Cytosolic Acidification Is the First Transduction Signal of Lactoferrin-induced Regulated Cell Death Pathway.

In yeast, we reported the critical role of K-efflux for the progress of the regulated cell death (RCD) induced by human lactoferrin (hLf), an antimicrobial protein of the innate immune system that blocks Pma1p H-ATPase. In the present study, the K channel Tok1p was identified as the K channel-mediating K-efflux, as indicated by the protective effect of extracellular K (30 mM), K-channel blockers, and the greater hLf-resistance of -disrupted strains. K-depletion was necessary but not sufficient to induce RCD as inferred from the effects of valinomycin, NHCl or nigericin which released a percentage of K similar to that released by lactoferrin without affecting cell viability. Cytosolic pH of hLf-treated cells decreased transiently (0.3 pH units) and its inhibition prevented the RCD process, indicating that cytosolic acidification was a necessary and sufficient triggering signal. The blocking effect of lactoferrin on Pma1p H-ATPase caused a transitory decrease of cytosolic pH, and the subsequent membrane depolarization activated the voltage-gated K channel, Tok1p, allowing an electrogenic K-efflux. These ionic events, cytosolic accumulation of H followed by K-efflux, constituted the initiating signals of this mitochondria-mediated cell death. These findings suggest, for the first time, the existence of an ionic signaling pathway in RCD.