The long-term regulation of skeletal muscle pyruvate dehydrogenase kinase by dietary lipid is dependent on fatty acid composition

The pyruvate dehydrogenase complex: Life's essential, vulnerable and druggable energy homeostat

Found in all organisms, pyruvate dehydrogenase complexes (PDC) are the keystones of prokaryotic and eukaryotic energy metabolism. In eukaryotic organisms these multi-component megacomplexes provide a crucial mechanistic link between cytoplasmic glycolysis and the mitochondrial tricarboxylic acid (TCA) cycle. As a consequence, PDCs also influence the metabolism of branched chain amino acids, lipids and, ultimately, oxidative phosphorylation (OXPHOS). PDC activity is an essential determinant of the metabolic and bioenergetic flexibility of metazoan organisms in adapting to changes in development, nutrient availability and various stresses that challenge maintenance of homeostasis. This canonical role of the PDC has been extensively probed over the past decades by multidisciplinary investigations into its causal association with diverse physiological and pathological conditions, the latter making the PDC an increasingly viable therapeutic target. Here we review the biology of the remarkable PDC and its emerging importance in the pathobiology and treatment of diverse congenital and acquired disorders of metabolic integration.

Effect of flaxseed oil on muscle protein loss and carbohydrate oxidation impairment in a pig model after lipopolysaccharide challenge

Flaxseed oil is rich in α-linolenic acid (ALA), which is the metabolic precursor of EPA and DHA. The present study investigated the effect of flaxseed oil supplementation on lipopolysaccharide (LPS)-induced muscle atrophy and carbohydrate oxidation impairment in a piglet model. Twenty-four weaned pigs were used in a 2 × 2 factorial experiment including dietary treatment (5 % maize oil v. 5 % flaxseed oil) and LPS challenge (saline v. LPS). On day 21 of treatment, the pigs were injected intraperitoneally with 100 μg/kg body weight LPS or sterile saline. At 4 h after injection, blood, gastrocnemius muscle and longissimus dorsi muscle were collected. Flaxseed oil supplementation increased ALA, EPA, total n-3 PUFA contents, protein:DNA ratio and pyruvate dehydrogenase complex quantity in muscles (P < 0·05). In addition, flaxseed oil reduced mRNA expression of toll-like receptor (TLR) 4 and nucleotide-binding oligomerisation domain protein (NOD) 2 and their downstream signalling molecules in muscles and decreased plasma concentrations of TNF-α, IL-6 and IL-8, and mRNA expression of TNF-α, IL-1β and IL-6 (P < 0·05). Moreover, flaxseed oil inclusion increased the ratios of phosphorylated protein kinase B (Akt) 1:total Akt1 and phosphorylated Forkhead box O (FOXO) 1:total FOXO1 and reduced mRNA expression of FOXO1, muscle RING finger (MuRF) 1 and pyruvate dehydrogenase kinase 4 in muscles (P < 0·05). These results suggest that flaxseed oil might have a positive effect on alleviating muscle protein loss and carbohydrates oxidation impairment induced by LPS challenge through regulation of the TLR4/NOD and Akt/FOXO signalling pathways.

Ketogenic diet and childhood neurological disorders other than epilepsy: an overview

INTRODUCTION

In the last years, ketogenic diet (KD) has been experimentally utilized in various childhood neurologic disorders such as mitochondriopathies, alternating hemiplegia of childhood (AHC), brain tumors, migraine, and autism spectrum disorder (ASD). The aim of this review is to analyze how KD can target these different medical conditions, highlighting possible mechanisms involved. Areas covered: We have conducted an analysis on literature concerning KD use in mitochondriopathies, AHC, brain tumors, migraine, and ASD. Expert commentary: The role of KD in reducing seizure activity in some mitochondriopathies and its efficacy in pyruvate dehydrogenase deficiency is known. Recently, few cases suggest the potentiality of KD in decreasing paroxysmal activity in children affected by AHC. A few data support its potential use as co-adjuvant and alternative therapeutic option for brain cancer, while any beneficial effect of KD on migraine remains unclear. KD could improve cognitive and social skills in a subset of children with ASD.

Skeletal Muscle Mitochondrial Bioenergetics and Morphology in High Fat Diet Induced Obesity and Insulin Resistance: Focus on Dietary Fat Source

It has been suggested that skeletal muscle mitochondria play a key role in high fat (HF) diet induced insulin resistance (IR). Two opposite views are debated on mechanisms by which mitochondrial function could be involved in skeletal muscle IR. In one theory, mitochondrial dysfunction is suggested to cause intramyocellular lipid accumulation leading to IR. In the second theory, excess fuel within mitochondria in the absence of increased energy demand stimulates mitochondrial oxidant production and emission, ultimately leading to the development of IR. Noteworthy, mitochondrial bioenergetics is strictly associated with the maintenance of normal mitochondrial morphology by maintaining the balance between the fusion and fission processes. A shift toward mitochondrial fission with reduction of fusion protein, mainly mitofusin 2, has been associated with reduced insulin sensitivity and inflammation in obesity and IR development. However, dietary fat source during chronic overfeeding differently affects mitochondrial morphology. Saturated fatty acids induce skeletal muscle IR and inflammation associated with fission phenotype, whereas ω-3 polyunsaturated fatty acids improve skeletal muscle insulin sensitivity and inflammation, associated with a shift toward mitochondrial fusion phenotype. The present minireview focuses on mitochondrial bioenergetics and morphology in skeletal muscle IR, with particular attention to the effect of different dietary fat sources on skeletal muscle mitochondria morphology and fusion/fission balance.

Pyruvate dehydrogenase complex deficiency and its relationship with epilepsy frequency--An overview

The pyruvate dehydrogenase complex (PDHc) is a member of a family of multienzyme complexes that provides the link between glycolysis and the tricarboxylic acid (TCA) cycle by catalyzing the physiologically irreversible decarboxylation of various 2-oxoacid substrates to their corresponding acyl-CoA derivatives, NADH and CO2. PDHc deficiency is a metabolic disorder commonly associated with lactic acidosis, progressive neurological and neuromuscular degeneration that vary with age and gender. In this review, we aim to discuss the relationship between occurrence of epilepsy and PDHc deficiency associated with the pyruvate dehydrogenase complex (E1α subunit (PDHA1) and E1β subunit (PDHB)) and PDH phosphatase (PDP) deficiency. PDHc plays a crucial role in the aerobic carbohydrate metabolism and regulates the use of carbohydrate as the source of oxidative energy. In severe PDHc deficiency, the energy deficit impairs brain development in utero resulting in physiological and structural changes in the brain that contributes to the subsequent onset of epileptogenesis. Epileptogenesis in PDHc deficiency is linked to energy failure and abnormal neurotransmitter metabolism that progressively alters neuronal excitability. This metabolic blockage might be restricted via inclusion of ketogenic diet that is broken up by β-oxidation and directly converting it to acetyl-CoA, and thereby improving the patient's health condition. Genetic counseling is essential as PDHA1 deficiency is X-linked. The demonstration of the X-chromosome localization of PDHA1 resolved a number of questions concerning the variable phenotype displayed by patients with E1 deficiency. Most patients show a broad range of neurological abnormalities, with the severity showing some dependence on the nature of the mutation in the Elα gene, while PDHB and PDH phosphatase (PDP) deficiencies are of autosomal recessive inheritance. However, in females, the disorder is further complicated by the pattern of X-chromosome inactivation, i.e., unfavorable lyonization. Furthermore research should focus on epileptogenic animal models; this might pave a new way toward identification of the pathophysiology of this challenging disorder.

Dietary enrichment with fish oil prevents high fat-induced metabolic dysfunction in skeletal muscle in mice

High saturated fat (HF-S) diets increase intramyocellular lipid, an effect ameliorated by omega-3 fatty acids in vitro and in vivo, though little is known about sex- and muscle fiber type-specific effects. We compared effects of standard chow, HF-S, and 7.5% HF-S replaced with fish oil (HF-FO) diets on the metabolic profile and lipid metabolism gene and protein content in red (soleus) and white (extensor digitorum longus) muscles of male and female C57BL/6 mice (n = 9-12/group). Weight gain was similar in HF-S- and HF-FO-fed groups. HF-S feeding increased mesenteric fat mass and lipid marker, Oil Red O, in red and mixed muscle; HF-FO increased interscapular brown fat mass. Compared to chow, HF-S and HF-FO increased expression of genes regulating triacylglycerol synthesis and fatty acid transport, HF-S suppressed genes and proteins regulating fatty acid oxidation, whereas HF-FO increased oxidative genes, proteins and enzymes and lipolytic gene content, whilst suppressing lipogenic genes. In comparison to HF-S, HF-FO further increased fat transporters, markers of fatty acid oxidation and mitochondrial content, and reduced lipogenic genes. No diet-by-sex interactions were observed. Neither diet influenced fiber type composition. However, some interactions between muscle type and diet were observed. HF-S induced changes in triacylglycerol synthesis and lipogenic genes in red, but not white, muscle, and mitochondrial biogenesis and oxidative genes were suppressed by HF-S and increased by HF-FO in red muscle only. In conclusion, HF-S feeding promotes lipid storage in red muscle, an effect abrogated by the fish oil, which increases mediators of lipolysis, oxidation and thermogenesis while inhibiting lipogenic genes. Greater storage and synthesis, and lower oxidative genes in red, but not white, muscle likely contribute to lipid accretion encountered in red muscle. Despite several gender-dimorphic genes, both sexes exhibited a similar HF-S-induced metabolic and gene expression profile; likewise fish oil was similarly protective in both sexes.

The relationship between human skeletal muscle pyruvate dehydrogenase phosphatase activity and muscle aerobic capacity

Pyruvate dehydrogenase (PDH) is a mitochondrial enzyme responsible for regulating the conversion of pyruvate to acetyl-CoA for use in the tricarboxylic acid cycle. PDH is regulated through phosphorylation and inactivation by PDH kinase (PDK) and dephosphorylation and activation by PDH phosphatase (PDP). The effect of endurance training on PDK in humans has been investigated; however, to date no study has examined the effect of endurance training on PDP in humans. Therefore, the purpose of this study was to examine differences in PDP activity and PDP1 protein content in human skeletal muscle across a range of muscle aerobic capacities. This association is important as higher PDP activity and protein content will allow for increased activation of PDH, and carbohydrate oxidation. The main findings of this study were that 1) PDP activity ( r2 = 0.399, P = 0.001) and PDP1 protein expression ( r2 = 0.153, P = 0.039) were positively correlated with citrate synthase (CS) activity as a marker for muscle aerobic capacity; 2) E1α ( r2 = 0.310, P = 0.002) and PDK2 protein ( r2 = 0.229, P =0.012) are positively correlated with muscle CS activity; and 3) although it is the most abundant isoform, PDP1 protein content only explained ∼18% of the variance in PDP activity ( r2 = 0.184, P = 0.033). In addition, PDP1 in combination with E1α explained ∼38% of the variance in PDP activity ( r2 = 0.383, P = 0.005), suggesting that there may be alternative regulatory mechanisms of this enzyme other than protein content. These data suggest that with higher muscle aerobic capacity (CS activity) there is a greater capacity for carbohydrate oxidation (E1α), in concert with higher potential for PDH activation (PDP activity).

Mechanisms for increased myocardial fatty acid utilization following short-term high-fat feeding

AIMS

Diet-induced obesity is associated with increased myocardial fatty acid (FA) utilization, insulin resistance, and cardiac dysfunction. The study was designed to test the hypothesis that impaired glucose utilization accounts for initial changes in FA metabolism.

METHODS AND RESULTS

Ten-week-old C57BL6J mice were fed a high-fat diet (HFD, 45% calories from fat) or normal chow (4% calories from fat). Cardiac function and substrate metabolism in isolated working hearts, glucose uptake in isolated cardiomyocytes, mitochondrial function, insulin-stimulated protein kinase B (Akt/PKB) and Akt substrate (AS-160) phosphorylation, glucose transporter 4 (GLUT4) translocation, pyruvate dehydrogenase (PDH) activity, and mRNA levels for metabolic genes were determined after 2 or 5 weeks of HFD. Two weeks of HFD reduced basal rates of glycolysis and glucose oxidation and prevented insulin stimulation of glycolysis in hearts and reduced insulin-stimulated glucose uptake in cardiomyocytes. Insulin-stimulated Akt/PKB and AS-160 phosphorylation were preserved, and PDH activity was unchanged. GLUT4 content was reduced by 55% and GLUT4 translocation was significantly attenuated. HFD increased FA oxidation rates and myocardial oxygen consumption (MVO2), which could not be accounted for by mitochondrial uncoupling or by increased expression of peroxisome proliferator activated receptor-alpha (PPAR-alpha) target genes, which increased only after 5 weeks of HFD.

CONCLUSION

Rates of myocardial glucose utilization are altered early in the course of HFD because of reduced GLUT4 content and GLUT4 translocation despite normal insulin signalling to Akt/PKB and AS-160. The reciprocal increase in FA utilization is not due to PPAR-alpha-mediated signalling or mitochondrial uncoupling. Thus, the initial increase in myocardial FA utilization in response to HFD likely results from impaired glucose transport that precedes impaired insulin signalling.

Elevated n-3 fatty acids in a high-fat diet attenuate the increase in PDH kinase activity but not PDH activity in human skeletal muscle

We tested the hypothesis that a high-fat diet (75% fat; 5% carbohydrates; 20% protein), for which 15% of the fat content was substituted with n-3 fatty acids, would not exhibit the diet-induced increase in pyruvate dehydrogenase kinase (PDK) activity, which is normally observed in human skeletal muscle. The fat content was the same in both the regular high-fat diet (HF) and in the n-3-substituted diet (N3). PDK activity increased after both high-fat diets, but the increase was attenuated after the N3 diet (0.051 +/- 0.007 and 0.218 +/- 0.047 min(-1) for pre- and post-HF, respectively; vs. 0.073 +/- 0.016 and 0.133 +/- 0.032 min(-1) for pre- and post-N3, respectively). However, the active form of pyruvate dehydrogenase (PDHa) activity decreased to a similar extent in both conditions (0.93 +/- 0.17 and 0.43 +/- 0.09 mmol/kg wet wt pre- and post-HF; vs. 0.87 +/- 0.19 and 0.39 +/- 0.05 mmol/kg wet wt pre- and post-N3, respectively). This suggested that the difference in PDK activity did not affect PDHa activation in the basal state, and it was regulated by intramitochondrial effectors, primarily muscle pyruvate concentration. Muscle glycogen content was consistent throughout the study, before and after both diet conditions, whereas muscle glucose-6-phosphate, glycerol-3-phosphate, lactate, and pyruvate were decreased after the high-fat diets. Plasma triglycerides decreased after both high-fat diets but decreased to a greater extent after the N3, whereas plasma free fatty acids increased after both diets, but to a lesser extent after the N3. In summary, PDK activity is decreased after a high-fat diet that is rich in n-3 fatty acids, although PDHa activity was unaltered. In addition, our data demonstrated that the hypolipidemic effect of n-3 fatty acids occurs earlier (3 days) than previously reported and is evident even when the diet has 75% of its total energy derived from fat.

Effects of dietary fatty acids on insulin sensitivity and secretion

Globalization and global market have contributed to increased consumption of high-fat, energy-dense diets, particularly rich in saturated fatty acids( SFAs). Polyunsaturated fatty acids (PUFAs) regulate fuel partitioning within the cells by inducing their own oxidation through the reduction of lipogenic gene expression and the enhancement of the expression of those genes controlling lipid oxidation and thermogenesis. Moreover, PUFAs prevent insulin resistance by increasing membrane fluidity and GLUT4 transport. In contrast, SFAs are stored in non-adipocyte cells as triglycerides (TG) leading to cellular damage as a sequence of their lipotoxicity. Triglyceride accumulation in skeletal muscle cells (IMTG) derives from increased FA uptake coupled with deficient FA oxidation. High levels of circulating FAs enhance the expression of FA translocase the FA transport proteins within the myocites. The biochemical mechanisms responsible for lower fatty acid oxidation involve reduced carnitine palmitoyl transferase (CPT) activity, as a likely consequence of increased intracellular concentrations of malonyl-CoA; reduced glycogen synthase activity; and impairment of insulin signalling and glucose transport. The depletion of IMTG depots is strictly associated with an improvement of insulin sensitivity, via a reduced acetyl-CoA carboxylase (ACC) mRNA expression and an increased GLUT4 expression and pyruvate dehydrogenase (PDH) activity. In pancreatic islets, TG accumulation causes impairment of insulin secretion. In rat models, beta-cell dysfunction is related to increased triacylglycerol content in islets, increased production of nitric oxide, ceramide synthesis and beta-cell apoptosis. The decreased insulin gene promoter activity and binding of the pancreas-duodenum homeobox-1 (PDX-1) transcription factor to the insulin gene seem to mediate TG effect in islets. In humans, acute and prolonged effects of FAs on glucose-stimulated insulin secretion have been widely investigated as well as the effect of high-fat diets on insulin sensitivity and secretion and on the development of type 2 diabetes.

Metabolic aspects of low carbohydrate diets and exercise

Following a low carbohydrate diet, there is a shift towards more fat and less carbohydrate oxidation to provide energy to skeletal muscle, both at rest and during exercise. This review summarizes recent work on human skeletal muscle carbohydrate and fat metabolic adaptations to a low carbohydrate diet, focusing mainly on pyruvate dehydrogenase and pyruvate dehydrogenase kinase, and how these changes relate to the capacity for carbohydrate oxidation during exercise.

Acute omega-3 fatty acid enrichment selectively reverses high-saturated fat feeding-induced insulin hypersecretion but does not improve peripheral insulin resistance

In rats fed a high-saturated fat diet, replacement of a small percentage of total fatty acids with long-chain omega-3 fatty acids from fish oil for the duration of high-fat feeding prevents the development of insulin resistance. We investigated the effect of acute (24-h) modulation of dietary fat composition on glucose-stimulated insulin secretion (GSIS) in rats made insulin resistant by high-saturated fat feeding for 4 weeks. Insulin secretion after an intravenous glucose challenge was greatly increased by high-saturated fat feeding. Glucose tolerance was minimally perturbed, demonstrating insulin hypersecretion compensated for insulin resistance. The effect of high-saturated fat feeding to enhance GSIS was retained in perifused islets, such that glucose stimulus-secretion coupling was potentiated. Acute replacement of 7% of dietary fatty acids with long-chain omega-3 fatty acids reversed insulin hypersecretion in vivo, and the effect of long-term high-saturated fat feeding to enhance insulin secretion by perifused islets was also completely reversed. Although a hyperbolic relationship existed between insulin secretion and action in the high-saturated fat and control groups, lowered insulin secretion in the acute fish oil-supplemented group was not accompanied by improved insulin action, and glucose tolerance was adversely affected. Our studies are important because they demonstrate that hyperinsulinemia can be rapidly reversed via the dietary provision of small amounts of long-chain omega-3 fatty acids. However, this "insulin sparing" action of acute dietary long-chain omega-3 fatty acids occurs in the absence of an acute improvement in insulin sensitivity and therefore at the expense of maintenance of glucose tolerance.

Diabetogenic impact of long-chain omega-3 fatty acids on pancreatic beta-cell function and the regulation of endogenous glucose production

In healthy individuals, peripheral insulin resistance evoked by dietary saturated lipid can be accompanied by increased insulin secretion such that glucose tolerance is maintained. Substitution of long-chain omega-3 fatty acids for a small percentage of dietary saturated fat prevents insulin resistance in response to high-saturated fat feeding. We substituted a small amount (7%) of dietary lipid with long-chain omega-3 fatty acids during 4 wk of high-saturated fat feeding to investigate the relationship between amelioration of insulin resistance and glucose-stimulated insulin secretion (GSIS). We demonstrate that, despite dietary delivery of saturated fat throughout, this manipulation prevents high-saturated fat feeding-induced insulin resistance with respect to peripheral glucose disposal and reverses insulin hypersecretion in response to glucose in vivo. Effects of long-chain omega-3 fatty acid enrichment to lower GSIS were also observed in perifused islets suggesting a direct effect on islet function. However, long-chain omega-3 fatty acid enrichment led to hepatic insulin resistance with respect to suppression of glucose output and impaired glucose tolerance in vivo. Our data demonstrate that the insulin response to glucose is suppressed to a greater extent than whole-body insulin sensitivity is enhanced by enrichment of a high-saturated fat diet with long-chain omega-3 fatty acids. Additionally, reduced GSIS despite glucose intolerance suggests that either long-chain omega-3 fatty acids directly impair the beta-cell response to saturated fat such that insulin secretion cannot be augmented to normalize glucose tolerance or beta-cell compensatory hypersecretion represents a response to insulin resistance at the level of peripheral glucose disposal but not endogenous glucose production.

Insulin resistance modifies plasma fatty acid distribution and decreases cardiac tolerance to in vivo ischaemia/reperfusion in rats

1. The early stage of insulin resistance, also termed the 'prediabetic state', is characterized by the development of hyperinsulinaemia, which maintains normoglycaemia under fasting conditions. The metabolic disorders induced in myocardial cells during this stage of the disease may constitute a basis for an alteration of the tolerance of the heart to ischaemia and reperfusion. 2. To test this hypothesis, male Wistar rats were fed a 66% fructose diet for 4 weeks, inducing a prediabetic state. Rats were then subjected to in vivo left coronary artery ligation followed by reperfusion. Blood samples were collected for plasma lipid profile determination. 3. The prediabetic state significantly increased the severity of ischaemia-induced arrhythmias (arrhythmia score 1.4 +/- 0.2 vs 2.0 +/- 0.0 in control and fructose-fed rats, respectively; P < 0.05) and the size of infarction (infarct size 41.2 +/- 3.0 vs 56.0 +/- 2.0% in control and fructose-fed rats, respectively; P < 0.01). This alteration of the tolerance to in vivo ischaemia/reperfusion may be the consequence of an increase in mono-unsaturated fatty acids and a decrease in omega3 polyunsaturated fatty acids in fructose-fed-rats. 4. In conclusion, because it is known that the prediabetic state increases the incidence of cardiovascular diseases by promoting coronaropathy, our study suggests that this metabolic disorder may also affect the prognosis of heart disease by decreasing the tolerance of cardiomyocytes to ischaemic insults.

Reduced PDK4 expression associates with increased insulin sensitivity in postobese patients

OBJECTIVE

The aim of this study was to verify whether changes in PDK4 mRNA expression in skeletal muscle in formerly obese subjects who underwent malabsorptive bariatric surgery [bilio-pancreatic diversion (BPD)] might be related to insulin sensitivity improvement, and if these possible modifications might correlate with a reduction of the intramyocytic lipid level.

RESEARCH METHODS AND PROCEDURES

Six obese women (body mass index 46.6 +/- 8.2 kg/m(2)) were enrolled in the study. Body composition, euglycemic-hyperinsulinemic clamp and muscle biopsies for skeletal muscle lipid analysis, and semiquantitative reverse transcriptase-polymerase chain reaction were performed before and 3 years after BPD.

RESULTS

The average weight loss observed after surgery was approximately 42%. Increased glucose uptake was accompanied by a significant decrease of PDK4 mRNA (R(2) = 0.71, p < 0.001). The amounts of intramyocytic triglycerides correlate directly with PDK4 mRNA (R(2) = 0.87, p = 0.005) and inversely with glucose uptake values (R(2) = 0.75, p < 0.001).

DISCUSSION

Our results support the concept that a reduced tissue availability of fatty acids consequent to a massive lipid malabsorption influences glucose metabolism acting through the regulation of PDH complex. In fact, as shown in animals, a higher level of FFA availability is likely to induce overexpression of PDK4 also in humans.

Evaluation of the role of peroxisome-proliferator-activated receptor alpha in the regulation of cardiac pyruvate dehydrogenase kinase 4 protein expression in response to starvation, high-fat feeding and hyperthyroidism

Inactivation of cardiac pyruvate dehydrogenase complex (PDC) after prolonged starvation and in response to hyperthyroidism is associated with enhanced protein expression of pyruvate dehydrogenase kinase (PDK) isoform 4. The present study examined the potential role of peroxisome-proliferator-activated receptor alpha (PPARalpha) in adaptive modification of cardiac PDK4 protein expression after starvation and in hyperthyroidism. PDK4 protein expression was analysed by immunoblotting in homogenates of hearts from fed or 48 h-starved rats, rats rendered hyperthyroid by subcutaneous injection of tri-iodothyronine and a subgroup of euthyroid rats maintained on a high-fat/low-carbohydrate diet, with or without treatment with the PPARalpha agonist WY14,643. In addition, PDK4 protein expression was analysed in hearts from fed, 24 h-starved or 6 h-refed wild-type or PPARalpha-null mice. PPARalpha activation by WY14,643 in vivo over the timescale of the response to starvation failed to up-regulate cardiac PDK4 protein expression in rats maintained on standard diet (WY14,643, 1.1-fold increase; starvation, 1.8-fold increase) or influence the cardiac PDK4 response to starvation. By contrast, PPARalpha activation by WY14,643 in vivo significantly enhanced cardiac PDK4 protein expression in rats maintained on a high-fat diet, which itself increased cardiac PDK4 protein expression. PPARalpha deficiency did not abolish up-regulation of cardiac PDK4 protein expression in response to starvation (2.9-fold increases in both wild-type and PPARalpha-null mice). Starvation and hyperthyroidism exerted additive effects on cardiac PDK4 protein expression, but PPARalpha activation by WY14,643 did not influence the response of cardiac PDK4 protein expression to hyperthyroidism in either the fed or starved state. Our data support the hypothesis that cardiac PDK4 protein expression is regulated, at least in part, by a fatty acid-dependent, PPARalpha-independent mechanism and strongly implicate a fall in insulin in either initiating or facilitating the response of cardiac PDK4 protein expression to starvation.

Human skeletal muscle PDH kinase activity and isoform expression during a 3-day high-fat/low-carbohydrate diet

The increase in skeletal muscle pyruvate dehydrogenase kinase (PDK) activity was measured in skeletal muscle of six healthy males after a eucaloric high-fat/low-carbohydrate (HF/LC; 5% carbohydrate, 73% fat, and 22% protein of total energy intake) diet compared with a standardized prediet (50% carbohdyrate, 30% fat, and 21% protein). Biopsies were obtained from the vastus lateralis muscle after 3 days on the prediet (day 0) and after 1, 2, and 3 days of the HF/LC diet. Intact mitchondria were extracted from fresh muscle and analyzed for PDK activity and Western blotting of PDK2 and PDK4 protein. A second biopsy was taken at each time point and frozen for Northern blot analysis of PDK2 and PDK4 mRNAs. PDK activity increased in a linear fashion over the 3-day HF/LC diet and was significantly higher than control by 1 day. PDK activity was 0.09 +/- 0.03, 0.18 +/- 0.05, 0.30 +/- 0.07, and 0.37 +/- 0.09 min(-1) at 0, 1, 2, and 3 days, respectively. PDK4 protein and mRNA increased maximally by day 1, and PDK2 protein and mRNA were unaffected by the HF/LC diet. Resting respiratory exchange ratios decreased after 1 day of the HF/LC diet (from 0.79 +/- 0.02 to 0.72 +/- 0.02) and remained depressed throughout the 3-day dietary intervention (0.68 +/- 0.01). The immediate shift to fat utilization was accompanied by increased blood glycerol, beta-hydroxybutyrate, and plasma free fatty acid concentrations. These results suggest that the continuing increase in PDK activity over the 3-day HF/LC diet is not due to increasing PDK protein beyond 1 day. This could be due to the contribution of another isoform to the total PDK activity or to a continual increase in PDK4 or PDK2 specific activity.

Caveats when considering ketogenic diets for the treatment of pyruvate dehydrogenase complex deficiency

OBJECTIVES

We conducted a critical assessment of the use of diets high in fat and low in carbohydrate ("ketogenic") in the treatment of children with congenital lactic acidosis caused by mutations in the mitochondrial pyruvate dehydrogenase complex (PDC).

STUDY DESIGN

The dietary composition of 18 subjects (11 from literature sources and 7 previously unpublished cases) was analyzed for nutrient composition. The biochemical and clinical responses to a long-term ketogenic regimen were also evaluated.

RESULTS

There was lack of uniformity in the proportion of fat calories administered and in the fatty acid composition of the diets. Ketogenic diets are also generally high in protein, compared with the recommended dietary allowance for age. Patient response to these regimens also varied considerably.

CONCLUSIONS

Although ketogenic diets have become the standard of care for the treatment of PDC deficiency, data to support their use are based on a few uncontrolled case reports in which dietary composition varied widely. Furthermore, there are several theoretical reasons for concern about the long-term safety of high-fat, low-carbohydrate diets. A controlled, prospective evaluation of the risks and benefits of these regimens for patients with PDC deficiency is required to establish rational nutritional guidelines.

Muscle fiber type comparison of PDH kinase activity and isoform expression in fed and fasted rats

Fiber type specificity for expression of all three rat skeletal muscle pyruvate dehydrogenase kinase (PDK) isoforms (PDK1, 2, and 4) was determined in fed and 24-h fasted rats. PDK activity and isoform protein and mRNA contents were determined in white gastrocnemius (WG; fast-twitch glycolytic), red gastrocnemius (RG; fast-twitch oxidative), and soleus (Sol; slow-twitch oxidative) muscles. PDK activity was lower in WG compared with oxidative muscles (RG, Sol) in both fed and fasted rats. PDK activities from fed muscles were 0.12 +/- 0.04, 0.30 +/- 0.01, and 0.36 +/- 0.08 min(-1) in WG, Sol, and RG, respectively, and increased in fasted muscles (0.36 +/- 0.09, 0.68 +/- 0.18, and 0.80 +/- 0.14 min(-1)). This correlated with increased PDK4 protein and to a lesser extent with PDK4 mRNA. PDK2 protein was not different between fiber types in fed or fasted rats, but PDK2 mRNA content was twofold greater in RG from fasted rats compared with fed rats. PDK1 was unaltered by fasting in all muscle types at both the protein and mRNA level, but in both fed and fasted rats had much greater protein and mRNA content in the oxidative vs. glycolytic muscles. In conclusion, PDK activity and PDK1 and 4 protein and mRNA were lower in glycolytic vs. oxidative muscles from fed and fasted rats. Fasting for 24 h induced a two- to threefold increase in PDK activity that was mainly due to increases in PDK4 protein and mRNA. PDK1 and 2 protein and mRNA were generally unaltered by fasting in all fiber types, except for increased PDK2 mRNA in the fast oxidative fibers. Because the PDK isoforms vary greatly in their kinetic properties, their relative proportions in the three fiber types at any given time during fasting could significantly alter the acute regulation of the pyruvate dehydrogenase complex.

Pyruvate overrides inhibition of PDH during exercise after a low-carbohydrate diet

The effects of carbohydrate deprivation on the regulation of pyruvate dehydrogenase (PDH) were studied at rest and during moderate-intensity exercise. An inhibitory effect of a chronic low-carbohydrate diet (LCD) on the active form of PDH (PDHa) mediated by a stable increase in PDH kinase (PDHK) activity has recently been reported (Peters SJ, Howlett RA, St. Amand TA, Heigenhauser GJF, and Spriet LL. Am J Physiol Endocrinol Metab 275: E980-E986, 1998.). In the present study, seven males cycled at 65% maximal O(2) uptake for 30 min after a 6-day LCD. Exercise was repeated 1 wk later after a mixed diet (MD). Muscle biopsies were sampled from the vastus lateralis at rest and at 2 and 30 min of exercise. At rest, PDHa activity (0.18 +/- 0.04 vs. 0.63 +/- 0.18 mmol x min(-1) x kg wet wt(-1)), muscle glycogen content (310.2 +/- 36.9 vs. 563.9 +/- 32.6 mmol/kg dry wt), and muscle lactate content (2.6 +/- 0.3 vs. 4.2 +/- 0.6 mmol/kg dry wt) were significantly lower after the LCD. Resting muscle acetyl-CoA (10.8 +/- 1.9 vs. 7.4 +/- 0.8 micromol/kg dry wt) and acetylcarnitine (5.3 +/- 1.4 vs. 1.6 +/- 0.3 mmol/kg dry wt) contents were significantly elevated after the LCD. During exercise, PDHa, glycogenolytic rate (LCD 5.8 +/- 0.4 vs. MD 6.9 +/- 0.2 mmol x min(-1) x kg dry wt(-1)), and muscle concentrations of acetylcarnitine, pyruvate, and lactate increased to the same extent in both conditions. The results of the present study suggest that inhibition of resting PDH by elevated PDHK activity after a LCD may be overridden by the availability of muscle pyruvate during exercise.

Antecedent protein restriction exacerbates development of impaired insulin action after high-fat feeding

The study investigated whether a persistent impairment of insulin secretion resulting from mild protein restriction predisposes to loss of glucoregulatory control and impaired insulin action after the subsequent imposition of the diabetogenic challenge of high-fat feeding. Offspring of dams provided with either control (20% protein) diet (C) or an isocaloric restricted (8%) protein diet (PR) were weaned onto the maintenance diet with which their mothers had been provided. At 20 wk of age, protein restriction enhanced glucose tolerance despite impaired insulin secretion and an augmented and sensitized lipolytic response to norepinephrine in adipocytes. C and PR rats were then transferred to a high-fat diet (HF, 19% protein, 22% lipid, 34% carbohydrate) and sampled after 8 wk. These groups are termed C-HF and PR-HF. Glucose tolerance was impaired in PR-HF, but not C-HF, rats. Insulin-stimulated glucose disposal rates were significantly lower (by 30%; P < 0.01) in the PR-HF group than in the C-HF group, and a specific impairment of antilipolytic response of insulin was unmasked in adipocytes from PR-HF, but not C-HF, rats. The study demonstrates that antecedent protein restriction accelerates and augments the development of impaired glucoregulation and insulin resistance after high-fat feeding.

Human skeletal muscle pyruvate dehydrogenase kinase activity increases after a low-carbohydrate diet

To characterize human skeletal muscle enzymatic adaptation to a low-carbohydrate, high-fat, and high-protein diet (LCD), subjects consumed a eucaloric diet consisting of 5% of the total energy intake from carbohydrate, 63% from fat, and 33% from protein for 6 days compared with their normal diet (52% carbohydrate, 33% fat, and 14% protein). Biopsies were taken from the vastus lateralis before and after 3 and 6 days on a LCD. Intact mitochondria were extracted from fresh muscle and analyzed for pyruvate dehydrogenase (PDH) kinase, total PDH, and carnitine palmitoyltransferase I activities and mitochondrial ATP production rate (using carbohydrate and fat substrates). beta-Hydroxyacyl CoA dehydrogenase, active PDH (PDHa), and citrate synthase activities were also measured on whole muscle homogenates. PDH kinase (PDHK) was calculated as the absolute value of the apparent first-order rate constant of the inactivation of PDH in the presence of 0.3 mM Mg2+-ATP. PDHK increased dramatically from 0.10 +/- 0.02 min-1 to 0.35 +/- 0.09 min-1 at 3 days and 0.49 +/- 0. 06 min-1 after 6 days. Resting PDHa activity decreased from 0.63 +/- 0.17 to 0.17 +/- 0.04 mmol. min-1. kg-1 after 6 days on the diet, whereas total PDH activity did not change. Activities for all other enzymes were unaltered by the LCD. In summary, severe deficiency of dietary carbohydrate combined with a twofold increase in dietary fat and protein caused a rapid three- to fivefold increase in PDHK activity in human skeletal muscle. The increased PDHK activity downregulated the amount of PDH in its active form at rest and decreased carbohydrate metabolism. However, an increase in the activities of enzymes involved in fatty acid oxidation did not occur.

Influence of diet on the metabolic responses to exercise

The relationship between dietary intake and skeletal-muscle exercise metabolism is central to the interests of exercise physiologists. This area has been examined experimentally for over 100 years. Classic studies with male subjects demonstrated the importance of dietary CHO in maximizing muscle and liver glycogen stores in an attempt to optimize exercise performance. CHO becomes the predominant fuel for exercise at power outputs above 50-60% Vo2max and its availability limits prolonged aerobic exercise at intensities corresponding to 65-85% VO2max. Recent information suggests that female subjects are less able to maximize muscle glycogen stores through dietary means. Contemporary studies have documented in more detail the greater reliance on CHO metabolism following a high-CHO-low-fat and -protein diet and the greater reliance on fat metabolism following a low-CHO-high-fat and protein diet. More emphasis on documenting key enzymic changes in the energy-producing pathways and transport proteins has appeared. However, very little is known regarding the mechanisms that induce these changes over the short or long term in human skeletal muscle. For example, the central role of PDH activity in the selection of intramuscular fuel during exercise and the role of carnitine palmitoyltransferase 1 in the entry of NEFA into the mitochondria, and the effects of diet on these enzymes has received little attention to date. Many research studies have examined extreme diet variations (% total energy; > 85% CHO v. < 5-10% CHO) for short periods of time in an attempt to maximize diet-induced alterations and study the mechanisms responsible for the changes. However, future studies will need to examine less-severe diet alterations for longer periods of time that more accurately reflect what the normal population might experience, such as a diet containing (% total energy) 60 fat, 20 CHO, 20 protein or the recently popular diet with (% total energy) 30 fat, 40 CHO, 30 protein.

Studies of the long-term regulation of hepatic pyruvate dehydrogenase kinase

The administration of a low-carbohydrate/high-saturated-fat (LC/HF) diet for 28 days or starvation for 48 h both increased pyruvate dehydrogenase kinase (PDHK) activity in extracts of rat hepatic mitochondria, by approx. 2.1-fold and 3.5-fold respectively. ELISAs of extracts of hepatic mitochondria, conducted over a range of pyruvate dehydrogenase (PDH) activities, revealed that mitochondrial immunoreactive PDHKII (the major PDHK isoform in rat liver) was significantly increased by approx. 1.4-fold after 28 days of LC/HF feeding and by approx. 2-fold after 48 h of starvation. The effect of LC/HF feeding to increase hepatic PDHK activity was retained through hepatocyte preparation, but was decreased on 21 h culture with insulin (100 micro-i.u./ml). A sustained (24 h) 2-4-fold elevation in plasma insulin concentration in vivo (achieved by insulin infusion via an osmotic pump) suppressed the effect of LC/HF feeding so that hepatic PDHK activities did not differ significantly from those of (insulin-infused) control rats. The increase in hepatic PDHK activity evoked by 28 days of LC/HF feeding was prevented and reversed (within 24 h) by the replacement of 7% of the dietary lipid with long-chain omega-3 fatty acids. Analysis of hepatic membrane lipid revealed a 1.9-fold increase in the ratio of total polyunsaturated omega-3 fatty acids to total mono-unsaturated fatty acids. The results indicate that the increased hepatic PDHK activities observed in livers of LC/HF-fed or 48 h-starved rats are associated with long-term actions to increase hepatic PDHKII concentrations. The long-term regulation of hepatic PDHK by LC/HF feeding might be achieved through an impaired action of insulin to suppress PDHK activity. In addition, the fatty acid composition of the diet, rather than the fat content, is a key influence.

The influence of sub-optimal protein nutrition on insulin hypersecretion evoked by high-energy/high-fat feeding in rats

Low (8%) protein feeding during pregnancy impairs the functional development of the fetal endocrine pancreas. Continued low-protein feeding post-natally decreases pancreatic insulin content and secretion, whereas transfer to standard diet evoked beta-cell recuperation. Glucose-stimulated insulin secretion (GSIS) and insulin action were examined in vivo at 28 days after transfer from 8% protein diet to a high-energy/high-fat/standard (20%)-protein diet (HEF diet). HEF feeding dramatically enhanced GSIS after intravenous glucose in control rats, but not in rats previously maintained on the low-protein diet. However, glucose disappearance after intravenous glucose, and glucose production and whole-body glucose disposal during euglycaemic-hyperinsulinaemic clamps were unaffected by prior protein malnutrition. In conclusion, impaired insulin secretion after protein malnutrition is exacerbated by high-energy/high-fat feeding, but this response is not linked to enhanced whole-body insulin resistance.