Constitutive regulation of cardiac fatty acid metabolism through peroxisome proliferator-activated receptor alpha associated with age-dependent cardiac toxicity

The peroxisome proliferator-activated receptor alpha (PPARalpha) is a member of the nuclear receptor superfamily and mediates the biological effects of peroxisome proliferators. To determine the physiological role of PPARalpha in cardiac fatty acid metabolism, we examined the regulation of expression of cardiac fatty acid-metabolizing proteins using PPARalpha-null mice. The capacity for constitutive myocardial beta-oxidation of the medium and long chain fatty acids, octanoic acid and palmitic acid, was markedly reduced in the PPARalpha-null mice as compared with the wild-type mice, indicating that mitochondrial fatty acid catabolism is impaired in the absence of PPARalpha. In contrast, constitutive beta-oxidation of the very long chain fatty acid, lignoceric acid, did not differ between the mice, suggesting that the constitutive expression of enzymes involved in peroxisomal beta-oxidation is independent of PPARalpha(.) Indeed, PPARalpha-null mice had normal levels of the peroxisomal beta-oxidation enzymes except the D-type bifunctional protein. At least seven mitochondrial fatty acid-metabolizing enzymes were expressed at much lower levels in the PPARalpha-null mice, whereas other fatty acid-metabolizing enzymes were present at similar or slightly lower levels in the PPARalpha-null, as compared with wild-type mice. Additionally, lower constitutive mRNA expression levels of fatty acid transporters were found in the PPARalpha-null mice, suggesting a role for PPARalpha in fatty acid transport and catabolism. Indeed, in fatty acid metabolism experiments in vivo, myocardial uptake of iodophenyl 9-methylpentadecanoic acid and its conversion to 3-methylnonanoic acid were reduced in the PPARalpha-null mice. Interestingly, a decreased ATP concentration after exposure to stress, abnormal cristae of the mitochondria, abnormal caveolae, and fibrosis were observed only in the myocardium of the PPARalpha-null mice. These cardiac abnormalities appeared to proceed in an age-dependent manner. Taken together, the results presented here indicate that PPARalpha controls constitutive fatty acid oxidation, thus establishing a role for the receptor in cardiac fatty acid homeostasis. Furthermore, altered expression of fatty acid-metabolizing proteins seems to lead to myocardial damage and fibrosis, as inflammation and abnormal cell growth control can cause these conditions.     

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.     

Inhibition of c-Jun-N-terminal kinase increases cardiac peroxisome proliferator-activated receptor alpha expression and fatty acid oxidation and prevents lipopolysaccharide-induced heart dysfunction

Septic shock results from bacterial infection and is associated with multi-organ failure, high mortality, and cardiac dysfunction. Sepsis causes both myocardial inflammation and energy depletion. We hypothesized that reduced cardiac energy production is a primary cause of ventricular dysfunction in sepsis. The JNK pathway is activated in sepsis and has also been implicated in impaired fatty acid oxidation in several tissues. Therefore, we tested whether JNK activation inhibits cardiac fatty acid oxidation and whether blocking JNK would restore fatty acid oxidation during LPS treatment. LPS treatment of C57BL/6 mice and adenovirus-mediated activation of the JNK pathway in cardiomyocytes inhibited peroxisome proliferator-activated receptor α expression and fatty acid oxidation. Surprisingly, none of the adaptive responses that have been described in other types of heart failure, such as increased glucose utilization, reduced αMHC:βMHC ratio or induction of certain microRNAs, occurred in LPS-treated mice. Treatment of C57BL/6 mice with a general JNK inhibitor (SP600125) increased fatty acid oxidation in mice and a cardiomyocyte-derived cell line. JNK inhibition also prevented LPS-mediated reduction in fatty acid oxidation and cardiac dysfunction. Inflammation was not alleviated in LPS-treated mice that received the JNK inhibitor. We conclude that activation of JNK signaling reduces fatty acid oxidation and prevents the peroxisome proliferator-activated receptor α down-regulation that occurs with LPS.     

Primary hepatocytes from mice lacking cysteine dioxygenase show increased cysteine concentrations and higher rates of metabolism of cysteine to hydrogen sulfide and thiosulfate

The oxidation of cysteine in mammalian cells occurs by two routes: a highly regulated direct oxidation pathway in which the first step is catalyzed by cysteine dioxygenase (CDO) and by desulfhydration-oxidation pathways in which the sulfur is released in a reduced oxidation state. To assess the effect of a lack of CDO on production of hydrogen sulfide (H₂S) and thiosulfate (an intermediate in the oxidation of H₂S to sulfate) and to explore the roles of both cystathionine γ-lyase (CTH) and cystathionine β-synthase (CBS) in cysteine desulfhydration by liver, we investigated the metabolism of cysteine in hepatocytes isolated from Cdo1-null and wild-type mice. Hepatocytes from Cdo1-null mice produced more H₂S and thiosulfate than did hepatocytes from wild-type mice. The greater flux of cysteine through the cysteine desulfhydration reactions catalyzed by CTH and CBS in hepatocytes from Cdo1-null mice appeared to be the consequence of their higher cysteine levels, which were due to the lack of CDO and hence lack of catabolism of cysteine by the cysteinesulfinate-dependent pathways. Both CBS and CTH appeared to contribute substantially to cysteine desulfhydration, with estimates of 56 % by CBS and 44 % by CTH in hepatocytes from wild-type mice, and 63 % by CBS and 37 % by CTH in hepatocytes from Cdo1-null mice.     

The peroxisome proliferator-activated receptor alpha (PPARalpha) agonist ciprofibrate inhibits apolipoprotein B mRNA editing in low density lipoprotein receptor-deficient mice: effects on plasma lipoproteins and the development of atherosclerotic lesions

Low density lipoprotein receptor (LDLR)-deficient mice fed a chow diet have a mild hypercholesterolemia caused by the abnormal accumulation in the plasma of apolipoprotein B (apoB)-100- and apoB-48-carrying intermediate density lipoproteins (IDL) and low density lipoproteins (LDL). Treatment of LDLR-deficient mice with ciprofibrate caused a marked decrease in plasma apoB-48-carrying IDL and LDL but at the same time caused a large accumulation of triglyceride-depleted apoB-100-carrying IDL and LDL, resulting in a significant increase in plasma cholesterol levels. These plasma lipoprotein changes were associated with an increase in the hepatic secretion of apoB-100-carrying very low density lipoproteins (VLDL) and a decrease in the secretion of apoB-48-carrying VLDL, accompanied by a significant decrease in hepatic apoB mRNA editing. Hepatic apobec-1 complementation factor mRNA and protein abundance were significantly decreased, whereas apobec-1 mRNA and protein abundance remained unchanged. No changes in apoB mRNA editing occurred in the intestine of the treated animals. After 150 days of treatment with ciprofibrate, consistent with the increased plasma accumulation of apoB-100-carrying IDL and LDL, the LDLR-deficient mice displayed severe atherosclerotic lesions in the aorta. These findings demonstrate that ciprofibrate treatment decreases hepatic apoB mRNA editing and alters the pattern of hepatic lipoprotein secretion toward apoB-100-associated VLDL, changes that in turn lead to increased atherosclerosis.     

Liver-specific knockdown of JNK1 up-regulates proliferator-activated receptor gamma coactivator 1 beta and increases plasma triglyceride despite reduced glucose and insulin levels in diet-induced obese mice

The c-Jun N-terminal kinases (JNKs) have been implicated in the development of insulin resistance, diabetes, and obesity. Genetic disruption of JNK1, but not JNK2, improves insulin sensitivity in diet-induced obese (DIO) mice. We applied RNA interference to investigate the specific role of hepatic JNK1 in contributing to insulin resistance in DIO mice. Adenovirus-mediated delivery of JNK1 short-hairpin RNA (Ad-shJNK1) resulted in almost complete knockdown of hepatic JNK1 protein without affecting JNK1 protein in other tissues. Liver-specific knockdown of JNK1 resulted in significant reductions in circulating insulin and glucose levels, by 57 and 16%, respectively. At the molecular level, JNK1 knockdown mice had sustained and significant increase of hepatic Akt phosphorylation. Furthermore, knockdown of JNK1 enhanced insulin signaling in vitro. Unexpectedly, plasma triglyceride levels were robustly elevated upon hepatic JNK1 knockdown. Concomitantly, expression of proliferator-activated receptor gamma coactivator 1 beta, glucokinase, and microsomal triacylglycerol transfer protein was increased. Further gene expression analysis demonstrated that knockdown of JNK1 up-regulates the hepatic expression of clusters of genes in glycolysis and several genes in triglyceride synthesis pathways. Our results demonstrate that liver-specific knockdown of JNK1 lowers circulating glucose and insulin levels but increases triglyceride levels in DIO mice.