JNK signaling promotes intestinal tumorigenesis through activation of mTOR complex 1 in Apc(Δ716) mice

BACKGROUND & AIMS

Signaling by the mammalian target of rapamycin complex 1 (mTORC1) has been implicated in various human cancers. mTORC1 signaling is activated in intestinal tumors of adenomatous polyposis coli (Apc(Δ716)) mice, a model of familial adenomatous polyposis; in these mice, the mTORC1 inhibitor RAD001 can block tumor formation. However, the precise mechanism of mTORC1 signaling in intestinal tumors is not clear. We investigated whether c-Jun-NH(2) terminal kinase (JNK) is involved in the mTORC1 activation.

METHODS

We investigated the effects of an inhibitor and an activator of JNK, as well as small interfering RNA against JNK, on mTORC1 in Apc(Δ716) mice and colon cancer cell lines. We also determined the role of JNK in mTORC1 signaling using in vitro kinase assays.

RESULTS

JNK was activated in intestinal polyps of Apc((Δ716) mice); the JNK inhibitor SP600125 significantly suppressed tumor formation. In colorectal cancer cell lines, the JNK activator anisomycin activated mTORC1, whereas SP600125 or small interfering RNAs against JNK suppressed signaling. Importantly, JNK stimulated the mTORC1 kinase activity in vitro, through direct phosphorylation of Raptor at serine 863.

CONCLUSIONS

JNK is required for activation of mTORC1 in intestinal tumor cells. JNK inhibitors might be developed as therapeutics or to prevent development of intestinal tumors.

Runx3 is required for full activation of regulatory T cells to prevent colitis-associated tumor formation

Inflammation is increasingly recognized as an essential component of tumorigenesis, which is promoted and suppressed by various T cell subsets acting in different ways. It was shown previously in Runx3-deficient mice that differentiation of CD8 T and NK cells is perturbed. In this study, we show that Runx3 is also required for proper differentiation and function of regulatory T cells. In Runx3-deficient mice, T cells were unable to inhibit inflammation and to suppress tumor development. As expected, recombination activating gene 2-deficient mice bearing Runx3-deficient lymphocytes spontaneously developed colon tumors. However, tumor formation was completely blocked by transfer of either regulatory T cells or CD8 T cells derived from wild-type mice to mutant mice or by housing mutant mice in a specific pathogen-free condition. These results indicate that Runx3-deficient lymphocytes and microorganisms act together to induce inflammation and consequently induce the development of colon tumors.

Ras mutation cooperates with β-catenin activation to drive bladder tumourigenesis

Mutations in the Ras family of proteins (predominantly in H-Ras) occur in approximately 40% of urothelial cell carcinoma (UCC). However, relatively little is known about subsequent mutations/pathway alterations that allow tumour progression. Indeed, expressing mutant H-Ras within the mouse bladder does not lead to tumour formation, unless this is expressed at high levels. The Wnt signalling pathway is deregulated in approximately 25% of UCC, so we examined if this correlated with the activation of MAPK signalling in human UCC and found a significant correlation. To test the functional significance of this association we examined the impact of combining Ras mutation (H-Ras(Q61L) or K-Ras(G12D)) with an activating β-catenin mutation within the mouse bladder using Cre-LoxP technology. Although alone, neither Ras mutation nor β-catenin activation led to UCC (within 12 months), mice carrying both mutations rapidly developed UCC. Mechanistically this was associated with reduced levels of p21 with dependence on the MAPK signalling pathway. Moreover, tumours from these mice were sensitive to MEK inhibition. Importantly, in human UCC there was a negative correlation between levels of p-ERK and p21 suggesting that p21 accumulation may block tumour progression following Ras mutation. Taken together these data definitively show Ras pathway activation strongly cooperates with Wnt signalling to drive UCC in vivo.

Suppression of colonic polyposis by homeoprotein CDX2 through its nontranscriptional function that stabilizes p27Kip1

Caudal-related homeoprotein CDX2 is expressed in intestinal epithelial cells, in which it is essential for their development and differentiation. A tumor suppressor function is suggested by evidence that CDX2 levels are decreased in human colon cancer specimens and that an inactivating mutation of Cdx2 in Apc(Δ716) mice markedly increases the incidence of colonic polyps. In this study, we investigated roles for transcriptional and nontranscriptional functions of CDX2 in suppression of colonic tumorigenesis. Mutagenic analysis of CDX2 revealed that loss of function stabilizes CDK inhibitor p27Kip1 by a nontranscriptional but homeodomain-dependent mechanism that inhibits cyclin E-CDK2 activity and blocks G0/G1-S progression in colon cancer cells. p27Kip1 stabilization was mediated by an inhibition of ubiquitylation-dependent proteolysis associated with decreased phosphorylation of Thr187 in p27Kip1. siRNA-mediated knockdown of p27Kip1 relieved the decrease in cyclin E-CDK2 activity and S-phase cell fraction elicited by CDX2 expression. Together, these results implicate a nontranscriptional function of CDX2 in tumor suppression mediated by p27Kip1 stabilization. Up to approximately 75% of low-CDX2 human colon cancer lesions show reduced levels of p27Kip1, whereas approximately 68% of high-CDX2 lesions retain expression of p27Kip1. These results show that low levels of CDX2 accelerate colon tumorigenesis by reducing p27Kip1 levels.

Significance and mechanism of lymph node metastasis in cancer progression

The effect of local therapy, such as surgical lymph node (LN) dissection and radiotherapy, on the survival of cancer patients has been debated for decades. Several lines of recent clinical evidence support that LN metastasis plays significant roles in systemic dissemination of cancer cells, although the effects of surgical LN dissection on survival was downplayed historically because of controversial data. Molecular studies of LN metastasis suggest that the microenvironment within LNs, including chemokines and lymphangiogenesis, can mediate the metastatic spread to the sentinel LNs, and beyond. It has been shown that chemokine receptor CXCR3 is involved in LN metastasis, and its inhibition may improve patient prognosis. Although it remains to be determined whether local therapy is best pursued through LN dissection or through a combination of resection with radiation, prevention of regional metastases is an important goal in the treatment of cancer patients to achieve a better survival.

Subtypes of medulloblastoma have distinct developmental origins

Medulloblastoma encompasses a collection of clinically and molecularly diverse tumor subtypes that together comprise the most common malignant childhood brain tumor1–4. These tumors are thought to arise within the cerebellum, with approximately 25% originating from granule neuron precursor cells (GNPCs) following aberrant activation of the Sonic Hedgehog pathway (hereafter, SHH-subtype)3–8. The pathological processes that drive heterogeneity among the other medulloblastoma subtypes are not known, hindering the development of much needed new therapies. Here, we provide evidence that a discrete subtype of medulloblastoma that contains activating mutations in the WNT pathway effector CTNNB1 (hereafter, WNT-subtype)1,3,4, arises outside the cerebellum from cells of the dorsal brainstem. We found that genes marking human WNT-subtype medulloblastomas are more frequently expressed in the lower rhombic lip (LRL) and embryonic dorsal brainstem than in the upper rhombic lip (URL) and developing cerebellum. Magnetic resonance imaging (MRI) and intra-operative reports showed that human WNT-subtype tumors infiltrate the dorsal brainstem, while SHH-subtype tumors are located within the cerebellar hemispheres. Activating mutations in Ctnnb1 had little impact on progenitor cell populations in the cerebellum, but caused the abnormal accumulation of cells on the embryonic dorsal brainstem that included aberrantly proliferating Zic1⁺ precursor cells. These lesions persisted in all mutant adult mice and in 15% of cases in which Tp53 was concurrently deleted, progressed to form medulloblastomas that recapitulated the anatomy and gene expression profiles of human WNT-subtype medulloblastoma. We provide the first evidence that subtypes of medulloblastoma have distinct cellular origins. Our data provide an explanation for the marked molecular and clinical differences between SHH and WNT-subtype medulloblastomas and have profound implications for future research and treatment of this important childhood cancer.

The Wnt/beta-catenin pathway modulates vascular remodeling and specification by upregulating Dll4/Notch signaling

The Wnt/beta-catenin pathway is evolutionary conserved signaling system that regulates cell differentiation and organogenesis. We show that endothelial specific stabilization of Wnt/beta-catenin signaling alters early vascular development in the embryo. The phenotype resembles that induced by upregulation of Notch signaling, including lack of vascular remodeling, altered elongation of the intersomitic vessels, defects in branching, and loss of venous identity. Both in vivo and in vitro data show that beta-catenin upregulates Dll4 transcription and strongly increases Notch signaling in the endothelium, leading to functional and morphological alterations. The functional consequences of beta-catenin signaling depend on the stage of vascular development and are lost when a gain-of-function mutation is induced at a late stage of development or postnatally. Our findings establish a link between Wnt and Notch signaling in vascular development. We propose that early and sustained beta-catenin signaling prevents correct endothelial cell differentiation, altering vascular remodeling and arteriovenous specification.

Abstract B23: Dominant stablized ß-catenin induces adenomyosis formation and ablation of Mig-6 accelerates progress of adenomyosis formation

Adenomyosis is a common gynecological disorder defined by the presence of endometrial glands and stroma within the myometrium. Despite its frequent occurrence, the precise etiology of adenomyosis is still unknown, although it has often been associated with endometrioid adenocarcinoma. β-catenin abnormalities are common in endometrioid type endometrial carcinomas. The expression of the dominant stabilized β-catenin in the murine uterus (PRcre/+ Ctnnb1f(ex3)/+) resulted in endometrial glandular hyperplasia. In addition to the glandular hyperplasia phenotype, uteri of PRcre/+ Ctnnb1f(ex3)/+ mice exhibited an abnormal myometrial structure and proceed to develop adenomyosis. Ablation of Mig-6 in the murine uterus (PRcre/+ Mig-6f/f) leads to the development of endometrial hyperplasia and estrogen-induced endometrial cancer. Concomitant stabilization of β -catenin and ablation of Mig-6 dramatically accelerated the development of adenomyosis and glandular hyperplasia compared to stablizing β-catenin alone. The adenomyosis phenotype of ovariectomized Pffre/+ Ctnnb1f<ex3)/+ and PRcre/+ Ctnnb1f(ex3)/+Mig-6f/f mice manifests in the presence of E2 and P4, but not, however, in the absence of ovarian hormones. Importantly, increased nuclear β-catenin expression and decreased MIG-6 expression was observed in women with adenomyosis, providing compelling support for and implicating an important role of β-catenin and MIG-6 in the etiology of adenomyosis in both humans and mice. We have demonstrated that abnormal activation of β-catenin induces adenomyosis formation and ablation of Mig-6 accelerates the progress of adenomyosis formation in the murine uterus. These mouse models are useful and allow us to investigate in detail the initiation and progression of adenomyosis because they mimic several features of human adenomyosis.

Citation Information: Clin Cancer Res 2010;16(14 Suppl):B23.

beta-Catenin initiates tooth neogenesis in adult rodent incisors

beta-Catenin signaling is required for embryonic tooth morphogenesis and promotes continuous tooth development when activated in embryos. To determine whether activation of this pathway in the adult oral cavity could promote tooth development, we induced mutation of epithelial beta-catenin to a stabilized form in adult mice. This caused increased proliferation of the incisor tooth cervical loop, outpouching of incisor epithelium, abnormal morphology of the epithelial-mesenchymal junction, and enhanced expression of genes associated with embryonic tooth development. Ectopic dental-like structures were formed from the incisor region following implantation into immunodeficient mice. Thus, forced activation of beta-catenin signaling can initiate an embryonic-like program of tooth development in adult rodent incisor teeth.

beta-Catenin regulates intercellular signalling networks and cell-type specific transcription in the developing mouse midbrain-rhombomere 1 region

β-catenin is a multifunctional protein involved in both signalling by secreted factors of Wnt family and regulation of the cellular architecture. We show that β-catenin stabilization in mouse midbrain-rhombomere1 region leads to robust up-regulation of several Wnt signalling target genes, including Fgf8. Suggestive of direct transcriptional regulation of the Fgf8 gene, β-catenin stabilization resulted in Fgf8 up-regulation also in other tissues, specifically in the ventral limb ectoderm. Interestingly, stabilization of β-catenin rapidly caused down-regulation of the expression of Wnt1 itself, suggesting a negative feedback loop. The changes in signal molecule expression were concomitant with deregulation of anterior-posterior and dorso-ventral patterning. The transcriptional regulatory functions of β-catenin were confirmed by β-catenin loss-of-function experiments. Temporally controlled inactivation of β-catenin revealed a cell-autonomous role for β-catenin in the maintenance of cell-type specific gene expression in the progenitors of midbrain dopaminergic neurons. These results highlight the role of β-catenin in establishment of neuroectodermal signalling centers, promoting region-specific gene expression and regulation of cell fate determination.

LKB1 suppresses p21-activated kinase-1 (PAK1) by phosphorylation of Thr109 in the p21-binding domain

The serine/threonine protein kinase LKB1 is a tumor suppressor gene mutated in Peutz-Jeghers syndrome patients. The mutations are found also in several types of sporadic cancer. Although LKB1 is implicated in suppression of cell growth and metastasis, the detailed mechanisms have not yet been elucidated. In this study, we investigated the effect of LKB1 on cell motility, whose acquisition occurs in early metastasis. The knockdown of LKB1 enhanced cell migration and PAK1 activity in human colon cancer HCT116 cells, whereas forced expression of LKB1 in Lkb1-null mouse embryonic fibroblasts suppressed PAK1 activity and PAK1-mediated cell migration simultaneously. Notably, LKB1 directly phosphorylated PAK1 at Thr(109) in the p21-binding domain in vitro. The phosphomimetic T109E mutant showed significantly lower protein kinase activity than wild-type PAK1, suggesting that the phosphorylation at Thr(109) by LKB1 was responsible for suppression of PAK1. Consistently, the nonphosphorylatable T109A mutant was resistant to suppression by LKB1. Furthermore, we found that PAK1 was activated in the hepatocellular carcinomas and the precancerous liver lesions of Lkb1(+/-) mice. Taken together, these results suggest that PAK1 is a direct downstream target of LKB1 and plays an essential role in LKB1-induced suppression of cell migration.

Wnt signaling and its downstream target N-myc regulate basal progenitors in the developing neocortex

Basal progenitors (also called non-surface dividing or intermediate progenitors) have been proposed to regulate the number of neurons during neocortical development through expanding cells committed to a neuronal fate, although the signals that govern this population have remained largely unknown. Here, we show that N-myc mediates the functions of Wnt signaling in promoting neuronal fate commitment and proliferation of neural precursor cells in vitro. Wnt signaling and N-myc also contribute to the production of basal progenitors in vivo. Expression of a stabilized form of beta-catenin, a component of the Wnt signaling pathway, or of N-myc increased the numbers of neocortical basal progenitors, whereas conditional deletion of the N-myc gene reduced these and, as a likely consequence, the number of neocortical neurons. These results reveal that Wnt signaling via N-myc is crucial for the control of neuron number in the developing neocortex.

Stabilised beta-catenin in postnatal ventricular myocardium leads to dilated cardiomyopathy and premature death

Beta-catenin is a component of the intercalated disc in cardiomyocytes, but can also be involved in signalling and activation of gene transcription. We wanted to determine how long-term changes in beta-catenin expression levels would affect mature cardiomyocytes. Conditional transgenic mice that either lacked beta-catenin or that expressed a non-degradable form of beta-catenin in the adult ventricle were created. While mice lacking beta-catenin in the ventricle do not have an overt phenotype, mice expressing a non-degradable form develop dilated cardiomyopathy and do not survive beyond 5 months. A detailed analysis could reveal that this phenotype is correlated with a distinct localisation of beta-catenin in adult cardiomyocytes, which cannot be detected in the nucleus, no matter how much protein is present. Our report is the first study that addresses long-term effects of either the absence of beta-catenin or its stabilisation on ventricular cardiomyocytes and it suggests that beta-catenin's role in the nucleus may be of little significance in the healthy adult heart.

Aberrant expression of a beta-catenin gain-of-function mutant induces hyperplastic transformation in the mouse cornea

Beta-catenin signaling has been shown to play a fundamental role in embryonic development and tumorigenesis. In this study, we investigated the role of beta-catenin (Ctnnb1) in corneal homeostasis and tumorigenesis. Conditional expression of a murine Ctnnb1 gain-of-function mutation alone caused corneal neoplasia and neovascularization, resembling human ocular surface squamous neoplasia (OSSN). These corneas displayed an upregulation of cell proliferative markers (PCNA and p63), while presenting downregulation of both the Pax-6 transcription factor and the corneal differentiation marker cytokeratin 12. In addition, the expression of limbal-type keratin 15 ectopically extended to cornea, but the pattern of conjunctival keratin 4 and epidermal keratin 10 were unchanged. Moreover, epithelial E-cadherin and laminins decreased concomitantly with elevated levels of MMP-7. We also noticed a dramatic upregulation of pro-angiogenic factors (Vegf-A, Vegfr1) and angiopoietins in these corneas. Interestingly, all human OSSN specimens examined revealed nuclear beta-catenin immunoreactivity. Taken together, these results argue that beta-catenin activation is a crucial step during OSSN pathogenesis. Thus, inhibition of beta-catenin might be beneficial for treating this disease.

CDX transcription factors positively regulate expression of solute carrier family 5, member 8 in the colonic epithelium

BACKGROUND & AIMS

Caudal-related homeodomain transcription factors CDX1 and CDX2 regulate gut development and differentiation of intestinal epithelial cells; they are candidate tumor suppressors of colorectal carcinomas. Because the functions of CDX1 and CDX2 in the colonic epithelium are not fully understood, we sought to identify genes that they target.

METHODS

We conducted a chromatin immunoprecipitation (ChIP) screen to identify genes that bind the CDX transcription factors. Expression of target genes was analyzed in colon cells and tissues from Cdx1(-/-), Cdx2(+/-), Apc(+/Delta716), and wild-type (control) mice.

RESULTS

Using the ChIP screen, we identified solute carrier family 5, member 8 (SLC5A8, also known as SMCT1) as a direct target of CDX1 and CDX2. CDX transcription factors bind to the promoter region of SLC5A8 and transactivate SLC5A8 reporter constructs. Overexpression of Cdx1 or Cdx2 in human colon cancer cell lines induced expression of endogenous SLC5A8, whereas CDX1 and CDX2 knockdowns reduced its level. Consistently, Slc5a8 expression was significantly reduced in colons of Cdx1(-/-) or Cdx2(+/-) mice compared with wild-type mice. Slc5a8 levels were also reduced in colonic adenomatous polyps and hamartomas from Apc(+/Delta716) and Cdx2(+/-) mutant mice, respectively, compared with adjacent normal colon tissues.

CONCLUSIONS

CDX1 and CDX2 bind the promoter region of SLC5A8 and up-regulate its expression in cultured cells and in colonic epithelium. SLC5A8 transports monocarboxylates such as pyruvate, lactate, and butyrate; CDX1 and CDX2 might therefore regulate the uptake of these substances in the colon.

Dosage-dependent hedgehog signals integrated with Wnt/beta-catenin signaling regulate external genitalia formation as an appendicular program

Embryonic appendicular structures, such as the limb buds and the developing external genitalia, are suitable models with which to analyze the reciprocal interactions of growth factors in the regulation of outgrowth. Although several studies have evaluated the individual functions of different growth factors in appendicular growth, the coordinated function and integration of input from multiple signaling cascades is poorly understood. We demonstrate that a novel signaling cascade governs formation of the embryonic external genitalia [genital tubercle (GT)]. We show that the dosage of Shh signal is tightly associated with subsequent levels of Wnt/beta-catenin activity and the extent of external genitalia outgrowth. In Shh-null mouse embryos, both expression of Wnt ligands and Wnt/beta-catenin signaling activity are downregulated. beta-catenin gain-of-function mutation rescues defective GT outgrowth and Fgf8 expression in Shh-null embryos. These data indicate that Wnt/beta-catenin signaling in the distal urethral epithelium acts downstream of Shh signaling during GT outgrowth. The current data also suggest that Wnt/beta-catenin regulates Fgf8 expression via Lef/Tcf binding sites in a 3' conserved enhancer. Fgf8 induces phosphorylation of Erk1/2 and cell proliferation in the GT mesenchyme in vitro, yet Fgf4/8 compound-mutant phenotypes indicate dispensable functions of Fgf4/8 and the possibility of redundancy among multiple Fgfs in GT development. Our results provide new insights into the integration of growth factor signaling in the appendicular developmental programs that regulate external genitalia development.

The role of mTORC1 pathway in intestinal tumorigenesis

Activation of the mTORC1 pathway has been implicated in many types of cancer, and several mTORC1 inhibitors are currently under clinical trials for treating various cancer patients. Notably, Temsirolimus has recently been approved for treatment of advanced renal cell carcinoma. However, the role of mTORC1 pathway in colorectal tumorigenesis remains largely unknown. We have recently found that the mTORC1 pathway is activated in intestinal adenomas of Apc mutant mice, accompanied by an elevated level of mTOR protein, and that treatment with RAD001, an mTORC1 inhibitor, suppresses the growth of these polyps. Our results suggest an important role of mTORC1 pathway in colorectal cancer, as well as a therapeutic possibility for mTORC1 inhibitors in its treatment.

Induction and down-regulation of Sox17 and its possible roles during the course of gastrointestinal tumorigenesis

BACKGROUND & AIMS

The activation of Wnt/beta-catenin signaling causes the development of gastric and colon cancers. Sox17 represses Wnt/beta-catenin signaling and is down-regulated in colon cancer. This study was designed to elucidate the role of Sox17 during the course of gastrointestinal tumorigenesis.

METHODS

Sox17 expression was examined in gastrointestinal tumors of mouse models and humans. The roles of Sox17 in gastric tumorigenesis were examined by cell culture experiments and by construction of Sox17 transgenic mice.

RESULTS

Sox17 was induced in K19-Wnt1/C2mE mouse gastric tumors and K19-Wnt1 preneoplastic lesions, where Wnt/beta-catenin signaling was activated. Consistently, Wnt activation induced Sox17 expression in gastric cancer cells. In contrast, Sox17 was rarely detected by immunohistochemistry in gastric and colon cancers, whereas strong nuclear staining of Sox17 was found in >70% of benign gastric and intestinal tumors. Treatment with a demethylating agent induced Sox17 expression in gastric cancer cells, thus indicating the down-regulation of Sox17 by methylation. Moreover, transfection of Sox17 in gastric cancer cells suppressed both the Wnt activity and colony formation efficiency. Finally, transgenic expression of Sox17 suppressed dysplastic tumor development in K19-Wnt1/C2mE mouse stomach.

CONCLUSIONS

Sox17 plays a tumor suppressor role through suppression of Wnt signaling. However, Sox17 is induced by Wnt activation in the early stage of gastrointestinal tumorigenesis, and Sox17 is down-regulated by methylation during malignant progression. It is therefore conceivable that Sox17 protects benign tumors from malignant progression at an early stage of tumorigenesis, and down-regulation of Sox17 contributes to malignant progression through promotion of Wnt activity.

Constitutive WNT/beta-catenin signaling in murine Sertoli cells disrupts their differentiation and ability to support spermatogenesis

Sertoli and germ cell interactions are essential for spermatogenesis and, thus, male fertility. Sertoli cells provide a specialized microenvironment for spermatogonial stem cells to divide, allowing both self-renewal and spermatogenesis. In the present study, we used mice with a conditional activated allele of the beta-catenin gene (Ctnnb1(tm1Mmt)(/+)) in Sertoli cells expressing Cre recombinase driven by the anti-Müllerian hormone (AMH; also known as Müllerian-inhibiting substance) type II receptor promoter (Amhr2(tm3(cre)Bhr)(/+)) to show that constitutively activated beta-catenin leads to their continuous proliferation and compromised differentiation. Compared to controls, Sertoli cells in mature mutant mice continue to express high levels of both AMH and glial cell-derived neurotrophic factor (GDNF), which normally are expressed only in immature Sertoli cells. We also show evidence that LiCl treatment, which activates endogenous nuclear beta-catenin activity, regulates both AMH and GDNF expression at the transcriptional level. The epididymides were devoid of sperm in the Amhr2(tm3(cre)Bhr)(/+);Ctnnb1(tm1Mmt)(/+) mice at all ages examined. We show that the mutant mice are infertile because of defective differentiation of germ cells and increased apoptosis, both of which are characteristic of GDNF overexpression in Sertoli cells. Constitutive activation of beta-catenin in Amhr2-null mice showed the same histology, suggesting that the phenotype was the result of persistent overexpression of GDNF. These results show that dysregulated wingless-related MMTV integration site/beta-catenin signaling in Sertoli cells inhibits their postnatal differentiation, resulting in increased germ cell apoptosis and infertility.

Reduced level of smoothened suppresses intestinal tumorigenesis by down-regulation of Wnt signaling

BACKGROUND & AIMS

Although the Hedgehog (Hh) pathway regulates development and progression of several types of cancer, its involvement in colon cancer remains unclear. We aimed to clarify the roles of Hh signaling in intestinal tumorigenesis.

METHODS

We studied expression of the Hh signaling components in the intestinal tumors of Apc(+/Delta716) mouse, a model for familial adenomatous polyposis. We used small interfering RNAs against Smoothened (SMO), which encodes the major signal transducer of the Hh pathway, to knockdown SMO expression and explore its function in human colon cancer cell lines. We also compared the intestinal tumor phenotypes of Apc(+/Delta716)Smo(+/-) mice with those of Apc(+/Delta716) mice.

RESULTS

Expression of Smo was markedly increased in the intestinal adenoma epithelium of Apc(+/Delta716) mice. Importantly, SMO knockdown in human colon cancer cell lines suppressed proliferation in culture; cells arrested at the G1/S phase. Furthermore, Apc(+/Delta716)Smo(+/-) mice had decreased numbers of polyps in the large size class (Phi >or= 1-2 mm) and recessed polyp morphology, accompanied by reduced proliferation of the tumor epithelial cells. Unexpectedly, reduced expression of Smo suppressed beta-catenin-dependent transcription, rather than Hh-responsive Gli-dependent transcription. Interestingly, SMO knockdown reduced protein levels of active beta-catenin and induced its nuclear exclusion.

CONCLUSIONS

Smo contributes to intestinal tumorigenesis by increasing Wnt signaling. SMO might be a good therapeutic target for patients with colorectal polyps and carcinomas, even in the absence of Hh signal activation.

Spatial and temporal regulation of Wnt/beta-catenin signaling is essential for development of the retinal pigment epithelium

Wnt/beta-catenin signaling is highly active in the dorsal retinal pigment epithelium (RPE) during eye development. To study the role of Wnt/beta-catenin signaling in the RPE development we used a conditional Cre/loxP system in mice to inactivate or ectopically activate Wnt/beta-catenin signaling in the RPE. Inactivation of Wnt/beta-catenin signaling results in transdifferentiation of RPE to neural retina (NR) as documented by downregulation of RPE-specific markers Mitf and Otx2 and ectopic expression of NR-specific markers Chx10 and Rx, respectively. In contrast, ectopic activation of Wnt/beta-catenin signaling results in the disruption of the RPE patterning, indicating that precise spatial and temporal regulation of Wnt/beta-catenin signaling is required for normal RPE development. Using chromatin immunoprecipitation (ChIP) and reporter gene assays we provide evidence that Otx2 and RPE-specific isoform of Mitf, Mitf-H, are direct transcriptional targets of Wnt/beta-catenin signaling. Combined, our data suggest that Wnt/beta-catenin signaling plays an essential role in development of RPE by maintaining or inducing expression of Mitf and Otx2.

Pax6 is essential for lens fiber cell differentiation

The developing ocular lens provides an excellent model system with which to study the intrinsic and extrinsic cues governing cell differentiation. Although the transcription factors Pax6 and Sox2 have been shown to be essential for lens induction, their later roles during lens fiber differentiation remain largely unknown. Using Cre/loxP mutagenesis, we somatically inactivated Pax6 and Sox2 in the developing mouse lens during differentiation of the secondary lens fibers and explored the regulatory interactions of these two intrinsic factors with the canonical Wnt pathway. Analysis of the Pax6-deficient lenses revealed a requirement for Pax6 in cell cycle exit and differentiation into lens fiber cells. In addition, Pax6 disruption led to apoptosis of lens epithelial cells. We show that Pax6 regulates the Wnt antagonist Sfrp2 in the lens, and that Sox2 expression is upregulated in the Pax6-deficient lenses. However, our study demonstrates that the failure of differentiation following loss of Pax6 is independent of beta-catenin signaling or Sox2 activity. This study reveals that Pax6 is pivotal for initiation of the lens fiber differentiation program in the mammalian eye.