p53 Induction: phosphorylation sites cooperate in regulating

FOXO3 signalling links ATM to the p53 apoptotic pathway following DNA damage

DNA damage as a result of environmental stress is recognized by sensor proteins that trigger repair mechanisms, or, if repair is unsuccessful, initiate apoptosis. Defects in DNA damage-induced apoptosis promote genomic instability and tumorigenesis. The protein ataxia-telangiectasia mutated (ATM) is activated by DNA double strand breaks and regulates apoptosis via p53. Here we show that FOXO3 interacts with the ATM-Chk2-p53 complex, augments phosphorylation of the complex and induces the formation of nuclear foci in cells upon DNA damage. FOXO3 is essential for DNA damage-induced apoptosis and conversely FOXO3 requires ATM, Chk2, and phosphorylated p53 isoforms to trigger apoptosis as a result of DNA damage. Under these conditions FOXO3 may also play a role in regulating chromatin retention of phosphorylated p53. These results suggest an essential link between FOXO3 and the ATM-Chk2-p53-mediated apoptotic program following DNA damage.

Regulation of cell proliferation and survival: convergence of protein kinases and caspases

Intricate networks of protein kinases are intimately involved in the regulation of cellular events related to cell proliferation and survival. In addition to protein kinases, cells also contain networks of proteases including aspartic-acid directed caspases organized in cascades that play a major role in the regulation of cell survival through their involvement in the initiation and execution phases of apoptosis. Perturbations in regulatory protein kinase and caspase networks induce alterations in cell survival and frequently accompany transformation and tumorigenesis. Furthermore, recent studies have documented that caspases or their substrates are subject to phosphorylation in cells illustrating a potential convergence of protein kinase and caspase signaling pathways. Interestingly, a number of caspase substrates are protected from cleavage when they are phosphorylated at sites that are adjacent to caspase cleavage sites. While it is theoretically possible that many distinct protein kinases could protect proteins from caspase-mediated cleavage, protein kinase CK2 is of particular interest because acidic amino acids, including aspartic acid residues that are recognized by caspases, are its dominant specificity determinants.

Estrogen inhibits ATR signaling to cell cycle checkpoints and DNA repair

DNA damage activates the ataxia telangiectasia-mutated and Rad3-related (ATR) kinase signal cascade. How this system is restrained is not understood. We find that in estrogen receptor (ER)-positive breast cancer cells, UV or ionizing radiation and hydroxyurea rapidly activate ATR-dependent phosphorylation of endogenous p53 and Chk1. 17-beta-estradiol (E(2)) substantially blocks ATR activity via plasma membrane-localized ERalpha. E(2)/ER reduces the enhanced association of ATR andTopBP1 proteins that follows DNA damage and strongly correlates to ATR activity. E(2) inhibits ATR activation through rapid PI3K/AKT signaling: AKT phosphorylates TopBP1 at Serine 1159, thereby preventing the enhanced association of ATR with TopBP1 after DNA damage. E(2) also inhibits Claspin:Chk1 protein association via AKT phosphorylation of Chk1, preventing Chk1 signaling to the G2/M checkpoint. ATR-phosphorylation of p53 induces p21 transcription, prevented by E(2)/ER. E(2) delays the assembly and prolongs the resolution of gammaH2AX and Rad51 nuclear foci and delays DNA repair. E(2)/ER also increases the chromosomal damage seen from cell exposure to IR. Therefore, the restraint of ATR cascade activation may be a novel estrogen action relevant to breast cancer.

Heat shock factor-1 modulates p53 activity in the transcriptional response to DNA damage

Here we define an important role for heat shock factor 1 (HSF1) in the cellular response to genotoxic agents. We demonstrate for the first time that HSF1 can complex with nuclear p53 and that both proteins are co-operatively recruited to p53-responsive genes such as p21. Analysis of natural and synthetic cis elements demonstrates that HSF1 can enhance p53-mediated transcription, whilst depletion of HSF1 reduces the expression of p53-responsive transcripts. We find that HSF1 is required for optimal p21 expression and p53-mediated cell-cycle arrest in response to genotoxins while loss of HSF1 attenuates apoptosis in response to these agents. To explain these novel properties of HSF1 we show that HSF1 can complex with DNA damage kinases ATR and Chk1 to effect p53 phosphorylation in response to DNA damage. Our data reveal HSF1 as a key transcriptional regulator in response to genotoxic compounds widely used in the clinical setting, and suggest that HSF1 will contribute to the efficacy of these agents.

Differential Regulation of Estrogen Receptor α Turnover and Transactivation by Mdm2 and Stress-Inducing Agents

In mammalian cells, the level of estrogen receptor alpha (ERalpha) is rapidly decreased upon estrogen treatment, and this regulation involves proteasome degradation. Using different approaches, we showed that the Mdm2 oncogenic ubiquitin-ligase directly interacts with ERalpha in a ternary complex with p53 and is involved in the regulation of ERalpha turnover (both in the absence or presence of estrogens). Several lines of evidence indicated that this effect of Mdm2 required its ubiquitin-ligase activity and involved the ubiquitin/proteasome pathway. Moreover, in MCF-7 human breast cancer cells, various p53-inducing agents (such as UV irradiation) or treatment with RITA (which inhibits the interaction of p53 with Mdm2) stabilized ERalpha and abolished its 17beta-estradiol-dependent turnover. Interestingly, our data indicated that ligand-dependent receptor turnover was not required for efficient transactivation. Altogether, our results indicate that the Mdm2 oncoprotein and stress-inducing agents complexly and differentially regulate ERalpha stability and transcriptional activity in human cancer cells.

Cdk9 phosphorylates p53 on serine 392 independently of CKII

The tumor suppressor p53 is an important cellular protein, which controls cell cycle progression. Phosphorylation is one of the mechanisms by which p53 is regulated. Here we report the interaction of p53 with another key regulator, cdk9, which together with cyclin T1 forms the positive transcription elongation complex, p-TEFb. This complex cooperates with the HIV-1 Tat protein to cause the phosphorylation of the carboxyl terminal domain (CTD) of RNA polymerase II and this facilitates the elongation of HIV-1 transcription. We demonstrate that cdk9 phosphorylates p53 on serine 392 through their direct physical interaction. Results from protein-protein interaction assays revealed that cdk9 interacts with the C-terminal domain (aa 361-393) of p53, while p53 interacts with the N-terminal domain of cdk9. Transfection and protein binding assays (EMSA and ChIP) demonstrated the ability of p53 to bind and activate the cdk9 promoter. Interestingly, cdk9 phosphorylates serine 392 of p53, which could be also phosphorylated by casein kinase II. Kinase assays demonstrated that cdk9 phosphorylates p53 independently of CKII. These studies demonstrate the existence of a feedback-loop between p53 and cdk9, pinpointing a novel mechanism by which p53 regulates the basal transcriptional machinery.

p53 α-Helix mimetics antagonize p53/MDM2 interaction and activate p53

Overexpression or hyperactivation of MDM2 contributes to functional inactivation of wild-type p53 in nearly 50% of tumors. Inhibition of p53 by MDM2 depends on binding between an NH(2)-terminal (residues 16-28) p53 alpha-helical peptide and a hydrophobic pocket on MDM2, presenting an attractive target for development of inhibitors against tumors expressing wild-type p53. Here we report that novel p53 alpha-helical peptide mimics based on a terphenyl scaffold can inhibit MDM2-p53 binding in vitro and activate p53 in vivo. Several active compounds have been identified that inhibit MDM2-p53 binding in an ELISA assay with IC(50) of 10 to 20 micromol/L and induce p53 accumulation and activation in cell culture at 15 to 40 micromol/L. These results suggest that p53 alpha-helical mimetics based on the terphenyl scaffold may be developed into potent p53 activators.