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Although cell death can arise as a default in the absence of survival factors, apoptosis can also be stimulated by positively acting death signals. For instance, tumor necrosis factor alpha (TNFα), which is released by macrophages, triggers the cell death and tissue destruction seen in certain chronic inflammatory diseases (see Chapter 23). Another important death-inducing signal, the Fas ligand, is a cell-surface protein produced by activated natural killer cells and cytotoxic T lymphocytes. This signal can trigger death of virus-infected cells, some tumor cells, and foreign graft cells. Depending on the type of cell, death can be by apoptosis or necroptosis.

Both TNFα, depicted in Figure 21-42, and the Fas ligand (also called CD95 ligand) are trimeric proteins present on the surface of one cell that bind to “death receptors” on an adjacent cell. These death receptors have a single transmembrane domain and are activated when binding of a trimeric ligand brings three receptor molecules into close proximity. The activated trimeric death receptor complex then binds cytosolic proteins termed Fas-associated death domain protein (FADD) and TNF receptor-associated death domain protein (TRADD), forming a large oligomeric complex that contains other signaling proteins as well. (TRADD is required for the induction of apoptosis by some death receptors, such as those for TNFα, but not by others.) FADD then serves as an adapter to recruit and activate caspase-8, an initiator caspase. Like the other initiator caspase, caspase-9, caspase-8 is activated by dimerization following binding of two molecules to the FADD proteins recruited to an active death receptor trimer. Once activated, caspase-8 activates several effector caspases and the amplification cascade begins.

Caspase-8 also cleaves the BH3-only protein BH3-interacting-domain death agonist (Bid). The resulting tBid fragment then binds to Bcl-2 on the outer mitochondrial membrane, leading to the formation of Bak/Bax pores and holes, release of cytochrome c into the cytosol, and activation of the intrinsic apoptosis pathway (see Figure 21-40) as well.

To test the ability of the death receptor for the Fas ligand to induce cell death, researchers incubated cells with antibodies against the receptor. These antibodies, which bind and cross-link their cognate receptors, were found to stimulate cell death, indicating that activation of this receptor by oligomerization is sufficient to trigger apoptosis.

It came as a surprise to many researchers that, in cells lacking initiator caspase-8, addition of TNFα triggered necroptosis rather than apoptosis. The pathway is initiated by the same protein complex—TNFα, the TNFα receptor and TRADD—depicted in Figure 21-42, but does not involve FADD or caspase-8. There are several signal transduction proteins involved in the pathway; one essential protein is the kinase RIP1 (Requiescat in pace 1). When activated, RIP1 phosphorylates a second kinase, RIP3, and RIP3 phosphorylates another essential protein termed MLKL. Phosphorylation causes MLKL to form an oligomer that inserts into the plasma membrane and forms a hole, allowing Ca²⁺ entry. The influx of Ca²⁺ causes the cell and its organelles to swell and burst, releasing its contents into the extracellular space. Some of these released intracellular proteins trigger activation of immune-system cells and cause tissue inflammation and damage. Inflammation due to necroptosis has been implicated in mediating several human diseases, including neurodegeneration and progressive atherosclerotic lesions. As we discussed in Chapter 16, protein inhibitors of TNFα are among the most widely used therapeutics for many inflammatory diseases; inhibiting RIP1 kinase is another promising approach to treat human diseases characterized by necrosis and inflammation.

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But why would such a harmful signaling pathway have evolved in the first place? One popular theory relates to the finding that several viruses and other pathogens encode proteins that inactivate caspase-8, thus preventing the infected cells from undergoing the apoptotic death that would otherwise prevent the virus from replicating and the infection from spreading to neighboring cells. Necroptosis, which occurs only in the absence of caspase-8, would provide an alternative pathway for cell death that also prevents pathogen spread, but at a cost to the host organism—inflammation.

Recall that TNFα activates multiple signal transduction pathways: one leads to activation of the transcription factor NF-κB (see Figure 16-35), a second to apoptosis (see Figure 21-42), and the third to necroptosis. Much work needs to be done to understand the regulation of each of these pathways and their interactions, as this hormone is involved in many inflammatory diseases.