Nuclear to cytoplasmic compartment shift of the p33ING1b tumour suppressor protein is associated with malignancy in melanocytic lesions

CG7379 and ING1 suppress cancer cell invasion by maintaining cell-cell junction integrity

Approximately 90% of cancer-related deaths can be attributed to a tumour's ability to spread. We have identified CG7379, the fly orthologue of human ING1, as a potent invasion suppressor. ING1 is a type II tumour suppressor with well-established roles in the transcriptional regulation of genes that control cell proliferation, response to DNA damage, oncogene-induced senescence and apoptosis. Recent work suggests a possible role for ING1 in cancer cell invasion and metastasis, but the molecular mechanism underlying this observation is lacking. Our results show that reduced expression of CG7379 promotes invasion in vivo in Drosophila, reduces the junctional localization of several adherens and septate junction components, and severely disrupts cell-cell junction architecture. Similarly, ING1 knockdown significantly enhances invasion in vitro and disrupts E-cadherin distribution at cell-cell junctions. A transcriptome analysis reveals that loss of ING1 affects the expression of several junctional and cytoskeletal modulators, confirming ING1 as an invasion suppressor and a key regulator of cell-cell junction integrity.

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.

Aberrant localization of signaling proteins in skin cancer: Implications for treatment

Aberrant subcellular localization of signaling proteins can provide cancer cells with advantages such as resistance to apoptotic cell death, increased invasiveness and more rapid proliferation. Nuclear to cytoplasmic shifts in tumor-promoting proteins can lead to worse patient outcomes, providing opportunities to target cancer-specific processes. Herein, we review the significance of dysregulated protein localization with a focus on skin cancer. Altered localization of signaling proteins controlling cell cycle progression or cell death is a common feature of cancer. In some instances, aberrant subcellular localization results in an acquired prosurvival function. Taking advantage of this knowledge reveals novel targets useful in the development of cancer therapeutics.

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.

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.

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

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

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

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

Down-regulation of ING4 is associated with initiation and progression of lung cancer

AIMS

Tumour suppressor ING4 is one of ING family genes, which are involved in cell cycle arrest, gene transcription regulation, DNA repair and apoptosis. ING4 inhibition has been reported in various tumours, including gliomas, breast tumours, and stomach adenocarcinoma. The aim was to evaluate ING4 expression in lung cancers.

METHOD AND RESULTS

By immunohistochemistry of 246 lung tumour tissues, reduced ING4 nuclear and cytoplasmic expression were both revealed in lung cancer and associated with tumour grade. Interestingly, compared with normal tissues, we found more tumours with ING4 expression in the cytoplasm higher than in the nucleus. Nuclear ING4 inhibition correlated with the tumour stage and lymph node metastasis. Consistent with these findings, semiquantitative reverse transcriptase-polymerase chain reaction and Western blotting demonstrated decreased ING4 mRNA and expression in 100% (50/50) tumour tissues. Furthermore, ING4 expression was lower in grade III than in grades I-II tumours. Reduced ING4 mRNA correlated with lymph node metastasis.

CONCLUSIONS

Our results indicate that overall inhibition of ING4 expression and ING4 expression higher in cytoplasm than in nucleus of tumour cells may be involved in the initiation and progression of lung cancers, and thus, analysis for ING4 expression may be useful as a clinical diagnostic and prognostic tool for lung cancer.

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.

Inhibitor of growth tumor suppressors in cancer progression

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

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

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

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

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

Histone H3K4me3 binding is required for the DNA repair and apoptotic activities of ING1 tumor suppressor

Inhibitor of growth 1 (ING1) is implicated in oncogenesis, DNA damage repair, and apoptosis. Mutations within the ING1 gene and altered expression levels of ING1 are found in multiple human cancers. Here, we show that both DNA repair and apoptotic activities of ING1 require the interaction of the C-terminal plant homeodomain (PHD) finger with histone H3 trimethylated at Lys4 (H3K4me3). The ING1 PHD finger recognizes methylated H3K4 but not other histone modifications as revealed by the peptide microarrays. The molecular mechanism of the histone recognition is elucidated based on a 2.1 A-resolution crystal structure of the PHD-H3K4me3 complex. The K4me3 occupies a deep hydrophobic pocket formed by the conserved Y212 and W235 residues that make cation-pi contacts with the trimethylammonium group. Both aromatic residues are essential in the H3K4me3 recognition, as substitution of these residues with Ala disrupts the interaction. Unlike the wild-type ING1, the W235A mutant, overexpressed in the stable clones of melanoma cells or in HT1080 cells, was unable to stimulate DNA repair after UV irradiation or promote DNA-damage-induced apoptosis, indicating that H3K4me3 binding is necessary for these biological functions of ING1. Furthermore, N216S, V218I, and G221V mutations, found in human malignancies, impair the ability of ING1 to associate with H3K4me3 or to induce nucleotide repair and cell death, linking the tumorigenic activity of ING1 with epigenetic regulation. Together, our findings reveal the critical role of the H3K4me3 interaction in mediating cellular responses to genotoxic stresses and offer new insight into the molecular mechanism underlying the tumor suppressive activity of ING1.

Role of ING4 in human melanoma cell migration, invasion and patient survival

Inhibitor of growth (ING) 4 has been reported as a tumor suppressor and shown to diminish colony-forming efficiency, induce p53-dependent apoptosis and arrest cell cycle at G(2)-M phase. In this study, we investigated the role of ING4 in human melanoma pathogenesis. Using the tissue microarray technology, we found that ING4 expression is significantly decreased in malignant melanoma compared with dysplastic nevi (P < 0.0001, chi(2) test) and reduced ING4 staining is associated with melanoma thickness, ulceration (P = 0.034 and 0.002, respectively, chi(2) test) as well as poor overall and disease-specific 5-year survival of primary melanoma patients (P = 0.0002 and 0.001, respectively, chi(2) test). Cox regression analysis revealed that reduced ING4 staining is an independent factor for the poor prognosis of patients with primary melanomas. Furthermore, we found that overexpression of ING4 suppressed cell migration by 63% and inhibited the activity of Ras homolog gene family member A (RhoA) small GTPase protein and Rho-associated kinase (ROCK)-mediated formation of stress fiber in melanoma cells. Moreover, our data showed that overexpression of ING4 inhibited melanoma cell invasion by 43% compared with the control (P = 0.006, t-test) and ING4-overexpressing melanoma cells showed significantly reduced activity of matrix metalloproteinase (MMP)-2 and MMP-9. Taken together, this study highlights the importance of ING4 in melanoma pathogenesis and ING4 may serve as a promising prognostic marker and a potential therapeutic target for human melanoma.

Nuclear to cytoplasmic shift of p33(ING1b) protein from normal oral mucosa to oral squamous cell carcinoma in relation to clinicopathological variables

PURPOSE

p33(ING1b), as a candidate tumour suppressor gene, has been found to be expressed a proportion of oral squamous cell carcinomas (OSCCs), however, its clinicopathological significance is not studied yet. Our aim was to investigate association of p33(ING1b) expression with clinicopathological variables and particularly interesting new cysteine-histidine rich protein (PINCH) in OSCCs.

METHODS

p33(ING1b) expression was immunohistochemically examined in 20 normal oral mucosa specimens and 49 OSCCs.

RESULTS

Normal squamous cells showed only p33(ING1b )nuclear expression (no cytoplasmic expression), with a rate of 90% positive cases. While 24% of OSCCs appeared cytoplasmic expression (11 of them with weak nuclear staining) and the rest tumours (76%) were negative for p33(ING1b). Furthermore, the cases having lymph node metastasis showed a higher frequency of positive cytoplasmic expression than those without metastasis (P = 0.03). The p33(ING1b) cytoplasmic expression was positively related to PINCH expression (P = 0.04), the cases positive for both proteins had a high rate of the metastasis (P = 0.03).

CONCLUSIONS

The transfer of p33(ING1b) protein from the nucleus to the cytoplasm may result in loss of normal cellular function of the protein, which might play a role in the tumourigenesis and metastasis of OSCCs.

Prognostic Significance of Nuclear ING3 Expression in Human Cutaneous Melanoma

PURPOSE

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

EXPERIMENTAL DESIGN

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

RESULTS

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

CONCLUSIONS

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

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.

NoING1 mutations in human brain tumours but reduced expression in high malignancy grades of astrocytoma

The ING1 family of proteins has been shown to have regulatory functions in oncogenesis, apoptosis, DNA repair and cell cycle regulation. Here we present the first report on LOH analysis of the ING1 locus, mutation analysis of the complete coding sequence including intron-exon boundaries and expression analysis of the different ING1 splice products and protein isoforms in primary brain tumours. No somatic ING1 mutations were detected. Semi-quantitative analysis revealed higher levels of p33ING1b RNA in benign than in malignant lesions. This correlation was significant in a subset of 37 astrocytic tumours WHO grades I to IV. ING1 protein isoforms p47ING1a, p33ING1b and p24ING1c were found to be expressed variably in this series. Our findings support a regulatory contribution of ING1 to the development or progression of brain tumours.

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

ING1b can stimulate cell cycle arrest, repair, senescence, and apoptosis. The actions of ING1b are attributed to its activation of the tumor suppressor p53. Here we investigate the more subtle effects of ING1b on the cell cycle and DNA damage responses in the absence of p53. To this end, we have generated isogenic cell lines that expressed ING1b and p53 either individually or in combination under the control of inducible promoters. A five- to 10-fold induction of ING1b over the endogenous protein in a p53-null H1299 background slightly impairs proliferation by increasing the doubling time by approximately 10%. Significantly, ectopic expression of ING1b enhanced the G(2)/M DNA damage checkpoint induced by adriamycin. We demonstrated that the DNA damage-induced cell death mediated by the cooperation between ING1b and p53 was more prominent than by the individual proteins alone. In adriamycin-treated cells, p53 was stabilized and induced the expression of p21(CIP1/WAF1), but the expression of ING1b was not affected. The exact targets of ING1b in the p53-null background are not known, but we demonstrated that the transcriptional activities of other members of the p53 family, p63alpha and p73alpha, could be activated by ING1b. These data indicate that ING1 has a subtle antiproliferative effect even in the absence of p53, and ING1b enhances the DNA damage responses through p53-dependent and -independent mechanisms.

Downregulation of nuclear expression of the p33(ING1b) inhibitor of growth protein in invasive carcinoma of the breast

BACKGROUND/AIMS

The inhibitor of growth gene 1 (ING1) is a modulator of cell cycle checkpoints, apoptosis, and cellular senescence. The most widely expressed ING1 isoform is p33(ING1b), which can modulate p53, a molecule that is frequently altered in breast cancer. Reduced ING1 mRNA expression has been observed in primary breast cancer expressing wild-type p53.

METHODS

p33(ING1b), p53, oestrogen receptor (ER), and progesterone receptor (PgR) expression was studied in 86 primary invasive breast cancers using immunohistochemistry.

RESULTS

Reduced nuclear expression of p33(ING1b) was found in cancer cells, both in intensity and the proportion of cells staining. This was associated with enhanced cytoplasmic p33(ING1b) expression in a proportion of cases. Analysis of several known biological factors indicated that high grade tumours were of larger size and more often negative for ER and PgR expression. However, larger tumours were more frequently p53 negative. These results provide evidence that p33(ING1b) alterations are associated with more poorly differentiated tumours. Positive correlations were found between nuclear p33(ING1b) expression and both ER and PgR expression.

CONCLUSIONS

Optimum function of p53 is dependent on p33(ING1b) so that a reduction of nuclear p33(ING1b) expression, as seen in this series, would be predicted to compromise p53 function. This study showed that p33(ING1b) alterations were associated with more poorly differentiated tumours. Therefore, p33(ING1b) expression could be used as a marker of differentiation in invasive breast cancer. These results support the view that loss of p33(ING1b) may be an important molecular event in the differentiation and pathogenesis of invasive breast cancer.

The role of the tumour suppressor p33 ING1b in human neoplasia

The inhibitor of growth (ING) genes (ING1-4) probably descend from tumour suppressor genes. ING1 was the first to be identified and later isolated using an approach to detect genes whose expression is suppressed in cancer. The others were isolated through homology and similarity searches in human and mouse databases. All members contain a plant homeodomain involved in macromolecule recognition. Apart from the extensively studied ING1, little is known about the number of transcripts encoded by the other members or their gene structure. ING1 encodes several differentially spliced mRNAs, which may produce a family of proteins. The most widely expressed protein isoform is p33(INGb1), which is involved in restriction of cell growth and proliferation, apoptosis, tumour anchorage independent growth, cellular senescence, maintenance of genomic stability, and modulation of cell cycle checkpoints. ING1 gene mutation is uncommon in cancer, although the subcellular localisation of p33(INGb1) may have an effect on its function. The p33(INGb1) cellular compartmental shift from the nucleus to the cytoplasm may cause loss of normal cellular function, and may play a central role in the pathogenesis of several cancers.