Interaction of lipoproteins with type II pneumocytes in vitro: morphological studies, uptake kinetics and secretion rate of cholesterol.

Apart from dipalmitoyl phosphatidylcholine, cholesterol is the most abundant surfactant lipid. About 90 to 99% of cholesterol of the alveolar surfactant is derived from serum lipoproteins. The aim of this study was to identify the lipoprotein which preferentially supplements type II pneumocytes with cholesterol destined for surfactant production. Ultrastructural investigations revealed that type II pneumocytes bind and take up HDL, LDL and VLDL. Binding and uptake of VLDL occurred even in the presence of excess LDL indicating that, besides LDL receptors, type II pneumocytes express additional binding sites for VLDL. Type II pneumocytes in primary culture are able to take up cholesterol added in the form of HDL, LDL and VLDL. Cholesterol uptake was lowest from HDL and highest from VLDL. The maximal velocity of cholesterol uptake from VLDL was more than three times that of cholesterol uptake from LDL. The half-maximal saturation of cholesterol uptake from VLDL was nearly half that of LDL. From these kinetic data and the distribution of free cholesterol among the serum lipoproteins, we calculated that the cholesterol uptake from VLDL is more than three times that of cholesterol uptake from LDL. In double-labeling experiments type II pneumocytes secreted palmitic acid-labeled phospholipids together with labeled free cholesterol taken up from lipoproteins. The secretion rates of both phospholipids and free cholesterol were stimulated to nearly the same extent by isoproterenol. From our results we conclude that type II pneumocytes interact specifically with HDL, LDL and VLDL. Cholesterol taken up in the form of the individual lipoproteins shows no difference in its availability for the formation of cholesterol ester and surfactant by type II pneumocytes in vitro. Based on the kinetic studies, it appears that VLDL is the major gateway through which cholesterol is provided to satisfy the cholesterol requirements of type II pneumocytes for the synthesis of surfactant.