Hydroxyacid Oxidase 1, a Glutamine Metabolism-Associated Protein, Predicts Poor Patient Outcome in Luminal Breast Cancer

Breast cancer (BC), which remains the most prevalent malignancy among women, is characterised by significant heterogeneity across its molecular subtypes. Oestrogen receptor-positive (ER+) (luminal) BC represents approximately 75% of cases, and despite advancements in treatment there remains around a 40% recurrence rate. Cellular uptake of glutamine is conducted by solute carriers (SLCs), which are significantly associated with outcome in luminal BC. In this study, differential gene expression analysis was carried out using The Cancer Genome Atlas BC dataset. This identified hydroxyacid oxidase 1 (<i>HAO1</i>) as significantly overexpressed in luminal BC with a high expression of SLCs. Extended analysis in the METABRIC (n = 1980) and Breast Cancer Gene-Expression Miner (n = 4421) transcriptomic databases and the Nottingham (n = 952) BC tissue cohort showed a varied survival outcome for HAO1 expression at the genomic, transcriptomic, and proteomic levels. <i>HAO1</i> copy number (CN) gain (<i>p</i> = 0.002) and high HAO1 protein expression (<i>p</i> = 0.019) were associated with poor prognosis in luminal BC, whereas high <i>HAO1</i> mRNA expression correlated with better survival outcomes (<i>p</i> = 0.023) suggesting a complex regulatory mechanism affecting HAO1 at different biological levels. Importantly, in luminal BC patients treated with endocrine therapy, high protein expression of HAO1 predicted shorter distant-metastasis free survival (<i>p</i> = 0.042). The knockdown of <i>SLC1A5</i> and <i>SLC7A5</i> significantly reduced HAO1 expression in MCF-7 and ZR-751 BC cell lines. Protein analysis confirmed significant associations between HAO1 and SLC7A5 and SLC1A5, emphasising a potential role for the enzyme in glutamine metabolism and its potential as a therapeutic target. This study underscores the prognostic significance of HAO1 in luminal BC and its relationship with patient outcomes.