Inhibition by chloroquine of the secretion of very low density lipoproteins by cultured rat hepatocytes

Genetically modified mouse models to study hepatic neutral lipid mobilization

Excessive accumulation of triacylglycerol is the common denominator of a wide range of clinical pathologies of liver diseases, termed non-alcoholic fatty liver disease. Such excessive triacylglycerol deposition in the liver is also referred to as hepatic steatosis. Although liver steatosis often resolves over time, it eventually progresses to steatohepatitis, liver fibrosis and cirrhosis, with associated complications, including liver failure, hepatocellular carcinoma and ultimately death of affected individuals. From the disease etiology it is obvious that a tight regulation between lipid uptake, triacylglycerol synthesis, hydrolysis, secretion and fatty acid oxidation is required to prevent triacylglycerol deposition in the liver. In addition to triacylglycerol, also a tight control of other neutral lipid ester classes, i.e. cholesteryl esters and retinyl esters, is crucial for the maintenance of a healthy liver. Excessive cholesteryl ester accumulation is a hallmark of cholesteryl ester storage disease or Wolman disease, which is associated with premature death. The loss of hepatic vitamin A stores (retinyl ester stores of hepatic stellate cells) is incidental to the onset of liver fibrosis. Importantly, this more advanced stage of liver disease usually does not resolve but progresses to life threatening stages, i.e. liver cirrhosis and cancer. Therefore, understanding the enzymes and pathways that mobilize hepatic neutral lipid esters is crucial for the development of strategies and therapies to ameliorate pathophysiological conditions associated with derangements of hepatic neutral lipid ester stores, including liver steatosis, steatohepatitis, and fibrosis. This review highlights the physiological roles of enzymes governing the mobilization of neutral lipid esters at different sites in liver cells, including cytosolic lipid droplets, endoplasmic reticulum, and lysosomes. This article is part of a Special Issue entitled Molecular Basis of Disease: Animal models in liver disease.

Complex role of autophagy in regulation of hepatic lipid and lipoprotein metabolism

Discovering new therapeutic interventions to treat lipid and lipoprotein disorders is of great interest and the discovery of autophagy as a regulator of lipid metabolism has opened up new avenues for targeting modulators of this pathway. Autophagy is a degradative process that targets cellular components to the lysosome and recent studies have indicated a role for autophagy in regulating hepatic lipid metabolism (known as lipophagy) as well as lipoprotein assembly. Autophagy directly targets apolipoprotein B-100 (apoB100), the structural protein component of very low-density lipoproteins (VLDLs), and further targets lipid droplets (LDs), the cellular storage for neutral lipids. Autophagy thus plays a complex and dual role in VLDL particle assembly by regulating apoB100 degradation as well as aiding the maturation of VLDL particles by hydrolyzing lipid from LDs. The purpose of this article is to review our current understanding of molecular and cellular mechanisms mediating authophagic control of hepatic lipid biogenesis and VLDL production as well as dysregulation in insulin resistance and dyslipidemia.

The frequency of high/moderate lipoprotein risk factor for coronary artery disease is significant in juvenile-onset systemic lupus erythematosus

To determine the prevalence of dyslipoproteinemia on a large juvenile systemic lupus erythematosus (jSLE) cohort, we selected 40 patients after rigorous exclusion criteria. Lipoprotein levels were determinated after 12 hours fast and risk levels for CAD were defined by standards of the Brazilian Guidelines for dyslipoproteinemia according to US-NCEP. All individuals were under steroid therapy and chloroquine and 43% had active disease. Thirty patients (75%) had high-risk levels for CAD (23 isolated low HDL, while in seven subjects low HDL was associated to high TG in four, high LDL in one, high TG/LDL in one and high TC/LDL in one). Remarkably, overall analysis revealed that 85% patients were included in high/medium risk levels group (29% for TC, 29% for LDL, 88% for HDL, and 18% for TG) and these disturbances occured mainly in the first four years of disease. Unexpectedly, one-third of the patients presented two or more high/medium lipoprotein risk factors. Independently, active jSLE was associated to TG (OR = 3.2; P < 0.001) and had a tendency towards reduction on HDL (OR = 8.5; P = 0.056). Considering improvements on jSLE outcome, the increased frequency of high/medium lipoprotein risk levels for CAD reinforces the need of prevention measures in order to minimize deleterious effects of this disturbance.

Changes in plasma lipids after a non-lethal dose of cycloheximide in rats

This paper describes a study of the effect of a single intraperitoneal non-lethal dose of cycloheximide (CHM; 2.0 mg/kg body weight) on the concentration of plasma lipids and lipoproteins in male rats killed one, two, three, four and nine days after receiving the dose. The concentration of triglycerides, total cholesterol, high-density lipoproteins (HDL)-cholesterol and low-density lipoproteins (LDL)-cholesterol was measured in treated and control animals. The effect of CHM on the concentration of triglycerides, total cholesterol, HDL-cholesterol, and LDL-cholesterol was visible in rat plasma throughout the study. Total cholesterol and HDL-cholesterol concentrations showed the same pattern of changes, probably due to the reversible inhibition of apolipoprotein apo A-I synthesis by CHM. The concentration of triglycerides decreased after a lag period of three days when the reserves of apolipoprotein apo B, the main apolipoprotein of very low-density lipoproteins (VLDL)-cholesterols produced in the liver, were consumed.

Polymorphism of the beta3-adrenergic receptor and lipid profile in patients with rheumatoid arthritis and systemic lupus erythematosus treated with chloroquine

We investigated the effect of beta 3-adrenergic receptor (beta(3)AR) polymorphism on lipid profiles in patients with rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) treated with chloroquine. One hundred sixty-eight subjects were classified into three groups: 61 RA patients, 57 SLE patients, and 50 healthy subjects. All patients fulfilled the 1987 and 1982 classification criteria for RA and SLE, respectively, of the American College of Rheumatology. Demographic data and clinical characteristics of the patients were registered. Fasting lipid profile determination and leukocyte genomic DNA isolation from peripheral blood was performed in all the participants. Screening of the beta(3)-AR gene polymorphic region (exon 1) was done by the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) technique. Quantitative and qualitative variables were analyzed using analysis of variance (ANOVA) with the LSD and chi(2) tests, respectively. An association between the arg64/arg64 beta(3)-AR genotype and high levels of triglycerides (TG) and very low-density lipoprotein cholesterol (VLDL-c) was found in three RA patients ( P=0.01), two of them taking chloroquine. Arg64/arg64 beta(3)-AR polymorphism may contribute to increased TG and VLDL-c in RA patients, independently of chloroquine treatment.

Hydroxychloroquine has no significant effect on lipids and apolipoproteins in Chinese systemic lupus erythematosus patients with mild or inactive disease

OBJECTIVE

To investigate the effects of hydroxychloroquine (HCQ) in Chinese patients with systemic lupus erythematosus who were either on a low dose of prednisone or not on prednisone.

METHODS

65 consecutive Chinese lupus patients without features of nephritis underwent assessment of the fasting serum lipid profile, including total cholesterol (TC), triglycerides (TG), HDL-cholesteorl (HDL-C), LDL-cholesterol (LDL-C), apolipoprotein A-1 (apo A-1), apolipoprotein B (apo B) and lipoprotein (a) (Lp(a)).

RESULTS

44/65 (67.7%) patients were on HCQ with a dose of 244 +/- 86 mg daily and 34/65 (52.3%) patients were on 4.3 +/- 2.6 mg daily of prednisone. The percentages of patients taking prednisone and the mean dose in those patients receiving this treatment were not different between the groups with or without HCQ treatment. TG tended to be lower in those patients on HCQ (median and interquartile range) (1.0 (0.7-1.4) mmol/L vs 1.2 (0.8 1.6) mmol/L), but there were no significant differences in TC or other lipid parameters including Lp(a) between the 2 groups of patients with or without HCQ treatment.

CONCLUSION

HCQ had no significant effect on the serum lipid profile in these lupus patients with mild or inactive disease. This finding probably reflects the low doses of corticosteroid used and a lower background lipid level in this population.

Differential effects of eicosapentaenoic acid on glycerolipid and apolipoprotein B metabolism in primary human hepatocytes compared to HepG2 cells and primary rat hepatocytes

We compared the effects of eicosapentaenoic acid (EPA) and oleic acid (OA) on glycerolipid and apolipoprotein B (apoB) metabolism in primary human hepatocytes, HepG2 cells and primary rat hepatocytes. Cells were incubated for 1 to 5 h with 0.25 mM bovine serum albumin in the absence (control) or presence of 1 mM of EPA or OA. Synthesis and secretion of [3H]glycerolipid were determined after 1 h incubation with [3H]glycerol. Cellular and medium apoB abundance was semi-quantitatively estimated in human cells by Western blotting. The following observations were made. (1) Compared to control, OA induced a 7-fold increase in [3H]triacylglycerol (TG) synthesis in human hepatocytes and a 4-fold increase in rat hepatocytes and HepG2 cells. EPA enhanced [3H]TG synthesis about 2-fold in all three cell types although it stimulated [3H]diacylglycerol (DG) synthesis to an extent (i.e., 2.5- to 5-fold) similar to OA. (2) In contrast to OA, which stimulated VLDL-associated [3H]TG secretion 2.5- to 3-fold in the three cell types relative to control, EPA did not alter [3H]TG secretion in HepG2 and rat hepatocytes and suppressed [3H]TG secretion by 75% in primary human hepatocytes. (3) In primary human hepatocytes, both OA and EPA did not alter cellular apoB abundance but EPA decreased apoB secretion by 44% as compared to control. In contrast, both EPA and OA increased cellular and medium apoB abundance 2- to 2.5-fold in HepG2 cells, although medium apoB tended to be lower in EPA-treated cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of 4,7,10,13,16,19-docosahexaenoic acid on triglyceride accumulation and secretion in rat hepatocytes in culture

We have investigated the effect of oleic (18:1) and 4,7,10,13,16,19-docosahexaenoic (22:6 omega 3) acids on triglyceride (TG) accumulation, secretion and reuptake in rat hepatocytes in culture. We also calculated the percentage of total TG, TG-esterified 18:1 and TG-esterified 22:6 omega 3 that were secreted relative to the total accumulation (intra + extracellular TG). Both fatty acids were incorporated mainly in the intracellular TG fraction. Treatment with 18:1 but not with 22:6 omega 3 increased the quantity of TG secreted into the culture medium relative to controls. Treatment with 22:6 omega 3 caused a greater accumulation of intracellular TG than 18:1. This arises in part from the preferential retention of 22:6 omega 3-enriched TG by the hepatocytes. At 24 hr, there was no longer any difference in the net secretion of TG by 18:1 and 22:6 omega 3-treated cells, which may be a consequence of the reuptake of TG-esterified 18:1. There was no reuptake of TG-esterified 22:6 omega 3. We conclude that inhibition of hepatocyte TG synthesis is not obligatory for 22:6 omega 3-induced diminution of TG secretion.

Absorption, transport and metabolism of vitamin E

Vitamin E includes eight naturally occurring fat-soluble nutrients called tocopherols and dietary intake of vitamin E activity is essential in many species. alpha-Tocopherol has the highest biological activity and the highest molar concentration of lipid soluble antioxidant in man. Deficiency of vitamin E may cause neurological dysfunction, myopathies and diminished erythrocyte life span. alpha-Tocopherol is absorbed via the lymphatic pathway and transported in association with chylomicrons. In plasma alpha-tocopherol is found in all lipoprotein fractions, but mostly associated with apo B-containing lipoproteins in man. In rats approximately 50% of alpha-tocopherol is bound to high density lipoproteins (HDL). After intestinal absorption and transport with chylomicrons alpha-tocopherol is mostly transferred to parenchymal cells of the liver were most of the fat-soluble vitamin is stored. Little vitamin E is stored in the non-parenchymal cells (endothelial, stellate and Kupffer cells). alpha-Tocopherol is secreted in association with very low density lipoprotein (VLDL) from the liver. In the rat about 90% of total body mass of alpha-tocopherol is recovered in the liver, skeletal muscle and adipose tissue. Most alpha-tocopherol is located in the mitochondrial fractions and in the endoplasmic reticulum, whereas little is found in cytosol and peroxisomes. Clinical evidence from heavy drinkers and from experimental work in rats suggests that alcohol may increase oxidation of alpha-tocopherol, causing reduced tissue concentrations of alpha-tocopherol. Increased demand for vitamin E has also been observed in premature babies and patients with malabsorption, but there is little evidence that the well balanced diet of the healthy population would be improved by supplementation with vitamin E.

Assembly and secretion of hepatic very-low-density lipoprotein

In contrast to water-soluble fuels such as glucose or ketone bodies, the use of lipids as an energy source for tissues has required the development of complex structures for their transport through the aqueous plasma. In the case of endogenously synthesized triacylglycerol this is achieved by the assembly and secretion of hepatic VLDL which provides the necessary stability in an aqueous medium. An essential component of this assembly process is apo B. Dietary changes which require an increase in hepatic VLDL secretion appear to be accompanied by increases in the availability of functional apo B. Interesting questions relate to: (a) the intracellular site(s) of triacylglycerol association with apo B, and (b) the mechanism(s) by which the availability of functional apo B at this site responds to metabolic and hormonal signals which reflect dietary status and, thus, the need to secrete triacylglycerol. As regards the latter, although in some cases changes in apo B synthesis occur in response to VLDL secretion hepatic apo B mRNA levels appear to be quite stable in vitro. Intracellular switching of apo B between the secretory and degradative pathways may be important in controlling VLDL assembly and post-translational modifications of the apoprotein may also play a role by influencing its ability to bind to triacylglycerol. Transport is not the only problem associated with the utilization of a concentrated energy source such as triacylglycerol and the complex problems of waste product disposal and recycling have to be dealt with. In the case of triacylglycerol, potentially toxic waste products include atherogenic remnants and LDL. The overall problem, then, in the long-term, involves the development of a 'safe' means of utilizing triacylglycerol and this requirement accounts for much of the complexity of plasma lipoprotein metabolism. In this area, the rat could teach the human a few tricks. One of these appears to be the utilization of hepatic apo B48 rather than apo B100 for VLDL assembly in response to increases in the extrahepatic utilization of hepatically synthesized triacylglycerol. Under these conditions, the remnants of hepatic triacylglycerol utilization by peripheral tissues are cleared from the plasma much more readily via a process which seems to involve the cycling of more triacylglycerol back to the liver than that which occurs in humans. The means by which this is achieved, though, are obscure and may involve a chylomicron remnant receptor, the nature of which, itself, remains controversial.(ABSTRACT TRUNCATED AT 400 WORDS)

Eicosapentaenoic acid inhibits hepatic production of very low density lipoprotein

Synthesis and secretion of triacylglycerol and cholesterol ester by cultured hepatocytes were highest in the presence of oleic, palmitic, stearic and linoleic acid, and lowest with eicosapentaenoic (EPA) and docosahexaenoic acid. Cellular formation and secretion of triacylglycerol and cholesterol ester were reduced by 50-80% in the presence of EPA in comparison with oleic acid. Reduced formation of triacylglycerol and cholesterol ester was also observed when EPA was given together with oleic acid. EPA caused higher incorporation of (3H)water into phospholipid and lower incorporation into triacylglycerol and cholesterol ester as compared with oleic acid. Rat liver microsomes incorporated EPA-CoA into triacylglycerol and cholesterol ester to a lesser extent than oleoyl-CoA. Decreased formation of triacylglycerol and cholesterol ester was also observed when EPA-CoA was given together with oleoyl-CoA, whereas palmitoyl-CoA, stearoyl-CoA, linolenoyl-CoA and arachidonoyl-CoA had no inhibitory effects. In conclusion, inhibition of acyl-CoA:1,2-diacylglycerol acyltransferase and acyl-CoA:cholesterol acyltransferase by EPA may be important for decreased hepatic synthesis and secretion of triacylglycerol and cholesterol ester.

Inhibition of apolipoprotein B net synthesis and secretion from cultured rat hepatocytes by the calcium-channel blocker diltiazem

1. The effect of the Ca2+-channel blocker diltiazem on hepatic apolipoprotein B (apo B) synthesis and secretion was studied in 12-18 h cultures of collagenase-dispersed rat hepatocytes. 2. The presence of diltiazem in the medium decreased apo B secretion by hepatocytes in a concentration-dependent manner. At 25 microM, diltiazem inhibited apo B secretion by approx. 36%, but there was no evidence of intracellular accumulation of apo B. 3. The inhibition of apo B secretion by hepatocytes was significantly correlated with cell-associated diltiazem (r = 0.72, P less than 0.01). 4. The rate of apo B secretion remained linear over 16 h even in the presence of 50 microM-diltiazem. 5. At diltiazem concentrations in the medium which were inhibitory for apo B secretion, [14C]acetate incorporation into cellular lipids and [35S]methionine incorporation into protein were enhanced. 6. Diltiazem inhibited the secretion of the apo B variants with a preferential inhibition of the higher-molecular-mass form of apo B (apo BH) over the lower-molecular-mass form (apo BL) at diltiazem concentrations in the medium greater than 25 microM. 7. Together, these results suggest that Ca2+ may play an important role in the synthesis and secretion of apo B-containing lipoproteins.

Contribution of cytoplasmic storage triacylglycerol to VLDL-triacylglycerol in isolated rat hepatocytes

The cytoplasmic triacylglycerol (TG) storage pool of isolated hepatocytes was labelled in order to evaluate its incorporation into very low density lipoproteins (VLDL). Rats were injected with [1-14C]oleate 2 min prior to surgery and cell incubations began 90-100 min thereafter. In keeping with the equilibration of the two TG pools (in smooth endoplasmic reticulum, SER, and cytoplasm) in 120 min (Stein, Y. and Shapiro, B. (1959) Am. J. Physiol. 196, 1238-1241) the bulk of radioactive TG at time zero was in the cytoplasm and TG specific activities were similar in cytoplasm and SER. Radioactive and total VLDL-TG secretions were greatly inhibited after 80 min by chloroquine which is assumed to block lysosomal hydrolysis of cytoplasmic TG. When the SER-TG pool was labelled by addition of [1-14C]oleate in vitro, chloroquine affected neither [1-14C]oleate uptake and esterification nor its incorporation into VLDL-TG from 15-20 min until 80 min. After 100 min, when [1-14C]oleate-TG was transferred back from cytoplasm to SER, chloroquine began to decrease radioactive VLDL-TG output and by 210 min caused the same inhibition as under the in vivo labelling condition. These results are consistent with an inhibition by chloroquine of the lysosomal hydrolysis of cytoplasmic TG resulting in a blockage of their back transfer to SER membranes whereas other steps of VLDL production were not affected, at least up to 100 min. This study also showed that stored TG is a quantitatively important VLDL precursor, sustaining VLDL production for several hours in the absence of exogenous fatty acids.

Acute and chronic effects of ethanol on secretion of alpha-tocopherol from primary cultures of rat hepatocytes

Primary cultures of rat hepatocytes were used to study the effect of acute and chronic ethanol exposure on alpha-tocopherol content in cells and media. Cells treated acutely with 60 mM ethanol secreted 74.5 +/- 18.0% (P less than 0.05), and their cellular alpha-tocopherol content was 85.7 +/- 15.4% (not significant) of controls after 20 h incubation. At this time total recovery of alpha-tocopherol was significantly reduced in ethanol-exposed cells (43.1 +/- 8.4%) as compared to control cells (52.8 +/- 5.0%, P less than 0.05). Hepatocytes isolated from chronic ethanol-fed rats (35% of total energy intake as ethanol for 5 weeks) secreted 41.9 +/- 12.7% less alpha-tocopherol than did cells of pair-fed controls during 20 h incubation (P less than 0.05). The amount of alpha-tocopherol secreted was then 15.6 +/- 4.2 and 19.8 +/- 3.8% of cell-associated alpha-tocopherol at start of incubation for chronic ethanol-fed and control rats, respectively (P less than 0.05). When 60 mM ethanol was added to the incubation medium, hepatocytes of control rats secreted significantly less alpha-tocopherol (about 30%, P less than 0.05), whereas alpha-tocopherol secretion was not significantly reduced in hepatocytes of chronic ethanol-fed rats. We conclude that both acute and chronic ethanol exposure reduce alpha-tocopherol secretion from rat hepatocytes.

Secretion of alpha-tocopherol from cultured rat hepatocytes

Primary cultures of rat hepatocytes and rat liver perfusions were used to study hepatic secretion of alpha-tocopherol. The secretion of alpha-tocopherol from hepatocytes in culture was linear with time for 4 h. Ultracentrifugation of the medium revealed that 89.4 +/- 2.1% of alpha-tocopherol secreted during 4 h incubation was associated with the very-low density lipoprotein fraction (VLDL, d less than 1.006 g/ml). Oleic acid had no significant effect on the secretory rate of alpha-tocopherol, whereas eicosapentaenoic acid reduced the amount of alpha-tocopherol secreted to 48.4 +/- 12.7% of the control value after 20 h incubation (P less than 0.01). Monensin, a known inhibitor of VLDL secretion, reduced the secretion of alpha-tocopherol to 14.1 +/- 4.3% of the control value (P less than 0.02). Colchicine and chloroquine inhibited the secretion of alpha-tocopherol in the same order of magnitude as monensin. Hepatic perfusion after intravenous injection of in vivo labeled alpha-[3H]tocopherol lymph, showed that about 75% of the secreted radioactivity was in the VLDL fraction. From these results we conclude that most alpha-tocopherol is secreted from the liver associated with nascent VLDL in rats.

Calcium-antagonists inhibit secretion of very-low-density lipoprotein from cultured rat hepatocytes

The effects of different calcium-antagonists on secretion of very-low-density lipoprotein (VLDL) from cultured rat hepatocytes were examined. Verapamil (an inhibitor of voltage-dependent calcium channels) and EGTA (a calcium chelator) decreased VLDL-triacylglycerol secretion in a concentration-dependent manner, with maximum inhibition (about 90%) at 0.2 mM-verapamil and 5 mM-EGTA. Inorganic calcium-antagonists such as lanthanum, nickel, cobalt and manganese decreased secretion of VLDL-triacylglycerol by 55-95%, whereas the calcium-agonist barium did not affect secretion. Inhibition of VLDL-triacylglycerol secretion appeared within 30 min, without inhibition of triacylglycerol synthesis. Pulse-chase experiments revealed that verapamil and cobalt inhibited the secretory pathway itself. Cobalt showed a concentration-dependent inhibition of VLDL-triacylglycerol secretion, with maximal effect at 8 mM. Although inhibition by cobalt was not completely reversible, Trypan Blue exclusion and lactate dehydrogenase leakage indicated that the hepatocytes were not injured by cobalt or any of the other calcium-antagonists tested. Inhibition of protein synthesis by cycloheximide did not affect triacylglycerol secretion (up to 2 h), and the observed effects were therefore probably not due to impaired production of apolipoproteins. Taken together, these results suggest that calcium is important for secretion of VLDL particles.

Inhibition of very-low-density lipoprotein secretion by chloroquine, verapamil and monensin takes place in the Golgi complex

The effects of chloroquine, verapamil and monensin on secretion of very-low-density lipoproteins (VLDLs) were studied in cultured rat hepatocytes. Maximum inhibition of VLDL-triacylglycerol secretion by 50-90% of control was reached at 200 microM chloroquine, 200 microM verapamil and 5 microM monensin, whereas no effect on cellular triacylglycerol synthesis was observed. The inhibition could be seen within 15 min and was reversible after washout of the drugs. Chloroquine and verapamil inhibited both cellular protein synthesis and protein secretion, whereas monensin reduced protein secretion without any effect on protein synthesis. Control experiments with cycloheximide revealed that intact protein synthesis was not necessary for secretion of VLDL-triacylglycerol during 2 h. Electron micrographs of cells treated with chloroquine, verapamil or monensin showed swollen Golgi cisternae containing VLDL-like particles. By morphometry, a more than 2-fold increase in volume fractions and size indices of Golgi complexes and secondary lysosomes was observed, except that monensin had no significant effect on these parameters of secondary lysosomes. These results suggest that the inhibition of VLDL secretion by chloroquine, verapamil and monensin which takes place in the Golgi complex might be due to disruption of trans-membrane proton gradients. An increase in pH of acidic Golgi vesicles may cause swelling and disturb sorting and membrane flow through this organelle.

The effect of carbon tetrachloride on the enzymatic hydrolysis of cellular triacylglycerol in adult rat hepatocytes in primary monolayer culture

The effect of carbon tetrachloride on the intracellular hydrolysis of triacylglycerol and on the activity of acid triacylglycerol lipase was investigated with primary cultured rat hepatocytes. It was found that the concentration of the precursors in the medium did not affect the time course of the synthesis, secretion and the hydrolysis of triacylglycerol, and that carbon tetrachloride significantly suppressed the hydrolysis of intracellular triacylglycerol and the activity of acid triacylglycerol lipase with a concomitant accumulation of triacylglycerol. The results indicate a possibility that the triacylglycerol accumulation in the cultured rat hepatocytes caused by carbon tetrachloride might be mediated by the suppression of lysosomal acid triacylglycerol lipase activity in addition to the suppression of the secretion of triacylglycerol.

Eicosapentaenoic acid inhibits synthesis and secretion of triacylglycerols by cultured rat hepatocytes

Primary cultures of rat hepatocytes were used to study the effects of eicosapentaenoic and oleic acid on synthesis and secretion of triacylglycerols associated with very low density lipoproteins. From the experiments the following was observed. Oleic acid markedly stimulates secretion as well as synthesis of triacylglycerols, whereas eicosapentaenoic acid causes very little or no increase in secretion or synthesis as compared to a fatty-acid-free medium. The effects could already be observed after 15 min incubation. The inhibitory effect of eicosapentaenoic acid is reversible within 1-2 h. Eicosapentaenoic acid inhibits much of the stimulatory effect of oleic acid on synthesis and secretion of triacylglycerols. The cellular uptake of eicosapentaenoic acid is somewhat higher than that of oleic acid and the metabolism of these fatty acids to acid-soluble materials is similar. Eicosapentaenoic acid does not affect the secretory pathway of triacylglycerols per se. From these results it may be concluded that the mechanism for the inhibitory effect of eicosapentaenoic acid on triacylglycerol secretion is probably via reduced triacylglycerol synthesis.

Effects of acidotropic compounds on the secretory pathway: inhibition of secretion and processing of the third and fourth components of complement

Acidotropic compounds (also termed lysosomotropic) such as chloroquine and amantadine interfered with processing of the single-chain precursors to the third and fourth components of complement (C3 and C4) by the human hepatoma-derived cell line HepG2. When these compounds were added to culture medium, the precursors of C3 and C4 became the major secretory forms in contrast to the normal secretion of C3 and C4 as their mature forms. In addition, secretion of C3, C4, and total protein was inhibited by these compounds. Our results indicate that lysosomotropic agents, in addition to their well recognized effects on lysosomes and endosomes, inhibit functions of the secretory pathway.

Release of hepatic lipase and very low density lipoprotein by cultured rat hepatocytes

Primary cultures of rat hepatocytes were used to study the release of hepatic lipase and very low density lipoprotein (VLDL). The presence of hepatic lipase activity was proved by salt-resistance, affinity chromatography and inactivation by a hepatic lipase antibody. Cellular rate of hepatic lipase release increased by prolonged time in culture, whereas VLDL secretion decreased. Oleic acid and dextran-70 had no effect on release of hepatic lipase, whereas VLDL secretion was increased and decreased, respectively. Calcium antagonists (cobalt and verapamil), monensin and cycloheximide inhibited both the release of hepatic lipase and VLDL. Colchicine and chloroquine, which decreased VLDL secretion, had no effect on release of hepatic lipase. The present results suggest that release of hepatic lipase and secretion of VLDL are not coordinated and exhibit different sensitivity towards certain compounds altering secretory functions.

Retinol esterification in cultured rat liver cells

Retinol esterification was examined in cultured hepatocytes and stellate cells from the rat. Esterification of [3H]retinol was linear for 2 h in both cell types. By increasing the concentration of retinol in the medium, there was a marked increase in retinol esterification in both cell types. The capacity for esterification of retinol was in the same order of magnitude in the two cell types at 3.5 microM-retinol in the medium. This represents a rate of retinol esterification which far exceeds that required to esterify the amount of retinol absorbed in the intestine. It was demonstrated in particulate homogenates from cultured hepatocytes that the esterification of retinol was dependent on acyl-CoA. Addition of 25-hydroxycholesterol or mevalonolactone promoted an increase in cholesterol esterification, whereas retinol esterification was unaffected, suggesting that cholesterol and retinol are esterified by two different enzymes. Some 80% of vitamin A in cultured hepatocytes is retinyl esters, mostly retinyl palmitate. By adding 87 microM-retinol in the medium the cells accumulated 100-fold free retinol and 2.5-3.0-fold retinyl esters within 1 h. When retinol-loaded cells were incubated without retinol, there was a marked decrease especially in free but also in esterified retinol. In the presence of 1 mM-oleic acid in the medium the amount of retinyl oleate was twice that in control cells.

The effects of monensin on secretion of very-low-density lipoprotein and metabolism of asialofetuin by cultured rat hepatocytes

Primary cultures of rat hepatocytes were used to study secretion of very-low-density lipoproteins and metabolism of asialofetuin. The ionophore monensin inhibited both secretion of very-low-density lipoproteins and binding and degradation of asialofetuin in a concentration-dependent manner. Secretion as well as receptor binding were markedly decreased after 15 min treatment with monensin. The inhibitory effect of the ionophore was fully reversible, and no effect on protein synthesis was observed at concentrations up to 50 microM. The secretion of apoproteins (B-small, B-large and E) and that of albumin were inhibited to the same extent as was triacylglycerol secretion. Secretion of very-low-density lipoproteins was more sensitive to low concentrations of monensin than was the metabolism of asialofetuin. Maximum inhibition of very-low-density-lipoprotein secretion was obtained at 5-10 microM-monensin, whereas 25 microM was required to obtain maximum inhibition of binding and degradation of asialofetuin. The number of surface receptors for asialofetuin decreased to about half when the cells were exposed to 25 microM-monensin. It is possible that monensin inhibits endo- and exo-cytosis via a similar mechanism, e.g. by disturbing proton gradients. Since secretion of very-low-density lipoproteins was more sensitive to low concentrations of monensin, it is likely that monensin independently inhibits endocytic and secretory functions in cultured hepatocytes.