Suppression of the growth factor-mediated induction of c-fos and down-modulation of AP-1-binding activity are not required for cellular senescence.

It has been suggested that the impaired response of the c-fos gene to serum growth factors and the concomitant loss of AP-1 activity may be a crucial step in the process of cellular senescence. In the present study, we provide evidence arguing against such a conclusion. Data obtained in five independent experiments showed that both c-fos RNA and protein expression were similar in «young» and in senescent serum-stimulated WI-38 cells, suggesting that the previously reported suppression of c-fos induction is not an obligatory event in the process of cellular senescence. Likewise, expression of fra-1, c-jun and junB continued to be high in serum-stimulated senescent cells, while induction of fosB was reduced approximately fivefold