Calpain-Associated Proteolytic Regulation of the Stromal Microenvironment in Cancer

Normalization of the stromal microenvironment is a promising strategy for cancer control. Cancer-associated fibroblasts, tumor-associated macrophages, and mesenchymal stromal cells have a central role in stromal functions. Accordingly, understanding these stromal cells is indispensable for the development of next-generation cancer therapies. Growing evidence suggests that calpain-induced intracellular proteolysis is responsible for cancer growth and stromal regulation. Calpain is a family of stress-responsive intracellular proteases and is inducible in cancer and stromal cells during carcinogenesis.

Here, we shed light on the recent advances that have been made in understanding how calpain contributes to stromal regulation in cancer.

Calpains are activated in stromal cells, including pancreatic stellate cells and mesenchymal cells. They induce fibrogenic responses in cancer stroma. Moreover, these molecules contribute to epithelial-mesenchymal transition and endothelial-mesenchymal transition to provide mesenchymal stromal cells in the microenvironment and concomitantly participate in cancer angiogenesis. In addition to the conventional calpains, the unconventional calpain-9 is associated with epithelial-mesenchymal transition. Animal experiments showed that targeting calpain systems antagonizes cancer development; thus, this approach is promising for cancer control.