The role of p53, p21WAF1/C1PI, and bcl-2 in radioresistant colorectal carcinoma

Genetic alterations in the p53 tumor suppressor gene are common in human colorectal cancers, occurring in approximately 70% of tumors. In vitro studies have shown that wild-type p53 is involved in controlling cell cycle checkpoint functions and apoptosis involved in the cytotoxic response induced by ionizing radiation and several anticancer chemotherapeutic agents. Wild-type p53 protein can transcriptionally activate the WAF gene, which encodes a cyclin-dependent kinase inhibitory protein, p21WAF1/C1PI protein, and transcriptionally repress the bcl-2 gene, which encodes an inhibitor of apoptosis. To learn more about the in vivo relationship between p53 protein and the expression of p21WAF1/C1PI and bcl-2 proteins in human colorectal cancers treated with radiation therapy, we examined the expression of these proteins by immunohistochemistry in pre-irradiated biopsy specimens and surgical specimens with residual tumor of 27 patients with colorectal carcinoma. Cell proliferation was measured using Ki-67 expression in the tumor cells. The p53 protein was not detected in normal colorectal mucosa, but it was expressed in 21 of 27 (78%) of pre-irradiated tumor samples and in 19 of 27 (70%) of post-irradiated tumors. Expression of the bcl-2 protein in normal colorectal mucosa was confined to the basal epithelial cells of the crypts. Diffuse bcl-2 staining was detected in tumor cells in 13 of 27 (48%) of pre-irradiated samples and in 14 of 27 (52%) of post-irradiated samples. p21WAF1/C1PI expression was detected in 14 of 27 (52%) of pre-irradiated samples but only in 7 of 27 (26%) of post-irradiated samples. No inverse relationship between expression of p53 protein and abnormal bcl-2 expression was apparent. p21WAF1/C1PI was expressed in most nonproliferating Ki-67-negative epithelial cells at the apical tips of the crypts in normal colorectal mucosa, but not in proliferating Ki-67-positive cells of adjacent adenomatous mucosa. An inverse relationship between Ki-67 and p21WAF1/C1PI expression was observed in normal colorectal mucosa and adjacent adenomatous mucosa. After radiation therapy, p53 protein accumulation did not change among residual tumors in 18 cases (three of which were initially negative and remained negative); in four cases there was a significant increase, and five cases had a substantial decrease of p53 expression. Aberrant bcl-2 expression is not correlated with expression of p53 and does not increase significantly in post-irradiated tumor cells. p21WAF1/C1PI expression is markedly reduced in tumor cells that survive radiation therapy.