Cells die for diverse reasons, and the consequences are just as distinct
The Scientist 16:27, Jun. 24, 2002
By Nicole Johnston
Cell death keeps the cellular balance in check by ridding organisms of unwanted cells, ensuring normal development and protecting against tumor formation and viral infection. Yet increasingly, cell death is being implicated in a number of disorders, including cancers, autoimmune diseases, and neurodegenerative diseases. In 2000, two groups of scientists, working on altogether different aspects of the issue, made two seminal discoveries that shed crucial light on the matter. They showed for the first time that all cell deaths are not equal; it means different things at different times and in different places. The findings presented in both these Hot Papers1,2 give insight into the effect of cell death on immune system initiation and the implications for vaccine development; and the way in which a certain programmed cell-death trigger may offer a clue to the mechanism of neurodegenerative diseases such as Alzheimer.
What triggers the immune system to react to certain cell-death situations but not others is baffling. Cell death by necrosis is a cellular explosion that sets off inflammation; in contrast, programmed cell suicide, or apoptosis, causes cell implosion but not inflammation. In the first of these Hot Papers, Nina Bhardwaj, associate professor of clinical investigation, Laboratory of Cellular Physiology, Rockefeller University, and her colleagues studied the consequences of cell death on the maturation of dendritic cells. They found that how a cell goes about dying means everything.1
Dendritic cells are the most potent antigen-presenting cells (APCs) within the immune system. APCs capture antigens such as tumor cells or microbes and degrade them into smaller protein fragments, which are then presented to the T cells, like a red cape before a bull, to initiate an immune response. But before the T cells can do that, immature dendritic cells must be activated through a process called maturation. Scientists did not know what triggered that maturation process.
Bhardwaj and colleagues studied the influence of apoptotic and necrotic cells on dendritic cell activation. The results were completely unexpected. "We actually thought that apoptotic cells induced maturation because virus-infected cells undergo apoptosis; therefore, we thought it was a typical way for activation of T cells to take place," explains Bhardwaj. "We were really surprised to find out that was not the case. Both apoptotic and necrotic cells are digested by dendritic cells, and antigens from both are presented to T cells. But only necrotic cells induce maturation [and initiate an immune response]," says Bhardwaj. Two other groups published papers that confirmed their findings; one while their paper was under review and the other following their publication.3,4
"[Bhardwaj's] paper is one of the first papers that shows that all types of cell death are not the same," explains immunology professor Pramod Srivastava, cancer center director at the University of Connecticut School of Medicine, and author of the work that followed Bhardwaj's paper. "It is important because cells die in the body all the time and we don't really have a clue yet what types of cell death trigger, or are ignored by, the immune system. Obviously it can't respond to every type of cell death."
Now, Bhardwaj and her colleagues believe that heat-shock proteins are an immune system trigger based on findings published by Srivastava's group, which showed that necrotic cells release heat-shock proteins and can directly induce maturation of murine dendritic cells.4 Last fall, Bhardwaj and Srivastava published additional findings that necrotic-transformed (immortal) cell lines are enriched in the heat-shock proteins hsp70 and gp96, compared to the supernatants from normal cells.5 The scientists prevented dendritic cell maturation by boiling, protein digestion, and treatment with a heat-shock protein inhibitor that either destroyed or inhibited heat-shock protein activity. In contrast, lysates from apoptotic cells, which were forced to similarly explode, induced dendritic cell maturation. They hypothesized that heat-shock proteins released from lysed cells are trafficking viral or tumor peptides into the dendritic cells, triggering maturation.
Bhardwaj's team is working on using different types of dendritic cells for vaccine purposes. "A number of studies have shown that you can induce an antitumor response that's quite effective," she explains. "When dendritic cells were fed tumor cells and given to humans, there was some clinical regression." They have also shown that the lysate from biopsied tumor cells obtained from patients with cancer can activate immature dendritic cells to undergo maturation. In contrast, immature dendritic cells exposed to lysate from normal cells did not mature.
"The application [of these findings] is in the vaccine realm," she notes. Furthermore, she believes that this will help scientists understand how the immune system reacts to antigens in different situations, which could shed light on autoimmune disorders. "Different forms of cell death have different consequences on dendritic cells. One may be important in acquired tolerance to an antigen; the other [to] activation," she says. Srivastava agrees. "The location, quantity, and mechanisms of cell death are all unresolved yet. That's where this area is going," he adds.
In the second Hot Paper, cell biology professor Junying Yuan, Harvard
Medical School, and her colleagues discovered a new caspase-12-mediated
pathway of apoptosis that may help to explain the neurodegeneration associated
with diseases such as Alzheimer.2 Caspases, enzymes that belong to a family
of cysteine proteases, play a role in mediating apoptotic signals when
cells are confronted with injury or stress. Yuan and colleagues have identified
a cell-death role for caspase-12, one that has so far been found only in
mice. "It was a tough paper; we didn't know what was going on for a long
time," explains Yuan. "Most caspases get cleaved; caspase-12 didn't and
we were frustrated and wondering if it was a real functional caspase."
Searching for the location of caspase-12 at the subcellular level, they were amazed to find it in the endoplasmic reticulum (ER), the site of protein assebly and secretion, as well as calcium homeostasis.2,6 Unlike other caspases that localize to the cytosol, they discovered that caspase-12 resided specifically in the endoplasmic reticulum and would trigger apoptosis in cells damaged by ER stress. "Anything that can disturb the ER's ability to modify proteins and store calcium induces ER stress," explains Yuan. "Similarly, ER stress is induced by viral infections, where cells start making lots of viral proteins causing the ER to be overloaded with protein."
Perhaps most significantly, their findings may shed some light on how neurodegenerative damage occurs in diseases such as Alzheimer. Working with caspase-12-deficient mice, researchers exposed the neurons to the amyloid-ß peptide associated with Alzheimer; the neurons were protected from the toxicity normally associated with Aß. To confirm this, they used a snippet of nucleic acid to physically shut off caspase-12 in normal cells and showed that they were still protected against neurotoxicity.
"It's exciting work in that it has brought forward a couple of different areas," says Dale Bredesen, a physician scientist and president of the Buck Institute for Age Research in Novato, Calif. "Caspase-12 has been shown by Junying [Yuan] to play an important role in converting ER stress signals into a death signal." Nonetheless, the role remains controversial, as a human equivalent of caspase-12 has not been found. "There have been claims at meetings that there is no such thing," says Bredesen. However, he says that antibodies against caspase-12 react with human material, but the gene itself remains elusive. Subsequently, he and his colleagues have shown that caspase-7 is involved in the activation of caspase-12, resulting in increased cell death.7
If a human equivalent of caspase-12 is identified, it may have promising therapeutic implications. "One of the major problems of using caspases as therapy is how do you differentiate good apoptosis from bad apoptosis," points out Yuan. Unlike other caspases, however, the specificity of caspase-12 avoids that problem. Furthermore, specifically targeting caspase-12 should not produce side effects, since mice lacking caspase-12 were perfectly normal. "This is likely going to be relevant in the neuropathology of all major neurodegenerative conditions such as Huntington disease, prion (proteinaceous infectious particle) diseases, and amyotrophic lateral sclerosis," says Bredesen. "It tells us something about how misfolded proteins trigger cell death."
Nicole Johnston (firstname.lastname@example.org) is a freelance writer in Hamilton,
1. B. Sauter, M.L. Albert, L. Francisco, M. Larsson, S. Somersan, N. Bhardwaj, "Consequences of cell death: exposure to necrotic tumor cells, but not primary tissue cells or apoptotic cells, induces the maturation of immunostimulatory dendritic cells," Journal of Experimental Medicine, 191:423-33, February 7, 2000. (Cited in 213 papers)
2. T. Nakagawa, H. Zhu, N. Morishima, E. Li, J. Xu, B. Yankner, J. Yuan, "Caspase-12 mediates endoplasmic-reticulum-specific apoptosis and cytotoxicity by amyloid-ß," Nature, 403:98-103, 2000. (Cited in 163 papers)
3. S. Gallucci et al., "Natural adjuvants: endogenous activators of dendritic cells," Nature Medicine, 5:1249-55, 1999.
4. S. Basu et al., "Necrotic but not apoptotic cell death releases heat shock proteins, which deliver a partial maturation signal to dendritic cells and activate the NF-B pathway," International Immunology, 12:1539-46, 2000.
5. S. Somersan et al., "Primary tumor tissue lysates are enriched in heat shock proteins and induce the maturation of human dendritic cells," Journal of Immunology, 167:4844-52, 2001.
6. H. Mehmet, "Caspases find a new place to hide," Nature, 403:29-30, 2000.
7. R.V. Rao et al., "Coupling endoplasmic reticulum stress to the cell
death program. Mechanism of caspase activation," Journal of Biological
Chemistry, 276:33869-74, 2001.
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