Hypothyroidism down-regulates mitochondrial citrate carrier activity and expression in rat liver

Effects of Thyroid Hormones on Lipid Metabolism Pathologies in Non-Alcoholic Fatty Liver Disease

The typical modern lifestyle contributes to the development of many metabolic-related disorders, as exemplified by metabolic syndrome. How to prevent, resolve, or avoid subsequent deterioration of metabolic disturbances and the development of more serious diseases has become an important and much-discussed health issue. Thus, the question of the physiological and pathological roles of thyroid hormones (THs) in metabolism has never gone out of fashion. Although THs influence almost all organs, the liver is one of the most important targets as well as the hub of metabolic homeostasis. When this homeostasis is out of balance, diseases may result. In the current review, we summarize the common features and actions of THs, first focusing on their effects on lipid metabolism in the liver. In the second half of the review, we turn to a consideration of non-alcoholic fatty liver disease (NAFLD), a disease characterized by excessive accumulation of fat in the liver that is independent of heavy alcohol consumption. NAFLD is a growing health problem that currently affects ~25% of the world’s population. Unfortunately, there are currently no approved therapies specific for NAFLD, which, if left uncontrolled, may progress to more serious diseases, such as cirrhosis or liver cancer. This absence of effective treatment can also result in the development of non-alcoholic steatohepatitis (NASH), an aggressive form of NAFLD that is the leading cause of liver transplantation in the United States. Because THs play a clear role in hepatic fat metabolism, their potential application in the prevention and treatment of NAFLD has attracted considerable research attention. Studies that have investigated the use of TH-related compounds in the management of NAFLD are also summarized in the latter part of this review. An important take-home point of this review is that a comprehensive understanding of the physiological and pathological roles of THs in liver fat metabolism is possible, despite the complexities of this regulatory axis—an understanding that has clinical value for the specific management of NAFLD.

Differential lipids in pregnant women with subclinical hypothyroidism and their correlation to the pregnancy outcomes

Subclinical hypothyroidism (SCH) has become a prevalent complication in pregnancy. Recent research links SCH to disturbed thyroid lipid profile; however, it is unclear how lipid metabolism disorders contribute to the pathogenesis of SCH during pregnancy. Thus, we used nontargeted lipidomics to identify and compare the lipids and metabolites expressed by pregnant women with SCH and healthy pregnant women. Multivariate analysis revealed 143 lipid molecules differentially expressed between the SCH group and the control group. Based on fold change, 30 differentially expressed lipid metabolites are potential biomarkers. KEGG pathway enrichment analysis showed that the differentially expressed metabolites participate in several pathways, including response to pathogenic Escherichia coli infection, regulation of lipolysis in adipocytes, metabolic pathways, glycerophospholipid metabolism, and fat digestion and absorption pathways. Correlation analyses revealed sphingomyelin (SM) and phosphatidylcholine (PC) positively correlate to tumor necrosis factor-α (TNF-α), C-reactive protein (CRP), and interleukin-6 (IL-6), while phosphatidylglycerol (PG), and phosphatidylinositol (PI) negatively correlate with them. In addition, PG positively correlates to birth weight. Thus, the lipid profile of pregnant women with SCH is significantly different from that of healthy pregnant women. Lipid molecules associated with the differential lipid metabolism, such as SM, phosphatidylethanolamine (PE), and PI, should be further investigated for their roles in the pathogenesis of SCH in pregnancy, as they might be targets for reducing the incidence of adverse pregnancy outcomes.

Analysis of metabolomics profile in hypothyroid patients before and after thyroid hormone replacement

PURPOSE

The serum metabolic changes occurring during the transition from hypothyroidism to euthyroidism are not known. This study aimed to determine the metabolomic profile in hypothyroid patients before (HypoT₀) and after (HypoT₁) euthyroidism achieved through levothyroxine (L-T4) treatment.

METHODS

Eighteen patients with overt primary hypothyroidism were recruited for the study. All patients were treated with L-T4 to achieve euthyroidism. Thyrotropin (TSH), free thyroxine (FT4), free triiodothyronine (FT3) and metabolomics profiles were measured before and after 3 months of treatment. The euthyroid control group consisted of 28 healthy volunteers. Metabolomics analysis was performed using Nuclear Magnetic Resonance (NMR) spectroscopy.

RESULTS

¹H NMR-based metabolomics profiling of patients with newly diagnosed hypothyroidism (HypoT₀) showed significantly higher levels of citrate, creatinine, glycerol, myo-inositol and serine, and lower levels of proline and taurine compared to controls. Interestingly, some metabolic changes were persistent three months after pharmacological treatments, despite normal serum TSH and thyroid hormone concentrations (HypoT₁). When an Orthogonal Partial Least Squares Discriminant Analysis (OPLS-DA) model was built to evaluate possible differences in the metabolic profile between HypoT₀ and HypoT₁, the data obtained were not significantly different.

CONCLUSION

These results suggest that metabolic changes in the patients with hypothyroidism may persist after normalization of serum levels of FT3, FT4, and TSH, which currently represent the gold standard in laboratory testing for diagnosis and evaluation of thyroid pathology. So, the metabolomics approach may contribute to integrate classical hormone assays and to determine the euthyroid status achievement with greater efficacy.

Pathogenesis of hypothyroidism-induced NAFLD: Evidence for a distinct disease entity?

Nonalcoholic fatty liver disease (NAFLD), the most common liver disease worldwide, may be associated with primary hypothyroidism. However, the pathogenesis underlying such an association is complex and not completely understood. Here, we specifically discuss the pathogenic mechanisms potentially involved in hypothyroidism-induced NAFLD. To this end, we summarize the general pathophysiology of thyroid hormones (TH). Next, we analyze the published data from rodent studies by discussing whether hypothyroid rats may develop NAFLD via hyperphagia; whether mitochondria become energetically more efficient; what the overall energy balance is and if diversion of fatty substrates occurs; and the latest advancements in molecular pathogenesis brought about by metabolomics, cell imaging, lipophagy, autophagy and genetically engineered mouse models. Moreover, we discuss the data published regarding humans on the pathogenic role of TH, metabolic syndrome and other risk factors in hypothyroidism-related NAFLD as well as the putative mechanisms underlying the development of NAFLD-related hepatocellular carcinoma in hypothyroidism. In conclusion, although many research questions still remain unanswered, the pathophysiology of hypothyroidism-induced NAFLD makes this a potentially curable and distinct disease entity. However, further studies are needed to better elucidate the underlying mechanisms, and to ascertain whether treatment with either TH or thyromimetic agents improves NAFLD.

3,5-diiodo-L-thyronine increases de novo lipogenesis in liver from hypothyroid rats by SREBP-1 and ChREBP-mediated transcriptional mechanisms

Hepatic de novo lipogenesis (DNL), the process by which carbohydrates are converted into lipids, is strictly controlled by nutritional and hormonal status. 3,5-Diiodo-L-thyronine (T2), a product of the 3,5,3'-triiodo-L-thyronine (T3) peripheral metabolism, has been shown to mimic some T3 effects on lipid metabolism by a short-term mechanism independent of protein synthesis. Here, we report that T2, administered for 1 week to hypothyroid rats, increases total fatty acid synthesis from acetate in isolated hepatocytes. Studies carried out on liver subcellular fractions demonstrated that T2 not only increases the activity and the expression of acetyl-CoA carboxylase and fatty acid synthase but also of other proteins linked to DNL such as the mitochondrial citrate carrier and the cytosolic ATP citrate lyase. Parallelly, T2 stimulates the activities of enzymes supplying cytosolic NADPH needed for the reductive steps of DNL. With respect to both euthyroid and hypothyroid rats, T2 administration decreases the hepatic mRNA level of SREBP-1, a transcription factor which represents a master regulator of DNL. However, when compared to hypothyroid rats T2 significantly increases, without bringing to the euthyroid value, the content of both mature (nSREBP-1), and precursor (pSREBP-1) forms of the SREBP-1 protein as well as their ratio. Moreover, T2 administration strongly augmented the nuclear content of ChREBP, another crucial transcription factor involved in the regulation of lipogenic genes. Based on these results, we can conclude that in the liver of hypothyroid rats the transcriptional activation by T2 of DNL genes could depend, at least in part, on SREBP-1- and ChREBP-dependent mechanisms. © 2019 IUBMB Life, 2019.

Action of Thyroid Hormones, T3 and T2, on Hepatic Fatty Acids: Differences in Metabolic Effects and Molecular Mechanisms

The thyroid hormones (THs) 3,3′,5,5′-tetraiodo-l-thyronine (T4) and 3,5,3′-triiodo-l-thyronine (T3) influence many metabolic pathways. The major physiological function of THs is to sustain basal energy expenditure, by acting primarily on carbohydrate and lipid catabolism. Beyond the mobilization and degradation of lipids, at the hepatic level THs stimulate the de novo fatty acid synthesis (de novo lipogenesis, DNL), through both the modulation of gene expression and the rapid activation of cell signalling pathways. 3,5-Diiodo-l-thyronine (T2), previously considered only a T3 catabolite, has been shown to mimic some of T3 effects on lipid catabolism. However, T2 action is more rapid than that of T3, and seems to be independent of protein synthesis. An inhibitory effect on DNL has been documented for T2. Here, we give an overview of the mechanisms of THs action on liver fatty acid metabolism, focusing on the different effects exerted by T2 and T3 on the regulation of the DNL. The inhibitory action on DNL exerted by T2 makes this compound a potential and attractive drug for the treatment of some metabolic diseases and cancer.

Dietary long-chain unsaturated fatty acids acutely and differently reduce the activities of lipogenic enzymes and of citrate carrier in rat liver

The activities of lipogenic enzymes appear to fluctuate with changes in the level and type of dietary fats. Polyunsaturated fatty acids (PUFAs) are known to induce on hepatic de novo lipogenesis (DNL) the highest inhibitory effect, which occurs through a long-term adaptation. Data on the acute effects of dietary fatty acids on DNL are lacking. In this study with rats, the acute 1-day effect of high-fat (15 % w/w) diets (HFDs) enriched in saturated fatty acids (SFAs) or unsaturated fatty acids (UFAs), i.e., monounsaturated (MUFA) and PUFA, of the ω-6 and ω-3 series on DNL and plasma lipid level was investigated; a comparison with a longer time feeding (21 days) was routinely carried out. After 1-day HFD administration UFA, when compared to SFA, reduced plasma triacylglycerol (TAG) level and the activities of the lipogenic enzymes acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS), a decreased activity of the citrate carrier (CIC), a mitochondrial protein linked to lipogenesis, was also detected. In this respect, ω-3 PUFA was the most effective. On the other hand, PUFA maintained the effects at longer times, and the acute inhibition induced by MUFA feeding on DNL enzyme and CIC activities was almost nullified at 21 days. Mitochondrial fatty acid composition was slightly but significantly changed both at short- and long-term treatment, whereas the early changes in mitochondrial phospholipid composition vanished in long-term experiments. Our results suggest that in the early phase of administration, UFA coordinately reduced both the activities of de novo lipogenic enzymes and of CIC. ω-3 PUFA showed the greatest effect.

Nutritional and Hormonal Regulation of Citrate and Carnitine/Acylcarnitine Transporters: Two Mitochondrial Carriers Involved in Fatty Acid Metabolism

The transport of solutes across the inner mitochondrial membrane is catalyzed by a family of nuclear-encoded membrane-embedded proteins called mitochondrial carriers (MCs). The citrate carrier (CiC) and the carnitine/acylcarnitine transporter (CACT) are two members of the MCs family involved in fatty acid metabolism. By conveying acetyl-coenzyme A, in the form of citrate, from the mitochondria to the cytosol, CiC contributes to fatty acid and cholesterol synthesis; CACT allows fatty acid oxidation, transporting cytosolic fatty acids, in the form of acylcarnitines, into the mitochondrial matrix. Fatty acid synthesis and oxidation are inversely regulated so that when fatty acid synthesis is activated, the catabolism of fatty acids is turned-off. Malonyl-CoA, produced by acetyl-coenzyme A carboxylase, a key enzyme of cytosolic fatty acid synthesis, represents a regulator of both metabolic pathways. CiC and CACT activity and expression are regulated by different nutritional and hormonal conditions. Defects in the corresponding genes have been directly linked to various human diseases. This review will assess the current understanding of CiC and CACT regulation; underlining their roles in physio-pathological conditions. Emphasis will be placed on the molecular basis of the regulation of CiC and CACT associated with fatty acid metabolism.

Hypothyroidism is a Risk Factor for New-Onset Diabetes: A Cohort Study

OBJECTIVE

To identify risk factors for the development of statin-associated diabetes mellitus (DM).

RESEARCH DESIGN AND METHODS

The study was conducted in two phases. Phase one involved high-throughput in silico processing of a large amount of biomedical data to identify risk factors for the development of statin-associated DM. In phase two, the most prominent risk factor identified was confirmed in an observational cohort study at Clalit, the largest health care organization in Israel. Time-dependent Poisson regression multivariable models were performed to assess rate ratios (RRs) with 95% CIs for DM occurrence.

RESULTS

A total of 39,263 statin nonusers were matched by propensity score to 20,334 highly compliant statin initiators in 2004-2005 and followed until the end of 2010. Within 59,597 statin users and nonusers in a multivariable model, hypothyroidism and subclinical hypothyroidism carried an increased risk for DM (RR 1.53 [95% CI 1.31-1.79] and 1.75 [1.40-2.18], respectively). Hypothyroidism increased DM risk irrespective of statin treatment (RR 2.06 [1.42-2.99] and 1.66 [1.05-2.64] in statin users and nonusers, respectively). Subclinical hypothyroidism risk for DM was prominent only upon statin use (RR 1.94 [1.13-3.34] and 1.20 [0.52-2.75] in statin users and nonusers, respectively). Patients with hypothyroidism treated with thyroid hormone replacement therapy were not at increased risk for DM.

CONCLUSIONS

Hypothyroidism is a risk factor for DM. Subclinical hypothyroidism-associated risk for DM is prominent only upon statin use. Identifying and treating hypothyroidism and subclinical hypothyroidism might reduce DM risk. Future clinical studies are needed to confirm the findings.

The WOMED model of benign thyroid disease: Acquired magnesium deficiency due to physical and psychological stressors relates to dysfunction of oxidative phosphorylation

BACKGROUND

The aim of this study was to discern whether a relation between biochemical parameters, sonography and musculoskeletal data exists in cases of hyperthyroidism and whether they are modifiable through supplementation with selenomethionine and magnesium citrate as well as by acupuncture and manual medicine methods.

RESULTS

A direct correlation between whole blood selenium and serum magnesium was found in subjects without thyroid disease and in menopausal women while it was reversed in cases of thyroid diseases as well as in patients with depression, infection, and in infertile women. Vascularization indices were elevated in cases of newly diagnosed benign thyroid diseases. Musculoskeletal changes i.e. lateral tension and idiopathic moving toes, as well as situations of physical and psychological stress and minor trauma and infection led to an increase of vascularization. Magnesium levels correlated negatively with these two conditions. The supplementation brought a reduction of the vascularization indices and reduced the incidence of idiopathic moving toes. Treatment of lateral tension required manual medicine methods and acupuncture (gastrocnemius). A small subgroup of patients showed a further reduction of hyper-vascularization after receiving coenzyme Q10.

CONCLUSIONS

We interpret the elevated thyroid vascularization and low magnesium levels as signs of an inflammatory process related to the musculoskeletal changes. Improvement of thyroid function and morphology can be achieved after correcting the influence of stressors together with the supplementation regime. We hypothesize that the central biochemical event in thyroid disease is that of an acquired, altered mitochondrial function due to deficiency of magnesium, selenium, and coenzyme Q10.

Physiological and pathological roles of mitochondrial SLC25 carriers

The mitochondrion relies on compartmentalization of certain enzymes, ions and metabolites for the sake of efficient metabolism. In order to fulfil its activities, a myriad of carriers are properly expressed, targeted and folded in the inner mitochondrial membrane. Among these carriers, the six-transmembrane-helix mitochondrial SLC25 (solute carrier family 25) proteins facilitate transport of solutes with disparate chemical identities across the inner mitochondrial membrane. Although their proper function replenishes building blocks needed for metabolic reactions, dysfunctional SLC25 proteins are involved in pathological states. It is the purpose of the present review to cover the current knowledge on the role of SLC25 transporters in health and disease.

Many faces of mitochondrial uncoupling during age: damage or defense?

An increased mitochondrial proton leak occurs in aging, but the origin of such modification remains unclear. This study defined the cause of mitochondrial uncoupling in mitotic (liver) and postmitotic (heart) rat tissues during aging and its effects on energy homeostasis and free radical production. Proton leak in old heart mitochondria was dependent on uncoupling proteins' upregulation, whereas it was caused by alterations in the mitochondrial membrane composition in old liver. ATP homeostasis was impaired in both tissues from old animals and was associated to disrupted F0F1-ATPase activity. H2O2 production rate and 4-hydroxy-2-nonenalprotein adducts were higher in old liver mitochondria compared with young liver mitochondria, but they were similar in heart mitochondria from both groups. Moreover, key mitochondrial biogenesis regulators were upregulated in old liver but downregulated in old heart. In conclusion, uncoupling proteins mediate proton leak and avoid oxidative damage in heart, acting as a protective mechanism. This does not occur in liver, where ATP depletion and oxidative stress may stimulate mitochondrial biogenesis and eliminate damaged cells.

Transcriptional Regulation of the Mitochondrial Citrate and Carnitine/Acylcarnitine Transporters: Two Genes Involved in Fatty Acid Biosynthesis and β-oxidation

Transcriptional regulation of genes involved in fatty acid metabolism is considered the major long-term regulatory mechanism controlling lipid homeostasis. By means of this mechanism, transcription factors, nutrients, hormones and epigenetics control not only fatty acid metabolism, but also many metabolic pathways and cellular functions at the molecular level. The regulation of the expression of many genes at the level of their transcription has already been analyzed. This review focuses on the transcriptional control of two genes involved in fatty acid biosynthesis and oxidation: the citrate carrier (CIC) and the carnitine/ acylcarnitine/carrier (CAC), which are members of the mitochondrial carrier gene family, SLC25. The contribution of tissue-specific and less tissue-specific transcription factors in activating or repressing CIC and CAC gene expression is discussed. The interaction with drugs of some transcription factors, such as PPAR and FOXA1, and how this interaction can be an attractive therapeutic approach, has also been evaluated. Moreover, the mechanism by which the expression of the CIC and CAC genes is modulated by coordinated responses to hormonal and nutritional changes and to epigenetics is highlighted.

3,5-Diiodo-L-thyronine administration to hypothyroid rats rapidly enhances fatty acid oxidation rate and bioenergetic parameters in liver cells

Growing evidence shows that, among triiodothyronine derivatives, 3,5 diiodo-L-thyronine (T(2)) plays an important role in energy metabolism and fat storage. In the present study, short-term effects of T(2) administration to hypothyroid rats on fatty acid oxidation rate and bioenergetic parameters were investigated. Within 1 h following T(2) injection, state 3 and state 4 respiration rates, which were reduced in hypothyroid mitochondria, were noticeably increased particularly in succinate- with respect to glutamate/malate-energized mitochondria. Maximal respiratory activity, observed when glutamate/malate/succinate were simultaneously present in the respiratory medium, was significantly stimulated by T(2) treatment. A T(2)-induced increase in respiratory rates was also observed when palmitoyl-CoA or L-palmitoylcarnitine were used as substrates. No significant change in respiratory control index and ADP/O ratio was observed. The activities of the mitochondrial respiratory chain complexes, especially Complex II, were increased in T(2)-treated rats. In the latter, Complex V activities, assayed in both ATP synthesis and hydrolysis direction, were enhanced. The rate of fatty acid oxidation, followed by conversion of [(14)C]palmitate to CO(2) and ketone bodies, was higher in hepatocytes isolated from T(2)-treated rats. This increase occurs in parallel with the raise in the activity of carnitine palmitoyltransferase-I, the rate limiting enzyme of fatty acid β-oxidation, assayed in situ in digitonin-permeabilized hepatocytes. Overall, these results indicate that T(2) rapidly increases the ability of mitochondria to import and oxidize fatty acids. An emerging idea in the literature is the ability of T(2) to reduce adiposity and dyslipidemia and to prevent the development in liver steatosis. The results of the present study, showing a rapid T(2)-induced increase in the ability of mitochondria to import and oxidize fatty acids, may contribute to understand the biochemical mechanisms of T(2)-metabolic effects.

3,5,3'triiodo-L-thyronine induces SREBP-1 expression by non-genomic actions in human HEP G2 cells

Liver is an important target for thyroid hormone actions. T(3) exerts its effects by two mechanisms: (i) Genomic actions consisting of T(3) link to nuclear receptors that bind responsive elements in the promoter of target genes, (ii) non-genomic actions including integrin αvb3 receptor-mediated MAPK/ERK and PI3K/Akt/mTOR-C1 activation. SREBP-1a, SREBP-1c, and SREBP-2 are transcription factors involved in the regulation of lipogenic genes. We show in Hep G2 cells that T(3) determined a dose- and time-dependent increase in the level of the precursor form of SREBP-1 without affecting SREBP-1 mRNA abundance. T(3) also induced phosphorylation of ERK1/2, Akt and of mTOR-C1 target S6K-P70, and the cytosol-to-membrane translocation of PKC-α. Modulation of SREBP-1 protein level by T(3) was dependent on MAPK/ERK, PI3K/Akt/mTOR-C1 pathway activation since the MEK inhibitor PD98059 or the PI3K inhibitor LY294002 abolished the stimulatory effect of T(3) . Conversely, the effect of T(3) on SREBP-1 level was enhanced by using rapamycin, mTOR-C1 inhibitor. These data suggest a negative control of mTOR-C1 target S6K-P70 on PI3K/Akt pathway. The effect of T(3) on SREBP-1 content increased also by using PKC inhibitors. These inhibitors increased the action of T(3) on Akt phosphorylation suggesting that conventional PKCs may work as negative regulators of the T(3) -dependent SREBP-1 increase. T(3) effects were partially abrogated by tetrac, an inhibitor of the T(3) -αvβ3 receptor interaction and partially evoked by T(3) analog T(3) -agarose. These findings support a model in which T(3) activates intracellular signaling pathways which may be involved in the increment of SREBP-1 level through an IRES-mediated translation mechanism.

Mitochondrial oxidative stress and respiratory chain dysfunction account for liver toxicity during amiodarone but not dronedarone administration

The role played by oxidative stress in amiodarone-induced mitochondrial toxicity is debated. Dronedarone shows pharmacological properties similar to those of amiodarone but several differences in terms of toxicity. In this study, we analyzed the effects of the two drugs on liver mitochondrial function by administering an equivalent human dose to a rat model. Amiodarone increased mitochondrial H(2)O(2) synthesis, which in turn induced cardiolipin peroxidation. Moreover, amiodarone inhibited Complex I activity and uncoupled oxidative phosphorylation, leading to a reduction in the hepatic ATP content. We also observed a modification of membrane phospholipid composition after amiodarone administration. N-acetylcysteine completely prevented such effects. Although dronedarone shares with amiodarone the capacity to induce uncoupling of oxidative phosphorylation, it did not show any of the oxidative effects and did not impair mitochondrial bioenergetics. Our data provide important insights into the mechanism of mitochondrial toxicity induced by amiodarone. These results may greatly influence the clinical application and toxicity management of these two antiarrhythmic drugs.

3,5-diiodo-L-thyronine increases FoF1-ATP synthase activity and cardiolipin level in liver mitochondria of hypothyroid rats

Short-term effects of 3,5-L-diiodothyronine (T(2)) administration to hypothyroid rats on F(o)F(1)-ATP synthase activity were investigated in liver mitochondria. One hour after T(2) injection, state 4 and state 3 respiration rates were noticeably stimulated in mitochondria subsequently isolated. F(o)F(1)-ATP synthase activity, which was reduced in mitochondria from hypothyroid rats as compared to mitochondria from euthyroid rats, was significantly increased by T(2) administration in both the ATP-synthesis and hydrolysis direction. No change in β-subunit mRNA accumulation and protein amount of the α-β subunit of F(o)F(1)-ATP synthase was found, ruling out a T(2) genomic effect. In T(2)-treated rats, changes in the composition of mitochondrial phospholipids were observed, cardiolipin (CL) showing the greatest alteration. In mitochondria isolated from hypothyroid rats the decrease in the amount of CL was accompanied by an increase in the level of peroxidised CL. T(2) administration to hypothyroid rats enhanced the level of CL and decreased the amount of peroxidised CL in subsequently isolated mitochondria, tending to restore the CL value to the euthyroid level. Minor T(2)-induced changes in mitochondrial fatty acid composition were detected. Overall, the enhanced F(o)F(1)-ATP synthase activity observed following injection of T(2) to hypothyroid rats may be ascribed, at least in part, to an increased level of mitochondrial CL associated with decreased peroxidation of CL.

Oxidation of hepatic carnitine palmitoyl transferase-I (CPT-I) impairs fatty acid beta-oxidation in rats fed a methionine-choline deficient diet

There is growing evidence that mitochondrial dysfunction, and more specifically fatty acid β-oxidation impairment, is involved in the pathophysiology of non-alcoholic steatohepatitis (NASH). The goal of the present study was to achieve more understanding on the modification/s of carnitinepalmitoyltransferase-I (CPT-I), the rate-limiting enzyme of the mitochondrial fatty acid β-oxidation, during steatohepatitis. A high fat/methionine-choline deficient (MCD) diet, administered for 4 weeks, was used to induce NASH in rats.We demonstrated that CPT-I activity decreased, to the same extent, both in isolated liver mitochondria and in digitonin-permeabilized hepatocytes from MCD-diet fed rats.At the same time, the rate of total fatty acid oxidation to CO(2) and ketone bodies, measured in isolated hepatocytes, was significantly lowered in treated animals when compared to controls. Finally, an increase in CPT-I mRNA abundance and protein content, together with a high level of CPT-I protein oxidation was observed in treated rats. A posttranslational modification of rat CPT-I during steatohepatitis has been here discussed.

Retinoylation reactions are inversely related to the cardiolipin level in testes mitochondria from hypothyroid rats

The effect of hypothyroidism, induced by 6-n-propyl-2-thiouracil (PTU) administration to rats, on the retinoylation reaction and oxidative status was investigated in rat-testes mitochondria. In hypothyroid mitochondria, when compared to euthyroid controls, we found a noticeable increase in the amount of all-trans-retinoic acid (atRA) bound to mitochondrial proteins by an acylation process (34.2 +/- 1.9 pmoles atRA/mg protein/360 min and 22.2 +/- 1.7 pmoles atRA/mg protein/360 min, respectively). This increase, which was time- and temperature-dependent, was accompanied by a strong reduction in the cardiolipin (CL) amount in the mitochondrial membranes of hypothyroid (2.6 +/- 0.2%) as compared to euthyroid rats (4.5 +/- 0.5%) Conversely, a decreased retinoylation reaction was observed when CL liposomes were added to mitochondria or mitoplasts from both euthyroid and hypothyroid rats, thus confirming a role of CL in the retinoylation process. In mitochondria from the latter animals an increase of the level of oxidized CL occurred. The ATP level, which was reduced in hypothyroid mitochondria (27.3 +/- 4.1 pmoles ATP/mg protein versus 67.1 +/- 8.3 pmoles ATP/mg protein of euthyroid animals), was surprisingly increased in mitochondria by the retinoylation reaction in the presence of 100 nM atRA (481.5 +/- 19.3 pmoles ATP/mg protein of hypothyroid animals versus 84.7 +/- 7.7 pmoles ATP/mg protein of euthyroid animals). Overall, in hypothyroid rat-testes mitochondria the increase in retinoylation activity correlates with a significant depletion of the CL level, due to a peroxidation of this lipid. In addition, an enhanced production of reactive oxygen species was observed.

Reduced activity and expression of mitochondrial citrate carrier in streptozotocin-induced diabetic rats

Citrate carrier (CiC), an integral protein of the mitochondrial inner membrane, plays an important role in hepatic intermediary metabolism, supplying the cytosol with acetyl-coenzyme A for fatty acid and cholesterol synthesis. Here, the effect of streptozotocin-induced diabetes on CiC activity and expression in rat liver was investigated. The rate of citrate transport was reduced by about 35% in mitochondria from diabetic vs. control rats. Kinetic studies in mitochondria from diabetic rats showed a reduction in maximum velocity and almost unchanged Michaelis-Menten constant of the CiC protein. Mitochondrial phospholipid amount was not significantly affected, whereas an increase in the cholesterol content and in the cholesterol/phospholipid ratio was observed. To thoroughly investigate the mechanism responsible for the reduced CiC activity in the diabetic state, molecular studies were performed. Ribonuclease protection assays and Western blotting analysis indicated that both hepatic CiC mRNA accumulation and protein level decreased similarly to the CiC activity. The reduced mRNA level and the lower content of the mitochondrial CiC protein, might account for the decline of CiC activity in diabetic animals. To discriminate between the role played by hyperglycemia from that of hypoinsulinemia in the reduction of CiC activity and expression, studies were conducted administrating phlorizin or insulin to streptozotocin-diabetic rats. Our data indicated that both insulin and glucose affect CiC activity and expression in diabetic rats, although they act at different regulatory steps.

A silybin-phospholipid complex prevents mitochondrial dysfunction in a rodent model of nonalcoholic steatohepatitis

Mitochondrial dysfunction and oxidative stress are determinant events in the pathogenesis of nonalcoholic steatohepatitis. Silybin has shown antioxidant, anti-inflammatory, and antifibrotic effects in chronic liver disease. We aimed to study the effect of the silybin-phospholipid complex (SILIPHOS) on liver redox balance and mitochondrial function in a dietary model of nonalcoholic steatohepatitis. To accomplish this, glutathione oxidation, mitochondrial oxygen uptake, proton leak, ATP homeostasis, and H(2)O(2) production rate were evaluated in isolated liver mitochondria from rats fed a methionine- and choline-deficient (MCD) diet and the MCD diet plus SILIPHOS for 7 and 14 weeks. Oxidative proteins, hydroxynonenal (HNE)- and malondialdehyde (MDA)-protein adducts, and mitochondrial membrane lipid composition were also measured. Treatment with SILIPHOS limited glutathione depletion and mitochondrial H(2)O(2) production. Moreover, SILIPHOS preserved mitochondrial bioenergetics and prevented mitochondrial proton leak and ATP reduction. Finally, SILIPHOS limited the formation of HNE- and MDA-protein adducts. In conclusion, SILIPHOS is effective in preventing severe oxidative stress and preserving hepatic mitochondrial bioenergetics in nonalcoholic steatohepatitis induced by the MCD diet. The modifications of mitochondrial membrane fatty acid composition induced by the MCD diet are partially prevented by SILIPHOS, conferring anti-inflammatory and antifibrotic effects. The increased vulnerability of lipid membranes to oxidative damage is limited by SILIPHOS through preserved mitochondrial function.

The mitochondrial citrate carrier: metabolic role and regulation of its activity and expression

The citrate carrier (CiC), a nuclear-encoded protein located in the mitochondrial inner membrane, is a member of the mitochondrial carrier family. CiC plays an important role in hepatic lipogenesis, which is responsible for the efflux of acetyl-CoA from the mitochondria to the cytosol in the form of citrate, the primer for fatty acid and cholesterol synthesis. In addition, CiC is a key component of the isocitrate-oxoglutarate and the citrate-malate shuttles. CiC has been purified from various species and its reconstituted function characterized as well as its cDNA isolated and sequenced. CiC mRNA and/or CiC protein levels are high in liver, pancreas, and kidney, but are low or absent in brain, heart, skeletal muscle, placenta, and lungs. A reduction of CiC activity was found in diabetic, hypothyroid, starved rats, and in rats fed on a polyunsaturated fatty acid (PUFA)-enriched diet. Molecular analysis suggested that the regulation of CiC activity occurs mainly through transcriptional and post-transcriptional mechanisms. This review begins with an assessment of the current understanding of CiC structural and biochemical characteristics, underlying the structure-function relationship. Emphasis will be placed on the molecular basis of the regulation of CiC activity in coordination with fatty acid synthesis.