Dishevelled-DEP domain interacting protein (DDIP) inhibits Wnt signaling by promoting TCF4 degradation and disrupting the TCF4/β-catenin complex

The TCF4/β-catenin complex, the executor of canonical Wnt/β-catenin signaling, is regulated by a variety of factors. Among these, Dishevelled (Dvl) is a critical regulator that releases β-catenin from degradation and stabilizes TCF4/β-catenin complex. Here, we report that DDIP (Dishevelled-DEP domain Interacting Protein, also named as Spats1, spermatogenesis associated, serine-rich 1), a novel protein that interacts with Dvl, regulates Wnt signaling. We provide evidence that DDIP suppresses Lef-1 luciferase reporter activity stimulated by Wnt1, Dvl2 or β-catenin, interacts with the TCF4/β-catenin complex, and disrupts the interaction of TCF4 and β-catenin by promoting TCF4 degradation through the proteasome pathway. Our results indicate that DDIP is a negative regulator of the canonical Wnt signaling.     

Analysing the impact of nucleo-cytoplasmic shuttling of β-catenin and its antagonists APC,Axin and GSK3 on Wnt/β-catenin signalling

The canonical Wnt signalling pathway plays a critical role in development and disease. The key player of the pathway is β-catenin. Its activity is mainly regulated by the destruction complex consisting of APC, Axin and GSK3. In the nucleus, the complex formation of β-catenin and TCF initiates target gene expression. Our study provides a comprehensive analysis of the role of nucleo-cytoplasmic shuttling of APC, Axin, and GSK3 and the inactivation of β-catenin by the destruction complex in Wnt/β-catenin signalling.We address the following questions: Can nucleo-cytoplasmic shuttling of APC, Axin and GSK3 increase the [β-catenin/TCF] concentration? And, how is the [β-catenin/TCF] concentration influenced by phosphorylation and subsequent degradation of nuclear β-catenin?Based on experimental findings, we develop a compartmental model and conduct several simulation experiments. Our analysis reveals the following key findings: 1) nucleo-cytoplasmic shuttling of β-catenin and its antagonists can yield a spatial separation between the said proteins, which results in a breakdown of β-catenin degradation, followed by an accumulation of β-catenin and hence leads to an increase of the [β-catenin/TCF] concentration. Our results strongly suggest that Wnt signalling can benefit from nucleo-cytoplasmic shuttling of APC, Axin and GSK3, although they are in general β-catenin antagonising proteins. 2) The total robustness of the [β-catenin/TCF] output is closely linked to its absolute concentration levels. We demonstrate that the compartmental separation of β-catenin and the destruction complex does not only lead to a maximization, but additionally to an increased robustness of [β-catenin/TCF] signalling against perturbations in the cellular environment. 3) A nuclear accumulation of the destruction complex renders the pathway robust against fluctuations in Wnt signalling and against changes in the compartmental distribution of β-catenin. 4) Elucidating the impact of destruction complex inhibition, we show that the [β-catenin/TCF] concentration is more effectively enhanced by inhibition of the kinase GSK3 rather than the binding of β-catenin to the destruction complex.     

Long non-coding RNA Linc00320 inhibits glioma cell proliferation through restraining Wnt/β-catenin signaling

Recent efforts have revealed that numerous oncogenic lncRNAs have been found play pivotal role in Glioma progression while there is little know about anti-oncogenic lncRNAs in Glioma. In current study, we found a HMGB1 regulated lncRNA, Linc00320, is significantly decreased in Glioma malignant tissues and its low expression predicts poor prognosis. Moreover, we found that the nucleus localized Linc00320 inhibits Glioma cell proliferation both in vitro and in vivo. In addition, we found that Linc00320 binds to β-catenin and inhibits the activity of Wnt/β-catenin signaling by disrupting β-catenin binds to TCF4 in Glioma cells. Taken together, we firstly demonstrated the tumor suppressive lncRNA, Linc00320, is down-regulated in Glioma tissues and inhibits Glioma cell proliferation by restraining Wnt/β-catenin signaling through segregating β-catenin and TCF4 and revealed the novel HMGB1/Linc00320/β-catenin axis in Glioma progression.     

Homeodomain-interacting protein kinase 2 (HIPK2) targets β-catenin for phosphorylation and proteasomal degradation

The regulation of intracellular β-catenin levels is central in the Wnt/β-catenin signaling cascade and the activation of the Wnt target genes. Here, we show that homeodomain-interacting protein kinase 2 (HIPK2) acts as a negative regulator of the Wnt/β-catenin pathway. Knock-down of endogenous HIPK2 increases the stability of β-catenin and results in the accumulation of β-catenin in the nucleus, consequently enhancing the expression of Wnt target genes and cell proliferation both in vivo and in cultured cells. HIPK2 inhibits TCF/LEF-mediated target gene activation via degradation of β-catenin. HIPK2 phosphorylates β-catenin at its Ser33 and Ser37 residues without the aid of a priming kinase. Substitutions of Ser33 and Ser37 for alanines abolished the degradation of β-catenin associated with HIPK2. In ex vivo mouse model, HIPK2 knock-down resulted in accumulation of β-catenin, thereby potentiated β-catenin-mediated cell proliferation and tumor formation. Furthermore, the axis duplication induced by the ectopic expression of β-catenin was blocked by co-injection of HIPK2 mRNAs into Xenopus embryos. Taken together, HIPK2 appears to function as a novel negative regulator of β-catenin through its phosphorylation and proteasomal degradation.     

Analysis of the Wnt/B-catenin/TCF4 pathway using SAGE,genome-wide microarray and promoter analysis: Identification of BRI3 and HSF2 as novel targets

The Wnt signaling pathway is involved in many differentiation events during embryonic development and can lead to tumor formation after aberrant activation of its components. β-catenin, a cytoplasmic component, plays a major role in the transduction of canonical Wnt signaling. The aim of this study was to identify novel genes that are regulated by active β-catenin/TCF signaling in hepatocellular carcinoma-derived Huh7 cells with high (transfected) and low β-catenin/TCF activities. High TCF activity Huh7 cells led to earlier and larger tumor formation when xenografted into nude mice. SAGE (Serial Analysis of Gene Expression), genome-wide microarray and in silico promoter analysis were performed in parallel, to compare gene expression between low and high β-catenin/TCF activity clones, and also those that had been rescued from the xenograft tumors. SAGE and genome-wide microarray data were compared and contrasted. BRI3 and HSF2 were identified as novel targets of Wnt/β-catenin signaling after combined analysis and confirming experiments including qRT-PCR, ChIP, luciferase assay and lithium treatment.