Fos family members: regulation, structure and role in oncogenic transformation.

The members of the Fos protein family might be subdivided in two groups, according to their ability to transform rodent fibroblasts, transforming (c-Fos and FosB) and non-transforming (Fra-1 and Fra-2) proteins. Members of these groups are differently activated in response to external stimuli and possess different structural features. Importantly, whilst c-Fos and FosB contain multiple transactivation modules in their N- and C-terminal parts, transactivation domains are absent in the non-transforming Fos proteins. As a result, Fra-1 and Fra-2 though efficiently form dimers with the Jun proteins, are weak transcriptional activators and inhibit the c-Fos-dependent activation in transient transfection assay. The numerous experiments performed with the different Fos mutant proteins with impaired transforming ability, as well as with chimeric proteins revealed the importance of the transactivation function for transformation. Fra-1 and Fra-2 proteins albeit ineffectively triggering oncogenic transformation, are abundant in ras- and src-transformed murine and chicken fibroblasts, in neoplastic thyroid cells and in highly malignant mouse adenocarcinoma cells, which underwent mesenchymal transition. The abundance of the non-transforming Fos proteins in these systems might be mediated by a positive AP-l-dependent feedback mechanism, as well as by wnt signals. Furthermore, the manipulation of the Fra-1 expression level in thyroid and mammary tumor cells modulated the transcription of several tumor progression markers and affected cell morphology and invasiveness. These recent data demonstrate a novel function of non-transforming Fos proteins in the maintenance and progression of the transformed state. Interestingly, this function is independent of the documented invalidity of the Fra-1 and Fra-2 proteins as transcriptional activators in rodent fibroblasts.