Abstract
Autophagy is a survival mechanism activated in response to metabolic stress. In normal tissues autophagy plays a major role in energy homeostasis through catabolic self-digestion of damaged proteins and organelles. Contrary to its survival function, autophagy defects are implicated in tumorigenesis suggesting that autophagy is a tumor suppression mechanism. Although the exact mechanism of this tumor suppressor function is not known, it likely involves mitigation of cellular damage leading to chromosomal instability. The complex role of functional autophagy in tumors calls for model systems that allow the assessment of autophagy status, stress management and the impact on oncogenesis both in vitro as well as in vivo. We developed model systems that involve generation of genetically defined, isogenic and immortal epithelial cells from different tissue types that are applicable to both wild-type and mutant mice. This permits the study of tissue- as well as gene-specific tumor promoting functions. We successfully employed this strategy to generate isogenic, immortal epithelial cell lines from wild-type and mutant mice deficient in essential autophagy genes such as beclin 1 (beclin 1(+/-)) and atg5 (atg 5(-/-)). As these cell lines are amenable to further genetic manipulation, they allowed us to generate cell lines with apoptosis defects and stable expression of the autophagy marker EGFP-LC3 that facilitate in vitro and in vivo assessment of stress-mediated autophagy induction. We applied this model system to directly monitor autophagy in cells and 3D-morphogenesis in vitro as well as in tumor allografts in vivo. Using this model system we demonstrated that autophagy is a survival response in solid tumors that co-localizes with hypoxic regions, allowing tolerance to metabolic stress. Furthermore, our studies have established that autophagy also protects tumor cells from genome damage and limits cell death and inflammation as possible means to tumor suppression. Additionally these cell lines provide an efficient way to perform biochemical analyses, and high throughput screening for modulators of autophagy for potential use in cancer therapy and prevention.