Riboflavin carrier protein: a serum and tissue marker for breast carcinoma

We have earlier shown that the estrogen-modulated riboflavin carrier protein (RCP) first isolated from the chicken egg is evolutionarily conserved in mammals and is elaborated by lactating mammary gland as demonstrated with rat mammary epithelial cells in culture and confirmed by isolation of the vitamin carrier from bovine milk. In view of several earlier reports that many milk proteins as well as other estrogen-inducible proteins are up-regulated and secreted into circulation in animal models and in women with neoplastic breast disease, we analyzed serum RCP levels in a double-blind study using a specific radioimmunoassay in pre- and post-menopausal women with clinically diagnosed breast cancer at early and advanced stages of the disease and compared these levels with those in normal age-matched control volunteers. Our data reveal that the serum RCP levels in cycling breast cancer patients are 3- to 4-fold higher (p < 0.01) than those in their normal counterparts. This difference in circulatory RCP levels between cancer patients and their age-matched normal counterparts is further magnified to 9- to 11-fold (p < 0.005) at the post-menopausal stage. In addition, there seems to be a good correlation between rising RCP levels and disease progression, since significantly higher RCP concentrations (p < 0.005) are encountered in patients with advanced metastasizing breast cancer versus those with early disease. Using specific monoclonal antibodies, RCP could be localized immunohistochemically in the cytoplasm of invading neoplastic cells of lobular and ductal carcinomas of the breast, indicating that the malignant cells are probably the source of the elevated serum RCP levels in breast cancer. These findings suggest that measurement of circulatory RCP and the immunohistochemical staining pattern of RCP in biopsy specimens could be exploited as an additional marker in diagnosis/prognosis of breast cancer in women.