Phosphorylation of HIC1 (Hypermethylated in Cancer 1) Ser694 by ATM is essential for DNA repair.

The tumor suppressor gene HIC1 (Hypermethylated in Cancer 1) encodes a transcriptional repressor involved in the DNA-damage response. A SUMOylation increase on HIC1 Lysine314 favors the direct transcriptional repression of SIRT1 and thus the P53-dependent apoptotic response to irreparable DNA double strand breaks (DSBs). HIC1 is also essential for DSBs repair but in a SUMOylation-independent manner. Here, we show that repairable DSBs induced by a 1 h Etoposide treatment results in three specific posttranslational modifications (PTMs) of HIC1. Two of these PTMs, phosphorylation of Serine 694 and Acetylation of Lysine 623 are located in the conserved HIC1 C-terminal region located downstream of the Zinc Finger DNA-binding domain. By contrast, phosphorylation of Serine 285 found in the poorly conserved central region is unique to the human protein. We showed that Ser694 phosphorylation is mediated mainly by the PIKK kinase ATM and is essential for the DNA repair activity of HIC1 as demonstrated by the lack of efficiency of the S694A point mutant in Comet assays. Thus, our results provide the first evidence for a functional role of the conserved HIC1 C-terminal region as a novel ATM substrate that plays an essential role in the cellular HIC1-mediated cellular response to repairable DSBs.

Evidence of a compensatory regulation of colonic O-GlcNAc transferase and O-GlcNAcase expression in response to disruption of O-GlcNAc homeostasis.

O-GlcNAcylation is a post-translational modification of thousands of intracellular proteins that dynamically regulates many fundamental cellular processes. Cellular O-GlcNAcylation levels are regulated by a unique couple of enzymes: O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), which adds and removes the GlcNAc residue, respectively. Maintenance of O-GlcNAc homeostasis is essential to ensure optimal cellular function and disruption of this homeostasis has been linked to the etiology of several human diseases including cancer. The mechanisms through which the cell maintains O-GlcNAc homeostasis are not fully understood but several studies have suggested that a reciprocal regulation of OGT and OGA expression could be one of them. In this study, we investigated the putative regulation of OGT and OGA expression in response to disruption in O-GlcNAc homeostasis in colon. We provide in vitro and in vivo evidences that in colon cells, modulation of O-GlcNAcylation levels leads to a compensatory regulation of OGT and OGA expression in an attempt to restore basal O-GlcNAcylation levels. Our results also suggests that the regulation of colonic OGA expression in response to changes in O-GlcNAc homeostasis occurs mostly at the transcriptional level whereas OGT regulation seems to rely mainly on post-transcriptional mechanisms.

DNA double-strand breaks lead to activation of hypermethylated in cancer 1 (HIC1) by SUMOylation to regulate DNA repair.

HIC1 (hypermethylated in cancer 1) is a tumor suppressor gene frequently epigenetically silenced in human cancers. HIC1 encodes a transcriptional repressor involved in the regulation of growth control and DNA damage response. We previously demonstrated that HIC1 can be either acetylated or SUMOylated on lysine 314. This deacetylation/SUMOylation switch is governed by an unusual complex made up of SIRT1 and HDAC4 which deacetylates and thereby favors SUMOylation of HIC1 by a mechanism not yet fully deciphered. This switch regulates the interaction of HIC1 with MTA1, a component of the NuRD complex and potentiates the repressor activity of HIC1. Here, we show that HIC1 silencing in human fibroblasts impacts the repair of DNA double-strand breaks whereas ectopic expression of wild-type HIC1, but not of nonsumoylatable mutants, leads to a reduced number of γH2AX foci induced by etoposide treatment. In this way, we demonstrate that DNA damage leads to (i) an enhanced HDAC4/Ubc9 interaction, (ii) the activation of SIRT1 by SUMOylation (Lys-734), and (iii) the SUMO-dependent recruitment of HDAC4 by SIRT1 which permits the deacetylation/SUMOylation switch of HIC1. Finally, we show that this increase of HIC1 SUMOylation favors the HIC1/MTA1 interaction, thus demonstrating that HIC1 regulates DNA repair in a SUMO-dependent way. Therefore, epigenetic HIC1 inactivation, which is an early step in tumorigenesis, could contribute to the accumulation of DNA mutations through impaired DNA repair and thus favor tumorigenesis.

The receptor tyrosine kinase EphA2 is a direct target gene of hypermethylated in cancer 1 (HIC1).

The tumor suppressor gene hypermethylated in cancer 1 (HIC1), which encodes a transcriptional repressor, is epigenetically silenced in many human tumors. Here, we show that ectopic expression of HIC1 in the highly malignant MDA-MB-231 breast cancer cell line severely impairs cell proliferation, migration, and invasion in vitro. In parallel, infection of breast cancer cell lines with a retrovirus expressing HIC1 also induces decreased mRNA and protein expression of the tyrosine kinase receptor EphA2. Moreover, chromatin immunoprecipitation (ChIP) and sequential ChIP experiments demonstrate that endogenous HIC1 proteins are bound, together with the MTA1 corepressor, to the EphA2 promoter in WI38 cells. Taken together, our results identify EphA2 as a new direct target gene of HIC1. Finally, we observe that inactivation of endogenous HIC1 through RNA interference in normal breast epithelial cells results in the up-regulation of EphA2 and is correlated with increased cellular migration. To conclude, our results involve the tumor suppressor HIC1 in the transcriptional regulation of the tyrosine kinase receptor EphA2, whose ligand ephrin-A1 is also a HIC1 target gene. Thus, loss of the regulation of this Eph pathway through HIC1 epigenetic silencing could be an important mechanism in the pathogenesis of epithelial cancers.

Hypermethylated in cancer 1 (HIC1) recruits polycomb repressive complex 2 (PRC2) to a subset of its target genes through interaction with human polycomb-like (hPCL) proteins.

HIC1 (hypermethylated in cancer 1) is a tumor suppressor gene epigenetically silenced or deleted in many human cancers. HIC1 is involved in regulatory loops modulating p53- and E2F1-dependent cell survival, growth control, and stress responses. HIC1 is also essential for normal development because Hic1-deficient mice die perinatally and exhibit gross developmental defects throughout the second half of development. HIC1 encodes a transcriptional repressor with five C(2)H(2) zinc fingers mediating sequence-specific DNA binding and two repression domains: an N-terminal BTB/POZ domain and a central region recruiting CtBP and NuRD complexes. By yeast two-hybrid screening, we identified the Polycomb-like protein hPCL3 as a novel co-repressor for HIC1. Using multiple biochemical strategies, we demonstrated that HIC1 interacts with hPCL3 and its paralog PHF1 to form a stable complex with the PRC2 members EZH2, EED, and Suz12. Confirming the implication of HIC1 in Polycomb recruitment, we showed that HIC1 shares some of its target genes with PRC2, including ATOH1. Depletion of HIC1 by siRNA interference leads to a partial displacement of EZH2 from the ATOH1 promoter. Furthermore, in vivo, ATOH1 repression by HIC1 is associated with Polycomb activity during mouse cerebellar development. Thus, our results identify HIC1 as the first transcription factor in mammals able to recruit PRC2 to some target promoters through its interaction with Polycomb-like proteins.

Loss of Hypermethylated in Cancer 1 (HIC1) in breast cancer cells contributes to stress-induced migration and invasion through ß-2 adrenergic receptor (ADRB2) misregulation.

The transcriptional repressor HIC1 (Hypermethylated in Cancer 1) is a tumor suppressor gene inactivated in many human cancers including breast carcinomas. In this study, we show that HIC1 is a direct transcriptional repressor of ß-2 adrenergic receptor (ADRB2). Through promoter luciferase activity, chromatin immunoprecipitation (ChIP) and sequential ChIP experiments, we demonstrate that ADRB2 is a direct target gene of HIC1, endogenously in WI-38 cells and following HIC1 re-expression in breast cancer cells. Agonist-mediated stimulation of ADRB2 increases the migration and invasion of highly malignant MDA-MB-231 breast cancer cells but these effects are abolished following HIC1 re-expression or specific down-regulation of ADRB2 by siRNA treatment. Our results suggest that early inactivation of HIC1 in breast carcinomas could predispose to stress-induced metastasis through up-regulation of the ß-2 adrenergic receptor.

Identification of p21 (CIP1/WAF1) as a direct target gene of HIC1 (Hypermethylated In Cancer 1).

The tumor suppressor gene HIC1 (Hypermethylated In Cancer 1) encodes a transcriptional repressor involved in the regulation of growth control and DNA damage response. We previously demonstrated that p57Kip2; a member of the CIP/KIP family of CDK (cyclin dependent kinase) inhibitors (CKI); is a direct target gene of HIC1 in quiescent cells. Here we show that ectopic expression of HIC1 in MDA-MB-231 cells or its overexpression in BJ-Tert fibroblasts induces decreased mRNA and protein expression of p21 (CIP1/WAF1) another member of this CKI family that plays essential roles in the p53-mediated DNA damage response. Conversely, knock-down of endogenous HIC1 in BJ-Tert through RNA interference up-regulates p21 in basal conditions and further potentiates this CKI in response to apoptotic etoposide-induced DNA damage. Through promoter luciferase activity and chromatin immunoprecipitation (ChIP), we demonstrate that HIC1 is a direct transcriptional repressor of p21. Thus, our results further demonstrate that HIC1 is a key player in the regulation of the DNA damage response.

The Reelin receptors ApoER2 and VLDLR are direct target genes of HIC1 (Hypermethylated In Cancer 1).

The tumor suppressor gene HIC1 (Hypermethylated In Cancer 1) is located in 17p13.3 a region frequently hypermethylated or deleted in tumors and in a contiguous-gene syndrome, the Miller-Dieker syndrome which includes classical lissencephaly (smooth brain) and severe developmental defects. HIC1 encodes a transcriptional repressor involved in the regulation of growth control, DNA damage response and cell migration properties. We previously demonstrated that the membrane-associated G-protein-coupled receptors CXCR7, ADRB2 and the tyrosine kinase receptor EphA2 are direct target genes of HIC1. Here we show that ectopic expression of HIC1 in U2OS and MDA-MB-231 cell lines decreases expression of the ApoER2 and VLDLR genes, encoding two canonical tyrosine kinase receptors for Reelin. Conversely, knock-down of endogenous HIC1 in BJ-Tert normal human fibroblasts through RNA interference results in the up-regulation of these two Reelin receptors. Finally, through chromatin immunoprecipitation (ChIP) in BJ-Tert fibroblasts, we demonstrate that HIC1 is a direct transcriptional repressor of ApoER2 and VLDLR. These data provide evidence that HIC1 is a new regulator of the Reelin pathway which is essential for the proper migration of neuronal precursors during the normal development of the cerebral cortex, of Purkinje cells in the cerebellum and of mammary epithelial cells. Deregulation of this pathway through HIC1 inactivation or deletion may contribute to its role in tumor promotion. Moreover, HIC1, through the direct transcriptional repression of ATOH1 and the Reelin receptors ApoER2 and VLDLR, could play an essential role in normal cerebellar development.

Molecular dissection of the interaction between HIC1 and SIRT1.

HIC1 (Hypermethylated in Cancer 1) is a tumor suppressor gene frequently epigenetically silenced in human cancers. HIC1 encodes a transcriptional repressor involved in the regulation of growth control, cell survival and DNA damage response. The deacetylase SIRT1 regulates the repressive capacity of HIC1 in several fashions. First SIRT1 interacts with the BTB/POZ domain of HIC1 to form a transcriptional repression complex that prevents the transcription of SIRT1 itself. SIRT1 is also responsible of the deacetylation of the lysine 314 of HIC1 that allows its subsequent SUMOylation which in turn favors its interaction with the NuRD complex. To better understand the interplay between HIC1 and SIRT1, we performed co-immunoprecipitation experiments to define the domains essential for the HIC1/SIRT1 interaction. We demonstrated that the isolated four last zinc fingers of HIC1 were capable to interact with SIRT1 and that the amino-acids 610-677 of SIRT1 encompassing the ESA region of the deacetylase were crucial for the HIC1/SIRT1 interaction and HIC1 deacetylation. Finally we demonstrated that this interaction mainly depends on CKII-mediated phosphorylation of SIRT1 serine 659/661 which occurs upon DNA damage. Therefore, our results demonstrate that the activating acetylation to SUMOylation switch of HIC1 is favored by genotoxic stresses to regulate the DNA damage response.

Functional characterization of human Polycomb-like 3 isoforms identifies them as components of distinct EZH2 protein complexes.

PcG (Polycomb group) proteins are conserved transcriptional repressors essential to regulate cell fate and to maintain epigenetic cellular memory. They work in concert through two main families of chromatin-modifying complexes, PRC1 (Polycomb repressive complex 1) and PRC2-4. In Drosophila, PRC2 contains the H3K27 histone methyltransferase E(Z) whose trimethylation activity towards PcG target genes is stimulated by PCL (Polycomb-like). In the present study, we have examined hPCL3, one of its three human paralogues. Through alternative splicing, hPCL3 encodes a long isoform, hPCL3L, containing an N-terminal TUDOR domain and two PHDs (plant homeodomains) and a smaller isoform, hPCL3S, lacking the second PHD finger (PHD2). By quantitative reverse transcription-PCR analyses, we showed that both isoforms are widely co-expressed at high levels in medulloblastoma. By co-immunoprecipitation analyses, we demonstrated that both isoforms interact with EZH2 through their common TUDOR domain. However, the hPCL3L-specific PHD2 domain, which is better conserved than PHD1 in the PCL family, is also involved in this interaction and implicated in the self-association of hPCL3L. Finally, we have demonstrated that both hPCL3 isoforms are physically associated with EZH2, but in different complexes. Our results provide the first evidence that the two hPCL3 isoforms belong to different complexes and raise important questions about their relative functions, particularly in tumorigenesis.

Generation of two modified mouse alleles of the Hic1 tumor suppressor gene.

HIC1 (hypermethylated in cancer 1) is a tumor suppressor gene located on chromosome 17p13.3, a region frequently hypermethylated or deleted in human neoplasias. In mouse, Hic1 is essential for embryonic development and exerts an antitumor role in adult animals. Since Hic1-deficient mice die perinatally, we generated a conditional Hic1 null allele by flanking the Hic1-coding region by loxP sites. When crossed to animals expressing Cre recombinase in a cell-specific manner, the Hic1 conditional mice will provide new insights into the function of Hic1 in developing and mature tissues. Additionally, we used gene targeting to replace sequence-encoding amino acids 186-893 of Hic1 by citrine fluorescent protein cDNA. We demonstrate that the distribution of Hic1-citrine fusion polypeptide corresponds to the expression pattern of wild-type Hic1. Consequently, Hic1-citrine "reporter" mice can be used to monitor the activity of the Hic1 locus using citrine fluorescence.

Scavenger chemokine (CXC motif) receptor 7 (CXCR7) is a direct target gene of HIC1 (hypermethylated in cancer 1).

The tumor suppressor gene HIC1 (Hypermethylated in Cancer 1) that is epigenetically silenced in many human tumors and is essential for mammalian development encodes a sequence-specific transcriptional repressor. The few genes that have been reported to be directly regulated by HIC1 include ATOH1, FGFBP1, SIRT1, and E2F1. HIC1 is thus involved in the complex regulatory loops modulating p53-dependent and E2F1-dependent cell survival and stress responses. We performed genome-wide expression profiling analyses to identify new HIC1 target genes, using HIC1-deficient U2OS human osteosarcoma cells infected with adenoviruses expressing either HIC1 or GFP as a negative control. These studies identified several putative direct target genes, including CXCR7, a G-protein-coupled receptor recently identified as a scavenger receptor for the chemokine SDF-1/CXCL12. CXCR7 is highly expressed in human breast, lung, and prostate cancers. Using quantitative reverse transcription-PCR analyses, we demonstrated that CXCR7 was repressed in U2OS cells overexpressing HIC1. Inversely, inactivation of endogenous HIC1 by RNA interference in normal human WI38 fibroblasts results in up-regulation of CXCR7 and SIRT1. In silico analyses followed by deletion studies and luciferase reporter assays identified a functional and phylogenetically conserved HIC1-responsive element in the human CXCR7 promoter. Moreover, chromatin immunoprecipitation (ChIP) and ChIP upon ChIP experiments demonstrated that endogenous HIC1 proteins are bound together with the C-terminal binding protein corepressor to the CXCR7 and SIRT1 promoters in WI38 cells. Taken together, our results implicate the tumor suppressor HIC1 in the transcriptional regulation of the chemokine receptor CXCR7, a key player in the promotion of tumorigenesis in a wide variety of cell types.

Arginine 469 is a pivotal residue for the Hsc70-GlcNAc-binding property.

The members of the 70kDa-heat shock proteins (HSP70) family play numerous fundamental functions in the cell such as promoting the assembly of multimeric complexes or helping the correct folding of nascent proteins to take place. In numerous previous studies we demonstrated that Hsp70 and its constitutive isoform Hsc70 are endowed of a GlcNAc-binding activity. The molecular modeling of the substrate binding domain of Hsc70 and in silico docking experiments using Ser/Thr-O-GlcNAc motifs allowed to define the potential carbohydrate-recognition region and to point out the crucial position of Arg469 as an amino-acid directly interacting with the sugar moiety. We cloned a flagged Hsc70 in a pCMV.SPORT6 vector and we showed that the mutation R469A decreased the GlcNAc-binding property of the chaperone of around 70%. This is the first work reporting the localization of the GlcNAc-binding domain of a member of the HSP70 family.

An acetylation/deacetylation-SUMOylation switch through a phylogenetically conserved psiKXEP motif in the tumor suppressor HIC1 regulates transcriptional repression activity.

Tumor suppressor HIC1 (hypermethylated in cancer 1) is a gene that is essential for mammalian development, epigenetically silenced in many human tumors, and involved in a complex pathway regulating P53 tumor suppression activity. HIC1 encodes a sequence-specific transcriptional repressor containing five Krüppel-like C(2)H(2) zinc fingers and an N-terminal BTB/POZ repression domain. Here, we show that endogenous HIC1 is SUMOylated in vivo on a phylogenetically conserved lysine, K314, located in the central region which is a second repression domain. K314R mutation does not influence HIC1 subnuclear localization but significantly reduces its transcriptional repression potential, as does the mutation of the other conserved residue in the psiKXE consensus, E316A, or the overexpression of the deSUMOylase SSP3/SENP2. Furthermore, HIC1 is acetylated in vitro by P300/CBP. Strikingly, the K314R mutant is less acetylated than wild-type HIC1, suggesting that this lysine is a target for both SUMOylation and acetylation. We further show that HIC1 transcriptional repression activity is positively controlled by two types of deacetylases, SIRT1 and HDAC4, which increase the deacetylation and SUMOylation, respectively, of K314. Knockdown of endogenous SIRT1 by the transfection of short interfering RNA causes a significant loss of HIC1 SUMOylation. Thus, this dual-deacetylase complex induces either a phosphorylation-dependent acetylation-SUMOylation switch through a psiKXEXXSP motif, as previously shown for MEF2, or a phosphorylation-independent switch through a psiKXEP motif, as shown here for HIC1, since P317A mutation severely impairs HIC1 acetylation. Finally, our results demonstrate that HIC1 is a target of the class III deacetylase SIRT1 and identify a new posttranslational modification step in the P53-HIC1-SIRT1 regulatory loop.

HIC1 (Hypermethylated in Cancer 1) modulates the contractile activity of prostate stromal fibroblasts and directly regulates CXCL12 expression.

HIC1 (Hypermethylated In Cancer 1) a tumor suppressor gene located at 17p13.3, is frequently deleted or epigenetically silenced in many human tumors. HIC1 encodes a transcriptional repressor involved in various aspects of the DNA damage response and in complex regulatory loops with P53 and SIRT1. HIC1 expression in normal prostate tissues has not yet been investigated in detail. Here, we demonstrated by immunohistochemistry that detectable HIC1 expression is restricted to the stroma of both normal and tumor prostate tissues. By RT-qPCR, we showed that HIC1 is poorly expressed in all tested prostate epithelial lineage cell types: primary (PrEC), immortalized (RWPE1) or transformed androgen-dependent (LnCAP) or androgen-independent (PC3 and DU145) prostate epithelial cells. By contrast, HIC1 is strongly expressed in primary PrSMC and immortalized (WMPY-1) prostate myofibroblastic cells. HIC1 depletion in WPMY-1 cells induced decreases in α-SMA expression and contractile capability. In addition to SLUG, we identified stromal cell-derived factor 1/C-X-C motif chemokine 12 (SDF1/CXCL12) as a new HIC1 direct target-gene. Thus, our results identify HIC1 as a tumor suppressor gene which is poorly expressed in the epithelial cells targeted by the tumorigenic process. HIC1 is expressed in stromal myofibroblasts and regulates CXCL12/SDF1 expression, thereby highlighting a complex interplay mediating the tumor promoting activity of the tumor microenvironment. Our studies provide new insights into the role of HIC1 in normal prostatic epithelial-stromal interactions through direct repression of CXCL12 and new mechanistic clues on how its loss of function through promoter hypermethylation during aging could contribute to prostatic tumors.

O-GlcNAcylation Links Nutrition to the Epigenetic Downregulation of UNC5A during Colon Carcinogenesis.

While it is now accepted that nutrition can influence the epigenetic modifications occurring in colorectal cancer (CRC), the underlying mechanisms are not fully understood. Among the tumor suppressor genes frequently epigenetically downregulated in CRC, the four related genes of the UNC5 family: UNC5A, UNC5B, UNC5C and UNC5D encode dependence receptors that regulate the apoptosis/survival balance. Herein, in a mouse model of CRC, we found that the expression of UNC5A, UNC5B and UNC5C was diminished in tumors but only in mice subjected to a High Carbohydrate Diet (HCD) thus linking nutrition to their repression in CRC. O-GlcNAcylation is a nutritional sensor which has enhanced levels in CRC and regulates many cellular processes amongst epigenetics. We then investigated the putative involvement of O-GlcNAcylation in the epigenetic downregulation of the UNC5 family members. By a combination of pharmacological inhibition and RNA interference approaches coupled to RT-qPCR (Reverse Transcription-quantitative Polymerase Chain Reaction) analyses, promoter luciferase assay and CUT&RUN (Cleavage Under Target & Release Using Nuclease) experiments, we demonstrated that the O-GlcNAcylated form of the histone methyl transferase EZH2 (Enhancer of Zeste Homolog 2) represses the transcription of UNC5A in human colon cancer cells. Collectively, our data support the hypothesis that O-GlcNAcylation could represent one link between nutrition and epigenetic downregulation of key tumor suppressor genes governing colon carcinogenesis including UNC5A.

hPCL3S promotes proliferation and migration of androgen-independent prostate cancer cells.

Polycomb repressive complex 2 (PRC2) allows the deposition of H3K27me3. PRC2 facultative subunits modulate its activity and recruitment such as hPCL3/PHF19, a human ortholog of Drosophila Polycomb-like protein (PCL). These proteins contain a TUDOR domain binding H3K36me3, two PHD domains and a "Winged-helix" domain involved in GC-rich DNA binding. The human PCL3 locus encodes the full-length hPCL3L protein and a shorter isoform, hPCL3S containing the TUDOR and PHD1 domains only. In this study, we demonstrated by RT-qPCR analyses of 25 prostate tumors that hPCL3S is frequently up-regulated. In addition, hPCL3S is overexpressed in the androgen-independent DU145 and PC3 cells, but not in the androgen-dependent LNCaP cells. hPCL3S knockdown decreased the proliferation and migration of DU145 and PC3 whereas its forced expression into LNCaP increased these properties. A mutant hPCL3S unable to bind H3K36me3 (TUDOR-W50A) increased proliferation and migration of LNCaP similarly to wt hPCL3S whereas inactivation of its PHD1 domain decreased proliferation. These effects partially relied on the up-regulation of genes known to be important for the proliferation and/or migration of prostate cancer cells such as S100A16, PlexinA2, and Spondin1. Collectively, our results suggest hPCL3S as a new potential therapeutic target in castration resistant prostate cancers.

HIC1 (Hypermethylated in Cancer 1) epigenetic silencing in tumors.

HIC1 (Hypermethylated in Cancer 1), as it name implied, was originally isolated as a new candidate tumor suppressor gene located at 17p13.3 because it resides in a CpG island that is hypermethylated in many types of human cancers. HIC1 encodes a transcription factor associating an N-terminal BTB/POZ domain to five C-terminal Krüppel-like C(2)H(2) zinc finger motifs. In this review, we will begin by providing an overview of the current knowledge on HIC1 function, mainly gained from in vitro studies, as a sequence-specific transcriptional repressor interacting with a still growing range of HDAC-dependent and HDAC-independent corepressor complexes. We will then summarize the studies that have demonstrated frequent hypermethylation changes or losses of heterozygosity of the HIC1 locus in human cancers. Next, we will review animal models which have firmly established HIC1 as a bona fide tumor suppressor gene epigenetically silenced and functionally cooperating notably with p53 within a complex HIC1-p53-SIRT1 regulatory loop. Finally, we will discuss how this epigenetic inactivation of HIC1 might "addict" cancer cells to altered survival and signaling pathways or to lineage-specific transcription factors during the early stages of tumorigenesis.

HIC1 interacts with a specific subunit of SWI/SNF complexes, ARID1A/BAF250A.

HIC1, a tumor suppressor gene epigenetically silenced in many human cancers encodes a transcriptional repressor involved in regulatory loops modulating p53-dependent and E2F1-dependent cell survival and stress responses. HIC1 is also implicated in growth control since it recruits BRG1, one of the two alternative ATPases (BRM or BRG1) of SWI/SNF chromatin-remodeling complexes to repress transcription of E2F1 in quiescent fibroblasts. Here, through yeast two-hybrid screening, we identify ARID1A/BAF250A, as a new HIC1 partner. ARID1A/BAF250A is one of the two mutually exclusive ARID1-containing subunits of SWI/SNF complexes which define subsets of complexes endowed with anti-proliferative properties. Co-immunoprecipitation assays in WI38 fibroblasts and in BRG1-/- SW13 cells showed that endogenous HIC1 and ARID1A proteins interact in a BRG1-dependent manner. Furthermore, we demonstrate that HIC1 does not interact with BRM. Finally, sequential chromatin immunoprecipitation (ChIP-reChIP) experiments demonstrated that HIC1 represses E2F1 through the recruitment of anti-proliferative SWI/SNF complexes containing ARID1A.

Regulation of Polycomb Repression by O-GlcNAcylation: Linking Nutrition to Epigenetic Reprogramming in Embryonic Development and Cancer.

Epigenetic modifications are major actors of early embryogenesis and carcinogenesis and are sensitive to nutritional environment. In recent years, the nutritional sensor O-GlcNAcylation has been recognized as a key regulator of chromatin remodeling. In this review, we summarize and discuss recent clues that OGT and O-GlcNAcylation intimately regulate the functions of the Polycomb group proteins at different levels especially during Drosophila melanogaster embryonic development and in human cancer cell lines. These observations define an additional connection between nutrition and epigenetic reprogramming associated to embryonic development and cancer.