Relationship Between p53 Mutations and Inducible Nitric Oxide Synthase Expression in Human Colorectal Cancer

Inducible Ca 2+ -independent nitric oxide synthase (NOS), also referred to as NOS2, which is expressed in a variety of human cancers ( 1 - 4 ), can generate mutagenic concentrations of nitric oxide (NO) in mice ( 5 ). NOS2 is the most active isoform among the three known nitric oxide synthases ( 6 ), which also include the neuronal (NOS1) and endothelial (NOS3) isoforms. Only NOS2 is capable of producing sustained NO concentrations in the micromolar range ( 7 ). We investigated the hypothesis that NO generated by NOS2 is capable of inducing mutations in the p53 (also known as TP53) gene and contributes to human colon carcinogenesis. We analyzed 118 sporadic colon tumors for NOS2 expression and p53 gene mutations. Colon tumors and surrounding normal tissues were collected from the Cooperative Human Tissue Network and the Department of Pathology, University of Baltimore, with the approval of local boards governing research on human subjects, as described previously ( 3 ). The expression of NOS2 was increased in various tumors (Fig. 1, A and B) throughout the right, left, and sigmoid colon. Adenomas showed the highest average NOS2 activity. The NOS2 activity declined with advancing tumor stage and was seen at the lowest level in metastatic tumors (Fig. 1 , A). NOS2 activity correlated with NOS2 protein expression. Immunohistochemical analysis localized NOS2 protein mainly in tumor-infiltrating mononuclear cells and less frequently in endothelial cells within the tumors and in the tumor cells (Fig. 1 , B). The decline in NOS2 activity with advancing tumor stage may be attributed to the tumor cell-induced immunosuppression of NOS2 expression in tumor-infiltrating mononuclear cells of advanced tumors ( 8 ).