Carboxypeptidase A4 accumulation is associated with an aggressive phenotype and poor prognosis in triple-negative breast cancer

Using whole transcriptome analysis and a lentiviral short hairpin RNA screening library, carboxypeptidase A4 (CPA4) was identified as a novel marker in breast cancer and a therapeutic target in triple-negative breast cancer (TNBC) in the present study. Immunohistochemistry was used to evaluate the presence of CPA4, estrogen receptor, progesterone receptor, human epidermal growth factor receptor 2, Ki67, epidermal growth factor receptor, cytokeratin 5/6, aldehyde dehydrogenase 1, cluster of differentiation (CD)44, CD24, claudins, E-cadherin, vimentin and androgen receptor in 221 cases of breast cancer, including 68 TNBC cases. The effects of CPA4 on the viability and migration ability of TNBC cells were analyzed using RNA interference methods. Increased CPA4 expression, specifically in the cytoplasm of cancer tissue cells, was detected. Furthermore, high CPA4 expression in TNBC cases was associated with low expression of E-cadherin and with the expression of cancer stem cell markers (high CD44/low CD24). Patients with TNBC and high levels of CPA4 expression had a significantly poorer prognosis compared with those with low CPA4 expression. Notably, viability and migration were reduced, but E-cadherin expression was upregulated in CPA4-suppressed TNBC cells. The present data suggested that CPA4 may be a novel inducer for epithelial-mesenchymal transition, which is characterized by the downregulation of E-cadherin and mesenchymal phenotypes. To conclude, CPA4 may be a marker for poor prognosis and a promising therapeutic target in TNBC with aggressive phenotypes.