To study the contribution of autocrine and paracrine TGF-pi to tumor progression in a well-defined system of multistage carcinogenesis, keratinocytes with a targeted deletion of the TGF-pi gene were initiated in vitro with the v-ras"^^ 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""-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^' gene, alterations in p53 protein, or loss of responsiveness to TGF-pi. 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-pi null fibroblasts formed the stroma. The null tumor cells could not accumulate TGF-pi from the host, but grafts of uninitiated null keratinocytes, which formed a normal epidermis, became TGF-pl positive even though they did not express TGF-pi mRNA. These results demonstrate that autocrine TGF-pl suppresses the frequency and rate of malignant progression, and that autocrine and paracrine TGF-pi 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-pi by a mechanism that operates in normal cells.

Ampli®cation and overexpression of genes involved in cellular growth control occur frequently in human cancers. Here, we report characterization of the full length OS4 cDNA derived from 12q13-q15 (Su et al., Proc. Natl. Acad. Sci. USA, 91: 9121 ± 9125, 1994), a region frequently ampli®ed in sarcomas and brain tumors. This cDNA consists of 4833 base pairs (bp) encoding an open reading frame (ORF) of 283 amino acids. The ORF predicts a water-soluble acidic (pI 5.50) polypeptide with a molecular weight of 31 759. Database searches revealed highly signi®cant similarity between OS4 and eight proteins predicted from genomic sequences of Caenorhabditis elegans, Schizosaccaharomyces pombe, and Saccharomyces cerevisiae. Thus, OS4 de®nes a novel evolutionarily conserved gene superfamily. Northern and database analyses revealed OS4 transcripts in numerous human tissues demonstrating its ubiquitous expression. We also observed overexpression of OS4 in three cancer cell lines with ampli®cation of this gene. Furthermore, we detected OS4 ampli®cation in 5/5 primary sarcomas with known ampli®cation of the closely linked marker CDK4. These results demonstrate that the highly conserved OS4 gene is frequently included in the 12q13-q15 amplicon and may contribute to the development of a subset of sarcomas.