Characterization of a response element for peroxisomal proliferator activated receptor (PPRE) in human muscle-type carnitine palmitoyltransferase I

Effects of medium chain triglycerides on hepatic fatty acid oxidation in clofibrate-fed newborn piglets

To investigate whether increasing tricarboxylic acid (TCA) cycle activity and ketogenic capacity would augment fatty acid (FA) oxidation induced by the peroxisome proliferator-activated receptor-alpha (PPARα) agonist clofibrate, suckling newborn piglets (n = 54) were assigned to 8 groups following a 2 ( ± clofibrate) × 4 (glycerol succinate [SUC], triglycerides of 2-methylpentanoic acid [T2M], valeric acid [TC5] and hexanoic acid [TC6]) factorial design. Each group was fed an isocaloric milk formula containing either 0% or 0.35% clofibrate (wt/wt, dry matter basis) with 5% SUC, T2M, TC5 or TC6 for 5 d. Another 6 pigs served as newborn controls. Fatty acid oxidation was examined in fresh homogenates of liver collected on d 6 using [1-¹⁴C] palmitic acid (1 mM) as a substrate (0.265 μCi/μmol). Measurements were performed in the absence or presence of L-carnitine (1 mM) or inhibitors of 3-hydroxy-3-methylglutaryl-CoA synthase (L659699, 1.6 μM) or acetoacetate-CoA deacylase (iodoacetamide, 50 μM). Without clofibrate stimulation, ¹⁴C accumulation in CO₂ was higher from piglets fed diets containing T2M and TC5 than SUC, but similar to those fed TC6. Under clofibrate stimulation, accumulation also was higher in homogenates from piglets fed TC5 than all other dietary treatments. Interactions between clofibrate and carnitine or the inhibitors were observed (P = 0.0004) for acid soluble products (ASP). In vitro addition of carnitine increased ¹⁴C-ASP (P < 0.0001) above all other treatments, regardless of clofibrate treatment. The percentage of ¹⁴C in CO₂ was higher (P = 0.0023) in TC5 than in the control group. From these results we suggest that dietary supplementation of anaplerotic and ketogenic FA could impact FA oxidation and modify the metabolism of acetyl-CoA (product of β-oxidation) via alteration of TCA cycle activity, but the modification has no significant impact on the hepatic FA oxidative capacity induced by PPARα. In addition, the availability of carnitine is a critical element to maintain FA oxidation during the neonatal period.

KetogenicHMGCS2Is a c-Myc Target Gene Expressed in Differentiated Cells of Human Colonic Epithelium and Down-Regulated in Colon Cancer

HMGCS2, the gene that regulates ketone body production, is expressed in liver and several extrahepatic tissues, such as the colon. In CaCo-2 colonic epithelial cells, the expression of this gene increases with cell differentiation. Accordingly, immunohistochemistry with specific antibodies shows that HMGCS2 is expressed mainly in differentiated cells of human colonic epithelium. Here, we used a chromatin immunoprecipitation assay to study the molecular mechanism responsible for this expression pattern. The assay revealed that HMGCS2 is a direct target of c-Myc, which represses HMGCS2 transcriptional activity. c-Myc transrepression is mediated by blockade of the transactivating activity of Miz-1, which occurs mainly through a Sp1-binding site in the proximal promoter of the gene. Accordingly, the expression of human HMGCS2 is down-regulated in 90% of Myc-dependent colon and rectum tumors. HMGCS2 protein expression is down-regulated preferentially in moderately and poorly differentiated carcinomas. In addition, it is also down-regulated in 80% of small intestine Myc-independent tumors. Based on these findings, we propose that ketogenesis is an undesirable metabolic characteristic of the proliferating cell, which is down-regulated through c-Myc-mediated repression of the key metabolic gene HMGCS2.

Effects of interleukin-15 on lipid oxidation

Interleukin 15 (IL-15) has previously been shown to have important effects on lipid metabolism in adipose tissue, particularly influencing the rate of the de novo fatty acid synthesis. The results presented here show that chronic administration to rats (100 microg/kg body weight) has important effects on the metabolic fate of an exogenous [(14)C]-triolein load, decreasing the incorporation of lipid into adipose tissue and significantly increasing the total (14)CO(2) formation from [(14)C]-triolein. Skeletal muscle and possibly liver seem to be the main organs involved in the action of IL-15 on lipid oxidation, since the presence of the cytokine in incubated EDL muscle with [(14)C]-palmitic acid increased (14)CO(2) formation by 39%. Concerning the mechanism, the results suggest that the transport of fatty acids into mitochondria could be involved in the action of IL-15 since the cytokine clearly increases the presence of L-CPT-I and CPT-II in liver tissue. In addition, IL-15 treatment resulted in a significant increment in the gene expression of PPARdelta, a transcription factor clearly related with lipid catabolism in many tissues. Altogether, the results presented here suggest that IL-15 alters exogenous lipid partitioning, limiting adipose tissue uptake and favouring oxidation.

Regulation of stearoyl-CoA desaturase expression

Stearoyl-CoA desaturase (SCD) is a regulatory enzyme in lipogenesis, catalyzing the rate-limiting step in the overall de novo synthesis of monounsaturated FA, mainly oleate and palmitoleate from stearoyl- and palmitoyl-CoA, respectively. Oleate and palmitoleate are the major monounsaturated FA of membrane phospholipids, TG, wax esters, cholesterol esters, and alkyldiacylglycerol. Several SCD gene isoforms (SCD1, SCD2, SCD3, and SCD4) exist in mice, and two have been characterized in humans. SCD1 gene expression in liver cells is regulated by numerous stimuli including diet and hormones. We are interested in why SCD is such a highly regulated enzyme even though oleate, the major product of this enzyme, is one of the most abundant FA in the diet and is therefore readily available. Dietary oleate is also well known for its TG-lowering effects and, as a major component of olive oil, is expected to have beneficial effects. However, high SCD activity has been implicated in diabetes, obesity, atherosclerosis, and cancer in several animal models; therefore, the role that de novo oleate plays in these disease states has to be carefully evaluated. By using SCD1-/- mice, which are deficient in tissue oleate, we begin to learn more about the physiological role of SCD gene expression and oleate in normal and disease states.

Emerging roles of PPARs in inflammation and immunity

Lipids and lipid metabolism have well-documented regulatory effects on inflammatory processes. Recent work has highlighted the role of the peroxisome proliferator-activated receptors (PPARs)--a subset of the nuclear-hormone-receptor superfamily that are activated by various lipid species--in regulating inflammatory responses. Here, we describe how the PPARs, through their interactions with transcription factors and other cell-signalling systems, have important regulatory roles in innate and adaptive immunity.