On the mechanism of the so-called uncoupling effect of medium- and short-chain fatty acids

Octanoate applied to rat liver mitochondria respiring with glutamate plus malate or succinate (plus rotenone) under resting-state (State 4) conditions stimulates oxygen uptake and decreases the membrane potential, both effects being sensitive to oligomycin but not to carboxyatractyloside. Octanoate also decreases the rate of pyruvate carboxylation under the same conditions, this effect being correlated with the decrease of intramitochondrial content of ATP and increase of AMP. The decrease of pyruvate carboxylation and the change of mitochondrial adenine nucleotides are both reversed by 2-oxoglutarate. Fatty acids of shorter chain length have similar effects, though at higher concentrations. Addition of octanoate in the presence of fluoride (inhibitor of pyrophosphatase) produces intramitochondrial accumulation of pyrophosphate, even under conditions when oxidation of octanoate is prevented by rotenone. In isolated hepatocytes incubated with lactate plus pyruvate, octanoate also increases oxygen uptake and produces a shift in the profile of adenine nucleotides similar to that observed in isolated mitochondria. It decreases the 'efficiency' of gluconeogenesis, as expressed by the ratio between an increase of glucose production and an increase of oxygen uptake upon addition of gluconeogenic substrates (lactate plus pyruvate), and increases the reduction state of mitochondrial NAD. These effects taken together are not compatible with uncoupling, but point to intramitochondrial hydrolysis of octanoyl-CoA and probably also shorter chain-length acyl-CoAs. This mechanism probably functions as a 'safety valve' preventing a drastic decrease of intramitochondrial free CoA under a large supply of medium- and short-chain fatty acids.

Effects of ethanol feeding on hepatic lipid synthesis

Rats were fed a high-fat, liquid diet containing either 36% of total calories as ethanol or an isocaloric amount of sucrose, for a period up to 35 days. At different time intervals we measured the effects of ethanol administration on the activities of a number of key enzymes involved in hepatic lipid synthesis. At the start of the experimental period the activities of acetyl-CoA carboxylase and fatty acid synthase, measured in liver homogenates, increased in the control as well as in the ethanol-fed group. After 35 days these enzyme activities were still elevated but there were no significant differences between the two groups. In hepatocytes isolated from controls as well as from ethanol-fed rats, short-term incubations with ethanol induced an increase in the rate of fatty acid synthesis and in the activities of acetyl-CoA carboxylase and fatty acid synthase. However, no alterations in the regulation of these enzymes by short-term modulators of lipogenesis were apparent in hepatocytes isolated from alcohol-treated animals. The results do not indicate a major role for the enzymes of de novo fatty acid synthesis in the development of the alcoholic fatty liver. The amount of liver triacylglycerols increased in ethanol-fed rats during the entire treatment period, whereas the hepatic levels of phosphatidylcholine and phosphatidylethanolamine were not affected by ethanol ingestion. Ethanol administration for less than 2 weeks increased the activities of phosphatidate phosphohydrolase, diacylglycerol acyltransferase, and microsomal phosphocholine cytidylyltransferase, whereas the cytosolic activity of phosphocholine cytidylyltransferase was slightly decreased. Upon prolonged ethanol administration the activities of these enzymes were slowly restored to control values after 35 days, suggesting development of some kind of adaptation. It is interesting that, although the activities of phosphatidate phosphohydrolase and diacylglycerol acyltransferase were restored to the levels found in the control rats, this effect was not accompanied by a stabilization or decrease of the concentration of hepatic triacylglycerols.

Effects of ethanol feeding on the activity and regulation of hepatic carnitine palmitoyltransferase I

The effects of ethanol administration on activity and regulation of carnitine palmitoyltransferase I (CPT-I) were studied in hepatocytes isolated from rats fed a liquid, high-fat diet containing 36% of total calories as ethanol or an isocaloric amount of sucrose. Cells were isolated at several time points in the course of a 5-week experimental period. Ethanol consumption markedly decreased CPT-I activity and increased enzyme sensitivity to inhibition by exogenously added malonyl-CoA. Changes in enzyme activity occurred sooner than those in enzyme sensitivity. Fatty acid oxidation to CO2 and ketone bodies was depressed in hepatocytes from ethanol-fed animals during the first part of the treatment. At the end of the 35-day period, there were no longer differences in the rate of ketogenesis between the two groups. At that time, however, the rate of CO2 formation was still impaired in the ethanol-fed animals. Furthermore, addition of ethanol or acetaldehyde to the incubation medium strongly depressed CPT-I activity and rates of fatty acid oxidation in hepatocytes from ethanol-treated rats, whereas these effects were much less pronounced in cells from control animals. The response of CPT-I activity to insulin, glucagon, vasopressin, and phorbol ester was blunted in cells derived from ethanol-fed rats. These changes in the regulation of CPT-I activity corresponded with those observed in the rate of fatty acid oxidation. It is concluded that CPT-I may play a role in the generation of the ethanol-induced fatty liver.

Short-term inhibition of carnitine palmitoyltransferase I activity in rat hepatocytes incubated with ethanol

Ethanol decreased the activity of carnitine palmitoyltransferase I and the rate of fatty acid oxidation in rat hepatocytes in short-term incubations. These effects were mimicked by acetaldehyde, the product of hepatic ethanol metabolism, and were absent when ethanol oxidation was prevented by 4-methylpyrazole. Ethanol was also able to increase intracellular malonyl-CoA levels. The results suggest that inhibition of fatty acid translocation into mitochondria may play an important role in the ethanol-induced inhibition of hepatic fatty acid oxidation.

Short-term regulation of carnitine palmitoyltransferase activity in isolated rat hepatocytes

An assay procedure for carnitine palmitoyltransferase is described which allows rapid measurement of the overt activity of this enzyme in isolated rat hepatocytes. In a one-step procedure digitonin permeabilizes the plasma membrane and at the same time carnitine palmitoyltransferase activity is measured. The use of the present procedure shows that carnitine palmitoyltransferase activity is regulated on the short term by different types of agonists. Thus, insulin, epidermal growth factor, vasopressin and the phorbol ester PMA inhibit carnitine palmitoyltransferase activity, whereas glucagon treatment renders the enzyme more active. These changes in enzyme activity coincide with corresponding changes in the rate of fatty acid oxidation.

Effects of dietary conditions on the pool sizes of precursors of phosphatidylcholine and phosphatidylethanolamine synthesis in rat liver

We developed a new method for the determination of choline- and ethanolamine-containing precursors of phosphatidylcholine and phosphatidylethanolamine after their separation by HPLC and we have studied the effects of different dietary conditions on the pool sizes of these metabolites in rat liver. Fasting for 48 h induced only a small decrease in the amounts of phosphatidylethanolamine and its water-soluble precursors. Upon refeeding with a high-sucrose, fat-free diet for 72 h, the levels of ethanolamine-containing compounds were only slowly restored. The effects of various dietary conditions on the amounts of phosphatidylcholine and its water-soluble precursors were much more pronounced. Fasting induced a sharp decrease, especially of the amount of cholinephosphate. However, the levels of phosphatidylcholine and the choline-containing precursors were rapidly restored upon refeeding for 24 h. Continued refeeding for an additional 48 h enhanced the cholinephosphate pool size to a level more than double that found in normally fed rats. The latter effect was accompanied by an inhibition of the enzyme CTP:choline-phosphate cytidylyltransferase. The results are discussed in view of a possible regulatory mechanism that may balance the amounts of phosphatidylcholine and phosphatidylethanolamine.

Dissimilar effects of 1-oleoyl-2-acetylglycerol and phorbol 12-myristate 13-acetate on fatty acid synthesis in isolated rat-liver cells

Exogenous 1-oleoyl-2-acetylglycerol (OAG) is known to mimic the action of tumour-promoting phorbol esters in various cell types. However, in isolated rat hepatocytes OAG depressed the rate of de novo fatty acid synthesis and the activity of the key enzyme acetyl-CoA carboxylase (EC 6.4.1.2), in contrast to the pronounced stimulation of both parameters by phorbol 12-myristate 13-acetate (PMA). The inhibition by OAG appeared to be dose- and time-dependent. On the other hand, medium-chain 1,2-diacylglycerols like 1,2-dioctanoyl-sn-glycerol did mimic the stimulatory action of PMA. The anomalous effect of OAG may well be explained by its metabolic breakdown leading to liberation of oleate and subsequent inhibition of acetyl-CoA carboxylase activity by endogenously formed oleoyl-CoA. The stimulatory effects of both PMA and medium-chain diacylglycerols are likely to be mediated by protein kinase C.

Effects of vasopressin on the synthesis of phosphatidylethanolamines and phosphatidylcholines by isolated rat hepatocytes

The effect of vasopressin on the biosynthesis of phosphatidylcholines and phosphatidylethanolamines was investigated in freshly isolated rat hepatocytes in suspension. Treatment of hepatocytes with vasopressin inhibits the incorporation of [Me-14C]choline into phosphatidylcholines in a dose-dependent manner. The hormone does not affect the uptake, phosphorylation or oxidation of choline. Pulse-chase studies indicate that CTP:cholinephosphate cytidylyltransferase might be subject to hormonal regulation by vasopressin. In contrast with the inhibitory effect of vasopressin on the synthesis of phosphatidylcholines, this hormone stimulates the incorporation of [1,2-14C]ethanolamine into phosphatidylethanolamines in a dose-dependent manner. Pulse and pulse-chase studies with labelled ethanolamine show that the conversion of ethanolaminephosphate to CDPethanolamine as well as the formation of phosphatidylethanolamines from CDPethanolamine and diacylglycerol are enhanced. Determination of the effect of vasopressin on the activity of the enzymes of the synthesis de novo of phosphatidylethanolamines demonstrates an increase of the activity of ethanolaminephosphotransferase, probably as a result of the increased amount of diacylglycerol in vasopressin-treated cells.

Measurement of acetyl-CoA carboxylase activity in isolated hepatocytes

An assay is described for acetyl-CoA carboxylase activity in isolated hepatocytes. The assay is based on two principles: The hepatocytes are made permeable by digitonin. 64 micrograms of digitonin per mg of cellular protein were most effective in exposing enzyme activity without a significant effect on mitochondrial permeability. Enzyme activity is measured by coupling the carboxylase reaction to the fatty acid synthase reaction. The advantages offered by this procedure over existing assays are: rapidity, no need to prepare cell extracts, absence of product inhibition, no interference by mitochondrial enzymes, useful in systems with bicarbonate buffers, and simple separation of radioactive substrate from labelled products. Using this coupled enzyme assay a good correlation was observed between changes in the activity of acetyl-CoA carboxylase and changes in the rate of fatty acid synthesis in hepatocytes as effected by short-term modulators.

Stimulation by a tumor-promoting phorbol ester of acetyl-CoA carboxylase activity in isolated rat hepatocytes

Acetyl-CoA carboxylase (EC 6.4.1.2) in hepatocytes from meal-fed rats was activated by phorbol myristate acetate (PMA) in a time- and concentration-dependent fashion. This activation can account for the PMA-induced stimulation of de novo fatty acid synthesis. Purified rat-liver acetyl-CoA carboxylase was found to be phosphorylated and activated by protein kinase C, thus providing a possible mechanism for the metabolic action of PMA in intact hepatocytes.

Cholesterol metabolism and sterol carrier protein-2 (non-specific lipid transfer protein)

Hepatic sterol carrier protein-2 significantly enhances the microsomal conversion of cholesterol to 7 alpha-hydroxy-cholesterol. In the present work we have attempted to correlate the hepatic content of sterol carrier protein-2 with bile acid formation. We have determined the amount of this protein in a variety of physiological and experimental conditions, in which the rate of bile acid synthesis varies over a wide range, viz. during fetal development, in inbred strains of rats with different rates of bile acid synthesis, and in rats fed diets containing drugs which modify the rate of bile acid synthesis. The outcome of these experiments does not support the idea that sterol carrier protein-2 has any association with bile acid synthesis. From our data we further conclude that hepatic sterol carrier protein-2 is an adaptable protein because its level increases during development from the fetal to the post-weaning stage of the rat and since it can be modulated by oral administration of certain drugs. Furthermore, it is demonstrated that the level of sterol carrier protein-2 varies between six inbred strains of rats.

No synergism between ionomycin and phorbol ester in fatty acid synthesis by isolated rat hepatocytes

With hepatocytes in suspension, freshly isolated from meal-fed rats, no significant effect of ionomycin on the rate of de novo fatty acid synthesis was observed, whereas phorbol myristate acetate (PMA) was strongly stimulatory. The combination of ionomycin and PMA produced the same stimulation as was seen with PMA alone. Stimulation of fatty acid synthesis by vasopressin was comparable and not additive to that observed with PMA, indicating that activation of protein kinase C is solely responsible for this metabolic effect of vasopressin. Both vasopressin and PMA increased acetyl-CoA carboxylase activity in isolated rat hepatocytes.

The synthesis and esterification of cholesterol by hepatocytes and H35 hepatoma cells are independent of the level of nonspecific lipid transfer protein

The level of the nonspecific lipid transfer protein (i.e., sterol carrier protein 2) is 16-fold lower in the Reuber H35 hepatoma cells as compared to the hepatocytes in culture (49 and 810 ng of protein per mg of 105000 X g supernatant protein, respectively). In order to establish whether there is a relationship between the level of nonspecific transfer protein and intracellular cholesterol metabolism, we have determined the biosynthesis and esterification of cholesterol in these hepatoma cells and hepatocytes. Both types of cells incorporated [3H]mevalonate into cholesterol and cholesterol ester. Incubation of both cell types with [3H]cholesterol in the medium resulted in a time-dependent uptake and subsequent conversion into cholesterol ester. In both instances, the amount of 3H label incorporated into cholesterol per mg of cellular protein was about 2-fold higher for the hepatoma cells. The kinetics of esterification of endogenously synthesized cholesterol were similar for both hepatoma cells and hepatocytes. Esterification of cholesterol derived from the medium proceeded 2-times faster in the hepatoma cells than in the hepatocytes. From the kinetics of cholesterol esterification we conclude that cells do not discriminate between cholesterol synthesized de novo and cholesterol derived from the medium. In addition, the proposition that the nonspecific lipid transfer protein is involved in cholesterol synthesis and esterification is not substantiated by this study.

Short-term stimulation of lipogenesis by triiodothyronine in maintenance cultures of rat hepatocytes

Within 4 h following the addition of 3,3',5 triiodo-L-thyronine to monolayer cultures of hepatocytes isolated from hypothyroid rats, a very distinct stimulation of fatty acid and cholesterol synthesis, measured as incorporation of either [1-14C]acetate or [3H]H2O into these lipid fractions, is observed. A smaller but significant increase in the rate of lipogenesis occurs in hepatocytes derived from euthyroid animals. These stimulatory effects of triiodothyronine are also observed in the presence of cycloheximide, indicating that the described early and direct stimulation of lipogenesis by the thyroid hormone is, at least in part, independent of protein synthesis.

Synthesis of phosphatidylcholines in rat hepatocytes. Possible regulation by norepinephrine via an alpha-adrenergic mechanism

The effect of norepinephrine on phosphatidylcholine and phosphatidylethanolamine formation was investigated in short-term incubations with freshly isolated rat hepatocytes. In the presence of dl-propranolol, norepinephrine decreases the incorporation of [methyl-14C]choline into phosphatidylcholines in a dose-dependent manner. At a concentration of 50 microM, norepinephrine (plus 20 microM propranolol) inhibits the incorporation of [methyl-14C]choline over a wide range of choline concentrations (59% inhibition at 5 microM choline; 34% inhibition at 1 mM choline). Norepinephrine also decreases the incorporation rates of [1-14C]palmitic acid and [1-14C]oleic acid into phosphatidylcholines. The effect of norepinephrine is mediated through an alpha-adrenergic receptor. Norepinephrine (plus propranolol) does not decrease the uptake or phosphorylation rate of [methyl-14C]choline. Pulse-label and pulse-chase studies indicate that the conversion rate of phosphocholine to CDP-choline, catalyzed by CTP:phosphocholine cytidylyltransferase, is diminished by norepinephrine. In contrast with the inhibitory effect of norepinephrine on phosphatidylcholine synthesis, this hormone stimulates the formation of phosphatidylethanolamines from [1,2-14C]ethanolamine. This increased incorporation rate is apparent at ethanolamine concentrations above 25 microM. A combination of norepinephrine and propranolol decreases, however, the synthesis of phosphatidylcholines from [1,2-14C]ethanolamine. The results indicate that alpha-adrenergic regulation dissociates the synthesis of phosphatidylcholines from that of phosphatidylethanolamines.

Studies on the substrate for hepatic lipogenesis in the rat

The contribution of hepatic glycogen to lipogenesis was studied in isolated, intact rat hepatocytes. To establish its importance as a substrate for lipogenesis, the glycogen of isolated hepatocytes was prelabelled with 14C from glucose. Evidence is presented that neither glucose nor glycogen constitute major sources of carbon for de novo synthesis of fatty acids and that less than 1% of glycogen is converted into fatty acids.

Studies on the assay, activity and sedimentation behaviour of acetyl-CoA carboxylase from isolated hepatocytes incubated with insulin or glucagon

The activity of acetyl-CoA carboxylase, measured in various ways, was studied in 15000g extracts of rat liver hepatocytes and compared with the rate of fatty acid synthesis in intact hepatocytes incubated with insulin or glucagon. Hepatocyte extracts were prepared by disruption of cells with a Dounce homogenizer or by solubilization with 1.5% (v/v) Triton X-100. Sucrose-density-gradient centrifugation demonstrated that the sedimentation coefficient of acetyl-CoA carboxylase from cell extracts was 30-35S, regardless of the conditions of incubation or disruption of hepatocytes. Solubilization of cells with 1.5% Triton X-100 yielded twice as much enzyme activity (measured by [14C]bicarbonate fixation) in the sucrose-gradient fractions as did cell disruption by the Dounce homogenizer. Analysis by high-performance liquid chromatography of acetyl-CoA carboxylase reaction mixtures showed that [14C]malonyl-CoA accounted for 10-60% of the total acid-stable radioactivity, depending on the method for disrupting hepatocytes and on the preincubation of the 15000g extract, with or without citrate, before assay. Under conditions in which incubation of cells with insulin or glucagon caused an activation or inhibition, respectively, of acetyl-CoA carboxylase, only 25% of the acid-stable radioactivity was [14C]malonyl-CoA and enzyme activity was only 13% (control), 16% (insulin), and 57% (glucagon) of the rate of fatty acid synthesis. Under conditions when up to 60% of the acid-stable radioactivity was [14C]malonyl-CoA and acetyl-CoA carboxylase activity was comparable with the rate of fatty acid synthesis, there was no effect of insulin or glucagon on enzyme activity.

Relations between fatty acid synthesis, pyruvate concentration and cell concentration of suspensions of isolated rat hepatocytes

The cell concentration of suspensions of isolated rat hepatocytes affects both the rate of pyruvate accumulation in the incubation medium and the rate of fatty acid synthesis. At low cell concentrations pyruvate accumulation is directly related to the cell concentration but levels off at higher concentrations even when maximum pyruvate concentrations in the medium are not yet reached. The rate of fatty acid synthesis in the 30-60-min incubation interval is proportional to the cell concentration. In contrast, the rate of fatty acid synthesis during the 0-30-min incubation period decreases with increasing cell concentrations and subsequently becomes independent of the cell concentration.

Effects of fenfluramine on hepatic intermediary metabolism

Effects of fenfluramine on hepatic intermediary metabolism have been studied using the isolated hepatocyte system. Fenfluramine inhibits the formation of glucose from lactate plus pyruvate and from alanine as well as the production of ketone bodies from added oleate. The latter observation suggests that inhibition of gluconeogenesis may result from a decrease in the level of mitochondrial acetyl-CoA. Fatty acid synthesis by hepatocytes is slightly stimulated by fenfluramine, perhaps as a consequence of the increase in cytosolic reducing equivalents, observed as an increase in the ratio of lactate/pyruvate.

Acetoacetate: a major substrate for the synthesis of cholesterol and fatty acids by isolated rat hepatocytes

Evidence is presented that isolated, intact rat hepatocytes can synthesize fatty acids and cholesterol from acetoacetate. The quantitative importance of these processes is evaluated by measuring total rates of fatty acid and cholesterol synthesis by incorporation of 3H from 3H2O. The contribution of acetoacetate varies from 14-54% and from 21-75% for de novo synthesized fatty acids and cholesterol, respectively, depending on the physiological condition of the donor rat. The relative contribution of acetoacetate to cholesterol synthesis is 1.4-2.3-times greater than to fatty acid synthesis.

Mechanisms responsible for the inhibitory effects of benfluorex on hepatic intermediary metabolism

The effects of benfluorex on hepatic intermediary metabolism have been studied using the isolated hepatocyte system. The drug inhibits the synthesis of both glucose and fatty acids by hepatocytes. Evidence is obtained that hepatocytes rapidly split benfluorex into benzoic acid and 1-(3-trifluoromethylphenyl)-2-[N-(2-hydroxyethyl)amino]propane (THEP). Comparison of the effects of the parent compound with the effects of THEP and benzoic acid on gluconeogenesis and on fatty acid synthesis indicates that different metabolites of the drug are responsible for its various actions: THEP inhibits gluconeogenesis, whereas benzoic acid inhibits fatty acid synthesis. The latter pathway appears to be inhibited at two sites: mitochondrial pyruvate uptake is inhibited by benfluorex itself, whereas fatty acid synthase is inhibited by benfluorex-derived benzoic acid.

Effects of insulin and glucagon on fatty acid synthesis from acetate by hepatocytes incubated with (--)-hydroxycitrate

(--)-Hydroxycitrate is a well-known inhibitor of the citrate cleavage enzyme (EC 4.1.3.8). In isolated hepatocytes it inhibits fatty acid synthesis from glucose, but it does not affect fatty acid synthesis from acetate. In its presence, insulin stimulates and glucagon inhibits incorporation of labelled acetate into fatty acids. This is evidence that both hormones directly influence fatty acid synthesis from acetate.

Short-term regulation of hepatic triacylglycerol metabolism by insulin and glucagon

This paper presents a review of our current knowledge of short-term regulation of triacylglycerol metabolism by insulin and glucagon in isolated rat hepatocytes. Insulin is devoted to rapidly improve the lipogenic climate of the hepatocyte. The hormone stimulates glycolysis, the conversion of pyruvate into acetyl-CoA, the synthesis of fatty acids and their esterification. The opposite pathways, hydrolysis of triacylglycerols and fatty acid oxidation, are depressed by insulin. Glucagon does all but deteriorate the lipogenic climate within the liver cell as the hormone inhibits triacylglycerol synthesis and stimulates the breakdown of triacylglycerols.

Inhibition of hepatic lipogenesis by salicylate

Salicylate has been found to be an inhibitor of fatty acid synthesis in isolated rat hepatocytes. Half-maximal inhibition of fatty acid synthesis occurs at approximately 2 mM. The inhibitory effect of salicylate on fatty acid synthesis is not relieve by the addition of acetate, suggesting that salicylate inhibits the conversion of acetate into fatty acids. Acetyl-CoA carboxylase activity in homogenates of hepatocytes is not influenced by previous exposure of the intact cells to salicylate. Partially purified acetyl-CoA carboxylase, isolated and assayed in the absence of citrate, is markedly inhibited by salicylate. However, in the presence of 0.5 mM citrate, which is the concentration of this metabolite in the cytosol of the liver cell, salicylate activates the enzyme. Upon treatment of acetyl-CoA carboxylase with salicylate (in the absence or presence of citrate), followed by separation of enzyme and effector on a Sephadex G-25 column, the enzyme activity is enhanced as compared to the salicylate-free control, demonstrating that the inhibitory effect of salicylate (in the absence of citrate) is reversible, but not the stimulatory effect (in the presence of citrate). Salicylate inhibition of fatty acid synthesis by hepatocytes is not rapidly reversible; hepatocytes preincubated with salicylate followed by a wash procedure (centrifugation and resuspension) still show depressed rates of fatty acid synthesis from acetate upon further incubation. Salicylate was found to prevent pyruvate accumulation in hepatocyte suspensions observed in the absence of this compound; salicylate even induces the disappearance of pyruvate and lactate initially present in the cell suspension. This suggests that salicylate activates pyruvate and lactate consumption, which is most likely related to the well-known fact that salicylate uncouples oxidative phosphorylation. The latter action of the drug will stimulate citric acid-cycle activity. This causes an inhibition of fatty acid and cholesterol synthesis since acetyl units will be specifically channelled into the citric acid cycle and not into the lipogenic pathway. It is concluded that part of the inhibitory effect of salicylate on fatty acid biosynthesis is exerted at (a) step(s) in the conversion of acetate into fatty acids, acetyl-CoA carboxylase not being a target of this compound. In addition, salicylate prevents that pyruvate, generated by glycolysis, enters the lipogenic pathway. The latter effect of salicylate would also explain the observed inhibition of cholesterol synthesis by this compound.

Short-term inhibition of fatty acid biosynthesis in isolated hepatocytes by mono-aromatic compounds

An overview is presented of a selected number of mono-aromatic derivatives and their short-term effects on hepatic fatty acid biosynthesis. The compounds discussed in this paper are ortho-hydroxybenzoate (salicylate), meta-hydroxybenzoate, para-hydroxybenzoate, benzoate, para-t-butylbenzoate, para-aminosalicylate, clofibrate, halofenate, alpha-cyano-4-hydroxycinnamate and benfluorex. All of these drugs inhibit fatty acid biosynthesis by isolated rat liver cells, albeit with different effectiveness. In contrast, the compounds have differential effects on fatty acid esterification and oxidation by isolated hepatocytes. An attempt is made to describe in molecular terms the underlying mechanisms of the acute inhibitory effects of the mono-aromatic derivatives on hepatic lipogenesis. It is proposed that all of the drugs exert an inhibitory action at the level of acetyl-CoA carboxylase, the enzyme generally considered to catalyse the rate-limiting step in hepatic fatty acid synthesis. This inhibitory effect may be either direct, i.e. by an alteration of the enzyme's structure as a result of interaction between drug and enzyme, or indirect, i.e. through a drug-induced change in the cellular levels of allosteric effectors of acetyl-CoA carboxylase.

The effects of lactate and acetate on fatty acid and cholesterol biosynthesis by isolated rat hepatocytes

1. The present study demonstrates that lactate and acetate stimulate fatty acid synthesis and inhibit cholesterogenesis by isolated rat hepatocytes. 2. Exposure of the intact cells to lactate increases the activity of acetyl-CoA carboxylase, as can be measured in homogenates of these cells. A similar effect by acetate was not observed. 3. Both acetate and lactate drastically increase the cellular level of citrate. 4. Possible mechanisms underlying the difference in response of fatty acid and cholesterol synthesis to an increase in substrate availability are discussed. Futhermore, a mechanism is proposed for the lactate effect on acetyl-CoA carboxylase.

The effects of insulin and glucagon on the release of triacylglycerols by isolated rat hepatocytes are mere reflections of the hormonal effects on the rate of triacylglycerol synthesis

1. Isolated hepatocytes from meal-fed donor rats secrete newly synthesized very-low-density lipoproteins (VLDL) when incubated in a simple bicarbonate buffer. When incubated with 3H2O for 2 h, 72-81% of the 3H-labelled triacylglycerols secreted by the hepatocytes were recovered in VLDL. The secretion of newly synthesized triacylglycerols shows a lag phase of about 30 min. 2. Insulin stimulates the secretion of newly synthesized VLDL triacylglycerols, whereas glucagon has an inhibitory effect on this process. 3. When hepatocytes triacylglycerols were labelled by preincubating the cells with 3H2O or [1-14C]oleate and the cells were subsequently washed and further incubated in radioisotope-free buffer containing hormones, it was observed that the release of the pre-labelled triacylglycerols is not hormone-sensitive. This suggests that insulin and glucagon do not affect the release of triacylglycerols per se. 4. It is concluded that the effects of insulin and glucagon on the overall process of triacylglycerol secretion are reflections of the hormone-determined rate of triacylglycerol synthesis.

Control of glycogen metabolism by insulin in isolated hepatocytes

Isolated rat hepatocytes incubated in a simple bicarbonate buffer, previously shown to display enhanced rates of fatty acid biosynthesis upon a brief exposure to insulin, were used to study acute effects of this hormone on glycogen metabolism. In hepatocytes prepared from fed rats insulin does not increase glycogen deposition as measured chemically, but the incorporation of labelled glucose is markedly accelerated. The rate of disappearance of radioactivity from glycogen, prelabelled with D[U-14C]glucose, is lowered by insulin. In hepatocytes from starved rats insulin promotes net glycogen synthesis. All metabolic parameters studied are affected by glucagon in manner opposite to that of insulin.

Inhibition of hepatic cholesterol biosynthesis by chloroquine

Chloroquine is shown to be a potent inhibitor of cholesterol biosynthesis by isolated rat hepatocytes. Half-maximal inhibition of cholesterogenesis occurs at ca. 10 micro M chloroquine. Chloroquine does not affect fatty acid synthesis by isolated hepatocytes. This suggests that chloroquine acts on the cholesterol biosynthetic pathway beyond the cytosolic acetyl-coA branchpoint of cholesterol and fatty acid synthesis.

Regulation of triacylglycerol synthesis in the liver: a decrease in diacylglycerol acyltransferase activity after treatment of isolated rat hepatocytes with glucagon

Isolated rat hepatocytes were used to investigate the possibility of a short-term effect of glucagon on the synthesis of triacylglycerols in the liver. Incubation of hepatocytes in the presence of glucagon, followed by homogenization in a buffer containing F- (50 mM) and EDTA (2.5 mM), resulted in a 53% decrease in activity of microsomal diacylglycerol acyltransferase (EC 2.3.1.20), the only enzyme that is exclusively involved in the synthesis of triacylglycerols. The activity of cholinephosphotransferase (EC 2.7.8.2), which also uses diacylglycerols as substrate, was not decreased after exposure of the hepatocytes to glucagon. This may imply that triacylglycerol synthesis can be regulated independently of phosphatidylcholine synthesis. The activity of diacylglycerol acyltransferase in microsomes isolated from a homogenate of whole liver could be reduced by preincubating the microsomes with Mg2+ (5 mM), ATP (1 mM) and 105 000 X g supernatant. The enzyme could be reactivated by incubation of the washed microsomes with a 105 000 X g supernatant in the presence of dithiothreitol (5 mM). Fluoride (50 mM) inhibited this reactivation. It is concluded that the activity of diacylglycerol acyltransferase is subject to hormonal short-term control, possibly via a phosphorylation-dephosphorylation mechanism.

Inhibition of lipogenesis in isolated hepatocytes by 3-amino-1,2,4-triazole

3-Amino-1,2,4-triazole has been found to be an inhibitor of fatty acid synthesis by isolated rat hepatocytes. Half-maximal inhibition of fatty acid synthesis occurs at approximately 20mM. Acetyl-CoA carboxylase activity in homogenates of hepatocytes is not affected by previous exposure of the intact cells to 3-amino-1,2,4-triazole. The drug opposes the activation of partially purified acetyl-CoA carboxylase by citrate, but does not influence enzyme activity in the absence of citrate. As compared to fatty acid synthesis, cholesterol synthesis by the hepatocytes is more drastically depressed by incubation of the cells with 3-amino-1,2,4-triazole. Half-maximal inhibition of cholesterogenesis occurs at approximately 5 mM 8-amino-1,2,4-triazole.

Acute effects of insulin on fatty acid metabolism in isolated rat hepatocytes

Isolated rat hepatocytes, previously shown to display enhanced rates of fatty acid biosynthesis upon a brief exposure to insulin, were used to study acute effects of this hormone on other aspects of hepatic fatty acid metabolism. Insulin activates the incorporation of exogenously added fatty acids into glycerolipids and depresses their utilization in the formation of ketone bodies. Insulin increases both the activity of acetyl-CoA carboxylase and the cellular content of malonyl-CoA. Evidence is presented that malonyl-CoA plays an important role in the insulin-mediated control of both ketogenesis and de novo fatty acid synthesis. All metabolic parameters studied are affected by glucagon in a manner opposite to that of insulin.

Persistent changes in the initial rate of pyruvate transport by isolated rat liver mitochondria after preincubation with adenine nucleotides and calcium ions

1. Preincubation of isolated rat-liver mitochondria in the presence of adenine nucleotides or Ca2+ results in definite and persistent changes in the initial rate of pyruvate transport. 2. These changes in the rate of pyruvate transport are accompanied by equally persistent changes in the opposite direction of the activity of pyruvate dehydrogenase (EC 1.2.4.1). 3. Changes of the transmembrane pH gradient and of the membrane potential, brought about by the pretreatments of the mitochondria, cannot account for the observed changes in the rate of pyruvate transport. 4. It is proposed that the pretreatment of the mitochondria directly modulates the activity of the mitochondrial pyruvate carrier. The possible regulatory role of such a modulation system is discussed.

Accumulation of pyruvate by isolated rat liver mitochondria

1. Various methods to measure the rate of accumulation of [3-14C]pyruvate in the sucrose-impermeable space of isolated rat liver mitochondria are tested and compared with respect to their ability to distinguish between carrier-linked pyruvate transport and non-carrier-linked processes (adsorption and diffusion). 2. Evidence is presented that the cinnamic acid derivatives commonly used as specific inhibitors of the pyruvate carrier (i) do not completely abolish all carrier-mediated pyruvate transport; (ii) inhibit pyruvate adsorption, and (iii) at higher concentrations lead to a removal of previously accumulated pyruvate from the mitochondria. It is concluded that procedures which avoid the use of transport inhibitors allow more reliable estimates of carrier-linked pyruvate transport. 3. It is proposed to measure pyruvate adsorption as the accumulation of pyruvate in the presence of an uncoupler. Using this procedure, it could be shown that, with 1 mM pyruvate, adsorption represents only a small part of the total pyruvate accumulation, the main part being carrier-linked transport driven by the pH gradient across the mitochondrial inner membrane.

Opposite effects of insulin and glucagon in acute hormonal control of hepatic lipogenesis

Conditions for the isolation of rat hepatocytes that are responsive to insulin with regard to fatty acid synthesis were explored. Cells prepared according to the procedure of Ingebretsen and Wagle require the presence of fetal calf serum for insulin expression. Cells isolated by the Seglen method are the preparation of choice, since they respond to insulin in a simple, well-defined medium and, moreover, show much higher basal rates of fatty acid synthesis. In the latter cells isolated from fed male rats, the rate of fatty acid synthesis, as determined by tritium incorporation from [3H]H2O at 37 degrees C, is enhanced within 30 min after addition of insulin to the incubation medium; with glucagon, it is depressed. In the presence of insulin, the cellular content of malonyl coenzyme A is noticeably increased, whereas the concentrations of pyruvate, lactate, and citrate are not markedly affected. Glucagon, on the other hand, decreases the concentrations of all four intermediates. The activity of acetyl-CoA carboxylase is stimulated and depressed after addition of insulin and glucagon, respectively. In all conditions tested, the activity of acetyl-CoA carboxylase correlates with the rate of fatty acid synthesis, which in turn correlates with the cellular level of malonyl-CoA.