Aspartate is a limiting metabolite for cancer cell proliferation under hypoxia and in tumors

As oxygen is essential for many metabolic pathways, tumor hypoxia may impair cancer cell proliferation (1–4). However, the limiting metabolites for proliferation under hypoxia and in tumors are unknown. Here, we assessed proliferation of a collection of cancer cells upon inhibition of the mitochondrial electron transport chain (ETC), a major metabolic pathway requiring molecular oxygen (5). Sensitivity to ETC inhibition varied across cell lines, and subsequent metabolomic analysis uncovered aspartate availability as a major determinant of sensitivity. Cell lines least sensitive to ETC inhibition maintain aspartate levels by importing it through an aspartate/glutamate transporter, SLC1A3. Genetic or pharmacologic modulation of SLC1A3 activity markedly altered cancer cell sensitivity to ETC inhibitors. Interestingly, aspartate levels also decrease under low oxygen, and increasing aspartate import by SLC1A3 provides a competitive advantage to cancer cells at low oxygen levels and in tumor xenografts. Finally, aspartate levels in primary human tumors negatively correlate with the expression of hypoxia markers, suggesting that tumor hypoxia is sufficient to inhibit ETC and, consequently, aspartate synthesis in vivo. Therefore, aspartate may be a limiting metabolite for tumor growth and aspartate availability could be targeted for cancer therapy.