Cyclooxygenase-2 activation mediates the proangiogenic effect of nitric oxide in colorectal cancer

Purpose. Up-regulation of both inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) enzymes has been reported in colorectal cancer. We aimed at evaluating the possible interaction between the NO and COX-2 pathways and its effect on promoting tumor angiogenesis. Experimental design. Expression of iNOS, COX-2, vascular endothelial growth factor (VEGF) and CD31 was analyzed in tumor samples and corresponding normal mucosa obtained from 46 surgical specimens. We also evaluated iNOS activity, prostaglandin E2 (PGE2), cGMP and cAMP production in the same specimens. Nitrite/nitrate levels and PGE2 and VEGF production were assessed in HCT116 and HT29 colon cancer cell lines after induction and selective inhibition of the two enzyme pathways . Results. A significant correlation was found between iNOS and COX-2 immunohistochemical expression. PGE2 production significantly correlated with iNOS activity and cGMP levels. A significant correlation was also found among PGE2 production, microvessel density (MVD) and VEGF expression. Co-induction of both iNOS and COX-2 activities occurred after lipopolysaccharide (LPS) and epidermal growth factor (EGF) treatment in HCT116 and HT29 cells. Inhibition of iNOS by 1400W significantly reduced both LPS- and EGF-induced PGE2 production. Treatment with LPS, EGF and arachidonic acid significantly increased VEGF production in the iNOS negative/COX-2 positive HT29 cells. This effect was completely reversed by treatment with the selective COX-2 inhibitor celecoxib. Conclusions. Our data showed a prominent role of NO in stimulating COX-2 activity in colorectal cancer. This interaction is likely to produce a cooperative effect in promoting angiogenesis through PGE2- mediated increase in VEGF production.