Polyamine pathway activity promotes cysteine essentiality in cancer cells

Cancer cells have high demands for non-essential amino acids (NEAA), which are precursors for anabolic and anti-oxidant pathways supporting cell survival and proliferation. It is well established that cancer cells consume the NEAA cysteine, and that cysteine deprivation can induce cell death, however, the specific factors governing acute sensitivity to cysteine starvation are poorly characterised. Here we show that that neither expression of enzymes for cysteine synthesis nor availability of the primary precursor methionine correlated with acute sensitivity to cysteine starvation. We observed a strong correlation between efflux of the methionine-derived metabolite methylthioadenosine (MTA), and cysteine starvation sensitivity. MTA efflux results from genetic deletion of methylthioadenosine phosphorylase (MTAP), which is frequently deleted in cancers. We show that MTAP loss up-regulates polyamine metabolism which, concurrent with cysteine withdrawal, promotes elevated ROS and prevents cell survival. Our results reveal an unexplored metabolic weakness at the intersection of polyamine and cysteine metabolism.