[Membrane cholesterol and insulin receptor in erythrocytes]

The author studied influence of membrane cholesterol on insulin receptor behavior using human erythrocytes after modification of their membrane cholesterol contents. Insulin binding to the erythrocytes containing higher membrane cholesterol which were obtained from alcoholic liver cirrhosis (ALC) and familial lecithin: cholesterol acyl transferase (LCAT) deficiency patients was also examined. 1) Influence of modified membrane cholesterol contents on insulin binding to erythrocytes (in vitro experiments): The cholesterol content of human erythrocyte membranes was modified by incubating the cells with phosphatidylcholine/cholesterol and phosphatidylcholine vesicles. Then insulin binding to the erythrocytes was measured as a function of membrane cholesterol content. Low membrane cholesterol decreased insulin binding at 10 degrees C and 15 degrees C, but increased insulin binding at 25 degrees C and 37 degrees C. Scatchard plot analysis showed that low membrane cholesterol decreased a number of the receptor without alteration of affinity. Phosphatidylcholine spin probe showed a dual effect of cholesterol on membrane fluidity, fluidizing below but rigidifying membranes above 20 degrees C. It is suggested that membrane cholesterol affects insulin receptor behavior through alteration of membrane fluidity, depending on the phase state of the membranes. 2) Insulin binding and membrane lipid composition of erythrocytes from ALC and LCAT deficiency patients: The erythrocytes from both ALC and LCAT deficiency patients are known to have higher content of membrane cholesterol. Insulin binding and membrane lipid composition of erythrocytes in both diseases were measured to see the influence of cholesterol on insulin binding in sites. a) Erythrocytes from ALC patients: In the erythrocytes from ALC patients, cholesterol to phospholipid molar ratio was significantly increased as compared with normal subjects. In phospholipid acyl chains, contents of polyunsaturated fatty acid significantly decreased in ALC patients. Insulin binding to erythrocytes was significantly increased in the patients of ALC. Scatchard plot analysis (at 15 degrees C) showed significant increase of the receptor number in erythrocytes from the ALC patients as compared to the normal subjects, while the affinity did not differ between the groups. The increased cholesterol content in the erythrocyte membranes was suggested to be responsible for the increased receptor number and insulin binding to the erythrocytes from ALC patients. b) Erythrocytes from LCAT deficiency patients: Membrane cholesterol to phospholipid molar ratio was increased in two patients of LCAT deficiency studied. In phospholipid classes, increase of phosphatidylcholine and decreases of both phosphatidylethanolamine and sphingomyelin were observed in the erythrocytes from both patients.(ABSTRACT TRUNCATED AT 400 WORDS)