Metastasis to lung and bone is one of the major causes of death associated with breast cancer. Using the immunocompetent 4T1 murine model of breast cancer metastasis, our laboratory has previously identified a role for cysteine cathepsins in promoting the spread of primary tumour cells to these distant sites. When tumour-bearing mice were treated with a small-molecule inhibitor of cathepsin B, CA-074, in both early and late treatment settings, incidence of lung and bone metastases was significantly reduced. Likewise, enforced tumour-cell expression of Stefin A, an endogenous inhibitor for cathepsin B, also significantly attenuated metastasis.
In the current study, we are using the small-molecule cathepsin inhibitors in combination with quenched activity-based probes (qABPs) to study the cell-specific function of cathepsins in this model. The qABP BMV109 binds exclusively to cathepsins in an activity-dependent manner, allowing a distinction between active enzymes and those that may be bound to endogenous inhibitors such as Stefin A. Using BMV109 for fluorescent imaging studies, we have evaluated the levels of active cathepsins at the whole-organism, tissue, cellular and biochemical levels. We are currently interrogating the specific function of cathepsins derived from myeloid-derived suppressor cells within the metastatic microenvironment and assessing their involvement in immune regulation, angiogenesis, bone degradation, and the establishment of a pre-metastatic niche.