c-Myc-induced aberrant DNA synthesis and activation of DNA damage response in p300 knockdown cells

Natesan Sankar, Ravi Kumar Kadeppagari, Bayar Thimmapaya*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

27 Scopus citations


We previously showed that in quiescent cells, p300/CBP (CREB-binding protein)family coactivators repress c-myc and prevent premature induction of DNA synthesis. p300/CBP-depleted cells exit G1 early and continue to accumulate in S phase but do not progress into G2/M, and eventually they die of apoptosis. Here, we show that the S-phase arrest in these cells is because of an intra-S-phase block. The inappropriate DNA synthesis that occurs as a result of forced expression of c-myc leads to the activation of the DNA damage response as evidenced by the phosphorylation of several checkpoint related proteins and the formation of foci containing γ-H2AX. The activation of checkpoint response is related to the induction of c-myc, as the phosphorylation of checkpoint proteins can be reversed when cells are treated with a c-Myc inhibitor or when Myc synthesis is blocked by short hairpin RNA. Using the DNA fiber assay, we show that in p300-depleted cells initiation of replication occurs from multiple replication origins. Chromatin loading of the Cdc45 protein also indicates increased origin activity in p300 knockdown cells. Immunofluorescence experiments indicate that c-Myc colocalizes with replication foci, consistent with the recently reported direct role of c-Myc in the initiation of DNA synthesis. Thus, the inappropriate S-phase entry of p300 downregulated cells is likely to be because of c-Myc-induced deregulated replication origin activity, which results in replicative stress, activation of aDNAdamage response, and S-phase arrest. Our results point to an important role for p300 in maintaining genomic integrity by negatively regulating c-myc.

Original languageEnglish (US)
Pages (from-to)15193-15205
Number of pages13
JournalJournal of Biological Chemistry
Issue number22
StatePublished - May 29 2009

ASJC Scopus subject areas

  • Molecular Biology
  • Biochemistry
  • Cell Biology


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