Overexpression of inducible cyclo-oxygense-2 (COX-2), which catalyses a key step in the conversion of arachidonic acid to prostaglandin H2, is found to be deeply associated with tumorigenesis of colorectal cancers in humans. More than 80 % of human colorectal cancers show higher expression of COX-2 than accompanying normal mucosa (Williams et al. 1999). Thromboxane (TX) synthase, a downstream enzyme of COX-2, catalyses the conversion of prostaglandin H2 to TXA2. Recently, the involvement of TXA2 in endothelial migration, angiogenesis and tumour metastasis has been reported in animal models and cultured cells (Nie et al. 2000).
Here we investigated whether TX synthase and TXA2 are related to human colorectal cancers. The specimens of colorectal well-differentiated adenocarcinomas and adjacent normal mucosa were obtained from surgical resection of patients in accordance with the recommendations of the Declaration of Helsinki. Informed consents were obtained from all patients at Toyama Medical and Pharmaceutical University Hospital. Data are shown as means ± S.E.M. The differences between groups were analysed by one-way ANOVA.
Northern blot analysis showed that TX synthase mRNA overexpressed in 17 of 20 cancers (85 %) compared with the normal mucosa, and average score of its expression in the carcinoma was 7.0 ± 1.5-fold higher than that of the normal mucosa (n = 20). The overexpression of TX synthase does not seem to correlate with age, sex, location and size of carcinoma, or clinical stage. COX-2 mRNA overexpressed in the carcinoma as previously reported; however, this overexpression was less consistent among the samples (40%) compared with the case of TX synthase mRNA. TX synthase mRNA also highly expressed in human colonic cancer cell lines such as HT-29, KM12-L4, T-84 and WiDr. In the immunohistochemistry of fresh-frozen tissues using the anti-human TX synthase monoclonal antibody, the cancer cell itself showed clear immunoreactivity of TX synthase. Such immunoreactivity was not observed in epithelial cells in the adjacent normal mucosa. 9,11-Epithio-11,12-methano-TXA2 (STA2; 0.1 µM), a stable analogue of TXA2, accelerated proliferation of HT-29 and KM12-L4 cells (P < 0.01, n = 6), whereas TXB2 (0.1 µM), a stable metabolite of TXA2, was ineffective (P > 0.05, n = 6).
These results suggest that overexpression of TX synthase and TXA2-induced cell proliferation are associated with human colorectal cancers.