This study aimed to clarify the clinical significance of the expression of KL-6 mucin, a type of MUC1, in primary liver cancer. Tissue specimens were collected from 21 patients with cholangiocarcinoma (CC), 78 with hepatocellular carcinoma (HCC), and 12 with combined hepatocellular and cholangiocarcinoma (cHCC-CC). Immunohistochemical analysis was done using a monoclonal antibody for KL-6 mucin as well as antibodies for Hep1 or CK7. KL-6 staining was positive in all the CC tissues examined, while it was not positive in any of the HCC tissues. Similar selectivity of KL-6 staining was also observed in the cHCC-CC specimens, and the cholangiocellular tissue could be clearly delineated by KL-6 staining. In contrast, 79.5% of HCC specimens and 25.0% of cHCC-CC specimens were positive for Hep1 in the hepatocellular tissues, while none of the CC or cHCC-CC specimens were positive in the cholangiocellular tissues. Staining for CK7 was positive in 95.2% of CC and 35.9% of HCC specimens, while 58.3 and 25.0% of cHCC-CC specimens displayed positivity for CK7 in the cholangiocellular and hepatocellular tissues, respectively. These results suggest that KL-6 may be a useful tumor marker for distinguishing CC from HCC. In addition, the high selectivity of KL-6 for cholangiocallular tissue may help to provide information for deciding the clinical strategy in cHCC-CC patients.

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