Signal Transduction

Edited by: Paul Smaglik H. Duan, A.M. Chinnaiyan, P.L. Hudson, J.P. Wing, W. He, V.M. Dixit, "ICE-LAP3, a novel mammalian homologue of the Caenorhabditis elegans cell death protein ced-3, is activated during Fas- and tumor necrosis factor-induced apoptosis," Journal of Biological Chemistry, 271:1621-5, 1996. (Cited in more than 125 publications since its publication in January 1996)Comments by Vishva M. Dixit, Director of Molecular Oncology, Genentech Inc., South San Francisco, Calif. THE EXECUTIONER UNCLOAKED: Genentech's Vishva Dixit identified ICE-LAP3 as the death effector, then used an antibody to pinpoint its location. Scientists searching for a mammalian genetic trigger for apoptosis turned to a worm for hints. Genetic analysis of the nematode Caenorhabditis elegans revealed three genes that regulate cell death-ced-3, ced-4, and ced-9. Both ced-3 and ced-4 encode proteins required for somatic cell death in the nematode (H.M. Ellis, H.R. Horowitz, Cell, 44:817-29, 1986). Scientists then found a mammalian protein similar to ced-3, which they named interleukin-1ß-converting enzyme (ICE). But a series of experiments suggested that an ICE-like enzyme, rather than ICE itself, played a key role in initiating mammalian apoptosis. However, the entire extended family of ICE had not yet been examined. "It was clear that there were likely many mammalian equivalents," says Vishva M. Dixit, director of molecular oncology at Genentech Inc. in South San Francisco, Calif. Dixit and colleagues identified ICE-LAP3 (caspase-7) as the death effector, then used an antibody to pinpoint its location. "The zymogen, or precursor form of the molecule, was found to exist in the cytoplasmic compartment," confirming colleagues' hunches that the mechanism should exist outside the nucleus, he explains. Dixit's team followed the processing of endogenous ICE-LAP3 using an antibody that could detect both the zymogen form and an active subunit. Dixit describes the chain of events: "On receipt of a death signal from either the Fas cytokine or the tumor necrosis factor (TNF) receptor, the zymogen form converted to the active protease well before there were any discernible morphological alterations. Inhibitors of cell death such as the virally encoded Serpin crmA blocked this conversion. Regardless, the observation of this conversion prior to the apoptotic demise was akin to the uncloaking of an executioner." Those observations confirmed the group's suspicion that ICE-LAP3 acts as a mediator early in the signaling process. "We were the first to show it by capturing the transition of the zymogen to the active protease," Dixit asserts. Interest in the apoptosis field has grown steadily since this discovery. Earlier work tended to focus on reactions near receptors. Since then, scientists have traced the trail of signals farther downstream. Although the complex cascades of reactions still need further elucidation, more scientists are joining the effort to sort them out. "Signal transduction as it relates to apoptosis is now a hot topic," Dixit notes. "You can't escape apoptosis because it's as fundamental to the survival of multicellular organisms as is cell proliferation." More recent studies have identified other death proteases. Those discoveries spur inquiries into their exact role. Some, Dixit thinks, may be peripheral to the apoptosis process. Others, he suspects, play more specialized roles. One study from a group at Yale University led by Richard Flavell hints that one death protease, caspase-3, is responsible primarily for apoptosis in central nervous system cells (K. Kuida et al., Nature, 384:368-72, 1996). In the study, scientists knocked out the gene encoding caspase-3. Cells that were destined to die in the mouse models survived and contributed to structural anomalies that interrupted the flow of cerebro-spinal fluid, leading to fatal hydrocephalus. Dixit suspects that different death proteases affect apoptosis in different organ systems. He forecasts that more knockout experiments will confirm that hunch. He also thinks that similar experiments would substantiate his hypothesis on the importance of ICE-LAP3. "I would predict that the knockout of ICE-LAP3 will give a death-defective phenotype."

Signal Transduction

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