SUMMARY Allelic loss of the essential autophagy gene beclin1 occurs in human cancers and renders mice tumor-prone suggesting that autophagy is a tumor-suppression mechanism. While tumor cells utilize autophagy to survive metabolic stress, autophagy also mitigates the resulting cellular damage that may limit tumorigenesis. In response to stress, autophagy-defective tumor cells preferentially accumulate p62/SQSTM1 (p62), endoplasmic reticulum (ER) chaperones, damaged mitochondria, reactive oxygen species (ROS), and genome damage. Moreover, suppressing ROS or p62 accumulation prevented damage resulting from autophagy defects indicating that failure to regulate p62 caused oxidative stress. Importantly, sustained p62 expression resulting from autophagy defects was sufficient to alter NF-κB regulation and gene expression and to promote tumorigenesis. Thus defective autophagy is a mechanism for p62 upregulation commonly observed in human tumors that contributes directly to tumorigenesis likely by perturbing the signal transduction adaptor function of p62 controlling pathways critical for oncogenesis.

Autophagy is a bulk degradation process that promotes survival under metabolic stress, but it can also be a means of cell death if executed to completion. Monoallelic loss of the essential autophagy gene beclin1 causes susceptibility to metabolic stress, but also promotes tumorigenesis. This raises the paradox that the loss of a survival pathway enhances tumor growth, where the exact mechanism is not known. Here, we show that compromised autophagy promoted chromosome instability. Failure to sustain metabolism through autophagy was associated with increased DNA damage, gene amplification, and aneuploidy, and this genomic instability may promote tumorigenesis. Thus, autophagy maintains metabolism and survival during metabolic stress that serves to protect the genome, providing an explanation for how the loss of a survival pathway leads to tumor progression. Identification of this novel role of autophagy may be important for rational chemotherapy and therapeutic exploitation of autophagy inducers as potential chemopreventive agents.[Keywords: Autophagy; beclin1; genomic instability; apoptosis; cancer] Received February 23, 2007; revised version accepted April 12, 2007. 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...

Due to an error in preparing the final revision of Figure 3A in the article above, the blot for calnexin was accidentally duplicated as PDI. A corrected version of Figure 3A is shown below. The original PDI blot, as published here, was included in the original submission that was evaluated by reviewers. This correction does not alter the conclusions of the paper in any way.

Autophagy is a crucial component of the cellular stress adaptation response that maintains mammalian homeostasis. Autophagy protects against neurodegenerative and inflammatory conditions, aging, and cancer. This is accomplished by the degradation and intracellular recycling of cellular components to maintain energy metabolism and by damage mitigation through the elimination of damaged proteins and organelles. How autophagy modulates oncogenesis is gradually emerging. Tumor cells induce autophagy in response to metabolic stress to promote survival, suggesting deployment of therapeutic strategies to block autophagy for cancer therapy. By contrast, defects in autophagy lead to cell death, chronic inflammation, and genetic instability. Thus, stimulating autophagy may be a powerful approach for chemoprevention. Analogous to infection or toxins that create persistent tissue damage and chronic inflammation that increases the incidence of cancer, defective autophagy represents a cell-intrinsic mechanism to create the damaging, inflammatory environment that predisposes to cancer. Thus, cellular damage mitigation through autophagy is a novel mechanism of tumor suppression.

Most tumors are epithelial-derived, and although disruption of polarity and aberrant cellular junction formation is a poor prognosticator in human cancer, the role of polarity determinants in oncogenesis is poorly understood. Using in vivo selection, we identified a mammalian orthologue of the Drosophila polarity regulator crumbs as a gene whose loss of expression promotes tumor progression. Immortal baby mouse kidney epithelial cells selected in vivo to acquire tumorigenicity displayed dramatic repression of crumbs3 (crb3) expression associated with disruption of tight junction formation, apicobasal polarity, and contactinhibited growth. Restoration of crb3 expression restored junctions, polarity, and contact inhibition while suppressing migration and metastasis. These findings suggest a role for mammalian polarity determinants in suppressing tumorigenesis that may be analogous to the well-studied polarity tumor suppressor mechanisms in Drosophila. [Cancer Res 2008;68(11):4105-15]