MYC overrides HIF-1 to regulate proliferating primary cell metabolism in hypoxia

Hypoxia requires metabolic adaptations to sustain energetically demanding cellular activities. While the metabolic consequences of hypoxia have been studied extensively in cancer cell models, comparatively little is known about how primary cell metabolism responds to hypoxia. Thus, we developed metabolic flux models for human lung fibroblast and pulmonary artery smooth muscle cells proliferating in hypoxia. Unexpectedly, we found that hypoxia decreased glycolysis despite activation of hypoxia-inducible factor 1 alpha (HIF-1 alpha) and increased glycolytic enzyme expression. While HIF-1 alpha activation in normoxia by prolyl hydroxylase (PHD) inhibition did increase glycolysis, hypoxia blocked this effect. Multi-omic profiling revealed distinct molecular responses to hypoxia and PHD inhibition, and suggested a critical role for MYC in modulating HIF-1 alpha responses to hypoxia. Consistent with this hypothesis, MYC knockdown in hypoxia increased glycolysis and MYC over-expression in normoxia decreased glycolysis stimulated by PHD inhibition. These data suggest that MYC signaling in hypoxia uncouples an increase in HIF-dependent glycolytic gene transcription from glycolytic flux. Editor's evaluation The manuscript by Copeland and colleagues describes the impact of HIF1a, MYC and metabolism in pulmonary lung fibroblast and pulmonary artery smooth muscle cell phenotype, which is highly relevant to pulmonary vascular disease. The work includes metabolic flux assays of cultured cells, using a combination of metabolite concentration assessments, stable isotope-labeled substrates coupled with mass spectrometry, mathematical modeling, and cell proliferation analysis. Overall the findings are that there is an unexpected drop in lactate production in hypoxia and with HIF augmentation. These studies will add to the field's understanding of the role of HIF and cellular metabolism in pulmonary hypertension.