REGULATION OF CELLULAR GENES IN A CHROMOSOMAL CONTEXT BY THE RETINOBLASTOMA TUMOR-SUPPRESSOR PROTEIN

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 beta 2 gene. However, th ese assays place the promoter in an artificial context and exclude the effects of far 5' upstream regions and chromosomal architecture on ge ne transcription. In these experiments, we have studied the role of pR b in the control of cell cycle-related genes within a chromosomal cont ext and within the context of the G(1) phase of the cell cycle. We hav e used adenovirus vectors to overexpress pRb in human osteosarcoma cel ls and breast cells synchronized in early G(1). By RNase protection as says, we have assayed the effects of this virus-produced pRb on gene e xpression in these cells. These results indicate that pRb is involved in the transcriptional downregulation of the E2F-1 E2F-2, dihydrofolat e reductase, thymidine kinase, c-myc, proliferating-cell nuclear antig en, p107, and p21/Cip1 genes. However, it has no effect on the transcr iption 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 transcrip tion of genes involved in S-phase promotion. They also suggest that pR b negatively regulates the transcription of two of the transcription f actors whose activity it also represses, E2F-1 and E2F-2, and that it plays a role in downregulating the immediate-early gene response to se rum stimulation.