There is evidence of other forms of non-apoptotic programmed cell death that should also be considered since they may lead to new insights into cell death programs and reveal their potentially unique roles in development, homeostasis, neoplasia and degeneration. It has become increasingly apparent that cell death mechanisms include a highly diverse array of phenotypes and molecular mechanisms. And there is evidence that modulation of one form of cell death may lead to another. Because other types of cell death may require gene activation and function in an energy dependent manner, they are also considered to be forms of “programmed cell death.” Therefore, there is some resistance to the exclusive use of the term “programmed cell death” to specifically describe apoptosis.
There are necrotic-like phenotypes that require gene activation and protein synthesis so they are, strictly speaking, forms of programmed cell death (Proskuryakov et al., 2003
). These forms of cell death that have certain morphological features of both necrosis and apoptosis have been given the term “aponecrosis” (Formigli et al., 2000
). By affecting the mitochondrial respiratory chain with antimycin A, Formigli and coworkers induced a type of cell death that shared dynamic, molecular, and morphological features with both apoptosis and necrosis.
Caspase-independent mechanisms of neuronal cell death have also been identified. This specific type of programmed cell death may involve specific mitochondrial factors. In experimental models, apoptosis-inducing factor (AIF) and endonuclease G promote this type of cell death; however, Smac/DIABLO and HtrA2/Omi may also contribute (Ravagnan et al., 2002
; van Loo et al., 2002b
). Oppenheim and coworkers (2001)
have shown that programmed cell death occurs in developing mammalian neurons, even after the genetic deletion of caspases. Other research has shown that inhibition of the caspase execution machinery may only temporarily rescue damaged neurons and that classical apoptotic features can still appear in caspase-inhibited neurons (Volbracht et al., 2001
). It appears that caspase-dependent and caspase-independent mechanisms of neuronal cell death may depend on brain region, cell type, and age.
Sperandio and coworkers (2000)
have described a form of programmed cell death that is morphologically and biochemically distinct from apoptosis, dubbed “paraptosis.” Although this form of cell death does not respond to caspase inhibitors or BCL-XL, it is driven by an alternative form of caspase-9 activity that is Apaf-1 independent. This alternative form of programmed cell death is reported to occur during development and in transgenic models of Huntington’s disease and human amyotrophic lateral sclerosis (Dal Canto and Gurney, 1994
; Turmaine et al., 2000
There is speculation that “autophagy” represents another mechanism for programmed cell death and, similar to apoptosis, has important roles in developmental processes, human diseases and cellular responses to nutrient deprivation (Schwartz et al., 1993
; Gozuacik and Kimchi, 2004
; Debnath et al., 2005
). Other terms used synonymously are “macroautophagy” and “autophagic type II cell death” (Klionsky and Emr, 2000
). Autophagic cell death is characterized by the sequestration of cytoplasm and organelles in double or multimembrane vesicles and delivery to the cells own lysosomes for subsequent degradation (Noda et al., 2002
). In a sense, the cell “cannibalizes” itself. The mechanisms and morphology of autophagy are evolutionarily conserved with strong similarities between organisms as diverse as animals, plants and yeast. The process of autophagy depends on both continuous protein synthesis and the continuous presence of ATP. The molecular mechanisms have been extensively studied in yeast and mammalian orthologues continue to be elucidated (Ohsumi, 2001
; Huang and Klionsky, 2002
This distinction of a autophagic programmed cell death was made because it was determined that some cells would undergo caspase-independent gene-activated cell death but would display few of the ultrastructural features characteristic of apoptosis () and would not exhibit DNA laddering (Cohen, 1991
). However these cells do require de novo gene expression with an increase in expression of the polyubiquitin gene, similar to apoptosis (Ohsumi, 2001
; Gozuacik and Kimchi, 2004
). Specifically, autophagy occurs in all eukaryotic cells and involves the dynamic rearrangement of subcellular membranes to sequester cytoplasm and organelles for delivery to the lysosome or vacuole where degradation occurs. This is considered to be the major inducible pathway for the general turnover of cytoplasmic components.
A unique ubiquitin-like protein conjugation system and a protein complex that directs membrane docking and fusion at the lysosome or vacuole are important components of autophagy. In general, the process of autophagy can be divided into 4 steps: (1) induction, (2) formation of the autophagosome, (3) fusion with the lysosome or vacuole, and (4) autophagic body breakdown and recycling. Although the molecular details are still being elucidated, the regulation of this process occurs through various kinases, phosphatases and guanosine triphosphatases (GTPases).
There are some settings where autophagy and apoptosis seem to be interconnected and the idea of “molecular switches” between the two processes has been introduced (Piacentini et al., 2003
). This is similar to the concept of the necrosis-apoptosis “continuum” or “paradox” which suggests that both apoptosis and necrosis represent morphologic expressions of a shared biochemical network in which the route of cell death depends on a variety of factors such as the physiologic milieu, developmental stage, tissue type, and the nature of the cell death signal (Hirsch et al., 1997
; Zeiss, 2003
). However, the core apoptotic pathway can be diverted to induce a necrotic phenotype by alteration of the availability of intracellular ATP and the availability of caspases.
A similar relationship may occur between apoptosis and autophagy. It has been suggested that mitochondria may be central organelles that integrate both apoptosis and autophagy (Elmore et al., 2001
). Moreover, some of the same signals that are involved in apoptosis may also be involved in autophagy. For example, in both apoptosis and autophagy, there is the coordinated regulation of Akt (protein kinase B) and p70S6 kinase. Other proteins that may be part of the network connecting the two types of cell death include DAPk, Beclin 1, BNIP3, HSpin1, or protymosin-α
(Klionsky and Emr, 2000
). Malignant transformation is another link between autophagy and apoptosis (Gozuacik and Kimchi, 2004
The role of genetic alterations in the pathways that control cellular proliferation/apoptosis in cancer development has been well established. Similarly, a correlation between reduced autophagy and cancer has also been documented. Studies have indicated that during malignant transformation several proteins and pathways that are related to autophagy signaling are deregulated resulting in reduced autophagocytic activity. This suggests that, in some circumstances, autophagy may function as a safeguard mechanism that restricts uncontrolled cell growth. Autophagy has also been considered a protective mechanism against apoptosis. Lemasters and coworkers (1998)
observed that depolarized mitochondria, a feature of apoptosis, are rapidly eliminated by autophagy in primary hepatocytes. Eliminating damaged mitochondria prevents the release of pro-apoptotic substances from the mitochondria, thus preventing apoptosis.
Autophagy is considered the major cellular mechanism for disposing of long-lived proteins and cytoplasmic organelles; however, the concept of autophagic cell death has been a matter of debate within the scientific community. Since there is a distinct advantage of increased autophagy in various physiological and stress conditions, it has been suggested that autophagy represents an important adaptive mechanism that attempts to rescue cells from death. In other words, the presence of autophagic vesicles in dying cells may reflect an adaptive response to maintain cell survival under stress conditions rather than a reflection of “autophagic cell death.” Alternatively, are apoptosis and autophagic cell death mutually exclusive or can both apply in a situation similar to the apoptosis-necrosis continuum? It may be that the type of cell death depends on the severity of the response, the influences of cellular constituents and/or the influences of other signaling pathways.
There are reports that apoptosis and autophagic programmed cell death are not mutually exclusive and the diverse morphologies are attributed, in part, to distinct biochemical and molecular events (Gozuacik and Kimchi, 2004
). In certain cells and under certain conditions the morphological features of autophagy may occur prior to apoptotic cell death, representing an early phase of apoptosis. The controversy still remains, however interest in the field of autophagic cell death is constantly increasing with the emergence of new assays and markers for elucidating the molecular basis of autophagy and its possible implications in programmed cell death and malignant cell transformation.