The frizzled-related sFRP2 gene is a target of thyroid hormone receptor alpha1 and activates beta-catenin signaling in mouse intestine

All-trans retinoic acid modulates Wnt3A-induced osteogenic differentiation of mesenchymal stem cells via activating the PI3K/AKT/GSK3β signalling pathway

Osteogenic differentiation of mesenchymal stem cells (MSCs) is a vital process for the maintenance of healthy bone tissue and is mediated by numerous factors. Canonical Wnt signalling is essential for MSC osteogenic differentiation, and it interacts with several nuclear receptors, including the retinoic acid receptor, vitamin D receptor, and glucocorticoid receptor. Here, we explored whether Wnt3A and all-trans-retinoic acid (ATRA) play synergistic roles in MSC osteogenic differentiation. We found that ATRA potentiated the Wnt3A-induced expression of early and late osteogenic markers as well as matrix mineralization and further confirmed the phenomena using foetal limb explant culture and MSC implantation experiments. Mechanistically, ATRA cooperated with Wnt3A to induce β-catenin translocation from cell-cell contacts into the cytosol and nucleus, thereby activating Wnt/β-catenin signalling. Additionally, Wnt3A attenuated ATRA-induced Cyp26a1 expression, inhibiting the degradation of ATRA into its oxidative forms. β-catenin silencing abolished the stimulatory effect of ATRA on Wnt3A-induced alkaline phosphatase (ALP) activity and reversed its inhibitory effect on Cyp26a1 expression. Furthermore, ATRA and Wnt3A synergistically promoted AKT phosphorylation, enhancing β-catenin-dependent transcription through GSK3β inhibition or direct β-catenin phosphorylation at Ser552. This event was largely abolished by LY294002 pre-treatment, suggesting that ATRA and Wnt3A at least partially promote osteogenic differentiation via activating the PI3K/AKT/GSK3β signalling pathway. Thus, crosstalk between the Wnt/β-catenin and retinoic acid signalling pathways may be an effective therapeutic target for bone diseases, such as osteoporosis.

Thyroid Hormone Receptor α Plays an Essential Role in Male Skeletal Muscle Myoblast Proliferation, Differentiation, and Response to Injury

Thyroid hormone plays an essential role in myogenesis, the process required for skeletal muscle development and repair, although the mechanisms have not been established. Skeletal muscle develops from the fusion of precursor myoblasts into myofibers. We have used the C2C12 skeletal muscle myoblast cell line, primary myoblasts, and mouse models of resistance to thyroid hormone (RTH) α and β, to determine the role of thyroid hormone in the regulation of myoblast differentiation. T3, which activates thyroid hormone receptor (TR) α and β, increased myoblast differentiation whereas GC1, a selective TRβ agonist, was minimally effective. Genetic approaches confirmed that TRα plays an important role in normal myoblast proliferation and differentiation and acts through the Wnt/β-catenin signaling pathway. Myoblasts with TRα knockdown, or derived from RTH-TRα PV (a frame-shift mutation) mice, displayed reduced proliferation and myogenic differentiation. Moreover, skeletal muscle from the TRα1PV mutant mouse had impaired in vivo regeneration after injury. RTH-TRβ PV mutant mouse model skeletal muscle and derived primary myoblasts did not have altered proliferation, myogenic differentiation, or response to injury when compared with control. In conclusion, TRα plays an essential role in myoblast homeostasis and provides a potential therapeutic target to enhance skeletal muscle regeneration.

Molecular and cytological analyses reveal distinct transformations of intestinal epithelial cells during Xenopus metamorphosis

Background

The thyroid hormone (T3)-induced formation of adult intestine during amphibian metamorphosis resembles the maturation of the mammalian intestine during postembryonic development, the period around birth when plasma T3 level peaks. This process involves de novo formation of adult intestinal stem cells as well as the removal of the larval epithelial cells through apoptosis. Earlier studies have revealed a number of cytological and molecular markers for the epithelial cells undergoing different changes during metamorphosis. However, the lack of established double labeling has made it difficult to ascertain the identities of the metamorphosing epithelial cells.

Results

Here, we carried out different double-staining with a number of cytological and molecular markers during T3-induced and natural metamorphosis in Xenopus laevis. Our studies demonstrated conclusively that the clusters of proliferating cells in the epithelium at the climax of metamorphosis are undifferentiated epithelial cells and express the well-known adult intestinal stem cell marker gene Lgr5. We further show that the adult stem cells and apoptotic larval epithelial cells are distinct epithelial cells during metamorphosis.

Conclusions

Our findings suggest that morphologically identical larval epithelial cells choose two alternative paths: programmed cell death or dedifferentiation to form adult stem cells, in response to T3 during metamorphosis with apoptosis occurring prior to the formation of the proliferating adult stem cell clusters (islets).

Orchestrating change: The thyroid hormones and GI-tract development in flatfish metamorphosis

Metamorphosis in flatfish (Pleuronectiformes) is a late post-embryonic developmental event that prepares the organism for the larval-to-juvenile transition. Thyroid hormones (THs) play a central role in flatfish metamorphosis and the basic elements that constitute the thyroid axis in vertebrates are all present at this stage. The advantage of using flatfish to study the larval-to-juvenile transition is the profound change in external morphology that accompanies metamorphosis making it easy to track progression to climax. This important lifecycle transition is underpinned by molecular, cellular, structural and functional modifications of organs and tissues that prepare larvae for a successful transition to the adult habitat and lifestyle. Understanding the role of THs in the maturation of organs and tissues with diverse functions during metamorphosis is a major challenge. The change in diet that accompanies the transition from a pelagic larvae to a benthic juvenile in flatfish is associated with structural and functional modifications in the gastrointestinal tract (GI-tract). The present review will focus on the maturation of the GI-tract during metamorphosis giving particular attention to organogenesis of the stomach a TH triggered event. Gene transcripts and biological processes that are associated with GI-tract maturation during Atlantic halibut metamorphosis are identified. Gene ontology analysis reveals core biological functions and putative TH-responsive genes that underpin TH-driven metamorphosis of the GI-tract in Atlantic halibut. Deciphering the specific role remains a challenge. Recent advances in characterizing the molecular, structural and functional modifications that accompany the appearance of a functional stomach in Atlantic halibut are considered and future research challenges identified.

Direct Activation of Amidohydrolase Domain-Containing 1 Gene by Thyroid Hormone Implicates a Role in the Formation of Adult Intestinal Stem Cells During Xenopus Metamorphosis

The T3-dependent anuran metamorphosis resembles postembryonic development in mammals, the period around birth when plasma T3 levels peak. In particular, the remodeling of the intestine during metamorphosis mimics neonatal intestinal maturation in mammals when the adult intestinal epithelial self-renewing system is established. We have been using intestinal metamorphosis to investigate how the organ-specific adult stem cells are formed during vertebrate development. Early studies in Xenopus laevis have shown that this process involves complete degeneration of the larval epithelium and de novo formation of adult stem cells. A tissue-specific microarray analysis of intestinal gene expression during Xenopus laevis metamorphosis has identified a number of candidate stem cell genes. Here we have carried out detailed analyses of one such gene, amidohydrolase domain containing 1 (AMDHD1) gene, which encodes an enzyme in the histidine catabolic pathway. We show that AMDHD1 is exclusively expressed in the proliferating adult epithelial stem cells during metamorphosis with little expression in other intestinal tissues. We further provide evidence that T3 activates AMDHD1 gene expression directly at the transcription level through T3 receptor binding to the AMDHD1 gene in the intestine. In addition, we have reported earlier that histidine ammonia-lyase gene, another gene in histidine catabolic pathway, is similarly regulated by T3 in the intestine. These results together suggest that histidine catabolism plays a critical role in the formation and/or proliferation of adult intestinal stem cells during metamorphosis.

The thyroid hormone nuclear receptor TRα1 controls the Notch signaling pathway and cell fate in murine intestine

Thyroid hormones control various aspects of gut development and homeostasis. The best-known example is in gastrointestinal tract remodeling during amphibian metamorphosis. It is well documented that these hormones act via the TR nuclear receptors, which are hormone-modulated transcription factors. Several studies have shown that thyroid hormones regulate the expression of several genes in the Notch signaling pathway, indicating a possible means by which they participate in the control of gut physiology. However, the mechanisms and biological significance of this control have remained unexplored. Using multiple in vivo and in vitro approaches, we show that thyroid hormones positively regulate Notch activity through the TRα1 receptor. From a molecular point of view, TRα1 indirectly controls Notch1, Dll1, Dll4 and Hes1 expression but acts as a direct transcriptional regulator of the Jag1 gene by binding to a responsive element in the Jag1 promoter. Our findings show that the TRα1 nuclear receptor plays a key role in intestinal crypt progenitor/stem cell biology by controlling the Notch pathway and hence the balance between cell proliferation and cell differentiation.

Tissue-nonspecific alkaline phosphatase as a target of sFRP2 in cardiac fibroblasts

Recent studies of myocardial infarction in secreted Frizzled-related protein 2 (sFRP2) knockout mice and our hamster heart failure therapy based on sFRP2 blockade have established sFRP2 as a key profibrotic cytokine in the heart. The failing hamster heart is marked by prominent fibrosis and calcification with elevated expression of sFRP2. Noting the involvement of tissue-nonspecific alkaline phosphatase (TNAP) in bone mineralization and vascular calcification, we determined whether sFRP2 might be an upstream regulator of TNAP. Biochemical assays revealed an approximately twofold increase in the activity of TNAP and elevated levels of inorganic phosphate (Pi) in the failing heart compared with the normal heart. Neither was this change detected in the liver or hamstring muscle nor was it associated with systemic hyperphosphatemia. TNAP was readily cloned from the hamster heart and upon overexpression increased the level of extracellular but not intracellular Pi, which is consistent with the cell surface location of the ectoenzyme. In line with the previous demonstration that sFRP2 blockade attenuated fibrosis, we show here that the therapy downregulated TNAP. This in vivo finding is corroborated by the in vitro study showing that cultured cardiac fibroblasts treated with recombinant sFRP2 protein exhibited progressive increase in the expression and activity of TNAP, which was completely abrogated by cycloheximide or tunicamycin. Induction of TNAP by sFRP2 is restricted to cardiac fibroblasts among the multiple cell types examined, and was not observed with sFRP4. The current work indicates that sFRP2 may promote cardiac fibrocalcification through coordinate activation of tolloid-like metalloproteinases and TNAP.

Chronic Ethanol-Induced Impairment of Wnt/β-Catenin Signaling is Attenuated by PPAR-δ Agonist

BACKGROUND

The Wnt/β-catenin pathway regulates liver growth, repair, and regeneration. Chronic ethanol (EtOH) exposure blunts normal liver regenerative responses, in part by inhibiting insulin/IGF signaling, and correspondingly, previous studies showed that EtOH-impaired liver regeneration could be restored by insulin sensitizer (proliferator-activated receptor [PPAR]-δ agonist) treatment. As Wnt/β-catenin functions overlap and cross talk with insulin/IGF pathways, we investigated the effects of EtOH exposure and PPAR-δ agonist treatment on Wnt pathway gene expression in relation to liver regeneration.

METHODS

Adult male Long Evans rats were fed with isocaloric liquid diets containing 0 or 37% EtOH for 8 weeks and also treated with vehicle or a PPAR-δ agonist during the last 3 weeks of the feeding regimen. The rats were then subjected to 70% partial hepatectomy (PH) and livers harvested at various post-PH time points were used to quantitate expression of 19 Wnt pathway genes using Quantigene 2.0 Multiplex Assay.

RESULTS

EtOH broadly inhibited expression of Wnt/β-catenin signaling-related genes, including down-regulation of Wnt1, Fzd3, Lef1, and Bcl9 throughout the post-PH time course (0 to 72 hours), and suppression of Wnt7a, Ccnd1, Fgf4, Wif1, Sfrp2, and Sfrp5 at 18- and 24-hour post-PH time points. PPAR-δ agonist treatments rescued the EtOH-induced suppression of Wnt1, Wnt7a, Fzd3, Lef1, Bcl9, Ccnd1, and Sfrp2 gene expression in liver, corresponding with the improvements in DNA synthesis and restoration of hepatic architecture.

CONCLUSIONS

Chronic high-dose EtOH exposures inhibit Wnt signaling, which likely contributes to the impairments in liver regeneration. Therapeutic effects of PPAR-δ agonists extend beyond restoration of insulin/IGF signaling mechanisms and are mediated in part by enhancement of Wnt pathway signaling. Future studies will determine the degree to which targeted restoration of Wnt signaling is sufficient to improve liver regeneration and remodeling in the context of chronic EtOH exposure.

Number and brightness analysis of sFRP4 domains in live cells demonstrates vesicle association signal of the NLD domain and dynamic intracellular responses to Wnt3a

The Wnts are secreted, lipidated glycoproteins that play a role in cellular processes of differentiation, proliferation, migration, survival, polarity and stem cell self-renewal. The majority of Wnts biological effects are through binding to specific frizzled (Fzd) receptor complexes leading to activation of downstream pathways. Secreted frizzled-related proteins (sFRPs) were first identified as antagonists of Wnt signalling by binding directly to Wnts. They comprise two domains, a Fzd-like cysteine rich domain (CRD) and a netrin-like domain (NLD). Subsequently sFRPs have been shown to also interact with Fzd receptors and more diverse functions have been identified, including potentiation of Wnt signalling. Many aspects of the biology of this family remain to be elucidated. We used the number and brightness (N&B) method, a technique based on fluorescence fluctuation analysis, to characterise the intracellular aggregation and trafficking of sFRP4 domains. We expressed sFRP4 and its' domains as EGFP fusions and then characterised the effect of endogenous Wnt3a by fluorescence confocal imaging. We observed vesicular trafficking of sFRP4 and that the NLD domain has a vesicular association signal. We found that sFRP4 and the CRD formed oligomeric aggregates in the perinuclear region while the NLD was distributed evenly throughout the cell with a larger proportion of aggregates. Most significantly we observed intracellular redistribution of sFRP4 in response to Wnt3a suggesting that Wnt3a can modulate intracellular localisation and secretion of sFRP4. Our results reveal a number of novel findings regarding sFRP4 which are likely to have relevance to this wider family.

Activation of Sox3 gene by thyroid hormone in the developing adult intestinal stem cell during Xenopus metamorphosis

The maturation of the intestine into the adult form involves the formation of adult stem cells in a thyroid hormone (T3)-dependent process in vertebrates. In mammals, this takes place during postembryonic development, a period around birth when the T3 level peaks. Due to the difficulty of manipulating late-stage, uterus-enclosed embryos, very little is known about the development of the adult intestinal stem cells. Interestingly, the remodeling of the intestine during the T3-dependent amphibian metamorphosis mimics the maturation of mammalian intestine. Our earlier microarray studies in Xenopus laevis revealed that the transcription factor SRY (sex-determining region Y)-box 3 (Sox3), well known for its involvement in neural development, was upregulated in the intestinal epithelium during metamorphosis. Here, we show that Sox3 is highly and specifically expressed in the developing adult intestinal progenitor/stem cells. We further show that its induction by T3 is independent of new protein synthesis, suggesting that Sox3 is directly activated by liganded T3 receptor. Thus, T3 activates Sox3 as one of the earliest changes in the epithelium, and Sox3 in turn may facilitate the dedifferentiation of the larval epithelial cells into adult stem cells.

Thyroid hormone-regulated Wnt5a/Ror2 signaling is essential for dedifferentiation of larval epithelial cells into adult stem cells in the Xenopus laevis intestine

Background and Aims

Amphibian intestinal remodeling, where thyroid hormone (T3) induces some larval epithelial cells to become adult stem cells analogous to the mammalian intestinal ones, serves as a unique model for studying how the adult stem cells are formed. To clarify its molecular mechanisms, we here investigated roles of non-canonical Wnt signaling in the larval-to-adult intestinal remodeling during Xenopus laevis metamorphosis.

Methods/Findings

Our quantitative RT-PCR (qRT-PCR) and immunohistochemical analyses indicated that the expressions of Wnt5a and its receptors, frizzled 2 (Fzd2) and receptor tyrosine kinase-like orphan receptor 2 (Ror2) are up-regulated by T3 and are spatiotemporally correlated with adult epithelial development in the X. laevis intestine. Notably, changes in morphology of larval absorptive epithelial cells expressing Ror2 coincide well with formation of the adult stem cells during metamorphosis. In addition, by using organ cultures of the tadpole intestine, we have experimentally shown that addition of exogenous Wnt5a protein to the culture medium causes morphological changes in the larval epithelium expressing Ror2 even in the absence of T3. In contrast, in the presence of T3 where the adult stem cells are formed in vitro, inhibition of endogenous Wnt5a by an anti-Wnt5a antibody suppressed the epithelial morphological changes, leading to the failure of stem cell formation.

Significance

Our findings strongly suggest that the adult stem cells originate from the larval absorptive cells expressing Ror2, which require Wnt5a/Ror2 signaling for their dedifferentiation accompanied by changes in cell morphology.

Thyroid hormones and their nuclear receptors: new players in intestinal epithelium stem cell biology?

Thyroid hormones participate in the development and homeostasis of several organs and tissues. It is well documented that they act via nuclear receptors, the TRs, which are transcription factors whose function is modulated by the hormone T3. Importantly, T3-induced physiological response within a cell depends on the specific TR expression and on the T3 bioavailability. However, in addition to this T3-dependent control of TR functionality, increasing data show that the action of TRs is coordinated and integrated with other signaling pathways, specifically at the level of stem/progenitor cell populations. By focusing on the intestinal epithelium of both amphibians and mammals we summarize here new data in support of a role for thyroid hormones and the TR nuclear receptors in stem cell biology. This new concept may be extended to other organs and have biological relevance in therapeutic approaches aimed to target stem cells such as tissue engineering and cancer.

Chondrocytes play a major role in the stimulation of bone growth by thyroid hormone

Thyroid hormone (T3) is required for postnatal skeletal growth. It exerts its effect by binding to nuclear receptors, TRs including TRα1 and TRβ1, which are present in most cell types. These cell types include chondrocytes and osteoblasts, the interactions of which are known to regulate endochondral bone formation. In order to analyze the respective functions of T3 stimulation in chondrocytes and osteoblasts during postnatal growth, we use Cre/loxP recombination to express a dominant-negative TRα1(L400R) mutant receptor in a cell-specific manner. Phenotype analysis revealed that inhibiting T3 response in chondrocytes is sufficient to reproduce the defects observed in hypothyroid mice, not only for cartilage maturation, but also for ossification and mineralization. TRα1(L400R) in chondrocytes also results in skull deformation. In the meantime, TRα1(L400R) expression in mature osteoblasts has no visible effect. Transcriptome analysis identifies a number of changes in gene expression induced by TRα1(L400R) in cartilage. These changes suggest that T3 normally cross talks with several other signaling pathways to promote chondrocytes proliferation, differentiation, and skeletal growth.

Knockdown of type 3 iodothyronine deiodinase severely perturbs both embryonic and early larval development in zebrafish

Exposure to appropriate levels of thyroid hormones (THs) at the right time is of key importance for normal development in all vertebrates. Type 3 iodothyronine deiodinase (D3) is the prime TH-inactivating enzyme, and its expression is highest in the early stages of vertebrate development, implying that it may be necessary to shield developing tissues from overexposure to THs. We used antisense morpholino knockdown to examine the role of D3 during early development in zebrafish. Zebrafish possess 2 D3 genes, dio3a and dio3b. Here, we show that both genes are expressed during development and both contribute to in vivo D3 activity. However, dio3b mRNA levels in embryos are higher, and the effects of dio3b knockdown on D3 activity and on the resulting phenotype are more severe. D3 knockdown induced an overall delay in development, as determined by measurements of otic vesicle length, eye and ear size, and body length. The time of hatching was also severely delayed in D3-knockdown embryos. Importantly, we also observed a severe disturbance of several aspects of development. Swim bladder development and inflation was aberrant as was the development of liver and intestine. Furthermore, D3-knockdown larvae spent significantly less time moving, and both embryos and larvae exhibited perturbed escape responses, suggesting that D3 knockdown affects muscle development and/or functioning. These data indicate that D3 is essential for normal zebrafish embryonic and early larval development and show the value of morpholino knockdown in this model to further elucidate the specific role of D3 in some aspects of vertebrate development.

Expression of sfrp2 is increased in catagen of hair follicles and inhibits keratinocyte proliferation

Background

Hair follicles undergo cycles of repeated growth and regression. The Wnt pathway plays an important role in the regeneration and differentiation of hair follicles. Sfrp2, a Wnt inhibitor, is involved in the developmental and disease processes of various cells and tissues by modulating the Wnt pathway.

Objective

The aim of this study was to understand the role of Sfrp2 in hair follicles through investigation of the Sfrp2 expression pattern in the skin and its effect on keratinocytes.

Methods

We investigated Sfrp2 mRNA expression and the expression of the wnt target genes, Ccnd1 and C-myc, at various mouse hair follicle developmental stages using Real-time polymerase chain reaction. We also investigated the effect of SFRP2 on the proliferation and differentiation of mouse keratinocyte cells by adding SFRP2 protein or overexpressing Sfrp2 using an in vitro culture system.

Results

Sfrp2 expression peaked in the catagen phase and remained high until telogen, and then declined at the beginning of the next anagen. An inverse relationship to Sfrp2 expression was found for the expression of the Wnt target genes, C-myc and Ccnd1. In addition, we also observed inhibited proliferation of mouse keratinocytes in the presence of SFRP2.

Conclusion

These results suggest that Sfrp2 may play a role in the catagen phase by inhibiting the proliferation of keratinocyte and functioning as a Wnt inhibitor in keratinocytes.

Secreted Frizzled-related protein 2 as a target in antifibrotic therapeutic intervention

Progressive fibrosis is a pathological hallmark of many chronic diseases responsible for organ failure. Although there is currently no therapy on the market that specifically targets fibrosis, the dynamic fibrogenic process is known to be regulated by multiple soluble mediators that may be therapeutically intervened. The failing hamster heart exhibits marked fibrosis and increased expression of secreted Frizzled-related protein 2 (sFRP2) amenable to reversal by mesenchymal stem cell (MSC) therapy. Given the previous demonstration that sFRP2-null mice subjected to myocardial infarction exhibited reduced fibrosis and improved function, we tested whether antibody-based sFRP2 blockade might counteract the fibrogenic pathway and repair cardiac injury. Cardiomyopathic hamsters were injected intraperitoneally twice a week each with 20 μg of sFRP2 antibody. Echocardiography, histology, and biochemical analyses were performed after 1 mo. sFRP2 antibody increased left ventricular ejection fraction from 40 ± 1.2 to 49 ± 6.5%, whereas saline and IgG control exhibited a further decline to 37 ± 0.9 and 31 ± 3.2%, respectively. Functional improvement is associated with a ∼ 50% reduction in myocardial fibrosis, ∼ 65% decrease in apoptosis, and ∼ 75% increase in wall thickness. Consistent with attenuated fibrosis, both MSC therapy and sFRP2 antibody administration significantly increased the activity of myocardial matrix metalloproteinase-2. Gene expression analysis of the hamster heart and cultured fibroblasts identified Axin2 as a downstream target, the expression of which was activated by sFRP2 but inhibited by therapeutic intervention. sFRP2 blockade also increased myocardial levels of VEGF and hepatocyte growth factor (HGF) along with increased angiogenesis. These findings highlight the pathogenic effect of dysregulated sFRP2, which may be specifically targeted for antifibrotic therapy.

Differential regulation of two histidine ammonia-lyase genes during Xenopus development implicates distinct functions during thyroid hormone-induced formation of adult stem cells

Background

Organ-specific, adult stem cells are essential for organ-homeostasis and tissue repair and regeneration. The formation of such stem cells during vertebrate development remains to be investigated. Frog metamorphosis offers an excellent opportunity to study the formation of adult stem cells as this process involves essentially the transformations of all larval tissues/organs into the adult form. Of particular interest is the remodeling of the intestine. Early studies in Xenopus laevis have shown that this process involves complete degeneration of the larval epithelium and de novo formation of adult stem cells through dedifferentiation of some larval epithelial cells. A major advantage of this metamorphosis model is its total dependence on thyroid hormone (T3). In an effort to identify genes that are important for stem cell development, we have previously carried out tissue-specific microarray analysis of intestinal gene expression during Xenopus laevis metamorphosis.

Results

We report the detailed characterization of one of the genes thus identified, the histidine ammonia-lyase (HAL) gene, which encodes an enzyme known as histidase or histidinase. We show that there are two duplicated HAL genes, HAL1 and HAL2, in both Xenopus laevis and Xenopus tropicalis, a highly related but diploid species. Interestingly, only HAL2 is highly upregulated by T3 and appears to be specifically expressed in the adult intestinal progenitor/stem cells while HAL1 is not expressed in the intestine during metamorphosis. Furthermore, when analyzed in whole animals, HAL1 appears to be expressed only during embryogenesis but not metamorphosis while the opposite appears to be true for HAL2.

Conclusions

Our results suggest that the duplicated HAL genes have distinct functions with HAL2 likely involved in the formation and/or proliferation of the adult stem cells during metamorphosis.

Defining suitable reference genes for RT-qPCR analysis on intestinal epithelial cells

The study of the mammalian intestinal epithelium concerns several aspects of cellular and molecular biology. In fact, most of these studies aim to define molecular components or mechanisms related with the control of stemness and the balance between cell proliferation and differentiation in physiopathological conditions. It is worth mentioning that real-time quantitative reverse transcription-polymerase chain reaction (RT-qPCR) approaches are commonly used, but only a few studies are available regarding suitable reference genes to normalize gene expression data. The present study was designed to validate potential reference genes in freshly isolated proliferating or differentiated epithelial cells from the mouse intestine. We also extended our analysis to the IEC6 intestinal epithelial cells, as a promising model to study intestinal physiopathology in vitro. The stability of six potential reference genes (Hprt1, Ppia, Gapdh, Rplp0, Ppib, and Vil1) has been tested both in epithelial cells isolated from the mouse intestine and in the IEC6 cell line. The software programs-geNorm and Normfinder-were used to obtain an estimation of the expression stability of each gene and, by comparing the results, to identify the most suitable genes for RT-qPCR data normalization. These multiple approaches allowed us to select different suitable reference genes for the correct quantification of mRNAs depending on the differentiated or proliferative nature of the cells.

Expression profiling of intestinal tissues implicates tissue-specific genes and pathways essential for thyroid hormone-induced adult stem cell development

The study of the epithelium during development in the vertebrate intestine touches upon many contemporary aspects of biology: to name a few, the formation of the adult stem cells (ASCs) essential for the life-long self-renewal and the balance of stem cell activity for renewal vs cancer development. Although extensive analyses have been carried out on the property and functions of the adult intestinal stem cells in mammals, little is known about their formation during development due to the difficulty of manipulating late-stage, uterus-enclosed embryos. The gastrointestinal tract of the amphibian Xenopus laevis is an excellent model system for the study of mammalian ASC formation, cell proliferation, and differentiation. During T3-dependent amphibian metamorphosis, the digestive tract is extensively remodeled from the larval to the adult form for the adaptation of the amphibian from its aquatic herbivorous lifestyle to that of a terrestrial carnivorous frog. This involves de novo formation of ASCs that requires T3 signaling in both the larval epithelium and nonepithelial tissues. To understand the underlying molecular mechanisms, we have characterized the gene expression profiles in the epithelium and nonepithelial tissues by using cDNA microarrays. Our results revealed that T3 induces distinct tissue-specific gene regulation programs associated with the remodeling of the intestine, particularly the formation of the ASCs, and further suggested the existence of potentially many novel stem cell-associated genes, at least in the intestine during development.

Secreted Frizzled-related protein potentiation versus inhibition of Wnt3a/β-catenin signaling

Wnt signaling regulates a variety of cellular processes during embryonic development and in the adult. Many of these activities are mediated by the Frizzled family of seven-pass transmembrane receptors, which bind Wnts via a conserved cysteine-rich domain (CRD). Secreted Frizzled-related proteins (sFRPs) contain an amino-terminal, Frizzled-like CRD and a carboxyl-terminal, heparin-binding netrin-like domain. Previous studies identified sFRPs as soluble Wnt antagonists that bind directly to Wnts and prevent their interaction with Frizzleds. However, subsequent observations suggested that sFRPs and Frizzleds form homodimers and heterodimers via their respective CRDs, and that sFRPs can stimulate signal transduction. Here, we present evidence that sFRP1 either inhibits or enhances signaling in the Wnt3a/β-catenin pathway, depending on its concentration and the cellular context. Nanomolar concentrations of sFRP1 increased Wnt3a signaling, while higher concentrations blocked it in HEK293 cells expressing a SuperTopFlash reporter. sFRP1 primarily augmented Wnt3a/β-catenin signaling in C57MG cells, but it behaved as an antagonist in L929 fibroblasts. sFRP1 enhanced reporter activity in L cells that were engineered to stably express Frizzled 5, though not Frizzled 2. This implied that the Frizzled expression pattern could determine the response to sFRP1. Similar results were obtained with sFRP2 in HEK293, C57MG and L cell reporter assays. CRDsFRP1 mimicked the potentiating effect of sFRP1 in multiple settings, contradicting initial expectations that this domain would inhibit Wnt signaling. Moreover, CRDsFRP1 showed little avidity for Wnt3a compared to sFRP1, implying that the mechanism for potentiation by CRDsFRP1 probably does not require an interaction with Wnt protein. Together, these findings demonstrate that sFRPs can either promote or suppress Wnt/β-catenin signaling, depending on cellular context, concentration and most likely the expression pattern of Fzd receptors.

The role of thyroid hormone signaling in the prevention of digestive system cancers

Thyroid hormones play a critical role in the growth and development of the alimentary tract in vertebrates. Their effects are mediated by nuclear receptors as well as the cell surface receptor integrin αVβ3. Systemic thyroid hormone levels are controlled via activation and deactivation by iodothyronine deiodinases in the liver and other tissues. Given that thyroid hormone signaling has been characterized as a major effector of digestive system growth and homeostasis, numerous investigations have examined its role in the occurrence and progression of cancers in various tissues of this organ system. The present review summarizes current findings regarding the effects of thyroid hormone signaling on cancers of the esophagus, stomach, liver, pancreas, and colon. Particular attention is given to the roles of different thyroid hormone receptor isoforms, the novel integrin αVβ3 receptor, and thyroid hormone-related nutrients as possible protective agents and therapeutic targets. Future investigations geared towards a better understanding of thyroid hormone signaling in digestive system cancers may provide preventive or therapeutic strategies to diminish risk, improve outcome and avert recurrence in afflicted individuals.

Thyroid hormone actions in liver cancer

The thyroid hormone 3,3',5-triiodo-L-thyronine (T3) mediates several physiological processes, including embryonic development, cellular differentiation, metabolism, and the regulation of cell proliferation. Thyroid hormone receptors (TRs) generally act as heterodimers with the retinoid X receptor (RXR) to regulate target genes. In addition to their developmental and metabolic functions, TRs have been shown to play a tumor suppressor role, suggesting that their aberrant expression can lead to tumor transformation. Conversely, recent reports have shown an association between overexpression of wild-type TRs and tumor metastasis. Signaling crosstalk between T3/TR and other pathways or specific TR coregulators appear to affect tumor development. Since TR actions are complex as well as cell context-, tissue- and time-specific, aberrant expression of the various TR isoforms has different effects during diverse tumorigenesis. Therefore, elucidation of the T3/TR signaling mechanisms in cancers should facilitate the identification of novel therapeutic targets. This review provides a summary of recent studies focusing on the role of TRs in hepatocellular carcinomas (HCCs).

Thyroid hormone-induced cell-cell interactions are required for the development of adult intestinal stem cells

The mammalian intestine has long been used as a model to study organ-specific adult stem cells, which are essential for organ repair and tissue regeneration throughout adult life. The establishment of the intestinal epithelial cell self-renewing system takes place during perinatal development when the villus-crypt axis is established with the adult stem cells localized in the crypt. This developmental period is characterized by high levels of plasma thyroid hormone (T3) and T3 deficiency is known to impair intestinal development. Determining how T3 regulates adult stem cell development in the mammalian intestine can be difficult due to maternal influences. Intestinal remodeling during amphibian metamorphosis resembles perinatal intestinal maturation in mammals and its dependence on T3 is well established. A major advantage of the amphibian model is that it can easily be controlled by altering the availability of T3. The ability to manipulate and examine this relatively rapid and localized formation of adult stem cells has greatly assisted in the elucidation of molecular mechanisms regulating their formation and further revealed evidence that supports conservation in the underlying mechanisms of adult stem cell development in vertebrates. Furthermore, genetic studies in Xenopus laevis indicate that T3 actions in both the epithelium and the rest of the intestine, most likely the underlying connective tissue, are required for the formation of adult stem cells. Molecular analyses suggest that cell-cell interactions involving hedgehog and BMP pathways are critical for the establishment of the stem cell niche that is essential for the formation of the adult intestinal stem cells.

Biological significance of a thyroid hormone-regulated secretome

The thyroid hormone, 3,3,5-triiodo-L-thyronine (T3), modulates several physiological processes, including cellular growth, differentiation, metabolism and proliferation, via interactions with thyroid hormone response elements (TREs) in the regulatory regions of target genes. Several intracellular and extracellular protein candidates are regulated by T3. Moreover, T3-regulated secreted proteins participate in physiological processes or cellular transformation. T3 has been employed as a marker in several disorders, such as cardiovascular disorder in chronic kidney disease, as well as diseases of the liver, immune system, endocrine hormone metabolism and coronary artery. Our group subsequently showed that T3 regulates several tumor-related secretory proteins, leading to cancer progression via alterations in extracellular matrix proteases and tumor-associated signaling pathways in hepatocellular carcinomas. Therefore, elucidation of T3/thyroid hormone receptor-regulated secretory proteins and their underlying mechanisms in cancers should facilitate the identification of novel therapeutic targets. This review provides a detailed summary on the known secretory proteins regulated by T3 and their physiological significance. This article is part of a Special Issue entitled: An Updated Secretome.

Establishment of intestinal stem cell niche during amphibian metamorphosis

In the amphibian intestine during metamorphosis, most of the larval epithelial cells undergo apoptosis, whereas a small number of them survive. These cells dedifferentiate into stem cells through interactions with the microenvironment referred to as "stem cell niche" and generate the adult epithelium analogous to the mammalian counterpart. Since all processes of the larval-to-adult intestinal remodeling can be experimentally induced by thyroid hormone (TH) both in vivo and in vitro, the amphibian intestine provides us a valuable opportunity to study how adult stem cells and their niche are formed during postembryonic development. To address this issue, a number of expression and functional analyses of TH response genes have been intensely performed in the Xenopus laevis over the past two decades, by using organ culture and transgenic techniques. We here review recent progress in this field, focusing on key signaling pathways involved in establishment of the stem cell niche and discuss their evolutionarily conserved roles in the vertebrate intestine.

Thyroid hormone regulation of adult intestinal stem cell development: mechanisms and evolutionary conservations

The adult mammalian intestine has long been used as a model to study adult stem cell function and tissue renewal as the intestinal epithelium is constantly undergoing self-renewal throughout adult life. This is accomplished through the proliferation and subsequent differentiation of the adult stem cells located in the crypt. The development of this self-renewal system is, however, poorly understood. A number of studies suggest that the formation/maturation of the adult intestine is conserved in vertebrates and depends on endogenous thyroid hormone (T3). In amphibians such as Xenopus laevis, the process takes place during metamorphosis, which is totally dependent upon T3 and resembles postembryonic development in mammals when T3 levels are also high. During metamorphosis, the larval epithelial cells in the tadpole intestine undergo apoptosis and concurrently, adult epithelial stem/progenitor cells are formed de novo, which subsequently lead to the formation of a trough-crest axis of the epithelial fold in the frog, resembling the crypt-villus axis in the adult mammalian intestine. Here we will review some recent molecular and genetic studies that support the conservation of the development of the adult intestinal stem cells in vertebrates. We will discuss the mechanisms by which T3 regulates this process via its nuclear receptors.

β-Catenin regulates deiodinase levels and thyroid hormone signaling in colon cancer cells

BACKGROUND & AIMS

Activation of the β-catenin/T-cell factor (TCF) complex occurs in most colon tumors, and its actions correlate with the neoplastic phenotype of intestinal epithelial cells. Type 3 deiodinase (D3), the selenoenzyme that inactivates thyroid hormone (3,5,3' triiodothyronine [T3]), is frequently expressed by tumor cells, but little is known about its role in the regulation of T3 signaling in cancer cells.

METHODS

We measured D3 expression in 6 colon cancer cell lines and human tumors and correlated it with the activity of the β-catenin/TCF complex. We also determined the effects of D3 loss on local thyroid hormone signaling and colon tumorigenesis.

RESULTS

We show that D3 is a direct transcriptional target of the β-catenin/TCF complex; its expression was higher in human intestinal adenomas and carcinomas than in healthy intestinal tissue. Experimental attenuation of β-catenin reduced D3 levels and induced type 2 deiodinase (the D3 antagonist that converts 3,5,3',5' tetraiodothyronine into active T3) thereby increasing T3-dependent transcription. In the absence of D3, excess T3 reduced cell proliferation and promoted differentiation in cultured cells and in xenograft mouse models. This occurred via induction of E-cadherin, which sequestered β-catenin at the plasma membrane and promoted cell differentiation.

CONCLUSIONS

Deiodinases are at the interface between the β-catenin and the thyroid hormone pathways. Their synchronized regulation of intracellular T3 concentration is a hitherto unrecognized route by which the multiple effects of β-catenin are generated and may be targeted to reduce the oncogenic effects of β-catenin in intestinal cells.

Thyroid hormone's action on progenitor/stem cell biology: new challenge for a classic hormone?

BACKGROUND

Thyroid hormones are involved in developmental and homeostatic processes in several tissues. Their action results in different outcomes depending on the developmental stage, tissue and/or cellular context. Interestingly, their pleiotropic roles are conserved across vertebrates. It is largely documented that thyroid hormones act via nuclear receptors, the TRs, which are transcription factors and whose activity can be modulated by the local availability of the hormone T3. In the "classical view", the T3-induced physiological response depends on the expression of specific TR isoforms and the iodothyronine deiodinase selenoenzymes that control the local level of T3, thus TR activity.

SCOPE OF THE REVIEW

Recent data have clearly established that the functionality of TRs is coordinated and integrated with other signaling pathways, specifically at the level of stem/progenitor cell populations. Here, we summarize these data and propose a new and intriguing role for thyroid hormones in two selected examples.

MAJOR CONCLUSIONS

In the intestinal epithelium and the retina, TRα1 and TRβ2 are expressed at the level of the precursors where they induce cell proliferation and differentiation, respectively. Moreover, these different functions result from the integration of the hormone signal with other intrinsic pathways, which play a fundamental role in progenitor/stem cell physiology.

GENERAL SIGNIFICANCE

Taken together, the interaction of TRs with other signaling pathways, specifically in stem/progenitor cells, is a new concept that may have biological relevance in therapeutic approaches aimed to target stem cells such as tissue engineering and cancer. This article is part of a Special Issue entitled Thyroid hormone signalling.

Multi-level interactions between the nuclear receptor TRα1 and the WNT effectors β-catenin/Tcf4 in the intestinal epithelium

Intestinal homeostasis results from complex cross-regulation of signaling pathways; their alteration induces intestinal tumorigenesis. Previously, we found that the thyroid hormone nuclear receptor TRα1 activates and synergizes with the WNT pathway, inducing crypt cell proliferation and promoting tumorigenesis. Here, we investigated the mechanisms and implications of the cross-regulation between these two pathways in gut tumorigenesis in vivo and in vitro. We analyzed TRα1 and WNT target gene expression in healthy mucosae and tumors from mice overexpressing TRα1 in the intestinal epithelium in a WNT-activated genetic background (vil-TRα1/Apc mice). Interestingly, increased levels of β-catenin/Tcf4 complex in tumors from vil-TRα1/Apc mice blocked TRα1 transcriptional activity. This observation was confirmed in Caco2 cells, in which TRα1 functionality on a luciferase reporter-assay was reduced by the overexpression of β-catenin/Tcf4. Moreover, TRα1 physically interacted with β-catenin/Tcf4 in the nuclei of these cells. Using molecular approaches, we demonstrated that the binding of TRα1 to its DNA target sequences within the tumors was impaired, while it was newly recruited to WNT target genes. In conclusion, our observations strongly suggest that increased β-catenin/Tcf4 levels i) correlated with reduced TRα1 transcriptional activity on its target genes and, ii) were likely responsible for the shift of TRα1 binding on WNT targets. Together, these data suggest a novel mechanism for the tumor-promoting activity of the TRα1 nuclear receptor.

Thyroid hormone receptors, cell growth and differentiation

BACKGROUND

Tissue homeostasis depends on the balance between cell proliferation and differentiation. Thyroid hormones (THs), through binding to their nuclear receptors, can regulate the expression of many genes involved in cell cycle control and cellular differentiation. This can occur by direct transcriptional regulation or by modulation of the activity of different signaling pathways.

SCOPE OF REVIEW

In this review we will summarize the role of the different receptor isoforms in growth and maturation of selected tissues and organs. We will focus on mammalian tissues, and therefore we will not address the fundamental role of the THs during amphibian metamorphosis.

MAJOR CONCLUSIONS

The actions of THs are highly pleiotropic, affecting many tissues at different developmental stages. As a consequence, their effects on proliferation and differentiation are highly heterogeneous depending on the cell type, the cellular context, and the developmental or transformation status. Both during development and in the adult, stem cells are essential for proper organ formation, maintenance and regeneration. Recent evidence suggests that some of the actions of the thyroid hormone receptors could be secondary to regulation of stem/progenitor cell function. Here we will also include the latest knowledge on the role of these receptors in proliferation and differentiation of embryonic and adult stem cells.

GENERAL SIGNIFICANCE

The thyroid hormone receptors are potent regulators of proliferation and differentiation of many cell types. This can explain the important role of the thyroid hormones and their receptors in key processes such as growth, development, tissue homeostasis or cancer. This article is part of a Special Issue entitled Thyroid hormone signalling.

Thyroid hormone activates protein arginine methyltransferase 1 expression by directly inducing c-Myc transcription during Xenopus intestinal stem cell development

Adult organ-specific stem cells are essential for organ homeostasis and tissue repair and regeneration. The formation of such stem cells during vertebrate development is poorly understood. Intestinal remodeling during thyroid hormone (T3)-dependent Xenopus metamorphosis resembles postembryonic intestinal maturation in mammals. During metamorphosis, the intestine is remodeled de novo via a yet unknown mechanism. Protein arginine methyltransferase 1 (PRMT1) is up-regulated in and required for adult intestinal stem cells during metamorphosis. PRMT1 up-regulation is the earliest known molecular event for the developing stem cells and is also conserved during zebrafish and mouse intestinal development. To analyze how PRMT1 is specifically up-regulated during the formation of the adult intestinal stem cells, we cloned the Xenopus PRMT1 promoter and characterized it in CaCo-2 cells, a human cell line with intestinal stem cell characteristics. Through a series deletion and mutational analyses, we showed that the stem cell-associated transcription factor c-Myc could bind to a conserved site in the first intron to activate the promoter. Furthermore, we demonstrated that during metamorphosis, both c-Myc and PRMT1 were highly up-regulated, specifically in the remodeling intestine but not the resorbing tail, and that c-Myc was induced by T3 prior to PRMT1 up-regulation. In addition, we showed that T3 directly activated the c-Myc gene during metamorphosis in the intestine via binding of the T3 receptor to the c-Myc promoter. These results suggest that T3 induces c-Myc transcription directly in the intestine, that c-Myc, in turn, activates PRMT1 expression, and that this is an important gene regulation cascade controlling intestinal stem cell development.

Yeast-based assay identifies novel Shh/Gli target genes in vertebrate development

Background

The increasing number of developmental events and molecular mechanisms associated with the Hedgehog (Hh) pathway from Drosophila to vertebrates, suggest that gene regulation is crucial for diverse cellular responses, including target genes not yet described. Although several high-throughput, genome-wide approaches have yielded information at the genomic, transcriptional and proteomic levels, the specificity of Gli binding sites related to direct target gene activation still remain elusive. This study aims to identify novel putative targets of Gli transcription factors through a protein-DNA binding assay using yeast, and validating a subset of targets both in-vitro and in-vivo. Testing in different Hh/Gli gain- and loss-of-function scenarios we here identified known (e.g., ptc1) and novel Hh-regulated genes in zebrafish embryos.

Results

The combined yeast-based screening and MEME/MAST analysis were able to predict Gli transcription factor binding sites, and position mapping of these sequences upstream or in the first intron of promoters served to identify new putative target genes of Gli regulation. These candidates were validated by qPCR in combination with either the pharmacological Hh/Gli antagonist cyc or the agonist pur in Hh-responsive C3H10T1/2 cells. We also used small-hairpin RNAs against Gli proteins to evaluate targets and confirm specific Gli regulation their expression. Taking advantage of mutants that have been identified affecting different components of the Hh/Gli signaling system in the zebrafish model, we further analyzed specific novel candidates. Studying Hh function with pharmacological inhibition or activation complemented these genetic loss-of-function approaches. We provide evidence that in zebrafish embryos, Hh signaling regulates sfrp2, neo1, and c-myc expression in-vivo.

Conclusion

A recently described yeast-based screening allowed us to identify new Hh/Gli target genes, functionally important in different contexts of vertebrate embryonic development.

The complex roles of Wnt antagonists in RCC

Renal cell carcinoma (RCC) is the most lethal of all the genitourinary cancers, as it is generally refractory to current treatment regimens, including chemotherapy and radiation therapy. Targeted therapies against critical signaling pathways associated with RCC pathogenesis, such as vascular endothelial growth factor, von Hippel-Lindau tumor suppressor and mammalian target of rapamycin, have shown limited efficacy so far. Thus, Wnt signaling, which is known to be intricately involved in the pathogenesis of RCC, has attracted much interest. Several Wnt signaling components have been examined in RCC, and, while studies suggest that Wnt signaling is constitutively active in RCC, the molecular mechanisms differ considerably from other human carcinomas. Increasing evidence indicates that secreted Wnt antagonists have important roles in RCC pathogenesis. Considering these vital roles, it has been postulated--and supported by experimental evidence--that the functional loss of Wnt antagonists, for example by promoter hypermethylation, can contribute to constitutive activation of the Wnt pathway, resulting in carcinogenesis through dysregulation of cell proliferation and differentiation. However, subsequent functional studies of these Wnt antagonists have demonstrated the inherent complexities underlying their role in RCC pathogenesis.

The development of the adult intestinal stem cells: Insights from studies on thyroid hormone-dependent amphibian metamorphosis

Adult organ-specific stem cells are essential for organ homeostasis and repair in adult vertebrates. The intestine is one of the best-studied organs in this regard. The intestinal epithelium undergoes constant self-renewal throughout adult life across vertebrates through the proliferation and subsequent differentiation of the adult stem cells. This self-renewal system is established late during development, around birth, in mammals when endogenous thyroid hormone (T3) levels are high. Amphibian metamorphosis resembles mammalian postembryonic development around birth and is totally dependent upon the presence of high levels of T3. During this process, the tadpole intestine, predominantly a monolayer of larval epithelial cells, undergoes drastic transformation. The larval epithelial cells undergo apoptosis and concurrently, adult epithelial stem/progenitor cells develop de novo, rapidly proliferate, and then differentiate to establish a trough-crest axis of the epithelial fold, resembling the crypt-villus axis in the adult mammalian intestine. We and others have studied the T3-dependent remodeling of the intestine in Xenopus laevis. Here we will highlight some of the recent findings on the origin of the adult intestinal stem cells. We will discuss observations suggesting that liganded T3 receptor (TR) regulates cell autonomous formation of adult intestinal progenitor cells and that T3 action in the connective tissue is important for the establishment of the stem cell niche. We will further review evidence suggesting similar T3-dependent formation of adult intestinal stem cells in other vertebrates.

Looking for the mechanism of action of thyroid hormone

The mechanisms of action of thyroid hormone (TH), characterized by multiple physiological activities, proposed over the last 80 years are a reflection of the progression of our knowledge about eukaryotic signalling processes. The cumulative knowledge gained raises the question as to what is so special about the action of this hormone. The discovery in the 1980s that TH receptors belong to the family of nuclear transcription factors that regulate the expression of hormonal target genes was an important milestone. TH receptors are highly organized within the chromatin structure, which itself is modified by several chromosomal and nonchromosomal factors, in the presence and absence of the hormone. Recently, some investigators have suggested that TH acts via both genomic and nongenomic mechanisms and introduced the concept of networking within cellular complexes. While one cannot as yet precisely describe the mechanism of thyroid hormone action, I will attempt here to point out the present thinking and future directions to achieve this goal in the light of the historical background.

Maternal thyroid hormones are transcriptionally active during embryo-foetal development: results from a novel transgenic mouse model

Even though several studies highlighted the role of maternal thyroid hormones (THs) during embryo-foetal development, direct evidence of their interaction with embryonic thyroid receptors (TRs) is still lacking. We generated a transgenic mouse model ubiquitously expressing a reporter gene tracing TH action during development. We engineered a construct (TRE2×) containing two TH-responsive elements controlling the expression of the LacZ reporter gene, which encodes β-galactosidase (β-gal). The specificity of the TRE2× activation by TH was evaluated in NIH3T3 cells by cotransfecting TRE2× along with TRs, retinoic or oestrogen receptors in the presence of their specific ligands. TRE2× transgene was microinjected into the zygotes, implanted in pseudopregnant BDF1 (a first-generation (F1) hybrid from a cross of C57BL/6 female and a DBA/2 male) mice and transgenic mouse models were developed. β-gal expression was assayed in tissue sections of transgenic mouse embryos at different stages of development. In vitro, TRE2× transactivation was observed only following physiological T3 stimulation, mediated exclusively by TRs. In vivo, β-gal staining, absent until embryonic day 9.5-10.5 (E9.5-E10.5), was observed as early as E11.5-E12.5 in different primordia (i.e. central nervous system, sense organs, intestine, etc.) of the TRE2× transgenic embryos, while the foetal thyroid function (FTF) was still inactive. Immunohistochemistry for TRs essentially colocalized with β-gal staining. No β-gal staining was detected in embryos of hypothyroid transgenic mice. Importantly, treatment with T3 in hypothyroid TRE2× transgenic mice rescued β-gal expression. Our results provide in vivo direct evidence that during embryonic life and before the onset of FTF, maternal THs are transcriptionally active through the action of embryonic TRs. This model may have clinical relevance and may be employed to design end-point assays for new molecules affecting THs action.

TRbeta is the critical thyroid hormone receptor isoform in T3-induced proliferation of hepatocytes and pancreatic acinar cells

BACKGROUND & AIMS

Thyroid hormones elicit many cellular and metabolic effects in various organs. Most of these actions, including mitogenesis, are mediated by the thyroid hormone 3,5,3'-triiodo-l-thyronine (T3) nuclear receptors (TRs). They are transcription factors, expressed as different isoforms encoded by the TRalpha and TRbeta genes. Here, experiments were performed to determine whether (i) T3-induces hepatocyte proliferation in mouse liver and pancreas, and, (ii) which TR isoform, is responsible for its mitogenic effect.

METHODS

Cell proliferation was measured by bromodeoxyuridine (BrdU) incorporation after T3 or the TRbeta agonist GC-1 in liver and pancreas of CD-1, C57BL, or TRalpha(0/0) mice. Cell cycle-associated proteins were measured by Western blot.

RESULTS

T3 added to the diet at a concentration of 4 mg/kg caused a striking increase in BrdU incorporation in mouse hepatocytes. Increased BrdU incorporation was associated with enhanced protein levels of cyclin D1 and PCNA and decreased levels of p27. Treatment with GC-1, a selective agonist of the TRbeta isoform, also induced a strong mitogenic response of mouse hepatocytes and pancreatic acinar cells which was similar to that elicited by T3. Finally, treatment with T3 of mice TRalpha(0/0) induced a proliferative response in the liver and pancreas, similar to that of their wild type counterpart.

CONCLUSIONS

These results demonstrate that T3 is a powerful inducer of cell proliferation in mouse liver and suggest that the beta-isoform is responsible for the hepatomitogenic activity of T3. The same isoform seems to also mediate the proliferation of mouse pancreatic acinar cells.

Secreted Frizzled-related protein-2 (sFRP2) augments canonical Wnt3a-induced signaling

Secreted Frizzled-related proteins (sFRP) are involved in embryonic development as well as pathological conditions including bone and myocardial disorders and cancer. Because of their sequence homology with the Wnt-binding domain of Frizzled, they have generally been considered antagonists of canonical Wnt signaling. However, additional activities of various sFRPs including both synergism and mimicry of Wnt signaling as well as functions other than modulation of Wnt signaling have been reported. Using human embryonic kidney cells (HEK293A), we found that sFRP2 enhanced Wnt3a-dependent phosphorylation of LRP6 as well as both cytosolic β-catenin levels and its nuclear translocation. While addition of recombinant sFRP2 had no activity by itself, Top/Fop luciferase reporter assays showed a dose-dependent increase of Wnt3a-mediated transcriptional activity. sFRP2 enhancement of Wnt3a signaling was abolished by treatment with the Wnt antagonist, Dickkopf-1 (DKK1). Wnt-signaling pathway qPCR arrays showed that sFRP2 enhanced the Wnt3a-mediated transcriptional up-regulation of several genes regulated by Wnt3a including its antagonists, DKK1, and Naked cuticle-1 homolog (NKD1). These results support sFRP2's role as an enhancer of Wnt/β-catenin signaling, a result with biological impact for both normal development and diverse pathologies such as tumorigenesis.

The overexpression of the putative gut stem cell marker Musashi-1 induces tumorigenesis through Wnt and Notch activation

The RNA-binding protein Musashi-1 (Msi1) has been proposed as a marker of intestinal epithelial stem cells. These cells are responsible for the continuous renewal of the intestinal epithelium. Although the function of Msi1 has been studied in several organs from different species and in mammalian cell lines, its function and molecular regulation in mouse intestinal epithelium progenitor cells are still undefined. We describe here that, in these cells, the expression of Msi1 is regulated by the canonical Wnt pathway, through a mechanism involving a functional Tcf/Lef binding site on its promoter. An in vitro study in intestinal epithelium primary cultures showed that Msi1 overexpression promotes progenitor proliferation and activates Wnt and Notch pathways. Moreover, Msi1-overexpressing cells exhibit tumorigenic properties in xenograft experiments. These data point to a positive feedback loop between Msi1 and Wnt in intestinal epithelial progenitors. They also suggest that Msi1 has oncogenic properties in these cells, probably through induction of both the Wnt and Notch pathways.

Cross-regulation of signaling pathways: an example of nuclear hormone receptors and the canonical Wnt pathway

Predicting the potential physiological outcome(s) of any given molecular pathway is complex because of cross-talk with other pathways. This is particularly evident in the case of the nuclear hormone receptor and canonical Wnt pathways, which regulate cell growth and proliferation, differentiation, apoptosis, and metastatic potential in numerous tissues. These pathways are known to intersect at many levels: in the intracellular space, at the membrane, in the cytoplasm, and within the nucleus. The outcomes of these interactions are important in the control of stem cell differentiation and maintenance, feedback loops, and regulating oncogenic potential. The aim of this review is to demonstrate the importance of considering pathway cross-talk when predicting functional outcomes of signaling, using nuclear hormone receptor/canonical Wnt pathway cross-talk as an example.

Oncogenic functions of secreted Frizzled-related protein 2 in human renal cancer

The secreted Frizzled-related proteins (sFRP) are modulators of the Wnt signaling pathway, which is involved in embryonic development and tumor progression. The functions of sFRP2 have not been studied in renal cancer. Transient transfection of sFRP2 promoted cell growth in renal carcinoma cells, whereby the largest effect was observed in A498 cells. To further study the functions of sFRP2 gene in renal carcinoma cells, we established A498 renal cancer cell lines, which stably expressed sFRP2. Stably expressed sFRP2 significantly promoted cell proliferation in vitro and in vivo tumor growth. The stably expressed sFRP2 cells were also found to have reduced UV-induced apoptosis and increased G(2) phase of the cell cycle. The phosphorylation level at Ser(33/37)/Thr(41) of beta-catenin was lower in the stable sFRP2 cell lines compared with the control cell line. sFRP2 significantly activated T-cell factor/lymphoid enhancer factor transcriptional activity. In the stable sFRP2 cell line, expression of c-Fos, Bcl2, Bcl-w, cyclin B2, and cyclin E2 genes was significantly increased and p53 expression was decreased. This is the first report documenting that sFRP2 activates the canonical Wnt pathway and promotes cell growth by evoking diverse signaling cascades in renal cancer cells. This study may provide better strategies for the management of renal cancer through regulation of sFRP2 pathways.

Cooperation between the thyroid hormone receptor TRalpha1 and the WNT pathway in the induction of intestinal tumorigenesis

BACKGROUND & AIMS

Colorectal tumorigenesis is a multistep process involving the alteration of oncogenes and tumor suppressor genes, leading to the deregulation of molecular pathways that govern intestinal homeostasis. We have previously shown that the thyroid hormone receptor alpha1 (TRalpha1) controls intestinal development and homeostasis through the WNT pathway. More precisely, TRalpha1 directly enhances the transcription of several components of this pathway, allowing increased expression of beta-catenin/Tcf4 target genes and stimulation of cell proliferation. Because the WNT pathway is a major player in controlling intestinal homeostasis, we addressed whether the TRalpha1 receptor has tumor-inducing potential.

METHODS

We generated mice overexpressing TRalpha1 specifically in the intestinal epithelium in a wild-type (vil-TRalpha1) or a WNT-activated (vil-TRalpha1/Apc(+/1638N)) genetic background.

RESULTS

The intestine of vil-TRalpha1 mice presents aberrant intestinal mucosal architecture and increased cell proliferation and develops adenoma at a low rate. However, TRalpha1 overexpression is unable to induce cancer development. On the contrary, we observed accelerated tumorigenesis in vil-TRalpha1/Apc(+/1638N) mice compared with the Apc(+/1638N) mutants.

CONCLUSION

Our results suggest that this phenotype is due to cooperation between the activated TRalpha1 and WNT pathways. This is the first report describing the tumor-inducing function of TRalpha1 in the intestine.