A novel repressor, par-4, modulates transcription and growth suppression functions of the Wilms' tumor suppressor WT1

Inactivation of the inhibitory kappaB protein kinase/nuclear factor kappaB pathway by Par-4 expression potentiates tumor necrosis factor alpha-induced apoptosis

Par-4 is a novel protein identified in cells undergoing apoptosis. The ability of Par-4 to promote apoptotic cell death is dependent on the binding and inactivation of the atypical protein kinases C (PKCs). This subfamily of kinases has been reported to control nuclear factor kappaB (NF-kappaB) through the regulation of the IkappaB kinase activity. NF-kappaB activation by tumor necrosis factor alpha (TNFalpha) provides a survival signal that impairs the TNFalpha-induced apoptotic response. We show here that expression of Par-4 inhibits the TNFalpha-induced nuclear translocation of p65 as well as the kappaB-dependent promoter activity. Interestingly, Par-4 expression blocks inhibitory kappaB protein (IkappaB) kinase activity, which leads to the inhibition of IkappaB phosphorylation and degradation, in a manner that is dependent on its ability to inhibit lambda/iotaPKC. Of potential functional relevance, the expression of Par-4 allows TNFalpha to induce apoptosis in NIH-3T3 cells. In addition, the down-regulation of Par-4 levels by oncogenic Ras sensitizes cells to TNFalpha-induced NF-kappaB activation.

Prostate apoptosis response-4 production in synaptic compartments following apoptotic and excitotoxic insults: evidence for a pivotal role in mitochondrial dysfunction and neuronal degeneration

Synapses are often located at great distances from the cell body and so must be capable of transducing signals into both local and distant responses. Although progress has been made in understanding biochemical cascades involved in neuronal death during development of the nervous system and in various neurodegenerative disorders, it is not known whether such cascades function locally in synaptic compartments. Prostate apoptosis response-4 (Par-4) is a leucine zipper and death domain-containing protein that plays a role in neuronal apoptosis. We now report that Par-4 levels are rapidly increased in cortical synaptosomes and in dendrites of hippocampal neurons in culture and in vivo, following exposure to apoptotic or excitotoxic insults. Par-4 expression is regulated at the translational level within synaptic compartments. Par-4 antisense treatment suppressed mitochondrial dysfunction and caspase activation in synaptosomes and prevented death of cultured hippocampal neurons following exposure to excitotoxic and apoptotic insults. Local translational regulation of death-related proteins in synaptic compartments may play a role in programmed cell death, adaptive remodeling of synapses, and neurodegenerative disorders.

Tumor-associated WT1 missense mutants indicate that transcriptional activation by WT1 is critical for growth control

The WT1 gene encodes a zinc finger DNA binding transcription factor and is mutated in up to 15% of Wilms tumor cases. The WT1 protein binds to the promoters of many genes through GC- or TC-rich sequences and can function both as a transcriptional repressor and an activator in co-transfection assays depending on the cell type, the structure of the test promoter, and even the expression vectors used. Engineered expression of WT1 can lead to growth suppression by both cell cycle arrest and induction of apoptosis. However, the transcriptional activity of WT1 that is required for growth control was not defined. We found that three N-terminal tumor-associated missense mutations of WT1 were defective for activation of both a synthetic reporter containing WT1-binding sites as well as the promoter of a WT1 responsive gene, p21. These mutants failed to inhibit cell growth but still retain their ability to repress several putative WT1 target promoters. These results indicate that activation and not repression by WT1 is the critical transcriptional activity of the protein responsible for its growth suppressing properties.

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.

Analysis of native WT1 protein from frozen human kidney and Wilms' tumors

The Wilms' tumor susceptibility gene, WT1, is altered in a subset of Wilms' tumors and encodes a transcription factor with four zinc fingers. Here we describe the isolation of native WT1 protein from frozen normal human kidney and Wilms' tumor samples. Through size exclusion chromatography and Western blot analysis we determined the elution pattern of WT1. The majority of WT1 from adult kidney and Wilms' tumor specimens was found to elute at a size of approximately 120 kDa, consistent with a WT1 homodimer and some WT1 protein was also found in a higher molecular weight complex. In 14 week fetal kidney the majority of the WT1 protein eluted at a size of 80 kDa, suggesting that at this developmental stage the WT1 protein is not present as a homodimer. The identity of complexing partners can now be studied using this approach.

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.

The Wilms' tumor suppressor gene (wt1) product regulates Dax-1 gene expression during gonadal differentiation

Gonadal differentiation is dependent upon a molecular cascade responsible for ovarian or testicular development from the bipotential gonadal ridge. Genetic analysis has implicated a number of gene products essential for this process, which include Sry, WT1, SF-1, and DAX-1. We have sought to better define the role of WT1 in this process by identifying downstream targets of WT1 during normal gonadal development. We have noticed that in the developing murine gonadal ridge, wt1 expression precedes expression of Dax-1, a nuclear receptor gene. We document here that the spatial distribution profiles of both proteins in the developing gonad overlap. We also demonstrate that WT1 can activate the Dax-1 promoter. Footprinting analysis, transient transfections, promoter mutagenesis, and mobility shift assays suggest that WT1 regulates Dax-1 via GC-rich binding sites found upstream of the Dax-1 TATA box. We show that two WT1-interacting proteins, the product of a Denys-Drash syndrome allele of wt1 and prostate apoptosis response-4 protein, inhibit WT1-mediated transactivation of Dax-1. In addition, we demonstrate that WT1 can activate the endogenous Dax-1 promoter. Our results indicate that the WT1-DAX-1 pathway is an early event in the process of mammalian sex determination.

Ornithine decarboxylase is a transcriptional target of tumor suppressor WT1

The product of the Wilm's tumor suppressor gene, WT1, is a zinc-finger DNA-binding protein, which is thought to be a transcription factor. Two genes, those encoding epidermal growth factor receptor and syndecan-1, are known to be endogenous targets of WT1. Previous studies had identified binding sites for WT1 in the promoter of the ornithine decarboxylase (ODC) gene. In this paper, we tested whether the endogenous ODC gene might be a target of WT1 by establishing lines of baby hamster kidney (BHK) cells that expressed WT1 isoform A under control of a tetracycline-regulated expression system. When expression of WT1 was activated in BHK cells, the cellular level of ODC mRNA declined, with kinetics that correlated with the increase in WT1 level, demonstrating that the endogenous ODC gene was indeed responsive to cellular level of WT1. WT1 isoforms A and B inhibited the activity of the ODC promoter by approximately fivefold in transiently transfected BHK cells, while isoforms C and D, which have altered DNA binding domains, had no significant effect. The sequence CTCCCCCGC, located at nucleotides -106 to -98 relative to the site of transcriptional initiation in the ODC gene, interacted with the zinc-finger domain of isoforms A and B of WT1 with high affinity and specificity. A mutation in the binding site that disrupted this interaction partially removed the inhibition of ODC promoter activity by WT1, as did mutation of the two E-box sequences in intron I of the ODC gene. Simultaneous mutation of the WT1-binding motif and the two E-boxes completely abolished inhibition by WT1 of ODC promoter activity. These results, taken together, implicate the ODC gene as a downstream target of the tumor suppressor WT1.

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.

Mutually exclusive expression patterns of Bcl-2 and Par-4 in human prostate tumors consistent with down-regulation of Bcl-2 by Par-4

Par-4 is a widely expressed protein that sensitizes both prostatic and non-prostatic cells to apoptosis. Constitutive- or regulated- overexpression of Par-4 caused a reduction in the levels of the anti-apoptotic protein Bcl-2. Replenishment of Bcl-2 levels abrogated susceptibility to Par-4-dependent apoptosis, suggesting that Par-4-mediated apoptosis requires downmodulation of Bcl-2 levels. The inverse correlation between Par-4 and Bcl-2 expression was recapitulated in human prostate tumors. Par-4 but not Bcl-2 was detected in the secretory epithelium of benign prostatic tumors and in primary and metastatic prostate cancers that are apt to undergo apoptosis. Moreover, xenografts of human, androgen-dependent CWR22 tumors showed Par-4 but not Bcl-2 expression. By contrast, androgen-independent CWR22R tumors derived from the CWR22 xenografts showed mutually exclusive expression patterns of Par-4 and Bcl-2. These findings suggest a mechanism by which Par-4 may sensitize prostate tumor cells to apoptosis.

WT1 expression induces features of renal epithelial differentiation in mesenchymal fibroblasts

The WT1 tumor suppressor gene, implicated in hereditofamilial and sporadic Wilms' tumor, is required for normal renal development and is up-regulated during the mesenchymal-epithelial transition. NIH3T3 fibroblasts overexpressing WT1 were less proliferative, larger in size and more firmly attached to tissue culture plastic, suggesting an alteration of their state of differentiation. These cells were studied in vivo by subcutaneous injection into nude mice. The resulting tumors exhibited epithelioid histopathology and formed desmosome-like structures. Molecular analyses of these WT1 expressing fibroblasts grown in culture and in nude mice revealed significant alterations in the expression of many kidney epithelial markers. These studies indicate that WT1 expression can initiate features of a program of epithelial differentiation consistent with a prominent role for WT1 in the mesenchymal epithelial transition that occurs during renal development. Through this work we identified a number of novel target genes for the WT1 transcription factor, including uvomorulin, integrin alpha8 and perlecan, and suggest that WTI may activate the IGF-II gene, also implicated in the development of Wilms' tumor.

WT1--more than a transcription factor?

The Wilms tumor-suppressor gene WT1 was originally identified through its involvement in the development of a pediatric kidney tumor. Recent genetic data show that mutations in the WT1 gene cause a variety of other diseases, and new biochemical evidence suggests that the WT1 protein is not only a transcription factor but might also act at the post-transcriptional level.

Par-4 is a mediator of neuronal degeneration associated with the pathogenesis of Alzheimer disease

Prostate apoptosis response-4 (Par-4) is a protein containing both a leucine zipper and a death domain that was isolated by differential screening for genes upregulated in prostate cancer cells undergoing apoptosis. Par-4 is expressed in the nervous system, where its function is unknown. In Alzheimer disease (AD), neurons may die by apoptosis, and amyloid beta-protein (A beta) may play a role in this. We report here that Par-4 expression is increased in vulnerable neurons in AD brain and is induced in cultured neurons undergoing apoptosis. Blockade of Par-4 expression or function prevented neuronal apoptosis induced by Ab and trophic factor withdrawal. Par-4 expression was enhanced, and mitochondrial dysfunction and apoptosis exacerbated, in cells expressing presenilin-1 mutations associated with early-onset inherited AD.

Adenovirus E1B 19,000-molecular-weight protein activates c-Jun N-terminal kinase and c-Jun-mediated transcription

Adenovirus E1B proteins (19,000-molecular-weight [19K] and 55K proteins) inhibit apoptosis and cooperate with adenovirus E1A to induce full oncogenic transformation of primary cells. The E1B 19K protein has previously been shown to be capable of activating transcription; however, the underlying mechanisms are unclear. Here, we show that adenovirus infection activates the c-Jun N-terminal kinase (JNK) and that the E1B gene products are necessary for adenovirus to activate JNK. In transfection assays, we show that the E1B 19K protein is sufficient to activate JNK and can strongly induce c-Jun-dependent transcription. Mapping studies show that the C-terminal portion of E1B 19K is necessary for induction of c-Jun-mediated transcription. Using dominant-negative mutants of several kinases upstream of JNK, we show that MEKK1 and MKK4, but not Ras, are involved in the induction of JNK activity by adenovirus infection. The same dominant-negative kinase mutants also block the ability of E1B 19K to induce c-Jun-mediated transcription. Taken together, these results suggest that E1B 19K may utilize the MEKK1-MKK4-JNK signaling pathway to activate c-Jun-dependent transcription and demonstrate a novel, kinase-activating activity of E1B 19K that may underlie its ability to regulate transcription.

novH: differential expression in developing kidney and Wilm's tumors

We previously established that the expression of the human nov gene (novH) was altered in Wilms' tumors and that levels of novH and WT1 mRNA were inversely correlated in individual Wilms' tumors. Insofar as novH has been shown to be a target for WT1 regulation, novH might play an important role during normal nephrogenesis and in the development of Wilms' tumors. We now show that during normal nephrogenesis novH protein is tightly associated with differentiation of glomerular podocytes. NovH expression is not restricted to renal differentiation but is also detected in endothelium and neural tissue of the kidney. Our results establish that alteration of novH expression in sporadic and heritable Wilms' tumors is associated with dysregulated expression of both novH mRNA and protein. In general, the highest novH expression was noted in the Wilms' tumor, genitourinary anomalies, aniridia, and mental retardation (WAGR)-associated Wilms' tumors. Expression in the Denys-Drash syndrome (DDS)-associated Wilms' tumors fell within the variable spectrum observed in sporadic Wilms' tumor cases. As in developing kidney podocytes, novH protein was also prominent in the abnormal hypoplastic podocytes from DDS cases and in kidney podocytes adjoining Wilms' tumors. In Wilms' tumors exhibiting heterotypic differentiation, novH protein was expressed at high levels in tumor-derived striated muscle and at lower levels in tumor-derived cartilage. These observations taken together indicate that novH may represent both a marker of podocytic differentiation in kidney and a marker of heterotypic mesenchymal differentiation in Wilms' tumors. In addition, absence or very low levels of WT1 are correlated with higher novH expression, and its variable expression in cases with mutant WT1 (sporadic and DDS) suggests that the potential activation and repression transcriptional functions possessed by WT1 are likely dependent on the specific mutation incurred.

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.

Ciao 1 is a novel WD40 protein that interacts with the tumor suppressor protein WT1

The Wilms tumor suppressor protein, WT1, is a transcription factor capable of activating or repressing transcription of various cellular genes. The mechanisms involved in regulating the transcriptional activities of WT1 are beginning to be unraveled. It appears that physical interactions of other cellular proteins (p53 and par-4) with WT1 can modulate the function of WT1. Here, we report the identification and cloning of a novel WT1-interacting protein termed Ciao 1, a member of the WD40 family of proteins. Ciao 1 specifically interacts with WT1 both in vitro and in vivo. This interaction alters the mobility of a WT1.DNA complex in gel shift assays, and results in a decrease in transcriptional activation mediated by WT1. Ciao 1 does not inhibit binding of WT1 to its consensus nucleotide sequence and does not affect the repression activity of WT1. Thus, Ciao 1 appears to specifically modulate the transactivation activity of WT1 and may function to regulate the physiological functions of WT1 in cell growth and differentiation.

Tumor suppressor p53 can participate in transcriptional induction of the GADD45 promoter in the absence of direct DNA binding

The GADD45 gene is a growth arrest-associated gene that is induced by certain DNA-damaging agents and other stresses, such as starvation, in all mammalian cells. In addition to a strong p53-binding element in an intronic sequence, we have recently found that p53, while not required or sufficient alone, may contribute to the stress responsiveness of the promoter. Much of the responsiveness was localized to a GC-rich motif in the proximal promoter which contains multiple Egr1 sites and a larger WT1 site; this 20-bp WT1 motif is identical to the WT1-binding site in the PDGF-A gene. In extracts from a human breast carcinoma cell line expressing p53 and WT1, which is known to associate with p53 in vivo, evidence was obtained that these proteins are in a complex that binds this 20-bp element. A combination of p53 and WT1 expression vectors strongly induced a GADD45-reporter construct, while mutation of the WT1-Egr1 site in the promoter prevented this induction. Abrogation of p53 function by a dominant-negative vector or abrogation of WT1 function by an antisense vector markedly reduced the induction of this promoter. Since p53 does not bind directly to the promoter, these results indicate that p53 can contribute to the positive regulation of a promoter by protein-protein interactions.

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.

Overlapping RNA and DNA binding domains of the wt1 tumor suppressor gene product

The Wilms' tumour suppressor gene (wt1) is mutated in a subset of patients with Wilms' tumour and has a critical role in urogenital development. wt1 encodes a zinc finger transcription factor which regulates expression of several genes involved in cellular proliferation and differentiation. Although a number of studies have characterized the DNA binding properties of the WT1 protein, recent evidence has suggested that WT1 may also have a role in RNA metabolism. We have used an RNA selection method to identify WT1 binding ligands from a random RNA pool. Three groups of RNA ligands specifically recognized by WT1 were identified. Mutational analysis pinpointed ribonucleotide sequences critical for binding. Analysis of truncated WT1 proteins demonstrated that three of four zinc fingers were necessary for RNA-protein interaction. The naturally occurring WT1 isoforms with insertion of lysine, threonine and serine between zinc fingers three and four were unable to bind the selected RNAs. The selected RNA ligands competed with the cognate WT1 DNA binding site for complex formation with WT1. Our findings suggest potential cellular RNA target sequences for WT1 and provide tools for studying the structural and functional properties of this tumour suppressor protein.

Rex-1, a gene encoding a transcription factor expressed in the early embryo, is regulated via Oct-3/4 and Oct-6 binding to an octamer site and a novel protein, Rox-1, binding to an adjacent site

The Rex-1 (Zfp-42) gene, which encodes an acidic zinc finger protein, is expressed at high levels in embryonic stem (ES) and F9 teratocarcinoma cells. Prior analysis identified an octamer motif in the Rex-1 promoter which is required for promoter activity in undifferentiated F9 cells and is involved in retinoic acid (RA)-associated reduction in expression. We show here that the Oct-3/4 transcription factor, but not Oct-1, can either activate or repress the Rex-1 promoter, depending on the cellular environment. Rex-1 repression is enhanced by E1A. The protein domain required for Oct-3/4 activation was mapped to amino acids 1 to 35, whereas the domain required for Oct-3/4 repression was mapped to amino acids 61 to 126, suggesting that the molecular mechanisms underlying transcriptional activation and repression differ. Like Oct-3/4, Oct-6 can also lower the expression of the Rex-1 promoter via the octamer site, and the amino-terminal portion of Oct-6 mediates this repression. In addition to the octamer motif, a novel positive regulatory element, located immediately 5' of the octamer motif, was identified in the Rex-1 promoter. Mutations in this element greatly reduce Rex-1 promoter activity in F9 cells. High levels of a binding protein(s), designated Rox-1, recognize this novel DNA element in F9 cells, and this binding activity is reduced following RA treatment. Taken together, these results indicate that the Rex-1 promoter is regulated by specific octamer family members in early embryonic cells and that a novel element also contributes to Rex-1 expression.

Positioning atypical protein kinase C isoforms in the UV-induced apoptotic signaling cascade

Recent studies have documented the involvement of the atypical protein kinase C (aPKC) isoforms in important cellular functions such as cell proliferation and survival. Exposure of cells to a genotoxic stimulus that induces apoptosis, such as UV irradiation, leads to a profound inhibition of the atypical PKC activity in vivo. In this study, we addressed the relationship between this phenomenon and different proteins involved in the apoptotic response. We show that (i) the inhibition of the aPKC activity precedes UV-induced apoptosis; (ii) UV-induced aPKC inhibition and apoptosis are independent of p53; (iii) Bcl-2 proteins are potent modulators of aPKC activity; and (iv) the aPKCs are located upstream of the interleukin-converting enzyme-like protease system, which is required for the induction of apoptosis by both Par-4 (a selective aPKC inhibitor) and UV irradiation. We also demonstrate here that inhibition of aPKC activity leads to a decrease in mitogen-activated protein (MAP) kinase activity and simultaneously an increase in p38 activity. Both effects are critical for the induction of apoptosis in response to Par-4 expression and UV irradiation. Collectively, these results clarify the position of the aPKCs in the UV-induced apoptotic pathway and strongly suggest that MAP kinases play a role in this signaling cascade.

Expression and function of the leucine zipper protein Par-4 in apoptosis

The prostate apoptosis response-4 (par-4) gene was identified by differential screening for genes that are upregulated when prostate cancer cells are induced to undergo apoptosis. The par-4 gene is induced by apoptotic signals but not by growth-arresting, necrotic, or growth-stimulatory signals. The deduced amino acid sequence of par-4 predicts a protein with a leucine zipper domain at its carboxy terminus. We have recently shown that the Par-4 protein binds, via its leucine zipper domain, to the zinc finger domain of Wilms' tumor protein WT1 (R. W. Johnstone et al., Mol. Cell. Biol. 16:6945-6956, 1996). In experiments aimed at determining the functional role of par-4 in apoptosis, an antisense par-4 oligomer abrogated par-4 expression and activator-driven apoptosis in rat prostate cancer cell line AT-3, suggesting that par-4 is required for apoptosis in these cells. Consistent with a functional role for par-4 in apoptosis, ectopic overexpression of par-4 in prostate cancer cell line PC-3 and melanoma cell line A375-C6 conferred supersensitivity to apoptotic stimuli. Transfection studies with deletion mutants of Par-4 revealed that full-length Par-4, but not mutants that lacked the leucine zipper domain of Par-4, conferred enhanced sensitivity to apoptotic stimuli. Most importantly, ectopic coexpression of the leucine zipper domain of Par-4 inhibited the ability of Par-4 to enhance apoptosis. Finally, ectopic expression of WT1 attenuated apoptosis, and coexpression of Par-4 but not a leucine zipperless mutant of Par-4 rescued the cells from the antiapoptotic effect of WT1. These findings suggest that the leucine zipper domain is required for the Par-4 protein to function in apoptosis.

Suppression of grp78 core promoter element-mediated stress induction by the dbpA and dbpB (YB-1) cold shock domain proteins

The highly conserved grp78 core promoter element plays an important role in the induction of grp78 under diverse stress signals. Previous studies have established a functional region in the 3' half of the core (stress-inducible change region [SICR]) which exhibits stress-inducible changes in stressed nuclei. The human transcription factor YY1 is shown to bind the SICR and transactivate the core element under stress conditions. Here we report that expression library screening with the core element has identified two new core binding proteins, YB-1 and dbpA. Both proteins belong to the Y-box family of proteins characterized by an evolutionarily conserved DNA binding motif, the cold shock domain (CSD). In contrast to YY1, which binds only double-stranded SICR, the Y-box/CSD proteins much prefer the lower strand of the SICR. The Y-box proteins can repress the inducibility of the grp78 core element mediated by treatment of cells with A23187, thapsigargin, and tunicamycin. In gel shift assays, YY1 binding to the core element is inhibited by either YB-1 or dbpA. A yeast interaction trap screen using LexA-YY1 as a bait and a HeLa cell cDNA-acid patch fusion library identified YB-1 as a YY1-interacting protein. In cotransfection experiments, the Y-box proteins antagonize the YY1-mediated enhancement of transcription directed by the grp78 core in stressed cells. Thus, the CSD proteins may be part of the stress signal transduction mechanism in the mammalian system.