ING1b decreases cell proliferation through p53-dependent and -independent mechanisms

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.

Regulat-INGs in tumors and diseases: Focus on ncRNAs

ING family genes (Inhibitor of Growth) are tumor suppressor genes that play a vital role in cell homeostasis. It has been shown that their expression is lost or diminished in many cancers and other diseases. The main mechanisms by which they are regulated in oncogenesis have not yet been fully elucidated. The involvement of non-coding RNAs (ncRNAs) and in particular microRNAs (miRNAs) in post-transcriptional gene regulation is well established. miRNAs are short sequences (18-25 nucleotides) that can bind to the 3 'UTR sequence of the targeted messenger RNA (mRNA), leading to its degradation or translational repression. Interactions between the ING family and miRNAs have been described in some cancers but also in other diseases. The involvement of miRNAs in ING family regulation opens up new fields of investigation, particularly for targeted therapies. In this review, we will summarize the regulatory mechanisms at the RNA and protein level of the ING family and focus on the interactions with ncRNAs.

The tumor suppressor ING1b is a novel corepressor for the androgen receptor and induces cellular senescence in prostate cancer cells

The androgen receptor (AR) signaling is critical for prostate cancer (PCa) progression to the castration-resistant stage with poor clinical outcome. Altered function of AR-interacting factors may contribute to castration-resistant PCa (CRPCa). Inhibitor of growth 1 (ING1) is a tumor suppressor that regulates various cellular processes including cell proliferation. Interestingly, ING1 expression is upregulated in senescent primary human prostate cells; however, its role in AR signaling in PCa was unknown. Using a proteomic approach by surface-enhanced laser desorption ionization-mass spectrometry (SELDI-MS) combined with immunological techniques, we provide here evidence that ING1b interacts in vivo with the AR. The interaction was confirmed by co-immunoprecipitation, in vitro GST-pull-down, and quantitative intracellular colocalization analyses. Functionally, ING1b inhibits AR-responsive promoters and endogenous key AR target genes in the human PCa LNCaP cells. Conversely, ING1b knockout (KO) mouse embryonic fibroblasts (MEFs) exhibit enhanced AR activity, suggesting that the interaction with ING1b represses the AR-mediated transcription. Also, data suggest that ING1b expression is downregulated in CRPCa cells compared with androgen-dependent LNCaP cells. Interestingly, its ectopic expression induces cellular senescence and reduces cell migration in both androgen-dependent and CRPCa cells. Intriguingly, ING1b can also inhibit androgen-induced growth in LNCaP cells in a similar manner as AR antagonists. Moreover, ING1b upregulates different cell cycle inhibitors including p27(KIP1), which is a novel target for ING1b. Taken together, our findings reveal a novel corepressor function of ING1b on various AR functions, thereby inhibiting PCa cell growth.

Ectopic expression of p33ING1b suppresses proliferation and induces apoptosis in colonic adenocarcinoma cells

Inhibitor of growth 1b (ING1b) is considered to be a class II tumor suppressor gene. Although decreased expression of p33^ING1b has previously been reported in colorectal cancer (CRC), its role in CRC has remained to be elucidated. The present study was designed to assess the function of p33^ING1b in CRC and to further evaluate its underlying mechanisms of action. Western blot analysis confirmed that ING1b gene expression was significantly decreased in CRC tissues compared with that of adjacent non-tumorous colorectal tissues. Furthermore, recombinant adenovirus-mediated ectopic expression of p33^ING1b resulted in growth inhibition, G1-phase cell cycle arrest and apoptosis in the SW480, HT29 and LoVo colorectal adenocarcinoma cell lines. The results suggested that the downregulation of ING1b contributes to colorectal carcinogenesis and that ectopic expression of ING1b may be a potentially useful therapeutic approach for CRC.

LincRNA-p21 acts as a mediator of ING1b-induced apoptosis

ING1b is a tumor suppressor that affects transcription, cell cycle control and apoptosis. ING1b is deregulated in disease, and its activity is closely linked to that of p53. In addition to regulating protein-coding genes, we found that ING1b also influences the expression of large intergenic non-coding RNAs (lincRNAs). In particular, lincRNA-p21 was significantly induced after DNA-damage stress or by ING1b overexpression. Furthermore, lincRNA-p21 expression in response to DNA damage was significantly attenuated in cells lacking ING1b. LincRNA-p21 is also a target of p53 and can trigger apoptosis in mouse cell models. We found that this function of lincRNA-p21 is conserved in human cell models. Moreover, ING1b and p53 could function independently to influence lincRNA-p21 expression. However, their effects become more additive under conditions of stress. In particular, ING1b regulates lincRNA-p21 levels by binding to its promoter and is required for induction of lincRNA-p21 by p53. The ability of ING1b to cause apoptosis is also impaired in the absence of lincRNA-p21. Surprisingly, deletion of the ING1b plant homeodomain, which allows it to bind histones and regulate chromatin structure, did not alter regulation of lincRNA-p21. Our findings suggest that ING1b induces lincRNA-p21 expression independently of histone 3 lysine 4 trimethylation mark recognition and that lincRNA-p21 functions downstream of ING1b. Thus, regulation at the level of lincRNA-p21 may represent the point at which ING1b and p53 pathways converge to induce apoptosis under specific stress conditions.

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.

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.

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.

Nucleolar protein CSIG is required for p33ING1 function in UV-induced apoptosis

Cellular senescence-inhibited gene (CSIG) protein, a nucleolar protein with a ribosomal L1 domain in its N-terminus, can exert non-ribosomal functions to regulate biological processes, such as cellular senescence. Here, we describe a previously unknown function for CSIG: promotion of apoptosis in response to ultraviolet (UV) irradiation-induced CSIG upregulation. We identified p33ING1 as a binding partner that interacts with CSIG. After UV irradiation, p33ING1 increases its protein expression, translocates into the nucleolus and binds CSIG. p33ING1 requires its nucleolar targeting sequence region to interact with CSIG and enhance CSIG protein stability, which is essential for activation of downstream effectors, Bcl-2-associated X protein, to promote apoptosis. Thus, our data imply that p33ING1–CSIG axis functions as a novel pro-apoptotic regulator in response to DNA damage.

Nuclear exclusion of p33ING1b tumor suppressor protein: explored in HCC cells using a new highly specific antibody

Mouse monoclonal antibodies (MAb) were generated against p33ING1b tumor suppressor protein. 15B9 MAb was highly specific in recognizing a single protein band of approximately 33 kDa endogenous p33ING1b protein from HCC cell lines and normal liver tissue by Western blot analysis and by immunoprecipitation. Although p33ING1b mutations are rarely observed in cancer, differential subcellular distribution and nuclear exclusion of p33ING1b were reported in different cancer types. Therefore we analyzed the expression and subcellular localization of p33ING1b in HCC cell lines using 15B9 MAb. So far, p33ING1b mutations or differential subcellular localization are not reported in HCC. In this study, by indirect immunofluorescence using MAb 15B9, we demonstrate that nuclear localization of p33ING1b was highly correlated with well-differentiated HCC cell lines whereas poorly differentiated HCC cells have nuclear exclusion of the protein. Moreover no association was observed between differential subcellular localization of p33ING1b and p53 mutation status of HCC cell lines. Hence our newly produced MAb 15B9 can be used for studying cellular activities of p33ING1b under normal and cancerous conditions.

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.

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.

Oxygen regulation of arterial smooth muscle cell proliferation and survival

The purpose of this study was to determine if hypoxia elicits different proliferative and apoptotic responses in systemic arterial smooth muscle cells incubated under conditions that do or do not result in cellular ATP depletion and whether these effects are relevant to vascular remodeling in vivo. Gene expression profiling was used to identify potential regulatory pathways. In human aortic smooth muscle cells (HASMCs) incubated at 3% O(2), proliferation and progression through the G1/S interphase are enhanced. Incubation at 1% O(2) reduced proliferation, delayed G1/S transition, increased apoptotic cell death, and is associated with mitochondrial membrane depolarization and reduced cellular ATP levels. In aorta and mesenteric artery from rats exposed to hypoxia (10% O(2), 48 h), both proliferation and apoptosis are increased, as are medial nuclear density and smooth muscle cell content. Although nuclear levels of hypoxia-inducible factor 1-alpha (HIF-1alpha) are increased to a similar extent in HASMCs incubated at 1 and 3% O(2), expression of tumor protein p53, its transcriptional target p21, as well as their regulatory factors and downstream effectors, are differentially affected under these two conditions, suggesting that the bidirectional effects of hypoxia are mediated by this pathway. We conclude that hypoxia induces a state of enhanced cell turnover through increased rates of both smooth muscle cell proliferation and death. This confers the ability to remodel the vasculature in response to changing tissue metabolic needs while avoiding the accumulation of mutations that may lead to malignant transformation or the formation of abnormal vascular structures.

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.

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.

Deletion of p37Ing1 in mice reveals a p53-independent role for Ing1 in the suppression of cell proliferation, apoptosis, and tumorigenesis

ING proteins have been proposed to alter chromatin structure and gene transcription to regulate numerous aspects of cell physiology, including cell growth, senescence, stress response, apoptosis, and transformation. ING1, the founding member of the inhibitor of growth family, encodes p37(Ing1), a plant homeodomain (PHD) protein that interacts with the p53 tumor suppressor protein and seems to be a critical cofactor in p53-mediated regulation of cell growth and apoptosis. In this study, we have generated and analyzed p37(Ing1)-deficient mice and primary cells to further explore the role of Ing1 in the regulation of cell growth and p53 activity. The results show that endogenous levels of p37(Ing1) inhibit the proliferation of p53-wild-type and p53-deficient fibroblasts, and that p53 functions are unperturbed in p37(Ing1)-deficient cells. In addition, loss of p37(Ing1) induces Bax expression and increases DNA damage-induced apoptosis in primary cells and mice irrespective of p53 status. Finally, p37(Ing1) suppresses the formation of spontaneous follicular B-cell lymphomas in mice. These results indicate that p53 does not require p37(Ing1) to negatively regulate cell growth and offers genetic proof that Ing1 suppresses cell growth and tumorigenesis. Furthermore, these data reveal that p37(Ing1) can negatively regulate cell growth and apoptosis in a p53-independent manner.

Grow-ING, Age-ING and Die-ING: ING proteins link cancer, senescence and apoptosis

The INhibitor of Growth (ING) family of plant homeodomain (PHD) proteins induce apoptosis and regulate gene expression through stress-inducible binding of phospholipids with subsequent nuclear and nucleolar localization. Relocalization occurs concomitantly with interaction with a subset of nuclear proteins, including PCNA, p53 and several regulators of acetylation such as the p300/CBP and PCAF histone acetyltransferases (HATs), as well as the histone deacetylases HDAC1 and hSir2. These interactions alter the localized state of chromatin compaction, subsequently affecting the expression of subsets of genes, including those associated with the stress response (Hsp70), apoptosis (Bax, MDM2) and cell cycle regulation (p21WAF1, cyclin B) in a cell- and tissue-specific manner. The expression levels and subcellular localization of ING proteins are altered in a significant number of human cancer types, while the expression of ING isoforms changes during cellular aging, suggesting that ING proteins may play a role in linking cellular transformation and replicative senescence. The variety of functions attributed to ING proteins suggest that this tumor suppressor serves to link the disparate processes of cell cycle regulation, cell suicide and cellular aging through epigenetic regulation of gene expression. This review examines recent findings in the ING field with a focus on the functions of protein-protein interactions involving ING family members and the mechanisms by which these interactions facilitate the various roles that ING proteins play in tumorigenesis, apoptosis and senescence.

ING3 promotes UV-induced apoptosis via Fas/caspase-8 pathway in melanoma cells

The novel ING tumor-suppressor family proteins (ING1-5) have been discovered during the past decade and are recognized as the regulators of transcription, cell cycle checkpoints, DNA repair, apoptosis, cellular senescence, angiogenesis, and nuclear phosphoinositide signaling. ING proteins contain a few conserved domains, including plant homeodomain motif, nuclear localization signal, and potential chromatin regulatory domain, suggesting that the ING family proteins may share common biological functions. ING3 has been shown to modulate p53-mediated transcription, cell cycle control, and apoptosis, possibly by modulating the NuA4 complex histone acetyltransferase activity. Because ING1b and ING2 have been shown to be involved in cellular stress responses such as nucleotide excision repair and apoptosis after UV irradiation, we investigated whether ING3 also mediated UV-induced apoptosis. We found that ING3 expression was rapidly induced by UV irradiation at both mRNA and protein levels. Using the stable clones of melanoma cells overexpressing ING3, we showed that overexpression of ING3 significantly promoted UV-induced apoptosis. Unlike its homologues ING1b and ING2, ING3-increased apoptosis was independent of functional p53. Furthermore, ING3 did not affect the expression of mitochondrial proteins but increased the cleavage of Bid and caspases-8, -9, and -3. Moreover, ING3-mediated apoptosis was blocked by inhibition of caspase-8 or Fas activation. In addition, ING3 up-regulated Fas expression at both mRNA and protein levels. Knock down of ING3 decreased UV-induced apoptosis remarkably. These data indicate that ING3 plays an important role in cellular response to UV irradiation by enhancing UV-induced apoptosis through the activation of Fas/caspase-8 pathway.

The p33ING1b tumor suppressor cooperates with p53 to induce apoptosis in response to etoposide in human osteosarcoma cells

p33ING1b induces cell cycle arrest and stimulates DNA repair, apoptosis and chemosensitivity. The magnitude of some p33ING1b effects may be due to activation of the tumor suppressor p53. To investigate if the p33ING1b protein affected chemosensitivity of osteosarcoma cells, we overexpressed p33ING1b in p53+/+ U2OS cells or in p53-mutant MG63 cells, and then assessed for growth arrest and apoptosis after treatment with etoposide. p33ING1b increased etoposide-induced growth inhibition and apoptosis to a much greater degree in p53+/+ U2OS cells than in p53-mutant MG63 cells. Moreover, ectopic expression of p33ING1b markedly upregulated p53, p21WAF1 and bax protein levels and activated caspase-3 protein kinase in etoposide-treated U2OS cells. Together, our data indicate that p33ING1b prominently enhances etoposide-induced apoptosis through p53-dependent pathways in human osteosarcoma cells. p33ING1b may be an important marker and/or therapeutic target in the prevention and treatment of metastatic osteosarcoma.

Cellular UV damage responses--functions of tumor suppressor p53

DNA damage, provoked by ultraviolet (UV) radiation, evokes a cellular damage response composed of activation of stress signaling and DNA checkpoint functions. These are translated to responses of replicative arrest, damage repair, and apoptosis aimed at cellular recovery from the damage. p53 tumor suppressor is a central stress response protein, activated by multiple endogenous and environmental insults, including UV radiation. The significance of p53 in the DNA damage responses has frequently been reviewed in the context of ionizing radiation or other double strand break (DSB)-inducing agents. Despite partly similar patterns, the molecular events following UV radiation are, however, distinct from the responses induced by DSBs and are profoundly coupled with transcriptional stress. These are illustrated, e.g., by the UV damage-specific translocations of Mdm2, promyelocytic leukemia protein, and nucleophosmin and their interactions with p53. In this review, we discuss UV damage-provoked cellular responses and the functions of p53 in damage recovery and cell death.

Distinctive gene expression pattern in VH3-21 utilizing B-cell chronic lymphocytic leukemia

The usage of the immunoglobulin (Ig) V(H)3-21 gene is associated with poor prognosis in B-cell chronic lymphocytic leukemia (B-CLL) despite V(H) gene mutation status. Many V(H)3-21+ patients also display restricted heavy- and light-chain Ig gene rearrangements, implying a role of antigen selection in disease development. To explore the specific phenotypic/genotypic features among V(H)3-21+ B-CLLs, we compared gene expression patterns in 15 V(H)3-21+ and 24 non-V(H)3-21 patients (11 with unmutated and 13 with mutated V(H) genes) using Affymetrix microarray analysis (approximately 12,500 genes). A distinct expression profile was identified for V(H)3-21+ patients in contrast to the Ig-unmutated and -mutated groups. By applying different algorithms, the data enabled an efficient class discrimination of the V(H)3-21+ subset based on 27 or 57 genes. A set of genes was sorted out which, using different analytical methods, consistently gave a distinction between V(H)3-21+ and non-V(H)3-21 samples. Several of these genes are involved in regulation of DNA replication/cell-cycle control, transcription and protein kinase activity, which may render the V(H)3-21+ cells with a higher proliferative drive. However, no clear evidence of increased B-cell receptor signaling was found in the V(H)3-21+ group. Altogether, our identification of a specific V(H)3-21 profile may provide insights into the pathogenesis of the V(H)3-21+ subgroup.

Growth inhibition by the tumor suppressor p33ING1 in immortalized and primary cells: involvement of two silencing domains and effect of Ras

ING1 was identified as an inhibitor of growth and has been described as a tumor suppressor. Furthermore, the expression of ING1 is induced in senescent cells and antisense ING1 extends the proliferative life span of primary human fibroblasts. Cooperation of p33ING1 with p53 has been suggested to be an important function of ING1 in cell cycle control. Intriguingly, it has been shown that p33ING1 is associated with histone acetylation as well as with histone deacetylation function. Here we show that p33ING1 is a potent transcriptional silencer in various cell types. However, the silencing function is independent of the presence of p53. By use of deletion mutants two potent autonomous and transferable silencing domains were identified, but no evidence of an activation domain was found. The amino (N)-terminal silencing domain is sensitive to the histone deacetylase inhibitor trichostatin A (TSA) whereas the carboxy-terminal silencing function is resistant to TSA, suggesting that p33ING1 confers gene silencing through both HDAC-dependent and -independent mechanisms. Interestingly, the presence of oncogenic Ras, which is able to induce premature senescence, increases the p33ING1-mediated silencing function. Moreover, ING1-mediated silencing was reduced by coexpressing dominant-negative Ras or by treatment with the mitogen-activated protein kinase inhibitor PD98059 but not by treatment with SB203580, an inhibitor of the p38 pathway. In addition, we show that both silencing domains of ING1 are involved in cell cycle control, as measured by inhibition of colony formation of immortalized cells and by thymidine incorporation of primary human diploid fibroblasts (HDF). Interestingly, p33ING1 expression induces features of cellular senescence in HDFs.

ING4 induces G2/M cell cycle arrest and enhances the chemosensitivity to DNA-damage agents in HepG2 cells

The known members of inhibitor of growth (ING) gene family are considered as candidate tumor suppressor genes. ING4, a novel member of ING family, is recently reported to interact with tumor suppressor p53, p300 (a major component of histone acetyl transferase complexes), and p65(RelA) subunit of NF-kappaB. In this study, we investigated the cellular behaviors of HepG2 cells with exogenous ING4. Interestingly, the overexpression of ING4 negatively regulated the cell growth with significant G2/M arrest of cell cycle, and moreover, enhanced the cell apoptosis triggered by serum starvation in HepG2 cells. Furthermore, the exogenous ING4 could upregulate endogenous p21 and Bax in HepG2 cells, not in p53-deficient Saos-2 cells, suggesting that G2/M arrest induced by ING4 could be mediated by the increased p21 expression in a p53-dependent manner, although there is no significant increase of p53 expression in HepG2 cells. Moreover, HepG2 cells with exogenous ING4 could significantly increase cell death, as exposed to some DNA-damage agents, such as etoposide and doxorubicin, implying that ING4 could enhance chemosensitivity to certain DNA-damage agents in HepG2 cells.