Evidence that cholesteryl ester hydrolase and triglyceride lipase are different enzymes in rat liver

Lipid metabolism in liver of rat exposed to cadmium

We investigated the effect of exposition to cadmium (Cd, 15ppm for 8 weeks) through drinking water on liver lipid metabolism in adult male Wistar rats. As compared to metal non-exposed (control) rats, the serum triglycerides, cholesterol and LDL+VLDL cholesterol concentrations increased. This was associated to a decrease of lipoprotein lipase activity in post heparinic plasma. The VLDL secretion from liver was not modified. Cd treatment increased triglycerides and decreased esterified cholesterol contents in liver. The high triglyceride mass was related to the increased glycerol-3-phosphate acyltransferase mRNA expression. In addition, the liver fatty acids synthesis increased, as determined by an increment of fatty acid synthetase and isocitrate dehydrogenase activities, and [(14)C]-acetate incorporation into saponifiable lipid fraction. The relative percentage of palmitic acid (16:0) and total saturated fatty acids were increased compared with control. Hepatic glucose-6-phosphate dehydrogenase, malic dehydrogenase and cholesteryl ester hydrolase activities were unchanged. In liver, the Cd treatment decreased triglyceride and cholesterol in mitochondria, also increased triglyceride in cytosol, and cholesterol and phospholipid contents in nuclei, compared with control. In addition, an increase of nuclei phosphatidylcholine synthesis was observed. Cd exposure alters directly or indirectly the serum lipid content and liver lipid metabolism.

Inhibition of neutral cholesteryl ester hydrolase by the glycolytic enzyme enolase. Is this a secondary function of enolase?

There is an accumulation of the glycolytic enzyme enolase and of cholesteryl esters in macrophages that have been converted into "foam" cells. In this study, we questioned whether enolase could be involved in this accumulation of cholesteryl esters by inhibiting the activity of neutral cholesteryl ester hydrolases. Enolase from both yeast and rabbit muscle were incubated with three different cholesteryl ester hydrolases and were shown to inhibit the hydrolysis of cholesteryl esters. Inhibition was dependent on the concentration of enolase and appeared to occur through binding of the enolase to the cholesteryl ester. Nevertheless, the yeast and rabbit muscle enolases differed in their efficiency of inhibition and in their mechanism of action. Purification of commercial enolase preparations by gel-filtration yielded single proteins with the same inhibitory activities as the originals, indicating that the inhibition was not due to the presence of an impurity. Partially purified alpha alpha- and gamma gamma-isoforms of the enzyme from rat brain also appear to have inhibitory effects on cholesteryl ester hydrolysis. Negative control of the hydrolytic phase of the cholesterol/cholesteryl ester cycle may be a secondary function of enolases which correlates with the accumulation of cholesteryl esters in a number of neuro-degenerative and demyelinating diseases.

Cytoplasmic triacylglycerols and cholesteryl esters are degraded in two separate catabolic pools in cultured human fibroblasts

The sources and the catabolic pathways of cytoplasmic pools of triacylglycerols and cholesteryl esters have been comparatively investigated in cultured fibroblasts from normal subjects and from patients affected with neutral lipid storage disease (NLSD) and Wolman disease (WD). (i) Endogenously biosynthesized triacylglycerols and cholesteryl esters were degraded extra-lysosomally since they were catabolized at similar rates in normal and in WD fibroblasts. In NLSD fibroblasts, the degradation of endogenous triacylglycerols was severely deficient, whereas that of endogenous cholesteryl esters was in the normal range. (ii) Reconstituted high density lipoproteins (HDL) containing radiolabelled [3H]triolein and cholesteryl [14C]oleate were taken up by cultured fibroblasts and rapidly degraded in a non-lysosomal compartment. In NLSD fibroblasts the degradation of HDL-[3H]triolein was blocked whereas that of HDL-[14C]cholesteryl oleate was in the normal range. These data suggest that: (i) the cytoplasmic pools of triacylglycerols and cholesteryl esters originate from HDL uptake and from endogenous biosynthesis as well; (ii) cytoplasmic (non-lysosomal) triacylglycerols and cholesteryl esters are degraded by two separate catabolic pathways.

Separation and differential activation of rat liver cytosolic cholesteryl ester hydrolase, triglyceride lipase and retinyl palmitate hydrolase by cholestyramine and protein kinases

Cholesteryl ester hydrolase (CEH), triacylglycerol lipase (TGL) and retinyl palmitate hydrolase (RPH) were measured in 104,000 x g supernatants from rat liver under optimal conditions for measurement of cytosolic CEH. Similar levels of hydrolytic activity were seen with oil droplet dispersions of cholesteryl oleate, trioleoylglycerol and retinyl palmitate. No cytosolic TGL activity was seen with substrate presented in the triton-albumin emulsion used for measurement of lipoprotein lipase-like TGL associated with hepatic plasma membrane. Cytosolic CEH, TGL and RPH were differentially partially purified by both ammonium sulfate precipitation and anion exchange fast protein liquid chromatography (FPLC). Of the tree activities, only CEH was stimulated by cholestyramine feeding and by activators of protein kinases A and C. All three activities were inhibited by alkaline phosphatase treatment, although to different degrees. It is concluded that these activities are catalyzed by at least three differentially regulated enzymes with a high degree of specificity for their respective substrates.

Fate of fatty acids taken up as cholesteryl ester by rat hepatocytes in primary culture from high-density lipoprotein

Primary cultures of rat hepatocytes were incubated in the presence of high-density lipoproteins (HDL) labelled with [1-14C]oleyl or [1-14C]linoleyl cholesteryl ester. Labelled HDL were prepared by selective delipidation with heptane, relipidation and sequential ultracentrifugations. Hepatocytes took up cholesteryl esters and cholesteryl ether their non-hydrolizable analog, at the same rate. The uptake increased with time, the cholesteryl ester/protein ratio and the amount of added HDL. It was not dependent on the nature of acyl chain or on the nature of the bond. The uptake did not depend on a specific interaction between HDL and cell membranes, since cholesteryl ester was taken up from HDL to the same extent as from albumin complexes. Linoleic and oleic acids released from cholesteryl esters taken up by hepatocytes were mainly reesterified into phosphatidylcholine and triacylglycerols. Linoleic acid was preferentially channelled into PC. A portion of these lipids were secreted by hepatocytes during a 24-h reincubation in a medium devoid of lipoprotein. Nearly the same amount of radioactivity was recovered in secreted phospholipids as in secreted triacylglycerols, in contrast with hepatocytes labelled with free fatty acids which secreted very little radioactivity as phospholipids. From these results and the high content in polyunsaturated fatty acids of cholesteryl esters, one can hypothesize that hepatic cholesteryl ester uptake may contribute to biliary phosphatidylcholine production, and therefore to polyunsaturated fatty acid sparing.

Activation of rat liver cholesterol ester hydrolase by cAMP-dependent protein kinase and protein kinase C

Short term regulation of hepatic cholesterol ester hydrolase by reversible phosphorylation is described. Two different kinase systems seem to be involved in this regulation. The addition of ATP, cyclic AMP and Mg2+ to rat liver 104,000 X g supernatant (S104) produced a 100-140% increase in cholesterol ester hydrolase activity. This stimulation was abolished when protein kinase inhibitor was added prior to the addition of ATP, cyclic AMP and Mg2+. Cholesterol ester hydrolase activity was also stimulated when calcium ions, phosphatidylserine, and diolein were added to S104 along with ATP and Mg2+. Diolein in this reaction could be substituted by phorbol 12-myristate 13-acetate. Preincubation of S104 with alkaline phosphatase resulted in a deactivation of cholesterol ester hydrolase. The addition of increasing concentrations of Mg2+ to S104 produced increasing inhibition of cholesterol ester hydrolase activity, and this effect was blocked by NaF. It is suggested that rat liver cholesterol ester hydrolase is activated by cyclic AMP dependent protein kinase and protein kinase C. Deactivation is accomplished by dephosphorylation catalyzed by a phosphoprotein phosphatase, dependent on Mg2+.

Distributions of retinoids, retinoid-binding proteins and related parameters in different types of liver cells isolated from young and old rats

The levels of retinoids, retinol-binding protein, cellular retinol-binding protein, cellular retinoic-acid-binding protein, transthyretin and the activities of retinyl palmitate hydrolase and cholesteryl oleate hydrolase were determined in purified parenchymal, fat-storing, endothelial and Kupffer cell preparations, and in liver homogenates from young adult (6-month-old) and old (36-month-old) rats. Retinoid levels were also determined in the plasma from young and old rats. Retinoid contents were determined by HPLC. The binding proteins and transthyretin were measured by specific radioimmunoassays; retinyl palmitate and cholesterol oleate hydrolases were measured by sensitive microassays. The retinoid content of both the liver homogenates and of the fat-storing, and parenchymal cell preparations increased between 6 months and 36 months of age. The cellular distribution of retinoids was similar for the two age groups analyzed with the fat-storing cells being the main retinoid storage sites in the rat liver. Concentrations of retinol-binding protein and transthyretin were high in parenchymal cell preparations. Cellular retinol-binding protein was enriched both in parenchymal and in fat-storing cell preparations; the highest concentrations of cellular retinoic-acid-binding protein were present in fat-storing cell preparations. No major differences were observed between the two age groups in the cellular concentrations and distributions of any of these binding proteins. High activity of cholesterol oleate hydrolase was measured in parenchymal and in Kupffer cell preparations; endothelial cell preparations also contained considerable activities. The distribution of this activity over the various cell types reflects their role in lipoprotein metabolism. Retinyl palmitate hydrolase activity was specifically enriched in parenchymal and in fat-storing cell preparations, consistent with the roles of these cells in retinoid metabolism. No major differences were observed between the two age groups in the cellular distributions of the two hydrolase activities. This study indicates that no major changes occur in the retinoid-related parameters analyzed with age, suggesting that rat liver retinoid metabolism does not change dramatically with age and that retinoid homeostasis is maintained.

Cholesterol esterases from normal and FEC hamster liver

1. Synthetic cholesteryl esters with various acyl chain length (C2-C18) are hydrolysed by several enzymes in hamster liver. 2. The comparison of effect of inhibitors, divalent cations, detergents, pH and substrate specificity allows discrimination between four enzymes hydrolyzing cholesteryl esters, which are characterized by their enzymatic properties, two cholesterol esterases (resistant to E600) hydrolyzed medium- and long-chain cholesteryl esters, whereas short-chain cholesteryl esters were hydrolyzed by two different carboxylesterases (dramatically inhibited by E600). 3. The acid cholesterol esterase (identical to the lysosomal lipase) exhibited a pH optimum at pH 5.0 and is activated by 1 mM taurocholate. 4. The alkaline cholesterol esterase (pH optimum 7.5) is not very sensitive to the tested effectors. 5. Both acid and alkaline carboxylesterases (pH optima 5.5 and 7.5), were characterized by their strict dependence on divalent cations (Mn2+ or Mg2+). 6. The acid carboxylesterase was inhibited by increasing concentrations of Triton X-100, whereas the alkaline carboxylesterase was dramatically activated by 2 g/l Triton X-100. 7. No significant difference was observed in activities of cholesterol esterases or carboxylesterases between normal and FEC hamster livers.