Regulation of the expression of the tissue transglutaminase gene by DNA methylation

Tissue transglutaminase: a multifunctional and multisite regulator in health and disease

Tissue transglutaminase (TG2) is a widely distributed multifunctional protein involved in a broad range of cellular and metabolic functions carried out in a variety of cellular compartments. In addition to transamidation, TG2 also functions as a Gα signaling protein, a protein disulfide isomerase (PDI), a protein kinase, and a scaffolding protein. In the nucleus, TG2 modifies histones and transcription factors. The PDI function catalyzes the trimerization and activation of heat shock factor-1 in the nucleus and regulates the oxidation state of several mitochondrial complexes. Cytosolic TG2 modifies proteins by the addition of serotonin or other primary amines and in this way affects cell signaling. Modification of protein-bound glutamines reduces ubiquitin-dependent proteasomal degradation. At the cell membrane, TG2 is associated with G protein-coupled receptors (GPCRs), where it functions in transmembrane signaling. TG2 is also found in the extracellular space, where it functions in protein cross-linking and extracellular matrix stabilization. Of particular importance in transglutaminase research are recent findings concerning the role of TG2 in gene expression, protein homeostasis, cell signaling, autoimmunity, inflammation, and hypoxia. Thus, TG2 performs a multitude of functions in multiple cellular compartments, making it one of the most versatile cellular proteins. Additional evidence links TG2 with multiple human diseases including preeclampsia, hypertension, cardiovascular disease, organ fibrosis, cancer, neurodegenerative diseases, and celiac disease. In conclusion, TG2 provides a multifunctional and multisite response to physiological stress.

Prognostic significance of factor XIIIA promoter methylation status in aneurysmal subarachnoid haemorrhage (aSAH)

BACKGROUND

Aneurysmal subarachnoid hemorrhage is a life- threatening condition with high rate of disability and mortality. Apolipoprotein E (APOE) and Factor XIIIA (F13A) genes are involved in the pathogenetic mechanism of aneurysmal subarachnoid haemorrhage (aSAH). We evaluated the association of promoter methylation status of APOE and F13A gene and risk of aSAH.

METHODS

For evaluating the effect of hypermethylation in the promoter region of these genes with risk of aSAH, we conducted a case -control study with 50 aSAH patients and 50 healthy control. The methylation pattern was analysed using methylation specific PCR. The risk factors associated with poor outcome after aSAH was also analysed in this study. The outcome was assessed using Glasgow outcome score (GOS) after 3 months from the initial bleed.

RESULTS

The frequency of APOE and F13A methylation pattern showed insignificant association with risk of aSAH in this study. Gender stratification analysis suggests that F13A promoter methylation status was significantly associated with the risk of aSAH in male gender. Age, aneurysm located at the anterior communicating artery and diabetes mellitus showed significant association with poor outcome after aSAH.

CONCLUSION

There was no significant association with APOE promoter methylation with the risk as well as outcome of patients after aSAH. F13A promoter methylation status was significantly associated with risk of aSAH in male gender, with no significant association with outcome after aSAH.

Spotlight on the transglutaminase 2 gene: a focus on genomic and transcriptional aspects

The type 2 isoenzyme is the most widely expressed transglutaminase in mammals displaying several intra- and extracellular activities depending on its location (protein modification, modulation of gene expression, membrane signalling and stabilization of cellular interactions with the extracellular matrix) in relation to cell death, survival and differentiation. In contrast with the appreciable knowledge about the regulation of the enzymatic activities, much less is known concerning its inducible expression, which is altered in inflammatory and neoplastic diseases. In this context, we first summarize the gene's basic features including single-nucleotide polymorphism characterization, epigenetic DNA methylation and identification of regulatory regions and of transcription factor-binding sites at the gene promoter, which could concur to direct gene expression. Further aspects related to alternative splicing events and to ncRNAs (microRNAs and lncRNAs) are involved in the modulation of its expression. Notably, this important gene displays transcriptional variants relevant for the protein's function with the occurrence of at least seven transcripts which support the synthesis of five isoforms with modified catalytic activities. The different expression of the TG2 (type 2 transglutaminase) variants might be useful for dictating the multiple biological features of the protein and their alterations in pathology, as well as from a therapeutic perspective.

A long non-coding RNA inside the type 2 transglutaminase gene tightly correlates with the expression of its transcriptional variants

The long non-coding RNAs (lncRNAs) are matter of intense investigation as potential regulators of gene expression. In the case of the transglutaminase 2 gene (TGM2) the databases of genome sequence indicate location of a lncRNA (LOC107987281) within the first intron. This lncRNA is 1000 bp long, arises from 2 exons and starts few nucleotides 3' of the first splicing site of translated TGM2. We have analysed correlations between expression of LOC107987281 lncRNA and TGM2 mRNA by real-time PCR in K562 cell line untreated or treated with the anticancer drugs TPA (12-O-tetradecanoylphorbol-13-acetate), Docetaxel and Doxorubicin. In the treated cells the lncRNA increase follows the trend of TGM2 transcript. To validate this finding we used HumanExon1_0ST Affymetrix; chip data were background-adjusted, quantile-normalized and summarized using robust multi-array average analysis implemented in the R package. The probesets recognize sequences inside each exon, near intronic splicing sites and others located in the untranslated regions of TGM2 gene. The analysis of total RNA samples in GEO datasets from K562, HL-60, THP-1 and U937 cell lines, untreated or treated with TPA in replicated experiments confirmed our earlier results. These demonstrate correlation between LOC107987281 and TGM2 mRNA in the cell lines (K562, HL60 and THP-1) where increased levels of TGM2 mRNA are produced. Additional array study on 358 samples of several normal and paired tumor tissues leads to the same conclusions, indicating a correlation between full-length TGM2 mRNA and LOC107987281 lncRNA in relation to the development of several tumors.

Age dictates a steroid-resistant cascade of Wnt5a, transglutaminase 2, and leukotrienes in inflamed airways

BACKGROUND

Airway remodeling is a detrimental and refractory process showing age-dependent clinical manifestations that are mechanistically undefined. The leukotriene (LT) and wingless/integrase (Wnt) pathways have been implicated in remodeling, but age-specific expression profiles and common regulators remained elusive.

OBJECTIVE

We sought to study the activation of the LT and Wnt pathways during early- or late-onset allergic airway inflammation and to address regulatory mechanisms and clinical relevance in normal human bronchial epithelial cells (NHBEs) and nasal polyp tissues.

METHODS

Mice were sensitized with house dust mite (HDM) allergens from days 3, 15, or 60 after birth. Remodeling factors in murine bronchoalveolar lavage fluid, lung tissue, or human nasal polyp tissue were analyzed by means of Western blotting, immunoassays, or histology. Regulatory mechanisms were studied in cytokine/HDM-stimulated NHBEs and macrophages.

RESULTS

Bronchoalveolar lavage fluid LT levels were increased in neonatal and adult but reduced in juvenile HDM-sensitized mice. Lungs of neonatally sensitized mice showed increased 5-lipoxygenase levels, whereas adult mice expressed more group 10 secretory phospholipase A₂, Wnt5a, and transglutaminase 2 (Tgm2). Older mice showed colocalization of Wnt5a and LT enzymes in the epithelium, a pattern also observed in human nasal polyps. IL-4 promoted epithelial Wnt5a secretion, which upregulated macrophage Tgm2 expression, and Tgm2 inhibition in turn reduced LT release. Tgm2, group 10 secretory phospholipase A₂, and LT enzymes in NHBEs and nasal polyps were refractory to corticosteroids.

CONCLUSION

Our findings reveal age differences in LT and Wnt pathways during airway inflammation and identify a steroid-resistant cascade of Wnt5a, Tgm2, and LTs, which might represent a therapeutic target for airway inflammation and remodeling.

Epigenetics and inheritance of phenotype variation in livestock

Epigenetic inheritance plays a crucial role in many biological processes, such as gene expression in early embryo development, imprinting and the silencing of transposons. It has recently been established that epigenetic effects can be inherited from one generation to the next. Here, we review examples of epigenetic mechanisms governing animal phenotype and behaviour, and we discuss the importance of these findings in respect to animal studies, and livestock in general. Epigenetic parameters orchestrating transgenerational effects, as well as heritable disorders, and the often-overlooked areas of livestock immunity and stress, are also discussed. We highlight the importance of nutrition and how it is linked to epigenetic alteration. Finally, we describe how our understanding of epigenetics is underpinning the latest cancer research and how this can be translated into directed efforts to improve animal health and welfare.

Epigenetics and inheritance of phenotype variation in livestock

Epigenetic inheritance plays a crucial role in many biological processes, such as gene expression in early embryo development, imprinting and the silencing of transposons. It has recently been established that epigenetic effects can be inherited from one generation to the next. Here, we review examples of epigenetic mechanisms governing animal phenotype and behaviour, and we discuss the importance of these findings in respect to animal studies, and livestock in general. Epigenetic parameters orchestrating transgenerational effects, as well as heritable disorders, and the often-overlooked areas of livestock immunity and stress, are also discussed. We highlight the importance of nutrition and how it is linked to epigenetic alteration. Finally, we describe how our understanding of epigenetics is underpinning the latest cancer research and how this can be translated into directed efforts to improve animal health and welfare.

Cellular functions of tissue transglutaminase

Transglutaminase 2 (TG2 or tissue transglutaminase) is a highly complex multifunctional protein that acts as transglutaminase, GTPase/ATPase, protein disulfide isomerase, and protein kinase. Moreover, TG2 has many well-documented nonenzymatic functions that are based on its noncovalent interactions with multiple cellular proteins. A vast array of biochemical activities of TG2 accounts for its involvement in a variety of cellular processes, including adhesion, migration, growth, survival, apoptosis, differentiation, and extracellular matrix organization. In turn, the impact of TG2 on these processes implicates this protein in various physiological responses and pathological states, contributing to wound healing, inflammation, autoimmunity, neurodegeneration, vascular remodeling, tumor growth and metastasis, and tissue fibrosis. TG2 is ubiquitously expressed and is particularly abundant in endothelial cells, fibroblasts, osteoblasts, monocytes/macrophages, and smooth muscle cells. The protein is localized in multiple cellular compartments, including the nucleus, cytosol, mitochondria, endolysosomes, plasma membrane, and cell surface and extracellular matrix, where Ca(2+), nucleotides, nitric oxide, reactive oxygen species, membrane lipids, and distinct protein-protein interactions in the local microenvironment jointly regulate its activities. In this review, we discuss the complex biochemical activities and molecular interactions of TG2 in the context of diverse subcellular compartments and evaluate its wide ranging and cell type-specific biological functions and their regulation.

Transglutaminase 2: biology, relevance to neurodegenerative diseases and therapeutic implications

Neurodegenerative disorders are characterized by progressive neuronal loss and the aggregation of disease-specific pathogenic proteins in hallmark neuropathologic lesions. Many of these proteins, including amyloid Αβ, tau, α-synuclein and huntingtin, are cross-linked by the enzymatic activity of transglutaminase 2 (TG2). Additionally, the expression and activity of TG2 is increased in affected brain regions in these disorders. These observations along with experimental evidence in cellular and mouse models suggest that TG2 can contribute to the abnormal aggregation of disease causing proteins and consequently to neuronal damage. This accumulating evidence has provided the impetus to develop inhibitors of TG2 as possible neuroprotective agents. However, TG2 has other enzymatic activities in addition to its cross-linking function and can modulate multiple cellular processes including apoptosis, autophagy, energy production, synaptic function, signal transduction and transcription regulation. These diverse properties must be taken into consideration in designing TG2 inhibitors. In this review, we discuss the biochemistry of TG2, its various physiologic functions and our current understanding about its role in degenerative diseases of the brain. We also describe the different approaches to designing TG2 inhibitors that could be developed as potential disease-modifying therapies.

Elimination of Colon Cancer Stem-Like Cells by the Combination of Curcumin and FOLFOX

5-Fluorouracil (5-FU) or 5-FU plus oxaliplatin (FOLFOX) remains the backbone of colorectal cancer chemotherapeutics but with limited success. This could partly be due to the enrichment of cancer stem cells (CSCs) that are resistant to conventional chemotherapy. Therefore, validation of a nontoxic agent that can either cause reversal of chemoresistance or promote the killing of CSCs would be highly desirable. The current study examines whether curcumin, the major active ingredient of turmeric, either alone or together with FOLFOX, would be an effective strategy to eliminate colon CSCs. Exposure of colon cancer HCT-116 or HT-29 cells to FOLFOX that inhibited their growth led to the enrichment of CSC phenotype as evidenced by increased proportion of CD133-, CD44-, and/or CD166-positive cells and epidermal growth factor receptor (EGFR) levels. Treatment of FOLFOX-surviving colon cancer cells with either curcumin alone or together with FOLFOX resulted in a marked reduction in CSCs, as evidenced by the decreased expression of CD44 and CD166 as well as EGFR and by their ability to form anchorage-dependent colonies. They also caused disintegration of colonospheres. Increased expression of EGFR in FOLFOX-surviving cells could be attributed to hypomethylation of the EGFR promoter, whereas an opposite phenomenon was observed when the FOLFOX-surviving cells were treated with curcumin and/or FOLFOX. These changes were accompanied by parallel alterations in the levels of DNA methyltransferase 1. In conclusion, our data suggest that curcumin by itself or together with the conventional chemotherapeutic could be an effective treatment strategy for preventing the emergence of chemoresistant colon cancer cells by reducing/eliminating CSCs.

Aberrant DNA methylation of matrix remodeling and cell adhesion related genes in pterygium

Background

Pterygium is a common ocular surface disease characterized by abnormal epithelial and fibrovascular proliferation, invasion, and matrix remodeling. This lesion, which migrates from the periphery to the center of the cornea, impairs vision and causes considerable irritation. The mechanism of pterygium formation remains ambiguous, and current treatment is solely surgical excision, with a significant risk of recurrence after surgery. Here, we investigate the role of methylation in DNA sequences that regulate matrix remodeling and cell adhesion in pterygium formation.

Methodology/Principal Findings

Pterygium and uninvolved conjunctiva samples were obtained from the same eye of patients undergoing surgery. The EpiTYPER Sequenom technology, based on differential base cleavage and bisulfite sequencing was used to evaluate the extent of methylation of 29 matrix and adhesion related genes. In pterygium, three CpG sites at −268, −32 and −29 bp upstream of transglutaminase 2 (TGM-2) transcription initiation were significantly hypermethylated (p<0.05), whereas hypomethylation was detected at CpGs +484 and +602 bp downstream of matrix metalloproteinase 2 (MMP-2) transcription start site, and −809, −762, −631 and −629 bp upstream of the CD24 transcription start site. RT-qPCR, western blot and immunofluorescent staining showed that transcript and protein expression were reduced for TGM-2 and increased for MMP-2 and CD24. Inhibition of methylation in cultured conjunctival epithelial cells increased these transcripts.

Conclusions/Significance

We found regions of aberrant DNA methylation which were consistent with alteration of TGM-2, MMP-2, and CD24 transcript and protein expression, and that inhibition of methylation in cultured cells can increase the expression of these genes. Since these genes were related to cell adhesion and matrix remodeling, dysregulation may lead to fibroblastic and neovascular changes and pterygium formation. These results have implications for the prognostication of pterygium in clinical practice, for example, detection of epigenetic changes may have a role in predicting post surgical recurrence of aggressive lesions.

MicroRNA expression in human retinal pigment epithelial (ARPE-19) cells: increased expression of microRNA-9 by N-(4-hydroxyphenyl)retinamide

PURPOSE

MicroRNAs (miRNAs) are important regulators of many cellular functions due to their ability to target mRNAs for degradation or translational inhibition. Previous studies have reported that the expression of microRNA-9 (miR-9) is regulated by retinoic acid and reactive oxygen species (ROS). We have previously shown that N-(4-hydroxyphenyl)-retinamide (4HPR), a retinoic acid derivative, induces ROS generation and apoptosis in cultured human retinal pigment epithelial (RPE) cells, known as ARPE-19 cells. The aim of the present study was to investigate the expression of miR-9 in ARPE-19 cells in response to 4HPR treatment, and to identify other miRNAs normally expressed in these cells.

METHODS

ARPE-19 cells in culture were treated with 4HPR, the total RNA fractions were isolated, and the expression of various miRNAs and mRNAs was analyzed using real-time PCR. The miRNA expression profile of ARPE-19 cells was analyzed using microarray hybridization.

RESULTS

Treatment of ARPE-19 cells with 4HPR resulted in apoptosis characterized by the increased expression of HMOX1 and GADD153 genes. A twofold increase in the expression of miR-9 was also observed during this response. Potential binding sites for the transcription factors encoded by CEBPA and CEBPB genes were found to be present in the putative promoter regions of all three genes encoding miR-9. 4HPR-induced miR-9 expression was associated with parallel increases in the expression of these transcription factor genes. 5-Aza-2'-deoxycytidine, a methyl transferase inhibitor, also increased the expression of miR-9 in ARPE-19 cells. Microarray hybridization analysis identified let-7b, let-7a, miR-125b, miR-24, miR-320, miR-23b, let-7e, and let-7d as the most abundant miRNAs normally expressed in ARPE-19 cells. These miRNAs are known to regulate cell growth, differentiation or development. The 4HPR treatment increased the expression of miR-16, miR-26b, miR-23a, and miR-15b in ARPE-19 cells, although these increases were modest when compared to the increase in the expression of miR-9.

CONCLUSIONS

Our studies demonstrate that miR-9 is expressed in the RPE cell line ARPE-19, and its expression is increased by a retinoic acid derivative and by an inhibitor of promoter hypermethylation. Several miRNAs with inherent ability to regulate cell growth, differentiation and development are also normally expressed in ARPE-19 cells. Thus, miR-9 and other miRNAs could be important in maintaining RPE cell function.

Folate regulation of axonal regeneration in the rodent central nervous system through DNA methylation

The folate pathway plays a crucial role in the regeneration and repair of the adult CNS after injury. Here, we have shown in rodents that such repair occurs at least in part through DNA methylation. In animals with combined spinal cord and sciatic nerve injury, folate-mediated CNS axon regeneration was found to depend on injury-related induction of the high-affinity folate receptor 1 (Folr1). The activity of folate was dependent on its activation by the enzyme dihydrofolate reductase (Dhfr) and a functional methylation cycle. The effect of folate on the regeneration of afferent spinal neurons was biphasic and dose dependent and correlated closely over its dose range with global and gene-specific DNA methylation and with expression of both the folate receptor Folr1 and the de novo DNA methyltransferases. These data implicate an epigenetic mechanism in CNS repair. Folic acid and possibly other nontoxic dietary methyl donors may therefore be useful in clinical interventions to promote brain and spinal cord healing. If indeed the benefit of folate is mediated by epigenetic mechanisms that promote endogenous axonal regeneration, this provides possible avenues for new pharmacologic approaches to treating CNS injuries.

Active demethylation of paternal genome in mammalian zygotes

Epigenetic reprogramming in early preimplantation embryos, that refers to erasing and remodeling epigenetic marks such as DNA methylation, is essential for differentiation and development. In many species, paternal genome is subjected to genome-wide active demethylation before the DNA replication commences, while maternal genome maintains its methylation status until being demethylated passively during the subsequent cleavage divisions. The purpose of this manuscript was to review the available knowledge about the paternal genome active demethylation process concerning the possible mechanisms, species variation and the factors affecting the active demethylation dynamics such as in vitro protocols for production of pronuclear-stage zygotes. Better understanding the mechanisms by which the epigenetic reprogramming is occurred may contribute to clarify the biological significance of this process.

Transglutaminase 2 as a cisplatin resistance marker in non-small cell lung cancer

PURPOSE

Recently, it was reported that expression of transglutaminase 2 plays an important role in doxorubicin/cisplatin resistance in breast and ovarian cancer. The aims of this study were to verify the role of transglutaminase 2 in cisplatin response in non-small cell lung cancer (NSCLC) and to study if transglutaminase 2 gene (TGM2) methylation can be a molecular marker for good response to cisplatin.

METHODS

TGM2 promoter methylation was analyzed by sodium bisulfite sequencing. Cisplatin sensitivity was analyzed by treatment of cisplatin in NSCLC cell lines with/without TGM2 or TGM2 siRNA transfection.

RESULTS

In one-third of NSCLC cell lines, TGase 2 gene (TGM2) was silenced by promoter methylation. The TGM2 promoter-methylated cell lines (HCC-95 and HCC-1588) showed relatively higher sensitivity to cisplatin than the TGM2-expressing cell lines (NCI-H1299 and HCC-1195). Down-regulation and over-expression of TGM2 in those NSCLC cells also suggested a positive correlation of cisplatin sensitivity and TGM2 inhibition. With doxorubicin, the relationship was quite similar.

CONCLUSIONS

We showed that good responders of cisplatin in NSCLC could be identified by the promoter methylation of TGM2 and that TGase 2 inhibition appears to be an effective cisplatin-sensitizing modality in NSCLC.

The transglutaminase 2 gene (TGM2), a potential molecular marker for chemotherapeutic drug sensitivity, is epigenetically silenced in breast cancer

Tissue transglutaminase (TG2) is a ubiquitously expressed enzyme capable of catalyzing protein cross-links. TG2-dependent cross-links are important in extracellular matrix integrity and it has been proposed that this TG2 activity establishes a barrier to tumor spread. Furthermore, TG2 controls sensitivity to the chemotherapeutic drug doxorubicin. Both doxorubicin sensitivity and TG2 expression are highly variable in cultured human breast cancer cell lines and inspection of the human gene (termed TGM2) determined that a canonical CpG island exists within its 5' flank. These features, when combined with its potential tumor suppressor activity, make TG2 an attractive candidate for epigenetic silencing. Consistent with this, we observed that culturing breast tumor cells with the DNA demethylating agent 5-aza-2'-deoxycytidine (5-azadC) resulted in a robust increase in TG2 expression. Analysis of DNA harvested from cultured lines and primary breast tumor samples indicated that TGM2 often displays aberrant hypermethylation and that there is a statistically significant correlation between gene methylation and reduced expression. Finally, we observed that doxorubicin-resistant MCF-7/ADR cells do not show TGM2 silencing but that doxorubicin-sensitive MCF-7 cells do and that culturing MCF-7 cells on 5-azadC and subsequently restoring TG2 expression reduced sensitivity to doxorubicin. This work indicates that the TGM2 gene is a target for epigenetic silencing in breast cancer and suggests that this aberrant molecular event is a potential marker for chemotherapeutic drug sensitivity.

Identification of two GTP-independent alternatively spliced forms of tissue transglutaminase in human leukocytes, vascular smooth muscle, and endothelial cells

Tissue transglutaminase (tTG) is a multifunctional enzyme with transglutaminase crosslinking (TGase), GTP binding, and hydrolysis activities that play a role in many different disorders. We identified, characterized, and investigated the function and stability of two alternatively spliced forms of tTG using biochemical, cellular, and molecular biological approaches. Using a human aortic vascular smooth muscle cells (VSMC) cDNA library, we identified two cDNAs encoding C-terminal truncated forms, tTG(V1) and tTG(V2). tTG(V1,2) mRNAs were synthesized by a rare splicing event using alternate splice sites within exons 12 and 13 of the tTG gene, respectively. Quantitative PCR and immunoblotting demonstrated that there was unique expression and localization of tTG(V1,2) compared with tTG in human umbilical vein endothelial cells (HUVECs), VSMC, and leukocytes. The loss of C-terminal 52 amino acid residues (AAs) in tTG(V1,2) altered GTP binding, enhanced GTP hydrolysis, rendered the variants insensitive to GTP inhibition, and resulted in <10% residual Ca(+2)-dependent TGase activity. Transfection in HEK293 demonstrated a 28- and 5-fold reduction in the expression of tTG(V1) and tTG(V2), respectively, demonstrating that the C-terminal GTP-binding domain is important in stabilizing and promoting the half-life of tTG. The altered affinity for GTP allowed tTG(V1,2) to exhibit enhanced TGase activity when there is a transient increase in Ca(+2) levels. The abundance of tTG(V1,2) and its distinct intracellular expression patterns in human vascular cells and leukocytes indicate these isoforms likely have unique physiological functions.

The language of methylation in genomics of eukaryotes

Background studies have shown that 6-methylaminopurine (m6A) and 5-methylcytosine (m5C), detected in DNA, are products of its post-synthetic modification. At variance with bacterial genomes exhibiting both, eukaryotic genomes essentially carry only m5C in m5CpG doublets. This served to establish that, although a slight extra-S phase asymmetric methylation occurs de novo on 5'-CpC-3'/3'GpG-5', 5'-CpT-3'/3'-GpA-5', and 5'-CpA-3'/3'-GpT-5' dinucleotide pairs, a heavy methylation during S involves Okazaki fragments and thus semiconservatively newly made chains to guarantee genetic maintenance of -CH3 patterns in symmetrically dimethylated 5'-m5CpG-3'/3'-Gpm5C-5' dinucleotide pairs. On the other hand, whilst inverse correlation was observed between bulk DNA methylation, in S, and bulk RNA transcription, in G1 and G2, probes of methylated DNA helped to discover the presence of coding (exon) and uncoding (intron) sequences in the eukaryotic gene. These achievements led to the search for a language that genes regulated by methylation should have in common. Such a deciphering, initially providing restriction minimaps of hypermethylatable promoters and introns vs. hypomethylable exons, became feasible when bisulfite methodology allowed the direct sequencing of m5C. It emerged that, while in lymphocytes, where the transglutaminase gene (hTGc) is inactive, the promoter shows two fully methylated CpG-rich domains at 5 and one fully unmethylated CpG-rich domain at 3' (including the site +1 and a 5'-UTR), in HUVEC cells, where hTGc is active, in the first CpG-rich domain of its promoter four CpGs lack -CH3: a result suggesting new hypotheses on the mechanism of transcription, particularly in connection with radio-induced DNA demethylation.

Developmental regulation of tissue transglutaminase in the mouse forebrain

Tissue transglutaminase (tTG) is a multifunctional enzyme that catalyzes both transamidation and GTPase reactions. In cell culture models tTG-mediated transamidation positively regulates many processes that occur in vivo during the mammalian brain growth spurt (BGS), including neuronal differentiation, neurite outgrowth, synaptogenesis and cell death mechanisms. However, little is known about the levels of tTG expression and transglutaminase (TG) activity during mammalian brain development. In this study, C57BL/6 mouse forebrains were collected at embryonic day (E) 12, E14, E17, postnatal day (P) 0, P7 and P56 and analyzed for tTG expression and TG activity. RT-PCR analysis demonstrated that tTG mRNA content increases during mouse forebrain development, whereas immunoblot analysis demonstrated that tTG protein content decreases during this time. TG activity was low in prenatal mouse forebrain but increased fivefold to peak at P0, which corresponds with the beginning of the mouse BGS. Further analysis demonstrated that the lack of temporal correlation between tTG protein content and TG activity is the result of an endogenous inhibitor of tTG that is present in prenatal but not postnatal mouse forebrain. These results demonstrate for the first time that tTG enzymatic activity in the mammalian forebrain is developmentally regulated by post-translational mechanisms.

Folic acid-mediated inhibition of serum-induced activation of EGFR promoter in colon cancer cells

Although accumulating evidence suggests a chemopreventive role for folic acid (FA) in colorectal carcinogenesis, the underlying mechanisms are largely unknown. Previously, we reported that supplemental FA inhibits the expression and activation of epidermal growth factor receptor (EGFR) in colon cancer cell lines. To determine the mechanism(s) by which FA affects EGFR function, we have examined whether and to what extent supplemental FA or its metabolites 5-methyltetrahydrofolate (MTF), dihydrofolate (DF), and tetrahydrofolate (TF) will modulate basal and serum-induced activation of the EGFR promoter in the HCT-116 colon cancer cell line. HCT-116 cells were preincubated with or without (control) FA or one of its metabolites (10 microg/ml) for 48 h, transfected with the EGFR promoter luciferase reporter construct, and incubated for 48 h with FA, DF, TF, or 5-MTF in the absence or presence of 10% FBS. Supplemental FA as well as its metabolites markedly inhibited EGFR promoter activity and its methylation status. Exposure of the cells to 10% FBS caused a marked stimulation of EGFR promoter activity and its expression, both of which were greatly abrogated by supplemental FA and 5-MTF. In contrast, serum-induced activation of c-fos promoter activity was unaffected by 5-MTF. The 5-MTF-induced inhibition of serum-mediated stimulation of EGFR promoter activity and EGFR expression was reversed when methylation was inhibited by 5-aza-2'-deoxycytidine. Our data suggest that FA and its metabolite 5-MTF inhibit EGFR promoter activity in colon cancer cells by enhancing methylation. This could partly be responsible for FA-mediated inhibition of growth-related processes in colorectal neoplasia.

Changes in expression level of genes as a function of time of day in the liver of rats

Daily, rhythmic variation in various biochemical, physiological, and behavioral events is a fundamental property of biological organization. Here, we report analysis of relative levels of gene expression in the liver of 16 Fischer 344 rats as a function of time of day. Expression levels were determined for 3906 genes using high-density oligonucleotide microarrays. Of the 3906 genes, 1171 (30%) were clearly expressed while 2735 (70%) were not expressed or the expression was too low to distinguish from background levels. The maximum estimated changes observed for most genes (1029, 88%) were less than 1.5-fold. Analysis of variance and the Kruskal-Wallis tests were used to identify 67 genes whose expression was significantly altered as a function of time of day. These significantly altered genes were classified according to their functions and fall into key cellular pathways including drug metabolism, ion transport, signal transduction, DNA binding and regulation of transcription, and immune response.

Opposing roles of Ras/Raf oncogenes and the MEK1/ERK signaling module in regulation of expression and adhesive function of surface transglutaminase

Tissue transglutaminase (tTG) serves as a potent and ubiquitous integrin-associated adhesion co-receptor for fibronectin on the cell surface and affects several key integrin functions. Here we report that in fibroblasts, activated H-Ras and Raf-1 oncogenes decrease biosynthesis, association with beta1 integrins, and surface expression of tTG because of down-regulation of tTG mRNA. In turn, the reduction of surface tTG inhibits adhesion of H-Ras- and Raf-1-transformed cells on fibronectin and, in particular, on its tTG-binding fragment I(6)II(1,2)I(7-9), which does not interact directly with integrins. Analysis of Ras/Raf downstream signaling with specific pharmacological inhibitors reveals that the decrease in tTG expression is mediated by the p38 MAPK, c-Jun NH2-terminal kinase, and phosphatidylinositol 3-kinase pathways. In contrast, increased activation of the ERK pathway by constitutively active MEK1 stimulates tTG mRNA expression, biosynthesis, and surface expression of tTG, whereas MEK inhibitors or dominant negative MEK1 exert an opposite effect. This modulation of surface tTG by ERK signaling alters adhesion of cells on fibronectin and its fragment that binds tTG. Furthermore, transient stimulation of ERK signaling in untransformed fibroblasts by adhesion on fibronectin or growth factors elevates tTG biosynthesis, increases complex formation with beta1 integrins, and raises surface expression of tTG. Finally, ERK activation is required for growth factor-induced redistribution of tTG on the surface of adherent fibroblasts and co-clustering of beta1 integrins and tTG at cell-matrix adhesion contacts. Together, our data indicate that down-regulation of surface tTG by Ras and Raf oncogenes contributes to adhesive deficiency of transformed fibroblasts, whereas stimulation of biosynthesis and surface expression of tTG by the MEK1/ERK module promotes and sustains cell-matrix adhesion of untransformed cells. Contrasting effects of Ras/Raf oncogenes and their immediate downstream signaling module, MEK1/ERK, on tTG expression are consistent with adhesive function of surface tTG.

Transglutaminases: nature's biological glues

Transglutaminases (Tgases) are a widely distributed group of enzymes that catalyse the post-translational modification of proteins by the formation of isopeptide bonds. This occurs either through protein cross-linking via epsilon-(gamma-glutamyl)lysine bonds or through incorporation of primary amines at selected peptide-bound glutamine residues. The cross-linked products, often of high molecular mass, are highly resistant to mechanical challenge and proteolytic degradation, and their accumulation is found in a number of tissues and processes where such properties are important, including skin, hair, blood clotting and wound healing. However, deregulation of enzyme activity generally associated with major disruptions in cellular homoeostatic mechanisms has resulted in these enzymes contributing to a number of human diseases, including chronic neurodegeneration, neoplastic diseases, autoimmune diseases, diseases involving progressive tissue fibrosis and diseases related to the epidermis of the skin. In the present review we detail the structural and regulatory features important in mammalian Tgases, with particular focus on the ubiquitous type 2 tissue enzyme. Physiological roles and substrates are discussed with a view to increasing and understanding the pathogenesis of the diseases associated with transglutaminases. Moreover the ability of these enzymes to modify proteins and act as biological glues has not gone unnoticed by the commercial sector. As a consequence, we have included some of the present and future biotechnological applications of this increasingly important group of enzymes.

Leptin gene expression in human preadipocytes is switched on by maturation-induced demethylation of distinct CpGs in its proximal promoter

The peptide hormone leptin plays a major role in the regulation of energy intake and expenditure and is predominantly expressed in mature adipocytes but not in preadipocytes. Using bisulfite genomic sequencing, we found that 32 CpGs, distributed within a 317-bp sequence of the proximal leptin promoter, were highly methylated in human preadipocytes (73.4% +/- 9.0%). During maturation toward terminally differentiated adipocytes, this promoter region was extremely demethylated (9.4% +/- 4.4%). CpG methylation-dependent transcriptional activity of the promoter fragment was determined in transfection experiments using a set of 5'-truncated mock-, HhaI-, and SssI-methylated promoter-reporter constructs. Whereas the methylated CpG within the CCAAT/enhancer-binding protein alpha recognition site down-regulated reporter expression, methylated CpGs proximal to the TATA motif and/or in a further upstream region abrogated promoter activity completely. These distinct promoter CpG sequences were found unmethylated in leptin-expressing mature adipocytes. As evidenced by electrophoretic mobility shift assays, nuclear protein complexes were specifically formed on methylated oligonucleotide probes corresponding to the dedicated promoter sequences, indicating that methyl-CpG binding proteins participate in transcriptional repression and regulation of the human leptin gene.

Specific methylation of the CpG-rich domains in the promoter of the human tissue transglutaminase gene

The activity of tissue transglutaminase is present in many cells and tissues but almost absent in leucocytes and lymphocytes. The present work describes the distribution of 5-methylcytosine along the bisulphite-converted promoter of the human tissue transglutaminase gene as being in an essentially repressed state. In this promoter, the chain-specific sequencing revealed the location of three CpG-rich domains whose methylation responds to an 'all or nothing' signal. While the CpGs of domain 1, at the 5'-end, and 2, in the mid-promoter, were fully methylated, those of domain 3, at the 3'-end, were fully unmethylated. Before the 5'-UTR sequence, from site+1 to site+67, also unmethylated, there was thus a striking contrast in the post-synthetic modification between the sequence, from -1594 to -436, containing domains 1 and 2, and the sequence, from -435 to -1, containing domain 3 with the core promoter.

FMR1 silencing and the signals to chromatin: a unified model of transcriptional regulation

Understanding the effect of DNA methylation requires an appreciation of the determinants responsible for gene expression. To this end, we and others have demonstrated the exquisite silencing properties of methylation-dependent repressors that belong to the methyl-CpG binding domain (MBD) protein family. It is crucial, to establish a direct connection between the remodelling events observed with the fragile X mental retardation gene 1 (FMR1) promoter if we are to understand the mechanism of control. In this paper contrasting but not mutually exclusive models are discussed, which conceptually and experimentally reflects an appreciation of the surrounding chromatin environment. Examining general transcription systems controlled by DNA methylation, proteins that recognize the methyl-CpG moiety and the enzyme that maintains methylation, a unified model of FMR1 gene silencing are discussed.

Functional analyses of MeCP2 mutations associated with Rett syndrome using transient expression systems

Rett syndrome, an X-linked neurodevelopmental disorder, is a major cause of mental retardation in females. Recent genetic analyses have revealed that mutations in the methyl-CpG-binding protein gene encoding MeCP2 are associated with Rett syndrome. In this study, we used transient expression systems to investigate the functional significance of mutations seen in patients with Rett syndrome. Missense mutations in the methyl-CpG-binding domain were analyzed by the transfection in mouse L929 cells and Drosophila SL2 cells. The L929 cells were utilized to investigate the effects of mutations on the affinity for heterochromatin, where methylated CpG dinucleotides are extremely enriched. The SL2 cells were utilized to analyze their effects on transcriptional repression activities. R106W and F155S mutations led to the substantial impairment of MeCP2 functions, showing the loss of accumulation of the mutated protein to mouse heterochromatin and the reduction of the transcriptional repressive activity in Drosophila SL2 cells. Intriguingly, the R133C mutant retained the functionality equivalent to MeCP2 in these analyses. On the other hand, the T158M mutation exhibited the intermediate level of the impairment of functions in both analyses. Thus, these functional assays are useful to evaluate the consequences of mutation in the methyl-CpG-binding domain of MeCP2 and provide an insight into the relationship between the genotype and the severity of Rett syndrome.

Hypomethylation of the MN/CA9 promoter and upregulated MN/CA9 expression in human renal cell carcinoma

MN/CA9 is a cancer-related gene, frequently activated in human renal cell carcinomas (RCCs). To reveal the activation mechanism, we investigated the relationship between methylation status of the MN/CA9 promoter region and gene expression using 13 human RCCs, and examined the effect of in vitro CpG methylation on the MN/CA9 promoter activity using a human RCC cell line (SK-RC-44), expressing MN/CA9. MN/CA9 expression was evaluated by RT-PCR and observed in 10 of 13 RCCs (77%). A total of 9 out of 10 MN/CA9 -positive RCCs (90%) contained clear cell components. Methylation status of 6 CpGs in the MN/CA9 promoter region was decided by using the bisulfite genomic sequencing protocol. Out of 13 RCCs 9 (69%) showed partial hypomethylation of the CpG at -74 bp, while the other 4 RCCs and 3 normal kidney tissue samples showed complete methylation. Hypomethylation of the CpG at -74 bp was strongly correlated with MN/CA9 expression. Luciferase assay revealed that the MN/CA9 promoter activity was strongly suppressed by methylation of the CpG at -74 bp. These findings suggest that hypomethylation of the CpG at -74 bp in the MN/CA9 promoter region might play an important role in this gene activation of human RCC.

Induction of transcription factor AP-2 by cytokines and prostaglandins in cultured mesangial cells

BACKGROUND/AIMS

Activator protein-2 (AP-2) is an important transcription factor for activation of growth- and inflammatory-associated genes. To detect AP-2 in the mesangium, the expression level of AP-2 was examined in cultured mesangial cells in response to various cytokines and prostaglandins. The level was also observed in kidney tissue samples obtained from patients with proteinuria and from a rat nephrosis model.

METHODS

AP-2 was immunohistochemically detected with a specific antibody. The expression level was analyzed by immunoblotting. Human tissue samples were obtained from patients with proteinuria. Kidney samples were also obtained from rats with puromycin aminonucleoside-induced nephrosis.

RESULTS

Pro-inflammatory cytokines, such as IL-6, IL-1 and IL-2, but not TNF-alpha, induced AP-2 expression in a time- and dose-dependent manner in cultured mesangial cells. PGE(2) and PGI(2) also induced AP-2 expression, while PGF(2 alpha) failed to induce this protein. High expression levels of AP-2 were observed in different cell types including mesangial cells of kidney samples from patients with proteinuria. Similar results were obtained from the rat nephrosis model.

CONCLUSION

These findings demonstrate that the primary cytokines induce AP-2 protein in mesangial cells. AP-2 may act as a transcription factor to produce additional cytokines and growth-associated gene products, suggesting an important role for AP-2 for the function of mesangial cells in glomerular disorders.

The dynamics of myogenin site-specific demethylation is strongly correlated with its expression and with muscle differentiation

The molecular mechanisms underlying the activation of tissue-specific genes have not yet been fully clarified. We analyzed the methylation status of specific CCGG sites in the 5'-flanking region and exon 1 of myogenin gene, a very important myogenic differentiation factor. We demonstrated a loss of methylation, at the onset of C2C12 muscle cell line differentiation, limited to the CCGG site of myogenin 5'-flanking region, which was strongly correlated with the transcriptional activation of this gene and with myogenic differentiation. The same CCGG site was also found to be hypomethylated, in vivo, in embryonic mouse muscle (a myogenin-expressing tissue), as opposed to nonmuscle (nonexpressing) tissues that had a fully methylated site. In a C2C12-derived clone with enhanced myogenic ability, demethylation occurred within 2 h of induction of differentiation, suggesting the involvement of some active demethylation mechanism(s) that occur in the absence of DNA replication. Exposure to drugs that inhibit DNA methylation by acting on the S-adenosylmethionine metabolism produced a further reduction, to a few minutes, in the duration of the demethylation dynamics. These effects suggest that the final site-specific DNA methylation pattern of tissue-specific genes is defined through a continuous, relatively fast interplay between active DNA demethylation and re-methylation mechanisms.

Intron-exon swapping of transglutaminase mRNA and neuronal Tau aggregation in Alzheimer's disease

In order to understand the mechanism for insoluble neurotoxic protein polymerization in Alzheimer's disease (AD) brain neurons, we examined protein and gene expression for transglutaminase (TGase 2; tissue transglutaminase (tTG)) in hippocampus and isocortex. We found co-localization of tTG protein and activity with tau-positive neurofibrillary tangles, whereas mRNA and sequence analysis indicated an absolute increase in tTG synthesized. Although apoptosis in AD hippocampus is now an established mode of neuronal cell death, no definite underlying mechanism(s) is known. Since TGase-mediated protein aggregation is implicated in polyglutamine ((CAG)(n)/Q(n) expansion) disorder apoptosis, and expanded Q(n) repeats are excellent TGase substrates, a role for TGase in AD is possible. However, despite such suggestions almost 20 years ago, the molecular mechanism remained elusive. We now present one possible molecular mechanism for tTG-mediated, neurotoxic protein polymerization leading to neuronal apoptosis in AD that involves not its substrates (like Q(n) repeats) but rather the unique presence of alternative transcripts of tTG mRNA. In addition to a full-length (L) isoform in aged non-demented brains, we found a short isoform (S) lacking a binding domain in all AD brains. Our current results identify intron-exon "switching" between L and S isoforms, implicating G-protein-coupled signaling pathways associated with tTG that may help to determine the dual roles of this enzyme in neuronal life and death processes.

Protein crosslinking in assembly and remodelling of extracellular matrices: the role of transglutaminases

Transglutaminases form a family of proteins that have evolved for specialized functions such as protein crosslinking in haemostasis, semen coagulation, or keratinocyte cornified envelope formation. In contrast to the other members of this protein family, tissue transglutaminase is a multifunctional enzyme apparently involved in very disparate biological processes. By virtue of its reciprocal Ca2+-dependent crosslinking activity or GTP-dependent signal transducing activity, tissue transglutaminase exhibits true multifunctionality at the molecular level. The crosslinking activity can subserve disparate biological phenomena depending on the location of the target proteins. Intracellular activation of tissue transglutaminase can give rise to crosslinked protein envelopes in apoptotic cells, whereas extracellular activation contributes to stabilization of the extracellular matrix and promotes cell-substrate interaction. While tissue transglutaminase synthesis and activation is normally part of a protective cellular response contributing to tissue homeostasis, the enzyme has also been implicated in a number of pathological conditions including fibrosis, atherosclerosis, neurodegenerative diseases, celiac disease, and cancer metastasis. This review discusses the role of transglutaminases in extracellular matrix crosslinking with a focus on the multifunctional enzyme tissue transglutaminase.

Tissue transglutaminase: a possible role in neurodegenerative diseases

Tissue transglutaminase is a multifunctional protein that is likely to play a role in numerous processes in the nervous system. Tissue transglutaminase posttranslationally modifies proteins by transamidation of specific polypeptide bound glutamines. This action results in the formation of protein crosslinks or the incorporation of polyamines into substrate proteins, modifications that likely have significant effects on neural function. Tissue transglutaminase is a unique member of the transglutaminase family as in addition to catalyzing the calcium-dependent transamidation reaction, it also binds and hydrolyzes ATP and Guanosine 5'-triphosphate and may play a role in signal transduction. Tissue transglutaminase is a highly regulated and inducible enzyme that is developmentally regulated in the nervous system. In vitro, numerous substrates of tissue transglutaminase have been identified, and several of these proteins have been shown to be in situ substrates as well. Several specific roles for tissue transglutaminase have been described and there is evidence that tissue transglutaminase may also play a role in apoptosis. Recent findings have provided evidence that dysregulation of tissue transglutaminase may contribute to the pathology of several neurodegenerative conditions including Alzheimer's disease and Huntington's disease. In both of these diseases tissue transglutaminase and transglutaminase activity are elevated compared to age-matched controls. Further, immunohistochemical studies have demonstrated that there is an increase in tissue transglutaminase reactivity in affected neurons in both Alzheimer's and Huntington's disease. Although intriguing, many issues remain to be addressed to definitively establish a role for tissue transglutaminase in these neurodegenerative diseases.

Loss of tissue transglutaminase as a biomarker for prostate adenocarcinoma

BACKGROUND

Additional molecular tissue biomarkers for prostate carcinoma are needed to stratify patients with clinically suspicious findings, such as an elevated prostate specific antigen (PSA) with a negative biopsy, according to risk.

METHODS

Prostate tissues from 43 cancer cases and 47 controls with no evidence of cancer were labeled for transglutaminase by immunohistochemistry. Immunoreactivity was quantified using the Autocyte Pathology Workstation. In addition, quantitative fluorescence image analysis was used to compare transglutaminase concentrations in cells obtained by fine-needle aspiration from excised prostates. Loss of gene expression was evaluated by reverse transcriptase-polymerase chain reaction and growth with 5-azacytidine.

RESULTS

Visually, benign glands from controls generally expressed tissue transglutaminase, whereas regions with adenocarcinoma generally were negative. With quantitative immunohistochemistry, 41 of 43 adenocarcinoma of the prostate (CaP) cases expressed lower mean percentage areas positive for transglutaminase than did 30 of 30 benign prostatic hyperplasia (BPH) and 17 of 17 prostatitis cases (P < 0.0001; odds ratio [OR], 1577; 95% confidence interval (CI), 74-33, 820; relative risk [RR], 25; 95% CI, 6-95). Quantitative immunofluorescence of 3277 cells collected by FNA from 19 CaP cases and 645 cells from 5 cases of BPH showed that the mean content of transglutaminase was 93 femtograms (fg) for the CaP-derived cells and 138 fg for the BPH cells (P < 0.0001). Receiver operating curve analysis of the immunohistochemistry data showed an optimized threshold produced 95% sensitivity with 100% specificity. Growth of LNCaP cells with 5-azacytidine failed to stimulate transglutaminase expression, suggesting that loss of expression was likely not attributable to promoter methylation.

CONCLUSIONS

Measurements of transglutaminase on tissue sections provides additional diagnostic information that is potentially useful for risk assessment of patients with suspicious clinical findings, such as nodules or positive PSA and negative biopsies, without overdetecting disease.

Tissue-specific N-terminal isoforms from overlapping alternate promoters of the human AE2 anion exchanger gene

Previously, we isolated the human AE2 (SLC4A2) gene, a member of the sodium-independent anion exchanger family. Rat ortholog of this gene was reported to drive alternative transcription yielding N-terminal variants of the AE2a message. We thus analyzed the human AE2 gene in this regard. Using HepG2 cells, two alternative first exons, each splicing to exon 3 in alternative transcripts, were found to be transcribed from overlapping sequences of intron 2. Exon 1b(1) corresponds to the rat variant "b" and encodes three initial residues (MTQ) in AE2b(1) isoform that replace the first 17 amino acids of AE2a protein, while the novel exon 1b(2) encodes eight initial residues (MDFLLRPQ) in AE2b(2) isoform. The relative abundance of AE2b(1) and AE2b(2) mRNAs was about 10% of AE2a mRNA each. Alternate promoter sequences have multiple potential binding motifs for liver-enriched factors, and dual-luciferase assays indicated that they possess the ability for driving transcription in transiently transfected HepG2 cells. Tissue survey showed that expression of human AE2b(1) and AE2b(2) transcripts is restricted to liver and kidney, while AE2a mRNA was encountered in all examined tissues. Our findings reveal a characteristic tissue-specific expression of two N-terminal variants of human AE2 from overlapping sequences within intron 2, one of which is a novel isoform.

Expression of human histo-blood group ABO genes is dependent upon DNA methylation of the promoter region

We have investigated the regulatory role of DNA methylation in the expression of the human histo-blood group ABO genes. The ABO gene promoter region contains a CpG island whose methylation status correlates well with gene expression in the cell lines tested. The CpG island was found hypomethylated in some cell lines that expressed ABO genes, whereas the other cell lines that did not express ABO genes were hypermethylated. Whereas constitutive transcriptional activity of the ABO gene promoter was demonstrated in both expressor and nonexpressor cell lines by transient transfection of reporter constructs containing the ABO gene promoter sequence, HhaI methylase-catalyzed in vitro methylation of the promoter region prior to DNA transfection suppressed the promoter activity when introduced into the expressor gastric cancer cell line KATOIII cells. On the other hand, in the nonexpressor gastric cancer cell line MKN28 cells, treatment with DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine resulted in demethylation of the ABO gene promoter and appearance of A-transferase messages, as well as A-antigens synthesized by A-transferase. Taken together, these studies suggest that DNA methylation of the ABO gene promoter may play an important role in the regulation of ABO gene expression.

Novel bimodal effects of the G-protein tissue transglutaminase on adrenoreceptor signalling

Tissue transglutaminase (tTG) is a novel G-protein that previous studies showed can couple ligand-bound activated alpha(1B) adrenoreceptors to phospholipase C-delta, resulting in phosphoinositide (PI) hydrolysis. In human neuroblastoma SH-SY5Y cells we found that although endogenous tTG can facilitate alpha(1B) adrenoreceptor-stimulated PI hydrolysis, its contribution is minor compared with the classical heterotrimeric G-protein G(q/11). Further, we show that the alpha(1B) adrenoreceptor recruits tTG to the membrane and that this recruitment is enhanced by agonist occupancy of the receptor. In addition, the effects of tTG on signalling are bimodal. At low expression levels, tTG enhanced alpha(1B) adrenoreceptor-stimulated PI hydrolysis, whereas at higher expression levels tTG attenuated significantly this response. These findings are the first to demonstrate that a protein can both facilitate and attenuate receptor-stimulated PI hydrolysis.

Induction of transcription factor AP-2 by inflammatory cytokines in human keratinocytes

Activator protein-2 is an important transcription factor for the activation of a number of genes. Here we report the induction of activator protein-2 in response to inflammatory cytokines such as interleukin-6 in keratinocytes. Immunoblotting and semiquantitative reverse transcriptase-polymerase chain reaction assays using normal human keratinocytes revealed that interleukin-6 caused a time- and concentration-dependent induction of activator protein-2 mRNA and protein. The increase of activator protein-2 mRNA was detected at 30 min after stimulation and that of activator protein-2 protein was at 2 h. Their levels were lower than the control levels at 24 h. The interleukin-6-dependent induction of activator protein-2 mRNA was completely blocked by adding actinomycin D, whereas it was approximately 50% affected by cycloheximide. Co-incubation with neutralizing antibodies against various inflammatory cytokines resulted in inhibition of the interleukin-6-dependent activator protein-2 induction at varying degrees, indicating an involvement of various cytokines in the activator protein-2 induction. The activator protein-2 induction was observed in keratinocytes derived from lesional skins with psoriasis or squamous cell carcinoma, and the high levels of activator protein-2 were histochemically detected in these lesions. Furthermore, a gel mobility shift assay using the nuclear extracts from interleukin-6-treated cells showed that interleukin-6 induced the functional activator protein-2 protein for the gene activation. These findings suggest a possible regulation mechanism of activator protein-2 through a complex cytokine system, which is conceivably the initial reaction leading to skin inflammation, and resultant keratinocyte growth and carcinogenesis.

Tissue transglutaminase: an enzyme with a split personality

Tissue transglutaminase (tTG) belongs to the family of transglutaminase enzymes that catalyze the posttranslational modification of proteins via Ca(2+)-dependent cross-linking reactions. The catalytic action of tTG results in the formation of an isopeptide bond that is of great physiological significance since it is highly resistant to proteolysis and denaturants. Although tTG-mediated cross-linking reactions have been implicated to play a role in diverse biological processes, the precise physiological function of the enzyme remains unclear. Recent data, however, suggest that the protein polymers resulting from tTG-catalyzed reactions may play a role in commitment of cells to undergo apoptosis. On the same token, tTG-mediated formation of insoluble protein aggregates may underlie the markers of numerous pathological conditions, such as the senile plaques in Alzheimer's disease and the Lewy bodies in Parkinson's disease. In addition to catalyzing Ca(2+)-dependent cross-linking reactions, tTG can also bind and hydrolyze guanosine triphosphate and adenosine triphosphate. By virtue of this ability, tTG has been identified as a novel G-protein that interacts and activates phospholipase C following stimulation of the alpha-adrenergic receptor. The ability of tTG to mediate signal transduction may contribute to its involvement in the regulation of cell cycle progression. The following review summarizes the important features of this multifunctional enzyme that have emerged as a result of recent work from different laboratories.

Transcriptional regulation of cell type-specific expression of the TATA-less A subunit gene for human coagulation factor XIII

To study the mechanism of gene regulation for coagulation factor XIII A subunit (FXIIIA), we characterized its 5'-flanking region using a monocytoid (U937), a megakaryocytoid (MEG-01), and other cells. Our results confirmed that U937 and MEG-01 contained FXIIIA mRNA. A tentative transcription start site was determined to be 76 bases upstream from the first exon/intron boundary. Reporter gene assays revealed that a 5'-fragment (-2331 to +75) was sufficient to support basal expression in U937 and MEG-01 but not in the other cells. Deletion analysis confined a minimal promoter sequence from -114 to +75. DNase footprinting, electrophoretic mobility shift, and reporter gene assays demonstrated that promoter elements for a myeloid-enriched transcription factor (MZF-1-like protein) and two ubiquitous transcription factors (NF-1 and SP-1) in this region were important for the basal FXIII expression. It was also revealed that an upstream region (-806 to -290) had enhancer activity in MEG-01 but silencer activity in U937. DNA sequences for binding of myeloid-enriched factors (GATA-1 and Ets-1) were recognized in this region, and the GATA-1 element was found to be responsible for the enhancer activity. These transcription factors play a major role in the cell type-specific expression of FXIIIA, which differs from other transglutaminases.

Regulation of connexin32 and connexin43 gene expression by DNA methylation in rat liver cells

Gap junction proteins (connexins) are expressed in a cell-specific manner and expression is often reduced in neoplastic cells. We investigated the mechanisms of connexin32 (Cx32) and connexin43 (Cx43) expression in hepatic cells using MH1C1 rat hepatoma cells and freshly isolated, adult rat hepatocytes that express Cx32 but not Cx43 and WB-F344 rat liver epithelial cells that express Cx43 but not Cx32. Southern blotting after DNA restriction with MspI and HpaII indicated that two MspI/HpaII restriction sites in the Cx32 promoter (positions -147 and -847) were methylated in WB-F344 cells, but not in MH1C1 cells or hepatocytes. In contrast, an MspI/HpaII restriction site in the Cx43 promoter (position -38) was methylated in MH1C1 cells, but not in WB-F344 cells or hepatocytes. Transient transfection of the cell lines with connexin promoter-luciferase constructs indicated that the Cx32 promoter was 7-fold more active in MH1C1 cells and the Cx43 promoter was 5-fold more active in WB-F344 cells. These results suggest that transcription of Cx32 and Cx43 in hepatic cells is controlled by promoter methylation and by cell-specific transcription factors. Similar mechanisms may be involved in the reduced expression of these genes frequently observed in neoplastic cells.

DNA methylation in transcriptional repression of two differentially expressed X-linked genes, GPC3 and SYBL1

Methylation of CpG islands is an established transcriptional repressive mechanism and is a feature of silencing in X chromosome inactivation. Housekeeping genes that are subject to X inactivation exhibit differential methylation of their CpG islands such that the inactive alleles are hypermethylated. In this report, we examine two contrasting X-linked genes with CpG islands for regulation by DNA methylation: SYBL1, a housekeeping gene in the Xq pseudoautosomal region, and GPC3, a tissue-specific gene in Xq26 that is implicated in the etiology of the Simpson-Golabi-Behmel overgrowth syndrome. We observed that in vitro methylation of either the SYBL1 or the GPC3 promoter resulted in repression of reporter constructs. In normal contexts, we found that both the Y and inactive X alleles of SYBL1 are repressed and hypermethylated, whereas the active X allele is expressed and unmethylated. Furthermore, the Y and inactive X alleles of SYBL1 were derepressed by treatment with the demethylating agent azadeoxycytidine. GPC3 is also subject to X inactivation, and the active X allele is unmethylated in nonexpressing leukocytes as well as in an expressing cell line, suggesting that methylation is not involved in the tissue-specific repression of this allele. The inactive X allele, however, is hypermethylated in leukocytes, presumably reflecting early X inactivation events that become important for gene dosage in expressing lineages. These and other data suggest that all CpG islands on Xq, including the pseudoautosomal region, are subject to X inactivation-induced methylation. Additionally, methylation of SYBL1 on Yq may derive from a process related to X inactivation that targets large chromatin domains for transcriptional repression.

Genomic structure of the human DNA methyltransferase gene

We determined the genomic structure of the gene encoding human DNA methyltransferase (DNA MTase). Six overlapping human genomic DNA clones which include all of the known cDNA sequence were isolated. Analysis of these clones demonstrates that the human DNA MTase gene consists of at least 40 exons and 39 introns spanning a distance of 60 kilobases. Elucidation of the chromosomal organization of the human DNA MTase gene provides the template for future structure-function analysis of the properties of mammalian DNA MTase.

Mouse alloantigen Ly10 is identical to murine fusion regulatory protein-1 (mFRP-1)/4F2/CD98: aberrant expression of mFRP-1/Ly10 allotypes in cells derived from CDF1 mice due to the gene deletion

Murine FRP-1 (mFRP-1) and Ly10 antigens are alloantigens. Anti-Ly10.1, monoclonal antibody (mAb) reacts to HeLa cells stably expressing mFRP-1.1 (HeLa cells/mFRP-1.1), but does not react to HeLa cells/ mFRP-1.2. On the other hand, anti-Ly10.2 mAb reacts to HeLa/mFRP-1.2 cells, but does not react to HeLa cells/mFRP-1.1. These findings indicate that Ly10.1 and Ly10.2 antigens are identical to the heavy chains of mFRP-1.1 and mFRP-1.2 molecules, respectively. Thymocytes and fibroblast cells obtained from CDF1 and BDF1 mice showed reactivity to both anti-Ly10.1 and anti-Ly10.2/FRP-2 antibodies, indicating that mFRP-1/Ly10 alloantigens are codominantly expressed in the F1 mice. Intriguingly, IMC carcinoma cells, derived from CDF1 mice, express mFRP-1.2, but do not express mFRP-1.1. Surprisingly, DBT cells, derived from CDF1 mice, consist of two different cell populations: one expresses both mFRP-1.1 and mFRP-1.2 antigens and the other expresses only mFRP-1.1 antigen. Consequently, we tried to isolate cloned DBT cell lines by limiting dilution. Finally, six cloned DBT cell lines were obtained, and three clones of these cloned DBT cells expressed both mFRP-1.1 and mFRP-1.2, and another three clones of these cloned DBT cells expressed both mFRP-1.1 and mFRP-1.2, and another three clones including clone 5 cells expressed only mFRP-1.1 antigen. No induction of mFRP-1.2/Ly10.2 by 5-azacytidine and sodium n-butyrate could be detected in DBT clone 5 cells. Neither mFRP-1.2 mRNA nor the genomic cDNA clone encoding mFRP-1.2 could be detected in DBT clone 5 cells, nor could the genomic cDNA encoding mFRP-1.1 be detected in IMC cells, indicating that aberrant expression of mFRP-1/Ly10 allotypes in CDF1 mice-derived cells is due to the gene deletion.

DNA methylation contributes to expression of the human neurotensin/neuromedin N gene

The gut and liver share a common embryological origin. The gene encoding the gut hormone neurotensin/neuromedin N (NT/N) is expressed in the adult small bowel, and NT/N is transiently expressed in the fetal liver, suppressed in the adult liver, and reexpressed in certain liver cancers. In our present study, we found that the NT/N gene was expressed at high levels in the human hepatoma cell line Hep 3B but was not expressed in Hep G2 cells. To further determine the mechanisms regulating NT/N expression, we performed Southern blotting and gene cloning techniques. Neither alteration nor mutation of the NT/N gene was responsible for this differential NT/N expression pattern. Human NT/N promoter constructs were transfected into either Hep 3B or Hep G2. Both cell lines supported NT/N transcription, indicating that the absence of NT/N expression in Hep G2 cells was due to mechanisms other than the absence of positive transcription factors. The role of DNA methylation was next assessed. Methylation of NT/N promoter constructs in vitro resulted in a 67-fold reduction in promoter activity, whereas treatment with the demethylating agent 5-azacytidine induced NT/N expression in Hep G2 cells, thus suggesting that DNA methylation plays a role in the expression of the gut endocrine gene NT/N. Defining the mechanisms regulating NT/N expression in these hepatic-derived cell lines will provide not only a better understanding of cell-specific and developmental regulation of a gut endocrine gene but also possible insight into liver cell lineage patterns and the derivation of certain hepatocellular cancers.