Expression of Aminopeptidase N on Human Choriocarcinoma Cells and Cell Growth Suppression by the Inhibition of Aminopeptidase N Activity

Abstract

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 Be Wo, had higher AP‐N activity than other cell lines (358.8 and 340.2 nmol/h/106 cells, respectively), as did the human myeloid leukemia cell line, HL‐60 (373.8 nmol/h/106 cells). These Choriocarcinoma and leukemia cell lines with abundant AP‐N activity showed much higher sensitivity to bestatin (IC50 = 0.5, 2.1 and l.0μg/ml, respectively) than the other cell lines. By imniuiioblotting 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.