Smooth muscle α-actin is a transformation-sensitive marker for mouse NIH 3T3 and Rat-2 cells

Abstract

Heteroploid mouse NIH 3T3 fibroblasts and several rat fibroblast strains (Rat-1, Rat-2 and REF-52) are cell lines of special interest in the field of carcinogenesis because of their extensive use as normal cells in transformation assays for putative cancer-causing genes. Exposure of these cells to carcinogenic chemicals or oncogenic DNA produces anchorage-independent cells with retracted cytoplasms that lack actin cables^1–7. All human fibroblast strains, normal and transformed, synthesize two electrophoretic forms of actin (β- and γ-actin)^8–10. In contrast, we discovered that early-passage mouse and rat strains synthesize abundant amounts of each of the three electrophoretic forms of actin (α-, β- and γ-actin) but mouse and rat cancer cells express only β- and γ-actins. We now show that in NIH 3T3 and Rat-2 fibroblasts a third actin, the smooth muscle α isoform, is abundantly co-expressed with β-and γ-actin. In every instance tested following transformation to tumorigenicity, the accumulation of a-actin messenger RNA and α-actin synthesis was greatly inhibited. Shutdown of α-actin expression thus appears to be a reproducible transformation-sensitive marker in rodent fibroblasts.