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This study was conducted to determine whether (1) silica particles induce morphological transformation in BALB/c 3T3 cells, (2) the transforming activity of silica is related to particle size, (3) silica-transformed cells possess pre-neoplastic properties, (4) an association exists between silica-induced cell transformation and proto-oncogene activation or tumor suppressor gene inactivation. BALB/c cells were treated with 3 particle-size distributions of quartz (Min-U-Sil 5, Min-U-Sil 10, and Min-U-Sil 30; U.S. Silica Company) in a transformation assay. Neoplastic properties were investigated using an anchorage-independence (soft-agar cloning) assay and transfection-mediated transformation of NIH 3T3 cells. Alterations in oncogenes and a tumor suppressor gene were investigated by dot-blot hybridization studies of transformed cell DNA and RNA with oncogene probes, and by SSCP (single strand conformation polymorphism) analysis and automated DNA sequencing. All 3 particle-size distributions of silica induced morphological transformation, but there was no clear difference in transforming activity among the samples. All cell lines derived from silica-induced transformed cells were capable of anchorage-independent growth, and DNA from all cell lines induced transfection-mediated transformation. There was no amplification of oncogene DNA noted in the dot blot studies, but an increase in H-ras and c-myc mRNA was observed in 2 of 5 transformed cell lines studied. There were no mutations in K-ras detected by SSCP analysis, but 7 of 10 transformed cell lines analyzed had point mutations in the p53 gene confirmed by DNA sequencing. There was no commonly observed pattern to the p53 mutations, and the mutations were confined to exons 2 and 3. It is possible that alterations in the p53 gene, and perhaps in the H-ras and c-myc genes as well, may be involved in silica-induced transformation. No clear conclusion could be drawn regarding the effect of particle size in these studies.