The Wnt Signaling Pathway Effector TCF7L2 and Type 2 Diabetes ...

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Molecular Endocrinology 22(11):2383–2392 Copyright © 2008 by The Endocrine Society doi: 10.1210/me.2008-0135

MINIREVIEW

The Wnt Signaling Pathway Effector TCF7L2 and Type 2 Diabetes Mellitus Tianru Jin and Ling Liu Departments of Medicine, Physiology, and Laboratory Medicine and Pathobiology, University of Toronto; and Division of Cell and Molecular Biology, Toronto General Research Institute, University Health Network, Toronto, Ontario, Canada M5G 1L7 Since the relationship between TCF7L2 (also known as TCF-4) polymorphisms and type 2 diabetes mellitus was identified in 2006, extensive genome-wide association examinations in different ethnic groups have further confirmed this relationship. As a component of the bipartite transcription factor ␤-catenin/ TCF, TCF7L2 is important in conveying Wnt signaling during embryonic development and in regulating gene expression during adulthood. Although we still do not know mechanistically how the polymorphisms within the intron regions of TCF7L2 affect the risk of type 2 diabetes, this transcriptional regulator

was shown to be involved in stimulating the proliferation of pancreatic ␤-cells and the production of the incretin hormone glucagon-like peptide-1 in intestinal endocrine L cells. In this review, we introduce background knowledge of TCF7L2 as a component of the Wnt signaling pathway, summarize recent findings demonstrating the association between TCF7L2 polymorphisms and the risk of type 2 diabetes, outline experimental evidence of the potential function of TCF7L2 in pancreatic and intestinal endocrine cells, and present our perspective views. (Molecular Endocrinology 22: 2383–2392, 2008)

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demonstrated that although nondiabetic carriers of the risk-TCF7L2-SNPs show impaired GLP-1-induced insulin secretion, their GLP-1 levels after glucose challenge were still within the normal range (10). Furthermore, a few recent studies have demonstrated the expression of TCF7L2 in human pancreatic islets, the rat pancreatic islet ␤-cell line Ins-1, as well as in human adipocytes (8, 28, 31). Moreover, TCF7L2 may stimulate ␤-cell proliferation (8, 28, 29). These observations suggest that TCF7L2 polymorphisms may directly affect the function of pancreatic ␤-cells. In this review, we will first briefly introduce the structure and function of TCF7L2 as a component of the Wnt signaling pathway. We will then present a summary of studies, confirming the relationship between TCF7L2 SNPs and the development of type 2 diabetes. Third, we will outline our current understanding of the expression and function of TCF7L2 in pancreatic and intestinal endocrine cells. Finally, we will present our own perspective view on this rapidly growing research topic. Reviews on genome-wide association studies of TCF7L2 and type 2 diabetes can be found elsewhere (2–4, 7, 9, 18).

N 2006, GRANT ET AL. (1) demonstrated a strong genetic association between polymorphisms of the TCF7L2 gene (also known as TCF-4) and an increased risk of type 2 diabetes mellitus in Icelandic individuals, a Danish cohort, and a cohort in the United States of America. This discovery rapidly drew attention globally (2–27). Extensive investigations have been conducted not only to assess the effect of TCF7L2 polymorphisms [single-nucleotide polymorphisms (SNPs)] on the risk of type 2 diabetes in other ethnic groups, but also to explore molecular mechanisms of how TCF7L2, as a component of the Wnt signaling pathway, exerts its physiological functions in pancreatic and intestinal endocrine cells (8, 28, 29). Because a previous study showed that TCF7L2 is involved in regulating proglucagon gene transcription and the production of glucagon-like peptide-1 (GLP-1) in the intestinal endocrine L cells (30), it had been suggested that TCF7L2 polymorphisms may increase the risk of type 2 diabetes by affecting the production of the incretin hormone GLP-1 (1). Later investigations First Published Online July 3, 2008 Abbreviations: ␤-cat, ␤-Catenin; CtBP1, C-terminal binding protein 1; GLP-1, glucagon-like peptide-1; GSK-3, glycogen synthase kinase-3; HDAC, histone deacetylase; HMG, high mobility group; SNP, single nucleotide polymorphism. Molecular Endocrinology is published monthly by The Endocrine Society (http://www.endo-society.org), the foremost professional society serving the endocrine community.

Wnt SIGNALING PATHWAY AND FUNCTION OF TCF7L2 The Wnt signaling pathway was initially characterized through studies on colon cancer and embryonic de2383

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velopment in Drosophila, Xenopus, and other organisms (33, 34). Wnt signaling exerts many important physiological and pathophysiological functions in different cell lineages and organs, including organogenesis and the development and progression of tumors (35–40). The key effector of the canonical Wnt signaling pathway (defined as Wnt pathway hereafter) is the bipartite transcription factor ␤-cat (␤-catenin)/TCF, formed by ␤-cat and a member of the TCF family [TCF-1/TCF7, LEF-1, TCF-3/TCF7L1 and TCF-4/ TCF7L2]. The concentration of ␤-cat in cytosol in a resting cell is tightly controlled by the proteasomemediated degradation process through the actions of adenomatous polyposis coli (encoded by the adenomatous polyposis coli gene), axin/conductin, the serine/threonine kinases glycogen synthase kinase-3 (GSK-3), and casein kinase I␣ (41, 42) (Fig. 1A). Wnt glycoproteins, as the ligands, exert their effect via the seven-transmembrane domain frizzled receptors and the LRP5/6 (low-density lipoprotein receptor-related proteins 5 and 6) coreceptors (Fig. 1). After receptor binding, Wnt signals are transmitted by an association between the Wnt receptors and Dishevelled (Dvl), an event that triggers the disruption of the complex that contains adenomatous polyposis coli, axin, GSK-3, and ␤-cat, thus preventing the phosphorylation-dependent degradation of ␤-cat (33). ␤-cat then enters the nucleus to form the ␤-cat/TCF complex and the activation of ␤-cat/ TCF (or Wnt) downstream target genes (38) (Fig. 1B). GSK-3 is an important negative modulator of the Wnt sig-

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naling pathway. Lithium and other inhibitors of GSK-3 have been shown to mimic the function of the Wnt ligands in stimulating the expression of Wnt downstream target genes (Fig. 1B) (43). Although the prototype of the TCF family, TCF7 (TCF1), was originally isolated as a lymphoid transcription factor (44), members of this family are now well recognized to be transcriptional regulators of many developmental processes, and in the adult organism to be functional in many other cell lineages and organs (37). Shortly after the identification of TCF-1/TCF7 (44), Castrop et al. (45) isolated cDNAs for TCF7L1 and TCF7L2, which they called TCF3 and TCF4, respectively. Because the highmobility group (HMG) boxes of the TCF7L1, TCF7L2, and TCF7 sequences show striking similarity, Castrop et al. (45) suggested that they represent a subfamily of TCF7-like HMG box-containing transcription factors. In 2000, Duval and colleagues (46) presented the genomic structure of the human TCF7L2 gene and mapped it to chromosome 10q25.3 by fluorescence in situ hybridization. In the absence of Wnt signaling, these HMG box TCF proteins function in the nucleus as transcriptional repressors of the Wnt target genes (47–50). TCF forms a complex with transcriptional corepressors, including Groucho (51) and C-terminal binding protein 1 (CtBP-1) (52). Both Groucho and CtBP-1 are able to recruit nuclear corepressors, such as histone deacetylases (HDACs) to the promoters of the Wnt target genes (53) (Fig. 1A). ␤-cat, however, converts TCF into a transcriptional

Fig. 1. Summary of Canonical Wnt Signaling Pathway A, In the absence of Wnt stimulation, ␤-cat is phosphorylated by GSK-3, casein kinase I␣, and pERK, and subsequently destroyed by the proteasome-mediated protein degradation process. TCF proteins will bind to the Wnt target gene promoters and repress their expression via recruiting Groucho, CtBP-1, and HDACs. B, After Wnt stimulation, the phosphorylation complex dissembles. Free ␤-cat will be accumulated and form the bipartite transcription factor ␤-cat/TCF, which is able to recruit nuclear coactivators, such as cAMP response element binding protein-binding protein (CBP), leading to enhanced expression of the Wnt target genes. ␤-TrCP, ␤-transducing repeat-containing protein. APC, Adenomatous polyposis coli; CK-1␣, casein kinase I␣; Dvl, dishevelled. pERK, phosphorylated ERK.


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Fig. 2. Structure of the Human TCF7L2 Protein Exact positions of domains that interact with Groucho and CtBP-1 have yet to be further mapped (55). Smad4, the major mediator of the TGF␤ signaling pathway (56). HBP1, HMG-box transcription factor 1, a transcriptional repressor (57). Smad, Smaand Mad-related protein.

activator for the same panel of genes that are repressed by TCF in the absence of ␤-cat (54, 55) (Fig. 1B). In Fig. 2 the functional domains of human TCF7L2 that interact with ␤-cat, DNA, Groucho, and CtBP-1 are illustrated. We have also learned that ␤-cat/TCF interacts with Smad4, an essential mediator of signals initiated by members of the TGF␤ growth factor superfamily through TGF␤ receptors (56). Furthermore, the HMG-box repressor HBP1 inhibits the Wnt signaling cascade by interaction with TCF7L2 (57) (Fig. 2). A dominant-negative form of TCF7L2 (TCF-4), namely Tcf-4⌬N31, was shown to inhibit the ability of constitutively active ␤-cat in the stimulation of Tcf-dependent transcription (58). This dominantnegative molecule was later shown to block both basal and lithium-stimulated proglucagon expression and GLP-1 production in a mouse intestinal endocrine L cell line (30). Korinek et al. (59) generated the TCF7L2⫺/⫺ mice by disrupting one of the exons that encodes the DNA binding HMG box using a conventional gene-targeting approach. These mice die shortly after their birth. Although an apparently normal transition of intestinal endoderm into epithelium occurred at embryonic d 14.5, no proliferative compartments that should have been present in the prospective crypt regions between the villi was found (59), suggesting that the genetic program controlled by TCF7L2 is responsible for maintaining the crypt stem cells of the small intestine (59). No abnormality in the pancreatic islets of the TCF7L2⫺/⫺ or the TCF7L2⫹/⫺ mice was reported, whereas the effect of TCF7L2 deletion on the genesis of intestinal endocrine L cells was not examined (59). TCF7L2⫺/⫺/TCF7⫺/⫺ mice show severe caudal truncations and neural tube duplications (60). However, examination of embryonic d 14.5 embryos showed that most internal organs, including lungs, heart, pancreas, and liver, were clearly distinguished and histologically appeared to be normal, suggesting that Tcf-4 (TCF7L2) and Tcf-1 (TCF7) are not required for the early morphogenesis of these organs (60).

THE RELATIONSHIP BETWEEN TCF7L2 POLYMORPHISMS AND THE RISK OF TYPE 2 DIABETES As early as 1999, Duggirala et al. (61) reported that a region on chromosome 10q was linked to type 2 diabetes in Mexican Americans. Reynisdottir et al. (62) also found evidence for a suggestive linkage of type 2 diabetes to 10q in an Icelandic population. In 2006, Grant et al. (1) reported their discovery of the potential linkage between the polymorphisms in TCF7L2 and the risk of type 2 diabetes. This group genotyped 228 microsatellite markers in Icelandic individuals with type 2 diabetes and healthy controls across a 10.5-Mb interval on chromosome 10q. Microsatellite, DG10S478, located within intron 3 of the TCF7L2 gene, was found to be associated with type 2 diabetes. This observation was replicated in a Danish cohort as well as a U.S. cohort (1). Two SNPs, rs12255372 and rs7903146, were found to be in strong linkage disequilibrium with DG10S478 and also showed similar robust associations with type 2 diabetes (1). When compared with noncarriers, heterozygous and homozygous carriers of the at-risk alleles (38% and 7% of the population, respectively) have relative risks of type 2 diabetes of 1.45 and 2.41, respectively, corresponding to a population-attributable risk of 21% (1). Florez et al. (6) assessed whether the two SNPs, rs12255372 and rs7903146, can predict the progression to diabetes in subjects with impaired glucose tolerance in a diabetes prevention program in which lifestyle intervention or treatment with metformin were compared with placebo. Both SNPs were shown to be associated with an increased risk of developing type 2 diabetes for those with impaired glucose tolerance. Sladek et al. (17) have examined 392,935 SNPs in a French case-control cohort searching for genetic variants that influence the risk of type 2 diabetes. Genetic markers with the highest significant difference in the genotype frequencies between cases of type 2 diabetes and healthy controls were then fast tracked for testing in a second cohort. This group identified five loci that contained variants that confer a risk of devel-

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Fig. 3. The Diagram Shows the Positions of Seven SNPs and the Microsatellite DG10S478 in the Human TCF7L2 Gene Among the seven SNPs, five (in bold) were initially studied by Grant et al. (1). The SNP rs290487 is the one that was identified by Chang et al. (67) in a Chinese population study. The SNP rs11196218 is the one that was identified by Ng et al. (26) in a Hong Kong Chinese study. Chr. 10, Chromosome 10; E1–14, exons 1–14.

oping type 2 diabetes, including confirmation of the association with TCF7L2 (17). Meigs et al. (63) used the Affymetrix 100K SNP array in the 1087 Framingham Offspring Study family members to examine genetic associations with diabetes-related quantitative glucose traits and insulin traits, as well as the risk of type 2 diabetes. Their data replicated the association of TCF7L2 polymorphisms with the risk of type 2 diabetes. To date, most studies with TCF7L2 have focused on the SNPs reported by Grant et al. (1), ignoring those in the remainder of the gene (64). In Asian populations, the frequencies of SNPs rs7903146 and rs12255372 are quite low, although an association of type 2 diabetes with these two SNPs was identified in two large Japanese cohorts (65, 66). Chang et al. (67) genotyped SNPs across the TCF7L2 gene in a Han Chinese population in Taiwan

and identified a novel SNP, rs290487, associated with type 2 diabetes. More recently, Ng et al. (26) demonstrated an association between another novel SNP, rs11196218, and the risk of type 2 diabetes in a Hong Kong Chinese study. Figure 3 presents the overall structure of the human TCF7L2 gene, the position of the microsatellite DG10S478, and the positions of the five SNPs that were initially investigated by Grant et al. (1), as well as two SNPs identified in studies from Taiwan (rs290487) and Hong Kong (rs11196218) (26, 67). The SNPs identified by Grant et al. have been extensively examined by other investigators within different ethnic groups (2–16, 68), and among them, rs12255372 and rs7903146 are most strongly associated with type 2 diabetes (21, 69). Subsequent reports have determined that rs7903146 has the greatest effect in white people (64, 69).


The contribution of TCF7L2 polymorphisms to other metabolic diseases has also been examined, as well as the combined effects of genetic and environmental factors. For example, Duan et al. (68) reported on the combined effect of obesity and genotype at DG10S478 and rs12255372 in predicting the risk of type 2 diabetes in a sample of French Canadian cardiac patients. Sale et al. (70) found that both rs7903146 and rs7901695 are associated with type 2 diabetes in African-American case subjects with type 2-diabetes enriched for nephropathy. Huertas-Vazquez et al. (71) investigated whether TCF7L2 variants contribute to a genetic susceptibility for dislipidemia. They assessed the effect of rs7903146 and rs12255372 on familial combined hyperlipidemia and its component traits triacylglycerol, total cholesterol, and apolipoprotein B in 759 individuals from 55 Mexican families. The data from this study showed a significant association between the two TCF7L2 SNPs with high triacylglycerol in familial combined hyperlipidemia families (71). Finally, an association between TCF7L2 polymorphisms and colorectal tumors has also been recently reported (72, 73).

IV. EXPRESSION AND FUNCTION OF TCF7L2 IN PANCREATIC AND INTESTINAL ENDOCRINE CELLS TCF7L2 in Intestinal Endocrine L Cells In the intestinal endocrine L cells, expression of the proglucagon gene leads to the production of the incretin hormone GLP-1 (74–76). In 2003, Ni et al. (77) examined whether proglucagon is a downstream target of the Wnt signaling pathway. They found that both lithium [which mimics the function of the Wnt ligands (43)] and the constitutively active ␤-cat (the S33Y mutant) stimulated the activity of the proglucagon promoter. Lithium was also shown to stimulate endogenous proglucagon mRNA expression and GLP-1 production in the mouse intestinal GLUTag and STC-1 cell lines, as well as in the fetal rat intestinal cell cultures (77). Yi et al. (30) found that the stimulatory effect of lithium on proglucagon expression occurred in intestinal endocrine L cells, but not in pancreatic ␣-cells. Activation of proglucagon promoter activity is dependent upon a TCF binding site within the G2 enhancer element of the proglucagon gene promoter (30). Because this region has been shown by Furstenau et al. (78) to mediate the stimulatory effects of both cAMP and calcium on proglucagon promoter activity, this observation raises the question as to whether cAMP activates proglucagon expression via cross talking with the Wnt signaling pathway (30, 77). With the approach of chromatin immunoprecipitation, Yi et al. (30) demonstrated an in vivo physical interaction between TCF7L2 and the G2 enhancer element. Western blotting, RT-PCR, and immunostaining, demonstrated that TCF7L2 is abundantly expressed in both cultured intestinal GLP-1 producing cell lines and in-


testinal epithelia of adult mice (30). Furthermore, dominant-negative TCF7L2 attenuated both basal and lithium-stimulated proglucagon mRNA expression in the intestinal endocrine cell line GLUTag (30). In adult mouse pancreatic islets, a TCF7L2 signal was not detected by immunohistochemical staining (30), consistent with a report by Barker et al. (79) in 1999. It is well known that insulin inhibits proglucagon expression in pancreatic ␣-cells (80, 81), and this inhibition is physiologically important because proglucagon expression in the pancreas leads to the production of glucagon, the primary counterregulatory hormone of insulin (80, 81). Yi et al. (82) found that at pathological concentrations, insulin caused a significant stimulation in proglucagon mRNA expression and GLP-1 production in intestinal endocrine L cells. First, they found that insulin used the same cis- and transelements that are employed by the Wnt signaling to stimulate intestinal proglucagon expression. Either knock-down ␤-cat or the expression of a dominantnegative TCF7L2 blocked insulin-stimulated proglucagon mRNA expression in the GLUTag cell line (82). They then showed a stimulatory effect of insulin on the binding of ␤-cat/TCF7L2 to the G2 enhancer element of the proglucagon gene promoter by quantitative chromatin immunoprecipitation. Finally, they observed that in hyperinsulinemic and insulin-resistant MKR mice (83, 84), both intestinal proglucagon mRNA expression and GLP-1 production were significantly higher than that in sex- and age-matched control mice (82). These observations suggest that increased insulin level affect the homeostasis of GLP-1 production via cross talking with the Wnt signaling pathway (82). TCF7L2 in Pancreatic ␤-Cells To investigate how TCF7L2 polymorphisms increase the risk of type 2 diabetes, Schafer et al. (10) genotyped 1100 nondiabetic German participants for the five known TCF7L2 SNPs and conducted oral glucose tolerance tests on these subjects. They then measured GLP-1 secretion and performed iv glucose tolerance tests in a portion of the participants (10). Their results confirmed that TCF7L2 SNPs are associated with reduced insulin secretion. Plasma GLP-1 concentrations during oral glucose tolerance tests, however, were not significantly influenced by the TCF7L2 variants (10). In a study of Scandinavian subjects, Lyssenko et al. (8) found that the CT/TT genotypes of SNP rs7903146 strongly predicted future type 2 diabetes in two independent cohorts. The risk T allele was associated with impaired insulin secretion, incretin effects, and enhanced rate of hepatic glucose production (8). Furthermore, these investigators found that the carriers of the TT alleles showed a 5-fold increase in TCF7L2 mRNA expression in their islets. Although TCF7L2 expression was positively correlated with insulin gene expression, it correlated inversely with glucose-stimulated insulin release (8). These in vivo human subject studies indi-


cate a potential physiological role of TCF7L2 in the pancreatic islet ␤-cells; however, the cause and effect relationship between the expression of TCF7L2 and insulin secretion is far from being clear. For example, could the altered expression of insulin and/or TCF7L2 be due to a compensatory response to insulin resistance? To examine whether TCF7L2 plays a role in pancreatic ␤-cells, Shu et al. (28) exposed isolated human pancreatic islets to a TCF7L2 small interfering RNA. TCF7L2 depletion resulted in a 5.1-fold increase in ␤-cell apoptosis, 2.2-fold decrease in ␤-cell proliferation, and 2.6-fold decrease in glucose-stimulated insulin secretion, and similar effects were seen with TCF7L2 depletion in mouse islets (28). In contrast, TCF7L2 overexpression protected islets from glucose- and cytokine-mediated apoptosis of pancreatic ␤-cells (28). Liu and Habener (29) recently detected the expression of TCF7L2 in the rat pancreatic ␤-cell line Ins-1. Exendin-4 is a long acting analog of the incretin hormone GLP-1, which exerts its stimulatory effect on pancreatic ␤-cell proliferation via binding to the GLP-1 receptor (85). In the TOPGAL transgenic mice, which were generated by Dagupta and Fuchs (86), the expression of the lacZ reporter is under the control of a regulatory sequence consisting of three consensus LEF/TCF binding sites upstream of a minimal c-fos gene promoter (86). Liu and Habener (29) found that islets from TOPGAL mice show increased lacZ expression in response to Exendin-4 treatment, although basal lacZ expression in islets of these mice was shown to be low. This observation would suggest that ␤-cat/TCF, the effector of the Wnt signaling, might have mediated the effect of GLP-1 in stimulating ␤-cell proliferation. By using an artificial ␤-cat/TCF-responsive reporter gene system (TOPFlash), Liu and Habener (29) demonstrated that both ␤-cat and TCF7L2 are involved in mediating the stimulatory effect of GLP-1 on ␤-cat/TCF activity.

SUMMARY AND PERSPECTIVES Although it is clear that TCF7L2 polymorphisms are strongly associated with the risk of type 2 diabetes in different ethnic groups, molecular mechanisms underlying this association are far from understood at this time. Because no risk SNP of type 2 diabetes has been identified within the coding region of TCF7L2, or a region that can be reliably determined to have an effect on the expression of this gene, we still cannot eliminate the possibility that the risk SNPs of TCF7L2 may affect the susceptibility of type 2 diabetes via a mechanism that has nothing to do with the TCF7L2 protein itself. The SNPs may be evolutionarily linked to the inheritance of a genetic defect elsewhere. In the future, more efforts should be made to assess whether polymorphisms within the coding region and the promoter region of TCF7L2 are associated with the risk of type 2 diabetes, other metabolic diseases, and colorectal tumors. In ad-


dition, alternative splicing of TCF7L2 in human colon cancer cell lines has been previously demonstrated (87). Whether some risk SNPs of type 2 diabetes affect TCF7L2 mRNA splicing and whether certain alternatively spliced TCF7L2 products possess deleterious effects on pancreatic and intestinal endocrine cells deserves to be examined. Several recent studies have indicated a potential role of TCF7L2 in human pancreatic ␤-cells and in a rat insulin-producing cell line, Ins-1 (8, 28, 29). A few important issues, however, need to be clarified. First, many of the examinations in these studies, rather than investigating the role of TCF7L2 polymorphisms in the risk of type 2 diabetes, assessed the role of Wnt signaling or the role of the cross talk between Wnt and other signaling pathways [such as GLP-1 signaling (29)] in the pancreatic ␤-cells. During the past few years we have come to recognize the role of the bipartite transcription factor ␤-cat/TCF in the genesis of pancreatic islets and the proliferation of pancreatic ␤-cells through studies utilizing various genetically altered mouse models (88–94). Although an early investigation showed that the loss of ␤-cat did not significantly perturb pancreatic islet endocrine cell mass or function (91), a few recent studies have revealed the complicated role of ␤-cat in the development of pancreatic islets (92–94). Utilizing the Pdx1Cre system to specifically delete the gene that encodes ␤-cat (Ctnnb1), Papadopoulou and Edlund (92) found that Ctnnb1-deleted cells had a competitive disadvantage during pancreas development. Interestingly, although there was a reduction in islet numbers during early embryonic development and the mice developed pancreatitis perinatally because of the disruption of acinar epithelial structure, the mice later recovered from the pancreatitis and regenerated normal pancreas and duodenal villi from the wild-type cells that escaped the ␤-cat deletion (92). These observations indicate that the mouse embryos are capable of overcoming a substantial reduction in ␤-cat via complicated compensatory mechanisms that generate a normal pancreas. Furthermore, to overexpress constitutively active ␤-cat at different developmental stages generated different effects (93). During the early stage of organogenesis, robust expression of constitutively active ␤-cat drives changes in Hedgehog and fibroblast growth factor signaling and blocks the expression of Pdx-1. Expression of the constitutively active ␤-cat at a later time point in pancreas development enhances proliferation and increases the size of this organ (93). Considering that contradictive results have been presented in detecting TCF7L2 expression in pancreatic islets (28–30, 79), detailed reassessment on the expression and participation of TCF7L2 during different developmental stages of pancreatic islets need to be conducted. Second, the activity of Wnt signaling could be attenuated during aging, due to the fact that FOXO proteins compete with TCF factors for the limited pool of free ␤-cat (32). Therefore, to understand the role of


TCF7L2 and other components of the Wnt signaling pathway in preventing the development of type 2 diabetes, it is necessary to assess the expression and function of TCF7L2 as well as other Wnt signaling components in pancreatic islets during aging. Third, from data presented by Shu et al. (28), it appears that TCF7L2 is expressed in only select insulin-producing cells. This raises the question of whether in other insulin-producing ␤-cells, other TCF factors (TCF-1/TCF7, LEF-1, TCF-3/TCF7L1) function as the partner of ␤-cat in mediating the effects of the Wnt signaling. Finally, in the study presented by Shu et al. (28), TCF7L2 depletion and TCF7L2 overexpression were shown to generate opposite effects in human pancreatic ␤-cells. This raises the question whether TCF7L2 in the pancreatic ␤-cells is, in fact, not a transcriptional modulator but a transcriptional activator. Early studies have shown that overexpression of a TCF factor would repress Wnt target gene expression, because without an interaction with ␤-cat, TCF factors recruit transcriptional corepressors including Groucho and CtBP-1, which leads to further recruitment of nuclear corepressors, such as HDACs (53–55). Whether TCF7L2 in pancreatic ␤-cells activates proliferative signaling without the participation of ␤-cat also deserves to be investigated. Schafer et al. (10) found that in nondiabetic TCF7L2 SNP carriers, although GLP-1 stimulated insulin secretion is reduced, GLP-1 levels after glucose challenge are within the normal range. Can this observation eliminate the possibility of a defect in GLP-1 production in the TCF7L2 SNP carriers? Schafer et al. have commented “… it cannot fully exclude an effect of these SNPs on GLP-1 levels. First, by measuring total GLP-1 levels, we may have missed a subtle defect in GLP-1 secretion, which may have been detected by measuring the active form of GLP-1. Second, systemic GLP-1 level may not adequately reflect the level of the active hormone acting in the gut wall on the autonomic nervous system. Third, impaired TCF7L2 activity might tissue-specifically reduce the GLP-1 levels in the brain, which are believed as well to be important for insulin secretion.” All of these are valid points. In addition, our knowledge about GLP-1 production and homeostasis of GLP-1 production, as well as its secretion in normal and diabetic subjects, needs to be further expanded. As mentioned above, we have learned recently that in a hyperinsulinemic and insulin-resistant mouse model, the MRK mouse, both intestinal proglucagon mRNA expression and GLP-1 production were significantly higher than that of control mice (82). These observations would suggest that during certain prediabetic status, that the existence of yet to be determined compensatory mechanism/s enables the body to produce sufficient amounts of GLP-1. To determine whether TCF7L2 polymorphisms affect the risk of type 2 diabetes via altering GLP-1 production, it would be necessary to examine both GLP-1


secretion and production during prediabetic as well as diabetic status in the TCF7L2 SNP carriers. Acknowledgments We thank Drs. Donald Branch, David Irwin, and Anthony Hanley for critical reading of the manuscript. Tianru Jin would like to thank Banting and Best Diabetes Centre for supporting his team in studying the role of the Wnt pathway in intestinal proglucagon expression and GLP-1 production.

Received April 22, 2008. Accepted June 23, 2008. Address all correspondence and requests for reprints to Tianru Jin: Room 10-354, Tenth Floor, Toronto Medical Discovery Tower, the MaRS Centre, University Health Network, 101 College Street, Toronto, Ontario, Canada M5G 1L7. E-mail: tianru.jin@utoronto.ca. This work was supported by Grant 68991 from the Canadian Institutes of Health Research (to T.J.). Disclosure Statement: The authors have nothing to disclose.

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