Prognostic impact of co-expression of LLCL2 and SLC7A5 in ERα-positive breast cancer patients

Background Lethal giant larvae homolog 2 (LLGL2) functions as a promoter of tumor growth and localizes at cell junctions and membranes with solute carrier family 7 member 5 (SLC7A5) in estrogen receptor α (ERα)-positive breast cancer. LLGL2 and SLC7A5 have been reported to be involved in resistance to endocrine therapy. This study aimed to assess the effects of LLGL2/SLC7A5 co-expression in predicting prognosis and response to endocrine therapy in ERα-positive breast cancer patients. Methods The associations of clinicopathological factors with LLGL2 and SLC7A5 expression or LLGL2/SLC7A5 co-expression at the mRNA and protein level were assessed in invasive breast cancer patients with long-term follow-up. The median follow-up period was approximately10 years. Survival curves were analyzed using the Kaplan–Meier method and verified by the log-rank test. A Cox proportional hazards regression analysis was used for univariate and multivariate analyses of prognostic values using stepwise linear regression. Results We identified a positive association between low mRNA expression of LLGL2 or SLC7A5 alone and longer disease-free survival (DFS) and overall survival (OS) in ERα-positive breast cancer patients, but not in ERα-negative patients. We also identified that low LLGL2/SLC7A5 mRNA co-expression (LLGL2low/SLC7A5low) was associated with longer survival compared with other combination groups in all breast cancer patients. In ERα-positive breast cancer patients, LLGL2low/SLC7A5low showed longer survival compared with LLGL2high/SLC7A5high and a positive trend of longer survival compared with other combination groups. We also observed that LLGL2low/SLC7A5low showed longer survival compared with LLGL2high/SLC7A5high in ERα-positive breast cancer patients receiving adjuvant tamoxifen therapy. Multivariate analysis demonstrated that LLGL2low/SLC7A5low was an independent favorable prognostic factor of both DFS and OS in ERα-positive breast cancer patients. High co-expression of LLGL2 and SLC7A5 protein showed a positive trend of shorter survival. Conclusions Our study showed that co-expression of LLGL2 and SLC7A5 mRNA is a promising candidate biomarker and suggested that the LLGL2–SLC7A5 axis may be a therapeutic target in early breast cancer patients, especially in those receiving adjuvant tamoxifen therapy.