The effect in vitro of high-density lipoprotein on hydrolysis of triacylglycerol by lipoprotein lipase

Silymarin downregulates COX-2 expression and attenuates hyperlipidemia during NDEA-induced rat hepatocellular carcinoma

Silymarin is a naturally available bioflavonoid and is a strong antioxidant with a capacity to inhibit the formation of tumors in several cancer models. In the present study, we investigated whether dietary supplementation of silymarin has any role in lipid components, lipid-metabolizing enzymes, free fatty acid profile, and expression of cyclooxygenase-2 (COX-2) in N-nitrosodiethylamine (NDEA)-induced hepatocellular carcinoma in rats. NDEA-induced rats showed severe hyperlipidemia along with upregulated expression of COX-2 as revealed by western blotting and immunohistochemistry. Dietary silymarin supplementation attenuated this hyperlipidemia and downregulated the expression of COX-2. Thus we conclude that compounds like silymarin with potent hypolipidemic effect are strong candidates as chemopreventive agents for the treatment of liver cancer.

The biochemical alterations following administration of Kalpaamruthaa and Semecarpus anacardium in mammary carcinoma

BACKGROUND

Breast cancer is one of the most common cancers in women of developed and developing countries. Lipids, lipoproteins and lipid-metabolizing enzymes have been associated with the risk of breast cancer. Kalpaamruthaa (KA) is a modified Siddha preparation, which contains Semecarpus anacardium Linn. (SA), Emblica officinalis (EO) and honey.

OBJECTIVE

The present study was embarked to study the variations in lipids, lipid-metabolizing enzymes and lipoproteins in cancerous animals and the effect of KA on the lipid metabolism.

MATERIALS AND METHODS

Breast cancer was induced in rats by administrating 7,12-dimethylbenz(a)anthracene orally (25 mg/kg body weight). After 90 days of induction, KA (300 mg/kg body weight) and SA (200 mg/kg body weight) were administered for 14 days, by gastric intubations. The levels of lipids and lipid-metabolizing enzymes were analysed in control and experimental animals.

RESULTS AND CONCLUSION

The increased levels of total cholesterol, free cholesterol, phospholipids, triglycerides and free fatty acids and decreased levels of ester cholesterol in plasma, liver and kidney found in cancer suffering animals were reverted back to near normal levels on treatment with KA and SA. In mammary carcinoma bearing animals, the activities of total lipase, cholesterol ester synthase, and cholesterol ester hydrolase were significantly (p < 0.05) increased whereas lipoprotein lipase and lecithin cholesterol-acyl transferase were decreased. The levels of very low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) were increased and the level of high-density lipoprotein (HDL) was decreased. These alterations were recouped back upon treatment with KA as well as SA when compared to cancer animals. The effects of KA were found to be more effective than SA. No significant alterations were observed in herbal preparation control animals when compared to control animals.

Preventive effects of a verapamil against tumor necrosis factor-alpha-induced shock symptoms: approached from lipoprotein metabolic disorders

We examined the role of intracellular Ca2+ in the mechanism of the preventive effects of the Ca2+-channel blocker verapamil against lipoprotein disturbances during tumor necrosis factor (TNFa)-induced shock syndrome. The heparin-releasable lipoprotein lipase (LPL) activity in plasma of TNFalpha (5 X 10(4) units/mouse, i.v.)-injected mice was markedly lower at 4 h post-intoxication than in the controls. In mice treated with verapamil (10 mg/kg, s.c.), the activity of LPL 4 h after TNFalpha injection was significantly higher than in mice treated with TNFalpha alone. On the other hand, on polyacrylamide gel disk electrophoresis, very low density lipoprotein (VLDL) and high density lipoprotein (HDL) fractions in the sera of TNFalpha-injected mice were increased and reduced, respectively, relative to the controls. The administration of verapamil clearly prevented the lipoprotein damage arising from TNFalpha challenge. We investigated whether verapamil could suppress TNFalpha generation in endotoxin-treated J774A.1 cells. Treatment with verapamil (30 microM) markedly inhibited endotoxin (1 microg/ml)-induced TNFalpha production in these cells. These findings suggest that the concentration of intracellular Ca2+ may contribute to the extent of lipoprotein disturbances in plasma, which results from LPL suppression in TNFalpha-induced shock syndrome. Verapamil may, therefore, protect against some of the various disturbances caused by changes in Ca2+ mobilization through its ability to inhibit TNFalpha production in septic shock.

Lipoprotein lipase in heart and myocytes: characteristics with intralipid as substrate

1. Intralipid is a suitable substrate for measuring lipoprotein lipase activity in the presence of other triacylglycerol lipases in heart and myocytes. 2. Triacylglycerol lipase activity in heart and myocytes was increased 10-fold in the presence of serum at pH 7.4 and 8.1. The serum-stimulated activity in myocytes was 95% inhibited by saturating concentrations of antiserum to lipoprotein lipase. 3. Both heparin-releasable and non-releasable lipoprotein lipase fractions had similar Km values for Intralipid and a similar pattern of inhibition by high density lipoprotein but different responses to heparin. 4. Isoproterenol did not alter lipoprotein lipase activity in cardiac myocytes.

Increasing effect of vanadate on lipoprotein lipase activity in isolated rat fat pads

Vanadate increased lipoprotein lipase (LPL) activity in the isolated fat pads in a time- and dose-dependent manner. The increasing effect of vanadate was inhibited by amiloride, similar to that of insulin, and it also was not additive to that of insulin. Although the increasing effects of vanadate and insulin were preserved in K(+)-free medium, appreciable decreases in both effects were observed by replacement of Na+ with choline ion or omission of Ca2+ in the medium. Vanadate showed the full effect in the presence of cycloheximide at concentrations that inhibited protein synthesis of the fat pads, suggesting that the action of vanadate is not due to the increase in protein synthesis. Tetrakis (acetoxymethyl) ester of quin 2 at 50 microM concentration never inhibited the action of vanadate though it showed a little inhibition at a concentration of 300 microM. No inhibition of the action of vanadate was observed with ruthenium red. These results suggest that vanadate increases the LPL activity via a process less sensitive to the intracellular Ca2+ concentration. Adrenaline, dibutyryl cyclic AMP, and 3-isobutyl-1-methylxanthine all inhibited the action of vanadate, suggesting that the action is inhibited with increase in the intracellular concentration of cyclic AMP. Monensin and carbonyl cyanide m-chlorophenylhydrazone inhibited the action of vanadate. In contrast, the action of insulin was never inhibited by monensin. Tunicamycin and 2-deoxyglucose, at rather high concentrations, inhibited both actions. These findings suggest that vanadate increases the LPL activity through mechanisms of action involving amiloride- and monensin-sensitive pathways dependent on energy.

Stimulatory release of lipoprotein lipase activity from rat fat pads by vanadate

Vanadate stimulated the release of lipoprotein lipase (LPL) activity from rat fat pads into the medium in a time- and dose-dependent manner. It exerted the synergetic effect with heparin. The stimulatory effects of vanadate and heparin were decreased by incubation in Na+- or Ca2+-free media but were well preserved in K+-free medium. Amiloride inhibited the vanadate-stimulated release of LPL activity in a dose-dependent manner, but did not inhibit the heparin-stimulated release of LPL activity. Colchicine, antimycin A, and carbonyl cyanide m-chlorophenylhydrazone suppressed the stimulatory effect of vanadate, but cycloheximide did not. Preincubation of the fat pads with the tetrakis (acetoxymethyl) ester of quin 2 (quin 2-AM) inhibited the vanadate-stimulatory release of LPL activity without affecting basal activity. The concentration required for half-maximal inhibition of the action of vanadate by quin 2-AM was calculated to be 39 microM, suggesting that the action of vandate was dependent on intracellular Ca2+ concentration. The heparin-stimulated release, on the other hand, was not inhibited even at higher concentrations of quin 2-AM (up to 200 microM). These findings suggest that vanadate stimulates the release of LPL activity through mechanisms of action involving amiloride-sensitive and calcium-dependent pathways with a requirement of metabolic energy.

Interfacial interactions between proteins and mammalian lipases

The effects of proteins, both endogenous and exogenous, on the activity of lipases against water soluble and water insoluble substrates have been reviewed. The enzymes considered are pancreatic and gastric lipases, and lipoprotein, bile-salt-stimulated human milk and pancreatic carboxyl ester lipases. A brief account is given of the function of each enzyme and of the physical properties of the interacting proteins, which include albumins, lysozymes, globulins and immunoglobulins, myoglobin, transferrins, alpha-lactalbumin and melittin. With few exceptions (for example, the effect of colipase on pancreatic lipase), the interaction of proteins with lipases which act at the lipid-water interface of water insoluble substrates results in deactivation of enzymic activity. It seems that the amphiphilic nature of proteins allows them to aggregate at interfaces, thereby altering the nature of the interface and decreasing accessibility of the substrate to the enzyme. This discussion gives consideration to association of the proteins with the enzyme or the interface and to whether the interactions with specific binding sites or interfacial inactivation are responsible for the observations. However, the effect of proteins on lipases acting against water soluble substrates varies from protein to protein. Activation of enzyme-activity occurs if the interacting proteins are able to act as acyl transfer agents and thus introduce another catalytic hydrolysis pathway into the reaction mechanism. Inhibition may be caused by specific interactions between the protein and the enzyme or the substrate.

Inhibition of human and rat lipoprotein lipase by high-density lipoprotein

The hydrolysis in vitro of preactivated Intralipid (an artificial triacylglycerol-phospholipid emulsion) by rat adipose tissue lipoprotein lipase is inhibited by rat high-density lipoprotein (HDL). The aim of this work was to investigate whether human lipoprotein lipase was also inhibited, the mechanism of inhibition of the rat enzyme by HDL, and the role of the various individual apolipoproteins. Both human and rat lipoprotein lipase from post-heparin plasma are inhibited by HDL. This inhibition is considerably decreased if the HDL is first made 'apolipoprotein poor' by removal of some transferable apolipoproteins. In contrast, both native and apolipoprotein poor HDL inhibit the hydrolysis of Intralipid by rat hepatic lipase. Apolipoproteins C and E, either free in solution or attached to lipid vesicles, inhibit the hydrolysis of activated Intralipid by rat lipoprotein lipase to a maximum of 85% and 50%, respectively. Apolipoprotein A attached to vesicles gives little inhibition. HDL apolipoprotein and apolipoprotein C compete with the substrate for binding to lipoprotein lipase with apolipoprotein C having a higher affinity for the enzyme than HDL apolipoprotein. The inhibition of lipoprotein lipase by HDL can be explained by the association of the constituent apolipoproteins, in particular apolipoprotein C, with the enzyme so that there is less enzyme available to act on substrate.

Isolated adipocytes: an assessment of cell surface changes during their preparation

A method is described, based on the detection of adipocyte-specific cell surface antigens, which allows assessment of the relative surface damage incurred by the cells when they are prepared under a variety of conditions. Using the method it is possible to develop, for any set of reagents, a set of cell isolation conditions (collagenase concentration, time of incubation) which will produce minimally damaged cells which exhibit high levels of specific cell surface immunoreactivity. Under certain conditions a recovery from limited surface damage can be achieved, although, when cells are prepared under more extreme conditions irreversible surface damage occurs. The surface morphology of the cells as revealed by scanning electron microscopy, is also clearly affected by the conditions of cell isolation. The method has been used to define the conditions necessary for the isolation of cells to be used in the study of subtle biochemical responses.

Reconstituted high density lipoprotein behaviour is modified by the nature of the lipid vesicle

Apo HDL is a more potent inhibitor of lipoprotein lipase than native HDL. HDL reconstituted from apo HDL and vesicles made from either HDL total lipids or HDL phospholipids inhibits lipoprotein lipase in the same manner as native HDL. HDL reconstituted from apo HDL and vesicles made from phosphatidylcholine and sphingomyelin still behaves like apo HDL.