CONSTITUTIVE EXPRESSION OF THE E2F1 TRANSCRIPTION FACTOR IN FIBROBLASTS ALTERS G0 AND S-PHASE TRANSIT FOLLOWING SERUM STIMULATION

The E2F1 transcription factor was constitutively expressed in NIH3T3 f ibroblasts to determine its effect on the cell cycle. These E2F1 cell lines were not tightly synchronized in GO phase of the cell cycle foll owing serum starvation, as are normal fibroblasts. Instead, the cells are spread throughout GO and G1 phase with a portion of the population initiating DNA synthesis. Upon serum stimulation, the remaining cells in G0/G1 begin to enter S phase immediately but with a reduced rate. Constitutive expression of E2F1 appears to primarily affect the GO pha se, since transit of proliferating E2F1 cell lines through G1 phase is the same as control cells. Consistent with a shortened GO phase, the E2F1 cell lines have a significantly reduced cellular volume. Addition ally, the first S phase after serum stimulation, but not subsequent S phases, is nearly doubled in the E2F1 cell lines compared with control cells. Cell lines expressing a deletion mutant of E2F1 (termed E2F1d8 7), known to significantly affect cell shape, have cell cycle and volu me characteristics similar to the E2F1 expressing cells. However, all S phase durations are considerably lengthened and the cells demonstrat e delayed growth after plating.