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.

The major cardiac syndromes, myocardial infarction and heart failure, are responsible for a large portion of deaths worldwide. Genetic and pharmacological manipulations indicate that cell death is an important component in the pathogenesis of both diseases. Cells die primarily by apoptosis or necrosis, and autophagy has been associated with cell death. Apoptosis has long been recognized as a highly regulated process. Recent data indicate that a significant subset of necrotic deaths is also highly programmed. In this review, we discuss the molecular mechanisms that underlie these forms of cell death and their interconnections. Because of their regulated nature, the possibility is raised that small molecules aimed at inhibiting cell death may provide novel therapies for these common and lethal heart syndromes.

The defining event in apoptosis is mitochondrial outer membrane permeabilization (MOMP), allowing apoptogen release. In contrast, the triggering event in primary necrosis is early opening of the inner membrane mitochondrial permeability transition pore (mPTP), precipitating mitochondrial dysfunction and cessation of ATP synthesis. Bcl-2 proteins Bax and Bak are the principal activators of MOMP and apoptosis. Unexpectedly, we find that deletion of Bax and Bak dramatically reduces necrotic injury during myocardial infarction in vivo. Triple knockout mice lacking Bax/Bak and cyclophilin D, a key regulator of necrosis, fail to show further reduction in infarct size over those deficient in Bax/Bak. Absence of Bax/Bak renders cells resistant to mPTP opening and necrosis, effects confirmed in isolated mitochondria. Reconstitution of these cells or mitochondria with wild-type Bax, or an oligomerization-deficient mutant that cannot support MOMP and apoptosis, restores mPTP opening and necrosis, implicating distinct mechanisms for Bax-regulated necrosis and apoptosis. Both forms of Bax restore mitochondrial fusion in Bax/Bak-null cells, which otherwise exhibit fragmented mitochondria. Cells lacking mitofusin 2 (Mfn2), which exhibit similar fusion defects, are protected to the same extent as Bax/Bak-null cells. Conversely, restoration of fused mitochondria through inhibition of fission potentiates mPTP opening in the absence of Bax/Bak or Mfn2, indicating that the fused state itself is critical. These data demonstrate that Bax-driven fusion lowers the threshold for mPTP opening and necrosis. Thus, Bax and Bak play wider roles in cell death than previously appreciated and may be optimal therapeutic targets for diseases that involve both forms of cell death.

In testing the hypothesis that interleukin-4 receptor alpha-subunit (IL-4R alpha)-coupled signaling mediates altered airway smooth muscle (ASM) responsiveness in the atopic sensitized state, isolated rabbit tracheal ASM segments were passively sensitized with immunoglobulin E (IgE) immune complexes, both in the absence and presence of an IL-4R alpha blocking antibody (anti-IL-4R alpha Ab). Relative to control ASM, IgE-sensitized tissues exhibited enhanced isometric constrictor responses to administered ACh and attenuated relaxation responses to isoproterenol. These proasthmatic-like effects were prevented in IgE-sensitized ASM that were pretreated with anti-IL-4R alpha Ab. In complementary experiments, IgE-sensitized cultured human ASM cells exhibited upregulated expression of IL-13 mRNA and protein, whereas IL-4 expression was undetected. Moreover, extended studies demonstrated that 1) exogenous IL-13 administration to naïve ASM elicited augmented contractility to ACh and impaired relaxation to isoproterenol, 2) these effects of IL-13 were prevented by pretreating the tissues with an IL-5 receptor blocking antibody, and 3) IL-13 administration induced upregulated mRNA expression and release of IL-5 protein from cultured ASM cells. Collectively, these findings provide new evidence demonstrating that the altered responsiveness of IgE-sensitized ASM is largely attributed to activation of an intrinsic Th2-type autocrine mechanism involving IL-13/IL-4R alpha-coupled release and action of IL-5 in the sensitized ASM itself.