Perilipin 5 Deletion Unmasks an Endoplasmic Reticulum Stress-Fibroblast Growth Factor 21 Axis in Skeletal Muscle.

Lipid droplets (LDs) are critical for the regulation of lipid metabolism, and dysregulated lipid metabolism contributes to the pathogenesis of several diseases, including type 2 diabetes. We generated mice with muscle-specific deletion of the LD-associated protein perilipin 5 (PLIN5, Plin5MKO ) and investigated PLIN5's role in regulating skeletal muscle lipid metabolism, intracellular signaling, and whole-body metabolic homeostasis. High-fat feeding induced changes in muscle lipid metabolism of Plin5MKO mice, which included increased fatty acid oxidation and oxidative stress but, surprisingly, a reduction in inflammation and endoplasmic reticulum (ER) stress. These muscle-specific effects were accompanied by whole-body glucose intolerance, adipose tissue insulin resistance, and reduced circulating insulin and C-peptide levels in Plin5MKO mice. This coincided with reduced secretion of fibroblast growth factor 21 (FGF21) from skeletal muscle and liver, resulting in reduced circulating FGF21. Intriguingly, muscle-secreted factors from Plin5MKO , but not wild-type mice, reduced hepatocyte FGF21 secretion. Exogenous correction of FGF21 levels restored glycemic control and insulin secretion in Plin5MKO mice. These results show that changes in lipid metabolism resulting from PLIN5 deletion reduce ER stress in muscle, decrease FGF21 production by muscle and liver, and impair glycemic control. Further, these studies highlight the importance for muscle-liver cross talk in metabolic regulation

The Long Life of Birds: The Rat-Pigeon Comparison Revisited

The most studied comparison of aging and maximum lifespan potential (MLSP) among endotherms involves the 7-fold longevity difference between rats (MLSP 5y) and pigeons (MLSP 35y). A widely accepted theory explaining MLSP differences between species is the oxidative stress theory, which purports that reactive oxygen species (ROS) produced during mitochondrial respiration damage bio-molecules and eventually lead to the breakdown of regulatory systems and consequent death. Previous rat-pigeon studies compared only aspects of the oxidative stress theory and most concluded that the lower mitochondrial superoxide production of pigeons compared to rats was responsible for their much greater longevity. This conclusion is based mainly on data from one tissue (the heart) using one mitochondrial substrate (succinate). Studies on heart mitochondria using pyruvate as a mitochondrial substrate gave contradictory results. We believe the conclusion that birds produce less mitochondrial superoxide than mammals is unwarranted

Mitochondrial Dysfunction and Diabetes: Is Mitochondrial Transfer a Friend or Foe?

Obesity, insulin resistance and type 2 diabetes are accompanied by a variety of systemic and tissue-specific metabolic defects, including inflammation, oxidative and endoplasmic reticulum stress, lipotoxicity, and mitochondrial dysfunction. Over the past 30 years, association studies and genetic manipulations, as well as lifestyle and pharmacological invention studies, have reported contrasting findings on the presence or physiological importance of mitochondrial dysfunction in the context of obesity and insulin resistance. It is still unclear if targeting mitochondrial function is a feasible therapeutic approach for the treatment of insulin resistance and glucose homeostasis. Interestingly, recent studies suggest that intact mitochondria, mitochondrial DNA, or other mitochondrial factors (proteins, lipids, miRNA) are found in the circulation, and that metabolic tissues secrete exosomes containing mitochondrial cargo. While this phenomenon has been investigated primarily in the context of cancer and a variety of inflammatory states, little is known about the importance of exosomal mitochondrial transfer in obesity and diabetes. We will discuss recent evidence suggesting that (1) tissues with mitochondrial dysfunction shed their mitochondria within exosomes, and that these exosomes impair the recipient’s cell metabolic status, and that on the other hand, (2) physiologically healthy tissues can shed mitochondria to improve the metabolic status of recipient cells. In this context the determination of whether mitochondrial transfer in obesity and diabetes is a friend or foe requires further studies

The long life of birds: an examination of the oxidative stress theory of aging

Birds, as a group, are long-living. Both for mammals and birds, the maximum lifespan potential (MLSP) of a species is correlated with body size, but birds live, on average, twice as long as mammals. Differences in longevity also exist within the group of birds, again as a correlation with body size, but there is also considerable mass-independent variation in MLSP. For example, Psittaciformes (parrots) are extremely long-living with an average MLSP of 25 years, and with some species living more than 100 years. In contrast, similar-sized Galliformes (fowl) are short-living, and the comparison of these bird orders might give considerable insights into the mechanisms of aging. Early attempts to understand the mechanisms determining maximum longevity were carried out in mammals and implicated differences in metabolic rate. Thus, the longevity differences between mammals and birds, as well as within birds, were surprising as the rate of living of birds (i.e. their metabolic rate) is generally higher than that of mammals of the same size, and birds usually have much higher resting body temperatures than mammals. Furthermore, similar-sized birds with extreme differences in MLSP (such as parrots and fowl) have very similar metabolic rates. While differences in metabolic rate per se cannot fully explain longevity differences among animals, there does appear to be some link between the „rate of living‟ and the „length of life‟. Oxygen-derived radicals (nowadays called reactive oxygen species; ROS) are produced as a normal by-product of mitochondrial respiration. These ROS cause oxidative damage to biological molecules and the accumulated damage, in turn, results in the breakdown of homeostatic regulatory systems, eventually causing an animal\u27s death and consequently determining the characteristic maximum longevity of the particular species. This “oxidative stress theory of aging” is currently the most widely accepted explanation of an animal‟s maximum lifespan, and can be divided into its functional components: (i) the mitochondrial production of ROS during normal respiration, (ii) the countervailing influence of an array of antioxidant systems (both enzymatic and non-enzymatic), and (iii) oxidative damage to a wide variety of bio-molecules. This study examined the biochemical mechanisms underlying the longevity differences, on the one hand between long-living pigeons (MLSP 35y) and short-living rats (MLSP 5y), and on the other hand between three species of long-living parrots (average MLSP 27y) and two species of short-living quails (average MLSP 5.5y). A number of functional aspects of the oxidative stress theory were investigated in the two species-comparisons. In the rat-pigeon comparison, total antioxidant status and non-enzymatic antioxidants are essentially the same in rats and pigeons. The enzymatic antioxidants (especially mitochondrial) suggest that the rats experience a much greater degree of oxidative stress in vivo than do pigeons. This is especially the case for the peroxidases (glutathione peroxidase and catalase). However, the results from in vitro measurements of mitochondrial ROS production (superoxide and hydrogen peroxide) do not support this interpretation. It is only heart mitochondria that suggest a greater oxidative stress in rats compared to pigeons, and only with succinate as a substrate. There was no convincing evidence of rat tissues having consistently higher in vivo mitochondrial superoxide production than tissues in pigeons. Yet, the higher enzymatic antioxidant content of rats compared to pigeons suggests that rat tissues in vivo may experience greater oxidative stress than pigeon tissues. Is this apparent contradiction real? In this respect it is of interest that the most consistent difference that was observed between rats and pigeons was in the peroxidation index of membrane lipids, with the rats having a significantly higher membrane susceptibility to oxidative damage than pigeons. Fatty acid peroxidation leads to the formation of harmful secondary lipid-based ROS. These secondary ROS might be as important as mitochondria-derived primary ROS in the determination of an animal‟s lifespan. Biomarkers of lipid peroxidation, as well as markers of protein and mitochondrial DNA damage, were determined for the same individuals. No differences were found between rats and pigeons which may be due to rapid repair and removal rates. A similar approach was used in the parrot-quail comparison, with the difference that all birds in this comparison were fed the same diet for two months prior to the beginning of the experiments, to exclude dietary effects on all variables examined. ROS production was determined in intact cells (erythrocytes), as well as in vitro in isolated mitochondria. Both methodological approaches revealed that parrots and quails have very similar levels of ROS generation. Mitochondrial ROS production, therefore, may not account for their longevity differences. Glutathione peroxidase (GPx) and glutathione (GSH) levels are higher in the long-living parrots and suggest higher protection against the harmful effects of hydroperoxides which might be important for parrot longevity. Parrots have a higher total antioxidant capacity, but only on a „per g tissue‟ basis. All other antioxidants show either no association, or a negative correlation with MLSP. Despite indications of higher protection against some aspects of oxidative stress in the parrots, overall antioxidant defence mechanisms do not account for their longevity. Besides maximum lifespan, basal metabolic rate (BMR) also varies with body size in mammals and birds and it has been suggested that both mass-related variations are mediated through differences in membrane fatty acid composition. BMR, the tissue phospholipid fatty acid composition of seven tissues, and fatty acid composition of mitochondrial membranes from two tissues, were evaluated in all parrots and quails. Whereas neither BMR nor the membrane susceptibilities to oxidative damage corresponded with the long MLSP of parrots, there was consistent demonstration that (i) all birds exclude n-3 polyunsaturated fatty acids from their mitochondria, and that (ii) independent of the mode of locomotion (flight vs. non-flight muscles) both pectoral and leg muscle have an almost identical membrane fatty acid composition in all birds. In agreement with the absence of differences in ROS production, antioxidants and membrane composition, the tissue levels of oxidative damage (mitochondrial DNA, protein and lipid damage) were similar in parrots and quails. The rat-pigeon comparison and the parrot-quail comparison can be regarded as two separate studies, with one investigating the mechanisms underlying the long MLSP of birds in general (in comparison to mammals), and the other examining the basis of lifespan differences within birds. Mitochondrial primary ROS generation is commonly regarded as an important determinant of longevity differences between species. However, both studies showed equally that mitochondria-derived ROS might not be as important as generally assumed. Instead, secondary lipid-based ROS, produced during lipid peroxidation of membrane fatty acids, might be the more important ROS with respect to oxidative stress. A low secondary lipid-based ROS production can to some extent explain the long lifespan of pigeons compared to rats due to much higher membrane susceptibility to oxidative damage in the rats. It can also to some extent explain the long lifespan of parrots because of their high GPx and GSH levels, which protect against hydroperoxides formed during lipid peroxidation of membrane fatty acids

Metabolic rate and membrane fatty acid composition in birds : a comparison between long-living parrots and short-living fowl

Both basal metabolic rate (BMR) and maximum lifespan potential (MLSP) vary with body size in mammals and birds and it has been suggested that these are mediated through size-related variation in membrane fatty acid composition. Whereas the physical properties of membrane fatty acids affect the activity of membrane proteins and, indirectly, an animal\u27s BMR, it is the susceptibility of those fatty acids to peroxidation which influence MLSP. Although there is a correlation between body size and MLSP, there is considerable MLSP variation independent of body size. For example, among bird families, Galliformes (fowl) are relatively short-living and Psittaciformes (parrots) are unusually long-living, with some parrot species reaching maximum lifespans of more than 100 years. We determined BMR and tissue phospholipid fatty acid composition in seven tissues from three species of parrots with an average MLSP of 27 years and from two species of quails with an average MLSP of 5. 5 years. We also characterised mitochondrial phospholipids in two of these tissues. Neither BMR nor membrane susceptibility to peroxidation corresponded with differences in MLSP among the birds we measured. We did find that (1) all birds had lower n-3 polyunsaturated fatty acid content in mitochondrial membranes compared to those of the corresponding tissue, and that (2) irrespective of reliance on flight for locomotion, both pectoral and leg muscle had an almost identical membrane fatty acid composition in all birds.<br /

Disparate metabolic response to fructose feeding between different mouse strains

Diets enriched in fructose (FR) increase lipogenesis in the liver, leading to hepatic lipid accumulation and the development of insulin resistance. Previously, we have shown that in contrast to other mouse strains, BALB/c mice are resistant to high fat diet-induced metabolic deterioration, potentially due to a lack of ectopic lipid accumulation in the liver. In this study we have compared the metabolic response of BALB/c and C57BL/6 (BL6) mice to a fructose-enriched diet. Both strains of mice increased adiposity in response to FR-feeding, while only BL6 mice displayed elevated hepatic triglyceride (TAG) accumulation and glucose intolerance. The lack of hepatic TAG accumulation in BALB/c mice appeared to be linked to an altered balance between lipogenic and lipolytic pathways, while the protection from fructose-induced glucose intolerance in this strain was likely related to low levels of ER stress, a slight elevation in insulin levels and an altered profile of diacylglycerol species in the liver. Collectively these findings highlight the multifactorial nature of metabolic defects that develop in response to changes in the intake of specific nutrients and the divergent response of different mouse strains to dietary challenges

Contrasting metabolic effects of medium- versus long-chain fatty acids in skeletal muscle

Dietary intake of long-chain fatty acids (LCFAs) plays a causative role in insulin resistance and risk of diabetes. Whereas LCFAs promote lipid accumulation and insulin resistance, diets rich in medium-chain fatty acids (MCFAs) have been associated with increased oxidative metabolism and reduced adiposity, with few deleterious effects on insulin action. The molecular mechanisms underlying these differences between dietary fat subtypes are poorly understood. To investigate this further, we treated C2C12 myotubes with various LCFAs (16:0, 18:1n9, and 18:2n6) and MCFAs (10:0 and 12:0), as well as fed mice diets rich in LCFAs or MCFAs, and investigated fatty acid-induced changes in mitochondrial metabolism and oxidative stress. MCFA-treated cells displayed less lipid accumulation, increased mitochondrial oxidative capacity, and less oxidative stress than LCFA-treated cells. These changes were associated with improved insulin action in MCFA-treated myotubes. MCFA-fed mice exhibited increased energy expenditure, reduced adiposity, and better glucose tolerance compared with LCFA-fed mice. Dietary MCFAs increased respiration in isolated mitochondria, with a simultaneous reduction in reactive oxygen species generation, and subsequently low oxidative damage. Collectively our findings indicate that in contrast to LCFAs, MCFAs increase the intrinsic respiratory capacity of mitochondria without increasing oxidative stress. These effects potentially contribute to the beneficial metabolic actions of dietary MCFAs