Overexpression of p21waf1/cip1 arrests the growth of chicken embryo fibroblasts that overexpress E2F1

P21(waf1/cip1) is a potent inhibitor of cell cycle progression and can inhibit the growth of both normal cells and cells transformed by a number of oncogenes. However, the ability of p21(waf1/cip1) to inhibit the growth of cells that overexpress the transcriptional transactivator E2F1 is controversial: it has been reported both that E2F1 can and cannot overcome the block in the cell cycle induced by p21(waf1/cip1). To avoid the complications that arise when such experiments are done with permanent cell lines, we tested the effects of overexpressing p21(waf1/cip1) and E2F1 in primary chicken embryo fibroblasts. In this system very high levels of E2F1 overexpression cause considerable apoptosis; however, the surviving cells still overexpress E2F1. These cells are transformed and their growth is blocked by overexpression of p21(waf1/cip1).