Inhibition of purified bovine milk lipoprotein lipase by propranolol and other beta-adrenergic blockers in vitro

Genetic variant showing a positive interaction with beta-blocking agents with a beneficial influence on lipoprotein lipase activity, HDL cholesterol, and triglyceride levels in coronary artery disease patients. The Ser447-stop substitution in the lipoprotein lipase gene. REGRESS Study Group

BACKGROUND

Lipoprotein lipase (LPL) is the rate-limiting enzyme in the lipolysis of triglyceride-rich lipoproteins, and the gene coding for LPL is therefore a candidate gene in atherogenesis. We previously demonstrated that two amino acid substitutions in LPL, the Asn291-Ser and the Asp9-Asn, are associated with elevated triglycerides and lower HDL cholesterol and are present with greater frequency in coronary artery disease (CAD) patients than in normolipidemic control subjects. Conversely, a third frequent mutation in this gene, the Ser447-Stop, is reported by some investigators to underlie higher HDL cholesterol levels and would represent a beneficial genetic variant in lipoprotein metabolism. We therefore sought conclusive evidence for these allegations by investigating the effects of the LPL Ser447-Stop mutation on LPL and hepatic lipase (HL) activity, HDL cholesterol, and triglycerides in a large group of CAD patients (n = 820) with normal to mildly elevated total and LDL cholesterol levels.

METHODS AND RESULTS

Carriers of the Ser447-Stop allele (heterozygotes and homozygotes) had significantly higher postheparin LPL activity (P = .034), normal postheparin HL activity (P = .453), higher HDL cholesterol levels (P = .013), and lower triglyceride levels (P = .044) than noncarriers. The influence of the Ser447-Stop allele on LPL activity was pronounced in patients using beta-blockers (P = .042) and not significant in those not using them (P = .881), suggesting a gene-environment interaction between the Ser447-Stop mutation and beta-blockers.

CONCLUSIONS

We conclude that the LPL Ser447-Stop mutation has a significant positive effect on LPL activity and HDL cholesterol and triglyceride levels and that certain subgroups of CAD patients carrying the Ser447-Stop mutation will have less adverse metabolic effects when placed on beta-blockers. The LPL Ser447-Stop mutation therefore should have a protective effect against the development of atherosclerosis and subsequent CAD.

Effects of adrenoreceptor antagonists and agonists on clearance of emulsion models of triacylglycerol-rich lipoproteins from plasma in rats

1. We previously found that adrenaline and noradrenaline exert essentially opposite effects on clearance from plasma of chylomicron-like emulsions injected intravenously in rats, suggesting mechanisms that may be implicated in the atherogenic effects of chronic stress and hypertension and conversely in the protective effect of regular exercise. 2. The mechanisms underlying the effects of adrenaline and noradrenaline have now been investigated. Chronic adrenergic blockade with either the alpha 1-receptor antagonist doxazosin or the beta-receptor antagonist propranolol slowed the clearance of labelled emulsion lipids from plasma of normal Wistar rats. The results with doxazosin were unexpected in view of its capacity to decrease plasma triglycerides in patients. 3. In spontaneously hypertensive rats (SHR) the clearance of triolein (TO) was very slow compared with normal Wistar rats. Emulsion TO clearance provides a measure of lipolysis by lipoprotein lipase, and a defect in clearance indicates either defective enzyme action or poor perfusion of capillary beds rich in enzyme. Defective enzyme activity in SHR was excluded, suggesting redistribution of blood flow away from skeletal muscle and adipose tissue. In SHR the TO clearance from injected chylomicron-like emulsions was improved by blockade with doxazosin compared with control untreated SHR. 4. The beta 2-adrenoreceptor agonist Fenoterol was infused intravenously during clearance of an injected lipid emulsion. Clearance of radiolabelled cholesteryl oleate (CO) was clearly slowed while there was a lesser reduction of TO clearance rate. Emulsion CO clearance provides a measure of the uptake of lipoprotein remnants by the liver, and a defect in clearance of CO indicates either defective ligand (apolipoprotein E)-receptor interaction or decreased perfusion of the splanchnic bed. Isoprenaline, a non-selective beta-adrenergic agonist, gave similar results. Both compounds reduced mean arterial pressure by about 20-40 mm Hg at the doses employed, indicating that the beta 1 (cardiac) effect of the isoprenaline was insufficient to offset its vasodilatatory effect on skeletal muscle arterioles (beta 2). 5. The alpha-agonist phenylephrine, at a dose which moderately raised mean arterial pressure, slowed clearance of both TO and CO for the first 12 min after injection of emulsion but at later time points clearances caught up with the controls. 6. Administration of a mixture of isoprenaline and phenylephrine produced definite enhancement of both TO clearance and CO clearance. The effect of the mixture was opposite to the effects of of either agonist alone, demonstrating clearly that direct effects on lipoprotein lipase activity or receptor mediated processes were not involved.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of purified human postheparin lipoprotein lipase by beta-adrenergic blockers in vitro

We examined the effects of five beta-adrenergic blockers on the hydrolysis of phosphatidylcholine-stabilized triolein particles by purified human postheparin lipoprotein lipase (PHLpL) in order to evaluate the possible role of direct inhibition as a mechanism of drug-induced hypertriglyceridemia. The relative inhibitory potencies were observed in the following order: propranolol much greater than pindolol greater than metoprolol greater than atenolol greater than nadolol. There was a positive correlation between the octanol/water partition coefficients of these agents and their inhibition of lipoprotein lipase, suggesting that hydrophobicity may be one of the major determinants for PHLpL inhibition. The amount of the beta-adrenergic blockers required to produce 50% inhibition of human PHLpL was much greater than that required to inhibit purified bovine lipoprotein lipase.

Propranolol inhibition of the neutral phospholipases A of rat heart mitochondria, sarcoplasmic reticulum and cytosol

Membrane damage caused by phospholipase A action is thought to be an important factor in ischemic myocardial injury. Propranolol has been shown previously to have beneficial effects in both animal experiments and clinical trials, and it has membrane-stabilizing properties in vitro. To investigate the possibility that these effects might be due, in part, to effects on phospholipases, we determined the effects of propranolol on rat heart phospholipases A at physiological pH using small unilamellar liposomes of di[1-14C]oleoylphosphatidylcholine as substrate. Propranolol inhibited heart phospholipases A in vitro. The concentration required to give 50% inhibition was 0.2 mM for the mitochondrial and cytosolic phospholipases A and 2.9 mM for sarcoplasmic reticulum phospholipase A. The binding of [4-3H]propranolol to fresh membrane preparations was studied using an ultracentrifugation method. Propranolol bound readily to both membrane fractions in vitro with no significant difference in the saturation number (0.20 to 0.28 mol drug per mol phospholipid) but the association constant, KA, was higher for mitochondrial membranes (3760 +/- 350) than for the sarcoplasmic reticulum (2190 +/- 390). Our results show that propranolol inhibited heart phospholipases A in vitro at physiological pH. The mitochondrial and cytosolic phospholipases A were more susceptible to inhibition than the phospholipase A of sarcoplasmic reticulum. Propranolol bound to mitochondria and sarcoplasmic reticulum in vitro, suggesting the possibility that propranolol binding to heart membranes in vivo could result in drug concentrations in these membranes high enough to inhibit phospholipase A. This could represent an additional mechanism by which propranolol exerts beneficial effects in myocardial ischemia.