Carnitine uptake and fatty acid utilization by diploid cells aging in culture

Carnitine is a naturally occurring inhibitor of thyroid hormone nuclear uptake

Carnitine (3-hydroxy-4N-trimethylammoniumbutanoate) is a naturally occurring quaternary amine that is ubiquitous in mammalian tissues (concentrations in the order of mM). Based on limited studies of approximately 40 years ago, carnitine was considered to be a peripheral antagonist of thyroid hormone (TH) action. These interesting observations have not been explored. To study the biologic basis of this effect, we tested the following possibilities in three TH-responsive cell lines: (1) inhibition of TH entry into cells; (2) inhibition of TH entry into the nucleus; (3) inhibition of TH interaction with the isolated nuclei; and (4) facilitated efflux of TH from cells. On a preliminary basis we had verified that these cell lines (human skin fibroblasts, human hepatoma cells HepG2, and mouse neuroblastoma cells NB 41A3) take up 14Ccarnitine; however, there was no 14Ccarnitine uptake into the nuclei. Concentrations of unlabeled carnitine as high as 100 mM did not affect (125I)T3 binding to isolated nuclei or exit of TH from cells, thus excluding possibilities numbered 3 and 4. At 10 mM camitine, (125I)T3 and (125I)T4 whole-cell uptake was inhibited by approximately 20% in fibroblasts and in HepG2, but by approximately 5% in NB 41A3 cells. Inhibition of T3 nuclear uptake was evaluated in HepG2 and NB 41A3 cells. At 10 mM carnitine, inhibition of T3 nuclear uptake was disproportionately higher, namely approximately 25% in neurons and 35% in hepatocytes. At 50 mM carnitine, there was a minimal additional decrease in whole-cell uptake of either hormone but a marked decrease in T3 nuclear uptake. The latter inhibition was approximately 60% in neurons and 70% in hepatocytes. We are aware of no inhibitor of TH uptake that has such a markedly different effect on the nuclear versus whole-cell uptake. Our data are consistent with carnitine being a peripheral antagonist of TH action, and they indicate a site of inhibition at or before the nuclear envelope.

Genetic disorders of carnitine metabolism and their nutritional management

Carnitine functions as a substrate for a family of enzymes, carnitine acyltransferases, involved in acyl-coenzyme A metabolism and as a carrier for long-chain fatty acids into mitochondria. Carnitine biosynthesis and/or dietary carnitine fulfill the body's requirement for carnitine. To date, a genetic disorder of carnitine biosynthesis has not been described. A genetic defect in the high-affinity plasma membrane carnitine-carrier(in) leads to renal carnitine wasting and primary carnitine deficiency. Myopathic carnitine deficiency could be due to an increase in efflux moderated by the carnitine-carrier(out). Defects in the carnitine transport system for fatty acids in mitochondria have been described and are being examined at the molecular and pathophysiological levels. the nutritional management of these disorders includes a high-carbohydrate, low-fat diet and avoidance of those events that promote fatty acid oxidation, such as fasting, prolonged exercise, and cold. Large-dose carnitine treatment is effective in systemic carnitine deficiency.

Characterization of L-carnitine transport by rat kidney brush-border-membrane vesicles

In the presence of a 100 mM Na+ gradient, transport of L-carnitine into rat renal brush-border-membrane vesicles was linear over 30 s and showed an overshoot at 5 min. The uptake of L-carnitine was clearly less active in the presence of other cations such as Li+, K+, Cs+ or choline. In the presence of a Na+ gradient, L-carnitine uptake after 20 s was much higher for chloride as an anion than for SCN-, NO3-, gluconate or SO4(2-). In comparison with conditions with inside positive or no membrane potential, transport was higher in vesicles with an inside negative membrane potential, suggesting an electrogenic mechanism. The kinetic characterization of the Na(+)-dependent portion of L-carnitine transport revealed two transport systems with Km values of 17.4 +/- 3.9 microM and 15.0 +/- 6.0 mM, respectively. The transport could be inhibited in a concentration-dependent fashion by structural analogues such as butyrobetaine, L-acetylcarnitine, trimethyl-lysine and D-carnitine, but not by L-arginine or glycinebetaine.

L-carnitine uptake in differentiating human cultured muscle

We studied carnitine uptake in human skeletal muscle growing in culture for up to 30 days, and correlated it to the degree of muscle differentiation revealed by myotube formation and muscle-specific creatine-kinase isozyme accumulation. In our study carnitine uptake was a saturable specific process with two distinct components: a high affinity uptake at carnitine concentration between 0.5 and 10 microM and a low affinity uptake at carnitine concentration between 25 and 200 microM. High affinity uptake (Km 4.17-5.50 microM, Vmax 11.78-19.6 pmol/h per mg protein) did not change during muscle maturation in culture. Low affinity uptake showed significant changes in Km and Vmax in the various stages of muscle differentiation. Our studies suggest the existence of a muscle-specific system, operating at physiological carnitine concentration, which gradually develops during muscle maturation in culture. We hypothesize that a defect of the low affinity muscle-specific uptake might be the cause of the primary muscle carnitine deficiency syndrome.

Chronic cardiomyopathy and weakness or acute coma in children with a defect in carnitine uptake

A defect in intracellular uptake of carnitine has been identified in patients with severe carnitine deficiency. To define the clinical manifestations of this disorder, the presenting features of 15 affected infants and children were examined. Progressive cardiomyopathy, with or without chronic muscle weakness, was the most common presentation (median age of onset, 3 years). Other patients presented with episodes of fasting hypoglycemia during the first 2 years of life before cardiomyopathy had become apparent. A defect in carnitine uptake was demonstrable in fibroblasts and leukocytes from patients. The defect also appears to be expressed in muscle and kidney. Concentrations of plasma carnitine and rates of carnitine uptake in parents were intermediate between affected patients and normal control subjects, consistent with recessive inheritance. Early recognition and treatment with high doses of oral carnitine may be life-saving in this disorder of fatty acid oxidation.

Transport of carnitine into cells in hereditary carnitine deficiency

Carnitine uptake has been studied in fibroblasts from a case of hereditary carnitine deficiency and in relatives. There was no evidence for carrier-dependent uptake in cells from the patient. The mother and probably the healthy sister had an impaired uptake. The results show that the defect in this form of carnitine deficiency is an inability to establish a concentration gradient over the cell membrane.

Effect of L-carnitine on cultured murine melanoma cells exposed to azelaic acid

The cytotoxic effect of azelaic acid on murine melanoma cells in culture is due, at least in part, to an antimitochondrial action. We investigated the possibility that the addition of carnitine to the medium may increase the transport of azelaic acid into the mitochondria and thereby increase its cytotoxic effect. Using mitochondrial cross-sectional area measured from electron micrographs as a criterion for mitochondrial damage, we found that the addition of L-carnitine to the culture medium had no effect either alone or with a low (10(-3) M) concentration of azelaic acid. At a high concentration (5 X 10(-2) M) azelaic acid caused swelling and disruption of the mitochondria to such an extent that this was not increased by carnitine. At 10(-2) M azelaic acid, however, some swelling of the mitochondria occurred which was significantly increased by the addition of carnitine. This indicates that carnitine-mediated transport of the diacid into the mitochondria had occurred. We conclude that carnitine may reduce the time or concentration needed for azelaic acid to have a toxic effect on the malignant melanocyte.

Serum factors that stimulate fatty acid oxidation: properties of factors

Cultured heart muscle cells, but not HeLa cells, oxidize long-chain fatty acids in medium containing dialyzed serum. Addition of chicken serum dialysate (or non-dialized serum) stimulated palmitic acid oxidation by HeLa cells 10 to 20 fold. This serum activity was not eliminated by lipid extraction, ethanol or acid precipitation, alkaline phosphatase treatment, or autoclaving. About 80% was lost after any one of the following treatments: 6N HCl at 110 degrees C for 16 hr, pepsin, Dowex cation exchange at pH 3, or 1N KOH at 100 degrees C for 1 hr. Serum activity was separated into five or more peaks by gel filtration with Sephadex G-10. Each of these peak fractions was further purified by HPLC using a cyanopropyl-bonded resin. Carnitine, which is important for the transport of long-chain fatty acids into mitochondria for oxidation, also stimulated the oxidation of palmitate. However, these serum factors are not known precursors to carnitine since its immediate precursor 4-n-trimethylaminobutyrate, did not stimulate palmitate oxidation. Total carnitine, including that in acylcarnitine compounds, was approximately 15 microM in the chicken sera to give approximately 0.7 microM in the medium. Based on the fraction of total activity accountable by carnitine and fractional stability to acid, alkali, and pepsin, about 75% of the activity is from non-carnitine compounds. Only one of the factors appears to be carnitine or an acylcarnitine derivative. Several lines of evidence suggest that the other factors are peptide compounds.