Proteomic analysis of lung metastases in a murine breast cancer model reveals divergent influence of CTSB and CTSL overexpression

Studies in the MMTV-PyMT (PyMT) breast cancer mouse model have shown a strong influence of the lysosomal cysteine cathepsins B or L on lung metastasis formation. Transgenic expression of human CTSB (tgCTSB) or CTSL (tgCTSL) both led to similar metastatic phenotypes with increased metastatic burden in the PyMT mice. However, recent studies in other tumor models proved marked differences in effects of either cathepsin on the proteome composition. We sought to analyze and compare proteome changes in the metastatic proteome of PyMT mice expressing either tgCTSB or tgCTSL to evaluate similarities and differences in those models. Performing an explorative, quantitative proteome comparison based on LC-MS/MS, we identified up to 3,000 proteins from murine lung metastases in three independent biological replicates per genotype. In both cases, when compared to wild-type (WT) mice, we noticed a pronounced impact of transgene cathepsin expression on the metastasis proteome. Highlights include increased moesin, integrin beta 1 and vinexin levels in the tgCTSB dataset and increased saposin and granulin levels in the tgCTSL dataset. Importantly, non-supervised hierarchical clustering clearly separated tgCTSB vs. tgCTSL induced proteome changes. In summary, tgCTSB and tgCTSL both display a strong and distinct impact on proteome composition of lung macrometastases in the PyMT model. Our observations suggest that they impact malignant behavior in distinct ways, thus further emphasizing interest into their tumor-contextual functionality.