Expression of c-fos in NIH3T3 cells is very low but inducible throughout the cell cycle.

It has previously been shown that the c-fos proto-oncogene is rapidly and transiently induced following growth factor stimulation of quiescent NIH3T3 mouse fibroblasts. To investigate a possible role of c-fos in growth control mechanisms we have studied its expression and inducibility during the NIH3T3 cell cycle. Two major conclusions can be drawn from this analysis. First, expression of c-fos is not cell cycle-regulated, and is barely detectable in all phases of the cycle. Second, cells at different stages of the cell cycle (except for mitosis) are as sensitive to c-fos induction by growth factors as quiescent cells. These observations suggest that induction of the c-fos gene does not play a role during the continuous cycling of NIH3T3 cells, but they are fully compatible with the hypothesis that a function of c-fos may be associated with the induction of competence in fibroblasts. Through such a function c-fos may contribute to moving cells out of the quiescent state.