Targeting ASCT2/SLC1A5 for inhibition of cellular growth and metabolism in triple-negative breast cancer In order to maintain nutrient supply to fuel their rapid growth, cancer cells selectively upregulate expression of cell-surface nutrient transporters. One such transporter is alanine, serine, cysteine-preferring transporter 2 (ASCT2), which catalyses the sodium-dependent uptake of small neutral amino acids, including glutamine, a “conditionally essential” amino acid in cancer cells. Glutamine promotes cell growth by supporting biosynthetic and bioenergetic metabolism, activating mTORC1 signalling, and inhibiting apoptosis. Blocking ASCT2 to prevent glutamine uptake has been shown to inhibit cell growth in prostate cancer, non-small cell lung cancer, melanoma, and acute myeloid leukaemia. As ASCT2 expression and glutamine reliance are increased in aggressive breast cancer subtypes, such as triple-negative (TN) breast cancer, targeting ASCT2 may be a viable therapeutic approach. Analysis of ASCT2 function showed that TN, but not Luminal A, breast cancer cells require ASCT2-mediated uptake of glutamine to sustain cell growth in vitro and in vivo. Gene expression analysis of TN xenografts and patient samples suggested dynamic regulation of glutamine metabolism by MYC and ATF4, and suggested that TN breast cancers may have unique metabolic signatures that could be exploited therapeutically. Using targeted metabolomics, TN cells were shown to have higher levels of glutamine-dependent anaplerosis and less flexibility in their use of glutamine as a mitochondrial fuel than Luminal A cells. This phenotype was mirrored in gene expression analysis of the TCGA provisional dataset, which revealed a cluster of highly expressed glutamine-metabolism genes in the TN samples. Despite this metabolic vulnerability, ASCT2 inhibition caused similar effects in Luminal A and TN cells, causing a global reduction in uptake and utilisation of glutamine. Finally, blocking ASCT2 in combination with the L-type amino acid transporter, LAT1, was able to inhibit cell growth in both Luminal A and TN breast cancer cells, extending the efficacy of targeting glutamine or leucine uptake alone. Taken together, these data provide preclinical evidence for targeting ASCT2 and downstream glutamine metabolism in breast cancer, particularly in the high-risk TN subgroup where there is increased glutamine dependence
