Expression of biliary glycoprotein (CD66a) in normal and malignant breast epithelial cells

Biliary glycoprotein (BGP, CD66a, or C-CAM-I) is a cell adhesion glycoprote in expressed in colon, liver, and hematopoietic tissues. Four major isoform s (a-d) of BGP are expressed in most epithelial tissues by alternative mRNA splicing from a single gene. Since BCP is down regulated in colon cancer a nd in premalignant colonic adenomas, it has been of interest to study its e xpression in other tumors. Using immunohistochemistry with a BGP specific a ntibody, and mRNA analysis by in situ hybridization, RNase protection, and RT-PCR we show here that BGP is expressed to the same extent in both normal and malignant breast, demonstrating that BGP is not down regulated in brea st cancer. In normal breast, BGP expression is confined to the apical surfa ce of ductal and lobular epithelial cells, while in invasive carcinoma of t he breast, BGP is expressed throughout the cytoplasm. In situ hybridization shows a specific pattern of BGP expression in both normal and malignant br east epithelium. RNase protection analysis confirms the immunohistochemistr y results and shows no quantitative differences between normal and malignan t breast. RT-PCR analysis agrees with these results and shows that only 3 o f the 4 major isoforms (a, c, d) of BGP are expressed in normal and maligna nt breast. Since recent studies by Turbide et al (Cancer Res 57: 2781-2788, 1997) have shown that the ratio of murine BGP isoforms may affect tumor su ppression in colonic cancer, it is proposed here that the isoform differenc e between human breast and colon may account for the observed lack of BGP d own-regulation in breast vs colon cancer.