Acetylation of p53 activates transcription through recruitment of coactivators/histone acetyltransferases

Tip60 and p400 are both required for UV-induced apoptosis but play antagonistic roles in cell cycle progression

The histone acetyl transferase Tip60 (HTATIP) belongs to a multimolecular complex involved in the cellular response to DNA damage. Tip60 participates in cell cycle arrest following DNA damage by allowing p53 to activate p21CIP (p21) expression. We show here that Tip60 and the E1A-associated p400 protein (EP400), which belongs to the Tip60 complex, are also required for DNA damage-induced apoptosis. Tip60 favours the expression of some proapoptotic p53 target genes most likely through the stimulation of p53 DNA binding activity. In contrast, p400 represses p21 expression in unstressed cells, thereby allowing cell cycle progression and DNA damage-induced apoptosis. Tip60 and p400 have thus opposite effects on p21 expression in the absence of DNA damage. We further found that this antagonism relies on the inhibition of Tip60 function by p400, a property that is abolished following DNA damage. Therefore, taken together, our results indicate that Tip60 and p400 play distinct roles in DNA damage-induced apoptosis and underline the importance of the Tip60 complex and its regulation in the proper control of cell fate.

Acetylation of p53 at lysine 373/382 by the histone deacetylase inhibitor depsipeptide induces expression of p21(Waf1/Cip1)

Generally, histone deacetylase (HDAC) inhibitor-induced p21(Waf1/Cip1) expression is thought to be p53 independent. Here we found that an inhibitor of HDAC, depsipeptide (FR901228), but not trichostatin A (TSA), induces p21(Waf1/Cip1) expression through both p53 and Sp1/Sp3 pathways in A549 cells (which retain wild-type p53). This is demonstrated by measuring relative luciferase activities of p21 promoter constructs with p53 or Sp1 binding site mutagenesis and was further confirmed by transfection of wild-type p53 into H1299 cells (p53 null). That p53 was acetylated after depsipeptide treatment was tested by sequential immunoprecipitation/Western immunoblot analysis with anti-acetylated lysines and anti-p53 antibodies. The acetylated p53 has a longer half-life due to a significant decrease in p53 ubiquitination. Further study using site-specific antiacetyllysine antibodies and transfection of mutated p53 vectors (K319/K320/K321R mutated and K373R/K382R mutations) into H1299 cells revealed that depsipeptide specifically induces p53 acetylation at K373/K382, but not at K320. As assayed by coimmunoprecipitation, the K373/K382 acetylation is accompanied by a recruitment of p300, but neither CREB-binding protein (CBP) nor p300/CBP-associated factor (PCAF), to the p53 C terminus. Furthermore, activity associated with the binding of the acetylated p53 at K373/K382 to the p21 promoter as well as p21(Waf1/Cip1) expression is significantly increased after depsipeptide treatment, as tested by chromatin immunoprecipitations and Western blotting, respectively. In addition, p53 acetylation at K373/K382 is confirmed to be required for recruitment of p300 to the p21 promoter, and the depsipeptide-induced p53 acetylation at K373/K382 is unlikely to be dependent on p53 phosphorylation at Ser15, Ser20, and Ser392 sites. Our data suggest that p53 acetylation at K373/K382 plays an important role in depsipeptide-induced p21(Waf1/Cip1) expression.

Inhibition of SIRT1 catalytic activity increases p53 acetylation but does not alter cell survival following DNA damage

Human SIRT1 is an enzyme that deacetylates the p53 tumor suppressor protein and has been suggested to modulate p53-dependent functions including DNA damage-induced cell death. In this report, we used EX-527, a novel, potent, and specific small-molecule inhibitor of SIRT1 catalytic activity to examine the role of SIRT1 in p53 acetylation and cell survival after DNA damage. Treatment with EX-527 dramatically increased acetylation at lysine 382 of p53 after different types of DNA damage in primary human mammary epithelial cells and several cell lines. Significantly, inhibition of SIRT1 catalytic activity by EX-527 had no effect on cell growth, viability, or p53-controlled gene expression in cells treated with etoposide. Acetyl-p53 was also increased by the histone deacetylase (HDAC) class I/II inhibitor trichostatin A (TSA). EX-527 and TSA acted synergistically to increase acetyl-p53 levels, confirming that p53 acetylation is regulated by both SIRT1 and HDACs. While TSA alone reduced cell survival after DNA damage, the combination of EX-527 and TSA had no further effect on cell viability and growth. These results show that, although SIRT1 deacetylates p53, this does not play a role in cell survival following DNA damage in certain cell lines and primary human mammary epithelial cells.

Histone deacetylase inhibitors repress the transactivation potential of hypoxia-inducible factors independently of direct acetylation of HIF-alpha

Hypoxia-inducible factors (HIFs) are heterodimeric transcription factors regulating the oxygen supply, glucose metabolism, and angiogenesis. HIF function requires the recruitment of p300/CREB-binding protein, two coactivators with histone acetyltransferase activity, by the C-terminal transactivation domain of HIF-alpha (HIF-alphaCAD). Histone deacetylase inhibitors (HDAIs) induce differentiation or apoptosis and repress tumor growth and angiogenesis, hence being explored intensively as anti-cancer agents. Using combined pharmacological, biochemical, and genetic approaches, here we show that HDAIs repress the transactivation potential of HIF-alphaCAD. This repression is independent of the function of tumor suppressors von Hippel-Lindau or p53 or the degradation of HIF-alpha. We also demonstrate the sufficiency of low concentrations of HDAIs in repression of HIF target genes in tumor cells. We further show that HDAIs induce hyperacetylation of p300 and repress the HIF-1alpha.p300 complex in vivo. In vitro acetylation analysis reveals that the p300CH1 region, but not HIF-alphaCAD, is susceptible to acetylation. Taken together, our data demonstrate that a deacetylase activity is indispensable for the transactivation potential of HIF-alphaCAD and support a model that acetylation regulates HIF function by targeting HIF-alpha.p300 complex, not by direct acetylating HIF-alpha. The demonstration that HDAIs repress both HIF-1alpha and HIF-2alpha transactivation potential independently of von Hippel-Lindau tumor suppressor and p53 function indicates that HDAIs may have biological effects in a broad range of tissues in addition to tumors.

The biological impact of the human master regulator p53 can be altered by mutations that change the spectrum and expression of its target genes

Human tumor suppressor p53 is a sequence-specific master regulatory transcription factor that targets response elements (REs) in many genes. p53 missense mutations in the DNA-binding domain are often cancer associated. As shown with systems based on the yeast Saccharomyces cerevisiae, p53 mutants can alter the spectra and intensities of transactivation from individual REs. We address directly in human cells the relationship between changes in the p53 master regulatory network and biological outcomes. Expression of integrated, tightly regulated DNA-binding domain p53 mutants resulted in many patterns of apoptosis and survival following UV or ionizing radiation, or spontaneously. These patterns reflected changes in the spectra and activities of target genes, as demonstrated for P21, MDM2, BAX, and MSH2. Thus, as originally proposed for "master genes of diversity," p53 mutations in human cells can differentially influence target gene transactivation, resulting in a variety of biological consequences which, in turn, might be expected to influence tumor development and therapeutic efficacy.

p53 regulates cellular resistance to complement lysis through enhanced expression of CD59

It has been recently hypothesized that the CD59 gene has two putative p53-responsive elements that may be involved in defense of host cells from damage by the complement system in inflammation. Here we have examined the roles of these putative p53-binding sequences within the CD59 gene in regulation of CD59 expression. We have shown that both of these potential responsive elements bind p53 in vitro. Knocking down expression of p53 using small interfering RNA led to a 6-fold decrease in CD59 protein expression in HeLa cells. We have previously observed a decrease of CD59 in camptothecin-induced apoptotic IMR32 cells, whereas expression was increased in the surviving fraction compared with untreated cells. Here, we have shown that these changes are associated with altered expression levels and acetylation status of p53. We have also shown that acetylation status of p53 regulates CD59 expression on cells exposed to inflammatory cytokines to model inflammation. Our data suggest that p53 and in vivo positive/negative regulators of p53 could be used to modulate susceptibility of tumor cells to complement lysis in chemotherapy.

Src family kinase members have a common response to histone deacetylase inhibitors in human colon cancer cells

Histone deacetylase inhibitors (HDIs) induce cell cycle arrest, differentiation and/or apoptosis in numerous cancer cell types and have shown promise in clinical trials. These agents are particularly novel, given their ability to selectively influence gene expression. Previously, we demonstrated that the HDIs butyrate and trichostatin A (TSA) directly repress c-Src proto-oncogene expression in many cancer cell lines. Activation and/or overexpression of c-Src have been frequently observed in numerous malignancies, especially of the colon. Therefore, our observation was particularly interesting since butyrate is a naturally abundant component of the large intestine and has been suggested to be a cancer-preventive agent. However, c-Src is not the only Src family kinase (SFK) member to be implicated in the development of human cancers, including those of the colon. Therefore, the relative expression levels of known SFKs were examined in a panel of human colon cancer cell lines. We found a surprisingly diverse expression pattern but noted that most cell lines expressed relatively high levels of at least 2 SFKs. When the effects of butyrate and TSA were examined in representative cell lines, the expression of all SFKs was repressed in a dose- and time-dependent manner. Further, detailed examination of Lck, Yes and Lyn demonstrated that this repression had a direct effect on transcription and was independent of new protein synthesis. These results mirror our earlier data obtained with c-Src and suggest that SFKs are a major target of HDIs and likely account in part for the anticancer effects of these promising new drugs.

Regulatory factor for X-box family proteins differentially interact with histone deacetylases to repress collagen alpha2(I) gene (COL1A2) expression

Our studies indicate that the regulatory factor for X-box (RFX) family proteins repress collagen alpha2(I) gene (COL1A2) expression (Xu, Y., Wang, L., Buttice, G., Sengupta, P. K., and Smith, B. D. (2003) J. Biol. Chem. 278, 49134-49144; Xu, Y., Wang, L., Buttice, G., Sengupta, P. K., and Smith, B. D. (2004) J. Biol. Chem. 279, 41319-41332). In this study, we examined the mechanism(s) underlying the repression of collagen gene by RFX proteins. Two members of the RFX family, RFX1 and RFX5, associate with distinct sets of co-repressors on the collagen transcription start site in vitro. RFX5 specifically interacts with histone deacetylase 2 (HDAC2) and the mammalian transcriptional repressor (mSin3B), whereas RFX1 preferably interacts with HDAC1 and mSin3A. HDAC2 cooperates with RFX5 to down-regulate collagen promoter activity, whereas HDAC1 enhances inhibition of collagen promoter activity by RFX1. Interferon-gamma promotes the recruitment of RFX5/HDAC2/mSin3B to the collagen transcription start site but decreases the occupancy by RFX1/mSin3A as manifested by chromatin immunoprecipitation assay. RFX1 binds to the methylated collagen sequence with much higher affinity than unmethylated sequence, recruiting more HDAC1 and mSin3A. The DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine, which inhibits DNA methylation, reduces RFX1/HDAC1 binding to the collagen transcription start site in chromatin immunoprecipitation assays. Finally, both RFX1 and RFX5 are acetylated in vivo. Trichostatin A stimulates the acetylation of RFX proteins and activates the collagen promoter activity. Collectively, our data strongly indicate two separate pathways for RFX proteins to repress collagen gene expression as follows: one for RFX5/HDAC2 in interferon-gamma-mediated repression, and the other for RFX1/HDAC1 in methylation-mediated collagen silencing.

DNA damage signaling and p53-dependent senescence after prolonged beta-interferon stimulation

Interferons are cytokines with potent antiviral and antiproliferative activities. We report that although a transient exposure to beta-interferon induces a reversible cell cycle arrest, a sustained treatment triggers a p53-dependent senescence program. Beta-interferon switched on p53 in two steps. First, it induced the acetylation of p53 at lysine 320 and its dephosphorylation at serine 392 but not p53 activity. Later on, it triggered a DNA signaling pathway, the phosphorylation of p53 at serine 15 and its transcriptional activity. In agreement, beta-interferon-treated cells accumulated gamma-H2AX foci and phosphorylated forms of ATM and CHK2. The DNA damage signaling pathway was activated by an increase in reactive oxygen species (ROS) induced by interferon and was inhibited by the antioxidant N-acetyl cysteine. More important, RNA interference against ATM inhibited p53 phosphorylation at serine 15, p53 activity and senescence in response to beta-interferon. Beta-interferon-induced senescence was more efficient in cells expressing either, p53, or constitutive allele of ERK2 or RasV12. Hence, beta-interferon-induced senescence targets preferentially cells with premalignant changes.

Regulation of the p53 transcriptional activity

In response to various stresses, p53 is rapidly activated and transcriptionally regulates a number of target genes by which p53 modulates a variety of cellular activities. The transcriptional activity of p53 is delicately regulated by a plethora of cellular factors, independently or synergistically, in multiple ways in order to achieve a specific response. This article reviewed the role of the basal transcriptional machinery, co-activators, and co-repressors involved in p53-dependent transcription, and the underlying mechanism by which the p53 transcriptional activity is regulated. We also discussed some potentially interesting questions and future directions in the field.

On the mechanism of sequence-specific DNA-dependent acetylation of p53: the acetylation motif is exposed upon DNA binding

P53 acetylation requires p300-docking to two contiguous sites in the activation domain that in turn mediates DNA-dependent acetylation of the tetramer. In an attempt to further define the mechanism of DNA-dependent acetylation of p53, an in vitro system has been reconstituted with distinct p53 isoforms and has been used to reveal conformational constraints on p53 acetylation. Two native p53 tetrameric isoforms purified from Sf9 cells differing by the extent of phosphorylation within the C-terminal acetylation site are both acetylated in a sequence-specific DNA-dependent manner. By contrast, p53 purified from an Escherichia coli expression system is in a largely denatured conformation and its acetylation is DNA-independent. Heating native p53 to destroy the folded structure restores DNA-independent acetylation similar to that seen with bacterially expressed p53. There are at least two sites of conformational flexibility in the p53 tetramer: the first in the flexible S10 beta-sheet within the MDM2 ubiquitination sequence and the second in the C-terminal regulatory domain. We analysed therefore whether DNA-dependent acetylation correlated with conformational changes in either of these two regions. DNA-dependent acetylation of p53 is maintained in a dose-dependent manner by low concentrations of consensus site DNA under conditions where flexibility in the S10 beta-sheet region is maintained. Oligonucleotide DNAs that promote acetylation stimulate the binding of monoclonal antibodies PAb421 and ICA-9; two antibodies whose contiguous epitopes overlap the C-terminal acetylation motif. By contrast, bent oligonucleotide DNAs that conceal both the S10 beta-sheet from binding of the monoclonal antibody DO-12 and attenuate binding of the monoclonal antibody PAb421 can preclude acetylation. These data suggest that, in the absence of DNA, the acetylation motif of p53 is in a cryptic state, but after DNA binding, allosteric effects mediate an exposure of the acetylation motif to allow DNA-dependent acetylation of the tetramer.

Regulation of Kruppel-like factor 6 tumor suppressor activity by acetylation

Krüppel-like factor 6 (KLF6) is a zinc finger transcription factor and tumor suppressor that is inactivated in a number of human cancers by mutation, allelic loss, and/or promoter methylation. A key mechanism of growth inhibition by wild-type KLF6 is through p53-independent up-regulation of p21(WAF1/cip1) (CDKN1A), which is abrogated in several tumor-derived mutants. Here we show by chromatin immunoprecipitation that transactivation of p21(WAF1/cip1) by KLF6 occurs through its direct recruitment to the p21(WAF1/cip1) promoter and requires acetylation by histone acetyltransferase activity of either cyclic AMP-responsive element binding protein-binding protein or p300/CBP-associated factor. Direct lysine acetylation of KLF6 peptides can be shown by mass spectrometry. A single lysine-to-arginine point mutation (K209R) derived from prostate cancer reduces acetylation of KLF6 and abrogates its capacity to up-regulate endogenous p21(WAF1/cip1) and reduce cell proliferation. These data indicate that acetylation may regulate KLF6 function, and its loss in some tumor-derived mutants could contribute to its failure to suppress growth in prostate cancer.

Acetylation and alternative splicing regulate ZNF76-mediated transcription

We previously showed that ZNF76 is a general transcription repressor targeting TATA-binding protein (TBP), through a process regulated by sumoylation [G. Zheng, Y.C. Yang, ZNF76, a novel transcriptional repressor targeting TATA-binding protein, is modulated by sumoylation, J. Biol. Chem. 279 (2004) 42410-42421]. In this study, two additional regulatory mechanisms for ZNF76 were identified. ZNF76 is acetylated by p300 and deacetylated by HDAC1, and acetylation of ZNF76 leads to its loss of sumoylation and attenuation of TBP interaction. Consistent with their physical antagonism, acetylation, and sumoylation play opposite roles in regulating the transactivation of ZNF76. Besides acetylation and sumoylation, ZNF76 is also regulated through mRNA splicing: two isoforms of ZNF76 have different abilities of interacting with TBP. Our study shows that ZNF76, a TBP-interacting transcriptional modulator, is regulated by both lysine modifications and alternative splicing.

GCN5 and p300 share essential functions during early embryogenesis

Previous studies revealed that deletion of genes encoding the histone acetyltransferases GCN5, p300, or CBP results in embryonic lethality in mice. PCAF and GCN5 physically interact with p300 and CBP in vitro. To determine whether these two groups of histone acetyltransferases interact functionally in vivo, we created mice lacking one or more alleles of p300, GCN5, or PCAF. As expected, we found that mice heterozygous for any single null allele are viable. The majority of GCN5(+/-)p300(+/-) mice also survive to adulthood with no apparent abnormalities. However, approximately 25% of these mice die before birth. These embryos are developmentally stunted and exhibit increased apoptosis compared with wild-type or single GCN5(+/-) or p300(+/-) littermates at embryonic day 8.5. In contrast, no abnormalities were observed in PCAF(-/-) p300(+/-) mice. Of interest, we find that p300 protein levels vary in different mouse genetic backgrounds, which likely contributes to the incomplete penetrance of the abnormal phenotype of GCN5(+/-) p300(+/-) mice. Our data indicate that p300 cooperates specifically with GCN5 to provide essential functions during early embryogenesis.

Phosphorylation, acetylation and ubiquitination: the molecular basis of RUNX regulation

The RUNX family members play pivotal roles in normal development and neoplasia. RUNX1 and RUNX2 are essential for hematopoiesis and osteogenesis, respectively, while RUNX3 is involved in neurogenesis, thymopoiesis and functions as a tumor suppressor. Inappropriate levels of RUNX activity are associated with leukemia, autoimmune disease, cleidocranial dysplasia, craniosynostosis and various solid tumors. Therefore, RUNX activity must be tightly regulated to prevent tumorigenesis and maintain normal cell differentiation. Recent work indicates that RUNX activity is controlled by various extracellular signaling pathways, and that phosphorylation, acetylation and ubiquitination are important post-translational modifications of RUNX that affect its stability and activity. Defining the precise roles, these modifications that play in the regulation of RUNX function may reveal not only how the RUNX proteins are regulated but also how they are assembled into other regulatory machineries.

Production of constitutively acetylated recombinant p53 from yeast and Escherichia coli by tethered catalysis

Post-translational modification of proteins is a dynamic way of generating new protein-protein interaction interfaces that are critical for signaling networks in diverse cellular functions. Purified recombinant proteins frequently lack these signature modifications. Using the tumor suppressor p53 as the model protein, we present here a tethered catalysis approach for the production of acetylated p53 in vivo. P53 is a major tumor suppressor protein that protects the cell from various oncogenic stresses. Upon DNA damage, p53 is stabilized and activated by a plethora of post-translational modifications, including acetylation. Here, we show that constitutively acetylated p53 can be expressed and purified from both yeast and Escherichia coli. This method is highly suitable for studying protein-protein interactions in the conventional yeast two-hybrid screen that requires a constitutively acetylated state of p53. Furthermore, effective production and purification of acetylated p53 from E. coli supports future biochemical and structural characterization. The method described in this work can be applied to other proteins and modifications, and thus has widespread use in the fields of signal transduction and proteomic research.

STAT3 NH2-terminal acetylation is activated by the hepatic acute-phase response and required for IL-6 induction of angiotensinogen

BACKGROUND & AIMS

The signal transducers and activators of transcription (STATs) are cytoplasmic transcription factors mediating acute-phase response (APR) of the human angiotensinogen (hAGT) gene in hepatocytes. The mechanisms of how STAT3 activates target genes are unknown. Here we analyzed the biochemistry of STAT3 activation by interleukin (IL)-6 in hepatocellular carcinoma HepG2 and Balb/C mice.

METHODS

Immunoprecipitation-Western assays and Matrix Assisted Laser Desorption-Time of Flight mass spectrometry determined sites of STAT3 acetylation by the 300-kilodalton target of E1A (p300) co-activator. The subcellular localization of acetylation-deficient STAT3 molecules were studied by microscopic imaging, effects on DNA binding measured by gel shift and chromatin immunoprecipitation (ChIP) assays, and gene transactivation by Northern blot and reporter assays.

RESULTS

Two Lys residues at amino acids 49 and 87 in the STAT3 NH2 terminus are acetylated by p300. Lys-to-Arg point mutations (STAT3 K49R/K87R) had no effect on inducible DNA binding, but blocked p300-mediated acetyl(Ac)-STAT3 formation and abrogated IL-6-induced hAGT activation. Although STAT3 K49R/K87R rapidly translocated into the nucleus, it did not bind p300 and had delayed cytoplasmic redistribution. ChIP assays show IL-6-inducible acetylated STAT3 and p300 binding to the native hAGT promoter. Activation of the APR in mice induces nuclear Tyr phosphorylated and acetylated STAT3 in hepatic nuclei. We also observed that STAT3 interacts with histone deacetylases (HDACs), specifically HDAC 1, that down-regulate IL-6-induced hAGT transactivation.

CONCLUSIONS

IL-6-induced target gene activation requires p300-mediated STAT3 acetylation, and HDACs are involved in the termination of STAT3 action. These studies indicate the acetylation-deacetylation reaction as a novel signaling mechanism controlling the IL-6-STAT3 pathway in the hepatic APR.

Smads and chromatin modulation

Smad proteins are critical intracellular effector proteins and regulators of transforming growth factor type beta (TGFbeta) modulated gene transcription. They directly convey signals that initiate at ligand-bound receptor complexes and end in the nucleus with changes in programs of gene expression. Activated Smad proteins seem to recruit chromatin modifying proteins to target genes besides cooperating with DNA-bound transcription factors. We survey here the current and still emerging knowledge on Smad-binding factors, and their different mechanisms of chromatin modification in particular, in Smad-dependent TGFbeta signaling.

Resveratrol and breast cancer chemoprevention: molecular mechanisms

Despite years of intensive research, breast cancer remains a major cause of death among women. New strategies to combat breast cancer are being developed, one of the most exciting of which is the use of chemopreventive agents. Resveratrol (RES) is a polyphenolic compound found in plants that seems to have a wide spectrum of biological activity. RES has been shown to afford protection against several types of cancer. This review summarizes the chemopreventive effects of RES at the three major stages of breast carcinogenesis: initiation, promotion, and progression. It has anti-oxidant and anti-inflammatory properties, and may induce apoptosis as well as modulate cell cycle and estrogen receptor function in breast cancer cell lines. Although RES has shown remarkable promise as a potent chemopreventive agent in breast cancer, further studies are needed to etablish its usefulness.

HIPK2 contributes to PCAF-mediated p53 acetylation and selective transactivation of p21Waf1 after nonapoptotic DNA damage

The p53 tumor suppressor gene is activated in response to DNA damage resulting in either growth arrest or apoptosis. We previously demonstrated the specific involvement of homeodomain interacting protein-kinase 2 (HIPK2), a nuclear serine/threonine kinase, in inducing p53-dependent apoptosis through selective p53 phosphorylation at serine 46 after severe genotoxic damage. Here we show that HIPK2 contributes to p53 regulation, independently from serine 46 phosphorylation upon nonapoptotic DNA damage such as that induced by cytostatic doses of cisplatin. We show that HIPK2 depletion by RNA interference inhibits p53 binding to the p21Waf1 promoter affecting its p53-induced transactivation thereby allowing cell proliferation. We found that nonapoptotic DNA damage induces p53 acetylation mediated by the HAT protein PCAF and this p53 post-translational modification is abolished by HIPK2 depletion. In this regard, we found that HIPK2 cooperates with PCAF to induce selectively p53 transcriptional activity toward the p21Waf1 promoter while depletion of either HIPK2 or PCAF abolished this function. These data show that HIPK2 regulates the p53 growth arrest function through its PCAF-mediated acetylation.

p300/CBP-associated factor drives DEK into interchromatin granule clusters

DEK is a mammalian protein that has been implicated in the pathogenesis of autoimmune diseases and cancer, including acute myeloid leukemia, melanoma, glioblastoma, hepatocellular carcinoma, and bladder cancer. In addition, DEK appears to participate in multiple cellular processes, including transcriptional repression, mRNA processing, and chromatin remodeling. Sub-nuclear distribution of this protein, with the attendant functional ramifications, has remained a controversial topic. Here we report that DEK undergoes acetylation in vivo at lysine residues within the first 70 N-terminal amino acids. Acetylation of DEK decreases its affinity for DNA elements within the promoter, which is consistent with the involvement of DEK in transcriptional repression. Furthermore, deacetylase inhibition results in accumulation of DEK within interchromatin granule clusters (IGCs), sub-nuclear structures that contain RNA processing factors. Overexpression of P/CAF acetylase drives DEK into IGCs, and addition of a newly developed, synthetic, cell-permeable P/CAF inhibitor blocks this movement. To our knowledge, this is the first reported example of acetylation playing a direct role in relocation of a protein to IGCs, and this may explain how DEK can function in multiple pathways that take place in distinct sub-nuclear compartments. These findings also suggest that DEK-associated malignancies and autoimmune diseases might be amenable to treatment with agents that alter acetylation.

Corepressor CtBP1 interacts with and specifically inhibits CBP activity

Transcription repression in eukaryotes is mediated by a wide variety of transcription factors that usually recruit corepressors and form corepressor complexes at the specific promoter sites. One of these corepressors is the C-terminal-binding protein (CtBP) which was first identified as a protein that binds to the C-terminal region of the adenovirus E1A protein. CtBP has a strong role in both development and oncogenesis. Till date, the mechanism of transcription repression by CtBP is unknown. Here, we report that CtBP physically interacts in vivo with HAT enzymes from different families. The vast majority of the HAT enzymes have a potential consensus site for CtBP binding within the bromodomain but we show that additional site(s) exists for CBP. The interaction between CtBP and CBP is functionally important and leads to impairment of histone H3 acetylation by CBP at specific lysine residues (Lys9, Lys14, and Lys18) in a dose-dependent and NADH-dependent manner. Based on these results, we propose that CtBP1 mediates repression by blocking histone acetylation by HAT complexes.

Depletion of Mutant p53 and Cytotoxicity of Histone Deacetylase Inhibitors

Mutant p53 is a cancer-specific target for pharmacologic intervention. We show that histone deacetylase inhibitors such as FR901228 and trichostatin A completely depleted mutant p53 in cancer cell lines. This depletion was preceded by induction of p53-regulated transcription. In cells with mutant p53 pretreated with histone deacetylase inhibitors, DNA damage further enhanced the p53 trans-function. Furthermore, histone deacetylase inhibitors were preferentially cytotoxic to cells with mutant p53 rather than to cells lacking wild-type p53. We suggest that, by either restoring or mimicking p53 trans-functions, histone deacetylase inhibitors initiate degradation of mutant p53. Because mutant p53 is highly expressed, a sudden restoration of p53-like functions is highly cytotoxic to cells with mutant p53. In a broader perspective, this shows how selectivity may be achieved by targeting a non-cancer-specific target, such as histone deacetylases, in the presence of a cancer-specific alteration, such as mutant p53.

Functional analysis of the roles of posttranslational modifications at the p53 C terminus in regulating p53 stability and activity

Posttranslational modification of the tumor suppressor p53 plays important roles in regulating its stability and activity. Six lysine residues at the p53 C terminus can be posttranslationally modified by various mechanisms, including acetylation, ubiquitination, neddylation, methylation, and sumoylation. Previous cell line transfection studies show that ubiquitination of these lysine residues is required for ubiquitin-dependent degradation of p53. In addition, biochemical and cell line studies suggested that p53 acetylation at the C terminus might stabilize p53 and activate its transcriptional activities. To investigate the physiological functional outcome of these C-terminal modifications in regulating p53 stability and activity, we introduced missense mutations (lysine to arginine) at the six lysine residues (K6R) into the endogenous p53 gene in mouse embryonic stem (ES) cells. The K6R mutation prevents all posttranslational modifications at these sites but conserves the structure of p53. In contrast to conclusions of previous studies, analysis of p53 stability in K6R ES cells, mouse embryonic fibroblasts, and thymocytes showed normal p53 stabilization in K6R cells both before and after DNA damage, indicating that ubiquitination of these lysine residues is not required for efficient p53 degradation. However, p53-dependent gene expression was impaired in K6R ES cells and thymocytes in a promoter-specific manner after DNA damage, indicating that the net outcome of the posttranslational modifications at the C terminus is to activate p53 transcriptional activities after DNA damage.

p300 Regulates p63 Transcriptional Activity

The transcriptional co-activator p300 has been reported to regulate the tumor suppressor p53 and its ortholog p73. Here we describe a study showing that this coactivator also regulates the transcriptional function of p63. p300 bound to the N-terminal domain of p63gamma, and p63gamma bound to the N terminus of p300 in vitro and in cells. p300, but not its acetylase-defective mutant AT2, stimulated p63gamma-dependent transcription and induction of p21 in cells, consequently leading to G1 arrest. Inversely, the deltaN-p63gamma isoform as well as p300AT2 inhibited the induction of p21 by p63gamma. These results suggest that p300 regulates p63-dependent transcription of p21.

The MLL partial tandem duplication: evidence for recessive gain-of-function in acute myeloid leukemia identifies a novel patient subgroup for molecular-targeted therapy

MLL (ALL-1) chimeric fusions and MLL partial tandem duplications (PTD) may have mechanistically distinct contributions to leukemogenesis. Acute myeloid leukemia (AML) blasts with the t(9;11)(p22; q23) express MLL-AF9 and MLL wild-type (WT) transcripts, while normal karyotype AML blasts with the MLL(PTD/WT) genotype express MLL PTD but not the MLL WT. Silencing of MLL WT in MLL(PTD/WT) blasts was reversed by DNA methyltransferase (DNMT) and histone deacetylase (HDAC) inhibitors, and MLL WT induction was associated with selective sensitivity to cell death. Reduction of MLL PTD expression induced MLL WT and reduced blast colony-forming units, supporting opposing functions for MLL PTD and MLL WT whereby the MLL PTD contributes to the leukemic phenotype via a recessive gain-of-function. The coincident suppression of the MLL WT allele with the expression of the MLL PTD allele, along with the functional data presented here, supports the hypothesis that loss of WT MLL function via monoallelic repression contributes to the leukemic phenotype by the remaining mutant allele. These data from primary AML and the pharmacologic reversal of MLL WT silencing associated with a favorable alteration in the threshold for apoptosis suggest that these patients with poor prognosis may benefit from demethylating or histone deacetylase inhibitor therapy, or both.

A Novel Human p53 Isoform Is an Essential Element of the ATR-Intra-S Phase Checkpoint

The archetypal human tumor suppressor p53 is considered to have unique transactivation properties. The assumption is based on the fact that additionally identified human p53 isoforms lack transcriptional activity. However, we provide evidence for the existence of an alternatively spliced p53 isoform (Deltap53) that exerts its transcriptional activity independent from p53. In contrast to p53, Deltap53 transactivates the endogenous p21 and 14-3-3sigma but not the mdm2, bax, and PIG3 promoter. Cell cycle studies showed that Deltap53 displays its differential transcriptional activity only in damaged S phase cells. Upon activation of the ATR-intra-S phase checkpoint, Deltap53, but not p53, transactivates the Cdk inhibitor p21. Induction of p21 results in downregulation of cyclin A-Cdk activity and accordingly attenuation of S phase progression. Data demonstrate that the Deltap53-p21-cyclin A-Cdk pathway is crucial to facilitate uncoupling of repair and replication events, indicating that Deltap53 is an essential element of the ATR-intra-S phase checkpoint.

Krüppel-like Factor-6 Promotes Preadipocyte Differentiation through Histone Deacetylase 3-dependent Repression of DLK1

Preadipocyte differentiation occurs during distinct periods of human development and is a key determinant of body mass. Transcriptional events underlying adipogenesis continue to emerge, but the link between chromatin remodeling of specific target loci and preadipocyte differentiation remains elusive. We have identified Krüppel-like factor-6 (KLF6), a recently described tumor suppressor gene, as a repressor of the proto-oncogene Delta-like 1 (Dlk1), a gene encoding a transmembrane protein that inhibits adipocyte differentiation. Forced expression of KLF6 strongly inhibits Dlk1 expression in preadipocytes and NIH 3T3 cells in vivo, whereas down-regulation of KLF6 in 3T3-L1 cells by small interfering RNA prevents adipogenesis. Repression of Dlk1 requires HDAC3 deacetylase activity, which is recruited to the endogenous Dlk1 promoter where it interacts with KLF6. Our studies identify the interaction between HDAC3 and KLF6 as a potential mechanism underlying human adipogenesis, and highlight the role of KLF6 as a multifunctional transcriptional regulator capable of mediating adipocyte differentiation through gene repression.

p53 must be competent for transcriptional regulation to suppress tumor formation

In vitro studies suggest that effective tumor suppression by p53 requires multiple domains to execute transcription-dependent and transcription-independent functions. We generated a mutant p53 allele in mice, p53(W25QL26S) (p53(QS)), containing an inactive transactivation domain to evaluate the importance of transactivation for p53-mediated tumor suppression. Recently, we discovered that the allele also contains a valine substitution for alanine at codon 135, which borders the DNA-binding domain. We found that p53(QSval135) bound to chromatin albeit less well than p53(QSala135), but both were equally deficient in transcriptional regulation, apoptosis induction in mouse embryo fibroblasts (MEFs), and suppression of tumor formation by E1A, Ha-Ras transformed MEFs. p53(QSval135) mice and p53-null mice exhibited identical tumor development kinetics and spectra in spontaneous and oncogene-initiated tumorigenicity assays, when tested in a homo- and heterozygous configuration. The p53(QSval135) allele did not have dominant negative functions and behaved as a null allele. Taken together, these data indicate that effective tumor suppression requires the transcriptional regulation function of p53, and they suggest that transactivation independent functions of p53 are unlikely to contribute significantly to tumor suppression in vivo.

AMP-activated protein kinase induces a p53-dependent metabolic checkpoint

Replicative cell division is an energetically demanding process that can be executed only if cells have sufficient metabolic resources to support a doubling of cell mass. Here we show that proliferating mammalian cells have a cell-cycle checkpoint that responds to glucose availability. The glucose-dependent checkpoint occurs at the G(1)/S boundary and is regulated by AMP-activated protein kinase (AMPK). This cell-cycle arrest occurs despite continued amino acid availability and active mTOR. AMPK activation induces phosphorylation of p53 on serine 15, and this phosphorylation is required to initiate AMPK-dependent cell-cycle arrest. AMPK-induced p53 activation promotes cellular survival in response to glucose deprivation, and cells that have undergone a p53-dependent metabolic arrest can rapidly reenter the cell cycle upon glucose restoration. However, persistent activation of AMPK leads to accelerated p53-dependent cellular senescence. Thus, AMPK is a cell-intrinsic regulator of the cell cycle that coordinates cellular proliferation with carbon source availability.

Purification of acetyl-p53 using p300 co-infection and the baculovirus expression system

As cells persist in their environment, they are exposed to harmful agents that can damage their genomic DNA. When DNA becomes damaged, p53, a tumor suppressor, is stabilized and acts as a transcription factor to cause either cell cycle arrest or apoptosis. Strict p53 regulatory mechanisms have been well characterized relative to phosphorylation and dephosphorylation, but acetylation of p53 in response to DNA damage has also been shown to participate in p53 function. Proper investigation of the many roles that acetylated p53 plays in the cell requires accurate in vitro studies, which can only be easily conducted if highly pure acetyl-p53 is available. Purified p53 that is acetylated in vitro can routinely achieve 10-20%. Separating this acetylated fraction from the undesired unacetylated fraction can be technically challenging, inefficient, and time consuming. We have developed an in vivo strategy to rapidly produce microgram quantities of p53 preparations that are greater than 60% acetylated using co-infection of p53 and p300 baculoviruses in Sf21 insect cell culture. Immunoaffinity recovery followed by further depletion of unacetylated p53 results in a preparation that is greater than 70-75% in acetyl-p53 after a single round, and undetectable levels of unacetylated p53 after two rounds. This approach to preparing acetylated protein in vivo may also extend to other acetylated transcription factors and histones.

The relationship among p53 oligomer formation, structure and transcriptional activity using a comprehensive missense mutation library

Tumor suppressor p53 forms a homo-tetramer through its COOH-terminal oligomerization domain and acts as a sequence-specific transcription factor. We have analysed the interrelation among the transcriptional activities, the structure and the cancer-related mutations in the oligomerization domain by using a comprehensive missense mutation library. Here, we examined the ability of 184 mutant p53s in the domain to form an oligomer by expressing these mutant p53s in yeast, and compared the data with the previous information. We showed that specific residues in the alpha-helix and the beta-strand of the oligomerization domain were critical for both oligomer formation and sequence-specific transactivation, and the activities were closely related. In particular, the alpha-helix was more sensitive to amino-acid substitutions than the beta-strand. We found identity in the interrelation between the two activities, that is, monomer mutants were transcriptionally inactive whereas dimer and tetramer mutants retained their transcriptional activities. In TP53 mutation databases, a small number of mutations have been reported in this domain. Surprisingly, most do not encode p53s defective in functional properties. These results indicate that, although oligomer formation is essential for p53 transactivation function, the inactivation of oligomer formation and therefore the inactivation of transactivation may not be essential for tumor suppression by p53 because they do not lead to oncogenic proteins.

Effects of glucocorticoids on gene transcription

Glucocorticoids bind to and activate a cytoplasmic glucocorticoid receptor. The activated glucocorticoid receptor translocates into the nucleus and binds to specific response elements in the promoter regions of anti-inflammatory genes such as lipocortin-1 and secretory leukocyte protease inhibitor (SLPI). However, the major anti-inflammatory effects of glucocorticoids appear to be due largely to interaction between the activated glucocorticoid receptor and transcription factors, notably nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1), that mediate the expression of inflammatory genes. NF-kappaB switches on inflammatory genes via a process involving recruitment of transcriptional co-activator proteins and changes in chromatin modifications such as histone acetylation. This process must occur in the correct temporal manner to allow for effective inflammatory gene expression to occur. The interactions between NF-kappaB and the glucocorticoid receptor result in differing effects on histone modifications and chromatin remodelling. Drugs that enhance glucocorticoid receptor nuclear translocation (long acting beta-agonists) and GR-associated histone deacetylases activity (theophylline) have been shown to be effective add-on therapies. In addition, dissociated glucocorticoids that target NF-kappaB preferentially have also been successful in the treatment of allergic disease.

Role of phosphorylation in p53 acetylation and PAb421 epitope recognition in baculoviral and mammalian expressed proteins

Post-translational modifications, such as phosphorylation and acetylation of the tumour suppressor protein p53, elicit important effects on the function and the stability of the resultant protein. However, as phosphorylation and acetylation are dynamic events subject to complex controls, elucidating the relationships between phosphorylation and acetylation is difficult. In the present study we sought to address this problem by comparing full-length wild-type p53 with full-length p53 proteins mutated at specific phosphorylation targets. Recombinant murine p53 proteins were expressed in insect cells (using the baculoviral expression vector system) and in a mammalian in vitro transcription/translation reticulocyte lysate system. In p53 proteins derived from baculoviral expression vectors, S37A (but not S37D) was found to abrogate phosphorylation at S15. Lysine 382 (K382) is constitutively acetylated and was shown to form part of the epitope recognized by PAb421. Lysine 373 (K373) was only acetylated following substitutions at S315 (S315A or S315D) or at S378 (S378A). Importantly, in baculoviral expressed proteins, PAb421 reactivity was independent of K373 acetylation status, indicating that acetylation at K382 specifically determines the PAb421 epitope.

The p53 tumor suppressor protein represses human snRNA gene transcription by RNA polymerases II and III independently of sequence-specific DNA binding

Human U1 and U6 snRNA genes are transcribed by RNA polymerases II and III, respectively. While the p53 tumor suppressor protein is a general repressor of RNA polymerase III transcription, whether p53 regulates snRNA gene transcription by RNA polymerase II is uncertain. The data presented herein indicate that p53 is an effective repressor of snRNA gene transcription by both polymerases. Both U1 and U6 transcription in vitro is repressed by recombinant p53, and endogenous p53 occupancy at these promoters is stimulated by UV light. In response to UV light, U1 and U6 transcription is strongly repressed. Human U1 genes, but not U6 genes, contain a high-affinity p53 response element located within the core promoter region. Nonetheless, this element is not required for p53 repression and mutant p53 molecules that do not bind DNA can maintain repression, suggesting a reliance on protein interactions for p53 promoter recruitment. Recruitment may be mediated by the general transcription factors TATA-box binding protein and snRNA-activating protein complex, which interact well with p53 and function for both RNA polymerase II and III transcription.

The coactivator p300 directly acetylates the forkhead transcription factor Foxo1 and stimulates Foxo1-induced transcription

The FOXO (Forkhead box class O) subgroup of forkhead transcription factors controls the expression of many genes involved in fundamental cellular processes. Until recently, studies conducted on posttranslational modifications of Forkhead proteins were restricted to their phosphorylation. In this report, we show that the coactivator p300 directly acetylates lysines in the carboxyl-terminal region of Foxo1 in vivo and in vitro, and potently stimulates Foxo1-induced transcription of IGF-binding protein-1 in transient transfection experiments. The intrinsic acetyltransferase activity of p300 is required for both activities. Our results suggest that acetylation of Foxo1 by p300 is responsible, at least in part, for its increased transactivation potency, although acetylation of histones cannot be excluded. Insulin, the major negative regulator of Foxo1-stimulated transcription, potently enhances p300 acetylation of Foxo1. Three consensus protein kinase B/Akt phosphorylation sites whose phosphorylation is stimulated by insulin are required for insulin-induced acetylation of Foxo1. In contrast to its importance in regulating the transcriptional activity of Foxo1 in the absence of insulin, acetylation plays only a minor role compared with phosphorylation in insulin inhibition of Foxo1 transcriptional activity.

HDM2 negatively affects the Chk2-mediated phosphorylation of p53

By GST pull downs and co-immunoprecipitation analyses we found that recombinant Chk2 and HDM2 can form stable complexes in vitro. Chk2/HDM2 complexes were also detected in transfected Cos-1 cells over-expressing both proteins. Furthermore, we show that HDM2, as would be expected, severely affects the Chk2-catalyzed phosphorylation of p53. HDM2 itself is only slightly phosphorylated by Chk2. However, whereas HDM2 inhibits the Chk2-catalyzed p53 phosphorylation, HDM2 phosphorylation by Chk2 doubles in the presence of p53. The significance of the HDM2 phosphorylation is unknown, but it is possible that it might influence the stability of the HDM2/p53 complex.

Direct p53 transcriptional repression: in vivo analysis of CCAAT-containing G2/M promoters

In response to DNA damage, p53 activates G(1)/S blocking and apoptotic genes through sequence-specific binding. p53 also represses genes with no target site, such as those for Cdc2 and cyclin B, key regulators of the G(2)/M transition. Like most G(2)/M promoters, they rely on multiple CCAAT boxes activated by NF-Y, whose binding to DNA is temporally regulated during the cell cycle. NF-Y associates with p53 in vitro and in vivo through the alphaC helix of NF-YC (a subunit of NF-Y) and a region close to the tetramerization domain of p53. Chromatin immunoprecipitation experiments indicated that p53 is associated with cyclin B2, CDC25C, and Cdc2 promoters in vivo before and after DNA damage, requiring DNA-bound NF-Y. Following DNA damage, p53 is rapidly acetylated at K320 and K373 to K382, histones are deacetylated, and the release of PCAF and p300 correlates with the recruitment of histone deacetylases (HDACs)-HDAC1 before HDAC4 and HDAC5-and promoter repression. HDAC recruitment requires intact NF-Y binding sites. In transfection assays, PCAF represses cyclin B2, and a nonacetylated p53 mutant shows a complete loss of repression potential, despite its abilities to bind NF-Y and to be recruited on G(2)/M promoters. These data (i) detail a strategy of direct p53 repression through associations with multiple NF-Y trimers that is independent of sequence-specific binding of p53 and that requires C-terminal acetylation, (ii) suggest that p53 is a DNA damage sentinel of the G(2)/M transition, and (iii) delineate a new role for PCAF in cell cycle control.

Combinatorial control of the bradykinin B2 receptor promoter by p53, CREB, KLF-4, and CBP: implications for terminal nephron differentiation

Despite a wealth of knowledge regarding the early steps of epithelial differentiation, little is known about the mechanisms responsible for terminal nephron differentiation. The bradykinin B2 receptor (B2R) regulates renal function and integrity, and its expression is induced during terminal nephron differentiation. This study investigates the transcriptional regulation of the B2R during kidney development. The rat B2R 5′-flanking region has a highly conserved cis-acting enhancer in the proximal promoter consisting of contiguous binding sites for the transcription factors cAMP response element binding protein (CREB), p53, and Krüppel-like factor (KLF-4). The B2R enhancer drives reporter gene expression in inner medullary collecting duct-3 cells but is considerably weaker in other cell types. Site-directed mutagenesis and expression of dominant negative mutants demonstrated the requirement of CREB DNA binding and Ser-133 phosphorylation for optimal enhancer function. Moreover, helical phasing experiments showed that disruption of the spatial organization of the enhancer inhibits B2R promoter activity. Several lines of evidence indicate that cooperative interactions among the three transcription factors occur in vivo during terminal nephron differentiation: 1) CREB, p53, and KLF-4 are coexpressed in B2R-positive differentiating cells; 2) the maturational expression of B2R correlates with CREB/p53/KLF-4 DNA-binding activity; 3) assembly of CREB, p53, and KLF-4 on chromatin at the endogenous B2R promoter is developmentally regulated and is accompanied by CBP recruitment and histone hyperacetylation; and 4) CREB and p53 occupancy of the B2R enhancer is cooperative. These results demonstrate that combinatorial interactions among the transcription factors, CREB, p53, and KLF-4, and the coactivator CBP, may be critical for the regulation of B2R gene expression during terminal nephron differentiation.

Multiple mixed lineage leukemia (MLL) fusion proteins suppress p53-mediated response to DNA damage

Chromosomal translocations involving the mixed lineage leukemia (MLL) gene are often observed in acute leukemias of both myeloid and lymphocytic origin. Expression of MLL fusion proteins is known to induce malignant transformation of normal blood progenitors; however, molecular mechanisms of this process are still poorly understood. In this study we investigated the effect of several frequently detected MLL fusion proteins on p53 transcriptional activity. Our data show that MLL-AF9, MLL-AF10, MLL-ENL, and MLL-ELL substantially down-regulate p53-mediated induction of p21, MDM2, and Bax in response to DNA damage. Furthermore, we identify the reduction in p53 acetylation by p300 as a major mechanism of the inhibitory effect of MLL leukemic fusions. Our data suggest that abrogation of p53 functional activity can be a common feature of MLL fusion-mediated leukemogenesis.

Modulation of Binding of DNA to the C-Terminal Domain of p53 by Acetylation

The binding of nonspecific DNA to the C-terminal negative regulatory domain (CTD) of p53 modulates its activity. The CTD is a natively unfolded region, which is subject to acetylation and phosphorylation at several residues as part of control. To measure the effect of covalent modification on binding to DNA, we synthesized a series of fluorescein-labeled CTD peptides with single and multiple acetylations at lysine residues that we had identified by NMR as making contact with DNA, and developed an analytical ultracentrifugation method to study their binding to DNA. Binding depended on ionic strength, indicating an electrostatic contribution. Monoacetylation weakened DNA binding at physiological ionic strength 2- to 3-fold, diacetylations resulted in further 2- to 3-fold decrease in the affinity, and tri- and tetraacetylations rendered DNA binding undetectable. Phosphorylation at S392 did not affect DNA binding. NMR spectroscopy showed binding to DNA did not induce significant structure into CTD, apart possibly from local helix formation.

p53 C-terminal phosphorylation by CHK1 and CHK2 participates in the regulation of DNA-damage-induced C-terminal acetylation

The tumor suppressor protein p53 mediates stress-induced growth arrest or apoptosis and plays a major role in safeguarding genome integrity. In response to DNA damage, p53 can be modified at multiple sites by phosphorylation and acetylation. We report on the characterization of p53 C-terminal phosphorylation by CHK1 and CHK2, two serine/threonine (Ser/Thr) protein kinases, previously implicated in the phosphorylation of the p53 N terminus. Using tryptic phosphopeptide mapping, we have identified six additional CHK1 and CHK2 sites residing in the final 100 amino acids of p53. Phosphorylation of at least three of these sites, Ser366, Ser378, and Thr387, was induced by DNA damage, and the induction at Ser366 and Thr387 was abrogated by small interfering RNA targeting chk1 and chk2. Furthermore, mutation of these phosphorylation sites has a different impact on p53 C-terminal acetylation and on the activation of p53-targeted promoters. Our results demonstrate a possible interplay between p53 C-terminal phosphorylation and acetylation, and they provide an additional mechanism for the control of the activity of p53 by CHK1 and CHK2.

Differential Regulation of MDR1 Transcription by the p53 Family Members

Although the p53 family members share a similar structure and function, it has become clear that they differ with respect to their role in development and tumor progression. Because of the high degree of homology in their DNA binding domains (DBDs), it is not surprising that both p63 and p73 activate the majority of p53 target genes. However, recent studies have revealed some differences in a subset of the target genes affected, and the mechanism underlying this diversity has only recently come under investigation. Our laboratory has demonstrated previously that p53 represses transcription of the P-glycoprotein-encoding MDR1 gene via direct DNA binding through a novel p53 DNA-binding site (the HT site). By transient transfection analyses, we now show that p63 and p73 activate rather than repress MDR1 transcription, and they do so through an upstream promoter element (the alternative p63/p73 element (APE)) independent of the HT site. This activation is dependent on an intact DNA binding domain, because mutations within the p63DBD or p73DBD are sufficient to prevent APE-mediated activation. However, neither p63 nor p73 directly interact with the APE, suggesting an indirect mechanism of activation through this site. Most interestingly, when the p53DBD is replaced by the p63DBD, p53 is converted from a repressor working through the HT site to an activator working through the APE. Taken together, these data indicate that, despite considerable homology, the DBD of the p53 family members have unique properties and can differentially regulate gene targeting and transcriptional output by both DNA binding-dependent and -independent mechanisms.

Histone deacetylase inhibitors down-regulate bcl-2 expression and induce apoptosis in t(14;18) lymphomas

Histone deacetylase (HDAC) inhibitors are promising antitumor agents, but they have not been extensively explored in B-cell lymphomas. Many of these lymphomas have the t(14;18) translocation, which results in increased bcl-2 expression and resistance to apoptosis. In this study, we examined the effects of two structurally different HDAC inhibitors, trichostatin A (TSA) and sodium butyrate (NaB), on the cell cycle, apoptosis, and bcl-2 expression in t(14;18) lymphoma cells. We found that in addition to potent cell cycle arrest, TSA and NaB also dramatically induced apoptosis and down-regulated bcl-2 expression, and overexpression of bcl-2 inhibited TSA-induced apoptosis. The repression of bcl-2 by TSA occurred at the transcriptional level. Western blot analysis and quantitative chromatin immunoprecipitation (ChIP) assay showed that even though HDAC inhibitors increased overall acetylation of histones, localized histone H3 deacetylation occurred at both bcl-2 promoters. TSA treatment increased the acetylation of the transcription factors Sp1 and C/EBPalpha and decreased their binding as well as the binding of CBP and HDAC2 to the bcl-2 promoters. Mutation of Sp1 and C/EBPalpha binding sites reduced the TSA-induced repression of bcl-2 promoter activity. This study provides a mechanistic rationale for the use of HDAC inhibitors in the treatment of human t(14;18) lymphomas.

The C-terminal sterile alpha motif and the extreme C terminus regulate the transcriptional activity of the alpha isoform of p73

p73, a member of the p53 family, is expressed from two separate promoters, generating TA and DeltaN variants. Each variant potentially encodes at least seven alternatively spliced isoforms (alpha-eta). Interestingly, we and others have shown that the alpha isoform of p73 has a weaker transcriptional activity than the beta isoform. Because the alpha isoform has an extended C terminus consisting of a sterile alpha motif (SAM) and an extreme C terminus, it appears that the C terminus is inhibitory. However, how the C terminus inhibits the transcriptional activity of p73 has not been determined. Here, we found that both the SAM and the extreme C terminus exert their inhibitory activity by preventing the accessibility of p300/CBP to the activation domain in p73. Specifically, we showed that the SAM and the extreme C terminus together or individually are capable of repressing the function of p73 activation domain, but neither interacts directly with the activation domain, or suppresses the DNA-binding activity, of the p73 protein. We also showed that the intact state of the SAM and the extreme C terminus is essential for their inhibitory functions such that a small deletion of either the SAM or the extreme C terminus abolishes its inhibitory activity. Furthermore, we showed that both inhibitory domains in the C terminus are capable of suppressing the function of a cis heterologous activation domain from p53 or Gal4. Finally, we showed that both inhibitory domains suppress the ability of p73 to interact with the transcriptional coactivators p300/CBP that are necessary for the initiation of transcription.

Human MUC1 oncoprotein regulates p53-responsive gene transcription in the genotoxic stress response

The MUC1 oncoprotein is aberrantly overexpressed by most human carcinomas. The present work demonstrates that MUC1 associates with the p53 tumor suppressor, and that this interaction is increased by genotoxic stress. The MUC1 cytoplasmic domain binds directly to p53 regulatory domain. Chromatin immunoprecipitation assays demonstrate that MUC1 coprecipitates with p53 on the p53-responsive elements of the p21 gene promoter and coactivates p21 gene transcription. Conversely, MUC1 attenuates activation of Bax transcription. In concert with these results, MUC1 promotes selection of the p53-dependent growth arrest response and suppresses the p53-dependent apoptotic response to DNA damage. These findings indicate that MUC1 regulates p53-responsive genes and thereby cell fate in the genotoxic stress response.

Downregulation of lipopolysaccharide response in drosophila by negative crosstalk between the AP1 and NF-κB signaling modules

IkappaB kinase (IKK) and Jun N-terminal kinase (Jnk) signaling modules are important in the synthesis of immune effector molecules during innate immune responses against lipopolysaccharide and peptidoglycan. However, the regulatory mechanisms required for specificity and termination of these immune responses are unclear. We show here that crosstalk occurred between the drosophila Jnk and IKK pathways, which led to downregulation of each other's activity. The inhibitory action of Jnk was mediated by binding of drosophila activator protein 1 (AP1) to promoters activated by the transcription factor NF-kappaB. This binding led to recruitment of the histone deacetylase dHDAC1 to the promoter of the gene encoding the antibacterial protein Attacin-A and to local modification of histone acetylation content. Thus, AP1 acts as a repressor by recruiting the deacetylase complex to terminate activation of a group of NF-kappaB target genes.

The c-MYC oncoprotein is a substrate of the acetyltransferases hGCN5/PCAF and TIP60

The c-MYC oncoprotein functions as a sequence-specific transcription factor. The ability of c-MYC to activate transcription relies in part on the recruitment of cofactor complexes containing the histone acetyltransferases mammalian GCN5 (mGCN5)/PCAF and TIP60. In addition to acetylating histones, these enzymes have been shown to acetylate other proteins involved in transcription, including sequence-specific transcription factors. This study was initiated in order to determine whether c-MYC is a direct substrate of mGCN5 and TIP60. We report here that mGCN5/PCAF and TIP60 acetylate c-MYC in vivo. By using nanoelectrospray tandem mass spectrometry to examine c-MYC purified from human cells, the major mGCN5-induced acetylation sites have been mapped. Acetylation of c-MYC by either mGCN5/PCAF or TIP60 results in a dramatic increase in protein stability. The data reported here suggest a conserved mechanism by which acetyltransferases regulate c-MYC function by altering its rate of degradation.

The acetyltransferase p300/CBP-associated factor is a p53 target gene in breast tumor cells

p300/CBP-associated factor (PCAF) is a coactivator of the tumor suppressor, p53. PCAF participates in p53's transactivation of target genes through acetylation of both bound p53 and histones within p53 target promoters. Using microarrays, we discovered that PCAF itself is induced by p53 in a panel of breast tumor cell lines. Two p53 mutant breast tumor cell lines, BT-549 and UACC-1179, were chosen for further study of PCAF induction by wild-type p53. PCAF induction following adenoviral transduction of p53 expression was confirmed with real-time polymerase chain reaction in a time course experiment. Chromatin immunoprecipitation experiments then showed that PCAF induction was associated with increased p53 binding to the PCAF promoter, which contains p53 consensus-binding sites. PCAF induction by p53 activity was further demonstrated in wild-type p53 MCF10A cells when PCAF expression was induced following activation of endogenous wild-type p53 with doxorubicin in a dose- and time-dependent manner. Furthermore, the doxorubicin-induced increase in PCAF expression was blocked by pretreatment of the MCF10A cells with siRNA (small interfering RNA) targeted against p53 mRNA. Taken together, the results show that PCAF expression can be induced by wild-type p53.