Procathepsin B interacts with the annexin II tetramer on the surface of tumor cells

Tumor cell invasion involves attachment of tumor cells to the underlying basement membrane, local proteolysis and migration of tumor cells through the proteolytically modified region. In general, high-metastatic cells synthesize degradative enzymes at higher levels than their low- or non-metastatic counterparts. The cysteine protease cathepsin B is upregulated in a variety of tumors, particularly at the invasive edges. Cathepsin B can degrade extracellular matrix proteins, such as collagen IV and laminin, and can activate the precursor form of urokinase plasminogen activator (uPA), perhaps thereby initiating an extracellular proteolytic cascade.;To study potential roles of plasma membrane-associated extracellular cathepsin B in tumor cell invasion and metastasis, we used the yeast two-hybrid system to screen for proteins that interact with procathepsin B. Annexin II light chain (p11), one of the two subunits of the annexin II tetramer, was one of the proteins which interacted with procathepsin B. We have shown that recombinant procathepsin B interacted with p11 as well as the annexin II tetramer in vitro. Furthermore, procathepsin B interacted with the annexin II tetramer in vivo as demonstrated by coimmunoprecipitation. Cathepsin B was also shown to colocalize with the annexin II tetramer on the cell surface of human breast carcinoma and glioma cell lines by immunofluorescent staining. In addition, procathepsin B can be cross-linked by sulfo-MBS to p11 on the surface of tumor cells. Taken together, our results indicate the annexin II tetramer serves as a binding protein for procathepsin B on the surface of tumor cells.;AIIt had previously been shown to interact with two serine proteases: plasminogen and tissue-type plasminogen activator (t-PA). The AIIt binding site for cathepsin B differs from that for either plasminogen or t-PA. AIIt also interacts with extracellular matrix proteins (e.g., collagen I and tenascin-C) forming a structural link between the tumor cell surface and the extracellular matrix. We determined that tenascin-C is a substrate for cathepsin B and can be degraded in vitro at pH 7.4 in a time- and concentration-dependent manner. Interestingly, cathepsin B, plasminogen, t-PA and tenascin-C have all been implicated in malignant development. We speculate that colocalization through AIIt of proteases and their substrates on the tumor cell surface may form a proteolytic center to facilitate: (1) activation of precursor form of proteases and initiation of proteolytic cascades; and (2) selective degradation of extracellular matrix proteins. Therefore, interaction between procathepsin B and the annexin II tetramer on the cell surface may play an important role in tumor cell invasion and metastasis.