Solid tumors exhibit heterogeneous microenvironments, often characterized by limiting concentrations of oxygen (O2), glucose, and other nutrients. How oncogenic mutations alter stress response pathways, metabolism, and cell survival in the face of these challenges is incompletely understood. Here, we report that constitutive mTORC1 activity renders hypoxic cells dependent on exogenous desaturated lipids, as levels of de novo synthesized unsaturated fatty acids are reduced under low O2. Specifically, we demonstrate that hypoxic Tsc2-/- cells deprived of serum lipids exhibit a magnified UPR response, but fail to appropriately expand their ER, leading to IRE1-dependent cell death that can be reversed by the addition of unsaturated lipids. UPR activation and apoptosis were also detected in Tsc2-deficient kidney tumors. Importantly, we observe this phenotype in multiple human cancer lines and suggest that cells committed to unregulated growth within ischemic tumor microenvironments are unable to balance lipid and protein synthesis due to a critical limitation in desaturated lipids.