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Enhancement of Experimental Colon Carcinogenesis by Dietary 6-Phenylhexyl Isothiocyanate¹

Divisions of Nutritional Carcinogenesis [C. V. R., B. S., B. S. R.], Pathology and Toxicology [A. R.], Research Animal Facility [R. H.], and Biostatistics and Computer Facility [E. Z.], American Health Foundation, Valhalla, New York 10595, and Chemoprevention Branch, National Cancer Institute, Bethesda, Maryland 20892 [G. J. K., V. S.]

Naturally occurring and related synthetic isothiocyanates are known to exert chemopreventive effects in several organs in rodent models. The present study was designed to investigate the efficacy of 6-phenylhexyl isothiocyanate (PHITC), a potent chemopreventive agent in the lung tumor model in strain A mice, on azoxymethane-induced colon tumorigenesis. Another aim was to study the modulating effect of PHITC on colonic mucosal and tumor phospholipase A₂ (PLA₂), phosphatidylinositol-specific phospholipase C (PI-PLC), lipoxygenase (LOX), and cyclooxygenase (COX) activities. At 5 weeks of age, groups of male F344 rats were fed control diet or diets containing 320 or 640 ppm of PHITC representing 40 and 80% maximum tolerated dose levels, respectively. At 7 weeks of age, all animals except those in the vehicle-treated groups were given two weekly s.c. injections of azoxymethane at a dose rate of 15 mg/kg body weight/week. All animals continued on their respective dietary regimen for 52 weeks after the carcinogen treatment; then the study was terminated. Colonic mucosa and tumors were analyzed for PLA₂, PI-PLC, prostaglandin (PG) E₂, COX, and LOX activities. Intestinal tumors were evaluated histopathologically and classified as invasive or noninvasive adenocarcinomas. Intestinal tumor incidence (percentage of animals with tumors) and tumor multiplicity (tumors/animal; tumors/tumor-bearing animal) were compared among the dietary groups. At the 640-ppm dose level, dietary PHITC significantly increased the incidence of intestinal (small intestine plus colon) adenocarcinomas (P < 0.05) as well as the multiplicities of invasive and noninvasive adenocarcinomas of the colon (P < 0.05 to 0.01). At the 320-ppm dose level, PHITC increased the multiplicity (tumors/animal) of noninvasive adenocarcinomas and total (invasive plus noninvasive) adenocarcinomas of the colon (P < 0.05). Dietary PHITC also increased the colon tumor volume (2- to 4.3-fold) in a dose-dependent manner. Moreover, PHITC significantly enhanced the activities of PLA₂ (50–100%) and levels of PGE₂ (2-fold) in the colonic mucosa and in tumors, but it had no significant effect (P > 0.05) on PI-PLC activity. The formation of COX metabolites, particularly PGE₂, PGF₂, PGD₂, 6-keto PGF₁, and thromboxane B₂, as well as LOX metabolites such as 8(S)-, 12(S)- and 15 (S)-hydroxyeicosatetraenoic acids, were significantly increased in the colonic mucosa and tumors of animals that were fed 640 ppm of PHITC. Although the exact mechanism by which PHITC promotes colon tumorigenesis remains to be elucidated, it is likely that the tumor-promoting effects of PHITC may, at least in part, be related to increased eicosanoid metabolism in the colon.

¹ Supported by USPHS Grant CN85095-05 and grant CA17613 from the National Cancer Institute.

² To whom requests for reprints should be addressed, at the Division of Nutritional Carcinogenesis, American Health Foundation, One Dana Road, Valhalla, NY 10595.

Received 4/18/95. Accepted 7/26/95.

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