SHP and Sin3A expression are essential for adamantyl-substituted retinoid-related molecule-mediated nuclear factor-kappaB activation, c-Fos/c-Jun expression, and cellular apoptosis

Hepatocyte-specific Sox9 knockout ameliorates acute liver injury by suppressing SHP signaling and improving mitochondrial function

BACKGROUND AND AIMS

Sex determining region Y related high-mobility group box protein 9 (Sox9) is expressed in a subset of hepatocytes, and it is important for chronic liver injury. However, the roles of Sox9+ hepatocytes in response to the acute liver injury and repair are poorly understood.

METHODS

In this study, we developed the mature hepatocyte-specific Sox9 knockout mouse line and applied three acute liver injury models including PHx, CCl4 and hepatic ischemia reperfusion (IR). Huh-7 cells were subjected to treatment with hydrogen peroxide (H2O2) in order to induce cellular damage in an in vitro setting.

RESULTS

We found the positive effect of Sox9 deletion on acute liver injury repair. Small heterodimer partner (SHP) expression was highly suppressed in hepatocyte-specific Sox9 deletion mouse liver, accompanied by less cell death and more cell proliferation. However, in mice with hepatocyte-specific Sox9 deletion and SHP overexpression, we observed an opposite phenotype. In addition, the overexpression of SOX9 in H2O2-treated Huh-7 cells resulted in an increase in cytoplasmic SHP accumulation, accompanied by a reduction of SHP in the nucleus. This led to impaired mitochondrial function and subsequent cell death. Notably, both the mitochondrial dysfunction and cell damage were reversed when SHP siRNA was employed, indicating the crucial role of SHP in mediating these effects. Furthermore, we found that Sox9, as a vital transcription factor, directly bound to SHP promoter to regulate SHP transcription.

CONCLUSIONS

Overall, our findings unravel the mechanism by which hepatocyte-specific Sox9 knockout ameliorates acute liver injury via suppressing SHP signaling and improving mitochondrial function. This study may provide a new treatment strategy for acute liver injury in future.

Novel adamantyl retinoid-related molecules with POLA1 inhibitory activity

Atypical retinoids (AR) or retinoid-related molecules (RRMs) represent a promising class of antitumor compounds. Among AR, E-3-(3'-adamantan-1-yl-4'-hydroxybiphenyl-4-yl)acrylic acid (adarotene), has been extensively investigated. In the present work we report the results of our efforts to develop new adarotene-related atypical retinoids endowed also with POLA1 inhibitory activity. The effects of the synthesized compounds on cell growth were determined on a panel of human and hematological cancer cell lines. The most promising compounds showed antitumor activity against several tumor histotypes and increased cytotoxic activity against an adarotene-resistant cell line, compared to the parent molecule. The antitumor activity of a selected compound was evaluated on HT-29 human colon carcinoma and human mesothelioma (MM487) xenografts. Particularly significant was the in vivo activity of the compound as a single agent compared to adarotene and cisplatin, against pleural mesothelioma MM487. No reduction of mice body weight was observed, thus suggesting a higher tolerability with respect to the parent compound adarotene.

Small heterodimer partner (SHP) contributes to insulin resistance in cardiomyocytes

Small heterodimer partner (SHP) is an atypical nuclear receptor expressed in heart that has been shown to inhibit the hypertrophic response. Here, we assessed the role of SHP in cardiac metabolism and inflammation. Mice fed a high-fat diet (HFD) displayed glucose intolerance accompanied by increased cardiac mRNA levels of Shp. In HL-1 cardiomyocytes, SHP overexpression inhibited both basal and insulin-stimulated glucose uptake and impaired the insulin signalling pathway (evidenced by reduced AKT and AS160 phosphorylation), similar to insulin resistant cells generated by high palmitate/high insulin treatment (HP/HI; 500μM/100nM). In addition, SHP overexpression increased Socs3 mRNA and reduced IRS-1 protein levels. SHP overexpression also induced Cd36 expression (~6.2 fold; p<0.001) linking to the observed intramyocellular lipid accumulation. SHP overexpressing cells further showed altered expression of genes involved in lipid metabolism, i.e., Acaca, Acadvl or Ucp3, augmented NF-κB DNA-binding activity and induced transcripts of inflammatory genes, i.e., Il6 and Tnf mRNA (~4-fold induction, p<0.01). Alterations in metabolism and inflammation found in SHP overexpressing cells were associated with changes in the mRNA levels of Ppara (79% reduction, p<0.001) and Pparg (~58-fold induction, p<0.001). Finally, co-immunoprecipitation studies showed that SHP overexpression strongly reduced the physical interaction between PPARα and the p65 subunit of NF-κB, suggesting that dissociation of these two proteins is one of the mechanisms by which SHP initiates the inflammatory response in cardiac cells. Overall, our results suggest that SHP upregulation upon high-fat feeding leads to lipid accumulation, insulin resistance and inflammation in cardiomyocytes.

Nuclear receptor 4A (NR4A) family - orphans no more

The orphan nuclear receptors NR4A1, NR4A2 and NR4A3 are immediate early genes induced by multiple stressors, and the NR4A receptors play an important role in maintaining cellular homeostasis and disease. There is increasing evidence for the role of these receptors in metabolic, cardiovascular and neurological functions and also in inflammation and inflammatory diseases and in immune functions and cancer. Despite the similarities of NR4A1, NR4A2 and NR4A3 and their interactions with common cis-genomic elements, they exhibit unique activities and cell-/tissue-specific functions. Although endogenous ligands for NR4A receptors have not been identified, there is increasing evidence that structurally-diverse synthetic molecules can directly interact with the ligand binding domain of NR4A1 and act as agonists or antagonists, and ligands for NR4A2 and NR4A3 have also been identified. Since NR4A receptors are key factors in multiple diseases, there are opportunities for the future development of NR4A ligands for clinical applications in treating multiple health problems including metabolic, neurologic and cardiovascular diseases, other inflammatory conditions, and cancer.

Structures and regulation of non-X orphan nuclear receptors: A retinoid hypothesis

Nuclear receptors are defined as a family of ligand regulated transcription factors [1-6]. While this definition reflects that ligand binding is a key property of nuclear receptors, it is still a heated subject of debate if all the nuclear receptors (48 human members) can bind ligands (ligands referred here to both physiological and synthetic ligands). Recent studies in nuclear receptor structure biology and pharmacology have undoubtedly increased our knowledge of nuclear receptor functions and their regulation. As a result, they point to new avenues for the discovery and development of nuclear receptor regulators, including nuclear receptor ligands. Here we review the recent literature on orphan nuclear receptor structural analysis and ligand identification, particularly on the orphan nuclear receptors that do not heterodimerize with retinoid X receptors, which we term as non-X orphan receptors. We also propose a speculative "retinoid hypothesis" for a subset of non-X orphan nuclear receptors, which we hope to help shed light on orphan nuclear receptor biology and drug discovery. This article is part of a Special Issue entitled 'Orphan Nuclear Receptors'.

Insights into Orphan Nuclear Receptors as Prognostic Markers and Novel Therapeutic Targets for Breast Cancer

There is emerging evidence asserting the importance of orphan nuclear receptors (ONRs) in cancer initiation and progression. In breast cancer, there is a lot unknown about ONRs in terms of their expression profile and their transcriptional targets in the various stages of tumor progression. With the classification of breast tumors into distinct molecular subtypes, we assess ONR expression in the different breast cancer subtypes and with patient outcomes. Complementing this, we review evidence implicating ONR-dependent molecular pathways in breast cancer progression to identify candidate ONRs as potential prognostic markers and/or as therapeutic targets.

MiR-210 up-regulation inhibits proliferation and induces apoptosis in glioma cells by targeting SIN3A

Background

The aim of this study was to determine whether miR-210 can affect the apoptosis and proliferation of human U251 glioma cells from down-regulating SIN3A.

Material/Methods

The expression of miRNA-210 was detected by quantitative real-time PCR in normal brain tissue and glioma samples. The apoptosis and proliferation ability of U251 cells were analyzed by MTT and flow cytometry assay after anti-miR-210 transfection. For the regulation mechanism analysis of miR-210, TargetScan, PicTar, and microRNA were selected to predict some potential target genes of miR-210. The predicted gene was identified to be the direct and specific target gene of miR-210 by luciferase activities assay and Western blot. RNA interference technology was used to confirm that the apoptosis and proliferation effects of miR-210 were directly induced by SIN3A.

Results

The expression of miR-210 increased significantly in glioma in comparison with normal brain tissue. The silence of miR-210 expression could inhibit the proliferation of U251 cells and induce the apoptosis. Mechanism analysis revealed that SIN3A was a specific and direct target gene of miR-210. The siRNA-SIN3A could down-regulate the expression of SIN3A protein, which was up-regulated in U251 cells by anti-miR-210 transfection, and our experiments found that silence of SIN3A could inhibit the apoptosis and sharply increase the proliferation of U251 cells. The regulation effects of anti-miR-210 on apoptosis and proliferation can be reversed respectively by the expression silence of SIN3A.

Conclusions

Aberrantly expressed miR-210 regulates human U251 glioma cells apoptosis and proliferation partly through directly down-regulating SIN3A protein expression. This might offer a new potential therapeutic stratagem for glioma.

SIN3A, generally regarded as a transcriptional repressor, is required for induction of gene transcription by the aryl hydrocarbon receptor

CYP1A1 bioactivates several procarcinogens and detoxifies several xenobiotic compounds. Transcription of CYP1A1 is highly induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) via the aryl hydrocarbon receptor. We recently described an RNAi high throughput screening performed in the Hepa-1 mouse hepatoma cell line, which revealed that SIN3A is necessary for the induction of CYP1A1-dependent ethoxyresorufin-o-deethylase (EROD) enzymatic activity by TCDD. In the current studies, we sought to provide insight into the role of SIN3A in this process, particularly because studies on SIN3A have usually focused on its repressive activity on transcription. We report that ectopic expression of human SIN3A in Hepa-1 cells enhanced EROD induction by TCDD and efficiently rescued TCDD induction of EROD activity in cells treated with an siRNA to mouse SIN3A, thus validating a role for SIN3A in CYP1A1 induction. We demonstrate that SIN3A is required for TCDD induction of the CYP1A1 protein in Hepa-1 cells but not for expression of the aryl hydrocarbon receptor protein. In addition, siRNAs for SIN3A decreased TCDD-mediated induction of CYP1A1 mRNA and EROD activity in human hepatoma cell line Hep3B. We establish that TCDD treatment of Hepa-1 cells rapidly increases the degree of SIN3A binding to both the proximal promoter and enhancer of the Cyp1a1 gene and demonstrate that increased binding to the promoter also occurs in human Hep3B, HepG2, and MCF-7 cells. These studies establish that SIN3A physically interacts with the CYP1A1 gene and extends the transcriptional role of SIN3A to a gene that is very rapidly and dramatically induced.

Inhibition of IκB kinase-β and IκB kinase-α by heterocyclic adamantyl arotinoids

We recently reported on a series of retinoid-related molecules containing an adamantyl group, a.k.a. adamantyl arotinoids (AdArs), that showed significant cancer cell growth inhibitory activity and activated RXRα (NR2B1) in transient transfection assays while devoid of RAR transactivation capacity. We have now explored whether these AdArs could also bind and inhibit IKKβ, a known target that mediates the induction of apoptosis and cancer cell growth inhibition by related AdArs containing a chalcone functional group. In addition, we have prepared and evaluated novel AdArs that incorporate a central heterocyclic ring connecting the adamantyl-phenol and the carboxylic acid at the polar termini. Our results indicate that the majority of the RXRα activating compounds lacked IKKβ inhibitory activity. In contrast, the novel heterocyclic AdArs containing a thiazole or pyrazine ring linked to a benzoic acid motif were potent inhibitors of both IKKα and IKKβ, which in most cases paralleled significant growth inhibitory and apoptosis inducing activities.

Minireview: role of orphan nuclear receptors in cancer and potential as drug targets

The nuclear orphan receptors for which endogenous ligands have not been identified include nuclear receptor (NR)0B1 (adrenal hypoplasia congenita critical region on chromosome X gene), NR0B2 (small heterodimer partner), NR1D1/2 (Rev-Erbα/β), NR2C1 (testicular receptor 2), NR2C2 (testicular receptor 4), NR2E1 (tailless), NR2E3 (photoreceptor-specific NR [PNR]), NR2F1 chicken ovalbumin upstream promoter transcription factor 1 (COUP-TFI), NR2F2 (COUP-TFII), NR2F6 (v-erbA-related protein), NR4A1 (Nur77), NR4A2 (Nurr1), NR4A3 (Nor1), and NR6A1 (GCNF). These receptors play essential roles in development, cellular homeostasis, and disease including cancer where over- or underexpression of some receptors has prognostic significance for patient survival. Results of receptor knockdown or overexpression in vivo and in cancer cell lines demonstrate that orphan receptors exhibit tumor-specific pro-oncogenic or tumor suppressor-like activity. For example, COUP-TFII expression is both a positive (ovarian) and negative (prostate and breast) prognostic factor for cancer patients; in contrast, the prognostic activity of adrenal hypoplasia congenita critical region on chromosome X gene for the same tumors is the inverse of COUP-TFII. Functional studies show that Nur77 is tumor suppressor like in acute leukemia, whereas silencing Nur77 in pancreatic, colon, lung, lymphoma, melanoma, cervical, ovarian, gastric, and some breast cancer cell lines induces one or more of several responses including growth inhibition and decreased survival, migration, and invasion. Although endogenous ligands for the orphan receptors have not been identified, there is increasing evidence that different structural classes of compounds activate, inactivate, and directly bind several orphan receptors. Thus, the screening and development of selective orphan receptor modulators will have important clinical applications as novel mechanism-based agents for treating cancer patients overexpressing one or more orphan receptors and also for combined drug therapies.

Adamantyl arotinoids that inhibit IκB kinase α and IκB kinase β

A series of analogues of the adamantyl arotinoid (AdAr) chalcone MX781 with halogenated benzyloxy substituents at C2' and heterocyclic derivatives replacing the chalcone group were found to inhibit IκBα kinase α (IKKα) and IκBα kinase β (IKKβ) activities. The growth inhibitory capacity of some analogues against Jurkat T cells as well as prostate carcinoma (PC-3) and chronic myelogenous leukemia (K562) cells, which contain elevated basal IKK activity, correlates with the induction of apoptosis and increased inhibition of recombinant IKKα and IKKβ in vitro, pointing toward inhibition of IKK/NFκB signaling as the most likely target of the anticancer activities of these AdArs. While the chalcone functional group present in many dietary compounds has been shown to mediate interactions with IKKβ via Michael addition with cysteine residues, AdArs containing a five-membered heterocyclic ring (isoxazoles and pyrazoles) in place of the chalcone of the parent system are potent inhibitors of IKKs as well, which suggests that other mechanisms for inhibition exist that do not depend on the presence of a reactive α,β-unsaturated ketone.

Adamantyl Retinoid-Related Molecules Induce Apoptosis in Pancreatic Cancer Cells by Inhibiting IGF-1R and Wnt/β-Catenin Pathways

Pancreatic carcinoma has a dismal prognosis as it often presents as locally advanced or metastatic. We have found that exposure to adamantyl-substituted retinoid-related (ARR) compounds 3-Cl-AHPC and AHP3 resulted in growth inhibition and apoptosis induction in PANC-1, Capan-2, and MiaPaCa-2 pancreatic cancer cell lines. In addition, AHP3 and 3-Cl-AHPC inhibited growth and induced apoptosis in spheres derived from the CD44⁺/CD24⁺ (CD133⁺/EpCAM⁺) stem-like cell population isolated from the pancreatic cancer cell lines. 3-Cl-AHPC-induced apoptosis was preceded by decreasing expression of IGF-1R, cyclin D1, β-catenin, and activated Notch-1 in the pancreatic cancer cell lines. Decreased IGF-1R expression inhibited PANC-1 proliferation, enhanced 3-Cl-AHPC-mediated apoptosis, and significantly decreased sphere formation. 3-Cl-AHPC inhibited the Wnt/β-catenin pathway as indicated by decreased β-catenin nuclear localization and inhibited Wnt/β-catenin activation of transcription factor TCF/LEF. Knockdown of β-catenin using sh-RNA also induced apoptosis and inhibited growth in pancreatic cancer cells. Thus, 3-Cl-AHPC and AHP3 induce apoptosis in pancreatic cancer cells and cancer stem-like cells and may serve as an important potential therapeutic agent in the treatment of pancreatic cancer.

Ligand-dependent regulation of the activity of the orphan nuclear receptor, small heterodimer partner (SHP), in the repression of bile acid biosynthetic CYP7A1 and CYP8B1 genes

Small heterodimer partner (SHP) plays important roles in diverse biological processes by directly interacting with transcription factors and inhibiting their activities. SHP has been designated an orphan nuclear receptor, but whether its activity can be modulated by ligands has been a long-standing question. Recently, retinoid-related molecules, including 4-[3-(1-adamantyl)-4-hydroxyphenyl]-3-chlorocinnamic acid (3Cl-AHPC), were shown to bind to SHP and enhance apoptosis. We have examined whether 3Cl-AHPC acts as an agonist and increases SHP activity in the repression of bile acid biosynthetic CYP7A1 and CYP8B1 genes and delineated the underlying mechanisms. Contrary to this expectation, micromolar concentrations of 3Cl-AHPC increased CYP7A1 expression but indirectly via p38 kinase signaling. Nanomolar concentrations, however, repressed CYP7A1 expression and decreased bile acid levels in HepG2 cells, and little repression was observed when SHP was down-regulated by small hairpin RNA. Mechanistic studies revealed that 3Cl-AHPC bound to SHP, increased the interaction of SHP with liver receptor homologue (LRH)-1, a hepatic activator for CYP7A1 and CYP8B1 genes, and with repressive cofactors, Brahma, mammalian Sin3a, and histone deacetylase-1, and, subsequently, increased the occupancy of SHP and these cofactors at the promoters. Mutation of Leu-100, predicted to contact 3Cl-AHPC within the SHP ligand binding pocket by molecular modeling, severely impaired the increased interaction with LRH-1, and repression of LRH-1 activity mediated by 3Cl-AHPC. 3Cl-AHPC repressed SHP metabolic target genes in a gene-specific manner in human primary hepatocytes and HepG2 cells. These data suggest that SHP may act as a ligand-regulated receptor in metabolic pathways. Modulation of SHP activity by synthetic ligands may be a useful therapeutic strategy.

Nuclear receptor small heterodimer partner in apoptosis signaling and liver cancer

Small heterodimer partner (SHP, NR0B2) is a unique orphan nuclear receptor that contains the dimerization and a putative ligand-binding domain, but lacks the conserved DNA binding domain. SHP exerts its physiological function as an inhibitor of gene transcription through physical interaction with multiple nuclear receptors and transcriptional factors. SHP is a critical transcriptional regulator affecting diverse biological functions, including bile acid, cholesterol and lipid metabolism, glucose and energy homeostasis, and reproductive biology. Recently, we and others have demonstrated that SHP is an epigenetically regulated transcriptional repressor that suppresses the development of liver cancer. In this review, we summarize recent major findings regarding the role of SHP in cell proliferation, apoptosis, and DNA methylation, and discuss recent progress in understanding the function of SHP as a tumor suppressor in the development of liver cancer. Future study will be focused on identifying SHP associated novel pro-oncogenes and anti-oncogenes in liver cancer progression and applying the knowledge gained on SHP in liver cancer prevention, diagnosis and treatment.

Maximal adamantyl-substituted retinoid-related molecule-induced apoptosis requires NF-κB noncanonical and canonical pathway activation

NF-κB transcription factors have a critical role in regulating cell survival and apoptosis. We have previously shown that 4-(3-Cl-(1-adamantyl)-4-hydroxyphenyl)-3-chlorocinnamic acid (3-Cl-AHPC), an adamantyl-substituted retinoid molecule, induced apoptosis and required NF-κB activation in prostate and breast carcinoma cells. Here, we show that 3-Cl-AHPC activated both IκB kinase (IKK)α and IKKβ with subsequent activation of the canonical and noncanonical NF-κB pathways in the human breast carcinoma and leukemia cell lines. 3-Cl-AHPC-mediated activation of the NF-κB canonical pathway occurred within 6 h, whereas maximal activation of the NF-κB noncanonical pathway required 48 h. Knockout of IKKα or IKKβ expression in mouse embryonic fibroblast cells and knockdown of IKKα or IKKβ in MDA-MB-468 cells resulted in the inhibition of 3-Cl-AHPC-mediated apoptosis, indicating that activation of canonical and noncanonical pathways are required for maximal 3-Cl-AHPC-mediated apoptosis. 3-Cl-AHPC activation of the noncanonical pathway was preceded by caspase-mediated decrease in the E3-ligase c-IAP1 with subsequent stabilization of NF-κB-inducing kinase (NIK) expression, increased binding of NIK by TRAF3, activation of IKKα, and the resultant increased levels of RelB and p52. Increased expression of c-IAP1 blocked 3-Cl-AHPC-mediated stabilization of NIK levels and 3-Cl-AHPC-mediated apoptosis. Cdc37 expression was required for activation of IKKα and IKKβ by 3-Cl-AHPC. These findings suggest that NF-κB pathways have an important role in 3-Cl-AHPC-mediated apoptosis.

Adamantyl-substituted retinoid-related molecules induce apoptosis in human acute myelogenous leukemia cells

The adamantyl-substituted retinoid-related (ARR) compounds 3-Cl-AHPC and AHP3 induce apoptosis in vitro and in vivo in a newly established human acute myelogenous leukemia (AML) cell line, FFMA-AML, and in the established TF(v-SRC) AML cell line. FFMA-AML and TF(v-SRC) cells displayed resistance to apoptosis mediated by the standard retinoids (including trans-retinoic acid, 9-cis-retinoic acid, and the synthetic retinoid TTNPB) but showed sensitivity to apoptosis mediated by 3-Cl-AHPC- and AHP3 in vitro and in vivo as documented by poly(ADP-ribose) polymerase (PARP) cleavage and apoptosis terminal deoxyribonucleotidyl transferase-mediated dUTP nick end labeling assay. 3-Cl-AHPC or AHP3 exposure in vitro resulted in decreased expression of the antiapoptotic proteins (cellular inhibitor of apoptosis 1, X-linked inhibitor of apoptosis protein) and phospho-Bad and activated the NF-κB canonical pathway. A significant prolongation of survival was observed both in nonobese diabetic severe combined immunodeficient mice carrying FFMA-AML cells and treated with either 3-Cl-AHPC or AHP3 and in severe combined immunodeficient mice carrying TF(v-SRC) cells and treated with AHP3. We have previously shown that ARRs bind to the orphan nuclear receptor small heterodimer partner (SHP) and that the expression of SHP is required for ARR-mediated apoptosis. Induced loss of SHP in these AML cells blocked 3-Cl-AHPC- and AHP3-mediated induction of apoptosis. These results support the further development of 3-Cl-AHPC and AHP3 as potential therapeutic agents in the treatment of AML patients.

Retinoid pathway and cancer therapeutics

The retinoids are a class of compounds that are structurally related to vitamin A. Retinoic acid, which is the active metabolite of retinol, regulates a wide range of biological processes including development, differentiation, proliferation, and apoptosis. Retinoids exert their effects through a variety of binding proteins including cellular retinol-binding protein (CRBP), retinol-binding proteins (RBP), cellular retinoic acid-binding protein (CRABP), and nuclear receptors i.e. retinoic acid receptor (RAR) and retinoid x receptor (RXR). Because of the pleiotropic effects of retinoids, understanding the function of these binding proteins and nuclear receptors assists us in developing compounds that have specific effects. This review summarizes our current understanding of how retinoids are processed and act with an emphasis on the application of retinoids in cancer treatment and prevention.

Role of nuclear receptor SHP in metabolism and cancer

Small heterodimer partner (SHP, NR0B2) is a unique member of the nuclear receptor (NR) superfamily that contains the dimerization and ligand-binding domain found in other family members, but lacks the conserved DNA-binding domain. The ability of SHP to bind directly to multiple NRs is crucial for its physiological function as a transcriptional inhibitor of gene expression. A wide variety of interacting partners for SHP have been identified, indicating the potential for SHP to regulate an array of genes in different biological pathways. In this review, we summarize studies concerning the structure and target genes of SHP and discuss recent progress in understanding the function of SHP in bile acid, cholesterol, triglyceride, glucose, and drug metabolism. In addition, we review the regulatory role of SHP in microRNA (miRNA) regulation, liver fibrosis and cancer progression. The fact that SHP controls a complex set of genes in multiple metabolic pathways suggests the intriguing possibility of developing new therapeutics for metabolic diseases, including fatty liver, dyslipidemia and obesity, by regulating SHP with small molecules. To achieve this goal, more progress regarding SHP ligands and protein structure will be required. Besides its metabolic regulatory function, studies by us and other groups provide strong evidence that SHP plays a critical role in the development of cancer, particularly liver and breast cancer. An increased understanding of the fundamental mechanisms by which SHP regulates the development of cancers will be critical in applying knowledge of SHP in diagnostic, therapeutic or preventive strategies for specific cancers. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Sumoylation of ING2 regulates the transcription mediated by Sin3A

ING2 (inhibitor of growth 2) is a candidate tumor-suppressor gene involved in cell cycle control, apoptosis and senescence. Although the functions of ING2 within the chromatin remodeling complex Sin3A/histone deacetylase (HDAC) and in the p53 pathway have been described, how ING2 itself is regulated remains unknown. In this study we report for the first time that ING2 can be sumoylated by small ubiquitin-like modifier 1 (SUMO1) on lysine 195 both in vitro and in vivo. Strikingly, ING2 sumoylation enhances its association with Sin3a. We provide evidences that ING2 can bind to the promoter of genes to mediate their expression and that sumoylation of ING2 is required for this binding to some of these genes. Among them, we identified the gene TMEM71 (transmembrane protein 71), whose expression is regulated by ING2 sumoylation. ING2 must be sumoylated to bind to the promoter of TMEM71 and to recruit the Sin3A chromatin-modifying complex to this promoter, in order to regulate TMEM71 transcription. Hence, sumoylation of ING2 enhances its binding to the Sin3A/HDAC complex and is required to regulate gene transcriptions.