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The Role of Calpain in the Proteolytic Cleavage of E-cadherin in Prostate and Mammary Epithelial Cells*

The E-cadherin protein mediates Ca2+-dependent interepithelial adhesion. Association of E-cadherin with the catenin family of proteins is critical for the maintenance of a functional adhesive complex. We have identified a novel truncated E-cadherin species of 100-kDa (E-cad100) in prostate and mammary epithelial cells. E-cad100 was generated by treatment of cells with ionomycin or TPA. Cell-permeable calpain inhibitors prevented E-cad100 induction by ionomycin. Immunoblotting for spectrin and μ-calpain confirmed calpain activation in response to ionomycin treatment. Both the μ- and m-isoforms of calpain efficiently generated E-cad100 in vitro. The E-cad100 fragment was unable to bind to β-catenin, γ-catenin, and p120, suggesting that this cleavage event would disrupt the E-cadherin adhesion complex. Mutational analysis localized the calpain cleavage site to the cytosolic domain upstream of the β- and γ-catenin binding motifs of E-cadherin. Because E-cadherin is inactivated in many adenocarcinomas we hypothesized that calpain may play a role in prostate tumorigenesis. A prostate cDNA microarray data base was analyzed for calpain expression in which it was found that m-calpain was up-regulated in localized prostate cancer, and to an even higher degree in metastatic prostate cancer compared with normal prostate tissue. Furthermore, we examined the cleavage of E-cadherin in prostate cancer specimens and found that E-cad100accumulated in both localized and metastatic prostate tumors, supporting the cDNA microarray data. These findings demonstrate a novel mechanism by which E-cadherin is functionally inactivated through calpain-mediated proteolysis and suggests that E-cadherin is targeted by calpain during the tumorigenic progression of prostate cancer.