An investigation into mechanisms of non-ionic surfactant effect on epithelial cells

Amphipathic, non-ionic surfactants are widely used in pharmaceutical and food industries to enhance product features; as pharmaceutical excipients they achieve increased cell membrane permeability and consequently can improve the oral absorption of drugs across the intestinal epithelial barrier. The use of non-ionic surfactants has grown rapidly, and is predicted to increase, however, the mechanism(s) surrounding the induction of surfactant toxicity is not well established and, consequently, the potential risks of surfactant exposure are not well understood. This work studies the concentration- and time-dependent succession of events that occur during and following exposure of an intestinal epithelial cell model to a ‘typical’ non-ionic surfactant – Solutol HS15. The resulted gathered demonstrate that prior to a significant increase in membrane permeability to a model drug (FITC-dextran 4kDa), non-ionic surfactant, at concentrations above its critical micellar concentration (CMC), produced almost immediate redox and mitochondrial effects manifested as an increased NADH pool, increased ROS levels, and hyperpolarisation of the mitochondrial membrane potential. Apoptosis was triggered early in this initial phase, and relied on mitochondrial hyperpolarisation as a crucial step leading to subsequent depolarisation and caspase-3/7 activation. Inhibition of mitochondrial hyperpolarisation prolonged cell survival, delayed the onset of metabolic reduction by the mitochondrial, and inhibited caspase activation. The apoptotic cell death pathway appears to be triggered prior to the emergence of substantial membrane damage by the surfactant: loss of plasma membrane integrity, nuclear membrane permeabilisation, and perturbations in calcium homeostasis - indicators of a necrotic process. It is proposed that the rapid cellular response is triggered via rapid surfactant-induced increases in plasma membrane fluidity; a phenomenon akin to the membrane-regulated stress response following membrane fluidisation by heat shock, and consequently cell death events. Furthermore, work performed on differentiated Caco-2 monolayers, alongside culture models replicating the basement membrane and paracrine signalling, demonstrate surfactant toxicity is reduced. Toxicity in vivo is therefore predicted to be less than measured on the standard model.