Cyclic depsipeptides are toxic compounds made by many microorganisms. A small number have been implicated in serious, even fatal, human and animal toxicity. It is understood that, because of inadequate testing and underreporting, the public health problem is more serious than it appears. The mechanism of action of cereulide, the depsipeptide most often associated with food poisoning, is unknown, but it is structurally similar to valinomycin, a depsipeptide potassium ionophore. Valinomycin, also a toxic microbial product, has been shown to induce apoptosis in a wide variety of cell lines. This project proposes to determine whether apoptosis is a general mechanism for depsipeptide toxicity. The role of the ionophoric property will be determined; it may be that apoptosis induction is independent of a depsipeptide's ability to bind the potassium ion. The apoptotic pathway induced by valinomycin will be studied, in view of preliminary evidence that it acts in a novel way that involves neither caspases 1 to 10, nor calpain. Similarly, upstream signaling by valinomycin, which seems to involve the activation of protein tyrosine kinase and phospholipase C activity, will be examined, so that a pathway of sequential or parallel events can be identified. The relevance of these findings to whole-animal toxicity will be tested in vivo, using inhibitors that are shown to block valinomycin-induced apoptosis in cell culture. The long-term goal of the project is to understand how these ubiquitous environmental toxins affect cells, so that better testing methods can be developed, a rational basis established for evaluating their public health impact, and specific interventions designed for use in cases of serious poisoning.