[Role of growth factor signaling in epithelial cell plasticity during development and in carcinogenesis].

The work described in this review addresses the multifunctional roles of growth factors and their cognate receptors in normal development and in tumorigenesis. The concept of epithelial cell plasticity is described in the context of embryonic development during which frequent remodeling occurs in epithelial tissues. The critical role of one member of the FGF family of growth factors is demonstrated in lung branching morphogenesis. Several members of this family have been shown to induce an epithelial-mesenchymal transition in a bladder carcinoma line. In vivo the same factors act in an autocrine or paracrine mode to favor tumor progression. It is suggested that an EGFR-ligand autocrine loop exerts a positive role in tumor progression of human bladder carcinoma whereas FGFR2 acts as a phenotypic tumor suppressor gene. Unexpectedly, constitutive activating mutations in FGFR3 have been uncovered in the majority of the Ta superficial tumors which progress only very rarely to the invasive stages. In contrast, in situ carcinoma, which are considered to be associated with a strong malignant potential, do not carry the FGFR3 mutations. The presence or absence of the mutations defines two distinct oncogenic pathways in bladder carcinogenesis. The studies reveal the complexities in defining the putative functions of growth factors at different times and differentiation stages during development and in tumor progression. These results emphasize the need for caution in the interpretation of studies evaluating the potential of novel anti-cancer agents and for better designs of in vitro and in vivo biological assays.