Fatty acid oxidation in embryonic chick tissues

Transcriptional profiling of liver in riboflavin-deficient chicken embryos explains impaired lipid utilization, energy depletion, massive hemorrhaging, and delayed feathering

BACKGROUND

A strain of Leghorn chickens (rd/rd), unable to produce a functional riboflavin-binding protein, lays riboflavin-deficient eggs, in which all embryos suddenly die at mid-incubation (days 13-15). This malady, caused by riboflavin deficiency, leads to excessive lipid accumulation in liver, impaired β-oxidation of lipid, and severe hypoglycemia prior to death. We have used high-density chicken microarrays for time-course transcriptional scans of liver in chicken embryos between days 9-15 during this riboflavin-deficiency-induced metabolic catastrophe. For comparison, half of rd/rd embryos (n = 16) were rescued from this calamity by injection of riboflavin just prior to incubation of fertile eggs from rd/rd hens.

RESULTS

No significant differences were found between hepatic transcriptomes of riboflavin-deficient and riboflavin-rescued embryos at the first two ages (days 9 and 11). Overall, we found a 3.2-fold increase in the number of differentially expressed hepatic genes between day 13 (231 genes) and day 15 (734 genes). Higher expression of genes encoding the chicken flavoproteome was more evident in rescued- (15 genes) than in deficient-embryos (4 genes) at day 15. Diminished activity of flavin-dependent enzymes in riboflavin-deficient embryos blocks catabolism of yolk lipids, which normally serves as the predominant source of energy required for embryonic development.

CONCLUSIONS

Riboflavin deficiency in mid-stage embryos leads to reduced expression of numerous genes controlling critical functions, including β-oxidation of lipids, blood coagulation and feathering. Surprisingly, reduced expression of feather keratin 1 was found in liver of riboflavin-deficient embryos at e15, which could be related to their delayed feathering and sparse clubbed down. A large number of genes are expressed at higher levels in liver of riboflavin-deficient embryos; these up-regulated genes control lipid storage/transport, gluconeogenesis, ketogenesis, protein catabolism/ubiquitination and cell death.

Effects of metabolic inhibition on conduction, Ca transients, and arrhythmia vulnerability in embryonic mouse hearts

Developing myocardium is more dependent on glycolysis than adult myocardium, yet the effects of selectively inhibiting glycolysis versus oxidative phosphorylation on embryonic heart function have not been well characterized. Accordingly, we investigated how selective metabolic inhibition affects membrane voltage and intracellular Ca (Ca(i)) transients in embryonic mouse hearts, including their susceptibility to arrhythmias. A total of 136 isolated embryonic mouse hearts were exposed to either 1) 2-deoxyglucose (2DG; 10 mM) or iodoacetate (IAA; 0.1 mM) with 10 mM pyruvate in place of glucose to selectively inhibit glycolysis or 2) the mitochondrial uncoupler protonophore carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP; 500 nM) with 10 mM glucose present to selectively inhibit oxidative phosphorylation. Using confocal imaging, we found that mitochondrial membrane potential monitored with tetramethylrhodamine methyl ester (200 nM) remained stable with 2DG or IAA but depolarized within 5 min after exposure to FCCP. IAA and FCCP decreased heart rate, inhibited Ca(i) transient amplitude, shortened action potential duration at 80% repolarization (APD(80)), and prolonged atrioventricular conduction time to similar extents. Although 2DG decreased heart rate and Ca(i) transient amplitude, it did not significantly affect APD(80) and AV conduction time. In addition, spontaneous arrhythmias occurred in 77 of 136 embryonic hearts (57%) after exposure to IAA (28/53) or FCCP (49/83). There were no significant differences in the types or incidence of arrhythmias induced by IAA and FCCP. These data support the idea that both glycolysis and oxidative phosphorylation play critical metabolic roles in regulating cardiac function in the embryonic mouse heart.

Mitochondrial effects with ceramide-induced cardiac apoptosis are different from those of palmitate

The objective of this study was to evaluate whether ceramide, palmitate, and inhibitors of mitochondrial electron transport chain shared similar effects on the mitochondria of intact cardiomyocytes in order to determine the likelihood that ceramide and palmitate utilize similar mitochondrial mechanisms or pathways to apoptosis. In embryonic chick cardiomyocytes, ceramide, 100 microM for 24h, induced a 42.9+/-5.8% increase in cell death assessed by the MTT assay, and a significant (P<0.01) 3.9+/-0.6-fold increase in apoptosis assessed by propidium iodide staining of permeabilized cells. Mitochondrial potential (delta psi (m)), as demonstrated microscopically and by flow cytometry of cardiomyocytes stained with a J-aggregate dye, was markedly and significantly reduced by ceramide, palmitate, and two different inhibitors of the mitochondrial electron transport chain-rotenone and antimycin A. In contrast, the effect on mitochondria as assessed by CMX-Ros oxidation was dramatically different, as palmitate, rotenone, and antimycin A each produced a reduction, while ceramide increased CMX-Ros fluorescence. Further ceramide-induced cardiomyocyte apoptosis and loss of delta psi (m) operated through a cyclosporine-insensitive pathway similar to rotenone and antimycin A but distinct from palmitate which induced apoptosis though a cyclosporine-sensitive mechanism in these cells. These data suggest that ceramide acts on the mitochondria of intact cells through a cyclosporine-insensitive mechanism likely from a combination of actions including production of mitochondrial oxidants. The discordant findings between ceramide and palmitate suggest that palmitate-induced cell death is not primarily mediated by de novo ceramide synthesis.

Lovastatin does not accentuate but is rather additive to palmitate-induced apoptosis in cardiomyocytes

Fatty acids such as palmitate have been observed to induce apoptosis in cardiomyocytes but the mechanism of this cytotoxicity is unresolved. The present study sought to determine whether an aspect of fatty acid metabolism is responsible for palmitate-induced apoptosis in cardiomyocytes. As palmitate metabolism increases acetyl CoA production via increased beta oxidation within the mitochondria, we hypothesized that increased acetyl CoA entering the cholesterol biosynthesis pathway might produce intermediates or end products that would be toxic to the cell. To test this hypothesis, cardiomyocytes from embryonic chick cardiomyocytes were treated with the 3-hydroxy-3-methylgutaryl CoA (HMG-CoA) reductase inhibitor lovastatin that inhibits the cholesterol biosynthesis pathway downstream of the acetyl CoA trimerization into HMG-CoA. Lovastatin did not inhibit palmitate-induced apoptosis. Rather, lovastatin induced significant apoptosis itself and when combined with palmitate, the level of apoptosis was equal to the sum of palmitate alone and lovastatin alone. This observation suggests that palmitate and lovastatin are inducing apoptosis by two independent mechanisms. A role for mitochondrial metabolism via carnitine palmitoyl transferase (CPT) in palmitate-induced apoptosis was suggested since capric acid, a fatty acid that is metabolized within the mitochondria but does not utilize CPT-1, did not induce apoptosis. Palmitate-induced apoptosis was further related to the metabolism of saturated fatty acids as the unsaturated fatty acid oleic acid did not induce apoptosis. These data suggest that a unique feature about palmitate metabolism independent of its role in cholesterol biosynthesis is responsible for palmitate-induced apoptosis and the effects of palmitate are additive to those of lovastatin to induce cardiac apoptosis.

Oxidative and glycogenolytic cCapacities within the developing chick heart

Cardiac morphogenesis and function are known to depend on both aerobic and anaerobic energy-producing pathways. However, the relative contribution of mitochondrial oxidation and glycogenolysis, as well as the determining factors of oxygen demand in the distinct chambers of the embryonic heart, remains to be investigated. Spontaneously beating hearts isolated from stage 11, 20, and 24HH chick embryos were maintained in vitro under controlled metabolic conditions. O(2) uptake and glycogenolytic rate were determined in atrium, ventricle, and conotruncus in the absence or presence of glucose. Oxidative capacity ranged from 0.2 to 0.5 nmol O(2)/(h.microg protein), did not depend on exogenous glucose, and was the highest in atria at stage 20HH. However, the highest reserves of oxidative capacity, assessed by mitochondrial uncoupling, were found at the youngest stage and in conotruncus, representing 75 to 130% of the control values. At stage 24HH, glycogenolysis in glucose-free medium was 0.22, 0.17, and 0.04 nmol glucose U(h.microg protein) in atrium, ventricle, and conotruncus, respectively. Mechanical loading of the ventricle increased its oxidative capacity by 62% without altering glycogenolysis or lactate production. Blockade of glycolysis by iodoacetate suppressed lactate production but modified neither O(2) nor glycogen consumption in substrate-free medium. These findings indicate that atrium is the cardiac chamber that best utilizes its oxidative and glycogenolytic capacities and that ventricular wall stretch represents an early and major determinant of the O(2) uptake. Moreover, the fact that O(2) and glycogen consumptions were not affected by inhibition of glyceraldehyde-3-phosphate dehydrogenase provides indirect evidence for an active glycerol-phosphate shuttle in the embryonic cardiomyocytes.

Acid soluble, short chain esterified and free carnitine in the liver, heart, muscle and brain of pre and post hatched chicks

1. The behaviour of total acid soluble, short chain esterified and free carnitine in the liver, heart, muscle and brain of chick embryos between 11th and 21st day of development and of 8 and 180-day-old chicks is described. 2. Total acid soluble carnitine fluctuates around the same levels in the brain, liver and muscle until 18th day of development, whereas it attains a peak on that day in the heart. At hatching compared to 18th day, it suddenly increases three times in the muscle, drops not significantly in the heart and brain, but sharply in the liver (-40%). However the levels are always higher than those of the grown chick in the brain but lower in the other tissues. 3. Free carnitine levels are almost constant in all tissues during the embryonic life; if compared to adult ones, they are very much lower in the liver, heart and muscle, but higher in the brain, even in 8 day-old chick. 4. Short chain esterified, carnitine reaches a maximum on 18th day of egg incubation in the liver, brain and heart; in the muscle it stays on constant levels until this day and then rapidly increases so that at hatching it doubles the values. 5. The short chain esterified to free carnitine percentage ratio peaks in all tissues on 18th day of development, attaining figures which are well above those determined in the grown chick.

The effects of L- and D-carnitine administration on cardiovascular development of the chick embryo

A single 1.0-ml volume of L- or D-carnitine solution, at several selected mmole concentrations, was applied to the extraembryonic membranes of 3- and 4-day chick embryos in ovo. Hamburger-Hamilton stages of chick development ranged from 17 to 23. During the 17-18th days of incubation, embryos were dissected, and both survival and intracardiac anomaly rates were determined. Only at extremely high doses, both stereoisomers of carnitine exhibited a statistically significant toxigenic effect (p less than 0.001) as measured by a sharp decrease in survival rate when compared to chick Ringer's saline controls. Furthermore, since the anomaly rates became significant only near the LD50's, this indicated that intracardiac anomalies were induced only at toxic doses. Therefore, it is suggested that cardiovascular teratogenicity may be the result of toxicity. Below the LD50, anomaly rates were not significantly different from those of control embryos. In comparison, L- and D-carnitine were significantly different from one another (p less than 0.001) both in survival rate and percent affected embryos at a dose of 0.5 mmole. In summary, exogenous carnitine administration to the chick embryo does not appear to be deleterious to the developing cardiovascular system.

Changes in carnitine levels in the embryonic chick heart during development

Carnitine levels in the embryonic chick heart were measured. The amount of total carnitine, free plus short chain acyl carnitine (acid-soluble fraction), and long chain acyl carnitine (acid-insoluble fraction) were examined at days 7, 11, 17, and 21 of incubation. These concentrations were found to correspond favorably with data from previous investigators with regard to variations in palmitoylcarnitine transferase enzyme activity, mitochondrial chain elongation activity, and palmitic acid oxidation.