In 16HBE14o- or in A549 cells, however, there is no overlap of alpha-toxin and Rab7 made up of vesicles (Determine 2A and Determine 4A) which indicates that toxin heptamers are either not internalized at all or that internalization and processing occurs via other pathways than the regular endosomal/lysosomal pathway. airway epithelial cells [10,11,12] which facilitates attachment of the toxin to the apical surface of the epithelial cells. Lateral movements of the toxin to plasma membrane areas of high sphingomyelin and phosphatidylcholine contents [13, 14] seems to favor multimerization of the toxin molecules and formation of heptameric non-lytic pre-pores [15]. Once the pre-pore is usually formed, Mequitazine each of the seven monomers rolls out a domain name comprised of two beta-sheets that penetrates the plasma membrane. These seven stem domains jointly form a transmembrane beta-barrel channel connecting the host cell cytosol to the extracellular space [15,16]. The alpha-toxin pore is usually permeable for ions like Na+, K+ or Ca2+ [9,17,18,19] and also for small organic molecules like ATP [20]. In airway epithelial cells, this induces alterations in membrane potential, cytosolic ion concentrations, cell signaling, actin cytoskeleton architecture, in losses of cell-cell and cell-matrix contacts, and, Mequitazine ultimately, in the formation of paracellular gaps in the epithelial cell layer [21,22,23]. In vivo, such effects of pore-formation would disrupt the barrier function of the respiratory epithelium [24]. Epithelial cell types exposed to alpha-toxin display largely different sensitivities toward the toxin. The immortalized human airway epithelial cell collection S9 copes well even when in contact with high concentrations of alpha-toxin (2000 ng/mL) while other immortalized cells (16HBE14o-) Mequitazine or lung malignancy cells (A549) are massively damaged at this concentration [22,25]. In recent studies, it was shown that this can be, at least in part, attributed to the different expression levels of ADAM10 [10,11,12] as well as the differences in sphingomyelin abundances in these cell types [12,14] which makes monomer binding and pre-pore formation more efficient in 16HBE14o- or A549 cells compared with S9 cells [12]. Different sensitivities of cells to pore-forming toxins, however, may also be affected by the different abilities of cells to internalize and degrade or dispose of toxin pores. Endocytosis of pore-containing plasma membrane areas in different types of cultured cells has been previously shown [26]. Especially in human transformed keratinocytes (HaCaT cells), release of pore-containing exosomes has been implicated as a potential pathway for disposal of toxin heptamers while lysosomal degradation was not observed in these cells [26]. The aim of this study was to elucidate whether the observed differences Mequitazine in toxin sensitivities in the airway epithelial cell types may also be attributable to different rates Synpo of intracellular processing of pore-containing plasma membrane material, or disposal of such material by release of toxin-containing vesicles to the extracellular space. 2. Results 2.1. Loss of Heptamers from Airway Epithelial Cells Transiently Exposed to Alpha-Toxin Pulse-chase experiments were performed by exposing 16HBE14o-, S9 or A549 cells, respectively, to 2000 (S9) or 500 ng/mL (16HBE14o- and A549) of recombinant alpha-toxin monomers (rHla) for 90 min and further cultivation of the cells for different periods up to 8 h. The residual amounts of toxin heptamers in plasma membrane preparations of these cells were determined by semi-quantitative Western blotting. The amounts of toxin heptamers remained more or less constant over the 8 h period following toxin exposure in plasma membrane extracts of 16HBE14o- cells (Physique 1A) or even showed a tendency to increase over time in A549 cells (Physique 1C). However, toxin heptamers were lost from cellular membranes in S9 cells following termination of toxin exposure (Physique 1B). These data indicated that this kinetics of toxin heptamer processing were different in these cell types. Open in a separate window Physique 1 Large quantity of residual rHla heptamers in plasma membranes of 16HBE14o-, S9, or A549 cells upon rHla-pulse treatment. Due to their different sensitivities against the toxin,16HBE14o- (A) were treated with 500 ng/mL rHla, S9 cells (B) with 2000 ng/mL rHla and A549 cells (C) with 500 ng/mL rHla for 90 min and then incubated for 0, 1, 2, 4 and 8 h in new toxin-free cell culture medium. Plasma membrane preparations.