Defective apoptosis renders immortalized epithelial cells highly tumorigenic, but how this is impacted by other common tumor mutations is not known. In apoptosis-defective cells, inhibition of autophagy by AKT activation or by allelic disruption of beclin1 confers sensitivity to metabolic stress by inhibiting an autophagy-dependent survival pathway. While autophagy acts to buffer metabolic stress, the combined impairment of apoptosis and autophagy promotes necrotic cell death in vitro and in vivo. Thus, inhibiting autophagy under conditions of nutrient limitation can restore cell death to apoptosis-refractory tumors, but this necrosis is associated with inflammation and accelerated tumor growth. Thus, autophagy may function in tumor suppression by mitigating metabolic stress and, in concert with apoptosis, by preventing death by necrosis.

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.

BACKGROUND A profound difference between cancer and normal tissues is the preferential utilization of glycolysis by cancer cells. To translate this paradigm in the clinic, we completed a phase I study of 2-deoxyglucose (2DG), and assessed 2DG uptake with fluorodeoxyglucose (FDG) positron emission tomography (PET) and the autophagy substrate p62 as a marker of 2DG resistance. METHODS Patients received 2DG orally on days 1–14 of a 21-day cycle in cohorts of three in a dose-escalating manner. Correlative assessments included PET scans at baseline and day 2 and p62 protein in peripheral blood mononuclear cells as a potential marker of 2DG resistance. RESULTS The dose of 45 mg/kg was defined as the recommended phase II dose, secondary to dose-limiting toxicity of grade 3 asymptomatic QTc prolongation at a dose of 60 mg/kg. PK evaluation of 2DG revealed linear pharmacokinetics with Cmax 45 μg/ml (277 μM), 73.7 μg/ml (449 μM), and 122 μg/ml (744 μM) in dose levels 30, 45, and 60 mg/kg, respectively. Five of eight patients assessed with FDG-PET scanning demonstrated decreased FDG uptake by day 2 of therapy, suggesting competition of 2DG with FDG. Five of six patients assessed for p62 had a decrease in p62 at 24 hr. CONCLUSIONS These data support the safety of 2DG, defined 2DG PK, demonstrated the effect of 2DG on FDG-PET imaging, and demonstrated the feasibility of assessment of p62 as an autophagic resistance marker. These data support future studies of 2DG alone or in combination with approaches to abrogate autophagy.