Alanine, serine, cysteine-preferring transporter 2 (ASCT2) mediates uptake of glutamine, a conditionally essential amino acid in rapidly proliferating tumour cells. Uptake of glutamine and subsequent glutaminolysis are critical for activation of mTORC1 signalling, which regulates cell growth and protein translation in cancer cells. This is of particular interest in breast cancer, as glutamine dependence is increased in high-risk breast cancer subtypes. Targeting ASCT2 is therefore a possible therapeutic approach in breast cancer to prevent tumour cell proliferation by inhibiting glutamine uptake and blocking mTORC1 growth signalling.
ASCT2 gene expression in primary breast tumour tissue was shown to correlate with increased invasiveness, tumour grade at diagnosis and poor overall survival. Competitive pharmacological inhibition of ASCT2-mediated transport in human breast cancer cell lines significantly reduced glutamine uptake, leading to suppression of mTORC1 signalling, cell growth and cell cycle progression. Notably, these effects were subtype-dependent, with ASCT2-mediated glutamine transport critical for triple-negative basal-like breast cancer cell growth compared to minimal effects in luminal breast cancer cells. Stable or inducible shRNA knockdown of ASCT2 confirmed that inhibiting ASCT2 function was sufficient to prevent cellular proliferation and induce rapid cell death in triple-negative basal-like HCC1806 breast cancer cells, but not in luminal MCF-7 cells. A bioluminescent orthotopic xenograft model to examine tumour regression after induction of ASCT2 knockdown was then developed to confirm these data in vivo and for testing of preclinical ASCT2 inhibitors.
This study provides evidence for the feasibility of therapeutically exploiting glutamine dependence and transporter activity in breast cancer. Furthermore, ASCT2 may be a suitable candidate for future targeted therapies in breast cancer, particularly in the high-risk triple-negative subgroup.