Colorectal cancer, comprising 85% sporadic non-hereditary tumors, is the third in cancer incidence and the second leading cause of cancer mortality in the United States. Initiating mutations in key genes, APC, KRAS and p53, determine transformation and early oncogenesis in the colon. The initiating mutation in the APC tumor suppressor gene activates Wnt signaling. Progression mostly requires mutation in the KRAS oncogene, facilitating cytoplasmic->nuclear translocation of β-catenin. Mutation in p53 coincides with the transition from adenoma->carcinoma. While the specific genetic mutations that determine oncogenesis in colon cancer are well documented, the fundamental biologic mechanisms that determine synergism in effecting transformation remain poorly understood. The GLI genes are transcription factors that regulate Hedgehog signaling, which is activated in colon cancer. GLI1 and GLI2 are oncogenes, induce transformation, and determine oncogenesis in glioma, medulloblastoma (GLI1), and basal cell carcinoma (GLI2). We have demonstrated in colon carcinoma cells that GLI2 is activated by oncogenic KRAS, activates GLI1, and determines cell survival. In immortalized HCEC cells with defined genetic mutations, we discovered that GLI2 is activated during transformation, dependent on synergistic interactions between mAPC and KRAS[G12V] pathways in the presence of inactive p53. This renders cells dependent on GLI for survival. In isogenic HCEC models, KRAS[G12V] determines β-catenin subcellular trafficking between membrane adherens junctions (AJ), cytoplasm and nucleus. This is apparent following inhibition of ERK (AJ->cytoplasm) and/or GSK3β (AJ->nucleus) that determine β-catenin subcellular localization. Further, we have identified GLI2 as a new Wnt target gene, transcriptionally regulated by Wnt-dependent TCF4, which binds to consensus sequences in the GLI2 promoter. Oncogenic KRAS signaling channels through and converges on GLI2 to drive GLI2 to a higher activating state. GLI2 is transcriptionally regulated by an ERK-dependent mechanism that requires the GLI cofactor, ZIC2. Thus, GLI2 is activated by both Wnt and oncogenic KRAS signaling pathways. In summary, we present a new molecular model that identifies the GLI2 oncogene as a critical determinant of human colonic epithelial cell transformation. These studies will: 1) define the key mutational events and biologic mechanisms that functionally activate the oncogene GLI2 during colonic epithelial cell transformation; 2) define the role of GLI2 in early oncogenesis; 3) identify key molecules and potential new targets that determine transformation; 4) lead to improved therapeutic strategies specific to colorectal cancer.
Citation Format: Tapati Mazumdar, Akwasi Agyeman, Jerry W. Shay, Janet A. Houghton. New molecular model identifying the critical role of the GLI2 oncogene in human colonic epithelial cell (HCEC) transformation. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 467. doi:10.1158/1538-7445.AM2014-467