Tissue Proteomics of Feline Mammary Carcinoma: Differences in Protein Profiles Among Histological Grades Using Liquid Chromatography–Tandem Mass Spectrometry

Mammary carcinomas are aggressive neoplasms and a significant cause of mortality in female cats. Despite surgical removal, feline mammary carcinoma (FMC) often recurs or metastasizes. Specific tumour biomarkers are necessary for early detection, prognosis and therapy selection. This study aims to identify candidate biomarkers for FMC by comparing tissue proteomic profiles among grades of 31 FMC cats and six normal mammary tissues (control) using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). Candidate proteins identified by LC-MS/MS were validated by Western blotting and LC-MS/MS. Prognostic values of candidate proteins were evaluated using immunohistochemistry and survival analysis. Protein-chemotherapy drug interaction networks were also evaluated. Among the 268 differential proteins observed, dermatopontin (DPT) expression was significantly downregulated, while sorting nexin 5 (SNX5) expression was elevated in cancerous tissues compared to controls (p < 0.05). Immunohistochemistry results revealed a significant association of DPT and SNX5 with stages (DPT, p < 0.0001; SNX5, p = 0.046) and grades of FMC (DPT, p < 0.0001; SNX5, p = 0.04). Low DPT expression was associated with poor overall survival (p = 0.02), along with Stage 4 FMC (p = 0.0001), high mitotic count (p = 0.003) and the presence of lymphovascular invasion (p = 0.003). Moreover, protein-chemotherapy drug interaction showed a relationship of DPT with doxorubicin, lapatinib and neratinib. This study identified DPT and SNX5 as potential diagnostic and therapeutic targets for FMC, with DPT emerging as a promising prognostic biomarker.