Expression of P-glycoprotein (pgp) was analyzed by immunohistochemical staining with monoclonal antibody JSB-1 in 145 frozen specimens (67 were samples of normal colorectal mucosa from sites adjacent to the tumor, 66 were colorectal carcinomas, 5 colorectal polyps, 5 metastatic lymph nodes, and 2 samples of metastatic liver tumors) of 67 patients with colorectal carcinoma and polyps. All 72 specimens of normal colorectal mucosa and adenomatous polyps expressed pgp to various degrees. By contrast, 18 of 39 (46.2%) samples from cases of well differentiated adenocarcinoma were positive for pgp but only 3 of 21 (14.3%) samples from cases of moderately differentiated adenocarcinoma and none of the 4 samples from cases of poorly differentiated adenocarcinoma were positive for pgp. There was no correlation between the clinicopathological stage of colorectal carcinoma and the expression of pgp. These findings indicate that the expression of P-glycoprotein is closely related to the differentiation of cells. In normal colorectal epithelium, pgp was expressed normally and in well differentiated adenocarcinomas, pgp was still expressed. However, expression of pgp was no longer detectable in carcinomas with moderate or poor differentiation.

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