LONDON - A molecule which plays a vital role in determining whether a cell lives or dies has been identified by an international team of researchers. Dubbed the "central executioner" of the cell, the molecule's more formal name reflects its role in bringing about programmed cell death: apoptosis inducing factor (AIF).
AIF normally is found in the mitochondria of the cell, where it is thought to play a part in the normal metabolism of these organelles. But when cell death is required, AIF moves from the mitochondria to the nucleus, where it causes condensation of chromatin and fragmentation of DNA.
The discovery of AIF could open the way for new types of gene therapy for cancer, or lead to new drugs to prevent the loss of cells by apoptosis in conditions such as ischemia and infarction. A joint patent application on AIF has been issued by Thousand Oaks, Calif.-based Amgen Inc.'s Amgen Institute and the Ontario Cancer Research Institute, of Toronto, and the Centre National de la Recherche Scientifique (CNRS), in Villejuif, France, where much of the research was carried out.
Santos Susin of the CNRS - together with colleagues from Paris' Institut Pasteur, from the Amgen Institute and the Ontario Cancer Institute, and from the University of Washington in Seattle - reports the findings in a paper in the Feb. 4 issue of Nature. Its title: "Molecular characterization of mitochondrial apoptosis-inducing factor."
Guido Kroemer, research director at CNRS, told BioWorld International AIF is "probably one of the most important proteins involved in the regulation of apoptosis. It is a bifunctional protein. It has one normal metabolic function in the intermembrane space of the mitochondrion, as an oxidoreductase, and a second function - causing apoptosis - that is acquired when it leaks out of the mitochondrion and moves into the nucleus."
Kroemer and his colleagues at the CNRS and the Ontario Cancer Institute have already embarked on work to produce a knockout mouse that lacks functional AIF. "We want to find out whether absence of AIF or mutation of AIF causing abolition of its apoptogenic function will change the regulation of cell death," he said.
Research by Kroemer's group had already established that the membrane of the cell's mitochondria had to become permeable for apoptosis to occur. In addition, several studies had shown that the intermembrane space of mitochondria contained a variety of proteins that are released through the outer membrane and take part in apoptosis.
Probably Works In Antioxidant Pathway
The Nature paper reports on how AIF was identified and its gene cloned. The team also carried out several studies of the protein's function. The scientists found that, when they added recombinant AIF to isolated nuclei, the chromatin in these nuclei condensed, and the DNA broke up into large fragments. "We also know," Kroemer added, "that once AIF is released from the mitochondrion, it acts in cooperation with additional cytosolic factors on other mitochondria, causing them to release more AIF and other apoptogenic factors, including cytochrome C and caspase 9."
In a "News and Views" article in the same issue of Nature, titled "A cellular poison cupboard," William Earnshaw, of the Institute of Cell and Molecular Biology at the University of Edinburgh, in the U.K., wrote that this paper, together with a second by Susin et al. (Journal of Experimental Medicine, 1999, 189:381-394), provides "convincing evidence that mitochondria are involved in many aspects of the death response. In fact, they lead to a view where the mitochondrion is both essential for a cell's life and the reservoir for key components involved in its death."
Kroemer said little is known about the function of AIF while it is within the mitochondrion. "We don't know exactly what it does," he said, "but it probably participates in some antioxidant protective pathway. Nor do we know the metabolic effect of absence of AIF from the mitochondria."
The team also wants to identify which proteins in the cytosol and the nucleus interact with AIF in order to bring about apoptosis.
It is possible, Kroemer said, that AIF is a tumor suppressor gene. If this turns out to be the case, he added, then AIF could be used for gene therapy of cancer. Another potential application could come from identifying compounds that inhibit AIF. "If we could find pharmacological inhibitors that selectively neutralize the apoptogenic effects of AIF, either by acting directly on AIF or by inhibiting the interaction of AIF with proteins in the cytosol or nucleus, then the result could be a class of cytoprotective drugs which could be used to treat conditions where there is an increased loss of cells, such as schema and infarction," Kroemer said.