Diurnal variations in the effects of an unsaturated-fat-containing diet on fatty acid and cholesterol synthesis in rat hepatocytes

Physiological Role of Glutamate Dehydrogenase in Cancer Cells

NH 4 + increased growth rates and final densities of several human metastatic cancer cells. To assess whether glutamate dehydrogenase (GDH) in cancer cells may catalyze the reverse reaction of NH 4 + fixation, its covalent regulation and kinetic parameters were determined under near-physiological conditions. Increased total protein and phosphorylation were attained in NH 4 + -supplemented metastatic cells, but total cell GDH activity was unchanged. Higher V _max values for the GDH reverse reaction vs. forward reaction in both isolated hepatoma (HepM) and liver mitochondria [rat liver mitochondria (RLM)] favored an NH 4 + -fixing role. GDH sigmoidal kinetics with NH 4 + , ADP, and leucine fitted to Hill equation showed n _H values of 2 to 3. However, the K _0.5 values for NH 4 + were over 20 mM, questioning the physiological relevance of the GDH reverse reaction, because intracellular NH 4 + in tumors is 1 to 5 mM. In contrast, data fitting to the Monod-Wyman-Changeux (MWC) model revealed lower K _m values for NH 4 + , of 6 to 12 mM. In silico analysis made with MWC equation, and using physiological concentrations of substrates and modulators, predicted GDH N-fixing activity in cancer cells. Therefore, together with its thermodynamic feasibility, GDH may reach rates for its reverse, NH 4 + -fixing reaction that are compatible with an anabolic role for supporting growth of cancer cells.

The effects of lovastatin and simvastatin on the diurnal periodicity of plasma mevalonate concentrations in patients with heterozygous familial hypercholesterolemia

Animal and human studies have shown that the biosynthesis of cholesterol exhibits diurnal periodicity with nocturnal increases in the level of cholesterol precursors. Dietary cholesterol, which increases the intracellular pool of cholesterol and plasma cholesterol levels, has been shown to blunt the nocturnal increases in cholesterol biosynthesis. Patients with heterozygous familial hypercholesterolemia (FH) have very high levels of plasma low-density lipoprotein cholesterol (LDL) due to their reduced ability to metabolize LDL particles. The present studies were carried out to determine whether diurnal variations in cholesterol synthesis occur in FH patients and to test the effects of 3-hydroxy-3-methyl glutaryl CoA (HMG CoA) reductase inhibitors on the diurnal cycle of cholesterol biosynthesis in these patients. Diurnal rates of cholesterol synthesis were assessed by measuring the plasma concentrations of mevalonate, an intermediate in the pathway of cholesterol biosynthesis. Female FH patients exhibited a diurnal pattern in plasma mevalonate levels similar to that previously reported in controls with peak values occurring at night. Treatment with lovastatin and simvastatin (40 mg b.i.d.) significantly reduced 24-h mean plasma mevalonate levels from baseline values. Administration of lovastatin in the evening reduced the nocturnal increases in mevalonate levels, and the administration of simvastatin completely abolished the nighttime rise. These results demonstrate that inhibition of cholesterol biosynthesis by lovastatin and simvastatin modifies the normal diurnal rhythm of cholesterol biosynthesis in female FH patients.

Effect of nutritional state on the utilization of fatty acids for hepatitic triacylglycerol synthesis and secretion as very-low-density lipoprotein

The mass of very-low-density-lipoproteins (VLDL) triacylglycerol secreted from isolated hepatocytes was dependent on the nutritional state of the donor rats, and declined in the order sucrose-fed greater than chow-fed greater than polyunsaturated-fat-fed greater than starved. This was the case irrespective of the presence or absence of exogenous oleate. The contribution of newly synthesized fatty acids to the total mass of VLDL triacylglycerol also declined in the above order, and reflected the relative rates of fatty acid synthesis de novo in each of the groups. The contribution of exogenous oleate to VLDL triacylglycerol varied in a manner similar to that for newly synthesized fatty acid. However, the contribution either of exogenous oleate or of newly synthesized fatty acid never exceeded 17-20% of the total VLDL triacylglycerol fatty acid even in the sucrose-fed animals. The increased contribution of newly synthesized fatty acids in the sucrose-fed group was not sufficient to account for the increase in the total mass of VLDL triacylglycerol secreted. These results suggest that: (a) changes in the rate of triacylglycerol secretion are not a direct consequence of variations in the rate of fatty acid synthesis de novo; (b) in the short term, most of the triacylglycerol required for VLDL assembly and secretion is derived from an intracellular storage source: (c) the distribution of newly synthesized triacylglycerol between the cytosolic and secretory pools was similar irrespective of the source of fatty acids (i.e. synthesized de novo or exogenous).

Regulation of hepatic synthesis and secretion of cholesterol and glycerolipids in animals maintained in different nutritional states

The distribution of newly synthesized and exogenous fatty acids and of newly synthesized cholesterol between cellular and very-low-density lipoprotein (VLDL) lipids was studied in hepatocytes derived from animals fed on a normal diet or on diets supplemented with polyunsaturated fat or sucrose. Phospholipid synthesis from either exogenous or endogenous (biosynthetic) fatty acids was unaffected by nutritional state. Cholesterol synthesis was decreased in the fat-fed animals, but sucrose feeding had no significant effect. In all nutritional states, newly synthesized rather than exogenous fatty acids were better substrates for phospholipid synthesis. In all groups, compared with newly synthesized triacylglycerol, smaller proportions of newly synthesized phospholipid and cholesterol were secreted as VLDLs. This was confirmed in intact animals by using Triton WR-1339. Newly synthesized phospholipid formed a greater proportion of the VLDL glycerolipid in the fat-fed than in the normal or sucrose-fed animals. In all groups, phospholipids labelled from endogenous fatty acids were secreted in preference to those labelled from exogenous fatty acids.

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)

Purification and characterization of the AMP-activated protein kinase. Copurification of acetyl-CoA carboxylase kinase and 3-hydroxy-3-methylglutaryl-CoA reductase kinase activities

1. We have purified the AMP-activated protein kinase 4800-fold from rat liver. The acetyl-CoA carboxylase kinase and 3-hydroxy-3-methylglutaryl-CoA(HMG-CoA) reductase kinase activities copurify through all six purification steps and are inactivated with similar kinetics by treatment with the reactive ATP analogue fluorosulphonylbenzoyladenosine. 2. The final preparation contains several polypeptides detectable by SDS/polyacrylamide gel electrophoresis, but only one of these, with an apparent molecular mass of 63 kDa, is labelled using [14C]fluorosulphonylbenzoyladenosine. This is also the only polypeptide in the preparation that becomes significantly labelled during incubation with [gamma 32P]ATP. This autophosphorylation reaction did not affect the AMP-stimulated kinase activity. 3. In the absence of AMP the purified kinase has apparent Km values for ATP and acetyl-CoA carboxylase of 86 microM and 1.9 microM respectively. AMP increases the Vmax 3-5-fold without a significant change in the Km for either protein or ATP substrates. 4. The response to AMP depends on the ATP concentration in the assay, but at a near-physiological ATP concentration the half-maximal effect of AMP occurs at 14 microM. Studies with a range of nucleoside monophosphates and diphosphates, and AMP analogues showed that the allosteric activation by AMP was very specific. ADP gave a small stimulation at low concentrations but was inhibitory at high concentrations. 5. These results show that the AMP-activated protein kinase is the major HMG-CoA reductase kinase detectable in rat liver under our assay conditions and that it is therefore likely to be identical to previously described HMG-CoA reductase kinase(s) which are activated by adenine nucleotides and phosphorylation. The AMP-binding and catalytic domains of the kinase are located on a 63-kDa polypeptide which is subject to autophosphorylation.

A common bicyclic protein kinase cascade inactivates the regulatory enzymes of fatty acid and cholesterol biosynthesis

A highly purified rat liver protein kinase phosphorylates and inactivates acetyl-CoA carboxylase, and causes rapid inactivation of microsomal HMG-CoA reductase in the presence of MgATP. Both effects are stimulated in an identical manner by AMP, and are greatly reduced by prior treatment of the kinase with purified protein phosphatase. The dephosphorylated kinase can be reactivated in the presence of MgATP, apparently due to a distinct kinase kinase, and this reactivation is stimulated by nanomolar concentrations of palmitoyl-CoA. These results show that a common, bicyclic protein kinase cascade can potently inactivate the regulatory enzymes of both fatty acid and cholesterol biosynthesis.

Regulation of hepatic very-low-density lipoprotein secretion in rats fed on a diet high in unsaturated fat

Rats were fed ad libitum on either a standard, high-carbohydrate, chow diet or a similar diet supplemented with 15% unsaturated fat (corn oil). Hepatocytes were prepared either during the dark phase (D6-hepatocytes) or during the light phase (L2-hepatocytes) of the diurnal cycle. In hepatocytes from rats fed on the unsaturated-fat-containing diet, secretion of very-low-density lipoprotein (VLDL) triacylglycerol was inhibited to a greater extent in the D6- than in the L2-hepatocytes. Plasma non-esterified fatty acid concentrations were elevated to the same extent at both D6 and L2 in the unsaturated-fat-fed animals. The secretion of VLDL esterified and non-esterified cholesterol was relatively insensitive to changes in the unsaturated-fat content of the diet. This resulted in proportionate increases in the content of these lipid constituents compared with that of triacylglycerol in the nascent VLDL. There was also an increase in the ratio of esterified to non-esterified cholesterol in the nascent VLDL produced by hepatocytes of the unsaturated-fat-fed animals. In the D6-hepatocytes from the unsaturated-fat-fed animals, the decrease in the secretion of VLDL triacylglycerol could not be reversed by addition of exogenous oleate (0.7 mM) to the incubation medium. In contrast, addition of a mixture of lactate (10 mM) and pyruvate (1 mM) stimulated both fatty acid synthesis de novo and the rate of VLDL triacylglycerol secretion. Secretion of esterified and non-esterified cholesterol also increased under these conditions. Insulin suppressed the secretion of VLDL triacylglycerol and cholesteryl ester under a wide range of conditions in all types of hepatocyte preparations. Non-esterified cholesterol secretion was unaffected. In hepatocytes prepared from the fat-fed animals, these effects of insulin were more pronounced at D6 than at L2. Glucagon also inhibited VLDL lipid secretion in all types of hepatocyte preparations. The decrease in cholesterol secretion was due equally to decreases in the rates of secretion of both esterified and non-esterified cholesterol.