Impacts of protease inhibitors on clathrin and fibronectin in cancer metastasis

Metastasis is a major cause of cancer deaths. Seeking alternative prognostic biomarkers may enable oncologists to make accurate therapeutic decisions to benefit cancer patients. Cumulated evidence reveals that fibronectin (FN) is highly correlated with cancer metastasis but has not been deemed as a prognostic biomarker due to its broad tissue distribution patterns and complicated physiological and pathological functionalities that significantly interfere with the judgmental accuracy. Combining other FN-related factors may make FN possible as a useful prognostic biomarker. Clathrin, a highly protease-susceptible cytoplasmic molecule, is known to affect pericellular FN (periFN) assembly via regulating cell surface FN receptors or FN matrix turnover by coating the endocytic vesicles. Researching our previously published proteomics database of 660 differential secretome proteins expressed in human lung adenocarcinoma cell lines and performing double immunofluorescent staining for periFN and clathrin, we recognized an inverse relationship between them. However, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) data contradicted this relationship, which could be corrected by the addition of a mixture of protease inhibitors into nonmetastatic cancer cell lysates. These results suggested that nonmetastatic cells express either higher levels of cellular proteases or less amounts of protease inhibitors. By examining our proteomic database and reviewing the literature, we conclude that clathrin expression and assembly is inversely correlated with metastatic potential of FN^high cancer cells mainly related to the expression of protease inhibitors, instead of proteases. It is worth investigating whether such an inverse relationship between FN/protease inhibitors and clathrin in human cancers could clinically be incorporated into the prognostic strategy for various cancer types.