Transient systemic autophagy ablation irreversibly inhibits lung tumor cell metabolism and promotes T-cell mediated tumor killing.

Macroautophagy/autophagy is a highly conserved catabolic process pivotal to cellular homeostasis and support of tumorigenesis. Being a potential therapeutic target for cancer, we have worked to understand the implications of autophagy inhibition both systemically, and tumor-specifically. We utilized inducible expression of Atg5 shRNA to temporally control autophagy levels in a reversible manner to study the effects of tumor-intrinsic and systemic autophagic loss and restoration on established KrasG12D/+;trp53-/- (KP) lung tumor growth. We reported that transient systemic ATG5 loss significantly reduces KP lung tumor growth. Through in vivo isotope tracing and metabolic flux analyses, we noted that systemic ATG5 knockdown significantly reduces the uptake of glucose and lactate in lung tumors, leading to impaired TCA cycle metabolism and biosynthesis. Additionally, we observed an increased tumor T cell infiltration in the absence of systemic ATG5, which is essential for T cell-mediated tumor killing. Moreover, the impaired tumor metabolism and increased T cell infiltration are sustained when autophagy is restored in a short term. Finally, we found that intermittent systemic ATG5 knockdown, a mock therapy situation, significantly prolongs the lifespan of mice bearing KP lung tumors. Our findings lay the proof of concept for inhibition of autophagy as a valid approach to cancer therapy.

Glucose feeds the TCA cycle via circulating lactate.

Mammalian tissues are fuelled by circulating nutrients, including glucose, amino acids, and various intermediary metabolites. Under aerobic conditions, glucose is generally assumed to be burned fully by tissues via the tricarboxylic acid cycle (TCA cycle) to carbon dioxide. Alternatively, glucose can be catabolized anaerobically via glycolysis to lactate, which is itself also a potential nutrient for tissues and tumours. The quantitative relevance of circulating lactate or other metabolic intermediates as fuels remains unclear. Here we systematically examine the fluxes of circulating metabolites in mice, and find that lactate can be a primary source of carbon for the TCA cycle and thus of energy. Intravenous infusions of 13C-labelled nutrients reveal that, on a molar basis, the circulatory turnover flux of lactate is the highest of all metabolites and exceeds that of glucose by 1.1-fold in fed mice and 2.5-fold in fasting mice; lactate is made primarily from glucose but also from other sources. In both fed and fasted mice, 13C-lactate extensively labels TCA cycle intermediates in all tissues. Quantitative analysis reveals that during the fasted state, the contribution of glucose to tissue TCA metabolism is primarily indirect (via circulating lactate) in all tissues except the brain. In genetically engineered lung and pancreatic cancer tumours in fasted mice, the contribution of circulating lactate to TCA cycle intermediates exceeds that of glucose, with glutamine making a larger contribution than lactate in pancreatic cancer. Thus, glycolysis and the TCA cycle are uncoupled at the level of lactate, which is a primary circulating TCA substrate in most tissues and tumours.