Bcl-2 and Bcl-2-related proteins in apoptosis regulation.

In this review we have discussed the importance of Bcl-2 and related proteins in the regulation of apoptotic cell death in mammalian systems. It is clear that Bcl-2 plays a critical role in controlling many forms of PCD. Bcl-2 seems to have particular significance in lymphocyte development and the function of the immune system. We have also discussed the increasing size of the newly identified Bcl-2 family. There are a number of Bcl-2 homologues in human, murine, avian, nematode, and viral systems. The evolutionary conservation of the function of the Bcl-2 homologues, reinforces the importance of PCD in all complex organisms. Some of these bcl-2-like genes function as agonists and others as antagonists. Despite the seemingly universal importance of Bcl-2, it is unable to prevent PCD in all systems. In addition, we have described a role for other Bcl-2 family members in systems in which Bcl-2 is ineffective and supplied a potential rationale for the large number of genes involved in the regulation of PCD. Identification and functional analysis of the Bcl-2 family members reveals the complex nature of cell death regulation. As we begin to appreciate the significance of PCD in the control of development and homeostasis, its regulation at the molecular level is becoming better understood. Bcl-2 has long been the only known intracellular regulator of the PCD pathway(s), although its ability to prevent apoptosis is not universal. We now know that bcl-2 is only one member of an evolutionary conserved family of genes which display different patterns of expression as well as function. At least two family members, Bcl-xs and Bax, act in opposition to Bcl-2. The discovery of these new family members, including those with Bcl-2-like function and antagonists, should help clear up the discrepancies seen in Bcl-2's ability to protect cells from PCD. In doing so, we will be able to further define the pathways associated with cell death signaling. The study of these family members, as well as the non-related genes of the PCD pathways (ced-3, ced-4, ice) should lead us to understanding of how cells of multicellular organisms make decisions to die.