Purification and molecular cloning of a secreted, Frizzled-related antagonist of Wnt action

Beyond Wnt inhibition: new functions of secreted Frizzled-related proteins in development and disease

The secreted Frizzled-related proteins (SFRPs) are a family of soluble proteins that are structurally related to Frizzled (Fz) proteins, the serpentine receptors that mediate the extensively used cell-cell communication pathway involving Wnt signalling. Because of their homology with the Wnt-binding domain on the Fz receptors, SFRPs were immediately characterised as antagonists that bind to Wnt proteins to prevent signal activation. Since these initial studies, interest in the family of SFRPs has grown progressively, offering new perspectives on their function and mechanism of action in both development and disease. These studies indicate that SFRPs are not merely Wnt-binding proteins, but can also antagonise one another's activity, bind to Fz receptors and influence axon guidance, interfere with BMP signalling by acting as proteinase inhibitors, and interact with other receptors or matrix molecules. Furthermore, their expression is altered in different types of cancers, bone pathologies, retinal degeneration and hypophosphatemic diseases, indicating that their activity is fundamental for tissue homeostasis. Here we review some of the debated aspects of SFRP-Wnt interactions and discuss the new and emerging roles of SFRPs.

DNA methylation and histone modifications cause silencing of Wnt antagonist gene in human renal cell carcinoma cell lines

Secreted frizzled-related protein 2 (sFRP2) is a negative modulator of the Wingless-type (Wnt) signaling pathway, and shown to be inactivated in renal cell carcinoma (RCC). However, the molecular mechanism of silencing of sFRP2 is not fully understood. Our study was designed to elucidate the silencing mechanism of sFRP2 in RCC. Expression of sFRP2 was examined in 20 pairs of primary cancers by immunohistochemistry. Kidney cell lines (HK-2, Caki-1, Caki-2, A-498 and ACHN) were analyzed for sFRP2 expression using real-time RT-PCR and Western blotting. The methylation status at 46 CpG sites of the 2 CpG islands in the sFRP2 promoter was characterized by bisulfite DNA sequencing. Histone modifications were assessed by chromatin immunoprecipitation (ChIP) assay using antibodies against AcH3, AcH4, H3K4 and H3K9. sFRP2 was frequently repressed in primary cancers and in RCC cells. The majority of sFRP2 negative cells had a methylated promoter. Meanwhile, sFRP2 expression was repressed by a hypomethylated promoter in Caki-1 cells, and these cells had a repressive histone modification at the promoter. In Caki-1 cells, sFRP2 was reactivated by trichostatin A (TSA). Repressive histone modifications were also observed in RCC cells with hypermethylated promoters, but sFRP2 was reactivated only by 5-aza-2'-deoxycytidine (DAC) and not by TSA. However, the activation of the silenced sFRP2 gene could be achieved in all cells using a combination of DAC and TSA. This is the first report indicating that aberrant DNA methylation and histone modifications work together to silence the sFRP2 gene in RCC cells.

Myeloma-derived Dickkopf-1 disrupts Wnt-regulated osteoprotegerin and RANKL production by osteoblasts: a potential mechanism underlying osteolytic bone lesions in multiple myeloma

Multiple myeloma (MM) is characterized by osteolytic bone lesions (OBL) that arise as a consequence of osteoblast inactivation and osteoclast activation adjacent to tumor foci within bone. Wnt signaling in osteoblasts regulates osteoclastogenesis through the differential activation and inactivation of Receptor Activator of Nuclear factor Kappa B Ligand (RANKL) and osteoprotegerin (OPG), positive and negative regulators of osteoclast differentiation, respectively. We demonstrate here that MM cell-derived DKK1, a soluble inhibitor of canonical Wnt signaling, disrupted Wnt3a-regulated OPG and RANKL expression in osteoblasts. Confirmed in multiple independent assays, we show that pretreatment with rDKK1 completely abolished Wnt3a-induced OPG mRNA and protein production by mouse and human osteoblasts. In addition, we show that Wnt3a-induced OPG expression was diminished in osteoblasts cocultured with a DKK1-expressing MM cell line or primary MM cells. Finally, we show that bone marrow sera from 21 MM patients significantly suppressed Wnt3a-induced OPG expression and enhanced RANKL expression in osteoblasts in a DKK1-dependent manner. These results suggest that DKK1 may play a key role in the development of MM-associated OBL by directly interrupting Wnt-regulated differentiation of osteoblasts and indirectly increasing osteoclastogenesis via a DKK1-mediated increase in RANKL-to-OPG ratios.

Hypermethylation and aberrant expression of secreted fizzled-related protein genes in pancreatic cancer

AIM: To determine the methylation status and aberrant expression of some secreted frizzled-related protein (SFRP) genes in pancreatic cancer and explore their role in pancreatic carcinogenesis.

METHODS: Methylation status and expression of SFRP genes were detected by methylation-specific PCR (MSPCR) and reverse-transcription PCR (RT-PCR) respectively.

RESULTS: The frequencies of methylation for SFRP genes 1, 2, 4, 5 were 70%, 48.3%, 60% and 76.7% in pancreatic cancer samples, and 21.7%, 20%, 10% and 36.7% in matched cancer adjacent normal tissue samples, respectively (χ² = 28.23, P < 0.0001 for SFRP gene 1; χ² = 10.71, P = 0.001 for SFRP gene 2; χ² = 32.97, P < 0.0001 for SFRP gene 4; χ² = 19.55, P < 0.0001 for SFRP gene 5). Expression loss of SFRP genes 1, 2, 4 and 5 was found in 65%, 40%, 55% and 71.7% of 60 pancreatic cancer samples, and 25%, 15%, 18.3% and 31.7% of matched cancer adjacent normal tissue samples, respectively (χ² = 19.39, P < 0.0001 for SFRP gene 1; χ² = 9.40, P = 0.002 for SFRP gene 2; χ² = 17.37, P < 0.0001 for SFRP gene 4; χ² = 19.22, P < 0.0001 for SFRP gene 5). SFRP gene 1 was methylated but not expressed in PC-3 and PANC-1, SFRP gene 2 was methylated but not expressed in PANC-1 and CFPAC-1, SFRP gene 4 was methylated but not expressed in PC-3, and SFRP gene 5 was methylated but not expressed in CFPAC-1.

CONCLUSION: Hypermethylation and aberrant expression of SFRP genes are common in pancreatic cancer, which may be involved in pancreatic carcinogenesis.

SFRP-4 abrogates Wnt-3a-induced beta-catenin and Akt/PKB signalling and reverses a Wnt-3a-imposed inhibition of in vitro mammary differentiation

Background

Conserved Wnt ligands are critical for signalling during development; however, various factors modulate their activity. Among these factors are the Secreted Frizzled-Related Proteins (SFRP). We previously isolated the SFRP-4 gene from an involuting rat mammary gland and later showed that transgenic mice inappropriately expressing SFRP-4 during lactation exhibited a high level of apoptosis with reduced survival of progeny.

Results

In order to address the questions related to the mechanism of Wnt signalling and its inhibition by SFRP-4 which we report here, we employed partially-purified Wnt-3a in a co-culture model system. Ectopic expression of SFRP-4 was accomplished by infection with a pBabe^puro construct. The co-cultures comprised Line 31E mouse mammary secretory epithelial cells and Line 30F, undifferentiated, fibroblast-like mouse mammary cells. In vitro differentiation of such co-cultures can be demonstrated by induction of the β-casein gene in response to lactogenic hormones.

We show here that treatment of cells with partially-purified Wnt-3a initiates Dvl-3, Akt/PKB and GSK-3β hyperphosphorylation and β-catenin activation. Furthermore, while up-regulating the cyclin D1 and connexin-43 genes and elevating transepithelial resistance of Line 31E cell monolayers, Wnt-3a treatment abrogates differentiation of co-cultures in response to the lactogenic hormones prolactin, insulin and glucocorticoid. Cells which express SFRP-4, however, are largely unaffected by Wnt-3a stimulation. Since a physical association between Wnt-3a and SFRP-4 could be demonstrated with immunoprecipitation/Western blotting experiments, this interaction, presumably owing to the Frizzled homology region typical of all SFRPs, explains the refractory response to Wnt-3a which was observed.

Conclusion

This study demonstrates that Wnt-3a treatment activates the Wnt signalling pathway and interferes with in vitro differentiation of mammary co-cultures to β-casein production in response to lactogenic hormones. Similarly, in another measure of differentiation, following Wnt-3a treatment mammary epithelial cells could be shown to up-regulate the cyclin D1 and connexin-43 genes while phenotypically they show increased transepithelial resistance across the cell monolayer. All these behavioural changes can be blocked in mammary epithelial cells expressing SFRP-4. Thus, our data illustrate in an in vitro model a mechanism by which SFRP-4 can modulate a differentiation response to Wnt-3a.

Wnt signalling and its impact on development and cancer

The Wnt signalling pathway is an ancient system that has been highly conserved during evolution. It has a crucial role in the embryonic development of all animal species, in the regeneration of tissues in adult organisms and in many other processes. Mutations or deregulated expression of components of the Wnt pathway can induce disease, most importantly cancer. The first gene to be identified that encodes a Wnt signalling component, Int1 (integration 1), was molecularly characterized from mouse tumour cells 25 years ago. In parallel, the homologous gene Wingless in Drosophila melanogaster, which produces developmental defects in embryos, was characterized. Since then, further components of the Wnt pathway have been identified and their epistatic relationships have been defined. This article is a Timeline of crucial discoveries about the components and functions of this essential pathway.

Increased expression of the WNT antagonist sFRP-1 in glaucoma elevates intraocular pressure

Elevated intraocular pressure (IOP) is the principal risk factor for glaucoma and results from excessive impedance of the fluid outflow from the eye. This abnormality likely originates from outflow pathway tissues such as the trabecular meshwork (TM), but the associated molecular etiology is poorly understood. We discovered what we believe to be a novel role for secreted frizzled-related protein-1 (sFRP-1), an antagonist of Wnt signaling, in regulating IOP. sFRP1 was overexpressed in human glaucomatous TM cells. Genes involved in the Wnt signaling pathway were expressed in cultured TM cells and human TM tissues. Addition of recombinant sFRP-1 to ex vivo perfusion-cultured human eyes decreased outflow facility, concomitant with reduced levels of beta-catenin, the Wnt signaling mediator, in the TM. Intravitreal injection of an adenoviral vector encoding sFRP1 in mice produced a titer-dependent increase in IOP. Five days after vector injection, IOP increased 2 fold, which was significantly reduced by topical ocular administration of an inhibitor of a downstream suppressor of Wnt signaling. Thus, these data indicate that increased expression of sFRP1 in the TM appears to be responsible for elevated IOP in glaucoma and restoring Wnt signaling in the TM may be a novel disease intervention strategy for treating glaucoma.

Wnt3a signaling within bone inhibits multiple myeloma bone disease and tumor growth

Canonical Wnt signaling is central to normal bone homeostasis, and secretion of Wnt signaling inhibitors by multiple myeloma (MM) cells contributes to MM-related bone resorption and disease progression. The aim of this study was to test the effect of Wnt3a on bone disease and growth of MM cells in vitro and in vivo. Although Wnt3a activated canonical signaling in the majority of MM cell lines and primary cells tested, Wnt3a had no effect on MM cell growth in vitro. Moreover, forced expression of Wnt3a in H929 MM cells conferred no growth advantage over empty vector-transfected cells in vitro or importantly when grown subcutaneously in severe combined immunodeficient (SCID) mice. Importantly, although H929 cells stably expressing an empty vector injected into human bone grew rapidly and induced a marked reduction in bone mineral density, bones engrafted with Wnt3a-expressing H929 cells were preserved, exhibited increased osteoblast-to-osteoclast ratios, and reduced tumor burden. Likewise, treatment of myelomatous SCID-hu mice, carrying primary disease, with recombinant Wnt3a stimulated bone formation and attenuated MM growth. These results provide further support of the potential anabolic and anti-MM effects of enhancing Wnt signaling in the bone.

Structure-function analysis of secreted frizzled-related protein-1 for its Wnt antagonist function

Secreted frizzled-related proteins (sFRPs) are glycoproteins that are recognized as Wnt antagonists. To identify the functional domains that are involved in Wnt antagonist function, several sFRP-1 mutants and sFRP-1/sFRP-2 chimeras were generated. These mutants were characterized in an optimized T-cell factor (TCF)-luciferase based assay in U2OS human osteosarcoma cells. Deletions of the sFRP-1 cysteine rich domain (CRD) lead to the complete loss of Wnt antagonist function. A region between amino acids 73-86 within the second loop of the CRD of sFRP-1 was necessary for the optimal Wnt inhibitory function. Within this region, a conserved tyrosine residue played a critical role, and its change to neutral or polar amino acids lead to decreased Wnt inhibitory activity. The sFRP-1/sFRP-2 chimeras with the netrin domain of sFRP-1 replaced by corresponding sFRP-2 sequences showed 40-70% loss of Wnt antagonist function. The sFRP-1/sFRP-2 chimera with the replacement of C-terminal 19 amino acids of sFRP-1 with 11 amino acids of sFRP-2 resulted in 70% loss of activity indicating that carboxyl-terminal region of sFRP-1 is important for its Wnt inhibitory activity. The structure-function analysis studies of sFRP-1 clearly demonstrate the interaction of several functional domains for its optimal Wnt antagonist function.

Secreted frizzled related protein 1 is a paracrine modulator of epithelial branching morphogenesis, proliferation, and secretory gene expression in the prostate

Previous in vitro studies identified secreted frizzled related protein 1 (SFRP1) as a candidate pro-proliferative signal during prostatic development and cancer progression. This study determined the in vivo roles of SFRP1 in the prostate using expression studies in mice and by creating loss- and gain-of-function mouse genetic models. Expression studies using an Sfrp1(lacZ) knock-in allele showed that Sfrp1 is expressed in the developing mesenchyme/stroma of the prostate. Nevertheless, Sfrp1 null prostates exhibited multiple prostatic developmental defects in the epithelium including reduced branching morphogenesis, delayed proliferation, and increased expression of genes encoding prostate-specific secretory proteins. Interestingly, over-expression of SFRP1 in the adult prostates of transgenic mice yielded opposite effects including prolonged epithelial proliferation and decreased expression of genes encoding secretory proteins. These data demonstrated a previously unrecognized role for Sfrp1 as a stromal-to-epithelial paracrine modulator of epithelial growth, branching morphogenesis, and epithelial gene expression. To clarify the mechanism of SFRP1 action in the prostate, the response of WNT signaling pathways to SFRP1 was examined. Forced expression of SFRP1 in prostatic epithelial cells did not alter canonical WNT/beta-catenin signaling or the activation of CamKII. However, forced expression of SFRP1 led to sustained activation of JNK, and inhibition of JNK activity blocked the SFRP1-induced proliferation of prostatic epithelial cells, suggesting that SFRP1 acts through the non-canonical WNT/JNK pathway in the prostate.

Osteoblast-targeted expression of Sfrp4 in mice results in low bone mass

Transgenic mice overexpressing Sfrp4 in osteoblasts were established. These mice exhibited low bone mass caused by a decrease in bone formation.

INTRODUCTION

We recently reported that single nucleotide polymorphisms in the secreted frizzled-related protein 4 (Sfrp4) gene are responsible for low peak BMD in senescence-accelerated mouse (SAM) P6. In vitro studies revealed inhibition of osteoblast proliferation by Sfrp4, which is supposed to be mediated by canonical Wnt signaling.

MATERIALS AND METHODS

We examined the expression of Sfrp4 in neonate long bones by in situ hybridization and generated transgenic mice in which Sfrp4 was specifically overexpressed in osteoblasts under the control of a 2.3-kb Col1a1 osteoblast-specific promoter. Next, we compared the phenotype of Sfrp4 transgenic (Sfrp4 TG) mice with that of mice in which one allele of beta-catenin was conditionally disrupted in osteoblasts (betaChet), and administered lithium chloride (LiCl) to Sfrp4 TG mice.

RESULTS

Hemizygous Sfrp4 TG mice exhibited a 30% reduction of trabecular bone mass compared with that in wildtype littermates at 8 wk of age, and histomorphometrical analysis showed decreases in both osteoblast numbers and bone formation rate. betaChet mice exhibited a 17% reduction of trabecular bone mass in distal femora caused by an increase in the osteoclast number and a decrease in bone formation rate. Furthermore, LiCl administration rescued the bone phenotype of Sfrp4 TG mice.

CONCLUSIONS

Expression of Sfrp4 in periosteum and bone tissues suggested the role of Sfrp4 in osteoblasts, and we identified that overexpression of Sfrp4 in osteoblasts suppressed osteoblast proliferation, resulting in a decrease in bone formation in vivo. Partial suppression of beta-catenin/canonical Wnt signaling also impaired bone formation, and activation of the signaling restored low bone mass of Sfrp4 TG mice. Thus, these results indicate that Sfrp4 decreases bone formation at least in part by attenuating canonical Wnt signaling in vivo.

Adrenal gland tumorigenesis after gonadectomy in mice is a complex genetic trait driven by epistatic loci

Postgonadectomy adrenocortical tumorigenesis is a strain-specific phenomenon in inbred mice, assumed to be caused by elevated LH secretion and subsequent ectopic LH receptor (LHR) overexpression in adrenal gland. However, the molecular mechanisms of this cascade of events remain unknown. In this study, we took advantage of the mouse strain dependency of the phenotype to unravel its genetic basis. Our results present the first genome-wide screening related to this pathology in two independent F2 and backcross populations generated between the neoplastic DBA/2J and the nonsusceptible C57BL/6J strains. Surprisingly, the postgonadectomy elevation of serum LH was followed by similar up-regulation of adrenal LHR expression in both parental strains and their crosses, irrespective of their tumor status, indicating that it is not the immediate cause of the tumorigenesis. Linkage analysis revealed one major significant locus for the tumorigenesis on chromosome 8, modulated by epistasis with another quantitative trait locus on chromosome 18. Weight gain, a secondary phenotype after gonadectomy, showed a significant but separate quantitative trait locus on chromosome 7. Altogether, postgonadectomy adrenocortical tumorigenesis in DBA/2J mice is a dominant trait that is not a direct consequence of adrenal LHR expression but is driven by a complex genetic architecture. Analysis of candidate genes in the tumorigenesis linkage region showed that Sfrp1 (secreted frizzled-related protein 1), a tumor suppressor gene, is differentially expressed in the neoplastic areas. These findings may have relevance to the human pathogenesis of macronodular adrenal hyperplasia and adrenocortical tumors in postmenopausal women and why some of them develop obesity.

Identification of genes that exhibit changes in expression on the 8p chromosomal arm by the Systematic Multiplex RT-PCR (SM RT-PCR) and DNA microarray hybridization methods

Losses of the p-arm of chromosome 8 are frequently observed in breast, prostate, and other types of cancers. Using the Systematic Multiplex RT-PCR (SM RT-PCR) method and the DNA microarray hybridization method, we examined the expression of 273 genes located on the p-arm of chromosome 8 in five breast and three prostate human cancer cell lines. We observed frequent decreases in expression of two dozen genes and increases in expression of several genes on this chromosomal arm. These changes in gene expression of the cell lines were later confirmed by real-time qRT-PCR. Additionally and more importantly, we found that a number of these variations were also observed in the majority of clinical cases of breast cancer we examined. These included downregulation of the MYOM2, NP_859074, NP_001034551, NRG1, PHYIP (PHYHIP), Q7Z2R7, SFRP1, and SOX7 genes, and upregulation of the ESCO2, NP_115712 (GINS4), Q6P464, and TOPK (PBK) genes.

Purification and Wnt-inhibitory activities of secreted frizzled-related proteins

Recombinant expression of secreted Frizzled-related proteins (sFRPs) in mammalian expression systems is a convenient source of these proteins for biological studies. Yields of protein vary; screening of clonal lines for high expression is usually worthwhile. Heparin affinity chromatography is an easy step that provides a major enrichment, particularly for sFRP-1 and sFRP-2. Alternatively, sFRP derivatives tagged with poly-histidine at their carboxyl termini are functional and can be readily isolated by chelating chromatography. Once purified, the proteins are stable indefinitely if stored frozen and they tolerate multiple rounds of freeze-thawing. Pre-incubation of Wnt samples with sFRP protein for 30 min at 37 degrees C is sufficient to inhibit Wnt activity in various assays. The concentration of sFRP required to block Wnt signaling should be determined empirically, as it will vary with the Wnt preparation and cellular context.

Wnt10b increases postnatal bone formation by enhancing osteoblast differentiation

Overexpression of Wnt10b from the osteocalcin promoter in transgenic mice increases postnatal bone mass. Increases in osteoblast perimeter, mineralizing surface, and bone formation rate without detectable changes in pre-osteoblast proliferation, osteoblast apoptosis, or osteoclast number and activity suggest that, in this animal model, Wnt10b primarily increases bone mass by stimulating osteoblastogenesis.

INTRODUCTION

Wnt signaling regulates many aspects of development including postnatal accrual of bone. Potential mechanisms for how Wnt signaling increases bone mass include regulation of osteoblast and/or osteoclast number and activity. To help differentiate between these possibilities, we studied mice in which Wnt10b is expressed specifically in osteoblast lineage cells or in mice devoid of Wnt10b.

MATERIALS AND METHODS

Transgenic mice, in which mouse Wnt10b is expressed from the human osteocalcin promoter (Oc-Wnt10b), were generated in C57BL/6 mice. Transgene expression was evaluated by RNase protection assay. Quantitative assessment of bone variables was done by radiography, muCT, and static and dynamic histomorphometry. Mechanisms of bone homeostasis were evaluated with assays for BrdU, TUNEL, and TRACP5b activity, as well as serum levels of C-terminal telopeptide of type I collagen (CTX). The endogenous role of Wnt10b in bone was assessed by dynamic histomorphometry in Wnt10b(-/-) mice.

RESULTS

Oc-Wnt10b mice have increased mandibular bone and impaired eruption of incisors during postnatal development. Analyses of femoral distal metaphyses show significantly higher BMD, bone volume fraction, and trabecular number. Increased bone formation is caused by increases in number of osteoblasts per bone surface, rate of mineral apposition, and percent mineralizing surface. Although number of osteoclasts per bone surface is not altered, Oc-Wnt10b mice have increased total osteoclast activity because of higher bone mass. In Wnt10b(-/-) mice, changes in mineralizing variables and osteoblast perimeter in femoral distal metaphyses were not observed; however, bone formation rate is reduced because of decreased total bone volume and trabecular number.

CONCLUSIONS

High bone mass in Oc-Wnt10b mice is primarily caused by increased osteoblastogenesis, with a minor contribution from elevated mineralizing activity of osteoblasts.

Molecular changes in the progression of Barrett's oesophagus

Barrett's oesophagus is a frequent complication of gastro-oesophageal reflux disease predicting oesophageal adenocarcinoma. The majority of Barrett's patients will not develop cancer, so that specific methods of identification of those at risk are required. Recent molecular studies have identified a selection of candidate biomarkers that need validation in prospective studies. They reflect various changes in cell behaviour during neoplastic progression. The ASPECT trial in the UK aims to establish whether chemoprevention with aspirin and a proton pump inhibitor will reduce adenocarcinoma development and mortality in patients with Barrett's oesophagus. It will also validate biomarkers for progression and clinical response and further study disease pathogenesis.

SFRP1 suppressed hepatoma cells growth through Wnt canonical signaling pathway

Oncogenic activation of the Wnt/beta-catenin signaling pathway is common in hepatocellular carcinoma (HCC). The secreted frizzled-related proteins (SFRPs) function as negative regulators of Wnt signaling and have important implications for carcinogenesis. Promoter hypermethylation of SFRP genes is common in human cancers. However, the role of SFRPs in HCC is not clear. Recently, we have shown that SFRP1 is frequently downregulated through promoter hypermethylation. To confirm and extend these findings, the methylation status of the other SFRP members, including SFRP2, SFRP4 and SFRP5, was examined by methylation-specific polymerase chain reaction (MS-PCR). Hypermethylation of SFRP genes, except for SFRP4, is frequent in HCCs and the levels found here were significantly higher than those seen in cirrhotic livers, chronic hepatitis livers and normal controls (p < 0.0001 for SFRP1 and SFRP2, p < 0.05 for SFRP5). To investigate the role of SFRP1 in HCCs, we used re-expression of SFRP1 in beta-catenin-dependent HCC cell lines: Huh6 and HepG2. Restoration of SFRP1 attenuated Wnt signaling in those Huh6 hepatoma cells with a beta-catenin gene point mutation, decreased abnormal accumulation of beta-catenin in the nucleus and suppressed cell growth. Conversely, restoration of SFRP1 in HepG2 hepatoma cells with truncated beta-catenin could not block the Wnt signaling pathway. Furthermore, knocking down SFRP1 by RNA interference in beta-catenin-deficient cell lines (SK-Hep1) stimulated Wnt signaling and promoted cell growth. Our data suggested that SFRP1 suppressed liver cancer cells growth through Wnt canonical signaling. Moreover, beta-catenin-independent noncanonical pathway might be involved in Wnt signaling activation through unknown molecules in HCC.

Independent functions and mechanisms for homeobox gene Barx1 in patterning mouse stomach and spleen

Homeobox genes convey positional information in embryos and their role in patterning the mammalian gut is a topic of considerable interest. Barx1 is expressed selectively in fetal stomach mesenchyme and directs differentiation of overlying endoderm. Recombinant tissue cultures and study of young mouse embryos previously suggested that Barx1 controls expression of secreted Wnt antagonists, which suppress endodermal Wnt signaling, to enable stomach epithelial differentiation. We overcame mid-gestational lethality of Barx1(-/-) mouse embryos and report here the spectrum of anomalies in a distinctive and unprecedented model of gastrointestinal homeotic transformation. Using various mouse models, we confirm the importance of attenuated Wnt signaling in stomach development and the role of Barx1 in suppressing endodermal Wnt activity. Absence of Barx1 also results in fully penetrant defects in positioning and expansion of the spleen, an organ that originates within the mesothelial lining of the stomach. Barx1 is absent from the spleen primordium but highly expressed in the mesogastrium, indicating an indirect effect on spleen development. However, our results argue against a role for Wnt antagonism in genesis of the spleen. Mouse spleen development relies on several homeodomain transcriptional regulators that are expressed in the spleen primordium. Loss of Barx1 does not affect expression of any of these genes but notably reduces expression of Wt1, a transcription factor implicated in spleen morphogenesis and expressed in the mesothelium. These observations place Barx1 proximally within a Wt1 pathway of spleen development and reveal how a homeotic regulator employs different molecular mechanisms to mold neighboring organs.

Wnt signaling and neural stem cells: caught in the Wnt web

Wnt proteins have now been identified as major physiological regulators of multiple aspects of stem cell biology, from self-renewal and pluripotency to precursor cell competence and terminal differentiation. Neural stem cells are the cellular building blocks of the developing nervous system and provide the basis for continued neurogenesis in the adult mammalian central nervous system. Here, we outline the most recent advances in the field about the critical factors and regulatory networks involved in Wnt signaling and discuss recent findings on how this increasingly intricate pathway contributes to the shaping of the developing and adult nervous system on the level of the neural stem cell.

R-Spondin1 regulates Wnt signaling by inhibiting internalization of LRP6

The R-Spondin (RSpo) family of secreted proteins act as potent activators of the Wnt/beta-catenin signaling pathway. We have previously shown that RSpo proteins can induce proliferative effects on the gastrointestinal epithelium in mice. Here we provide a mechanism whereby RSpo1 regulates cellular responsiveness to Wnt ligands by modulating the cell-surface levels of the coreceptor LRP6. We show that RSpo1 activity critically depends on the presence of canonical Wnt ligands and LRP6. Although RSpo1 does not directly activate LRP6, it interferes with DKK1/Kremen-mediated internalization of LRP6 through an interaction with Kremen, resulting in increased LRP6 levels on the cell surface. Our results support a model in which RSpo1 relieves the inhibition DKK1 imposes on the Wnt pathway.

Roles of secreted frizzled-related proteins in cancer

The Wnt signaling pathway is implicated in a variety of biological processes ranging from developmental cell fate to human disease. The components involved in Wnt signaling have been under intense investigation over the last 2 decades. Aberrant canonical Wnt activation has been linked to tumor formation and involves activation of effector molecules or loss of tumor suppressor function. Secreted frizzled-related proteins (sFRPs) are Wnt antagonists. In recent years, accumulating evidence has suggested that sFRPs act as tumor suppressors because their expression is frequently silenced in cancer by promoter hypermethylation. However, sFRPs may also promote cell growth in some contexts. Here, we focus on the known knowledge of sFRPs in tumorigenesis.

Down-regulation of SFRP1 as a putative tumor suppressor gene can contribute to human hepatocellular carcinoma

Background

Hepatocellular carcinoma (HCC) is one of the most common cancers in the world. SFRP1 (the secreted frizzled-related protein 1), a putative tumor suppressor gene mapped onto chromosome 8p12-p11.1, the frequent loss of heterozygosity (LOH) region in human HCC, encodes a Wingless-type (Wnt) signaling antagonist and is frequently inactivated by promoter methylation in many human cancers. However, whether the down-regulation of SFRP1 can contribute to hepatocarcinogenesis still remains unclear.

Methods

We investigated the expression of SFRP1 through real time RT-PCR and immunohistochemistry staining. The cell growth and colony formation were observed as the overexpression and knockdown of SFRP1. The DNA methylation status within SFRP1 promoter was analyzed through methylation-specific PCR or bisulphate-treated DNA sequencing assays. Loss of heterozygosity was here detected with microsatellite markers.

Results

SFRP1 was significantly down-regulated in 76.1% (35/46) HCC specimens at mRNA level and in 30% (30/100) HCCs indicated by immunohistochemistry staining, as compared to adjacent non-cancerous livers. The overexpression of SFRP1 can significantly inhibit the cell growth and colony formation of YY-8103, SMMC7721, and Hep3B cells. The RNA interference against the constitutional SFRP1 in the offspring SMMC7721 cells, which were stably transfected by ectopic SFRP1, can markedly promote cell growth of these cells. LOH of both microsatellite markers D8S532 and D8SAC016868 flanking the gene locus was found in 13% (6 of 46 HCCs) and 6.5% (3 of 46 HCCs) of the informative cases, respectively, where 5 of 8 HCC specimens with LOH showed the down-regulation of SFRP1. DNA hypermethylation within SFRP1 promoter was identified in two of three HCC specimens without SFRP1 expression. Moreover, the DNA methylation of SFRP1 promoter was significantly reduced, along with the re-expression of the gene, in those HCC cell lines, Bel7404, QGY7701, and MHCC-H, as treated by DAC.

Conclusion

Our data suggested that the down-regulation of SFRP1 as a candidate tumor suppressor gene, triggered by the epigenetic and/or genetic events, could contribute to the oncogenesis of HCC.

Regulation of Secreted Frizzled-related Protein-1 by Heparin

Secreted Frizzled-related protein-1 (sFRP-1) belongs to a class of extracellular antagonists that modulate Wnt signaling pathways by preventing ligand-receptor interactions among Wnts and Frizzled membrane receptor complexes. sFRP-1 and Wnts are heparin-binding proteins, and their interaction can be stabilized by heparin in vitro. Here we report that heparin can specifically enhance recombinant sFRP-1 accumulation in a cell type-specific manner. The effect requires O-sulfation in heparin, and involves fibroblast growth factor-2 as well as fibroblast growth factor receptor-1. Interestingly, further investigation uncovers that heparin can also affect the post-translational modification of sFRP-1. We demonstrate that sFRP-1 is post-translationally modified by tyrosine sulfation at tyrosines 34 and 36, which is inhibited by the treatment of heparin. The results suggest that accumulation of sFRP-1 induced by heparin is in part due to the relative stabilization of unsulfated sFRP-1 and the direct stabilization by heparin. The study has revealed a multifaceted regulation on sFRP-1 protein by heparin.

WNTs in the ovine uterus: potential regulation of periimplantation ovine conceptus development

WNTs (Wingless-type MMTV integration site family member) are involved in critical developmental and growth processes in animals. These studies investigated WNT pathways in the ovine uterus and conceptus during the periimplantation period of pregnancy. WNT2 and WNT2B mRNAs were detected in endometrial stroma. WNT5A and WNT5B mRNAs were most abundant in the stroma and less so in the luminal epithelium, whereas WNT11 mRNA was detected primarily in the glands. WNT7A mRNA was present in the luminal epithelium on d 10, absent on d 12 and 14, and increased between d 16 and 20. Only WNT2, WNT2B, and WNT4 were detected in conceptus trophectoderm. FZD6/8 (frizzled receptor) and GSK3B (glycogen synthase kinase 3beta) mRNAs were detected primarily in endometrial epithelia and conceptus trophectoderm, whereas the LRP5/6 (low-density lipoprotein receptor-related proteins 5 and 6) coreceptor was present in all endometrial cells and the trophectoderm. DKK1 (Dickkopf), a WNT signaling inhibitor, increased in the endometrium from d 16-20. CTNNB1 [catenin (cadherin associated protein) beta1] and CDH1 (E-cadherin) mRNAs were most abundant in the endometrial epithelia and trophectoderm. LEF1 (lymphoid enhancer-binding factor 1) mRNA was expressed primarily in uterine epithelia, whereas TCF7L2 [(transcription factor 7-like 2 (T-cell specific, HMG-box)] was primarily in the conceptus. CTNNB1 and TCF7L2 proteins were both abundant in the nuclei of trophoblast giant binucleate cells. WNT7A stimulated a TCF/LEF-luciferase reporter activity in ovine trophectoderm cells that was inhibited by dominant-negative TCF and Sfrp2 (secreted FZD-related protein 2). WNT7A increased trophectoderm cell proliferation as well as MSX2 (msh homeobox 2) and MYC (myelocytomatosis oncogene) mRNA levels. Wnt5a increased trophectoderm cell migration in a Rho kinase-dependent manner. These results support the hypotheses that canonical and noncanonical WNT signaling pathways are conserved regulators of conceptus-endometrial interactions in mammals and regulate periimplantation ovine conceptus development.

Epigenetic changes (aberrant DNA methylation) in colorectal neoplasia

Both genetic and epigenetic events have been implicated in the stepwise histological progression involving adenoma-carcinoma and hyperplastic polyp/serrated adenoma-carcinoma sequences in the development of colorectal cancer. Genetic changes have been observed at each step in the initiation and progression of polyps to adenocarcinomas. Epigenetic changes also occur at each step in the pathogenesis of colorectal cancers and include CpG island DNA hypermethylation in the promoter region of genes resulting in transcriptional silencing through associated changes in chromatin structure and effects on binding of transcription factors, and DNA global hypomethylation which leads to chromosomal instability. Recent studies on MLH1 and APC genes indicate that epigenetic and genetic changes cooperate to facilitate tumor initiation and progression. Since aberrant CGI DNA promoter hypermethylation can be detected not only in colorectal polyps and cancers, but also in sera and stool, hypermethylated genes may serve as molecular markers for early detection, risk assessment and diagnosis. In addition, silenced genes caused by CGI DNA promoter hypermethylation can be reactivated by demethylating agents and also by both the inhibitors of DNA methyltransferases and histone deacetylases. Therefore, these epigenetically acting drugs should be evaluated for their chemopreventive and therapeutic potential for colorectal cancers.

Hypermethylation and aberrant expression of Wnt antagonist secreted frizzled-related protein 1 in gastric cancer

AIM: To identify the methylation of secreted frizzled-related protein 1 (SFRP1) in gastric cancer and to investigate the aberrant expression of SFRP1 and its correlation with the clinical pathological features of patients.

METHODS: We determined SFRP1 methylation and SFRP1 mRNA expression in 3 gastric cancer cell lines SGC-7901, BGC-823, HGC-27, from 52 primary gastric cancer specimens and matched tumor adjacent tissue specimens by methylation-specific (MSP) PCR and RT-PCR respectively. Fisher’s exact test was used to analyze the statistical association between clinical pathological data and aberrant expression of SFRP1.

RESULTS: In 3 cancer cell lines, BGC-823 and HGC-27 had methylated SFRP1 and lost SFRP1 mRNA expression. After treatment of BGC-823 and HGC-27 with the demethylating agent, 5-aza-2’-deoxycytidine, SFRP1 was re-expressed. In 52 primary gastric cancer specimens and matched tumor adjacent tissue specimens, hypermethylation of SFRP1 was detected in 23 (44%) and 8 (15%) specimens respectively (χ² = 10.34, P < 0.01). Loss of SFRP1 expression was detected in 17(33%) and 6 (12%) specimens respectively (χ² = 6.75, P < 0.01). There was a significant correlation between SFRP1 hypermethylation and SFRP1 expression loss. SFRP1 expression was also correlated significantly with tumor stage and lymph node status, but not with patient sex, age and histological type.

CONCLUSION: SFRP1 inactivation is a common and early event caused mainly by hypermethylation in gastric cancer. SFRP1 expression loss may be correlated with tumor metastasis in primary gastric cancer.

Sexually dimorphic expression of secreted frizzled-related (SFRP) genes in the developing mouse Müllerian duct

In developing male embryos, the female reproductive tract primordia (Müllerian ducts) regress due to the production of testicular anti-Müllerian hormone (AMH). Because of the association between secreted frizzled-related proteins (SFRPs) and apoptosis, their reported developmental expression patterns and the role of WNT signaling in female reproductive tract development, we examined expression of Sfrp2 and Sfrp5 during development of the Müllerian duct in male (XY) and female (XX) mouse embryos. We show that expression of both Sfrp2 and Sfrp5 is dynamic and sexually dimorphic. In addition, the male-specific expression observed for both genes prior to the onset of regression is absent in mutant male embryos that fail to undergo Müllerian duct regression. We identified ENU-induced point mutations in Sfrp5 and Sfrp2 that are predicted to severely disrupt the function of these genes. Male embryos and adults homozygous for these mutations, both individually and in combination, are viable and apparently fertile with no overt abnormalities of reproductive tract development.

Frequent loss of SFRP1 expression in multiple human solid tumours: association with aberrant promoter methylation in renal cell carcinoma

Oncogenic wingless-related mouse mammary tumour virus (Wnt) signalling, caused by epigenetic inactivation of specific pathway regulators like the putative tumour suppressor secreted frizzled-related protein 1 (SFRP1), may be causally involved in the carcinogenesis of many human solid tumours including breast, colon and kidney cancer. To evaluate the incidence of SFRP1 deficiency in human tumours, we performed a large-scale SFRP1 expression analysis using immunohistochemistry on a comprehensive tissue microarray (TMA) comprising 3448 tumours from 36 organs. This TMA contained 132 different tumour subtypes as well as 26 different normal tissues. Although tumour precursor stages of, for example kidney, colon, endometrium or adrenal gland still exhibited moderate to abundant SFRP1 expression, this expression was frequently lost in the corresponding genuine tumours. We defined nine novel tumour entities with apparent loss of SFRP1 expression, i.e., cancers of the kidney, stomach, small intestine, pancreas, parathyroid, adrenal gland, gall bladder, endometrium and testis. Renal cell carcinoma (RCC) exhibited the highest frequency of SFRP1 loss (89% on mRNA level; 75% on protein level) and was selected for further analysis to investigate the cause of SFRP1 loss in human tumours. We performed expression, mutation and methylation analysis in RCC and their matching normal kidney tissues. SFRP1 promoter methylation was frequently found in RCC (68%, n=38) and was correlated with loss of SFRP1 mRNA expression (p<0.05). Although loss of heterozygosity was found in 16% of RCC, structural mutations in the coding or promoter region of the SFRP1 gene were not observed. Our results indicate that loss of SFRP1 expression is a very common event in human cancer, arguing for a fundamental role of aberrant Wnt signalling in the development of solid tumours. In RCC, promoter hypermethylation seems to be the predominant mechanism of SFRP1 gene silencing and may contribute to initiation and progression of this disease.

The cancer epigenome--components and functional correlates

It is increasingly apparent that cancer development not only depends on genetic alterations but on an abnormal cellular memory, or epigenetic changes, which convey heritable gene expression patterns critical for neoplastic initiation and progression. These aberrant epigenetic mechanisms are manifest in both global changes in chromatin packaging and in localized gene promoter changes that influence the transcription of genes important to the cancer process. An exciting emerging theme is that an understanding of stem cell chromatin control of gene expression, including relationships between histone modifications and DNA methylation, may hold a key to understanding the origins of cancer epigenetic changes. This possibility, coupled with the reversible nature of epigenetics, has enormous significance for the prevention and control of cancer.

WNTS and WNT receptors as therapeutic tools and targets in human disease processes

The body of scientific literature linking Wnts and Wnt-associated proteins to human disease processes continues to grow in parallel with new discoveries from basic science laboratories that further characterize the elaborate cellular events following the binding of Wnts to their receptors. While Wnt-mediated signaling has long been known to play a major role in human carcinogenesis, accumulating evidence indicates that Wnts are also important mediators of inflammation and recovery from injury. The binding of secreted Wnt ligands to their receptors offers an attractive and accessible target for therapeutic regulation of these signaling pathways. Several promising preliminary studies have already addressed potential avenues for the manipulation of Wnt signaling in disease processes. This review will focus on disease processes involving the regulation of Wnt signaling at the level of Wnt binding to its target receptors. Wnt proteins, Wnt receptors, and secreted Wnt inhibitors are attractive as potential therapeutic agents and targets due to their extracellular location. In addition, since Wnt signaling results in a diverse array of downstream intracellular events, many of which are not fully understood, the targeting of this pathway at the most upstream site of pathway activation also provides a strategic advantage for therapy. As the list of Wnt-related diseases continues to grow, advances in our understanding of the biochemical and molecular mechanisms underlying Wnt signaling may ultimately translate into innovative ways to treat Wnt-related disease processes in patients.

Eye field requires the function of Sfrp1 as a Wnt antagonist

Wnts have been shown to provide a posteriorizing signal that has to be repressed in the specification of vertebrate forebrain region. Previous studies have shown that Wnt activation by LiCl treatment causes an expansion of optic stalk and mid-hindbrain boundary, whereas eye and ventral diencephalon in the forebrain region were reduced. However, the molecular mechanism, by which inhibits Wnt activity in the forebrain remains poorly defined. To investigate relationship between forebrain specification and Wnt signaling, the zebrafish homologue of secreted frizzled related protein1 (sfrp1) has been characterized. The transcripts of sfrp1 are detected in the presumptive forebrain at gastrula and in the ventral telencephalon, ventral diencephalon, midbrain and optic vesicles at 24h after postfertilization (hpf). Overexpression of sfrp1 causes an anteriorization of embryo, with enlarged head and reduced posterior structure as in the embryo overexpressing dominant-negative form of Frizzled8a or Dkk1. Its overexpression restored the eye defects in the Wnt8b-overexpressing embryos, but not in the LiCl-treated embryos. These results suggest that Sfrp1 expressed in the forebrain and eye field plays a critical role in the extracellular events of antagonizing Wnt activity for the forebrain specification.

Genetic analysis of anterior posterior expression gradients in the developing mammalian forebrain

Intrinsic regulatory factors play critical roles in early cortical patterning, including the development of the anteroposterior (A-P) axis. To identify genes that are differentially expressed along the A-P axis of the developing cerebral cortex, we analyzed gene expression in presumptive frontal, parietal, and occipital cerebral walls of E12.5 mouse using complementary DNA microarrays. We identified 106 genes, including expressed sequence tags (ESTs), expressed in an A-P gradient in the embryonic brain and screened 88 by in situ hybridization for confirmation. Central nervous system (CNS) expression patterns of many of these genes were previously unknown. Others, such as Sfrp1, CoupTF1, and FABP7, were expressed in a manner consistent with previous studies, providing independent confirmation. Two related transcription factors, previously not implicated in CNS development, Fhl1 and Fhl2, were observed to be enriched in posterior and anterior telencephalon, respectively. We studied patterning gradients in Fhl1 knockout mice but observed no changes in gene expression related to A-P regionalization in the Fhl1 knockout mice. These data provide an important set of new candidates for studies of cortical patterning and maturation.

Suppressing Wnt signaling by the hedgehog pathway through sFRP-1

The hedgehog (Hh) signaling pathway is essential for embryonic development and carcinogenesis. Activation of Hh signaling has been identified in several types of gastrointestinal cancers, including esophageal, gastric, pancreatic, and liver cancers. Several recent studies suggest that Hh signaling activation can inhibit Wnt signaling. However, the molecular basis underlying this inhibition remains unclear. As transcription factors in the Hh signaling pathway, Gli molecules transform cells in culture, and their expression are associated with cancer development. Here we report that expression of a secreted frizzled-related protein-sFRP-1 in mouse embryonic fibroblasts is dependent on Gli1 and Gli2. In human gastric cancer cells, inhibition of Hh signaling reduces the level of sFRP-1 transcript, whereas ectopic expression of Gli1 increases the level of sFRP-1 transcript. Results from chromatin immunoprecipitation indicate that Gli1 is involved in transcriptional regulation of sFRP-1. In 293 cells with Gli1 expression, Wnt-1-mediated beta-catenin accumulation in the cytosol and DKK1 expression are all abrogated, which can be reversed by inhibiting sFRP-1 expression. Furthermore, while SIIA cells do not respond to Wnt-1-conditioned medium, inhibition of Hh signaling by smoothened (SMO) antagonist KAAD-cyclopamine (keto-N-aminoethylaminocaproyldihydrocinnamoylcyclopamine) leads to Wnt1-mediated beta-catenin accumulation in the cytosol. These data indicate that sFRP-1, a target gene of the hedgehog pathway, is involved in cross-talk between the hedgehog pathway and the Wnt pathway.

Promoter methylation of the secreted frizzled-related protein 1 gene SFRP1 is frequent in hepatocellular carcinoma

BACKGROUND

The secreted frizzled-related protein 1 gene (SFRP1) encodes a Wnt/beta-catenin signaling antagonist and frequently is inactivated by promoter methylation in many tumors. However, the role of SFRP1 in hepatocellular carcinoma (HCC) is not clear. Therefore, the authors investigated whether methylation of the SFRP1 promoter is common in HCC and whether it may influence SFRP1 expression.

METHODS

Four HCC cell lines, 54 HCCs, 42 cirrhotic livers, 21 livers with chronic hepatitis, and 15 normal control tissues were analyzed for 1) SFRP1 promoter methylation by using methylation-specific polymerase chain reaction analysis and bisulfite sequencing, 2) SFRP1 messenger RNA expression by using quantitative reverse transcriptase-polymerase chain reaction analysis, and 3) loss of heterozygosity (LOH) by using microsatellite markers flanking the SFRP1 locus. HCC cells were treated with the demethylating agent 5-aza-2'-deoxycytidine to determine whether it could restore SFRP1 expression.

RESULTS

SFRP1 promoter methylation was observed in 75%, 48.2%, 21.4%, 14.3% and 0% in HCC cell lines, primary HCCs, cirrhotic livers, livers with chronic hepatitis, and normal control tissues, respectively. Methylation of the SFRP1 promoter region in HCCs increased significantly compared with control tissues. All samples with SFRP1 methylation showed down-regulation of SFRP1 expression. Demethylation treatment with 5-aza-2'-deoxycytidine in HCC cells restored SFRP1 expression. Moreover, LOH of markers D8S505 and D8S1722 was found in 25% and 27.6% of the informative samples, respectively.

CONCLUSIONS

The current results suggested that promoter hypermethylation of SFRP1 is a common event in HCC and plays an important role in the regulation of SFRP1 expression. In addition to methylation-mediated down-regulation of SFRP1, LOH also may play a role.

Signaling across the synapse: a role for Wnt and Dishevelled in presynaptic assembly and neurotransmitter release

Proper dialogue between presynaptic neurons and their targets is essential for correct synaptic assembly and function. At central synapses, Wnt proteins function as retrograde signals to regulate axon remodeling and the accumulation of presynaptic proteins. Loss of Wnt7a function leads to defects in the localization of presynaptic markers and in the morphology of the presynaptic axons. We show that loss of function of Dishevelled-1 (Dvl1) mimics and enhances the Wnt7a phenotype in the cerebellum. Although active zones appear normal, electrophysiological recordings in cerebellar slices from Wnt7a/Dvl1 double mutant mice reveal a defect in neurotransmitter release at mossy fiber-granule cell synapses. Deficiency in Dvl1 decreases, whereas exposure to Wnt increases, synaptic vesicle recycling in mossy fibers. Dvl increases the number of Bassoon clusters, and like other components of the Wnt pathway, it localizes to synaptic sites. These findings demonstrate that Wnts signal across the synapse on Dvl-expressing presynaptic terminals to regulate synaptic assembly and suggest a potential novel function for Wnts in neurotransmitter release.

Lhx5 promotes forebrain development and activates transcription of secreted Wnt antagonists

In vertebrate embryos, induction and patterning of the forebrain require the local inhibition of caudalizing signals, such as Wnts, emanating from the mesendoderm and caudal brain. Here, we report that Lhx5, expressed in the rostral neuroectoderm, regulates the local inhibition of Wnts. Activation of Lhx5 expands forebrain structures, whereas inhibition of Lhx5 function compromises forebrain development in zebrafish embryos. Lhx5 can rescue forebrain deficiencies caused by excess Wnt activity, and inhibition of Lhx5 function results in ectopic activation of Wnt signaling. Lhx5 regulates the expression of two secreted Frizzled-related Wnt antagonists, Sfrp1a and Sfrp5. These Sfrps can reduce the ectopic activation of Wnt signaling and rescue the forebrain deficiencies caused by inhibition of Lhx5 function. Our results demonstrate that Lhx5 is a required factor that promotes forebrain development and inhibits Wnt signaling by activating the transcription of secreted Wnt antagonists.

The Wnt antagonist secreted frizzled-related protein-1 controls osteoblast and osteocyte apoptosis

Mechanisms controlling human bone formation remain to be fully elucidated. We have used differential display-polymerase chain reaction analysis to characterize osteogenic pathways in conditionally immortalized human osteoblasts (HOBs) representing distinct stages of differentiation. We identified 82 differentially expressed messages and found that the Wnt antagonist secreted frizzled-related protein (sFRP)-1 was the most highly regulated of these. Transient transfection of HOBs with sFRP-1 suppressed canonical Wnt signaling by 70% confirming its antagonistic function in these cells. Basal sFRP-1 mRNA levels increased 24-fold during HOB differentiation from pre-osteoblasts to pre-osteocytes, and then declined in mature osteocytes. This expression pattern correlated with levels of cellular viability such that the pre-osteocytes, which had the highest levels of sFRP-1 mRNA, also had the highest rate of cell death. Basal sFRP-1 mRNA levels also increased 29-fold when primary human mesenchymal stem cells were differentiated to osteoblasts supporting the developmental regulation of the gene. Expression of sFRP-1 mRNA was induced 38-fold following prostaglandin E2 (PGE2) treatment of pre-osteoblasts and mature osteoblasts that had low basal message levels. In contrast, sFRP-1 expression was down-regulated by as much as 80% following transforming growth factor (TGF)-beta1 treatment of pre-osteocytes that had high basal mRNA levels. Consistent with this, treatment of pre-osteoblasts and mature osteoblasts with PGE(2) increased apoptosis threefold, while treatment of pre-osteocytes with TGF-beta1 decreased cell death by 50%. Likewise, over-expression of sFRP-1 in HOBs accelerated the rate of cell death threefold. These results establish sFRP-1 as an important negative regulator of human osteoblast and osteocyte survival.

Secreted frizzled related protein 2 (sFRP2) decreases susceptibility to UV-induced apoptosis in primary culture of canine mammary gland tumors by NF-kappaB activation or JNK suppression

Tumor formation can result from a decrease in cell death, as well as an increase in cell proliferation. In spite of the high incidence of mammary gland tumors (MGTs) in female dogs, the understanding of its etiology is still poor. Consistent with several proto-oncogenes (such as Wnt) for the mammary gland, sFRP2 is expressed in canine MGTs which is normally silent in the mammary gland. To elucidate the roles of SFRP2 in the tumorigenesis of MGTs, apoptosis regulation mediated by sFRP2 was investigated by overexpression of sFRP2 in MGT cells. DNA fragmentation and TUNEL assays showed a decreased susceptibility of the cells to UV-induced apoptosis in the context of sFRP2 overexpression. To analyze the pathways through which sFRP2 transduces anti-apoptosis signals, multiple-color immunofluorescence staining, immunoprecipitation, and immunoblotting were carried out. sFRP2 was found co-localized in the extracellular matrix of MGTs and the tyrosine phosphorylation of FAK was enhanced. Moreover, JNK was suppressed and NF-kB was activated in the cells expressing sFRP2 after UV-induced apoptosis analyzed by immunoblotting and electrophoretic mobility shift assay (EMSA). Taken together, these results suggest that sFRP2 exerts its anti-apoptotic function in mammary cancer cells through NF-kappaB activation or JNK suppression.

Differential inhibition of Wnt-3a by Sfrp-1, Sfrp-2, and Sfrp-3

Secreted frizzled related proteins (Sfrps) are extracellular attenuators of Wnt signaling that play important roles in both embryogenesis and oncogenesis. Although Sfrps are generally thought to bind and sequester Wnts away from active receptor complexes, very little is known about the specificity of Sfrp family members for various Wnts. In the developing chick neural tube, sfrp-1, 2, and 3 transcripts are expressed in and adjacent to the dorsal neural tube, where Wnt-1 and Wnt-3a are expressed. To better define the possible roles of Sfrp-1, 2, and 3 in the neural tube, we first tested the ability of purified Sfrps to inhibit Wnt-3a-induced accumulation of beta-catenin in L cells. We find that both Sfrp-1 and Sfrp-2 can inhibit Wnt-3a activity while Sfrp-3 cannot. To determine where Sfrp-1 and Sfrp-2 impinge on the Wnt signaling pathway, we tested the ability of these Sfrps to inhibit Wnt signaling induced by the addition of LiCl, an inhibitor of GSK-3. Sfrp-1 and Sfrp-2 are unable to inhibit the accumulation of beta-catenin in LiCl-treated cells, suggesting that the ability of Sfrps to inhibit the accumulation of beta-catenin is GSK-3 dependent. We have further shown that Sfrp-2 inhibits the ability of ectopic Wnt-3a to stimulate proliferation in the developing chick neural tube. These results provide the framework for understanding how Sfrps function to regulate Wnt-3a activity in developing embryos and in cancer.

Regulation of prostate cell growth and morphogenesis by Dickkopf-3

Wnt signalling plays a critical role in the development of cancer. Recent studies indicate that Wnt signalling is negatively regulated by secreted Wnt antagonists such as secreted frizzled related proteins (sFRPs) and Dickkopfs (Dkks). We compared Dkk family expression levels in normal prostate and prostate cancer cells and found a reduction in Dkk-3 expression in cancer cells. Ectopic expression of Dkk-3 inhibited colony formation in LNCaP and PC3 prostate cancer cell lines and inducible expression of Dkk-3 reduced LNCaP cell proliferation. Moreover, small interfering RNA-mediated downregulation of Dkk-3 enhanced cell cycle progression in untransformed RWPE-1 prostate epithelial cells. Immunohistochemical analysis revealed that Dkk-3 is expressed in a subset of normal prostate gland acini and that Dkk-3 expression is reduced in prostate tumours, particularly those with a high Gleason grade, suggesting a role for Dkk-3 in postmitotic differentiation. Consistent with this, depletion of Dkk-3 disrupted acinar morphogenesis of RWPE-1 cells in a three-dimensional cell culture model. Our results are consistent with the loss of Dkk-3 expression resulting in impairment of glandular structure and uncontrolled prostate epithelial cell (PrEC) proliferation, both of which are crucial for prostate cancer progression.

Genetic alterations in urothelial bladder carcinoma: an updated review

New oncogenes and tumor suppressor genes that play an important role in the pathogenesis of urothelial bladder carcinoma have been discovered. The objectives of this review were to summarize the most important oncogenes and tumor suppressor genes involved in urothelial carcinoma and to address their role in pathogenesis, their prognostic value, and their potential use as therapeutic targets. The collected data led the authors to propose a common pathway in which the fibroblastic growth factor receptor 3 (FGFR3) mutation seems to be the earliest genetic abnormality responsible for the transformation from normal tissue to atypia and dysplasia. Three different progression pathways were proposed: The first operative pathway is from dysplasia to superficial papillary pathologic Ta (pTa) tumors to pT1 tumors and, ultimately, to pT2 tumors with FGFR3 and tuberous sclerosis complex 1 (TSC1) the responsible genes. The second major operative pathway is from dysplasia, to carcinoma in situ, and to solid pT1 and pT2 tumors. The third pathway of progression is from dysplasia to papillary T1 and pT2 tumors. The genes involved in the last 2 pathways are the p53, serine threonine protein kinase 15 (STK15), triple-function domain (TRIO), fragile histidine triad (FHIT), p63 genes; and alterations of 20q and 5p, alterations of adhesions, angiogenesis, and matrix-remodeling gene products also are involved. Finally, murine leukemia viral oncogene homologue 1 (RAF1) and CD9 are involved in the progression from papillary pT1 tumors to pT2 tumors.

1,25-Dihydroxyvitamin D3 regulates the expression of low-density lipoprotein receptor-related protein 5 via deoxyribonucleic acid sequence elements located downstream of the start site of transcription

The skeleton is a direct target of vitamin D action, where the hormone modulates the proliferation of osteoblast precursors, their differentiation into mature osteoblasts, and their functional activity. Some of these effects of vitamin D are reminiscent of those orchestrated by the Wnt signaling pathway wherein stimulation of the membrane receptor Frizzled and its coreceptor LRP5 leads to activation of beta-catenin and subsequent transcription-mediated changes in osteoblast biology. Indeed, LRP5 is now known to play a particularly important role in bone formation such that the loss of this component results in a reduction in osteoblast number, a delay in mineralization, and a reduction in peak bone mineral density. Interestingly, we discovered during the course of a vitamin D receptor (VDR) chromatin immunoprecipitation/DNA microarray analysis that 1,25-(OH)2D3 could induce binding of the VDR to sites within the Lrp5 gene locus. VDR and retinoid X receptor binding was evident both in primary osteoblasts as well as in osteoblasts of cell line origin. Importantly, this interaction between 1,25-(OH)2D3-activated VDR and the Lrp5 gene led to both a modification in chromatin structure within the Lrp5 locus and the induction of Lrp5 mRNA transcripts in vivo as well as in vitro. One of these sites within the Lrp5 locus was discovered to confer vitamin D response to a heterologous promoter when introduced into osteoblastic cells, permitting both the identification and characterization of the vitamin D response element located within. Interestingly, additional studies revealed that whereas the regulatory region in the mouse Lrp5 gene was highly conserved in the human genome, the vitamin D response element was not. Our studies show that 1,25-(OH)2D3 can enhance the expression of a critical component of the Wnt signaling pathway that is known to impact osteogenesis.

Repressor roles for TCF-4 and Sfrp1 in Wnt signaling in breast cancer

Mutations in Wnt pathway genes are rare in human breast cancer, yet activation of the pathway is evident from the misolocalization of beta-catenin. We searched for relationships in the expression of Wnt pathway genes and found that both secreted frizzled related protein 1 (Sfrp1) and TCF-4 transcripts were all highly downregulated in a common subset of breast cancers relative to normal breast tissue. Sfrp1 has been previously characterized as a Wnt inhibitor, and we found that interfering with its expression in the human mammary epithelial cell line MCF10A activated Wnt signaling. Reduction of TCF-4 levels in breast cancer was surprising as it is a transcription factor that is responsive to Wnt signaling. Therefore, we investigated a possible inhibitory role for TCF-4 in human breast cells as well as further characterizing Sfrp1. We identified CD24 as a Wnt target in MCF10A cells and used its expression a marker of Wnt signaling. Interfering with either Sfrp1 or TCF-4 in this cell line enhanced CD24 expression. Furthermore, removal of TCF/LEF binding sites in a CD24-luciferase reporter resulted in elevated reporter gene expression. Our results indicate that both Sfrp1 and TCF-4 repress Wnt signaling in breast tissue and their downregulation contributes to the activation of Wnt signaling.

The Transcripts of SFRP1,CEP63 and EIF4G2 Genes Are Frequently Downregulated in Transitional Cell Carcinomas of the Bladder

OBJECTIVE

The aim of the present study was to identify differentially expressed genes that might be associated with the phenotype of superficial and invasive bladder cancer.

METHODS

Differential display reverse transcriptase PCR (DDRT-PCR) was used to compare the expression pattern between normal bladder tissue and 4 groups of transitional cell carcinomas of the bladder regarding clinical stage and grade.

RESULTS

We were able to identify 72 different transcripts, of which 57 (79%) showed homology to known genes, 12 (17%) to hypothetical proteins and 3 (4%) to human expressed sequence tags. Among the differentially expressed genes, SFRP1,CEP63 and EIF4G2 were further validated by quantitative RT-PCR in a series of 50 transitional cell carcinomas. Overall, the transcripts of these three genes were shown to be downregulated in the bladder tumors analyzed. In accordance with the DDRT-PCR results, the SFRP1 transcripts were shown to be downregulated in 90% (45/50) of the bladder tumors as compared with the normal bladder tissue. Although EIF4G2 and CEP63 transcripts exhibited three different expression patterns, downregulation was found in about 50% of the cases analyzed. In addition, downregulation of both CEP63 and EIF4G2 gene transcription was associated with invasive tumors.

CONCLUSION

The use of DDRT-PCR analysis to compare expression patterns among different subgroups of bladder tumors allowed us to identify a significant number of genes implicated in different cellular pathways that, when up- or downregulated, might play a role in the tumorigenic process of the bladder.