Cecal Tumorigenesis in Aryl Hydrocarbon Receptor–Deficient Mice Depends on Cecum-Specific Mitogen-Activated Protein Kinase Pathway Activation and Inflammation

The aryl hydrocarbon receptor (AhR) is a transcription factor known as a dioxin receptor. Recently, Ahr−/− mice were revealed to develop cecal tumors with inflammation and Wnt/β-catenin pathway activation. However, whether β-catenin degradation is AhR dependent remains unclear. To determine whether other signaling pathways function in Ahr−/− cecal tumorigenesis, we investigated histologic characteristics of the tumors and cytokine/chemokine production in tumors and Ahr−/− peritoneal macrophages. AhR expression was also assessed in human colorectal carcinomas. Of the 28 Ahr−/− mice, 10 developed cecal lesions by 50 weeks of age, an incidence significantly lower than previously reported. Cecal lesions of Ahr−/− mice developed from serrated hyperplasia to adenoma/dysplasia-like neoplasia with enhanced proliferation. Macrophage and neutrophil infiltration into the lesions was also observed early in serrated hyperplasia, although adjacent mucosa was devoid of inflammation. Il1b, Il6, Ccl2, and Cxcl5 were up-regulated at lesion sites, whereas only IL-6 production increased in Ahr−/− peritoneal macrophages after lipopolysaccharide + ATP stimulation. Neither Myc (alias c-myc) up-regulation nor β-catenin nuclear translocation was observed, unlike previously reported. Interestingly, enhanced phosphorylation of extracellular signal-regulated kinase, Src, and epidermal growth factor receptor and Amphiregulin up-regulation at Ahr−/− lesion sites were detected. In human serrated lesions, however, AhR expression in epithelial cells was up-regulated despite morphologic similarity to Ahr−/− cecal lesions. Our results suggest novel mechanisms underlying Ahr−/− cecal tumorigenesis, depending primarily on cecum-specific mitogen-activated protein kinase pathway activation and inflammation.