Ca2+-dependent activation of the malate-aspartate shuttle by norepinephrine and vasopressin in perfused rat liver

Glucagon: acute actions on hepatic metabolism

Type 2 diabetes mellitus is the result of impaired systemic control of glucose homeostasis, in part through the dysregulation of the hormone glucagon. Glucagon acts on the liver to increase glucose production through alterations in hepatic metabolism, and reducing the elevated glucagon signalling in diabetic patients is an attractive strategy for the treatment of hyperglycaemia. Here we review the actions of the hormone in the liver, focusing on the acute alterations of metabolic pathways. This review summarises a presentation given at the 'Novel data on glucagon' symposium at the 2015 annual meeting of the EASD. It is accompanied by two other reviews on topics from this symposium (by Mona Abraham and Tony Lam, DOI: 10.1007/s00125-016-3950-3 , and by Young Lee and colleagues, DOI: 10.1007/s00125-016-3965-9 ) and an overview by the Session Chair, Isabel Valverde (DOI: 10.1007/s00125-016-3946-z ).

Role of NADH/NAD+ transport activity and glycogen store on skeletal muscle energy metabolism during exercise: in silico studies

Skeletal muscle can maintain ATP concentration constant during the transition from rest to exercise, whereas metabolic reaction rates may increase substantially. Among the key regulatory factors of skeletal muscle energy metabolism during exercise, the dynamics of cytosolic and mitochondrial NADH and NAD+ have not been characterized. To quantify these regulatory factors, we have developed a physiologically based computational model of skeletal muscle energy metabolism. This model integrates transport and reaction fluxes in distinct capillary, cytosolic, and mitochondrial domains and investigates the roles of mitochondrial NADH/NAD+ transport (shuttling) activity and muscle glycogen concentration (stores) during moderate intensity exercise (60% maximal O2 consumption). The underlying hypothesis is that the cytosolic redox state (NADH/NAD+) is much more sensitive to a metabolic disturbance in contracting skeletal muscle than the mitochondrial redox state. This hypothesis was tested by simulating the dynamic metabolic responses of skeletal muscle to exercise while altering the transport rate of reducing equivalents (NADH and NAD+) between cytosol and mitochondria and muscle glycogen stores. Simulations with optimal parameter estimates showed good agreement with the available experimental data from muscle biopsies in human subjects. Compared with these simulations, a 20% increase (or approximately 20% decrease) in mitochondrial NADH/NAD+ shuttling activity led to an approximately 70% decrease (or approximately 3-fold increase) in cytosolic redox state and an approximately 35% decrease (or approximately 25% increase) in muscle lactate level. Doubling (or halving) muscle glycogen concentration resulted in an approximately 50% increase (or approximately 35% decrease) in cytosolic redox state and an approximately 30% increase (or approximately 25% decrease) in muscle lactate concentration. In both cases, changes in mitochondrial redox state were minimal. In conclusion, the model simulations of exercise response are consistent with the hypothesis that mitochondrial NADH/NAD+ shuttling activity and muscle glycogen stores affect primarily the cytosolic redox state. Furthermore, muscle lactate production is regulated primarily by the cytosolic redox state.

Mitochondrial Transporters as Novel Targets for Intracellular Calcium Signaling

Ca2+signaling in mitochondria is important to tune mitochondrial function to a variety of extracellular stimuli. The main mechanism is Ca2+entry in mitochondria via the Ca2+uniporter followed by Ca2+activation of three dehydrogenases in the mitochondrial matrix. This results in increases in mitochondrial NADH/NAD ratios and ATP levels and increased substrate uptake by mitochondria. We review evidence gathered more than 20 years ago and recent work indicating that substrate uptake, mitochondrial NADH/NAD ratios, and ATP levels may be also activated in response to cytosolic Ca2+signals via a mechanism that does not require the entry of Ca2+in mitochondria, a mechanism depending on the activity of Ca2+-dependent mitochondrial carriers (CaMC). CaMCs fall into two groups, the aspartate-glutamate carriers (AGC) and the ATP-Mg/Picarriers, also named SCaMC (for short CaMC). The two mammalian AGCs, aralar and citrin, are members of the malate-aspartate NADH shuttle, and citrin, the liver AGC, is also a member of the urea cycle. Both types of CaMCs are activated by Ca2+in the intermembrane space and function together with the Ca2+uniporter in decoding the Ca2+signal into a mitochondrial response.

Pyruvate ameliorates the defect in ureogenesis from ammonia in citrin-deficient mice

BACKGROUND/AIMS

Mutations in SLC25A13, encoding the mitochondrial aspartate-glutamate carrier citrin, cause adult-onset type II citrullinemia (CTLN2) in humans. We have previously reported that although citrin-knockout (Ctrn-/-) mice fail to display symptoms of CTLN2, liver perfusion revealed a deficit in ureogenesis from ammonia accompanied by an increase in the perfusate lactate-to-pyruvate (L/P) ratio. The present study explores the effects of pyruvate, aspartate and citrate on improving the abnormalities observed in the Ctrn-/- liver.

METHODS

We measured the rate of ureogenesis from ammonium chloride using the liver-perfusion system.

RESULTS

Pyruvate infusion lowered the L/P ratio and corrected the deficit in ureogenesis in the Ctrn-/- liver. This effect was found to be dose-dependent in both instances. Phenazine methosulfate, a cytosolic oxidant, also improved the rate of ureogenesis in the Ctrn-/- liver and led to a fall in the L/P ratio. The addition of aspartate or citrate did not change either the rate of ureogenesis or the L/P ratio in the Ctrn-/- liver.

CONCLUSIONS

Citrin deficiency disturbs urea synthesis primarily as a result of an elevated cytosolic NADH/NAD+ ratio owing to limited reoxidation of reducing equivalents. Clinically, pyruvate may have a therapeutic benefit for CTLN2 patients.

Role of calcium signaling in the activation of mitochondrial nitric oxide synthase and citric acid cycle

An apparent discrepancy arises about the role of calcium on the rates of oxygen consumption by mitochondria: mitochondrial calcium increases the rate of oxygen consumption because of the activation of calcium-activated dehydrogenases, and by activating mitochondrial nitric oxide synthase (mtNOS), decreases the rates of oxygen consumption because nitric oxide is a competitive inhibitor of cytochrome oxidase. To this end, the rates of oxygen consumption and nitric oxide production were followed in isolated rat liver mitochondria in the presence of either L-Arg (to sustain a mtNOS activity) or N(G)-monomethyl-L-Arg (NMMA, a competitive inhibitor of mtNOS) under State 3 conditions. In the presence of NMMA, the rates of State 3 oxygen consumption exhibited a K(0.5) of 0.16 microM intramitochondrial free calcium, agreeing with those required for the activation of the Krebs cycle. By plotting the difference between the rates of oxygen consumption in State 3 with L-Arg and with NMMA at various calcium concentrations, a K(0.5) of 1.2 microM intramitochondrial free calcium was obtained, similar to the K(0.5) (0.9 microM) of the dependence of the rate of nitric oxide production on calcium concentrations. The activation of dehydrogenases, followed by the activation of mtNOS, would lead to the modulation of the Krebs cycle activity by the modulation of nitric oxide on the respiratory rates. This would ensue in changes in the NADH/NAD and ATP/ADP ratios, which would influence the rate of the cycle and the oxygen diffusion.

Recombinant expression of the Ca(2+)-sensitive aspartate/glutamate carrier increases mitochondrial ATP production in agonist-stimulated Chinese hamster ovary cells

The Ca(2+)-sensitive dehydrogenases of the mitochondrial matrix are, so far, the only known effectors to allow Ca2+ signals to couple the activation of plasma membrane receptors to the stimulation of aerobic metabolism. In this study, we demonstrate a novel mechanism, based on Ca(2+)-sensitive metabolite carriers of the inner membrane. We expressed in Chinese hamster ovary cells aralar1 and citrin, aspartate/glutamate exchangers that have Ca(2+)-binding sites in their sequence, and measured mitochondrial Ca2+ and ATP levels as well as cytosolic Ca2+ concentration with targeted recombinant probes. The increase in mitochondrial ATP levels caused by cell stimulation with Ca(2+)-mobilizing agonists was markedly larger in cells expressing aralar and citrin (but not truncated mutants lacking the Ca(2+)-binding site) than in control cells. Conversely, the cytosolic and the mitochondrial Ca2+ signals were the same in control cells and cells expressing the different aralar1 and citrin variants, thus ruling out an indirect effect through the Ca(2+)-sensitive dehydrogenases. Together, these data show that the decoding of Ca2+ signals in mitochondria depends on the coordinate activity of mitochondrial enzymes and carriers, which may thus represent useful pharmacological targets in this process of major pathophysiological interest.

Citrin and aralar1 are Ca(2+)-stimulated aspartate/glutamate transporters in mitochondria

The mitochondrial aspartate/glutamate carrier catalyzes an important step in both the urea cycle and the aspartate/malate NADH shuttle. Citrin and aralar1 are homologous proteins belonging to the mitochondrial carrier family with EF-hand Ca(2+)-binding motifs in their N-terminal domains. Both proteins and their C-terminal domains were overexpressed in Escherichia coli, reconstituted into liposomes and shown to catalyze the electrogenic exchange of aspartate for glutamate and a H(+). Overexpression of the carriers in transfected human cells increased the activity of the malate/aspartate NADH shuttle. These results demonstrate that citrin and aralar1 are isoforms of the hitherto unidentified aspartate/glutamate carrier and explain why mutations in citrin cause type II citrullinemia in humans. The activity of citrin and aralar1 as aspartate/glutamate exchangers was stimulated by Ca(2+) on the external side of the inner mitochondrial membrane, where the Ca(2+)-binding domains of these proteins are localized. These results show that the aspartate/glutamate carrier is regulated by Ca(2+) through a mechanism independent of Ca(2+) entry into mitochondria, and suggest a novel mechanism of Ca(2+) regulation of the aspartate/malate shuttle.

Fatty acids and glycerol or lactate are required to induce gluconeogenesis from alanine in isolated rabbit renal cortical tubules

In isolated rabbit renal cortical tubules, glucose synthesis from 1 mM alanine is negligible, while the amino acid is metabolized to glutamine and glutamate. The addition of 0.5 mM octanoate plus 2 mM glycerol induces incorporation of [U-14C]alanine into glucose and decreases glutamine synthesis, whereas oleate and palmitate in the presence of glycerol are less potent than octanoate. Gluconeogenesis is also significantly accelerated when glycerol is substituted by lactate. In view of an increase in 14CO2 fixation and elevation of both cytosolic and mitochondrial NADH/NAD+ ratios, the activation of glucose formation from alanine upon the addition of glycerol and octanoate is likely due to (i) stimulation of pyruvate carboxylation, (ii) increased availability of NADH for glyceraldehyde-3-phosphate dehydrogenase and (iii) elevation of mitochondrial redox state causing a diminished provision of ammonium for glutamine synthesis. The induction of gluconeogenesis in the presence of alanine, glycerol and octanoate is not related to cell volume changes. The results presented in this paper show the importance of free fatty acids and glycerol for regulation of renal gluconeogenesis from alanine. The possible physiological significance of the data is discussed.

Integrating cytosolic calcium signals into mitochondrial metabolic responses

Stimulation of hepatocytes with vasopressin evokes increases in cytosolic free Ca2+ ([Ca2+]c) that are relayed into the mitochondria, where the resulting mitochondrial Ca2+ ([Ca2+]m) increase regulates intramitochondrial Ca2+-sensitive targets. To understand how mitochondria integrate the [Ca2+]c signals into a final metabolic response, we stimulated hepatocytes with high vasopressin doses that generate a sustained increase in [Ca2+]c. This elicited a synchronous, single spike of [Ca2+]m and consequent NAD(P)H formation, which could be related to changes in the activity state of pyruvate dehydrogenase (PDH) measured in parallel. The vasopressin-induced [Ca2+]m spike evoked a transient increase in NAD(P)H that persisted longer than the [Ca2+]m increase. In contrast, PDH activity increased biphasically, with an initial rapid phase accompanying the rise in [Ca2+]m, followed by a sustained secondary activation phase associated with a decline in cellular ATP. The decline of NAD(P)H in the face of elevated PDH activity occurred as a result of respiratory chain activation, which was also manifest in a calcium-dependent increase in the membrane potential and pH gradient components of the proton motive force (PMF). This is the first direct demonstration that Ca2+-mobilizing hormones increase the PMF in intact cells. Thus, Ca2+ plays an important role in signal transduction from cytosol to mitochondria, with a single [Ca2+]m spike evoking a complex series of changes to activate mitochondrial oxidative metabolism.

The role of intracellular Ca2+ in the regulation of gluconeogenesis

A hypothesis for the hormonal regulation of gluconeogenesis, in which increases in cytosolic free-Ca2+ levels ([Ca2+]i) play a major role, is presented. This hypothesis is based on the observation that gluconeogenic hormones evoke a common pattern of Ca2+ redistribution, resulting in increases in [Ca2+]i. Current concepts of hormonally evoked Ca2+ fluxes are presented and discussed. It is suggested that the increase in [Ca2+]i is functionally linked to stimulation of gluconeogenesis. The stimulation of gluconeogenesis is accomplished in two ways: (1) by increasing the activities of the Krebs cycle and the electron-transfer chain, thereby supplying adenosine triphosphates (ATP) and reducing equivalents to the process; and (2) by stimulating the activities of key gluconeogenic enzymes, such as pyruvate carboxylase. The hypothesis presents a conceptual framework that ties together two interrelated manifestations of hormone action: signal transduction and metabolism.

Abolition of the inhibitory effect of ethanol oxidation on gluconeogenesis from lactate by asparagine or low concentrations of ammonia

When isolated hepatocytes from fasted rats were incubated with 10 mM lactate, the [lactate]/[pyruvate] ratio measured at the beginning of the incubation was raised above 70:1 but declined to a steady level of about 8:1 within 40 min. The rate of gluconeogenesis from lactate was initially slow but gradually increased over the incubation period becoming maximal by 30 min. The simultaneous addition of lactate and ethanol resulted in an initial [lactate]/[pyruvate] ratio above 250:1 which by 60 min had declined to a new steady-state level of approx. 60:1. The lactate, ethanol combination also brought about a prolongation of the lag phase before glucose synthesis became maximal; however, by 40 min the rate of gluconeogenesis was independent of the presence of ethanol. Thus the inhibitory effect of ethanol on glucose synthesis was manifest only over the early portion of the incubation period. When asparagine, a precursor of malate/aspartate components, was added to the incubation mixture, the lag before maximal rates of glucose formation from lactate in the absence or presence of ethanol was almost abolished. The presence of asparagine also rapidly lowered the [lactate]/[pyruvate] ratio of hepatocytes incubated with lactate plus ethanol establishing a steady-state level of 15:1 within 10-15 min. Asparagine enhanced the rate of lactate-stimulated ethanol oxidation, particularly during the early part of the incubation. In endeavouring to elucidate which of the products of asparagine catabolism (i.e. ammonia and aspartate) were responsible for these effects, we found that a small and constant level of ammonia, formed by the degradation of urea by urease, almost reproduced the effects of asparagine on the [lactate]/[pyruvate] ratio, glucose synthesis and ethanol oxidation. A bolus addition of 10 mM aspartate or 4 mM ammonia to cells metabolising lactate and ethanol were less effective than a steady-state low ammonia concentration, generated from urea/urease. Our studies suggest that asparagine or a low concentration of ammonia, by providing components of the malate/aspartate shuttle, can ameliorate some of the metabolic effects of ethanol on the liver.

Effects of calmodulin antagonists on hydrogen-translocating shuttles in perfused rat liver

The effects of calmodulin antagonists on the capacity of hydrogen-translocating shuttles were studied in the perfused rat liver. The capacity was estimated by measuring the changes in the rate of production of glucose from sorbitol during the oxidation of ethanol [T. Sugano, T. Ohta, A. Tarui, and Y. Miyamae. Am. J. Physiol. 251 (Endocrinol. Metab. 14): E385-E392, 1986]. Thyroxine given to intact rats increased the activity of alpha-glycerophosphate dehydrogenase (alpha-GPD). Glucocorticoid replacement in adrenalectomized rats decreased the activity of the alpha-GPD to values obtained after treatment with PTU. In either thyroxine-treated or steroid-replaced rats, the capacity of hydrogen-translocating shuttles increased markedly. However, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), trifluoperazine, and chlorpromazine inhibited the increased capacity in steroid-replaced rats and had no effect on the increased capacity in thyroxine-treated rats. W-7 inhibited the stimulatory effects of norepinephrine on the capacity of the malate-aspartate shuttle without inhibition of efflux of intracellular Ca2+. The stimulatory effects of vasopressin on the malate-aspartate shuttle were also inhibited by W-7, trifluoperazine, and chlorpromazine. The results suggest that the malate-aspartate shuttle may be regulated by Ca(2+)-calmodulin.