Biosynthesis of fatty acids and cholesterol in isolated rat-liver parenchymal cells. Effect of albumin-bound fatty acids

Transport and metabolism of radiolabeled choline in hepatocellular carcinoma

Altered choline (Cho) metabolism in cancerous cells can be used as a basis for molecular imaging with PET using radiolabeled Cho. In this study, the metabolism of tracer Cho was investigated in a woodchuck hepatocellular carcinoma (HCC) cell line (WCH17) and in freshly derived rat hepatocytes. The transporter responsible for [(11)C]-Cho uptake in HCC was also characterized in WCH17 cells. The study helped to define the specific mechanisms responsible for radio-Cho uptake seen on the PET images of primary liver cancer such as HCC. Cells were pulsed with [(14)C]-Cho for 5 min and chased for varying durations in cold media to simulate the rapid circulation and clearance of [(11)C]-Cho. Radioactive metabolites were extracted and analyzed by radio-HPLC and radio-TLC. The Cho transporter (ChoT) was characterized in WCH17 cells. WCH17 cells showed higher (14)C uptake than rat primary hepatocytes. [(14)C]-Phosphocholine (PC) was the major metabolite in WCH17. In contrast, the intracellular Cho in primary hepatocytes was found to be oxidized to betaine (partially released into media) and, to a lesser degree, phosphorylated to PC. [(14)C]-Cho uptake by WCH17 cells was found to have both facilitative transport and nonfacilitative diffusion components. The facilitative transport was characterized by Na(+) dependence and low affinity (K(m) = 28.59 ± 6.75 μM) with partial energy dependence. In contrast, ChoT in primary hepatocytes is Na(+) independent and low affinity. Our data suggest that transport and phosphorylation of Cho are responsible for the tracer accumulation during [(11)C]-Cho PET imaging of HCC. WCH17 cells incorporate [(14)C]-Cho preferentially into PC. Conversion of [(14)C]-PC into phosphatidylcholine occurred slowly in vitro. Basal oxidation and phosphorylation activities in surrounding hepatic tissue contribute to the background seen in [(11)C]-Cho PET images.

Time course changes in glycogen accretion, 6-phosphogluconate, fructose-2,6-bisphosphate, and lipogenesis upon refeeding a high sucrose diet to starved rats

1. Starved rats refed 60% sucrose diets were used to determine in vivo lipogenesis and levels of hepatic metabolites. 2. Fatty acid synthesis increased 11-fold 4 hr after refeeding. 3. Glycogen rose from 3 to 100 mg/g liver after 8 hr. 4. Fructose-2,6-bisphosphate rose to 6 nmol/g at 1 hr and remained constant. 5. 6-Phosphogluconate increased from 10 to 45 nmol/g liver after 2 hr and remained constant.

Relationship between lipogenesis, ketogenesis, and malonyl-CoA content in isolated hepatocytes from the obese Zucker rat adapted to a high-fat diet

The relationship between lipogenesis and ketogenesis and the concentration of malonyl coenzyme A (CoA) was investigated in hepatocytes from adult obese Zucker rats and their lean littermates fed either a control low-fat diet or a high-fat diet (30% lard in weight). With the control diet, lipogenesis--although strongly inhibited in the presence of either 1 mmol/L oleate, 10(-6) mol/L glucagon or 0.1 mmol/L TOFA (a hypolipidemic drug)--remained about fifteen-fold higher in the obese rats than in the lean rats. In contrast, ketogenesis under some conditions (oleate + TOFA) was not significantly lower (30%) as compared with the lean rats. After adaptation to the high-fat diet, lipogenesis was depressed fourfold in the lean rats and ninefold in the obese ones; however its magnitude remained significantly higher in the latter, namely at a value close to that measured in control-fed lean rats. Ketogenesis was comparable in lean and obese rats and much higher in the presence of 1 mmol/L oleate than of 0.3 mmol/L oleate, whereas lipogenesis did not vary with increasing oleate concentration in the medium. Acetyl-CoA carboxylase activity measured in liver homogenates was higher in the obese group, but was stepwise inhibited by increasing concentrations of oleyl-CoA regardless of the diet for both lean and obese rats, thus showing no abnormality of in vitro responsiveness to this inhibitor. With the control diet, hepatocyte malonyl-CoA levels were significantly higher in the obese rats, both in the basal state and after inhibition of lipogenesis by oleate and TOFA.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipogenesis from U14C lactate in obese Zucker rat hepatocytes. Effect of albumin-bound oleate

Lipogenesis from U(14C) lactate was studied in hepatocytes isolated from obese Zucker rats (fa/fa) their lean littermates (Fa/?) and Sprague Dawley rats. The distribution of radioactive carbon between the glycerol and the fatty acid moieties of the acylglycerols were studied. Radioactive lactate was better utilized for glycerol formation than it was for fatty acid formation in the obese rats. However, when oleate was added to the hepatocytic incubation medium, radioactive lactate was preferentially incorporated into the fatty acid moiety of the acylglycerols. Zucker obesity classified as a "metabolic obesity" by Meyer (1) depends upon abnormalities in carbohydrate metabolism associated with increased lipogenesis. This might be explained by biochemical shifts in the utilization of nutrients (2). Among the nutrients, lactate seems to be a better source of carbon than glucose for lipid synthesis (3). It has been shown that there is an increased hepatic portal blood concentration of lactate several hours after eating: about 4 mM in Wistar rats (4) and 10-15 mM in obese Zucker rats (3). We are interested in determinating the incorporation of carbon from lactate either into glycerol or into fatty acyl moieties of hepatic acylglycerols, and in determining the influence of exogenous fatty acids on acylglycerol synthesis, since a high level of circulating fatty acids in Zucker obese rats has been reported (5). Our purpose was to determine the incorporation of lactate into glycerol and fatty acyl moieties of acylglycerols, under the influence of oleate. Hepatocytes were isolated from ad libitum fed obese Zucker rats (fa/fa), their lean littermates (Fa/?) and Sprague-Dawley rats (SD). Incorporation of lactate was studied for three hours, in order to exclude short-term regulation effects and to allow oleate to be distributed into all cellular compartments.

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.

Short-term regulation of fatty acid synthesis in isolated alveolar type II cells from adult rat lung. Effects of free fatty acids and hormones

We measured the total rate of fatty acid synthesis in alveolar type II cells freshly isolated from the lungs of adult rats. The rate of incorporation of 3H from 3H2O into cellular fatty acids was linear during 3-hr incubations and was very brisk [18 ng-atoms 3H/10(6) cells/hr +/- 2 (mean +/- SD, n = 23)]. When the nutrient medium (Minimum Essential Medium) was supplemented with various hormones or free fatty acids, the long-chain fatty acids (C14-C20) caused a decrease in the rate of 3H incorporation to a variable degree depending on the species of fatty acid. Stearate (10(-4) M) and palmitate (10(-4) M) caused the greatest inhibition of de novo cellular fatty acid synthesis, followed by myristate, arachidonate, and oleate. Insulin (10(-7) M), glucagon (10(-8) M), terbutaline (10(-5) M), 8-bromo-cyclic AMP (10(-3) M), the essential fatty acid linoleate (10(-4) M), and the medium-chain-free fatty acids laurate and octanoate (10(-4) M) did not alter the rate of fatty acid synthesis in type II cells. We demonstrated that the alveolar type II cell is a major lipogenic cell type in the rat. We also demonstrated that the availability of preformed fatty acids in the extracellular milieu is one factor regulating the rate of fatty acid synthesis in these cells.

Effect of dexamethasone on 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity and cholesterol synthesis in rat liver

Dexamethasone increases reductase activity in cultured liver cells after a lag period of 2 h. The increases of activity are linear from 10(-9) to 10(-5) M dexamethasone, the maximum responses ranging from 2- to 4-fold. The increased reductase activity after dexamethasone treatment is not due to a change of the state of phosphorylation/dephosphorylation of the enzyme nor to an increase of cytosolic activating factor(s) for the reductase. Cholesterol synthesis, measured by incorporation of either [14C]acetate or 3H2O, increases 3-fold after dexamethasone (10(-6) M) treatment, as does the hydroxymethylglutaryl-CoA reductase activity, confirming that this enzyme is rate-controlling for cholesterol synthesis in cultured liver cells as it is in vivo. Dexamethasone (10 micrograms/100 g rat), given after onset of the light cycle, increases reductase activity over control rats at the nadir of the circadian cycle of this enzyme. When given after onset of the dark cycle, dexamethasone does not increase reductase activity over controls at the peak of their circadian cycle. Thus, physiologic doses of glucocorticoids partially reverse the decline in reductase activity due to the circadian rhythm.

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.

Sulfonamide acetylation in isolated rat liver cells

Primary suspension of isolated liver cells, prepared from rat livers perfused with Ca++ free buffer and 0.05% collagenase, were used for studies of sulfadimidine uptake and metabolism at various temperatures (29 degrees -41 degrees) and pH (6.4 - 7.8). The intracellular: extracellular ratio for sulfanilamide was found to be insensitive to both temperature and pH variations, while the corresponding ratio for sulfadimidine was pH dependent but insensitive to temperature variation. Decreasing pH increased the cell content of sulfadimidine. Sulfanilamide was metabolized by acetylation only, while sulfadimidine gave rise to several metabolites. The rate of sulfanilamide acetylation in primary cell suspensions was of the same order of magnitude as the acetylation rate of sulfanilamide published previously for the intact organ. Sulfanilamide was acetylated at the highest rate in the pH-region from 7.0 to 7.5, while sulfadimidine was metabolized most rapidly between pH 6.7 and 7.3. At pH 7.5 sulfadimidine was metabolized at a rate only 35% of the rate at pH 7.3. The acetylation rate of both drugs increased with increasing temperature, approximately 5% per degree celcius, and exhibited a Q10 of 1.5.

Suppression of hepatocyte fatty acid synthesis by albumin-bound linoleate involves depolymerization of acetyl-CoA carboxylase filaments

Digitonin treatment of chick liver cells in monolayer culture results in plasma membrane perforations due to digitonin removal of membrane cholesterol. The amount and rate of acetyl-CoA carboxylase activity that escapes from the hepatocyte during digitonin treatment is positively related to the amount of protomeric carboxylase in the cells. Incubation of chick liver cells in culture with albumin-bound linoleate (60 min) caused a 3-fold increase in the amount of carboxylase activity released during exposure of cells to digitonin. Concomitant with the enhanced release of carboxylase activity was an 85% reduction in fatty acid synthesis induced by linoleate. Apparently, acute suppression of hepatocyte fatty acid synthesis by media free fatty acids resulted, in part, from a change in carboxylase conformation from the active polymeric state to the inactive protomeric form.

Effect of different fatty acids on triacylglycerol secretion in isolated rat hepatocytes

Lipoprotein triacylglycerol secretion was studied in isolated rat hepatocytes incubated with different albumin-bound fatty acids and labeled glycerol. The release of labeled triacylglycerol was stimulated more by unsaturated fatty acids than by saturated ones. When lipoprotein secretion was related to cell triacylglycerol synthesis, an effect of unsaturation was no longer observed. Instead the secretion rate, expressed in this manner, increased with increasing fatty acid chain length. For the first time, the secretion of molecular species of triacylglycerol has been studied. The distribution of labeled glycerol among different species was the same in the cells and in th secreted product, indicating that different triacylglycerols were secreted without selectivity. It is concluded that the fatty acid structure influences lipoprotein triacylglycerol secretion and it is emphasized that the effects observed depend on the method of quantitation of the secretion rate.

Regulation of hepatic free fatty acid metabolism by glucagon and insulin

The roles of glucagon and insulin in the direct short-term regulation of hepatic free fatty acid (FFA) metabolism were studied in hepatocytes isolated from fed, fasted, and streptozotocin-induced diabetic rats. In fed animals, the principal metabolic product of palmitate metabolism was triglyceride, whereas ketones were the major product in fasted and diabetic animals. Glucagon at physiological concentrations increased ketogenesis and decreased triglyceride synthesis from palmitate in hepatocytes from fed rats at FFA concentrations 1.0 mM or less. Insulin had no effect on FFA metabolism when present as the sole hormone, but could antagonize the actions of submaximal concentrations of glucagon. The metabolism of palmitate in fasted or diabetic hepatocytes was unaffected by either hormone. Ketogenesis from octanoate was also unaffected by hormone addition in all cell types. These data are consistent with a locus of hormonal regulation at a step prior to beta-oxidation of fatty acid. Glucagon and insulin may modulate FFA metabolism by both intrahepatic and extrahepatic mechanisms.

Effects of vitamin B12 deficiency on lipid metabolism of the rat liver and nervous system

1. Rats bred from vitamin B12-depleted dams were fed on a vitamin B12-deficient diet for 12-15 months and developed a severe vitamin B12 deficiency, as judged from methylmalonic acid excretion and tissue vitamin B12 levels at slaughter. Control rats were supplemented with vitamin B12 in the drinking-water. 2. Neurological signs were recorded after 7 months but the motor nerve conduction velocities remained normal. Neuropathological examination revealed mild changes in the peripheral nerves but no changes in the central nervous system. 3. The amounts of total lipids and phospholipids were normal, but in all examined tissues the proportions of pentadecanoate (C15 fatty acid) and heptadecanoate (C17 fatty acid) were considerably increased in vitamin B12 deficiency. 4. 3H2O was incorporated to the same extent into the fatty acids of nervous tissue from vitamin B12-deficient and control rats after 48 h. Less 3H was found in the liver fatty acids of the vitamin B12-deficient rats. 5. Neurological dysfunction can be demonstrated in the vitamin B12-deficient rat; the relation of the biochemical and neuropathological changes to the neurological signs needs further study.

Fatty acid synthesis and metabolism of phospholipid acyl groups in strain L mouse fibroblasts

Strain L mouse fibroblasts grown in medium supplemented with 2.5% delipidized horse serum were found capable of desaturating oleic and linoleic acid to dienoic and trienoic acid(s), respectively. Although 40-60% of de novo fatty acid synthesis from [2-3H]acetate was inhibited by the administration of exogenous oleic or linoleic acid, sterole synthesis was only slightly affected. Within 24-48 h after incorporation, phospholipid fatty acyl groups could undergo active exchange between phospholipids. After this dynamic transition period was over, not only were the phospholipid acyls retained but some vicinal fatty acyl pairs of phospholipid also appeared to be stable and remained together throughout the depletion period. At any time in the experiment, however, introduction of exogenous fatty acid perturbed this phospholipid acyl retention, delayed the time at which the phospholipid acyl groups no longer moved between phospholipids and also decreased the ultimate number of phospholipid acyl groups retained by strain L mouse fibroblasts.

Effects of analogles of ethanolamine and choline on phospholipid metabolism in rat hepatocytes

1. Analogues of ethanolamine and choline were incubated with different labelled precursors of phospholipids and isolated hepatocytes and the effects on phospholipid synthesis were studied. 2. 2-Aminopropan-1-ol and 2-aminobutan-1-ol were the most efficient inhibitors of [(14)C]ethanolamine incorporation into phospholipids, whereas the incorporation of [(3)H]choline was inhibited most extensively by NN-diethylethanolamine and NN-dimethylethanolamine. 3. When the analogues were incubated with [(3)H]glycerol and hepatocytes, the appearance of (3)H in unnatural phospholipids indicated that they were incorporated, at least in part, via CDP-derivatives. The distribution of [(3)H]glycerol among molecular species of phospholipids containing 2-aminopropan-1-ol and 1-aminopropan-2-ol was the same as in phosphatidylethanolamine. In other phospholipid analogues the distribution of (3)H was more similar to that in phosphatidylcholine. 4. NN-Diethylethanolamine stimulated both the conversion of phosphatidylethanolamine into phosphatidylcholine and the incorporation of [Me-(14)C]methionine into phospholipids. Other N-alkyl- or NN-dialkyl-ethanolamines also stimulated [(14)C]methionine incorporation, but inhibited the conversion of phosphatidylethanolamine into phosphatidylcholine. This indicates that phosphatidyl-NN-diethylethanolamine is a poor methyl acceptor, in contrast with other N-alkylated phosphatidylethanolamines. 5. These results on the regulation of phospholipid metabolism in intact cells are discussed with respect to the possible control points. They also provide guidelines for future experiments on the manipulation of phospholipid polar-headgroup composition in primary cultures of hepatocytes.

Metabolism of 14C-antipyrine in suspensions of isolated rat liver cells

Suspensions of liver cells isolated from perfused rat livers were incubated with antipyrine-N-methyl-14C. Antipyrine was eliminated by first-order kinetics during incubations for 3 hours with primary suspensions (parenchymal cells + non-parenchymal cells) and suspensions of purified parenchymal cells. Antipyrine concentrations were unchanged when incubated with suspensions of non-parenchymal cells, dead cells or medium only. At the end of incubation period, 4-OH-antipyrine and 3-CH2OH-antipyrine were detected mainly as the glucuronide or sulphate conjugates, and evidence for the N-demethylation of antipyrine was also obtained. Half-lives for elimination of antipyrine in primary cell suspensions were not significantly different from the half-lives measured in parenchymal cell suspensions. This finding together with the lack of metabolism of antipyrine found in non-parenchymal cell suspensions suggest that oxidation and conjugation of antipyrine is mainly confined to the parenchymal cells. There was significant inhibition of antipyrine metabolism in primary suspensions by phenylbutazone (1.6 X 10(-3)M), dexamethasone (2 X 10(-4)M) and ethanol (1.3 X 10(-2)M, 0.75%). We suggest that the use of primary suspensions of isolated rat liver cells provide a rapid and simple method for the study of factors influencing drug metabolism in the liver.

Phospholipid degradation in isolated rat hepatocytes. Metabolism of intracellularly formed dilinoleoyl-dipalmitoyl- and dimyristoylglycerophosphocholine

The aim of this work is to describe the role of different phospholipases in hepatic phospholipid catabolism. Therefore isolated rat hepatocytes enriched in labeled dilinoleoyl-, dipalmitoyl- or dimyristoylglycerophosphocholine were prepared by pulse incubation with [3H]glycerol and 14C-labeled fatty acid. The labeled cells were chased up to 4 h in a tracer-free medium and the degradation of different phosphatidylcholines studied. After a 2-h chase about 40% of dilinoleoyl-, 70% of dipalmitoyl- and 30% of dimyristoylglycerophosphocholine were degraded. From the positional distribution of 14C-labeled fatty acid and the change in the doubly labeled molecular species of phospholipids, it was concluded that tb degradation of dilinoleoylglycerophosphocholine and that of phosphatidylethanolamine could be accounted for by the action of phospholipase A1, while the degradation of dipalmitoylglycerophosphocholine proceeded through the action of phospholipase A2. Dimyristoylglycerophosphocholine was probably cleaved by the combined action of both phospholipases A1 and A2. Up to 10 mM tetracain, added to the chase medium, effectively blocked the action of both phospholipase activities. A considerable part of 2-linoleoyl- and 1-palmitoylglycerophosphocholine liberated during the chase was reutilized for phosphatidylcholine synthesis without further degradation.

Measurement of the absolute rates of cholesterol biosynthesis in isolated rat liver cells

Triparanol [2-(4-chlorophenyl)-1-(4-diethylaminoethoxyphenyl)-1-p-tolylethanol] at a concentration of 2 micronm has no effect on the overall conversion of [2=14C]acetate into C27 sterols by isolated liver cells. In the presence of triparanol, however, the formation of radioactive cholesterol is inhibited by 85-90% and the balance of radioactivity appears in the C27 sterol desmosterol (cholesta-5,24-dien-3beta-ol). The very small weights of desmosterol which accumulate under these conditions were, as a routine, quantitatively converted into the heptafluorobutyrate 3-enol ester of cholesta-4,24-dien-3-one. This derivative has a high electron-capturing capability, a property that enables extremely small quantities (less than 0.25pmol) of the material to be accurately measured by gas chromatography with electron-capture detection. Measurements of the mass and specific radioactivity of the newly biosynthesized desmosterol formed in the presence of triparanol provides an accurate assessment of the amount of cholesterol that would be synthesized by the liver cells in the absence of the drug.

Metabolism of different monoacylphospholipids in isolated hepatocytes and the intact rat

The 1-[3H] palmitoyl, 2-[3H] oleoyl, and 2-[14C] linoleoyl derivatives of sn-glycero-3-phosphoethanolamine and the corresponding derivatives of sn-glycero-3-phosphocholine were injected intraportally to rats and their incorporation into liver lipids was studied 15 min thereafter. Both the uptake by the liver and the degree of acylation was higher for the unsaturated compounds. The uptake of lysophosphatidylethanolamine was higher than that of lysophosphatidlycholine. The metabolism of 1-lysophosphatidylethanolamine was also studied in isolated hepatocytes. The degree of hydrolysis was much more prominent than in vivo. After injecting 2-[14C] linoleoyl derivatives, a large part of the 14C was recovered in the dienoic phospholipids. Subfractionation by reversed-phase partition chromatography showed that the isotope was located in the palmitoyllinoleoyl and stearoyl-linoleoyl fraction. The 100 X stearoly/(palmitoyl + stearoyl) ratio was 84 in dienoic phosphatidylethanolamine and 59 in dienoic phosphatidylcholine. This preference for stearic acid is significantly larger than in other pathways yielding dienoic phospholipids. It can be concluded that the monoacylphospholipid acyltransferase reactions operating at positions 1 or 2 yield different saturated acyl chain profiles in phosphatidylethanolamine and phosphatidylcholine of a specific unsaturation. This may be important in the regulation of the fatty acid composition of the membrane phospholipids.

The effect of two isomeric octadecenoic acids on the lipid metabolism and growth of Novikoff hepatoma cells

The origin and metabolism of octadecenoic acid (18 : 1) was examined in intact Novikoff rat hepatoma cells by using labeled precursors and two isomeric octadecenoic acids which differed in their abilities to stimulate cell growth in a serum-free medium. The isomers (ci-6-18 : 1 and cis-9-18 : 1) were measured in the cellular lipid by ozonolysis and reduction of the ozonides. The results indicate that the 18 : 1 fatty acid accumulated in the cell lipid by uptake of the preformed acid from the medium. The cis-9-18 : 1 to 16 : 1 and 20 : 1 fatty acids by chain shortening and chain elongation. Both isomers inhibited de novo fatty acid synthesis from acetate by cells suspended in a serum-free medium. The isomers did not exert coordinate control of both fatty acid and cholesterol biosynthesis in the Novikoff cells.

Correlation of rat liver chromatin-bound free and esterified cholesterol with the circadian rhythm of cholesterol biosynthesis in the rat

Cholesterol has been shown to be present in rat liver chromatin isolated by methods designed to avoid contamination by membrane fragments. Evidence that the cholesterol was actually a component of chromatin includes (a) the constancy of the amount (1.30 +/- 0.14 mug per mg DNA), (b) the striking difference in the ratio of free (i.e. unesterified) to esterified cholesterol between that in chromatin and that in membrane, and (c) the rapid and marked changes which occurred in this ratio during the circadian cycle in chromatin but not in membranes. Although the total amount of chromatin-bound cholesterol did not change throughout the circadian cycle, the concentration of free cholesterol increased sharply a short time before the peak of cholesterol synthetic activity was reached at about midnight; it reached a basal level about 6 h later at approximately the same time the rate of synthesis returned to its basal level. When labelled cholesterol was administered by stomach tube, it was detectable within 2 h in whole nuclei and in chromatin, indicating that chromatin-bound cholesterol is rapidly exchangeable with that in liver cytoplasm and in blood plasma. Removal of basic proteins from chromatin did not result in the loss of any cholesterol, but removal of most of the acidic as well as the basic proteins resulted in loss of most of the chromatin-bound cholesterol. These results indicate that cholesterol is bound either to the acidic proteins or to both the acidic proteins and DNA. The data are compatible with the hypothesis that cholesterol biosynthesis controlled at the nuclear level and suggest that the relative amounts of free and esterified cholesterol associated with chromatin may play a role.

Adaptive changes in delta 9 desaturase activity in rat liver

The delta 9 desaturase activity and the 14-C radioactivity of the de novo synthesized fatty acids incorporated into microsomal lipids and serum triglycerides were measured under different nutritional conditions. The results obtained indicate a correlation between the values of the three parameters studied after starvation or after refeeding Purina chow or either a high carbohydrate or a high protein diet. These data suggest that liver lipogenesis and delta 9 desaturase activities respond to the same regulatory factors.

Increased cholesterol-ester formation during forced cholesterol synthesis in rat hepatocytes

By comparing the incorporation of 3H20 and [14C]mevalonate into cholesterol in suspensions of rat hepatocytes, it was calculated that the cholesterol biosynthesis could be stimulated 4--7-fold by addition of mevalonate. The addition of 3.3--6.7 mM mevalonate also caused a 5--6-fold increase in the proportion of newly synthesized cholesterol that was esterifield. The esterification of radioactive cholesterol, entering the cells by exchange with surrounding plasma lipoproteins was also increased, indicating that a true increase in the rate of cholesterol ester formation rather than a more selective utilization of newly synthesized cholesterol for esterification, occurred. The increase in cholesterol esterification was not abolished by cycloheximide, indicating that it did not require an increased synthesis of cholesterol esterifying enzyme. Instead the data suggest that the supply of cholesterol to the esterifiable pool may be an important factor, regulating the rate of cholesterol ester formation in rat liver. The addition of 0.5 mM oleic acid to the medium did not increase the degree of cholesterol esterification significantly, whereas 2 mM oleic acid bound to 1% albumin increased the proportion of newly synthesized cholesterol that was esterified, by about 70%. The cells secreted radioactive cholesterol esters into the medium. Cycloheximide inhibited this secretion to about 80% but did not affect the rate at which newly synthesized cholesterol was transferred to surrounding plasma lipoproteins.