Chimeric Wnt proteins define the amino-terminus of Wnt-1 as a transformation-specific determinant

The WNT7B protein promotes the migration and differentiation of human dental pulp cells partly through WNT/beta-catenin and c-Jun N-terminal kinase signalling pathways

OBJECTIVE

The aim of this study is to investigate the role of the WNT7B protein in the migration and differentiation of human dental pulp cells (HDPCs).

DESIGN

The effect of recombinant human WNT7B (rhWNT7B) on the proliferation and migration of HDPCs was evaluated by 5-ethynyl-2'-deoxyuridine (EdU), immunofluorescence staining of Ki67, flow cytometry and scratch assay; the differentiation of HDPCs was measured by alkaline phosphatase (ALP) staining, alizarin red staining, ALP activity, qPCR and western blot. The activation of the WNT/beta-catenin (WNT/β-catenin) and c-Jun N-terminal kinase (JNK) pathways was analysed by western blot, immunocytochemistry and dual luciferase assays. XAV939 and SP600125，the inhibitors of the WNT/β-catenin and JNK pathways, were further applied to verify the mechanism.

RESULTS

rhWNT7B repressed the proliferation but did not affect the apoptosis of HDPCs. In the presence of rhWNT7B, ALP and alizarin red staining were increased substantially in the HDPCs with osteogenic induction; the gene expression of Runx2 and Col1 in HDPCs was quite elevated compared with that induced in osteogenic medium without WNT7B measured by qPCR; The ALP activity was also increased with rhWNT7B stimulation in HDPCs after 7-day odontogenic culture; Western blot revealed that the expression of dentin sialophosphoprotein (DSPP) of HDPCs was up-regulated significantly with the addition of WNT7B as well. Further study showed that rhWNT7B activated the WNT/β-catenin and JNK signalling pathways in the differentiation of HDPCs. XAV939 and SP600125 can partly offset the effect of the WNT7B-induced differentiation of HDPCs.

CONCLUSION

WNT7B promoted the differentiation of HDPCs partly through the WNT/β-catenin and JNK signalling pathways.

Threonine 393 of beta-catenin regulates interaction with Axin

CK2 is a regulatory kinase implicated in embryonic development and in cancer. Among the CK2 substrates is beta-catenin, a protein with dual function in Wnt signaling and cell adhesion. Previously, we reported that CK2 activity is required for beta-catenin stability and we identified threonine (T) 393 as a major CK2 phosphorylation site in beta-catenin. However, it is not known whether phosphorylation at T393 increases beta-catenin stability and if so, what is the mechanism. In this study we investigate the molecular mechanism of beta-catenin stabilization through phosphorylation at T393. We found that pseudophosphorylation of beta-catenin at T393 resulted in a stable activated form of beta-catenin with decreased affinity for Axin in vitro. This phosphomimetic mutant also displayed decreased regulation by Axin in vivo in a bioassay in Xenopus laevis embryos. In contrast, the binding of T393 pseudophosphorylated beta-catenin to E-cadherin was unaffected. Further analysis showed that pseudophosphorylation at T393 did not prevent beta-catenin phosphorylation by GSK3beta. Interestingly, we found that in the presence of pseudophophorylated beta-catenin and another activated form of beta-catenin, the recruitment of GSK3beta to Axin is enhanced. These findings indicate that phosphorylation of T393 by CK2 may affect the stability of beta-catenin through decreased binding to Axin. In addition, the increased recruitment of GSK3beta to the destruction complex in the presence of activated beta-catenin mutants could be a feedback mechanism to suppress overactive Wnt signaling.

Mitochondrial and nuclear forms of Wnt13 are generated via alternative promoters, alternative RNA splicing, and alternative translation start sites

Wnt proteins play a key role in cell survival, cell proliferation, and cell fate during development. In endothelial cells, we identified the expression of Wnt13A, Wnt13B, and Wnt13C mRNAs, which are generated by alternative promoters and alternative RNA splicing. Wnt13A and Wnt13B proteins differ only in their N-terminal sequences. Wnt13A, a typical Wnt, is N-glycosylated and localized in the endoplasmic reticulum, with only a small fraction being secreted. Wnt13B proteins appear as a protein doublet, L-Wnt13B and S-Wnt13B, which are neither N-glycosylated nor secreted. Wnt13B proteins localized mainly to mitochondria, as demonstrated using detection in mitochondria enriched fractions and colocalization with Mitotracker and HSP60. A nuclear localization was also observed in 20% of Wnt13B-expressing cells. Both the N-terminal hydrophobic stretch (residues 1-17) and alpha-helix (residues 26-50) were the main determinants for Wnt13B mitochondrial targeting. Serial deletions of Wnt13B N-terminal sequences abolished its association with mitochondria and favored instead a nuclear localization. The production of S-Wnt13B was independent of the mitochondrial targeting but dependent on an alternative translation start corresponding to Met(74) in L-Wnt13B. The same translation start is used in Wnt13C mRNA to encode a protein undistinguishable from S-Wnt13B. S-Wnt13B when expressed alone localized to the nucleus like Wnt13C, whereas L-Wnt13B localized to mitochondria. Wnt13 nuclear forms increased the beta-catenin/T-cell factor activity in HEK293 cells and increased apoptosis in bovine aortic endothelial cells. Altogether our results demonstrate that, in addition to alternative promoters and RNA splicing, an alternative translation start in Wnt13B and Wnt13C mRNAs increases the complexity of both human wnt13 expression and functions.

Wnt7b activates canonical signaling in epithelial and vascular smooth muscle cells through interactions with Fzd1, Fzd10, and LRP5

Wnt7b is a Wnt ligand that has been demonstrated to play critical roles in several developmental processes, including lung airway and vascular development and chorion-allantois fusion during placental development. Wnt signaling involves the binding of Wnt ligands to cell surface receptors of the frizzled family and coreceptors of the LRP5/6 family. However, little is known of the ligand-receptor specificity exhibited by different Wnts, Fzds, and LRPs in Wnt signaling. Expression analysis of Fzds and LRP5/6 in the developing lung and vasculature showed that Fzd1, -4, -7, and -10 and LRP5/6 are expressed in tissue-specific patterns during lung development. Fzd1, -4, and -7 are expressed primarily in the developing lung mesenchyme, and Fzd10 is expressed in airway epithelium. LRP5 and LRP6 are expressed in airway epithelium during lung development, whereas LRP5 but not LRP6 expression is observed in the muscular component of large blood vessels, including the aorta. Cell transfection studies demonstrate that Wnt7b can activate the canonical Wnt pathway but not the noncanonical Wnt pathway in a cell-specific manner. Biochemical analysis demonstrates that Wnt7b can bind to Fzd1 and -10 on the cell surface and cooperatively activate canonical Wnt signaling with these receptors in the presence of LRP5. Together, these data demonstrate that Wnt7b signals through Fzd1 and -10 and LRP5 and implicate these Wnt coreceptors in the regulation of lung airway and vascular development.

In vitro analysis of partial loss-of-function ZIC2 mutations in holoprosencephaly: alanine tract expansion modulates DNA binding and transactivation

Heterozygous loss-of-function mutations in ZIC2 result in the severe brain malformation known as holoprosencephaly (HPE), indicating that forebrain development is exquisitely sensitive to the activity of this poorly understood transcription factor. To identify the regions of ZIC2 that are essential for activity, we have assessed the ability of a variety of ZIC2 mutant proteins to function in in vitro assays. Two sources of information were used to design relevant mutations. First, phenotype producing mutations in human and in mouse ZIC2 were mimicked and secondly, a comparative sequence analysis of the C-terminal was carried out. Analysis of these mutations suggests that either a decrease or an increase in ZIC2 mediated transcriptional activity can produce a forebrain phenotype. In addition, the analysis reveals that the C-terminal of ZIC2 contains both activation and repression domains. This region of ZIC2 contains an alanine-tract, and expansion of this domain is associated with HPE. In vitro analysis of proteins with alterations in alanine-tract length illustrates that the C-terminal alanine-tract of ZIC2 influences the strength of DNA binding and alters transcriptional activity in a promoter-specific manner. This finding provides a possible mechanism by which alanine-tract expansion mutations could alter the function of other transcription factors.

Beta-catenin/Tcf activation partially mimics the transforming activity of Wnt-1 in Rat-1 fibroblasts

We have previously demonstrated that Wnt-1 induction of cytosolic beta-catenin and Tcf/Lef transcriptional activation correlate with enhanced proliferation, survival, and post-confluent growth in Rat-1 fibroblasts. To examine whether beta-catenin mediates the biological responses to Wnt-1 in this context, we characterized Rat-1 clonal cell lines expressing different levels of a mutant, stabilized beta-catenin (beta-cateninS37A). Clonal lines exhibit elevated cytosolic and nuclear beta-cateninS37A and Tcf transcriptional activation, comparable to that elicited by Wnt-1 expression. However, expression of beta-cateninS37A does not promote growth of Rat-1 cells in serum-free conditions and only partially promotes growth post-confluence, when compared to that induced by Wnt-1 expression. As ectopic expression of beta-cateninS37A only partially mimics Wnt-1 effects on Rat-1 cells, we conclude that Wnt-1 signaling elicits biochemical events that act in addition to beta-catenin/Tcf signaling to promote cell growth.

Wnt/beta-catenin/Tcf signaling induces the transcription of Axin2, a negative regulator of the signaling pathway

Axin2/Conductin/Axil and its ortholog Axin are negative regulators of the Wnt signaling pathway, which promote the phosphorylation and degradation of beta-catenin. While Axin is expressed ubiquitously, Axin2 mRNA was seen in a restricted pattern during mouse embryogenesis and organogenesis. Because many sites of Axin2 expression overlapped with those of several Wnt genes, we tested whether Axin2 was induced by Wnt signaling. Endogenous Axin2 mRNA and protein expression could be rapidly induced by activation of the Wnt pathway, and Axin2 reporter constructs, containing a 5.6-kb DNA fragment including the promoter and first intron, were also induced. This genomic region contains eight Tcf/LEF consensus binding sites, five of which are located within longer, highly conserved noncoding sequences. The mutation or deletion of these Tcf/LEF sites greatly diminished induction by beta-catenin, and mutation of the Tcf/LEF site T2 abolished protein binding in an electrophoretic mobility shift assay. These results strongly suggest that Axin2 is a direct target of the Wnt pathway, mediated through Tcf/LEF factors. The 5.6-kb genomic sequence was sufficient to direct the tissue-specific expression of d2EGFP in transgenic embryos, consistent with a role for the Tcf/LEF sites and surrounding conserved sequences in the in vivo expression pattern of Axin2. Our results suggest that Axin2 participates in a negative feedback loop, which could serve to limit the duration or intensity of a Wnt-initiated signal.

A possible role for the WNT-1 pathway in oral carcinogenesis

Reductions in cell-cell adhesion and stromal and vascular invasion are essential steps in the progression from localized malignancy to metastatic disease for all cancers. Proteins involved in intercellular adhesion, such as E-cadherin and catenin, probably play an important role in metastatic processes and cellular differentiation. While E-cadherin and beta-catenin expression has been extensively studied in many forms of human cancers, less is known about the role of the Wingless-Type-1 (WNT-1) pathway in human tumors. A large body of genetic and biochemical evidence has identified beta-catenin as a key downstream component of the WNT signaling pathway, and recent studies of colorectal tumors have shown a functional link among beta-catenin, adenomatous polyposis coli gene product (APC), and other components of the WNT-1 pathway. WNT-1 pathway signaling is thought to be mediated via interactions between beta-catenin and members of the LEF-1/TCF family of transcription factors. The WNT signal stabilizes beta-catenin protein and promotes its accumulation in the cytoplasm and nucleus. In the nucleus, beta-catenin associates with TCF to form a functional transcription factor which mediates the transactivation of target genes involved in the promotion of tumor progression, invasion, and metastasis, such as C-Myc, cyclin D1, c-jun, fra-1, and u-PAR. There is a strong correlation between the ability of the WNT-1 gene to induce beta-catenin accumulation and its transforming potential in vivo, suggesting that the WNT-1 gene activates an intracellular signaling pathway that can induce the morphological transformation of cells. For these reasons, data obtained from the study of the WNT-1 pathway could be important in our understanding of the mechanisms of epithelial tumors, in general, and probably also of oral squamous cell carcinoma, in particular.

Transcriptional regulation of topoisomerase II alpha at confluence and pharmacological modulation of expression by bis-benzimidazole drugs

Topoisomerase II alpha is a critical gene involved in DNA replication and maintenance of genomic stability. Several chemotherapeutic agents target topoisomerase II and levels of expression are an important factor in chemosensitivity. Transcriptional regulation has been demonstrated to regulate topoisomerase II alpha levels under several circumstances, including cellular confluence, heat shock, and expression of oncogenes including ras and myb. Expression of topoisomerase II alpha is regulated by cellular proliferation; transcriptional down-regulation in confluent cells is modulated through sequences within the promoter. In this study, we examined DNA-protein interactions within the topoisomerase II alpha promoter in exponential and confluent phase NIH3T3 cells. Using electrophoretic mobility shift assay and in vitro DNase I footprint experiments, the involvement of NF-Y in transcriptional regulation was established. Incubation of the DNA minor groove-binding agents Hoechst 33342 and Hoechst 33258 with nuclear extracts revealed drug binding to regions surrounding the inverted CCAAT boxes within the topoisomerase II alpha promoter and displacement of proteins binding to these elements. Addition of both Hoechst 33342 and Hoechst 33258 to NIH3T3 cells at confluence resulted in increased expression of topoisomerase II alpha. In addition, MTT cytotoxicity assays in confluent cells showed an additive effect of incubation with Hoechst 33342 and the topoisomerase II alpha poison etoposide. Therefore, DNA binding drugs which block transcription factor activation of the promoter may deregulate topoisomerase II alpha and this strategy may be of value in modifying gene expression and modulating chemosensitivity.

A stat-responsive element in the promoter of the episialin/MUC1 gene is involved in its overexpression in carcinoma cells

The mucin-like glycoprotein episialin (MUC1) is highly overproduced by a number of human carcinomas. We have shown previously in a variety of mammalian cell lines that overexpression of this very large transmembrane molecule diminishes cellular adhesion, suggesting that episialin/MUC1 overexpression may play an important role in tumor invasion and metastasis. By using in situ hybridization, we show here that episialin/MUC1 mRNA expression can be increased more than 10-fold in breast carcinoma cells relative to the expression in adjacent normal breast epithelium. In search of the molecular mechanism of this overexpression, we observed that the episialin/MUC1 promoter contains a candidate binding site for transcription factors of the STAT family approximately 500 base pairs upstream of the transcription start site. Cytokines and/or growth factors such as interleukin-6 or interferon-gamma can activate STATs. In the human breast carcinoma cell line T47D, both compounds are able to stimulate transcription of a luciferase reporter gene under the control of a 750-base pair MUC1 promoter fragment proximal to the transcription start site. The observed increase is entirely mediated by the single STAT-binding site, since mutation of this site abolishes stimulation of the reporter by interleukin-6 and interferon-gamma. In addition, mutation of the STAT site also decreased the promoter activity in nonstimulated T47D cells, suggesting that the STAT-binding site is among the elements that are involved in the overexpression of MUC1 in tumor cells.

Domains of axin and disheveled required for interaction and function in wnt signaling

Disheveled blocks the degradation of beta-catenin in response to Wnt signal by interacting with the scaffolding protein, Axin. To define this interaction in detail we undertook a mutational and binding analysis of the murine Axin and Disheveled proteins. The DIX domain of Axin was found to be important for association with Disheveled and two other regions of Axin (between residues 1-168 and 600-810) were identified that can promote the association of Axin and Disheveled. We found that the DIX domain of Disheveled is critical for association with Axin in vivo and for Disheveled activity. The Disheveled DIX domain controlled the ability of Disheveled to induce the accumulation of cytosolic beta-catenin whereas the PDZ domain was not essential to this function.

Cooperative induction of mammary tumorigenesis by TGFalpha and Wnts

We previously reported that multiparous WAP-TGFalpha transgenic mice develop mammary gland carcinomas with complete incidence. TGFalpha-induced tumors appear stochastically and with relatively long latency, indicating an additional requirement for other genetic alterations. To identify genes that cooperate with TGFalpha in mammary tumorigenesis, we used a retroviral insertion approach featuring a cloned and infectious hybrid MMTV (C3H/Mtv-1; (Shackleford and Varmus, 1988)). Tumor latency was decreased approximately 30% in MMTV-infected WAP-TGFalpha transgenic animals compared to noninfected transgenic controls, and > 30% of the corresponding tumors displayed evidence of integrated C3H/Mtv-1 DNA. PCR-based analyses of DNAs from two virus-infected, transgenic tumors revealed integration of hybrid MMTV in 3' untranslated exons of the Wnt-1 or Wnt-3 oncogenes. Moreover, Northern blots confirmed dramatic induction of Wnt-1 or Wnt-3 transcripts in the respective tumors, indicating that MMTV integration resulted in activated expression of these genes. Semiquantitative RT-PCR analyses showed that overexpression of Wnt-1 or Wnt-3 was a common occurrence in MMTV-infected WAP-TGFalpha tumors, and some noninfected WAP-TGFalpha tumors also showed evidence of elevated Wnt-3 transcripts. Collectively, these results reveal cooperative induction of mammary gland tumorigenesis by simultaneous deregulation of EGF-like (TGFalpha) and Wnt growth factors.

A GSK3beta phosphorylation site in axin modulates interaction with beta-catenin and Tcf-mediated gene expression

Upon binding of a Wnt to its receptor, GSK3beta is inhibited through an unknown mechanism involving Dishevelled (Dsh), resulting in the dephosphorylation and stabilization of beta-catenin, which translocates to the nucleus and interacts with Lef/Tcf transcription factors to activate target gene expression. Axin is a scaffold protein which binds beta-catenin and GSK3beta (as well as several other proteins) and thus promotes the phosphorylation of beta-catenin. Here we report that Axin is phosphorylated on Ser and Thr residues in several regions in vivo, while only one region (amino acids 600-672) is efficiently phosphorylated by GSK3beta in vitro. Site-directed mutagenesis, together with in vitro and in vivo phosphorylation assays, demonstrates that Axin residues T609 and S614 are physiological GSK3beta targets. Substitutions for one or more of these residues, which lie within a beta-catenin binding site, reduce the ability of Axin to modulate Wnt-induced signaling in a Lef/Tcf reporter assay. These amino acid substitutions also reduce the binding between Axin and beta-catenin. We propose a model in which inhibition of GSK3beta activity upon Wnt signaling leads to the dephosphorylation of GSK3beta sites in Axin, resulting in the release of beta-catenin from the phosphorylation complex.