cis regulation of the keratin 18 gene in transgenic mice

Differential sensitivity of mouse epithelial tissues to the polyomavirus middle T oncogene

To determine how different epithelial cell types respond to the same oncogenic stimulation, we have used a modified human keratin 18 gene to conditionally express the polyomavirus middle T antigen (PyMT) oncogene in simple epithelial tissues of transgenic mice. Activation of PyMT expression by transgenic Cre recombinase in mammary epithelial cells resulted in carcinomas in all bitransgenic females. PyMT expression induced by K18-driven Cre in internal epithelial organs resulted in pancreatic acinar metaplasia and ductal dysplasia with remarkable desmoplastic stromal responses in all 25 bitransgenic mice. Hepatoma formation with altered lipid metabolism and gastric adenocarcinoma occurred in 96 and 54% of these mice, respectively. Elevated PyMT RNA expression also correlated with intraepithelial neoplasia in the prostate. Activated Erk2 was found in mammary tumors, pancreatic tissues, and affected livers. Hes1 RNA, a target of Notch signaling that has been implicated downstream of Ras pathway activation, was elevated in pancreatic and liver lesions. The variety of responses of different epithelia to PyMT demonstrates the importance of the differentiated state in interpreting oncogenic signals.

The mouse sino-atrial node expresses both the type 2 and type 3 Ca(2+) release channels/ryanodine receptors

Ca(2+) released from intracellular Ca(2+) stores is shown to be involved in pacemaker activity in the sino-atrial (SA)-node. However, little is known about the molecular identity of the Ca(2+) release channel/ryanodine receptor (RYR) involved in pacemaker activity. We examined the mRNA distribution of three different RYR isoforms (RYR1, RYR2, and RYR3) in the mouse SA-node. RNase protection assay and in situ hybridization revealed that RYR2 mRNA expresses widely in the heart including the SA-node, while RYR3 mRNA expression is limited to the SA-node and to the right atrium. Thus, not only RYR2 but also RYR3 may participate in pacemaker activity.

Ets2 is not required for Ras or Neu/ErbB-2 mediated cellular transformation in vitro

Ets2 is a widely expressed Ets family transcription factor which is activated by Ras signaling and has been reported to transform fibroblasts. Expression of a dominant inhibitory Ets2 construct consisting of just the Ets2 DNA binding domain (Ets2DBD), reverses Ras transformation of NIH3T3 cells and the transformed characteristics of several human tumor cells. However, the Ets2DBD may interfere with multiple Ets family members. We have now utilized cell lines with a disrupted ets2 gene to determine whether Ets2 is required to mediate oncogenic signaling. Expression of the Ets2DBD in an Ets2-deficient cell line dramatically inhibited Ets-dependent (but not AP-1-dependent) reporter gene expression, revealing that the Ets2DBD does inhibit additional Ets family members. The transformation efficiency of Ets2-deficient cell lines by oncogenic Ras or Neu/ErbB-2 was similar to that of control cells in several in vitro assays, and was not enhanced by re-expression of Ets2. Finally, overexpression of Ets2 was not sufficient to induce focus formation in NIH3T3 cells, nor to enhance transformation by oncogenic Ras. Thus, Ets2 is not an essential mediator of Ras or Neu/ErbB-2 transformation in these cells. Our results illustrate the importance of utilizing specific approaches for analyzing the function of individual members of large gene families.

DNase I hypersensitivity patterns of the serglycin proteoglycan gene in resting and phorbol 12-myristate 13-acetate-stimulated human erythroleukemia (HEL), CHRF 288-11, and HL-60 cells compared with neutrophils and human umbilical vein endothelial cells

We mapped the DNase I-hypersensitive sites (DHSS) of the serglycin gene in resting and phorbol 12-myristate 13-acetate (PMA)-stimulated human erythroleukemia (HEL) and CHRF 288-11 cells, which have megakaryocytic characteristics, and HL-60 promyelocytic leukemia cells. We compared these DHSS with those of normal primary neutrophils and human umbilical vein endothelial cells. Several DHSS appear to be involved in regulating the level of endogenous expression and in the PMA response of hematopoietic cell lines. A DHSS unique to resting HL-60 cells and induced in CHRF 288-11 by PMA may explain the high degree of endogenous expression in HL-60 relative to HEL and CHRF (Schick, B. P., Petrushina, I., Brodbeck, K. C., and Castronuevo, P. (2001) J. Biol. Chem. 276, 24726-24735). A total of 4 DHSS in intron 1 and 6 in intron 2 are associated with the PMA response in a cell-specific manner. A DHSS in the 5'-flanking region and another in intron 1 lie in areas that have high homology with the orthologous murine serglycin locus and are rich in potential transcription factor binding sites. One DHSS in intron 1 and one in intron 2 are located within Alu repeats. Two DHSS found in DNA of normal primary neutrophils were different from those of the cell lines. One DHSS in exon 2 unique to neutrophils correlated with a previously unrecognized alternative splicing that removes exon 2. Human umbilical vein endothelial cells had a DHSS in intron 1 that was common with the cell lines. The different patterns of DHSS exhibited by the cells studied suggest that cell- and differentiation-specific alterations in chromatin structure may control serglycin gene expression.

Definition of transcriptional promoters in the human beta globin locus control region

Our previous studies on the human beta globin gene cluster revealed the presence of intergenic transcripts throughout the locus, and demonstrated that transcription of the locus control region (LCR) initiates within an ERV9 endogenous retroviral long-terminal repeat (LTR) upstream of DNase I hypersensitive site 5. We show, using a combination of assays, that there are additional sites of transcription initiation within the LCR at hypersensitive sites 2 and 3. We have defined sites of transcription initiation, which occurs at discrete positions in a direction towards the globin genes. In addition, we show that mutation of specific transcription factor binding sites within HS2 leads to a reduction in transcription levels from within this site. We propose that these initiation events within the LCR can account for the observed orientation dependence of LCR function, and contribute to the open chromatin configuration of the beta globin locus. In addition, transcription from within the LCR hypersensitive sites could compensate for the absence of the ERV9 LTR in many transgenic mice lines, which nevertheless regulate their globin clusters correctly.

An Alu element from the K18 gene confers position-independent expression in transgenic mice

We have identified a 323-base pair fragment of the 5'-flanking sequence of the K18 gene, which confers position-independent and copy number-dependent expression on two heterologous transgenes. This fragment is composed primarily of an Alu repetitive element. Its activity in mice is correlated with its RNA polymerase III promoter activity and its orientation-dependent ability to inhibit potential transcriptional interference in a transfection assay. However, the activity of the Alu element is not correlated with its enhancer blocking activity, a characteristic of insulator elements. In addition, this Alu element did not block the suppressive effect of co-injecting mouse alpha satellite DNA with the transgene. This Alu element is likely responsible for at least part of the protective effects of the sequences flanking the K18. These results suggest that transcriptionally active Alu elements may eliminate transcriptional interference of neighboring genes. This Alu element is one component of the locus control region associated with the K18 gene. Other Alu repetitive elements may also function to define regulatory domains.

RNAs from all categories generate retrosequences that may be exapted as novel genes or regulatory elements

While the significance of middle repetitive elements had been neglected for a long time, there are again tendencies to ascribe most members of a given middle repetitive sequence family a functional role--as if the discussion of SINE (short interspersed repetitive elements) function only can occupy extreme positions. In this article, I argue that differences between the various classes of retrosequences concern mainly their copy numbers. Consequently, the function of SINEs should be viewed as pragmatic such as, for example, mRNA-derived retrosequences, without underestimating the impact of retroposition for generation of novel protein coding genes or parts thereof (exon shuffling by retroposition) and in particular of SINEs (and retroelements) in modulating genes and their expression. Rapid genomic change by accumulating retrosequences may even facilitate speciation [McDonald, J.F., 1995. Transposable elements: possible catalysts of organismic evolution. Trends Ecol. Evol. 10, 123-126.] In addition to providing mobile regulatory elements, small RNA-derived retrosequences including SINEs can, in analogy to mRNA-derived retrosequences, also give rise to novel small RNA genes. Perhaps not representative for all SINE/master gene relationships, we gained significant knowledge by studying the small neuronal non-messenger RNAs, namely BC1 RNA in rodents and BC200 RNA in primates. BC1 is the first identified master gene generating a subclass of ID repetitive elements, and BC200 is the only known Alu element (monomeric) that was exapted as a novel small RNA encoding gene.

Control of genes by mammalian retroposons

Available data on possible genetic impacts of mammalian retroposons are reviewed. Most important is the growing number of established examples showing the involvement of retroposons in modulation of expression of protein-coding genes transcribed by RNA polymerase II (Pol II). Retroposons contain conserved blocks of nucleotide sequence for binding of some important Pol II transcription factors as well as sequences involved in regulation of stability of mRNA. Moreover, these mobile genes provide short regions of sequence homology for illegitimate recombinations, leading to diverse genome rearrangements during evolution. Therefore, mammalian retroposons representing a significant fraction of noncoding DNA cannot be considered at present as junk DNA but as important genetic symbionts driving the evolution of regulatory networks controlling gene expression.

A regulatory element of the human keratin 18 gene with AP-1-dependent promoter activity

The human keratin 18 (K18) gene is expressed in a restricted but diverse subset of differentiated epithelial tissues and carcinomas. The 10-kilobase pair K18 gene contains all of the genetic information necessary for tissue-specific, copy number-dependent and integration site-independent expression in transgenic mice. We identified a 100-base pair regulatory element that activates the K18 proximal promoter in the presence of the previously identified first intron enhancer. Deletion of the element greatly diminished K18 expression. This regulatory element also has cryptic, AP-1-dependent promoter activity in the absence of the normal promoter, which results in 10-40-fold higher levels of K18 RNA expression in transgenic mice. The high activity of this cryptic promoter is dependent upon the first intron enhancer. These experiments define interactive regulatory regions of the K18 gene that modulate expression in diverse epithelial cell types and identify an unusual regulatory element with promoter activity that may be useful for high level heterologous gene expression in transgenic animals.

Elevated expression of Ets2 or distinct portions of Ets2 can reverse Ras-mediated cellular transformation

Ets transcription factors are important downstream targets of oncogenic Ras. The transcriptional activity of several Ets family members is regulated by Ras, and interfering with Ets-dependent transcription by expression of just the Ets2 DNA binding domain can inhibit or reverse Ras-mediated cellular transformation. To better understand the role of Ets proteins in Ras transformation, we have now analyzed the effects of stably expressing a variety of Ets2 constructs in Ras-transformed NIH3T3 (DT) cells. Expression of only the Ets2 transactivation domains, which also inhibits Ras or Neu/ErbB-2-mediated activation of Ets-dependent transcription, strongly inhibited anchorage-independent growth, but did not revert the transformed DT cell morphology. Unexpectedly, high expression of full-length Ets2, a transcriptional activator, broadly reversed the transformed properties of DT cells, including anchorage-independent growth, transformed morphology, and tumorigenicity, but did not impair attached cell growth. Increasing full-length Ets2 transcriptional activity by fusing it to the VP16 transactivation domain enhanced its ability to reverse DT cell transformation. Mutational analysis revealed that the mitogen-activated protein kinase phosphorylation site required for Ras-mediated activation, Ets2(T72), was not essential for Ets2 reversion activity. The distinct reversion activities of the highly expressed Ets2 transactivation domains or full-length Ets2, along with the specific reversion activity by Ets2 constructs that either inhibit or activate Ets-dependent transcription, suggests multiple roles for Ets factors in cellular transformation. These results indicate that several distinct approaches for modulating Ets activity may be useful for intervention in human cancers.

Expression of rat thyrotropin-releasing hormone (TRH) gene in TRH-producing tissues of transgenic mice requires sequences located in exon 1

TRH, an amidated tripeptide secreted by certain hypothalamic neurons, is a principal regulator of TSH secretion and thyroid hormone release. TRH is also produced by other neurons in the central nervous system, where it appears to function as a neuromodulator or neurotransmitter, and by certain endocrine cells, where it may act as an autocrine or paracrine factor. The genomic organization of the rat TRH (rTRH) gene is well understood; however, the domains of the rTRH gene that regulate expression are less well characterized. We observed that the region between -47 and +6 of the rTRH gene (relative to the transcription start site at +1) was active in CA-77 cells, a medullary thyroid carcinoma cell line model of TRH production, but was not active in transgenic mice. Inclusion of most of exon 1 (84 out of 103 bp; -47 to +84) increased promoter activity in CA-77 cells and was active in transgenic mice, principally in tissues that normally express the TRH gene. Further lengthening of the 5' end to -243, -547, or -776 retained this expression in TRH-producing tissues in transgenic mice, while further increasing activity in CA-77 cells. These results suggest that cis element(s) located within exon 1 are necessary for the expression of the rTRH gene in vivo.

A regulatory element within a coding exon modulates keratin 18 gene expression in transgenic mice

Multiple tissue-specific, DNase-hypersensitive sites are correlated with known or potential regulatory regions of the human keratin 18 (K18) gene. One of these sites is found within exon 6, close to a potential AP-1 binding site. Footprint analysis confirmed that this site is capable of binding c-Jun and c-Fos in vitro. However, exon 6 can stimulate expression of a reporter gene driven by the K18 proximal promoter independent of AP-1 in F9 cells and additionally modulates AP-1 responsiveness when in combination with an intron enhancer. Analysis in transgenic mice and by transient transfections of mutant forms of the K18 gene showed that exon 6 contributes to the expression of the K18 gene. However, substitution of part of exon 6 with the corresponding part of the keratin 19 gene which lacks an AP-1 site decreased but did not destroy the regulatory activity of the exon. Furthermore, this mutation did not alter either the tissue specificity or the position-independent and copy number-dependent behavior of the K18 gene. In contrast, a frameshift mutation within exon 6 dramatically decreased the expression of the gene. K18 RNA expression from the frameshift mutation was less than 10% of the wild type K18 transgene. This decline in expression was the result of a combination of decreased stability of mutant K18 RNA and the creation of a negative regulatory element that can interact with the first intron regulatory elements and actively suppress K18 expression. These results demonstrate that a protein-coding portion of the K18 gene also has a regulatory function.

Towards a molecular definition of keratinocyte activation after acute injury to stratified epithelia

While in recent years we have come to increasingly appreciate the multifaceted role of skin, probably none of these novel contributions is as vital as its barrier function, inferred centuries ago. In human skin this function is fulfilled nearly entirely by the epidermis, a thin stratified squamous epithelium made up primarily of keratinocytes and located at the skin surface. Disruption of the integrity of epidermis triggers a homeostatic response involving blood-derived elements and resident skin cell types that is designed to rapidly restore a functional epithelial lining over the wound site. This article is focused on the process of recruitment of keratinocytes from intact skin tissue at the proximal wound edges to participate in re-epithelialization.

Influence of oxygen radical injury on DNA methylation

One of the most prevalent products of oxygen radical injury in DNA is 8-hydroxyguanosine. Cells must be able to withstand damage by oxygen radicals and possess specific repair mechanisms that correct this oxidative lesion. However, when these defenses are oversaturated, such as under conditions of high oxidative stress, or when repair is inefficient, the miscoding potential of this lesion can result in mutations in the mammalian genome. In addition to causing genetic changes, active oxygen species can lead to epigenetic alterations in DNA methylation, without changing the DNA base sequence. Such changes in DNA methylation patterns can strongly affect the regulation of expression of many genes. Although DNA methylation patterns have been found to be altered during carcinogenesis, little is known about the mechanism(s) that produce this loss of epigenetic controls of gene expression in tumors. Replacement of guanine with the oxygen radical adduct 8-hydroxyguanine profoundly alters methylation of adjacent cytosines, suggesting a role for oxidative injury in the formation of aberrant DNA methylation patterns during carcinogenesis. In this paper, we review both the genetic and epigenetic mechanisms of oxidative DNA damage and its association with the carcinogenic process, with special emphasis on the influence of free radical injury on DNA methylation.

Oncogenic regulation and function of keratins 8 and 18

Keratin 8 (K8) and keratin 18 (K18) are the most common and characteristic members of the large intermediate filament gene family expressed in 'simple' or single layer epithelial tissues of the body. Their persistent expression in tumor cells derived from these epithelia has led to the wide spread use of keratin monoclonal antibodies as aids in the detection and identification of carcinomas. Oncogenes which activate ras signal transduction pathways stimulate expression of the K18 gene through transcription factors including members of the AP-1 (jun and fos) and ETS families. The persistent expression of K8 and K18 may reflect the integrated transcriptional activation of such transcription factors and, in the cases of ectopic expression, an escape from the suppressive epigenetic mechanisms of DNA methylation and chromatin condensation. Comparison of the mechanisms of transcriptional control of K18 expression with expression patterns documented in both normal and pathological conditions leads to the proposal that persistent K8 and K18 expression is a reflection of the action of multiple different oncogenes converging on the nucleus through a limited number of transcription factors to then influence the expression of a large number of genes including these keratins. Furthermore, correlation of various tumor cell characteristics including invasive behavior and drug sensitivity with K8 and K18 expression has stimulated consideration of the possible functions of these proteins in both normal development and in tumorigenesis. Recent developments in the analysis of the functions of these intermediate filament proteins provide new insights into diverse functions influenced by K8 and K18.

Transcriptional silencer of the Wilms' tumor gene WT1 contains an Alu repeat

Expression of the Wilms' tumor gene WT1 is tightly regulated throughout development. In contrast, the WT1 promoter is promiscuous, functioning in all cell lines tested. We have cloned a transcriptional silencer that is involved in regulation of the WT1 gene. The transcriptional silencer is located in the third intron of the WT1 gene, approximately 12 kilobases from the promoter, and functions to repress transcription from the WT1 promoter in cell lines of non-renal origin. The 460-base pair silencer region is unusual in that it contains a full-length Alu repeat. We have also cloned an enhancer like-element located 1.3 kilobases upstream of the WT1 promoter.

Nucleosome positioning by human Alu elements in chromatin

Alu sequences are interspersed throughout the genomes of primate cells, occurring singly and in clusters around RNA polymerase II-transcribed genes. Because these repeat elements are capable of positioning nucleosomes in in vitro reconstitutes (Englander, E. W., Wolffe, A. P., and Howard, B. H. (1993) J. Biol. Chem. 268, 19565-19573), we investigated whether they also influence in vivo chromatin structure. When assayed collectively using consensus sequence probes and native chromatin as template, Alu family members were found to confer rotational positioning on nucleosomes or nucleosome-like particles. In particular, a 10-base pair pattern of DNase I nicking that spanned the RNA polymerase III box A promoter motif extended upstream to cover diverse 5'-flanking sequences, suggesting that Alu repeats may influence patterns of nucleosome formation over neighboring regions. Computational analysis of a set of naturally occurring Alu sequences indicated that nucleosome positioning information is intrinsic to these elements. Inasmuch as local chromatin organization influences gene expression, the capacity of Alu sequences to affect chromatin structure as demonstrated here may help to clarify some features of these elements.

Specific combinations of human serum albumin introns direct high level expression of albumin in transfected COS cells and in the milk of transgenic mice

A new series of expression vectors, each comprised of the beta-lactoglobulin (BLG) promoter driving one of a variety of human serum albumin (HSA) minigenes or the entire gene, were evaluated for their ability to direct expression of HSA in vitro in COS tissue culture cells and into the milk of transgenic mice. Vectors directed a hierarchy of expression levels in vitro, dependent upon the specific complement of HSA introns included. HSA introns acted in a synergistic manner. In addition, minigenes comprised of specific subsets of introns were more efficacious than the entire HSA gene with all of its introns. Transgenic mice expressed as much as 10 mg ml-1 of HSA in their milk. Vectors comprised of specific intron subsets directed levels at 1 mg ml-1 or greater in the milk of 20% of generated transgenics. A statistical correlation between the expression level trend in vitro with the trend of expression in vivo (% which express) at detectable levels (p = 0.0015) and at the level of greater than 0.1 mg ml-1 (p = 0.0156) was demonstrated. A weak correlation existed (p = 0.0526) at in vivo levels of 1 mg ml-1 or greater. These new vectors are expected to direct the production of high levels of HSA in the milk of a large percentage of generated transgenic dairy animals.

Activation of the silent human cytokeratin 17 pseudogene-promoter region by cryptic enhancer elements of the cytokeratin 17 gene

We have previously described the three loci CK-CA, CK-CB and CK-CC in the human genome that contain clustered type-I cytokeratin genes and reported the complete nucleic acid sequences of the functional cytokeratin 17 gene located in CK-CA and two closely related pseudogenes present in CK-CB and CK-CC [Troyanovsky, S.M., Leube, R.E. & Franke, W.W. (1992) Eur. J. Cell Biol. 59, 127-137]. By nucleic acid sequence analysis, we now show that extensive similarities between the functional gene and the pseudogenes exist in the 5'-upstream region. However, despite the high degree of nucleic acid identity (94%), only the 5'-upstream region of the functional gene was able to induce significant transcriptional activity in transfected cells of epithelial origin. Using chimeric upstream regions consisting of different fragments from the pseudogene and the functional gene, we made the surprising observation that cis elements in the proximal 5'-upstream region of the pseudogene promoter can cooperate with distal enhancer elements of the functional gene to induce strong chloramphenicol-O-acetyltransferase activity in transfected HeLa cells. A major site in the proximal upstream region was identified by deoxyribonuclease protection experiments to be necessary for this cooperative effect. The structure and properties of this element were further analysed by transfection of different chloramphenicol-O-acetyltransferase gene constructs, and by nucleic acid sequence comparison to corresponding regions of the related cytokeratins 14 and 16. It is concluded that the upstream regions identified in this study contribute to the strong expression of the human cytokeratin 17 gene in a coordinated fashion.

Structure and transcriptional regulation of the GFAP gene

Transcriptional regulation of the GFAP gene is intimately connected with astrocyte function: its initial activation marks the differentiation of astrocytes, and its up-regulation accompanies the reactive response to CNS injury. Studies of GFAP transcription should thus provide insights into multiple regulatory pathways operating in these cells. In addition, they should identify DNA elements that could be used to direct synthesis of other proteins to astrocytes in transgenic animals, permitting creation of disease models, and the testing of cause and effect relationships. This review describes several GFAP cDNA and genomic clones that have been isolated, including homology comparisons of the encoded RNAs and proteins. Cell transfection studies by several laboratories are summarized that have identified a DNA segment immediately upstream of the RNA start site that is essential for transcriptional activity, but which have yielded conflicting results concerning the importance of other segments located both further upstream and downstream of the RNA start site. Two procedures are recounted that have led to the successful expression of GFAP-transgenes in astrocytes in mice. One of these incorporates the transgene into the first exon of a fragment spanning the entire GFAP gene, while the other links it to a 2 kb 5'-flanking segment. Results already produced by GFAP-transgenic studies include demonstration of a neurotoxic effect of the HIV-1 gp120 coat protein, and creation of a hydrocephalic mouse model.