Human platelet-derived growth factor A chain is transcriptionally repressed by the Wilms tumor suppressor WT1

Tumor suppressor genes in normal and malignant hematopoiesis

Over the last decade, a growing number of tumor suppressor genes have been discovered to play a role in tumorigenesis. Mutations of p53 have been found in hematological malignant diseases, but the frequency of these alterations is much lower than in solid tumors. These mutations occur especially as hematopoietic abnormalities become more malignant such as going from the chronic phase to the blast crisis of chronic myeloid leukemia. A broad spectrum of tumor suppressor gene alterations do occur in hematological malignancies, especially structural alterations of p15(INK4A), p15(INK4B) and p14(ARF) in acute lymphoblastic leukemia as well as methylation of these genes in several myeloproliferative disorders. Tumor suppressor genes are altered via different mechanisms, including deletions and point mutations, which may result in an inactive or dominant negative protein. Methylation of the promoter of the tumor suppressor gene can blunt its expression. Chimeric proteins formed by chromosomal translocations (i.e. AML1-ETO, PML-RARalpha, PLZF-RARalpha) can produce a dominant negative transcription factor that can decrease expression of tumor suppressor genes. This review provides an overview of the current knowledge about the involvement of tumor suppressor genes in hematopoietic malignancies including those involved in cell cycle control, apoptosis and transcriptional control.

Humoral immune responses against Wilms tumor gene WT1 product in patients with hematopoietic malignancies

Wilms tumor gene WT1 is expressed at high levels in hematopoietic malignancies, such as leukemias and myelodysplastic syndromes (MDS), and in various kinds of solid tumors, including lung cancer, and it exerts an oncogenic function in these malignancies. IgM and IgG WT1 antibodies were measured by means of dot blot assay in 73 patients with hematopoietic malignancies (16 acute myeloid leukemia [AML], 11 acute lymphoid leukemia [ALL], 13 chronic myeloid leukemia [CML], and 33 MDS) and 43 healthy volunteers. Immunoglobulin IgM, IgG, and IgM+IgG WT1 antibodies were detected in 40 (54.8%), 40 (54.8%), and 24 (32.8%), respectively, of the 73 patients with hematopoietic malignancies, whereas 7 (16.2%), 2 (4.7%), and none of the 43 healthy volunteers had IgM, IgG, or IgM+IgG WT1 antibodies, respectively. Furthermore, immunoglobulin isotype class switching of WT1 antibodies from IgM to IgG occurred in conjunction with disease progression from refractory anemia (RA) to RA with excess of blasts (RAEB), and further to RAEB in transformation (RAEB-t) in MDS patients. These results showed that humoral immune responses against the WT1 protein could be elicited in patients with WT1-expressing hematopoietic malignancies, and they suggested that the helper T-cell responses needed to induce humoral immune responses and immunoglobulin isotype class switching from IgM to IgG were also generated in these patients. Our findings may provide new insight into the rationale for elicitation of cytotoxic T-cell responses against the WT1 protein in cancer immunotherapy using the WT1 vaccine.

NF1/X represses PDGF A-chain transcription by interacting with Sp1 and antagonizing Sp1 occupancy of the promoter

The regulatory mechanisms mediating basal and inducible platelet-derived growth factor (PDGF)-A expression have been the focus of intense recent investigation, but repression of PDGF-A expression is largely unexplored. Here we isolated a nuclear factor that interacts with the proximal region of the PDGF-A promoter using bulk binding assays and chromatography techniques. Peptide mass fingerprint and supershift analysis revealed this DNA-binding protein to be NF1/X. NF1/X repressed PDGF-A promoter-dependent transcription and endogenous mRNA expression, which was reversible by oligonucleotide decoys bearing an NF1/X-binding site. Mutation in the DNA-binding domain of NF1/X abolished its repression of PDGF-A promoter. NF1/X antagonized the activity of a known activator of the PDGF-A chain, Sp1, by inhibiting its occupancy of the proximal PDGF-A promoter. NF1/X physically and specifically interacts with Sp1 via its subtype-specific domain and blocks Sp1 induction of the promoter. NF1/X residues 311-416 mediated NF1/X suppression of basal PDGF-A transcription, whereas residues 243-416 were required for NF1/X repression of Sp1-inducible promoter activity. These findings demonstrate that repression of PDGF-A gene transcription is governed by interplay between NF1/X and Sp1.

PDGF and the testis

Testicular development is controlled by a complex hierarchy of gene regulatory proteins, growth factors, cell adhesion molecules, signaling molecules and hormones that interact, often acting within short time windows, via reciprocal control relationships. The identification in the testis of platelet-derived growth factor (PDGF), a key regulator of connective tissue cells in embryogenesis and pathogenesis, has focused attention on the role of this growth factor in testicular pathophysiology. This review summarizes recent advances in the study of the actions of PDGF in the male gonad, and attempts to incorporate complex in vitro and in vivo experimental data into a model that might clarify the role played by PDGF in the mammalian testis.

Wilms' tumor gene WT1: its oncogenic function and clinical application

The Wilms' tumor gene WT1 is a gene responsible for the childhood renal tumor. Wilms' tumor, and is defined as a tumor suppressor gene. However, the wild-type WT1 gene is highly expressed in leukemic blast cells of myeloid and lymphoid origin, and thus, WT1 messenger RNA provides a novel tumor marker for detection of minimal residual disease of leukemias and for monitoring disease progression of myelodysplastic syndromes. The WT1 gene exerts an oncogenic function rather than a tumor-suppressor gene function in solid tumors as well as leukemias, and the WT1 gene product is an attractive tumor antigen capable of eliciting cytotoxic T lymphocytes against WT1-expressing tumors.

WT1 proteins: functions in growth and differentiation

The Wilms' tumor 1 gene (WT1) has been identified as a tumor suppressor gene involved in the etiology of Wilms' tumor. Approximately 10% of all Wilms' tumors carry mutations in the WT1 gene. Alterations in the WT1 gene have also been observed in other tumor types, such as leukemia, mesothelioma and desmoplastic small round cell tumor. Dependent on the tumor type, WT1 proteins might either function as tumor suppressor proteins or as survival factors. Mutations in the WT1 gene can also result in congenital abnormalities as observed in Denys-Drash and Frasier syndrome patients. Mouse models have proven the critical importance of WT1 expression for the development of several organs, including the kidneys, the gonads and the spleen. The WT1 proteins seem to perform two main functions. They regulate the transcription of a variety of target genes and may be involved in post-transcriptional processing of RNA. The WT1 gene encodes at least 24 protein forms. These isoforms have partially distinct biological functions and effects, which in many cases are also specific for the model system in which WT1 is studied. This review discusses the molecular mechanisms by which the various WT1 isoforms exert their functions in normal development and how alterations in WT1 may lead to developmental abnormalities and tumor growth.

Immunotherapy for leukemia targeting the Wilms' tumor gene

The Wilms' tumor (WT1) gene encodes a zinc finger transcription factor, which is preferentially expressed in acute leukemia cells and chronic myelogenous leukemia cells in blast crisis, but not in most normal cells. These findings strongly suggest that WT1 is a potential target of immunotherapy for human leukemia. We have established a CD8+ cytotoxic T lymphocyte (CTL) clone, designated TAK-1, which is specific for a WT1-derived 9-mer peptide consisting of HLA-A24-binding anchor motifs. TAK-1 lysed both HLA-A24-positive allogeneic cells and autologous cells that were loaded with a WT1-derived peptide. TAK-1 was cytotoxic to HLA-A24-positive leukemia cells, but not to HLA-A24-positive lymphoma cells that did not express WT1, to HLA-A24-negative leukemia cells, or to HLA-A24-positive normal cells. Treating leukemia cells with an antisense oligonucleotide complementary to WT1 reduced TAK-1-mediated cytotoxicity. TAK-1 did not inhibit colony formation of HLA-A24-positive normal bone marrow cells. Recently, other groups have also reported the establishment of HLA-A2-restricted anti-leukemic CTLs specific for WT1-derived peptide. In addition, a murine model of immunotherapy against WT1-expressing tumors has been reported. Recent studies have demonstrated that WT1 is also aberrantly expressed in various kinds of cancer cells. Taken together, these results suggest that immunotherapy targeting WT1 should be effective against both solid tumors and leukemia.

A functional interaction with CBP contributes to transcriptional activation by the Wilms tumor suppressor WT1

The Wilms tumor gene WT1 encodes a zinc finger transcription factor that is required for normal kidney development. WT1 was identified as a transcriptional repressor, based on its suppression of promoter reporters, but analysis of native transcripts using high density microarrays has uncovered transcriptional activation, rather than repression, of potential target genes. We report here that WT1 binds to the transcriptional coactivator CBP, leading to synergistic activation of a physiologically relevant promoter. The physical interaction between WT1 and CBP is evident in vitro and in vivo, and the two proteins are co-immunoprecipitated from embryonic rat kidney cells. The WT1-CBP association requires the first two zinc fingers of WT1 and the adenovirus 5 E1A-binding domain of CBP. Overexpression of this domain of CBP is sufficient to inhibit WT1-mediated transcriptional activation of a promoter reporter, as is co-transfection of E1A. Retrovirally driven expression of either the CBP fragment or of E1A in human hematopoietic cells suppresses the induction by WT1 of its endogenous target gene, p21(Cip1). These observations support a model of WT1 as a transcriptional activator of genes required for cellular differentiation.

Wilms tumor and the WT1 gene

Wilms tumor or nephroblastoma is a pediatric kidney cancer arising from pluripotent embryonic renal precursors. Multiple genetic loci have been linked to Wilms tumorigenesis; positional cloning strategies have led to the identification of the WT1 tumor suppressor gene at chromosome 11p13. WT1 encodes a zinc finger transcription factor that is inactivated in the germline of children with genetic predisposition to Wilms tumor and in a subset of sporadic cancers. When present in the germline, specific heterozygous dominant-negative mutations are associated with severe abnormalities of renal and sexual differentiation, pointing to the essential role of WT1 for normal genitourinary development. The role of this tumor suppressor in normal organ-specific differentiation is also supported by the highly restricted temporal and spatial expression of WT1 in glomerular precursors of the developing kidney and by the failure of kidney development in wt1-null mice. Of two major alternative splicing products encoded by WT1, the (-KTS) isoform appears to mediate transcriptional activation of genes implicated in cellular differentiation, possibly also repressing proliferation-associated genes. The (+KTS) isoform, whose DNA-binding domain is disrupted by the insertion of three amino acids, may be involved in some aspect of mRNA processing. In addition to its function in genitourinary development, a role for WT1 in hematopoiesis is suggested by its aberrant expression and/or mutation in a subset of acute human leukemias. WT1 is also expressed in mesothelial cells; a specific oncogenic chromosomal translocation fusing the N-terminal domain of the Ewing sarcoma gene EWS to the three C-terminal zinc fingers of WT1 underlies desmoplastic small round cell tumor, an abdominal tumor thought to arise from the peritoneal lining. Understanding the distinct functional properties of WT1 isoforms and tumor-associated variants will provide unique insight into the link between normal organ-specific differentiation and malignancy.

Par4 is a coactivator for a splice isoform-specific transcriptional activation domain in WT1

The Wilms' tumor suppressor protein WT1 is a transcriptional regulator involved in differentiation and the regulation of cell growth. WT1 is subject to alternative splicing, one isoform including a 17-amino acid region that is specific to mammals. The function of this 17-amino acid insertion is not clear, however. Here, we describe a transcriptional activation domain in WT1 that is specific to the WT1 splice isoform that contains the 17-amino acid insertion. We show that the function of this domain in transcriptional activation is dependent on a specific interaction with the prostate apoptosis response factor par4. A mutation in WT1 found in Wilms' tumor disturbs the interaction with par4 and disrupts the function of the activation domain. Analysis of WT1 derivatives in cells treated to induce par4 expression showed a strong correlation between the transcription function of the WT1 17-amino acid insertion and the ability of WT1 to regulate cell survival and proliferation. Our results provide a molecular mechanism by which alternative splicing of WT1 can regulate cell growth in development and disease.

Principles of Wilms' tumor biology

The last few years have provided dramatic breakthroughs in understanding the genetic factors involved in Wilms' tumorigenesis and normal kidney development. The implications of these findings for the clinical management of children with Wilms' tumor are only now becoming apparent. Over 80% of patients with Wilms' tumor can be cured using contemporary multimodality therapy. As a consequence, the current NWTSG is attempting to intensify treatment for patients with poor prognostic features while decreasing therapy, and thereby adverse late effects, for patients with favorable prognosticators.

Initial differentiation of the metanephric mesenchyme is independent of WT1 and the ureteric bud

The early development of the metanephric kidney is characterized by the induced differentiation of mesenchymal cells into a stem cell population that undergoes a mesenchymal to epithelial transformation in response to stimuli from the ureteric bud. The Wilms' tumor suppressor gene, Wt1, is required for mesenchymal cells to complete this developmental program. In the absence of WT1, a prospective metanephric mesenchyme appears, but becomes apoptotic, and outgrowth of the ureteric bud from the Wolffian duct does not occur. Therefore, the examination of Wt1 -/- embryos allows the determination of those markers of early metanephric differentiation that do not require the ureteric bud or WT1 for their expression. Here, we demonstrate that several markers, including Pax-2, Six-2, and GDNF, were present as RNAs in the metanephric mesenchyme of Wt1 -/- embryos. These findings demonstrate that the metanephric mesenchyme in mutant embryos has begun to differentiate towards the nephrogenic lineage, and that this early differentiation does not require either WT1 or the presence of the ureteric bud. To determine whether WT1 functions other than to induce expression of factors that stimulate ureteric bud outgrowth, Wt1 -/- metanephric mesenchymes were recombined with wild-type ureteric buds in organ culture, but this failed to rescue tubulogenesis. However, the Wolffian duct from Wt1 -/- embryos was a competent inducer of wild-type metanephric mesenchyme.

Receptor tyrosine kinase inhibition suppresses growth of pediatric renal tumor cells in vitro

PURPOSE

Children who undergo standard therapy for renal tumors are at an increased risk for treatment sequelae such as congestive heart failure, abnormal trunk development, and secondary malignancies. Therefore, research on the use of novel chemotherapeutic agents with fewer side effects is justified. Recent experimental evidence suggests that growth factor receptors such as epidermal growth factor receptor (EGFR) and platelet-derived growth factor receptor (PDGFR) play an important role in growth and development of pediatric renal tumors especially that of Wilms' tumor. In this study we investigated the effects of genistein, AG1478, and AG1295, from the class of growth factor receptor tyrosine kinase (GFR-TK) inhibitors, on proliferation and colonigenic growth of 2 pediatric renal tumor cell lines.

METHODS

The authors studied the effect of genistein (broad-spectrum GFR-TK inhibitor), AG1478 (EGFR-specific GFR-TK inhibitor), and AG1295 (PDGFR-specific GFR-TK inhibitor) on proliferation and colonigenic growth of rhabdoid tumor of the kidney and Wilms' tumor cell lines: G-401 and SK-NEP-1, respectively. The effect of genistein at concentrations of 0 to 200 micromol/L, and AG1478 and AG1295 at 0 to 10,000 nmol/L were tested on proliferation by using a growth inhibition assay. Viable cell counts at each concentration were obtained by hemocytometer and trypan blue exclusion, and percent growth inhibition was calculated based on control cultures at the same time-point. As a measure of colonigenic survival, the percent inhibition of colony formation in drug-treated dishes was calculated based on the number of colonies (>50 cells) in control dishes.

RESULTS

Genistein at concentrations of 25 and 50 micromol/L inhibited the colonigenic growth of G-401 by 37% and 79% (P = .01 and 5E-06, 2-tailed t test, respectively) and that of SK-NEP-1 by 44% and 74% (P = .0001 and 9.9E-07). The mean percent growth inhibition at the above doses was 57% +/- 7.9% and 96% +/- 0.2% for G-401, and 47% +/- 11.2% and 60% +/- 2.7% for SK-NEP-1. AG1478 at concentrations of 1,000 and 5,000 nmol/L inhibited the colonigenic growth of G401 by 75% and 78% (P = .0005 and 7.38E-06, respectively) and that of SK-NEP-1 by 19% and 40% (P = .02 and .0001). The percent growth inhibition at the mentioned concentrations for G-401 were 53% +/- 9.3% and 63% +/- 6.3%, and for SK-NEP-1 were 55% +/- 14.5% and 65% +/- 20.1%, respectively. AG1295 did not appear to be as effective as AG1478.

CONCLUSIONS

This is the first experimental study on the use of GFR-TK inhibitors as a potential treatment for pediatric renal tumors. GFR-TK inhibitors such as genistein occur naturally in soybean foods and have been shown to reach therapeutic levels in blood after consuming a soybean-based diet. Considering the significance of growth factor receptor activity in Wilms' tumor development, inhibition of GFR-TKs should be investigated as effective and potentially nontoxic adjunctive treatment for this childhood tumor. Furthermore, GFR-TK inhibitors may offer an effective alternative to the treatment of commonly fatal rhabdoid tumor of the kidney in children.

Leydig cell loss and spermatogenic arrest in platelet-derived growth factor (PDGF)-A-deficient mice

Platelet-derived growth factor (PDGF)- A-deficient male mice were found to develop progressive reduction of testicular size, Leydig cells loss, and spermatogenic arrest. In normal mice, the PDGF-A and PDGF-Ralpha expression pattern showed positive cells in the seminiferous epithelium and in interstitial mesenchymal cells, respectively. The testicular defects seen in PDGF-A-/- mice, combined with the normal developmental expression of PDGF-A and PDGF-Ralpha, indicate that through an epithelial-mesenchymal signaling, the PDGF-A gene is essential for the development of the Leydig cell lineage. These findings suggest that PDGF-A may play a role in the cascade of genes involved in male gonad differentiation. The Leydig cell loss and the spermatogenic impairment in the mutant mice are reminiscent of cases of testicular failure in man.

Eel WT1 sequence and expression in spontaneous nephroblastomas in Japanese eel

Nephroblastomas spontaneously developing in Japanese eel reared at farms for 5 to 9months after collection from the wild [Masahito et al., Cancer Res., 52 (1992) 2575-2579] were investigated to cast light on the role of Wilms' tumor 1 gene (WT1) in eel kidney tumorigenesis. Cloning of the WT1 counterpart, EWT1, revealed that conservation of an alternative splice II site, located between the third and fourth zinc fingers, was conserved. The zinc finger domain was highly conserved. The transregulator region, sequences corresponding to exons 4 and 5 in WT1, were lacking in EWT1 cDNA. EWT1 was found to be expressed in kidney, testis and spleen and in situ hybridization revealed dark-stained immature cells in elver kidney to be positive. Although no EWT1 gene mutations were found in 38 eel nephroblastomas, 26 polymorphic nucleic acid changes were observed. Aberrant WT1 expression was noted in epithelial (12 out of 27; 44%) and nephroblastic cell histological types (three out of five; 60%) of eel nephroblastomas. On in situ hybridization the EWT1 expressive cells resembled human blastema cells, similar to those in human Wilms' tumor. These data demonstrated strong signals that the EWT1 protein may function in the development of eel kidney and play a role in genesis of nephroblastomas as in mammals.

Analysis of Wilms tumor gene (WT1) expression in acute leukemia patients with special reference to the differential diagnosis between eosinophilic leukemia and idiopathic hypereosinophilic syndromes

Continuous Wilms' tumor gene (WT1) expression is a typical feature of leukemic blasts in AML, ALL, and blast crisis CML patients. It is easily detectable by a variety of RT-PCR protocols, which differ mainly in their sensitivity. The nuclear WT1 protein can be found in blasts of approximately 50-60% of acute leukemia patients at diagnosis. Conversely, WT1 is only transiently expressed in normal hemopoiesis. Early CD34+ hemopoietic progenitors express WT1, whereas no WT1 mRNA transcripts can be found in mature blood cells and differentiation-induced committed CD34- progenitors. As a powerful complementary diagnostic tool, testing for WT1 expression can be helpful to discriminate between eosinophilic leukemia (EoL) patients and patients with idiopathic hypereosinophilic syndromes. Conflicting data about the usefulness of testing for WT1 expression to monitor minimal residual disease (MRD) in treated leukemia patients will be discussed. Finally, research strategies to circumvent shortcomings in detecting leukemia-associated WT1 expression will be outlined.

New DNA enzyme targeting Egr-1 mRNA inhibits vascular smooth muscle proliferation and regrowth after injury

Early growth response factor-1 (Egr-1) binds to the promoters of many genes whose products influence cell movement and replication in the artery wall. Here we targeted Egr-1 using a new class of DNA-based enzyme that specifically cleaved Egr-1 mRNA, blocked induction of Egr-1 protein, and inhibited cell proliferation and wound repair in culture. The DNA enzyme also inhibited Egr-1 induction and neointima formation after balloon injury to the rat carotid artery wall. These findings demonstrate the utility of DNA enzymes as biological tools to delineate the specific functions of a given gene, and implicate catalytic nucleic acid molecules composed entirely of DNA as potential therapeutic agents.

Transcriptional regulation of PDGF-A and TGF-beta by +KTS WT1 deletion mutants and a mutant mimicking Denys-Drash syndrome

Denys-Drash syndrome (DDS) and Frasier syndrome (FS) are rare diseases caused by the mutations of Wilms tumor gene, WT1. The common denominator in these syndromes is a nephropathy which is manifested by early-onset proteinuria, nephrotic syndrome and end stage renal failure. Although these syndromes are genetic models of nephropathy and the mutations of WT1 gene are characterized in these patients the mechanism how mutations of WT1 gene affect the embryonic kidney adversely has not been elucidated. Recently, there was a report that FS is caused by mutations in the donor splice site of WT1. These mutations predicted loss of +KTS isoform, which is one of the four splicing variants of WT1. In this study, two +KTS deletion mutants of WT1 were made as well as a WT1 mutant mimicking a mutation found in a patient who had diffuse mesangial sclerosis, end stage renal failure and Wilms tumor. Mutant embryonic kidney cell lines were established by transfection of 293 embryonic kidney cells with WT1 mutants. We investigated the transcription regulation of mutant WT1 among these cell lines using the reporter vectors containing PDGF-A and TGF-beta promoter sequence. Our results showed that the promoter activity of PDGF-A and TGF-beta, which are related to the progression of glomerular diseases, was modestly increased in the mutant cell mimicking the patent, while those activities were markedly increased in other two deletion mutant cell lines. This study demonstrated that +KTS WT1 mutation found in DDS affected the cytokine expression adversely in vitro. From these results, we suggest that the alteration of +KTS WT1 expression be responsible for the rapid progression of renal diseases in DDS and FS.

Mechanism of action and in vivo role of platelet-derived growth factor

Platelet-derived growth factor (PDGF) is a major mitogen for connective tissue cells and certain other cell types. It is a dimeric molecule consisting of disulfide-bonded, structurally similar A- and B-polypeptide chains, which combine to homo- and heterodimers. The PDGF isoforms exert their cellular effects by binding to and activating two structurally related protein tyrosine kinase receptors, denoted the alpha-receptor and the beta-receptor. Activation of PDGF receptors leads to stimulation of cell growth, but also to changes in cell shape and motility; PDGF induces reorganization of the actin filament system and stimulates chemotaxis, i.e., a directed cell movement toward a gradient of PDGF. In vivo, PDGF has important roles during the embryonic development as well as during wound healing. Moreover, overactivity of PDGF has been implicated in several pathological conditions. The sis oncogene of simian sarcoma virus (SSV) is related to the B-chain of PDGF, and SSV transformation involves autocrine stimulation by a PDGF-like molecule. Similarly, overproduction of PDGF may be involved in autocrine and paracrine growth stimulation of human tumors. Overactivity of PDGF has, in addition, been implicated in nonmalignant conditions characterized by an increased cell proliferation, such as atherosclerosis and fibrotic conditions. This review discusses structural and functional properties of PDGF and PDGF receptors, the mechanism whereby PDGF exerts its cellular effects, and the role of PDGF in normal and diseased tissues.

Vascular smooth muscle cells express the transcriptional corepressor NAB2 in response to injury

The early growth response 1 (Egr-1 or NGFI-A) gene product is a zinc finger protein transcription factor which has been implicated in the regulation of genes differentially expressed during the development of vascular disease. Egr-1 activity is regulated by alterations in the amount of protein, as well as protein-protein interactions with positive and negative transcriptional cofactors. NGFI-A-binding protein 2 (NAB2) is an example of a negative transcriptional cofactor capable of binding directly to Egr-1 and repressing Egr-1-mediated transcription. In this study, we show that NAB2 is rapidly and transiently expressed in vascular smooth muscle cells (VSMC) in response to the model agonist phorbol 12-myristate 13-acetate (PMA). This induction occurs at the protein as well as mRNA level, and the time course of induction trails closely behind that of Egr-1. NAB2 expression in VSMC is capable of inhibiting Egr-1 dependent gene expression in response to either PMA or fibroblastic growth factor-2 (FGF-2). In an in vivo model of mechanical arterial injury NAB2 levels also increase transiently in VSMC at a time when Egr-1 is elevated. It is possible that NAB2 is part of a negative-feedback mechanism which serves to down-regulate Egr-1-mediated gene transcription in injured VSMC.

Embryonic renal epithelia: induction, nephrogenesis, and cell differentiation

Embryonic metanephroi, differentiating into the adult kidney, have come to be a generally accepted model system for organogenesis. Nephrogenesis implies a highly controlled series of morphogenetic and differentiation events that starts with reciprocal inductive interactions between two different primordial tissues and leads, in one of two mainstream processes, to the formation of mesenchymal condensations and aggregates. These go through the intricate process of mesenchyme-to-epithelium transition by which epithelial cell polarization is initiated, and they continue to differentiate into the highly specialized epithelial cell populations of the nephron. Each step along the developmental metanephrogenic pathway is initiated and organized by signaling molecules that are locally secreted polypeptides encoded by different gene families and regulated by transcription factors. Nephrogenesis proceeds from the deep to the outer cortex, and it is directed by a second, entirely different developmental process, the ductal branching of the ureteric bud-derived collecting tubule. Both systems, the nephrogenic (mesenchymal) and the ductogenic (ureteric), undergo a repeat series of inductive signaling that serves to organize the architecture and differentiated cell functions in a cascade of developmental gene programs. The aim of this review is to present a coherent picture of principles and mechanisms in embryonic renal epithelia.

Angiotensin II (ATII)-inducible platelet-derived growth factor A-chain gene expression is p42/44 extracellular signal-regulated kinase-1/2 and Egr-1-dependent and mediated via the ATII type 1 but not type 2 receptor. Induction by ATII antagonized by nitric oxide

Angiotensin II (ATII) and platelet-derived growth factor (PDGF) are two vasoconstrictors implicated in the maintenance of normal vascular homeostasis. PDGF A-chain levels increase in cultured vascular smooth muscle cells (SMCs) exposed to ATII. The molecular mechanisms underlying this induction are not known. We used transient transfection analysis to show that ATII can increase reporter gene activity driven by fragments of the PDGF-A promoter bearing recognition elements for the transcription factor, Egr-1. Nuclear run-off experiments indicate that ATII induces Egr-1 expression at the level of transcription. Gel shift and supershift studies show that Egr-1 protein accumulates in the nuclei of SMCs exposed to ATII and binds to the proximal region of the PDGF-A promoter in a specific, time-dependent manner. ATII induced extracellular-signal regulated kinase (p42/44 ERK) activity as did phorbol 12-myristate 13-acetate. The specific MEK1/2 inhibitor, PD98059, suppressed both PDGF-A and Egr-1 endogenous and promoter-dependent expression inducible by ATII. The ATII type 1 receptor (AT1) antagonist, Losartan, inhibited ATII-induction of p42/44 ERK, as well as Egr-1 and PDGF-A, whereas neither PD123319, an AT2 receptor antagonist, nor wortmannin, an inhibitor of phosphatidylinositol 3-kinase and c-Jun N-terminal kinase, had any effect. ATII-induction of Egr-1 and PDGF-A was blocked by SIN-1, a NO donor. In addition, this pathway was blocked by overexpression of NO synthase. Collectively, these findings demonstrate that ATII activation of the PDGF-A promoter is mediated via the MEK/ERK/Egr-1 pathway and AT1 receptor and that this process is antagonized by NO.

Internal translation initiation generates novel WT1 protein isoforms with distinct biological properties

The Wilms' tumor 1 gene, WT1, is homozygously mutated in a subset of Wilms' tumors. Heterozygous mutations in WT1 give rise to congenital anomalies. During embryogenesis, WT1 is expressed mainly in the kidneys, uterus, and testes. Alternative splicing of the WT1 mRNA results in synthesis of four main WT1 protein isoforms with molecular masses of 52-54 kDa. In addition, translation initiation at a CUG upstream of the initiator AUG generates four larger WT1 proteins of 60-62 kDa. We describe here the existence of novel WT1 isoforms and demonstrate that they are derived from translation initiation at the second in-frame AUG of the WT1 mRNA. These N-terminally truncated WT1 proteins of 36-38 kDa can be detected in several cell lines, mouse testes, and Wilms' tumor specimens. They can bind to DNA and direct transcription from reporter constructs. The shorter WT1 protein lacking the two splice inserts has a greater transcription activation potential than the corresponding main WT1 protein isoform but shows no transcription repression potential. Overexpression of full-length or N-terminally truncated WT1 efficiently induces apoptosis. These data show that additional WT1 isoforms with distinct transcription-regulatory properties exist, which further increases the complexity of WT1 expression and activity.

GC factor 2 represses platelet-derived growth factor A-chain gene transcription and is itself induced by arterial injury

Platelet-derived growth factor (PDGF) is a mitogen and chemoattractant for a wide variety of cell types. The genes encoding PDGF A chain (PDGF-A) and PDGF B chain (PDGF-B) reside on separate chromosomes and are independently regulated at the level of transcription. Regulatory events underlying inducible PDGF-A expression have been the focus of much investigation. However, mechanisms that inhibit transcription of this gene are not well understood. In this study, we report the capacity of a newly cloned DNA binding factor, GC factor 2 (GCF2), to repress expression driven by the human PDGF-A promoter. 5' Deletion and transient cotransfection analysis in vascular endothelial cells revealed that GCF2 repression is mediated by a nucleotide region located in the proximal region of the PDGF-A promoter. Electrophoretic mobility shift assays demonstrate that GCF2 binds to this region in a specific and dose-dependent manner. Interestingly, the site bound by GCF2 overlaps those for specificity protein-1 (Sp1) and early growth response factor-1 (Egr-1), zinc finger transcription factors that direct basal and inducible expression of the PDGF-A gene. Gel shift experiments revealed that GCF2 competes with these factors for interaction with the PDGF-A promoter. Overexpression of GCF2 suppressed endogenous PDGF-A expression in vascular endothelial cells and smooth muscle cells. GCF2 was induced on mechanical injury of cells in culture as well as after balloon injury of the rat carotid artery wall. Time course studies revealed the sustained induction of GCF2 after injury while PDGF-A levels sharply returned to baseline. Smooth muscle cell proliferation was inhibited by GCF2, an effect reversed by the addition of exogenous PDGF-AA. These findings demonstrate negative regulation of PDGF-A expression by GCF2. This is the first report of the induction of an endogenous transcriptional repressor in the rat vessel wall.

Constitutive expression of the Wilms' tumor gene WT1 inhibits the differentiation of myeloid progenitor cells but promotes their proliferation in response to granulocyte-colony stimulating factor (G-CSF)

Bone marrow (BM) cells that were concentrated for hematopoietic progenitor cells by in vivo treatment with 5-FU were infected with a recombinant retrovirus containing a human full-sized, non-spliced type WT1 (Wilms' tumor gene 1) cDNA and then colony-assayed in the presence of granulocyte-colony stimulating factor (G-CSF). Significantly more colony-forming units granulocyte-monocyte (CFU-GM), colony-forming units granulocyte (CFU-G), and colony-forming units monocyte (CFU-M) colonies were formed in response to G-CSF from the BM cells infected with the WT1-containing retrovirus than from the control BM cells infected with an empty vector. Furthermore, FACS analysis of cell surface differentiation markers showed the inhibition of differentiation by constitutive WT1 expression resulting from the infection with the WT1-containing retrovirus. These results thus showed that the constitutive WT1 expression promoted the proliferation of myeloid progenitor cells but inhibited their differentiation in response to G-CSF, suggesting the alteration of G-CSF signaling pathway. The results also supported our hypothesis that the WT1 gene performs an oncogenic rather than a tumor suppressor gene function in hematopoietic progenitor cells, although the WT1 gene potentially performs both functions. This finding implies an important role of the WT1 gene in leukemogenesis.

Advances in the molecular basis of renal neoplasia

The past 2 years have provided exciting progress in elucidating the molecular basis of renal cancer. Work on the von Hippel-Lindau tumor suppressor, pVHL, in clear-cell renal cancer is already suggesting new potential therapies, and should have important implications in the pathogenesis of renal cystic disease and tumor angiogenesis. In addition, study of the Wilms' tumor suppressor, WT1, is revealing much about the pathogenesis of Wilms' tumor, urogenital development, and glomerular podocyte biology. c-met, the gene encoding the hepatocyte growth factor receptor, has recently been identified as a causative gene for hereditary papillary renal cancer. This review will highlight these and other new molecular advances in the renal cancer field.

Coordinate action of Wt1 and a modifier gene supports embryonic survival in the oviduct

The Wt1 gene, originally identified as a tumor suppressor gene associated with Wilms' tumors, encodes a zinc finger containing transcription factor expressed during gonadal and kidney development. Although Wt1 appears to be required for gonadal and kidney development, no reproductive defects were observed in outbred females heterozygous for a targeted mutation in Wt1. In contrast, no litters were obtained from Wt1 +/- females on a strain 129/Sv inbred genetic background. Ovaries were smaller in Wt1 +/- 129/Sv mice and produced fewer ova, but transplanted Wt1 +/- ovaries from 129/Sv females were able to support successful pregnancies. The inability of Wt1 +/- 129/Sv females to produce successful implantations after ovulation and fertilization appeared to be due to the failure of one-cell embryos to undergo mitosis, such that they were lost in the oviduct before reaching the uterus. Approximately 50% of Wt1 +/- females generated from a backcross of Wt1 +/- 129/Sv:C57BI/6 F1 hybrids to 129/Sv were fertile, indicating the presence of a Wt1 modifier gene that affects survival of the preimplantation embryo. Neither levels of WT1 protein nor the ratio of WT1 spice forms were significantly altered in Wt1 +/- reproductive organs, suggesting that this modifier effect acts downstream of WT1. Wt1 is therefore among a small subset of genes required for survival of the pre-implantation embryo, and appears to function non-autonomously.

Expression of the Wilms' tumor gene WT1 in solid tumors and its involvement in tumor cell growth

To determine the role of the Wilms' tumor gene WT1 in tumorigenesis of solid tumors, expression of the WT1 gene was examined in 34 solid tumor cell lines (four gastric cancer cell lines, five colon cancer cell lines, 15 lung cancer cell lines, four breast cancer cell lines, one germ cell tumor cell line, two ovarian cancer cell lines, one uterine cancer cell line, one thyroid cancer cell line, and one hepatocellular carcinoma cell line) by means of quantitative reverse transcriptase-polymerase chain reaction. WT1 gene expression was detected in three of the four gastric cancer cell lines, all of the five colon cancer cell lines, 12 of the 15 lung cancer cell lines, two of the four breast cancer cell lines, the germ cell tumor cell line, the two ovarian cancer cell lines, the uterine cancer cell line, the thyroid cancer cell line, and the hepatocellular carcinoma cell line. Therefore, of the 34 solid tumor cell lines examined, 28 (82%) expressed WT1. Three cell lines expressing WT1 (gastric cancer cell line AZ-521, lung cancer cell line OS3, and ovarian cancer cell line TYK-nu) were further analyzed for mutations and/or deletions in the WT1 gene by means of single-strand conformation polymorphism analysis. However, no mutations or deletions were detected in the region of the WT1 gene ranging from the 3' end of exon 1 to exon 10 (the WT1 gene consists of 10 exons) in these three cell lines. Furthermore, when AZ-521, OS3, and TYK-nu cells were treated with WT1 antisense oligomers, the growth of these cells was significantly inhibited in association with a reduction in WT1 protein levels. Furthermore, constitute expression of the transfected WT1 gene in cancer cells inhibited the antisense effect of WT1 antisense oligomer on cell growth. These results indicated that the WT1 gene plays an essential role in the growth of solid tumors and performs an oncogenic rather than a tumor-suppressor gene function.

WT1 and PAX-2 podocyte expression in Denys-Drash syndrome and isolated diffuse mesangial sclerosis

Denys-Drash syndrome is a rare disorder of urogenital development characterized by the association of early onset glomerulopathy caused by diffuse mesangial sclerosis, gonadal dysgenesis leading to pseudohermaphroditism in males, and a high risk of developing Wilms' tumor. The syndrome is caused by dominant negative point mutations in the WT1 gene that encodes a tumor suppressor transcription factor normally expressed in podocytes. Mutations usually affect the zinc fingers of the WT1 protein. The basic defect is unknown in most cases of isolated diffuse mesangial sclerosis, a disease characterized by the same glomerular changes as in Denys-Drash syndrome but possibly transmitted as an autosomal recessive trait. Here we show that the distribution of WT1 is abnormal in most patients with Denys-Drash syndrome : WT1 nuclear staining of podocytes is decreased or absent. This finding is consistent with the decreased DNA binding capacity of the mutated protein. One target gene of WT1 is PAX2, the expression of which is down-regulated in podocytes during early stages of nephrogenesis. We demonstrate that WT1 mislocalization is associated with abnormal podocyte expression of PAX2 protein and RNA. We suggest that persistent expression of PAX2 is likely to result from the loss of WT1 dependent transcriptional repression and may participate in the pathological mechanisms leading to glomerular dysfunction. Abnormal distribution of WT1 and PAX2 was also observed in isolated diffuse mesangial sclerosis suggesting that a defect in WT1 could also be operative in isolated diffuse mesangial sclerosis. Primary involvement of PAX2 is an alternative hypothesis because persistent expression of PAX2 in transgenic mice is associated with the occurrence of early and severe glomerulopathy.

Cloning of cDNA for newt WT1 and the differential expression during spermatogenesis of the Japanese newt, Cynops pyrrhogaster

To investigate the function of Wilms' tumor 1 (WT1) during spermatogenesis, cDNA for newt WT1 homolog was cloned and the expression of WT1 in newt testes was examined. The cDNA is 2089 bp in length and encodes 426 amino acid (aa) residues. The deduced aa sequence shares 76 and 79% homology with human and Xenopus WT1, respectively. Northern blot analysis shows that WT1 mRNA, 3.2 and 4.5 kb in length, are expressed in the testis and kidney. Both WT1 mRNA species are detected in various stages of spermatogenesis, but the 3.2 kb mRNA is highly expressed in spermatogonia and mature sperm stages, while the amount of 4.5 kb mRNA is almost constant throughout spermatogenesis. In situ hybridization reveals that WT1 mRNA is localized in Sertoli cells. Moreover, immunohistochemical analysis shows that WT1 protein is highly expressed in the nuclei of Sertoli cells in early spermatogonia and mature sperm stages, but not in pericystic cells or germ cells. These results suggest that WT1 is involved in the regulation of gene expression in Sertoli cells, depending on the spermatogenic stage.

Antisense WT1 transcription parallels sense mRNA and protein expression in fetal kidney and can elevate protein levels in vitro

Recent studies have identified antisense WT1 mRNAs whose expression is regulated by a promoter located in the first intron of the WT1 gene. Transcription directed by the antisense promoter is positively autoregulated by the WT1 protein implicating the antisense RNA in the control of WT1 gene expression. To elucidate further the biological role of the antisense RNA in the developing kidney, its distribution of expression has been examined relative to WT1 sense mRNA and WT1 protein. Using strand-specific WT1 riboprobes, the expression of WT1 and the antisense message were examined by in situ hybridization in the developing human fetal kidney at different gestational ages. The expression of the antisense strand was strongest in the podocytes and glomeruli and also in the S-form nephrons and the condensing blastema in the developing kidney. Expression was also seen in the podocytes of the mature kidney. The WT1 protein and sense mRNA for WT1 also showed a similar pattern, suggesting that the antisense transcript does not function simply as a downregulator of protein production. Expression of antisense WT1 exon 1 in cells constitutively producing high levels of WT1 also demonstrated no downregulation of protein and in most cases actually showed upregulated WT1 protein expression. These results strongly suggest that WT1 antisense transcripts positively modulate WT1 protein levels in vivo.

Wilms tumor gene (WT1) as a new marker for the detection of minimal residual disease in leukemia

WT1 (Wilms tumor gene) expression is a new tumor marker of leukemic blast cells of AML, ALL, and CML. Minimal residual disease (MRD) of leukemia can be detected at frequencies as low as 1 in 10(3) to 10(4) normal bone marrow (BM) cells and 1 in 10(5) normal peripheral blood (PB) cells by means of the quantitation of expression levels of the WT1 gene using reverse transcriptase-polymerase chain reaction (RT-PCR). This is regardless of the types of leukemia or the presence or absence of tumor-specific DNA markers. Thus, the WT1 assay makes it possible to rapidly assess the effectiveness of treatment and to evaluate the degree of eradication of leukemic cells in individual leukemia patients. Moreover, molecular relapse using PCR can be diagnosed by the monitoring of WT1 expression levels in BM or PB 1-24 months (means, 7 months for BM and 8 months for PB) before the clinical relapse became apparent. In case of rapid or gradual increase in WT1 expression levels to or over 10(-2) after return to normal BM levels during CR; or retention of the WTI expression at levels near or over 10(-2) in BM without return to normal BM levels even in CR (WT1 expression level in K562 cells was defined as 1.0), it seems that clinical relapse is impending. Since WT1 antisense oligomers inhibit the growth of leukemic cells, it is apparent that the WT1 gene plays an important role in leukemogenesis.

The Wilms' tumor suppressor WT1: approaches to gene function

Occurring with a frequency of 1 in 10,000 live births, Wilms' tumor is one of the most common solid tumors of children. The genetic basis of this tumor is highly complex and several loci have been shown to be associated with tumor formation. Thus far, however, WT1 is the only gene that has been isolated and proven to carry mutations within Wilms' tumors. During the last few years, a wealth of experiments has been carried out to address the function of WT1 as a tumor suppressor and developmental regulator. This review focuses on studies addressing WT1 function; new approaches to understand WT1 function in vivo and present transgenic data in which WT1 was driven ectopically using a CMV promoter are discussed. Our results suggest that ubiquitous expression of WT1 is not compatible with embryonic development.

Multiple roles for the Wilms' tumour suppressor gene, WT1 in genitourinary development

Wilms' tumour is a childhood kidney cancer, and a classic example of cancer arising through disrupted development (Armstrong et al., 1992). It is one of the most common solid paediatric malignancies, affecting one in 10000 children. The genetics of Wilms' tumour is complicated, with several different genes or chromosomal regions being implicated (Armstrong et al., 1992). However, the gene we know most about is the Wilms' tumour predisposition gene, WT1 (Bickmore et al., 1992; Bruening and Pelletier, 1996). It is now clear that mutations in this gene in humans can lead to abnormalities of the kidneys and gonads, as well as to the eponymous tumour. Also, as discussed below, WT1 is essential for kidney, testis and ovary development, as revealed in knockout mice.

Wilms' Tumor Gene (WT1) Competes With Differentiation-Inducing Signal in Hematopoietic Progenitor Cells

The WT1 gene is a tumor-suppressor gene that was isolated as a gene responsible for Wilms' tumor, a childhood kidney neoplasm. We have previously reported that the WT1 gene is strongly expressed in leukemia cells with an increase in its expression levels at relapse and an inverse correlation between its expression levels and prognosis, thus making it a novel tumor marker for leukemic blast cells. Furthermore, WT1 antisense oligomers have been found to inhibit the growth of leukemic cells. These results strongly suggested the involvement of the WT1 gene in human leukemogenesis. The present study was performed to prove our hypothesis that the WT1 gene plays a key role in leukemogenesis and performs an oncogenic function in hematopoietic progenitor cells, rather than a tumor-suppressor gene function. 32D cl3, an interleukin-3–dependent myeloid progenitor cell line, differentiates into mature neutrophils in response to granulocyte colony-stimulating factor (G-CSF). However, when transfected wild-type WT1 gene was constitutively expressed in 32D cl3, the cells stopped differentiating and continued to proliferate in response to G-CSF. As for signal transduction mediated by G-CSF receptor (G-CSFR), Stat3α was constitutively activated in wild-type WT1-infected 32D cl3 in response to G-CSF, whereas, in WT1-uninfected 32D cl3, activation of Stat3α was only transient. However, most interesting was the fact that G-CSF stimulation resulted in constitutive activation of Stat3β only in wild-type WT1-infected 32D cl3, but not in WT1-uninfected 32D cl3. Thus, WT1 expression constitutively activated both Stat3α and Stat3β. A transient activation of Stat1 was detected in both wild-type WT1-infected and uninfected 32D cl3 after G-CSF stimulation, but no difference in its activation was found. No activation of MAP kinase was detected in both wild-type WT1-infected and uninfected 32D cl3 after G-CSF stimulation. These results demonstrated that WT1 expression competed with the differentiation-inducing signal mediated by G-CSFR and constitutively activated Stat3, resulting in the blocking of differentiation and subsequent proliferation. Therefore, the data presented here support our hypothesis that the WT1 gene plays an essential role in leukemogenesis and performs an oncogenic function in hematopoietic progenitor cells and represent the first demonstration of an important role of the WT1 gene in signal transduction in hematopoietic progenitor cells.

The Wilms' tumor 1 gene: oncogene or tumor suppressor gene?

The Wilms' tumor 1 (wt1) gene is one of at least three genes that are involved in the development of Wilms' tumor, a pediatric kidney cancer. The expression pattern of the gene indicates that wt1 not only plays a role during kidney development but is also involved in the development and homeostasis of several other tissues. The physiological function of the gene, however, remains to be elucidated. The gene products have been implicated in many processes like proliferation, differentiation, and programmed cell death (apoptosis). The WT1 proteins function as transcription factors but may additionally be involved in splicing. Disruption of these activities may lead to aberrant development. In this paper we will discuss the role of the wt1 gene during normal development and homeostasis of several tissues. In addition, we will address the involvement of the gene products in processes like apoptosis and tumorigenesis.

Expression of the Wilms' Tumor Suppressor Gene, WT1, Is Upregulated by Leukemia Inhibitory Factor and Induces Monocytic Differentiation in M1 Leukemic Cells

The Wilms' tumor gene, WT1, encodes a transcription factor of the Cys2-His2 zinc finger type. The functional significance of WT1 expression in leukemias, in addition to tissues and cell lines of hematopoietic origin, has not been determined. Using the murine myeloblastic leukemia cell line M1 as a model for macrophage differentiation, expression of WT1 is shown to be activated in M1 cells 24 hours after differentiation induction by leukemia inhibitory factor (LIF). Upregulation ofWT1 in these cells is associated with cellular differentiation, coinciding with expression of the monocyte/macrophage marker c-fms, and the appearance of mature cells. WT1 isoforms lacking the KTS insert are unable to be ectopically expressed in M1 cells. Stable expression of the WT1 isoforms containing the KTS insert leads to spontaneous differentiation of the M1 myeloblasts through the monocytic differentiation pathway. These cells express c-fms,in addition to the myeloid-specific cell surface marker Mac-1. Exposure of these cells to LIF results in the rapid onset of terminal macrophage differentiation, accompanied by apoptotic cell death. These results show that the WT1 gene is an important regulator of M1 cell monocytic differentiation in vitro, and suggests a potential role for this gene in the molecular control of hematopoiesis.

Expression of the Wilms' Tumor Suppressor Gene, WT1, Is Upregulated by Leukemia Inhibitory Factor and Induces Monocytic Differentiation in M1 Leukemic Cells

The Wilms' tumor gene, WT1, encodes a transcription factor of the Cys2-His2 zinc finger type. The functional significance of WT1 expression in leukemias, in addition to tissues and cell lines of hematopoietic origin, has not been determined. Using the murine myeloblastic leukemia cell line M1 as a model for macrophage differentiation, expression of WT1 is shown to be activated in M1 cells 24 hours after differentiation induction by leukemia inhibitory factor (LIF). Upregulation ofWT1 in these cells is associated with cellular differentiation, coinciding with expression of the monocyte/macrophage marker c-fms, and the appearance of mature cells. WT1 isoforms lacking the KTS insert are unable to be ectopically expressed in M1 cells. Stable expression of the WT1 isoforms containing the KTS insert leads to spontaneous differentiation of the M1 myeloblasts through the monocytic differentiation pathway. These cells express c-fms,in addition to the myeloid-specific cell surface marker Mac-1. Exposure of these cells to LIF results in the rapid onset of terminal macrophage differentiation, accompanied by apoptotic cell death. These results show that the WT1 gene is an important regulator of M1 cell monocytic differentiation in vitro, and suggests a potential role for this gene in the molecular control of hematopoiesis.

Wilms' tumour-suppressor protein isoforms have opposite effects on Igf2 expression in primary embryonic cells, independently of p53 genotype

The p53 protein has been proposed as a modulator of the Wilms' tumour-suppressor protein (WT1) transcriptional regulation activity. To investigate this putative p53 role, the promoter P3 of the mouse insulin-like growth factor II gene (Igf2) was used as a target for WT1 regulation in primary cell cultures derived from p53 wild-type (p53+/+) and knock-out (p53-/-) mouse embryos. In these cells, the WT1 transcriptional activity was observed to be independent of p53 genotype. Furthermore, the two WT1 zinc finger (ZF) isoforms were for the first time found to have opposite effects on gene expression from a single promoter in the same cell type, WT1[-KTS] activating Igf2 P3, whereas WT1[+KTS] repressed its activity. In addition, we have mapped the WT1 binding sites and investigated the effect on WT1 binding activity of individual ZF deletions and Denys-Drash syndrome point mutations to this target.

Strain-responsive regions in the platelet-derived growth factor-A gene promoter

Proliferation of cultured neonatal vascular smooth muscle (VSM) cells is enhanced by exposure to cyclic mechanical strain, in part through autocrine action of secreted platelet-derived growth factor (PDGF). We examined transcription factors and DNA response elements that may participate in the induction of PDGF-A gene transcription by mechanical strain. PDGF-A mRNA increased gradually over 4 to 24 hours exposure to cyclic (1 Hz) strain. This was due, at least in part, to increased transcription since a full length (890 bp) PDGF-A promoter reporter construct was induced 3.5-fold in transfected VSM cells exposed to strain for 24 hours. A series of PDGF-A promoter truncation reporter constructs was used to identify potential regions of the promoter involved in regulation by strain. Strain-responsive regions were found between -262 bp and -92 bp and between -92 bp and -41 bp of the promoter. Since these regions are GC-rich and contain response elements for Egr-1 and Sp-1, we examined expression of these transcription factors in response to strain. mRNA for both factors increased over 0.5 to 4 hours of strain, while protein expression for both increased gradually over a 24 hours period. Gel shift assays with a probe specific for Egr-1 demonstrated at least 1 prominent new shifted band after 4 to 12 hours exposure to strain. An Sp-1 probe demonstrated constitutive shifted bands that did not change in response to strain. Thus, GC-rich regions in the proximal 92 bp of the PDGF-A promoter contain mechanical strain-responsive elements that bind Egr-1 and possibly Sp-1.

A far upstream cis-element is required for Wilms' tumor-1 (WT1) gene expression in renal cell culture

To identify novel cis-regulatory elements responsible for the tissue-restricted expression pattern of the Wilms' tumor-1 (WT1) gene, we mapped a total of 11 DNase I-hypersensitive sites in the 5'-flanking region and first intron of the human gene, six of which were specific for WT1 expressing cell lines. A 1.4-kilobase (kb) fragment from the mouse wt1 5'-flanking region contained cross-hybridizing sequence with significant homology to a region of DNase I hypersensitivity in the human WT1 gene which bound to nuclear matrix in human fetal kidney 293 cells. None of the DNase I-hypersensitive sites/matrix attachment regions, either alone or in combination, were sufficient for tissue-specific WT1 expression in transient and stably transfected cell lines. However, stable transfection of an approximately 620-kb yeast artificial chromosome (YAC) that carried the entire mouse wt1 locus into 293 cells resulted in wt1 (trans)gene expression at a level of approximately 30% of the endogenous human gene. Deletion of the 1.4-kb cross-hybridizing mouse fragment, located approximately 15 kb upstream of the transcription start site, caused complete loss of wt1 gene expression in the YAC-transfected 293 cells. In summary, we have identified a far upstream element that contains a region of DNase I hypersensitivity and that binds to nuclear matrix. This element includes phylogenetically conserved sequence and is required, although not sufficient, for mouse wt1 gene expression in human fetal kidney cells in culture.

PAX 8 regulates human WT1 transcription through a novel DNA binding site

The Wilms' tumor gene (WT1) is an essential gene for kidney and gonadal development, although how WT1 expression is induced in these tissues is not known. One kidney transcription factor likely to play a role in this regulation is PAX 8. The co-expression of WT1 and PAX 8 during kidney development and in Wilms' tumors with an epithelium predominant histology suggested a possible interaction, and indeed, we identified potential core PAX-binding sites in the WT1 promoter. Endogenous PAX 8 plays an important role in the activation of the WT1 promoter, since promoter activity is much stronger in cells with PAX 8 than without. Using binding assays, we searched for evidence of PAX 8-DNA interactions throughout the 652-base pair human WT1 promoter and found only one functional PAX 8 site with DNA binding activity, located 250 base pairs 5' of the minimal promoter. The responsiveness of the PAX 8 site was confirmed by assessing its ability to function as an enhancer significantly activating the minimal promoter in a position- and orientation-independent manner. Using transfection assays, we demonstrated that either endogenous or exogenously added PAX 8 activated the WT1 promoter and that this promoter up-regulation depended upon the presence of an intact PAX 8-binding site. In contrast, the previously reported core PAX 8-binding sites identified by computer analysis of the WT1 promoter failed to specifically bind in vitro translated PAX 8 protein or activate the minimal promoter. Thus, we identified a novel functional binding site for the transcription factor PAX 8, suggesting that part of its role in kidney development may be as a modulator of WT1 expression in the kidney.

Differential effects of Wilms tumor WT1 splice variants on the insulin receptor promoter

The Wilms tumor gene WT1 has been implicated in the early development of the kidney. Mutations in WT1 are found in a small fraction of Wilms tumor, a pediatric nephroblastoma, and Denys-Drash syndrome, characterized by genitourinary abnormalities. The WT1 gene product functions as a transcriptional repressor of growth factor-related genes. The kidney is one of the major sites of insulin action in vivo and expresses high levels of insulin receptors (IR). IR expression has been detected during early embryogenesis, suggesting that it may play a role in development. We investigated whether two WT1 splice variants lacking or including a three-amino-acid (KTS) insertion between the third and fourth zinc finger in the DNA-binding domain could repress the IR promoter in vitro. We show that the +KTS variant effectively represses promoter activity under all conditions tested but the -KTS variant was only able to repress in the presence of cotransfected C/EBP beta or a dominant-negative p53 mutation. Deletional mapping indicated that distinct regions of the IR promoter mediated the effects of the two isoforms and DNaseI footprint analysis identified potential WT1 binding sites within these regions.

The EWS-WT1 translocation product induces PDGFA in desmoplastic small round-cell tumour

Chromosomal translocations resulting in chimaeric transcription factors underlie specific malignancies, but few authentic target genes regulated by these fusion proteins have been identified. Desmoplastic small round-cell tumour (DSRT) is a multiphenotypic primitive tumour characterized by massive reactive fibrosis surrounding nests of tumour cells. The t(11;22)(p13;q12) chromosomal translocation that defines DSRT produces a chimaeric protein containing the potential transactivation domain of the Ewing-sarcoma protein (EWS) fused to zinc fingers 2-4 of the Wilms tumour suppressor and transcriptional repressor WT1 (refs 2,3). By analogy with other EWS fusion products, the EWS-WT1 chimaera may encode a transcriptional activator whose target genes overlap with those repressed by WT1 (ref. 4). To characterize its functional properties, we generated osteosarcoma cell lines with tightly regulated inducible expression of EWS-WT1. Expression of EWS-WT1 induced the expression of endogenous platelet-derived growth factor-A (PDGFA), a potent secreted mitogen and chemoattractant whose promoter contains the many potential WT1-binding sites. Native PDGFA was not regulated by wild-type WT1, indicating a difference in target gene specificity between this tumour suppressor and its oncogenic derivative. PDGFA was expressed within tumour cells in primary DSRT specimens, but it was absent in Wilms tumours expressing WT1 and Ewing sarcomas with an EWS-Fli translocation. We conclude that the oncogenic fusion of EWS to WT1 in DSRT results in the induction of PDGFA, a potent fibroblast growth factor that contributes to the characteristic reactive fibrosis associated with this unique tumour.

The Krüppel-associated box (KRAB)-zinc finger protein Kid-1 and the Wilms' tumor protein WT1, two transcriptional repressor proteins, bind to heteroduplex DNA

Zinc finger proteins of the Cys2His2 class represent a large group of DNA-binding proteins. A major subfamily of those proteins, the Krüppel-associated box (KRAB) domain-containing Cys2His2-zinc finger proteins, have been described as potent transcriptional repressors. So far, however, no DNA-binding sites for KRAB domain-containing zinc finger proteins have been isolated. Using a polymerase chain reaction-based selection strategy with double- and single-stranded DNA, we failed to reveal a binding site for Kid-1, one member of KRAB-zinc finger proteins. Binding of Kid-1 both to single- and homoduplex double-stranded DNA was negligible. We now present evidence that Kid-1 binds to heteroduplex DNA. Similar to Kid-1, the non-KRAB-zinc finger protein WT1 also bound avidly to heteroduplex DNA (both the -KTS and +KTS splice variant of WT1), whereas the POU domain protein Oct-6, the ets domain protein Ets-1 and the RING finger of BRCA-1 did not bind to heteroduplex DNA. Binding of WT1 to heteroduplex DNA was markedly reduced in naturally occurring mutants. The recognition of certain DNA structures by transcriptional repressor proteins may therefore represent a more common phenomenon than previously thought.

High Levels of Wilms' Tumor Gene (wt1) mRNA in Acute Myeloid Leukemias Are Associated With a Worse Long-Term Outcome

The tumor suppressor gene wt1 (Wilms' tumor gene) encodes for a zinc finger DNA-binding protein with predominantly transcription repressing properties. Because wt1 has been shown to be expressed in the vast majority of patients with acute myeloid leukemias (AML), we investigated the relevance of wt-1 mRNA expression regarding prognosis and possible prediction of relapse during follow-up. Totally bone marrow-derived blasts of 139 AML patients (129 newly diagnosed AML patients, 22 AML patients again in first relapse, and 10 AML patients analyzed primarily in first relapse) were studied for wt1 mRNA expression. Seventy-seven patients were analyzed for wt1 mRNA expression during follow-up. wt1-specific reverse transcription-polymerase chain reaction (RT-PCR) was performed and the amplification product was visually classified as not, weakly, moderately, or strongly amplified, as described previously. PCR products were quantitated by competitive PCR using a shortened homologous wt1 construct standard in representative cases. The expression of wt1 transcripts was correlated to age, French-American-British (FAB) subtype, phenotype, karyotype, and long-term survival. wt1 mRNA was detectable in 124 of 161 (77%) samples at diagnosis and in first relapse. wt1 expression was independent from age, antecedent myelodysplastic syndrome or FAB subtype, with the exception of a significant difference in M5 leukemias showing wt1 transcripts in only 40% (P = .0025). There was no correlation between the level of wt1 mRNA and response to treatment or the prognostic groups defined by the karyotype. Concerning long-term survival, patients with high levels of wt1 had a significantly worse overall survival (OS) than those with not detectable or low levels. The 3-year OS for all newly diagnosed AMLs was 13% and 38% (P = .038), respectively, and 12% and 43% (P = .014) for de novo AMLs. The difference was more distinct in patients less than 60 years of age. During follow-up, all patients achieving complete remission became wt1 negative. Reoccurrence of wt1 transcripts predicted relapse. The data indicate that high expression of wt1 mRNA is associated with a worse long-term prognosis.