Plasma membrane fluidity affects transient immobilization of oxidized phospholipids in endocytotic sites for subsequent uptake

Abstract

Oxidized phospholipids in serum initiate severe pathophysiological responses during the process of atherogenesis. On the cellular level it is known that these lipids induce apoptosis; however, the uptake mechanism remains enigmatic. We investigated here the behavior of the fluorescent oxidized phospholipid 1-palmitoyl-2-glutaroyl-sn-glycero-3-phospho-N-Alexa647- ethanolamine (PGPE-Alexa647) in the plasma membrane of various cell lines. The probe was taken up by the cells unspecifically via caveolae or clathrin-coated pits. Interestingly, we found the uptake to be facilitated by the overexpression of the scavenger receptor class B type I. Ultra-sensitive microscopy allowed us to follow the uptake process at the single molecule level; we observed rapid diffusion of PGPE-Alexa647 in the plasma membrane, interrupted by transient halts with duration of ∼0.9 s at endocytotic sites. Scavenger receptor class B type I overexpression yielded a pronounced increase in the single molecule mobility, and in consequence an increased frequency of immobilization. Alternatively, the plasma membrane fluidity could also be increased by treating cells with high levels of the unlabeled oxidized phospholipid 1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphocholine; also in this case, the immobilization frequency of PGPE-Alexa647 was concomitantly increased. The data demonstrate the relevance of plasma membrane properties for uptake of oxidized phospholipids, and indicate a novel indirect mechanism for the control of endocytosis. © 2009 by The American Society for Biochemistry and Molecular Biology, Inc.