Cell surface aminopeptidase A and N activities in human glomerular epithelial cells

Cell surface aminopeptidases N (APN) and A (APA) have been characterized on cultured human glomerular epithelial cells and a SV40-transformed cell line derived from them. APN had a wide substrate specificity whereas APA only attacked peptides with an acidic N terminal amino acid. Both enzymes also differed by their sensitivity to divalent cations and to aminopeptidase inhibitors. Phorbolmyristate acetate (PMA) stimulated APN but not APA expression after a lag time of 12 hours. An increase of twice the basal value was observed with 10 ng.ml-1 PMA. This effect was confirmed by immunofluorescence staining using a specific anti-APN monoclonal antibody. Both ecto- and total enzyme activities were stimulated by PMA. The effect of PMA was suppressed by H7, a PKC inhibitor, and cycloheximide, an inhibitor of protein synthesis. Thrombin (1 to 2.5 U.ml-1) and interferon (IFN)-gamma (100 U.ml-1) also stimulated APN activity, the latter after longer exposure of the cells. APA activity was increased by 8-bromo-cAMP and two cAMP-stimulating agents, forskolin and isobutylmethylxanthine (IBMX). A twofold increase above basal value was obtained with 100 microM forskolin after 72 hours of treatment. cAMP-stimulated APA activity was suppressed by cycloheximide. Dexamethasone also stimulated APA activity. The effects of forskolin and dexamethasone were additive. These results demonstrate that APN and APA in glomerular epithelial cells are under different regulations: mitogens and IFN-gamma for APN, cAMP and glucocorticoids for APA. This selective expression may imply possible functional consequences in glomerular diseases.