Excess cysteine drives conjugate formation and impairs proliferation of NRF2-activated cancer cells.

Cancer cells with constitutive NRF2 activation take up excess cystine beyond the cysteine demands of conventional pathways, implying unknown metabolic fates. Here, we develop an unbiased approach for the identification of cysteine metabolic fates and find that both known and previously uncharacterized cysteine-derived metabolites accumulate in NRF2-activated cancer cells. We identify many of these unknown metabolites as conjugates formed between cysteine and endogenous sugar metabolites, which can also be generated in vitro. We confirm the presence of these cysteine-derived conjugates in murine lung cancer models and primary human lung cancer samples, and their enrichment in NRF2-activated tumours in each context. Mechanistically, NRF2 promotes cystine uptake by driving SLC7A11 expression, which increases intracellular cysteine levels to promote these cysteine fates in a panel of cancer cell lines. Finally, we show that NRF2 activation creates a sensitivity to high environmental cystine, which impairs cell proliferation through excess free cysteine, and can be mitigated by sequestration into cysteine-derived conjugates. Overall, these findings reveal a cancer-associated metabolic vulnerability to excess cysteine stress, and reveal unrecognized routes of cysteine metabolism.