NO-donating aspirin inhibits angiogenesis by suppressing VEGF expression in HT-29 human colon cancer mouse xenografts.

The inhibitory effect of NO-donating aspirin (NO-ASA) on colon cancer has been demonstrated in vivo and in vitro but its mechanism is still obscure. We investigated the effect of NO-ASA on angiogenesis. Four groups of athymic mice (N = 12) bearing subcutaneous xenotransplants of HT-29 human colon cancer cells were injected intratumorally twice a week for 3 weeks with vehicle or m-NO-ASA or p-NO-ASA; the fourth group received no injections. The necrotic area of tumors, expressed as percentage of total area, was similar in the non-injected and vehicle-injected groups (51.8 +/- 2.8 versus 52.2 +/- 4.1, P > 0.05; mean +/- SEM for these and subsequent values). Compared with the vehicle group, the necrotic area of tumors was higher in the m-NO-ASA-treated (61.0 +/- 2.7, P < 0.02) and p-NO-ASA (65.8 +/- 2.4, P < 0.001)-treated groups. NO-ASA decreased microvessel density: vehicle = 11.7 +/- 0.8; m-NO-ASA = 7.8 +/- 0.6 (P = 0.0003 versus vehicle) and p-NO-ASA 6.2 +/- 0.7 (P = 0.0001 versus vehicle). The expression of vascular endothelial growth factor (VEGF) was significantly reduced in response to NO-ASA, with the p- isomer being more potent than the m-. NO-ASA altered the spatial distribution of VGEF expression, with 16.7% of the vehicle-treated xenografts displaying diminished VEGF in the inner region of the area between necrosis and the outer perimeter of the tumor, compared with those treated with m- (58.3%) or p-NO-ASA (75%, P < 0.01 for both versus control). Our findings indicate that NO-ASA suppresses the expression of VEGF, which leads to suppressed angiogenesis. The antiangiogenic activity of NO-ASA may be part of its antineoplastic effect.