Differential roles of E-type cyclins during transformation of murine E2F-1-deficient cells.

Deregulation of retinoblastoma gene product (pRB) is a hallmark of cancer, which acts as a transcriptional repressor by targeting E2F transcription factors. A transcription factor E2F-1 is not only important for S phase entry of cell cycle, but also stimulates gene expression of pro-apoptotic molecules. To investigate roles of E2F-1 and its target genes in cellular transformation, we studied murine E2F-1-deficient embryonic fibroblasts. Compared with control wild-type cells, E2F-1-deficient cells at early passages were less sensitive to exposure to gamma-radiation and showed an increase of colony formation, while their growth was slow. After sequential passages, the growth of E2F-1-deficient cells reached closely to that of wild-type cells. Immunoblot study of E2F target genes showed that multiple passages of E2F-1-deficient cells resulted in preferential increase of cyclin E2 expression. Furthermore, carcinogenicity study using N-nitrosomethylbenzylamine demonstrated that, compared to wild-type mice, fore-stomach tumors in E2F-1-deficient mice expressed an increased amount of cyclin E2, but not cyclin E1. Taken together, the present study shows that differential roles of E-type cyclins are involved at least partially in the process of cellular transformation, supporting the concept of important roles of the E2F regulatory pathway in carcinogenesis.