APOPTOSIS A Bit of Anoikis

Many cells undergo apoptosis (programmed cell death) after loss of attachment to the extracellular matrix. This form of apoptosis, called anoikis, is suppressed by integrin-mediated signaling pathways. One of these pathways involves enhanced transcription of the antiapoptotic protein Bcl-2. Jan et al. used a portion of integrin 5ß1 to disable integrin signaling and screened a cDNA library for proteins that affected Bcl-2 expression. They identified Bit1 (Bcl-2 inhibitor of transcription), a protein that was unrelated to any other protein in the human or mouse genomes. The authors used a combination of immunostaining, fluorescent labeling, and Western analysis to show that Bit1 was localized in the mitochondria of HeLa cells. Bit1 was released from the mitochondria of cells cultured under apoptosis-promoting conditions. Furthermore, overexpression of Bit1 produced apoptosis, with apoptosis occurring at lower levels of expression if the Bit1 was mutated to prevent mitochondrial targeting. Bit1 associated with the Groucho-related protein AES and transfection experiments in cell lines that did or did not express one or the other of the two proteins, which indicated that the two proteins interacted with each other to promote apoptosis. Bit1-mediated apoptosis was suppressed by cell attachment to fibronectin through integrin 5ß1; moreover, increased expression of Bit1 enhanced sensitivity to anoikis, whereas knockdown decreased sensitivity to anoikis. Bit1-mediated apoptosis did not involve caspase activity. Bit1 or AES expressed in Chinese hamster ovary cells inhibited transcription from a Bcl-2 gene reporter. Thus, Bit1 appears to represent a previously unrecognized proapoptotic mitochondrial protein whose activity is subject to regulation through integrin-mediated signaling pathways.

Y. Jan, M. Matter, J.-t. Pai, Y.-L. Chen, J. Pilch, M. Komatsu, E. Ong, M. Fukuda, E. Ruoslahti, A mitochondrial protein, Bit1, mediates apoptosis regulated by integrins and Groucho/TLE corepressors. Cell 116, 751-762 (2004). [Online Journal]