Regulation of pathways of extramitochondrial fatty acid oxidation and liver fatty acid-binding protein by long-chain monocarboxylic fatty acids in hepatocytes. Effect of inhibition of carnitine palmitoyltransferase I

Tubular CPT1A deletion minimally affects aging and chronic kidney injury

Kidney tubules use fatty acid oxidation (FAO) to support their high energetic requirements. Carnitine palmitoyltransferase 1A (CPT1A) is the rate-limiting enzyme for FAO, and it is necessary to transport long-chain fatty acids into mitochondria. To define the role of tubular CPT1A in aging and injury, we generated mice with tubule-specific deletion of Cpt1a (Cpt1aCKO mice), and the mice were either aged for 2 years or injured by aristolochic acid or unilateral ureteral obstruction. Surprisingly, Cpt1aCKO mice had no significant differences in kidney function or fibrosis compared with wild-type mice after aging or chronic injury. Primary tubule cells from aged Cpt1aCKO mice had a modest decrease in palmitate oxidation but retained the ability to metabolize long-chain fatty acids. Very-long-chain fatty acids, exclusively oxidized by peroxisomes, were reduced in kidneys lacking tubular CPT1A, consistent with increased peroxisomal activity. Single-nuclear RNA-Seq showed significantly increased expression of peroxisomal FAO enzymes in proximal tubules of mice lacking tubular CPT1A. These data suggest that peroxisomal FAO may compensate in the absence of CPT1A, and future genetic studies are needed to confirm the role of peroxisomal β-oxidation when mitochondrial FAO is impaired.

Comparative liver transcriptome analysis of duck reveals potential genes associated with egg production

BACKGROUND

Molecular studies on egg production in ducks were mostly focused on brain and ovaries as they are directly involved in egg production. Liver plays a vital role in cellular lipid metabolism. It also plays a decisive role in reproductive organ development, including yolk generation in laying ducks at sexual maturity. However, the precise molecular mechanism involved in the liver-blood-ovary axis in ducks remains elusive.

METHODS AND RESULTS

In this study, we analysed the liver transcriptome of laying (LA), immature (IM) and broody (BR) ducks using RNA sequencing to understand the role of genes expressed in the liver. The comparative transcriptome analysis revealed 82 DEGs between LA and IM ducks, 47 DEGs between LA and BR ducks and 51 DEGs between IM and BR ducks. GO analysis of DEGs, showed that DEGs were mainly involved in cellular anatomical entity, intracellular, metabolic process, and binding. Furthermore, pathway analysis indicated the important role of Wnt signaling pathway in egg formation and embryo development. Our study showed several candidate genes including vitellogenin-1, vitellogenin-2, riboflavin binding protein, G protein subunit gamma 4, and fatty acid binding protein 3 that are potentially related to egg production in ducks.

CONCLUSIONS

The study provides valuable information on the genes responsible for egg production and thus, pave the way for further investigation on the molecular mechanisms of egg production in duck.

The Role of Elevated Liver-Type Fatty Acid-Binding Proteins in Liver Diseases

Liver-type fatty acid-binding protein (L-FABP) is mainly expressed in the liver as well as the proximal tubular epithelial cells in the kidney. In general, the proteins and enzymes existing within the hepatocytes have the potential to become biomarkers, for instance alanine aminotransferase, which reflects hepatocellular damage. However, due to reduced hepatocellular function in late stage of chronic liver diseases (e.g. cirrhosis), proteins and enzymes relating to hepatocellular damage are not always accurate measures of disease progression. Recently, several publications have demonstrated elevated serum L-FABP levels during the progression of human liver diseases, including hepatocellular carcinoma (HCC), and were a prognostic factor for survival in acute and chronic liver disease patients. However, the study regarding serum L-FABP levels and hepatic L-FABP expression in liver diseases is not sufficient to understand the molecular mechanism of L-FABP during the progression of these disease states. In this review, we focus on the use of serum and/or hepatic L-FABP expression as a biomarker in human liver diseases, including mechanistic potential in HCC.

Maternal vitamin B12 deficiency in rats alters DNA methylation in metabolically important genes in their offspring

Vitamin B₁₂ deficiency is a critical problem worldwide and peri-conceptional deficiency of this vitamin is associated with the risk of complex cardio-metabolic diseases. Nutritional perturbations during these stages of development may lead to changes in the fetal epigenome. Using Wistar rat model system, we have earlier shown that low maternal B₁₂ levels are associated with low birth weight, adiposity, insulin resistance, and increased triglyceride levels in the offspring, which might predispose them to the risk of cardio-metabolic diseases in adulthood. In this study, we have investigated the effects of maternal B₁₂ deficiency on genome-wide DNA methylation profile of the offspring and the effect of rehabilitation of mothers with B₁₂ at conception. We have performed methylated DNA immunoprecipitation sequencing of liver from pups in four groups of Wistar rats: Control (C), B₁₂-restricted (B₁₂R), B₁₂-rehabilitated at conception (B₁₂RC), and B₁₂-rehabilitated at parturition (B₁₂RP). We have analyzed differentially methylated signatures between the three groups as compared to controls. We have identified a total of 214 hypermethylated and 142 hypomethylated regions in the 10 kb upstream region of transcription start site in pups of B₁₂-deficient mothers, which are enriched in genes involved in fatty acid metabolism and mitochondrial transport/metabolism. B₁₂ rehabilitation at conception and parturition is responsible for reversal of methylation status of many of these regions to control levels suggesting a causal association with metabolic phenotypes. Thus, maternal B₁₂ restriction alters DNA methylation of genes involved in important metabolic processes and influences the offspring phenotype, which is reversed by B₁₂ rehabilitation of mothers at conception.

Serum Liver-Type Fatty Acid-Binding Protein Is a Possible Prognostic Factor in Human Chronic Liver Diseases From Chronic Hepatitis to Liver Cirrhosis and Hepatocellular Carcinoma

Liver‐type fatty acid–binding protein (L‐FABP) is a key regulator of fatty acid metabolism, but serum L‐FABP levels are not well investigated in chronic liver diseases. We aimed to elucidate the prognostic ability of serum L‐FABP in human chronic liver diseases and compare it with the albumin‐bilirubin (ALBI) score. In 242 chronic liver disease patients, including chronic hepatitis (CH, n = 100), liver cirrhosis (LC, n = 142), and presence of hepatocellular carcinoma (HCC, n = 144), serum L‐FABP levels were correlated with liver function (P < 0.0001), increased in LC compared with CH (P < 0.01), and correlated to ALBI score (P < 0.0001). Serum L‐FABP levels were increased in the presence of HCC (P < 0.0001), correlating to des‐gamma‐carboxy prothrombin (P < 0.0001), alpha‐fetoprotein (P = 0.009), and Barcelona‐Clinic Liver Cancer stage. In the average follow‐up period of 1,054 days, serum L‐FABP levels were elevated (P < 0.0001) in patients who eventually died. The area under the curve (AUC) of serum L‐FABP (0.764) was higher than that of ALB (0.709), and the patients with serum L‐FABP ≤ 6.8 ng/mL had significantly longer rates of survival (P < 0.0001). Serum L‐FABP (hazard ratio [HR] 4.0; P < 0.001), HCC (HR 3.7; P = 0.001), ALBI score (HR 2.7; P < 0.001), and age (HR 1.0; P = 0.049) were independent predictors of survival. In the subgroup who maintained liver function, the AUC of serum L‐FABP (0.751) was higher than that of ALB (0.643). In this subgroup, serum L‐FABP (HR 4.4; P = 0.002) and HCC (HR 13.9; P < 0.001) were independent predictors of survival. Conclusion: Serum L‐FABP is a possible predictor of survival in chronic liver diseases from CH to LC and HCC, including any subgroup that maintains liver function.

Effect of Toxicants on Fatty Acid Metabolism in HepG2 Cells

Impairment of hepatic fatty acid metabolism can lead to liver steatosis and injury. Testing drugs for interference with hepatic fatty acid metabolism is therefore important. To find out whether HepG2 cells are suitable for this purpose, we investigated the effect of three established fatty acid metabolism inhibitors and of three test compounds on triglyceride accumulation, palmitate metabolism, the acylcarnitine pool and dicarboxylic acid accumulation in the cell supernatant and on ApoB-100 excretion in HepG2 cells. The three established inhibitors [etomoxir, methylenecyclopropylacetic acid (MCPA), and 4-bromocrotonic acid (4-BCA)] depleted mitochondrial ATP at lower concentrations than cytotoxicity occurred, suggesting mitochondrial toxicity. They inhibited palmitate metabolism at similar or lower concentrations than ATP depletion, and 4-BCA was associated with cellular fat accumulation. They caused specific changes in the acylcarnitine pattern and etomoxir an increase of thapsic (C18 dicarboxylic) acid in the cell supernatant, and did not interfere with ApoB-100 excretion (marker of VLDL export). The three test compounds (amiodarone, tamoxifen, and the cannabinoid WIN 55,212-2) depleted the cellular ATP content at lower concentrations than cytotoxicity occurred. They all caused cellular fat accumulation and inhibited palmitate metabolism at similar or higher concentrations than ATP depletion. They suppressed medium-chain acylcarnitines in the cell supernatant and amiodarone and tamoxifen impaired thapsic acid production. Tamoxifen and WIN 55,212-2 decreased cellular ApoB-100 excretion. In conclusion, the established inhibitors of fatty acid metabolism caused the expected effects in HepG2 cells. HepG cells proved to be useful for the detection of drug-associated toxicities on hepatocellular fatty acid metabolism.

Transcriptome profile of liver at different physiological stages reveals potential mode for lipid metabolism in laying hens

Background

Liver is an important metabolic organ that plays a critical role in lipid synthesis, degradation, and transport; however, the molecular regulatory mechanisms of lipid metabolism remain unclear in chicken. In this study, RNA-Seq technology was used to investigate differences in expression profiles of hepatic lipid metabolism-related genes and associated pathways between juvenile and laying hens. The study aimed to broaden the understanding of liver lipid metabolism in chicken, and thereby to help improve laying performance in the poultry industry.

Results

RNA-Seq analysis was carried out on total RNA harvested from the liver of juvenile (n = 3) and laying (n = 3) hens. Compared with juvenile hens, 2567 differentially expressed genes (1082 up-regulated and 1485 down-regulated) with P ≤ 0.05 were obtained in laying hens, and 960 of these genes were significantly differentially expressed (SDE) at a false discovery rate (FDR) of ≤0.05 and fold-change ≥2 or ≤0.5. In addition, most of the 198 SDE novel genes (91 up-regulated and 107 down-regulated) were discovered highly expressed, and 332 SDE isoforms were identified. Gene ontology (GO) enrichment and KEGG (Kyoto Encyclopedia of Genes and Genomes) pathway analysis showed that the SDE genes were most enrichment in steroid biosynthesis, PPAR signaling pathway, biosynthesis of unsaturated fatty acids, glycerophospholipid metabolism, three amino acid pathways, and pyruvate metabolism (P ≤ 0.05). The top significantly enriched GO terms among the SDE genes included lipid biosynthesis, cholesterol and sterol metabolic, and oxidation reduction, indicating that principal lipogenesis occurred in the liver of laying hens.

Conclusions

This study suggests that the majority of changes at the transcriptome level in laying hen liver were closely related to fat metabolism. Some of the SDE uncharacterized novel genes and alternative splicing isoforms that were detected might also take part in lipid metabolism, although this needs further investigation. This study provides valuable information about the expression profiles of mRNAs from chicken liver, and in-depth functional investigations of these mRNAs could provide new insights into the molecular networks of lipid metabolism in chicken liver.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1943-0) contains supplementary material, which is available to authorized users.

Molecular cloning and functional analysis of the fatty acid-binding protein (Sp-FABP) gene in the mud crab (Scylla paramamosain)

Intracellular fatty acid-binding proteins (FABPs) are multifunctional cytosolic lipid-binding proteins found in vertebrates and invertebrates. In this work, we used RACE to obtain a full-length cDNA of Sp-FABP from the mud crab Scylla paramamosain. The open reading frame of the full length cDNA (886 bp) encoded a 136 amino acid polypeptide that showed high homology with related genes from other species. Real-time quantitative PCR identified variable levels of Sp-FABP transcripts in epidermis, eyestalk, gill, heart, hemocytes, hepatopancreas, muscle, ovary, stomach and thoracic ganglia. In ovaries, Sp-FABP expression increased gradually from stage I to stage IV of development and decreased in stage V. Sp-FABP transcripts in the hepatopancreas and hemocytes were up-regulated after a bacterial challenge with Vibrio alginnolyficus. These results suggest that Sp-FABP may be involved in the growth, reproduction and immunity of the mud crab.

Mechanisms of triglyceride accumulation in activated macrophages

LPS treatment of macrophages induces TG accumulation, which is accentuated by TG-rich lipoproteins or FFA. We defined pathways altered during macrophage activation that contribute to TG accumulation. Glucose uptake increased with activation, accompanied by increased GLUT1. Oxidation of glucose markedly decreased, whereas incorporation of glucose-derived carbon into FA and sterols increased. Macrophage activation also increased uptake of FFA, associated with an increase in CD36. Oxidation of FA was markedly reduced, whereas the incorporation of FA into TGs increased, associated with increased GPAT3 and DGAT2. Additionally, macrophage activation decreased TG lipolysis; however, expression of ATGL or HSL was not altered. Macrophage activation altered gene expression similarly when incubated with exogenous FA or AcLDL. Whereas activation with ligands of TLR2 (zymosan), TLR3 (poly I:C), or TLR4 (LPS) induced alterations in macrophage gene expression, leading to TG accumulation, treatment of macrophages with cytokines had minimal effects. Thus, activation of TLRs leads to accumulation of TG in macrophages by multiple pathways that may have beneficial effects in host defense but could contribute to the accelerated atherosclerosis in chronic infections and inflammatory diseases.

Molecular cloning and tissue expression of the fatty acid-binding protein (Es-FABP) gene in female Chinese mitten crab (Eriocheir sinensis)

Background

Fatty acid-binding proteins (FABPs), small cytosolic proteins that function in the uptake and utilization of fatty acids, have been extensively studied in higher vertebrates while invertebrates have received little attention despite similar nutritional requirements during periods of reproductive activity.

Results

Therefore, a cDNA encoding Eriocheir sinensis FABP (Es-FABP) was cloned based upon EST analysis of a hepatopancreas cDNA library. The full length cDNA was 750 bp and encoded a 131 aa polypeptide that was highly homologous to related genes reported in shrimp. The 9108 bp Es-FABP gene contained four exons that were interrupted by three introns, a genomic organization common among FABP multigene family members in vertebrates. Gene expression analysis, as determined by RT-PCR, revealed the presence of Es-FABP transcripts in hepatopancreas, hemocytes, ovary, gills, muscle, thoracic ganglia, heart, and intestine, but not stomach or eyestalk. Real-time quantitative RT-PCR analysis revealed that Es-FABP expression in ovary, hemocytes, and hepatopancreas was dependent on the status of ovarian development, with peak expression observed in January.

Conclusions

Evidence provided in the present report supports a role of Es-FABP in lipid transport during the period of rapid ovarian growth in E. sinensis, and indirectly confirms the participation of the hepatopancreas, ovary, and hemocytes in lipid nutrient absorption and utilization processes.

Differential expression of lumican and fatty acid binding protein-1: new insights into the histologic spectrum of nonalcoholic fatty liver disease

The basis of hepatocellular injury and progressive fibrosis in a subset of patients with nonalcoholic fatty liver disease (NAFLD) is poorly understood. We sought to identify hepatic proteins that are differentially abundant across the histologic spectrum of NAFLD. Hepatic protein abundance was measured in liver samples from four groups (n = 10 each) of obese (body mass index >30 kg/m(2)) patients: (1) obese normal group (normal liver histology), (2) simple steatosis (SS), (3) nonalcoholic steatohepatitis (NASH)-mild (steatohepatitis with fibrosis stage 0-1), and (4) NASH-progressive (steatohepatitis with fibrosis stage 2-4). Hepatic peptides were analyzed on an API Qstar XL quadrupole time-of-flight mass spectrometer using Analyst QS software. Linear trends tests were performed and used to screen for differential abundance. Nine known proteins were expressed with differential abundance between study groups. For seven proteins differential abundance is likely to have been on the basis increased hepatic lipid content and/or inflammation. Lumican, a 40-kDa keratin sulfate proteoglycan that regulates collagen fibril assembly and activates transforming growth factor-beta and smooth muscle actin, was expressed similarly in obese normal and SS but was overexpressed in a progressive manner in NASH-mild versus SS (124%, P < 0.001), NASH-progressive versus NASH-mild (156%, P < 0.001) and NASH-progressive versus obese normal (178%, P < 0.001). Fatty acid binding protein-1 (FABP-1), which is protective against the detergent effects of excess free fatty acids, facilitates intracellular free fatty acid transport and is an important ligand for peroxisome proliferator-activated receptor-mediated transcription, was overexpressed in SS when compared to the obese normal group (128%, P < 0.001), but was paradoxically underexpressed in NASH-mild versus SS (73%, P < 0.001), NASH-progressive versus NASH-mild (81%, P < 0.001), and NASH-progressive versus obese normal (59%, P < 0.001).

CONCLUSION

Histologically progressive NAFLD is associated with overexpression of lumican, an important mediator of fibrosis in nonhepatic tissues, whereas FABP-1 is paradoxically underexpressed in NASH, suggesting a new potential mechanism of lipotoxicity in NAFLD. Further studies are needed to determine the biologic basis of lumican and/or FABP-1 dysregulation in NAFLD.

Ethanol and fish oil induce NFkappaB transactivation of the collagen alpha2(I) promoter through lipid peroxidation-driven activation of the PKC-PI3K-Akt pathway

To analyze whether fish oil, as a source of polyunsaturated fatty acids from the n-3 series, could synergize with ethanol to promote collagen I upregulation in vivo, collagen alpha2(I) promoter-betaGal (COL1A2-betaGal) transgenic mice were fed a diet enriched in fish oil in the presence of ethanol (ethanol group) or dextrose (control group). Ethanol-fed mice showed mild steatosis, increased alanine aminotransferase (ALT), aspartate aminotransferase (AST), nonsterified fatty acids, and plasma alcohol levels along with elevated cytochrome P450 2E1 activity, lipid peroxidation end products, and low glutathione (GSH) levels, which suggested enhanced oxidant stress and liver injury. Increased transactivation of the COL1A2 promoter assessed by betaGal activity was shown in vivo and by transfection with deletion constructs for the collagen alpha1(I) promoter (COL1A1) and COL1A2 promoters in vitro. Transcriptional regulation of both COL1A1 and COL1A2 promoters was validated by nuclear in vitro transcription run-on, northern blot analysis, and quantitative polymerase chain reaction, which was followed by the subsequent upregulation of collagen I protein with no changes in matrix metalloproteinase 13 (MMP 13). To further analyze the potential mechanism for collagen I upregulation, an in vitro coculture model was designed with primary stellate cells seeded on the bottom plate of a Boyden chamber and the rest of the liver cells plated on a cell culture insert, and fish oil or fish oil plus ethanol were added. The combination of fish oil plus ethanol increased nuclear factor kappaB binding to the COL1A2 promoter both in vivo and in the cocultures and also resulted in increased phosphorylation of protein kinase C, activation of PI3 kinase, and phosphorylation of Akt. The in vitro addition of vitamin E prevented such activation and collagen I increase. Furthermore, inhibitors of all 3 kinases blocked the increase in collagen I and NFkappaB binding to the COL1A2 promoter; the latter was also prevented by vitamin E.

CONCLUSION

These results suggest that fish oil (mainly n-3 polyunsaturated fatty acids [PUFAs]) can synergize with ethanol to induce collagen I, transactivating the COL1A2 promoter through a lipid peroxidation-PKC-PI3K-Akt-NFkappaB-driven mechanism in the absence of overt steatosis and inflammation.

Expression and antioxidant function of liver fatty acid binding protein in normal and bile-duct ligated rats

Liver fatty acid binding protein has recently been shown to possess antioxidant properties but its role in liver disease, such as cholestasis, is not known. Since oxidative stress has been recognized as an important contributing factor in liver disease, we investigated the expression and antioxidative function of this protein using the bile-duct ligated model of cholestasis. Rats were divided into 3 groups: sham, bile-duct ligated and bile-duct ligated plus clofibrate. Animals were sacrificed at various time points after bile-duct ligation. RT-PCR and Western blot were used to analyze liver fatty acid binding protein expression. Cellular lipid peroxidation products were assessed by measuring thiobarbituric acid-reactive substances. Liver function was evaluated by measuring serum total bilirubin, alanine aminotransferase and ammonia. Liver fatty acid binding protein mRNA and protein levels were reduced to 51% and 20% of sham, respectively at 2 weeks following bile-duct ligation (p<0.05). The decreased liver fatty acid binding protein was associated with a statistical increase in hepatic lipid peroxidation products (224%) and decrease in hepatic function. Clofibrate treatment restored protein level and improved hepatic function. Clofibrate treatment also reduced hepatic lipid peroxidation products by 68% as compared with the bile-duct ligated group (p<0.05). Liver fatty acid binding protein likely has important antioxidant function during hepatocellular oxidative stress.

Alcoholic liver injury in the rat is associated with reduced expression of peroxisome proliferator-alpha (PPARalpha)-regulated genes and is ameliorated by PPARalpha activation

Alcoholic liver disease is associated with a state of hepatic fatty acid overload. We examined the effect of ethanol and different types of dietary fat on the expression of mRNA for liver fatty acid binding protein (L-FABP), peroxisome proliferator-activated receptor-alpha (PPARalpha), and peroxisomal fatty acyl CoA oxidase (FACO). Four groups of rats (n = 5) were fed intragastrically, a liquid diet with or without ethanol (10-16 g/kg/day) for 4 weeks. Pair-fed controls received isocaloric amounts of dextrose. The source of fat was either corn oil or fish oil. Ethanolfed rats developed fatty liver, necrosis, and inflammation; the changes were more severe in the fish oil-ethanol (FE) rats. PPARalpha mRNA levels were not different between groups, although there was a trend toward increased levels in ethanol-fed rats. We calculated L-FABP/PPARalpha and FACO/PPARalpha ratios as a measure of FACO and L-FABP up-regulation relative to PPARalpha expression. Both FACO/PPARalpha and L-FABP/PPARalpha ratios were significantly decreased in FE rats. However, only L-FABP/PPARalpha was decreased in corn oil plus ethanol rats. Also, the level of L-FABP/mRNA correlated inversely with the degree of fatty liver in ethanol-fed rats. Since expression of PPARalpha response genes was impaired in ethanol-fed rats, we determined whether activation of PPARalpha would normalize the PPARalpha response and prevent the pathological changes in ethanol-fed rats. Treatment with clofibrate, a PPARalpha-activating ligand, led to a marked decrease in fatty liver and complete abrogation of necroinflammatory changes in FE rats. Also, nuclear factor kappaB activation and up-regulation of tumor necrosis factor-alpha and cyclooxygenase-2 was also abolished in clofibrate-treated rats. We conclude that adaptive gene regulation of FACO and L-FABP by PPARalpha is impaired in ethanol-fed rats and that treatment with clofibrate, a PPARalpha ligand, prevents alcohol-induced pathological liver injury, possibly by reversing the above changes.

The biochemistry of hypo- and hyperlipidemic fatty acid derivatives: metabolism and metabolic effects

A selection of amphipatic hyper- and hypolipidemic fatty acid derivatives (fibrates, thia- and branched chain fatty acids) are reviewed. They are probably all ligands for the peroxisome proliferation activation receptor (PPARalpha) which has a low selectivity for its ligands. These compounds give hyper- or hypolipidemic responses depending on their ability to inhibit or stimulate mitochondrial fatty acid oxidation in the liver. The hypolipidemic response is explained by the following metabolic effects: Lipoprotein lipase is induced in liver where it is normally not expressed. Apolipoprotein CIII is downregulated. These two effects in liver lead to a facilitated (re)uptake of chylomicrons and VLDL, thus creating a direct transport of fatty acids from the gut to the liver. Fatty acid metabolizing enzymes in the liver (CPT-I and II, peroxisomal and mitochondrial beta-oxidation enzymes, enzymes of ketogenesis, and omega-oxidation enzymes) are induced and create an increased capacity for fatty acid oxidation. The increased oxidation of fatty acids "drains" fatty acids from the body, reduces VLDL formation, and ultimately explains the antiadiposity and improved insulin sensitivity observed after administration of peroxisome proliferators.

The fatty acid transport function of fatty acid-binding proteins

The intracellular fatty acid-binding proteins (FABPs) comprise a family of 14-15 kDa proteins which bind long-chain fatty acids. A role for FABPs in fatty acid transport has been hypothesized for several decades, and the accumulated indirect and correlative evidence is largely supportive of this proposed function. In recent years, a number of experimental approaches which more directly examine the transport function of FABPs have been taken. These include molecular level in vitro modeling of fatty acid transfer mechanisms, whole cell studies of fatty acid uptake and intracellular transfer following genetic manipulation of FABP type and amount, and an examination of cells and tissues from animals engineered to lack expression of specific FABPs. Collectively, data from these studies have provided strong support for defining the FABPs as fatty acid transport proteins. Further studies are necessary to elucidate the fundamental mechanisms by which cellular fatty acid trafficking is modulated by the FABPs.

Effects of carnitine palmitoyltransferase I inhibitors on hepatic hypertrophy

We investigated the effect of two types of carnitine palmitoyltransferase I inhibitors, ethyl 2-(6-(4-chlorophenoxy)hexyl)oxirane-2-carboxylate (etomoxir) and (R)-3-carboxy-N,N, N-trimethyl-2-¿[hydroxy(tetradecyloxy)phosphinyl]oxy¿-1-propana minium hydroxide (SDZ CPI 975), on cardiac and hepatic hypertrophy in ddY mice. One-week administration of etomoxir caused cardiac and hepatic hypertrophy, 19% and 22% as a ratio to body weight, respectively. Although 4-week administration of etomoxir caused hepatic hypertrophy, there was no significant change in liver triglyceride content in the first or second week. In cultured HepG(2) cells, etomoxir treatment (1 week) did not cause triglyceride to accumulate. One-week administration of SDZ CPI 975 caused neither cardiac nor hepatic hypertrophy. In vitro, neither drug had selectivity for carnitine palmitoyltransferase I isozymes. These findings suggest that the hepatic hypertrophy following 1- or 2-week treatment with etomoxir is caused by mechanisms different from those responsible for triglyceride accumulation, and that inhibition of carnitine palmitoyltransferase I may not necessarily induce hepatic hypertrophy.

Etomoxir-induced PPARalpha-modulated enzymes protect during acute renal failure

Regulation of fatty acid beta-oxidation (FAO) represents an important mechanism for a sustained balance of energy production/utilization in kidney tissue. To examine the role of stimulated FAO during ischemia, Etomoxir (Eto), clofibrate, and WY-14,643 compounds were given 5 days prior to the induction of ischemia/reperfusion (I/R) injury. Compared with rats administered vehicle, Eto-, clofibrate-, and WY-treated rats had lower blood urea nitrogen and serum creatinines following I/R injury. Histological analysis confirmed a significant amelioration of acute tubular necrosis. I/R injury led to a threefold reduction of mRNA and protein levels of acyl CoA oxidase (AOX) and cytochrome P4A1, as well as twofold inhibition of their enzymatic activities. Eto treatment prevented the reduction of mRNA and protein levels and the inhibition of the enzymatic activities of these two peroxisome proliferator-activated receptor-alpha (PPARalpha) target genes during I/R injury. PPARalpha null mice subjected to I/R injury demonstrated significantly enhanced cortical necrosis and worse kidney function compared with wild-type controls. These results suggest that upregulation of PPARalpha-modulated FAO genes has an important role in the observed cytoprotection during I/R injury.

Stimulation of oxygen consumption following addition of lipid substrates in liver and skeletal muscle from rats fed a high-fat diet

We studied hepatic and skeletal muscle metabolic activity in rats fed a high-fat diet. Rats were fed a low-fat or high-fat diet for 15 days. At the end of the experimental period, full energy-balance determinations together with serum free triiodothyronine (FT3), leptin, and free fatty acid (FFA) measurements were performed. In addition, we assessed fatty acid-stimulated oxygen consumption in perfused liver and in skeletal muscle homogenate. Rats fed a high-fat diet showed a significant increase in energy intake but no variation in body energy gain, due to a significant increase in energy expenditure. Serum FT3 and FFA levels significantly increased in rats fed a high-fat diet versus rats fed a low-fat diet, while no variation was found in serum leptin levels. Perfused livers and skeletal muscle homogenates from rats fed a high-fat diet exhibited a significant increase in fatty acid-stimulated oxygen consumption. Our results suggest that the enhanced fatty acid oxidation rates in liver and skeletal muscle contribute to the maintenance of fat balance in response to increased fat intake, preventing excess fat deposition.

Eicosapentaenoic and docosahexaenoic acid affect mitochondrial and peroxisomal fatty acid oxidation in relation to substrate preference

Decreased triacylglycerol synthesis within hepatocytes due to decreased diacylglycerol acyltransferase (DGAT) activity has been suggested to be an important mechanism by which diets rich in fish oil lower plasma triacylglycerol levels. New findings suggest that eicosapentaenoic acid (EPA), and not docosahexaenoic acid (DHA), lowers plasma triacylglycerol by increased mitochondrial fatty acid oxidation and decreased availability of fatty acids for triacylglycerol synthesis. To contribute to the understanding of the triacylglycerol-lowering mechanism of fish oil, the different metabolic properties of EPA and DHA were studied in rat liver parenchymal cells and isolated rat liver organelles. EPA-CoA was a poorer substrate than DHA-CoA for DGAT in isolated rat liver microsomes, and in the presence of EPA, a markedly lower value for the triacyl[3H]glycerol/diacyl[3H]glycerol ratio was observed. The distribution of [1-14C]palmitic acid was shifted from incorporation into secreted glycerolipids toward oxidation in the presence of EPA (but not DHA) in rat liver parenchymal cells. [1-14C]EPA was oxidized to a much greater extent than [1-14C]DHA in rat liver parenchymal cells, isolated peroxisomes, and especially in purified mitochondria. As the oxidation of EPA was more effective and sensitive to the CPT-I inhibitor, etomoxir, when measured in a combination of both mitochondria and peroxisomes, we hypothesized that both are involved in EPA oxidation, whereas DHA mainly is oxidized in peroxisomes. In rats, EPA treatment lowered plasma triacylglycerol and increased hepatic mitochondrial fatty acid oxidation and carnitine palmitoyltransferase (CPT)-I activity in both the presence and absence of malonyl-CoA. Whereas only EPA treatment increased the mRNA levels of CPT-I, DHA treatment increased the mRNA levels of peroxisomal fatty acyl-CoA oxidase and fatty acid binding protein more effectively than EPA treatment. In conclusion, EPA and DHA affect cellular organelles in relation to their substrate preference. The present study strongly supports the hypothesis that EPA, and not DHA, lowers plasma triacylglycerol by increased mitochondrial fatty acid oxidation.

Down-regulation of liver and heart specific fatty acid binding proteins by endotoxin and cytokines in vivo

Fatty acid binding proteins (FABPs) are abundantly present in tissues that actively metabolize fatty acids (FA). While their precise physiological function is not known, FABPs have been shown to play a role in the uptake and/or utilization of FA within the cell. FA metabolism is markedly altered during the host response to infection and inflammation. Previous studies have demonstrated that endotoxin or bacterial lipopolysaccharide (LPS) enhances hepatic FA synthesis and re-esterification while inhibiting FA oxidation in liver, heart and muscle. Now, we have examined the in vivo effects of LPS and cytokines on FABPs in liver (L-FABP), heart and muscle (H-FABP). Syrian hamsters were injected with LPS, tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) and the mRNA and protein content for L-FABP and H-FABP were analyzed. 16 h after administration, LPS (100 microg/100 g body weight) produced a 72% decrease in L-FABP mRNA levels in liver and this effect was sustained for 24 h. LPS also produced a 41% decrease in the protein content of L-FABP in liver after 24 h of treatment. TNF-alpha and IL-1beta decreased L-FABP mRNA levels in liver by 30 and 45%, respectively. LPS decreased H-FABP mRNA levels in skeletal muscle by 60% and in heart by 65%. LPS also produced a 49% decrease in H-FABP protein content in muscle. Neither TNF-alpha nor IL-1beta had any significant effect on H-FABP mRNA expression in heart and muscle. Taken together, these results indicate that LPS decreases FABP mRNA and protein levels in liver, heart and muscle, tissues that normally utilize FA as their primary fuel, whereas the inhibitory effect of cytokines is limited to the liver. The LPS-induced decrease in L-FABP and H-FABP may be an additional mechanism contributing to the decrease in FA oxidation that is associated with the host response to infection and inflammation.

Permeation of dicarboxylic acids with different terminal position of two carboxylic groups through planar bilayer lipid membranes

Electrically silent hydrogen ion fluxes across a planar bilayer lipid membrane (BLM) induced by an addition of dicarboxylic (DC) acids at one side of BLM are monitored by measuring pH changes in the unstirred layers near the BLM surface via recording protonophore-dependent potentials. Two groups of DC acids are studied: (1) 2-n-alkylmalonic acids with an alkyl chain of different length which carry both carboxylic groups at one terminus of the hydrocarbon chain (alpha,alpha-DC acids); and (2) dicarboxylic acids of different linear chain length having carboxylic groups at the opposite ends of the hydrocarbon chain (alpha,omega-DC acids). It is shown that the pH optimum of hydrogen ion fluxes for the DC acids is shifted considerably to acidic pH values compared to monocarboxylic acids and is located near pH 5. For both types of DC acids at pH&z. Lt;5, the total transport is limited by diffusion of the anionic forms of the acids across the unstirred layers, while at pH&z.Gt;5 the transport is limited by diffusion of the neutral form across the membrane. The fluxes of alpha,alpha-DC acids are similar to those of alpha,omega-DC acids provided that the acids have the similar number of carbon atoms, the fluxes grow with the increase in the chain length of the alkyl radical.

Mitochondrial 3-hydroxy-3-methylglutaryl coenzyme A synthase and carnitine palmitoyltransferase II as potential control sites for ketogenesis during mitochondrion and peroxisome proliferation

3-Thia fatty acids are potent hypolipidemic fatty acid derivatives and mitochondrion and peroxisome proliferators. Administration of 3-thia fatty acids to rats was followed by significantly increased levels of plasma ketone bodies, whereas the levels of plasma non-esterified fatty acids decreased. The hepatic mRNA levels of fatty acid binding protein and formation of acid-soluble products, using both palmitoyl-CoA and palmitoyl-L-carnitine as substrates, were increased. Hepatic mitochondrial carnitine palmitoyltransferase (CPT) -II and 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase activities, immunodetectable proteins, and mRNA levels increased in parallel. In contrast, the mitochondrial CPT-I mRNA levels were unchanged and CPT-I enzyme activity was slightly reduced in the liver. The CoA ester of the monocarboxylic 3-thia fatty acid, tetradecylthioacetic acid, which accumulates in the liver after administration, inhibited the CPT-I activity in vitro, but not that of CPT-II. Acetoacetyl-CoA thiolase and HMG-CoA lyase activities involved in ketogenesis were increased, whereas the citrate synthase activity was decreased. The present data suggest that 3-thia fatty acids increase both the transport of fatty acids into the mitochondria and the capacity of the beta-oxidation process. Under these conditions, the regulation of ketogenesis may be shifted to step(s) beyond CPT-I. This opens the possibility that mitochondrial HMG-CoA synthase and CPT-II retain some control of ketone body formation.

Arachidonic and docosahexaenoic acids differentially affect the expression of fatty acyl-CoA oxidase, protein kinase C and lipid peroxidation in HepG2 cells

Arachidonic (AA) and docosahexaenoic (DHA) acids (5-20 microM), when supplemented to human hepatoma HepG2 cells, which are depleted in these long-chain polyunsaturated fatty acids in conventional culture conditions, enhance the expression of acyl-CoA oxidase (ACOX), the first enzyme in the peroxisomal beta-oxidation cycle. DHA is effective at lower concentrations (at 5 microM) and to a greater extent (about 60% increment) than AA (about 40%) at 20 microM. Protein kinase C (PKC) appears to be involved in the activity of AA on ACOX, but not in that of DHA, since only the effect of AA is prevented by the PKC inhibitor Staurosporine, and since a remarkable elevation of the PKC activator diacylglycerol occurs only after AA supplementation. AA also induces elevation of lipoperoxides, favoured by the relative vitamin E deficiency occurring in cultured cells, and this effect, which is prevented by supplementation of the vitamin, may contribute to PKC activation.

Polyunsaturated fatty acid suppression of hepatic fatty acid synthase and S14 gene expression does not require peroxisome proliferator-activated receptor alpha

Dietary polyunsaturated fatty acids (PUFA) induce hepatic peroxisomal and microsomal fatty acid oxidation and suppress lipogenic gene expression. The peroxisome proliferator-activated receptor alpha (PPARalpha) has been implicated as a mediator of fatty acid effects on gene transcription. This report uses the PPARalpha-deficient mouse to examine the role of PPARalpha in the PUFA regulation of mRNAs encoding hepatic lipogenic (fatty acid synthase (FAS) and the S14 protein (S14)), microsomal (cytochrome P450 4A2 (CYP4A2)), and peroxisomal (acyl-CoA oxidase (AOX)) enzymes. PUFA ingestion induced mRNAAOX (2.3-fold) and mRNACYP4A2 (8-fold) and suppressed mRNAFAS and mRNAS14 by >/=80% in wild type mice. In PPARalpha-deficient mice, PUFA did not induce mRNAAOX or mRNACYP4A2, indicating a requirement for PPARalpha in the PUFA-mediated induction of these enzymes. However, PUFA still suppressed mRNAFAS and mRNAS14 in the PPARalpha-deficient mice. Studies in rats provided additional support for the differential regulation of lipogenic and peroxisomal enzymes by PUFA. These studies provide evidence for two distinct pathways for PUFA control of hepatic lipid metabolism. One requires PPARalpha and is involved in regulating peroxisomal and microsomal enzymes. The other pathway does not require PPARalpha and is involved in the PUFA-mediated suppression of lipogenic gene expression.

Long-term effects of fatty acids on cell viability and gene expression of neonatal cardiac myocytes

Fatty acids are the most important source of energy for the adult heart. However, cardiac substrate preference changes during development and alters in pathophysiological states. Fatty acids have also been shown to be involved in signal transduction pathways, thereby affecting gene expression in various cell systems. In the present paper the significance of changes in substrate preference and the potential role of fatty acids in signal transduction in the cardiomyocyte are briefly reviewed. Furthermore, the development of a cellular model system, useful in exploring the long-term effects of fatty acids on the normal and hypertrophic cardiomyocyte, is described. Some aspects of this model system are illustrated by showing the effects of different fatty acid species on cell viability and the effects of fatty acids on the expression of heart type fatty acid-binding protein (H-FABP), a 15 kDa protein thought to be involved in intracellular trafficking of fatty acids. To this end primary cultures of rat neonatal ventricular myocytes were kept in defined medium containing various (combinations of) substrates for up to 48 h. First, the effects of prolonged exposure to different fatty acid species, complexed to BSA, on cell viability were investigated. Exposure of the cells to saturated fatty acids (C16:0 or C18:0), but not mono-unsaturated (C16:1 or C18:1) fatty acids, resulted in cell death, as evidenced by the release of intracellular proteins like lactate dehydrogenase. The detrimental effects of saturated fatty acids were nullified by the co-addition of mono-unsaturated fatty acids. Accordingly, the combination of C16:0/C18:1 was used to examine the effects of fatty acids on the expression of H-FABP. Therefore, the cells were incubated with either (i) glucose only, (ii) fatty acids only, or (iii) glucose plus fatty acids. Incubation with fatty acids (with or without glucose) resulted in a nearly four-fold increase of the H-FABP mRNA level. Similarly, at the protein level the cellular H-FABP/LDH ratio increased almost two-fold. In hypertrophic cardiomyocytes (stimulated with the alpha1-adrenergic agonist phenylephrine) the stimulatory effect of fatty acids on H-FABP expression was mitigated. These findings strongly suggest that fatty acids are able to modulate gene expression in the context of the cardiac muscle cell.

Peroxisome proliferator-activated receptor alpha inhibits hepatic S14 gene transcription. Evidence against the peroxisome proliferator-activated receptor alpha as the mediator of polyunsaturated fatty acid regulation of s14 gene transcription

The peroxisome proliferator-activated receptor (PPARalpha) has been implicated in fatty acid regulation of gene transcription. Lipogenic gene transcription is inhibited by polyunsaturated fatty acids (PUFA). We have used the PUFA-sensitive rat liver S14 gene as a model to examine the role PPARalpha plays in fatty acid regulation of hepatic lipogenic gene transcription. Both PPARalpha and the potent peroxisome proliferator, WY14643, inhibit S14CAT activity in transfected primary hepatocytes. WY14643 and PPARalpha target the S14 T3 regulatory region (TRR, -2.8 to -2.5 kilobases), a region containing 3 T3 response elements (TRE). Transfer of the TRR to the thymidine kinase (TK) promoter conferred negative control to the TKCAT gene following WY14643 and PPARalpha treatment. Gel shift analysis showed that PPARalpha, either alone or with RXRalpha, did not bind the S14TRR. However, PPARalpha interfered with TRbeta/RXRalpha binding to a TRE (DR+4). Functional studies showed that co-transfected RXRalpha, but not T3 receptor beta1 (TRbeta1), abrogated the inhibitory effect of PPARalpha on S14 gene transcription. These results suggest that WY14643 and PPARalpha functionally interfere with T3 regulation of S14 gene transcription by inhibiting TRbeta1/RXR binding to S14 TREs. Previous studies had established that the cis-regulatory targets of PUFA control were located within the proximal promoter region of the S14 gene, i.e. between -220 and -80 bp. Finding that the cis-regulatory elements for WY14643/PPARalpha and PUFA are functionally and spatially distinct argues against PPARalpha as the mediator of PUFA suppression of S14 gene transcription.

Influence of etomoxir on the expression of several genes in liver, testis and heart

1. The effect of ethyl-2-[6-(4-chlorophenoxy)hexyl) oxirane-2-carboxylate (etomoxir) and its oxirane analogues on the expression of several genes from liver and testis as well as the beneficial effect of etomoxir on heart performance and myosin isozyme expression is reviewed. 2. In liver, the effect of etomoxir, alone or in combination with fat or di-(2-ethylhexyl)phthalate (DEHP) on the expression of several genes related to lipid metabolism has been studied. The simultaneous addition of etomoxir and a fat diet produces an increase in the expression of carnitine palmitoyl transferase (CPT) I, cytochrome P-450 4A1 omega-hydroxylase and fatty acid binding protein (L-FABP). The mRNA levels of other genes such as CPT II, 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, and fatty acid synthase (FAS) are increased by etomoxir alone. Neither cytosolic nor mitochondrial HMG-CoA synthase have any significant effect on the mRNA levels induced by etomoxir. A probably frequent mechanism for the action of etomoxir may involve the overload of non-metabolized fatty acids produced after the inhibition of CPT I by the oxirane compounds. There is some speculation as to whether the peroxisome proliferator activated receptor (PPAR) increases its participation in the expression, under the action of etomoxir. 3. In testis, the changes in several genes related to cholesterogenesis, ketogenesis, fatty acid synthesis and transport of fatty acids into mitochondria have also been reviewed. Etomoxir in testis does not appear to produce any effect either alone or in combination with DEHP or a fat diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of rat liver apolipoprotein A-I, apolipoprotein A-II and acyl-coenzyme A oxidase gene expression by fibrates and dietary fatty acids

The regulation by fibrates and dietary fatty acids of the hepatic gene expression of apolipoproteins (apo) A-I and A-II, the major protein constituents of high-density lipoproteins, as well as of acyl-CoA oxidase, the rate-limiting enzyme of the peroxisomal beta-oxidation pathway, was studied in vivo in the rat and in vitro in primary cultures of rat hepatocytes. In primary hepatocytes, different fibrates decreased apo A-I and increased acyl-CoA oxidase mRNA levels, whereas apo A-II mRNA only decreased in level after treatment with fenofibric acid, but not after bezafibrate, gemfibrozil or Wy-14643 treatment. Treatment with fenofibric acid counteracted the increase in apo A-I mRNA levels observed after dexamethasone or all-trans retinoic acid treatment, whereas simultaneous addition of fenofibric acid together with all-trans retinoic acid or dexamethasone resulted in a superinduction of acyl-CoA oxidase mRNA. Addition of the n-3 polyunsaturated fatty acids (PUFAs), docosanohexaenoic acid and eicosanopentaenoic acid, or the fatty acid derivative alpha-bromopalmitate, decreased apo A-I and increased acyl-CoA oxidase mRNA in a dose-dependent and time-dependent manner, whereas apo A-II mRNA did not change significantly. Nuclear run-on experiments demonstrated that fenofibric acid and alpha-bromopalmitate decreased apo A-I and increased acyl-CoA oxidase gene expression at the transcriptional level. When rats were fed isocaloric diets enriched in saturated fat (hydrogenated coconut oil), n-6 PUFAs (safflower oil) or n-3 PUFAs (fish oil), a significant decrease in liver apo A-I and apo A-II mRNA levels was only observed after fish oil feeding. Compared to feeding low fat, liver acyl-CoA oxidase mRNA increased after fat feeding, but this effect was most pronounced (twofold) in rats fed fish oil. Results from these studies indicate that fish oil feeding reduces rat liver apo A-I and apo A-II gene expression, similar to results obtained after feeding fenofibrate. Fibrates and n-3 fatty acids (and the fatty acid derivative, alpha-bromopalmitate) down-regulate apo A-I and induce acyl-CoA oxidase gene expression through a direct transcriptional action on the hepatocyte. In contrast, only fenofibric acid, but not the other fibrates or fatty acids tested, decrease apo A-II gene expression in vitro.

Hypolipidemic agents alter hepatic mitochondrial respiration in vitro

The direct effects of three different classes of structurally diverse hypolipidemic agents on respiration were studied in mitochondria isolated from donor Sprague-Dawley rats. Two classes of peroxisome proliferators (i.e. plasticizers and hypolipidemic hormones and drugs) and one class of peroxisome inhibitors (i.e. anti-psychotic drugs) were studied. The phthalate ester plasticizers dibutylphthalate, ethylhexanoic acid and di(2-ethylhexyl) adipate, the hypolipidemic hormones or drugs dehydro-epiandrosterone (DHEA), thyroxine (T4), triiodothyronine (T3), gemfibrozil, clofibrate and naphthoflavone, and the anti-psychotic drugs chlorpromazine, thioridazine and fluphenazine were studied. As the dose of the plasticizer dibutylphthalate increased from 8 to 200 mumol/l, there was a decrease (P < 0.05) in state 3 (+ADP) respiration and in the respiratory control ratio for both substrates tested. The anti-psychotic drug chlorpromazine decreased state 3 malate + pyruvate-supported respiration and increased state 3 succinate-supported respiration. As the concentration of all three anti-psychotic drugs increased, there was a linear increase in state 4 respiration (-ADP) and a decrease in the respiratory control ratio for both substrates tested. As the dose of the hypolipidemic agents DHEA, gemfibrozil and T4 increased, there was a linear reduction in state 3 malate + pyruvate-supported respiration. However, when succinate was used as the substrate to support respiration, only the thyroid hormones significantly decreased state 3 respiration. Gemfibrozil, T4 and T3 increased state 4 respiration, regardless of the substrate used. As the dose of clofibrate, gemfibrozil, and the thyroid hormones increased, there was a linear reduction in the respiratory control ratio for both substrates tested.(ABSTRACT TRUNCATED AT 250 WORDS)

Fatty acids in cell signalling: modulation by lipid binding proteins

Long-chain fatty acids and several of their metabolites have now been shown to be involved as primary or secondary messengers in specific cell signalling pathways. In view of their extremely low aqueous solubility, the extracellular as well as intracellular transport of these compounds is assumed to be facilitated by specific lipid binding proteins, such as cytoplasmic fatty acid-binding protein (FABP). In this paper a survey is given on the biological significance and possible modulatory action of intracellular lipid binding proteins for fatty acid-mediated signal transduction pathways.

Modulation of mitogenesis by liver fatty acid binding protein

Liver fatty acid binding protein (L-FABP), a cytoplasmic 14 kDa protein previously termed Z protein, is conventionally considered to be an intracellular carrier of fatty acids in rat hepatocytes. The following evidence now indicates that L-FABP is also a specific mediator of mitogenesis of rat hepatocytes: a. the synergy between the action of L-FABP and unsaturated fatty acids, especially linoleic acid, in the promotion of cell proliferation; b. the specific requirement for L-FABP in induction of mitogenesis by two classes of nongenotoxic hepatocarcinogenic peroxisome proliferators (amphipathic carboxylates and tetrazole-substituted acetophenones); c. the direct correlation between the binding avidities of different prostaglandins for L-FABP and their relative growth inhibitory activities toward cultured rat hepatocytes; d. the temporal coincidences between the covalent binding to L-FABP by chemically reactive metabolites of the genotoxic carcinogens, 2-acetylaminofluorene and aminoazo dyes, and their growth inhibitions of hepatocytes during liver carcinogenesis in rats; e. and f. the marked elevations of L-FABP in rat liver during mitosis in normal and regenerating hepatocytes, and during the entire cell cycle in the hyperplastic and malignant hepatocytes that are produced by the genotoxic carcinogens, 2-acetylaminofluorene and aminoazo dyes. These actions of L-FABP are consistent with those of a protein involved in regulation of hepatocyte multiplication. Discovery that L-FABP, the target protein of the two types of genotoxic carcinogens, is required for the mitogenesis induced by two classes of nongenotoxic carcinogens points to a common process by which both groups of carcinogens promote hepatocyte multiplication. The implication is that during tumor promotion of liver carcinogenesis, these genotoxic and nongenotoxic carcinogens modify the normal process by which L-FABP, functioning as a specific receptor of unsaturated fatty acids or their metabolites, promotes the multiplication of hepatocytes.