Functional studies of a germ-line polymorphism at codon 47 within the p53 gene.

A rare germ-line polymorphism in codon 47 of the p53 gene replaces the wild-type proline (CCG) with a serine (TCG). Restriction analysis of 101 human samples revealed the frequency of the rare allele to be 0% (n = 69) in Caucasians and 4.7% (3/64, n = 32) among African-Americans. To investigate the consequence of this amino acid substitution, a cDNA construct (p53 mut47ser) containing the mutation was introduced into a lung adenocarcinoma cell line (Calu-6) that does not express p53. A growth suppression similar to that obtained after introduction of a wild-type p53 cDNA construct was observed, in contrast to the result obtained by introduction of p53 mut143ala. Furthermore, expression of neither p53 mut47ser nor wild-type p53 was tolerated by growing cells. In transient expression assays, both mut47ser and wild-type p53 activated the expression of a reporter gene linked to a p53 binding sequence (PG13-CAT) and inhibited the expression of the luciferase gene under the control of the Rous sarcoma virus promoter (RSVluc). In the same assay, mut143ala did not activate the expression of PG13-CAT and produced only a slight inhibitory effect on RSVluc. These findings indicate that the p53 variant with a serine at codon 47 should be considered as a rare germ-line polymorphism that does not alter the growth-suppression activity of p53.     

The transforming and suppressor functions of p53 alleles: effects of mutations that disrupt phosphorylation, oligomerization and nuclear translocation.

Mutant p53 alleles that have a recessive phenotype in human tumors can, in cooperation with an activated H-ras gene, transform rat embryo fibroblasts (REFs). Mutant p53 proteins differ from wild type, and from each other in conformation, localization and transforming potential. Missense mutations in codons 143, 175 and 275 confer strong transforming potential. A serine 135 p53 mutant has an intermediate transforming potential, while the histidine codon 273 allele transforms weakly, if at all. In contrast to the wild type p53 gene, mutant p53 alleles with strong transforming ability cannot suppress the transformation of REFs by other oncogenes. The His273 allele retains partial suppressor function in this assay. The relevance of p53 oligomerization, phosphorylation and nuclear translocation to the transforming potential of mutant p53 and to wild type p53 suppressor function were examined. The inability of mutant p53 polypeptides to form homodimers correlates with loss of transforming function. Monomeric variants of wild type p53 protein, however, retain the ability to suppress focus formation. Phosphorylation of serine residues 315 and 392 is not required for the transforming function of mutant p53, nor is serine 315 required for suppressor function when these alleles are constitutively expressed in REF assays. Nuclear translocation-defective mutant and wild type p53 proteins retain transforming and suppressor function in REF assays.     

p53 acetylation enhances Taxol-induced apoptosis in human cancer cells

Microtubule inhibitors (MTIs) such as Taxol have been used for treating various malignant tumors. Although MTIs have been known to induce cell death through mitotic arrest, other mechanisms can operate in MTI-induced cell death. Especially, the role of p53 in this process has been controversial for a long time. Here we investigated the function of p53 in Taxol-induced apoptosis using p53 wild type and p53 null cancer cell lines. p53 was upregulated upon Taxol treatment in p53 wild type cells and deletion of p53 diminished Taxol-induced apoptosis. p53 target proteins including MDM2, p21, BAX, and β-isoform of PUMA were also upregulated by Taxol in p53 wild type cells. Conversely, when the wild type p53 was re-introduced into two different p53 null cancer cell lines, Taxol-induced apoptosis was enhanced. Among post-translational modifications that affect p53 stability and function, p53 acetylation, rather than phosphorylation, increased significantly in Taxol-treated cells. When acetylation was enhanced by anti-Sirt1 siRNA or an HDAC inhibitor, Taxol-induced apoptosis was enhanced, which was not observed in p53 null cells. When an acetylation-defective mutant of p53 was re-introduced to p53 null cells, apoptosis was partially reduced compared to the re-introduction of the wild type p53. Thus, p53 plays a pro-apoptotic role in Taxol-induced apoptosis and acetylation of p53 contributes to this pro-apoptotic function in response to Taxol in several human cancer cell lines, suggesting that enhancing acetylation of p53 could have potential implication for increasing the sensitivity of cancer cells to Taxol.     

Mechanism of p53-dependent apoptosis induced by 3-methylcholanthrene: involvement of p53 phosphorylation and p38 MAPK.

Polycyclic aromatic hydrocarbons (PAHs) such as 3-methylcholanthrene (MC) cause untoward effects including carcinogenesis. Here we investigated the effect of MC on apoptosis. MC induced apoptosis, preceded by serine 15 phosphorylation and accumulation of p53. MC failed to cause apoptosis in p53-deficient MG63 cells, whereas ectopic expression of p53 in MG63 cells restored the response to MC. Therefore, MC-induced apoptosis was dependent on p53. MC also activated p38 mitogen-activated protein kinase (MAPK) at 16-24 h. Accumulation of p53 and p53 phosphorylated at serine 15 was not changed by SB203580, a specific inhibitor of p38 MAPK or overexpression of a dominant negative mutant of p38 MAPK at 8 h after MC treatment, whereas the accumulation was suppressed at 24 h. These results suggest that MC induces accumulation and phosphorylation of p53 via a p38 MAPK-independent (early) and p38 MAPK-dependent (late) pathway. SB203580 repressed MC-induced apoptosis. MC induced p38 MAPK activation in p53 expressing cells but not in p53-deficient cells, indicating that the p38 MAPK activation was dependent on early p53 activation. The current study shows that both p53 and p38 MAPK activation are required for MC-induced apoptosis and provides a novel model of a functional regulation between p53 and p38 MAPK in chemical stress-induced apoptosis.     

Phosphorylation of Tip60 by GSK-3 determines the induction of PUMA and apoptosis by p53

Activation of p53 by DNA damage results in either cell-cycle arrest, allowing DNA repair and cell survival, or induction of apoptosis. As these opposite outcomes are both mediated by p53 stabilization, additional mechanisms to determine this decision must exist. Here, we show that glycogen synthase kinase-3 (GSK-3) is required for the p53-mediated induction of the proapoptotic BH3 only-protein PUMA, an essential mediator of p53-induced apoptosis. Inhibition of GSK-3 protected from cell death induced by DNA damage and promoted increased long-term cell survival. We demonstrate that GSK-3 phosphorylates serine 86 of the p53-acetyltransferase Tip60. A Tip60(S86A) mutant was less active to induce p53 K120 acetylation, histone 4 acetylation, and expression of PUMA. Our data suggest that GSK-3 mediated Tip60S86 phosphorylation provides a link between PI3K signaling and the choice for or against apoptosis induction by p53.     

ATM mediates phosphorylation at multiple p53 sites, including Ser(46), in response to ionizing radiation

The p53 tumor suppressor protein preserves genome integrity by regulating growth arrest and apoptosis in response to DNA damage. In response to ionizing radiation (IR), ATM, the gene product mutated in ataxia telangiectasia, stabilizes and activates p53 through phosphorylation of Ser(15) and (indirectly) Ser(20). Here we show that phosphorylation of p53 on Ser(46), a residue important for p53 apoptotic activity, as well as on Ser(9), in response to IR also is dependent on the ATM protein kinase. IR-induced phosphorylation at Ser(46) was inhibited by wortmannin, a phosphatidylinositol 3-kinase inhibitor, but not PD169316, a p38 MAPK inhibitor. p53 C-terminal acetylation at Lys(320) and Lys(382), which may stabilize p53 and activate sequence-specific DNA binding, required Ser(15) phosphorylation by ATM and was enhanced by phosphorylation at nearby residues including Ser(6), Ser(9), and Thr(18). These observations, together with the proposed role of Ser(46) phosphorylation in mediating apoptosis, suggest that ATM is involved in the initiation of p53-dependent apoptosis after IR in human lymphoblastoid cells.     

Serine 123 Phosphorylation Modulates p21 Protein Stability and Activity by Suppressing Ubiquitin-independent Proteasomal Degradation

The cyclin-dependent kinase inhibitor p21^Waf1/Cip1 is a major regulator of the cell cycle and plays an important role in many cellular processes, including differentiation, stress response, apoptosis, and tumorigenesis. We previously cloned the gene encoding dog p21 and found that unlike its human ortholog, dog p21 is expressed as two isoforms, one high molecular mass band of 23 kDa and one low molecular mass band of 19 kDa. In the current study, we found that the high molecular mass band is phosphorylated, whereas the low molecular mass band is hypophosphorylated. Moreover, by generating multiple mutants of dog p21, we found that serine 123 and proline 124, which form a consensus site for proline-directed phosphorylation, are required for expression of the high molecular mass p21 isoform through phosphorylation at serine 123. Most importantly, we showed that serine 123 phosphorylation inhibits ubiquitin-independent proteasomal degradation of p21 protein and subsequently, prolongs p21 protein half-life and enhances the ability of p21 to suppress cell proliferation. Taken together, these data reveal that serine 123 phosphorylation modulates p21 protein stability and activity by suppressing ubiquitin-independent proteasomal degradation.     

Critical involvement of the ATM-dependent DNA damage response in the apoptotic demise of HIV-1-elicited syncytia

DNA damage can activate the oncosuppressor protein ataxia telangiectasia mutated (ATM), which phosphorylates the histone H2AX within characteristic DNA damage foci. Here, we show that ATM undergoes an activating phosphorylation in syncytia elicited by the envelope glycoprotein complex (Env) of human immunodeficiency virus-1 (HIV-1) in vitro. This was accompanied by aggregation of ATM in discrete nuclear foci that also contained phospho-histone H2AX. DNA damage foci containing phosphorylated ATM and H2AX were detectable in syncytia present in the brain or lymph nodes from patients with HIV-1 infection, as well as in a fraction of blood leukocytes, correlating with viral status. Knockdown of ATM or of its obligate activating factor NBS1 (Nijmegen breakage syndrome 1 protein), as well as pharmacological inhibition of ATM with KU-55933, inhibited H2AX phosphorylation and prevented Env-elicited syncytia from undergoing apoptosis. ATM was found indispensable for the activation of MAP kinase p38, which catalyzes the activating phosphorylation of p53 on serine 46, thereby causing p53 dependent apoptosis. Both wild type HIV-1 and an HIV-1 mutant lacking integrase activity induced syncytial apoptosis, which could be suppressed by inhibiting ATM. HIV-1-infected T lymphoblasts from patients with inactivating ATM or NBS1 mutations also exhibited reduced syncytial apoptosis. Altogether these results indicate that apoptosis induced by a fusogenic HIV-1 Env follows a pro-apoptotic pathway involving the sequential activation of ATM, p38MAPK and p53.     

Association of TP53 mutation, p53 overexpression, and p53 codon 72 polymorphism with susceptibility to apoptosis in adult patients with diffuse astrocytomas

Clarification of TP53 alterations is important to understand the mechanisms underlying the development of diffuse astrocytomas. It has been suggested that the alleles of TP53 at codon 72 differ in their ability to induce apoptosis in human cancers. The aim of this study was to analyze the possible association of TP53 mutation, p53 overexpression, and p53 codon 72 polymorphism with susceptibility to apoptosis in adult Brazilian patients with diffuse astrocytomas. We analyzed 56 surgical specimens of diffuse astrocytomas for alterations of TP53, using polymerase chain reaction single-strand conformation polymorphism (PCR-SSCP) direct sequencing. p53 and cleaved caspase 3 protein expression were assessed by immunohistochemistry. We found TP53 mutations in 19.6% (11 out of 56) of tumors tested, with the lowest mutation rate found in the cases of glioblastomas (8.8%) (p = 0.03). Only 16.1% of tumors tested showed cleaved caspase 3-positive staining, demonstrating that apoptosis is very inhibited in these tumors. All tumors having TP53 mutation and p53 accumulation had no expression of cleaved caspase 3. Additionally, no association was observed in tumors having proline and arginine alleles and expression of cleaved caspase 3. We concluded that clarification of the TP53 alterations allows a better understanding of the mechanisms involved in the progression of diffuse astrocytomas, and the allele status at codon 72 was not associated with apoptosis in these tumors.     

The p53 R72P polymorphism does not affect the physiological response to ionizing radiation in a mouse model

Tissue culture and mouse model studies show that the presence of the arginine (R) or proline (P) coding single nucleotide polymorphism (SNP) of the tumor suppressor gene p53 at codon 72 (p53 R72P) differentially affects the responses to genotoxic insult. Compared to the P variant, the R variant shows increased apoptosis in most cell cultures and mouse model tissues in response to genotoxins, and epidemiological studies suggest that the R variant may enhance cancer survival and reduce the risks of adverse reactions to genotoxic cancer treatment. As ionizing radiation (IR) treatment is often used in cancer therapy, we sought to test the physiological effects of IR in mouse models of the p53 R72P polymorphism. By performing blood counts, immunohistochemical (IHC) staining and survival studies in mouse populations rigorously controlled for strain background, sex and age, we discovered that p53 R72P polymorphism did not differentially affect the physiological response to IR. Our findings suggest that genotyping for this polymorphism to personalize IR therapy may have little clinical utility.     

Regulation of p53 stability and function in HCT116 colon cancer cells

We have used a lentiviral vector to stably express p53 at a physiological level in p53 knockout HCT116 cells. Cells transduced with wild type p53 responded to genotoxic stress by stabilizing p53 and expressing p53 target genes. The reconstituted cells underwent G(1) arrest or apoptosis appropriately depending on the type of stress, albeit less efficiently than parental wild type cells. Compared with cells expressing exogenous wild type p53, the apoptotic response to 5-fluorouracil (5FU) was >50% reduced in cells expressing S15A or S20A mutant p53, and even more reduced by combined mutation of serines 6, 9, 15, 20, 33, and 37 (N6A). Among a panel of p53 target genes tested by quantitative PCR, the gene showing the largest defect in induction by 5FU was BBC3 (PUMA), which was induced 4-fold by wild type p53 and 2-fold by the N6A mutant. Mutation of N-terminal phosphorylation sites did not prevent p53 stabilization by doxorubicin or 5FU. MDM2 silencing by RNA interference activated p53 target gene expression in normal fibroblasts but not in HCT116 cells, and exogenous p53 could be stabilized in HCT116 knockout cells despite combined mutation of p53 phosphorylation sites and silencing of MDM2 expression. The MDM2 feedback loop is thus defective, and other mechanisms must exist to regulate p53 stability and function in this widely used tumor cell line.     

Phosphorylation of human p53 at serine 46 determines promoter selection and whether apoptosis is attenuated or amplified

The capacity of DNA damaging agents to induce apoptosis is regulated by target gene induction by p53. We found that p53 targeted MDM2 in cells in which DNA repair was occurring, but persistent DNA damage induced by chemotherapy led p53 to selectively target PTEN. High dose chemotherapy induced the phosphorylation of p53 on serine 46, whereas low dose chemotherapy did not. A nonphosphorylatable serine 46 to alanine p53 mutant (S46A) targeted the MDM2 promoter in preference to that for PTEN. A serine 46 to aspartate mutant (S46D, a phosphorylation mimic) targeted PTEN in preference to MDM2. These observations show that phosphorylation of serine 46 in p53 is sufficient for it to induce the PTEN (phosphatase and tensin homolog deleted on chromosome ten) tumor suppressor protein in preference to MDM2. S46A induced significantly less cell death than the S46D in cells. The phosphorylation-induced change of p53 promoter targeting suppresses the induction of MDM2 and the formation of the autoregulatory feedback loop. Induction of PTEN by p53 followed by expression of PTEN inhibits AKT-induced translocation of MDM2 into the nucleus and sustains p53 function. The protection of p53 from MDM2 by PTEN and the damage-induced activation of PTEN by phosphorylated p53 leads to the formation of an apoptotic amplification cycle in which p53 and PTEN coordinately increase cellular apoptosis.     

The involvement of MAPK signaling pathways in determining the cellular response to p53 activation: cell cycle arrest or apoptosis

The effect of ERK, p38, and JNK signaling on p53-dependent apoptosis and cell cycle arrest was investigated using a Friend murine erythroleukemia virus (FVP)-transformed cell line that expresses a temperature-sensitive p53 allele, DP16.1/p53ts. In response to p53 activation at 32 degrees C, DP16.1/p53ts cells undergo p53-dependent G(1) cell cycle arrest and apoptosis. As a result of viral transformation, these cells express the spleen focus forming env-related glycoprotein gp55, which can bind to the erythropoietin receptor (EPO-R) and mimics many aspects of EPO-induced EPO-R signaling. We demonstrate that ERK, p38 and JNK mitogen-activated protein kinases (MAPKs) are constitutively active in DP16.1/p53ts cells. Constitutive MEK activity contributes to p53-dependent apoptosis and phosphorylation of p53 on serine residue 15. The pro-apoptotic effect of this MAPK kinase signal likely reflects an aberrant Ras proliferative signal arising from FVP-induced viral transformation. Inhibition of MEK alters the p53-dependent cellular response of DP16.1/p53ts from apoptosis to G(1) cell cycle arrest, with a concomitant increase in p21(WAF1), suggesting that the Ras/MEK pathway may influence the cellular response to p53 activation. p38 and JNK activity in DP16.1/p53ts cells is anti-apoptotic and capable of limiting p53-dependent apoptosis at 32 degrees C. Moreover, JNK facilitates p53 protein turnover, which could account for the enhanced apoptotic effects of inhibiting this MAPK pathway in DP16.1/p53ts cells. Overall, these data show that intrinsic MAPK signaling pathways, active in transformed cells, can both positively and negatively influence p53-dependent apoptosis, and illustrate their potential to affect cancer therapies aimed at reconstituting or activating p53 function.     

Site-specific phosphorylation of tyrosine hydroxylase after KCl depolarization and nerve growth factor treatment of PC12 cells

The phosphorylation and activation of tyrosine hydroxylase was examined in PC12 cells following depolarization with KCl or treatment with nerve growth factor. Both treatments activate tyrosine hydroxylase (TH) and increase enzyme phosphorylation. Site-specific analysis of the tryptic phosphopeptides of TH isolated from [32P]phosphate-labeled PC12 cells demonstrated that the major phosphorylated peptide (termed "H25") did not contain any of the previously reported phosphorylation sites. Phosphoamino acid analysis of this peptide demonstrated that the phosphorylated residue was a serine. Synthetic tryptic peptides containing putative phosphorylation sites were prepared, and subjected to high performance liquid chromatography analysis and isoelectric focusing. The tryptic phosphopeptide containing serine 31 comigrated with the H25 peptide during both of these analytical techniques. The tryptic phosphopeptide produced by the phosphorylation of tyrosine hydroxylase by the recently discovered proline-directed protein kinase and the phosphorylated synthetic phosphopeptide TH2-12 are clearly separated from H25 by this analysis. We conclude that serine 31 is phosphorylated during KCl depolarization and nerve growth factor treatment of PC12 cells and that this phosphorylation is responsible for the activation of tyrosine hydroxylase. Since this site is not located in a sequence selective for any of the "classical" protein kinases, we suggest that a novel protein kinase may be responsible for the phosphorylation of this site. Since serine 31 has a proline residue on the carboxyl-terminal side, the possibility that this kinase may be related to the recently reported proline-directed protein kinase is discussed. Other sites that are also phosphorylated on TH during KCl depolarization include serine 19, which is known to be phosphorylated by calmodulin-dependent protein kinase II. A schematic model for the regulation of tyrosine hydroxylase activity by phosphorylation of the NH2-terminal regulatory domain is presented.