Constitutive expression of FosB and its short form, FosB/SF, induces malignant cell transformation in rat-1A cells.

We have established Rat-1A cell lines constitutively expressing c-Fos and the two products of the fosB gene, FosB and its short form, FosB/SF. The expressed proteins in the different stable transfectants have been characterized by immunofluorescence and immunoprecipitation analysis. Our results demonstrate that constitutive expression of FosB, like the constitutive expression of c-Fos and, to a lesser extent, FosB/SF, results in cells that grow to increased saturation densities and have the ability to grow in an anchorage-independent manner. Most important is the finding that expression of these proteins augments the tumorigenic potential of Rat-1A cells. These results show that both forms of FosB have a stimulatory effect on cell proliferation.