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194 Part II Cellular Basis of Hematology

TNFR1 and DR3 is predominantly proinflammatory, while the also been reported. Thus the Fas–FasL system provides an upper limit
remaining death receptors principally activate cell death pathways. on the density of activated T cells at sites of inflammation. Lympho-
The extracellular segments contain several cysteine-rich domains cyte cell death is also directed by FasL expression on dissimilar cells.
forming an extended structure stabilized by disulfide bonds. Death Fas expression in germinal center B-lymphocytes appears to play a
receptor ligands share a TNF homology domain and bind as trimers role in eliminating cells bearing self-reactive surface immunoglobulin,
to the corresponding receptors. All known ligands are expressed as as mice expressing Fas only on T-lymphocytes acquire high levels
type II transmembrane proteins and are subject to limited proteolysis of auto-antibodies. In this case, FasL expression on T cells may
generating soluble forms. In most cases, soluble ligands are inferior deliver the fatal blow. T-lymphocytes can also be eliminated by FasL
to membrane-bound forms for receptor activation. Thus cell–cell expressed on nonlymphoid cell types.
contacts are necessary for death-receptor signaling, justifying the
characterization of subsequent apoptotic deaths as “fratricides.”
In the simplest example, binding of Fas ligand to CD95/Fas CELL SURVIVAL/DEATH PATHWAYS AND ADAPTIVE
receptor triggers clustering and allosteric conformational activation RESPONSES TO STRESS
of a trimeric receptor. An adaptor protein, FADD, binds at the Fas
cytoplasmic domain using homotypic DD associations. Similarly, Autophagy
procaspase-8 (or procaspase-10) is bound to FADD by homotypic
DED interactions. The induced proteolytic activity of procaspase-8 The main function of survival factor signaling is to support growth and
associated with the DISC appears to be sufficient for autoprocessing proliferation through activation of metabolism, including regulation
in trans of neighboring procaspase molecules. An NH 2-proximal of glucose uptake, glycolysis, and mitochondrial membrane poten-
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cleavage separates the caspase-8 prodomain from the catalytic sub- tial. Phosphatidylinositol 3-kinase (PI3K) activation downstream of
units, allowing untethering of active caspase-8 from the DISC and growth factor receptors, including activation of the serine/threonine
initiation of a cascade of effector caspase processing. Certain cells kinase AKT, is essential for mediating the metabolic effect of growth
(e.g., thymocytes) can bypass BCL-2 interdiction at the mitochondria factors. Consequently, growth factor withdrawal is associated with
and activate sufficient effector caspases downstream of death receptor metabolic decline, including a drop in cellular ATP levels, blunted
signaling to kill cells (type I cells). Others (e.g., hepatocytes) rely on glycolytic rates, decrease in O 2 consumption, inhibition of protein
an amplification loop in which BID cleavage triggers mitochondrial synthesis, and induction of apoptosis.
apoptosis (type II cells). In response to such metabolic stress and nutrient starvation, the
Superimposed on this three-component model are additional cell activates a homeostatic pathway known as autophagy (from Greek
factors that can substitute for one of the core components. FLICE/ meaning to eat [“phagy”] oneself [“auto”]). Autophagy is primarily a
caspase-8 inhibitory protein (FLIP) is homologous to caspase-8 but housekeeping mechanism that normally serves to degrade long-lived
devoid of protease activity (the active site cysteine is replaced). Dif- proteins and damaged organelles. It involves the formation of a
ferent splice forms of FLIP retain the DED motif and compete with double-membrane vesicle termed the autophagosome, which engulfs
caspase-8 for binding to FADD. The long splice variant of FLIP, cytoplasmic cargo followed by fusion with the lysosome and subse-
FLIP L , forms heterodimers with caspase-8. Caspase-8 bound to FLIP L quent degradation of internal contents. Autophagy is best known as a
has catalytic activity, but is processed inefficiently and remains associ- response to starvation, where the recycling of proteins and organelles
ated with the DISC. Importantly, the caspase-8–FLIP L heterodimer supplies required nutrients to the cell. 22,23 This process is regulated by
is unable to cleave caspase-3 or BID. Thus FLIP can either suppress an evolutionarily conserved set of proteins that ultimately orchestrate
caspase-8 activation or allow local activity. Moreover, FLIP L is also the recruitment of protein/organelle cargo to vesicles that will deliver
a substrate for caspase-mediated cleavage in the DISC. The cleaved their contents to lysosomes. Autophagy serves multiple functions,
product may assist with the recruitment of RIP1 kinase to the DISC, including tissue remodeling during development in addition to
promoting activation of NFκB and MAP kinases. Rapid turnover of survival in the face of nutrient starvation or other environmental
FLIP explains the sensitization of death receptor–induced apoptosis stress. 22,23
by protein synthesis inhibition. The survival signaling pathway and autophagy are hard wired
Recent evidence indicates that the signaling output of TNFR1 to preserve the cellular bioenergetic balance. Survival signaling
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and other death receptors arises from distinct complexes. The inhibits autophagy. Downstream of AKT, the mammalian target
TNFR1-bound complex triggers NFκB/MAPK signaling. Alterna- of rapamycin (mTOR) kinase (a mechanistic target of rapamycin)
tively, a cytosolic complex lacking the TNFR1 is established following serves as a nutrient sensor that is activated by high levels of ATP,
deubiquitination of RIPK1 kinase by the CYLD deubiquitinase and glucose, or amino acids, and in turn stimulates protein synthesis and
ubiquitin-editing functions of A20. This complex, designated the inhibits autophagy. In the presence of growth factors and extracellular
ripoptosome, is capable of apoptosis signaling via caspase-8. Notably, nutrients, mTOR inhibits autophagy through inactivation of unc-51
NFκB upregulates expression of FLIP to inhibit ripoptosome-triggered like autophagy activating kinase 1 (ULK1), an autophagy-related
apoptosis. In the absence of caspase-8 (or presence of caspase inhibi- serine/threonine kinase that is important for autophagy induction.
tors), a third complex known as the necrosome promotes necroptosis, During nutrient starvation, the activity of mTOR is inhibited
a programmed necrosis pathway characterized by cell swelling and by adenosine monophosphate-activated protein kinase (AMPK),
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rupture. This requires association with another RIP kinase family another nutrient sensor kinase that is activated when the ratio of
member, RIPK3. Curiously, deletion of both caspase-8 and RIPK1/ AMP to ATP increases during metabolic stress. AMPK activates
RIPK3 are synthetically viable in mice, indicating that the embryonic autophagy by inhibiting mTOR and phosphorylating ULK1. Upon
lethality associated with caspase-8 is due to RIPK1/RIPK3-dependent cellular metabolic decline and nutrient starvation, breakdown of
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necroptosis. The downstream target of RIPK1/RIPK3 in necrop- organelles and proteins in autophagosomes produces amino acids
tosis, the pseudokinase mixed lineage kinase domain-like protein and metabolites that can then feed into the mitochondrial TCA
(MLKL), oligomerizes after phosphorylation by RIPK3 and promotes cycle, sustaining the production of FADH 2 and NADH, ensuring
plasma membrane permeabilization. Caspase-8 suppresses necroptosis that the flow of electrons through the mitochondrial respiratory
by cleaving RIPK1 and RIPK3. Necrostatin 1, a small-molecule RIP1 chain complexes remains uninterrupted. The bioenergetic benefits
kinase inhibitor, is a potent inhibitor of necroptosis. 20 of autophagy are temporary until the metabolic stress is eliminated
Two arenas where death receptors have physiologic roles involve (e.g., growth factor or oxygen availability). Inactivation of autophagy
lymphocytes. Activation-induced cell death upon antigen restimula- during metabolic decline and nutrient stress leads to apoptosis, unless
tion involves Fas receptor signaling. Fas ligand (FasL) and Fas are apoptosis is inactivated (e.g., BAX/BAK deficiency), in which case
induced during T-cell activation downstream of Lck and NFκB. cell death occurs through necrosis. Whether autophagy is primarily
Engagement of Fas on one cell by Fas ligand on a second cell triggers a means of cellular survival or, under certain contexts, can promote
apoptosis. Autocrine suicide from FasL and Fas on the same cell have cell death, is the subject of intense investigation. Current findings