Stimulation of phospholipid N-methylation by isoproterenol in rat hearts

Phosphatidylethanolamine N-methyltransferase: from Functions to Diseases

Locating on endoplasmic reticulum and mitochondria associated membrane, Phosphatidylethanolamine N-methyltransferase (PEMT), catalyzes phosphatidylethanolamine methylation to phosphatidylcholine. As the only endogenous pathway for choline biosynthesis in mammals, the dysregulation of PEMT can lead to imbalance of phospholipid metabolism. Dysregulation of phospholipid metabolism in the liver or heart can lead to deposition of toxic lipid species that adversely result in dysfunction of hepatocyte/cardiomyocyte. Studies have shown that PEMT^-/- mice increased susceptibility of diet-induced fatty liver and steatohepatitis. However, knockout of PEMT protects against diet-induced atherosclerosis, diet-induced obesity, and insulin resistance. Thus, novel insights to the function of PEMT in various organs should be summarized. Here, we reviewed the structural and functional properties of PEMT, highlighting its role in the pathogenesis of obesity, liver diseases, cardiovascular diseases, and other conditions.

Homocysteine as a risk factor for atherosclerosis: is its conversion to s-adenosyl-L-homocysteine the key to deregulated lipid metabolism?

Homocysteine (Hcy) has been recognized for the past five decades as a risk factor for atherosclerosis. However, the role of Hcy in the pathological changes associated with atherosclerosis as well as the pathological mechanisms triggered by Hcy accumulation is poorly understood. Due to the reversal of the physiological direction of the reaction catalyzed by S-adenosyl-L-homocysteine hydrolase Hcy accumulation leads to the synthesis of S-adenosyl-L-homocysteine (AdoHcy). AdoHcy is a strong product inhibitor of S-adenosyl-L-methionine (AdoMet)-dependent methyltransferases, and to date more than 50 AdoMet-dependent methyltransferases that methylate a broad spectrum of cellular compounds including nucleic acids, proteins and lipids have been identified. Phospholipid methylation is the major consumer of AdoMet, both in mammals and in yeast. AdoHcy accumulation induced either by Hcy supplementation or due to S-adenosyl-L-homocysteine hydrolase deficiency results in inhibition of phospholipid methylation in yeast. Moreover, yeast cells accumulating AdoHcy also massively accumulate triacylglycerols (TAG). Similarly, Hcy supplementation was shown to lead to increased TAG and sterol synthesis as well as to the induction of the unfolded protein response (UPR) in mammalian cells. In this review a model of deregulation of lipid metabolism in response to accumulation of AdoHcy in Hcy-associated pathology is proposed.

Effect of beta-Adrenoceptor Antagonists on Phospholipid N-Methylation Activities of Cardiac Sarcolemma

BACKGROUND: Some beta-adrenoceptor antagonists exert a negative inotropic action by affecting Ca(2+) fluxes in the myocardial cell as a consequence of their interaction with sarcolemmal and sarcoplasmic reticular membranes. This action may be caused by their effects on the chemicophysical properties of membranes phospholipids. Because phosphatidylethanolamine (PE) N-methylation can influence the chemicophysical properties of membranes, these agents may affect PE N-methylation. This study was undertaken to examine the effects of propranolol, acebutolol, and atenolol on PE-N-methylation in rat heart sarcolemma (SL). METHODS AND RESULTS: Sarcolemmal membrane was isolated from rat hearts by the hypotonic shock LiBr method. Incorporation of radiolabeled methyl groups from S-adenosyl-l-methionine was assayed at three catalytic sites involved in the PE N-methylation reaction in the presence and absence of these drugs. A biphasic effect of propranolol at site I was noted; low concentrations (10(-8) M) were inhibitor. Acebutolol (10(-9)-10(-3) M) depressed methyl group incorporation in SL at site II in a dose-dependent manner, whereas atenolol showed no effect. Propranolol also exerted a biphasic effect on sarcoplasmic reticular (SR) methylation at site I, whereas acebutolol depressed the SR enzyme activity at site II and atenolol had no effect. The mitochondrial methyltransferase activities at sites I, II, and III were unaltered by any of these drugs. CONCLUSIONS: It is suggested that propranolol and acebutolol alter SL and SR PE N-methyltransferase activity at site I and site II, respectively, either by affecting the enzyme directly or by changing the physiochemical properties of the membrane.

Interactions between cyclic AMP-dependent protein phosphorylation and lipid transmethylation reactions in isolated porcine cardiac sarcolemma

Premethylation of purified porcine cardiac sarcolemma (SL) in the presence of 0.15, 10 and 150 microM S-adenosyl-L-methionine (AdoMet) did not change the phosphorylation of SL proteins catalyzed either by intrinsic cyclic AMP-dependent protein kinase (cAK) or by added catalytic (C) subunit of this enzyme. On the other hand, membrane exhibited increased lipid methyltransferase activity after preincubation with MgATP and C subunit. Prephosphorylation of membranes stimulated the total [3H]-methyl incorporation into SL lipids assayed at 0.15 microM [3H]AdoMet due to an enhancement of Vmax and without changes in the Km value for AdoMet. Analysis of the methylated lipid products revealed an increased methyl group incorporation into a nonpolar lipid fraction whereas phosphatidylethanolamine-N-methylation was not affected by phosphorylation. The results suggest that the cyclic AMP-mediated signal transduction at the level of cardiac SL is not affected by methylation-induced modifications of the membrane lipid microdomains. On the other hand, an intrinsic SL lipid methyltransferase activity is apparently not related to the N-methylation of phospholipids, is modulated by cyclic AMP-dependent protein phosphorylation.