Subcellular targeting of p33ING1b by phosphorylation-dependent 14-3-3 binding regulates p21WAF1 expression

In silico analysis prediction of HepTH1-5 as a potential therapeutic agent by targeting tumour suppressor protein networks

Many studies reported that the activation of tumour suppressor protein, p53 induced the human hepcidin expression. However, its expression decreased when p53 was silenced in human hepatoma cells. Contrary to Tilapia hepcidin TH1-5, HepTH1-5 was previously reported to trigger the p53 activation through the molecular docking approach. The INhibitor of Growth (ING) family members are also shown to directly interact with p53 and promote cell cycle arrest, senescence, apoptosis and participate in DNA replication and DNA damage responses to suppress the tumour initiation and progression. However, the interrelation between INGs and HepTH1-5 remains unknown. Therefore, this study aims to identify the mechanism and their protein interactions using in silico approaches. The finding revealed that HepTH1-5 and its ligands had interacted mostly on hotspot residues of ING proteins which involved in histone modifications via acetylation, phosphorylation, and methylation. This proves that HepTH1-5 might implicate in an apoptosis signalling pathway and preserve the protein structure and function of INGs by reducing the perturbation of histone binding upon oxidative stress response. This study would provide theoretical guidance for the design and experimental studies to decipher the role of HepTH1-5 as a potential therapeutic agent for cancer therapy. Communicated by Ramaswamy H. Sarma.

Inhibitor of Growth Factors Regulate Cellular Senescence

Simple Summary

Five members of the Inhibitor of Growth (ING) family share a highly conserved plant homeodomian with affinity to the specific histone modification H3K4me3. Since some ING family members are preferentially associated with histone acetyltransferaseactivity while other members with histone deacetlyse activity, the ING family membres are epigenetic regulators. Interestingly, ING members can regulate the induction cellular senescence in both primray untransformed human cells as well as human cancer cells. We discuss here the up-to-date knowledge about their regulatory activity within the cellular senescent program.

Abstract

The Inhibitor of Growth (ING) proteins are a group of tumor suppressors with five conserved genes. A common motif of ING factors is the conserved plant homeodomain (PHD), with which they bind to chromatin as readers of the histone mark trimethylated histone H3 (H3K4me3). These genes often produce several protein products through alternative splicing events. Interestingly, ING1 and ING2 participate in the establishment of the repressive mSIN3a-HDAC complexes, whereas ING3, ING4, and ING5 are associated with the activating HAT protein complexes. In addition to the modulation of chromatin’s structure, they regulate cell cycle transition, cellular senescence, repair of DNA damage, apoptosis, and angiogenic pathways. They also have fundamental effects on regulating cellular senescence in cancer cells. In the current review, we explain their role in cellular senescence based on the evidence obtained from cell line and animal studies, particularly in the context of cancer.

The effect of cullin 4A on lung cancer cell chemosensitivity to paclitaxel through p33ING1b regulation

Cullin 4A (Cul4A) reportedly has oncogenic roles in several cancer types by regulating tumor suppressors through the ubiquitination and proteolysis of the tumor suppressor. In addition, Cul4A is associated with chemosensitivity to chemotherapy drugs. This study investigated the association between Cul4A and lung cancer cell chemosensitivity to paclitaxel, particularly with respect to the role of the p33 inhibitor of the growth 1 (p33ING1b) tumor suppressor. The results showed that the Cul4A knockdown upregulated the p33ING1b expression in lung cancer cells and increased the lung cancer cell and mice tumor xenograft chemosensitivity to paclitaxel. The Cul4A knockdown also inhibited the growth and increased the apoptosis in the tumor xenografts treated with paclitaxel. Notably, the p33ING1b overexpression increased the lung cancer cell chemosensitivity to paclitaxel, but the p33ING1b knockdown reduced the chemosensitivity. A further analysis demonstrated that Cul4A regulates the expression of p33ING1b through protein-protein interactions, ubiquitination, and protein degradation. In conclusion, the present findings suggest that Cul4A mediates the chemosensitivity of lung cancer cells to paclitaxel by regulating p33ING1b. These findings may offer novel insights into future therapeutic strategies for lung cancer that target Cul4A.

ING2 tumor suppressive protein translocates into mitochondria and is involved in cellular metabolism homeostasis

ING2 (Inhibitor of Growth 2) is a tumor suppressor gene that has been implicated in critical biological functions (cell-cycle regulation, replicative senescence, DNA repair and DNA replication), most of which are recognized hallmarks of tumorigenesis occurring in the cell nucleus. As its close homolog ING1 has been recently observed in the mitochondrial compartment, we hypothesized that ING2 could also translocate into the mitochondria and be involved in new biological functions. In the present study, we demonstrate that ING2 is imported in the inner mitochondrial fraction in a redox-sensitive manner in human cells and that this mechanism is modulated by 14-3-3η protein expression. Remarkably, ING2 is necessary to maintain mitochondrial ultrastructure integrity without interfering with mitochondrial networks or polarization. We observed an interaction between ING2 and mtDNA under basal conditions. This interaction appears to be mediated by TFAM, a critical regulator of mtDNA integrity. The loss of mitochondrial ING2 does not impair mtDNA repair, replication or transcription but leads to a decrease in mitochondrial ROS production, suggesting a detrimental impact on OXPHOS activity. We finally show using multiple models that ING2 is involved in mitochondrial respiration and that its loss confers a protection against mitochondrial respiratory chain inhibition in vitro. Consequently, we propose a new tumor suppressor role for ING2 protein in the mitochondria as a metabolic shift gatekeeper during tumorigenesis.

p33ING1b regulates acetylation of p53 in oral squamous cell carcinoma via SIR2

Background

Oral squamous cell carcinoma (OSCC), a form of head and neck squamous cell carcinoma (HNSCC) has a poor 5-year survival rate. OSCC patients are often treated with cisplatin but resistance to chemotherapy is often observed. This makes it important identification of alternative therapeutic targets which will result in more favorable outcome in OSCC patients. The plant homeodomain (PHD)-containing protein Inhibitor of Growth family of tumor suppressor proteins (p33ING1b) has been indicated as a tumor suppressor in different cancers including OSCC. This protein has been shown to function by modulating transcriptional activity of p53; however, the exact mechanism(s) are not well defined.

Methods

Expression of total and acetylated p53 and p33ING1b protein was determined in OSCC cell lines YD-9, YD-8, and YD-38 by immunoblot analysis. Effect of modulation of p33ING1b protein expression on acetylation of p53 and cell proliferation was determined by immunoblot and MTT assay. Effect of modulation of p33ING1b protein expression on transactivation of p53 was assessed by heterologous promoter-based reporter and chromatin immunoprecipitation. Effect of modulation of expression of p33ING1b on SIR2 mRNA and protein was determined by quantitative real-time PCR and immunoblot analyses. Impact of modulation of p33ING1b alone or in combination with SIR2 on chemosensitivity of YD-9 and YD-8 cells to cisplatin was determined in time and dose-dependent cell proliferation assays.

Results

Here, using a panel of OSCC cell lines with wild type or mutant p53, we show that p33ING1b expression is correlated to acetylation of p53 at lysine 382 residue. Increased acetylation of p53 following overexpression of p33ING1b was associated with increased expression of the pro-apoptotic proteins BAX, p21, and cleaved-Caspase 3, and decreased cell proliferation. Reporter assays with p21 and BAX promoters showed that p33ING1b expression levels directly correlated to promoter activity of these 2 genes. Chromatin immunoprecipitation assay showed that transcriptional regulation of p21 and BAX by acetylated p53 is dependent on expression level of p33ING1b. Differential acetylation of p53 following modulation of p33ING1b expression was indirect. Expression of p33ING1b was found to be inversely correlated to the NAD-dependent deacetylase silent information regulator 2 (SIR2). SIR2 was transcriptionally regulated by p33ING1b. Relative expression of p33ING1b was found to dictate chemosensitivity of OSCC cell lines to cisplatin treatment. Concomitant overexpression of p33ING1b and knockdown of SIR2 had a synergistic effect on chemosensitivity of OSCC cell lines to cisplatin, compared to either overexpression of p33ING1b or knockdown of SIR2 alone.

Conclusions

The results from the current study thus elucidate that p33ING1b regulates p53 acetylation irrespective of p53 mutation and subsequent transactivation by transcriptional regulation of SIR2 expression. The results also indicate that p33ING1b and SIR2 are potentially attractive therapeutic targets.

mRNA and protein of p33ING1 in normal and cancer tissues

Background

Inhibitor growth protein 1 (ING1) is a tumor suppressor, and its down-regulation is involved in the progression and aggressive phenotypes of human malignancies through its interactions with the H3K4me3 and p53.

Methods

We collected datasets to analyze the relationship between ING1b mRNA expression and accumulative survival rate, and carried out immunohistochemistry analyses to determine the expression profiles of the p33ING1 protein on the mouse, normal human, and human cancer tissue microarrays.

Results

Compared with normal tissues, the ING1b mRNA was highly expressed in various types of cancer tissues, including, colorectal, lung, and breast cancers, and was positively correlated with the overall survival rate of gastric cancer patients. In mouse tissues, the subcellular location of p33ING1 was frequently nuclear; however, it was occasionally cytoplasmic or nucleocytoplasmic. There was a positive detection in the neuron body, a part of glial cells, the glandular epithelium of the stomach, intestines, breast, hepatocytes, heart, skeletal muscle cells, the bronchial and alveolar epithelium, and nephric tubules. In human tissues, the p33ING1 protein, apart from its cytoplasmic distribution, was distributed in the nuclei of the tongue, esophagus, stomach, intestine, lung, trachea, skin, appendix, cervix, endometrium, ovary, and breast. p33ING1 immunoreactivity was strongly detected in the stomach, trachea, skin, cervix, and breast, while it was weak in the other tissues. The positive rate of p33ING1 was 41.0% in the tested cancer entities (489/1,194). In general, p33ING1 expression was restricted to only the cytoplasm for all cancers, whereas it was found in the nucleus of renal clear cells, ovarian and colorectal cancers. Among them, p33ING1 was expressed in more than half of squamous cell carcinomas derived from the esophagus and cervix, while it was rarely expressed in hepatocellular (21.0%) and renal clear cell carcinoma (19.4%).

Conclusions

The findings suggest that p33ING1 might be participated in the repair and regeneration of organs or tissues the repair and regeneration of organs or tissue, and the carcinogenesis of the highly proliferative epithelium.

Nuclear localization of ING3 is required to suppress melanoma cell migration, invasion and angiogenesis

Inhibitor of growth family member 3 (ING3), a tumor suppressor, plays crucial roles in cell cycle regulation, apoptosis and transcription. Previous studies suggest important roles of nuclear ING3, however, the nuclear localization sequence (NLS) of ING3 is not defined and its biological functions remain to be elucidated. In this study, various ING3 mutants were generated to identify its NLS. The NLS of ING3 was determined as KKFK between 164 and 167 amino acids. More intriguingly, replacement of Lysine 164 residue of ING3 with alanine (K164A) resulted in retention of ING3 in the cytoplasm. Overexpression of ING3 led to inhibition of melanoma cell migration, invasion, and angiogenesis respectively, however, this inhibition was abrogated in cells with overexpression of ING3-K164A mutant. In conclusion, this study identified the NLS of ING3 and demonstrated the significance of ING3 nuclear localization for tumor suppressive functions of ING3, and future studies await to elucidate the role of ING3 (K164) post-modificaton in its nuclear transportation and cancer development.

Downregulation of nuclear ING3 expression and translocalization to cytoplasm promotes tumorigenesis and progression in head and neck squamous cell carcinoma (HNSCC)

ING3 (inhibitor of growth gene 3) is a member of the ING gene family, and is considered as a candidate tumor suppressor gene. In order to explore the roles of ING3 in tumorigenesis and cancer progression of head and neck squamous cell carcinoma (HNSCC), ING3 expression was assessed in 173 cases of HNSCC by immunohistochemistry. The expression of ING3 was also compared to clinicopathological variables, and the expression of several tumorigenic markers. Nuclear expression of ING3 in HNSCC was significantly lower than that in dysplasia and normal epithelium, and was negatively correlated with a poor-differentiated status, T staging and TNM staging. In contrast, cytoplasmic expression of ING3 was significantly increased in HNSCC, and was statistically associated with lymph node metastasis and 14-3-3η expression. In addition, nuclear expression of ING3 was positively correlated with the expression of p300, p21 and acetylated p53. In conclusion, decreases in nuclear ING3 may play important roles in tumorigenesis, progression and tumor differentiation in HNSCC. Increases in cytoplasmic ING3 may be due to 14-3-3η binding and may also be involved in malignant progression. Nuclear ING3 may modulate the transactivation of target genes, promoting apoptosis through interactions with p300 and p21. Moreover, ING3 may interact with p300 to upregulate the level of acetylation of p53, and promote p53-mediated cell cycle arrest, senescence and/or apoptosis. Therefore, ING3 may be a potential tumor suppressor and a possible therapeutic target in HNSCC.

Focus-ING on DNA Integrity: Implication of ING Proteins in Cell Cycle Regulation and DNA Repair Modulation

The ING family of tumor suppressor genes is composed of five members (ING1-5) involved in cell cycle regulation, DNA damage response, apoptosis and senescence. All ING proteins belong to various HAT or HDAC complexes and participate in chromatin remodeling that is essential for genomic stability and signaling pathways. The gatekeeper functions of the INGs are well described by their role in the negative regulation of the cell cycle, notably by modulating the stability of p53 or the p300 HAT activity. However, the caretaker functions are described only for ING1, ING2 and ING3. This is due to their involvement in DNA repair such as ING1 that participates not only in NERs after UV-induced damage, but also in DSB repair in which ING2 and ING3 are required for accumulation of ATM, 53BP1 and BRCA1 near the lesion and for the subsequent repair. This review summarizes evidence of the critical roles of ING proteins in cell cycle regulation and DNA repair to maintain genomic stability.

The Biological and Clinical Relevance of Inhibitor of Growth (ING) Genes in Non-Small Cell Lung Cancer

Carcinogenic mutations allow cells to escape governing mechanisms that commonly inhibit uncontrolled cell proliferation and maintain tightly regulated homeostasis between cell death and survival. Members of the inhibition of growth (ING) family act as tumor suppressors, governing cell cycle, apoptosis and cellular senescence. The molecular mechanism of action of ING genes, as well as their anchor points in pathways commonly linked to malignant transformation of cells, have been studied with respect to a variety of cancer specimens. This review of the current literature focuses specifically on the action mode of ING family members in lung cancer. We have summarized data from in vitro and in vivo studies, highlighting the effects of varying levels of ING expression in cancer cells. Based on the increasing insight into the function of these proteins, the use of ING family members as clinically useful biomarkers for lung cancer detection and prognosis will probably become routine in everyday clinical practice.

Expression pattern and level of ING5 protein in normal and cancer tissues

Inhibitor of growth family 5 (ING5) functions as a type-II tumor suppressor gene and exerts an important role in DNA repair, apoptotic induction, proliferative inhibition, chromatin remodeling and the invasion process. In the present study, immunohistochemistry was performed to characterize the expression profile of ING5 protein on a tissue microarray containing mouse and human normal tissues, and human cancer tissues, including hepatocellular (n=62), renal clear cell (n=62), pancreatic (n=62), esophageal squamous cell (n=45), cervical squamous cell (n=31), breast (n=144), gastric (n=196), colorectal (n=96), endometrial (n=96) and lung carcinoma (n=192). In the mouse tissues, ING5 expression was detected in the cytoplasm of neurons, the nephric tubule and glomerulus, alveolar epithelium, gastrointestinal glands, squamous epithelium of the skin and skeletal muscles. By contrast, ING5 was localized to the cell nucleus in breast tissues. In human tissues, ING5 protein was primarily localized in the cytoplasm. However, ING5 was detected in the cytoplasm and nucleus in various types of normal tissues, including the tongue, stomach, intestine, lung and breast. In total, ING5 expression was detected in 400/986 cancer tissues (40.6%). In the majority of cases, ING5 expression was observed to be restricted to the cytoplasm. However, ING5 was also detected in the nucleus in a number of cancer tissues, including gastric, colorectal and lung carcinoma. Notably, ING5 was more frequently expressed in breast (79.9%), colorectal (56.3%) and endometrial carcinoma (50.0%). The incidence of ING5 expression in hepatocellular carcinoma (14.5%) and pancreatic carcinoma (22.6%) was low. These findings indicate that ING5 may be involved in cell regeneration and be associated with colorectal carcinogenesis.

ING3 promotes prostate cancer growth by activating the androgen receptor

BACKGROUND

The androgen receptor (AR) is a major driver of prostate cancer, and increased AR levels and co-activators of the receptor promote the development of prostate cancer. INhibitor of Growth (ING) proteins target lysine acetyltransferase or lysine deacetylase complexes to the histone H3K4Me3 mark of active transcription, to affect chromatin structure and gene expression. ING3 is a stoichiometric member of the TIP60 lysine acetyltransferase complex implicated in prostate cancer development.

METHODS

Biopsies of 265 patients with prostate cancer were stained for ING3, pan-cytokeratin, and DNA. LNCaP and C4-2 androgen-responsive cells were used for in vitro assays including immunoprecipitation, western blotting, Luciferase reporter assay and quantitative polymerase chain reaction. Cell viability and migration assays were performed in prostate cancer cell lines using scrambled siRNA or siRNA targeting ING3.

RESULTS

We find that ING3 levels and AR activity positively correlate in prostate cancer. ING3 potentiates androgen effects, increasing expression of androgen-regulated genes and androgen response element-driven reporters to promote growth and anchorage-independent growth. Conversely, ING3 knockdown inhibits prostate cancer cell growth and invasion. ING3 activates the AR by serving as a scaffold to increase interaction between TIP60 and the AR in the cytoplasm, enhancing receptor acetylation and translocation to the nucleus. Activation is independent of ING3's ability to target the TIP60 complex to H3K4Me3, identifying a previously unknown chromatin-independent cytoplasmic activity for ING3. In agreement with in vitro observations, analysis of The Cancer Genome Atlas (TCGA) data (n = 498) and a prostate cancer tissue microarray (n = 256) show that ING3 levels are higher in aggressive prostate cancers, with high levels of ING3 predicting shorter patient survival in a low AR subgroup. Including ING3 levels with currently used indicators such as the Gleason score provides more accurate prognosis in primary prostate cancer.

CONCLUSIONS

In contrast to the majority of previous reports suggesting tumor suppressive functions in other cancers, our observations identify a clear oncogenic role for ING3, which acts as a co-activator of AR in prostate cancer. Data from TCGA and our previous and current tissue microarrays suggest that ING3 levels correlate with AR levels and that in patients with low levels of the receptor, ING3 level could serve as a useful prognostic biomarker.

INGs are potential drug targets for cancer

PURPOSE

The inhibitor of growth (ING) family consists of ING1, ING2, ING3, ING4 and ING5, which function as the type II tumor suppressors. INGs regulate cell proliferation, senescence, apoptosis, differentiation, angiogenesis, DNA repair, metastasis, and invasion by multiple pathways. In addition, INGs increase cancer cell sensitivity for chemotherapy and radiotherapy, while clinical observations show that INGs are frequently lost in some types of cancers. The aim of the study was to summarize the recent progress regarding INGs regulating tumor progression.

METHODS

The literatures of INGs regulating tumor progression were searched and assayed.

RESULTS

The regulating signaling pathways of ING1, ING2, ING3 or ING4 on tumor progression were shown. The mechanisms of INGs on tumor suppression were also assayed.

CONCLUSIONS

This review better summarized the signaling mechanism of INGs on tumor suppression, which provides a candidate therapy strategy for cancers.

Immunohistochemical profile of ING3 protein in normal and cancerous tissues

The inhibitor of growth family, member 3 (ING3) protein may be capable of blocking the cell cycle via activating p53-transactivated promoters of p21 and Bcl2-associated X protein, and may induce apoptosis via a Fas/caspase-8-dependent signaling pathway. In the present study, immunohistochemistry was performed in order to characterize the expression profile of ING3 protein in tissue microarrays containing mouse and human normal tissue, human hepatocellular (n=62), renal clear cell (n=62), pancreatic (n=62), esophageal squamous cell (n=45), cervical squamous cell (n=31), breast (n=144), gastric (n=196), colorectal (n=96), ovarian (n=208), endometrial (n=96) and lung carcinoma (n=192). In mouse tissue, ING3 protein was positively detected in the cytoplasm of cardiomyocytes, kidney and skeletal muscle cells, and was additionally detected in the cytoplasm and nucleus of bronchial and alveolar epithelium, gastric and intestinal gland, and mammary gland cells. In human tissues, ING3 protein was principally distributed in the cytoplasm, but was observed in the cytoplasm and nucleus of tongue, esophagus, stomach, intestine, lung, skin, appendix, bladder, cervix and breast cells. ING3 immunoreactivity was strongly detected in the stomach, skin and cervical tissues, whereas a weak signal was detected in the cerebellum, brain stem, thymus, liver, skeletal muscle, testis and prostate. In total, ING3-positive specimens were identified in 424 of 1,194 tested cancer entities (35.5%). In a number of cases, ING3 expression was observed to be restricted to the cytoplasm and nucleus, excluding the cytoplasmic distribution identified in breast and hepatocellular carcinoma. Among these cases, ING3 was more frequently expressed in breast and gynecological types of cancer, including ovarian (59.2%), endometrial (47.9%), breast (38.9%) and cervical (35.5%) cancer. ING3-positive cases were more rare in renal clear cell (17.7%), hepatocellular (16.1%) and esophageal carcinoma (17.8%). It is suggested that ING3 may be involved in the repair and regeneration of organs or tissues, and may be closely associated with gynecological carcinogenesis.

Isolation of Nuclei

The isolation of nuclei is often the first step in studying processes such as nuclear-cytoplasmic shuttling, subcellular localization of proteins, and protein-chromatin or nuclear protein-protein interactions in response to diverse stimuli. Therefore, rapidly obtaining nuclei from cells with relatively high purity and minimal subcellular contamination, protein degradation, or postharvesting modification is highly desirable. Historically, the isolation of nuclei involved a homogenization step followed by centrifugation through high-density glycerol or sucrose. Although clean nuclei with little cytoplasmic contamination can be prepared using this method, it is typically time consuming and can allow protein degradation, protein modification, and leaching of components from the nuclei to occur. We have developed a rapid and simple fractionation method that is based on the selective dissolution of the cytoplasmic membrane (but not the nuclear membrane) using a low concentration of a nonionic detergent and rapid centrifugation steps. Here we describe important considerations when isolating nuclei from cells, introduce our rapid method, and compare this method to a more traditional protocol for isolating nuclei, noting the strengths and limitations of each approach.

ING5 is phosphorylated by CDK2 and controls cell proliferation independently of p53

Inhibitor of growth (ING) proteins have multiple functions in the control of cell proliferation, mainly by regulating processes associated with chromatin regulation and gene expression. ING5 has been described to regulate aspects of gene transcription and replication. Moreover deregulation of ING5 is observed in different tumors, potentially functioning as a tumor suppressor. Gene transcription in late G1 and in S phase and replication is regulated by cyclin-dependent kinase 2 (CDK2) in complex with cyclin E or cyclin A. CDK2 complexes phosphorylate and regulate several substrate proteins relevant for overcoming the restriction point and promoting S phase. We have identified ING5 as a novel CDK2 substrate. ING5 is phosphorylated at a single site, threonine 152, by cyclin E/CDK2 and cyclin A/CDK2 in vitro. This site is also phosphorylated in cells in a cell cycle dependent manner, consistent with it being a CDK2 substrate. Furthermore overexpression of cyclin E/CDK2 stimulates while the CDK2 inhibitor p27^KIP1 represses phosphorylation at threonine 152. This site is located in a bipartite nuclear localization sequence but its phosphorylation was not sufficient to deregulate the subcellular localization of ING5. Although ING5 interacts with the tumor suppressor p53, we could not establish p53-dependent regulation of cell proliferation by ING5 and by phospho-site mutants. Instead we observed that the knockdown of ING5 resulted in a strong reduction of proliferation in different tumor cell lines, irrespective of the p53 status. This inhibition of proliferation was at least in part due to the induction of apoptosis. In summary we identified a phosphorylation site at threonine 152 of ING5 that is cell cycle regulated and we observed that ING5 is necessary for tumor cell proliferation, without any apparent dependency on the tumor suppressor p53.

Tumor cell apoptosis mediated by cytoplasmic ING1 is associated with improved survival in oral squamous cell carcinoma patients

The ING1 epigenetic regulator and tumor suppressor plays a central role in apoptosis. The Ing1 gene is functionally inactivated in many cancer types but is rarely mutated. Although most studies have implicated the major ING1 isoform, p33ING1b, in nuclear apoptotic signalling, we recently discovered a novel and potent apoptosis-inducing effect of p33ING1b translocation to the mitochondria in response to DNA damage. In the present study, we examined the impact of cytoplasmic/mitochondrial localization of p33ING1b in oral squamous cell carcinoma (OSCC) patient samples and explored the therapeutic potential of adenovirally-overexpressed p33ING1b in OSCC cell lines in combination with ionizing radiation (IR) treatment. In contrast with previous reports, we found that p33ING1b protein and mRNA levels are higher in OSCC compared to normal epithelial cells. In OSCC patient samples, higher levels of intra-tumoral cytoplasmic p33ING1b correlated with increased apoptotic markers and significantly better patient survival. This association was strongest in patients who received post-operative radiotherapy. IR treatment induced p33ING1b translocation to the mitochondria and adenoviral-p33ING1b synergized with IR to kill OSCC cells. Our results identify a novel functional relationship between cytoplasmic p33ING1b and patient survival and highlight the potential for the use of p33ING1b as a therapeutic agent in combination with adjuvant radiotherapy in OSCC.

Defining the minimal peptide sequence of the ING1b tumour suppressor capable of efficiently inducing apoptosis

The ING1b protein is a type-II tumour suppressor and stoichiometric member of the Sin3 histone deacetylase (HDAC) protein complex in which it acts to target HDAC activity to regulate chromatin structure. Altering ING1 levels by ectopic expression of ING1b in cancer cells promotes apoptosis, whereas altering levels by knockout in normal murine fibroblasts alters sensitivity to doxorubicin-induced apoptosis. We have identified a minimal region of ING1b capable of inducing levels of apoptosis in targeted cells as effectively as full-length ING1b, using transient overexpression of ING1b fragments followed by the Annexin V assay. We observed high levels of apoptosis in 14 of 14 cancer cell lines tested. Infecting triple-negative tumorigenic MDA-MB-468 breast cancer, U2OS or Saos-2 cells at multiplicities of infection (MOIs) ranging from 10 to 20 rapidly triggered apoptosis in ~80% of infected cells within 48 h. This was not due to the effects of virus, as infection at the same MOI with a control adenovirus expressing GFP was not effective in inducing apoptosis. When used at low MOIs, the ING1b fragment showed a cell-killing efficacy that was higher than native, full-length ING1b. Using a doxycycline-regulated inducible p53 expression system demonstrated that apoptosis induced by the ING1b fragment was p53 independent. Given the growing importance of combination therapies, we evaluated whether there was synergism between the ING1b fragment and HDAC inhibitors. Combination treatments with TSA, LBH 589 and SAHA reduced cancer cell survival by 3.9–4.7-fold as compared with single-drug treatment, and resulted in ~90% reduction in cell survival. Normalized isobologram analysis confirmed strong synergism between the ING1b fragment and drugs tested. These findings provide support for using ING1b-derived therapeutics as adjuvant treatments in combination with existing epigenetic therapies.

Anticancer drug mithramycin interacts with core histones: An additional mode of action of the DNA groove binder

Mithramycin (MTR) is a clinically approved DNA-binding antitumor antibiotic currently in Phase 2 clinical trials at National Institutes of Health for treatment of osteosarcoma. In view of the resurgence in the studies of this generic antibiotic as a human medicine, we have examined the binding properties of MTR with the integral component of chromatin - histone proteins - as a part of our broad objective to classify DNA-binding molecules in terms of their ability to bind chromosomal DNA alone (single binding mode) or both histones and chromosomal DNA (dual binding mode). The present report shows that besides DNA, MTR also binds to core histones present in chromatin and thus possesses the property of dual binding in the chromatin context. In contrast to the MTR-DNA interaction, association of MTR with histones does not require obligatory presence of bivalent metal ion like Mg(2+). As a consequence of its ability to interact with core histones, MTR inhibits histone H3 acetylation at lysine 18, an important signature of active chromatin, in vitro and ex vivo. Reanalysis of microarray data of Ewing sarcoma cell lines shows that upon MTR treatment there is a significant down regulation of genes, possibly implicating a repression of H3K18Ac-enriched genes apart from DNA-binding transcription factors. Association of MTR with core histones and its ability to alter post-translational modification of histone H3 clearly indicates an additional mode of action of this anticancer drug that could be implicated in novel therapeutic strategies.

SUMOylation of the ING1b tumor suppressor regulates gene transcription

The INhibitor of Growth (ING) proteins are encoded as multiple isoforms in five ING genes (ING1 -5) and act as type II tumor suppressors. They are growth inhibitory when overexpressed and are frequently mislocalized or downregulated in several forms of cancer. ING1 and ING2 are stoichiometric members of histone deacetylase complexes, whereas ING3-5 are stoichiometric components of different histone acetyltransferase complexes. The INGs target these complexes to histone marks, thus acting as epigenetic regulators. ING proteins affect angiogenesis, apoptosis, DNA repair, metastasis and senescence, but how the proteins themselves are regulated is not yet clear. Here, we find a small ubiquitin-like modification (SUMOylation) of the ING1b protein and identify lysine 193 (K193) as the preferred ING1b SUMO acceptor site. We also show that PIAS4 is the E3 SUMO ligase responsible for ING1b SUMOylation on K193. Sequence alignment reveals that the SUMO consensus site on ING1b contains a phosphorylation-dependent SUMOylation motif (PDSM) and our data indicate that the SUMOylation on K193 is enhanced by the S199D phosphomimic mutant. Using an ING1b protein mutated at the major SUMOylation site (ING1b E195A), we further demonstrate that ING1b SUMOylation regulates the binding of ING1b to the ISG15 and DGCR8 promoters, consequently regulating ISG15 and DGCR8 transcription. These results suggest a role for ING1b SUMOylation in the regulation of gene transcription.

ING1 and ING2: multifaceted tumor suppressor genes

Inhibitor of Growth 1 (ING1) was identified and characterized as a "candidate" tumor suppressor gene in 1996. Subsequently, four more genes, also characterized as "candidate" tumor suppressor genes, were identified by homology search: ING2, ING3, ING4, and ING5. The ING proteins are characterized by a high homology in their C-terminal domain, which contains a Nuclear Localization Sequence and a Plant HomeoDomain (PHD), which has a high affinity to Histone 3 tri-methylated on lysine 4 (H3K4Me3). The ING proteins have been involved in the control of cell growth, senescence, apoptosis, chromatin remodeling, and DNA repair. Within the ING family, ING1 and ING2 form a subgroup since they are evolutionarily and functionally close. In yeast, only one gene, Pho23, is related to ING1 and ING2 and possesses also a PHD. Recently, the ING1 and ING2 tumor suppressor status has been fully established since several studies have described the loss of ING1 and ING2 protein expression in human tumors and both ING1 and ING2 knockout mice were reported to have spontaneously developed tumors, B cell lymphomas, and soft tissue sarcomas, respectively. In this review, we will describe for the first time what is known about the ING1 and ING2 genes, proteins, their regulations in both human and mice, and their status in human tumors. Furthermore, we explore the current knowledge about identified functions involving ING1 and ING2 in tumor suppression pathways especially in the control of cell cycle and in genome stability.

ING1 induces apoptosis through direct effects at the mitochondria

The ING family of tumor suppressors acts as readers and writers of the histone epigenetic code, affecting DNA repair, chromatin remodeling, cellular senescence, cell cycle regulation and apoptosis. The best characterized member of the ING family, ING1,interacts with the proliferating cell nuclear antigen (PCNA) in a UV-inducible manner. ING1 also interacts with members of the14-3-3 family leading to its cytoplasmic relocalization. Overexpression of ING1 enhances expression of the Bax gene and was reported to alter mitochondrial membrane potential in a p53-dependent manner. Here we show that ING1 translocates to the mitochondria of primary fibroblasts and established epithelial cell lines in response to apoptosis inducing stimuli, independent of the cellular p53 status. The ability of ING1 to induce apoptosis in various breast cancer cell lines correlates well with its degree of translocation to the mitochondria after UV treatment. Endogenous ING1 protein specifically interacts with the pro-apoptotic BCL2 family member BAX, and colocalizes with BAX in a UV-inducible manner. Ectopic expression of a mitochondria-targeted ING1 construct is more proficient in inducing apoptosis than the wild type ING1 protein. Bioinformatic analysis of the yeast interactome indicates that yeast ING proteins interact with 64 mitochondrial proteins. Also, sequence analysis of ING1 reveals the presence of a BH3-like domain. These data suggest a model in which stress-induced cytoplasmic relocalization of ING1 by14-3-3 induces ING1-BAX interaction to promote mitochondrial membrane permeability and represent a paradigm shift in our understanding of ING1 function in the cytoplasm and its contribution to apoptosis [corrected].

Src regulates the activity of the ING1 tumor suppressor

The INhibitor of Growth 1 (ING1) is stoichiometric member of histone deacetylase (HDAC) complexes and functions as an epigenetic regulator and a type II tumor suppressor. It impacts cell growth, aging, apoptosis, and DNA repair, by affecting chromatin conformation and gene expression. Down regulation and mislocalization of ING1 have been reported in diverse tumor types and Ser/Thr phosphorylation has been implicated in both of these processes. Here we demonstrate that both in vitro and in vivo, the tyrosine kinase Src is able to physically associate with, and phosphorylate ING1, which results in a nuclear to cytoplasmic relocalization of ING1 in cells and a decrease of ING1 stability. Functionally, Src antagonizes the ability of ING1 to induce apoptosis, most likely through relocalization of ING1 and down regulation of ING1 levels. These effects were due to both kinase-dependent and kinase-independent properties of Src, and were most apparent at elevated levels of Src expression. These findings suggest that Src may play a major role in regulating ING1 levels during tumorigenesis in those cancers in which high levels of Src expression or activity are present. These data represent the first report of tyrosine kinase-mediated regulation of ING1 levels and suggest that kinase activation can impact chromatin structure through the ING1 epigenetic regulator.

RegulatING chromatin regulators: post-translational modification of the ING family of epigenetic regulators

The five human ING genes encode at least 15 splicing isoforms, most of which affect cell growth, differentiation and apoptosis through their ability to alter gene expression by epigenetic mechanisms. Since their discovery in 1996, ING proteins have been classified as type II tumour suppressors on the basis of reports describing their down-regulation and mislocalization in a variety of cancer types. In addition to their regulation by transcriptional mechanisms, understanding the range of PTMs (post-translational modifications) of INGs is important in understanding how ING functions are fine-tuned in the physiological setting and how they add to the repertoire of activities affected by the INGs. In the present paper we review the different PTMs that have been reported to occur on INGs. We discuss the PTMs that modulate ING function under normal conditions and in response to a variety of stresses. We also describe the ING PTMs that have been identified by several unbiased MS-based PTM enrichment techniques and subsequent proteomic analysis. Among the ING PTMs identified to date, a subset has been characterized for their biological significance and have been shown to affect processes including subcellular localization, interaction with enzymatic complexes and ING protein half-life. The present review aims to highlight the emerging role of PTMs in regulating ING function and to suggest additional pathways and functions where PTMs may effect ING function.

Cytoplasmic expression of p33(ING1b) is correlated with tumorigenesis and progression of human esophageal squamous cell carcinoma

p33(ING1b), a newly discovered candidate tumor suppressor gene and a nuclear protein, belongs to the inhibitor of growth gene family. Previous studies have shown that p33(ING1b) is involved in the restriction of cell growth and proliferation, apoptosis, tumor anchorage-independent growth, cellular senescence, maintenance of genomic stability and modulation of cell cycle checkpoints. Loss of nuclear p33(ING1b) has been observed in melanoma, seminoma, papillary thyroid carcinoma, oral squamous cell carcinoma, breast ductal cancer and acute lymphoblastic leukemia. Inactivation and/or decreased expression of p33(ING1b) have been reported in various types of cancer, including head and neck squamous cell, breast, lung, stomach, blood and brain malignancies. Since little is known about the clinicopathological significance of p33(ING1b) in esophageal squamous cell carcinoma (ESCC), this study aimed to investigate the association of p33(ING1b) expression with clinicopathological variables and particularly interesting new cysteine-histidine rich protein (PINCH) in patients with ESCC. p33(ING1b) expression was examined by immunohistochemistry in 20 normal esophageal mucosa and in 64 ESCC specimens. The results revealed that the positive expression of p33(ING1b) protein in normal squamous cells was localized in the nucleus alone and the positive rate was 95%, while in ESCCs, the positive expression was mainly in the cytoplasm, together with nuclear expression, and the positive rate was 36% (P<0.0001). Furthermore, the cases with lymph node metastasis showed a higher frequency of positive cytoplasmic expression than those without metastasis (P=0.001). The cytoplasmic expression of p33(ING1b) was positively related to PINCH expression (P<0.0001) in ESCC, and the cases positive for both proteins had a high lymph node metastasis rate (P=0.001). In conclusion, p33(ING1b) cellular compartmental shift from the nucleus to the cytoplasm may cause loss of normal cellular function and play a central role in the tumorigenesis and metastasis of ESCC.

Adenovirus-mediated expression of p33(ING1b) induces apoptosis and inhibits proliferation in gastric adenocarcinoma cells in vitro

BACKGROUND

Inhibitor of growth 1b (ING1b) is considered to be a class II tumor suppressor gene. Although reduced expression of p33(ING1b) has been reported in many human malignancies, including gastric cancers, the effect of p33(ING1b) on gastric cancer cells has yet to be investigated.

METHODS

Expression of p33(ING1b) in gastric adenocarcinoma tissues and their adjacent non-malignant gastric mucosa, as well as in gastric adenocarcinoma cell lines and normal gastric epithelial cells, was detected by using Western blotting. Recombinant adenoviruses were prepared to mediate the ectopic expression of p33(ING1b) (Ad-ING1b) and green fluorescent protein (GFP)(Ad-GFP) in the gastric adenocarcinoma cell lines, SGC-7901, MKN28, and MKN45 and the normal gastric epithelial cell line GES-1. Alterations in the proliferation and apoptosis of the cells after adenoviral infection were determined by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and flow cytometry, respectively, and cell cycle distribution was analyzed in a fluorescence-activated cell sorter.

RESULTS

Western blotting confirmed the reduced expression of p33(ING1b) in gastric adenocarcinoma tissues and gastric adenocarcinoma cell lines. The ectopic expression of p33(ING1b) mediated by Ad-ING1b resulted in decreased growth, increased apoptosis, and cell cycle arrest at the G1 phase in both benign and malignant gastric epithelial cells regardless of their p53 status. Addition of a p53 inhibitor, pifithrin-α, did not abolish the pro-apoptotic and cell cycle-arresting effects of p33(ING1b) in p53 wild-type cells.

CONCLUSIONS

Down-regulation of p33(ING1b) might play an important role in the development of gastric adenocarcinoma. Targeted local expression of p33(ING1b) may offer a promising alternative therapeutic measure for gastric cancer.

Regulation of p53 by ING family members in suppression of tumor initiation and progression

The INhibitor of Growth (ING) family is an evolutionarily conserved set of proteins, implicated in suppression of initiation and progression of cancers in various tissues. They promote cell cycle arrest, cellular senescence and apoptosis, participate in stress responses, regulate DNA replication and DNA damage responses, and inhibit cancer cell migration, invasion, and angiogenesis of the tumors. At the molecular level, ING proteins are believed to participate in chromatin remodeling and transcriptional regulation of their target genes. However, the best known function of ING proteins is their cooperation with p53 tumor suppressor protein in tumor suppression. All major isoforms of ING family members can promote the transactivition of p53 and the majority of them are shown to directly interact with p53. In addition, ING proteins are thought to interact with and modulate the function of auxiliary members of p53 pathway, such as MDM2, ARF , p300, and p21, indicating their widespread involvement in the regulation and function of this prominent tumor suppressor pathway. It seems that p53 pathway is the main mechanism by which ING proteins exert their functions. Nevertheless, regulation of other pathways which are not relevant to p53, yet important for tumorigenesis such as TGF-β and NF-κB, by ING proteins is also observed. This review summarizes the current understanding of the mutual interactions and cooperation between different members of ING family with p53 pathway and implications of this cooperation in the suppression of cancer initiation and progression.

Chaperone receptors: guiding proteins to intracellular compartments

Despite mitochondria and chloroplasts having their own genome, 99% of mitochondrial proteins (Rehling et al., Nat Rev Mol Cell Biol 5:519-530, 2004) and more than 95% of chloroplast proteins (Soll, Curr Opin Plant Biol 5:529-535, 2002) are encoded by nuclear DNA, synthesised in the cytosol and imported post-translationally. Protein targeting to these organelles depends on cytosolic targeting factors, which bind to the precursor, and then interact with membrane receptors to deliver the precursor into a translocase. The molecular chaperones Hsp70 and Hsp90 have been widely implicated in protein targeting to mitochondria and chloroplasts, and receptors capable of recognising these chaperones have been identified at the surface of both these organelles (Schlegel et al., Mol Biol Evol 24:2763-2774, 2007). The role of these chaperone receptors is not fully understood, but they have been shown to increase the efficiency of protein targeting (Young et al., Cell 112:41-50, 2003; Qbadou et al., EMBO J 25:1836-1847, 2006). Whether these receptors contribute to the specificity of targeting is less clear. A class of chaperone receptors bearing tetratricopeptide repeat domains is able to specifically bind the highly conserved C terminus of Hsp70 and/or Hsp90. Interestingly, at least of one these chaperone receptors can be found on each organelle (Schlegel et al., Mol Biol Evol 24:2763-2774, 2007), which suggests a universal role in protein targeting for these chaperone receptors. This review will investigate the role that chaperone receptors play in targeting efficiency and specificity, as well as examining recent in silico approaches to find novel chaperone receptors.

Downregulation and translocation of nuclear ING4 is correlated with tumorigenesis and progression of head and neck squamous cell carcinoma

ING4 (inhibitor of growth gene 4) is a new member of the ING gene family and is implicated in chromatin remodeling and repression of cell growth. In order to explore the roles of ING4 in head and neck squamous cell carcinoma (HNSCC), ING4 expression was assessed in 214 cases of HNSCC by immunohistochemistry using tissue microarray, and in three oral SCC cell lines by immunohistochemistry and Western blotting. Expression of ING4 was also compared to clinicopathological variables, TUNEL assay staining, and the expression of several tumorigenic markers. We found nuclear expression of ING4 was gradually decreased from non-cancerous epithelium and dysplasia to HNSCC and was negatively correlated with a poorly-differentiated status, T staging, and TNM staging in HNSCC. In contrast, cytoplasmic expression of ING4 was significantly increased in HNSCC and was significantly associated with lymph node metastasis and 14-3-3η expression. In addition, nuclear expression of ING4 was positively correlated with p21 and p300 expression and with the apoptotic index. These results suggest that the decreases in nuclear ING4 may play important roles in tumorigenesis, progression and tumor differentiation in HNSCC. Increases in cytoplasmic ING4 may be due to 14-3-3η binding and may also be involved in malignant progression. Nuclear ING4 may modulate the transactivation of target genes, promoting apoptosis and cell cycle arrest through interactions with p300 and p21.

The p53 tumor suppressor is stabilized by inhibitor of growth 1 (ING1) by blocking polyubiquitination

The INhibitor of Growth tumor suppressors (ING1-ING5) affect aging, apoptosis, DNA repair and tumorigenesis. Plant homeodomains (PHD) of ING proteins bind histones in a methylation-sensitive manner to regulate chromatin structure. ING1 and ING2 contain a polybasic region (PBR) adjacent to their PHDs that binds stress-inducible phosphatidylinositol monophosphate (PtIn-MP) signaling lipids to activate these INGs. ING1 induces apoptosis independently of p53 but other studies suggest proapoptotic interdependence of ING1 and p53 leaving their functional relationship unclear. Here we identify a novel ubiquitin-binding domain (UBD) that overlaps with the PBR of ING1 and shows similarity to previously described UBDs involved in DNA damage responses. The ING1 UBD binds ubiquitin with high affinity (K(d)∼100 nM) and ubiquitin competes with PtIn-MPs for ING1 binding. ING1 expression stabilized wild-type, but not mutant p53 in an MDM2-independent manner and knockdown of endogenous ING1 depressed p53 levels in a transcription-independent manner. ING1 stabilized unmodified and six multimonoubiquitinated forms of wild-type p53 that were also seen upon DNA damage, but not p53 mutants lacking the six known sites of ubiquitination. We also find that ING1 physically interacts with herpesvirus-associated ubiquitin-specific protease (HAUSP), a p53 and MDM2 deubiquitinase (DUB), and knockdown of HAUSP blocks the ability of ING1 to stabilize p53. These data link lipid stress signaling to ubiquitin-mediated proteasomal degradation through the PBR/UBD of ING1 and further indicate that ING1 stabilizes p53 by inhibiting polyubiquitination of multimonoubiquitinated forms via interaction with and colocalization of the HAUSP-deubiquitinase with p53.

The ING family tumor suppressors: from structure to function

The Inhibitor of Growth (ING) proteins belong to a well-conserved family which presents in diverse organisms with several structural and functional domains for each protein. The ING family members are found in association with many cellular processes. Thus, the ING family proteins are involved in regulation of gene transcription, DNA repair, tumorigenesis, apoptosis, cellular senescence and cell cycle arrest. The ING proteins have multiple domains that are potentially capable of binding to many partners. It is conceivable, therefore, that such proteins could function similarly within protein complexes. In this case, within this family, each function could be attributed to a specific domain. However, the role of ING domains is not definitively clear. In this review, we summarize recent advances in structure-function relationships in ING proteins. For each domain, we describe the known biological functions and the approaches utilized to identify the functions associated with ING proteins.

Nuclear receptor coactivators: structural and functional biochemistry

Transcription of eukaryotic cell is a multistep process tightly controlled by concerted action of macromolecules. Nuclear receptors are ligand-activated sequence-specific transcription factors that bind DNA and activate (or repress) transcription of specific sets of nuclear target genes. Successful activation of transcription by nuclear receptors and most other transcription factors requires "coregulators" of transcription. Coregulators make up a diverse family of proteins that physically interact with and modulate the activity of transcription factors and other components of the gene expression machinery via multiple biochemical mechanisms. The coregulators include coactivators that accomplish reactions required for activation of transcription and corepressors that suppress transcription. This review summarizes our current knowledge of nuclear receptor coactivators with an emphasis on their biochemical mechanisms of action and means of regulation.

REAP: A two minute cell fractionation method

BACKGROUND

The translocation or shuttling of proteins between the nucleus and cytoplasm (nucleocytoplasmic transport [NCPT]) is often a rapid event following stimulation with growth factors or in response to stress or other experimental manipulations. Commonly used methods to separate nuclei from cytoplasm employ lengthy steps such as density gradient centrifugation which exposes cells to non-physiological hyperosmotic conditions for extended time periods resulting in varying degrees of leakage between the nucleus and cytoplasm. To help maintain and quantify nuclear:cytoplasmic ratios of proteins, agents such as leptomycin B have been employed to be able to better analyze NCPT by inhibiting nuclear export. To track NCPT in the absence of these experimental manipulations that could introduce unknown artefacts, we have developed a rapid method that appears to produce pure nuclear and cytoplasmic fractions, suitable for obtaining accurate estimates of the nuclear:cytoplasmic ratios of proteins known to undergo NCPT.

FINDINGS

We have developed a Rapid, Efficient And Practical (REAP) method for subcellular fractionation of primary and transformed human cells in culture. The REAP method is a two minute non-ionic detergent-based purification technique requiring only a table top centrifuge, micro-pipette and micro-centrifuge tubes. This inexpensive method has proven to efficiently separate nuclear from cytoplasmic proteins as estimated by no detectible cross-contamination of the nucleoporin and lamin A nuclear markers or the pyruvate kinase and tubulin cytoplasmic markers. REAP fractions also mirrored TNFα induced NF-κB NCPT observed in parallel by indirect immunofluorescence.

CONCLUSIONS

This method drastically reduces the time needed for subcellular fractionation, eliminates detectable protein degradation and maintains protein interactions. The simplicity, brevity and efficiency of this procedure allows for tracking ephemeral changes in subcellular relocalization of proteins while maintaining protein integrity and protein complex interactions.

ING Genes Work as Tumor Suppressor Genes in the Carcinogenesis of Head and Neck Squamous Cell Carcinoma

Head and neck squamous cell carcinoma (HNSCC) is the sixth most common cancer in the world. The evolution and progression of HNSCC are considered to result from multiple stepwise alterations of cellular and molecular pathways in squamous epithelium. Recently, inhibitor of growth gene (ING) family consisting of five genes, ING1 to ING5, was identified as a new tumor suppressor gene family that was implicated in the downregulation of cell cycle and chromatin remodeling. In contrast, it has been shown that ING1 and ING2 play an oncogenic role in some cancers, this situation being similar to TGF-β. In HNSCC, the ING family has been reported to be downregulated, and ING translocation from the nucleus to the cytoplasm may be a critical event for carcinogenesis. In this paper, we describe our recent results and briefly summarize current knowledge regarding the biologic functions of ING in HNSCC.

Sumoylation of ING2 regulates the transcription mediated by Sin3A

ING2 (inhibitor of growth 2) is a candidate tumor-suppressor gene involved in cell cycle control, apoptosis and senescence. Although the functions of ING2 within the chromatin remodeling complex Sin3A/histone deacetylase (HDAC) and in the p53 pathway have been described, how ING2 itself is regulated remains unknown. In this study we report for the first time that ING2 can be sumoylated by small ubiquitin-like modifier 1 (SUMO1) on lysine 195 both in vitro and in vivo. Strikingly, ING2 sumoylation enhances its association with Sin3a. We provide evidences that ING2 can bind to the promoter of genes to mediate their expression and that sumoylation of ING2 is required for this binding to some of these genes. Among them, we identified the gene TMEM71 (transmembrane protein 71), whose expression is regulated by ING2 sumoylation. ING2 must be sumoylated to bind to the promoter of TMEM71 and to recruit the Sin3A chromatin-modifying complex to this promoter, in order to regulate TMEM71 transcription. Hence, sumoylation of ING2 enhances its binding to the Sin3A/HDAC complex and is required to regulate gene transcriptions.

Inhibitor of growth tumor suppressors in cancer progression

The inhibitor of growth (ING) family of tumor suppressors has five members and is implicated in the control of apoptosis, senescence, DNA repair, and cancer progression. However, little is known about ING activity in the regulation of cancer progression. ING members and splice variants seem to behave differently with respect to cancer invasion and metastasis. Interaction with histone trimethylated at lysine 4 (H3K4me3), hypoxia inducible factor-1 (HIF-1), p53, and nuclear factor kappa-B (NF-kappaB) are potential mechanisms by which ING members exert effects on invasion and metastasis. Subcellular mislocalization, rapid protein degradation, and to a lesser extent ING gene mutation are among the mechanisms responsible for inappropriate ING levels in cancer cells. The aim of this review is to summarize the different roles of ING family tumor suppressors in cancer progression and the molecular mechanisms involved.

14-3-3 proteins, FHA domains and BRCT domains in the DNA damage response

The DNA damage response depends on the concerted activity of protein serine/threonine kinases and modular phosphoserine/threonine-binding domains to relay the damage signal and recruit repair proteins. The PIKK family of protein kinases, which includes ATM/ATR/DNA-PK, preferentially phosphorylate Ser-Gln sites, while their basophilic downstream effecter kinases, Chk1/Chk2/MK2 preferentially phosphorylate hydrophobic-X-Arg-X-X-Ser/Thr-hydrophobic sites. A subset of tandem BRCT domains act as phosphopeptide binding modules that bind to ATM/ATR/DNA-PK substrates after DNA damage. Conversely, 14-3-3 proteins interact with substrates of Chk1/Chk2/MK2. FHA domains have been shown to interact with substrates of ATM/ATR/DNA-PK and CK2. In this review we consider how substrate phosphorylation together with BRCT domains, FHA domains and 14-3-3 proteins function to regulate ionizing radiation-induced nuclear foci and help to establish the G(2)/M checkpoint. We discuss the role of MDC1 a molecular scaffold that recruits early proteins to foci, such as NBS1 and RNF8, through distinct phosphodependent interactions. In addition, we consider the role of 14-3-3 proteins and the Chk2 FHA domain in initiating and maintaining cell cycle arrest.

Reviewing the current classification of inhibitor of growth family proteins

Inhibitor of growth (ING) family proteins have been defined as candidate tumor suppressors for more than a decade. Recent emerging results using siRNA and knockout mice are expanding the previous understanding of this protein family. The results of ING1 knockout mouse experiments revealed that ING1 has a protective effect on apoptosis. Our recent results showed that ING2 is overexpressed in colorectal cancer, and induces colon cancer cell invasion through an MMP13-dependent pathway. Knockdown of ING2 by siRNA induces premature senescence in normal human fibroblast cells, and apoptosis or cell cycle arrest in various adherent cancer cells. Taken together, these results suggest that ING2 may also have roles in cancer progression and/or malignant transformation under some conditions. Additionally, knockdown of ING4 and ING5 by siRNA shows an inhibitory effect on the transition from G(2)/M to G(1) phase and DNA replication, respectively, suggesting that these proteins may play roles during cell proliferation in some context. ING family proteins may play dual roles, similar to transforming growth factor-beta, which has tumor suppressor-like functions in normal epithelium and also oncogenic functions in invasive metastatic cancers. In the present article, we briefly review ING history and propose a possible interpretation of discrepancies between past and recent data.

The ING gene family in the regulation of cell growth and tumorigenesis

The five members of the inhibitor of growth (ING) gene family have garnered significant interest due to their putative roles as tumor suppressors. However, the precise role(s) of these ING proteins in regulating cell growth and tumorigenesis remains uncertain. Biochemical and molecular biological analysis has revealed that all ING members encode a PHD finger motif proposed to bind methylated histones and phosphoinosital, and all ING proteins have been found as components of large chromatin remodeling complexes that also include histone acetyl transferase (HAT) and histone deacetylase (HDAC) enzymes, suggesting a role for ING proteins in regulating gene transcription. Additionally, the results of forced overexpression studies performed in tissue culture have indicated that several of the ING proteins can interact with the p53 tumor suppressor protein and/or the nuclear factor-kappa B (NF-kappaB) protein complex. As these ING-associated proteins play well-established roles in numerous cell processes, including DNA repair, cell growth and survival, inflammation, and tumor suppression, several models have been proposed that ING proteins act as key regulators of cell growth not only through their ability to modify gene transcription but also through their ability to alter p53 and NF-kappaB activity. However, these models have yet to be substantiated by in vivo experimentation. This review summarizes what is currently known about the biological functions of the five ING genes based upon in vitro experiments and recent mouse modeling efforts, and will highlight the potential impact of INGs on the development of cancer.

Interspecies data mining to predict novel ING-protein interactions in human

BACKGROUND

The INhibitor of Growth (ING) family of type II tumor suppressors (ING1-ING5) is involved in many cellular processes such as cell aging, apoptosis, DNA repair and tumorigenesis. To expand our understanding of the proteins with which the ING proteins interact, we designed a method that did not depend upon large-scale proteomics-based methods, since they may fail to highlight transient or relatively weak interactions. Here we test a cross-species (yeast, fly, and human) bioinformatics-based approach to identify potential human ING-interacting proteins with higher probability and accuracy than approaches based on screens in a single species.

RESULTS

We confirm the validity of this screen and show that ING1 interacts specifically with three of the three proteins tested; p38MAPK, MEKK4 and RAD50. These novel ING-interacting proteins further link ING proteins to cell stress and DNA damage signaling, providing previously unknown upstream links to DNA damage response pathways in which ING1 participates. The bioinformatics approach we describe can be used to create an interaction prediction list for any human proteins with yeast homolog(s).

CONCLUSION

None of the validated interactions were predicted by the conventional protein-protein interaction tools we tested. Validation of our approach by traditional laboratory techniques shows that we can extract value from the voluminous weak interaction data already elucidated in yeast and fly databases. We therefore propose that the weak (low signal to noise ratio) data from large-scale interaction datasets are currently underutilized.

The new tumor suppressor genes ING: genomic structure and status in cancer

The Inhibitor of Growth 1 (ING1) gene has been identified and characterized as a Type-II tumor suppressor gene (TSG). Subsequently, 4 additional members of the family were identified by homology search. ING proteins contain a nuclear localization sequence (NLS) and a plant homeo domain (PHD) finger motif in their C-terminus. These proteins are involved in numerous signaling pathways especially in 2 tumor suppressor pathways: apoptosis and senescence. In human tumors, several studies have shown that the expression of ING1 is frequently lost or downregulated. It occurs most frequently at the RNA level, and thus epigenetics mechanism could be involved. We summarize the current knowledge on ING proteins functions and their involvement in various signaling pathways. We also review the studies that have investigated the ING protein status in human tumors. The interest of ING proteins as biomarkers and their role in tumor initiation and progression is discussed.

ING1 protein targeting to the nucleus by karyopherins is necessary for activation of p21

ING1 proteins affect apoptosis, growth, and DNA repair by binding histones and regulating chromatin structure and gene expression. ING1 is downregulated in cancers and cytoplasmic localization is associated with poor prognosis. Here, we report that ING1b interacts with karyopherins alpha2 and beta1 through several basic nuclear localization sequences (NLS) located adjacent to the ING1b PHD region. Deletion of NLS motifs resulted in failure of ING1b to completely localize to the nucleus and inhibited its ability to induce p21WAF1 expression. These observations support a general mechanism by which ING1b activity is regulated, in part, through dynamic subcellular partitioning between the nucleus and cytoplasm.

After a decade of study-ING, a PHD for a versatile family of proteins

The INhibitor of Growth (ING) family of type II tumour suppressors are encoded by five genes in mammals (ING1-ING5), most of which encode multiple isoforms via splicing, and all of which contain a highly conserved plant homeodomain (PHD) finger motif. Since their discovery approximately ten years ago, significant progress has been made in understanding their subcellular targeting, their relationship to p53, their activation by bioactive phospholipids, and their key role in reading the histone code via PHD fingers, with subsequent effects on histone acetylation and transcriptional regulation. In the past year, we have begun to understand how ING proteins integrate stress signals with interpretation and modification of the histone epigenetic code to function as tumour suppressors.

Prognostic Significance of Nuclear ING3 Expression in Human Cutaneous Melanoma

PURPOSE

The novel tumor-suppressor ING3 has been shown to modulate transcription, cell cycle control, and apoptosis. Our previous study showed that ING3 promotes UV-induced apoptosis via the Fas/caspase-8-dependent pathway in melanoma cells. To investigate the putative role of ING3 in the development of melanoma, we examined the expression of ING3 in melanocytic lesions at different stages and analyzed the correlation between ING3 expression and clinicopathologic variables and patient survival.

EXPERIMENTAL DESIGN

Using tissue microarray and immunohistochemistry, we evaluated nuclear and cytoplasmic ING3 staining in 58 dysplastic nevi, 114 primary melanomas, and 50 metastatic melanomas.

RESULTS

Nuclear ING3 expression was remarkably reduced in malignant melanomas compared with dysplastic nevi (P<0.001), which was significantly correlated with the increased ING3 level in cytoplasm (P<0.05). Furthermore, the reduced nuclear ING3 expression was significantly correlated with a poorer disease-specific 5-year survival of patients with primary melanoma, especially for the high-risk melanomas (thickness >or=2.0 mm) with the survival rate reducing from 93% for patients with strong nuclear ING3 staining in their tumor biopsies to 44% for those with negative-to-moderate nuclear ING3 staining (P=0.004). Strikingly, our multivariate Cox regression analysis revealed that reduced nuclear ING3 expression is an independent prognostic factor to predict patient outcome in primary melanomas (P=0.038).

CONCLUSIONS

Our data indicate that ING3 may be an important marker for human melanoma progression and prognosis as well as a potential therapeutic target.

Phosphorylation of the tumor suppressor p33(ING1b) at Ser-126 influences its protein stability and proliferation of melanoma cells

ING (inhibitor of growth) tumor suppressors regulate cell-cycle checkpoints, apoptosis, and ultimately tumor suppression. Among the ING family members, p33(ING1b) is the most intensively studied and plays an important role in the cellular stress response to DNA damage. Here we demonstrate that there is basal phosphorylation of p33(ING1b) at Ser-126 in normal physiological conditions and that this phosphorylation is increased on DNA damage. The mutation of Ser-126 to alanine dramatically shortened the half-life of p33(ING1b). Furthermore, we found that both Chk1 and Cdk1 can phosphorylate this residue. Interestingly, while Cdk1 can phosphorylate p33(ING1b) at Ser-126 in nonstress conditions, Chk1 predominantly phosphorylates this residue on DNA damage, which suggests that p33(ING1b) is a downstream target of the ATM/ATR response cascade to genotoxic stress. More importantly, our data indicate that the Ser-126 residue plays a key role in regulating the expression of cyclin B1 and proliferation of melanoma cells.