Suppression of azoxymethane-induced colonic aberrant crypt foci by a nitric oxide synthase inhibitor Toshihiko Kawamori * , Mami Takahashi, Kouji Watanabe, Toshihisa Ohta, Seiichi Nakatsugi, Takashi Sugimura, Keiji Wakabayashi Cancer Prevention Division, National Cancer Center Research Institute, 1-1 Tsukiji 5-chome, Chuo-ku, Tokyo 104 -0045, Japan Received 5 July 1999; received in revised form 26 July 1999; accepted 2 August 1999 Abstract Nitric oxide synthase (NOS), an important bioregulator of a variety of biological processes, is overexpressed in colonic tumors of humans and rodents. In this study, effects of l-N G -nitroarginine methyl ester (L-NAME), a NOS inhibitor, on development of aberrant crypt foci (ACF) induced by azoxymethane (AOM) in F344 male rats were investigated. Six- week-old male F344 rats were fed diets containing 0 or 100 ppm L-NAME, and given s.c. injections of AOM at 15 mg/kg body wt. once a week for 2 weeks. At 17 weeks of age, all animals were sacri®ced and their colons were evaluated for numbers of ACF. Feeding of 100 ppm L-NAME inhibited the development of ACF in different sizes by 24±39%, those containing four or more crypts being most markedly affected. Assessment of silver-stained nucleolar organizer regions protein (AgNORs)/nucleus further revealed a 44% reduction by administration of L-NAME. These results suggest that the NOS inhibitor, L-NAME, may be an effective chemopreventive agent against colon carcinogenesis due to depression of cell proliferation. q 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Azoxymethane; Aberrant crypt foci; Nitric oxide synthase inhibitor; Colon; Chemoprevention 1. Introduction Colorectal cancer is one of the leading causes of cancer deaths in both men and women in Western countries, including the United States [1]. In Japan, the progressive introduction of Western dietary habits, especially an increasing fat intake and decreas- ing carbohydrate intake, has been paralleled by an increase in colon and breast cancers [2]. Development of chemopreventive approaches is a high priority. NO is endogenously produced by a family of three distinctive nitric oxide synthase (NOS) isoforms [3,4]. Two are constitutive and calcium-dependent, one being present in vascular endothelial cells (eNOS), and the other in the neurons of the central and periph- eral nervous system (nNOS). The third is inducible (iNOS) and calcium-independent and produced in response to bacterial endotoxins and cytokines to provide a sustained release of NO, which mediates immune cell cytotoxicity. Recent studies have reported an increased level of NOS expression in human cancers including ovarian [5], breast [6], central nervous system [7] and colon [8±10] tumors. Also, in a rodent model, we previously found that azoxymethane (AOM)-induced colon tumors show increased expression of both iNOS and eNOS proteins as compared with normal colonic mucosa using Cancer Letters 148 (2000) 33±37 0304-3835/00/$ - see front matter q 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3835(99)00310-9 www.elsevier.com/locate/canlet * Corresponding author. Tel.: 181-3-3542-2511 (ext. 4501); fax: 181-3-3543-9305. E-mail address: tkawamor@gan2.ncc.go.jp (T. Kawamori)