Regulation of mammalian cell growth and death by bacterial redox proteins: relevance to ecology and cancer therapy.

Recent evidence indicates that bacterial redox proteins such as cupredoxins and cytochromes, that are normally involved in electron transfer during respiration, can enter mammalian cells and induce either apoptosis or inhibition of cell cycle progression. Such proteins have also been shown to demonstrate a good deal of specificity for entry and induction of cytotoxic effects in cancer cells, allowing both in vitro cell death and in vivo inhibition of cancer progression. An alteration in the hydrophobicity of the bacterial redox proteins can lead to a switch from apoptosis to growth arrest and vice versa through modulation of the intracellular levels of tumor suppressors. The preferential entry and cytotoxicity of these redox proteins in cancer cells raises interesting questions about the presence of other bacterial proteins that may affect cell cycle at the G(2)/M phase, thereby potentially arresting cancer growth. The intracellular localization of the bacterial redox proteins in nonpathogenic soil bacteria similarly raises questions about their possible role in allowing various nonpathogenic soil bacteria to defend themselves from environmental predators by inducing cytotoxicity when engulfed in large numbers. A new role of the redox proteins in soil bacteria in maintaining an ecological balance among the predators and preys is proposed.