Molecular mechanism of Fas-mediated apoptosis in colon cancer cell line HT-29 : Apoptosis-related gene expression and role of protein kinase C and protein phosphatase

BACKGROUND /AIMS: Physiologic cell death occurs primarily through an evolutionary conserved form of cellular suicide termed apoptosis. Recent evidence suggests that alterations in regulation of apoptosis contribute to the pathogenesis of a number of human diseases, including cancer, viral infections, autoimmune diseases, degenerative diseases and inflammatory diseases. Fas antigen(APO-1, CD95) is a cell surface receptor protein that is broadly expressed in normal and neoplastic cells and can mediate apoptosis in susceptible cells. Fas is involved in immune-related apoptosis including T-cell selection in thymus, down regulation of immune response and cytotoxic T-cell mediated cytotoxicity. In contrast to immune system, little is known about the function of Fas antigen expressed on epithelial cells. Recently, however, it has been shown that Fas is also important for the pathogenesis of liver disease and inflammatory skin disease. We have recently reported that although colon cancer cells HT-29 express Fas antigen on their surface, Fas ligation using IgM anti-Fas monoclonal antibody(CH-11) is not sufficient to induce apoptosis. In addition, cellular activation by IFN-gamma not only enhances Fas expression but also sensitizes HT-29 to apoptosis induced by Fas ligation as well as treatment with cycloheximide and actinomycin D. However, molecular mechanisms of Fas-mediated apoptosis are yet far from complete understanding. We, therefore, studied the functional role of Fas and apoptosis-related gene expression in apoptosis of colon cancer cell line HT-29 and signal transduction pathways including protein kinase C as well as protein phosphatase I and 2A. METHODS Fas, Fas ligand and apoptosis related gene mRNA expression was measured by RT-PCR. Cytotoxicity and cell survival were assessed by LDH assay and MTT assay, respectively. Apoptosis was detected by confocal microscopic observation of chromatin condensation after DAPI stain and confirmed by demonstration of DNA fragmentation in agarose gel electrophoresis as well as TUNEL assay. DNA content was deteunined by flow cytometry after staining with propidium iodide and sub-G1 peak was considered as apoptotic cells. Results Fas ligation by IgM anti-Fas monoclonal antibody(CH-11) tailed to induce poptosis in control HT-29. However, Fas ligation in IFN- gamma pretreated HT-29 induced apoptosis dose-dependently. HT-29 expressed very low level of bcl-2 mRNA, which was not changed by IFN-gamma pretreatment. IFN-gamma pretreatment did not alter the mRNA expression levels of bax, c-myc, p53, and caspases such as ICE, hich and cpp32. Protein kinase C inhibitor such as staurosporine and H7 did not inhibit Fas-mediated apoptosis of IFN-gamma pretreated HT-29. Fas-mediated apoptosis of IFN-gamma pretreated HT-29 was not suppressed as well by protein phosphatase 1 and 2A inhibitor calyculin A. Conclusions Colon cancer cell line HT-29 expresses Fas antigen on the surface which is not sufficient to induce apoptosis. IFN-gamma pretreatment sensitizes HT-29 to Fas-mediated apoptosis, but dose not alter the expression of apoptosis-related genes including bcl-2, bax, p53 and caspases. Fas-mediated apoptotic signal in IFN-gamma pretreated HT-29 maybe independent with protein kinase C as well as with protein phosphatase 1 and 2A.ope