Secretion of protease nexin-II/amyloid beta 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 w ere analyzed by reverse zymography. Among eleven cell lines analyzed, the m ajor inhibitor secreted was protease nexin-ll (PN-II), a secreted form of a myloid beta protein precursor (APP) containing a Kunitz-type serine protein ase 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 exp ression was constant during cell growth in vitro. On the other hand, the ra te of extracellular PN-II accumulation markedly increased after long-term s erum-free maintenance of the confluent culture. The extracellular accumulat ion 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 facto r. Neither serum depletion- nor IL-1 beta-induced stimulation of extracellu lar PN-H accumulation were accompanied by obvious alteration of the levels of APP mRNA and cellular APP holoprotein, suggesting that the enhanced extr acellular accumulation of PN-II might result from up-regulation of the secr etory pathway of APP. The IL-1 beta-induced PN-II secretion was significant ly inhibited by relatively high concentrations (50-200 mu g/ml) of aprotini n, a serine proteinase inhibitor, in a dose-dependent manner without obviou s cell-toxic effects.