Expression of Bcl-2 protein in hyperplastic polyps, adenomas, and carcinomas of the colon

The proto-oncogene Bcl-2 encodes a protein that protects cells from programmed cell death (apoptosis). The protein is expressed in the proliferative compartment of several normal tissues, including normal colonic crypts. The aim of this study was to test Bcl-2 expression in colorectal neoplasms, assuming that, as a regulator of apoptosis, it might be involved in the progression from adenoma to carcinoma. To this end, Bcl-2 reactivity was tested by immunohistochemistry in hyperplastic polyps, colonic adenomas, and carcinomas and its expression was compared with staining for the proliferation-associated Ki-67 antigen, using the MIB-1 antibody. Bcl-2 expression occurred in 2 out of 10 hyperplastic polyps and in 31 out of 35 (tubular, villous, and tubulovillous) adenomas, irrespective of their degree of dysplasia. Of ten carcinomas, only three were focally Bcl-2-positive, all moderately to well differentiated. In two of four carcinomas in Bcl-2-positive adenomas, no Bcl-2 staining was observed. High numbers of MIB-1-positive cells were found in all hyperplastic and neoplastic lesions, without apparent correlation between proliferation and Bcl-2 expression. These findings suggest that in the pathogenesis of hyperplastic polyps, increased crypt cell proliferation is primarily involved, but in some lesions decreased apoptosis may play a role. Furthermore, the increased Bcl-2 expression in adenomas but not in the majority of the carcinomas suggests either that decreased apoptosis is not usually involved in the pathogenesis of these lesions or that the regulation of apoptosis in colorectal epithelia involves additional regulatory factors.