Pivalate affects carnitine status but causes no severe metabolic changes in rat liver

Limited Impact of Pivalate-Induced Secondary Carnitine Deficiency on Hepatic Transcriptome and Hepatic and Plasma Metabolome in Nursery Pigs

Administration of pivalate has been demonstrated to be suitable for the induction of secondary carnitine deficiency (CD) in pigs, as model objects for humans. In order to comprehensively characterize the metabolic effects of secondary CD in the liver of pigs, the present study aimed to carry out comparative analysis of the hepatic transcriptome and hepatic and plasma metabolome of a total of 12 male 5-week-old pigs administered either pivalate (group PIV, n = 6) or vehicle (group CON, n = 6) for 28 days. Pigs of group PIV had approximately 40-60% lower concentrations of free carnitine and acetylcarnitine in plasma, liver and different skeletal muscles than pigs of group CON (p < 0.05). Transcript profiling of the liver revealed 140 differentially expressed genes (DEGs) between group PIV and group CON (fold change > 1.2 or <-1.2, p-value < 0.05). Biological process terms dealing with the innate immune response were found to be enriched with the DEGs (p < 0.05). Using a targeted metabolomics approach for the simultaneous quantification of 630 metabolites, 9 liver metabolites and 18 plasma metabolites were identified to be different between group PIV and group CON (p < 0.05). Considering the limited alterations of the hepatic transcriptome and of the liver and plasma metabolome, it can be concluded that pivalate-induced secondary CD is not associated with significant hepatic metabolism changes in pigs.

Role of carnitine in the regulation of glucose homeostasis and insulin sensitivity: evidence from in vivo and in vitro studies with carnitine supplementation and carnitine deficiency

BACKGROUND

Although carnitine is best known for its role in the import of long-chain fatty acids (acyl groups) into the mitochondrial matrix for subsequent β-oxidation, carnitine is also necessary for the efflux of acyl groups out of the mitochondria. Since intracellular accumulation of acyl-CoA derivatives has been implicated in the development of insulin resistance, carnitine supplementation has gained attention as a tool for the treatment of insulin resistance. More recent studies even point toward a causative role for carnitine insufficiency in developing insulin resistance during states of chronic metabolic stress, such as obesity, which can be reversed by carnitine supplementation.

METHODS

The present review provides an overview about data from both animal and human studies reporting effects of either carnitine supplementation or carnitine deficiency on parameters of glucose homeostasis and insulin sensitivity in order to establish the less well-recognized role of carnitine in regulating glucose homeostasis.

RESULTS

Carnitine supplementation studies in both humans and animals demonstrate an improvement of glucose tolerance, in particular during insulin-resistant states. In contrast, less consistent results are available from animal studies investigating the association between carnitine deficiency and glucose intolerance. The majority of studies dealing with this question could either find no association or even reported that carnitine deficiency lowers blood glucose and improves insulin sensitivity.

CONCLUSIONS

In view of the abovementioned beneficial effect of carnitine supplementation on glucose tolerance during insulin-resistant states, carnitine supplementation might be an effective tool for improvement of glucose utilization in obese type 2 diabetic patients. However, further studies are necessary to explain the conflicting observations from studies dealing with carnitine deficiency.

A moderate excess of dietary lysine lowers plasma and tissue carnitine concentrations in pigs

This study was performed to investigate whether dietary lysine concentration influences the carnitine status of pigs. Therefore, an experiment with twenty young pigs with an average body weight of 21 kg was performed which were fed either a control diet (9.7 g lysine/kg) or a diet with a moderate excess of lysine (16.8 g lysine/kg). Concentrations of all the other amino acids did not differ between the diets. Pigs fed the high-lysine diet had lower concentrations of free and total carnitine in plasma, liver, kidney and skeletal muscle than control pigs (P<0.05). Pigs fed the high-lysine diet moreover had an increased concentration of trimethyllysine (TML), a reduced mRNA abundance of TML dioxygenase and reduced concentrations of gamma-butyrobetaine (BB) in muscle, indicating that the conversion of TML into BB in muscle was impaired. Concentrations of BB, the metabolic precursor of carnitine, in plasma, liver and kidney were also reduced in pigs fed the high-lysine diet while the activity of BB dioxygenase in kidney was not different and that in liver was even increased compared to control pigs (P<0.05). In conclusion, this study shows that a moderate dietary excess of lysine lowers plasma and tissue carnitine concentrations in pigs. Reduced concentrations of BB in liver and kidney suggest that the depressed carnitine status was likely caused by a decreased rate of carnitine synthesis due to a diminished availability of carnitine precursor, probably mainly as a result of an impaired BB formation in muscle.

Dietary oxidised fat up regulates the expression of organic cation transporters in liver and small intestine and alters carnitine concentrations in liver, muscle and plasma of rats

It has been shown that treatment of rats with clofibrate, a synthetic agonist of PPARalpha, increases mRNA concentration of organic cation transporters (OCTN)-1 and -2 and concentration of carnitine in the liver. Since oxidised fats have been demonstrated in rats to activate hepatic PPARalpha, we tested the hypothesis that they also up regulate OCTN. Eighteen rats were orally administered either sunflower-seed oil (control group) or an oxidised fat prepared by heating sunflower-seed oil, for 6 d. Rats administered the oxidised fat had higher mRNA concentrations of typical PPARalpha target genes such as acyl-CoA oxidase, cytochrome P450 4A1 and carnitine palmitoyltransferases-1A and -2 in liver and small intestine than control rats (P < 0.05). Furthermore, rats treated with oxidised fat had higher hepatic mRNA concentrations of OCTN1 (1.5-fold) and OCTN2 (3.1-fold), a higher carnitine concentration in the liver and lower carnitine concentrations in plasma, gastrocnemius and heart muscle than control rats (P < 0.05). Moreover, rats administered oxidised fat had a higher mRNA concentration of OCTN2 in small intestine (2.4-fold; P < 0.05) than control rats. In conclusion, the present study shows that an oxidised fat causes an up regulation of OCTN in the liver and small intestine. An increased hepatic carnitine concentration in rats treated with the oxidised fat is probably at least in part due to an increased uptake of carnitine into the liver which in turn leads to reduced plasma and muscle carnitine concentrations. The present study supports the hypothesis that nutrients acting as PPARalpha agonists influence whole-body carnitine homeostasis.

PPARalpha agonists up-regulate organic cation transporters in rat liver cells

It has been shown that clofibrate treatment increases the carnitine concentration in the liver of rats. However, the molecular mechanism is still unknown. In this study, we observed for the first time that treatment of rats with the peroxisome proliferator activated receptor (PPAR)-alpha agonist clofibrate increases hepatic mRNA concentrations of organic cation transporters (OCTNs)-1 and -2 which act as transporters of carnitine into the cell. In rat hepatoma (Fao) cells, treatment with WY-14,643 also increased the mRNA concentration of OCTN-2. mRNA concentrations of enzymes involved in carnitine biosynthesis were not altered by treatment with the PPARalpha agonists in livers of rats and in Fao cells. We conclude that PPARalpha agonists increase carnitine concentrations in livers of rats and cells by an increased uptake of carnitine into the cell but not by an increased carnitine biosynthesis.

Oxidative stress-mediated macromolecular damage and dwindle in antioxidant status in aged rat brain regions: Role of l-carnitine and dl-α-lipoic acid

BACKGROUND

The free radical theory of aging has significant relevance in a number of age-related neurological disorders. Too many free radicals create cellular pollution that shuts down energy levels. They have also been implicated in the loss of physiological functioning associated with the aging of post mitotic cells such as the brain. The activities of enzymatic antioxidative defenses decrease in rat brain may be possible causes of age-associated increase in oxidative damage to macromolecules.

METHODS

We determined whether DL-alpha-lipoic acid (100 mg/kg body weight/day), and L-carnitine (300 mg/kg body weight/day) together when administered for 30 days declines the rate of oxidative stress-mediated macromolecular damages such as lipid peroxidation (LPO), protein carbonyl (PCO) and DNA protein cross-links in different anatomic regions (cortex, striatum and hippocampus). The activities of antioxidant enzymes in programmed aging were evaluated in the cortex, striatum and hippocampus of young and aged rat brain regions.

RESULTS

Aged rats elicited a significant decline in the antioxidant status and increase in LPO, PCO and DNA protein cross-links as compared to young rats in all the 3 brain regions. The increase in LPO, PCO and DNA protein cross-links were (35.8%, 35.6%, 43.5%) in cortex, (32.5%, 40.3%, 29.8%) in striatum and (62.7%, 42.4%, 34.9%) in hippocampus, respectively, in aged rats as compared to young rats. Co-supplementation of carnitine and lipoic acid was found to be effective in reducing brain regional LPO, PCO and DNA protein cross-links and in increasing the activities of enzymatic antioxidants in aged rats to near normalcy.

CONCLUSION

The combination of l-carnitine and lipoic acid overcame the oxidative stress induced rate of lipid peroxidation, protein carbonyl formation, accumulation of DNA protein cross-links and deficits in antioxidant enzyme activities in various brain regions of aged rats.

Carnitine depletion in rat pups from mothers given mildronate: a model of carnitine deficiency in late fetal and neonatal life

Mildronate (3-(2,2,2,-trimethylhydrazinium)propionate), is a butyrobetaine analogue that is known to inhibit gamma-butyrobetaine hydroxylase, the enzyme catalyzing the last step of carnitine biosynthesis. When administered to adult rats it determines a systemic carnitine deficiency and may therefore serve as an animal model for human carnitine depletion. The aim of this study was to evaluate the effect of mildronate administration to pregnant and lactating rats on tissue carnitine concentrations in 4- and 13-day-old rat pups. At 14 days of gestation female rats began to receive mildronate in the diet (200 mg/kg/d) and this continued for entire lactation period. Mildronate treatment determined a large reduction of carnitine levels in the milk of lactating dams. Because organ carnitine concentrations in neonatal rats are directly related to dietary supply, pups from mildronate group had significantly depleted levels of total carnitine in serum, heart, liver, muscle, brain and pancreas relative to controls, at 4 and 13 days of age. Correspondingly, an increase in triglyceride levels was observed in liver, heart and muscle of mildronate pups. This is in agreement with a reduction of basal rate of oxidation of [U-(14)C]-palmitate to (14)CO(2) and (14)C-acid-soluble products observed in liver homogenates from carnitine-deficient pups. All functional and biochemical modifications were compatible with a carnitine deficiency status. In conclusion our results describe a model of carnitine depletion in pups, suitable for the investigation of carnitine deficiency in fetal-neonatal nutrition, without any concomitant mildronate-mediated metabolic alterations.

Control of mitochondrial beta-oxidation flux

The control of mitochondrial beta-oxidation, including the delivery of acyl moieties from the plasma membrane to the mitochondrion, is reviewed. Control of beta-oxidation flux appears to be largely at the level of entry of acyl groups to mitochondria, but is also dependent on substrate supply. CPTI has much of the control of hepatic beta-oxidation flux, and probably exerts high control in intact muscle because of the high concentration of malonyl-CoA in vivo. beta-Oxidation flux can also be controlled by the redox state of NAD/NADH and ETF/ETFH(2). Control by [acetyl-CoA]/[CoASH] may also be significant, but it is probably via export of acyl groups by carnitine acylcarnitine translocase and CPT II rather than via accumulation of 3-ketoacyl-CoA esters. The sharing of control between CPTI and other enzymes allows for flexible regulation of metabolism and the ability to rapidly adapt beta-oxidation flux to differing requirements in different tissues.

Carnitine import to isolated hepatocytes and synthesis are accelerated in pivalate-treated rats

To investigate the effect of pivalate on carnitine import and carnitine synthesis in the liver, we measured carnitine uptake in isolated rat hepatocytes with L-[(14)C] carnitine and concentrations of free carnitine, gamma-butyrobetaine and acylcarnitines using tandem mass spectrometry. Hepatocytes from rats treated with 20 mmol/L of pivalate for 4 wk had greater L-[(14)C] carnitine uptake than those of unsupplemented rats after 5, 10, 30 and 90 min. Addition of 1 mmol/L of pivalate or 1 mmol/L of pivaloylcarnitine to control cell suspensions did not affect L-[(14)C] carnitine uptake. The K(m) values for L-[(14)C] carnitine uptake for pivalate-treated rats were significantly lower than control (2.9 +/- 0.7 mmol/L for pivalate-treated rats, 6.2 +/- 1.1 mmol/L for controls). The concentration of free carnitine was not reduced in the liver of pivalate-treated rats, whereas the concentrations of acetylcarnitine and gamma-butyrobetaine were significantly lower than controls. In the heart and muscle the concentration of free carnitine was significantly lower and that of gamma-butyrobetaine was higher than controls. These results suggest that carnitine transport from plasma into the liver and synthesis in the liver are accelerated in rats with secondary carnitine deficiency induced by the administration of pivalate.