Susceptibility to apoptosis is restored in human leukemia HCW-2 cells following induction and stabilization of the apoptotic effector Bak

Apoptosis-inducing human-origin Fcepsilon-Bak chimeric proteins for targeted elimination of mast cells and basophils: a new approach for allergy treatment

During the past few years, many chimeric proteins have been developed to specifically target and kill cells expressing specific surface molecules. Generally these molecules carry a bacterial or plant toxin to destroy the unwanted cells. The major obstacle regarding these molecules in their clinical application is the immunogenicity and nonspecific toxicity associated with bacterial or plant toxins. We lately reported a new approach for construction of chimeric proteins: we successfully replaced bacterial or plant toxins with human apoptosis-inducing proteins. The resulting chimeras were shown to specifically induce apoptosis in the target cells. Taking advantage of the human apoptosis inducing proteins Bak and Bax as novel killing components, we have now constructed new chimeric proteins targeted against the human FcepsilonRI, expressed mainly on mast cells and basophils. These cells are the main effectors of the allergic response. Treatment of the target cells with the new chimeric proteins, termed Fcepsilon-Bak/Bax, had a dramatic effect on cell survival, causing apoptosis. The effect was specific to cells expressing the FcepsilonRI of both human and, very unexpectedly, also of mouse origin. Moreover, interaction of the chimeric proteins with the mast cells did not cause degranulation. Fcepsilon-Bak/Bax are new chimeric proteins of human origin and, as such, are expected to be both less immunogenic and less toxic and, thus, may be specific and efficient reagents for the treatment of allergic diseases.

Bax and Bak coalesce into novel mitochondria-associated clusters during apoptosis

Bax is a member of the Bcl-2 family of proteins known to regulate mitochondria-dependent programmed cell death. Early in apoptosis, Bax translocates from the cytosol to the mitochondrial membrane. We have identified by confocal and electron microscopy a novel step in the Bax proapoptotic mechanism immediately subsequent to mitochondrial translocation. Bax leaves the mitochondrial membranes and coalesces into large clusters containing thousands of Bax molecules that remain adjacent to mitochondria. Bak, a close homologue of Bax, colocalizes in these apoptotic clusters in contrast to other family members, Bid and Bad, which circumscribe the outer mitochondrial membrane throughout cell death progression. We found the formation of Bax and Bak apoptotic clusters to be caspase independent and inhibited completely and specifically by Bcl-X(L), correlating cluster formation with cytotoxic activity. Our results reveal the importance of a novel structure formed by certain Bcl-2 family members during the process of cell death.