Skeletal muscle type comparison of pyruvate dehydrogenase phosphatase activity and isoform expression: effects of obesity and endurance training

Metabolomics analysis reveals altered metabolites in lean compared with obese adolescents and additional metabolic shifts associated with hyperinsulinaemia and insulin resistance in obese adolescents: a cross-sectional study

INTRODUCTION

Hyperinsulinaemia and insulin resistance (IR) are strongly associated with obesity and are forerunners of type 2 diabetes. Little is known about metabolic alterations separately associated with obesity, hyperinsulinaemia/IR and impaired glucose tolerance (IGT) in adolescents.

OBJECTIVES

To identify metabolic alterations associated with obesity, hyperinsulinaemia/IR and hyperinsulinaemia/IR combined with IGT in obese adolescents.

METHODS

81 adolescents were stratified into four groups based on body mass index (lean vs. obese), insulin responses (normal insulin (NI) vs. high insulin (HI)) and glucose responses (normal glucose tolerance (NGT) vs. IGT) after an oral glucose tolerance test (OGTT). The groups comprised: (1) healthy lean with NI and NGT, (2) obese with NI and NGT, (3) obese with HI and NGT, and (4) obese with HI and IGT. Targeted nuclear magnetic resonance-based metabolomics analysis was performed on fasting and seven post-OGTT plasma samples, followed by univariate and multivariate statistical analyses.

RESULTS

Two groups of metabolites were identified: (1) Metabolites associated with insulin response level: adolescents with HI (groups 3-4) had higher concentrations of branched-chain amino acids and tyrosine, and lower concentrations of serine, glycine, myo-inositol and dimethylsulfone, than adolescents with NI (groups 1-2). (2) Metabolites associated with obesity status: obese adolescents (groups 2-4) had higher concentrations of acetylcarnitine, alanine, pyruvate and glutamate, and lower concentrations of acetate, than lean adolescents (group 1).

CONCLUSIONS

Obesity is associated with shifts in fat and energy metabolism. Hyperinsulinaemia/IR in obese adolescents is also associated with increased branched-chain and aromatic amino acids.

Overexpression of pyruvate dehydrogenase phosphatase 1 promotes the progression of pancreatic adenocarcinoma by regulating energy-related AMPK/mTOR signaling

Background

Human pyruvate dehydrogenase phosphatase 1 (PDP1) plays an important physiological role in energy metabolism; however, its expression and function in human pancreatic adenocarcinoma (PDAC) remain unknown. This study aimed to investigate the expression pattern and mechanisms of action of PDP1 in human PDAC.

Methods

The expression pattern of PDP1 in PDAC was determined, and its correlation with patient survival was analyzed. Ectopic expression or knockdown of PDP1 was performed, and in vitro proliferation and migration, as well as in vivo tumor growth of PDAC, were measured. The mechanism was studied by biochemical approaches.

Results

PDP1 was overexpressed in human PDAC samples, and high expression of PDP1 correlated with poor overall and disease-free survival of PDAC patients. PDP1 promoted the proliferation, colony formation, and invasion of PDAC cells in vitro and facilitated orthotopic tumor growth in vivo. PDP1 accelerated intracellular ATP production, leading to sufficient energy to support rapid cancer progression. mTOR activation was responsible for the PDP1-induced tumor cell proliferation and invasion in PDAC. AMPK was downregulated by PDP1 overexpression, resulting in mTOR activation and cancer progression.

Conclusion

Our findings suggested that PDP1 could be a promising diagnostic and therapeutic target for anti-PDAC treatment.

Effects of adding sodium dichloroacetate to low-protein diets on nitrogen balance and amino acid metabolism in the portal-drained viscera and liver of pigs

Background

Identifying regulatory measures to promote glucose oxidative metabolism while simultaneously reducing amino acid oxidative metabolism is one of the foremost challenges in formulating low-protein (LP) diets designed to reduce the excretion of nitrogen-containing substances known to be potential pollutants. In this study, we investigated the effects of adding sodium dichloroacetate (DCA) to a LP diet on nitrogen balance and amino acid metabolism in the portal-drained viscera (PDV) and liver of pigs.To measure nitrogen balance, 18 barrows (40 ± 1.0 kg) were fed one of three diets (n = 6 per group): 18% crude protein (CP, control), 13.5% CP (LP), and 13.5% CP + 100 mg DCA/kg dry matter (LP-DCA). To measure amino acid metabolism in the PDV and liver, 15 barrows (40 ± 1.0 kg) were randomly assigned to one of the three diets (n = 5 per group). Four essential amino acids (Lys, Met, Thr, and Trp) were added to the LP diets such that these had amino acid levels comparable to those of the control diet.

Results

The LP-DCA diet reduced nitrogen excretion in pigs relative to that of pigs fed the control diet (P < 0.05), without any negative effects on nitrogen retention (P > 0.05). There were no differences between the control and LP-DCA groups with respect to amino acid supply to the liver and extra-hepatic tissues in pigs (P > 0.05). The net release of ammonia into the portal vein and production rate of urea in the liver of pigs fed the LP-DCA diet was reduced relative to that of pigs fed the control and LP diets (P < 0.05).

Conclusion

The results indicated that addition of DCA to a LP diet can efficiently reduce nitrogen excretion in pigs and maximize the supply of amino acids to the liver and extra-hepatic tissues.

Activation of Pyruvate Dehydrogenase by Sodium Dichloroacetate Shifts Metabolic Consumption from Amino Acids to Glucose in IPEC-J2 Cells and Intestinal Bacteria in Pigs

The extensive metabolism of amino acids (AA) as fuel is an important reason for the low use efficiency of protein in pigs. In this study, we investigated whether regulation of the pyruvate dehydrogenase kinase (PDK)/pyruvate dehydrogenase alpha 1 (PDHA1) pathway affected AA consumption by porcine intestinal epithelial (IPEC-J2) cells and intestinal bacteria in pigs. The effects of knockdown of PDHA1 and PDK1 with small interfering RNA (siRNA) on nutrient consumption by IPEC-J2 cells were evaluated. IPEC-J2 cells were then cultured with sodium dichloroacetate (DCA) to quantify AA and glucose consumption and nutrient oxidative metabolism. The results showed that knockdown of PDHA1 using siRNA decreased glucose consumption but increased total AA (TAA) and glutamate (Glu) consumption by IPEC-J2 cells ( P < 0.05). Opposite effects were observed using siRNA targeting PDK1 ( P < 0.05). Additionally, culturing IPEC-J2 cells in the presence of 5 mM DCA markedly increased the phosphorylation of PDHA1 and PDH phosphatase 1, but inhibited PDK1 phosphorylation ( P < 0.05). DCA treatment also reduced TAA and Glu consumption and increased glucose depletion ( P < 0.05). These results indicated that PDH was the regulatory target for shifting from AA metabolism to glucose metabolism and that culturing cells with DCA decreased the consumption of AAs by increasing the depletion of glucose through PDH activation.

The down-regulated ING5 expression in lung cancer: a potential target of gene therapy

ING5 can interact with p53, thereby inhibiting cell growth and inducing apoptosis. We found that ING5 overexpression not only inhibited proliferation, migration, and invasion, but also induced G2 arrest, differentiation, autophagy, apoptosis, glycolysis and mitochondrial respiration in lung cancer cells. ING5 transfection up-regulated the expression of Cdc2, ATG13, ATG14, Beclin-1, LC-3B, AIF, cytochrome c, Akt1/2/3, ADFP, PFK-1 and PDPc, while down-regulated the expression of Bcl-2, XIAP, survivin,β-catenin and HXK1. ING5 transfection desensitized cells to the chemotherapy of MG132, paclitaxel, and SAHA, which paralleled with apoptotic alteration. ING5 overexpression suppressed the xenograft tumor growth by inhibiting proliferation and inducing apoptosis. ING5 expression level was significantly higher in normal tissue than that in lung cancer at both protein and mRNA levels. Nuclear ING5 expression was positively correlated with ki-67 expression and cytoplasmic ING5 expression. Cytoplasmic ING5 expression was positively associated with lymph node metastasis, and negatively with age, lymphatic invasion or CPP32 expression. ING5 expression was different in histological classification: squamous cell carcinoma > adenocarcinoma > large cell carcinoma > small cell carcinoma. Taken together, our data suggested that ING5 downregulation might involved in carcinogenesis, growth, and invasion of lung cancer and could be considered as a promising marker to gauge the aggressiveness of lung cancer. It might be employed as a potential target for gene therapy of lung cancer.

The roles of ING5 expression in ovarian carcinogenesis and subsequent progression: a target of gene therapy

Here, we found that ING5 overexpression suppressed cell viability, glucose metabolism, migration, invasion and epithelial-mesenchymal transition, and induced cell arrest, apoptosis, senescence, autophagy and fat accumulation in ovarian cancer cells. ING5-mediated chemoresistance was positively linked to apoptotic resistance and chemoresistance-related gene expression. ING5 overexpression suppressed tumor growth of ovarian cancer by decreasing proliferation, and inducing apoptosis and autophagy. ING5 mRNA level was lower in ovarian cancer than normal ovary, and borderline than benign tumors (p < 0.05), and negatively correlated with vascular invasion, lymphatic invasion and FIGO staging of ovarian cancer (p < 0.05). ING5 protein was less expressed in primary cancer than normal ovary (p < 0.05). There was a negative correlation between ING5 mRNA expression and the overall or progression-free survival time of the cancer patients with Grade 2, Grade 3, and stage I cancer (p < 0.05). Immunohistochemically, ING5 was less expressed in serous and mucinous adenocarcinoma than miscellaneous subtypes, and positively correlated with dedifferentiation and ki-67 expression of ovarian cancer (p < 0.05). These data suggested that down-regulated ING5 expression might be involved in ovarian carcinogenesis possibly by suppressing aggressive phenotypes, including proliferation, tumor growth, migration, invasion, and anti-apoptosis.

Mitochondrial pyruvate dehydrogenase phosphatase 1 regulates the early differentiation of cardiomyocytes from mouse embryonic stem cells

Mitochondria are crucial for maintaining the properties of embryonic stem cells (ESCs) and for regulating their subsequent differentiation into diverse cell lineages, including cardiomyocytes. However, mitochondrial regulators that manage the rate of differentiation or cell fate have been rarely identified. This study aimed to determine the potential mitochondrial factor that controls the differentiation of ESCs into cardiac myocytes. We induced cardiomyocyte differentiation from mouse ESCs (mESCs) and performed microarray assays to assess messenger RNA (mRNA) expression changes at differentiation day 8 (D8) compared with undifferentiated mESCs (D0). Among the differentially expressed genes, Pdp1 expression was significantly decreased (27-fold) on D8 compared to D0, which was accompanied by suppressed mitochondrial indices, including ATP levels, membrane potential, ROS and mitochondrial Ca²⁺. Notably, Pdp1 overexpression significantly enhanced the mitochondrial indices and pyruvate dehydrogenase activity and reduced the expression of cardiac differentiation marker mRNA and the cardiac differentiation rate compared to a mock control. In confirmation of this, a knockdown of the Pdp1 gene promoted the expression of cardiac differentiation marker mRNA and the cardiac differentiation rate. In conclusion, our results suggest that mitochondrial PDP1 is a potential regulator that controls cardiac differentiation at an early differentiation stage in ESCs.

Age-related impairments in skeletal muscle PDH phosphorylation and plasma lactate are indicative of metabolic inflexibility and the effects of exercise training

The purpose of this study was to determine whether plasma lactate and skeletal muscle glucose regulatory pathways, specifically PDH dephosphorylation, are impaired during hyperinsulinemic conditions in middle- to older-aged individuals and determine whether exercise training could improve key variables responsible for skeletal muscle PDH regulation. Eighteen young (19-29 yr; n = 9 males and 9 females) and 20 middle- to older-aged (57-82 yr; n = 10 males and 10 females) individuals underwent a 2-h euglycemic hyperinsulinemic clamp. Plasma samples were obtained at baseline and at 30, 50, 90, and 120 min for analysis of lactate, and skeletal muscle biopsies were performed at 60 min for analysis of protein associated with glucose metabolism. In response to insulin, plasma lactate was elevated in aged individuals when normalized to insulin action. Insulin-stimulated phosphorylation of skeletal muscle PDH on serine sites 232, 293, and 300 decreased in young individuals only. Changes in insulin-stimulated PDH phosphorylation were positively related to changes in plasma lactate. No age-related differences were observed in skeletal muscle phosphorylation of LDH, GSK-3α, or GSK-3β in response to insulin or PDP1, PDP2, PDK2, PDK4, or MPC1 total protein. Twelve weeks of endurance- or strength-oriented exercise training improved insulin-stimulated PDH dephosphorylation, which was related to a reduced lactate response. These findings suggest that impairments in insulin-induced PDH regulation in a sedentary aging population contribute to impaired glucose metabolism and that exercise training is an effective intervention for treating metabolic inflexibility.

Voluntary physical activity abolishes the proliferative tumor growth microenvironment created by adipose tissue in animals fed a high fat diet

The molecular mechanisms behind the obesity-breast cancer association may be regulated via adipokine secretion by white adipose tissue. Specifically, adiponectin and leptin are altered with adiposity and exert antagonistic effects on cancer cell proliferation. We set out to determine whether altering adiposity in vivo via high fat diet (HFD) feeding changed the tumor growth supporting nature of adipose tissue and whether voluntary physical activity (PA) could ameliorate these HFD-dependent effects. We show that conditioned media (CM) created from the adipose tissue of HFD fed animals caused an increase in the proliferation of MCF7 cells compared with cells exposed to CM prepared from the adipose of lean chow diet fed counterparts. This increased proliferation was driven within the MCF7 cells by an HFD-dependent antagonism between AMP-activated protein kinase (AMPK) and protein kinase B (Akt) signaling pathways, decreasing p27 protein levels via reduced phosphorylation at T198 and downregulation of adiponectin receptor 1 (AdipoR1). PA can ameliorate these proliferative effects of HFD-CM on MCF7 cells, increasing p27(T198) by AMPK, reducing pAkt(T308), and increasing AdipoR1, resulting in cell cycle withdrawal in a manner that depends on the PA intensity. High physical activity (>3 km/day) completely abolished the effects of HFD feeding. In addition, AdipoR1 overexpression mimics the effects of exercise, abolishing the proliferative effects of the HFD-CM on MCF7 cells and further enhancing the antiproliferative effects of PA on the HFD-CM. Thus voluntary PA represents a means to counteract the proliferative effects of adipose tissue on breast cancers in obese patients.

Adipose tissue depot specific differences of PLIN protein content in endurance trained rats

Adipose tissue is classified as either white (WAT) or brown (BAT) and differs not only by anatomical location but also in function. WAT is the main source of stored energy and releases fatty acids in times of energy demand, whereas BAT plays a role in regulating non-shivering thermogenesis and oxidizes fatty acids released from the lipid droplet. The PLIN family of proteins has recently emerged as being integral in the regulation of fatty acid storage and release in adipose tissue. Previous work has demonstrated that PLIN protein content varies among adipose tissue depots, however an examination of endurance training-induced depot specific changes in PLIN protein expression has yet to be done. Male Sprague-dawley rats (n = 10) underwent 8-weeks of progressive treadmill training (18-25 m/min for 30-60 min at 10% incline) or remained sedentary as control. Following training, under isoflurane induced anesthesia epidydmal (eWAT), inguinal subcutaneous (iWAT) and intrascapular brown adipose tissue (BAT) was excised, and plasma was collected. Endurance training resulted in an increase in BAT PLIN5 and iWAT PLIN3 content, while there was no difference in PLIN protein content in endurance trained eWAT. Interestingly, endurance training resulted in a robust increase in ATGL and CGI-58 in eWAT alone. Together these results suggest the potential of a depot specific function of PLIN3 and PLIN5 in adipose tissue in response to endurance training.

Increases in skeletal muscle ATGL and its inhibitor G0S2 following 8 weeks of endurance training in metabolically different rat skeletal muscles

Adipose triglyceride lipase (ATGL) catalyzes the rate-limiting removal of the first fatty acid from a triglyceride. ATGL is activated by comparative gene identification-58 and inhibited by G(0)/G(1) switch gene-2 protein (G0S2). Research in other tissues and cell culture indicates that inhibition is dependent on relative G0S2-to-ATGL protein content. G0S2 may also have several roles within mitochondria; however, this has yet to be observed in skeletal muscle. The purpose of this study was to determine if muscle G0S2 relative to ATGL content would decrease to facilitate intramuscular lipolysis following endurance training. Male Sprague-Dawley rats (n = 10; age 51-53 days old) were progressively treadmill trained at a 10% incline for 8 wk ending with 25 m/min for 1 h compared with control. Sciatic nerve stimulation for hind-limb muscle contraction (and lipolysis) was administered for 30 min to one leg, leaving the opposing leg as a resting control. Soleus (SOL), red gastrocnemius (RG), and white gastrocnemius were excised from both legs following stimulation or control. ATGL protein increased in all trained muscles. Unexpectedly, G0S2 protein was greater in the trained SOL and RG. In RG-isolated mitochondria, G0S2 also increased with training, yet mitochondrial G0S2 content was unaltered with acute contraction; therefore, any role of G0S2 in the mitochondria does not appear to be acutely mediated by content alone. In summary, G0S2 increased with training in oxidative muscles and mitochondria but not following acute contraction, suggesting that inhibition is not through relative G0S2-to-ATGL content but through more complicated intracellular mechanisms.

Characterization of lipolytic inhibitor G(0)/G(1) switch gene-2 protein (G0S2) expression in male Sprague-Dawley rat skeletal muscle compared to relative content of adipose triglyceride lipase (ATGL) and comparitive gene identification-58 (CGI-58)

The rate-limiting enzyme in lipolysis, adipose triglyceride lipase (ATGL), is activated by comparative gene identification-58 (CGI-58) and inhibited by the G(0)/G(1) switch gene-2 (G0S2) protein. It is speculated that inhibition of ATGL is through a dose dependent manner of relative G0S2 protein content. There is little work examining G0S2 expression in lipolytic tissues, and the relative expression across oxidative tissues such as skeletal muscle has not yet been described. Three muscles, soleus (SOL), red gastrocnemius (RG), and white gastrocnemius (WG) were excised from 57-day old male Sprague-Dawley rats (n = 9). QRT-PCR was used for mRNA analysis, and western blotting was conducted to determine protein content. ATGL and G0S2 protein content were both greatest in the lipolytic SOL, with the least amount of both ATGL and G0S2 protein content found in the WG. CGI-58 protein content however did not mirror ATGL and G0S2 protein content, since the RG had the greatest CGI-58 protein content when compared to the SOL and WG. When comparing our tissues based on CGI-58-to-ATGL ratio and G0S2-to-ATGL ratio, it was discovered that contrary to oxidative demand, the glycolytic WG had the greatest activator CGI-58-to-ATGL ratio with the oxidative SOL having the least, and no differences in G0S2-to-ATGL across the three muscle types. These data suggest that the content of G0S2 relative to the lipase in skeletal muscle would not predict lipolytic potential.

Changes in mitochondrial perilipin 3 and perilipin 5 protein content in rat skeletal muscle following endurance training and acute stimulated contraction

NEW FINDINGS

What is the central question of this study? The aim was to determine whether mitochondrial protein content of perilipin 3 (PLIN3) and perilipin 5 (PLIN5) is increased following endurance training and whether mitochondrial PLIN5 protein is increased to a greater extent in endurance-trained rats when compared with sedentary rats following acute contraction. What is the main finding and its importance? Mitochondrial PLIN3 but not PLIN5 protein was increased in endurance-trained compared with sedentary rats, suggesting a mitochondrial role for PLIN3 due to chronic exercise. Contrary to our hypothesis, acute mitochondrial PLIN5 protein was similar in both sedentary and endurance-trained rats. Endurance training results in an increased association between skeletal muscle lipid droplets and mitochondria. This association is likely to be important for the expected increase in intramuscular fatty acid oxidation that occurs with endurance training. The perilipin family of lipid droplet proteins, PLIN(2-5), are thought to play a role in skeletal muscle lipolysis. Recently, results from our laboratory demonstrated that skeletal muscle mitochondria contain PLIN3 and PLIN5 protein. Furthermore, 30 min of stimulated contraction induces an increased mitochondrial PLIN5 content. To determine whether mitochondrial content of PLIN3 and PLIN5 is altered with endurance training, Sprague-Dawley rats were randomized into sedentary or endurance-trained groups for 8 weeks of treadmill running followed by an acute (30 min) sciatic nerve stimulation to induce lipolysis. Mitochondrial PLIN3 protein was ∼1.5-fold higher in red gastrocnemius of endurance-trained rats compared with sedentary animals, with no change in mitochondrial PLIN5 protein. In addition, there was an increase in plantaris intramuscular lipid storage. Acute electrically stimulated contraction in red gastrocnemius from sedentary and endurance-trained rats resulted in a similar increase of mitochondrial PLIN5 between these two groups, with no net change in PLIN3 in either group. Plantaris intramuscular lipid content decreased to a similar extent in sedentary and endurance-trained rats. These results suggest that while total mitochondrial PLIN5 content is not altered by endurance training, PLIN5 does have an acute role in the mitochondrial fraction during muscle contraction. Conversely, mitochondrial PLIN3 does not change acutely with muscle contraction, but PLIN3 content was increased following endurance training, indicating a role in chronic adaptations of skeletal muscle.

Calcium in health and disease

Evolution has exploited the chemical properties of Ca(2+), which facilitate its reversible binding to the sites of irregular geometry offered by biological macromolecules, to select it as a carrier of cellular signals. A number of proteins bind Ca(2+) to specific sites: those intrinsic to membranes play the most important role in the spatial and temporal regulation of the concentration and movements of Ca(2+) inside cells. Those which are soluble, or organized in non-membranous structures, also decode the Ca(2+) message to be then transmitted to the targets of its regulation. Since Ca(2+) controls the most important processes in the life of cells, it must be very carefully controlled within the cytoplasm, where most of the targets of its signaling function reside. Membrane channels (in the plasma membrane and in the organelles) mediate the entrance of Ca(2+) into the cytoplasm, ATPases, exchangers, and the mitochondrial Ca(2+) uptake system remove Ca(2+) from it. The concentration of Ca(2+) in the external spaces, which is controlled essentially by its dynamic exchanges in the bone system, is much higher than inside cells, and can, under conditions of pathology, generate a situation of dangerous internal Ca(2+) overload. When massive and persistent, the Ca(2+) overload culminates in the death of the cell. Subtle conditions of cellular Ca(2+) dyshomeostasis that affect individual systems that control Ca(2+), generate cell disease phenotypes that are particularly severe in tissues in which the signaling function of Ca(2+) has special importance, e.g., the nervous system.

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).

PDH activation during in vitro muscle contractions in PDH kinase 2 knockout mice: effect of PDH kinase 1 compensation

Pyruvate dehydrogenase (PDH) plays an important role in regulating carbohydrate oxidation in skeletal muscle. PDH is deactivated by a set of PDH kinases (PDK1, PDK2, PDK3, PDK4), with PDK2 and PDK4 being the most predominant isoforms in skeletal muscle. Although PDK2 is the most abundant isoform, few studies have examined its physiological role. The role of PDK2 on PDH activation (PDHa) at rest and during muscle stimulation at 10 and 40 Hz (eliciting low- and moderate-intensity muscle contractions, respectively) in isolated extensor digitorum longus muscles was studied in PDK2 knockout (PDK2KO) and wild-type (WT) mice (n = 5 per group). PDHa activity was unexpectedly 35 and 77% lower in PDK2KO than WT muscle (P = 0.043), while total PDK activity was nearly fourfold lower in PDK2KO muscle (P = 0.006). During 40-Hz contractions, initial force was lower in PDK2KO than WT muscle (P < 0.001) but fatigued similarly to ∼75% of initial force by 3 min. There were no differences in initial force or rate of fatigue during 10-Hz contractions. PDK1 compensated for the lack of PDK2 and was 1.8-fold higher in PDK2KO than WT muscle (P = 0.019). This likely contributed to ensuring that resting PDHa activity was similar between the groups and accounts for the lower PDH activation during muscle contraction, as PDK1 is a very potent inhibitor of the PDH complex. Increased PDK1 expression appears to be regulated by hypoxia inducible factor-1α, which was 3.5-fold higher in PDK2KO muscle. It is clear that PDK2 activity is essential, even at rest, in regulation of carbohydrate oxidation and production of reducing equivalents for the electron transport chain. In addition, these results underscore the importance of the overall kinetics of the PDK isoform population, rather than total PDK activity, in determining transformation of the PDH complex and PDHa activity during muscle contraction.

Regulation of mitochondrial dehydrogenases by calcium ions

Studies in Bristol in the 1960s and 1970s, led to the recognition that four mitochondrial dehydrogenases are activated by calcium ions. These are FAD-glycerol phosphate dehydrogenase, pyruvate dehydrogenase, NAD-isocitrate dehydrogenase and oxoglutarate dehydrogenase. FAD-glycerol phosphate dehydrogenase is located on the outer surface of the inner mitochondrial membrane and is influenced by changes in cytoplasmic calcium ion concentration. The other three enzymes are located within mitochondria and are regulated by changes in mitochondrial matrix calcium ion concentration. These and subsequent studies on purified enzymes, mitochondria and intact cell preparations have led to the widely accepted view that the activation of these enzymes is important in the stimulation of the respiratory chain and hence ATP supply under conditions of increased ATP demand in many stimulated mammalian cells. The effects of calcium ions on FAD-isocitrate dehydrogenase involve binding to an EF-hand binding motif within this enzyme but the binding sites involved in the effects of calcium ions on the three intramitochondrial dehydrogenases remain to be fully established. It is also emphasised in this article that these three dehydrogenases appear only to be regulated by calcium ions in vertebrates and that this raises some interesting and potentially important developmental issues.