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

Every Beat You Take-The Wilms' Tumor Suppressor WT1 and the Heart

Nearly three decades ago, the Wilms’ tumor suppressor Wt1 was identified as a crucial regulator of heart development. Wt1 is a zinc finger transcription factor with multiple biological functions, implicated in the development of several organ systems, among them cardiovascular structures. This review summarizes the results from many research groups which allowed to establish a relevant function for Wt1 in cardiac development and disease. During development, Wt1 is involved in fundamental processes as the formation of the epicardium, epicardial epithelial-mesenchymal transition, coronary vessel development, valve formation, organization of the cardiac autonomous nervous system, and formation of the cardiac ventricles. Wt1 is further implicated in cardiac disease and repair in adult life. We summarize here the current knowledge about expression and function of Wt1 in heart development and disease and point out controversies to further stimulate additional research in the areas of cardiac development and pathophysiology. As re-activation of developmental programs is considered as paradigm for regeneration in response to injury, understanding of these processes and the molecules involved therein is essential for the development of therapeutic strategies, which we discuss on the example of WT1.

A new role for wilms tumor protein 1: differential activities of + KTS and -KTS variants to regulate LHβ transcription

Luteinizing hormone (LH) is synthesized and secreted throughout the reproductive cycle from gonadotrope cells in the anterior pituitary, and is required for steroidogenesis and ovulation. LH contains an α-subunit common with FSH, and a unique LHβ subunit that defines biological activity. Basal LHβ transcription is low and stimulated by hypothalamic GnRH, which induces synthesis of early growth response protein-1 (Egr1), and stimulates binding of transcription factors Egr1 and steroidogenic factor-1 (SF1) on the promoter. WT1 (Wilms tumor protein1) is a zinc finger transcription factor with an essential role in urogenital system development, and which regulates several reproductive genes via interactions with SF1 or binding to GC-rich elements such as Egr1 binding sites. We investigated a potential role for WT1 in LHβ transcription in clonal mouse gonadotrope LβT2 cells. WT1 was present in LβT2 and mouse pituitary cells, and protein bound to the endogenous LHβ promoter. Interestingly, mRNAs for WT1(+KTS), which contains a three amino-acid insertion between the 3rd and 4th zinc fingers, and the WT1 (-KTS) variant were both expressed at significant levels. WT1 mRNAs and protein were decreased approximately 50% by GnRH treatment, under conditions where Egr1 mRNA and protein, and LHβ transcription, were stimulated. Decreasing expression of mRNA for WT1 (-KTS) decreased stimulation of LHβ and Egr1 by GnRH, whereas decreasing both WT1 (-KTS) and (+KTS) increased endogenous LHβ transcription, and prevented LHβ but not Egr1 stimulation by GnRH, suggesting differing biological activities for the WT1 isoforms. Overexpression of WT1 showed that WT1(-KTS) enhanced LHβ promoter GnRH stimulation 2-to-3-fold and required the 3'Egr1 site, but WT1(+KTS) repressed both basal and GnRH-stimulated LHβ promoter activity by approximately 70%. Our data suggest that WT1 can modulate LHβ transcription, with differential roles for the two WT1 variants; WT1 (-KTS) enhances and WT1 (+KTS) suppresses transcription.

Mechanisms of transcriptional regulation by WT1 (Wilms' tumour 1)

The WT1 (Wilms' tumour 1) gene encodes a zinc finger transcription factor and RNA-binding protein that direct the development of several organs and tissues. WT1 manifests both tumour suppressor and oncogenic activities, but the reasons behind these opposing functions are still not clear. As a transcriptional regulator, WT1 can either activate or repress numerous target genes resulting in disparate biological effects such as growth, differentiation and apoptosis. The complex nature of WT1 is exemplified by a plethora of isoforms, post-translational modifications and multiple binding partners. How WT1 achieves specificity to regulate a large number of target genes involved in diverse physiological processes is the focus of the present review. We discuss the wealth of the growing molecular information that defines our current understanding of the versatility and utility of WT1 as a master regulator of organ development, a tumour suppressor and an oncogene.

Splicing regulator SLU7 is essential for maintaining liver homeostasis

A precise equilibrium between cellular differentiation and proliferation is fundamental for tissue homeostasis. Maintaining this balance is particularly important for the liver, a highly differentiated organ with systemic metabolic functions that is endowed with unparalleled regenerative potential. Carcinogenesis in the liver develops as the result of hepatocellular de-differentiation and uncontrolled proliferation. Here, we identified SLU7, which encodes a pre-mRNA splicing regulator that is inhibited in hepatocarcinoma, as a pivotal gene for hepatocellular homeostasis. SLU7 knockdown in human liver cells and mouse liver resulted in profound changes in pre-mRNA splicing and gene expression, leading to impaired glucose and lipid metabolism, refractoriness to key metabolic hormones, and reversion to a fetal-like gene expression pattern. Additionally, loss of SLU7 also increased hepatocellular proliferation and induced a switch to a tumor-like glycolytic phenotype. Slu7 governed the splicing and/or expression of multiple genes essential for hepatocellular differentiation, including serine/arginine-rich splicing factor 3 (Srsf3) and hepatocyte nuclear factor 4α (Hnf4α), and was critical for cAMP-regulated gene transcription. Together, out data indicate that SLU7 is central regulator of hepatocyte identity and quiescence.

Insulin and insulin-like growth factor II differentially regulate endocytic sorting and stability of insulin receptor isoform A

The insulin receptor isoform A (IR-A) binds both insulin and insulin-like growth factor (IGF)-II, although the affinity for IGF-II is 3-10-fold lower than insulin depending on a cell and tissue context. Notably, in mouse embryonic fibroblasts lacking the IGF-IR and expressing solely the IR-A (R-/IR-A), IGF-II is a more potent mitogen than insulin. As receptor endocytosis and degradation provide spatial and temporal regulation of signaling events, we hypothesized that insulin and IGF-II could affect IR-A biological responses by differentially regulating IR-A trafficking. Using R-/IR-A cells, we discovered that insulin evoked significant IR-A internalization, a process modestly affected by IGF-II. However, the differential internalization was not due to IR-A ubiquitination. Notably, prolonged stimulation of R-/IR-A cells with insulin, but not with IGF-II, targeted the receptor to a degradative pathway. Similarly, the docking protein insulin receptor substrate 1 (IRS-1) was down-regulated after prolonged insulin but not IGF-II exposure. Similar results were also obtained in experiments using [NMeTyr(B26)]-insulin, an insulin analog with IR-A binding affinity similar to IGF-II. Finally, we discovered that IR-A was internalized through clathrin-dependent and -independent pathways, which differentially regulated the activation of downstream effectors. Collectively, our results suggest that a lower affinity of IGF-II for the IR-A promotes lower IR-A phosphorylation and activation of early downstream effectors vis à vis insulin but may protect IR-A and IRS-1 from down-regulation thereby evoking sustained and robust mitogenic stimuli.

Mutations of the Wilms tumor 1 gene (WT1) in older patients with primary cytogenetically normal acute myeloid leukemia: a Cancer and Leukemia Group B study

We previously reported the adverse prognostic impact of Wilms tumor 1 gene (WT1) mutations in younger adult cytogenetically normal acute myeloid leukemia (CN-AML). Here, we investigated 243 older (> or = 60 years) primary CN-AML patients. WT1 mutated (WT1mut) patients (7%) had FLT3-ITD more frequently (P < .001), lower hemoglobin (P = .01), higher white blood cell count (P = .03) and percentage blood blasts (P = .03), and a shorter overall survival (P = .08) than WT1 wild-type (WT1wt) patients. Comparing older and younger WT1mut patients, they had similar pretreatment characteristics and outcome. By contrast, among WT1wt CN-AML, younger patients had a significantly better outcome. A WT1 mutation-associated gene-expression signature, reported here for the first time, included CD96, a leukemia stem cell-specific marker, and genes involved in gene regulation (eg, MLL, PML, and SNRPN) and in proliferative and metabolic processes (eg, INSR, IRS2, and PRKAA1), supporting the role of mutated WT1 in deregulating multiple homeostatic processes. Our results indicate that WT1mut CN-AML represents a distinct entity with poor treatment response across age groups. This study has been registered at www.clinicaltrials.gov as #NCT00900224.

Differential signaling activation by insulin and insulin-like growth factors I and II upon binding to insulin receptor isoform A

A variety of human malignancies overexpresses isoform A of the insulin receptor (IR-A) and produces IGFs (IGF-I and/or IGF-II). IR-A binds IGF-II with high affinity (although 4-fold lower than that for insulin), whereas it binds IGF-I with low affinity (approximately 30-fold lower than that for insulin). However, in engineered cells expressing only the IR-A, but not IGF-I receptor (R(-)/IR-A cells), IGF-II is a more potent mitogen than insulin. Herein, we investigated downstream signaling of IGF-II, IGF-I, and insulin in R(-)/IR-A cells to better understand their role in cell growth. We found that despite inducing a lower IR-A autophosphorylation than insulin, IGF-II was more potent than insulin for activating p70S6 kinase (p70S6K) and approximately equally potent in activating the early peaks of ERK1/2 and Akt. However, ERK1/2 activation persisted longer after IGF-II, whereas Akt activation persisted longer after insulin. Therefore, cells stimulated with IGF-II had a higher p70S6K/Akt activation ratio than cells stimulated with insulin. Remarkably, IGF-I also elicited a similar signaling pattern as IGF-II, despite inducing minimal IR-A autophosphorylation. ERK1/2 and protein kinase C seem to be involved in the preferential stimulation of p70S6K by IGFs. In conclusion, our study has identified a novel complex role of IR-A, which not only elicits a unique signaling pattern after IGF-II binding but also induces substantial downstream signaling upon binding to the low-affinity ligand IGF-I. These results underline the role of IR-A in physiology and disease.

The role of the Wilms tumour gene (WT1) in normal and malignant haematopoiesis

In addition to its loss playing a pivotal role in the development of a childhood kidney malignancy, the Wilms tumour 1 gene (WT1) has emerged as an important factor in normal and malignant haematopoiesis. Preferentially expressed in CD34+ haematopoietic progenitors and down-regulated in more-differentiated cells, the WT1 transcription factor has been implicated in regulation of apoptosis, proliferation and differentiation. Putative target genes, such as BCL2, MYC, A1 and cyclin E, may cooperate with WT1 to modulate cell growth. However, the effects of WT1 on target gene expression appear to be isoform-specific. Certain WT1 isoforms are over-represented in leukaemia, but the exact mechanisms underlying the role of WT1 in transformation remain unclear. The ubiquity of WT1 in haematological malignancies has led to efforts to exploit it as a marker for minimal residual disease and as a prognostic factor, with conflicting results. In vitro killing of tumour cells by WT1-specific CD8+ cytotoxic T lymphocytes facilitated design of Phase I vaccine trials that showed clinical regression of WT1-positive tumours. Alternative methods employing WT1-specific immunotherapy are being investigated and might ultimately be used to optimise multimodal therapy of haematological malignancies.

A tumor suppressor and oncogene: the WT1 story

The Wilms' tumor 1 (WT1) gene encodes a transcription factor important for normal cellular development and cell survival. The initial discovery of WT1 as the causative gene in an autosomal-recessive condition identified it as a tumor suppressor gene whose mutations are associated with urogenital disease and the development of kidney tumors. However, this view is not in keeping with the frequent finding of wild-type, full-length WT1 in human leukemia, breast cancer and several other cancers including the majority of Wilms' tumors. Rather, these observations suggest that in those conditions, WT1 has an oncogenic role in tumor formation. In this review, we explore the literature supporting both views of WT1 in human cancer and in particular human leukemias. To understand the mechanism by which WT1 can do this, we will also examine its functional activity as a transcription factor and the influence of protein partners on its dual behavior.

Insulin receptor activation in solitary fibrous tumours

Solitary fibrous tumours (SFTs) are known to overexpress insulin-like growth factor 2 (IGF-2). The down-stream oncogenic pathways of IGF-2, however, are not clear. Here we report uniform activation of the insulin receptor (IR) pathway in SFTs, which are mesenchymal tumours frequently associated with hypoglycaemia. Whereas the IR and its downstream signalling pathways were constitutively activated in SFTs, insulin-like growth factor 1 receptor (IGF-1R) was not expressed in these tumours. We also find that SFT cells secrete IGF-2 and proliferate in serum-free medium, consistent with an IGF-2/IR autocrine loop. The aetiological relevance of IGF-2 is supported by expression of IR-A, the IR isoform with high affinity for IGF-2, in all SFTs. Our studies suggest that IR activation plays an oncogenic role in SFTs.

The antiapoptotic gene A1/BFL1 is a WT1 target gene that mediates granulocytic differentiation and resistance to chemotherapy

Previous work has demonstrated that WT1 (-Ex5/-KTS) potentiates granulocyte colony-stimulating factor (G-CSF)-mediated granulocytic differentiation. This WT1 isoform suppresses cyclin E, which may contribute to the prodifferentiation effect by slowing proliferation, but WT1 target genes that affect survival might also be involved. We screened a cDNA array and identified the bCL2 family member A1/BFL1 as a new WT1 target gene in 32D cl3 murine myeloblast cells. Induction of WT1 (-Ex5/-KTS) expression is accompanied by up-regulation of A1 on the cDNA array, and this up-regulation was confirmed by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR). Moreover, both promoter-reporter assays and chromatin immunoprecipitation assays suggest that this isoform of WT1 activates the promoter directly. Constitutive expression of A1 in 32D cl3 cells induces spontaneous granulocytic differentiation, with both morphologic and cell-surface antigen changes, as well as resistance both to chemotherapy and to withdrawal of interleukin-3 (IL-3). Finally, we note an association between WT1 expression and A1 expression in primary acute myeloid leukemia samples. Taken together, these results demonstrate that A1 is a new WT1 target gene involved in both granulocytic differentiation and resistance to cell death, and suggests that these genes might play an important role in the biology of high-risk leukemias.

WT1-p53 interactions in insulin-like growth factor-I receptor gene regulation

The insulin-like growth factor-I receptor (IGF-IR) plays a critical role in transformation. The expression of the IGF-IR gene is negatively regulated by a number of transcription factors, including the WT1 and p53 tumor suppressors. Previous studies have suggested both physical and functional interactions between the WT1 and p53 proteins. The potential functional interactions between WT1 and p53 in control of IGF-IR promoter activity were addressed by transient coexpression of vectors encoding different isoforms of WT1, together with IGF-IR promoter-luciferase reporter constructs, in p53-null osteosarcoma-derived Saos-2 cells, wild-type p53-expressing kidney tumor-derived G401 cells, and mutant p53-expressing, rhabdomyosarcoma-derived RD cells. Similar studies were also performed to compare p53-expressing Balb/c-3T3 and clonally derived p53-null, (10)1 fibroblasts and the colorectal cancer cell line HCT116 +/+, which expresses a wild-type p53 gene, and its HCT116 -/- derivative, in which the p53 gene has been disrupted by homologous recombination. WT1 splice variants lacking a KTS insert between zinc fingers 3 and 4 suppressed IGF-IR promoter activity in the absence of p53 or in the presence of wild-type p53. WT1 variants that contain the KTS insert are impaired in their ability to bind to the IGF-IR promoter and are unable to suppress IGF-IR promoter. In the presence of mutant p53, WT1 cannot repress the IGF-IR promoter. Coimmunoprecipitation experiments showed that p53 and WT1 physically interact, whereas electrophoretic mobility shift assay studies revealed that p53 modulates the ability of WT1 to bind to the IGF-IR promoter. In summary, the transcriptional activity of WT1 proteins and their ability to function as tumor suppressors or oncogenes depends on the cellular status of p53.

A review of the Wilms' tumor 1 gene (WT1) and its role in hematopoiesis and leukemia

One of the first clones of the Wilms tumor 1 (WT1) gene, WT33, was isolated from a B cell leukemia cell line in 1990. Now, 12 years on, WT1 has emerged as a potentially important target for antileukemic therapies. Our understanding of the role that WT1 plays during normal hematopoiesis is still limited, and there is a large amount of conflicting data concerning the precise manner in which WT1 gene expression contributes to leukemogenesis. However, interest in this field has intensified in the past 5 years. This review surveys the progress made in this area.

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.

Overexpression of murine WT1 + / + and - / - isoforms has no effect on chemoresistance but delays differentiation in the K562 leukemia cell line

The Wilms' tumor gene (WT1) encodes a zinc-finger transcription factor that is expressed as four distinct isoforms designated as, + / +, + / -, - / + and - / -. It is expressed in leukemic cells, and is proposed to play a role in their proliferation and differentiation. In this study we have shown that cell lines of the erythroleukemia, K562, overexpressing the murine + / + and - / - WT1 isoforms grow normally and do not exhibit altered responses to the induction of apoptosis by the reagents cisplatin and adriamycin, or to serum withdrawal. However, differentiation of K562 cells with 12-O-tetradecanoylphorbol 13-acetate, modeling aspects of megakaryopoiesis, was partially inhibited by the persistent expression of both the murine + / + and - / - WT1 isoforms. This finding suggests that WT1 plays a role in the regulation of hematopoietic differentiation and is consistent with an oncogenic role for WT1 in leukemogenesis.

The Wilms' tumor gene product represses the transcription of thrombospondin 1 in response to overexpression of c-Jun

Thrombospondin 1 (TSP1) is known for its significant anti-angiogenic properties. In a previous study, we have shown that transient or stable overexpression of the transcription factor c-Jun, in rat fibroblasts, leads to repression of TSP1. We now demonstrate that the c-Jun-induced repression of TSP1 does not occur directly and does not require binding of c-Jun to the TSP1 promoter. Instead, repression involves a factor secreted by c-Jun-overexpressing cells. This secreted factor triggers a signal transduction pathway from the membrane to the nucleus, and these signals lead to the binding of the product of the Wilms' tumor suppressor gene, WT1, to the -210 region of the TSP1 promoter. This region binds WT1 and SP1, but not EGR1, although its sequence fits the consensus binding site for this transcription factor. WT1 overexpression in transfected cells inhibits endogenous TSP1 gene expression and TSP1 transcription in experiments using TSP1 promoter-reporter constructs. The WT1 - KTS isoform is more active in repressing TSP1 transcription than WT1 + KTS, while EGR1 is inactive. Enhancement of WT1 binding to DNA in response to c-Jun does not require de novo protein synthesis. The above mechanism for TSP1 repression could apply to other genes, thus coordinating their regulation in the vicinity of a c-Jun-overexpressing cell. We conclude that WT1, which was discovered as a result of its tumor suppressor properties, may also possess oncogenic characteristics in the c-Jun transformation process, and thus repress the anti-angiogenic protein, TSP1.

WT1: what has the last decade told us?

When positionally cloned in late 1989, it was anticipated that mutations within the Wilms' tumour suppressor gene (WT1) would prove responsible for this common solid kidney cancer of childhood. Characterisation of the WT1 expression pattern and of the structure of the encoded protein isoforms and their mode of action has now spanned almost a decade. WT1 proteins act as nucleic acid-binding zinc finger-containing transcription factors involved in both transactivation and repression. These activities are facilitated and constrained by interactions with other proteins. Expression analyses and knockout mice indicate that WT1 protein plays a critical role in normal kidney and gonad development. Specific constitutional WT1 mutations results in several urogenital anomaly syndromes. While only 10% of sporadic Wilms' tumours do display WT1 mutation, WT1 is mutated in other cancers, including acute myeloid leukaemia. Much is still to be determined in WT1 biology. The next decade will see at least three streams of attention. The first two, elucidation of the role of WT1 in RNA metabolism and the characterisation of further protein partners, may together explain the distinct tissue-specific functions of WT1. Finally, further research into the role of WT1 in haematopoiesis will improve our understanding of WT1 in leukaemia.