EXPRESSION OF AMINOPEPTIDASE-N ON HUMAN CHORIOCARCINOMA CELLS AND CELL-GROWTH SUPPRESSION BY THE INHIBITION OF AMINOPEPTIDASE-N ACTIVITY

We previously found that an aminopeptidase inhibitor, ubenimex (bestatin), had a growth suppressive effect on choriocarcinoma cell lines in vitro. To clarify the mechanism of this action, we investigated the expression of aminopeptidase N (AP-N/CD13) on choriocarcinoma cells and other human tumor cells. Two choriocarcinoma cell lines, NaUCC-4 and BeWo, had higher AP-N activity than other cell lines (358.8 and 340.2 nmol/h/10(6) cells, respectively) as did the human myeloid leukemia cell line, HL-60 (373.8 nmol/h/10(6) cells). These choriocarcinoma and leukemia cell lines with abundunt AP-N activity showed much higher sensitivity to bestatin (IC50 = 0.5, 2.1 and 1.0 mu g/ml, respectively) than the other cell lines. By immunoblotting and immunocytochemical staining, AP-N was detected as an approximately 165-kDa protein and localized on the cell membrane in choriocarcinoma cells. We also examined the effects of two other aminopeptidase inhibitors and three anti-CD13 monoclonal antibodies (MAbs) (WM15, MCS2 and MY7) on the growth of NaUCC-4 cells. Cell growth was markedly suppressed by the AP-N inhibitor actinonin as well as bestatin, but not by the AP-B inhibitor arphamenine. Of the three MAbs, only WM15, which is able to inhibit AP-N activity, suppressed cell growth in a dose-dependent manner. These results indicate that AP-N inhibitors show a growth-suppressive effect, presumably through inhibition of the enzymatic activity of AP-N on tumor cells, and suggest that AP-N may play important roles in the growth of certain tumors, such as choriocarcinoma and leukemia.