Short-term hypothermia activates hepatic mitochondrial sn-glycerol-3-phosphate dehydrogenase and thermogenic systems

The contribution of the sn-glycerol-3-phosphate (G-3-P) shuttle in the control of energy metabolism is well established. It is also known that its activity may be modulated by hormones involved in thermogenesis, such as thyroid hormones or dehydroepiandrosterone and its metabolites, that act by inducing de novo synthesis of mitochondrial G-3-P dehydrogenase (mGPDH). However, little is known as to the factors that may influence the activity without enzyme induction. In the present study we investigated the possible role of the G-3-P shuttle in the thermogenic response to different hypothermic stresses. It was found that a decrease of body temperature causes the liver rapidly to enhance mGPDH activity and G-3-P-dependent respiration. The enhancement, which does not result from de novo synthesis of enzymes, has the potential of increasing heat production both by decreased ATP synthesis during the oxidation of G-3-P and by activation of the glycolytic pathway.

Opposite role of changes in mitochondrial membrane potential in different apoptotic processes

We have studied the role of changes in mitochondrial membrane potential (DeltaPsi) in two widely-used models of apoptosis, such as dexamethasone-treated rat thymocytes and U937 human cells treated with tumor necrosis factor-alpha and cycloheximide. To dissipate DeltaPsi, we used low concentrations of valinomycin, unable per se to induce apoptosis, and demonstrated that the decline in DeltaPsi exerts opposite effects in the two models. Indeed, in U937 cells, depolarization of mitochondria increased apoptosis, which decreased in rat thymocytes. This leads to the suggestion that disruption of DeltaPsi plays opposite roles depending on the experimental model. In U937 cells, the drop of DeltaPsi is a possible contributory cause for the apoptotic process; in rat thymocytes, it could be a limiting factor. We propose that these opposite effects could be due to the different ATP requirement of each apoptotic pathway.

Decrease in mitochondrial energy coupling by thyroid hormones: a physiological effect rather than a pathological hyperthyroidism consequence

The effect of the in vivo thyroid status on mitochondrial membrane potential (delta psi(m)) in isolated rat hepatocytes was studies by means of a cytofluorimetric technique and the delta psi(m)-specific probe JC-1. It is shown that the delta psi(m) level decreases in the order hypothyroid > euthyroid > hyperthyroid. Polarographic measurement of the hepatocyte respiratory rates revealed an opposite trend of values: the highest respiratory rate in hepatocytes from hyperthyroid animals, the lowest in those from hypothyroid ones. This means that mitochondrial energy coupling is highest in hypothyroid hepatocytes and lowest in hyperthyroid hepatocytes. 6-Ketocholestanol added to hepatocytes failed to counterbalance the uncoupling effect of thyroid hormones on delta psi(m) and respiration rate. Under the same conditions, 6-ketocholestanol appeared to be effective in recoupling of respiration uncoupled by low concentrations of the artificial protonophore FCCP. The mechanism and possible physiological functions of the thyroid hormone-induced decrease in mitochondrial energy coupling are discussed.

Use of flow cytometry as a tool to study mitochondrial membrane potential in isolated, living hepatocytes

The present paper describes the possibility of determination of mitochondrial membrane potential (Deltapsi) in isolated hepatocytes making use of a Deltapsi-sensitive dye, i.e., the lipophilic cationic probe 5,5',6,6'-tetrachloro-1,1',3, 3'-tetraethylbenzimidazolcarbocyanine iodide (JC-1) and of cytofluorimetry. The validity of the method was proved by treating hepatocytes with FCCP (decrease of Deltapsi) and subsequent addition of 6-ketocholestanol (increase of Deltapsi). The results indicate that the proposed method may be used in laboratory practice.

The effects of the ergosteroid 7-oxo-dehydroepiandrosterone on mitochondrial membrane potential: possible relationship to thermogenesis

Administered 3 beta-hydroxyandrost-5-ene-7,17-dione (7-oxo-DHEA) is more effective than 3 beta-hydroxyandrost-5-en-7-one (DHEA) as an inducer of liver mitochondrial sn-glycerol-3-phosphate dehydrogenase and cytosolic malic enzyme in rats. Like DHEA, the 7-oxo metabolite enhances liver catalase, fatty acylCoA oxidase, cytosolic sn-glycerol-3-phosphate dehydrogenase, mitochondrial substrate oxidation rate, and the reconstructed sn-glycerol 3-phosphate shuttle. The mitochondrial adenine nucleotide carrier is diminished by thyroidectomy and is restored to normal activity by administering 7-oxo-DHEA. The relationship between respiratory rate and proton motive force across the mitochondrial membrane was measured in the nonphosphorylating state. When treated with increasing concentrations of respiratory inhibitors liver mitochondria from rats treated with 7-oxo-DHEA or thyroid hormones show a more rapid decline of membrane potential than do normal liver mitochondria. Thus 7-oxo-DHEA induces an increased proton leak or slip as has been reported for the thyroid hormone by M.D. Brand [(1990) Biochem. Biophys. Acta 1018, 128-133]. This process may contribute to the enhanced thermogenesis caused by ergosteroids as well as by thyroid hormones.

Effect of cardiolipin on functional properties of isolated rat liver mitochondria

The specific involvement of cardiolipin in modulating and/or controlling the activity of a number of mitochondrial carriers, enzymes and receptors is well documented; however, comparatively less understood is its role for the integrated functions of intact mitochondria. The aim of the present research was to get a better insight into this problem by investigating the effect of in vitro addition of cardiolipin on the properties of isolated liver mitochondria. The results obtained show that cardiolipin induces extensive structural and functional perturbations at the level of the inner mitochondrial membrane. In fact, addition of cardiolipin to intact mitochondria causes a significant increase of proton leak associated with a parallel increase of respiratory rate in State 4. Concomitantly, a slight uncoupling of phosphorylation associated with a moderate increase in ATPase activity is observed. Furthermore, the pore-mediated membrane permeability to calcium is drastically modified, an effect that can be reversed by addition of cyclosporin.

Effects of dehydroepiandrosterone and carnitine treatment on rat liver

It is well established that DHEA treatment is associated in the rat to an increase in fatty acids metabolism. This condition would require levels of L-carnitine much higher than those physiologically present in the liver. The possibility thus exist that during DHEA treatment the concentration of L-carnitine may become a limiting factor for fatty acids oxidation and therefore responsible of some of the effects observed after administration of the hormone. The present experiments were designed to test this hypothesis. The results show that the increase in the levels of peroxisomal enzymes induced in hepatocytes by DHEA, is greatly reduced by parallel administration of L-carnitine. Furthermore, L-carnitine administration counteracts the effect of DHEA on mitochondrial structure. On the contrary, carnitine has no significant effect on the reduction in weight gain observed upon short- or long-term treatment with DHEA.

Comparative studies of effects of dehydroepiandrosterone on rat and chicken liver

1. An attempt to identify the cause of decrease of gain in body weight during dehydroepiandrosterone (DHEA) treatment was made comparing the effects of hormone treatment on chickens and rats. 2. Chickens treated with DHEA for 7-10 days do not change their weight gain with respect to controls although their mitochondrial respiration and peroxisomal catalase (index of peroxisomal mass) were increased. 3. Liver cytosolic malic enzyme and sn-glycerol-3-phosphate dehydrogenase were depressed in chickens treated with DHEA in comparison with activities in untreated controls. DHEA treatment did not increase the activity of mitochondrial sn-glycerol 3-phosphate dehydrogenase. 4. In contrast to rat liver cytosolic sn-glycerol-3-phosphate dehydrogenase this enzyme in chicken liver was inactive with NADPH.

Concerning the mechanism of increased thermogenesis in rats treated with dehydroepiandrosterone

Dehydroepiandrosterone (DHEA) treatment of rats decreases gain of body weight without affecting food intake; simultaneously, the activities of liver malic enzyme and cytosolic glycerol-3-P dehydrogenase are increased. In the present study experiments were conducted to test the possibility that DHEA enhances thermogenesis and decreases metabolic efficiency via transhydrogenation of cytosolic NADPH into mitochondrial FADH2 with a consequent loss of energy as heat. The following results provide evidence which supports the proposed hypothesis: (a) the activities of cytosolic enzymes involved in NADPH production (malic enzyme, cytosolic isocitrate dehydrogenase, and aconitase) are increased after DHEA treatment; (b) cytosolic glycerol-3-P dehydrogenase may use both NAD+ and NADP+ as coenzymes; (c) activities of both cytosolic and mitochondrial forms of glycerol-3-P dehydrogenase are increased by DHEA treatment; (d) cytosol obtained from DHEA-treated rats synthesizes more glycerol-3-P during incubation with fructose-1,6-P2 (used as source of dihydroxyacetone phosphate) and NADP+; the addition of citrate in vitro further increases this difference; (e) mitochondria prepared from DHEA-treated rats more rapidly consume glycerol-3-P added exogenously or formed endogenously in the cytosol in the presence of fructose-1,6-P2 and NADP+.

Interaction of carnitine with mitochondrial cardiolipin

The physiological role of L-carnitine is to determine the transport of acyl-CoA through the mitochondrial membrane. However, some observations may also suggest a direct effect of the molecule per se on the physical properties of the membrane, most probably at the level of the binding site. This possibility has been investigated by studying the influence of adriamycin, a drug that binds to cardiolipin, on the effect of carnitine on isolated rat liver mitochondria. It has been found that adriamycin almost abolishes the activating effect of carnitine on state 2 respiration. The effect and its inhibition is seen by using either the L-form of carnitine or the D-form or both. Cardiolipin removes the effect of adriamycin and restores the activation by carnitine. It is proposed that some effects of carnitine on mitochondrial properties may be the result of interaction of carnitine with cardiolipin at the membrane level.

Changes in liver structure and function after short-term and long-term treatment of rats with dehydroepiandrosterone

The effects on the liver of feeding a diet containing 0.2% dehydroepiandrosterone were studied after short (7 d) and long (100 d) periods of treatment in rats. The short-term treatment caused hypertrophy of the hepatocytes that, at the ultrastructural level, seemed to be due to proliferation of peroxisomes and (to a minor extent) of mitochondria. The mitochondria seemed to have undergone transition from expanded to condensed configuration; accordingly, after isolation, their rate of coupled respiration was greater than that of control mitochondria. After long-term treatment, the structure of the hepatocytes reverted toward normal. In fact, at the ultrastructural level, the number and the size of peroxisomes was not significantly different from those of the controls, but degenerative phenomena were observed in the mitochondria. Attempts are made to explain the above ultrastructural and biochemical findings in view of the effects of dehydroepiandrosterone on the energy metabolism of liver.

The protecting effect of L-carnitine on Ca(2+)-loaded rat liver mitochondria

It is shown that L-carnitine strongly increases the ability of rat liver mitochondria to respond to the train of Ca2+ additions by a transient stimulation of the State-4 respiration rate. Such an effect requires ATP and the L-carnitine efficiency strongly decreases when ATP is omitted. Oleate influences the mitochondria in a fashion opposite to that of L-carnitine. The oleate effect is strongly diminished by L-carnitine. Again, the L-carnitine effect requires ATP, and D-carnitine fails to substitute for L-carnitine. It is suggested that L-carnitine removes, in an ATP-dependent manner, endogenous or added fatty acids, which are involved in oxidative damage of Ca(2+)-loaded mitochondria.