The experimental manipulation of cholesterol

Experimentally, the cholesterol content of cell surface membranes may be modified in vivo e.g. in the guinea pig by dietary means [131] or by modification of the culture media of micro-organisms [104]. In vitro, cholesterol modulation has also been achieved by the use of inhibitors of sterol synthesis [95] or liposomes [97,103,132-134]. 

Pagano and Weinstein [135] suggest that there are 4 possible types of liposome-cell interaction, namely stable adsorption, fusion, endocytosis and hpid transfer. These can all occur depending upon the experimental conditions and the hpid used for the construction of the hposomes. 

A number of studies have also attempted to measure the transmembrane movement, or flip-flop , of cholesterol. Poznansky and Lange [142] and Lenard and Rothman [143] have shown that in sonicated dipalmitoylglycerophospho-choline-cholesterol liposomes and in influenza virus membranes, respectively, the half-time for flip-flop of cholesterol was in excess of 6 days, if it occurred at all. However, other workers have reported rapid transmembrane and intermembrane movement of cholesterol in small unilamellar liposomes [144-147]. 

It is concluded that under equihbrium conditions Upids do not easily undergo transbilayer movement in liposomes or membranes [149-151]. On the other hand, phospholipids and cholesterol have been shown to undergo transmembrane movement in erythrocytes under non-equilibrium conditions or following membrane perturbation such as ghost formation or phospholipase treatment [152-154]. More recently Lange and co-workers have made studies of the rate of transmembrane movement of cholesterol in the membranes of human erythrocytes. Normally, cholesterol in intact erythrocytes is not accessible to cholesterol oxidase [155]. 

However, foUowing enrichment of the cells with exogenous cholesterol or incubation with 0.1 mM chlorpromazine [156], the whole of the cholesterol pool becomes available to the cholesterol oxidase. This means that the time-course of cholesterol oxidation can be determined and hence the rate of flip-flop. Based on such studies, a half-time of less than 3 sec at 37°C has been found for transposition of cholesterol across the bilayers [157]. These studies have also been extended to probe features of lipid-cholesterol organisation in the human erythrocyte membrane [158]. Clearly, such studies are relevant to our understanding of the mechanisms of cholesterol loss from cells in vivo by the methods outhned earlier. 

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