SMAD3 in Embryonic Patterning, Mesoderm Induction, and Colorectal Cancer in the Mouse
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Smad3 transduces TGF-β/nodal/activin signals. We ventured to define the roles of Smad3 in development and colorectal cancer in the mouse. Embryos deficient in Smad3 and a related molecule, Smad2, developed defects in anterior morphogenesis, left-right determination, and anterior primitive streak induction indicating that these molecules function redundantly in transducing nodal signals in the mouse embryo. In addition, loss of Smad2 and Smad3 in embryonic stem cells led to impaired mesoderm formation by these cells. Smad3 mutant mice develop colon cancer (Zhu et al., 1998). We show that Smad3 loss in the mouse promotes the transition of benign Apc deficient intestinal polyps to invasive cancers. Using a conditional Smad3 mutant, nullizygosity of Smad3 in the colonic epithelium is demonstrated to be non-essential for the development of colorectal cancer in Smad3 mutant mice. We show that Smad3 mutant mice have gene expression changes in the colon consistent with bowel inflammation, potentially in response to intestinal flora antigens. Suppression of intestinal bacterial flora in Smad3 mutant mice with neomycin plus metronidazole led to nearly complete suppression of colorectal tumorigenesis. These data indicate that Smad3 has important immunosuppressive and tumor suppressive functions in vivo consistent with its role in TGF-β signaling. Lastly, we investigated the role of cyclooxygenase-2 (Cox-2) in modifying tumorigensis in Smad3 mutant mice. Contrary to previous reports in Apc mutant mice, Cox-2 heterozygosity did not effect tumorigenesis in Smad3 mutant mice. In carcinogen treated and ApcMin mice, Cox-2 heterozygosity had no effect on tumorigenesis whereas nullizygosity for Cox-2 suppressed colonic tumor number. Small intestinal polyp number, a sensitive indicator of tumor initiation, was unchanged in Cox-2 deficient ApcMin mice. Treatment of Cox-2 wild type or heterozygous ApcMin mice with the NSAID sulindac suppressed intestinal tumor number and size to a greater extent than Cox-2 nullizygosity suggesting that NSAIDs target Cox-2 independent pathways in vivo. To test this, we treated Cox-2 null ApcMin mice with sulindac and found significant, large magnitude decreases in polyp number and polyp size. These data suggest that using selective Cox-2 inhibitors as chemotherapeutic agents for colorectal cancer may not yield the maximal anti-neoplastic effects of NSAIDs.