SLC7A11-mediated cystine uptake is critical for maintaining redox balance and cell survival. Here, we show that this comes at a significant cost for cancer cells with high SLC7A11 expression. Actively importing cystine is potentially toxic due to its low solubility, forcing SLC7A11-high cancer cells to constitutively reduce cystine to the more soluble cysteine. This presents a substantial drain on the cellular NADPH pool and renders such cells dependent on the pentose phosphate pathway (PPP). Limiting glucose supply to SLC7A11-high cancer cells results in marked accumulation of intracellular cystine, redox system collapse, and rapid cell death, which can be rescued by treatments that prevent disulfide accumulation. We further show that glucose transporter (GLUT) inhibitors selectively kill SLC7A11-high cancer cells and suppress SLC7A11-high tumor growth. Our results identify a coupling between SLC7A11-associated cystine metabolism and the PPP, and uncover an accompanying metabolic vulnerability for therapeutic targeting in SLC7A11-high cancers.

Summary Serine, glycine, and other non-essential amino acids are critical for tumor progression, and strategies to limit their availability are emerging as potential cancer therapies 1 – 3 . However, the molecular mechanisms driving this response remain unclear, and the impact on lipid metabolism is relatively unexplored. Serine palmitoyltransferase (SPT) catalyzes the de novo biosynthesis of sphingolipids but also produces non-canonical 1-deoxysphingolipids (doxSLs) when using alanine as a substrate 4 , 5 . DoxSLs accumulate in the context of SPTLC1 or SPTLC2 mutations 6 , 7 or low serine availability 8 , 9 to drive neuropathy, and deoxysphinganine (doxSA) has been investigated as an anti-cancer agent 10 . Here we exploit amino acid metabolism and SPT promiscuity to modulate the endogenous synthesis of toxic doxSLs and slow tumor progression. Anchorage-independent growth reprograms a metabolic network involving serine, alanine, and pyruvate resulting in increased susceptibility to endogenous doxSL synthesis. Targeting the mitochondrial pyruvate carrier (MPC) promotes alanine oxidation to mitigate doxSL synthesis and improves spheroid growth, while direct inhibition of doxSL synthesis drives similar phenotypes. Restriction of dietary serine/glycine potently induces accumulation of doxSLs in xenografts while decreasing tumor growth. Pharmacological modulation of SPT rescues xenograft growth on serine/glycine-restricted diets, while reduction of circulating serine by inhibition of phosphoglycerate dehydrogenase (PHGDH) leads to doxSL accumulation and mitigates tumor growth. SPT promiscuity therefore links serine and mitochondrial alanine metabolism to membrane lipid diversity, which sensitizes tumors to metabolic stress.

Macular telangiectasia type 2 (MacTel) is a progressive, late-onset retinal degenerative disease linked to decreased serum levels of serine that elevate circulating levels of a toxic ceramide species, deoxysphingolipids (deoxySLs); however, causal genetic variants that reduce serine levels in patients have not been identified. Here, we identify rare, functional variants in the gene encoding the rate-limiting serine biosynthetic enzyme, phosphoglycerate dehydrogenase (PHGDH), as the single locus accounting for a significant fraction of MacTel. Under a dominant collapsing analysis model of a genome-wide enrichment analysis of rare variants predicted to impact protein function in 793 MacTel cases and 17,610 matched controls, the PHGDH gene achieves genome-wide significance (p=1.2×10 −13 ) with variants explaining ~3.2% of the affected