Depression by ethionine of phosphorylating oxidation in hepatic mitochondria

Induction of hepatic steatosis and suppression of hepatic ATP levels, protein synthesis and gluconeogenesis subsequent to administration of ethionine may be consequences of interference by this compound with mitochondrial phosphorylation of ADP. The mitochondrial dysfunction is not a direct action of ethionine on the organelle.     

In vitro metabolism of ethionine, N-acetyl-ethionine and N-acetylmethionine by rat liver microsomes.

Rat liver microsomes deacetylated N-acetylethionine and N-acetyl-methionine to ethionine and methionine. The deacetylation of N-acetylmethionine was more rapid than the deacetylation of N-acetylethionine. Ethionine was slowly converted to ethionine sulfoxide by the microsomal preparations. N-Acetylethionine and N-acetylmethionine were not oxidized by the microsomes.     

The relationship between membrane potential and ATP content in rat liver during ischemia

Summary Both ischemia and ethionine decreased the membrane potential and ATP content in rat liver. In ethioninetreated rat liver, ischemia brought about a further decrease in membrane potential without significant decrease in ATP content.     

Influence of pregnancy and fibrosarcoma on hepatic mitochondrial proteins of mice

Summary Non-pregnant, pregnant and fibrosarcoma-bearing non-pregnant mice differ in their total hepatic mitochondrial protein content, as well as the electrophoretic pattern following separation on SDS acrylamide gels.     

Effect of trypsin,S-adenosylmethionine and ethionine on L-serine sulfhydrase activity

Summary Trypsin causes an activation of serine sulfhydrase in the liver extracts from intact animals, but inhibits enzyme activity in the liver of ethionine treated rats. Trypsin also decreases an elevation of serine sulfhydrase activity caused by S-adenosylmethionine.This work was supported by the Serbian Medical Research Foundation.     

5'-Nucleotidase activity in liver homogenates of rats treated with CCl4, colchicine, cycloheximide, emetine, ethanol, ethionine and 5-fluorotryptophan

5'-Nucleotidase activity an enzyme marker of the plasma membranes, increases in female rat liver homogenates following ethionine administration, while homogenates from males show no changes. Treatment with CCl4, colchicine, cycloheximide, emetine, ethanol and 5-fluorotryptophan does not significantly modify the 5'-nucleotidase activity of liver homogenates of either female or male rats.     

Modulation of hepatic mitochondrial energy efficiency with age

This study was designed to examine the effect of youth-adulthood transition on hepatic mitochondrial energy efficiency. The changes in basal and palmitate-induced proton leak, which contribute to mitochondrial efficiency, were evaluated in mitochondria isolated from the liver of young and adult rats. Alterations in mitochondrial cytochrome oxidase and aconitase specific activities, and in adenine nucleotide translocator content were also assessed. There was no difference in basal proton leak or thermodynamic coupling and efficiency of oxidative phosphorylation in liver mitochondria between the two rat groups. On the other hand, palmitate-induced proton leak increased significantly in adult rats. The function of this uncoupling could be avoidance of elevated formation of reactive oxygen species, which are known to accelerate ageing.Received 17 February 2004; received after revision 30 March 2004; accepted 1 April 2004     

Molecular mechanisms of hepatic lipid accumulation in non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is currently the world’s most common liver disease, estimated to affect up to one-fourth of the population. Hallmarked by hepatic steatosis, NAFLD is associated with a multitude of detrimental effects and increased mortality. This narrative review investigates the molecular mechanisms of hepatic steatosis in NAFLD, focusing on the four major pathways contributing to lipid homeostasis in the liver. Hepatic steatosis is a consequence of lipid acquisition exceeding lipid disposal, i.e., the uptake of fatty acids and de novo lipogenesis surpassing fatty acid oxidation and export. In NAFLD, hepatic uptake and de novo lipogenesis are increased, while a compensatory enhancement of fatty acid oxidation is insufficient in normalizing lipid levels and may even promote cellular damage and disease progression by inducing oxidative stress, especially with compromised mitochondrial function and increased oxidation in peroxisomes and cytochromes. While lipid export initially increases, it plateaus and may even decrease with disease progression, sustaining the accumulation of lipids. Fueled by lipo-apoptosis, hepatic steatosis leads to systemic metabolic disarray that adversely affects multiple organs, placing abnormal lipid metabolism associated with NAFLD in close relation to many of the current life-style-related diseases.     

Oxygen consumption and biosynthetic function in perfused liver from rats at different stages of development

Changes in mitochondrial function were studied in perfused liver from rats aged 24 – 365 days. Oxygen consumption together with the rates of gluconeogenesis, urea synthesis and ketogenesis were determined. Basal mitochondrial respiration as well as the ability of the liver to synthesize glucose, urea and ketone bodies declined from 24- to 365-day-old rats. On the other hand, on transition from 24 to 60 days the liver oxidation rate of hexanoate, sorbitol and glycerol is enhanced, but not of ketone bodies or palmitate. Our results show that the transition from weaning to middle age is accompanied by defined changes in hepatic substrate oxidation. From the observed time course of the decrease in basal and substrate-stimulated oxygen consumption, it is concluded that in rat liver cells a decline in respiratory chain function, long-chain fatty acid and ketone body metabolism, gluconeogenesis and ureogenesis occurs at a relatively early life stage. Received 19 June 1998; received after revision 11 September 1998; accepted 11 September 1998     

5′-Nucleotidase activity in liver homogenates of rats treated with CCl4, colchicine,cycloheximide, emetine,ethanol, ethionine and 5-fluorotryptophan

Summary 5-Nucleotidase activity, an enzyme marker of the plasma membranes, increases in female rat liver homogenates following ethionine administration, while homogenates from males show no changes. Treatment with CCl₄, colchicine, cycloheximide, emetine, ethanol and 5-fluorotryptophan does not significantly modify the 5-nucleotidase activity of liver homogenates of either female or male rats.     

Temperature-dependence of stress-induced hepatic autophagy

Summary In rats, restraint for 48 h elicits hepatic glycogen depletion, autophagy and other ultrastructural changes (e.g. mitochondrial enlargement and rough endoplasmic reticulum disorganization) associated with marked hypothermia. By restoring the body temperature of these animals, all the hepatocytic alterations are abolished.Supported in part by the Medical Research Council of Canada (Block Term Grant MT-1829) and the Conseil de la recherche en santé du Québec. A preliminary report of this work was presented at the 19th Annual Meeting of the Canadian Federation of Biological Societies, Halifax, 15–18 June 1976.Recipient of a studentship from the Conseil de la recherche en santé du Québec.The authors thank Mr M. Audet, Mrs A. Perea and Miss F. Dionne, for their excellent technical assistance.     

Thyroid hormone controls carnitine status through modifications of γ-butyrobetaine hydroxylase activity and gene expression

The carnitine system plays a key role in β-oxidation of long-chain fatty acids by permitting their transport into the mitochondrial matrix. The effects of hypothyroidism and hyperthyroidism were studied on γ-butyrobetaine hydroxylase (BBH), the enzyme responsible for carnitine biosynthesis in the rat. In rat liver, BBH activity was decreased in the hypothyroid state and increased in hyperthyroid animals. The modifications in BBH activity correlated with changes in the enzyme Vmax values. These changes were shown to be related to hepatic BBH mRNA abundance. Thyroid hormones are known to interact with lipid metabolism, in particular by increasing long-chain fatty acid oxidation through activation of carnitine-dependent fatty acid import into mitochondria. Our study showed that thyroid hormones also increased carnitine bioavailability. Received 23 October 2001; received after revision 11 January 2002; accepted 15 January 2002     

Acetyl-L-carnitine treatment stimulates oxygen consumption and biosynthetic function in perfused liver of young and old rats

The effect of treatment with acetyl-L-carnitine on hepatic mitochondrial respiration and biosynthetic function in perfused liver from young (90 days) and old (22-24 months) rats was studied. Rats were given a 1.5% (w/v) solution of acetyl-L-carnitine in their drinking water for 1 month and oxygen consumption together with the rate of gluconeogenesis, urea synthesis, and ketogenesis with and without added substrates were measured in perfused liver. Mitochondrial oxygen consumption was also assessed in liver homogenate and isolated mitochondria to determine the maximal capacity for oxidative phosphorylation. Acetyl-L-carnitine treatment almost completely restored the age-dependent decline in oxygen consumption, gluconeogenesis, urea synthesis, and ketogenesis found in perfused liver of old rats to the levels found in young rats. In addition, acetyl-L-carnitine treatment increased oxygen consumption and biosynthetic function in perfused liver from young rats. After acetyl-L-carnitine treatment, we found detectable 3-oxoacyl-CoA-transferase activity associated with a consumption of ketone bodies in young and old rats. Finally, oxygen consumption measured in homogenate and isolated mitochondria did not change with age and acetyl-L-carnitine treatment. Our results show that in perfused liver, acetyl-L-carnitine treatment slows the age-associated decline in mitochondrial respiration and biosynthetic function. In addition, treatment of young rats with acetyl-L-carnitine has a stimulating effect on liver metabolism, probably through an increase in ATP production. Received 25 October 2000; received after revision 14 December 2000; accepted 11 January 2001     

Significance and molecular targets of protein kinase A during cAMP-mediated protection of cold stored liver grafts

The use of marginal donor livers is followed by a higher frequency of primary dys- or nonfunction after transplantation. The present study was designed to test the hypothesis that stimulation of the cAMP second-messenger signal pathway might protect the liver from ischemic injury, laying emphasis on the role of protein kinase A-mediated signal transduction.?Rat livers were harvested after 45 min of cardiac arrest and preserved in HTK solution for 24 h. Hepatic integrity was assessed thereafter using a blood-free reperfusion model.?Supplementation of the preservation solution with dibutyryl-cAMP (db-cAMP) promoted phosphorylation of BAD at Ser 112 and concomitantly mitigated mitochondrial release of cytochrome c into the cytosol. Apoptotic cell transformation was evident in reperfused livers by positive TUNEL-staining of sinusoidal lining cells and the detection of cleaved poly(ADP-ribose) polymerase (PARP) in tissue homogenates by western analysis. Treatment with db-cAMP was effective in minimizing both TUNEL staining and PARP cleavage and significantly reduced postischemic enzyme leakage of alanine aminotransferase to one half, while hepatic bile production was enhanced by approximately 60% when compared to untreated livers. This functional improvement was accompanied by a net amelioration of portal vascular conductivity. Inhibition of A kinase-anchoring protein with HT31 completely reversed any of the observed effects obtained by db-cAMP.?We conclude that enhancement of cellular cAMP signal maintains hepatic integrity during and after ischemic preservation which may be attributed to protein kinase A dependent phosphorylation of BAD in line with subsequent inhibition of mitochondria-initiated apoptosis of sinusoidal lining cells. Received 12 July 2001; received after revision 14 August 2001; accepted 14 August 2001     

Parvalbumin alters mitochondrial dynamics and affects cell morphology

The Ca²⁺-binding protein parvalbumin (PV) and mitochondria play important roles in Ca²⁺ signaling, buffering and sequestration. Antagonistic regulation of PV and mitochondrial volume is observed in in vitro and in vivo model systems. Changes in mitochondrial morphology, mitochondrial volume and dynamics (fusion, fission, mitophagy) resulting from modulation of PV were investigated in MDCK epithelial cells with stable overexpression/downregulation of PV. Increased PV levels resulted in smaller, roundish cells and shorter mitochondria, the latter phenomenon related to reduced fusion rates and decreased expression of genes involved in mitochondrial fusion. PV-overexpressing cells displayed increased mitophagy, a likely cause for the decreased mitochondrial volumes and the smaller overall cell size. Cells showed lower mobility in vitro, paralleled by reduced protrusions. Constitutive PV down-regulation in PV-overexpressing cells reverted mitochondrial morphology and fractional volume to the state present in control MDCK cells, resulting from increased mitochondrial movement and augmented fusion rates. PV-modulated, bi-directional and reversible mitochondrial dynamics are key to regulation of mitochondrial volume.     

Mitochondrial bioenergetics decay in aging: beneficial effect of melatonin

Aging is a biological process characterized by progressive decline in physiological functions, increased oxidative stress, reduced capacity to respond to stresses, and increased risk of contracting age-associated disorders. Mitochondria are referred to as the powerhouse of the cell through their role in the oxidative phosphorylation to generate ATP. These organelles contribute to the aging process, mainly through impairment of electron transport chain activity, opening of the mitochondrial permeability transition pore and increased oxidative stress. These events lead to damage to proteins, lipids and mitochondrial DNA. Cardiolipin, a phospholipid of the inner mitochondrial membrane, plays a pivotal role in several mitochondrial bioenergetic processes as well as in mitochondrial-dependent steps of apoptosis and in mitochondrial membrane stability and dynamics. Cardiolipin alterations are associated with mitochondrial bienergetics decline in multiple tissues in a variety of physiopathological conditions, as well as in the aging process. Melatonin, the major product of the pineal gland, is considered an effective protector of mitochondrial bioenergetic function. Melatonin preserves mitochondrial function by preventing cardiolipin oxidation and this may explain, at least in part, the protective role of this compound in mitochondrial physiopathology and aging. Here, mechanisms through which melatonin exerts its protective role against mitochondrial dysfunction associated with aging and age-associated disorders are discussed.     

Mitochondrial network remodeling: an important feature of myogenesis and skeletal muscle regeneration

The remodeling of the mitochondrial network is a critical process in maintaining cellular homeostasis and is intimately related to mitochondrial function. The interplay between the formation of new mitochondria (biogenesis) and the removal of damaged mitochondria (mitophagy) provide a means for the repopulation of the mitochondrial network. Additionally, mitochondrial fission and fusion serve as a bridge between biogenesis and mitophagy. In recent years, the importance of these processes has been characterised in multiple tissue- and cell-types, and under various conditions. In skeletal muscle, the robust remodeling of the mitochondrial network is observed, particularly after injury where large portions of the tissue/cell structures are damaged. The significance of mitochondrial remodeling in regulating skeletal muscle regeneration has been widely studied, with alterations in mitochondrial remodeling processes leading to incomplete regeneration and impaired skeletal muscle function. Needless to say, important questions related to mitochondrial remodeling and skeletal muscle regeneration still remain unanswered and require further investigation. Therefore, this review will discuss the known molecular mechanisms of mitochondrial network remodeling, as well as integrate these mechanisms and discuss their relevance in myogenesis and regenerating skeletal muscle.

     

The ever-growing complexity of the mitochondrial fission machinery

The mitochondrial network constantly changes and remodels its shape to face the cellular energy demand. In human cells, mitochondrial fusion is regulated by the large, evolutionarily conserved GTPases Mfn1 and Mfn2, which are embedded in the mitochondrial outer membrane, and by OPA1, embedded in the mitochondrial inner membrane. In contrast, the soluble dynamin-related GTPase Drp1 is recruited from the cytosol to mitochondria and is key to mitochondrial fission. A number of new players have been recently involved in Drp1-dependent mitochondrial fission, ranging from large cellular structures such as the ER and the cytoskeleton to the surprising involvement of the endocytic dynamin 2 in the terminal abscission step. Here we review the recent findings that have expanded the mechanistic model for the mitochondrial fission process in human cells and highlight open questions.     

Melatonin,mitochondria, and the cancer cell

The long-recognized fact that oxidative stress within mitochondria is a hallmark of mitochondrial dysfunction has stimulated the development of mitochondria-targeted antioxidant therapies. Melatonin should be included among the pharmacological agents able to modulate mitochondrial functions in cancer, given that a number of relevant melatonin-dependent effects are triggered by targeting mitochondrial functions. Indeed, melatonin may modulate the mitochondrial respiratory chain, thus antagonizing the cancer highly glycolytic bioenergetic pathway of cancer cells. Modulation of the mitochondrial respiratory chain, together with Ca²⁺ release and mitochondrial apoptotic effectors, may enhance the spontaneous or drug-induced apoptotic processes. Given that melatonin may efficiently counteract the Warburg effect while stimulating mitochondrial differentiation and mitochondrial-based apoptosis, it is argued that the pineal neurohormone could represent a promising new perspective in cancer treatment strategy.