Expression of putative fatty acid transporter genes are regulated by peroxisome proliferator-activated receptor alpha and gamma activators in a tissue- and inducer-specific manner

Regulation of gene expression of three putative long-chain fatty acid transport proteins, fatty acid translocase (FAT), mitochondrial aspartate aminotransferase (mAspAT), and fatty acid transport protein (FATP), by drugs that activate peroxisome proliferator-activated receptor (PPAR) alpha and gamma were studied using normal and obese mice and rat hepatoma cells. FAT mRNA was induced in liver and intestine of normal mice and in hepatoma cells to various extents only by PPARalpha-activating drugs. FATP mRNA was similarly induced in liver, but to a lesser extent in intestine. The induction time course in the liver was slower for FAT and FATP mRNA than that of an mRNA encoding a peroxisomal enzyme. An obligatory role of PPARalpha in hepatic FAT and FATP induction was demonstrated, since an increase in these mRNAs was not observed in PPARalpha-null mice. Levels of mAspAT mRNA were higher in liver and intestine of mice treated with peroxisome proliferators, while levels in hepatoma cells were similar regardless of treatment. In white adipose tissue of KKAy obese mice, thiazolidinedione PPARgamma activators (pioglitazone and troglitazone) induced FAT and FATP more efficiently than the PPARalpha activator, clofibrate. This effect was absent in brown adipose tissue. Under the same conditions, levels of mAspAT mRNA did not change significantly in these tissues. In conclusion, tissue-specific expression of FAT and FATP genes involves both PPARalpha and -gamma. Our data suggest that among the three putative long-chain fatty acid transporters, FAT and FATP appear to have physiological roles. Thus, peroxisome proliferators not only influence the metabolism of intracellular fatty acids but also cellular uptake, which is likely to be an important regulatory step in lipid homeostasis.     

Secondary abnormality of carnitine biosynthesis results from carnitine reabsorptional system defect in juvenile visceral steatosis mice

We characterized the L-carnitine transport system which is defective in the kidney of juvenile visceral steatosis (JVS) mice by using kidney slices and carnitine-related compounds, and evaluated the influence of the transport defect on the biosynthetic pathway of carnitine. The JVS mouse transport system defect, calculated as the difference in the transport activity between control and JVS mice, was simulated in control by gamma-butyrobetaine (gamma-BB) and acetyl L-carnitine. gamma-BB hydroxylase activity in the liver of JVS mice was double that of control mice, but the hepatic level of gamma-BB in JVS mice was lower than in control mice, suggesting that the conversion of gamma-BB to carnitine is not activated in the liver of JVS mice. JVS mice showed higher fractional excretions not only of L-carnitine but also of gamma-BB and acetyl L-carnitine than control mice, indicating disturbed reabsorption of gamma-BB and acetyl L-carnitine. The disturbed reabsorption of gamma-BB in JVS mice is consistent with the fact that the amount of urinary gamma-BB in JVS mice was four times that of control. The sum of the concentrations of L-carnitine, acetyl L-carnitine and gamma-BB in the urine of JVS mice was not significantly different from that of the control, suggesting no remarkable increase of biosynthesis of gamma-BB and carnitine in JVS mice. All these findings suggest that the carnitine transport system plays a role in the transport of gamma-BB and that carnitine deficiency is aggravated by the disturbed reabsorption of gamma-BB in the kidney.     

Peroxisome proliferator-activated receptor alpha controls the hepatic CYP4A induction adaptive response to starvation and diabetes

The hepatic CYP4A enzymes are important fatty acid and prostaglandin omega-hydroxylases that are highly inducible by fibric acid hypolipidemic agents and other peroxisome proliferators. Induction of the CYP4A enzymes by peroxisome proliferators is mediated through the nuclear peroxisome proliferator-activated receptor alpha (PPARalpha). Fatty acids have recently been identified as endogenous ligands of PPARalpha, and this receptor has been implicated in the regulation of lipid homeostasis. In the present report we characterized the induction of the hepatic CYP4A genes in rats during the altered lipid metabolism associated with starvation and diabetes. The mRNA levels of CYP4A1, CYP4A2, and CYP4A3 were induced 7-17-fold in the livers of fasted animals and 3-8-fold in the livers of diabetic animals. This was accompanied by corresponding changes in CYP4A protein levels and arachidonic and lauric acid omega-hydroxylase activity. Interestingly, feeding animals after the fasting period caused as much as an 80% suppression of CYP4A mRNA levels, whereas CYP4A protein levels and functional activity returned to control values. A second PPARalpha-responsive gene, acyl-CoA oxidase, was also induced in rat liver by diabetes and fasting. By using PPARalpha-deficient mice, we unambiguously demonstrated that PPARalpha is strictly required for hepatic CYP4A induction by starvation and diabetes. Similarly, induction of hepatic thiolase and bifunctional enzyme also required expression of PPARalpha. This represents the first evidence for the pathophysiologically induced activation of a nuclear receptor.     

The effect of starvation on obese mice

Plasma level of total and acylcarnitine and the activities of carnitine acetyltransferase (CAT) and carnitine palmitoyltransferase (PCT) in liver and CAT in brown fat were determined in young obese (ob/ob) mice and their littermates during starvation. Plasma levels of acylcarnitine and beta-hydroxybutyrate rose equally in both groups. Total carnitine levels, however, decreased in lean and rose in obese animals. Hepatic PCT and phosphoenolpyruvate carboxykinase activities rose more in lean than obese mice and brown fat CAT activity decreased in the obese group. Fatty acid synthetase activity decreased equally in the liver in obese mice and their lean littermates.     

Ornithine decarboxylase and polyamines in familial adenomatous polyposis

The effects of concanavalin A (Con A) on liver cytokine gene expression was studied in mice. The CD4 mRNA expression in normal liver suggests the presence of CD4+ T cells. The administration of Con A induced interleukin (IL)-1beta, IL-2 and IL-2 receptor mRNAs, which implies lymphocyte activation in the liver. Interferon-gamma and tumor necrosis factor-alpha mRNA expressions were increased gradually. The present results showed that Con A induced liver cytokine genes. This cytokine gene induction might have been the result of lymphocyte activation in the liver.     

Alpha and beta glucocorticoid receptor mRNA expression in skeletal muscle

The aim of the present study was to investigate the occurrence and autoregulation of both glucocorticoid receptor mRNAs in rat gastrocnemius muscle. The expression of both receptor forms was studied 1, 4 or 12 hours after intra-tracheal instillation of a high dose (100 micrograms) of budesonide; muscular expression was compared with glucocorticoid receptor expression in lung tissue. After Northern blot analysis, hybridization was performed with glucocorticoid receptor, glyceraldehyde-3-phosphate dehydrogenase and glutamine synthetase probes. In the gastrocnemius muscle, both the alpha and beta glucocorticoid receptor mRNA forms were detected and found to be downregulated four hours after the budesonide instillation. alpha/beta glucocorticoid receptor ratios were lower in the gastrocnemius (1.1 +/- 0.2) than in the lungs (2.6 +/- 0.6). In the lungs, at all time points, the average alpha glucocorticoid receptor mRNA levels did not differ from controls, although glutamine synthetase mRNA levels were upregulated. The beta glucocorticoid receptor mRNA was slightly reduced at 1 and 4 hours. In conclusion, after intra-tracheal instillation of budesonide, both alpha and beta glucocorticoid receptor forms were downregulated in muscle tissue. The difference in alpha/beta glucocorticoid receptor mRNA ratios and concentrations between lung and gastrocnemius muscle supports the hypothesis of differential gene regulation by glucocorticoids in different cell types.     

Molecular mechanisms involved in receptor editing at the Ig heavy chain locus

The Anorexia (anx) mutation causes reduced food intake in preweanling mice, resulting in death from starvation within 3-4 weeks. We have found serotonin (5HT) hyperinnervation in the anx brain; altered noradrenergic (NE) innervation may also mediate eating disorders. We examined the expression of synthetic or catabolic monoamine enzyme genes in brainstem nuclei: serotonin transporter (5HTT) and monoamine oxidase A (MAOA) in the raphe nuclei (RN), and MAOA, norepinephrine transporter (NET), and tyrosine hydroxylase (TH) in the locus ceruleus (LC). We compared 3-week old anx with control and 24-h food-deprived wildtype littermates using in situ hybridization to measure mRNA levels by quantitative autoradiography. The anx mutation was correlated with decreased MAOA mRNA in the LC (but not RN), decreased 5HTT mRNA in the RN, and a trend towards lower NET mRNA in the LC. Food deprivation decreased MAOA mRNA in the LC (but not RN), increased TH mRNA in the LC, and did not alter NET or 5HTT mRNA levels. Thus, the effect of the anx mutation on MAOA expression in the LC paralleled the effect of food-deprivation, but the anx mutation and food-deprivation had differential effects on the expression of TH, NET, and 5HTT genes. Decreased 5HTT expression in the anx RN is consistent with upregulation of serotonergic neurotransmission that may accompany 5HT hyperinnervation. Central NE levels or innervation may be altered in anx mice by decreased expression of NET and MAOA and a lack of TH upregulation induced by food deprivation as in wild-type mice.