Dominik Pesta

Cellular mechanisms by which FGF21 improves insulin sensitivity in male mice.

Fibroblast growth factor 21 (FGF21) is a potent regulator of glucose and lipid metabolism and is currently being pursued as a therapeutic agent for insulin resistance and type 2 diabetes. However, the cellular mechanisms by which FGF21 modifies insulin action in vivo are unclear. To address this question, we assessed insulin action in regular chow- and high-fat diet (HFD)-fed wild-type mice chronically infused with FGF21 or vehicle. Here, we show that FGF21 administration results in improvements in both hepatic and peripheral insulin sensitivity in both regular chow- and HFD-fed mice. This improvement in insulin responsiveness in FGF21-treated HFD-fed mice was associated with decreased hepatocellular and myocellular diacylglycerol content and reduced protein kinase Cε activation in liver and protein kinase Cθ in skeletal muscle. In contrast, there were no effects of FGF21 on liver or muscle ceramide content. These effects may be attributed, in part, to increased energy expenditure in the liver and white adipose tissue. Taken together, these data provide a mechanism by which FGF21 protects mice from lipid-induced liver and muscle insulin resistance and support its development as a novel therapy for the treatment of nonalcoholic fatty liver disease, insulin resistance, and type 2 diabetes.

Reversal of hypertriglyceridemia, fatty liver disease, and insulin resistance by a liver-targeted mitochondrial uncoupler.

Nonalcoholic fatty liver disease (NAFLD) affects one in three Americans and is a major predisposing condition for the metabolic syndrome and type 2 diabetes (T2D). We examined whether a functionally liver-targeted derivative of 2,4-dinitrophenol (DNP), DNP-methyl ether (DNPME), could safely decrease hypertriglyceridemia, NAFLD, and insulin resistance without systemic toxicities. Treatment with DNPME reversed hypertriglyceridemia, fatty liver, and whole-body insulin resistance in high-fat-fed rats and decreased hyperglycemia in a rat model of T2D with a wide therapeutic index. The reversal of liver and muscle insulin resistance was associated with reductions in tissue diacylglycerol content and reductions in protein kinase C epsilon (PKCε) and PKCθ activity in liver and muscle, respectively. These results demonstrate that the beneficial effects of DNP on hypertriglyceridemia, fatty liver, and insulin resistance can be dissociated from systemic toxicities and suggest the potential utility of liver-targeted mitochondrial uncoupling agents for the treatment of hypertriglyceridemia, NAFLD, metabolic syndrome, and T2D.

A high-protein diet for reducing body fat: mechanisms and possible caveats

High protein diets are increasingly popularized in lay media as a promising strategy for weight loss by providing the twin benefits of improving satiety and decreasing fat mass. Some of the potential mechanisms that account for weight loss associated with high-protein diets involve increased secretion of satiety hormones (GIP, GLP-1), reduced orexigenic hormone secretion (ghrelin), the increased thermic effect of food and protein-induced alterations in gluconeogenesis to improve glucose homeostasis. There are, however, also possible caveats that have to be considered when choosing to consume a high-protein diet. A high intake of branched-chain amino acids in combination with a western diet might exacerbate the development of metabolic disease. A diet high in protein can also pose a significant acid load to the kidneys. Finally, when energy demand is low, excess protein can be converted to glucose (via gluconeogenesis) or ketone bodies and contribute to a positive energy balance, which is undesirable if weight loss is the goal. In this review, we will therefore explore the mechanisms whereby a high-protein diet may exert beneficial effects on whole body metabolism while we also want to present possible caveats associated with the consumption of a high-protein diet.

The effects of caffeine, nicotine, ethanol, and tetrahydrocannabinol on exercise performance

Caffeine, nicotine, ethanol and tetrahydrocannabinol (THC) are among the most prevalent and culturally accepted drugs in western society. For example, in Europe and North America up to 90% of the adult population drinks coffee daily and, although less prevalent, the other drugs are also used extensively by the population. Smoked tobacco, excessive alcohol consumption and marijuana (cannabis) smoking are addictive and exhibit adverse health effects. These drugs are not only common in the general population, but have also made their way into elite sports because of their purported performance-altering potential. Only one of the drugs (i.e., caffeine) has enough scientific evidence indicating an ergogenic effect. There is some preliminary evidence for nicotine as an ergogenic aid, but further study is required; cannabis and alcohol can exhibit ergogenic potential under specific circumstances but are in general believed to be ergolytic for sports performance. These drugs are currently (THC, ethanol) or have been (caffeine) on the prohibited list of the World Anti-Doping Agency or are being monitored (nicotine) due to their potential ergogenic or ergolytic effects. The aim of this brief review is to evaluate the effects of caffeine, nicotine, ethanol and THC by: 1) examining evidence supporting the ergogenic or ergolytic effects; 2) providing an overview of the mechanism(s) of action and physiological effects; and 3) where appropriate, reviewing their impact as performance-altering aids used in recreational and elite sports.

Genetic activation of pyruvate dehydrogenase alters oxidative substrate selection to induce skeletal muscle insulin resistance

Significance Defects in mitochondrial substrate selection, mediated by inhibition of the pyruvate dehydrogenase complex (PDH), have been proposed to be a major contributor to lipid-induced muscle insulin resistance. To examine this hypothesis, we assessed insulin action in a genetic mouse model of constitutive PDH activation. Surprisingly, we found that preferential glucose oxidation in skeletal muscle in this mouse was accompanied by muscle insulin resistance. Muscle insulin resistance could be attributed to increased glucose oxidation at the expense of reduced fatty acid oxidation, leading to increased intramyocellular lipid accumulation and diacylglycerol-PKC-θ–mediated reductions in proximal insulin signaling. These findings have important clinical implications for novel antidiabetic therapies currently in development that activate PDH and enhance glucose oxidation in muscle. The pyruvate dehydrogenase complex (PDH) has been hypothesized to link lipid exposure to skeletal muscle insulin resistance through a glucose-fatty acid cycle in which increased fatty acid oxidation increases acetyl-CoA concentrations, thereby inactivating PDH and decreasing glucose oxidation. However, whether fatty acids induce insulin resistance by decreasing PDH flux remains unknown. To genetically examine this hypothesis we assessed relative rates of pyruvate dehydrogenase flux/mitochondrial oxidative flux and insulin-stimulated rates of muscle glucose metabolism in awake mice lacking pyruvate dehydrogenase kinase 2 and 4 [double knockout (DKO)], which results in constitutively activated PDH. Surprisingly, increased glucose oxidation in DKO muscle was accompanied by reduced insulin-stimulated muscle glucose uptake. Preferential myocellular glucose utilization in DKO mice decreased fatty acid oxidation, resulting in increased reesterification of acyl-CoAs into diacylglycerol and triacylglycerol, with subsequent activation of PKC-θ and inhibition of insulin signaling in muscle. In contrast, other putative mediators of muscle insulin resistance, including muscle acylcarnitines, ceramides, reactive oxygen species production, and oxidative stress markers, were not increased. These findings demonstrate that modulation of oxidative substrate selection to increase muscle glucose utilization surprisingly results in muscle insulin resistance, offering genetic evidence against the glucose-fatty acid cycle hypothesis of muscle insulin resistance.

Neuronal UCP1 expression suggests a mechanism for local thermogenesis during hibernation

Significance Mammalian hibernators can reduce their metabolic rate by 95% and body temperature to 2 °C. However, their central and peripheral nervous systems retain activity even in cold, through unknown mechanisms. We report here that neurons from hibernating squirrels express uncoupling protein 1 (UCP1), a protein known as a heat generator in brown adipose tissue. We show that squirrel UCP1 acts as the typical thermogenic protein and is up-regulated during torpor, suggesting its thermogenic capability is important during hibernation. Accordingly, we found that the temperature of squirrel brain during the deep torpor associated with hibernation is warmer than the surrounding tissues. We hypothesize that neuronal UCP1 allows squirrels to withstand the long hibernation season and tolerate temperatures prohibitively low for survival and neuronal function in nonhibernating species. Hibernating mammals possess a unique ability to reduce their body temperature to ambient levels, which can be as low as −2.9 °C, by active down-regulation of metabolism. Despite such a depressed physiologic phenotype, hibernators still maintain activity in their nervous systems, as evidenced by their continued sensitivity to auditory, tactile, and thermal stimulation. The molecular mechanisms that underlie this adaptation remain unknown. We report, using differential transcriptomics alongside immunohistologic and biochemical analyses, that neurons from thirteen-lined ground squirrels (Ictidomys tridecemlineatus) express mitochondrial uncoupling protein 1 (UCP1). The expression changes seasonally, with higher expression during hibernation compared with the summer active state. Functional and pharmacologic analyses show that squirrel UCP1 acts as the typical thermogenic protein in vitro. Accordingly, we found that mitochondria isolated from torpid squirrel brain show a high level of palmitate-induced uncoupling. Furthermore, torpid squirrels during the hibernation season keep their brain temperature significantly elevated above ambient temperature and that of the rest of the body, including brown adipose tissue. Together, our findings suggest that UCP1 contributes to local thermogenesis in the squirrel brain, and thus supports nervous tissue function at low body temperature during hibernation.

Hypophosphatemia promotes lower rates of muscle ATP synthesis

Hypophosphatemia can lead to muscle weakness and respiratory and heart failure, but the mechanism is unknown. To address this question, we noninvasively assessed rates of muscle ATP synthesis in hypophosphatemic mice by using in vivo saturation transfer [31P]‐magnetic resonance spectroscopy. By using this approach, we found that basal and insulin‐stimulated rates of muscle ATP synthetic flux (VATP) and plasma inorganic phosphate (Pi)were reduced by 50% in mice with diet‐induced hypophosphatemia as well as in sodium‐dependent Pi transporter solute carrier family 34, member 1 (NaPi2a)‐knockout (NaPi2a‐/‐) mice compared with their wild‐type littermate controls. Rates of VATP normalized in both hypophosphatemic groups after restoring plasma Pi concentrations. Furthermore, VATP was directly related to cellular and mitochondrial Pi uptake in L6 and RC13 rodent myocytes and isolated muscle mitochondria. Similar findings were observed in a patient with chronic hypophosphatemia as a result of a mutation in SLC34A3 who had a 50% reduction in both serum Pi content and muscle VATP. After oral Pi repletion and normalization of serum Pi levels, muscle VATP completely normalized in the patient. Taken together, these data support the hypothesis that decreased muscle ATP synthesis, in part, may be caused by low blood Pi concentrations, which may explain some aspects of muscle weakness observed in patients with hypophosphatemia.—Pesta, D. H., Tsirigotis, D.N., Befroy, D.E., Caballero, D., Jurczak, M. J., Rahimi, Y., Cline, G.W., Dufour, S., Birkenfeld, A. L., Rothman, D.L., Carpenter, T. O., Insogna, K., Petersen, K. F., Bergwitz, C., Shulman, G. I. Hypophosphatemia promotes lower rates of muscle ATP synthesis. FASEB J. 30, 3378–3387 (2016). www.fasebj.org

Resistance training to improve type 2 diabetes: working toward a prescription for the future

The prevalence of type 2 diabetes (T2D) is rapidly increasing, and effective strategies to manage and prevent this disease are urgently needed. Resistance training (RT) promotes health benefits through increased skeletal muscle mass and qualitative adaptations, such as enhanced glucose transport and mitochondrial oxidative capacity. In particular, mitochondrial adaptations triggered by RT provide evidence for this type of exercise as a feasible lifestyle recommendation to combat T2D, a disease typically characterized by altered muscle mitochondrial function. Recently, the synergistic and antagonistic effects of combined training and Metformin use have come into question and warrant more in-depth prospective investigations. In the future, clinical intervention studies should elucidate the mechanisms driving RT-mitigated mitochondrial adaptations in muscle and their link to improvements in glycemic control, cholesterol metabolism and other cardiovascular disease risk factors in individuals with T2D.

Reply to Constantin-Teodosiu et al.: Mice with genetic PDH activation are not protected from high-fat diet–induced muscle insulin resistance

We thank Constantin-Teodosiu et al. (1) for their comments on our manuscript (2). Constantin-Teodosiu et al. interpret our data to indicate that mice with genetic activation of pyruvate dehydrogenase (PDH) by deletion of pyruvate dehydrogenase kinase isoforms 2 and 4 (DKO mice) are protected from high-fat diet (HFD)-induced muscle insulin resistance. Although Constantin-Teodosiu et al. correctly point out that HFD feeding did not worsen insulin-stimulated muscle glucose uptake in DKO mice (compare figure 2h and figure S4f from ref. 2), comparison with chow-fed WT control mice reveals that insulin-stimulated muscle glucose uptake was decreased by 50% in DKO mice on both regular chow and HFD (compare figure 2h and figure S4f of ref. 2). Therefore, to claim that DKO mice are protected from HFD-induced muscle insulin resistance is misleading. Such a claim overlooks the critical point that both chow-fed and HFD-fed DKO mice exhibit profound muscle insulin resistance. With respect to the therapeutic potential of pharmacological PDH activators, these data are therefore somewhat inauspicious.

Does mild resistance training resemble a similar stimulus compared to aerobic training

and postprandial hepatic venous pressure gradient (HVPG) were also measured. In agreement to what was described by Arena et al., showing that most patients irrespective of the stage of fibrosis had a peak increase of LS 30 minutes after the meal, in our series postprandial hyperemia (confirmed by a marked increase in PBF, 133 6 31%, P < 0.0001 versus baseline) was accompanied by a marked increase in LS (127 6 33%; P < 0.0001). However, we observed that postprandial changes in LS did not correlate with the changes in PBF. Similarly, in patients in whom HVPG was measured, LS changes did not mirror the HVPG increase after the standardized meal. In contrast, postprandial changes in LS were directly correlated with changes in hepatic artery blood flow (r 5 0.658; P 5 0.002), so that in patients showing the expected postprandial decrease in hepatic artery blood flow (reflecting the “buffer response” to increased PBF after a meal) the LS increase was significantly attenuated as compared with patients lacking this adaptive mechanism (112 6 21% versus 162 6 29%, P < 0.0001). Altogether, our results suggest that the postprandial increase in LS in cirrhosis cannot be explained by the increase in PBF and portal pressure, while it seems at least in part dependent on changes in the arterial component of hepatic blood flow. Since extensive formation of collaterals in advanced portal hypertension leads to increased dependence of the hepatic blood flow on its arterial component, and since hepatic artery buffer response is reduced in cirrhosis, a greater postprandial increase in LS should be anticipated in overt cirrhosis.