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 (bestat in), had a growth suppressive effect on choriocarcinoma cell lines in vitro. To clarify the mechanism of this action, we investigated the ex pression of aminopeptidase N (AP-N/CD13) on choriocarcinoma cells and other human tumor cells. Two choriocarcinoma cell lines, NaUCC-4 and B eWo, had higher AP-N activity than other cell lines (358.8 and 340.2 n mol/h/10(6) cells, respectively) as did the human myeloid leukemia cel l line, HL-60 (373.8 nmol/h/10(6) cells). These choriocarcinoma and le ukemia cell lines with abundunt AP-N activity showed much higher sensi tivity to bestatin (IC50 = 0.5, 2.1 and 1.0 mu g/ml, respectively) tha n the other cell lines. By immunoblotting and immunocytochemical stain ing, AP-N was detected as an approximately 165-kDa protein and localiz ed on the cell membrane in choriocarcinoma cells. We also examined the effects of two other aminopeptidase inhibitors and three anti-CD13 mo noclonal antibodies (MAbs) (WM15, MCS2 and MY7) on the growth of NaUCC -4 cells. Cell growth was markedly suppressed by the AP-N inhibitor ac tinonin 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 ind icate that AP-N inhibitors show a growth-suppressive effect, presumabl y 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 certa in tumors, such as choriocarcinoma and leukemia.