An optimized mouse parabiosis protocol for investigation of aging and rejuvenative mechanisms.

Surgical parabiosis enables sharing of the circulating milieu between two organisms. This powerful model presents diverse complications based on age, strain, sex, and other experimental parameters. Here, we provide an optimized parabiosis protocol for the surgical union of two mice internally at the elbow and knee joints with continuous external joining of the skin. This protocol incorporates guidance and solutions to complications that can occur, particularly in aging studies, including non-cohesive pairing, variable anesthesia sensitivity, external and internal dehiscence, dehydration, and weight loss. We also offer a straightforward method for validating postoperative blood chimerism and confirming its time course using flow cytometry. Utilization of our optimized protocol can facilitate reproducible parabiosis experimentation to dynamically explore mechanisms of aging and rejuvenation.

Health benefits attributed to 17α-estradiol, a lifespan-extending compound, are mediated through estrogen receptor α.

Metabolic dysfunction underlies several chronic diseases, many of which are exacerbated by obesity. Dietary interventions can reverse metabolic declines and slow aging, although compliance issues remain paramount. 17α-estradiol treatment improves metabolic parameters and slows aging in male mice. The mechanisms by which 17α-estradiol elicits these benefits remain unresolved. Herein, we show that 17α-estradiol elicits similar genomic binding and transcriptional activation through estrogen receptor α (ERα) to that of 17ß-estradiol. In addition, we show that the ablation of ERα completely attenuates the beneficial metabolic effects of 17α-E2 in male mice. Our findings suggest that 17α-E2 may act through the liver and hypothalamus to improve metabolic parameters in male mice. Lastly, we also determined that 17α-E2 improves metabolic parameters in male rats, thereby proving that the beneficial effects of 17α-E2 are not limited to mice. Collectively, these studies suggest ERα may be a drug target for mitigating chronic diseases in male mammals.

The enigmatic role of growth hormone in age-related diseases, cognition, and longevity.

Growth hormone (GH) is secreted by the anterior pituitary gland and regulates various metabolic processes throughout the body. GH and IGF-1 levels are markedly reduced in older humans, leading some to hypothesize GH supplementation could be a viable "anti-aging" therapy. However, there is still much debate over the benefits and risks of GH administration. While an early study of GH administration reported reduced adiposity and lipid levels and increased bone mineral density, subsequent studies failed to show significant benefits. Conversely, other studies found positive effects of GH deficiency including extended life span, improved cognitive function, resistance to diseases such as cancer and diabetes, and improved insulin sensitivity despite a higher fat percentage. Thus, the roles of GH in aging and cognition remain unclear, and there is currently not enough evidence to support use of GH as an anti-aging or cognitive impairment therapy. Additional robust and longer-duration studies of efficacy and safety of GH administration are needed to determine if modulating GH levels could be a successful strategy for treating aging and age-related diseases.

Central IGF-1 protects against features of cognitive and sensorimotor decline with aging in male mice.

Disruptions in growth hormone/insulin-like growth factor-1 (GH/IGF-1) signaling have been linked to improved longevity in mice and humans. Nevertheless, while IGF-1 levels are associated with increased cancer risk, they have been paradoxically implicated with protection from other age-related conditions, particularly in the brain, suggesting that strategies aimed at selectively increasing central IGF-1 action may have favorable effects on aging. To test this hypothesis, we generated inducible, brain-specific (TRE-IGF-1 × Camk2a-tTA) IGF-1 (bIGF-1) overexpression mice and studied effects on healthspan. Doxycycline was removed from the diet at 12 weeks old to permit post-development brain IGF-1 overexpression, and animals were monitored up to 24 months. Brain IGF-1 levels were increased approximately twofold in bIGF-1 mice, along with greater brain weights, volume, and myelin density (P < 0.05). Age-related changes in rotarod performance, exercise capacity, depressive-like behavior, and hippocampal gliosis were all attenuated specifically in bIGF-1 male mice (P < 0.05). However, chronic brain IGF-1 failed to prevent declines in cognitive function or neurovascular coupling. Therefore, we performed a short-term intranasal (IN) treatment of either IGF-1 or saline in 24-month-old male C57BL/6 mice and found that IN IGF-1 treatment tended to reduce depressive (P = 0.09) and anxiety-like behavior (P = 0.08) and improve motor coordination (P = 0.07) and unlike transgenic mice improved motor learning (P < 0.05) and visuospatial and working memory (P < 0.05). These data highlight important sex differences in how brain IGF-1 action impacts healthspan and suggest that translational approaches that target IGF-1 centrally can restore cognitive function, a possibility that should be explored as a strategy to combat age-related cognitive decline.

Intestinal crypts recover rapidly from focal damage with coordinated motion of stem cells that is impaired by aging.

Despite the continuous renewal and turnover of the small intestinal epithelium, the intestinal crypt maintains a 'soccer ball-like', alternating pattern of stem and Paneth cells at the base of the crypt. To study the robustness of the alternating pattern, we used intravital two-photon microscopy in mice with fluorescently-labeled Lgr5+ intestinal stem cells and precisely perturbed the mosaic pattern with femtosecond laser ablation. Ablation of one to three cells initiated rapid motion of crypt cells that restored the alternation in the pattern within about two hours with only the rearrangement of pre-existing cells, without any cell division. Crypt cells then performed a coordinated dilation of the crypt lumen, which resulted in peristalsis-like motion that forced damaged cells out of the crypt. Crypt cell motion was reduced with inhibition of the ROCK pathway and attenuated with old age, and both resulted in incomplete pattern recovery. This suggests that in addition to proliferation and self-renewal, motility of stem cells is critical for maintaining homeostasis. Reduction of this newly-identified behavior of stem cells could contribute to disease and age-related changes.

Dietary Walnuts Protect Against Obesity-Driven Intestinal Stem Cell Decline and Tumorigenesis.

Obesity can negatively impact intestinal homeostasis, and increase colon cancer risk and related mortality. Thus, given the alarmingly high rates of obesity in the US and globally, it is critical to identify practical strategies that can break the obesity-cancer link. Walnuts have been increasingly recognized to mitigate cancer risk, and contain many bioactive constituents with antioxidant and anti-inflammatory properties that could potentially counteract pathways thought to be initiators of obesity-related cancer. Therefore, the purpose of this study was to determine if walnuts could preserve intestinal homeostasis, and attenuate tumorigenesis and growth in the context of obesity and a high calorie diet. To this end, we studied effects of walnuts on these parameters under different dietary conditions in wildtype mice, two independent Apc models (Apc1638N/+ and ApcΔ14), and in MC38 colon cancer cells in vivo, respectively. Walnuts did not alter the metabolic phenotype or intestinal morphology in normal mice fed either a low-fat diet (LFD), LFD with 6% walnuts (LFD+W), high-fat diet (HFD), or HFD with 7.6% walnuts (HFD+W). However, walnuts did lead to a significant reduction in circulating CCL5 and preserved intestinal stem cell (ISC) function under HFD-fed conditions. Furthermore, walnuts reduced tumor multiplicity in Apc1638N/+ male HFD+W animals, as compared to HFD controls (3.7 ± 0.5 vs. 2.5 ± 0.3; P = 0.015), tended to reduce the number of adenocarcinomas (0.67 ± 0.16 vs. 0.29 ± 0.12; P = 0.07), and preferentially limited tumor growth in ApcΔ14 male mice (P = 0.019) fed a high-calorie western-style diet. In summary, these data demonstrate that walnuts confer significant protection against intestinal tumorigenesis and growth and preserve ISC function in the context of a high-calorie diet and obesity. Thus, these data add to the accumulating evidence connecting walnuts as a potentially effective dietary strategy to break the obesity-colon cancer link.

Late-life targeting of the IGF-1 receptor improves healthspan and lifespan in female mice.

Diminished growth factor signaling improves longevity in laboratory models, while a reduction in the somatotropic axis is favorably linked to human aging and longevity. Given the conserved role of this pathway on lifespan, therapeutic strategies, such as insulin-like growth factor-1 receptor (IGF-1R) monoclonal antibodies (mAb), represent a promising translational tool to target human aging. To this end, we performed a preclinical study in 18-mo-old male and female mice treated with vehicle or an IGF-1R mAb (L2-Cmu, Amgen Inc), and determined effects on aging outcomes. Here we show that L2-Cmu preferentially improves female healthspan and increases median lifespan by 9% (P = 0.03) in females, along with a reduction in neoplasms and inflammation (P ≤ 0.05). Thus, consistent with other models, targeting IGF-1R signaling appears to be most beneficial to females. Importantly, these effects could be achieved at advanced ages, suggesting that IGF-1R mAbs could represent a promising therapeutic candidate to delay aging.

Mesenteric visceral lipectomy using tissue liquefaction technology reverses insulin resistance and causes weight loss in baboons.

BACKGROUND: Visceral obesity is associated with diabetogenic and atherogenic abnormalities, including insulin resistance and increased risk for cardiometabolic diseases and mortality. Rodent lipectomy studies have demonstrated a causal link between visceral fat and insulin resistance, yet human omentectomy studies have failed to replicate this metabolic benefit, perhaps owing to the inability to target the mesentery. OBJECTIVES: We aimed to demonstrate that safe and effective removal of mesenteric fat could be achieved in obese insulin-resistant baboons using tissue liquefaction technology. SETTING: Southwest National Primate Research Center, San Antonio, Texas. METHODS: Tissue liquefaction technology has been developed to enable mesenteric visceral lipectomy (MVL) to be safely performed without disturbing the integrity of surrounding nerves and vessels in the mesentary. After an initial MVL optimization study (n = 3), we then performed MVL (n = 4) or sham surgery (n = 2) in a cohort of insulin-resistant baboons, and the metabolic phenotype was assessed via hyperinsulinemic-euglycemic clamps at baseline and 6 weeks later. RESULTS: MVL led to a 75% improvement in glucose disposal at 6-weeks follow-up (P = .01). Moreover, despite removing only an average of 430 g of mesenteric fat (~1% of total body mass), MVL led to a 14.4% reduction in total weight (P = .001). Thus, these data demonstrate that mesenteric fat can be safely targeted for removal by tissue liquefaction technology in a nonhuman primate, leading to substantial metabolic improvements, including reversal of insulin resistance and weight loss. CONCLUSIONS: These data provide the first demonstration of successful adipose tissue removal from the mesentery in a mammal. Importantly, we have demonstrated that when MVL is performed in obese, insulin-resistant baboons, insulin resistance is reversed, and significant weight loss occurs. Therefore, trials performing MVL in humans with abdominal obesity and related metabolic sequelae should be explored as a potential clinical tool to ameliorate insulin resistance and treat type 2 diabetes.

Apc inactivation, but not obesity, synergizes with Pten deficiency to drive intestinal stem cell-derived tumorigenesis.

Obesity is a major risk factor for colorectal cancer and can accelerate Lgr5+ intestinal stem cell (ISC)-derived tumorigenesis after the inactivation of Apc However, whether non-canonical pathways involving PI3K-Akt signaling in ISCs can lead to tumor formation, and if this can be further exacerbated by obesity is unknown. Despite the synergy between Pten and Apc inactivation in epithelial cells on intestinal tumor formation, their combined role in Lgr5+-ISCs, which are the most rapidly dividing ISC population in the intestine, is unknown. Lgr5+-GFP mice were provided low-fat diet (LFD) or high-fat diet (HFD) for 8 months, and the transcriptome was evaluated in Lgr5+-ISCs. For tumor studies, Lgr5+-GFP and Lgr5+-GFP-Ptenflox/flox mice were tamoxifen treated to inactivate Pten in ISCs and provided LFD or HFD until 14-15 months of age. Finally, various combinations of Lgr5+-ISC-specific, Apc- and Pten-deleted mice were generated and evaluated for histopathology and survival. HFD did not overtly alter Akt signaling in ISCs, but did increase other metabolic pathways. Pten deficiency, but not HFD, increased BrdU-positive cells in the small intestine (P < 0.05). However, combining Pten and Apc deficiency synergistically increased proliferative markers, tumor pathology and mortality, in a dose-dependent fashion (P < 0.05). In summary, we show that HFD alone fails to drive Akt signaling in ISCs and that Pten deficiency is dispensable as a tumor suppressor in Lgr5+-ISCs. However, combining Pten and Apc deficiency in ISCs synergistically increases proliferation, tumor formation and mortality. Thus, aberrant Wnt/ß-catenin, rather than PI3K-Akt signaling, is requisite for obesity to drive Lgr5+ ISC-derived tumorigenesis.

A simplified characterization of S-adenosyl-l-methionine-consuming enzymes with 1-Step EZ-MTase: a universal and straightforward coupled-assay for in vitro and in vivo setting.

Methyltransferases use S-adenosyl-l-methionine (SAM) to deposit methyl marks. Many of these epigenetic 'writers' are associated with gene regulation. As cancer etiology is highly correlated with misregulated methylation patterns, methyltransferases are emerging therapeutic targets. Successful assignment of methyltransferases' roles within intricate biological networks relies on (1) the access to enzyme mechanistic insights and (2) the efficient screening of chemical probes against these targets. To characterize methyltransferases in vitro and in vivo, we report a highly-sensitive one-step deaminase-linked continuous assay where the S-adenosyl-l-homocysteine (SAH) enzyme-product is rapidly and quantitatively catabolized to S-inosyl-l-homocysteine (SIH). To highlight the broad capabilities of this assay, we established enzymatic characteristics of two protein arginine methyltransferases (PRMT5 and PRMT7), a histone-lysine N-methyltransferase (DIM-5) and a sarcosine/dimethylglycine N-methyltransferase (SDMT). Since the coupling deaminase TM0936 displays robust activity over a broad pH-range we determined the pH dependence of SDMT reaction rates. TM0936 reactions are monitored at 263 nm, so a drawback may arise when methyl acceptor substrates absorb within this UV-range. To overcome this limitation, we used an isosteric fluorescent SAM-analog: S-8-aza-adenosyl-l-methionine. Most enzymes tolerated this probe and sustained methyltransfers were efficiently monitored through loss of fluorescence at 360 nm. Unlike discontinuous radioactive- and antibody-based assays, our assay provides a simple, versatile and affordable approach towards the characterization of methyltransferases. Supported by three logs of linear dynamic range, the 1-Step EZ-MTase can detect methylation rates as low as 2 µM h-1, thus making it possible to quantify low nanomolar concentrations of glycine N-methyltransferase within crude biological samples. With Z'-factors above 0.75, this assay is well suited to high-throughput screening and may promote the identification of novel therapeutics.

Evaluating Health Span in Preclinical Models of Aging and Disease: Guidelines, Challenges, and Opportunities for Geroscience.

Life extension is no longer considered sufficient evidence of delayed aging in research animals. It must also be demonstrated that a broad swathe of health indicators have been extended. During a retreat of the Geroscience Network, a consortium of basic and clinical aging researchers, potential measures of mouse health were considered for their potential as easily standardized, highly informative metrics. Major health domains considered were neuromuscular, cognitive, cardiovascular, metabolic, and inflammatory functions as well as body composition and energetics and a multitude of assays interrogating these domains. A particularly sensitive metric of health is the ability to respond to, and recover, from stress. Therefore, the Network also considered stresses of human relevance that could be implemented in mouse models to assess frailty and resilience. Mouse models already exist for responses to forced immobility, cancer chemotherapy, infectious diseases, dietary challenges, and surgical stress, and it was felt that these could be employed to determine whether putative senescence-retarding interventions increased and extended organismal robustness. The Network discussed challenges in modeling age-related human chronic diseases and concluded that more attention needs to be paid to developing disease models with later age of onset, models of co- and multimorbidity, diversifying the strains and sexes commonly used in aging research, and considering additional species.

The Somatotropic Axis in Human Aging: Framework for the Current State of Knowledge and Future Research.

Mutations resulting in reduced signaling of the growth hormone/insulin-like growth factor-1 (GH/IGF-1) axis are associated with increased life- and healthspan across model organisms. Similar findings have been noted in human cohorts with functional mutations in the somatotropic axis, suggesting that this pathway may also be relevant to human aging and protection from age-related diseases. While epidemiological data indicate that low circulating IGF-1 level may protect aging populations from cancer, results remain inconclusive regarding most other diseases. We propose that studies in humans and animals need to consider differences in sex, pathway function, organs, and time-specific effects of GH/IGF-1 signaling in order to better define the role of the somatotropic axis in aging. Agents that modulate signaling of the GH/IGF-1 pathway are available for human use, but before they can be implemented in clinical studies that target aging and age-related diseases, researchers need to address the challenges discussed in this Review.

Energetic interventions for healthspan and resiliency with aging.

Several behavioral and pharmacological strategies improve longevity, which is indicative of delayed organismal aging, with the most effective interventions extending both life- and healthspan. In free living creatures, maintaining health and function into old age requires resilience against a multitude of stressors. Conversely, in experimental settings, conventional housing of rodents limits exposure to such challenges, thereby obscuring an accurate assessment of resilience. Caloric restriction (CR) and exercise, as well as pharmacologic strategies (resveratrol, rapamycin, metformin, senolytics), are well established to improve indices of health and aging, but some paradoxical effects have been observed on resilience. For instance, CR potently retards the onset of age-related diseases, and improves lifespan to a greater extent than exercise in a variety of models. However, exercise has proven more consistently beneficial to organismal resilience against a broad array of stressors, including infections, surgery, wound healing and frailty. CR can improve cellular stress defenses and protect from frailty, but also impairs the response to infections, bed rest and healing. How an intervention will impact not only longevity, health and function, but also resiliency, is critical to better understanding translational implications. Thus, organismal robustness represents a critical, albeit understudied aspect of aging, which needs more careful attention in order to better inform on how putative age-delaying strategies will impact preservation of health and function in response to stressors with aging in humans.

Naturally occurring mitochondrial-derived peptides are age-dependent regulators of apoptosis, insulin sensitivity, and inflammatory markers.

Mitochondria are key players in aging and in the pathogenesis of age-related diseases. Recent mitochondrial transcriptome analyses revealed the existence of multiple small mRNAs transcribed from mitochondrial DNA (mtDNA). Humanin (HN), a peptide encoded in the mtDNA 16S ribosomal RNA region, is a neuroprotective factor. An in silico search revealed six additional peptides in the same region of mtDNA as humanin; we named these peptides small humanin-like peptides (SHLPs). We identified the functional roles for these peptides and the potential mechanisms of action. The SHLPs differed in their ability to regulate cell viability in vitro. We focused on SHLP2 and SHLP3 because they shared similar protective effects with HN. Specifically, they significantly reduced apoptosis and the generation of reactive oxygen species, and improved mitochondrial metabolism in vitro. SHLP2 and SHLP3 also enhanced 3T3-L1 pre-adipocyte differentiation. Systemic hyperinsulinemic-euglycemic clamp studies showed that intracerebrally infused SHLP2 increased glucose uptake and suppressed hepatic glucose production, suggesting that it functions as an insulin sensitizer both peripherally and centrally. Similar to HN, the levels of circulating SHLP2 were found to decrease with age. These results suggest that mitochondria play critical roles in metabolism and survival through the synthesis of mitochondrial peptides, and provide new insights into mitochondrial biology with relevance to aging and human biology.

Low insulin-like growth factor-1 level predicts survival in humans with exceptional longevity.

Attenuated growth hormone and insulin-like growth factor-1 (GH/IGF-1) signaling is associated with extended lifespan in several animal models. However, the effect of diminished GH/IGF-1 activity on survival in humans has not been confirmed. We tested the hypothesis that IGF-1 levels in nonagenarians (n = 184), measured at study enrollment, predict the duration of their incremental survival. In the Kaplan-Meier analysis, females with IGF-1 levels below the median (≤ 96 ng mL(-1) ) had significantly longer survival compared with females with levels above the median, P < 0.01. However, this survival advantage was not observed in males (P = 0.83). On the other hand, in both males and females with a history of cancer, lower IGF-1 levels predicted longer survival (P < 0.01). IGF-1 level remained a significant predictor of survival duration in linear regression models after multivariable adjustment in females (P = 0.01) and individuals with a history of cancer (P < 0.01). We show for the first time that low IGF-1 levels predict life expectancy in exceptionally long-lived individuals.

Abdominal obesity, independent from caloric intake, accounts for the development of intestinal tumors in Apc(1638N/+) female mice.

To determine whether visceral fat (VF), independent of other confounders, is causally linked to intestinal tumorigenesis, we surgically removed visceral fat in Apc(1638/N+) mice. At 15 weeks of age, male and female Apc(1638/N+) mice were randomized to one of three groups: ad libitum, visceral fat removal (VF-) and ad libitum fed, or caloric restriction, and were studied for effects on tumorigenesis and survival. As compared with ad libitum, VF- and caloric restriction reduced macroadenomas to a similar extent (P < 0.05), but only caloric restriction significantly improved survival (P < 0.05). Given that a significant group × gender interaction was observed, we next examined males and females separately. In females, macroadenomas were markedly attenuated by VF- (1.33 ± 0.23 mean ± SE; P < 0.05), but not by caloric restriction (2.35 ± 0.25; P = 0.71), as compared with ad libitum (2.50 ± 0.34). In males, however, caloric restriction (1.71 ± 0.26; P < 0.01), but not VF- (2.94 ± 0.42; P = 0.29), reduced macroadenomas, as compared with ad libitum males (3.47 ± 0.30). In females, both VF- (P = 0.05) and caloric restriction (P < 0.01) improved survival, but not in male mice (P = 0.15). The benefits observed with caloric restriction were consistent with favorable metabolic adaptations, but protection conferred in VF- females was despite lower adiponectin levels (P < 0.05), and failure to reduce body mass, total adiposity, glucose, insulin, leptin, and chemokine (C-X-C motif) ligand 1 (CXCL-1) levels. In conclusion, these data provide the first causal evidence linking visceral fat to intestinal cancer risk, and suggest that factors, other than known metabolic mediators, may impact tumor development. Furthermore, these data emphasize that strategies designed to deplete visceral fat stores in humans should be considered in the prevention of intestinal cancer. Cancer Prev Res; 6(3); 177-87. ©2012 AACR.

Acute humanin therapy attenuates myocardial ischemia and reperfusion injury in mice.

OBJECTIVE: Humanin (HN), an endogenous antiapoptotic peptide, has previously been shown to protect against Alzheimer's disease and a variety of cellular insults. We evaluated the effects of a potent analog of HN (HNG) in an in vivo murine model of myocardial ischemia and reperfusion. METHODS AND RESULTS: Male C57BL6/J mice (8 to 10 week old) were subjected to 45 minutes of left coronary artery occlusion followed by a 24-hour reperfusion. HNG or vehicle was administered IP 1 hour prior or at the time of reperfusion. The extent of myocardial infarction per area-at-risk was evaluated at 24 hours using Evans Blue dye and 2-3-5-triphenyl tetrazolium chloride staining. Left ventricular function was evaluated at 1 week after ischemia using high-resolution, 2D echocardiography (VisualSonics Vevo 770). Myocardial cell signaling pathways and apoptotic markers were assessed at various time points (0 to 24 hours) following reperfusion. Cardiomyocyte survival and apoptosis in response to HNG were assessed in vitro. HNG reduced infarct size relative to the area-at-risk in a dose-dependent fashion, with a maximal reduction at the dose of 2 mg/kg. HNG therapy enhanced left ventricular ejection fraction and preserved postischemic left ventricular dimensions (end-diastolic and end-systolic), resulting in improved cardiac function. Treatment with HNG significantly increased phosphorylation of AMPK and phosphorylation of endothelial nitric oxide synthase in the heart and attenuated Bcl-2-associated X protein and B-cell lymphoma-2 levels following myocardial ischemia and reperfusion. HNG improved cardiomyocyte survival and decreased apoptosis in response to daunorubicin in vitro. CONCLUSIONS: These data show that HNG provides cardioprotection in a mouse model of myocardial ischemia and reperfusion potentially through activation of AMPK-endothelial nitric oxide synthase-mediated signaling and regulation of apoptotic factors. HNG may represent a novel agent for the treatment of acute myocardial infarction.

Aging per se increases the susceptibility to free fatty acid-induced insulin resistance.

Elevations in systemic free fatty acids (FFA) contribute to insulin resistance. To determine the effects of an acute elevation in FFA on insulin action with aging, we infused saline or intralipid (IL) during a hyperinsulinemic-euglycemic clamp in three groups of rats: young ad libitum-fed (YAL), old ad libitum-fed (OAL), and old on lifelong calorie restriction (OCR). The OCR group was included to distinguish between aging per se and age-related changes in body fat distribution. IL induced marked insulin resistance in both YAL and OCR, but the onset of insulin resistance was approximately two to three times more rapid in OCR as compared with YAL. In response to IL infusion, plasminogen-activating inhibitor-1 (PAI-1) expression was increased in subcutaneous fat from OAL animals. In visceral fat, a marked increase in PAI-1 and interleukin-6 expression was observed in OAL and OCR rats, but not YAL, in response to IL treatment. Thus, aging per se increases the inflammatory response to excess nutrients and vulnerability to FFA-induced insulin resistance with aging.

Role of visceral adipose tissue in aging.

BACKGROUND: Visceral fat (VF) accretion is a hallmark of aging in humans. Epidemiologic studies have implicated abdominal obesity as a major risk factor for insulin resistance, type 2 diabetes, cardiovascular disease, metabolic syndrome and death. METHODS: Studies utilizing novel rodent models of visceral obesity and surgical strategies in humans have been undertaken to determine if subcutaneous (SC) abdominal or VF are causally linked to age-related diseases. RESULTS: Specific depletion or expansion of the VF depot using genetic or surgical tools in rodents has been shown to have direct effects on disease risk. In contrast, surgically removing large quantities of SC fat does not consistently improve metabolic parameters in humans or rodents, while benefits were observed with SC fat expansion in mice, suggesting that SC fat accrual is not an important contributor to metabolic decline. There is also compelling evidence in humans that abdominal obesity is a stronger risk factor for mortality risk than general obesity. Likewise, we have shown that surgical removal of VF improves mean and maximum lifespan in rats, providing the first causal evidence that VF depletion may be an important underlying cause of improved lifespan with caloric restriction. GENERAL SIGNIFICANCE: This review provides both corollary and causal evidence for the importance of accounting for body fat distribution, and specifically VF, when assessing disease and mortality risk. Given the hazards of VF accumulation on health, treatment strategies aimed at selectively depleting VF should be considered as a viable tool to effectively reduce disease risk in humans.