Cell death in the pathogenesis of heart disease: mechanisms and significance

Abstract

Cell death was once viewed as unregulated. It is now clear that at least a portion of cell death is a regulated cell suicide process. This type of death can exhibit multiple morphologies. One of these, apoptosis, has long been recognized to be actively mediated, and many of its underlying mechanisms have been elucidated. Moreover, necrosis, the traditional example of unregulated cell death, is also regulated in some instances. Autophagy is usually a survival mechanism but can occur in association with cell death. Little is known, however, about how autophagic cells die. Apoptosis, necrosis, and autophagy occur in cardiac myocytes during myocardial infarction, ischemia/reperfusion, and heart failure. Pharmacological and genetic inhibition of apoptosis and necrosis lessens infarct size and improves cardiac function in these disorders. The roles of autophagy in ischemia/reperfusion and heart failure are unresolved. A better understanding of these processes and their interrelationships may allow for the development of novel therapies for the major heart syndromes.

Figure 1

Evolutionary conservation of apoptosis pathways. (a) Programmed cell death during Caenorhabditis elegans development. Precisely 131 cells (out of the 1090 somatic cells in the adult hermaphrodite) die at specific times during nematode development. Mutagenesis studies have revealed genes that regulate these deaths (162). These genes are termed cell death abnormal, or ced, and include ced-3 and ced-4, which promote death, and ced-9, which inhibits death. Loss-of-function mutations of ced-9 result in widespread death, which can be rescued by loss-of-function mutations of either ced-4 or ced-3. Thus, ced-9 is upstream of and inhibits ced-4 and ced-3. The relationship between ced-4 and ced-3 was elucidated in studies in which ced-4 killing was shown to require ced-3 (163). (b) Mammalian apoptosis. Although apoptosis in worms and mammals differs in some respects, the genetic blueprint has been conserved over 600 million years of evolution from worms to mammals. The ortholog of Ced-3 is the caspase family of cysteine proteases. Ced-4 is represented by a single protein, Apaf-1, which functions as an adaptor in the apoptosome. Orthologs of Ced-9 are the antiapoptotic branch of the Bcl-2 family. The mammalian Bcl-2 family also contains two subfamilies of proapoptotic proteins, one of which, the BH3-only branch, is present in C. elegans. The worm BH3-only protein Egl-1 (Egg laying defective-1) inhibits Ced-9 to promote apoptosis. The equivalent step in mammals is the inhibition of Bcl-2 by various BH3-only proteins. Mammalian BH3-only proteins may also induce apoptosis through additional mechanisms (, ; see text).

Figure 2

Apoptosis pathways. Apoptosis is mediated by an extrinsic pathway involving cell surface death receptors and by an intrinsic pathway that utilizes the mitochondria and endoplasmic reticulum. The extrinsic pathway is activated by binding of death ligand to its receptor, which triggers formation of the DISC. Caspase-8 is activated by forced proximity within the DISC and then cleaves and activates downstream procaspases. Caspase-8 can also cleave the BH3-only protein Bid, the carboxyl portion of which translocates to the mitochondria to trigger apoptotic mitochondrial events. The intrinsic pathway is activated by diverse biological, chemical, and physical stimuli. These signals are transduced to the mitochondria and ER (not shown) by proapoptotic Bcl-2 proteins: Bax (a multidomain protein) and BH3-only proteins. These death signals trigger the release of apoptogens from the mitochondria into the cytosol, one of which, cytochrome c, is depicted here. Cytosolic cytochrome c triggers the formation of a second multiprotein complex, the apoptosome, in which procaspase-9 undergoes activation. Caspase-9 then cleaves and activates downstream procaspases. Downstream caspases cleave several hundred cellular proteins to bring about the apoptotic death of the cell. See text for details.

Figure 3

Necrosis pathways. Information about necrosis signaling is currently limited to two pathways. The first involves death receptors, as exemplified by TNFR1 (tumor necrosis factor-α receptor 1). Depending on context, activation of TNFR1 can promote cell survival or either apoptotic or necrotic cell death. These choices are mediated by multiprotein complexes I and II. The binding of TNF-α to TNFR1 stimulates formation of complex I, which contains TNFR1, TRADD, RIP1, TRAF2, and cIAP1/2. The exact relationships among these proteins has not yet been defined, but one model postulates that TNFR1-TRADD-RIP1 proteins are linked through their respective death domains (DDs). RIP1 and TRAF2 undergo K63 polyubiquitination by cIAP1/2 in conjunction with TRAF2 (not shown). Polyubiquitinated RIP1 and TRAF2 recruit TAK1, which activates NF-κB, thereby stimulating transcription of survival genes. Death effects of TNFR1 signaling are mediated via complex II, which forms following endocytosis of complex I, the dissociation of TNFR1, and the deubiquitination of RIP1 by CYLD and A20 (not shown). TRADD recruits FADD (DD-DD interactions), and FADD recruits procaspase-8 (DED-DED interactions). Unless inhibited, procaspase-8 undergoes activation and cleaves RIP1, rendering RIP1 unable to signal either survival or necrosis. Caspase-8 also activates downstream caspases inducing apoptosis. In contrast, if procaspase-8 is inhibited (genetically or pharmacologically), RIP1 is not cleaved and instead recruits RIP3. RIP1 and RIP3 undergo a complex set of phosphorylation events, and necrosis ensues through unclear mechanisms. One potential mechanism may involve the activation of catabolic pathways and ROS production as shown. A second necrosis pathway involves the mitochondrial permeability transition pore (MPTP) in the inner mitochondrial membrane and its regulation by cyclophilin D (CypD). This pore may be opened by increased Ca²⁺, oxidative stress, decreased ATP, and other stimuli that operate during ischemia/reperfusion and heart failure. Ischemia/reperfusion can lead to increased Ca²⁺ and ROS as depicted. MPTP opening results in profound alterations in mitochondrial structure and function as described in text, which results in decreased ATP. No definitive connections have been delineated between death receptor and mitochondrial necrosis pathways. A possible connection is RIP3-induced ROS generation.

Figure 4

Model of potential relationships between apoptosis and necrosis. Apoptosis and necrosis are postulated to be linked in series as well as to exist in parallel. (a) Necrosis triggering downstream apoptotic events. A necrotic death stimulus, acting through yet-to-be-defined upstream pathways, triggers mitochondrial permeability transition pore (MPTP) opening. This results in redistribution of solutes and water down their respective gradients, inner mitochondrial membrane swelling, and rupture of the outer mitochondrial membrane (OMM), all manifestations of necrosis. Due to OMM rupture, apoptogens are released to the cytoplasm and trigger apoptosome assembly and activation of procaspases-9 and -3 (67). This sequence of events places necrosis upstream of apoptosis signaling. Given the initial MPTP insult, it is unclear if the activation of downstream apoptotic events/caspases contributes significantly to cell death. (b) Apoptosis triggering necrosis. An apoptotic death stimulus triggers Bax/Bak-dependent permeabilization of the OMM (without rupture), resulting in release of apoptogens to the cytosol and leading to procaspase-3 activation. Caspase-3 is then presumed to cross the permeabilized OMM and enter the mitochondrial intermembrane space. Caspase-3 then cleaves NDUFS1 (161), an inner mitochondrial membrane protein that faces the mitochondrial intermembrane space. NDUFS1 is a component of respiratory complex 1, and its cleavage interrupts electron transport, leading to loss of inner mitochondrial membrane potential (Δψm), decreased ATP levels, and increased reactive oxygen species (ROS) levels. Mitochondria then become swollen, suggesting MPTP opening, although the latter was not assessed (161). In addition, the plasma membrane becomes leaky through a yet-to-be-determined mechanism. Mitochondrial swelling, ATP depletion, and plasma membrane failure are all hallmarks of necrosis. Expression of a noncleavable NDUFS1 mutant partially rescues this phenotype. In this scenario, apoptosis leads to necrosis.