Secretion of protease nexin-II/amyloid β protein precursor by human colorectal carcinoma cells and its modulation by cytokines/growth factors and proteinase inhibitors

Trypsin inhibitors secreted by human colorectal adenocarcinoma cell lines were analyzed by reverse zymography. Among eleven cell lines analyzed, the major inhibitor secreted was protease nexin-ll (PN-II), a secreted form of amyloid beta protein precursor (APP) containing a Kunitz-type serine proteinase inhibitor domain. Expression of the APP gene was also confirmed in the cell lines and the main APP mRNA species were PN-II types. The APP gene expression was constant during cell growth in vitro. On the other hand, the rate of extracellular PN-II accumulation markedly increased after long-term serum-free maintenance of the confluent culture. The extracellular accumulation of PN-II was also strongly stimulated either by interleukin-1 beta (IL-1 beta) treatment or to a lesser extent by basic fibroblast growth factor, tumor necrosis factor-a, hepatocyte growth factor or epidermal growth factor. Neither serum depletion- nor IL-1 beta-induced stimulation of extracellular PN-H accumulation were accompanied by obvious alteration of the levels of APP mRNA and cellular APP holoprotein, suggesting that the enhanced extracellular accumulation of PN-II might result from up-regulation of the secretory pathway of APP. The IL-1 beta-induced PN-II secretion was significantly inhibited by relatively high concentrations (50-200 mu g/ml) of aprotinin, a serine proteinase inhibitor, in a dose-dependent manner without obvious cell-toxic effects.