Mechanisms during cathepsin inhibition and the effects on substrate degradation and breast cancer cell invasion
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Cysteine cathepsin proteases are powerful collagenases and gelatinases that play an important role in matrix remodeling and are upregulated in various diseases including breast cancer. During cancer progression, tumor cells upregulate cysteine cathepsins to assist with the invasion and metastasis of the tumor. This has motivated pharmaceutical companies to develop protease inhibitors, but many have not passed through clinical trials not due to the lack of efficacy, but due to adverse side effects. Currently there is limited research investigating cellular feedback mechanisms caused by cathepsin inhibitors. This highlights a need to understand how cathepsin inhibition affects cathepsin production in breast cancer cells. To accomplish this, we investigated the effects two cathepsin inhibitors, the small molecule E-64 and cystatin C protein, have on the cellular regulation of cathepsins. Interestingly, both E-64 and cystatin C caused an upregulation of the amount of active cathepsin S. This was in contrast to cathepsin L which was either downregulated or unchanged. This was due to differences in cellular localization of cathepsins S and L. Inhibitor-induced co-localization of cathepsin S with gelatin substrate and cystatin C inhibitor occurred in the breast cancer cells. However, cathepsin L was located in the cytoplasm instead of with gelatin or cystatin C. This work demonstrates the need to better understand feedback mechanisms within the cathepsin proteolytic network and the cellular responses due to inhibitors for developing effective therapeutics and dosing strategies targeting cysteine cathepsins for the prevention of cancer invasion and metastasis.