Abstract
Background: Actinin-4 was originally identified as an actin-binding protein associated with cell motility and cancer invasion and metastasis. However, actinin-4 is speculated to exert several distinct functions. Method and Result: The expression level of actinin-4 of prostate cancer cell lines (22RV1, PC-3 and LNCaP) were all decreased compared to that of normal prostate epithelial cells (PrEC) by Western blotting. The expression level of actinin-4 was significantly decreased in prostate cancer cells compared to that of non-cancerous basal cells in surgical prostate tissue by immunohistochemistry. Restoration of actinin-4 expression significantly inhibited its cell proliferation of 22RV1 in colony formation assay. Immunoprecipitation and MS analysis revealed that actinin-4 makes native complexes with several partner proteins in 22RV1 cells, including actin, clathrin heavy chain etc.. Clathrin is a coat protein that covers the internalized membrane pit and plays a central role in early endocytosis. We found the other endocytosis-related proteins interacted with actinin-4. Immunofluorescence microscopy revealed that dynamin and clathrin were co-localized with actinin-4 at dorsal sites of membrane ruffling, and the transfection of actinin-4 cDNA facilitated the transport of transferrin into peri-nuclear endosomes. We identified a panel of proteins whose expression and/or subcellular localization was regulated by actinin-4 using organelle fractionation and ICAT-MS. Conclusion: Disturbance of actinin-4-mediated endocytosis may be involved in prostate carcinogenesis.
