Many cells produce inhibitory proteins that act to restrain the extrinsic pathway. For example, some cells produce the protein FLIP, which resembles an initiator caspase but has no protease activity because it lacks the key cysteine in its active site. FLIP dimerizes with caspase-8 in the DISC; although caspase-8 appears to be active in these heterodimers, it is not cleaved at the site required for its stable activation, and the apoptotic signal is blocked. Such inhibitory mechanisms help prevent the inappropriate activation of the extrinsic pathway of apoptosis.
the Intrinsic pathway of Apoptosis Depends on Mitochondria
Cells can also activate their apoptosis program from inside the cell, often in response to stresses, such as DNA damage, or in response to developmental signals. In vertebrate cells, these responses are governed by the intrinsic, or mitochondrial, pathway of apoptosis, which depends on the release into the cytosol of mitochondrial proteins that normally reside in the intermembrane space of these organelles (see Figure 12–19). Some of the released proteins activate a caspase proteolytic cascade in the cytoplasm, leading to apoptosis.
A key protein in the intrinsic pathway is cytochrome c, a water-soluble component of the mitochondrial electron-transport chain. When released into the cytosol (Figure 18–6), it takes on a new function: it binds to an adaptor protein called Apaf1 (apoptotic protease activating factor-1), causing the Apaf1 to oligomerize into a wheel-like heptamer called an apoptosome. The Apaf1 proteins in the apoptosome then recruit initiator caspase-9 proteins, which are thought to be activated by proximity in the apoptosome, just as caspase-8 is activated in the DISC. The activated caspase-9 molecules then activate downstream executioner caspases to induce apoptosis (Figure 18–7).
Bcl2 proteins regulate the Intrinsic pathway of Apoptosis
The intrinsic pathway of apoptosis is tightly regulated to ensure that cells kill themselves only when it is appropriate. A major class of intracellular regulators of the intrinsic pathway is the Bcl2 family of proteins, which, like the caspase family, has been conserved in evolution from worms to humans; a human Bcl2 protein, for example, can suppress apoptosis when expressed in the worm Caenorhabditis elegans.
Figure 18–5 The extrinsic pathway of apoptosis activated through Fas death receptors. trimeric Fas ligands on the surface of a killer lymphocyte interact with trimeric Fas receptors on the surface of the target cell, leading to clustering of several ligand-bound receptor trimers (only one trimer is shown here for clarity). receptor clustering activates death domains on the receptor tails, which interact with similar domains on the adaptor protein FADD (FADD stands for Fas-associated death domain). Each FADD protein then recruits an initiator caspase (caspase-8) via a death effector domain on both FADD and the caspase, forming a death-inducing signaling complex (DISC). Within the DISC, two adjacent initiator caspases interact and cleave one another to form an activated protease dimer, which then cleaves itself in the region linking the protease to the death effector domain. this stabilizes and releases the active caspase dimer into the cytosol, where it activates executioner caspases by cleaving them.