Breast cancer (BC) is a heterogeneous disease consisting of various subtypes, with different prognostic and therapeutic outcomes. The amino acid transporter, SLC7A8, is overexpressed in oestrogen receptor-positive BC. However, the consequence of this overexpression, in terms of disease prognosis, is still obscure. This study aimed to evaluate the biological and prognostic value of SLC7A8 in BC with emphasis on the intrinsic molecular subtypes.
SLC7A8 was assessed at the genomic, using METABRIC data (n = 1980), and proteomic, using immunohistochemistry and TMA (n = 1562), levels in well-characterised primary BC cohorts. SLC7A8 expression was examined with clinicopathological parameters, molecular subtypes, and patient outcome.
SLC7A8 mRNA and SLC7A8 protein expression were strongly associated with good prognostic features, including small tumour size, low tumour grade, and good Nottingham Prognostic Index (NPI) (all P < 0.05). Expression of SLC7A8 mRNA was higher in luminal tumours compared to other subtypes (P < 0.001). High expression of SLC7A8 mRNA and SLC7A8 protein was associated with good patient outcome (P ≤ 0.001) but only in the low proliferative ER+/luminal A tumours (P = 0.01). In multivariate analysis, SLC7A8 mRNA and SLC7A8 protein were independent factors for longer breast cancer specific survival (P = 0.01 and P = 0.03), respectively.
SLC7A8 appears to play a role in BC and is a marker for favourable prognosis in the most predominant, ER+ low proliferative/luminal A, BC subtype. Functional assessment is necessary to reveal the specific role played by SLC7A8 in ER+ BC.

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