Chemoprevention of colon cancer by raloxifene, a selective estrogen receptor modulator (SERM).

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Recent studies suggest that estrogen receptor-β (ER-β) over-expression is positively associated with colon tumor development and evidences from estrogen replacement therapies also indicate a lowered risk of colon cancer. Therefore, in the present study we tested the chemopreventive potential of raloxifene (Evista), a selective estrogen receptor modulator (SERM) in a well-established model of colon cancer. Prior to the efficacy study, we assessed the maximum tolerated dose (MTD) and the dose-response effects of raloxifene on AOM-induced colonic aberrant crypt foci (ACF) in F344 rats. For MTD and ACF evaluation, seven week-old male F344 rats were fed the control diet (modified AIN-76A), or the experimental diets containing five different dose levels (0.31, 0.62, 1.25, 2.5 or 5 ppm) of raloxifene. One week later, all animals were s.c. injected with azoxymethane (AOM) (15 mg/kg body wt., once weekly for 2 weeks). At 15 weeks of age, all rats were killed and evaluated for MTD and colonic ACF. For colon tumor efficacy study, rats were fed diets containing 0 or 1 ppm raloxifene until termination, i.e., 48 weeks after carcinogen treatment. Colonic ACF and tumors were evaluated histopathologically. Expression levels of ER-β and markers associated with proliferation and apoptosis were determined in colon tumors and adjacent mucosa by Western blot and immunohistochemistry. Based on the body weight gain, the MTD of raloxifene in male F344 rats was found to be 5 ppm when administered in the AIN-76A diet. Administration of raloxifene at five different dose levels significantly inhibited AOM-induced total colonic ACF (31–36%, p<0.01) and multi-crypt (4 or more) aberrant foci (23–45%, p<0.05–0.01). Raloxifene at dose ranges of 0.31 –2.5 ppm did not show any dose response effect. Administration of raloxifene at the 1 ppm level significantly suppressed AOM-induced adenocarcinoma incidence by >36% (p<0.02). Similarly, administration of raloxifene suppressed AOM-induced colon adenocarcinoma multiplicity by 63% (p<0.001). AOM-induced colon adenocarcinomas showed significant up-regulation of ER-β and PCNA expression levels when compared to normal appearing colonic mucosa. Administration of raloxifene significantly suppressed AOM-induced colon tumor cell proliferation (PCNA-labeling, p<0.01), and an increase in apoptosis (TUNEL, p<0.05) compared to colon tumors of rats fed the control diet. The results of this study provide clear evidence that ER-β antagonist raloxifene prevents the AOM-induced colon carcinogenesis and its effect is associated with suppression of proliferation and induction of apoptosis in colon tumors. [Supported by NCI-CN-25114, NCI-CN-43300 and 1R01-CA-109247]