Targeted deletion of the TGF-β1 gene causes rapid progression to squamous cell carcinoma

To study the contribution of autocrine and paracrine TGF-β1 to tumor progression in a well-defined system of multistage carcinogenesis, keratinocytes with a targeted deletion of the TGF-β1 gene were initiated in vitro with the v-ras(Ha) oncogene and their in vivo tumorigenic properties were determined by skin grafting initiated cells onto athymic mice in combination with either wild-type or null dermal fibroblasts. Grafts of v- ras(Ha)-initiated null keratinocytes progressed rapidly to multifocal squamous cell carcinomas within dysplastic papillomas irrespective of the fibroblast genotype, whereas the initiated control genotypes formed well- differentiated papillomas. Malignant progression was not associated with mutations in the c-ras(Ha) gene, alterations in p53 protein, or loss of responsiveness to TGF-β1. The tumor cell labeling index was elevated in grafts of initiated null keratinocytes with wild-type fibroblasts compared to tumors of other genotypes. However, labeling index in all tumors was reduced when TGF-β1 null fibroblasts formed the stroma. The null tumor cells could not accumulate TGF-β1 from the host, but grafts of uninitiated null keratinocytes, which formed a normal epidermis, became TGF-β1 positive even though they did not express TGF-β1 mRNA. These results demonstrate that autocrine TGF-β1 suppresses the frequency and rate of malignant progression, and that autocrine and paracrine TGF-β1 can have opposing effects on tumor cell proliferation. The lack of paracrine inhibition of tumor cell progression appears to result from the inability of tumor cells to localize host-derived TGF-β1 by a mechanism that operates in normal cells.