Role of autophagy in K-RAS- and B-RAF-driven lung cancers

Abstract

Autophagy degrades and recycles proteins and organelles to support metabolism and survival starvation. Oncogenic RAS upregulates autophagy required for mitochondrial function, stress survival, and engrafted tumor growth. We deleted an essential autophagy gene, autophagy-related-7 (Atg7), concurrently with Kras G12D activation with or without intact Trp53 in two mouse models for non-small-cell lung cancer (NSCLC). In both models, Atg7 deficiency caused tumor cells to accumulate dysfunctional mitochondria, and acquire metabolic, growth and survival defects associated with reduction in tumor burden. Importantly, Atg7 deficiency altered the fate of Kras G12D -induced carcinomas to that of oncocytomas, rare, predominantly benign tumors characterized by the accumulation of defective mitochondria. Surprisingly, lipid accumulation was observed in Atg7-deficient tumors only when Trp53 was deleted. Atg7-deficient tumor-derived cell lines (TDCLs) had compromised starvation survival and formed lipidic cysts instead of tumors, suggesting defective utilization of lipid stores. Atg7 deficiency reduced fatty acid oxidation (FAO) and increased sensitivity to FAO inhibition, indicating that with Trp53 loss, RAS-driven tumors require autophagy for mitochondrial function and lipid catabolism. Thus, autophagy is required for carcinoma fate, and cancers require autophagy for distinct roles in metabolism that are oncogene- and tumor suppressor gene-specific. To test the role of autophagy in oncogenic signaling pathways downstream of RAS, Atg7 was deleted in a mouse model of BRAF V600E -induced lung cancer in the presence or absence of the tumor suppressor Trp53. Atg7 deletion initially induced oxidative stress and accelerated tumor cell proliferation in a manner indistinguishable from Nrf2 ablation. Compound deletion of Atg7 and Nrf2 had no additive effect suggesting that both genes modulate tumorigenesis by regulating oxidative stress, revealing a potential mechanism of autophagy-mediated tumor suppression. At later stages of tumorigenesis, Atg7 deficiency resulted in an accumulation of defective mitochondria, proliferative defects, reduced tumor burden, conversion of adenomas and adenocarcinomas to oncocytomas, and increased mouse lifespan. Autophagy-defective tumor-derived cell lines were defective in their ability to respire, survive starvation and were glutamine-dependent, suggesting that autophagy-supplied substrates from protein degradation sustains Braf V600E -tumor growth and metabolism.