Modulation of the Binding and Endocytosis of Concanavalin A by Guinea Pig Keratinocytes: Reversible Antagonistic Effects of Cholesterol and Phospholipid-Liposomes

Abstract

Alteration of guinea pig keratinocyte membrane microviscosities (eta) by liposomes of varying composition was determined by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene. Measurements performed either with whole cell suspensions or Percoll-separated cell subpopulations, indicate a similar membrane microviscosity (eta = 3.37 poise +/- 10%) compared to those microviscosities reported for other cell types. Our findings show that treatment of guinea pig keratinocytes with liposomes composed of phospholipids results in a decreased membrane microviscosity (1.95 poise), whereas treatment of the cells with an emulsion of cholesterol hemisuccinate, or liposomes composed of cerebrosides, causes an increase in membrane microviscosity (3.85 poise and 5.55 poise +/- 10%, respectively). Changes in membrane fluidity had no significant effect on cell viability. A reduced membrane microviscosity resulted in a decrease in the binding of Concanavalin A to keratinocytes, whereas an increased microviscosity resulted in an increased binding of Concanavalin A. Furthermore, endocytosis of Concanavalin A bound to keratinocytes plasma membranes was not significantly affected by a reduced membrane microviscosity, whereas an increased membrane microviscosity completely blocked the endocytosis of Concanavalin A. Another novel observation was that membranes "fluidified" by phospholipid liposomes could be "rigidified" by treatment with cholesterol hemisuccinate and vice versa. Moreover, these alternate changes in membrane microviscosity resulted in simultaneous alternate changes in the binding of Concanavalin A to the keratinocyte surface.