Comparison of metabolism of free fatty acid by isolated perfused livers from male and female rats

Metabolic effects of the schizophrenia-associated 3q29 deletion

The 1.6 Mb 3q29 deletion is associated with developmental and psychiatric phenotypes, including a 40-fold increased risk for schizophrenia. Reduced birth weight and a high prevalence of feeding disorders in patients suggest underlying metabolic dysregulation. We investigated 3q29 deletion-induced metabolic changes using our previously generated heterozygous B6.Del16^+/Bdh1-Tfrc mouse model. Animals were provided either standard chow (STD) or high-fat diet (HFD). Growth curves were performed on HFD mice to assess weight change (n = 30-50/group). Indirect calorimetry and untargeted metabolomics were performed on STD and HFD mice to evaluate metabolic phenotypes (n = 8-14/group). A behavioral battery was performed on STD and HFD mice to assess behavior change after the HFD challenge (n = 5-13/group). We found that B6.Del16^+/Bdh1-Tfrc animals preferentially use dietary lipids as an energy source. Untargeted metabolomics of liver tissue showed a strong sex-dependent effect of the 3q29 deletion on fat metabolism. A HFD partially rescued the 3q29 deletion-associated weight deficit in females, but not males. Untargeted metabolomics of liver tissue after HFD revealed persistent fat metabolism alterations in females. The HFD did not affect B6.Del16^+/Bdh1-Tfrc behavioral phenotypes, suggesting that 3q29 deletion-associated metabolic and behavioral outcomes are uncoupled. Our data suggest that dietary interventions to improve weight phenotypes in 3q29 deletion syndrome patients are unlikely to exacerbate behavioral manifestations. Our study also highlights the importance of assessing sex in metabolic studies and suggests that mechanisms underlying 3q29 deletion-associated metabolic phenotypes are sex-specific.

Phytosterols as bioactive food components against nonalcoholic fatty liver disease

Phytosterols are bioactive food components widely present in cell membranes of plants, especially in nuts and oilseeds. In recent years, many studies have shown that phytosterols possess therapeutic potentials for nonalcoholic fatty liver disease (NAFLD). This review summarizes the effects of phytosterols from in vitro and in vivo studies to lower the levels of total cholesterol (TC) and triglycerides (TG), and the evidence supporting the potential of phytosterols against NAFLD. The potential mechanisms by which phytosterols improve NAFLD may include (i) competition with cholesterol; (ii) regulation of key factors involved in cholesterol and TG metabolism; and (iii) inhibition of liver inflammation and (iv) regulation of liver fatty acid composition. In summary, phytosterols are potential natural ingredients with good safety profile against NAFLD, which deserve more future studies.

Computational Hypothesis: How Intra-Hepatic Functional Heterogeneity May Influence the Cascading Progression of Free Fatty Acid-Induced Non-Alcoholic Fatty Liver Disease (NAFLD)

Non-Alcoholic Fatty Liver Disease (NAFLD) is the most common type of chronic liver disease in developed nations, affecting around 25% of the population. Elucidating the factors causing NAFLD in individual patients to progress in different rates and to different degrees of severity, is a matter of active medical research. Here, we aim to provide evidence that the intra-hepatic heterogeneity of rheological, metabolic and tissue-regenerating capacities plays a central role in disease progression. We developed a generic mathematical model that constitutes the liver as ensemble of small liver units differing in their capacities to metabolize potentially cytotoxic free fatty acids (FFAs) and to repair FFA-induced cell damage. Transition from simple steatosis to more severe forms of NAFLD is described as self-amplifying process of cascading liver failure, which, to stop, depends essentially on the distribution of functional capacities across the liver. Model simulations provided the following insights: (1) A persistently high plasma level of FFAs is sufficient to drive the liver through different stages of NAFLD; (2) Presence of NAFLD amplifies the deleterious impact of additional tissue-damaging hits; and (3) Coexistence of non-steatotic and highly steatotic regions is indicative for the later occurrence of severe NAFLD stages.

Environmental estrogen exposure converts lipid metabolism in male fish to a female pattern mediated by AMPK and mTOR signaling pathways

Environmental estrogens, including bisphenol A (BPA) and 17β-estradiol (E2), which are widely used in industries and medicine, pose a severe ecological threat to fish due to feminization induction. However, the related metabolic basis for reproductive feminization in male fish has not been well addressed. We first found that female zebrafish exhibited higher lipid accumulation and lipogenesis activity than males. Next, we exposed male and female zebrafish to E2 (200 ng/L) or BPA (100 μg/L) for six weeks, and observed an early-phase reproductive feminization in males, accompanied with reduced spermatids, significant fat deposition and lipogenic gene expressions that mimicked female patterns. Cellular signaling assays revealed that, E2 or BPA modulated lipid metabolism in males mainly through lowering 5' AMP-activated protein kinase (AMPK) and upregulating the lipogenic mechanistic target of rapamycin (mTOR) pathways. For the first time, we show that environmental estrogens could alter lipid metabolism in male fish to a female pattern (metabolic feminization) prior to gonad feminization in male fish, to allows males to accumulate efficiently lipids to harmonize with the feminized gonads. This study suggests that negative effects of environmental estrogens, as hazardous materials, on vertebrate health are more complicated than originally thought.

Zonation of hepatic fatty acid metabolism - The diversity of its regulation and the benefit of modeling

A pronounced heterogeneity between hepatocytes in subcellular structure and enzyme activities was discovered more than 50years ago and initiated the idea of metabolic zonation. In the last decades zonation patterns of liver metabolism were extensively investigated for carbohydrate, nitrogen and lipid metabolism. The present review focuses on zonation patterns of the latter. We review recent findings regarding the zonation of fatty acid uptake and oxidation, ketogenesis, triglyceride synthesis and secretion, de novo lipogenesis, as well as bile acid and cholesterol metabolism. In doing so, we expose knowledge gaps and discuss contradictory experimental results, for example on the zonation pattern of fatty acid oxidation and de novo lipogenesis. Thus, possible rewarding directions of further research are identified. Furthermore, recent findings about the regulation of metabolic zonation are summarized, especially regarding the role of hormones, nerve innervation, morphogens, gender differences and the influence of the circadian clock. In the last part of the review, a short collection of models considering hepatic lipid metabolism is provided. We conclude that modeling, despite its proven benefit for understanding of hepatic carbohydrate and ammonia metabolisms, has so far been largely disregarded in the study of lipid metabolism; therefore some possible fields of modeling interest are presented.

Sex-dependent hepatic transcripts and metabolites in the development of glucose intolerance and insulin resistance in Zucker diabetic fatty rats

Male Zucker diabetic fatty (mZDF) rats spontaneously develop type 2 diabetes, whereas females only become diabetic when fed a diabetogenic high-fat diet (high-fat-fed female ZDF rat, HF-fZDF). The aim of this study was to investigate if differences in liver functions could provide clues to this sex difference. Non-diabetic obese fZDF rats were compared with either mZDF or HF-fZDF regarding hepatic molecular profiles, to single out those components that might be protective in the females. High-fat feeding in fZDF led to enhanced weight gain, increased blood glucose and insulin levels, reduced insulin sensitivity and a trend towards reduced glucose tolerance, indicative of a prediabetic state. mZDF rats were diabetic, with low levels of insulin, high levels of glucose, reduced insulin sensitivity and impaired glucose tolerance. Transcript profiling and capillary electrophoresis time-of-flight mass spectrometry were used to indentify hepatic transcripts and metabolites that might be related to this. Many diet-induced alterations in transcript and metabolite levels in female rats were towards a 'male-like' phenotype, including reduced lipogenesis, increased fatty acid (FA) oxidation and increased oxidative stress responses. Alterations detected at the level of hepatic metabolites, indicated lower capacity for glutathione (GSH) production in male rats, and higher GSH turnover in females. Taken together, this could be interpreted as if anabolic pathways involving lipogenesis and lipid output might limit the degree of FA oxidation and oxidative stress in female rats. Together with a greater capacity to produce GSH, these hepatic sex differences might contribute to the sex-different development of diabetes in ZDF rats.

Sex-different hepaticglycogen content and glucose output in rats

Background

Genes involved in hepatic metabolism have a sex-different expression in rodents. To test whether male and female rat livers differ regarding lipid and carbohydrate metabolism, whole-genome transcript profiles were generated and these were complemented by measurements of hepatic lipid and glycogen content, fatty acid (FA) oxidation rates and hepatic glucose output (HGO). The latter was determined in perfusates from in situ perfusion of male and female rat livers. These perfusates were also analysed using nuclear magnetic resonance (NMR) spectroscopy to identify putative sex-differences in other liver-derived metabolites. Effects of insulin were monitored by analysis of Akt-phosphorylation, gene expression and HGO after s.c. insulin injections.

Results

Out of approximately 3 500 gene products being detected in liver, 11% were significantly higher in females, and 11% were higher in males. Many transcripts for the production of triglycerides (TG), cholesterol and VLDL particles were female-predominant, whereas genes for FA oxidation, gluconeogenesis and glycogen synthesis were male-predominant. Sex-differences in mRNA levels related to metabolism were more pronounced during mild starvation (12 h fasting), as compared to the postabsorptive state (4 h fasting). No sex-differences were observed regarding hepatic TG content, FA oxidation rates or blood levels of ketone bodies or glucose. However, males had higher hepatic glycogen content and higher HGO, as well as higher ratios of insulin to glucagon levels. Based on NMR spectroscopy, liver-derived lactate was also higher in males. HGO was inhibited by insulin in parallel with increased phosphorylation of Akt, without any sex-differences in insulin sensitivity. However, the degree of Thr172-phosphorylated AMP kinase (AMPK) was higher in females, indicating a higher degree of AMPK-dependent actions.

Conclusions

Taken together, males had higher ratios of insulin to glucagon levels, higher levels of glycogen, lower degree of AMPK phosphorylation, higher expression of gluconeogenic genes and higher hepatic glucose output. Possibly these sex-differences reflect a higher ability for the healthy male rat liver to respond to increased energy demands.

Sequential changes in the expression of genes involved in lipid metabolism in adipose tissue and liver in response to fasting

The aim of this study was to provide a sequential analysis of the expression patterns of key genes involved in lipid metabolism in white adipose tissue (WAT) and liver and their relationship with blood parameters in response to fasting. Adult male rats were studied under different feeding conditions: feeding state, after 4, 8, or 24 h fasting, and after 3 h refeeding after 8 h fasting. Blood parameters and the expression of genes involved in lipogenesis and lipolysis in WAT and liver were analyzed. mRNA levels of genes involved in lipogenesis in liver (SREBP1c, FAS, and GPAT) had already decreased after 4 h fasting, as well as those of PPARgamma in WAT, whereas the decrease in SREBP1c, FAS, GPAT, and GLUT4 mRNA levels in WAT was observed after 8 h. Concerning lipolytic and fatty-acid-oxidation-related genes, liver PPARalpha, FGF21, CPT1, and PDK4 mRNA levels increased after 8 h fasting and those of ACOX1 after 24 h, and in WAT, ATGL, and CPT1 mRNA levels were greater after 24 h. Three hours refeeding increased the expression levels of PPARgamma in WAT, SREBP1c in both liver and WAT, and GPAT in liver, and decreased the expression levels of PPARalpha, CPT1, and PDK4 in liver. These results give new insight into the different adaptive time course response to fasting in the expression of genes involved in lipid metabolism, thus pointing out the very rapid response of lipogenic genes, particularly in liver, and the later response of lipolytic genes, particularly in WAT.

Women produce fewer but triglyceride-richer very low-density lipoproteins than men

CONTEXT

Very low-density lipoproteins (VLDL) are a major risk factor for cardiovascular disease. The concentrations of VLDL particles and VLDL-triglyceride (TG) in plasma are lower in women than men, but the mechanisms responsible for these differences are unclear.

OBJECTIVE

The objective of the study was to investigate the effects of sex on VLDL-TG and VLDL-apolipoprotein B-100 (apoB-100) metabolism. EXPERIMENTAL DESIGN AND MAIN OUTCOME MEASURES: We measured basal VLDL-TG and VLDL-apoB-100 kinetics by using stable isotope labeled tracers.

SETTING/PARTICIPANTS

Twenty-six healthy, lean subjects (13 men, aged 29+/-5 yr; 13 women, aged 28+/-6 yr) were studied in the General Clinical Research Center at Washington University School of Medicine.

RESULTS

VLDL-TG and VLDL-apoB-100 concentrations were less in women than men (P<0.05). The secretion rate of VLDL-TG was approximately 70% greater (P<0.05), whereas the secretion rate of VLDL-apoB-100 (i.e. VLDL particles) was approximately 20% less (P<0.05) in women than men. The molar ratio of VLDL-TG and VLDL-apoB-100 secretion rates was therefore more than double (P<0.05) in women than men. VLDL-TG plasma clearance rate was approximately 70% greater in women than men (P<0.05), whereas VLDL-apoB-100 plasma clearance rate was not different between sexes. However, VLDL-TG and VLDL-apoB-100 mean residence times in plasma were both shorter (by 45 and 25%, respectively; P<0.05) in women than men.

CONCLUSIONS

Increased VLDL-TG plasma clearance is responsible for lower VLDL-TG concentration, whereas decreased VLDL-apoB-100 secretion rate, combined with shorter VLDL-apoB-100 residence time in plasma, is responsible for lower VLDL-apoB-100 concentration in women than men. The greater molar ratio of VLDL-TG and VLDL-apoB-100 secretion rates suggests that the liver in women secretes fewer but TG-richer VLDL particles than the liver in men.

Fatty acid binding protein is a major determinant of hepatic pharmacokinetics of palmitate and its metabolites

Disposition kinetics of [(3)H]palmitate and its low-molecular-weight metabolites in perfused rat livers were studied using the multiple-indicator dilution technique, a selective assay for [(3)H]palmitate and its low-molecular-weight metabolites, and several physiologically based pharmacokinetic models. The level of liver fatty acid binding protein (L-FABP), other intrahepatic binding proteins (microsomal protein, albumin, and glutathione S-transferase) and the outflow profiles of [(3)H]palmitate and metabolites were measured in four experimental groups of rats: 1) males; 2) clofibrate-treated males; 3) females; and 4) pregnant females. A slow-diffusion/bound model was found to better describe the hepatic disposition of unchanged [(3)H]palmitate than other pharmacokinetic models. The L-FABP levels followed the order: pregnant female > clofibrate-treated male > female > male. Levels of other intrahepatic proteins did not differ significantly. The hepatic extraction ratio and mean transit time for unchanged palmitate, as well as the production of low-molecular-weight metabolites of palmitate and their retention in the liver, increased with increasing L-FABP levels. Palmitate metabolic clearance, permeability-surface area product, retention of palmitate by the liver, and cytoplasmic diffusion constant for unchanged [(3)H]palmitate also increased with increasing L-FABP levels. It is concluded that the variability in hepatic pharmacokinetics of unchanged [(3)H]palmitate and its low-molecular-weight metabolites in perfused rat livers is related to levels of L-FABP and not those of other intrahepatic proteins.

Targeted disruption of peroxisomal proliferator-activated receptor beta (delta) results in distinct gender differences in mouse brain phospholipid and esterified FA levels

The peroxisomal proliferator-activated receptor beta (delta) (PPARbeta) is a nuclear hormone receptor that is ubiquitously expressed and that regulates the transcription of genes involved in lipid metabolism. A homozygous PPARbeta-null mouse has been developed in which the ligand-binding domain of the PPARbeta receptor is disrupted. Analysis of brains from these animals shows that female null mice have 24 and 17% increases in plasmenylethanolamine and phosphatidylserine and a 9% decrease in the level of phosphatidylinositol when compared to controls. The phospholipid changes found in female null mice were associated with increased levels of esterified 18:1n-9, 20:1n-9, 20:4n-6, and 22:5n-3 FA in plasmenylethanolamine, 20:1n-9 in phosphaticlylinositol, and 18:0, 18:1n-9, 18:3n-6, 20:1 n-9, and 20:4n-6 in phosphatidylserine. Increased levels of esterified 18:1n-9, 18:2n-6, 18:3n-6, and 20:1n-9 were also found in the phosphatidylethanolamine fraction despite its cellular content remaining unchanged. Brain phospholipid content in male PPARbeta-null mice did not differ from controls, but increased levels of 20:1n-9 in the phosphatidylinositol and 18:1n-9 in the phosphatidylserine fractions were observed. No changes were found in the content of brain cholesterol, TAG, and FFA in either female or male PPARbeta-null mice. These data suggest that PPARbeta is involved in maintaining FA and phospholipid levels in adult female mouse brain and provide strong evidence that suggests a role for PPARbeta in brain peroxisomal acyl-CoA utilization.

Induction of hepatic cytosolic fatty acid binding protein with clofibrate accelerates both membrane and cytoplasmic transport of palmitate

The role of liver cytosolic fatty acid binding protein (L-FABP) in fatty acid transport and metabolism is unclear. Female liver contains substantially more L-FABP than male liver. Female liver also has a different fatty acid transport phenotype, including more rapid uptake, efflux and cytoplasmic transport. However, it is not known if the greater levels of L-FABP are responsible for these differences. We therefore determined whether increasing L-FABP using clofibrate causes male liver to acquire a female transport phenotype. The multiple indicator dilution (MID) method was used to estimate the rate constants for influx, efflux and cytoplasmic diffusion of palmitate in isolated perfused rat livers. Clofibrate treatment increased cytosolic concentrations of L-FABP 4.2+/-0.8-fold, the rate of cytoplasmic diffusion of palmitate 4.3+/-1.7-fold, and the steady-state palmitate extraction 1.5+/-0.3-fold (mean+/-S.E.). Influx and efflux constants were both increased (by 44% and 79%, respectively) to levels typical of female livers. These data suggest that clofibrate-induced elevation of cytosolic L-FABP not only stimulates intracellular diffusion but also influx and efflux of fatty acids. Possible mechanisms include reducing fatty acid binding to cytoplasmic membranes, induction of membrane fatty acid carriers, and catalyzing fatty acid exchange between aqueous cytoplasm and the plasma membrane.

Defect in peroxisome proliferator-activated receptor alpha-inducible fatty acid oxidation determines the severity of hepatic steatosis in response to fasting

Fasting causes lipolysis in adipose tissue leading to the release of large quantities of free fatty acids into circulation that reach the liver where they are metabolized to generate ketone bodies to serve as fuels for other tissues. Since fatty acid-metabolizing enzymes in the liver are transcriptionally regulated by peroxisome proliferator-activated receptor alpha (PPARalpha), we investigated the role of PPARalpha in the induction of these enzymes in response to fasting and their relationship to the development of hepatic steatosis in mice deficient in PPARalpha (PPARalpha(-/-)), peroxisomal fatty acyl-CoA oxidase (AOX(-/-)), and in both PPARalpha and AOX (double knock-out (DKO)). Fasting for 48-72 h caused profound impairment of fatty acid oxidation in both PPARalpha(-/-) and DKO mice, and DKO mice revealed a greater degree of hepatic steatosis when compared with PPARalpha(-/-) mice. The absence of PPARalpha in both PPARalpha(-/-) and DKO mice impairs the induction of mitochondrial beta-oxidation in liver following fasting which contributes to hypoketonemia and hepatic steatosis. Pronounced steatosis in DKO mouse livers is due to the added deficiency of peroxisomal beta-oxidation system in these animals due to the absence of AOX. In mice deficient in AOX alone, the sustained hyperactivation of PPARalpha and up-regulation of mitochondrial beta-oxidation and microsomal omega-oxidation systems as well as the regenerative nature of a majority of hepatocytes containing numerous spontaneously proliferated peroxisomes, which appear refractory to store triglycerides, blunt the steatotic response to fasting. Starvation for 72 h caused a decrease in PPARalpha hepatic mRNA levels in wild type mice, with no perceptible compensatory increases in PPARgamma and PPARdelta mRNA levels. PPARgamma and PPARdelta hepatic mRNA levels were lower in fed PPARalpha(-/-) and DKO mice when compared with wild type mice, and fasting caused a slight increase only in PPARgamma levels and a decrease in PPARdelta levels. Fasting did not change the PPAR isoform levels in AOX(-/-) mouse liver. These observations point to the critical importance of PPARalpha in the transcriptional regulatory responses to fasting and in determining the severity of hepatic steatosis.

Differential expression of apolipoprotein E messenger RNA within the rat liver lobule determined by in situ hybridization

Apolipoprotein (Apo) E plays a key role in the metabolism of lipoproteins. It also modulates immunoregulation, cell growth and differentiation and the response to nerve injury. The liver is a major site of ApoE synthesis. Most of the circulating ApoE is thought to be of hepatic origin with most synthesized in hepatocytes. We showed that total liver ApoE messenger RNA (mRNA) levels were greater in normal adult female rats than in male and that gender-specific patterns of liver ApoE mRNA expression were present by in situ hybridization. In the male liver, the signal was strongest in the portal area, decreasing toward the central vein with the weakest signal in pericentral hepatocytes, resulting in a hepatic lobular gradient of expression. In female liver, a strong periportal signal also was observed that decreased in Zone 2, similar to that in males, but which then increased in pericentral hepatocytes resulting in a bowl-like distribution in marked contrast with that of the male. The results suggest that ApoE mRNA level is regulated differentially in hepatocytes within the liver plate and that the regulation is gender-dependent. Further, the results suggest that in males, hepatocytes in the portal area are the major contributors of ApoE to the plasma and/or sinusoidal pool, whereas in females, both portal and central area hepatocytes play an equal role.

Characterization of the mechanisms involved in the gender differences in hepatic taurocholate uptake

Gender differences in the hepatic transport of organic anions is well established. Although uptake of many organic anions is greater in females, sodium-dependent taurocholate uptake is greater in hepatocytes from male rats. We examined the hypothesis that endogenous estrogens alter the number of sinusoidal bile acid transporters and/or decrease membrane lipid fluidity. The initial sodium-dependent uptake of [3H]taurocholate was 75% greater in hepatocytes from males than from either intact or oophorectomized females rats. Taurocholate maximal uptake was increased twofold (P < 0.03) without a significant change in the Michaelis-Menten constant. Sinusoidal membrane fractions were isolated from male and female rat livers with equal specific activities and enrichments of Na+-K+-ATPase. Males had a significant (P < 0.05) increase in cholesterol esters and phosphatidylethanolamine-to-phosphatidylcholine ratio. Fluorescence polarization indicated decreased lipid fluidity in females. In females, expression of the sodium-dependent taurocholate peptide (Ntcp) and mRNA were selectively decreased to 46 +/- 9 and 54 +/- 4% (P < 0.01), respectively, and the organic anion transporter peptide (Oatp) and Na+-K+-ATPase alpha-subunit were not significantly different. Nuclear run-on analysis indicated a 47% (P < 0.05) decrease in Ntcp transcription, without a significant change in Oatp. In conclusion, these studies demonstrated that decreased sodium-dependent bile salt uptake in female hepatocytes was due to decreased membrane lipid fluidity and a selective decrease in Ntcp.

Sex differences in multiple steps in hepatic transport of palmitate support a balanced uptake mechanism

Hepatic clearance of long-chain fatty acids is substantially faster in females than in males, a fact that may underlie known gender-related differences in lipoprotein metabolism and associated disease states. To further investigate the transport steps responsible for this difference, we used a novel method combining multiple-indicator dilution and steady-state measurements of palmitate extraction from albumin solutions. We found that cytoplasmic transport of palmitate is sufficiently slow (diffusion constants 9.0 and 5.9 x 10(-9) cm2/s for male and female liver, respectively) that the steady-state concentration of palmitate in the center of the cell should be approximately 0.5 of that found in the cytoplasm just beneath the plasma membrane. Previous studies in cultured liver cells using nonphysiological fatty acids have shown more rapid cytoplasmic transport in females. This sex difference reflects higher concentrations of cytosolic fatty acid-binding protein, which acts as a carrier system to transport fatty acids across cell water layers. The current study confirmed slow cytoplasmic diffusion rates in intact perfused rat liver using a physiological fatty acid and found a similar female-to-male ratio. Female liver also had a greater influx rate constant and a larger vascular volume than male liver but had a similar rate of metabolism. Rapid cytoplasmic diffusion enhances movement of palmitate into deeper layers of the cell cytoplasm, thus reducing efflux. The larger sinusoidal volume in females not only permits more dissociation of palmitate from albumin within the sinusoids but also may generate a greater permeability-surface area product. These multiple sex-related differences combine to produce a nearly twofold greater steady-state uptake rate by female liver.

Effect of increased uptake of plasma fatty acids by the liver on lipid metabolism in the hepatocellular nuclei

The nucleus contains different lipids. The aim of the present study was to examine whether increased uptake of free fatty acids by the liver affects lipid metabolism in the hepatocellular nuclei. The experiments were carried out on three groups of Wistar rats: I - male, control; II - male, heparin-treated, and III - female. [14C]-palmitic acid suspended in rat donor serum was administered intravenously 5 and 30 min before tissue samples were taken. Lipids were extracted from isolated liver nuclei and separated into different fractions (phospholipids - PH, monoacylglycerols - MG, diacylglycerols - DG, cholesterol - CH, free fatty acids - FFA, triacylglycerols - TG and cholesterol esters - CHE). It was found that 5 min after administration of the label all isolated nuclear lipid fractions were radioactive. Most of the radioactivity was located in the fraction of PH, TG and FFA. Elevation in the plasma FFA concentration (heparin-treated group) resulted in increased incorporation of [14C]-palmitic acid into the nuclear lipids and changes in its distribution. In the female rats the radioactivity of nuclear lipids was higher than in the male-controls. There were also differences in the percentage distribution of the radioactivity in different lipid fractions between the two groups. The concentration of PH and TG in the nuclei increased only in the heparin-treated but not in the female rats. However, specific activity of the nuclear PH and TG increased in with both groups compared to the male-control group. It is concluded that (a) the blood-borne FFA rapidly enter the nuclear lipid pool and (b) increased uptake of the plasma-borne FFA by the liver affects the nuclear lipid metabolism.

Effects of adrenergic agonists and antagonists on the metabolism of [1-14C]oleic acid by rat hepatocytes

The possibility that the antihypertensive adrenoceptor antagonists (propranolol, phentolamine and metoprolol) may alter hepatic lipid metabolism was examined in freshly dispersed rat hepatocytes with [1-14C]oleate. Propranolol (1.8 x 10(-4) M) and phentolamine (1.4 x 10(-4) M) increased incorporation of [1-14C]oleate into cholesteryl esters by 51 and 92%, respectively, and decreased ketogenesis by 46 and 62%, respectively. While neither drug affected incorporation into total phospholipid, propranolol decreased triglyceride synthesis by 37%. These effects of propranolol and phentolamine may not occur through beta- or alpha-receptor inhibition since neither epinephrine nor norepinephrine reversed the effects of the adrenoceptor antagonists. Although epinephrine and norepinephrine per se did not alter the incorporation of [1-14C]oleate into triglyceride, phospholipid, cholesteryl esters or ketone bodies, they stimulated the production of 14CO2 (control 5.6 +/- 1.3; epinephrine 7.6 +/- 1.1; norepinephrine 9.1 +/- 0.2 nmol oleate incorporated/mg protein), and these effects were reversed by phentolamine and propranolol. The data suggest that adrenoceptor antagonists exert direct effects on hepatic metabolism of oleate.

Relationships between fatty acid synthesis and lipid secretion in the isolated perfused rat liver: effects of hyperthyroidism, glucose and oleate

Various studies on the effects of thyroid status on hepatic fatty acid synthesis have produced conflicting results. Several variables (e.g., plasma free fatty acid and glucose concentrations) are altered simultaneously by thyroid status and can affect fatty acid synthesis. To evaluate the effects of these variables, hepatic fatty acid synthesis (lipogenesis) was studied in isolated perfused livers from normal and triiodothyronine-treated rats. Livers were perfused with media containing either 5.5 or 25 mM glucose without fatty acid, or 5.5 mM glucose and 0.7 mM oleate. Rates of lipogenesis were determined by measurement of incorporation of 3H2O into fatty acids. Lipogenesis in livers from hyperthyroid animals exceeded that of controls, when perfused with 5.5 mM glucose with or without oleate. Perfusion with 25 mM glucose increased lipogenesis in both euthyroid and hyperthyroid groups to the same level, abolishing this difference between them. Perfusion with oleate reduced rates of lipogenesis by livers from euthyroid and hyperthyroid rats to a similar extent, but stimulated secretion of radioactive fatty acid in phospholipid and free fatty acid fractions. Oleate increased ketogenesis by livers from normal and triiodothyronine-treated rats, with higher rates of ketogenesis in the triiodothyronine-treated group. When oleate was omitted, ketogenesis in the presence of 5.5 mM glucose by the hyperthyroid group was similar to that of euthyroid controls, while ketogenesis was decreased in the hyperthyroid group relative to controls when perfused with 25 mM glucose. About 30% of the radioactivity incorporated into the total fatty acid of both groups was recovered in palmitate, with the remainder in longer chain saturated and unsaturated fatty acids. In both euthyroid and hyperthyroid groups, the ratio of triacylglycerol:phospholipid fatty acid radioactivity was not only less than predicted (based on synthetic rates of PL and TG) but also was decreased in perfusions with exogenous oleate compared to perfusions without oleate. In perfusions with oleate, both groups incorporated twice as much radioactivity into phospholipid as into triacylglycerol. The data suggest the following concepts: while hepatic fatty acid synthesis and oxidation are increased simultaneously in the hyperthyroid state, de novo synthesized fatty acids seem to be poorer substrates for oxidation than are exogenous fatty acids, and are preferentially incorporated into phospholipid, while exogenous fatty acids are better substrates for oxidation and esterification to triacylglycerol. The preferential utilization of de novo synthesized fatty acid for phospholipid synthesis may be an important physiologic adaptation insuring a constant source of fatty acid for membrane synthesis.

In situ binding of fatty acids to the liver fatty acid binding protein: analysis using 3-[125I]iodo-4-azido-N-hexadecylsalicylamide

A photoactivatable radioiodinated fatty acid analogue, 3-[125I]iodo-4-azido-N-hexadecylsalicylamide (125I-AHS) has been synthesized and used to investigate the involvement of cellular lipid carriers in hepatic fatty acid utilization. Photoactivation of Hep G2 internalized 125I-AHS revealed that several cellular proteins were crosslinked with the radiolabeled fatty acid analogue. Three predominant proteins in the membrane fraction of the cell with molecular masses 17, 50 and 127 kDa were crosslinked with the lipid analogue, as determined using autoradiography after SDS-PAGE. Three other proteins in the soluble fraction of the cell, with molecular masses 14, 24 and 35 kDa, were also labeled in situ. In contrast to the other labeled proteins, the fatty acid analogue accumulated on the cytoplasmic 14 kDa protein in a time and temperature dependent fashion. The in situ-labeled 14 kDa protein was identified from primary rat hepatocytes as the liver fatty acid binding protein by partial purification and its ability to be immunoprecipitated with immunospecific L-FABP antiserum. Collectively the results indicate that fatty acids traverse the plasma membrane and are bound cytoplasmically by the liver fatty acid binding protein, as well as other proteins in the cell. This represents the first demonstration in intact hepatocytes that the liver fatty acid binding protein participates in the process of intracellular fatty acid trafficking, and supports a model in which cytoplasmic lipid carriers solubilize fatty acids as a step in their metabolic utilization.

Secretion of triglyceride and ketogenesis by livers from spontaneous diabetic BB Wistar rats

Livers from male and female BB Wistar spontaneously diabetic rats were perfused in vitro to determine the effects of spontaneously occurring insulin-dependent diabetes on the metabolism of fatty acid. The secretion of triglyceride and the incorporation of [1-14C] oleic acid into perfusate and hepatic triglyceride was reduced by the diabetic state, whereas beta-hydroxybutyrate production and output of total ketone bodies were increased. The spontaneous diabetic Wistar rat clearly is a suitable model to study the derangements induced in lipid/plasma lipoprotein metabolism by the insulin-dependent diabetic state; the data obtained with this model confirm our earlier observations on experimental insulin deficiency induced with alloxan, streptozotocin, and anti-insulin serum.

Treatment with anabolic steroids increases the activity of the mitochondrial outer carnitine palmitoyltransferase in rat liver and fast-twitch muscle

Treatment of male rats with the anabolic steroids fluoxymesterone or methylandrostanolone increased the activity of the outer carnitine palmitoyltransferase in liver and fast-twitch muscle mitochondria. This effect was not potentiated by physical exercise and was not observed in heart and slow-twitch muscle mitochondria. Anabolic steroids did not affect the sensitivity of the liver enzyme to inhibition by malonyl-CoA. The data presented herein suggest that androgens may have an important physiological role in the regulation of fatty acid oxidation in liver and fast-twitch muscle mitochondria. In addition, our results are at odds with the notion that (most of) the metabolic effects of anabolic steroids on muscle are only evident when physical training is parallely performed.

The physical-chemical basis for sex-related differences in uptake of fatty acids by the liver

The uptake of fatty acids by the liver was shown previously to be a non-catalyzed process, and rates of uptake were correlated to the affinity of the plasma membranes of liver cells for fatty acids. The experiments in this paper were designed to test whether the known differences in uptake and metabolism of free fatty acids by the livers of male and female rats could be understood based on differences in the affinities of the corresponding plasma membranes for these substrates. The relative affinities for palmitate and oleate of 'male' plasma membranes were found to be lower versus 'female' membranes. Measurements of uptake of palmitate from albumin-palmitate complexes by 'male' and 'female' perfused livers showed higher uptake rates by the latter when correlated with the concentration of the complex. However, the rates of uptake were identical when the concentrations of the fatty acid in the plasma membranes of male and female liver cells were the same.

Sex differences in sulfobromophthalein-glutathione transport by perfused rat liver

Sex differences have been described in the hepatic transport of many organic anions. Proposed mechanisms include differences in the rate of metabolism, in the degree of binding to cytoplasmic proteins, and in the rate of membrane transport. To better define these factors, we used the perfused rat liver to study the hepatic transport of the glutathione conjugate of sulfobromophthalein (BSP-GSP), a model compound that does not require metabolism for excretion. Hepatic transport of BSP-GSH was saturable for both sexes. Clearance of BSP-GSH from 1% albumin solutions at steady-state was 35-52% greater in female livers than in male livers, and reflected a 47% larger apparent Vmax with no change in the apparent Km. Analysis of the rate of disappearance of BSP-GSH from recirculating perfusate and its appearance in bile using a simple two-compartment model indicated that the ratio of influx to efflux was greater in female livers. These findings are compatible with sex-related differences in the electrochemical driving forces for BSP-GSH uptake.

Effects of 8-N,N-diethylamino-octyl-3,4,5-trimethoxybenzoate (TMB-8) HCl and verapamil on the metabolism of free fatty acid by hepatocytes

The influence of calcium antagonists on hepatic lipid metabolism was investigated in freshly dispersed rat hepatocytes incubated with [1-14C]oleate and verapamil or 8-N,N-diethylamino-octyl-3,4,5-trimethoxybenzoate (TMB-8). Synthesis of triglyceride was calculated from the specific radioactivity of [1-14C]oleate in extracted total lipid, after separation of each lipid class by thin-layer chromatography. Ketogenesis was measured enzymatically or as the amount of radioactivity incorporated into neutralized acid-soluble extracts. Neither verapamil nor TMB-8 affected triglyceride synthesis. In contrast, TMB-8 and verapamil exerted a concentration-dependent inhibition of ketogenesis, with TMB-8 being more potent than verapamil (inhibition by 50 microM TMB-8 was 73 +/- 9% versus 38 +/- 2% inhibition by 50 microM verapamil). Increasing the concentrations of calcium (0 to 4.2 mM) or oleate (0 to 2.0 mM) increased the rate of ketogenesis but did not alter the antiketogenic potency of TMB-8 or verapamil, indicating that inhibition of ketogenesis by these drugs was not calcium dependent. Since the calcium antagonists did not affect ketogenesis from octanoic acid, and since carnitine stimulated ketogenesis from [1-14C]oleate by 25% and reversed the antiketogenic effects of TMB-8 and verapamil, it appeared that the two calcium antagonists inhibited ketogenesis by interfering with the activity of the outer mitochondrial carnitine palmitoyltransferase. In assays of the outer carnitine palmitoyltransferase in isolated mitochondria, both TMB-8 and verapamil were inhibitory. TMB-8 was the more potent inhibitor of this enzyme, and carnitine was able to overcome inhibition by each of the inhibitors. These results suggest that verapamil and TMB-8 may inhibit ketogenesis by mechanisms independent of their well known effects on cellular calcium concentrations, and that inhibition may be competitive with respect to carnitine concentration. However, direct inhibition of carnitine palmitoyltransferase may not explain completely the inhibition of ketogenesis by these drugs, since concentrations required for enzyme inhibition were greater than those required for inhibition of ketogenesis in isolated hepatocytes.

Effects of the antiestrogen LY 117018 on the modulation by ethinyl estradiol of the metabolism of [1-14C]oleic acid by perfused livers from normal and ovariectomized rats

Intact female Sprague-Dawley rats (195-249 g) and rats that had been ovariectomized (210-285 g) were injected subcutaneously for 14 days with ethinyl estradiol (15 micrograms/kg), the antiestrogen LY 117018 (500 micrograms/kg), both drugs simultaneously, or the vehicle (sesame oil) alone. Livers were removed and perfused in vitro in a recycling system. The administration of LY 117018 alone did not affect the secretion of triacylglycerol by livers from normal rats but decreased the secretion of triacylglycerol by livers from ovariectomized rats. When the drugs were administered concurrently to either normal or ovariectomized animals, the increase in the concentration of triacylglycerol and the decrease in the concentration of cholesteryl esters in the plasma produced by ethinyl estradiol were prevented. When administered to either intact or ovariectomized rats, ethinyl estradiol alone stimulated the synthesis and secretion of triacylglycerol and cholesteryl esters by perfused livers isolated from these animals. The simultaneous administration of LY 117018 with ethinyl estradiol prevented this stimulation of the hepatic synthesis and secretion of triacylglycerol and cholesteryl esters. The depression of ketogenesis observed with livers from rats administered ethinyl estradiol alone was reversed by concurrent administration of LY 117018. The concurrent administration of the estrogen and antiestrogen did not result, however, in a complete blockade of the estrogen-induced elevation of hepatic triacylglycerol synthesis and depression of ketogenesis. The incorporation of [1-14C]oleic acid into the triacylglycerol and ketone bodies by livers from ovariectomized rats was less than that of livers from normal rats. It is clear that the antiestrogen antagonizes the actions of ethinyl estradiol on hepatic lipid metabolism. Furthermore, the use of the antiestrogen LY 117018 in these experiments allows the probable conclusion that the modulation of hepatic metabolism of fatty acid by estrogen is mediated by conventional estrogenic receptors.

Deprivation and repletion of androgen in vivo modifies triacylglycerol synthesis by rat hepatocytes

Given the same quantity of fatty acid, livers from male rats esterify less fatty acid and secrete less triacylglycerol in very-low-density lipoprotein than do livers from female animals. To elucidate the role of testosterone in maintenance of this male pattern, conversion of [1-14C]oleic acid into triacylglycerol was assessed in vitro by rat hepatocytes (male) following gonadectomy and replacement with testosterone. Following castration, incorporation of fatty acid into triacylglycerol was increased. In contrast, esterification of exogenous fatty acid into phospholipid, cholesteryl esters, and diacylglycerol was unchanged. Treatment with testosterone (75 micrograms/day) reduced incorporation of exogenous fatty acid into triacylglycerol. Higher doses of testosterone (200 or 100 micrograms/day) modified the effect, such that inhibition was observed only at low oleate (0.5 mM) concentrations. At higher substrate concentrations (1.0-2.0 mM) the inhibitory effect was no longer observed. Further, a similar dose-dependent effect of testosterone was observed following in vivo treatment of castrate females with testosterone. These data support the concept of a regulatory role of testosterone in hepatic triacylglycerol synthesis. These findings also demonstrate a biphasic effect of testosterone, an effect that is dependent not only upon the dose of testosterone administered, but also on the concentration of fatty acid to which the hepatocyte is exposed in vitro.

Sex-related differences in desaturation and chain elongation of essential fatty acids studied in isolated rat hepatocytes

When [14C]linoleic acid (18:2(n-6)) or [14C]dihomogammalinolenic acid (20:3(n-6)) was incubated with isolated liver cells from rats fed an essential fatty acid deficient diet, delta 6- and delta 5-desaturation, chain elongation and synthesis of 14C-labelled C14-C18 fatty acids (from [14C]acetate) were enhanced in female cells compared with male ones. No sex difference in total secretion of very low density lipoproteins (VLDL) was observed. However, VLDL secreted from female cells contained significantly more C16-C18 fatty acids than male cells. It is suggested that the observed sex differences, at least in part, may be related to the different content of fatty acid binding proteins in female cells compared with males.

In vitro effects of alpha-bromopalmitate on metabolism of essential fatty acids studied in isolated rat hepatocytes: sex differences

alpha-Bromopalmitate was shown to have a far more pronounced effect on metabolism of labelled linoleic acid (18:2, n-6) and arachidonic acid (20:4, n-6) in isolated liver cells from female rats than in those from males. alpha-Bromopalmitate decreased triacylglycerol synthesis with a concomitant accumulation of fatty acid in diacylglycerol, indicating that the acylation of diacylglycerol is affected by alpha-bromopalmitate.

Stimulation by acetoacetate of hepatic triacylglycerol synthesis

The factors responsible for the huge accumulation of hepatic triacylglycerols in the ketotic diabetic state are not established. Our earlier work suggested a role for ketone bodies in the increased hepatic triacylglycerol synthesis observed in the ketotic diabetic state. Isolated hepatocytes obtained from normal fed rats were incubated with sodium acetoacetate or sodium chloride (control) and [1-14C]palmitate in Krebs-albumin buffer. Acetoacetate stimulated triacylglycerol synthesis in a concentration-dependent manner without increasing palmitate uptake or inhibiting palmitate oxidation. Beta hydroxybutyrate showed no effect on palmitate esterification to triacylglycerols. Isolated hepatocytes of normal fed rats were incubated with either sodium acetoacetate or sodium chloride and the nuclear-free homogenate was incubated with [U-14C]glycero-3-phosphate and cofactors. The synthesis of triacylglycerol and the activity of the cytosolic phosphatidate phosphohydrolase were increased in the cells pre-incubated with acetoacetate. The results of this study demonstrate that the increases in triacylglycerol synthesis and the cytosolic activity of phosphatidate phosphohydrolase previously observed by us in the ketotic diabetic liver, could be reproduced in normal fed rat liver cells by incubating them with acetoacetate. The results identify acetoacetate as a potential factor, in the regulation of hepatic triacylglycerol synthesis and for hepatic accumulation of triacylglycerols observed in the ketotic diabetic state.

Effect of sulfonylureas on hepatic fatty acid oxidation

In isolated rat livers perfused with oleic acid (0.1 mM), infusion of tolbutamide or glyburide decreased the rate of ketogenesis in a dose-dependent manner. The inhibition of fatty acid oxidation was maximal at 2.0 mM and 10 microM concentrations of tolbutamide and glyburide, respectively. Neither tolbutamide nor glyburide inhibited ketogenesis in livers perfused with octanoate. The inhibition of hepatic ketogenesis by sulfonylureas was independent of perfusate oleic acid concentration. Additionally, in rat livers perfused with oleic acid in the presence of L-(-)-carnitine (10 mM), submaximal concentrations of tolbutamide and glyburide did not inhibit hepatic ketogenesis. Finally, glyburide infusion into livers perfused with [U-14C]oleic acid (0.1 mM) increased the rate of 14C label incorporation into hepatic triglycerides by 2.5-fold. These data suggest that both tolbutamide and glyburide inhibit long-chain fatty acid oxidation by inhibiting the key regulatory enzyme, carnitine palmitoyltransferase I, most probably by competing with L-(-)-carnitine.

Influence of dietary fat on metabolism of (14-14C)erucic acid in the perfused rat liver. Distribution of metabolites in lipid classes

Two groups of rats were fed diets containing 20% by weight of either partially hydrogenated marine oil supplemented with sunflower seed oil (PHMO) or palm oil (PO) for 8 wk. Using a liver perfusion system, the effect of dietary long chain monoenoic fatty acids on the uptake and metabolism of [14-14C]erucic acid was studied. The perfusion times were 15 and 60 min, respectively. The two groups showed equal ability for erucic acid uptake in the liver but differed in the channeling of the fatty acids into various metabolic pathways. A higher metabolic turnover of 22:1 in the PHMO livers relative to the PO livers was demonstrated by an increased recovery of total [14C]labeling in the triglyceride (TG) and phospholipid (PL) fractions, already evident after 15 min of perfusion. The chain-shortening capacity was highest in the PHMO group, reflected by a higher [14C]18:1 incorporation in both TG and PL, and increasing from 15 to 60 min of perfusion. The amount of [14C]18:1 found in PL and TG after 60 min of perfusion of livers from rats fed PO corresponded to that shown for the PHMO group after 15 min. The PL demonstrated a discrimination against 22:1 compared to TG, and, when available, 18:1 was highly preferred for PL-synthesis. The total fatty acid distribution in the TG, as determined by gas liquid chromatography (GLC), reflected the composition of the dietary fats. In the total liver PL, 22:1 and 20:1 were present in negligible amounts, although the PHMO diet contained 12-13% of both 22:1 and 20:1. In the free fatty acid fraction (FFA), the major part of the radioactivity (approximately 80%) was [14-14C]erucic acid, and only small amounts of [14C]18:1 (less than 2%) were present, even after 60 min of perfusion. The shortened-chain 18:1 was readily removed from the FFA pool and preferentially used for lipid esterification.

Sex differences in the metabolism of essential fatty acids studied in isolated rat liver cells

The triacylglycerol synthesis from exogenous linoleic acid (18:2(n-6], linolenic acid (18:3(n-3], dihomogammalinolenic acid (20:3(n-6], eicosapentaenoic acid (20:5(n-3] and oleic acid (18:1(n-9] was observed to be significantly increased in isolated liver cells from female rats compared with males. The rate of fatty acid oxidation and phospholipid biosynthesis was concomitantly more important in male cells. With the C22-polyenoic fatty acids, adrenic acid (22:4(n-6] and docosahexaenoic acid (22:6(n-3), only a minor sex-related difference in fatty acid metabolism was found.

Hepatic lipoprotein biosynthesis

Methods for the study of hepatic lipoprotein synthesis and secretion have been described, and the advantages of each system discussed. Attention has been focused on intact cell systems. The isolated perfused liver constitutes a standard of comparison for all others, even though it, too, has limitations. Table IV below gives our assessment of some of the advantages and disadvantages of the techniques for studying hepatic lipoprotein biosynthesis.

Differential effects of alanine on ketogenesis and triacylglycerol formation by isolated perfused livers from euthyroid and hyperthyroid rats

We examined the effects of infusion of alanine on hepatic concentration of glycero-3-phosphate (glycero-3-P) and output of triacylglycerol (TG) by isolated perfused livers from triiodothyronine (T3)-treated rats. It was expected that because of its gluconeogenic and antiketogenic properties, alanine might stimulate accumulation of glycero-3-P, which in turn might result in enhanced TG production by the hyperthyroid livers. The hepatic concentration of glycero-3-P is lower in livers from T3-treated rats than in euthyroid rats. Infusion of 1.83 and 4.23 mmol alanine/4 h did not alter the hepatic concentration of glycero-3-P and output of triacylglycerol by livers from T3-treated rats. However in these livers, the two concentrations of infused alanine increased the output of glucose and decreased the output of ketone bodies. In livers from euthyroid animals, the infusion of 1.83 mmol alanine/4 h had no effect, whereas 4.23 mmol/4 h decreased hepatic concentration of glycero-3-P. These concentrations of alanine did not alter the output of ketone bodies or TG, but progressively decreased the output of glucose by the euthyroid livers. Our data suggested that the decreased availability of glycero-3-P in livers from T3-treated rats is rate-limiting for TG production and correlated with the diminished output of TG, whereas in livers from euthyroid rats, the glycero-3-P concentration is sufficient to maintain maximal synthesis and secretion of TG. Furthermore, the effects of alanine on the output of ketone bodies or glucose appear to be independent of its effects on hepatic glycero-3-P.

Requirements of glycerol and fatty acid for triglyceride synthesis and ketogenesis by hepatocytes from normal and triiodothyronine-treated rats

Hepatocytes from T3-treated rats synthesized less triglyceride and more ketone bodies from [1-14C]oleate at all concentrations from 0-2 mM, than did hepatocytes from euthyroid animals; addition of 1.0 mM glycerol increased triglyceride synthesis and reduced ketogenesis in hepatocytes from T3-treated rats to the rates observed in euthyroid hepatocytes in the absence of added glycerol. Glycerol did not alter triglyceride synthesis, but reduced ketogenesis genesis by euthyroid hepatocytes. It is probable from these and other data (J. Biol. Chem. 259, 8857-8862 (1985)) that, in the hyperthyroid rat, glycero-3-P, and not fatty acid, is rate limiting for synthesis of triglyceride, and, secondarily for reducing rates of ketogenesis in the hepatocyte.

Analyzing the structures, functions and evolution of two abundant gastrointestinal fatty acid binding proteins with recombinant DNA and computational techniques

The structures of intestinal and liver fatty acid binding proteins (FABPs) have been determined from an analysis of the nucleotide sequences of cloned cDNAs. The primary translation product of intestinal FABP mRNA contains 132 residues (Mr = 15 124). Liver FABP mRNA encodes a 127 amino acid polypeptide (Mr = 14 273). In vitro co-translational cleavage and translocation assays showed that neither sequence has a cleavable signal peptide or signal peptide equivalent - suggesting that the FABPs do not enter the secretory apparatus but rather are targeted to the cytoplasm. A variety of computational techniques were used to compare the two FABP sequences. The results indicate that liver and intestinal FABP are paralogous homologues. A superfamily of proteins was defined which includes the FABPs, the cellular retinol and retinoic acid binding proteins, the P2 protein of peripheral nerve myelin, and a polypeptide known as 422 whose synthesis is induced during differentiation of 3T3-L1 cells to adipocytes. No sequence homologies were noted between any of these small molecular weight cytosolic proteins and nonspecific lipid transfer protein (sterol carrier protein 2), phosphatidylcholine transfer protein, serum albumin or apolipoprotein AI. The FABPs may have structural features responsible for lipid-protein interactions that are not present in these non-homologous sequences. The distribution of intestinal and liver FABP mRNAs in adult rat tissues and the changes in FABP gene expression which occur during gastrointestinal development support the notion that these proteins are involved in fatty acid uptake, transport and/or compartmentalization. However, differences in tissue distribution and periods of non-coordinate expression during gastrointestinal ontogeny suggest that the two FABPs have distinct functions. The relationship between intestinal and liver FABPs and similar sized cytosolic FABPs isolated from brain, skeletal and cardiac muscle remains unclear. Recombinant DNA techniques combined with comparative sequence analyses offer a useful approach for defining unique as well as general structure-function relationships in this group of fatty acid binding proteins.

Regulation of hepatic triacylglycerol synthesis and secretion

Triacylglycerols are the most concentrated storage form of energy for the mammalian organism. These lipids are synthesized and secreted by the liver and serve as a fuel for other tissues. This paper presents a brief review of the regulation of hepatic triacylglycerol synthesis and secretion. Particular attention will be given to the dissociation of the synthesis of triacylglycerols from that of the metabolically closely related nitrogenous phospholipids. Recent evidence is presented which suggests that triacylglycerol synthesis (and secretion) is regulated, at least partially, at the diacylglycerol branchpoint.

Oleate metabolism in isolated hepatocytes from lean and obese Zucker rats. Influence of a high fat diet and in vitro response to glucagon

The uptake and metabolism of [1-14C]oleate (0.3 mmol/L) were studied in isolated hepatocytes from lean and obese Zucker rats fed either a control (low-fat) diet or a high-fat diet. With the control diet, [1-14C]oleate uptake was increased by 70% in the obese rats, and fat-feeding decreased this uptake to values comparable to that of their lean littermates. Interestingly, the hepatocyte mean surface area was increased in the obese mutants by 21% with the control diet and by 30% with the high-fat diet. The possible reasons for the differences in oleate uptake are discussed. With the control diet, cells from the obese rats showed a four-fold rise in [1-14C]oleate esterification, while ketogenesis (beta-hydroxybutyrate + acetoacetate production) as well as the radioactive acid-soluble products were greatly depressed. Production of CO2 was very low and similar in both groups of animals. Adaptation to the high-fat diet in the obese rats resulted in a reversal between esterification and oxidation of oleate: the latter became the major metabolic pathway as in the lean rats. The ketogenic capacity was greatly if not completely restored. In the lean animals, glucagon stimulated ketogenesis both in the presence or absence of oleate and decreased [1-14C]oleate esterification. In the obese rats, the hormone exerted a significant ketogenic effect only if oleate was present and did not influence its esterification. The data demonstrate the following abnormalities in the hepatocytes of obese Zucker rats: (1) an enlargement of cell size, (2) an increased oleate uptake, (3) a virtual absence of a ketogenic response to exogenous oleate, and (4) a markedly increased esterification of the latter. The metabolic defects, but not the cell size, appear to be largely corrected by an adaptation to a high-fat diet. The hepatic response to glucagon was decreased in the obese rats at the level of endogenous ketogenesis.