A folding variant of human alpha-lactalbumin induces mitochondrial permeability transition in isolated mitochondria

A human milk fraction containing multimeric alpha -lactalbumin (MAL) is abl e to kill cells via apoptosis. MAL is a protein complex of a folding varian t of alpha -lactalbumin and lipids. Previous results have shown that upon t reatment of transformed cells, MAL localizes to the mitochondria and cytoch rome c is released into the cytosol. This is followed by activation of the caspase cascade. In this study, we further investigated the involvement of mitochondria in apoptosis induced by the folding variant of alpha -lactalbu min. Addition of MAL to isolated rat liver mitochondria induced a loss of t he mitochondrial membrane potential (Delta psi (m)), mitochondrial swelling and the release of cytochrome c. These changes were Ca2+-dependent and wer e prevented by cyclosporin A, an inhibitor of mitochondrial permeability tr ansition. MAL also increased the rate of state 4 respiration in isolated mi tochondria by exerting an uncoupling effect. This effect was due to the pre sence of fatty acids in the MAL complex because it was abolished completely by BSA. BSA delayed, but failed to prevent, mitochondrial swelling as well as dissipation of Delta psi (m), indicating that the fatty acid content of MAL facilitated, rather than caused, these effects. Similar results were o btained with HAMLET (human alpha -lactalbumin made lethal to tumour cells), which is native alpha -lactalbumin converted in vitro to the apoptosis-ind ucing folding variant of the protein in complex with oleic acid. Our findin gs demonstrate that a folding variant of alpha -lactalbumin induces mitocho ndrial permeability transition with subsequent cytochrome c release, which in transformed cells may lead to activation of the caspase cascade and apop totic death.