diseases, while increase of subcutaneous fat is associated with favourable plasma lipid profiles. Visceral adipocytes show higher lipogenic and lipolytic activities and produce more pro-inflammatory cytokines, while subcutaneous adipocytes are the main source of leptin and adiponectin. Moreover, adipose tissue associated with skeletal muscles (intramyocellular and intermuscular fat) and with the epicardium is believed to provide fuels for skeletal and cardiac muscle contraction. However, increased mass of either epicardial or intermuscular adipose tissue correlates with cardiovascular risk, while the presence of the intramyocellular fat is a risk factor for the development of insulin resistance. This review summarizes results of mainly human studies related to the differential characteristics of various WAT depots.

Role of β-adrenergic receptors in regulation of hepatic fat accumulation during aging

Excessive fat accumulation in liver (hepatic steatosis) predisposes to hepatic functional and structural impairment and overall metabolic risk. Previous studies noted an association between hepatic steatosis and age in humans and rodents. However, the mechanisms leading to age-associated hepatic fat accumulation remain unknown. Earlier work from our group showed that β-adrenergic receptor (β-AR) levels and β-AR-stimulated adenylyl cyclase activity increase in rat liver during aging. Here we investigated whether age-associated increases in β-AR signaling play a role in augmenting hepatic lipid accumulation. We demonstrate an increase in hepatic lipid content during senescence and a significant correlation between hepatic fat content and stimulation of adenylyl cyclase activity by the β-AR agonist isoproterenol in rat liver. Isoproterenol administration to young and old rodents in vivo increased hepatic lipid accumulation. Furthermore, in vitro overexpression of β1- and β2-AR subtypes in hepatocytes from young rodents increased cellular lipid content, whereas inhibition of β-ARs by receptor subtype-specific inhibitors reduced lipid levels in hepatocytes from senescent animals. Isoproterenol-induced hepatic lipid accumulation in vivo was prevented by the β-AR nonselective blocker propranolol, suggesting a novel therapeutic effect of this class of drugs in hepatic steatosis. Acipimox, which inhibits adipose tissue lipolysis, did not alter isoproterenol-mediated hepatic fat accumulation; thus β-AR responsive hepatic lipid accumulation does not appear to be related primarily to altered lipolysis. These findings suggest that augmented hepatic β-AR signaling during aging may increase lipid accumulation in liver and advocate a possible role for β-adrenergic blockers in preventing or retarding the development of hepatic steatosis.

The critical role of metabolic pathways in aging

Aging is characterized by a deterioration in the maintenance of homeostatic processes over time, leading to functional decline and increased risk for disease and death. The aging process is characterized metabolically by insulin resistance, changes in body composition, and physiological declines in growth hormone (GH), insulin-like growth factor-1 (IGF-1), and sex steroids. Some interventions designed to address features of aging, such as caloric restriction or visceral fat depletion, have succeeded in improving insulin action and life span in rodents. Meanwhile, pharmacologic interventions and hormonal perturbations have increased the life span of several mammalian species without necessarily addressing features of age-related metabolic decline. These interventions include inhibition of the mammalian target of rapamycin and lifetime deficiency in GH/IGF-1 signaling. However, strategies to treat aging in humans, such as hormone replacement, have mostly failed to achieve their desired response. We will briefly discuss recent advances in our understanding of the complex role of metabolic pathways in the aging process and highlight important paradoxes that have emerged from these discoveries. Although life span has been the major outcome of interest in the laboratory, a special focus is made in this study on healthspan, as improved quality of life is the goal when translated to humans.

Stem cell senescence and regenerative paradigms

The term "cellular senescence" denotes a cellular response to several stressors that results in irreversible growth arrest, alterations of the gene expression profile, epigenetic modifications, and an altered secretome, all of which eventually impair the reparative properties of primitive cells, adding a layer of complexity to the field of regenerative medicine. The purpose of this review is to illustrate how cellular senescence could affect tissue repair and to propose interventions that aim at interfering with it.

Growth hormone and adipose tissue: beyond the adipocyte

The last two decades have seen resurgence in research focused on adipose tissue. In part, the enhanced interest stems from an alarming increase in obesity rates worldwide. However, an understanding that this once simple tissue is significantly more intricate and interactive than previously realized has fostered additional attention. While few would argue that growth hormone (GH) radically alters fat mass, newer findings revealing the complexity of adipose tissue requires that GH's influence on this tissue be reexamined. Therefore, the objective of this review is to describe the more recent understanding of adipose tissue and to summarize our current knowledge of how GH may influence and contribute to these newer complexities of this tissue with special focus on the available data from mice with altered GH action.

Age Related Shift in Visceral Fat

Fat distribution, especially increased visceral fat, may be as important as overall obesity in increasing risk of heart disease, type 2 diabetes and certain cancers. Risk of disease, as well as visceral fat, increases dramatically with age. Cross-sectional data suggests that increased risk of disease may be largely prevented if the age related increase in visceral fat does not occur. The objective of this short review is to present data that shows visceral fat increasing over 200% in men and 400% in women between the 3rd and 7th decades, show that a combination of weight gain, loss of muscle, and a shift from peripheral to central fat patterning contributes to this increase, and identify hormones that may be responsible for the shift. Finally, the review will show how participation in exercise can slow the age related shift in visceral fat.

Metabolic syndrome, hormones, and maintenance of T cells during aging

Although the phenotype of T-cell senescence has been extensively investigated, few studies have analyzed the factors that promote the generation and maintenance of naïve and memory T cells that exist throughout the lifespan of the individuals. Unlike senescent T cells, naïve and memory T cells are able to participate in useful immune responses as well as respond to new activation. Hormones such as leptin, ghrelin, insulin-like growth factor 1, IGFBP3, and cytokines, including IL-7, regulate both thymopoiesis and maintenance of naïve T cells in the periphery. Although chronic viruses such as cytomegalovirus (CMV) are thought to drive T-cell senescence, other microbes may be important for the maintenance of nonsenescent T cells. Microbiota of the gut can induce metabolic syndrome as well as modulate T-cell development into specific subpopulations of effector cells. Finally, T-cell generation, maintenance, and apoptosis depend upon pathways of energy utilization within the T cells, which parallel those that regulate overall metabolism. Therefore, better understanding of metabolic syndrome, T-cell metabolism, hormones, and microbiota may lead to new insights into the maintenance of proper immune responses in old age.

Transplantation or removal of intra-abdominal adipose tissue prevents age-induced glucose insensitivity

Increases in intra-abdominal fat, a common feature associated with aging, is an established risk factor for insulin resistance, diabetes and the metabolic syndrome. To examine the direct contribution of intra-abdominal fat in the pathophysiology of insulin resistance we altered fat volume via removal or transplantation in a naturally occurring age-induced moderate model of obesity and insulin resistance. This was accomplished by bilateral removal of epididymal white adipose tissue (Lipx) or transplantation of donor fat into the intra-abdominal side of the peritoneal cavity of 28-week old rats. Control animals received sham surgery. Glucose tolerance was evaluated at baseline and 4 and 8weeks post-surgery in all groups, and fasting insulin and leptin were additionally measured in 28-week old rats. In addition, fasted and fed triglyceride, cholesterol and fatty acid concentrations were measured. Before surgery 28-week old rats weighed more and were glucose intolerant compared with 8-week old controls. Both Lipx and transplantation significantly prevented age-induced decreases in glucose tolerance, with Lipx causing improvement at 4weeks which declined by 8weeks; and with a significant transplantation improvement at 8weeks only. Lipx significantly increased insulin secretion 15min after a bolus injection of 0.75mg/kg dextrose at 4 and 8weeks compared with controls, while transplantation caused a significant ( approximately 220%) increase in fasted leptin level at 4weeks only. Taken together, these data suggest that surgical removal or addition of intra-abdominal fat prevents age-induced insulin resistance by different mechanisms and is a suitable model to investigate naturally occurring obesity.