Cell surface remodeling by plasmin: a new function for an old enzyme

Plasmin, one of the most potent and reactive serine proteases, is involved in various physiological processes, including embryo development, thrombolysis, wound healing and cancer progression. The proteolytic activity of plasmin is tightly regulated through activation of its precursor, plasminogen, only at specific times and in defined locales as well as through inhibition of active plasmin by its abundant natural inhibitors. By exploiting the plasminogen activating system and overexpressing distinct components of the plasminogen activation cascade, such as pro-uPA, uPAR and plasminogen receptors, malignant cells can enhance the generation of plasmin which in turn, modifies the tumor microenvironment to sustain cancer progression. While plasmin-mediated degradation and modification of extracellular matrix proteins, release of growth factors and cytokines from the stroma as well as activation of several matrix metalloproteinase zymogens, all have been a focus of cancer research studies for decades, the ability of plasmin to cleave transmembrane molecules and thereby to generate functionally important cleaved products which induce outside-in signal transduction, has just begun to receive sufficient attention. Herein, we highlight this relatively understudied, but important function of the plasmin enzyme as it is generated de novo at the interface between cross-talking cancer and host cells.