Cell-line specific role of Cathepsin B in triple-negative breast cancer growth, invasion and response to chemotherapy.

Cathepsins are papain-family cysteine proteases known to play a cell-intrinsic role in protein degradation in the lysosome, as well as in digesting ECM and surface proteins after being secreted. Both of these functions are known to mediate pro-tumorigenic effects of CTSB in a range of cancers. Here, we specifically investigate the role of CTSB in TNBC, an aggressive subtype of breast cancer, where we find that high expression of CTSB in TNBC is associated with better outcomes. We used CRISPR to knockout CTSB in two highly metastatic TNBC cell lines, MDA-MB-231 and MDA-MB-468, and find different effects. In MDA-MB-231 cells, knockout of CTSB has no effect on cell viability, increases tumor cell 3D invasion in an ECM-independent manner, and increases sensitivity to many standard of care chemotherapy drugs. However, in MDA-MB-468 cells, knockout of CTSB increases cell viability, decreases tumor cell 3D invasion, in an ECM-independent manner, and drives resistance to certain chemotherapy drugs without affecting response to others. We find that in these cells, CTSB is not secreted, and that differential downstream mTOR and Akt activation can explain the differences seen in these phenotypes. Overall, our studies demonstrate that CTSB can regulate TNBC cell phenotypes via its lysosomal cell-intrinsic role, but that effects are cell-line specific, suggesting potential heterogeneity in the role of CTSB in TNBC.