Deactivation of peroxisome proliferator-activated receptor-alpha during cardiac hypertrophic growth.

We sought to delineate the molecular regulatory events involved in the energy substrate preference switch from fatty acids to glucose during cardiac hypertrophic growth. alpha(1)-adrenergic agonist-induced hypertrophy of cardiac myocytes in culture resulted in a significant decrease in palmitate oxidation rates and a reduction in the expression of the gene encoding muscle carnitine palmitoyltransferase I (M-CPT I), an enzyme involved in mitochondrial fatty acid uptake. Cardiac myocyte transfection studies demonstrated that M-CPT I promoter activity is repressed during cardiac myocyte hypertrophic growth, an effect that mapped to a peroxisome proliferator-activated receptor-alpha (PPARalpha) response element. Ventricular pressure overload studies in mice, together with PPARalpha overexpression studies in cardiac myocytes, demonstrated that, during hypertrophic growth, cardiac PPARalpha gene expression falls and its activity is altered at the posttranscriptional level via the extracellular signal-regulated kinase mitogen-activated protein kinase pathway. Hypertrophied myocytes exhibited reduced capacity for cellular lipid homeostasis, as evidenced by intracellular fat accumulation in response to oleate loading. These results indicate that during cardiac hypertrophic growth, PPARalpha is deactivated at several levels, leading to diminished capacity for myocardial lipid and energy homeostasis.

PPARalpha deficiency reduces insulin resistance and atherosclerosis in apoE-null mice.

PPARalpha is a ligand-dependent transcription factor expressed at high levels in the liver. Its activation by the drug gemfibrozil reduces clinical events in humans with established atherosclerosis, but the underlying mechanisms are incompletely defined. To clarify the role of PPARalpha in vascular disease, we crossed PPARalpha-null mice with apoE-null mice to determine if the genetic absence of PPARalpha affects vascular disease in a robust atherosclerosis model. On a high-fat diet, concentrations of atherogenic lipoproteins were higher in PPARalpha(-/-)apoE(-/-) than in PPARalpha(+/+)apoE(-/-) mice, due to increased VLDL production. However, en face atherosclerotic lesion areas at the aortic arch, thoracic aorta, and abdominal aorta were less in PPARalpha-null animals of both sexes after 6 and 10 weeks of high-fat feeding. Despite gaining as much or more weight than their PPARalpha(+/+)apoE(-/-) littermates, PPARalpha(-/-)apoE(-/-) mice had lower fasting levels of glucose and insulin. PPARalpha-null animals had greater suppression of endogenous glucose production in hyperinsulinemic clamp experiments, reflecting less insulin resistance in the absence of PPARalpha. PPARalpha(-/-)apoE(-/-) mice also had lower blood pressures than their PPARalpha(+/+)apoE(-/-) littermates after high-fat feeding. These results suggest that PPARalpha may participate in the pathogenesis of diet-induced insulin resistance and atherosclerosis.

Transcriptional control of a nuclear gene encoding a mitochondrial fatty acid oxidation enzyme in transgenic mice: role for nuclear receptors in cardiac and brown adipose expression.

Expression of the gene encoding medium-chain acyl coenzyme A dehydrogenase (MCAD), a nuclearly encoded mitochondrial fatty acid beta-oxidation enzyme, is regulated in parallel with fatty acid oxidation rates among tissues and during development. We have shown previously that the human MCAD gene promoter contains a pleiotropic element (nuclear receptor response element [NRRE-1]) that confers transcriptional activation or repression by members of the nuclear receptor superfamily. Mice transgenic for human MCAD gene promoter fragments fused to a chloramphenicol acetyltransferase gene reporter were produced and characterized to evaluate the role of NRRE-1 and other promoter elements in the transcriptional control of the MCAD gene in vivo. Expression of the full-length MCAD promoter-chloramphenicol acetyltransferase transgene (MCADCAT.371) paralleled the known tissue-specific differences in mitochondrial beta-oxidation rates and MCAD expression. MCADCAT.371 transcripts were abundant in heart tissue and brown adipose tissue, tissues with high-level MCAD expression. During perinatal cardiac developmental stages, expression of the MCADCAT.371 transgene paralleled mouse MCAD mRNA levels. In contrast, expression of a mutant MCADCAT transgene, which lacked NRRE-1 (MCADCATdeltaNRRE-1), was not enriched in heart or brown adipose tissue and did not exhibit appropriate postnatal induction in the developing heart. Transient-transfection studies with MCAD promoter-luciferase constructs containing normal or mutant NRRE-1 sequences demonstrated that the nuclear receptor binding sequences within NRRE-1 are necessary for high-level transcriptional activity in primary rat cardiocytes. Electrophoretic mobility shift assays demonstrated that NRRE-1 was bound by several cardiac and brown adipose nuclear proteins and that these interactions required the NRRE-1 receptor binding hexamer sequences. Antibody supershift studies identified the orphan nuclear receptor COUP-TF as one of the endogenous cardiac proteins which bound NRRE-1. These results dictate an important role for nuclear receptors in the transcriptional control of a nuclear gene encoding a mitochondrial fatty acid oxidation enzyme and identify a gene regulatory pathway involved in cardiac energy metabolism.

The role of the peroxisome proliferator-activated receptor alpha (PPAR alpha) in the control of cardiac lipid metabolism.

The postnatal mammalian heart uses mitochondrial fatty acid oxidation (FAO) as the chief source of energy to meet the high energy demands necessary for pump function. Flux through the cardiac FAO pathway is tightly controlled in accordance with energy demands dictated by diverse physiologic and dietary conditions. In this report, we demonstrate that the lipid-activated nuclear receptor, peroxisome proliferator-activated receptor alpha (PPARalpha), regulates the expression of several key enzymes involved in cardiac mitochondrial FAO. In response to the metabolic stress imposed by pharmacologic inhibition of mitochondrial long-chain fatty acid import with etomoxir, PPARa serves as a molecular 'lipostat' factor by inducing the expression of target genes involved in fatty acid utilization including enzymes involved in mitochondrial and peroxisomal beta-oxidation pathways. In mice lacking PPARalpha (PPARalpha-/- mice), etomoxir precipitates a cardiac phenotype characterized by myocyte lipid accumulation. Surprisingly, this metabolic regulatory response is influenced by gender as demonstrated by the observation that male PPARalpha-/- mice are more susceptible to the metabolic stress compared to female animals. These results identify an important role for PPARalpha in the control of cardiac lipid metabolism.

Transcriptional control of cardiac fuel metabolism and mitochondrial function.

As a persistent pump, the mammalian heart demands a high-capacity mitochondrial system. Significant progress has been made in delineating the gene regulatory networks that control mitochondrial biogenesis and function in striated muscle. The PPARγ coactivator-1 (PGC-1) coactivators serve as inducible boosters of downstream transcription factors that control the expression of genes involved in mitochondrial energy transduction, ATP synthesis, and biogenesis. PGC-1 gain-of-function and loss-of-function studies targeting two PGC-1 family members, PGC-1α and PGC-1ß, have provided solid evidence that these factors are both necessary and sufficient for perinatal mitochondrial biogenesis and maintenance of high-capacity mitochondrial function in postnatal heart. In humans, during the development of heart failure owing to hypertension or obesity-related diabetes, the activity of the PGC-1 coactivators, and several downstream target transcription factors, is altered. Gene targeting studies in mice have demonstrated that loss of PGC-1α and PGC-1ß in heart leads to heart failure. Interestingly, the pattern of dysregulation within the PGC-1 transcriptional regulatory circuit distinguishes the heart disease caused by hypertension from that caused by diabetes. This transcriptional regulatory cascade and downstream metabolic pathways should be considered as targets for novel etiology-specific therapeutics aimed at the early stages of heart failure.

A critical role for the peroxisome proliferator-activated receptor alpha (PPARalpha) in the cellular fasting response: the PPARalpha-null mouse as a model of fatty acid oxidation disorders.

We hypothesized that the lipid-activated transcription factor, the peroxisome proliferator-activated receptor alpha (PPARalpha), plays a pivotal role in the cellular metabolic response to fasting. Short-term starvation caused hepatic steatosis, myocardial lipid accumulation, and hypoglycemia, with an inadequate ketogenic response in adult mice lacking PPARalpha (PPARalpha-/-), a phenotype that bears remarkable similarity to that of humans with genetic defects in mitochondrial fatty acid oxidation enzymes. In PPARalpha+/+ mice, fasting induced the hepatic and cardiac expression of PPARalpha target genes encoding key mitochondrial (medium-chain acyl-CoA dehydrogenase, carnitine palmitoyltransferase I) and extramitochondrial (acyl-CoA oxidase, cytochrome P450 4A3) enzymes. In striking contrast, the hepatic and cardiac expression of most PPARalpha target genes was not induced by fasting in PPARalpha-/- mice. These results define a critical role for PPARalpha in a transcriptional regulatory response to fasting and identify the PPARalpha-/- mouse as a potentially useful murine model of inborn and acquired abnormalities of human fatty acid utilization.

Effect of endurance training on lipid metabolism in women: a potential role for PPARalpha in the metabolic response to training.

Endurance training increases fatty acid oxidation (FAO) and skeletal muscle oxidative capacity. However, the source of the additional fat and the mechanisms for increasing FAO capacity in muscle are not clear. We measured whole body and regional lipolytic activity and whole body and plasma FAO in six lean women during 90 min of bicycling exercise (50% pretraining peak O(2) consumption) before and after 12 wk of endurance training. We also assessed skeletal muscle content of peroxisome proliferator-activated receptor-alpha (PPARalpha) and its target proteins that regulate FAO [medium-chain and very long chain acyl-CoA dehydrogenase (MCAD and VLCAD)]. Despite a 25% increase in whole body FAO during exercise after training (P < 0.05), training did not alter regional adipose tissue lipolysis (abdominal: 0.56 +/- 0.26 and 0.57 +/- 0.10 micromol x 100 g(-1) x min(-1); femoral: 0.13 +/- 0.07 and 0.09 +/- 0.02 micromol x 100 g(-1) x min(-1)), whole body palmitate rate of appearance in plasma (168 +/- 18 and 150 +/- 25 micromol/min), and plasma FAO (554 +/- 61 and 601 +/- 45 micromol/min). However, training doubled the levels of muscle PPARalpha, MCAD, and VLCAD. We conclude that training increases the use of nonplasma fatty acids and may enhance skeletal muscle oxidative capacity by PPARalpha regulation of gene expression.

Mitochondrial transcription factor A is increased but expression of ATP synthase beta subunit and medium-chain acyl-CoA dehydrogenase genes are decreased in hearts of copper-deficient rats.

The mechanism(s) by which impaired mitochondrial respiratory function and the accumulation of lipid droplets and mitochondria in hearts of copper-deficient rats occur remains unclear. It is not known whether specific components of the regulatory pathway involved in mitochondrial biogenesis, such as mitochondrial transcription factor A (mtTFA) and nuclear respiratory factors 1 and 2 (NRF-1 and NRF-2), are activated in copper deficiency. Little is known about gene expression of enzymes involved in fatty acid oxidation (FAO) in hearts of copper-deficient rats. Male weanling rats were fed copper-adequate (CuA), copper-deficient (CuD) or pair-fed (CuP) diets for 5 wk. Mitochondria and lipid droplet volume densities from electron micrographs were greater and there was an elevation in the mtTFA protein level in hearts of copper-deficient rats. DNA binding activities of NRF-1 and NRF-2 did not differ among the groups. Northern blot analysis of cardiac tissue revealed that transcripts of F(1)F(0)-ATP synthase subunit c were greater, but mRNA levels of ATP synthase beta subunit and the FAO enzyme, medium-chain acyl-CoA dehydrogenase (MCAD), were lower in hearts of copper-deficient rats. Long-chain acyl-CoA dehydrogenase (LCAD) mRNA levels did not differ among treatment groups. These results suggest that certain components of the mitochondrial biogenesis program are activated in hearts of copper-deficient rats. F(1)F(0)-ATP synthase beta subunit and MCAD transcript levels remain low, which may contribute to impaired mitochondrial respiratory function, decreased fatty acid utilization and lipid droplet accumulation in hearts of copper-deficient rats.

The human medium chain Acyl-CoA dehydrogenase gene promoter consists of a complex arrangement of nuclear receptor response elements and Sp1 binding sites.

Expression of the gene encoding the mitochondrial fatty acid. beta-oxidation enzyme, medium-chain acyl-CoA dehydrogenase (MCAD), is regulated among tissues during development and in response to alterations in substrate availability. To identify and characterize cis-acting MCAD gene promoter regulatory elements and corresponding transcription factors, DNA-protein binding studies and mammalian cell transfection analyses were performed with hjman MCAD gene promoter fragments. DNA:protein binding studies with nuclear protein extracts prepared from hepatoma G2 cells, 3T3 fibroblasts, or Y-1 adrenal tumor cells identified three sequences (nuclear receptor response element 1 or NRRE-1, NRRE-2, and NRRE-3) that bind orphan members of the steroid/thyroid nuclear receptor superfamily including chicken ovalbumin upstream promoter transcription factor and steroidogenic factor 1. Sp1 binding sites (A-C) were identified in close proximity to each of the NRREs. NRRE-3 conferred cell line-specific transcriptional repression by interacting with chicken ovalbumin upstream promoter transcription factor or activation via steroidogenic factor 1. In contrast, the Sp1 binding site A behaved as a transcriptional activator in all cell lines examined. We propose that multiple nuclear receptor transcription factors interact with MCAD gene promoter elements to differentially regulate transcription among a variety of cell types.