Ionomycin causes susceptibility to phospholipase A2 while temperature-induced increases in membrane fluidity fail: Possible involvement of actin fragmentation

Abstract

A diminution in the order of membrane lipids, which occurs during apoptosis, has been shown to correlate with increased membrane susceptibility to hydrolysis by secretory phospholipase A2. Studies with artificial membranes, however, have demonstrated that the relationship between membrane order and hydrolysis is more complex than suggested thus far by cell studies. To better resolve this relationship, this study focused on comparisons between increasing temperature and calcium ionophore as means of decreasing membrane order in S49 cells. Although these two treatments caused comparable changes in apparent membrane order as detected by steady-state fluorescence measurements, only ionophore treatment enhanced phospholipase activity. Experiments with exogenously-added phosphatidylserine indicated that the difference was not due to the presence of that anionic phospholipid in the outer membrane leaflet. Instead, analysis of the equilibration kinetics of various cationic membrane probes revealed that the difference could relate to the spacing of membrane lipids. Specifically, ionophore treatment increased that spacing while temperature only affected overall membrane order and fluidity. To consider the possibility that the distinction with ionophore might relate to the actin cytoskeleton, cells were stained with phalloidin and imaged via confocal microscopy. Ionophore caused disruption of actin fibers while increased temperature did not. This apparent connection between membrane hydrolysis and the cytoskeleton was further corroborated by examining the relationship among these events during apoptosis stimulated by thapsigargin.

High temperature affects the phagocytic activity of human peripheral blood mononuclear cells

“…It has been shown that a decrease in temperature of growing bacteria causes incorporation of more unsaturated fatty acids in their membranes followed by remodeling and change in cell fluidity [14]. a dependence of cell membrane fluidity on surrounding temperature expressed by changes in membrane lipid composition has been reported in other studies [15,16]. Increased cell membrane fluidity with subsequent enhanced phagocytosis may explain the promoting engulfment of latex particles by PBMC incubated with hydrophobic statins [12].…”

Annexin-A5 organized in 2D-network at the plasmalemma eases human trophoblast fusion

“…This ionophore disrupts cell-cell adhesion, cleaves E-Cadherin, modifies biochemical characteristics of lipids membrane ( i.e. fluidity and distribution) and enhances PS externalisation, which might interfere with our model of cell fusion 27 28 . Thus, the mammalian expression vector encoding for E-Cadherin fused to GFP (E-Cadherin-GFP) was introduced in primary human trophoblasts depleted or not of endogenous AnxA5 by siRNA transfection and rescued with either mCherry-AnxA5* or mCherry-AnxA5*-2Dmut ( Fig.…”

“…small unilamellar vesciles or planar supported lipid bilayers). Artificial membranes lack physiological processes that control biochemical properties of the membrane and observations performed with these models might differ from physiological conditions 27 . Interestingly, intracellular Ca 2+ concentration in subplasmalemma microdomains fits with the lowest concentration of Ca 2+ that have been described to promote AnxA5 binding to PS and 2D-network assembly 18 34 35 .…”