Purification and characterization of novel plasma membrane phosphatidate phosphohydrolase from rat liver

An N-ethylmaleimide-insensitive phosphatidate phosphohydrolase, which also hydrolyzes lysophosphatidate, was isolated from the plasma membranes of rat liver. The specific activity of an anionic form of the enzyme (53 kDa, pI < 4) was increased 2700-fold. A cationic form of enzyme (51 kDa, pI = 9) was purified to homogeneity, but the -fold purification was low because the activity of the highly purified enzyme was unstable. Immunoprecipitating antibodies raised against the homogeneous protein confirmed the identity of the cationic protein as the phosphohydrolase and were used to identify the anionic enzyme. Both forms are integral membrane glycoproteins that were converted to 28-kDa proteins upon treatment with N-glycanase F. Treatment of the anionic form with neuraminidase allowed it to be purified in the same manner as the cationic enzyme and yielded an immunoreactive protein with a molecular mass identical to the cationic protein. Thus, the two ionic forms most likely represent different sialated states of protein. An immunoreactive 51-53-kDa protein was detected in rat liver, heart, kidney, skeletal muscle, testis, and brain. Little immunoreactive 51-53-kDa protein was detected in rat thymus, spleen, adipose, or lung tissue. This work provides the tools for determining the regulation and function of the phosphatidate phosphohydrolase in signal transduction and cell activation.

Role of glucocorticoids and fatty acids in the impairment of lipid metabolism observed in the metabolic syndrome

The metabolic syndrome is discussed in terms of insulin resistance linked to an increased regulation of metabolism by cortisol and fatty acids. This change in hormonal balance is associated with diabetes, android (visceral) obesity, hypertension, hypertriglyceridemia, hyperapobetalipoproteinemia and low concentrations of HDL; a cluster of risk-factors that predisposes to the development of premature atherosclerosis. It is proposed that the metabolic syndrome is accompanied by a derangement in the hypothalamic-pituitary-adrenal-axis such that the effects of cortisol are exaggerated relative to those of CRF. Excessive action of fatty acids and cortisol causes insulin resistance and increase the hepatic secretion of glucose and VLDL. Furthermore, cortisol can decrease the uptake of LDL by the liver. Cortisol in the presence of relatively high insulin concentrations can promote the deposition of energy and lead to obesity. Chronic treatment of rats with D-fenfluramine has been shown to decrease the release of cortisol and fatty acids in response to stress, and to improve insulin sensitivity. The effects of D-fenfluramine were also tested in male JCR:LA corpulent rats which are prone to develop atherosclerosis and myocardial lesions. D-fenfluramine improved insulin sensitivity, decreased the hypertriglyceridemia, and prevented the development of necrotic myocardial lesions caused by ischemia. The data presented demonstrates a link between excessive action of cortisol and fatty acids in predisposing to insulin resistance and the pathologies that are associated with the metabolic syndrome.

Short-chain ceramide-1-phosphates are novel stimulators of DNA synthesis and cell division: antagonism by cell-permeable ceramides

Ceramide and ceramide-1-phosphate are sphingolipid analogues of diacylglycerol and phosphatidate, respectively, and they are putative second messengers of agonist-stimulated sphingomyelin metabolism. The interactions of exogenous cell-permeable ceramides and ceramide-1-phosphates in modifying DNA synthesis and signal transduction were investigated in Rat-1 fibroblasts. C2- and C8-Ceramide-1-phosphates (N-acetylsphingosine-1-phosphate and N-octanoylsphingosine-1-phosphate, respectively) at 1-10 microM stimulated DNA synthesis and cell division. This effect was blocked by cell-permeable ceramides. C2-Ceramide stimulated the conversion of exogenous C8-ceramide-1-phosphate to C8-ceramide, with very little production of sphingosine or sphingosine-1-phosphate. This mechanism may be partly responsible for preventing the stimulation of DNA synthesis. Unlike phosphatidate or lyso-phosphatidate, concentrations of C8-ceramide-1-phosphate that stimulated DNA synthesis did not inhibit adenylate cyclase activity, nor did they increase the activities of phospholipase D or mitogen-activated protein kinases (42- and 44 kDa isoforms). Although ceramide-1-phosphate can be considered as an analogue of phosphatidate, the effects of this compound on signal transduction differ considerably from those of phosphatidate. This work demonstrates that short-chain ceramide-1-phosphates can be used as novel external agonists that can stimulate DNA synthesis. This effect can be counteracted by short-chain ceramides.

Metabolic abnormalities linked to obesity: effects of dexfenfluramine in the corpulent rat

The JCR:LA-corpulent rat is a useful experimental model for the obese-diabetic-dyslipidemic syndrome that mimics the human condition and exhibits spontaneous development of atherosclerosis and myocardial lesions. A 30-day treatment of 6-month-old rats with dexfenfluramine 1, 2.5, and 5 mg per kilogram decreased body weight through loss of adipose tissue mass. The effect is caused primarily by the ability of dexfenfluramine to reduce food intake. The maximum depression of food intake and greatest weight loss is seen during the first 10 days of treatment in this experimental model; thereafter, body weight stabilizes. However, during this period, there is a marked decrease in serum concentrations of triglycerides, cholesterol, and insulin. Corpulent male rats were also treated from 6 to 37 weeks of age with dexfenfluramine 2.5 mg/kg. This also produces a sustained decrease in body weight and a decrease in circulating insulin concentrations. Preliminary evidence demonstrates a substantial decrease in the incidence of necrotic myocardial lesions produced by ischemic events. This study establishes that dexfenfluramine treatment can decrease the severity of associated risk factors for cardiovascular disease, namely obesity, diabetes, and dyslipidemias. Furthermore, we report the first evidence that long-term treatment with dexfenfluramine can largely prevent the occurrence of myocardial lesions and end-stage cardiovascular disease in this animal model prone to atherosclerosis.

Activity and subcellular distribution of phosphatidate phosphohydrolase (EC 3.1.3.4) in alcoholic liver disease

A micromethod was developed to assay the hepatic activity and subcellular distribution of phosphatidate phosphohydrolase (PAH, EC 3.1.3.4), an important regulatory enzyme in triacylglycerol synthesis, in human needle biopsy specimens. In normal liver PAH is predominantly cytosolic in distribution, but on treatment with oleic acid it shifts to the membranous compartments of the cell, its physiologically active site. The hepatic specific activity of PAH was similar in controls and patients with fatty liver but significantly more of the enzyme was associated with the membranous compartments in patients with severe alcoholic fatty liver. These observations may explain the enhanced rates of triacylglycerol synthesis observed in these patients and may be implicated in the pathogenesis of alcoholic fatty liver.

Effect of alcohol on the activity and subcellular distribution of phosphatidate phosphohydrolase in rat liver

The activity and subcellular distribution, before and after translocation with oleic acid, of hepatic phosphatidate phosphohydrolase (PAH: E.C.3.1.3.4) was measured following acute and chronic administration of ethanol with either 18% or 4.4% total calories as fat, to male Wistar rats. Acute injection produced a significant increase in PAH activity, without affecting the subcellular distribution or translocation of activity from the cytosol into the membraneous compartments induced by oleic acid. PAH activity, subcellular distribution and the translocation of activity by oleic acid was similar in all groups following chronic liquid feeding. Chronic ethanol feeding was associated with significantly increased plasma corticosterone in the ethanol-fed rats compared with controls, independent of the fat content of the diet. Plasma insulin was similar in chronic alcohol fed rats and controls. This suggests that enhanced PAH activity may contribute to triacylglycerol accumulation following acute alcohol injection, but activity normalises as hepatic lipid accumulation progresses and this effect is not due to attenuation of the hormonal effects of ethanol.

Cell-permeable ceramides inhibit the stimulation of DNA synthesis and phospholipase D activity by phosphatidate and lysophosphatidate in rat fibroblasts

The interactions of ceramides with phosphatidate and lysophosphatidate in the regulation of signal transduction in rat fibroblasts were examined. C2- and C6-ceramides (N-acetylsphingosine and N-hexanoylsphingosine, respectively) at 10 microM inhibited the stimulation of DNA synthesis that was produced by 50-100 microM phosphatidate, or lysophosphatidate, or by exogenous phospholipase D. Sphingosine (10 microM) had the opposite effect to the ceramides on DNA synthesis. C2- or C6-ceramides failed to inhibit the stimulation of DNA synthesis by insulin or serum. The ceramides did not modify the actions of phosphatidate, or lysophosphatidate, in decreasing the forskolin-induced increase in cAMP. C2- and C6-ceramides inhibited the stimulation of phospholipase D activity by: (a) phosphatidate, lysophosphatidate, phorbol ester, thrombin, or serum in intact fibroblasts and (b) phorbol ester or guanosine 5'-3-O-(thio)triphosphate in permeabilized fibroblasts. The ceramides can therefore modify cell signaling via phospholipase D, but this effect alone could not explain the decreased DNA synthesis. Incubation of fibroblasts with C2- or C6-ceramides or sphingomyelinase inhibited the interaction of exogenous phosphatidate or lysophosphatidate with the fibroblasts by 42 and 53%, respectively. Furthermore, a greater proportion of the phosphatidate, or lysophosphatidate, that was associated with the fibroblasts was metabolized further when the cells were pretreated with ceramides or sphingomyelinase. This effect was accompanied by an increased activity of N-ethylmaleimide-insensitive phosphatidate phosphohydrolase. Ceramides may therefore produce part of their growth inhibitory effects by blocking some of the signal transducing effects of phosphatidate and lysophosphatidate.

Decreased activities of phosphatidate phosphohydrolase and phospholipase D in ras and tyrosine kinase (fps) transformed fibroblasts

The activity of N-ethylmaleimide-insensitive phosphatidate phosphohydrolase (PAP-2) was characterized in control, ras-transformed, and tyrosine kinase-(fps) transformed rat fibroblasts. PAP-2 was assayed in two different ways: 1) within its natural membrane using liposomes of phosphatidate and 2) in the presence of sufficient Triton X-100 to solubilize PAP-2, and to form mixed micelles with the phosphatidate. Harvesting the fibroblasts in medium containing orthovanadate and Zn2+ gave up to 3-fold higher PAP-2 activities when measured in the absence, but not in the presence, of Triton X-100. PAP-2-specific activities from both assays increased in the control fibroblasts as the cells reached confluence. Both specific activities were lower in the oncogenically transformed fibroblasts than in controls at all cell densities tested. The specific activities of PAP-2 did not increase with time in culture in transformed cells which continued to divide. The relative increase in activity of phospholipase D after stimulation with serum or phorbol myristate acetate was lower in the transformed fibroblasts compared to control cells. This indicates a coordinated decrease in the phospholipase D/phosphatidate phosphohydrolase pathway at the level of both enzymes in ras and fps transformed fibroblasts. The ratio of the production of diacylglycerol relative to phosphatidate, after stimulation with serum, or phorbol ester, was lower in both transformed fibroblasts relative to the controls. This is compatible with the decreased specific activity of PAP-2 and indicates functional significance for the differences in PAP-2 activity in regulating the balance between the two mitogenic lipids, phosphatidate and diacylglycerol. Control of PAP-2 activity could be an important factor in regulating appropriate signals for cell division.

Cholesterol feeding induces hypertriglyceridaemia in hamsters and increases the activity of the Mg(2+)-dependent phosphatidate phosphohydrolase in the liver

(1) Feeding increased cholesterol to hamsters resulted in a dose-dependent increase in cholesterol and triacylglycerol in very-low-density lipoprotein (VLDL), in serum non-esterified fatty acids and in the activity of the Mg(2+)-dependent phosphatidate phosphohydrolase in liver. (2) The effects of increasing dietary cholesterol by 0.12% (w/w) in addition to feeding fat (14%, w/w) were dependent upon the nature of the fat. Lard in the presence of 0.12% (w/w) cholesterol increased serum triacylglycerols as did olive oil. By contrast, sunflower oil did not cause a significant change in serum triacylglycerol concentrations. (3) There was a highly positive correlation between VLDL triacylglycerol and VLDL cholesterol concentrations suggesting that, at least in this model, there is a close relationship between hypertriglyceridaemia and hypercholesterolaemia.

Neuroendocrine regulation and obesity

The combined syndrome of android (upper body) obesity, diabetes, hyperlipidaemia and hypertension is discussed in terms of a deranged endocrine regulation of metabolism. The syndrome is characterized by insulin insensitivity and an increased control of metabolism by cortisol. The antagonism between the two hormones appears to be partly responsible for the hyperglycaemia, hypertriglyceridaemia and hypercholesterolaemia. The synergism between insulin and cortisol in stimulating energy deposition, associated with a decreased effect of corticotropin-releasing factor in stimulating energy expenditure, is likely to contribute to the development of obesity. The efficacy of D-fenfluramine in treating the obese-diabetic-hyperlipidaemic-hypertensive syndrome probably depends on its actions on the serotoninergic system in the hypothalamus which both decreases food consumption and tends to normalize hormonal balance through the hypothalamic-pituitary-adrenal axis.

Phosphatidate phosphohydrolases in liver, heart and adipose tissue of the JCR:LA corpulent rat and the lean genotypes: implications for glycerolipid synthesis and signal transduction

The activities of two distinct phosphatidate phosphohydrolases (PAP) were measured in livers, hearts and adipose tissues of the JCR:LA corpulent rat which is hyperphagic, hypertriglyceridaemic and insulin resistant. The specific activity of PAP-1, which requires Mg2+, was similar in the livers of lean and corpulent female rats and in male corpulent rats, but these activities were about 1.6-fold higher than in lean males. There was a correlation between the specific activity of PAP-1 and the concentrations of hepatic and serum triacylglycerols in the males, but not in the females. Chronic treatment of the corpulent rats with ethanol did not significantly alter the hepatic activity of PAP-1, or the concentrations of hepatic or serum triacylglycerols. Specific activities of PAP-1 in the heart were higher in the lean compared to the corpulent males. There was no significant difference for the females. Specific activities of PAP-1 were over 5-fold higher in the subcutaneous adipose tissue of the corpulent males and females compared to the lean genotypes. The differences were smaller (1.6-1.9-fold) in the gonadal adipose tissue of both sexes and in the peri-renal depot for the males. PAP-1 activity in the peri-renal depots of corpulent females was 23% lower than in lean females. PAP-2 activity was insensitive to N-ethylmaleimide and did not require Mg2+ for activity. Its activity was 1.5-2.0-fold higher in the livers and hearts of the lean male and female rats than in the corpulent genotypes. Chronic treatment with ethanol increased the activity of PAP-2 in the hearts of the corpulent males, but had no effect in the corpulent females. The specific activity of PAP-2 was higher in subcutaneous, gonadal and peri-renal adipose depots in the females and in the peri-renal depot of the corpulent males compared with the lean genotypes. Lean males had higher specific activities in all three depots compared to lean females. The tissue specificity and the sex differences in the specific activities of PAP-1 and PAP-2 are discussed in terms of their proposed functions in glycerolipid biosynthesis and signal transduction. It is proposed that a decreased activity of PAP-2 could be involved in the insulin insensitivity in the corpulent rats.

[Mode of action of benfluorex. Recent data]

An increased risk of developing premature atherosclerosis is associated with stress, diabetes, obesity, and hypertension. These conditions are associated with insulin resistance, hyperglycemia, hypertriglyceridemia and hypercholesterolemia. An alternative way of interpreting insulin resistance is to consider that metabolism in this condition would be regulated to a greater extent by stress hormones and in particular by cortisol. Glucocorticoids and fatty acids (which are produced in response to stress) antagonise the actions of insulin in promoting glucose uptake and protein synthesis, in decreasing gluconeogenesis and protein catabolism, and promoting the clearance of intermediate density lipoprotein and low density lipoprotein from the circulation by the liver. They also promote the secretion of very low density lipoprotein thus producing hypertriglyceridemia and hypercholesterolemia. By contrast to this antagonism, cortisol can also facilitate the action of insulin in stimulating the storage of energy via glycogen and fatty acid synthesis and through lipoprotein lipase in adipose tissue. These effects are significant in relation to obesity and to weight gain. An increased control of metabolism by cortisol therefore produces changes in metabolism that are potentially atherogenic and it is associated with insulin resistance and the other risk factors for atherosclerosis. Benfluorex treatment improves insulin sensitivity and has antihyperglycemic and hypolipidemic effects in human beings and in experimental animals. These effects can be observed independently of weight loss, but lowering food intake also produces a metabolic benefit. Long-term treatment with benfluorex can also decrease stress responses in terms of glucocorticoid release and the stimulation of lipolysis probably by its serotoninergic control of the hypothalamic-pituitary-adrenal axis. Such an action provides for an integrated treatment of the obese-diabetic-hyperlipidemic syndrome. Benfluorex produces overall changes in metabolism that tend to normalise the major risk factors associated with premature atherosclerosis. This provides a potential advantage over other therapies for atherosclerosis which may ameliorate a symptom (e.g., hyperlipidemia) without treating the underlying metabolic disturbance that predisposes to atherogenesis.

Effects of sphingosine, albumin and unsaturated fatty acids on the activation and translocation of phosphatidate phosphohydrolases in rat hepatocytes

The activities of two phosphatidate phosphohydrolases were measured in cultured rat hepatocytes incubated with 0.1 mM albumin. The activity, which is inhibited by N-ethylmaleimide (PAP-1) is located in the cytosolic and membrane fractions. PAP-1 activity is stimulated by Mg2+ and it can be translocated from the cytosol to the membranes by relatively low (0.5-1 mM) concentrations of fatty acids. In addition, higher concentrations (1-3 mM) of fatty acids cause an increase in the total PAP-1 activity. Translocation of PAP-1 activity in the hepatocytes is preferentially promoted by unsaturated fatty acids (C18:1, C18:2, C18:3, C20:4 and C20:5), rather than by saturated acids (C14:0, C16:0, C18:0). Increasing the extracellular concentration of albumin from 30 microM to 1 mM displaces PAP-1 activity from the membrane fraction. Sphingosine, but not staurosporine, can inhibit the redistribution of PAP-1 activity induced by oleate. The amphiphilic amines, sphingosine, chlorpromazine and propranolol, also decrease membrane-bound PAP-1 activity in the absence of fatty acids, but they do not alter, significantly, the activity of the cytosolic PAP-1. In the presence of 1 mM oleate, sphingosine, chlorpromazine and propranolol decrease the translocation of PAP-1 from the cytosol to the membranes. The phosphohydrolase activity, which is insensitive to N-ethylmaleimide (PAP-2), is specifically located in the plasma membrane (Jamal, Z., Martin, A., Gomez-Muñoz, A. and Brindley, D.N. (1991) J. Biol. Chem. 266, 2988-2996) and it is not stimulated by Mg2+. Saturated fatty acids, albumin, sphingosine and propranolol have no significant effects on PAP-2 activity. However, chlorpromazine decreases PAP-2 activity by about 14%. Linolenate, arachidonate and eicosapentaenoate at 1 mM also produced small (7-10%) decreases in PAP-2 activity. It is proposed that both PAP-1 and PAP-2 activities may be involved in signal transduction, although the main function of PAP-1 seems to be involved in the synthesis of glycerolipids.

Effects of okadaic acid on the activities of two distinct phosphatidate phosphohydrolases in rat hepatocytes

Incubation of hepatocytes with okadaic acid displaced the N-ethylmaleimide-sensitive phosphatidate phosphohydrolase from the membrane fraction into the cytosol and partially prevented the oleate-induced movement of phosphohydrolase from cytosol to membranes. However, higher concentrations of oleate still caused translocation and activation of the phosphohydrolase. This enzyme is stimulated by Mg2+, and is probably involved in glycerolipid synthesis. Okadaic acid also decreased the concentration of diacylglycerol within the hepatocytes. Okadiac acid had no observable effect on the activity of an N-ethylmaleimide-insensitive phosphatidate phosphohydrolase which remained firmly attached to membranes. This activity is not stimulated by Mg2+ and is probably involved in signal transduction by the phospholipase D pathway.

Sustained decreases in weight and serum insulin, glucose, triacylglycerol and cholesterol in JCR:LA-corpulent rats treated with D-fenfluramine

1. The effects of D-fenfluramine were studied in the JCR:LA-corpulent rat that is grossly obese, hyperphagic, hyperlipidaemic, hyperinsulinaemic and atherosclerosis-prone. 2. Daily doses of 1, 2.5 and 5 mg kg-1 of D-fenfluramine produced sustained decreases in body weight and food intake over a period of 30 days in 6 month old female rats fed ad libitum. This was accompanied by decreases in the circulating concentrations of glucose, triacylglycerol, free cholesterol and insulin. 3. Food restriction imposed by meal feeding also decreased circulating glucose, triacylglycerols, cholesterol and insulin and diminished the effect of D-fenfluramine on these parameters in male and female rats. 4. Addition of D-fenfluramine to drinking water to give a dose of about 0.25 mg kg-1 daily produced a sustained decrease in body weight and food intake of male and female rats over a nine week period. 5. The results show that the JCR:LA-corpulent rat is very sensitive to the pharmacological effects of D-fenfluramine. These rats should provide an appropriate animal model for determining the mechanisms of action of this anti-obesity agent and whether apparently beneficial changes in metabolism translate into long-term protection against premature atherosclerosis.

Biphasic effects of glucagon and cyclic AMP on the synthesis and secretion of lipids by rat hepatocytes

Cultured rat hepatocytes were preincubated with glucagon or a cyclic AMP analogue for up to 24 h and lipid synthesis and secretion were determined during the next 2 h. Glucagon or cyclic AMP did not change the incorporation of choline or glycerol into phosphatidylcholine, or choline into sphingomyelin, in the cells after 0-12 h of preincubation. After 12 h these incorporations were increased. Incorporations into hepatic lysophosphatidylcholine were decreased after preincubation with glucagon or cyclic AMP for 0-12 h, but by 24 h they increased. There was no change in the lysophosphatidylcholine in the medium after preincubation with glucagon or cyclic AMP for up to 6 h, but increases occurred after preincubation from 12 to 24 h. The secretion of triacylglycerol was decreased after preincubation for 0-1 h, but it returned to control values after 4 h. After preincubation for 18-24 h the incorporation of glycerol into secreted triacylglycerol was increased. The results are discussed in relation to the control of lipid metabolism in starvation and diabetes.

Decreased serum lipids, serum insulin and triacylglycerol synthesis in adipose tissue of JCR:LA-corpulent rats treated with benfluorex

Rats of the JCR:LA-corpulent strain were treated with benfluorex daily at a dose of 25 mg/kg body weight. This strain of rat, if homozygous for the cp gene (cp/cp), is hyperphagous, obese, hypertriglyceridemic, insulin resistant and in the case of male rats, atherosclerosis prone. The benfluorex treatment produced a sharp reduction in food intake which remained suppressed despite recovery toward normal after 2 weeks of treatment. This was accompanied by sustained decreases in body weight and adipose tissue mass. The ability of adipose tissue from female rats to take up glucose and convert it to lactate, glyceride-glycerol and fatty acids was decreased. This decrease was largely due to decreased adipose tissue mass. The serum concentrations of glucose, lactate, triacylglycerol, cholesterol, phospholipids and insulin were decreased in both sexes. The treatment also improved glucose tolerance and decreased corticosterone concentrations in male rats only. While reduction of food consumption contributes to the effects seen, benfluorex clearly had significant direct metabolic effects. The effects are consistent with an improved insulin sensitivity leading to a decrease in circulating triacylglycerol. The changes produced by benfluorex are all in directions that should inhibit atherogenesis in this animal model for the human obesity/hypertriglyceridemia/insulin resistant syndrome.

Effects of the lipase inhibitors, Triton WR-1339 and tetrahydrolipstatin, on the synthesis and secretion of lipids by rat hepatocytes

The lipase inhibitors, Triton WR-1339 and tetrahydrolipstatin, were incubated with rat hepatocytes. Triton WR-1339 increased the recovery of triacylglycerol in the hepatocytes and incubation medium by 31% and 38%, respectively. Tetrahydrolipstatin decreased the accumulation of newly synthesized, and of total triacylglycerol in the medium. This compound might be useful in determining mechanisms involved in intracellular triacylglycerol metabolism and the secretion of very low density lipoproteins.

Effects of hypothyroidism and high-fat feeding on mRNA concentrations for the low-density-lipoprotein receptor and on acyl-CoA:cholesterol acyltransferase activities in rat liver

1. Induction of hypothyroidism in rats by feeding propylthiouracil (PTU) significantly increased serum cholesterol concentrations, and the effect was more pronounced for cholesterol in low-density lipoproteins (LDL) rather than high-density lipoproteins (HDL). The concentrations of serum triacylglycerol were decreased in hypothyroidism. These effects on serum lipids were also seen when the normal rats were pair-fed with the PTU-treated group. 2. Feeding a diet rich in saturated fat and cholesterol further increased cholesterol concentrations in LDL and also elevated that in very-low-density lipoprotein (VLDL) of hypothyroid rats. In euthyroid rats such a diet resulted in a relatively small increase in VLDL cholesterol, whereas LDL cholesterol was decreased. 3. Steady-state concentrations of mRNA for the hepatic LDL receptor were significantly decreased in the livers of hypothyroid rats, but were not significantly changed by high-fat feeding in euthyroid or hypothyroid rats. 4. The expression of the LDL receptor in hepatocytes cultured from hypothyroid rats was decreased relative to the euthyroid controls. 5. Whereas the esterification of cholesterol with oleate in hepatocytes cultured from hypothyroid rats was decreased, the activity of acyl-CoA:cholesterol acyltransferase (ACAT) in the livers of these animals was not changed. 6. High-fat feeding increased the hepatic ACAT activity in normal and hypothyroid rats. 7. Incubation of rat hepatocytes with 10 nM-tri-iodothyronine for 4 h increased the relative concentration of the mRNA for the LDL receptor by 25%. 8. It is therefore concluded that thyroid hormones stimulate the synthesis and expression of the hepatic LDL receptor. Elevated cholesterol concentrations in LDL in hypothyroidism probably result from a primary defect in the expression of the hepatic receptor, rather than indirectly via changes in ACAT activity.

Unsaturated fatty acids activate glycogen phosphorylase in cultured rat hepatocytes

Oleate, linoleate, linolenate, arachidonate and eicosapentaenoate, but not myristate, palmitate and stearate, stimulated glycogen phosphorylase activity by 2-8-fold when added to cultured rat hepatocytes. Addition of BSA or Ca2- to the incubation medium decreased the stimulating effects of the unsaturated fatty acids. The combination of oleate or linolenate, with corticosterone, testosterone or estradiol produced synergistic stimulations of phosphorylase activity. The stimulation of glycogen phosphorylase activity by linolenate was inhibited by staurosporine or sphingosine. Staurosporine (80 nM) alone also decreased basal phosphorylase activities by about 60%. The results show that unsaturated fatty acids can be used as model agonists to stimulate phosphorylase activity by a mechanism that probably involves protein kinase C. On the basis of the fatty acid: BSA ratios used, this stimulation should only occur in vivo at high fatty acid concentrations when accompanied by hypoalbuminaemia.

Plasma membrane fractions from rat liver contain a phosphatidate phosphohydrolase distinct from that in the endoplasmic reticulum and cytosol

Assays for two distinct phosphatidate phosphohydrolase activities were established based upon a differential inhibition by N-ethylmaleimide (NEM). The activity that is insensitive to this reagent in rat liver is predominantly in the plasma membrane fraction, whereas the NEM-sensitive activity is in the cytosolic and microsomal fractions. The NEM-insensitive activity is further distinguished from the NEM-sensitive phosphohydrolase by: (a) being relatively stable to heat; (b) not being inhibited by phenylglyoxal, butane-2,3-dione, cyclohexane-1,2-dione, 2,4-dinitrofluorobenzene, 7-chloro-4-nitrobenz-2-oxa-1,3-diazole, and diethyl pyrocarbonate; (c) being inhibited by NaF and phosphatidylcholine; and (d) not being stimulated by Mg2+. The NEM-insensitive activity was specific for phosphatidate. Both phosphohydrolase activities could be inhibited by chlorpromazine, propranolol, sphingosine, and spermine. The NEM-sensitive phosphatidate phosphohydrolase activity was increased by incubating hepatocytes for 12 h with glucagon and dexamethasone, and this effect was antagonized by insulin. The NEM-sensitive phosphohydrolase is concluded to be involved in glycerolipid synthesis. The activity of the NEM-insensitive phosphohydrolase was not altered by preincubation of rat hepatocytes in the short or long term with vasopressin, glucagon, insulin, triiodothyronine, or dexamethasone, but it might be modulated indirectly by sphingosine. The NEM-insensitive enzyme of the plasma membranes could be involved in signal transduction via the agonist-stimulated degradation of phosphatidylcholine through the phospholipase D pathway.

Sexual dimorphism in the preferential secretion of unsaturated lysophosphatidylcholine by rat hepatocytes but no secretion by sheep hepatocytes

(1) Rat and ovine hepatocytes were incubated in monolayer culture with various fatty acids to determine their effects on the composition of the lysophosphatidylcholine that was secreted. (2) No lysophosphatidylcholine was detected in the medium from the ovine hepatocytes even though these cells were hormonally responsive and they secreted phosphatidylcholine and triacylglycerol in very-low-density lipoprotein. (3) Lysophosphatidylcholine was readily detected in the incubation medium of rat hepatocytes. The predominant fatty acids in this lipid were unsaturated. Stearate and arachidonate contributed 15 and 34%, and 24 and 26% of the total fatty acids when hepatocytes from male and female rats were used, respectively. The relative proportions of stearate and arachidonate in the phosphatidylcholine secreted from the hepatocytes were 20 and 14%, and 28 and 21% for the males and females, respectively. The equivalent values for stearate and arachidonate for phosphatidylcholine in the hepatocytes were 18 and 17% and 33 and 22% for male and female rats. These results provide further indications of sex differences in hepatic phospholipid metabolism and extend this to the secretion of phosphatidylcholine and lysophosphatidylcholine. (4) The addition of 1 mM stearate to the incubation medium did not significantly decrease the proportion of arachidonate in the lysophosphatidylcholine obtained from the hepatocytes of the male rats. However, the relative proportion of arachidonate was decreased in incubations that contained 1 mM oleate or linoleate. (5) The results provide evidence that the preferential secretion of unsaturated lysophosphatidylcholine by the liver may provide a system for transporting unsaturated fatty acids and choline to other organs in non-ruminant animals. However, this mechanism may not operate for ruminants.

Stimulation of apolipoprotein secretion in very-low-density and high-density lipoproteins from cultured rat hepatocytes by dexamethasone

The effects of dexamethasone (a synthetic glucocorticoid) and insulin on the secretion of very-low-density lipoprotein (VLDL) and high-density lipoprotein (HDL) were investigated. Rat hepatocytes in monolayer culture were preincubated for 15 h in the presence or absence of combinations of 100 nM-dexamethasone and 2 nM-, 10 nM- or 50 nM-insulin. Dexamethasone increased [3H]oleate incorporation into secreted triacylglycerol by 2.7-fold and the mass of triacylglycerol secreted by 1.5-fold. Insulin alone decreased these parameters and antagonized the effect of dexamethasone. Dexamethasone increased the secretion of [3H]leucine in apolipoprotein (apo) E, and in the large (BH) and small (BI) forms of apo B in VLDL by about 7.1-, 3.6- and 4.0-fold respectively. Insulin alone decreased the secretion of these 3H-labelled apolipoproteins in VLDL. However, 2 nM-insulin with dexamethasone increased the secretion of 3H-labelled apo BH and apo BL by a further 0.8- and 3.2-fold respectively; 50 nM-insulin decreased the secretions of apo E, apo BH and apo BL in VLDL. Similar effects for dexamethasone or insulin alone were also obtained for the masses of apo E and apo BL + H secreted in VLDL. Albumin secretion was not significantly altered by either dexamethasone or insulin alone, but in combination they stimulated by 2.1-2.6-fold. Insulin or dexamethasone alone had little effect on the secretion of apolipoproteins in the HDL fraction. However, dexamethasone plus 2 nM-insulin increased the incorporation of [3H]leucine into apo AI, apo AH plus apo C, apo AIV and apo E of HDL by about 1.8-, 1.6-, 1.7- and 2.0-fold respectively. The apo E in the bottom fraction represented about 69% of the total 3H-labelled apo E secreted. The responses in the total secretion of apo E from the hepatocytes resembled those seen in HDL. The interactions of insulin and dexamethasone are discussed in relation to the general regulation of lipoprotein metabolism, the development of hyperlipidaemias and the predisposition to premature atherosclerosis.

Possible connections between stress, diabetes, obesity, hypertension and altered lipoprotein metabolism that may result in atherosclerosis

This review has discussed some metabolic and endocrine changes that can be associated with a stress type of metabolism, diabetes, obesity, hypertension, smoking and the consumption of diets rich in fat and refined sugar, or poor in ascorbate. These are some of the risk factors associated with premature atherosclerosis, coronary thrombosis and stroke. It has been proposed that an increased control of metabolism by the 'stress' or counter-regulatory hormones, relative to insulin, is a common feature of these risk factors. Particular emphasis was placed upon the action of the glucocorticoids which can produce insulin insensitivity, leading to hyperglycaemia, hypertriglyceridaemia, hypercholesterolaemia and hyperinsulinaemia. Furthermore, glucocorticoids can decrease energy expenditure and, together with insulin, promote energy deposition. These observations provide a partial explanation for the metabolic changes that can accompany the risk factors and clarify why they interact in promoting atherosclerosis.

Decreased incorporation of glucose into lipids and increased lactate production by adipose tissue after long-term treatment of rats with D-fenfluramine

Male rats were treated with ten daily doses of 10 mg of D-fenfluramine/kg. Body weight decreased after days 1 and 2, but thereafter the weight gain paralleled that of the control rats. After the tenth injection there were decreases in the weights of the epididymal fat pads, their fat content, and the average size of the adipocytes after collaginase digestion. The rate of glucose uptake by incubated pieces of adipose tissue was maintained after D-fenfluramine treatment, and the production of lactate increased. The incorporation of glucose into fatty acids by adipose tissue pieces decreased by 65-74% after treatment with D-fenfluramine. This effect was not reversed by adding insulin or phenylisopropyladenosine to the incubations. D-Fenfluramine also decreased the incorporation of glucose into glyceride-glycerol, but this effect was less pronounced than that for fatty acid synthesis. Direct addition of D-fenfluramine to the incubation inhibited lipid synthesis from [14C]glucose but only at drug concentrations above 1 mM. It is concluded that the treatment of rats with D-fenfluramine modifies the metabolic balance of adipose tissue so as to direct glucose metabolism away from lipid synthesis and towards lactate production. This could be a significant mechanism in the overall loss of adipose tissue mass caused by the administration of D-fenfluramine.

Cholesterol esterification plays a major role in determining low-density-lipoprotein receptor activity in primary monolayer cultures of rat hepatocytes

1. We have previously shown that the capacity for specific binding of human 125I-labelled low-density lipoprotein (LDL) to rat hepatocytes increases with time in culture [Salter, Bugaut, Saxton, Fisher & Brindley (1987) Biochem. J. 247, 79-84]. 2. In the present study we show that this up-regulation is accompanied by a rise in the cholesterol ester content of the cells. 3. Inhibition of cholesterol esterification with the drug 58-035 (Sandoz) significantly decreases the time-dependent 'up-regulation' of LDL receptors. 4. Incubation of hepatocytes with LDL itself has little effect on subsequent LDL binding. However, when cholesterol esterification is inhibited, incubation with LDL decreases binding below that attained with the drug alone. 5. Inhibition of cholesterol synthesis with Lovastatin significantly increases LDL binding and antagonizes the effect of 58-035. 6. We conclude that in hepatocytes the rate of cellular cholesterol esterification can become the major determinant of LDL-receptor activity.

Glucagon, cyclic AMP and adrenaline stimulate the degradation of low-density lipoprotein by cultured rat hepatocytes

Rat hepatocytes were preincubated for 16 h with hormones or drugs and then for a further 8 h with 125I-human low-density lipoprotein (LDL). Glucagon (via cyclic AMP) and adrenaline (via cyclic AMP and alpha-effects) increased the binding of 125I-LDL to the LDL receptor, and the degradation of LDL to [125I]iodotyrosine. The effects on degradation were antagonized by dexamethasone, and the action of cyclic AMP on binding and degradation was inhibited by actinomycin D. The results are discussed in relation to the control of lipoprotein metabolism in diabetes.

Rapid activation of glycogen phosphorylase by steroid hormones in cultured rat hepatocytes

Testosterone (40-300 microM), oestradiol (20-500 microM), progesterone (20-500 microM), dexamethasone (10 nM-1 microM) and corticosterone (1-10 microM) activate glycogen phosphorylase rapidly when added directly to hepatocytes. The activation of phosphorylase was concentration-dependent and occurred after 10 min for dexamethasone, 30 min for testosterone and 60 min for oestradiol and progesterone. This rapid effect does not appear to be dependent on a stimulation of protein synthesis, it is independent of an increase in cyclic AMP, and it is not diminished by the presence of ornithine decarboxylase inhibitors. The stimulation of phosphorylase activity is diminished by depleting the incubation medium of Ca2+ in the presence of 0.5 mM-EGTA, and therefore it may involve changes in the distribution of Ca2+ in the hepatocytes. These results may explain some of the pharmacological effects of sex steroids, and also might contribute to the physiological actions of glucocorticoids.

Translocation of phosphatidate phosphohydrolase from the cytosol to microsomal membranes in thioacetamide-induced liver tumours in rats

The translocation of phosphatidate phosphohydrolase induced by oleate was higher (two-fold) in liver homogenates obtained from long-term thioacetamide-treated rats than from control rats. These differences between thioacetamide-treated and control livers were noticeably higher (four-fold) in the presence of physiological concentrations of salt (0.15 M KCl). In homogenates from control rats, there was a lack of response when physiological concentrations of the salt were present. The enhanced response to translocate phosphatidate phosphohydrolase activity in liver homogenates from thioacetamide-treated rats was due to an increased binding ability of microsomal membranes.

Age-related changes in the translocation of phosphatidate phosphohydrolase from the cytosol to microsomal membranes in rat liver

The effects of oleate, spermine and chlorpromazine were assayed in the presence or absence of 0.15 M KCl on the translocation of phosphatidate phosphohydrolase activity from cytosol to endoplasmic reticulum membranes in liver homogenates obtained from rats aged 1, 30, 60, 180 and 360 days. Marked age-associated decreases in phosphatidate phosphohydrolase distribution onto the membranes were demonstrated under nearly all conditions. In liver homogenates taken from 1-day-old rats and incubated with 0.15 M KCl, most of the enzyme was active (associated with the membranes). Physiological salt concentration (0.15 M KCl) produced a 2-fold increase of oleate-induced translocation of phosphatidate phosphohydrolase activity in liver homogenates from 1-day-old rats; it had no effect on those from 60-day-old rats, and produced a notable decline in liver homogenates obtained from 180- and 360-day-old rats. The promoting effect of spermine on oleate-induced translocation of this enzyme activity was higher in younger rats when incubated in the absence of 0.15 M KCl. Chlorpromazine did not show its usual antagonizing effect on oleate-induced translocation of phosphatidate phosphohydrolase when added to homogenates taken from 1-day-old rats. The antagonizing effect was slightly apparent in liver homogenates from 30-day-old rats and was more pronounced in those from 60-day-old rats in which the values diminished to one-half and to one-third either in the presence or absence of 0.15 M KCl.

Factors regulating the secretion of lysophosphatidylcholine by rat hepatocytes compared with the synthesis and secretion of phosphatidylcholine and triacylglycerol. Effects of albumin, cycloheximide, verapamil, EGTA and chlorpromazine

1. The synthesis and secretion of glycerolipid by monolayer cultures of rat hepatocytes was measured by determining the incorporations of [3H]glycerol, [3H]oleate and [14C]choline and by the absolute concentration of triacylglycerol. 2. The presence of albumin in the medium stimulated the accumulation of lysophosphatidylcholine in the medium by 11-13-fold. 3. Cycloheximide did not significantly alter the accumulation of lysophosphatidylcholine. 4. This process was particularly sensitive to inhibition by chlorpromazine and verapamil, compared with the secretion of triacylglycerol and phosphatidylcholine. By contrast, it was relatively less sensitive to EGTA. 5. It is suggested that intracellular Ca2+ may be important in the production of lysophosphatidylcholine, which then accumulates in the medium by binding to albumin. In vivo this lysophosphatidycholine may be a means of delivering choline and polyunsaturated fatty acids to other organs.

Interactions of triiodothyronine, insulin and dexamethasone on the binding of human LDL to rat hepatocytes in monolayer culture

Rat hepatocytes were maintained for the first 24 h in culture in the presence of 10% (v/v) newborn calf serum and then for a further 16 h in serum-free medium containing 2 g bovine serum albumin per litre. The presence of 1-100 nM triiodothyronine (T3) in the second incubation significantly increased binding of human 125I-LDL to the LDL receptor. Unlike insulin, T3 was unable to reverse the decrease in binding brought about by dexamethasone. The increased binding to the LDL receptor produced by insulin and T3 was additive. We conclude that T3, insulin and glucocorticoids may play important roles in regulating plasma LDL concentrations by direct effect on LDL uptake by the liver.

Effects of chronic administration of benfluorex to rats on the metabolism of corticosterone, glucose, triacylglycerols, glycerol and fatty acid

(1) Rats were fed on diets enriched with sucrose, beef tallow or corn oil and treated for 11-16 days with 50 mg of benfluorex per kg of body weight. By these times the growth rate and food intake were not significantly different from those of control rats. (2) Benfluorex approximately halved the concentration of circulating triacylglycerol in rats fed the beef tallow or sucrose diets. (3) It did not significantly alter the total lipoprotein lipase activity in diaphragm, heart and adipose tissue. (4) The clearance of triacylglycerols from chylomicrons exhibited two t 1/2 values of about 0.6 and 6.9 min in rats fed the beef tallow diet. Benfluorex did not significantly alter these values. (5) Benfluorex did not significantly alter the rate of appearance of triacylglycerol in the blood of rats injected with Triton WR 1339 to block triacylglycerol uptake. It did, however, decrease the rise in circulating glucose which presumably resulted from the stress of the procedure. (6) Benfluorex decreased the extent and duration of the rise in serum corticosterone when rats maintained on the corn oil diet were fed acutely with fructose. It also decreased the circulating concentrations of glycerol, triacylglycerol and glucose after fructose feeding. (7) Rats fed on the corn oil diet and then treated with benfluorex had lower concentrations of circulating glucose, triacylglycerol, glycerol and fatty acids after being injected with 2-deoxyglucose. (8) It is proposed that some of the long-term hypoglycaemic and hypotriglyceridaemic effects of benfluorex could be mediated indirectly through changes in endocrine balance, perhaps via the serotonergic system and in particular, by decreasing the effects of stress hormones relative to insulin. The implications of these findings are discussed in relation to controlling metabolism in stress conditions and for the management of obesity, diabetes and atherosclerosis.

Fatty acid specificity for the synthesis of triacylglycerol and phosphatidylcholine and for the secretion of very-low-density lipoproteins and lysophosphatidylcholine by cultures of rat hepatocytes

1. The synthesis and secretion of glycerolipids by monolayer cultures of rat hepatocytes was measured by using radioactive choline, glycerol and fatty acids and by measuring the concentration of triacylglycerols in the cells. 2. The incorporation of glycerol into triacylglycerol and the accumulation of this lipid in hepatocytes showed little specificity for fatty acids, except for eicosapentaenoate, which stimulated least. Oleate was more effective at stimulating triacylglycerol secretion than were palmitate, stearate, arachidonate and eicosapentaenoate. 3. Linoleate, linolenate, arachidonate and eicosapentaenoate stimulated the incorporation of glycerol and choline into phosphatidylcholine that was secreted into the medium. By contrast, palmitate and stearate produced relatively high incorporations into the phosphatidylcholine that remained in the cells. 4. The incorporation of glycerol and choline into lysophosphatidylcholine in the medium was stimulated 2-3-fold by all of the unsaturated fatty acids tested, whereas palmitate and stearate failed to stimulate if the acids were added separately. When 1 mM-stearate was added with 1 mM-linoleate, the incorporation of linoleate into lysophosphatidylcholine was about 4 times higher than that of stearate. 5. It is proposed that the secretion of lysophosphatidylcholine by the liver could provide a transport system for choline and essential unsaturated fatty acids to other organs.

The biochemistry of lipoproteins

Lipids are transported in the blood in four major classes of lipoproteins. The triacylglycerol-rich lipoproteins are chylomicrons and very-low-density lipoproteins (VLDL) which are produced by the small intestine and liver, respectively. These lipoproteins mainly carry fatty acids to adipose tissue and muscle where the triacylglycerol is hydrolysed by lipoprotein lipase. The resulting particles that remain in the blood are chylomicron remnants and low-density lipoprotein (LDL), respectively. The remnant is taken up by the liver via endocytosis which is mediated by a specific receptor for apolipoprotein E (apoE). LDL, which are rich in cholesterol, can also be taken up by the liver or extrahepatic tissues by a receptor-mediated endocytosis that specifically recognises apoB or apoE. 'Nascent' high-density lipoprotein (HDL) particles are secreted by the liver and intestine and then undergo modification to become HDL3 and then HDL2 as they acquire cholesterol ester. They facilitate the reverse transport of cholesterol back to the liver. Little is known of the hormonal regulation of lipoprotein uptake by the liver. Recently, we have shown that insulin and tri-iodothyronine (T3) increase the specific binding of LDL to cultured hepatocytes whereas dexamethasone (a synthetic glucocorticoid) has the opposite effect. The changes in binding produced by insulin and dexamethasone are paralleled by alterations in the rate of degradation of apoB. These findings may in part explain the hypercholesterolaemia and increased risk of premature atherosclerosis that can be associated with poorly controlled diabetes or hypothyroidism.

Effect of a rhodium complex on alterations of hepatic function in thioacetamide-induced hyperplastic noduligenesis in rats

An in vivo model of liver hyperplastic noduligenesis was induced in rats by long-term administration of thioacetamide (TAM) (50 mg/kg/day i.p.). Three doses of 50 mg/kg of an antitumoral Rh(III) complex were administered at 14, 9 and 5 days before the end of TAM treatment. Plasma and urine were obtained from either TAM or Rh(III) complex or TAM plus Rh(III) complex treated rats to determine the interactions of both substances with the biochemical parameters related to liver function. The rise in alkaline phosphatase (ALP), leucine aminopeptidase (LAP), gamma-glutamyl transferase (GGT) and the unchanged activities in the aspartate and alanine aminotransferases (AST, ALT) in plasma of TAM-treated rats indicated that the disease induced by this substance can be considered as a chronic obstructive biliary disease with indices of cell proliferation and tumors. The increased concentration of bilirubin both in the plasma and urine of TAM-treated rats suggested liver cholestasis and hepatobiliary obstruction. The very low values of creatinine clearance indicated that there was some degree of kidney failure due to the effect of TAM. The increased concentration of ammonia both in plasma and urine were probably a consequence of the decreased flux in the urea cycle in the liver. The Rh(III) complex alone did not produce significant changes in the plasma enzyme activities. The only significant changes were found in the concentrations of uric acid and ammonia in the urine. When the Rh(III) complex was administered to TAM-treated rats, significant restoration of the following parameters were observed: plasma enzymatic activities, blood bilirubin and ammonia, uric acid and creatinine in the urine and the creatinine clearance. These results suggest that the altered liver function induced by TAM can be restored by Rh(III) complex. The mechanisms by which this complex acts to counteract the TAM-induced changes are not yet established.

Interactions of insulin and dexamethasone in the control of pyruvate kinase activity and glucose metabolism in sheep adipose tissue

1. The Vmax. activity of pyruvate kinase of sheep adipose tissue increased during tissue culture up to 48 h; the increase was blocked by actinomycin D (an inhibitor of transcription) and was promoted by insulin and antagonized by dexamethasone. 2. In contrast with their effects on pyruvate kinase, insulin and dexamethasone acted synergistically to increase the activity of glucose-6-phosphate dehydrogenase of sheep adipose tissue maintained in culture. 3. Insulin stimulated, whereas dexamethasone inhibited, glucose utilization by sheep adipose tissue maintained in culture; the two agents were mutually antagonistic, and their effects were prevented by actinomycin D. 4. Antimycin A (an inhibitor of the electron-transport chain) stimulated glucose uptake and lactate output by sheep adipose tissue in the presence of dexamethasone and insulin, suggesting that the effects of dexamethasone on glucose utilization by sheep adipose tissue were not due to an inhibition of glucose transport. 5. Comparison of these findings with previous studies on the endocrine control of hepatic pyruvate kinase shows that there are major differences in the control of these Vmax. activities in liver and adipose tissue. 6. Although glucocorticoid hormones inhibit glucose utilization themselves and can antagonize the stimulatory effects of insulin on glucose utilization in adipose tissue from both sheep and rats, there appear to be major differences in the sites of action of these hormones in the two species.

Effects of preincubation of primary monolayer cultures of rat hepatocytes with low- and high-density lipoproteins on the subsequent binding and metabolism of human low-density lipoprotein

1. There are two distinct binding sites (Site 1 and Site 2) for human low-density lipoprotein (LDL) on rat hepatocytes in monolayer culture [Salter, Saxton & Brindley (1986) Biochem. J. 240, 549-557]. 2. Binding of 125I-LDL to Site 1, but not to Site 2, is up-regulated between 20 and 44 h in culture by preincubation of the cells with human high-density lipoprotein 3 (HDL3). 3. A similar preincubation with HDL2 had no significant effect on binding to either site. 4. Preincubation with human LDL led to a partial down-regulation of subsequent binding of 125I-LDL to Site 1. Since binding after incubation with LDL was measured at 37 degrees C, binding to Site 2 could not be distinguished from LDL that had been internalized by the cells. 5. Hepatocytes were shown to degrade 125I-LDL, resulting in the accumulation of [125I]iodotyrosine in the medium. Evidence was found that iodotyrosine may be further degraded by deiodinase produced by the cells. 6. Regulation of binding to Site 1 by preincubation with LDL or HDL3 was found to lead to a parallel regulation of LDL degradation. 7. It is concluded that rat hepatocytes not only bind but also metabolize human LDL and that these processes are under metabolic regulation.

Binding of low-density lipoprotein to monolayer cultures of rat hepatocytes is increased by insulin and decreased by dexamethasone

Rat hepatocytes were maintained in monolayer culture for 20 h in the presence of 10% (v/v) newborn calf serum and then for a further 1-24 h in serum-free medium containing 2 g bovine serum albumin/l. The specific binding of human 125I-LDL to two distinct sites was then measured at 4 degrees C. Binding to site 1 was displaced by dextran sulphate while that to site 2 was not. The presence of 1-100 nM insulin for 24 h in the second incubation significantly increased binding to site 1. Significant increases were also seen when cells were incubated with 10 nM insulin for 1 h. No significant effects of insulin on binding to site 2 were observed. In contrast, 10 nM-1 microM dexamethasone decreased binding to both sites. The effects of these hormones were mutually antagonistic.

A rapid assay for measuring the activity and the Mg2+ and Ca2+ requirements of phosphatidate phosphohydrolase in cytosolic and microsomal fractions of rat liver

1. A rapid extraction and purification scheme was designed for the recovery of [3H]diacylglycerol formed during the assay of phosphatidate phosphohydrolase. 2. The importance of removing polyvalent cations, particularly Ca2+, from the phosphatidate and other reagents used in the assay of the phosphohydrolase activity was demonstrated. This was achieved mainly by treating the phosphatidate with a chelating resin and by adding 1 mM-EGTA and 1 mM-EDTA to the assays. 3. The activity of the phosphohydrolase in dialysed samples of the soluble and microsomal fractions of rat liver was very low. 4. Addition of optimum concentrations of MgCl2 resulted in a 110-167-fold stimulation in activity. 5. CaCl2 was also able to stimulate phosphohydrolase activity, but to a much smaller extent than MgCl2. 6. Chlorpromazine, an amphiphilic cation, inhibited the reaction when it was measured in these experiments by using a mixed emulsion of phosphatidylcholine and phosphatidate at pH 7.4. 7. Microsomal fractions that were preincubated with albumin contained very low activities of the Mg2+-dependent phosphohydrolase. When these were then incubated with the soluble fraction in the presence of oleate, the soluble phosphohydrolase attached to the microsomal membranes, and it retained its high dependency on Mg2+.