Overexpression of human aspartyl (asparaginyl) beta-hydroxylase is associated with malignant transformation

The human aspartyl (asparaginyl) beta-hydroxylase (HAAH) is a highly conser ved enzyme that hydroxylates epidermal growth factor-like domains in transf ormation-associated proteins. We previously reported overexpression of the HAAH gene in human hepatocellular carcinomas and cholangiocarcinomas (L. La vaissiere et at, J. Clin, Investig., 98: 1313-1323, 1996), In the present s tudy, we determined whether HAAH protein overexpression was linked to cellu lar proliferation or malignant transformation of bile ducts by using a huma n disease and rat model of bile duct proliferation. In addition, the transf orming properties of the AAH genes were assessed by transient and stable tr ansfection of NIH-3T3 cells with human and murine wild-type as well as muta nt cDNA constructs that lacked hydroxylation activity. Cellular characteris tics of the malignant phenotype were assessed by formation of transformed f oci, growth in soft agar, and tumor development in nude mice. We found that HAAH gene expression was undetectable during bile duct proliferation in bo th human disease and rat models as compared with cholangiocarcinoma. Overex pression of HAAH in NIH-3T3 cells was associated with generation of a malig nant phenotype, and enzymatic activity was required for cellular transforma tion. These findings suggest that overexpression of HAAH is linked to cellu lar transformation of biliary epithelial cells.