Chemical inhibition reveals differential requirements of signaling pathways in kras(V12)- and Myc-induced liver tumors in transgenic zebrafish

Previously we have generated inducible liver tumor models by transgenic expression of an oncogene and robust tumorigenesis can be rapidly induced by activation of the oncogene in both juvenile and adult fish. In the present study, we aimed at chemical intervention of tumorigenesis for understanding molecular pathways of tumorigenesis and for potential development of a chemical screening tool for anti-cancer drug discovery. Thus, we evaluated the roles of several major signaling pathways in kras(V12)- or Myc-induced liver tumors by using several small molecule inhibitors: SU5402 and SU6668 for VEGF/FGF signaling; IWR1 and cardionogen 1 for Wnt signaling; and cyclopamine and Gant61 for Hedgehog signaling. Inhibition of VEGF/FGF signaling was found to deter both Myc- and kras(V12)-induced liver tumorigenesis while suppression of Wnt signaling relaxed only Myc- but not kras(V12)-induced liver tumorigenesis. Inhibiting Hedgehog signaling did not suppress either kras(V12) or Myc-induced tumors. The suppression of liver tumorigenesis was accompanied with a decrease of cell proliferation, increase of apoptosis, distorted liver histology. Collectively, our observations suggested the requirement of VEGF/FGF signaling but not the hedgehog signaling in liver tumorigenesis in both transgenic fry. However, Wnt signaling appeared to be required for liver tumorigenesis only in Myc but not kras(V12) transgenic zebrafish.