Spontaneous apoptosis in human colon tumor cell lines and the relation of wt p53 to apoptosis

Objective. To examine spontaneous apoptosis of cultured human colon tumor cell lines in vitro and to investigate the role of wild type (wt) p53 in regulation of apoptosis induced by DNA-damaging treatment. Methods. A model system of human tumor progression involving three cell lines was used in this study for examination of apoptosis. They were originally established from human colon villous adenoma, including an early passage of non-tumorigenic cell line, V235E; a late passage of weakly tumorigenic cell line, V235L; and a spontaneous progressing highly tumorigenic cell line, V411. All of them maintain wt p53 expression. For identification of apoptosis, two tests were performed: 1. morphology study using acridine orange (AO) / ethidium bromide (EB) stainning by fluorescence microscopy; 2. DNA electrophoresis on agarose gel. P53 and WAF-1 (a downstream gene of p53) expressions were analysed at mRNA level using Northern blot technique. Apoptotic index of cell lines examined was measured by DNA fluorescence assay. Results. Spontaneous apoptosis was demonstrated in cell lines of all stages of progression by both morphology and DNA agarose gel electrophoresis. Apoptosis was further induced in V411 after treatment of cells with ¹³⁷Cs γ-irradiation and accompanied by increases in p53 and WAF-1 expression. In contrast, a mutant p53 bearing human colon cancer cell line, sw480, lacked spontaneous apoptosis, and upon irradiation neither induction of apoptosis nor increase expression of p53 and WAF-1 were seen. Conclusions. Apoptosis can be maintained in some human tumor cell lines despite transformation and carcinogenesis. Wt p53 and WAF-1 products are two of the potential mediators which effect apoptosis. Additionally, since apoptosis was enhanced by irradiation in V411, but not in sw480, it suggests that wt p53 cancer cells are more sensi-live to DNA-damaging treatment than mutant p53 cancer cells. These finding may have implications for cancer therapy.