The Wnt signaling pathway effector TCF7L2 is upregulated by insulin and represses hepatic gluconeogenesis.

Certain single nucleotide polymorphisms (SNPs) in transcription factor 7-like 2 (TCF7L2) are strongly associated with the risk of type 2 diabetes. TCF7L2 and ß-catenin (ß-cat) form the bipartite transcription factor cat/TCF in stimulating Wnt target gene expression. cat/TCF may also mediate the effect of other signaling cascades, including that of cAMP and insulin in cell-type specific manners. As carriers of TCF7L2 type 2 diabetes risk SNPs demonstrated increased hepatic glucose production, we aimed to determine whether TCF7L2 expression is regulated by nutrient availability and whether TCF7L2 and Wnt regulate hepatic gluconeogenesis. We examined hepatic Wnt activity in the TOPGAL transgenic mouse, assessed hepatic TCF7L2 expression in mice upon feeding, determined the effect of insulin on TCF7L2 expression and ß-cat Ser675 phosphorylation, and investigated the effect of Wnt activation and TCF7L2 knockdown on gluconeogenic gene expression and glucose production in hepatocytes. Wnt activity was observed in pericentral hepatocytes in the TOPGAL mouse, whereas TCF7L2 expression was detected in human and mouse hepatocytes. Insulin and feeding stimulated hepatic TCF7L2 expression in vitro and in vivo, respectively. In addition, insulin activated ß-cat Ser675 phosphorylation. Wnt activation by intraperitoneal lithium injection repressed hepatic gluconeogenic gene expression in vivo, whereas lithium or Wnt-3a reduced gluconeogenic gene expression and glucose production in hepatic cells in vitro. Small interfering RNA-mediated TCF7L2 knockdown increased glucose production and gluconeogenic gene expression in cultured hepatocytes. These observations suggest that Wnt signaling and TCF7L2 are negative regulators of hepatic gluconeogenesis, and TCF7L2 is among the downstream effectors of insulin in hepatocytes.

TCF4 induces enzalutamide resistance via neuroendocrine differentiation in prostate cancer.

In treating patients with castration resistant prostate cancer (CRPC), enzalutamide, the second-generation androgen receptor (AR) antagonist, is an accepted standard of care. However, clinical benefits are limited to a median time of 4.8 months because resistance inevitably emerges. To determine the mechanism of treatment resistance, we carried out a RNA sequence analysis and found increased expression levels of neuroendocrine markers in the enzalutamide-resistant LNCaP human prostate cancer (CaP) cell line when compared to the parental cell line. Subsequent studies demonstrated that Transcription Factor-4 (TCF4), a transcription factor implicated in WNT signaling, mediated neuroendocrine differentiation (NED) in response to enzalutamide treatment and was elevated in the enzalutamide-resistant LNCaP. In addition, we observed that PTHrP mediated enzalutamide resistance in tissue culture and inducible TCF4 overexpression resulted in enzalutamide-resistance in a mouse xenograft model. Finally, small molecule inhibitors of TCF4 or PTHrP partially reversed enzalutamide resistance in CaP cells. When tissues obtained from men who died of metastatic CaP were examined, a positive correlation was found between the expression levels of TCF4 and PTHrP. Taken together, the current results indicate that TCF4 induces enzalutamide resistance via NED in CaP.

Expression of µ-protocadherin is negatively regulated by the activation of the ß-catenin signaling pathway in normal and cancer colorectal enterocytes.

Mu-protocadherin (MUCDHL) is an adhesion molecule predominantly expressed by colorectal epithelial cells which is markedly downregulated upon malignant transformation. Notably, treatment of colorectal cancer (CRC) cells with mesalazine lead to increased expression of MUCDHL, and is associated with sequestration of ß-catenin on the plasma membrane and inhibition of its transcriptional activity. To better characterize the causal relationship between ß-catenin and MUCDHL expression, we performed various experiments in which CRC cell lines and normal colonic organoids were subjected to culture conditions inhibiting (FH535 treatment, transcription factor 7-like 2 siRNA inactivation, Wnt withdrawal) or stimulating (LiCl treatment) ß-catenin activity. We show here that expression of MUCDHL is negatively regulated by functional activation of the ß-catenin signaling pathway. This finding was observed in cell culture systems representing conditions of physiological stimulation and upon constitutive activation of ß-catenin in CRC. The ability of MUCDHL to sequester and inhibit ß-catenin appears to provide a positive feedback enforcing the effect of ß-catenin inhibitors rather than serving as the primary mechanism responsible for ß-catenin inhibition. Moreover, MUCDHL might have a role as biomarker in the development of CRC chemoprevention drugs endowed with ß-catenin inhibitory activity.

Acute Wnt pathway activation positively regulates leptin gene expression in mature adipocytes.

Genome-wide association studies (GWAS) have revealed the implication of several Wnt signaling pathway components, including its effector transcription factor 7-like 2 (TCF7L2) in diabetes and other metabolic disorders. As TCF7L2 is expressed in adipocytes, we investigated its expression and function in rodent fat tissue and mature adipocytes. We found that TCF7L2 mRNA expression in C57BL/6 mouse epididymal fat tissue was up-regulated by feeding but down-regulated by intraperitoneal insulin injection. In high-fat diet (HFD) fed mice, db/db mice and Zucker (fa/fa) rats, epididymal fat TCF7L2 mRNA levels were lower than the corresponding controls. Treating rat adipocytes with 100nM insulin repressed TCF7L2 mRNA and protein levels, associated with the repression of leptin mRNA level. The treatment with 1nM insulin, however, stimulated TCF7L2 and leptin mRNA levels. This stimulation could be attenuated by iCRT14, an inhibitor of ß-catenin/TCF-responsive transcription. Wnt3a stimulated leptin mRNA level, which was also blocked by iCRT14 co-treatment. Utilizing the leptin-expressing cell line HTR8 as a tool, we defined an evolutionarily conserved CREB binding motif that mediated Wnt3a activation. Although Wnt activation is known to repress the differentiation of 3T3-L1 cells towards mature adipocytes, short-term Wnt3a treatment of differentiated 3T3-L1 cells stimulated leptin mRNA levels. Thus, wnt pathway plays a dual function in adipocytes, including the well-known repressive effect on adipogenesis and the stimulation of leptin production in mature adipocytes in response to nutritional status.

Wnt-dependent T-cell factor-4 controls human etravillous trophoblast motility.

Formation of migratory extravillous trophoblasts (EVTs) is critical for human placentation and hence embryonic development. However, key regulatory growth factors, hormones, and nuclear proteins controlling the particular differentiation process remain poorly understood. Here, the role of the Wingless (Wnt)-dependent transcription factor T-cell factor-4 (TCF-4) in proliferation and motility was investigated using different trophoblast cell models. Immunofluorescence of first-trimester placental tissues revealed induction of TCF-4 and nuclear recruitment of its coactivator ß-catenin in nonproliferating EVTs, whereas membrane-associated ß-catenin decreased upon differentiation. In addition, EVTs expressed the TCF-4/ß-catenin coactivator Pygopus 2 as well as repressors of the Groucho/transducin-like enhancer of split family. Western blotting revealed Pygopus 2 expression and up-regulation of integrin α1 and nuclear TCF-4 in purified first-trimester cytotrophoblasts (CTBs) differentiating on fibronectin. Concomitantly, elevated TCF-4 mRNA, quantitated by real-time PCR, and increased TCF-dependent luciferase reporter activity were noticed in EVTs of villous explant cultures and differentiated primary CTBs. Gene silencing using specific small interfering RNA decreased TCF-4 transcript and protein levels, TCF-dependent reporter activity as well as basal and Wnt3a-stimulated migration of trophoblastic SGHPL-5 cells and primary CTBs through fibronectin-coated transwells. In contrast, proliferation of SGHPL-5 cells and primary cells, measured by cumulative cell numbers and 5-bromo-2'-deoxy-uridine labeling, respectively, was not affected. Moreover, siRNA-mediated down-regulation of TCF-4 in primary CTBs diminished markers of the differentiated EVT, such as integrin α1 and α5, Snail1, and Notch2. In summary, the data suggest that Wnt/TCF-4-dependent signaling could play a role in EVT differentiation promoting motility and expression of promigratory genes.

DIF-1 inhibits the Wnt/ß-catenin signaling pathway by inhibiting TCF7L2 expression in colon cancer cell lines.

We previously reported that differentiation-inducing factor-1 (DIF-1), a morphogen in Dictyostelium discoideum, inhibits the proliferation of human cancer cell lines by inducing ß-catenin degradation and suppressing the Wnt/ß-catenin signaling pathway. To determine whether ß-catenin degradation is essential for the effect of DIF-1, we examined the effect of DIF-1 on human colon cancer cell lines (HCT-116, SW-620 and DLD-1), in which the Wnt/ß-catenin signaling pathway is constitutively active. DIF-1 strongly inhibited cell proliferation and arrested the cell cycle in the G(0)/G(1) phase via the suppression of cyclin D1 expression at mRNA and protein levels without reducing ß-catenin protein. TCF-dependent transcriptional activity and cyclin D1 promoter activity were revealed to be inhibited via suppression of transcription factor 7-like 2 (TCF7L2) expression. Luciferase reporter assays and EMSAs using the TCF7L2 promoter fragments indicated that the binding site for the transcription factor early growth response-1 (Egr-1), which is located in the -609 to -601 bp region relative to the start codon in the TCF7L2 promoter, was involved in DIF-1 activity. Moreover, RNAi-mediated depletion of endogenous TCF7L2 resulted in reduced cyclin D1 promoter activity and protein expression, and the overexpression of TCF7L2 overrode the inhibition of the TCF-dependent transcriptional activity and cyclin D1 promoter activity induced by DIF-1. Therefore, DIF-1 seemed to inhibit the Wnt/ß-catenin signaling pathway by suppressing TCF7L2 expression via reduced Egr-1-dependent transcriptional activity in these colon cancer cell lines. Our results provide a novel insight into the mechanisms by which DIF-1 inhibits the Wnt/ß-catenin signaling pathway.