5-azaC treatment enhances expression of transforming growth factor-beta receptors through down-regulation of Sp3

Transforming growth factor-β suppresses metastasis in a subset of human colon carcinoma cells

Background

TGFβ signaling has typically been associated with suppression of tumor initiation while the role it plays in metastasis is generally associated with progression of malignancy. However, we present evidence here for an anti-metastatic role of TGFβ signaling.

Methods

To test the importance of TGFβ signaling to cell survival and metastasis we compared human colon carcinoma cell lines that are either non-tumorigenic with TGFβ response (FET), or tumorigenic with TGFβ response (FETα) or tumorigenic with abrogated TGFβ response via introduction of dominant negative TGFβRII (FETα/DN) and their ability to metastasize. Metastatic competency was assessed by orthotopic transplantation. Metastatic colony formation was assessed histologically and by imaging.

Results

Abrogation of TGFβ signaling through introduction of a dominant negative TGFβ receptor II (TGFβRII) in non-metastatic FETα human colon cancer cells permits metastasis to distal organs, but importantly does not reduce invasive behavior at the primary site. Loss of TGFβ signaling in FETα-DN cells generated enhanced cell survival capabilities in response to cellular stress in vitro. We show that enhanced cellular survival is associated with increased AKT phosphorylation and cytoplasmic expression of inhibitor of apoptosis (IAP) family members (survivin and XIAP) that elicit a cytoprotective effect through inhibition of caspases in response to stress. To confirm that TGFβ signaling is a metastasis suppressor, we rescued TGFβ signaling in CBS metastatic colon cancer cells that had lost TGFβ receptor expression due to epigenetic repression. Restoration of TGFβ signaling resulted in the inhibition of metastatic colony formation in distal organs by these cells. These results indicate that TGFβ signaling has an important role in the suppression of metastatic potential in tumors that have already progressed to the stage of an invasive carcinoma.

Conclusions

The observations presented here indicate a metastasis suppressor role for TGFβ signaling in human colon cancer cells. This raises the concern that therapies targeting inhibition of TGFβ signaling may be imprudent in some patient populations with residual TGFβ tumor suppressor activity.

Histone deacetylase inhibitor belinostat represses survivin expression through reactivation of transforming growth factor beta (TGFbeta) receptor II leading to cancer cell death

Survivin is a cancer-associated gene that functions to promote cell survival, cell division, and angiogenesis and is a marker of poor prognosis. Histone deacetylase inhibitors induce apoptosis and re-expression of epigenetically silenced tumor suppressor genes in cancer cells. In association with increased expression of the tumor suppressor gene transforming growth factor β receptor II (TGFβRII) induced by the histone deacetylase inhibitor belinostat, we observed repressed survivin expression. We investigated the molecular mechanisms involved in survivin down-regulation by belinostat downstream of reactivation of TGFβ signaling. We identified two mechanisms. At early time points, survivin protein half-life was decreased with its proteasomal degradation. We observed that belinostat activated protein kinase A at early time points in a TGFβ signaling-dependent mechanism. After longer times (48 h), survivin mRNA was also decreased by belinostat. We made the novel observation that belinostat mediated cell death through the TGFβ/protein kinase A signaling pathway. Induction of TGFβRII with concomitant survivin repression may represent a significant mechanism in the anticancer effects of this drug. Therefore, patient populations exhibiting high survivin expression with epigenetically silenced TGFβRII might potentially benefit from the use of this histone deacetylase inhibitor.

Identification of a novel TGFβ/PKA signaling transduceome in mediating control of cell survival and metastasis in colon cancer

Background

Understanding drivers for metastasis in human cancer is important for potential development of therapies to treat metastases. The role of loss of TGFβ tumor suppressor activities in the metastatic process is essentially unknown.

Methodology/Principal Findings

Utilizing in vitro and in vivo techniques, we have shown that loss of TGFβ tumor suppressor signaling is necessary to allow the last step of the metastatic process - colonization of the metastatic site. This work demonstrates for the first time that TGFβ receptor reconstitution leads to decreased metastatic colonization. Moreover, we have identified a novel TGFβ/PKA tumor suppressor pathway that acts directly on a known cell survival mechanism that responds to stress with the survivin/XIAP dependent inhibition of caspases that effect apoptosis. The linkage between the TGFβ/PKA transduceome signaling and control of metastasis through induction of cell death was shown by TGFβ receptor restoration with reactivation of the TGFβ/PKA pathway in receptor deficient metastatic colon cancer cells leading to control of aberrant cell survival.

Conclusion/Significance

This work impacts our understanding of the possible mechanisms that are critical to the growth and maintenance of metastases as well as understanding of a novel TGFβ function as a metastatic suppressor. These results raise the possibility that regeneration of attenuated TGFβ signaling would be an effective target in the treatment of metastasis. Our work indicates the clinical potential for developing anti-metastasis therapy based on inhibition of this very important aberrant cell survival mechanism by the multifaceted TGFβ/PKA transduceome induced pathway. Development of effective treatments for metastatic disease is a pressing need since metastases are the major cause of death in solid tumors.

Enforced SOCS1 and SOCS3 expression attenuates Lck-mediated cellular transformation

Lck is an Src family protein tyrosine kinase with predominant T cell expression. Aberrant expression or activation of Lck kinase has been reported in both lymphoid and non-lymphoid malignancies. We showed previously that the signal transduction pathway involving Janus kinase (JAK) and signal transducer and activator of transcription (STAT) is constitutively activated and contributes to Lck-mediated oncogenesis. Under normal physiological conditions, active STAT proteins induce the expression of suppressor of cytokine signaling (SOCS) family proteins to inhibit further JAK/STAT signaling. It is not fully understood whether and how SOCS-mediated negative feedback control is dysregulated in Lck-transformed cells. Here we report that two SOCS family members, SOCS1 and SOCS3, are not expressed in Lck-transformed LSTRA leukemia. While SOCS1 gene is silenced by DNA hypermethylation, loss of SOCS3 expression is through a mechanism independent of epigenetic silencing by DNA methylation. Furthermore, ectopic expression of SOCS1 or SOCS3 leads to reduced cell proliferation and increased apoptosis in Lck-transformed cells. This is consistent with the attenuation of Lck kinase activity by exogenous SOCS1 or SOCS3 expression. Downstream STAT5 activity is also inhibited as shown by reduced STAT5 tyrosine phosphorylation and in vitro DNA binding. All together, our data highlight the importance of silencing multiple SOCS genes in tumorigenesis and support the roles of SOCS1 and SOCS3 as tumor suppressors toward oncogenic Lck kinase.

Mechanisms of decreased expression of transforming growth factor-beta receptor type I at late stages of HPV16-mediated transformation

Transforming growth factor-beta (TGF-beta) signaling is disrupted in many cancers, including cervical cancer, leading to TGF-beta resistance. Although initially sensitive, human papillomavirus type 16 (HPV16) immortalized human keratinocytes (HKc/HPV16) become increasingly resistant to the growth inhibitory effects of TGF-beta during in vitro progression to a differentiation resistant phenotype (HKc/DR). We have previously shown that loss of TGF-beta sensitivity in HKc/DR is attributed to decreased expression of TGF-beta receptor type I (TGF-beta RI), while the levels of TGF-beta receptor type II (TGF-beta RII) remain unchanged. The present study explored molecular mechanisms leading to reduced TGF-beta RI expression in HKc/DR. Using TGF-beta RI and TGF-beta RII promoter reporter constructs, we determined that acute expression of the HPV16 oncogenes E6 and E7 decreased the promoter activity of TGF-beta RI and TGF-beta RII by about 50%. However, promoter activity of TGF-beta RI is decreased to a greater extent than TGF-beta RII as HKc/HPV16 progress to HKc/DR. Reduced TGF-beta RI expression in HKc/DR was found not to be linked to mutations within the TGF-beta RI promoter or to promoter methylation. Electrophoretic mobility shift and supershift assays using probes encompassing Sp1 binding sites in the TGF-beta RI promoter found no changes between HKc/HPV16 and HKc/DR in binding of the transcription factors Sp1 or Sp3 to the probes. Also, Western blots determined that protein levels of Sp1 and Sp3 remain relatively unchanged between HKc/HPV16 and HKc/DR. Overall, these results demonstrate that mutations in or hypermethylation of the TGF-beta RI promoter, along with altered levels of Sp1 or Sp3, are not responsible for the reduced expression of TGF-beta RI we observe in HKc/DR. Rather the HPV16 oncogenes E6 and E7 themselves exhibit an inhibitory effect on TGF-beta receptor promoter activity.

Epigenetic Targeting of Transforming Growth Factor β Receptor II and Implications for Cancer Therapy

The transforming growth factor (TGF) β signaling pathway is involved in many cellular processes including proliferation, differentiation, adhesion, motility and apoptosis. The loss of TGFβ signaling occurs early in carcinogenesis and its loss contributes to tumor progression. The loss of TGFβ responsiveness frequently occurs at the level of the TGFβ type II receptor (TGFβRII) which has been identified as a tumor suppressor gene (TSG). In keeping with its TSG role, the loss of TGFβRII expression is frequently associated with high tumor grade and poor patient prognosis. Reintroduction of TGFβRII into tumor cell lines results in growth suppression. Mutational loss of TGFβRII has been characterized, particularly in a subset of colon cancers with DNA repair enzyme defects. However, the most frequent cause of TGFβRII silencing is through epigenetic mechanisms. Therefore, re-expression of TGFβRII by use of epigenetic therapies represents a potential therapeutic approach to utilizing the growth suppressive effects of the TGFβ signaling pathway. However, the restoration of TGFβ signaling in cancer treatment is challenging because in late stage disease, TGFβ is a pro-metastatic factor. This effect is associated with increased expression of the TGFβ ligand. In this Review, we discuss the mechanisms associated with TGFβRII silencing in cancer and the potential usefulness of histone deacetylase (HDAC) inhibitors in reversing this effect. The use of HDAC inhibitors may provide a unique opportunity to restore TGFβRII expression in tumors as their pleiotropic effects antagonize many of the cellular processes, which mediate the pro-metastatic effects associated with increased TGFβ expression.

The human receptor tyrosine kinase Axl gene--promoter characterization and regulation of constitutive expression by Sp1, Sp3 and CpG methylation

Axl is a receptor tyrosine kinase which promotes anti-apoptosis, mitogenesis, invasion, angiogenesis and metastasis, and is highly expressed in cancers. However, the transcriptional regulation of this important gene has never been characterized. The present study was initiated to characterize the promoter, cis-acting elements and promoter methylation driving expression of Axl. The 2.4 kb sequence upstream of the translational start site, and sequential 5'-deletions were cloned and revealed a minimal GC-rich region (-556 to +7) to be sufficient for basal Axl promoter activity in Rko, HCT116 and HeLa cells. Within this minimal region, five Sp (specificity protein)-binding sites were identified. Two sites (Sp a and Sp b) proximal to the translation start site were indispensable for Axl promoter activity, whereas mutation of three additional upstream motifs (Sp c, Sp d and Sp e) was of additional relevance. Gel-shift assays and chromatin immunoprecipitation identified that Sp1 and Sp3 bound to all five motifs, and mutation of all motifs abolished binding. Mithramycin, which inhibits binding of Sp factors to GC-rich sites, dramatically reduced Axl promoter activity and Axl, Sp1 and Sp3 expression. In Drosophila Schneider SL2-cells, exogenous expression of Sp1/Sp3 increased Axl promoter activity. Use of Sp1/Sp3 siRNAs (small interfering RNAs) significantly reduced Axl promoter activity and protein levels in Rko and HeLa cells. Methylation-bisulfite sequencing detected methylated CpG sites within three Sp motifs (Sp a, Sp b and Sp c) and GC-rich flanking sequences, and demethylation by 5-aza-2'-deoxycytidine up-regulated Axl and Sp3 expression in low-Axl-expressing Colo206f/WiDr cells, but not in high-Axl-expressing Rko cells. The results of the present study suggest that Axl gene expression in cancer cells is (1) constitutively driven by Sp1/Sp3 bound to five core promoter motifs, and (2) restricted by methylation within/around Sp-binding sites. This might enhance the understanding and treatment of essential mechanisms associated with cancer and other diseases.

Identification of quantitative trait loci affecting murine long bone length in a two-generation intercross of LG/J and SM/J Mice

INTRODUCTION

Study of mutations with large phenotypic effects has allowed the identification of key players in skeletal development. However, the molecular nature of variation in large, phenotypically normal populations tends to be characterized by smaller phenotypic effects that remain undefined.

MATERIALS AND METHODS

We use interval mapping and quantitative trait locus (QTL) mapping techniques in the combined F2-F3 populations (n = 2111) of an LG/J x SM/J mouse intercross to detect QTLs associated with the lengths of the humerus, ulna, femur, and tibia.

RESULTS

Seventy individual trait QTLs affecting long bone lengths were detected, with several chromosomes harboring multiple QTLs. The genetic architecture suggests mainly small, additive effects on long bone length, with roughly one third of the QTLs displaying dominance. Sex interactions were common, and four sex-specific QTLs were observed. Pleiotropy could not be rejected for most of the QTLs identified. Thirty-one epistatic interactions were detected, almost all affecting regions including or immediately adjacent to QTLs.

CONCLUSIONS

A complex regulatory network with many gene interactions modulates bone growth, possibly with integrated skeletal modules that allow fine-tuning of developmental processes present. Candidate genes in the QTL CIs include many genes known to affect endochondral bone growth and genes that have not yet been associated with bone growth or body size but have a strong potential to influence these traits.

Concordant epigenetic silencing of transforming growth factor-beta signaling pathway genes occurs early in breast carcinogenesis

Human mammary epithelial cells (HMEC) grown under standard cell culture conditions enter a growth phase referred to as selection, but a subpopulation is able to escape from arrest and continue to proliferate. These cells, called post-selection or variant HMECs, may be derived from progenitor cells found in normal mammary epithelium that subsequently acquire premalignant lesions, including p16(INK4A) promoter hypermethylation. Epigenetic silencing of tumor suppressor genes through DNA methylation and histone modification is an early event in tumorigenesis. A major challenge is to find genes or gene pathways that are commonly silenced to provide early epigenetic diagnostic and therapeutic cancer targets. To identify very early epigenetic events that occur in breast cancer, we used microarrays to screen for gene pathways that were suppressed in post-selection HMECs but reactivated after treatment with the demethylation agent 5-aza-2'-deoxycytidine. We found that several members of the transforming growth factor beta (TGF-beta) signaling pathway were consistently down-regulated in the post-selection HMEC populations, and this was associated with a marked decrease in Smad4 nuclear staining. Gene suppression was not associated with DNA methylation but with chromatin remodeling, involving a decrease in histone H3 lysine 27 trimethylation and an increase in histone H3 lysine 9 dimethylation and deacetylation. These results show for the first time that TGF-beta2, its receptors TGF-beta R1 and TGF-beta R2, and activator thrombospondin-1 are concordantly suppressed early in breast carcinogenesis by histone modifications and indicate that the TGF-beta signaling pathway is a novel target for gene activation by epigenetic therapy.

NFkappaB mediates IL-1beta-induced down-regulation of TbetaRII through the modulation of Sp3 expression

We previously showed that interleukin-1beta (IL-1beta) down-regulation of type II TGFbeta receptor (TbetaRII) involves NFkappaB pathway and requires de novo synthesis of a yet unknown protein. Here, we demonstrate that this effect is mediated through Sp1 site located at position -25 of human TbetaRII promoter. Inhibition of transcription factors binding (decoy oligonucleotides or mithramycin) abolished IL-1beta effect. EMSA and ChIP revealed that this treatment induced Sp3 binding to cis-sequence whereby IL-1beta exerts its transcriptional effects whereas it decreased that of Sp1. Moreover, although the cytokine did not modulate Sp1 expression, it increased that of Sp3 via NFkappaB pathway. Experiments of gain and loss of function clearly showed that Sp3 inhibited TbetaRII expression whereas its silencing abolished IL-1beta effect. In addition, both Sp1 and Sp3 were found to interact with NFkappaB, which therefore may indirectly interact with TbetaRII pro moter. Altogether, these data suggest that IL-1beta decreases TbetaRII expression by inducing Sp3 via NFkappaB and its binding on core promote at the expense of Sp1, which could explain the loss of cell responsiveness in certain conditions. These findings bring new insights in the knowledge of the interference between two antagonistic transduction pathways involved in multiple physiopathological processes.

Coordinated changes in DNA methylation and histone modifications regulate silencing/derepression of luteinizing hormone receptor gene transcription

We have previously demonstrated that transcription of the luteinizing hormone receptor (LHR) gene is subject to repression by histone deacetylation at its promoter region, where a histone deacetylase (HDAC)/mSin3A complex is anchored at a proximal Sp1 site. The present studies have shown that epigenetic silencing and activation of the LHR gene is achieved through coordinated regulation at both the histone and DNA levels. The HDAC inhibitor trichostatin A (TSA) evoked robust but significantly lower activation of the LHR gene in JAR than in MCF-7 cells. This effect was localized to the 176-bp promoter region, which is highly methylated in JAR and lightly methylated in MCF-7 cells. Consequently, TSA and the DNA demethylating reagent 5-azacytidine (5-AzaC) caused marked synergistic activation of the LHR gene in JAR but not in MCF-7 cells. Multiple site-specific lysine acetylation of H3/H4 is associated with such LHR gene activation. Methylation or acetylation of H3 at K9 is present at the silenced and derepressed LHR promoter, respectively. While DNA methylation levels did not affect the histone code of the LHR gene promoter, demethylation of the promoter CpG sites was necessary for maximal stimulation of this gene. Mechanistically, the combined actions of TSA and 5-AzaC, but not either 5-AzaC or TSA alone, resulted in complete demethylation of the LHR gene promoter in JAR cells. Release of the repressive HDAC/mSin3A complex from the LHR gene promoter in both cell types required both TSA-induced changes of histone modifications and, concurrently, a demethylated promoter. Also, Dnmt1 was largely dissociated from the LHR gene promoter in the presence of TSA or TSA plus 5-AzaC, and binding of MBD2 in JAR cells was diminished upon conversion of the promoter to a demethylated state. Such changes induced a more permissive chromatin where recruitment of polymerase II and TFIIB to the promoter was significantly increased. The activated state of the LHR gene induced by TSA and 5-AzaC in JAR and MCF-7 cells was observed basally in LHR-expressing PLC cells, in which the promoter is unmethylated and associated with hyperacetylated histones. Consequently, PLC cells are unresponsive to drug treatment. These findings have elucidated a regulatory mechanism whereby concurrent dissociation of repressors and association of activators and basal transcriptional components, resulting from coordinated histone hyperacetylation and DNA demethylation, lead to derepression of the LHR gene expression.

Down-Regulation of Overexpressed Sp1 Protein in Human Fibrosarcoma Cell Lines Inhibits Tumor Formation

Sp1 is a transcription factor for many genes, including genes involved in tumorigenesis. We found that human fibroblast cells malignantly transformed in culture by a carcinogen or by stable transfection of an oncogene express Sp1 at 8-fold to 18-fold higher levels than their parental cells. These cell lines form fibrosarcomas in athymic mice with a very short latency, and the cells from the tumors express the same high levels of Sp1. Similar high levels of Sp1 were found in the patient-derived fibrosarcoma cell lines tested, and in the tumors formed in athymic mice by these cell lines. To investigate the role of overexpression of Sp1 in malignant transformation of human fibroblasts, we transfected an Sp1 U1snRNA/Ribozyme into two human cell lines, malignantly transformed in culture by a carcinogen or overexpression of an oncogene, and into a patient-derived fibrosarcoma cell line. The level of expression of Sp1 in these transfected cell lines was reduced to near normal. The cells regained the spindle-shaped morphology and exhibited increased apoptosis and decreased expression of several genes linked to cancer, i.e., epithelial growth factor receptor, urokinase plasminogen activator, urokinase plasminogen activator receptor, and vascular endothelial growth factor. When injected into athymic mice, these cell lines with near normal levels of Sp1 failed to form tumors or did so only at a greatly reduced frequency and with a much longer latency. These data indicate that overexpression of Sp1 plays a causal role in malignant transformation of human fibroblasts and suggest that for cancers in which it is overexpressed, Sp1 constitutes a target for therapy.

5-aza-2'-deoxycytidine upregulates caspase-9 expression cooperating with p53-induced apoptosis in human lung cancer cells

Treating lung cancer cell lines using low-dose 5-aza-2'-deoxycytidine (DAC) caused an accumulation of procaspase-9 through mRNA upregulation, but the cells did not undergo apoptosis. However, when cells were treated with DAC and infected with a low dose of a recombinant wild-type p53 adenovirus vector (Ad-p53), a synergistic growth inhibitory effect was observed. Combination treatment induced Apaf-1 and procaspase-9 expression in which cytochrome c releases by Ad-p53 triggered the mitochondrial pathway of apoptosis. Selective blockage of caspase-9 activities by Z-LEHD-FMK completely attenuated DAC-induced enhancement of apoptosis mediated by Ad-p53 infection, and ectopic overexpression of procaspase-9 sensitized cells to Ad-p53-induced apoptosis in p53-null cells. In addition, DAC sensitized lung cancer cells to cisplatin and paclitaxel. Induction of the mitochondrial pathway of apoptosis using a slightly toxic dose of DAC may therefore be a strategy for treating lung cancer, and DAC treatment may have clinical implications when combined with chemotherapy or apoptosis-inducing gene therapy.

Restoration of transforming growth factor-beta signaling through receptor RI induction by histone deacetylase activity inhibition in breast cancer cells

The loss of transforming growth factor-beta (TGF-beta) response due to the dysregulation of TGF-beta receptors type I (RI) and type II (RII) is well known for its contribution to oncogenesis. Estrogen receptor-expressing breast cancer cells are refractory to TGF-beta-mediated growth control because of the reduced expression of TGF-beta receptors. Although RII is required for the binding of TGF-beta to RI, RI is responsible for directly transducing TGF-beta signals through the Smad protein family. Treatment of estrogen receptor-expressing MCF-7L and ZR75 breast cancer cells with the histone deacetylase (HDAC) inhibitor suberoylanilide hydroxamic acid (SAHA) led to a dramatic induction of RI. Accumulation of acetylated histones H3 and H4 was observed in the SAHA-treated cells. Chromatin immunoprecipitation analysis followed by PCR with RI promoter-specific primers indicated an accumulation of acetylated histones in chromatin associated with the RI gene, suggesting that histone deacetylation was involved in the transcriptional inactivation of RI. SAHA treatment stimulated RI promoter activity through the inhibition of Sp1/Sp3-associated HDAC activity. Histone acetyltransferase p300 stimulated RI promoter activity, thus further confirming the involvement of HDAC activity in the transcriptional repression of RI. Significantly, SAHA-mediated RI regeneration restored the TGF-beta response in breast cancer cells.

DNA methyltransferase 1 knock down induces gene expression by a mechanism independent of DNA methylation and histone deacetylation

DNA methyltransferase 1 (DNMT1) catalyzes the post-replication methylation of DNA and is responsible for maintaining the DNA methylation pattern during cell division. A long list of data supports a role for DNMT1 in cellular transformation and inhibitors of DNMT1 were shown to have antitumorigenic effects. It was long believed that DNMT1 promoted tumorigenesis by maintaining the hypermethylated and silenced state of tumor suppressor genes. We have previously shown that DNMT1 knock down by either antisense oligonucleotides directed at DNMT1 or expressed antisense activates a number of genes involved in stress response and cell cycle arrest by a DNA methylation-independent mechanism. In this report we demonstrate that antisense knock down of DNMT1 in human lung carcinoma A549 and embryonal kidney HEK293 cells induces gene expression by a mechanism that does not involve either of the known epigenomic mechanisms, DNA methylation, histone acetylation, or histone methylation. The mechanism of activation of the cell cycle inhibitor p21 and apoptosis inducer BIK by DNMT1 inhibition is independent of the mechanism of activation of the same genes by histone deacetylase inhibition. We determine whether DNMT1 knock down activates one of the nodal transcription activation pathways in the cell and demonstrate that DNMT1 activates Sp1 response elements. This activation of Sp1 response does not involve an increase in either Sp1 or Sp3 protein levels in the cell or the occupancy of the Sp1 elements with these proteins. The methylation-independent regulation of Sp1 elements by DNMT1 unravels a novel function for DNMT1 in gene regulation. DNA methylation was believed to be a mechanism for suppression of CG-rich Sp1-bearing promoters. Our data suggest a fundamentally different and surprising role for DNMT1 regulation of CG-rich genes by a mechanism independent of DNA methylation and histone acetylation. The implications of our data on the biological roles of DNMT1 and the therapeutic potential of DNMT1 inhibitors as anticancer agents are discussed.

CpG methylation and transcription factors Sp1 and Sp3 regulate the expression of the human secretin receptor gene

The human secretin receptor (hSR) is an important glycoprotein receptor for regulating the secretion of pancreatic bicarbonate, water, and electrolytes. In this study we investigated the transcriptional regulation of the hSR gene. A minimal 106-bp promoter was identified, and it contains two GC boxes (GC box-A, -240 to -226; and GC box-B, -203 to -194, from the translation start site). EMSA and supershift analyses showed that both GC boxes interact with Sp1 and Sp3 transcription factors. Transient transfection in pancreas-derived human pancreatic ductule carcinoma (PANC)-1 and bovine pancreatic duct-1 cells showed that mutation of either GC box-A or -B reduced the promoter strength by 56-67%, whereas mutation of both GC boxes caused more than 90% reduction of promoter activity. Cotransfections of the hSR promoter with Sp1 and Sp3 expression vectors in Sp-deficient Drosophila SL-2 Schneider cells further demonstrated that the ratio of Sp1 to Sp3 is the key mechanism to modulate hSR gene expression. The methylation statuses of 27 CpG sites within the promoter region (-400 to -151 bp) were assessed in various human pancreas and liver cell lines. The hSR promoter is unmethylated (CAPAN-1, human pancreatic adenocarcinoma) or partially methylated (PANC-1 and HPAC, human pancreatic adenocarcinoma) in hSR-expressing cell lines but is completely methylated in hSR nonexpressing HepG2 cells. Methyltransferase inhibitor 5-aza-2'deoxycytidine increased hSR gene expression level in PANC-1 cells and induced hSR gene expression in HepG2 cells. Together, our study shows that, in addition to Sp1 and Sp3, promoter methylation also plays a role in the regulation of hSR gene expression.

Acetylated sp3 is a transcriptional activator

Sp3 transcription factor can either activate or repress target gene expression. However, the molecular event that controls this dual function is unclear. We previously reported (Ammanamanchi, S., and Brattain, M. G. (2001) J. Biol. Chem. 276, 3348-3352) that unmodified Sp3 acts as a transcriptional repressor of transforming growth factor-beta receptors in MCF-7L breast cancer cells. We now report that histone deacetylase inhibitor trichostatin A (TSA) induces acetylation of Sp3, which acts as a transcriptional activator of transforming growth factor-beta receptor type II (RII) in MCF-7L cells. Mutation analysis indicated the TSA response is mediated through a GC box located on the RII promoter, which was previously identified as an Sp1/Sp3-binding site that was critical for RII promoter activity. Ectopic Sp3 expression in Sp3-deficient MCF-7E breast cancer cells repressed RII promoter activity in the absence of TSA. However, in the TSA-treated MCF-7E cells ectopic Sp3 activated RII promoter. Histone acetyltransferase p300 was shown to acetylate Sp3. Sp3-mediated RII promoter activity was stimulated by wild type p300 but not the histone acetyltransferase domain-deleted mutant p300 in MCF-7L cells, suggesting the positive effect of p300 acetylase activity on Sp3. Consequently, the results presented in this manuscript demonstrate that acetylation acts as a switch that controls the repressor and activator role of Sp3.

Reconstitution of TIMP-2 expression in SH-SY5Y neuroblastoma cells by 5-azacytidine is mediated transcriptionally by NF-Y through an inverted CCAAT site

Advanced stage neuroblastomas (NB) exhibit a tissue inhibitor of metalloproteinase (TIMP)-2/matrix metalloproteinase (MMP) imbalance, considered a prerequisite for MMP involvement in tumor progression in vivo. Human SH-SY5Y NB cells exhibit a similar TIMP-2/MMP imbalance that promotes in vitro invasive behavior that is inhibited by exogenous TIMP-2. The DNA methyltransferase inhibitor 5-azacytidine (5-AzaC) redresses this TIMP-2/MMP imbalance, reconstituting TIMP-2 expression, without effecting that of MMP-2, by stimulating TIMP-2 transcription and inhibiting in vitro invasivity of SH-SY5Y cells. 5-AzaC stimulated transcription from a nonmethylated TIMP-2 promoter reporter gene construct consistent with regulation of a TIMP-2 transactivator. Promoter deletion and point-mutation analysis localized this effect to an inverted CCAAT element at position -73. This element bound specific complexes containing NF-YA and NF-YB proteins in SH-SY5Y nuclear extracts, the binding of which was augmented by 5-AzaC in association with enhanced levels of NF-YB protein and the function of which was confirmed by inhibition using dominant-negative NF-YA. The data highlight a novel indirect methylation-mediated mechanism for regulating the TIMP/MMP equilibrium in NB cells, involving repression of TIMP-2 relative to MMP-2 expression, dependent upon suboptimal NF-Y transcription factor function, which can be reversed by methyltransferase inhibition.

Transcriptional and post-transcriptional regulation of transforming growth factor beta type II receptor expression in osteoblasts

Variations in transforming growth factor beta (TGF-beta) activity depend on the expression of specific receptors in normal as well as transformed cells. For example, in addition to mutations in TGF-beta type II receptor (TbetaRII) that abrogate normal TGF-beta function, its expression decreases during the transition from replication to extracellular matrix production, or in response to other growth regulators in bone. Therefore, to understand how TbetaRII expression is controlled, we cloned the rat TbetaRII gene promoter and defined basic aspects of its structure and activity. Among several cis-acting elements, mutations within an upstream E-box that specifically binds USF nuclear factors or a downstream Sp1 binding site significantly reduced TbetaRII promoter activity in primary cultures of fetal rat osteoblasts. Treatment with bone morphogenetic protein 2 (BMP-2), which induces further osteoblast differentiation, significantly reduced cell surface TbetaRII. However, BMP-2 did not alter TbetaRII promoter activity, steady state TbetaRII mRNA, or total TbetaRII protein, but caused an intracellular relocation of TbetaRII. Select transcriptional elements thus regulate TbetaRII gene expression, whereas post-translational events controlled by BMP-2 rapidly modify the amount of TbetaRII protein on the bone cell surface. Consequently, several processes can alter functional TbetaRII levels in order to regulate the biological effects of this important growth factor.

5-Azacytidine and 5-aza-2'-deoxycytidine as inhibitors of DNA methylation: mechanistic studies and their implications for cancer therapy

5-Azacytidine was first synthesized almost 40 years ago. It was demonstrated to have a wide range of anti-metabolic activities when tested against cultured cancer cells and to be an effective chemotherapeutic agent for acute myelogenous leukemia. However, because of 5-azacytidine's general toxicity, other nucleoside analogs were favored as therapeutics. The finding that 5-azacytidine was incorporated into DNA and that, when present in DNA, it inhibited DNA methylation, led to widespread use of 5-azacytidine and 5-aza-2'-deoxycytidine (Decitabine) to demonstrate the correlation between loss of methylation in specific gene regions and activation of the associated genes. There is now a revived interest in the use of Decitabine as a therapeutic agent for cancers in which epigenetic silencing of critical regulatory genes has occurred. Here, the current status of our understanding of the mechanism(s) by which 5-azacytosine residues in DNA inhibit DNA methylation is reviewed with an emphasis on the interactions of these residues with bacterial and mammalian DNA (cytosine-C5) methyltransferases. The implications of these mechanistic studies for development of less toxic inhibitors of DNA methylation are discussed.

Methyl-CpG binding domain protein 2 represses transcription from hypermethylated pi-class glutathione S-transferase gene promoters in hepatocellular carcinoma cells

During the pathogenesis of human hepatocellular carcinoma (HCC), the CpG island encompassing the pi-class glutathione S-transferase gene (GSTP1) becomes hypermethylated. Repression of transcription accompanying CpG island hypermethylation has been proposed to be mediated by methyl-CpG binding domain (MBD) proteins. We report here that inhibition of transcription from hypermethylated GSTP1 promoters in Hep3B HCC cells, which fail to express GSTP1 mRNA or GSTP1 polypeptides, appears to be mediated by MBD2. Treatment of Hep3B cells with 5-azadeoxycytidine (5-aza-dC), a methyltransferase inhibitor, activated GSTP1 expression, whereas treatment with trichostatin A, a histone deacetylase inhibitor, had little effect. To more precisely assess the contribution of the pattern of GSTP1 CpG island methylation on GSTP1 mRNA expression, Hep3B cells were treated for 72 h with 5-aza-dC and then subjected to limiting dilution cloning. Bisulfite sequencing was used to map the methylation patterns of the GSTP1 promoter region in GSTP1-expressing and -non-expressing clones. In the clone that expressed GSTP1 mRNA determined by Northern blot analysis and quantitative reverse transcriptase (RT)-PCR, widespread demethylation of at least one GSTP1 allele was evident. Chromatin immunoprecipitation experiments revealed the presence of MBD2, but not Sp1, at the GSTP1 promoter in Hep3B cells. In contrast, Sp1 was detected at the GSTP1 promoter in a GSTP1-expressing Hep3B 5-aza-dC subclone. To test whether MBD2 might be responsible for the inhibition of GSTP1 transcription from hypermethylated GSTP1 promoters, siRNAs were used to reduce MBD2 polypeptide levels in Hep3B cells. SssI-catalyzed methylation of GSTP1 promoter sequences resulted in diminished luciferase reporter activity after transfection into Hep3B cells. However, when hypermethylated GSTP1 promoter sequences were transfected into Hep3B cells that had been treated with siRNA-targeting MBD2 mRNA, no repression of luciferase reporter expression was evident. These findings implicate MBD2 in the repression of GSTP1 expression associated with GSTP1 CpG island hypermethylation in HCC cells.