Dynamic differences in oxidative stress and the regulation of metabolism with age in visceral versus subcutaneous adipose.

Once thought only as storage for excess nutrients, adipose tissue has been shown to be a dynamic organ implicated in the regulation of many physiological processes. There is emerging evidence supporting differential roles for visceral and subcutaneous white adipose tissue in maintaining health, although how these roles are modulated by the aging process is not clear. However, the proposed beneficial effects of subcutaneous fat suggest that targeting maintenance of this tissue could lead to healthier aging. In this study, we tested whether alterations in adipose function with age might be associated with changes in oxidative stress. Using visceral and subcutaneous adipose from C57BL/6 mice, we discovered effects of both age and depot location on markers of lipolysis and adipogenesis. Conversely, accumulation of oxidative damage and changes in enzymatic antioxidant expression with age were largely similar between these two depots. The activation of each of the stress signaling pathways JNK and MAPK/ERK was relatively suppressed in subcutaneous adipose tissue suggesting reduced sensitivity to oxidative stress. Similarly, pre-adipocytes from subcutaneous adipose were significantly more resistant than visceral-derived cells to cell death caused by oxidative stress. Cellular respiration in visceral-derived cells was dramatically higher than in cells derived from subcutaneous adipose despite little evidence for differences in mitochondrial density. Together, our data identify molecular mechanisms by which visceral and subcutaneous adipose differ with age and suggest potential targetable means to preserve healthy adipose aging.

Complex IV-deficient Surf1(-/-) mice initiate mitochondrial stress responses.

Mutations in SURF1 (surfeit locus protein 1) COX (cytochrome c oxidase) assembly protein are associated with Leigh's syndrome, a human mitochondrial disorder that manifests as severe mitochondrial phenotypes and early lethality. In contrast, mice lacking the SURF1 protein (Surf1-/-) are viable and were previously shown to have enhanced longevity and a greater than 50% reduction in COX activity. We measured mitochondrial function in heart and skeletal muscle, and despite the significant reduction in COX activity, we found little or no difference in ROS (reactive oxygen species) generation, membrane potential, ATP production or respiration in isolated mitochondria from Surf1-/- mice compared with wild-type. However, blood lactate levels were elevated and Surf1-/- mice had reduced running endurance, suggesting compromised mitochondrial energy metabolism in vivo. Decreased COX activity in Surf1-/- mice is associated with increased markers of mitochondrial biogenesis [PGC-1α (peroxisome-proliferator-activated receptor γ co-activator 1α) and VDAC (voltage-dependent anion channel)] in both heart and skeletal muscle. Although mitochondrial biogenesis is a common response in the two tissues, skeletal muscle has an up-regulation of the UPRMT (mitochondrial unfolded protein response) and heart exhibits induction of the Nrf2 (nuclear factor-erythroid 2-related factor 2) antioxidant response pathway. These data are the first to show induction of the UPRMT in a mammalian model of decreased COX activity. In addition, the results of the present study suggest that impaired mitochondrial function can lead to induction of mitochondrial stress pathways to confer protective effects on cellular homoeostasis.

Decreased in vitro mitochondrial function is associated with enhanced brain metabolism, blood flow, and memory in Surf1-deficient mice.

Recent studies have challenged the prevailing view that reduced mitochondrial function and increased oxidative stress are correlated with reduced longevity. Mice carrying a homozygous knockout (KO) of the Surf1 gene showed a significant decrease in mitochondrial electron transport chain Complex IV activity, yet displayed increased lifespan and reduced brain damage after excitotoxic insults. In the present study, we examined brain metabolism, brain hemodynamics, and memory of Surf1 KO mice using in vitro measures of mitochondrial function, in vivo neuroimaging, and behavioral testing. We show that decreased respiration and increased generation of hydrogen peroxide in isolated Surf1 KO brain mitochondria are associated with increased brain glucose metabolism, cerebral blood flow, and lactate levels, and with enhanced memory in Surf1 KO mice. These metabolic and functional changes in Surf1 KO brains were accompanied by higher levels of hypoxia-inducible factor 1 alpha, and by increases in the activated form of cyclic AMP response element-binding factor, which is integral to memory formation. These findings suggest that Surf1 deficiency-induced metabolic alterations may have positive effects on brain function. Exploring the relationship between mitochondrial activity, oxidative stress, and brain function will enhance our understanding of cognitive aging and of age-related neurologic disorders.

Reduced mitochondrial ROS, enhanced antioxidant defense, and distinct age-related changes in oxidative damage in muscles of long-lived Peromyscus leucopus.

Comparing biological processes in closely related species with divergent life spans is a powerful approach to study mechanisms of aging. The oxidative stress hypothesis of aging predicts that longer-lived species would have lower reactive oxygen species (ROS) generation and/or an increased antioxidant capacity, resulting in reduced oxidative damage with age than in shorter-lived species. In this study, we measured ROS generation in the young adult animals of the long-lived white-footed mouse, Peromyscus leucopus (maximal life span potential, MLSP = 8 yr) and the common laboratory mouse, Mus musculus (C57BL/6J strain; MLSP = 3.5 yr). Consistent with the hypothesis, our results show that skeletal muscle mitochondria from adult P. leucopus produce less ROS (superoxide and hydrogen peroxide) compared with M. musculus. Additionally, P. leucopus has an increase in the activity of antioxidant enzymes superoxide dismutase 1, catalase, and glutathione peroxidase 1 at young age. P. leucopus compared with M. musculus display low levels of lipid peroxidation (isoprostanes) throughout life; however, P. leucopus although having elevated protein carbonyls at a young age, the accrual of protein oxidation with age is minimal in contrast to the linear increase in M. musculus. Altogether, the results from young animals are in agreement with the predictions of the oxidative stress hypothesis of aging with the exception of protein carbonyls. Nonetheless, the age-dependent increase in protein carbonyls is more pronounced in short-lived M. musculus, which supports enhanced protein homeostasis in long-lived P. leucopus.

Mitochondrial dysfunction in aging and longevity: a causal or protective role?

SIGNIFICANCE: Among the most highly investigated theories of aging is the mitochondrial theory of aging. The basis of this theory includes a central role for altered or compromised mitochondrial function in the pathophysiologic declines associated with aging. In general, studies in various organisms, including nematodes, rodents, and humans, have largely upheld that aging is associated with mitochondrial dysfunction. However, results from a number of studies directly testing the mitochondrial theory of aging by modulating oxidant production or scavenging in vivo in rodents have generally been inconsistent with predictions of the theory. RECENT ADVANCES: Interestingly, electron transport chain mutations or deletions in invertebrates and mice that causes mitochondrial dysfunction paradoxically leads to enhanced longevity, further challenging the mitochondrial theory of aging. CRITICAL ISSUES: How can mitochondrial dysfunction contribute to lifespan extension in the mitochondrial mutants, and what does it mean for the mitochondrial theory of aging? FUTURE DIRECTIONS: It will be important to determine the potential mechanisms that lead to enhanced longevity in the mammalian mitochondrial mutants.

Comparative studies of oxidative stress and mitochondrial function in aging.

The oxidative stress theory and its correlate the mitochondrial theory of aging are among the most studied and widely accepted of all hypotheses of the mechanism of aging. To date, most of the supporting evidence for these theories has come from investigations using common model organisms such as Caenorhabditis elegans, Drosophila melanogaster, and laboratory rodents. However, comparative data from a wide range of endotherms provide equivocal support as to whether oxidative stress is merely a correlate, rather than a determinant, of species' maximum lifespan. The great majority of studies in this area have been devoted to the relationship between reactive oxygen species and maximal longevity in young adult organisms, with little emphasis on mitochondrial respiratory efficiency, age-related alterations in mitochondrial physiology or oxidative damage. The advantage of studying a broader spectrum of species is the broad range of virtually every biological phenotype/trait, such as lifespan, body weight and metabolic rate. Here we summarize the results from a number of comparative studies in an effort to correlate oxidant production and oxidative damage among many species with their maximal lifespan and briefly discuss the pitfalls and limitations. Based on current information, it is not possible to accept or dispute the oxidative stress theory of aging, nor can we exclude the possibility that private mechanisms might offer an explanation for the longevity of exceptionally long-lived animal models. Thus, there is need for more thorough and controlled investigations with more unconventional animal models for a deeper understanding of the role of oxidative stress in longevity.