Intermediate Filament Proteins of Lamin A/C and Cytokeratin 18 are involved in Apoptotic Induction by Photodynamic Therapy with Hexaminolevulinate in Human Colon Carcinoma Cells

Abstract

Hexaminolevulinate (HAL), a hexylester of 5-aminolevulinic acid, is a precursor of the photosensitizer protoporphyrin IX (PpIX) and clinically used for photodynamic therapy (PDT) of cancer, an established modality with a light-activated drug. However, the mechanism involved in the killing cancer cells is still not fully understood. Our previous report (Cancer Lett., 2013; 339: 25-32) has shown a crucial role of caspase-6-mediated cleavage of lamin A/C, a member of type-V intermediate filaments (IFs), in the HAL-PDT-induced apoptosis in human B-cell lymphoma cells. Lamin A/C is present in all cell types; while cytokeratin 18, a major component of type-I IFs, is expressed only in the epithelial cells and carcinoma. This study has focused upon the roles of the two IF proteins, lamin A/C and cytokeratin 18 in the HAL-PDT-mediated apoptotic induction in the human colon carcinoma COLO 205 and HCC2998 cell lines. HAL-PDT-induce apoptosis was confirmed by fluorescence microscopy, electron microscopy and M30 CytoDeath™ ELISA in both cell lines. Fluorescence microscopy, immunoblots and immunocytochemistry showed that both lamin A/C and cytokeratin 18 were involved in the apoptotic induction and the specific caspase-6 inhibitor stopped not only the cleavages of the two IF proteins, but also the apoptotic induction. Knockdown of both lamin A/C and cytokeratin 18 by siRNAs induced the cells to be apoptotic, further supporting the hypothesis that the disruption of both lamin A/C and cytokeratin 18 is required in the apoptotic induction by HAL-PDT in the human carcinoma cells.