Targeted disruption of the mouse ing1 locus results in reduced body size, hypersensitivity to radiation and elevated incidence of lymphomas

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.

ING Tumour Suppressors and ING Splice Variants as Coregulators of the Androgen Receptor Signalling in Prostate Cancer

Prevention and overcoming castration resistance of prostate cancer (PC) remains one of the main unsolved problems in modern oncology. Hence, many studies are focused on the investigation of novel androgen receptor (AR) regulators that could serve as potential drug targets in disease therapy. Among such factors, inhibitor of growth (ING) proteins were identified. Some ING proteins act as AR transcriptional coregulators, indicating their relevance for PC research. The ING family consists of five protein-coding genes from ING1 to ING5 and pseudogene INGX. The ING genes were revealed through their sequence homology to the first identified ING1 from an in vivo screen. ING factors are a part of histone modification complexes. With the help of the conserved plant homeodomain (PHD) motif, ING factors bind to Histone 3 Lysine 4 (H3K4) methylation mark with a stronger affinity to the highest methylation grade H3K4me3 and recruit histone acetyltransferases (HAT) and histone deacetylases (HDAC) to chromatin. ING1 and ING2 are core subunits of mSIN3a-HDAC corepressor complexes, whereas ING3–5 interact with different HAT complexes that serve as coactivators. ING members belong to type II tumour suppressors and are frequently downregulated in many types of malignancies, including PC. As the family name indicates, ING proteins are able to inhibit cell growth and tumour development via regulation of cell cycle and cancer-relevant pathways such as apoptosis, cellular senescence, DNA repair, cell migration, invasion, and angiogenesis. Many ING splice variants that enhance the diversity of ING activity were discovered. However, it seems that the existence of multiple ING splice variants is underestimated, since alternative splice variants, such as the AR coregulators ING1 and ING3, counteract full-length ING and thus play an opposite functional role. These results open a novel prospective investigation direction in understanding ING factors biology in PC and other malignancies.

Altered endocytosis in cellular senescence

Cellular senescence occurs in response to diverse stresses (e.g., telomere shortening, DNA damage, oxidative stress, oncogene activation). A growing body of evidence indicates that alterations in multiple components of endocytic pathways contribute to cellular senescence. Clathrin-mediated endocytosis (CME) and caveolae-mediated endocytosis (CavME) represent major types of endocytosis that are implicated in senescence. More recent research has also identified a chromatin modifier and tumor suppressor that contributes to the induction of senescence via altered endocytosis. Here, molecular regulators of aberrant endocytosis-induced senescence are reviewed and discussed in the context of their capacity to serve as senescence-inducing stressors or modifiers.

Roles of the tumor suppressor inhibitor of growth family member 4 (ING4) in cancer

Inhibitor of growth family member 4 (ING4) is best known as a tumor suppressor that is frequently downregulated, deleted, or mutated in many cancers. ING4 regulates a broad array of tumor-related processes including proliferation, apoptosis, migration, autophagy, invasion, angiogenesis, DNA repair and chromatin remodeling. ING4 alters local chromatin structure by functioning as an epigenetic reader of H3K4 trimethylation histone marks (H3K4Me3) and regulating gene transcription through directing histone acetyltransferase (HAT) and histone deacetylase (HDAC) protein complexes. ING4 may serve as a useful prognostic biomarker for many cancer types and help guide treatment decisions. This review provides an overview of ING4's central functions in gene expression and summarizes current literature on the role of ING4 in cancer and its possible use in therapy.

ING Proteins: Tumour Suppressors or Oncoproteins

The INhibitor of Growth family was defined in the mid-1990s by the identification of a tumour suppressor, ING1, and subsequent expansion of the family based essentially on sequence similarities. However, later work and more recent investigations demonstrate that at least a few ING proteins are actually required for normal proliferation of eukaryotic cells, from yeast to human. ING proteins are also part of a larger family of chromatin-associated factors marked by a plant homeodomain (PHD), which mediates interactions with methylated lysine residues. Herein, we discuss the role of ING proteins and their various roles in chromatin signalling in the context of cancer development and progression.

Senescence and Apoptosis: Architects of Mammalian Development

Mammalian development involves an exquisite choreography of cell division, differentiation, locomotion, programmed cell death, and senescence that directs the transformation of a single cell zygote to a mature organism containing on the order of 40 trillion cells in humans. How a single totipotent zygote undergoes the rapid stages of embryonic development to form over 200 different cell types is complex in the extreme and remains the focus of active research. Processes such as programmed cell death or apoptosis has long been known to occur during development to help sculpt organs and tissue systems. Other processes such as cellular senescence, long thought to only occur in pathologic states such as aging and tumorigenesis have been recently reported to play a vital role in development. In this review, we focus on apoptosis and senescence; the former as an integral mechanism that plays a critical role not only in mature organisms, but that is also essential in shaping mammalian development. The latter as a well-defined feature of aging for which some reports indicate a function in development. We will dissect the dual roles of major gene families, pathways such as Hox, Rb, p53, and epigenetic regulators such as the ING proteins in both early and the late stages and how they play antagonistic roles by increasing fitness and decreasing mortality early in life but contribute to deleterious effects and pathologies later in life.

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.

ING5 inhibits cancer aggressiveness by inhibiting Akt and activating p53 in prostate cancer

Prostate cancer (PCa) is one of the most common types of cancer in men. In several recent studies, chromosomal deletions in the q arm of chromosome 2, where ING5 resides within, have been identified in various cancer types including PCa. In this study, we investigate the role of ING5 as a tumor suppressor in PCa. We examined the expression level of ING5 in tissue samples and cell lines using quantitative real-time polymerase chain reaction and western blot analysis. We tested the in vitro tumor suppressor potential of ING5 in PC3 and LNCaP cells stably overexpressing it using cell viability, colony formation, migration, invasion, and apoptosis assays. We then investigated the effects of ING5 on the Akt and p53 signaling using western blot analysis. We show that ING5 is significantly downregulated in PCa tumor tissue samples and cell lines compared with the corresponding controls. In vitro assays demonstrate that ING5 effectively suppresses proliferative, clonogenic, migratory, and invasive potential and induce apoptosis in PCa cells. ING5 may potentially exert its anti-tumor potential by inhibiting AKT and inducing p53 signaling pathways. Our findings demonstrate that ING5 possesses tumor suppressor roles in vitro, pointing its importance during the prostatic carcinogenesis processes.

Loss of Ing3 Expression Results in Growth Retardation and Embryonic Death

The ING3 candidate tumour suppressor belongs to a family of histone modifying proteins involved in regulating cell proliferation, senescence, apoptosis, chromatin remodeling, and DNA repair. It is a stoichiometric member of the minimal NuA4 histone acetyl transferase (HAT) complex consisting of EAF6, EPC1, ING3, and TIP60. This complex is responsible for the transcription of an essential cascade of genes involved in embryonic development and in tumour suppression. ING3 has been linked to head and neck and hepatocellular cancers, although its status as a tumour suppressor has not been well established. Recent studies suggest a pro-metastasis role in prostate cancer progression. Here, we describe a transgenic mouse strain with insertional mutation of an UbC-mCherry expression cassette into the endogenous Ing3 locus, resulting in the disruption of ING3 protein expression. Homozygous mutants are embryonically lethal, display growth retardation, and severe developmental disorders. At embryonic day (E) 10.5, the last time point viable homozygous embryos were found, they were approximately half the size of heterozygous mice that develop normally. µCT analysis revealed a developmental defect in neural tube closure, resulting in the failure of formation of closed primary brain vesicles in homozygous mid-gestation embryos. This is consistent with high ING3 expression levels in the embryonic brains of heterozygous and wild type mice and its lack in homozygous mutant embryos that show a lack of ectodermal differentiation. Our data provide direct evidence that ING3 is an essential factor for normal embryonic development and that it plays a fundamental role in prenatal brain formation.

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.

Biological Functions of the ING Proteins

The proteins belonging to the inhibitor of growth (ING) family of proteins serve as epigenetic readers of the H3K4Me3 histone mark of active gene transcription and target histone acetyltransferase (HAT) or histone deacetylase (HDAC) protein complexes, in order to alter local chromatin structure. These multidomain adaptor proteins interact with numerous other proteins to facilitate their localization and the regulation of numerous biochemical pathways that impinge upon biological functions. Knockout of some of the ING genes in murine models by various groups has verified their status as tumor suppressors, with ING1 knockout resulting in the formation of large clear-cell B-lymphomas and ING2 knockout increasing the frequency of ameloblastomas, among other phenotypic effects. ING4 knockout strongly affects innate immunity and angiogenesis, and INGs1, ING2, and ING4 have been reported to affect apoptosis in different cellular models. Although ING3 and ING5 knockouts have yet to be published, preliminary reports indicate that ING3 knockout results in embryonic lethality and that ING5 knockout may have postpartum effects on stem cell maintenance. In this review, we compile the known information on the domains of the INGs and the effects of altering ING protein expression, to better understand the functions of this adaptor protein family and its possible uses for targeted cancer therapy.

Exploiting ING2 Epigenetic Modulation as a Therapeutic Opportunity for Non-Small Cell Lung Cancer

Non-small cell lung cancer (NSCLC) has been the leading cause of cancer-related death worldwide, over the last few decades. Survival remains extremely poor in the metastatic setting and, consequently, innovative therapeutic strategies are urgently needed. Inhibitor of Growth Gene 2 (ING2) is a core component of the mSin3A/Histone deacetylases complex (HDAC), which controls the chromatin acetylation status and modulates gene transcription. This gene has been characterized as a tumor suppressor gene and its status in cancer has been scarcely explored. In this review, we focused on ING2 and other mSin3A/HDAC member statuses in NSCLC. Taking advantage of existing public databases and known pharmacological properties of HDAC inhibitors, finally, we proposed a therapeutic model based on an ING2 biomarker-guided strategy.

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.

Impaired DNA demethylation of C/EBP sites causes premature aging

Here, Schäfer et al. investigated whether DNA methylation alterations are involved in aging. Using knockout mice for adapter proteins for site-specific demethylation by TET methylcytosine dioxygenases Gadd45a and Ing1, they show that enhancer methylation can affect aging and imply that C/EBP proteins play an unexpected role in this process.

Changes in DNA methylation are among the best-documented epigenetic alterations accompanying organismal aging. However, whether and how altered DNA methylation is causally involved in aging have remained elusive. GADD45α (growth arrest and DNA damage protein 45A) and ING1 (inhibitor of growth family member 1) are adapter proteins for site-specific demethylation by TET (ten-eleven translocation) methylcytosine dioxygenases. Here we show that Gadd45a/Ing1 double-knockout mice display segmental progeria and phenocopy impaired energy homeostasis and lipodystrophy characteristic of Cebp (CCAAT/enhancer-binding protein) mutants. Correspondingly, GADD45α occupies C/EBPβ/δ-dependent superenhancers and, cooperatively with ING1, promotes local DNA demethylation via long-range chromatin loops to permit C/EBPβ recruitment. The results indicate that enhancer methylation can affect aging and imply that C/EBP proteins play an unexpected role in this process. Our study suggests a causal nexus between DNA demethylation, metabolism, and organismal aging.

Human ex vivo prostate tissue model system identifies ING3 as an oncoprotein

BACKGROUND

Although the founding members of the INhibitor of Growth (ING) family of histone mark readers, ING1 and ING2, were defined as tumour suppressors in animal models, the role of other ING proteins in cellular proliferation and cancer progression is unclear.

METHODS

We transduced ex vivo benign prostate hyperplasia tissues with inducible lentiviral particles to express ING proteins. Proliferation was assessed by H3S10phos immunohistochemistry (IHC). The expression of ING3 was assessed by IHC on a human prostate cancer tissue microarray (TMA). Gene expression was measured by DNA microarray and validated by real-time qPCR.

RESULTS

We found that ING3 stimulates cellular proliferation in ex vivo tissues, suggesting that ING3 could be oncogenic. Indeed, ING3 overexpression transformed normal human dermal fibroblasts. We observed elevated levels of ING3 in prostate cancer samples, which correlated with poorer patient survival. Consistent with an oncogenic role, gene-silencing experiments revealed that ING3 is required for the proliferation of breast, ovarian, and prostate cancer cells. Finally, ING3 controls the expression of an intricate network of cell cycle genes by associating with chromatin modifiers and the H3K4me3 mark at transcriptional start sites.

CONCLUSIONS

Our investigations create a shift in the prevailing view that ING proteins are tumour suppressors and redefine ING3 as an oncoprotein.

The histone code reader Spin1 controls skeletal muscle development

While several studies correlated increased expression of the histone code reader Spin1 with tumor formation or growth, little is known about physiological functions of the protein. We generated Spin1^M5 mice with ablation of Spin1 in myoblast precursors using the Myf5-Cre deleter strain. Most Spin1^M5 mice die shortly after birth displaying severe sarcomere disorganization and necrosis. Surviving Spin1^M5 mice are growth-retarded and exhibit the most prominent defects in soleus, tibialis anterior, and diaphragm muscle. Transcriptome analyses of limb muscle at embryonic day (E) 15.5, E16.5, and at three weeks of age provided evidence for aberrant fetal myogenesis and identified deregulated skeletal muscle (SkM) functional networks. Determination of genome-wide chromatin occupancy in primary myoblast revealed direct Spin1 target genes and suggested that deregulated basic helix-loop-helix transcription factor networks account for developmental defects in Spin1^M5 fetuses. Furthermore, correlating histological and transcriptome analyses, we show that aberrant expression of titin-associated proteins, abnormal glycogen metabolism, and neuromuscular junction defects contribute to SkM pathology in Spin1^M5 mice. Together, we describe the first example of a histone code reader controlling SkM development in mice, which hints at Spin1 as a potential player in human SkM disease.

The emerging role of alternative splicing in senescence and aging

Deregulation of precursor mRNA splicing is associated with many illnesses and has been linked to age-related chronic diseases. Here we review recent progress documenting how defects in the machinery that performs intron removal and controls splice site selection contribute to cellular senescence and organismal aging. We discuss the functional association linking p53, IGF-1, SIRT1, and ING-1 splice variants with senescence and aging, and review a selection of splicing defects occurring in accelerated aging (progeria), vascular aging, and Alzheimer's disease. Overall, it is becoming increasingly clear that changes in the activity of splicing factors and in the production of key splice variants can impact cellular senescence and the aging phenotype.

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.

Keep-ING balance: tumor suppression by epigenetic regulation

Cancer cells accumulate genetic and epigenetic changes that alter gene expression to drive tumorigenesis. Epigenetic silencing of tumor suppressor, cell cycle, differentiation and DNA repair genes contributes to neoplastic transformation. The ING (inhibitor of growth) proteins (ING1-ING5) have emerged as a versatile family of growth regulators, phospholipid effectors, histone mark sensors and core components of HDAC1/2 - and several HAT chromatin-modifying complexes. This review will describe the characteristic pathways by which ING family proteins differentially affect the Hallmarks of Cancer and highlight the various epigenetic mechanisms by which they regulate gene expression. Finally, we will discuss their potentials as biomarkers and therapeutic targets in epigenetic treatment strategies.

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.

Metastasis suppression by BRMS1 associated with SIN3 chromatin remodeling complexes

Epigenetic regulation of gene transcription by histone modification and chromatin remodeling has been linked to many biological and pathological events including cancer metastasis. Breast cancer metastasis suppressor 1 (BRMS1) interacts with SIN3 chromatin remodeling complexes, and, upon forced expression in metastatic cells, a nearly complete suppression of metastasis is noted without preventing primary tumor growth. The data for BRMS1-mediated metastasis suppression and SIN3 interaction are clear; however, connecting the inhibition directly to the association of BRMS1 with SIN3 complexes is currently not well defined. Considering the recent advancements in developing epigenetic drugs for cancer therapy, an improved understanding of how the interactions between BRMS1 and SIN3 regulate the process of metastasis should lead to novel therapies specifically targeting the most deadly aspect of tumor progression. In this article, the data for BRMS1-mediated metastasis suppression are reviewed with a focus on how the SIN3 chromatin remodeling complexes may be functionally involved.

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.

Ing1 functions in DNA demethylation by directing Gadd45a to H3K4me3

Active DNA demethylation regulates epigenetic gene activation in numerous processes, but how the target site specificity of DNA demethylation is determined and what factors are involved are still poorly understood. Here we show that the tumor suppressor inhibitor of growth protein 1 (Ing1) is required for targeting active DNA demethylation. Ing1 functions by recruiting the regulator of DNA demethylation growth arrest and DNA damage protein 45a (Gadd45a) to histone H3 trimethylated at Lys 4 (H3K4me3). We show that reduced H3K4 methylation impairs recruitment of Gadd45a/Ing1 and gene-specific DNA demethylation. Our results indicate that histone methylation directs DNA demethylation.

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.

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.

Modulation of thyroid hormone-dependent gene expression in Xenopus laevis by INhibitor of Growth (ING) proteins

BACKGROUND

INhibitor of Growth (ING) proteins belong to a large family of plant homeodomain finger-containing proteins important in epigenetic regulation and carcinogenesis. We have previously shown that ING1 and ING2 expression is regulated by thyroid hormone (TH) during metamorphosis of the Xenopus laevis tadpole. The present study investigates the possibility that ING proteins modulate TH action.

METHODOLOGY/PRINCIPAL FINDINGS

Tadpoles expressing a Xenopus ING2 transgene (Trans(ING2)) were significantly smaller than tadpoles not expressing the transgene (Trans(GFP)). When exposed to 10 nM 3,5,3'-triiodothyronine (T(3)), premetamorphic Trans(ING2) tadpoles exhibited a greater reduction in tail, head, and brain areas, and a protrusion of the lower jaw than T(3)-treated Trans(GFP) tadpoles. Quantitative real time polymerase chain reaction (QPCR) demonstrated elevated TH receptor β (TRβ) and TH/bZIP transcript levels in Trans(ING2) tadpole tails compared to Trans(GFP) tadpoles while TRα mRNAs were unaffected. In contrast, no difference in TRα, TRβ or insulin-like growth factor (IGF2) mRNA abundance was observed in the brain between Trans(ING2) and Trans(GFP) tadpoles. All of these transcripts, except for TRα mRNA in the brain, were inducible by the hormone in both tissues. Oocyte transcription assays indicated that ING proteins enhanced TR-dependent, T(3)-induced TRβ gene promoter activity. Examination of endogenous T(3)-responsive promoters (TRβ and TH/bZIP) in the tail by chromatin immunoprecipitation assays showed that ING proteins were recruited to TRE-containing regions in T(3)-dependent and independent ways, respectively. Moreover, ING and TR proteins coimmunoprecipitated from tail protein homogenates derived from metamorphic climax animals.

CONCLUSIONS/SIGNIFICANCE

We show for the first time that ING proteins modulate TH-dependent responses, thus revealing a novel role for ING proteins in hormone signaling. This has important implications for understanding hormone influenced disease states and suggests that the induction of ING proteins may facilitate TR function during metamorphosis in a tissue-specific manner.

The ING tumor suppressors in cellular senescence and chromatin

The Inhibitor of Growth (ING) proteins represent a type II tumor suppressor family comprising five conserved genes, ING1 to ING5. While ING1, ING2 and ING3 proteins are stable components of the mSIN3a-HDAC complexes, the association of ING1, ING4 and ING5 with HAT protein complexes was also reported. Among these the ING1 and ING2 have been analyzed more deeply. Similar to other tumor suppressor factors the ING proteins are also involved in many cellular pathways linked to cancer and cell proliferation such as cell cycle regulation, cellular senescence, DNA repair, apoptosis, inhibition of angiogenesis and modulation of chromatin.

A common structural feature of ING factors is the conserved plant homeodomain (PHD), which can bind directly to the histone mark trimethylated lysine of histone H3 (H3K4me3). PHD mutants lose the ability to undergo cellular senescence linking chromatin mark recognition with cellular senescence. ING1 and ING2 are localized in the cell nucleus and associated with chromatin modifying enzymes, linking tumor suppression directly to chromatin regulation. In line with this, the expression of ING1 in tumors is aberrant or identified point mutations are mostly localized in the PHD finger and affect histone binding. Interestingly, ING1 protein levels increase in replicative senescent cells, latter representing an efficient pathway to inhibit cancer proliferation. In association with this, suppression of p33ING1 expression prolongs replicative life span and is also sufficient to bypass oncogene-induced senescence. Recent analyses of ING1- and ING2-deficient mice confirm a tumor suppressive role of ING1 and ING2 and also indicate an essential role of ING2 in meiosis.

Here we summarize the activity of ING1 and ING2 as tumor suppressors, chromatin factors and in development.

The tumor suppressor ING1 contributes to epigenetic control of cellular senescence

Cellular senescence is an effective tumor-suppressive mechanism that causes a stable proliferative arrest in cells with potentially oncogenic alterations. Here, we have investigated the role of the p33ING1 tumor suppressor in the regulation of cellular senescence in human primary fibroblasts. We show that p33ING1 triggers a senescent phenotype in a p53-dependent fashion. Also, endogenous p33ING1 protein accumulates in chromatin in oncogene-senescent fibroblasts and its silencing by RNA interference impairs senescence triggered by oncogenes. Notably, the ability to induce senescence is lost in a mutant version of p33ING1 present in human tumors. Using specific point mutants, we further show that recognition of the chromatin mark H3K4me3 is essential for induction of senescence by p33ING1. Finally, we demonstrate that ING1-induced senescence is associated to a specific genetic signature with a strong representation of chemokine and cytokine signaling factors, which significantly overlaps with that of oncogene-induced senescence. In summary, our results identify ING1 as a critical epigenetic regulator of cellular senescence in human fibroblasts and highlight its role in control of gene expression in the context of this tumor-protective response.

Tumour suppressor ING1b maintains genomic stability upon replication stress

The lesion bypass pathway, which is regulated by monoubiquitination of proliferating cell nuclear antigen (PCNA), is essential for resolving replication stalling due to DNA lesions. This process is important for preventing genomic instability and cancer development. Previously, it was shown that cells deficient in tumour suppressor p33ING1 (ING1b) are hypersensitive to DNA damaging agents via unknown mechanism. In this study, we demonstrated a novel tumour suppressive function of ING1b in preserving genomic stability upon replication stress through regulating PCNA monoubiquitination. We found that ING1b knockdown cells are more sensitive to UV due to defects in recovering from UV-induced replication blockage, leading to enhanced genomic instability. We revealed that ING1b is required for the E3 ligase Rad18-mediated PCNA monoubiquitination in lesion bypass. Interestingly, ING1b-mediated PCNA monoubiquitination is associated with the regulation of histone H4 acetylation. Results indicate that chromatin remodelling contributes to the stabilization of stalled replication fork and to the regulation of PCNA monoubiquitination during lesion bypass.

Targeted disruption of Ing2 results in defective spermatogenesis and development of soft-tissue sarcomas

ING2 (inhibitor of growth family, member 2) is a member of the plant homeodomain (PHD)-containing ING family of putative tumor suppressors. As part of mSin3A-HDAC corepressor complexes, ING2 binds to tri-methylated lysine 4 of histone H3 (H3K4me3) to regulate chromatin modification and gene expression. ING2 also functionally interacts with the tumor suppressor protein p53 to regulate cellular senescence, apoptosis and DNA damage response in vitro, and is thus expected to modulate carcinogenesis and aging. Here we investigate the developmental and physiological functions of Ing2 through targeted germline disruption. Consistent with its abundant expression in mouse and human testes, male mice deficient for Ing2 showed abnormal spermatogenesis and were infertile. Numbers of mature sperm and sperm motility were significantly reduced in Ing2^−/− mice (∼2% of wild type, P<0.0001 and ∼10% of wild type, P<0.0001, respectively). Their testes showed degeneration of seminiferous tubules, meiotic arrest before pachytene stage with incomplete meiotic recombination, induction of p53, and enhanced apoptosis. This phenotype was only partially abrogated by concomitant loss of p53 in the germline. The arrested spermatocytes in Ing2^−/− testes were characterized by lack of specific HDAC1 accumulation and deregulated chromatin acetylation. The role of Ing2 in germ cell maturation may extend to human ING2 as well. Using publicly available gene expression datasets, low expression of ING2 was found in teratozoospermic sperm (>3-fold reduction) and in testes from patients with defective spermatogenesis (>7-fold reduction in Sertoli-cell only Syndrome). This study establishes ING2 as a novel regulator of spermatogenesis functioning through both p53- and chromatin-mediated mechanisms, suggests that an HDAC1/ING2/H3K4me3-regulated, stage-specific coordination of chromatin modifications is essential to normal spermatogenesis, and provides an animal model to study idiopathic and iatrogenic infertility in men. In addition, a bona fide tumor suppressive role of Ing2 is demonstrated by increased incidence of soft-tissue sarcomas in Ing2^−/− mice.

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.

Regulation of the microRNA processor DGCR8 by the tumor suppressor ING1

The ING family of tumor suppressor proteins controls several cellular functions relevant to antitumor protection, such as cell cycle control, apoptosis, senescence, or migration. ING proteins are functionally linked to the p53 pathway, and they participate in transcriptional control via the recognition of histone marks and recruitment of protein complexes with chromatin-modifying activity to specific promoters. Here, we have investigated the global effect of ING1 in gene regulation through genome-wide analysis of expression profiles in primary embryonic fibroblasts deficient for the Ing1 locus. We find that Ing1 has a predominant role as transcriptional repressor in this setting, affecting the expression of genes involved in a variety of cellular functions. Within the subset of genes showing differential expression, we have identified DGCR8, a protein involved in the early steps of microRNA biogenesis. We show that ING1 binds to the DGCR8 promoter and controls its transcription through chromatin regulation. We also find that ING1 and DGCR8 can cooperate in restraining proliferation. In summary, this study reveals a novel connection between ING1 and a regulator of microRNA biogenesis and identifies new links between tumor suppressor proteins and the microRNA machinery.

Polycomb group proteins: navigators of lineage pathways led astray in cancer

The Polycomb group (PcG) proteins are transcriptional repressors that regulate lineage choices during development and differentiation. Recent studies have advanced our understanding of how the PcG proteins regulate cell fate decisions and how their deregulation potentially contributes to cancer. In this Review we discuss the emerging roles of long non-coding RNAs (ncRNAs) and a subset of transcription factors, which we call cell fate transcription factors, in the regulation of PcG association with target genes. We also speculate about how their deregulation contributes to tumorigenesis.

ING proteins as potential anticancer drug targets

Recent emerging evidence suggests that ING family proteins play roles in carcinogenesis both as oncogenes and tumor suppressor genes depending on the family members and on cell status. Previous results from non-physiologic overexpression experiments showed that all five family members induce apoptosis or cell cycle arrest, thus it had been thought until very recently that all of the family members function as tumor suppressor genes. Therefore restoration of ING family proteins in cancer cells has been proposed as a treatment for cancers. However, ING2 knockdown experiments showed unexpected results: ING2 knockdown led to senescence in normal human fibroblast cells and suppressed cancer cell growth. ING2 is also overexpressed in colorectal cancer, and promotes cancer cell invasion through an MMP13 dependent pathway. Additionally, it was reported that ING2 has two isoforms, ING2a and ING2b. Although expression of ING2a predominates compared with ING2b, both isoforms confer resistance against cell cycle arrest or apoptosis to cancer cells, thus knockdown of both isoforms is critical to remove this resistance. Taken together, these results suggest that ING2 can function as an oncogene in some specific types of cancer cells, indicating restoration of this gene in cancer cells could cause cancer progression. Because knockdown of ING2 suppresses cancer cell invasion and induces apoptosis or cell cycle arrest, ING2 may be an anticancer drug target. In this brief review, we discuss possible clinical applications of ING2 with the latest knowledge of molecular targeted therapies.

Structural insight into histone recognition by the ING PHD fingers

The Inhibitor of Growth (ING) tumor suppressors are implicated in oncogenesis, control of DNA damage repair, cellular senescence and apoptosis. All members of the ING family contain unique amino-terminal regions and a carboxy-terminal plant homeodomain (PHD) finger. While the amino-terminal domains associate with a number of protein effectors including distinct components of histone deacetylase (HDAC) and histone acetyltransferase (HAT) complexes, the PHD finger binds strongly and specifically to histone H3 trimethylated at lysine 4 (H3K4me3). In this review we describe the molecular mechanism of H3K4me3 recognition by the ING1-5 PHD fingers, analyze the determinants of the histone specificity and compare the biological activities and structures within subsets of PHD fingers. The atomic-resolution structures of the ING PHD fingers in complex with a H3K4me3 peptide reveal that the histone tail is bound in a large and deep binding site encompassing nearly one-third of the protein surface. An extensive network of intermolecular hydrogen bonds, hydrophobic and cation-pi contacts, and complementary surface interactions coordinate the first six residues of the H3K4me3 peptide. The trimethylated Lys4 occupies an elongated groove, formed by the highly conserved aromatic and hydrophobic residues of the PHD finger, whereas the adjacent groove accommodates Arg2. The two grooves are connected by a narrow channel, the small size of which defines the PHD finger's specificity, excluding interactions with other modified histone peptides. Binding of the ING PHD fingers to H3K4me3 plays a critical role in regulating chromatin acetylation. The ING proteins function as tethering molecules that physically link the HDAC and HAT enzymatic complexes to chromatin. In this review we also highlight progress recently made in understanding the molecular basis underlying biological and tumorigenic activities of the ING tumor suppressors.

ING function in apoptosis in diverse model systems

Genetic studies in model organisms have shown that programmed cell death (apoptosis) plays a significant role during development, where a deficiency in apoptosis results in severe and diverse diseases. Dysregulation of apoptosis also contributes to a variety of human diseases, such as cancer and autoimmune diseases. ING family proteins (ING1-ING5) are involved in many cellular processes, and appear to play a significant role in apoptosis. Loss or downregulation of ING protein function is frequently observed in different tumour types, many of which are resistant to apoptosis, thus warranting their classification as type II tumour suppressors. Several different in vitro and in vivo models have explored the role of ING proteins in regulating apoptosis. In this review, we discuss the progress that has been made in understanding ING protein function in apoptosis using in vitro studies and Mus musculus, Xenopus laevis, and Caenorhabditis elegans experimental models, with an emphasis on ING1 and ING3.

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.

p37Ing1b regulates B-cell proliferation and cooperates with p53 to suppress diffuse large B-cell lymphomagenesis

The Inhibitor of Growth (ING) gene family encodes structurally related proteins that alter chromatin to regulate gene expression and cell growth. The initial member, ING1, has also been proposed to function as a tumor suppressor in human cancer based on its ability to suppress cell growth and transformation in vitro. Mouse Ing1 produces two proteins (p31 and p37) from differentially spliced transcripts. We have recently generated p37(Ing1b)-null mice and observed spontaneous follicular B-cell lymphomagenesis in this model to show that ING proteins can function in vivo as tumor suppressors. In this present report, we examine the role of p37(Ing1b) in the regulation of B-cell growth and explore the relationship between p37(Ing1b) and p53-mediated tumor suppression. Our results indicate that p37(Ing1b) inhibits the proliferation of B cells and follicular B cells regardless of p53 status, and loss of p53 greatly accelerates the rate of B-cell lymphomagenesis in p37(Ing1b)-null mice. However, in contrast to the highly penetrant follicular B-cell lymphomas observed in p37(Ing1b)-null mice, mice lacking both p37(Ing1b) and p53 typically present with aggressive diffuse large B-cell lymphomas (DLBL). Analysis of marker gene expression in p37(Ing1b)/p53 null tumors indicates that the double-null mice develop both nongerminal center and germinal center B-cell-like DLBL, and also documents up-regulation of nuclear factor-kappaB activity in p37(Ing1b)/p53-null B cells and B-cell tumors. These results confirm that p53 mutation is an important mechanistic step in the formation of diffuse large B-cell lymphomas and reveals a p53-independent role for Ing1b in suppressing B-cell tumorigenesis.

PHD fingers in human diseases: disorders arising from misinterpreting epigenetic marks

Histone covalent modifications regulate many, if not all, DNA-templated processes, including gene expression and DNA damage response. The biological consequences of histone modifications are mediated partially by evolutionarily conserved "reader/effector" modules that bind to histone marks in a modification- and context-specific fashion and subsequently enact chromatin changes or recruit other proteins to do so. Recently, the Plant Homeodomain (PHD) finger has emerged as a class of specialized "reader" modules that, in some instances, recognize the methylation status of histone lysine residues, such as histone H3 lysine 4 (H3K4). While mutations in catalytic enzymes that mediate the addition or removal of histone modifications (i.e., "writers" and "erasers") are already known to be involved in various human diseases, mutations in the modification-specific "reader" proteins are only beginning to be recognized as contributing to human diseases. For instance, point mutations, deletions or chromosomal translocations that target PHD fingers encoded by many genes (such as recombination activating gene 2 (RAG2), Inhibitor of Growth (ING), nuclear receptor-binding SET domain-containing 1 (NSD1) and Alpha Thalassaemia and Mental Retardation Syndrome, X-linked (ATRX)) have been associated with a wide range of human pathologies including immunological disorders, cancers, and neurological diseases. In this review, we will discuss the structural features of PHD fingers as well as the diseases for which direct mutation or dysregulation of the PHD finger has been reported. We propose that misinterpretation of the epigenetic marks may serve as a general mechanism for human diseases of this category. Determining the regulatory roles of histone covalent modifications in the context of human disease will allow for a more thorough understanding of normal and pathological development, and may provide innovative therapeutic strategies wherein "chromatin readers" stand as potential drug targets.

The Caenorhabditis elegans ing-3 gene regulates ionizing radiation-induced germ-cell apoptosis in a p53-associated pathway

The inhibitor of growth (ING) family of type II tumor suppressors are encoded by five genes in mammals and by three genes in Caenorhabditis elegans. All ING proteins contain a highly conserved plant homeodomain (PHD) zinc finger. ING proteins are activated by stresses, including ionizing radiation, leading to the activation of p53. ING proteins in mammals and yeast have recently been shown to read the histone code in a methylation-sensitive manner to regulate gene expression. Here we identify and characterize ing-3, the C. elegans gene with the highest sequence identity to the human ING3 gene. ING-3 colocalizes with chromatin in embryos, the germline, and somatic cells. The ing-3 gene is part of an operon but is also transcribed from its own promoter. Both ing-3(RNAi) and ing-3 mutant strains demonstrate that the gene likely functions in concert with the C. elegans p53 homolog, cep-1, to induce germ-cell apoptosis in response to ionizing radiation. Somatically, the ing-3 mutant has a weak kinker uncoordinated (kinker Unc) phenotype, indicating a possible neuronal function.

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.

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.

Chromatin remodeling and cancer, Part I: Covalent histone modifications

Dynamic chromatin remodeling underlies many, if not all, DNA-templated biological processes, including gene transcription; DNA replication and repair; chromosome condensation; and segregation and apoptosis. Disruption of these processes has been linked to the development and progression of cancer. The mechanisms of dynamic chromatin remodeling include the use of covalent histone modifications, histone variants, ATP-dependent complexes and DNA methylation. Together, these mechanisms impart variation into the chromatin fiber, and this variation gives rise to an 'epigenetic landscape' that extends the biological output of DNA alone. Here, we review recent advances in chromatin remodeling, and pay particular attention to mechanisms that appear to be linked to human cancer. Where possible, we discuss the implications of these advances for disease-management strategies.

Ing1 mediates p53 accumulation and chromatin modification in response to oncogenic stress

ING proteins are putative tumor suppressor proteins linked to the p53 pathway and to the chromatin modification machinery. Here we have analyzed the role of the products of the murine Ing1 locus in cellular tumor-protective responses, using mouse primary fibroblasts where the Ing1 locus has been inactivated by the integration of a betageo cassette. We show that Ing1-deficient mouse embryonic fibroblasts display a defective senescence-like antiproliferative response against oncogenic Ras, affecting several senescence-specific markers. This phenotype is accompanied by a reduced accumulation of p53, which can be explained by the reduced basal p53 protein stability in the Ing1-deficient background. Ing1 deficiency also results in defects in the appearance of heterochromatic marks upon expression of oncogenic Ras, suggestive of impaired heterochromatin formation during oncogene-induced senescence. Our results support an important role for the Ing1 locus in protection against oncogenic stress in vivo, both as a mediator of p53 activation and as a regulator of chromatin remodeling processes.

Expression profiles of mRNA transcript variants encoding the human inhibitor of growth tumor suppressor gene family in normal and neoplastic tissues

The INhibitor of Growth (ING) tumor suppressor gene family is important in regulating cell fate and reads the epigenetic code by interacting specifically with methylated histone H3. Several transcript variants are expressed from the five ING genes but nomenclature for these variants are not consistent in the literature, and very little is known regarding transcript variant expression in normal human tissues and during development. Here we propose a standardized nomenclature for human ING gene family transcript variants and present an expression analysis using real-time quantitative PCR. We establish the steady-state levels of eleven human ING mRNA transcript variants across several fetal, adult, and tumor tissues as well as in cancer-derived cell lines. Consistent with their roles as type II tumor suppressors, we find up to 10,000-fold reduction in many transcript variants in a subset of neoplastic cells. We also find considerable variation in expression levels in different tissues, with up to 1 million-fold higher expression of some ING transcripts in adult, compared to fetal counterparts, particularly in the brain cerebral cortex. These results show differential expression of specific subsets of ING1-5 transcript variants in tissues that may influence the degree to which these variants contribute to epigenetic regulation in cancer and development.