Persistence of the effect of insulin on pyruvate dehydrogenase activity in rat white and brown adipose tissue during the preparation and subsequent incubation of mitochondria

Urinary microRNA and mRNA in Tumors

Liquid biopsy is gaining importance in the context of analysis of circulating subcellular components, such as exosomes and nucleic acids, and the investigation of biological fluids is increasing because they express features common to the tissue of origin. Particularly, urine has become one of the most attractive biofluids in clinical practice due to its easy collection approach, its availability of large quantities, and its noninvasiveness. Furthermore, a peculiarity is that, compared to serum or plasma, urine is characterized by a simpler composition that improves isolation and identification of biomarkers. Recent studies have been associated with the investigation of mRNAs and microRNAs as potential noninvasive cancer biomarkers in urine, and to date, several approaches for isolating and measuring urinary nucleic acids have been established, despite still developing. This chapter aims at giving some main published evidences on urinary microRNAs and mRNAs, with the intent to consider their potential translational use in clinical practice.

Role of mitochondrial Ca2+ in the regulation of cellular energetics

Calcium is an important signaling molecule involved in the regulation of many cellular functions. The large free energy in the Ca(2+) ion membrane gradients makes Ca(2+) signaling inherently sensitive to the available cellular free energy, primarily in the form of ATP. In addition, Ca(2+) regulates many cellular ATP-consuming reactions such as muscle contraction, exocytosis, biosynthesis, and neuronal signaling. Thus, Ca(2+) becomes a logical candidate as a signaling molecule for modulating ATP hydrolysis and synthesis during changes in numerous forms of cellular work. Mitochondria are the primary source of aerobic energy production in mammalian cells and also maintain a large Ca(2+) gradient across their inner membrane, providing a signaling potential for this molecule. The demonstrated link between cytosolic and mitochondrial Ca(2+) concentrations, identification of transport mechanisms, and the proximity of mitochondria to Ca(2+) release sites further supports the notion that Ca(2+) can be an important signaling molecule in the energy metabolism interplay of the cytosol with the mitochondria. Here we review sites within the mitochondria where Ca(2+) plays a role in the regulation of ATP generation and potentially contributes to the orchestration of cellular metabolic homeostasis. Early work on isolated enzymes pointed to several matrix dehydrogenases that are stimulated by Ca(2+), which were confirmed in the intact mitochondrion as well as cellular and in vivo systems. However, studies in these intact systems suggested a more expansive influence of Ca(2+) on mitochondrial energy conversion. Numerous noninvasive approaches monitoring NADH, mitochondrial membrane potential, oxygen consumption, and workloads suggest significant effects of Ca(2+) on other elements of NADH generation as well as downstream elements of oxidative phosphorylation, including the F(1)F(O)-ATPase and the cytochrome chain. These other potential elements of Ca(2+) modification of mitochondrial energy conversion will be the focus of this review. Though most specific molecular mechanisms have yet to be elucidated, it is clear that Ca(2+) provides a balanced activation of mitochondrial energy metabolism that exceeds the alteration of dehydrogenases alone.

Regulation of oxidative phosphorylation complex activity: effects of tissue-specific metabolic stress within an allometric series and acute changes in workload

The concentration of mitochondrial oxidative phosphorylation complexes (MOPCs) is tuned to the maximum energy conversion requirements of a given tissue; however, whether the activity of MOPCs is altered in response to acute changes in energy conversion demand is unclear. We hypothesized that MOPCs activity is modulated by tissue metabolic stress to maintain the energy-metabolism homeostasis. Metabolic stress was defined as the observed energy conversion rate/maximum energy conversion rate. The maximum energy conversion rate was assumed to be proportional to the concentration of MOPCs, as determined with optical spectroscopy, gel electrophoresis, and mass spectrometry. The resting metabolic stress of the heart and liver across the range of resting metabolic rates within an allometric series (mouse, rabbit, and pig) was determined from MPOCs content and literature respiratory values. The metabolic stress of the liver was high and nearly constant across the allometric series due to the proportional increase in MOPCs content with resting metabolic rate. In contrast, the MOPCs content of the heart was essentially constant in the allometric series, resulting in an increasing metabolic stress with decreasing animal size. The MOPCs activity was determined in native gels, with an emphasis on Complex V. Extracted MOPCs enzyme activity was proportional to resting metabolic stress across tissues and species. Complex V activity was also shown to be acutely modulated by changes in metabolic stress in the heart, in vivo and in vitro. The modulation of extracted MOPCs activity suggests that persistent posttranslational modifications (PTMs) alter MOPCs activity both chronically and acutely, specifically in the heart. Protein phosphorylation of Complex V was correlated with activity inhibition under several conditions, suggesting that protein phosphorylation may contribute to activity modulation with energy metabolic stress. These data are consistent with the notion that metabolic stress modulates MOPCs activity in the heart.

Is pyruvate an endogenous anti-inflammatory molecule?

Pyruvic acid is an effective scavenger of reactive oxygen species. Ethyl pyruvate has demonstrated anti-inflammatory actions and improved hyperpermeability and bacterial translocation due to endotoxemia and is of benefit in animal models of sepsis and septic shock. Ethyl pyruvate specifically inhibits tumor necrosis factor-alpha production and decreases circulating levels of high-mobility group box-1 and nuclear factor-kappaB signaling pathways by specifically targeting its p65 subunit in animals with established endotoxemia or sepsis and in macrophage cultures. Ethyl pyruvate also decreases cyclo-oxygenase-2, inducible nitric oxide synthase, and interleukin-6 mRNA expression in the liver, ileal mucosa, and colonic mucosa in animal models with hemorrhagic shock. Similar beneficial actions have been seen in endotoxemia. These and other studies suggest that ethyl pyruvate could be of significant benefit in the treatment of patients who are critically ill and have sepsis/septic shock.

Distinct regulatory properties of pyruvate dehydrogenase kinase and phosphatase isoforms

The mammalian pyruvate dehydrogenase complex (PDC) plays central and strategic roles in the control of the use of glucose-linked substrates as sources of oxidative energy or as precursors in the biosynthesis of fatty acids. The activity of this mitochondrial complex is regulated by the continuous operation of competing pyruvate dehydrogenase kinase (PDK) and pyruvate dehydrogenase phosphatase (PDP) reactions. The resulting interconversion cycle determines the fraction of active (nonphosphorylated) pyruvate dehydrogenase (E1) component. Tissue-specific and metabolic state-specific control is achieved by the selective expression and distinct regulatory properties of at least four PDK isozymes and two PDP isozymes. The PDK isoforms are members of a family of serine kinases that are not structurally related to cytoplasmic Ser/Thr/Tyr kinases. The catalytic subunits of the PDP isoforms are Mg2+-dependent members of the phosphatase 2C family that has binuclear metal-binding sites within the active site. The dihydrolipoyl acetyltransferase (E2) and the dihydrolipoyl dehydrogenase-binding protein (E3BP) are multidomain proteins that form the oligomeric core of the complex. One or more of their three lipoyl domains (two in E2) selectively bind each PDK and PDP1. These adaptive interactions predominantly influence the catalytic efficiencies and effector control of these regulatory enzymes. When fatty acids are the preferred source of acetyl-CoA and NADH, feedback inactivation of PDC is accomplished by the activity of certain kinase isoforms being stimulated upon preferentially binding a lipoyl domain containing a reductively acetylated lipoyl group. PDC activity is increased in Ca2+-sensitive tissues by elevating PDP1 activity via the Ca2+-dependent binding of PDP1 to a lipoyl domain of E2. During starvation, the irrecoverable loss of glucose carbons is restricted by minimizing PDC activity due to high kinase activity that results from the overexpression of specific kinase isoforms. Overexpression of the same PDK isoforms deleteriously hinders glucose consumption in unregulated diabetes.

The hormonal regulation of pyruvate dehydrogenase complex

The pyruvate dehydrogenase complex has a central role in the regulation of mammalian metabolism as it represents the point-of-no-return in the utilization of carbohydrate. This article summarizes our studies into how signalling systems initiated by hormones binding to cell surface receptors can reach the pyruvate dehydrogenase system which is located within the inner mitochondrial membrane. One class of hormones which activate pyruvate dehydrogenase are those that increase cytoplasmic Ca2+. A wide range of studies on isolated enzymes, separated mitochondria and intact cell preparations have shown that the activation is due to the stimulation of pyruvate dehydrogenase phosphatase. Two other intramitochondrial dehydrogenases which regulate the citrate acid cycle are activated in parallel and this is an important means of balancing the supply of ATP to increasing cell demand. Insulin is also able to activate pyruvate dehydrogenase, but this is restricted to fat and other cells capable of lipogenesis. Insulin acts by stimulating pyruvate dehydrogenase phosphatase, but the activation does not involve alterations in Ca2+. The signalling pathway involved has not been established, but it appears to be quite distinct from those involved in many other actions of insulin.

Heterogeneous responses to an intravenous glucose load

The aim of this study was to determine how the insulin sensitive enzymes pyruvate dehydrogenase (PDH) complex and glycogen synthase (GS) of different tissues respond to an endogenous pulse of insulin elicited by an intravenous infusion of glucose. An infusion of glucose (0.5 g/kg) into conscious, unrestrained animals via an indwelling cannula rapidly elevated plasma insulin concentration (to approx. 600 microU/ml after 10 min). The animals were sacrificed at selected time points after the commencement of infusion. Samples of heart, red quadriceps muscle, white adipose tissue (WAT) and brown adipose tissue (BAT) were excised and assayed for PDH complex and GS activities. The glucose infusion elicited a rapid (< 5-10 min) increase in both PDH complex and GS activities in heart, BAT and WAT. The maximum rise in the activity of PDH and GS above basal were (respectively) 2- and 8-fold for heart, 5.5- and 5-fold for BAT, and 3.5- and 4-fold for WAT. The return of PDH complex activity to basal values was also very rapid (occurring over the next 20 min). The glucose infusion also stimulated GS activity in red quadriceps muscle but was, however without effect on PDH complex activity in this tissue. We conclude that although insulin stimulates PDH and GS with the same time course and magnitude in many insulin sensitive tissues, the time course and magnitude of insulin stimulation of these enzymes can vary between tissues. These results may mean that the stimulation of PDH complex and GS by insulin occurs via different receptor-effector pathways.

Effect of sulfonylurea agents on pyruvate dehydrogenase activity in circulating lymphocytes from patients with non-insulin-dependent diabetes mellitus (NIDDM)

In circulating lymphocytes from patients with non-insulin-dependent diabetes mellitus (NIDDM) subnormal pyruvate dehydrogenase (PDH) activity returns to normal following patient treatment with sulfonylurea (gliclazide, 80 mg twice daily/5 weeks). Moreover, in vitro in cells from diabetic patients exposed to insulin at 50 microU/mL PDH activation also occurs; in cells of controls the same happens for insulin at 5 microU/mL, whereas at 50 microU/mL inhibition takes place. Therefore, the low PDH activity in cells of NIDDM patients might be caused by defective insulin control on the enzyme and its recovery in gliclazide-treated patients by drug-mediated removal of the defect. The validity of the hypothesis was verified in this study where cells of NIDDM patients before and after gliclazide treatment were exposed, in vitro, to insulin at 5 and 50 microU/mL and then tested for PDH activity. In such conditions, the profile of PDH behavior in treated patients was no longer comparable to that in untreated patients but closer to that in euglycemic controls, thus supporting the view that the recovery of PDH activity in NIDDM patients following gliclazide treatment might be the expression of an additional effect that the drug would have in these patients, aimed to renew cell responsiveness to insulin.

Physiological modulation of the uptake and fate of glucose in brown adipose tissue

Glucose utilization indices (GUI values) and rates of fatty acid synthesis in interscapular brown adipose tissue (IBAT) varied during the diurnal cycle in virgin and late-pregnant rats permitted unrestricted access to food. In virgin rats, peak GUI values and lipogenic rates were observed at the end of the dark (feeding) phase, but were not sustained during the light phase. Whereas peak GUI values were comparable with those observed during re-feeding after 24 h starvation, maximum rates of IBAT fatty acid synthesis in virgin rats during the diurnal cycle were only approx. 25% of those measured during re-feeding after 24 h starvation. Despite hyperphagia, GUI values during the diurnal cycle in late-pregnant rats fed ad libitum were generally lower than those of age-matched virgin controls. The percentage of pyruvate dehydrogenase complex present in the active form (PDHa) was also significantly decreased. Suppression of GUI and PDHa was not parallelled by suppression of fatty acid synthesis. IBAT GUI values in late-pregnant rats during chow re-feeding ad libitum after 24 h starvation were only 25% of those of corresponding virgin controls, and stimulation of fatty acid synthesis was also dramatically attenuated. The suppression of IBAT GUI values after re-feeding in pregnancy was not due to depletion of GLUT 4 protein. The results are discussed in relation to the importance of glucose as a precursor for fatty acid synthesis in IBAT.

In vivo insulin sensitivity of the pyruvate dehydrogenase complex in tissues of the rat

Activity of the insulin-activated pyruvate dehydrogenase complex (PDHC) is necessary for the complete oxidation of glucose to carbon dioxide or the conversion of glucose to fatty acids in lipogenic tissues. To determine the in vivo insulin sensitivity of PDHC activity in rat tissues, we measured the amount of PDHC in the active form in heart, diaphragm, red quadriceps, white adipose tissue (WAT), and brown adipose tissue (BAT) of rats exposed to five different circulating insulin concentrations under euglycemic clamp conditions. PDHC was measured in mitochondrial extracts of tissues rapidly dissected from rats in the starved state or after euglycemic clamp (4 mM) at insulin infusion rates of 0, 0.125, 0.25, and 2.0 U.kg-1.h-1. Increasing the insulin concentration increased the PDHC activity in all tissues, but the magnitude of this activation was different in different tissues (heart: 3.5-fold; diaphragm: 2.5-fold; red quadriceps: 1.8-fold; WAT: 3.4-fold; and BAT: 10.5-fold). Calculation of the half-maximal effective dose (ED50) for the activation of PDHC produced values that were similar in all tissues (heart: 112 pM; diaphragm: 108 pM; red quadriceps: 146 pM; WAT: 120 pM; and BAT: 118 pM). The insulin sensitivity of PDHC in these tissues correlated particularly well with the ED50 for the insulin effect of decreasing circulating nonesterified fatty acids (NEFA; 122 pM). The differences in the magnitude of the effect of increasing insulin on PDHC activity implies a tissue difference in the requirement for an increased capacity for glucose oxidation after insulin stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of pyruvate dehydrogenase kinase activity from pig kidney cortex

The activity of pyruvate dehydrogenase (PDH) kinase in the purified PDH complex from pig kidney is sensitive to changes in ionic strength. The enzyme has optimum activity within a small range of ionic strength (0.03-0.05 M). An increase in ionic strength from 0.04 M to 0.2 M lowers the activity of PDH kinase by 32% and decreases the Km for ATP from 25 microM to 10 microM. At constant ionic strength (0.15 M) the enzyme has optimum activity over a broad pH range (7.2-8.0). The PDH kinase is stimulated 2.2-fold by 20 mM-K+, whereas Na+ even at high concentration (80 mM) has no effect on the enzyme activity. The stimulation of PDH kinase by K+ is not dependent on pH and ionic strength. PDH kinase is inhibited by HPO4(2-) in the presence of K+, whereas HPO4(2-) has no effect on the activity of this enzyme in the absence of K+. HPO4(2-) at concentrations of 2 and 10 mM inhibits PDH kinase by 28% and 55% respectively. The magnitude of this inhibition is not dependent on the ATP/ADP ratio. Inhibition by HPO4(2-) in the concentration range 0-10 mM is non-competitive with respect to ATP, and becomes mixed-type at concentrations over 10 mM. The Ki for HPO4(2-) is 10 mM. When HPO4(2-) is replaced by SO4(2-), the same effects on the activity of PDH kinase are observed. PDH kinase is also inhibited by Cl-. In the presence of 80 mM-Cl- the PDH kinase is inhibited by 40%. The inhibition by Cl- is not dependent on K+. In conclusion, we postulate that changes in phosphate concentrations may play a significant role in the regulation of PDH kinase activity in vivo.

Regulation of pyruvate dehydrogenase by insulin and polyamines within electropermeabilized fat-cells and isolated mitochondria

1. Regulation of the mammalian pyruvate dehydrogenase (PDH) complex by insulin and polyamines has been examined by using electropermeabilized rat epididymal fat-cells and isolated mitochondria. The complex could be regulated within the permeabilized cells not only by insulin, but also by certain low-M(r) species, including Ca2+ and the polyamine spermidine. 2. Both spermine and spermidine increased the level of active dephosphorylated PDH (PDHa) in isolated adipose-tissue mitochondria 2-3-fold, with half-maximal effects at 0.9 mM and 1.7 mM respectively. By contrast, PDH activity in rat heart mitochondria was essentially insensitive to the effects of these polyamines. 3. The effects on PDH activity of incubation of adipose-tissue mitochondria with spermine persisted through re-isolation and re-incubation of the mitochondria in the absence of the polyamine. 4. No evidence was found of any increase in the concentration of spermine associated with purified mitochondrial fractions prepared from insulin-treated tissue. 5. Overall, the data provide further evidence against a role for polyamines in the rapid stimulation of PDH by insulin, but suggest that polyamines may be important in mediating longer-term changes in the activity of the complex.

Does the glycogen synthase (EC 2.4.1.21) of brown adipose tissue play a regulatory role in glucose homeostasis?

Glycogen synthase (GS) activity was characterized in rat brown adipose tissue (BAT) and the activity was found to be much higher than that in white adipose tissue. Prolonged starvation had no effect on the active form of GS, as found in the liver and muscle. The GS activity was similar in BAT of rats housed in an animal room (21 +/- 1 degree) whether they were fed on high-carbohydrate, high-fat, or stock diets. Acclimatization of rats to cold (4 +/- 1 degree) for 2 weeks significantly increased GS activity. This increase in the cold was fivefold greater when rats were fed on high-carbohydrate diets than in control rats at room temperature fed on an identical diet. The increase was accompanied by a large accumulation of glycogen in BAT. It was concluded that GS may play an important role in BAT and may contribute to the control of blood glucose in a cold environment. Its relevance to thermogenesis requires further elucidation.

Effects of protein phosphatase inhibitors on the regulation of insulin-sensitive enzymes within rat epididymal fat-pads and cells

1. The effects of the protein phosphatase inhibitors okadaic acid and microcystin LR on the regulation by insulin of pyruvate dehydrogenase and acetyl-CoA carboxylase have been studied in rat epididymal fat-pads and isolated cells. These inhibitors both completely blocked the phosphatase activity (against phosphorylase a) present in extracts of epididymal fat-pads, with half-maximal effects in the nanomolar range. 2. Okadaic acid treatment of pads and cells lowered the activity of acetyl-CoA carboxylase assayed in tissue extracts, both before and after treatment of the extracts with the activator, citrate. Further, okadaic acid treatment abolished the 2-3-fold difference in activity observed between extracts from control and insulin-treated tissues, assayed without prior treatment with citrate. 3. Incubation of pads with [32P]Pi, sufficient to label the intracellular pool of ATP, demonstrated that okadaic acid increased the overall phosphorylation of acetyl-CoA carboxylase on a number of distinct sites, as judged by two-dimensional mapping of tryptic peptides. These included the 'I-peptide' [Brownsey & Denton (1982) Biochem. J. 202, 77-86], the phosphorylation of which may be associated with the stimulation of the activity of the enzyme by insulin, as well as inhibitory phosphorylation sites. 4. Incubation with 1 microM-okadaic acid had no effect on the basal level of active pyruvate dehydrogenase apparent after tissue extraction, but abolished the 2-3-fold increase in this parameter which was elicited by insulin in the absence of okadaic acid. However, okadaic acid treatment did not affect the persistent increase in active pyruvate dehydrogenase levels which was apparent in mitochondria subsequently isolated from insulin-treated pads and re-incubated with an oxidizable substrate. It is concluded that the effects of okadaic acid are exerted through changes in metabolite concentrations rather than some direct action on the signalling pathway whereby insulin stimulates pyruvate dehydrogenase. 5. Microcystin LR did not mimic the effects of okadaic acid on intact cells and pads described above.

Mammalian hibernation

In mammalian hibernation, the body temperature approaches that of the surroundings, allowing large savings in energy costs of basal metabolism and eliminating the need for heat production to compensate for heat loss. During entry into hibernation, heat production ceases while the body temperature set-point gradually decreases during slow-wave sleep. In the hibernating phase, the animal copes with problems concerning the maintenance of ion gradients, possible membrane phase transitions and the risk of ventricular fibrillation. In the arousal phase, the main part of the heat and practically all the necessary substrate comes from brown adipose tissue. The hibernation season is preceded by a preparatory phase. It may be concluded that hibernation is a practical, and perhaps even enviable, solution to a mammalian problem.

Effect of starvation and insulin in vivo on the activity of the pyruvate dehydrogenase complex in rat skeletal muscles

The in vivo responses of pyruvate dehydrogenase (PDH) complex to starvation and insulin was assessed in heart, diaphragm and red quadriceps muscle. PDH complex activity was decreased by starvation (3.4-10.2-fold), the magnitude of change depending on muscle type. Insulin increased PDH activity in all muscle types. In fed rats, this effect was relatively small (1.25-1.29-fold). In starved rats there were effects in heart (4.3-fold) and red quadriceps (1.7-fold) but no effect in diaphragm. These results demonstrate that PDH complex in different groups of muscle has different insulin sensitivity (particularly in tissues from starved animals).

Effect of nutritional status on insulin sensitivity in vivo and tissue enzyme activities in the rat

The hyperinsulinaemic-glucose-clamp technique, in combination with measurement of glucose turnover in conscious unrestrained rats, was used to assess the effects of nutritional status on insulin sensitivity in vivo and glucose metabolism. Liver, heart and quadriceps skeletal-muscle glycogen content and activities of pyruvate dehydrogenase (PDH) and glycogen synthase were measured both basally and at the end of a 2.5 h glucose clamp (insulin 85 munits/h) in rats 6, 24 and 48 h after food withdrawal. Clamp glucose requirement and glucose turnover were unchanged by fasting. Activation of glycogen synthase and glycogen deposition in liver and skeletal muscle during the clamps were also not impaired in rats after a prolonged fast. By contrast with skeletal muscle, activation of cardiac-muscle glycogen synthase and glycogen deposition during the clamps were markedly impaired by 24 h of fasting and were undetectable at 48 h. Skeletal-muscle PDH activity fell with more prolonged fasting (6 h, 15.3 +/- 3.4%; 24 h, 4.7 +/- 0.7%; 48 h, 4.3 +/- 0.6% active; P less than 0.005), but at 24 and 48 h was stimulated by the clamp to values unchanged by the duration of fasting. Stimulation of cardiac PDH activity by the clamp was, however, impaired in rats fasted for 24 or 48 h. Basal hepatic PDH did not change significantly with fasting (6 h, 5.3 +/- 1.1%; 24 h, 4.6 +/- 0.7%; 48 h, 3.9 +/- 0.5%), and, although it could be partly restored at 24 h, very little stimulation occurred at 48 h. Hepatic pyruvate kinase and acetyl-CoA carboxylase activity were both stimulated by the clamps, and this was not impaired with more prolonged fasting. During the glucose clamps, blood concentrations of lactate, pyruvate and alanine were increased to a greater extent in rats fasted for 24 and 48 h than in rats studied 6 h after food withdrawal. The findings suggest that, although sensitivity to insulin of whole-body glucose disposal is unchanged with fasting, there may be qualitative differences in the metabolism of glucose.

Insulin regulation of the activity and phosphorylation of branched-chain 2-oxo acid dehydrogenase in adipose tissue

The activity of the intramitochondrial branched-chain 2-oxo acid dehydrogenase (BCDH), like that of pyruvate dehydrogenase, is regulated, at least in part, by interconversion between the active dephosphorylated enzyme and its inactive phosphorylated form. The stimulatory effect of insulin on BCDH activity was compared with its effect on phosphorylation of the enzyme. Intact tissues were incubated in the presence or the absence of insulin, and then mitochondria were isolated and disrupted before assaying for enzyme activity or estimating the extent of enzyme phosphorylation. Tissues were incubated in either the presence or the absence of leucine, which also stimulated BCDH activity up to 10-fold. Insulin (1 munit/ml) doubled the activity of BCDH in the absence and in the presence of leucine. Together, 1 mM-leucine and insulin appeared to stimulate BCDH activity fully. Phosphorylation of BCDH was estimated indirectly by measuring the incorporation of 32P into phosphorylation sites that remained unesterified after preparing mitochondrial extracts under conditions that preserved the effect of insulin on BCDH activity. Increased incorporation of 32P in these experiments implies decreased phosphorylation in situ when tissues were incubated with insulin and leucine. In the absence of leucine, little incorporation of 32P into BCDH was detected. In the presence of leucine, however, incorporation of 32P into BCDH was markedly increased, and insulin increased 32P incorporation still further. The results support the hypothesis that leucine and insulin both stimulate the activity of BCDH by promoting its dephosphorylation.

Effects of growth hormone on pyruvate dehydrogenase activity in intact rat liver and in isolated hepatocytes: comparison with insulin

The effects of growth hormone and insulin on the activity of pyruvate dehydrogenase were examined in the rat, both in vivo and in isolated hepatocytes. Liver mitochondria isolated from rats killed from five to 45 minutes after injection of 50 micrograms/100 g human growth hormone (hGH) or 25 micrograms/100 g insulin displayed a significant increase in the activity of basal pyruvate dehydrogenase (38% and 48% above control at ten minutes, respectively). These changes probably result from the conversion of the phosphorylated form to the nonphosphorylated form of pyruvate dehydrogenase since total enzyme activity was unaffected. Treatment of isolated hepatocytes by hGH or insulin also led to an increase in pyruvate dehydrogenase activity which was maximal (25% above control value) at 15 minutes. Later, activation progressively decreased and was no longer detectable at 60 minutes. The concentrations of hGH or insulin required for maximal activation were 100 nmol/L and 20 nmol/L, respectively, and the concentration required for half-maximal stimulation was 2 nmol/L for both hormones. The effects of 100 nmol/L hGH and 100 nmol/L insulin on pyruvate dehydrogenase activity were not additive. Basal pyruvate dehydrogenase activity in hepatocytes exhibited linear kinetics; hGH or insulin increased the Vmax of the enzyme without changing its Km and did not affect the Vmax of the total enzyme activity. It is concluded that growth hormone is as potent and as efficient as insulin in its ability to stimulate the activity of liver pyruvate dehydrogenase, and thus may be a physiological activator of this enzyme.

"Stable" effects of insulin and isoproterenol on adipocyte pyruvate dehydrogenase

Insulin, at a concentration of 1 mU/ml, stimulated glycogen synthase and pyruvate dehydrogenase about threefold in isolated rat adipocytes. Upon the removal of insulin, glycogen synthase activity remained in the activated state for 10 min and thereafter rapidly returned to basal level. On the other hand, insulin-stimulated pyruvate dehydrogenase activity remained elevated for at least 30 min. Isoproterenol (10(-8) M) stimulated phosphorylase and inhibited pyruvate dehydrogenase through the activation of beta-adrenergic receptors. Addition of the beta-antagonist, propranolol (10(-5) M), after isoproterenol reversed the action of isoproterenol on phosphorylase but not its action on pyruvate dehydrogenase. Dibutyryl cyclic AMP, when added to intact adipocytes, produced an effect on pyruvate dehydrogenase similar to that induced by isoproterenol. Our results indicate that both insulin and the beta-agonist have a unique action on pyruvate dehydrogenase which is different from their effects on other enzymes such as glycogen synthase and phosphorylase.

Regulation of renal and hepatic pyruvate dehydrogenase complex on carbohydrate re-feeding after starvation. Possible mechanisms and a regulatory role for thyroid hormone

The work investigated the mechanisms for modulation of renal and hepatic pyruvate dehydrogenase complex (PDH) activities after carbohydrate re-feeding of 48 h-starved rats, and identified a regulatory role for tri-iodothyronine. Glucose re-feeding decreased blood concentrations of lipid fuels in both euthyroid and hyperthyroid rats. This treatment was not associated with re-activation of hepatic PDH in either group of rats, or of renal PDH in hyperthyroid rats (where activity was already high), but it increased renal PDH in euthyroid rats. Dichloroacetate (DCA), an activator of PDH kinase, increased renal PDH activities in euthyroid rats, but not hyperthyroid rats, and effects of glucose re-feeding or hyperthyroidism were no longer apparent. These treatments therefore exert their effects on renal PDH through changes in PDH kinase. DCA re-activation of hepatic PDH was more marked in hyperthyroid than in euthyroid rats, suggesting that, under conditions of inhibited kinase activity, PDH phosphatase is more active in livers of hyperthyroid rats. The limited effect of DCA on hepatic PDH in euthyroid rats was potentiated by glucose re-feeding or insulin, but not by inhibition of lipolysis, demonstrating a direct effect of insulin to increase hepatic PDH phosphatase. Glucose re-feeding, inhibition of lipolysis or insulin administration did not increase hepatic PDH in DCA-treated hyperthyroid rats, indicating that effects of hyperthyroidism and of insulin on PDH phosphatase are not additive.

Effects of Ca2+ and Mg2+ on the activity of pyruvate dehydrogenase phosphate phosphatase within toluene-permeabilized mitochondria

Mitochondria from rat epididymal white adipose tissue were made permeable to small molecules by toluene treatment and were used to investigate the effects of Mg2+ and Ca2+ on the re-activation of pyruvate dehydrogenase phosphate by endogenous phosphatase. Re-activation of fully phosphorylated enzyme after addition of 0.18 mM-Mg2+ showed a marked lag of 5-10 min before a maximum rate of reactivation was achieved. Increasing the Mg2+ concentration to 1.8 mM (near saturating) or the addition of 100 microM-Ca2+ resulted in loss of the lag phase, which was also greatly diminished if pyruvate dehydrogenase was not fully phosphorylated. It is concluded that, within intact mitochondria, phosphatase activity is highly sensitive to the degree of phosphorylation of pyruvate dehydrogenase and that the major effect of Ca2+ may be to overcome the inhibitory effects of sites 2 and 3 on the dephosphorylation of site 1. Apparent K0.5 values for Mg2+ and Ca2+ were determined from the increases in pyruvate dehydrogenase activity observed after 5 min. The K0.5 for Mg2+ was diminished from 0.60 mM at less than 1 nM-Ca2+ to 0.32 mM at 100 microM-Ca2+; at 0.18 mM-Mg2+, the K0.5 for Ca2+ was 0.40 microM. Ca2+ had little or no effect at saturating Mg2+ concentrations. Since effects of Ca2+ are readily observed in intact coupled mitochondria, it follows that Mg2+ concentrations within mitochondria are sub-saturating for pyruvate dehydrogenase phosphate phosphatase and hence less than 0.5 mM.

Use of toluene-permeabilized mitochondria to study the regulation of adipose tissue pyruvate dehydrogenase in situ. Further evidence that insulin acts through stimulation of pyruvate dehydrogenase phosphate phosphatase

Rat epididymal-adipose-tissue mitochondria were made selectively permeable to small molecules without the loss of matrix enzymes by treating the mitochondria with toluene under controlled conditions. With this preparation the entire pyruvate dehydrogenase system was shown to be retained within the mitochondrial matrix and to retain its normal catalytic activity. By using dilute suspensions of these permeabilized mitochondria maintained in the cuvette of a spectrophotometer, it was possible to monitor changes of pyruvate dehydrogenase activity continuously while the activities of the interconverting kinase and phosphatase could be independently manipulated. Permeabilized mitochondria were prepared from control and insulin-treated adipose tissue, and the properties of both the pyruvate dehydrogenase kinase and the phosphatase were compared in situ. No difference in kinase activity was detected, but increases in phosphatase activity were observed in permeabilized mitochondria from insulin-treated tissue. Further studies showed that the main effect of insulin treatment was a decrease in the apparent Ka of the phosphatase for Mg2+, in agreement with earlier studies with mitochondria made permeable to Mg2+ by using the ionophore A23187 [Thomas, Diggle & Denton (1986) Biochem. J. 238, 83-91]. No effects of spermine were detected, although spermine diminishes the Ka of purified phosphatase preparations for Mg2+. Since effects of insulin on pyruvate dehydrogenase phosphatase activity are not evident in mitochondrial extracts, it is concluded that insulin may act by altering some high-Mr component which interacts with the pyruvate dehydrogenase system within intact or permeabilized mitochondria, but not when the mitochondrial membranes are disrupted.

Sensitivity of pyruvate dehydrogenase phosphate phosphatase to magnesium ions. Similar effects of spermine and insulin

The effects of Mg2+ on the activity of pyruvate dehydrogenase phosphate phosphatase within intact mitochondria prepared from control and insulin-treated rat epididymal adipose tissue was explored by incubating the mitochondria in medium containing the ionophore A23187. The apparent Ka for Mg2+ was approximately halved in the mitochondria derived from insulin-treated tissue in both the absence and the presence of Ca2+. In this system, the major effect of Ca2+ was also to decrease the apparent Ka for Mg2+, rather than to change the Vmax. of the phosphatase. Damuni, Humphreys & Reed [(1984) Biochem. Biophys. Res. Commun. 124, 95-99] have reported that spermine activates ox kidney pyruvate dehydrogenase phosphate phosphatase. Studies were carried out on phosphatase from pig heart and rat epididymal adipose tissue which confirm and extend this observation. The major effect of spermine is shown to be a decrease in the Ka for Mg2+, which is apparent in both the presence and the absence of Ca2+. Spermine did not affect the sensitivity of the phosphatase to Ca2+ at saturating concentrations of Mg2+. Other polyamines tested were not as effective as spermine. No alteration in the maximum activity or Mg2+-sensitivity of pyruvate dehydrogenase phosphate phosphatase was apparent in extracts of mitochondria from insulin-treated tissue. The close similarity of the effects of spermine and the changes in kinetic properties of pyruvate dehydrogenase phosphate phosphatase within mitochondria from insulin-treated adipose tissue suggests that insulin may activate pyruvate dehydrogenase by increasing the concentration of spermine within the mitochondria. However, it is concluded that insulin is more likely to alter the interaction of the pyruvate dehydrogenase system with some other polybasic intramitochondrial component whose action can be mimicked by spermine.

Pyruvate inhibition of pyruvate dehydrogenase kinase is a physiological variable

Pyruvate inhibited pyruvate dehydrogenase kinase activity in mitochondria from adipose tissue, heart, brain and kidney of fed rats. Starvation for 24 h led to increased kinase activity in mitochondria from adipose tissue and heart but not from brain or kidney and to reduction of pyruvate inhibition of the enzyme from adipose tissue, heart and brain. Insulin injection into starved animals rapidly restored pyruvate inhibition without alteration of kinase activity in adipose tissue and heart mitochondria. Induction of streptozotocin diabetes resulted in loss of pyruvate inhibition of the kinase in heart mitochondria at 48 h but not at 24 h whereas a significant increase of kinase activity was seen at 24 h. It is concluded that the mechanisms which control fluctuations of pyruvate sensitivity of the kinase are different from the mechanisms which control fluctuations of the uninhibited kinase activity.

Insulin mediator causes dephosphorylation of the alpha subunit of pyruvate dehydrogenase by stimulating phosphatase activity

Insulin treatment of rats results in an increased amount or activity of insulin mediators in liver and skeletal muscle. These mediators stimulated pyruvate dehydrogenase and inhibited adenylate cyclase. The insulin-generated mediators caused dephosphorylation of the alpha subunit of pyruvate dehydrogenase in mitochondria prelabeled with [gamma-32P]ATP. An assay was developed which quantitatively measured mediator activity by determining the rate of alpha-subunit dephosphorylation. The dephosphorylation was directly proportional to the amount of mediator added and was directly related to activation of pyruvate dehydrogenase. The decrease of alpha-subunit phosphorylation resulted from stimulation of pyruvate dehydrogenase phosphatase, since it occurred in the absence of ATP and was inhibited by NaF. These data further delineate the mechanism of insulin mediator activation of pyruvate dehydrogenase.

Insulin activates phospholipase C in fat cells: similarity with the activation of pyruvate dehydrogenase

Phospholipase C (PHL-C) activity determined in homogenates of fat cells treated with physiological concentrations of insulin showed a 2-3-fold increase as compared to controls in the absence of insulin. The changes of PHL-C and pyruvate dehydrogenase (PDH) activity which was measured concomitantly exhibited very similar characteristics as to insulin sensitivity, saturability, time dependence and glucose requirement. Exogenous PHL-C as an activator of PDH in fat cells (Honeyman et al., 1983) also showed a striking similarity to insulin. Our findings strongly suggest that, in fat cells, PHL-C is susceptible to short-term activation by insulin. This effect may be relevant to the mechanism of PDH activation and perhaps to other metabolic actions of insulin.

Role of Ca2+ ions in the regulation of intramitochondrial metabolism in rat epididymal adipose tissue. Evidence against a role for Ca2+ in the activation of pyruvate dehydrogenase by insulin

The sensitivity of rat epididymal-adipose-tissue pyruvate dehydrogenase phosphate phosphatase, NAD+-isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase to Ca2+ ions was studied both in mitochondrial extracts and within intact coupled mitochondria. It is concluded that all three enzymes may be activated by increases in the intramitochondrial concentration of Ca2+ and that the distribution of Ca2+ across the mitochondrial inner membrane is determined, as in rat heart mitochondria, by the relative activities of a uniporter (which transports Ca2+ into mitochondria and is inhibited by Mg2+ and Ruthenium Red) and an antiporter (which allows Ca2+ to leave mitochondria in exchange for Na+ and is inhibited by diltiazem). Previous studies with incubated fat-cell mitochondria have indicated that the increases in the amount of active non-phosphorylated pyruvate dehydrogenase in rat epididymal tissue exposed to insulin are the result of activation of pyruvate dehydrogenase phosphate phosphatase. In the present studies, no changes in the activity of the phosphatase were found in extracts of mitochondria, and thus it seemed likely that insulin altered the intramitochondrial concentration of some effector of the phosphatase. Incubation of rat epididymal adipose tissue with medium containing a high concentration of CaCl2 (5mM) was found to increase the active form of pyruvate dehydrogenase to much the same extent as insulin. However, the increases caused by high [Ca2+] in the medium were blocked by Ruthenium Red, whereas those caused by insulin were not. Moreover, whereas the increases resulting from both treatments persisted during the preparation of mitochondria and their subsequent incubation in the absence of Na+, only the increases caused by treatment of the tissue with insulin persisted when the mitochondria were incubated in the presence of Na+ under conditions where the mitochondria are largely depleted of Ca2+. It is concluded that insulin does not act by increasing the intramitochondrial concentration of Ca2+. This conclusion was supported by finding no increases in the activities of the other two Ca2+-responsive intramitochondrial enzymes (NAD+-isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase) in mitochondria prepared from insulin-treated tissue compared with controls.