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Previous studies have shown that UV-induced binding of p21(WAF1) to PCNA through the PCNA-interacting protein (PIP) domain in p21(WAF1) promotes a switch from DNA replication to DNA repair by altering the PCNA protein complex. Here we show that the p33(ING1b) isoform of the ING1 candidate tumour suppressor contains a PIP domain. UV rapidly induces p33(ING1b) to bind PCNA competitively through this domain, a motif also found in DNA ligase, the DNA repair-associated FEN1 and XPG exo/endonucleases, and DNA methyltransferase. Interaction of p33(ING1b) with PCNA occurs between a significant proportion of ING1 and PCNA, increases more than tenfold in response to UV and is specifically inhibited by overexpression of p21(WAF1), but not by p16(MTS1), which has no PIP sequence. In contrast to wild-type p33(ING1b), ING1 PIP mutants that do not bind PCNA do not induce apoptosis, but protect cells from UV-induced apoptosis, suggesting a role for this PCNA-p33(ING1b) interaction in eliminating UV-damaged cells through programmed cell death. These data indicate that ING1 competitively binds PCNA through a site used by growth regulatory and DNA damage proteins, and may contribute to regulating the switch from DNA replication to DNA repair by altering the composition of the PCNA protein complex.

The ING1 candidate tumor suppressor is downregulated in a variety of primary tumors and established cancer cell lines. Blocking its expression experimentally promotes unregulated growth in vitro and in vivo, using cell and animal models. Alternative splicing products encode proteins that localize to the nucleus, inhibit cell cycle progression and affect apoptosis in different model systems. Here we show that ING1 proteins translocate to the nucleolus 12–48 h after UV-induced DNA damage. When a small 50 amino acid portion of ING1 was fused to green fluorescent protein, the fusion protein was efficiently targeted to the nucleolus, indicating that ING1 possesses an intrinsic nucleolar targeting sequence (NTS). We mapped this activity to two distinct 4 amino acid regions, which individually direct fused heterologous proteins to the nucleolus. Overexpression of ING1 induced apoptosis of primary fibroblasts in the presence and absence of UV exposure. In contrast, NTS mutants of ING1 that were not targeted to the nucleolus did not efficiently induce apoptosis when overexpressed and instead protected cells from UV-induced apoptosis. Taken together, these results indicate that UV induces ING1 to translocate to the nucleolus and that this translocation may facilitate apoptosis.

Nine monoclonal antibodies (MAbs) against human and rodent ING1 protein have been generated using an IL6-secreting mouse myeloma line. These antibodies are all effective in recognizing ING1 protein in ELISAs, Western blot assays, and by indirect immunofluorescence. Combining different CAb monoclonal antibodies in a Western blot assay also allows detection of the very low levels of endogenous ING1 found in fibroblast cells in culture and the identification of at least two isoforms of ING1 in normal human diploid fibroblasts and established brain cancer cell lines.

Clinical and experimental evidence suggest that the p33ING1b candidate tumor suppressor functionally cooperates with p53 in controlling biochemical and biological functions. Because p53 is frequently mutated in brain tumors and the ING1 locus maps to a site of which the loss is associated with gliomas, we analyzed the mutation and expression profiles of ING1B in human brain tumors. Here we present the first report of ING1 expression and mutation analyses in human brain tumor samples and malignant glioma cell lines.Expression and mutation analyses of ING1B together with subcellular localization studies of ING1 proteins were performed on 29 brain tumor specimens and 6 human glioma cell lines.A single point mutation (3.5%) was detected in the 29 brain tumor specimens analyzed. This missense mutation occurred in a sequence reported previously to confer nuclear translocation properties to p33ING1b. Interestingly, overexpression and subcellular mislocalization of p33ING1b were observed in all 29 of the brain tumor specimens and some glioma cell lines. In tumor samples, ING1 proteins aberrantly localized to the cytoplasm, and to a lesser extent, to the nucleus of glioma cells.Our data indicate that although mutations of ING1 seem to be infrequent in human brain tumors, deregulated expression and mislocalization of ING1 proteins, particularly the p33ING1b isoform, are common events in gliomas and glioblastomas.

The ING family of proteins is involved in the regulation of diverse processes ranging from cell cycle and cellular senescence to apoptosis. These effects are most likely through activation of acetylation-dependent pathways that ultimately alter gene expression. Despite reports linking ING to p53 activation, the molecular basis of how ING activates p53 function has not been elucidated. In this study, we found that a subset of ING family members strongly repressed human alpha-fetoprotein (AFP) promoter activity but stimulated the p21(WAF1) promoter in parallel experiments in the same cell type, similar to the effects of p53. The p47(ING1a) isoform also repressed AFP promoter activity, but in contrast to other ING isoforms, it repressed the p21(WAF1) promoter. p47(ING3) up-regulated p21(WAF1) promoter activity, but it did not have any effect on the AFP promoter. ING1b and ING2 also repressed the AFP promoter in Hep3B p53-null cell lines, and p53 coexpression enhanced this transcriptional repression. Suppression of AFP gene transcription by ING was strongly dependent on AT-motifs that bind to the hepatocyte nuclear factor 1 (HNF1) transcription factor. Indeed, electrophoretic mobility shift assays confirmed that HNF1 binds to AT-motifs, but we found, surprisingly, that the ING1 complexes binding to these AT-motifs were devoid of HNF1 protein. Both ING1 and p53 were able to suppress AFP transcription and cause p21 induction; hSIR2, a negative regulator of the p53 protein, showed the opposite effects on the AFP promoter and, like HDAC1, repressed p21 promoter activity. In addition, we found that p33(ING1b) physically interacts with hSIR2, reverses its ability to induce the AFP promoter, and induces acetylation of p53 residues at Lys(373) and/or Lys(382). These findings provide novel evidence that p33(ING1b) represses AFP transcription by at least two mechanisms, one of which includes p53. The first is by binding to the AT-motif and excluding HNF1 binding while possibly targeting HAT activity to promoter regions, and the second is by increasing the levels of active, acetylated p53 via binding and inhibiting the ability of hSIR2 to deacetylate p53 protein.

Recently, several novel human ING1 isoforms have been cloned. However,the biochemical functions and the involvement of these proteins in apoptosis remain uncharacterized. We have examined the apoptotic effects and biochemical functions of the two major human ING1 isoforms p47(ING1a) and p33(ING1b) in young and senescent human diploid fibroblasts induced to enter into apoptosis by diverse treatments. We have found that ING1 displayed isoform-, stimulus- and cell age-dependent apoptotic properties. We present evidence indicating that ING1 proteins bind to chromatin and are regulated in a manner related to their apoptotic properties. In agreement with previous reports, we have found that only young but not senescent fibroblasts were able to enter into apoptosis induced by growth factor deprivation. This effect was accompanied by up-regulation of endogenous p33(ING1b). Ectopic up-regulation of p33(ING1b), but not p47(ING1a), also induced apoptosis and sensitized young but not senescent cells to UV irradiation and hydrogen peroxide-mediated apoptosis. Cotransfection of p33(ING1b) and the tumor suppressor p53 increased the percentage of apoptotic cells yielded by either of these two proteins alone, in agreement with data from tumor cell models. Finally, we found that the chromatin binding affinity of p33(ING1b) was increased in senescent cells, which were resistant to apoptosis. Together, these data support the idea that the apoptotic functions of ING1 may be exerted by chromatin-related functions that are subject to cell age-dependent mechanisms of regulation.

ING1 proteins are nuclear, growth inhibitory, and regulate apoptosis in different experimental systems. Here we show that similar to their yeast homologs, human ING1 proteins interact with proteins associated with histone acetyltransferase (HAT) activity, such as TRRAP, PCAF, CBP, and p300. Human ING1 immunocomplexes contain HAT activity, and overexpression of p33ING1b, but not of p47ING1a, induces hyperacetylation of histones H3 and H4, in vitro and in vivo at the single cell level. p47ING1a inhibits histone acetylation in vitro and in vivo and binds the histone deacetylase HDAC1. Finally, we present evidence indicating that p33ING1b affects the degree of physical association between proliferating cell nuclear antigen (PCNA) and p300, an association that has been proposed to link DNA repair to chromatin remodeling. Together with the finding that human ING1 proteins bind PCNA in a DNA damage-dependent manner, these data suggest that ING1 proteins provide a direct linkage between DNA repair, apoptosis, and chromatin remodeling via multiple HAT·ING1·PCNA protein complexes.