Autophagy suppresses tumor progression by limiting chromosomal instability

Abstract

Autophagy is an evolutionarily conserved catabolic process involving regulated turnover and elimination of proteins and cellular organelles, such as peroxisomes, mitochondria, and endoplasmic reticulum, through the lysosomal degradation pathway (Mizushima 2005). The process of autophagy is characterized by the formation of double-membrane cytosolic vesicles, known as autophagosomes, that are essential for the lysosomal targeting of these organelles. In yeast, a number of autophagy-related genes (referred to as atg) have been identified that regulate the formation of autophagosomes and the autophagy process (Klionsky et al. 2003). Several mammalian homologs of these yeast genes have been identified (Levine and Klionsky 2004), among which the essential autophagy genes atg5 and atg7 have been most informative in demonstrating a role for autophagy in maintaining metabolism and homeostasis in mammalian development.

Autophagy, constitutively active at low levels, is robustly activated under metabolic stress. Autophagy plays an important role in development, as mice deficient in autophagy due to complete deficiency of beclin1 (atg6/vps30), another essential autophagy gene, die early in embryogenesis (Yue et al. 2003). Mice lacking atg5 fail to survive the neonatal starvation period and die perinatally, suggesting that autophagy plays an important role in the maintenance of energy homeostasis (Kuma et al. 2004). Thus, autophagy functions as an alternative cellular energy source to maintain normal metabolism during development and starvation by recycling cytoplasm and macromolecules (Jin and White 2007). Furthermore, targeted deletion of atg5 and atg7 in the central nervous system results in accumulation of polyubiquitinated proteins leading to neurodegeneration, revealing a housekeeping role for autophagy in the regulation of long-lived or damaged proteins to prevent neurodegeneration (Komatsu et al. 2005; Hara et al. 2006). The protective effects of autophagy also include immune-related functions such as defense against pathogens (Kirkegaard et al. 2004) and T lymphocyte development (Pua et al. 2006). Thus, autophagy plays a critical role in development, survival, and maintenance of homeostasis.

Despite its predominant role as a survival pathway, progressive autophagy can result in cell death if allowed to proceed to completion under persistent stress and during development. In Drosophila, autophagic cell death plays an important role during salivary gland development (Baehrecke 2003). In mammals, autophagy proteins Beclin1 and Atg7 are necessary for the autophagic cell death induced by the caspase-8 inhibitor zVAD-fmk in L929 cells as well as in human U937 cells (Yu et al. 2004). Furthermore, apoptosis-resistant bax^/bak ^{mouse embryonic fibroblasts (MEFs) undergo autophagic cell death in response to apoptosis inducers in a Beclin1- and Atg5-dependent manner (}Shimizu et al. 2004). Autophagy-like cell death is visualized in Alzheimer’s (Cataldo et al. 1994) and Parkinson’s (Anglade et al. 1997) and other neurodegenerative diseases (Klionsky and Emr 2000; Gomez-Santos et al. 2003). However, evidence in support of autophagy as a cell death mechanism is limited mostly to pharmacological induction of autophagy in cell lines in vitro, and direct evidence in a physiological context is frequently lacking. Moreover, it is not clear whether the induction of autophagy under the above circumstances represents the activation of a specific pathway for cell death or merely a failed attempt to promote cell survival under stress.

Quite contradictory to its survival-promoting function, defective autophagy is implicated in tumorigenesis, as haploinsuffieciency in the essential autophagy gene beclin1 is commonly observed in sporadic human ovarian, breast, and prostate cancers (Aita et al. 1999), and allelic loss of beclin1 promotes the incidence of sporadic malignancies in mice (Qu et al. 2003; Yue et al. 2003). In addition, oncogenic signals such as that of the Class I phosphatidylinositol-3-kinase (PI3K) pathway suppress autophagy, but if or how this contributes to tumorigenesis is not known.

We have shown that defects in apoptosis due to either Bax/Bak deficiency or Bcl-2 expression reveal autophagy in response to metabolic stress, which is required for cell survival (Degenhardt et al. 2006). Allelic loss of beclin1 impairs both constitutive and stress-induced autophagy, causing susceptibility to metabolic stress in immortalized baby mouse kidney epithelial (iBMK) cells (Degenhardt et al. 2006). Despite impairment in autophagy-mediated survival, the autophagy-defective beclin1 ^{iBMK cells are more tumorigenic than the wild-type cells, and the tumorigenicity is further accentuated by defects in apoptosis conferred by Bcl-2 expression (}Degenhardt et al. 2006). In tumors, autophagy localizes to metabolically stressed regions, suggesting that tumor cells exploit autophagy to survive metabolic stress (Degenhardt et al. 2006). Thus, defects in apoptosis manifest an autophagy-mediated survival pathway, and concomitant defects in apoptosis and autophagy render cells susceptible to metabolic stress while promoting tumorigenesis (Nelson et al. 2004; Degenhardt et al. 2006). However, these two apparently contradictory roles of autophagy lead to the paradox that the loss of a survival pathway promotes tumorigenesis.

Despite several studies linking autophagy to development, protein turnover, maintenance of cell organelles, and energy homeostasis, there is no satisfactory explanation for the intuitively contradictory roles of autophagy as a survival pathway and a tumor suppressor mechanism. We report here for the first time that autophagy can function to protect the genome. Defects in autophagy, conferred by either allelic loss of beclin1 or deficiency in atg5, lead to impaired survival in metabolic stress and an enhanced DNA damage response. Furthermore, loss of Beclin1 function promotes gene amplification and chromosomal instability, resulting in aneuploidy, all of which are hallmarks of cancer, suggesting that autophagy plays a major role in the maintenance of genomic integrity.

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