A GC-rich element confers epidermal growth factor responsiveness to transcription from the gastrin promoter

Activation of pro-oncogenic pathways in colorectal hyperplastic polyps

Background

In contrast to sessile serrated adenomas and traditional serrated adenomas which are associated with a significant cancer risk, the role of hyperplastic polyps (HP) in colorectal carcinogenesis as well as the molecular mechanisms underlying their development remain controversial and still need to be clarified. Several reports suggest that a subset of HP may represent precursor lesions of some colorectal cancers. However, biomarkers are needed to identify the subset of HP that may have a malignant potential. The hormone precursor, progastrin (PG) has been involved in colon carcinogenesis and is known to activate pro-oncogenic pathways such as the ERK or the STAT3 pathway. We therefore analyzed PG expression and the activation of these signaling factors in HP.

Methods

We retrospectively analyzed PG expression as well as the phosphorylation of ERK and STAT3 by immunohistochemistry in HP from 48 patients.

Results

Mean percentages of epithelial cells positive for PG or phospho-ERK were respectively, 31% and 33% in HP and were significantly higher in these lesions compared to normal colon (3%, p = 0.0021 and 7%, p = 0.0008, respectively). We found a significant correlation between PG and phospho-ERK expression in HP with ERK activation significantly stronger in lesions with high progastrin expression (p = 0.015). In contrast, STAT3 was not significantly activated in HP compared to normal colon and we did not observe a significant correlation with PG expression.

Conclusions

HP overexpressing PG that have the highest activation of the ERK pathway might reflect less latent lesions that might have a malignant potential.

Role of gastrin-peptides in Barrett's and colorectal carcinogenesis

Gastrin is the main hormone responsible for the stimulation of gastric acid secretion; in addition, gastrin and its derivatives exert proliferative and antiapoptotic effects on several cell types. Gastrin synthesis and secretion are increased in certain situations, for example, when proton pump inhibitors are used. The impact of sustained hypergastrinemia is currently being investigated. In vitro experiments and animal models have shown that prolonged hypergastrinemia may be related with higher cancer rates; although, this relationship is less clear in human beings. Higher gastrin levels have been shown to cause hyperplasia of several cell types; yet, the risk for developing cancer seems to be the same in normo- and hypergastrinemic patients. Some tumors also produce their own gastrin, which can act in an autocrine manner promoting tumor growth. Certain cancers are extremely dependent on gastrin to proliferate. Initial research focused only on the effects of amidated gastrins, but there has been an interest in intermediates of gastrin in the last few decades. These intermediates aren’t biologically inactive; in fact, they may exert greater effects on proliferation and apoptosis than the completely processed forms. In certain gastrin overproduction states, they are the most abundant gastrin peptides secreted. The purpose of this review is to examine the gastrin biosynthesis process and to summarize the results from different studies evaluating the production, levels, and effects of the main forms of gastrin in different overexpression states and their possible relationship with Barrett’s and colorectal carcinogenesis.

A gastrin transcript expressed in gastrointestinal cancer cells contains an internal ribosome entry site

As the hormone gastrin promotes gastrointestinal (GI) cancer progression by triggering survival pathways, regulation of gastrin expression at the translational level was explored. Sequence within the 5' untranslated region of a gastrin transcript expressed in GI cancer cells was investigated, then cloned into a bicistronic vector upstream of firefly luciferase and transfected into a series of GI cancer cell lines. Firefly luciferase activity was measured relative to that of a cap-dependent Renilla luciferase. A gastrin transcript that was different from that described in Ensembl was expressed in GI cancer cells. Its transcription appears to be initiated within the region designated as the gene's first intron. In GI cancer cells transfected with the bicistronic construct, firefly luciferase activity increased 8-15-fold compared with the control vector, and there was a further induction of the signal (up to 25-fold) following exposure of the cells to genotoxic stress or hypoxia, suggesting that the sequence acts as an internal ribosome entry site. These data suggest that the gastrin transcript within GI cancer cells contains an internal ribosome entry site that may allow continued expression of gastrin peptides when normal translational mechanisms are inactive, such as in hypoxia, thereby promoting cancer cell survival.

DNA methylation in the promoter region of the p16 (CDKN2/MTS-1/INK4A) gene in human breast tumours

The p16 (CDKN2/MTS-1/INK4A) gene is one of several tumour-suppressor genes that have been shown to be inactivated by DNA methylation in various human cancers including breast tumours. We have used bisulphite genomic sequencing to examine the detailed sequence specificity of DNA methylation in the CpG island promoter/exon 1 region in the p16 gene in DNA from a series of human breast cancer specimens and normal human breast tissue (from reductive mammaplasty). The p16 region examined was unmethylated in the four normal human breast specimens and in four out of nine breast tumours. In the other five independent breast tumour specimens, a uniform pattern of DNA methylation was observed. Of the nine major sites of DNA methylation in the amplified region from these tumour DNAs, four were in non-CG sequences. This unusual concentration of non-CG methylation sites was not a general phenomenon present throughout the genome of these tumour cells because the methylated CpG island regions of interspersed L1 repeats had a pattern of (almost exclusively) CG methylation similar to that found in normal breast tissue DNA and in DNA from tumours with unmethylated p16 genes. These data suggest that DNA methylation of the p16 gene in some breast tumours could be the result of an active process that generates a discrete methylation pattern and, hence, could ultimately be amenable to therapeutic manipulation.

Regulation of transforming growth factor alpha expression in a growth factor-independent cell line

Aberrant transcriptional regulation of transforming growth factor alpha (TGF alpha) appears to be an important contributor to the malignant phenotype and the growth factor independence with which malignancy is frequently associated. However, little is known about the molecular mechanisms responsible for dysregulation of TGF alpha expression in the malignant phenotype. In this paper, we report on TGF alpha promoter regulation in the highly malignant growth factor-independent cell line HCT116. The HCT116 cell line expresses TGF alpha and the epidermal growth factor receptor (EGFR) but is not growth inhibited by antibodies to EGFR or TGF alpha. However, constitutive expression of TGF alpha antisense RNA in the HCT116 cell line resulted in the isolation of clones with markedly reduced TGF alpha mRNA and which were dependent on exogenous growth factors for proliferation. We hypothesized that if TGF alpha autocrine activation is the major stimulator of TGF alpha expression in this cell line, TGF alpha promoter activity should be reduced in the antisense TGF alpha clones in the absence of exogenous growth factor. This was the case. Moreover, transcriptional activation of the TGF alpha promoter was restored in an antisense-TGF alpha-mRNA-expressing clone which had reverted to a growth factor-independent phenotype. Using this model system, we were able to identify a 25-bp element within the TGF alpha promoter which conferred TGF alpha autoregulation to the TGF alpha promoter in the HCT116 cell line. In the TGF alpha-antisense-RNA-expressing clones, this element was activated by exogenous EGF. This 25-bp sequence contained no consensus sequences of known transcription factors so that the TGF alpha or EGF regulatory element within this 25-bp sequence represents a unique element. Further characterization of this 25-bp DNA sequence by deletion analysis revealed that regulation of TGF alpha promoter activity by this sequence is complex, as both repressors and activators bind in this region, but the overall expression of the activators is pivotal in determining the level of response to EGF or TGF alpha stimulation. The specific nuclear proteins binding to this region are also regulated in an autocrine-TGF alpha-dependent fashion and by exogenous EGF in EGF-deprived TGF alpha antisense clone 33. This regulation is identical to that seen in the growth factor-dependent cell line FET, which requires exogenous EGF for optimal growth. Moreover, the time response of the stimulation of trans-acting factor binding by EGF suggests that the effect is directly due to growth factor and not mediated by changes in growth state. We conclude that this element appears to represent the major positive regulator of TGF alpha expression in the growth factor-independent HCT116 cell line and may represent the major site of transcriptional dysregulation of TGF alpha promoter activity in the growth factor-independent phenotype.

EGF receptor activation stimulates endogenous gastrin gene expression in canine G cells and human gastric cell cultures

Gastrin release from the antral gastrin-expressing cell (G cell) is regulated by bombesin and luminal factors. Yet, these same extracellular regulators do not stimulate expression of the gene. Since the gastric mucosa expresses large quantities of EGF receptor ligands such as TGFalpha, we examined whether EGF receptor ligands stimulate gastrin gene expression in gastrin-expressing cell cultures. EGF receptor activation of primary cultures stimulated gastrin gene expression about twofold; whereas bombesin treatment of antral G cell cultures stimulated gastrin release but not gene expression. EGF and TGFalpha were weak stimulants of gastrin release. EGF receptor activation of AGS human gastric adenocarcinoma cell line stimulated gastrin gene expression nearly fourfold; and gastrin reporter constructs transfected into AGS cells were stimulated more than fourfold by EGF. EGF induction was conferred by the previously defined GC-rich gastrin EGF response element (gERE) element located at -68 to -53 bp upstream from the cap site since a mutation of the gERE element abolished both basal and EGF induction. Moreover, EGF treatment of AGS cells stimulated binding of the transcription factor Sp1 to this element. Collectively, these results demonstrate that gastrin gene expression and gastrin release are regulated by different signaling pathways: gene expression by EGF receptor activation and gastrin secretion by neuropeptides and luminal factors.

ZBP-89, a Krüppel-like zinc finger protein, inhibits epidermal growth factor induction of the gastrin promoter

We have shown previously that a GC-rich element (GGGGCGGGGTGGGGGG) conferring epidermal growth factor (EGF) responsiveness to the human gastrin promoter binds Sp1 and additional undefined complexes. A rat GH4 cell line expression library was screened by using a multimer of the gastrin EGF response element, and three overlapping cDNA clones were identified. The full-length rat cDNA encoded an 89-kDa zinc finger protein (ZBP-89) that was 89% identical to a 49-kDa human factor, ht(beta), that binds a GTGGG/CACCC element in T-cell receptor promoters. The conservation of amino acids between the zinc fingers indicates that ZBP-89 is a member of the C2H2 zinc finger family subclass typified by the Drosophila Krüppel protein. ZBP-89 is ubiquitously expressed in normal adult tissues. It binds specifically to the gastrin EGF response element and inhibits EGF induction of the gastrin promoter. Collectively, these results demonstrate that ZBP-89 functions as a repressor of basal and inducible expression of the gastrin gene.

Homologous DNA sequences and cellular factors are implicated in the control of glucagon and insulin gene expression

The glucagon gene is specifically expressed in the alpha cells of pancreatic islets. The promoter of the glucagon gene is responsible for this specificity. Within the promoter, the upstream promoter element G1 is critical to restrict expression to the alpha cells. We define here a composite DNA control element, G4, localized upstream of G1 between nucleotides -100 and -140 which functions as an islet-specific activator in both glucagon- and insulin-producing cells but not in nonislet cells. G4 contains at least three protein binding sites. The most proximal site, E2, is highly homologous to the E1, SMS-UE, and B elements of the rat insulin I, somastatin, and elastase I genes, respectively, and interacts with a pancreas-specific complex; the distal site, E3, represents an E box which is identical to the E boxes of the rat insulin I and II genes and binds to a complex similar or identical to IEF1 which has been implicated in the tissue-specific control of insulin gene expression. These two sites necessitate a third element, the intervening sequence, to activate transcription. We conclude that the first 140 bp of the glucagon gene promoter contains at least two DNA control elements responsible for pancreatic alpha-cell-specific expression: G4, an islet cell-specific element sharing common binding sites with the insulin gene, and G1, which restricts glucagon gene expression to the alpha cells. This double control of specificity might have relevance during islet cell differentiation.

Epidermal growth factor-stimulated parathyroid hormone-related protein expression involves increased gene transcription and mRNA stability

Epidermal growth factor (EGF) produced rapid and striking effects on parathyroid hormone-related protein (PTHrP) gene expression in the immortalized human keratinocyte cell line, HaCaT. Steady-state levels of PTHrP mRNA and secreted PTHrP were increased 10-fold by maximally effective concentrations of EGF. EGF increased both PTHrP gene transcription and PTHrP mRNA stability. Nuclear run-on assays demonstrated a 4-fold increase in transcriptional rate in EGF-stimulated cells while transient transfection analysis indicated that the action of EGF on transcription involved both the GC-rich promoter, P2, and the downstream TATA promoter, P3, but apparently not the upstream TATA promoter, P1. In experiments where EGF treatment produced more stable PTHrP transcripts, the half-life of c-fos mRNA was unaltered, suggesting a relatively specific effect of EGF. Moreover, only those species of PTHrP mRNA containing two of the alternative 3' exons (exons VII and VIII) were stable, those containing exon IX were not. Reverse-transcription PCR demonstrated that EGF produced differential increases in the abundance of PTHrP mRNA species initiated by the three PTHrP promoters. The major effect was seen on the abundance of transcripts initiated by P1 and P2, with less marked regulation of P3-initiated transcripts. Thus EGF regulation of PTHrP gene expression in HaCaT cells is multifactorial and the combination of its actions at the 5' and 3' ends of the gene favours the accumulation of subpopulations of PTHrP mRNA containing exons I, VII and VIII.

Tumors in hepatobiliary tract and pancreatic islet tissues of transgenic mice harboring gastrin simian virus 40 large tumor antigen fusion gene

Gastrin is expressed in the gastric antrum and in fetal pancreatic islets but not in adult islets. We have now identified the hepatobiliary tract as another, previously unknown, potential site of gastrin gene expression. Two human gastrin simian virus 40 large tumor antigen (SV40 T antigen) fusion genes containing 1.5 kb of 5' flanking sequence and 10.5 kb that included 5.5 kb upstream, 1.5 kb downstream, and the entire transcribed region were used to generate transgenic mice. Analysis of several transgenic lines, derived from both fusion genes, revealed development of transmissible hepatobiliary tract tumors and pancreatic islet cell tumors. Analysis of each of the tumor cells demonstrates expression of SV40 T antigen but no expression of gastrin. Of the two fusion genes, only the 10.5-kb sequence induces hyperplasia of gastrin-producing cells in the antrum. Analysis of these cells demonstrates expression of SV40 T antigen and gastrin, suggesting that the 10.5-kb sequence is sufficient for gastrin cell hyperplasia in the antrum. These data raise the possibility that gastrin is transiently expressed in the hepatobiliary tract.

Epidermal growth factor and transforming growth factor alpha specifically induce the activation- and hyperproliferation-associated keratins 6 and 16

Epidermal injury results in activation of keratinocytes which produce and respond to growth factors and cytokines and become migratory. Activated keratinocytes express a specific pair of keratin proteins, K6 and K16, distinct from the keratins in the healthy epidermis. Keratinocytes can be activated, for example, by binding of the appropriate ligands to the epidermal growth factor receptor (EGFR). We have analyzed the effects of EGFR activation on keratin gene transcription by transfecting DNAs containing keratin promoters linked to a reporter gene into primary cultures of human epidermal keratinocytes in the presence or absence of EGF or transforming growth factor alpha (TGF alpha), two growth factors that activate EGFR. The activation of EGFR had no effect on the promoters of simple epithelial, basal-layer-specific, or differentiation-specific keratins. In contrast, the expression of K6 and K16 was strongly and specifically induced. A 20-bp DNA segment of the K16 gene promoter conveyed the EGF regulation, functioned in a heterologous construct, and therefore constituted an EGF-responsive element. A nuclear protein specifically bound to this element and to the analogous sequence of the K6 promoter. Thus, EGF specifically induces K6 and K16, markers of activated keratinocytes, via nuclear proteins that bind to EGF-responsive elements in the promoters of these keratin genes.

A mitogen-responsive promoter region that is synergistically activated through multiple signalling pathways

A regulatory region of the human transferrin receptor gene promoter was found to be required for increased expression in response to serum or growth factors. This region contains two elements that appear to cooperate for full responsiveness. We found that sodium orthovanadate treatment of cells significantly activated expression of promoter constructs containing these elements. 12-O-Tetradecanoylphorbol-13-acetate alone induced a twofold increase in expression but acted synergistically with vanadate to generate a highly elevated level of expression. Dibutyryl cyclic AMP alone had no effect on expression, but when added together with vanadate and 12-O-tetradecanoylphorbol-13-acetate, led to superinduction of the promoter construct. Induction of expression by these reagents was delayed several hours, and the kinetics were identical to those observed for serum induction.

A mitogen-responsive promoter region that is synergistically activated through multiple signalling pathways

A regulatory region of the human transferrin receptor gene promoter was found to be required for increased expression in response to serum or growth factors. This region contains two elements that appear to cooperate for full responsiveness. We found that sodium orthovanadate treatment of cells significantly activated expression of promoter constructs containing these elements. 12-O-Tetradecanoylphorbol-13-acetate alone induced a twofold increase in expression but acted synergistically with vanadate to generate a highly elevated level of expression. Dibutyryl cyclic AMP alone had no effect on expression, but when added together with vanadate and 12-O-tetradecanoylphorbol-13-acetate, led to superinduction of the promoter construct. Induction of expression by these reagents was delayed several hours, and the kinetics were identical to those observed for serum induction.

Identification of a transcriptional enhancer important for enteroendocrine and pancreatic islet cell-specific expression of the secretin gene

It is well established that the gene encoding the hormone secretin is expressed in a specific enteroendocrine cell, the S cell. We now show that the secretin gene is transiently expressed in insulin-producing B cells of the developing pancreatic islets in addition to the intestine. Furthermore, secretin is produced by most established islet cell lines. In order to identify and characterize the regulatory elements within the secretin gene that control tissue-specific expression, we have introduced secretin reporter gene constructions into the secretin-producing HIT and STC-1 cell lines as well as the nonexpressing INR1-G9 glucagonoma line. Analysis of deletion mutants revealed that sequences between 174 and 53 bp upstream from the transcriptional start site are required for maximal expression in secretin-producing cells. This positive element functioned independently of position and orientation. Further deletions into the enhancer resulted in a stepwise loss of transcriptional activity, suggesting the presence of several discrete control elements. The sequence CAGCTG within the secretin enhancer closely resembles that of the core of the B-cell-specific enhancer in the insulin gene. Point mutations introduced into this putative element led to greater than 85% reduction in transcriptional activity. Gel mobility shift assays suggested that a factor in B cells closely related or identical to proteins that bind to the insulin enhancer interacts with the CAGCTG motif in the secretin gene.

Evolutionary consequences of nonrandom damage and repair of chromatin domains

Some evolutionary consequences of different rates and trends in DNA damage and repair are explained. Different types of DNA damaging agents cause nonrandom lesions along the DNA. The type of DNA sequence motifs to be preferentially attacked depends upon the chemical or physical nature of the assaulting agent and the DNA base composition. Higher-order chromatin structure, the nonrandom nucleosome positioning along the DNA, the absence of nucleosomes from the promoter regions of active genes, curved DNA, the presence of sequence-specific binding proteins, and the torsional strain on the DNA induced by an increased transcriptional activity all are expected to affect rates of damage of individual genes. Furthermore, potential Z-DNA, H-DNA, slippage, and cruciform structures in the regulatory region of some genes or in other genomic loci induced by torsional strain on the DNA are more prone to modification by genotoxic agents. A specific actively transcribed gene may be preferentially damaged over nontranscribed genes only in specific cell types that maintain this gene in active chromatin fractions because of (1) its decondensed chromatin structure, (2) torsional strain in its DNA, (3) absence of nucleosomes from its regulatory region, and (4) altered nucleosome structure in its coding sequence due to the presence of modified histones and HMG proteins. The situation in this regard of germ cell lineages is, of course, the only one to intervene in evolution. Most lesions in DNA such as those caused by UV or DNA alkylating agents tend to diminish the GC content of genomes. Thus, DNA sequences not bound by selective constraints, such as pseudogenes, will show an increase in their AT content during evolution as evidenced by experimental observations. On the other hand, transcriptionally active parts may be repaired at rates higher than inactive parts of the genome, and proliferating cells may display higher repair activities than quiescent cells. This might arise from a tight coupling of the repair process with both transcription and replication, all these processes taking place on the nuclear matrix. Repair activities differ greatly among species, and there is a good correlation between life span and repair among mammals. It is predicted that genes that are transcriptionally active in germ-cell lineages have a lower mutation rate than bulk DNA, a circumstance that is expected to be reflected in evolution. Exception to this rule might be genes containing potential Z-DNA, H-DNA, or cruciform structures in their coding or regulatory regions that appear to be refractory to repair.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of a transcriptional enhancer important for enteroendocrine and pancreatic islet cell-specific expression of the secretin gene

It is well established that the gene encoding the hormone secretin is expressed in a specific enteroendocrine cell, the S cell. We now show that the secretin gene is transiently expressed in insulin-producing B cells of the developing pancreatic islets in addition to the intestine. Furthermore, secretin is produced by most established islet cell lines. In order to identify and characterize the regulatory elements within the secretin gene that control tissue-specific expression, we have introduced secretin reporter gene constructions into the secretin-producing HIT and STC-1 cell lines as well as the nonexpressing INR1-G9 glucagonoma line. Analysis of deletion mutants revealed that sequences between 174 and 53 bp upstream from the transcriptional start site are required for maximal expression in secretin-producing cells. This positive element functioned independently of position and orientation. Further deletions into the enhancer resulted in a stepwise loss of transcriptional activity, suggesting the presence of several discrete control elements. The sequence CAGCTG within the secretin enhancer closely resembles that of the core of the B-cell-specific enhancer in the insulin gene. Point mutations introduced into this putative element led to greater than 85% reduction in transcriptional activity. Gel mobility shift assays suggested that a factor in B cells closely related or identical to proteins that bind to the insulin enhancer interacts with the CAGCTG motif in the secretin gene.

Molecular and functional analysis of the XPBC/ERCC-3 promoter: transcription activity is dependent on the integrity of an Sp1-binding site

The human XPBC/ERCC-3 gene, which corrects the excision-repair defect in xeroderma pigmentosum group B cells and the UV-sensitive CHO mutant 27-1 cells, appears to be expressed constitutively in various cell types and tissues. We have analysed the structure and functionality of the XPBC/ERCC-3 promoter. Transcription of the XPBC/ERCC-3 gene is initiated from heterogeneous sites, with a major startpoint mapped at position -54 (relative to the translation start codon ATG). The promoter region does not possess classical TATA and CAAT elements, but it is GC-rich and contains three putative Sp1-binding sites. In addition, there are two elements related to the cyclic AMP (cAMP)-response element (CRE) and the 12-O-tetradecanoyl phorbol-13-acetate-response element (TRE) in the 5'-flanking region. Transient expression analysis of XPBC/ERCC-3 promoter-CAT chimeric plasmids revealed that a 127-bp fragment, spanning position -129 to -3, is minimally required for the promoter activity. Transcription of the XPBC/ERCC-3 promoter depends on the integrity of a putative Sp1-binding site in close proximity to the major cap site. Band shift assays showed that this putative Sp1-binding site can interact specifically with a nuclear factor, most likely transcription factor Sp1 (or an Sp1-like factor) in vitro.