Autophagy Is Required for Glucose Homeostasis and Lung Tumor Maintenance

Abstract

Macroautophagy (autophagy hereafter) recycles intracellular components to sustain mitochondrial metabolism that promotes the growth, stress tolerance and malignancy of lung cancers, suggesting that autophagy inhibition may have antitumor activity. To assess the functional significance of autophagy in both normal and tumor tissue, we conditionally deleted the essential autophagy gene, autophagy-related-7, Atg7, throughout adult mice. Here we report that systemic ATG7 ablation caused susceptibility to infection and neurodegeneration that limited survival to 2–3 months. Moreover, upon fasting, autophagy-deficient mice suffered fatal hypoglycemia. Prior autophagy ablation did not alter the efficiency of non-small-cell lung cancer (NSCLC) initiation by activation of oncogenic KrasG12D and deletion of the Trp53 tumor suppressor. Acute autophagy ablation in mice with pre-existing NSCLC, however, blocked tumor growth, promoted tumor cell death, and generated more benign disease (oncocytomas). This anti-tumor activity occurred prior to destruction of normal tissues, suggesting that, acute autophagy inhibition may be therapeutically beneficial in cancer.

A novel Atg5-shRNA mouse model enables temporal control of Autophagy in vivo

“…This is consistent with the reported inefficient expression of shRNA in the brain using this system [12]. Additionally, in contrast to whole-body somatic atg7 KO mice, which develop lethal neurodegeneration [10], ATG5i mice displayed no evidence of overt neurological or motor phenotypes, and presented with normal limb clasping reflexes and brain histology when treated with dox for up to 8 months (Fig. S3).…”

“…Using a tamoxifen-inducible Cre (Cre-ERT2) system, Atg7 was recently knocked out systemically in adult mice [10]. To test the extent to which ATG5i mice recapitulate gross phenotypes of the whole-body somatic atg7 knockout (KO) mice, 8-week old ATG5i mice were placed on a dox-containing diet for 6 weeks.…”

“…Eighty percent of whole-body somatic atg7 KO mice die with lethal hypoglycemia during a 24-h fasting period [10]. To test whether the ATG5i mice also recapitulate this phenotype, 8-week old ATG5i mice were treated with dox for a period of 2 weeks and then fasted for 24 h with free access to water, to replicate the same experimental design as previously reported [10]. At this time point, diminished expression of ATG5 in comparison to control mice was associated with SQSTM1 aggregation in the liver (Figure 4(a,b)).…”

“…At this time point, diminished expression of ATG5 in comparison to control mice was associated with SQSTM1 aggregation in the liver (Figure 4(a,b)). Unlike whole-body somatic atg7 KO mice [10], ATG5i mice displayed no evidence of fasting-induced death (Figure 4(c)) and maintained blood glucose levels similar to that of control mice, despite continued suppression of ATG5 at least in the liver (Figure 4(d)). These results indicate that both embryonic and somatic ATG5i mice are highly robust under metabolic stress conditions, although it remains to be elucidated whether or not the unaltered autophagy activity in the brain of ATG5i mice is also responsible for rescuing starvation-induced death in the somatic model.…”

“…Additionally, conditional whole-body knockout of Atg7 in adult mice has provided further evidence for the requirement of autophagy to survive acute metabolic stress. These mice also experience premature death due to neurotoxicity in metabolically unstressed states [10]. …”