UCSF

Hypoxia within solid tumors is a hallmark of the tumor microenvironment and a long-recognized negative prognostic factor, yet how oxygen availability shapes tumor metabolism and the response to therapy remains incompletely understood. This dissertation examines that question at two scales: hypoxia within the tumor microenvironment, and hypoxia imposed at the level of the whole organism.To define how microenvironmental hypoxia affects chemotherapy efficacy, we screen FDA-approved anticancer drugs across oxygen tensions and identify several nucleotide antimetabolites that are less effective in hypoxia. Focusing on 5-fluorouracil (5-FU), we show that hypoxia impairs prodrug activation through a metabolic bottleneck in cytidine/uridine monophosphate kinase 1 (CMPK1)-dependent phosphorylation. This bottleneck arises from reduced intracellular ATP availability rather than canonical hypoxia-inducible factor (HIF)- mediated transcriptional responses. The result is reduced incorporation of cytotoxic metabolites into RNA. Parallel CRISPRi screening across oxygen conditions identifies phosphoglucomutase 2 (PGM2) inhibition as a strategy to enhance 5-FU activation and efficacy in vitro and in vivo, regardless of oxygen tension.Turning to the whole organism, we find that systemic hypoxia decreases tumor growth in vivo across multiple cancer types and preclinical models, in contrast to the liability seen locally. Reduced glucose and caloric intake, lower tumor-intrinsic insulin signaling, and increased tumor HIF activation are each insufficient to explain this effect. Stable-isotope tracing instead reveals suppression of de novo purine synthesis: of the perturbations tested, reduced ATP availability alone recapitulates it, and nucleotide supplementation partially restores tumor growth. As a therapeutic strategy, systemic hypoxia is both durable and complementary to existing therapies: tumors do not acquire resistance to it, and pairing it with gemcitabine or immunotherapy suppresses growth. The same hypoxic state can be induced pharmacologically with the small molecule HypoxyStat.These two settings appear to point in opposite directions, yet they share a common mechanistic cause. In both, hypoxic ATP depletion constrains nucleotide metabolism: locally, it limits the ATP-dependent phosphorylation that activates 5-FU; systemically, it starves the tumor's ATP-dependent purine biosynthesis. Together, these studies identify a shared constraint that can undermine chemotherapies within the hypoxic microenvironment yet can be exploited when imposed at the level of the organism.