Defective p53 Post-translational Modification Required for Wild Type p53 Inactivation in Malignant Epithelial Cells with mdm2 Gene Amplification

Unconventional protein post-translational modifications: the helmsmen in breast cancer

Breast cancer is the most prevalent malignant tumor and a leading cause of mortality among females worldwide. The tumorigenesis and progression of breast cancer involve complex pathophysiological processes, which may be mediated by post-translational modifications (PTMs) of proteins, stimulated by various genes and signaling pathways. Studies into PTMs have long been dominated by the investigation of protein phosphorylation and histone epigenetic modifications. However, with great advances in proteomic techniques, several other PTMs, such as acetylation, glycosylation, sumoylation, methylation, ubiquitination, citrullination, and palmitoylation have been confirmed in breast cancer. Nevertheless, the mechanisms, effects, and inhibitors of these unconventional PTMs (particularly, the non-histone modifications other than phosphorylation) received comparatively little attention. Therefore, in this review, we illustrate the functions of these PTMs and highlight their impact on the oncogenesis and progression of breast cancer. Identification of novel potential therapeutic drugs targeting PTMs and development of biological markers for the detection of breast cancer would be significantly valuable for the efficient selection of therapeutic regimens and prediction of disease prognosis in patients with breast cancer.

Akt1-mediated Gata3 phosphorylation controls the repression of IFNγ in memory-type Th2 cells

Th2 cells produce Th2 cytokines such as IL-4, IL-5 and IL-13, but repress Th1 cytokine IFNγ. Recent studies have revealed various distinct memory-type Th2 cell subsets, one of which produces a substantial amount of IFNγ in addition to Th2 cytokines, however it remains unclear precisely how these Th2 cells produce IFNγ. We herein show that phosphorylation of Gata3 at Ser308, Thr315 and Ser316 induces dissociation of a histone deacetylase Hdac2 from the Gata3/Chd4 repressive complex in Th2 cells. We also identify Akt1 as a Gata3-phosphorylating kinase, and the activation of Akt1 induces derepression of Tbx21 and Ifng expression in Th2 cells. Moreover, T-bet-dependent IFNγ expression in IFNγ-producing memory Th2 cells appears to be controlled by the phosphorylation status of Gata3 in human and murine systems. Thus, this study highlights the molecular basis for posttranslational modifications of Gata3 that control the regulation of IFNγ expression in memory Th2 cells.

Prevention of liver carcinogenesis by amarogentin through modulation of G1/S cell cycle check point and induction of apoptosis

Amarogentin, a secoiridoid glycoside, is an active component of the medicinal plant Swertia chirata. In this study, chemopreventive and chemotherapeutic actions of amarogentin were evaluated in a carbon tetrachloride (CCl(4))/N-nitrosodiethylamine (NDEA)-induced liver carcinogenesis mouse model system during continuous and posttreatment schedule. Better survival, no toxicity and increased body weight were noted in amarogentin-treated mice. Reduction in proliferation and increase in apoptosis frequency were evident in amarogentin-treated groups. In carcinogen control group moderate dysplasia, severe dysplasia and hepatocellular carcinoma were evident at 10th, 20th and 30th week, respectively. Amarogentin was found to prevent progression of liver carcinogenesis at mild dysplastic stage. Exposure to CCl(4)/NDEA resulted in upregulation of ppRb807/811, cyclinD1 and cdc25A at 10th week and additional activation of cMyc and mdm2 along with downregulation of LIMD1, p53 and p21 at 20th week. This was followed by activation of ppRb567 and downregulation of Rbsp3 at 30th week. Prevention of carcinogenesis by amarogentin in both groups might be due to cumulative upregulation of LIMD1, RBSP3, p16 and downregulation of cdc25A at 10th week along with activation of p53 and p21 and downregulation of ppRb807/811 and ppRb567 at 20th week, followed by downregulation of cyclinD1, cMyc and mdm2 at 30th week. During carcinogenesis reduction of apoptosis was evident since 20th week. However, amarogentin treatment could significantly induce apoptosis through upregulation of the Bax-Bcl2 ratio, activation of caspase-3 and poly ADP ribose polymerase cleavage. This is the first report of chemopreventive/therapeutic role of amarogentin during liver carcinogenesis through modulation of cell cycle and apoptosis.

Large-scale phosphoproteome of human whole saliva using disulfide-thiol interchange covalent chromatography and mass spectrometry

To date, only a handful of phosphoproteins with important biological functions have been identified and characterized in oral fluids, and these include some of the abundant protein constituents of saliva. Whole saliva (WS) samples were trypsin digested, followed by chemical derivatization using dithiothreitol (DTT) of the phospho-serine/threonine-containing peptides. The DTT-phosphopeptides were enriched by covalent disulfide-thiol interchange chromatography and analysis by nanoflow liquid chromatography and electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS). The specificity of DTT chemical derivatization was evaluated separately under different base-catalyzed conditions with NaOH and Ba(OH)(2), blocking cysteine residues by iodoacetamide and enzymatic O-deglycosylation prior to DTT reaction. Further analysis of WS samples that were subjected to either of these conditions provided supporting evidence for phosphoprotein identifications. The combined chemical strategies and mass spectrometric analyses identified 65 phosphoproteins in WS; of these, 28 were based on two or more peptide identification criteria with high confidence and 37 were based on a single phosphopeptide identification. Most of the identified proteins (∼80%) were previously unknown phosphoprotein components. This study represents the first large-scale documentation of phosphoproteins of WS. The origins and identity of WS phosphoproteome suggest significant implications for both basic science and the development of novel biomarkers/diagnostic tools for systemic and oral disease states.

Regulation by phosphorylation of the relative affinities of the N-terminal transactivation domains of p53 for p300 domains and Mdm2

The transcriptional activity of the tumour suppressor, p53, requires direct binding between its transactivation domain (TAD, 1-57) and the transcriptional coactivator, p300. We systematically assessed the role of TAD phosphorylation on binding of the p300 domains CH3, Taz1, Kix and IBiD. Thr18 phosphorylation increased the affinity up to sevenfold for CH3 and Taz1, with smaller increases from phosphorylation of Ser20, Ser15, Ser37, Ser33, Ser46 and Thr55. Binding of Kix and IBiD was less sensitive to phosphorylation. Strikingly, hepta-phosphorylation of all Ser and Thr residues increased binding 40- and 80-fold with CH3 and Taz1, respectively, but not with Kix or IBiD. Substitution of all phospho-sites with aspartates partially mimicked the effects of hepta-phosphorylation. Mdm2, the main negative regulator of p53, competes with p300 for binding to TAD. Binding of Mdm2 to TAD was reduced significantly only on phosphorylation of Thr18 (sevenfold) or by hepta-phosphorylation (24-fold). The relative affinities of Mdm2 and p300 for p53 TAD can thus be changed by up to three orders of magnitude by phosphorylation. Accordingly, phosphorylation of Thr18 and hepta-phosphorylation dramatically shifts the balance towards favouring the binding of p300 with p53, and is thus likely to be an important factor in its regulation.

A function for the RING finger domain in the allosteric control of MDM2 conformation and activity

The MDM2 oncoprotein plays multiple regulatory roles in the control of p53-dependent gene expression. A picture of MDM2 is emerging where structurally discrete but interdependent functional domains are linked through changes in conformation. The domain structure includes: (i) a hydrophobic pocket at the N terminus of MDM2 that is involved in both its transrepressor and E3-ubiqutin ligase functions, (ii) a central acid domain that recognizes a ubiquitination signal in the core DNA binding domain of p53, and (iii) a C-terminal C2H2C4 RING finger domain that is required for E2 enzyme-binding and ATP-dependent molecular chaperone activity. Here we show that the binding affinity of MDM2s hydrophobic pocket can be regulated through the RING finger domain and that increases in pocket affinity are reflected by a gain in MDM2 transrepressor activity. Thus, mutations within the RING domain that affect zinc coordination, but not one that inhibits ATP binding, produce MDM2 proteins that have a higher affinity for the BOX-I transactivation domain of p53 and a reduced I(0.5) for p53 transrepression. An allosteric model for regulation of the hydrophobic pocket is supported by differences in protein conformation and pocket accessibility between wild-type and the RING domain mutant MDM2 proteins. Additionally the data demonstrate that the complex relationship between different domains of MDM2 can impact on the efficacy of anticancer drugs directed toward its hydrophobic pocket.

Proteome- and microarray-based expression analysis of lymphoma cell lines identifies a p53-centered cluster of differentially expressed proteins in mantle cell and follicular lymphoma

We used a standardized electrophoresis protocol to identify differentially expressed proteins based on a sample pooling approach comparing three follicular lymphoma and three mantle cell lymphoma-derived cell lines. One hundred and seventy-five consistently differentially expressed proteins were identified out of more than 1600 protein spots per gel. Of these 175 protein spots, 38 of the 41 most highly expressed proteins were identified by MS analysis (MALDI-TOF), involving different cellular programs such as DNA repair (Rad50), cell cycle control (Mad1L1), transcription (SAFB), and apoptosis (Luca-15 protein). Expression data were confirmed by Western blot analysis of identified proteins and 2-D gel hybridization of proteins with known overexpression (G1/S-specific cyclin-D1, apoptosis regulator Bcl-2). Comparison of proteome analysis to RNA expression array data revealed only a modest correlation of RNA and protein level emphasizing the relevance of post-translational regulation in lymphomagenesis (p = 0.36). Most interestingly, additional data bank search identified 13 out of 17 referenced proteins (76%) as members of a TP53-dependent network of cell regulation.

p73 expression modulates p63 and Mdm2 protein presence in complex with p53 family-specific DNA target sequence in squamous cell carcinogenesis

The expression of p73 and p63 isoforms is frequently deregulated in human epithelial tumors. We previously showed that loss of p73 protein expression associates with malignant conversion in vivo and ionizing radiation (IR) resistance in vitro in a clonal model of mouse epidermal carcinogenesis. Here we show that loss of endogenous p73 expression in squamous cell carcinoma (SCC) cells and tumors was concomitant with preferential DNA binding of the inhibitory DeltaNp63alpha isoform and reduction of transcriptionally active p63gamma isoforms binding to a p21 promoter sequence in vitro. Reconstitution of TAp73alpha in malignant cells increased the steady state DNA-binding capabilities of the endogenous transcriptionally active TAp63gamma and DeltaNp63gamma isoforms, correlating with restoration of tumor suppression but not IR sensitivity. Loss of p73 in malignant cells also coincided with increased presence of p53 family inhibitor Mdm2 in p53-specific DNA-bound complexes, whereas reconstitution of TAp73alpha expression resulted in exclusion of Mdm2 from these complexes. These results suggest a dual mechanism for TAp73alpha to foster tumor suppression through enhancement of the DNA-binding activity of p63gamma isoforms, and through inhibition of transcriptional repressors Mdm2 or DeltaNp63alpha.

p73 loss triggers conversion to squamous cell carcinoma reversible upon reconstitution with TAp73alpha

The expression level of the p53 family member, p73, is frequently deregulated in human epithelial cancers, correlating with tumor invasiveness, therapeutic resistance, and poor patient prognosis. However, the question remains whether p73 contributes directly to the process of malignant conversion or whether aberrant p73 expression represents a later selective event to maintain tumor viability. We explored the role of p73 in malignant conversion in a clonal model of epidermal carcinogenesis. Whether sporadic or small interfering RNA (siRNA) induced, loss of p73 in initiated p53+/+ keratinocytes leads to loss of cellular responsiveness to DNA damage by ionizing radiation (IR) and conversion to squamous cell carcinoma (SCC). Reconstitution of TAp73alpha but not DeltaNp73alpha reduced tumorigenicity in vivo, but did not restore cellular sensitivity to IR, uncoupling p73-mediated DNA damage response from its tumor-suppressive role. These studies provide direct evidence that loss of p73 can contribute to malignant conversion and support a role for TAp73alpha in tumor suppression of SCC. The results support the activation of TAp73alpha as a rational mechanism for cancer therapy in solid tumors of the epithelium.

Novel initiation genes in squamous cell carcinomagenesis: a role for substrate-specific ubiquitylation in the control of cell survival

The study of experimental epidermal carcinogenesis offers several advantages over other epithelial carcinogenesis models, including easy accessibility and a database of research findings spanning over a century. Our studies make use of a clonal in vitro/in vivo keratinocyte carcinogenesis model with low frequency of ras mutation and derivative clonal-initiated lineages with distinct tumor fate. Analysis of this model has yielded candidate genes involved in the stages of initiation and tumorigenic progression, and has revealed novel roles for ubiquitylation in transcriptional control of survival and apoptotic pathways during the early stages of carcinogenesis. The expression of a recently described E3-ubiquitin ligase, Trim32, is elevated during initiation, and ectopic expression of Trim32 confers extended survival in response to terminal differentiation and ultraviolet light (UV) B/TNF-alpha death signals. Trim32 binds and ubiquitylates Piasy, controlling its stability and accumulation. Piasy is a SUMOylation factor involved in the control of apoptosis, senescence, and NF-kappaB activation. NF-kappaB is a survival factor for keratinocytes in response to UV irradiation, the main carcinogenic stimulus for the epidermis. Piasy inhibits NF-kappaB activity, and promotes keratinocyte apoptosis in response to UV and TNF-alpha. In human skin squamous cell carcinoma (SCC) samples, we found an inverse correlation between Trim32 and Piasy expression supporting a role for Trim32-Piasy interaction in human epidermal carcinogenesis. Our hypothesis is that increased expression of Trim32 may enhance epidermal carcinogenesis, by increasing the threshold of NF-kappaB activity through Piasy downmodulation.

Circumvention and reactivation of the p53 oncogene checkpoint in mouse colon tumors

The p53 tumor suppressor protein is sequence-normal in azoxymethane (AOM)-induced mouse colon tumors, making them a good model for human colon cancers that retain a wild type p53 gene. Cellular localization and co-immunoprecipitation experiments using a cell line derived from an AOM-induced colon tumor (AJ02-NM(0) cells) pointed to constitutively expressed Mdm2 as being an important negative regulator of p53 in these cells. Although the Mdm2 inhibitory protein p19/ARF was expressed in AJ02-NM(0) cells, its level of expression was not sufficient for p53 activation. We tested the response of AJ02-NM(0) cells to the recently developed Mdm2 inhibitor, Nutlin-3. Nutlin-3 was found to activate p53 DNA binding in AJ02-NM(0) cells, to a level comparable to doxorubicin and 5-fluorouracil (5-FU). In addition, Nutlin-3 increased expression of the p53 target genes Bax and PERP to a greater extent than doxorubicin or 5-FU, and triggered a G2/M phase arrest in these cells, compared to a G1 arrest triggered by doxorubicin and 5-FU. The differences in the cellular response may be related to differences in the kinetics of p53 activation and/or its post-translational modification status. In an ex vivo experiment, Nutlin-3 was found to activate p53 target gene expression and apoptosis in AOM-induced tumor tissue, but not in normal adjacent mucosa. Our data indicate that Mdm2 inhibitors may be an effective means of selectively targeting colon cancers that retain a sequence-normal p53 gene while sparing normal tissue and that the AOM model is an appropriate model for the preclinical development of these drugs.

The subcellular localization of vaccinia-related kinase-2 (VRK2) isoforms determines their different effect on p53 stability in tumour cell lines

VRK is a new kinase family of unknown function. Endogenous human vacinia-related kinase 2 (VRK2) protein is present in both the nucleus and the cytosol, which is a consequence of alternative splicing of two VRK2 messages coding for proteins of 508 and 397 amino acids, respectively. VRK2A has a C-terminal hydrophobic region that anchors the protein to membranes in the endoplasmic reticulum (ER) and mitochondria, and it colocalizes with calreticulin, calnexin and mitotracker; whereas VRK2B is detected in both the cytoplasm and the nucleus. VRK2A is expressed in all cell types, whereas VRK2B is expressed in cell lines in which VRK1 is cytoplasmic. Both VRK2 isoforms have an identical catalytic N-terminal domain and phosphorylate p53 in vitro uniquely in Thr18. Phosphorylation of the p53 protein in response to cellular stresses results in its stabilization by modulating its binding to other proteins. However, p53 phosphorylation also occurs in the absence of stress. Only overexpression of the nuclear VRK2B isoform induces p53 stabilization by post-translational modification, largely due to Thr18 phosphorylation. VRK2B may play a role in controlling the binding specificity of the N-terminal transactivation domain of p53. Indeed, the p53 phosphorylated by VRK2B shows a reduction in ubiquitination by Mdm2 and an increase in acetylation by p300. Endogenous p53 is also phosphorylated in Thr18 by VRK2B, promoting its stabilization and transcriptional activation in A549 cells. The relative phosphorylation of Thr18 by VRK2B is similar in magnitude to that induced by taxol, which might use a different signalling pathway. In this context, VRK2B kinase might functionally replace nuclear VRK1. Therefore, these kinases might be components of a new signalling pathway that is likely to play a role in normal cell proliferation.

Destabilizing missense mutations in the tumour suppressor protein p53 enhance its ubiquitination in vitro and in vivo

p53 ubiquitination catalysed by MDM2 (murine double minute clone 2 oncoprotein) provides a biochemical assay to dissect stages in E3-ubiquitin-ligase-catalysed ubiquitination of a conformationally flexible protein. A mutant form of p53 (p53(F270A)) containing a mutation in the second MDM2-docking site in the DNA-binding domain of p53 (F270A) is susceptible to modification of long-lived and high-molecular-mass covalent adducts in vivo. Mutant F270A is hyperubiquitinated in cells as defined by immunoprecipitation and immunoblotting with an anti-ubiquitin antibody. Transfection of His-tagged ubiquitin along with p53(R175H) or p53(F270A) also results in selective hyperubiquitination in cells under conditions where wild-type p53 is refractory to covalent modification. The extent of mutant p53(R175H) or p53(F270A) unfolding in cells as defined by exposure of the DO-12 epitope correlates with the extent of hyperubiquitination, suggesting a link between substrate conformation and E3 ligase function. The p53(F270A:6KR) chimaeric mutant (where 6KR refers to the simultaneous mutation of lysine residues at positions 370, 372, 373, 381, 382 and 386 to arginine) maintains the high-molecular-mass covalent adducts and is modified in an MDM2-dependent manner. Using an in vitro ubiquitination system, mutant p53(F270A) and the p53(F270A:6KR) chimaeric mutant is also subject to hyperubiquitination outwith the C-terminal domain, indicating direct recognition of the mutant p53 conformation by (a) factor(s) in the cell-free ubiquitination system. These data identify an in vitro and in vivo assay with which to dissect how oligomeric protein conformational alterations are linked to substrate ubiquitination in cells. This has implications for understanding the recognition of misfolded proteins during aging and in human diseases such as cancer.

Distinct p53 acetylation cassettes differentially influence gene-expression patterns and cell fate

The activity of the p53 gene product is regulated by a plethora of posttranslational modifications. An open question is whether such posttranslational changes act redundantly or dependently upon one another. We show that a functional interference between specific acetylated and phosphorylated residues of p53 influences cell fate. Acetylation of lysine 320 (K320) prevents phosphorylation of crucial serines in the NH₂-terminal region of p53; only allows activation of genes containing high-affinity p53 binding sites, such as p21/WAF; and promotes cell survival after DNA damage. In contrast, acetylation of K373 leads to hyperphosphorylation of p53 NH₂-terminal residues and enhances the interaction with promoters for which p53 possesses low DNA binding affinity, such as those contained in proapoptotic genes, leading to cell death. Further, acetylation of each of these two lysine clusters differentially regulates the interaction of p53 with coactivators and corepressors and produces distinct gene-expression profiles. By analogy with the “histone code” hypothesis, we propose that the multiple biological activities of p53 are orchestrated and deciphered by different “p53 cassettes,” each containing combination patterns of posttranslational modifications and protein–protein interactions.

Excess HDM2 impacts cell cycle and apoptosis and has a selective effect on p53-dependent transcription

Mutational inactivation of p53 is only one of the ways that tumors lose p53 function. An alternate route is through overexpression of HDM2, the negative regulator of p53. To further understand how excess HDM2 regulates p53-mediated functions, we generated H1299 cell clones that constitutively express both ectopic HDM2 and tetracycline-regulated inducible p53. We found that over a range of p53 concentrations constitutively expressed HDM2 did not affect the levels of p53 protein. Nevertheless, cells with excess HDM2 displayed numerous changes in their response to p53. After DNA damage, such cells had both increased p53-mediated G2 arrest and reduced cell death. They also showed selective impairment of p53 target gene induction in that some p53 targets were unaffected whereas others were markedly less well induced in the presence of extra HDM2 protein. We also found that excess HDM2 was correlated with reduced p53 acetylation but did not affect p53 association with target promoters in vivo. Indeed, there was no significant difference in the amount of HDM2 associated with p53 at target promoters that differed in their expression depending on the presence of extra HDM2. Thus, HDM2 can selectively down-regulate the transcription function of p53 without either degrading p53 or affecting the interaction of p53 with target promoters.

Melanocyte and keratinocyte carcinogenesis: p53 family protein activities and intersecting mRNA expression profiles

Melanocytes and keratinocytes were analyzed for potential roles of p53, p73, and p63 tumor suppressor family proteins and of malignancy-specific gene expression changes in the etiology of multi-step cancer. Melanocytes expressed deltaNp73alpha, two p63 isoforms and p53. Although p21 and Noxa mRNA levels increased following DNA damage, p53 family member binding to p21 and Noxa DNA probes was undetectable, suggesting p53 family-independent responses. In contrast, keratinocytes expressed multiple isoforms each of p73 and p63 that were induced to bind p21 and Noxa DNA probes after ionizing (IR) or after ultraviolet B (UVB) irradiation, correlating with p21 and Noxa mRNA induction and with apoptosis. Interestingly, IR-resistant malignant melanocytes and keratinocytes both exhibited Noxa mRNA induction after UVB treatment, correlating with DNA binding of p53 family proteins to the Noxa probe only in keratinocytes. To uncover other malignancy-specific events, we queried mouse initiated keratinocyte clones for early changes that were exacerbated in malignant derivatives and also differentially expressed in human advanced melanoma versus normal melanocytes. Using a new method for ranking and normalization of microarray data for 5000 probe sets, 27 upregulated and 13 downregulated genes satisfied our query. Of these, the majority was associated with late-stage human cancers and six were novel genes. Thus, clonal lineage mouse models representing early through late cancer progression stages may inform the focus on early, potentially causal events from microarray studies of human cancers, facilitating prognosis and molecular therapy.

Post-translational modification of p53 in tumorigenesis

Interest in the tumour suppressor p53 has generated much information regarding the complexity of its function and regulation in carcinogenesis. However, gaps still exist in our knowledge regarding the role of p53 post-translational modifications in carcinogenesis and cancer prevention. A thorough understanding of p53 will be extremely useful in the development of new strategies for treating and preventing cancer, including restoration of p53 function and selective killing of tumours with mutant TP53.

Expression of proopiomelanocortin, corticotropin-releasing hormone (CRH), and CRH receptor in melanoma cells, nevus cells, and normal human melanocytes

Proopiomelanocortin (POMC) is a 31 kDa prohormone that is processed to various bioactive peptides, including adrenocorticotropin (ACTH), melanotropins (alpha, beta, gamma-MSH), lipotropins, and endorphins. POMC is expressed not only in the pituitary gland but also in a variety of nonpituitary organs and tumors, including melanomas. We previously showed that normal human melanocytes produce and secrete alpha-MSH and ACTH, and furthermore, that advanced melanoma cells generally produce higher amounts of POMC peptides that correlate with tumor progression. To elucidate the mechanism of this upregulation, the expression of genes encoding corticotropin-releasing hormone (CRH) and its receptor, CRH-R, as well as POMC and the MSH receptor (MC1-R), was evaluated by reverse transcriptase-polymerase chain reaction using cultured human melanoma cells, nevus cells, and normal melanocytes. Our results show that all melanocytic cells express CRH, CRH-R, POMC, and MC1-R, with highest intensities in melanoma cells. Furthermore, immunohistochemistry shows that CRH as well as POMC is strongly expressed in advanced melanomas, such as vertically growing lesions of acral lentiginous, nodular and metastatic melanomas, in contrast to negative expression in nevus cells. These results indicate that tumor progression accentuates CRH, CRH-R, and POMC expression by melanoma cells.