An in vivo model for receptor tyrosine kinase autocrine/paracrine activation: auto-stimulated KIT receptor acts as a tumor promoting factor in papillomavirus-induced tumorigenesis.

Constitutive overactivation of growth factor receptors through autocrine/paracrine mechanisms occurs frequently in cancer cells and are thought to play a critical role in carcinogenesis. In the present report, we propose a refined in vivo model which explains the significance of these mechanisms in tumour development. We have previously established transgenic mouse lines containing human papillomavirus type 16 (HPV16) E6E7 oncogenes, in male mice of which a Leydig cell tumor developes with a very high incidence. Not only HPV transgene but also the c-kit proto-oncogene receptor tyrosine kinase and its ligand Steel Factor (SLF) were coexpressed in all tumors analysed. This coexpression of c-kit/SLF was also found in two other Leydig cell tumor lines. Moreover, the proliferation of transgenic tumor cells was attenuated by treatment with a c-kit neutralizing antibody in vitro, strongly suggesting that tumorigenesis is closely related to stimulation of receptors through ligand induction. To confirm the significance of these findings, a defective mutation of the SLF gene in a laboratory mouse, the Steel-Dickey (Sld) mutation, was introduced into a line of transgenic mice showing 100% incidence of the tumor. In Sld-E6E7 transgenic mice, tumorigenesis was initiated but numbers of tumor cells were markedly reduced compared with transgenic mice carrying both wild type SLF allele, showing that c-kit activation through the induction of SLF is essential for testicular tumorigenesis, especially in tumour promotion. This transgenic mice system should be a useful in vivo model for clarifying the implication of growth factor autostimulation in carcinogenesis.