Phospholipid methylation in skeletal muscle membranes

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.

Inhibition of methyltransferase reduces the turnover of acetylcholine receptors

Because of the putative rule of phospholipid methyltransferase reactions in many important membrane and receptor translocation processes, we studied the effect of methyltransferase inhibitors on acetylcholine receptor (AcChoR) turnover in cultured rat skeletal muscle. Inhibition of methyltransferase significantly reduced the normal rate of degradation of AcChoRs, a process that involves endocytosis. Further, under conditions that greatly accelerate the rate of degradation of AcChoRs--i.e., by addition of anti-AcChoR antibody--methyltransferase inhibitors again significantly reduced receptor turnover. AcChoR synthesis was unaffected. Thus, the net effect of this treatment was slowing of the antibody-induced loss of surface AcChoRs. That this drug effect was mediated specifically by inhibition of methylation reactions was suggested by certain additional pharmacologic features: partial reversibility of the effect by methionine, enhancement by homocysteine, and correspondence with marked inhibition of phospholipid methylation. The substrate specificity of the methyltransferase inhibitors capable of reducing AcChoR degradation suggests that phospholipid methylation reactions may be most relevant. Methyltransferase inhibitor drugs may provide a therapeutic strategy in receptor disorders such as myasthenia gravis, in which accelerated receptor endocytosis plays a major role.

Lipid-protein interactions and the function of the Ca2+-ATPase of sarcoplasmic reticulum

Regardless of the nature of the protein constituents of membranes, the molecular arrangement of lipids interacting with them must satisfy hydrophobic, ionic, and steric requirements. Biological membranes have a great diversity of lipid constituents, and this diversity might have functional roles. It has been proposed, for example, that the hydrophobic regions of membrane proteins are stabilized in the membrane through interactions with lipids able to adopt configurations other than the bilayer structure. Progress in understanding at the molecular level how lipid-protein interactions control the properties of membrane proteins has been hindered by the lack of information concerning the structure of the hydrophobic regions of membrane proteins. Nevertheless, there are many examples in the literature describing how changes in the lipid environment affect physical and biochemical properties of membrane proteins. From these studies, discussed in this review, an overall picture of how lipids and proteins interact in membranes is beginning to emerge.