Palmitic acid-1- 14 C oxidation by skeletal muscle mitochondria of dystrophic mice

Essential role for uncoupling protein-3 in mitochondrial adaptation to fasting but not in fatty acid oxidation or fatty acid anion export

Uncoupling protein-3 (UCP3) is a mitochondrial inner membrane protein expressed most abundantly in skeletal muscle and to a lesser extent in heart and brown adipose tissue. Evidence supports a role for UCP3 in fatty acid oxidation (FAO); however, the underlying mechanism has not been explored. In 2001 we proposed a role for UCP3 in fatty acid export, leading to higher FAO rates (Himms-Hagen, J., and Harper, M. E. (2001) Exp. Biol. Med. (Maywood) 226, 78-84). Specifically, this widely held hypothesis states that during elevated FAO rates, UCP3 exports fatty acid anions, thereby maintaining mitochondrial co-enzyme A availability; reactivation of exported fatty acid anions would ultimately enable increased FAO. Here we tested mechanistic aspects of this hypothesis as well as its functional implications, namely increased FAO rates. Using complementary mechanistic approaches in mitochondria from wild-type and Ucp3(-/-) mice, we find that UCP3 is not required for FAO regardless of substrate type or supply rate covering a 20-fold range. Fatty acid anion export and reoxidation during elevated FAO, although present in skeletal muscle mitochondria, are independent of UCP3 abundance. Interestingly, UCP3 was found to be necessary for the fasting-induced enhancement of FAO rate and capacity, possibly via mitigated mitochondrial oxidative stress. Thus, although our observations indicate that UCP3 can impact FAO rates, the mechanistic basis is not via an integral function for UCP3 in the FAO machinery. Overall our data indicate a function for UCP3 in mitochondrial adaptation to perturbed cellular energy balance and integrate previous observations that have linked UCP3 to reduced oxidative stress and FAO.

Fatty acid oxidation abnormalities in childhood-onset spinal muscular atrophy: primary or secondary defect(s)?

The purpose of this study was to further identify and quantify the fatty acid oxidation abnormalities in spinal muscular atrophy, correlate these with disease severity, and identify specific underlying defect(s). Fifteen children with spinal muscular atrophy (3 type I, 8 type II, 4 type III) were studied. Serum carnitine total/free ratios demonstrated a tendency toward an increased esterified fraction ranging 35-58% of total carnitine (normal: 25-30% of total) in younger children with types I and II. The remaining type II and III patients, older than 23 months of age at sampling, had normal esterified carnitine levels. Urinary organic acid analysis demonstrated mild to moderate medium-chain dicarboxylic aciduria in type I patients and normal, mild, or moderate increases in short-chain and medium-chain organic acids in type II patients. In the type III group, the organic acids were normal except for one patient with mild medium-chain dicarboxylic aciduria. Muscle intramitochondrial beta-oxidation was measured in 5 children (2 type I, 2 type II, and 1 type III) and a significant reduction in the activities of short-chain L-3-hydroxyacyl-CoA dehydrogenase, long-chain L-3-hydroxyacyl-CoA dehydrogenase, acetoacetyl-CoA thiolase, and 3-ketoacyl-CoA thiolase were found; however, normal crotonase activity was documented. Most strikingly, there was a marked increase (3- to 5-fold) in the activity ratios of crotonase to L-3-hydroxyacyl-CoA dehydrogenase and thiolase activities with both short- and long-chain substrates. The combined abnormalities suggest a defect in a mitochondrial multifunctional enzyme complex, distinct from the trifunctional enzyme. These abnormalities may be either primary or secondary and may respond to dietary measures to reduce the dependence on fatty acid oxidation.

Studies on mitochondria from dystrophic skeletal muscle of mice

Mitochondrial respiration and oxidative phosphorylation were compared in normal and dystrophic mouse skeletal muscles. To obtain the maximum respiration control ratio (RCR) and adenosine diphosphate/oxygen (ADP/O) ratio from isolated muscle mitochondria, it is found that there is an advantage in having a low concentration of proteinase and EGTA present in the medium during preparation of mitochondria by centrifugation fractionation. Using pyruvate, acetylcarnitine, and palmitylcarnitine as substrates for oxidation, a highly significant reduction (40-60%) is shown in oxygen uptake by dystrophic muscle mitochondria as compared to normal muscle mitochondria. Studies of the integrity of the oxidative phosphorylation apparatus in these samples showed that there is a reduction of the RCR and ADP/O ratio in dystrophic muscle mitochondria as compared to normal muscle mitochondria.

Alterations in lipid incorporation in Duchenne muscular dystrophy. Studies of fresh and cultured muscle

The incorporation of [3H] glycerol into lipids of fresh and cultured skeletal muscle obtained from patients with Duchenne muscular dystrophy (DMD), patients with myotonic dystrophy (My Dyst), controls, and aborted fetuses (10-12 weeks old) was studied. A significant increase of specific incorporation of [3H] glycerol into phosphatidylcholine (PCh), phosphatidylserine (PS), phosphatidylinositol (PI), and triglycerides (TRI) was found in DMD and fetal muscle in both fresh and cultured muscle. No significant differences, however, were noted between the values of glycerol incorporation in DMD and fetal muscle. The ratio between phospholipids and TRI changed significantly for fresh muscle in DMD (3.5) and fetal muscle (4.9) versus controls (24). The incorporation of glycerol into these lipids in My Dyst fresh and cultured muscle showed the same value as controls when expressed both as incorporation/mg protein and ratio between phospholipids and TRI.

Impaired substrate utilization in mitochondria from strain 129 dystrophic mice

Mitochondria from skeletal muscle, heart and liver of strain 129/ReJ-dy dystrophic mice and their littermate controls were characterized with respect to their respiratory and phosphorylating activities. Skeletal muscle mitochondria from dystrophic mice showed significantly lower state 3 respiratory rates than controls with both pyruvate + malate and succinate as substrates (P less than 0.01). ADP/O and Ca2+/O ratios were found to be normal. A decreased rate of NADH oxidation (0.01 less than P less than 0.05) by sonicated mitochondrial suspensions from dystrophic mice was also seen. High respiratory rates with ascorbate + phenazine methosulfate as substrates indicated that cytochrome oxidase was not rate limiting in the oxidation of either pyruvate + malate or succinate. Skeletal muscle mitochondria from dystrophic mice showed no deficiency in any of the cytochromes or coenzyme Q. Mg2+-stimulated ATPase activity was higher in dystrophic muscle mitochondria than in controls, but basal and oligomycin-insensitive activities were virtually identical to those of controls. A significant reduction inthe intramitochondrial NAD+ content (0.01 less than P less than 0.02) was seen in dystrophic skeletal muscle as compared to controls. Heart mitochondria from dystrophic mice showed similar, though less extensive abnormalities while liver mitochondria were essentially normal. We concluded from these results that skeletal muscle mitochondria from strain 129 dystrophic mice possess impairments in substrate utilization which may result from (1) an abnormality in the transfer of electrons on the substrate side of coenzyme Q in the case of succinate oxidation; (2) a defect on the path of electron flow from NADH to cytochrome c, and (3) a deficiency of NAD+ in the case of NAD+-linked substrates.

Defective oxidative metabolism of myodystrophic skeletal muscle mitochondria

A small-scale procedure for preparing tightly coupled intact skeletal muscle mitochondria from myodystrophic (myd/myd) mice is described. Mitochondrial preparations derived from heart, liver, and skeletal muscle of myd/myd and their littermate (+/?) controls are characterized with respect to their cytochrome content and their oxidative and phosphorylative capacities. Our data indicate that there is an impairment in the NADH CoQ region of the respiratory chain of myodystrophic skeletal muscle mitochondria. Both heart and liver mitochondria of myd/myd exhibited normal activities of respiratory chain-linked oxidative phosphorylation.

Glyceride metabolism in the myopathic hamster

Analysis of plasma lipids of 30- and 185-day-old BIO 82.62 myopathic hamsters and age-matched normal controls revealed a decrease in only the concentration of cholesteryl esters of 185-day-old diseased animals. Measurement of lipoprotein lipase (LPL) activity in heart, muscle, and adipose tissue showed no difference between the activity of the enzyme in the heart and muscle of the cardiomyopathic hamsters and that of the age-matched controls. In adipose tissue, however, LPL activity was depressed in the diseased animals in both age groups. No difference was found in the activity of hormone sensitive lipase. Incorporation of sn[U-14C] glycerol-3-phosphate into total lipids was found to be depressed in homogenates of heart, muscle, and adipose tissue but unchanged in liver homogenates of diseased animals. It was concluded that the decrease in the capacity to synthesize glycerides, rather than limiting substrate concentrations, could be the cause of the decrease in the lipid content in some tissues of the cardiomyopathic hamster.

Comparison of the intermediary metabolism of fatty acids in denervated and dystrophic murine skeletal muscle

Certain aspects of lipid metabolism have been examined in denervated muscle from normal mice and in dystrophic muscle from mice of the Bar Harbor strain 129. A number of parameters show no change or similar changes. For example, the utilization of palmitate-[1-14C] and palmitylcarnitine by mitochondria from denervated and dystrophic hind leg skeletal muscle showed parallel decreased in the oxidation of palmitate (30-42%) and palmitylcarnite (37-66%). A comparable study with acetylcarnitine showed a striking difference with no change evident in mitochondria from denervated muscle and 80-85% decrease in dystrophic muscle. The study of succinate dehydrogenase and the enzymes of beta-oxidation in the above mitochondrial preparation showed similar findings except for acyl CoA dehydrogenase activity (an enzyme with a regulatory role in beta-oxidation) which was significantly diminished (29%) in denervated muscle, whereas no change was observed in dystrophic muscle. The findings show a close parallel in a number of parameters but distinct differences were observed in denervated as compared with dystrophic muscle. It is unlikely that the muscular disorder in murine muscular dystrophy can be explained solely on the basis of denervation or the loss of a neural trophic factor.

Depressed fatty acid and acetate oxidation and other metabolic defects in homogenates from hearts of hamsters with hereditary cardiomyopathy

Metabolic changes in heart homogenate of Syrian hamsters with hereditary cardiomyopathy (BIO 14.6 strain) were examined. Oxidation of labeled fatty acids and acetate by myopathic homogenates was severely depressed; CO 2 production from labeled acetate was only one-fifteenth of the control value, and butyrate oxidation was suppressed to one-fourth of the control value. Although the addition of carnitine enhanced the oxidation of palmitate and octanoate by homogenates from both healthy and cardiomyopathic hamsters, the magnitude of the depression of oxidation (60-80%) in the myopathic homogenates was not influenced by carnitine. No change occurred in the rate of formation of palmitoyl-CoA from palmitoyl- l -carnitine or in the activity of acyl-CoA synthetases; similarly, the one-step oxidation of [1- 14 C]pyruvate, [1- 14 C]α-ketoglutarate, and succinate was unimpaired in cardiomyopathy, and the oxidation of [2- 14 C]pyruvate and [U- 14 C]α-ketoglutarate was maintained at a relatively high level (63-71% of control). In contrast, oxidation of [1, 4- 14 C]- and [2, 3- 14 C]succinate and [1- 14 C]acetyl-CoA by the myopathic homogenates was depressed to only 40% of the control values. The mechanism responsible for these defects in substrate oxidation by tissue homogenates from cardiomyopathic hearts has not been elucidated; however, significant increases in the activities of two of five hydrolytic enzymes indicate the possible involvement of lysosomes. Ventricular RNA and DNA, the ratio of RNA to DNA and the ratio of heart weight to body weight were all significantly increased, but myocardial water and triglyceride content and esterification of [1- 14 C]palmitate into neutral lipids showed no significant change.