Regulation of Cellular Genes in a Chromosomal Context by the Retinoblastoma Tumor Suppressor Protein

Abstract

ABSTRACT The retinoblastoma tumor suppressor gene product (pRb) is involved in controlling cell cycle progression from G 1 into S. pRb functions, in part, by regulating the activities of several transcription factors, making pRb involved in the transcriptional control of cellular genes. Transient-transfection assays have implicated pRb in the transcription of several genes, including c- fos , the interleukin-6 gene, c- myc , cdc -2, c- neu , and the transforming growth factor β2 gene. However, these assays place the promoter in an artificial context and exclude the effects of far 5′ upstream regions and chromosomal architecture on gene transcription. In these experiments, we have studied the role of pRb in the control of cell cycle-related genes within a chromosomal context and within the context of the G 1 phase of the cell cycle. We have used adenovirus vectors to overexpress pRb in human osteosarcoma cells and breast cells synchronized in early G 1 . By RNase protection assays, we have assayed the effects of this virus-produced pRb on gene expression in these cells. These results indicate that pRb is involved in the transcriptional downregulation of the E2F-1, E2F-2, dihydrofolate reductase, thymidine kinase, c- myc , proliferating-cell nuclear antigen, p107, and p21/Cip1 genes. However, it has no effect on the transcription of the E2F-3, E2F-4, E2F-5, DP-1, DP-2, or p16/Ink4 genes. The results are consistent with the notion that pRb controls the transcription of genes involved in S-phase promotion. They also suggest that pRb negatively regulates the transcription of two of the transcription factors whose activity it also represses, E2F-1 and E2F-2, and that it plays a role in downregulating the immediate-early gene response to serum stimulation.