Changes in oleic acid oxidation and incorporation into lipids of differentiating L6 myoblasts cultured in normal or fatty acid-supplemented growth medium

Essential role of the PGC-1α/PPARβ axis in Ucp3 gene induction

KEY POINTS

We report that the peroxisome proliferator-activated receptor (PPAR)γ coactivator 1-α (PGC-1α)/PPARβ axis is a crucial mediator of uncoupling protein 3 (UCP3) expression in skeletal muscle cells via the transactivativation of a distal PPAR response element at the Ucp3 gene promoter. This mechanism is activated during the myogenic process and by high concentrations of fatty acids independent of PGC-1α protein levels. Ucp3 is essential for PGC-1α-induced oxidative capacity and the adaptive mitochondrial response to fatty acid exposure. These findings provide further evidence for the broad spectrum of the coactivator action in mitochondrial homeostasis, positioning the PGC-1ɑ/PPARβ axis as an essential component of the molecular regulation of Ucp3 gene in skeletal muscle cells.

ABSTRACT

Uncoupling protein 3 (UCP3) has an essential role in fatty acid metabolism and mitochondrial redox regulation in skeletal muscle. However, the molecular mechanisms involved in the expression of Ucp3 are poorly known. In the present study, we show that the peroxisome proliferator-activated receptor (PPAR)γ coactivator 1-α (PGC-1α)/PPARβ axis is a crucial mediator of Ucp3 expression in skeletal muscle cells. In silico analysis of the UCP3 promoter and quantitative chromatin immunoprecipitation experiments revealed that the induction of the UCP3 transcript is mediated by the transactivation of a distal PPAR response element at the Ucp3 gene promoter by the coactivator PGC-1α. This mechanism is activated during myogenesis and during metabolic stress induced by fatty acids independent of PGC-1α protein levels. We also provide evidence that Ucp3 is essential for PGC-1α-induced oxidative capacity. Taken together, our results highlight PGC-1ɑ/PPARβ as an essential component of the molecular regulation of Ucp3 gene in skeletal muscle cells.

Regulation of muscle plasticity and trophism by fatty acids: A short review

The skeletal muscle tissue has a remarkable ability to alter its plastic structural and functional properties after a harmful stimulus, regulating the expression of proteins in complex events such as muscle regeneration. In this context, considering that potential therapeutic agents have been widely studied, nutritional strategies have been investigated in order to improve the regenerative capacity of skeletal muscle. There is evidence of the modulatory action of fatty acids, such that oleic and linoleic acids, that are abundant in Western diets, on muscle function and trophism. Thus, fatty acids appear to be potential candidates to promote or impair the recovery of muscle mass and function during regeneration, since they modulate intracellular pathways that regulate myogenesis. This study is the first to describe and discuss the effect of fatty acids on muscle plasticity and trophism, with emphasis on skeletal muscle regeneration and in vitro differentiation of muscle cells.

Analysis of the bioactivity of magnetically immunoisolated peroxisomes

Peroxisomes produce reactive oxygen species which may participate in biotransformations of innate biomolecules and xenobiotics. Isolating functional peroxisomes with low levels of contaminants would be a useful tool to investigate biotransformations occurring in these organelles that are usually confounded with biotransformations occurring in other co-isolated organelles. Here, we immunoisolate peroxisomes and demonstrate that the impurity level after isolation is low and that peroxisomes retain their biological activity. In this method, an antibody targeting a 70-kDa peroxisomal membrane protein was immobilized to silanized magnetic iron oxide beads (1-4 μm in diameter) coated with Protein A. Peroxisomes from L6 rat myoblast homogenates were magnetically captured, washed, and then analyzed for subcellular composition using enzymatic assays. Based on the ratio of peroxisomal to lysosomal activity, the retained fraction is 70-fold enriched relative to the unretained fraction. Similarly, the ratio of peroxisomal activity to mitochondrial content suggests that the retained fraction is >30-fold enriched relative to the unretained fraction. H(2)O(2) production from the β-oxidation of palmitoyl-CoA demonstrated that the isolated peroxisomal fraction was biologically active. Capillary electrophoresis with laser-induced fluorescence detection (CE-LIF) analysis confirmed that the immunopurified fractions were capable of transforming the anticancer drug doxorubicin and the fatty acid analog, BODIPY 500/510 C1C12. Besides its use to investigate peroxisome biotransformations in health and disease, the combination of magnetic immunoisolation with CE-LIF could be widely applicable to investigate subcellular-specific biotransformations of xenobiotics occurring at immunoisolated subcellular compartments.

The effect of insulin on delta5 desaturation in hepG2 human hepatoma cells and L6 rat muscle myoblasts

In humans there is a correlation between the ratio of arachidonic acid (20:4n-6) to cis 8,11,14 eicosatrienoic acid (20:3n-6) in skeletal muscle phospholipids and insulin sensitivity. This has been interpreted as indicating a link between the activity of the delta5 desaturase enzyme and muscle insulin sensitivity. The present study addressed the possibility that insulin regulates delta5 desaturase activity using L6 rat myoblasts and hepG2 human hepatoma cells. Both cell lines responded to insulin by increasing the amount of D-[U-14C] glucose incorporated into glycogen. In L6 cells, insulin stimulated cis 8,11,14 eicosatrienoic acid uptake and arachidonic acid production but had no effect on the percentage conversion of cis 8,11,14 eicosatrienoic acid to arachidonic acid. In hepG2 cells, insulin had no effect on cis 8,11,14 eicosatrienoic acid uptake or arachidonic acid production. These results suggest that insulin has no direct effect on delta5 desaturase activity in the liver but can alter arachidonic acid production in muscle by altering substrate availability.

The effects of selenium deficiency on differentiation, degradation, and cell lysis of L8 rat skeletal muscle cells

We investigated the effects of selenium (Se) deficiency on differentiation, protein degradation, and cell lysis in cultured skeletal muscle cells, using L8 rat skeletal muscle cells cultured in serum-free (SF) medium to induce differentiation and to maintain myotubes. Creatine kinase activity was reduced (p < 0.05) by approximately 15% without Se supplementation for 96 h. Confluent myoblasts were treated with SF media with four different levels of vitamin E (0, 10, 35, and 100 microM) in the absence and presence of Se (0 and 0.25 microM, respectively). After 96 h, vitamin E at a high dose (100 microM) was effective in the prevention of the decrease of differentiation caused by Se deficiency (p < 0.05). Following differentiation, the effects of three Se concentrations (0, 0.25, and 2.5 microM) on degradation of proteins as assessed by release of 3H-labeled free amino acids secreted into the media were studied. Selenium supplementation did not affect (p > 0.05) total protein degradation. However, Se deficiency increased (p < 0.05) lactate dehydrogenase released from lyzed dead cells. The results indicate that Se is required to maintain an optimal rate of muscle cell differentiation and health of myotube cultures.

Expression of the extracellular fatty acid binding protein (Ex-FABP) during muscle fiber formation in vivo and in vitro

We report that Ex-FABP, an extracellular protein belonging to the lipocalin family and involved in the extracellular transport of long-chain fatty acids, is expressed in the forming myotubes both in vivo and in vitro. The presence of the protein and of the mRNA was observed in newly formed myotubes at early stages of chick embryo development by immunohistochemistry and by in situ hybridization. At later stages of development myofibers still expressed both the mRNA and the protein. Ex-FABP expression was observed also in the developing myocardium and the muscular layer of large blood vessels. In agreement with these findings, an initial expression of the mRNA and protein secretion by cultured chicken myoblasts were observed only after the onset of myoblast fusion. Double-immunofluorescence staining of these cultured cells revealed that multinucleate myotubes were stained by antibodies directed against both the Ex-FABP and the sarcomeric myosin, whereas immature myotubes and single myoblasts were not. When added to cultured myoblasts, antibodies against the Ex-FABP induced a strong enhancement of the production of the same protein. In all experiments some cell sufferance and a transient impairment of myotube formation were also observed. The finding that the continuous removal of the Ex-FABP from the culture medium of myoblasts, due to the formation of immune complexes, resulted in an overproduction of the protein suggests a feedback (autocrine) control during myotube differentiation and maturation. We propose that the requirement for increased transport and metabolism of free fatty acid released from the membrane phospholipids and storage lipids, mediated by Ex-FABP, may be essential during differentiation of multinucleated myotubes or that an increased local demand of fatty acids and metabolites may act as a local hormone in tissues differentiating and undergoing morphogenesis.

Evidence that acetyl-CoA carboxylase isoforms play different biological roles in H9c2 cardiomyocyte

The present work was performed to identify the possible roles of acetyl-CoA carboxylase isoforms (ACC-alpha and ACC-beta). Two forms show 70% amino acid identity, but N-terminal regions share no homology, indicating that these may be uniquely related to the specific role of each ACC form. Thus, we investigated whether introduction of the exogenous ACC N-terminus into H9c2 cardiomyocytes that express both ACC forms causes a noticeable change in a specific pathway of fatty acid metabolism. The effect of ACC-alpha N-terminus overexpression was specific to the fatty acid synthesis rate resulting in an 80% induction, whereas overexpression of the ACC-beta N-terminus increased fatty acid oxidation rate 50% without affecting the fatty acid synthesis rate. These results suggest that ACC-alpha and beta are involved in the regulation of fatty acid synthesis and oxidation, respectively, and that the N-terminus plays an important role in the process. We further demonstrated that novel proteins specifically bound to the ACC N-terminus. This interaction may mediate the involvement of each ACC form in different cellular activities.

Transfection of L6 myoblasts with adipocyte fatty acid-binding protein cDNA does not affect fatty acid uptake but disturbs lipid metabolism and fusion

We studied the involvement of fatty acid-binding protein (FABP) in growth, differentiation and fatty acid metabolism of muscle cells by lipofection of rat L6 myoblasts with rat heart (H) FABP cDNA or with rat adipocyte (A) FABP cDNA in a eukaryotic expression vector which contained a puromycin acetyltransferase cassette. Stable transfectants showed integration into the genome for all constructs and type-specific overexpression at the mRNA and protein level for the clones with H-FABP and A-FABP cDNA constructs. The rate of proliferation of myoblasts transfected with rat A-FABP cDNA was 2-fold higher compared with all other transfected cells. In addition, these myoblasts showed disturbed fusion and differentiation, as assessed by morphological examination and creatine kinase activity. Uptake rates of palmitate were equal for all clone types, in spite of different FABP content and composition. Palmitate oxidation over a 3 h period was similar in all clones from growth medium. After being cultured in differentiation medium, mock- and H-FABP-cDNA-transfected cells showed a lower fatty acid-oxidation rate, in contrast with A-FABP-cDNA-transfected clones. The ratio of [14C]palmitic acid incorporation into phosphatidylcholine and phosphatidylethanolamine of A-FABP-cDNA-transfected clones changed in the opposite direction in differentiation medium from that of mock- and H-FABP-cDNA-transfected clones. In conclusion, transfection of L6 myoblasts with A-FABP cDNA does not affect H-FABP content and fatty acid uptake, but changes fatty acid metabolism. The latter changes may be related to the observed fusion defect.

Myogenic differentiation of the muscle clonal cell line BC3H-1 is accompanied by changes in its lipid composition

Phospholipid and neutral lipid composition was studied in the course of myogenic differentiation of the clonal cell line BC3H-1. Total phospholipid content increased during differentiation, predominantly in the major classes of choline and ethanolamine glycerophospholipids. The contents of other lipids, such as triacylglycerols, diminished more than 50% during this period. The content and distribution of fatty acids also underwent marked differentiation-dependent changes. The polyunsaturated (tetrapenta- and hexaenoic) fatty acid species of several phospholipid classes diminished during differentiation, especially those in choline, serine and inositol glycerophospholipids. Most noticeable were the changes in phosphatidylserine; long-chain fatty acids having 20 to 22 carbon atoms and 4 to 6 double bonds decreased from about 30 to about 10 mol%. Although increased levels of saturation in other phospholipid fatty acyl chains appear to accompany the myogenic changes of BC3H-1 cells, some unsaturated fatty acids, such as oleic acid (18:1), increased by as much as 80% during the same period, suggesting the activation of a delta 9 desaturase. Sphingomyelin contained only saturated and monoenoic fatty acids and exhibited a four- to five-fold decrease in its content of monoenoic acyl groups. Diacylglycerols became enriched in arachidonate and docosahexaenoate. The amount of cholesterol and its esters increased slightly during differentiation of BC3H-1 cells. The data show that several metabolic pathways change during myogenic differentiation of the BC3H-1 clonal cell line, particularly de novo biosynthetic pathways, elongation/desaturation reactions, and acyl chain turnover.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of human monocytic cell line, U937, in taking up acetylated low-density lipoprotein and cholesteryl ester accumulation. A flow cytometric and HPLC study

The uptake of LDL and acetylated LDL and the ability of cholesteryl ester accumulation by cells of a human monocytic cell line, U937, has been characterized by flow cytometric assay using a fluorescent probe, DiI, and by high-performance liquid chromatography (HPLC). The increase of mean fluorescence intensity of U937 incubated with DiI-labeled lipoproteins demonstrates that this cell line could incorporate DiI-AcLDL, as well as DiI-labeled LDL. Competition and saturation studies indicate that the manner of taking up DiI-AcLDL is receptor-mediated. While differentiated U937 incubated with 16 nM phorbol myristate acetate for 24 h took up little DiI-AcLDL, HPLC analysis confirmed that intracellular free and esterified cholesterols significantly increase in the U937 cells incubated with AcLDL or LDL. The ability of mouse peritoneal macrophage to abundantly accumulate at least five kinds of cholesteryl ester were also shown in this analysis. In contrast, in U937 cells, free fatty acids are incorporated into various substances rather than into cholesteryl esters (as revealed by HPLC analysis), so that the cholesterol in AcLDL taken up by U937 cells is not synthesized into cholesteryl esters to any great extent.