title: Effects of depletion of CREB-binding protein on c-Myc regulation and cell cycle G1-S transition.
abstract: We recently reported that the transcriptional coactivator and histone acetyltransferase p300 plays an important role in the G(1) phase of the cell cycle by negatively regulating c-myc and thereby preventing premature G(1) exit (Kolli, et al. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 4646-4651; Baluchamy, et al. (2003) Proc. Natl. Acad. Sci. U. S. A. 100, 9524-9529). Because p300 does not substitute for all CREB-binding protein (CBP) functions, we investigated whether CBP also negatively regulates c-myc and prevents premature DNA synthesis. Here, we show that antisense-mediated depletion of CBP in serum-deprived human cells leads to induction of c-myc and that such cells emerge from quiescence without growth factors at a rate comparable with that of p300-depleted cells. The CBP-depleted cells contained significantly reduced levels of the cyclin-dependent kinase inhibitor p21 and low levels of p107 and p130 (but not pRb) phosphorylation, suggesting that these factors, along with elevated levels of c-Myc, contribute to induction of DNA synthesis. Antisense c-Myc reversed the phosphorylation of p107 and p130 and the induction of S phase in CBP-depleted cells, indicating that up-regulation of c-myc is directly responsible for the induction of S phase. Furthermore, the serum-stimulated p300/CBP-depleted cells did not traverse beyond S phase, and a significant number of these cells died of apoptosis, which was not related to p53 levels. These cells also contained significantly higher levels of c-Myc compared with normal cells. When c-myc expression was blocked by antisense c-Myc, the apoptosis of the serum-stimulated CBP-depleted cells was reversed, indicating that high levels of c-Myc contribute to apoptosis. Thus, despite their high degree of structural and functional similarities, normal levels of both p300 and CBP are essential for keeping c-myc in a repressed state in G(1) and thereby preventing inappropriate entry of cells into S phase. In addition, both these proteins also provide important functions in coordinated cell cycle progression.
authors.affiliation: Department of Microbiology-Immunology, Feinberg School of Medicine, Northwestern University, Chicago, Illinois 60611, USA.
authors.forename: Hasan N
authors.initials: HN
authors.lastname: Rajabi
authors.forename: Sudhakar
authors.initials: S
authors.lastname: Baluchamy
authors.forename: Sivanagarani
authors.initials: S
authors.lastname: Kolli
authors.forename: Alo
authors.initials: A
authors.lastname: Nag
authors.forename: Rampalli
authors.initials: R
authors.lastname: Srinivas
authors.forename: Pradip
authors.initials: P
authors.lastname: Raychaudhuri
authors.forename: Bayar
authors.initials: B
authors.lastname: Thimmapaya
mesh_terms: D050882:CREB-Binding Protein; D002453:Cell Cycle / Q000235:genetics*; D002460:Cell Line; D016193:G1 Phase / Q000235:genetics; D005786:Gene Expression Regulation; D016259:Genes, myc; D006801:Humans; D009687:Nuclear Proteins / Q000037:antagonists & inhibitors / Q000235:genetics*; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics*; D016196:S Phase / Q000235:genetics; D015534:Trans-Activators / Q000037:antagonists & inhibitors / Q000235:genetics*
chemical_list: D009687:Nuclear Proteins; D016271:Proto-Oncogene Proteins c-myc; D015534:Trans-Activators; D050882:CREB-Binding Protein; C494363:CREBBP protein, human