Current Drug Targets - Volume 10, Issue 5, 2009

The newly emerged Inhibitor of Growth (ING) family of tumor suppressors contains five evolutionary conserved members. The ING1-ING5 proteins are involved in numerous fundamental nuclear processes, including control of cell growth and DNA damage repair, cellular senescence and apoptosis. They also play a pivotal role in cancer development and progression, as the expression of ING proteins is often reduced or misregulated in human malignancies. Although mechanisms underlying the biological, physiological and tumorigenic activities of this family remain unclear, the recent studies have shed light on several important functions of the ING proteins. ING1-ING5 have been characterized as native subunits of large multiprotein histone deacetylase (HDAC) and histone acetyltransferase (HAT) complexes and are now viewed as essential regulators of chromatin acetylation. All five ING members contain a carboxy-terminal plant homeodomain (PHD) finger that recognizes histone H3 trimethylated at lysine 4, and unique amino-terminal regions that bind distinct components of the HDAC and HAT complexes. The ING proteins function as tethering molecules that stabilize the enzymatic complexes at chromatin to promote local histone acetylation or deacetylation and regulate gene expression. These properties of ING1-ING5 could bridge epigenetic control, transcriptional regulation and chromatin remodeling with neoplastic transformation. Various ING-mediated cellular processes are tightly linked to p53-dependent signaling pathways, and therefore ING1-ING5 may also act as tumor suppressors through altering the activity of p53. The specific interactions of the amino-terminal regions with other effectors, including Lamin A and PCNA, implicate the ING members in even more diverse biological processes that are associated with different forms of cancer. Thus, the ING proteins are engaged in multiple contacts with key players that collectively regulate a wide variety of cancer critical pathways. In this issue of Current Drug Targets we summarize the current knowledge of ING biology, highlight the remarkable progress recently made in understanding the tumorigenic activities of the ING family, and discuss the potential of using the ING proteins as biomarkers and/or targets for drug development.

Members of the ING family of type II tumor suppressors reside in different chromatin regulatory complexes and are stoichiometeric members of histone acetyltransferase (HAT) and histone deacetylase (HDAC) complexes. It has been frequently observed that expressing ING proteins promotes apoptosis in both normal and transformed cells of different species. They have also been reported to either rely upon p53, or to add to its ability to promote programmed cell death (apoptosis) although whether ING proteins require p53 to induce apoptosis is now questionable based upon observations using knockout cell lines and animal models. Genetic studies in model organisms, and particularly in Caenorhabditis elegans, have identified different pathways involved in apoptosis during development, in the germ line and in response to various forms of stress including DNA damage. In this review we summarize structural features of the INGs and recent observations made in knockout models of Mus musculus and Caenorhabditis elegans that have helped to further clarify the functions of the ING proteins in biochemical pathways leading to apoptosis. Based upon these observations we propose a model for how ING proteins may act both independently and in concert with p53 to promote apoptosis.

The inhibitor of growth (ING) gene family proteins regulate many critical cellular processes such as cell proliferation and growth, apoptosis, DNA repair, senescence, angiogenesis, and drug resistance. Their transcripts and proteins are differentially expressed in health and disease and there is evidence for developmental regulation. The vast majority of studies have characterized ING levels in the context of cancer. However, relatively little attention has been paid to the expression of ING family members in other contexts. This review summarizes the findings from human and animal model systems that provide insight into the factors influencing the expression of these important proteins. We examine the influence of cell cycle and aging as well as genotoxic stress on ING expression levels and evaluate several emerging areas of inquiry demonstrating that ING gene activity may be modulated by factors such as the p53 tumor suppressor, DNA methylation, and ING proteins themselves with external factors such as hormones, reactive oxygen species, TGFβ signalling, and other proteins of pathological significance also influencing ING levels. We then briefly discuss the influence of posttranslational modification and changes in subcellular localization as it pertains to modulation of ING expression. Understanding how ING expression is modulated represents a vital aspect of effective drug targeting strategies.

Cellular senescence is an effective anti-tumor barrier that acts by restraining the uncontrolled proliferation of cells carrying potentially oncogenic alterations. ING proteins are putative tumor suppressor proteins functionally linked to the p53 pathway and to chromatin regulation. ING proteins exert their tumor-protective action through different types of responses. Here, we review the evidence on the participation of ING proteins, mainly ING1 and ING2, in the implementation of the senescent response. The currently available data support an important role of ING proteins as regulators of senescence, in connection with the p53 pathway and chromatin organization.

Since their discovery, the members of the ING (inhibitor of growth) family of tumor suppressors have emerged as essential and core components of chromatin modifying complexes. Recent work has identified the ING family as histone mark sensors that orchestrate cellular responses to genotoxic insults and regulate chromatin homeostasis. Dysregulation of chromatin homeostasis is implicated in tumorigenesis through mechanisms such as silencing of tumor suppressor genes, inappropriate activation of oncogenes, and genomic instability due to failure to repair DNA damage. This review will concentrate on the chromatin signaling aspects of the ING proteins, focusing on how recognition of histone H3 trimethylated at lysine 4 (H3K4me3) by the PHD (plant homeodomain) finger of ING proteins is critical for regulating cellular functions such as gene expression. We will also discuss how H3K4me3-recognition by ING proteins plays a critical role in their tumor suppressive functions. Finally, we will discuss the relevance of the association between one ING protein (ING2) and the nuclear phosphoinositide, phosphatidylinositol-5-phosphate (PtdIns(5)P). Interestingly, the ING2- PtdIns(5)P interaction involves the PHD finger and an adjacent polybasic region. The level of nuclear PtdIns(5)P sharply increases upon genotoxic stress, and this increase positively regulates ING2-mediated responses. Thus, the PHD finger of ING2 integrates phosphoinositide and chromatin signaling networks to prevent unchecked cell growth.

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-π 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.

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.

The INhibitor of Growth (ING) genes were discovered during the past decade and identified as type II tumor suppressor genes. Previous studies demonstrated that ING family members participate in various cellular stress responses and thus play important roles in the pathogenesis of various types of cancers, including melanoma. Epidemiological studies showed that UV radiation is the primary etiological factor in melanoma development. Here we review the studies on the role of ING proteins in cellular responses to UV irradiation, melanoma cell motility, and melanoma progression.

The Inhibitor of Growth (ING) gene family is an emerging putative type II tumor suppressor gene (TSG). Proteins of INGs (ING1-5), critical modulator of the histone code via PHD fingers, are able to suppress cell growth and proliferation, induce apoptosis, and modulate cell cycle progression. ING proteins are involved in transcriptional regulation of genes, such as the p53-inducible gene p21. ING proteins also serve as shuttling proteins between nucleus and cytoplasm, and dysregulation of this nucleocytoplasmic traffic has been shown in some cancer cells. In cancer cells, ING mRNA levels are often lost or suppressed but the genes are rarely mutated. Recently the potential roles of ING proteins as prognostic biomarkers, detection of aggressive behavior of the tumor as well as prediction of chemo-radiotherapy response have also emerged. In this review, we summarize the up-to-date knowledge on functions of the ING proteins, the protein status in human tumors and discuss as a potential target in the molecular diagnostics and therapy of cancer.