INCREASED EXPRESSION OF NEPRILYSIN (NEUTRAL ENDOPEPTIDASE-24.11) IN RAT AND HUMAN HEPATOCELLULAR CARCINOMAS

BACKGROUND: Neprilysin (EC 3.4.24.11) (NEP), a membrane metallopeptida se, is identical with common acute lymphoblastic leukemia antigen or c luster differentiation antigen 10. This antigen is present in blast ce lls in acute lymphoblastic leukemias and is implicated in differentiat ion of B lymphocytes. NEP cleaves a variety of peptides including brad ykinin, substance P, bombesin, enkephalins, and atrial natriuretic pep tide. We investigated its expression in several variants of rat hepato mas and a human hepatocellular carcinoma cell line. Normal rat and hum an livers were used as controls. EXPERIMENTAL DESIGN: The expression o f NEP (common acute lymphoblastic leukemia antigen) was determined wit h: (a) enzyme assays; (b) high performance liquid chromatography analy sis of bradykinin metabolism; (c) immunoprecipitation; and (d) mRNA ch aracterization. RESULTS: NEP activity increased by 2 to 3 orders of ma gnitude in all rat hepatomas and in the human SK-HEP1 cell line, compa red with normal tissues. Antiserum against rat NEP precipitated 93% of endopeptidase activity in rat hepatomas, whereas monoclonal antibody to common acute lymphoblastic leukemia antigen immunoprecipitated 99% of that in humanhepatocarcinoma cells. Solubilized rat hepatoma membra nes cleaved bradykinin to a hepta- and dipeptide; the reaction was inh ibited by an NEP inhibitor. Activity of three other membrane peptidase s-did not increase in rat hepatomas. Northern hybridization revealed t he presence of NEP mRNA in rat hepatoma, but not in normal liver. Reve rse transcriptase-polymerase chain reaction showed that hepatomas have higher amounts of NEP mRNA than normal liver of the same strain. CONC LUSIONS: Rat hepatomas and a human hepatocarcinoma cell line express h igh amounts of NEP, in contrast to normal rat and human livers, which have very little. The increase in NEP activity could be due to increas ed transcription by tumor cells and may signal malignant transformatio n of liver cells.