Identification of the gene transcription repressor domain of Gli3

Inside the stemness engine: Mechanistic links between deregulated transcription factors and stemness in cancer

Cell identity is largely determined by its transcriptional profile. In tumour, deregulation of transcription factor expression and/or activity enables cancer cell to acquire a stem-like state characterised by capacity to self-renew, differentiate and form tumours in vivo. These stem-like cancer cells are highly metastatic and therapy resistant, thus warranting a more complete understanding of the molecular mechanisms downstream of the transcription factors that mediate the establishment of stemness state. Here, we review recent research findings that provide a mechanistic link between the commonly deregulated transcription factors and stemness in cancer. In particular, we describe the role of master transcription factors (SOX, OCT4, NANOG, KLF, BRACHYURY, SALL, HOX, FOX and RUNX), signalling-regulated transcription factors (SMAD, β-catenin, YAP, TAZ, AP-1, NOTCH, STAT, GLI, ETS and NF-κB) and unclassified transcription factors (c-MYC, HIF, EMT transcription factors and P53) across diverse tumour types, thereby yielding a comprehensive overview identifying shared downstream targets, highlighting unique mechanisms and discussing complexities.

Timing is everything: Transcriptional repression is not the default mode for regulating Hedgehog signaling

Hedgehog (HH) signaling is a conserved pathway that drives developmental growth and is essential for the formation of most organs. The expression of HH target genes is regulated by a dual switch mechanism where GLI proteins function as bifunctional transcriptional activators (in the presence of HH signaling) and transcriptional repressors (in the absence of HH signaling). This results in a tight control of GLI target gene expression during rapidly changing levels of pathway activity. It has long been presumed that GLI proteins also repress target genes prior to the initial expression of HH in a given tissue. This idea forms the basis for the limb bud pre-patterning model for regulating digit number. Recent findings indicate that GLI repressor proteins are indeed present prior to HH signaling but contrary to this model, GLI proteins are inert as they do not regulate transcriptional responses or enhancer chromatin modifications at this time. These findings suggest that GLI transcriptional repressor activity is not a default state as assumed, but is itself regulated in an unknown fashion. We discuss these findings and their implications for understanding pre-patterning, digit regulation, and HH-driven disease.

Evaluation of Hedgehog Pathway Inhibition on Nevoid Basal Cell Carcinoma Syndrome Fibroblasts and Basal Cell Carcinoma-Associated Fibroblasts: Are Vismodegib and Sonidegib Useful to Target Cancer-Prone Fibroblasts?

Activating mutations in the Hh pathway underlies the development of sporadic and familial skin BCC. For these oncogenic proliferations displaying ligand-independent activation of the intracellular pathway, two molecules have been approved for therapeutic purposes: vismodegib and sonidegib. Improper Hh signalling occurs in many human tumours also via a paracrine mechanism (ligand-dependent) in which the secretion of Hh ligands by stromal cells support tumour growth. On the other hand, the mobilization of neoplastic stroma by cancer cells is sustained by the activation of Hh signalling in surrounding fibroblasts suggesting a central role of this bidirectional crosstalk in carcinogenesis. Additionally, loss-of-function mutations in the PTCH1 gene in the context of NBCCS, an autosomal dominant disorder predisposing to multiple BCCs, determine tumour permissive phenotypes in dermal fibroblasts. Here, profiling syndromic and BCC-associated fibroblasts unveiled an extraordinary similarity characterized by overexpression of several Hh target genes and a marked pro-inflammatory outline. Both cell types exposed to Hh inhibitors displayed reversion of the tumour-prone phenotype. Under vismodegib and sonidegib treatment, the Wnt/β-catenin pathway, frequently over-active in tumour stroma, resulted down-regulated by pAKT-GSK3β axis and consequent increase of β-catenin turnover. Overall, this study demonstrated that vismodegib and sonidegib impacting on fibroblast tumour supportive functions might be considered in therapy for BCC independently to the mutation status of Hh components in neoplastic cells.

GLI1: A Therapeutic Target for Cancer

GLI1 is a transcriptional effector at the terminal end of the Hedgehog signaling (Hh) pathway and is tightly regulated during embryonic development and tissue patterning/differentiation. GLI1 has low-level expression in differentiated tissues, however, in certain cancers, aberrant activation of GLI1 has been linked to the promotion of numerous hallmarks of cancer, such as proliferation, survival, angiogenesis, metastasis, metabolic rewiring, and chemotherapeutic resistance. All of these are driven, in part, by GLI1’s role in regulating cell cycle, DNA replication and DNA damage repair processes. The consequences of GLI1 oncogenic activity, specifically the activity surrounding DNA damage repair proteins, such as NBS1, and cell cycle proteins, such as CDK1, can be linked to tumorigenesis and chemoresistance. Therefore, understanding the underlying mechanisms driving GLI1 dysregulation can provide prognostic and diagnostic biomarkers to identify a patient population that would derive therapeutic benefit from either direct inhibition of GLI1 or targeted therapy towards proteins downstream of GLI1 regulation.

Hedgehog Signaling and Truncated GLI1 in Cancer

The hedgehog (HH) signaling pathway regulates normal cell growth and differentiation. As a consequence of improper control, aberrant HH signaling results in tumorigenesis and supports aggressive phenotypes of human cancers, such as neoplastic transformation, tumor progression, metastasis, and drug resistance. Canonical activation of HH signaling occurs through binding of HH ligands to the transmembrane receptor Patched 1 (PTCH1), which derepresses the transmembrane G protein-coupled receptor Smoothened (SMO). Consequently, the glioma-associated oncogene homolog 1 (GLI1) zinc-finger transcription factors, the terminal effectors of the HH pathway, are released from suppressor of fused (SUFU)-mediated cytoplasmic sequestration, permitting nuclear translocation and activation of target genes. Aberrant activation of this pathway has been implicated in several cancer types, including medulloblastoma, rhabdomyosarcoma, basal cell carcinoma, glioblastoma, and cancers of lung, colon, stomach, pancreas, ovarian, and breast. Therefore, several components of the HH pathway are under investigation for targeted cancer therapy, particularly GLI1 and SMO. GLI1 transcripts are reported to undergo alternative splicing to produce truncated variants: loss-of-function GLI1ΔN and gain-of-function truncated GLI1 (tGLI1). This review covers the biochemical steps necessary for propagation of the HH activating signal and the involvement of aberrant HH signaling in human cancers, with a highlight on the tumor-specific gain-of-function tGLI1 isoform.

Lineage-specific roles of hedgehog-GLI signaling during mammalian kidney development

Aberrant hedgehog (Hh) signaling during embryogenesis results in various severe congenital abnormalities, including renal malformations. The molecular mechanisms that underlie congenital renal malformations remain poorly understood. Here, we review the current understanding of the lineage-specific roles of Hh signaling during renal morphogenesis and how aberrant Hh signaling during embryonic kidney development contributes to renal malformation.

GLI3: a mediator of genetic diseases, development and cancer

The transcription factor GLI3 is a member of the Hedgehog (Hh/HH) signaling pathway that can exist as a full length (Gli3-FL/GLI3-FL) or repressor (Gli3-R/GLI3-R) form. In response to HH activation, GLI3-FL regulates HH genes by targeting the GLI1 promoter. In the absence of HH signaling, GLI3 is phosphorylated leading to its partial degradation and the generation of GLI3-R which represses HH functions. GLI3 is also involved in tissue development, immune cell development and cancer. The absence of Gli3 in mice impaired brain and lung development and GLI3 mutations in humans are the cause of Greig cephalopolysyndactyly (GCPS) and Pallister Hall syndromes (PHS). In the immune system GLI3 regulates B, T and NK-cells and may be involved in LPS-TLR4 signaling. In addition, GLI3 was found to be upregulated in multiple cancers and was found to positively regulate cancerous behavior such as anchorage-independent growth, angiogenesis, proliferation and migration with the exception in acute myeloid leukemia (AML) and medulloblastoma where GLI plays an anti-cancerous role. Finally, GLI3 is a target of microRNA. Here, we will review the biological significance of GLI3 and discuss gaps in our understanding of this molecule. Video Abstract.

Non-canonical Hedgehog Signaling Pathway in Cancer: Activation of GLI Transcription Factors Beyond Smoothened

The Hedgehog-GLI (HH-GLI) pathway is a highly conserved signaling that plays a critical role in controlling cell specification, cell–cell interaction and tissue patterning during embryonic development. Canonical activation of HH-GLI signaling occurs through binding of HH ligands to the twelve-pass transmembrane receptor Patched 1 (PTCH1), which derepresses the seven-pass transmembrane G protein-coupled receptor Smoothened (SMO). Thus, active SMO initiates a complex intracellular cascade that leads to the activation of the three GLI transcription factors, the final effectors of the HH-GLI pathway. Aberrant activation of this signaling has been implicated in a wide variety of tumors, such as those of the brain, skin, breast, gastrointestinal, lung, pancreas, prostate and ovary. In several of these cases, activation of HH-GLI signaling is mediated by overproduction of HH ligands (e.g., prostate cancer), loss-of-function mutations in PTCH1 or gain-of-function mutations in SMO, which occur in the majority of basal cell carcinoma (BCC), SHH-subtype medulloblastoma and rhabdomyosarcoma. Besides the classical canonical ligand-PTCH1-SMO route, mounting evidence points toward additional, non-canonical ways of GLI activation in cancer. By non-canonical we refer to all those mechanisms of activation of the GLI transcription factors occurring independently of SMO. Often, in a given cancer type canonical and non-canonical activation of HH-GLI signaling co-exist, and in some cancer types, more than one mechanism of non-canonical activation may occur. Tumors harboring non-canonical HH-GLI signaling are less sensitive to SMO inhibition, posing a threat for therapeutic efficacy of these antagonists. Here we will review the most recent findings on the involvement of alternative signaling pathways in inducing GLI activity in cancer and stem cells. We will also discuss the rationale of targeting these oncogenic pathways in combination with HH-GLI inhibitors as a promising anti-cancer therapies.

Gli Proteins: Regulation in Development and Cancer

Gli proteins are transcriptional effectors of the Hedgehog signaling pathway. They play key roles in the development of many organs and tissues, and are deregulated in birth defects and cancer. We review the molecular mechanisms of Gli protein regulation in mammals, with special emphasis on posttranslational modifications and intracellular transport. We also discuss how Gli proteins interact with co-activators and co-repressors to fine-tune the expression of Hedgehog target genes. Finally, we provide an overview of the regulation of developmental processes and tissue regeneration by Gli proteins and discuss how these proteins are involved in cancer progression, both through canonical regulation via the Hedgehog pathway and through cross-talk with other signaling pathways.

Survivin, a novel target of the Hedgehog/GLI signaling pathway in human tumor cells

Survivin, an important antiapoptotic protein, is expressed in tumors, whereas in normal tissues the expression of this protein is extremely low, defining a role for survivin as a cancer gene. Survivin exhibits multifunctional activity in tumor cells. However, why survivin expression is sharply and invariably restricted to tumor tissue remains unclear. Here, we identified 11 putative consensus binding sites for GLI transcription factors in the survivin promoter and characterized the promoter activity. Inhibitors of the Hedgehog/GLI pathway, cyclopamine and GANT61, decreased the promoter activity in reporter assays. ΔNGLI2 (which lacks the repressor domain) was the most potent vector in activating the survivin promoter–reporter. Moreover, GANT61, a GLI1/2 inhibitor, repressed endogenous survivin protein and mRNA expression in most cells across a large panel of tumor cell lines. Chromatin immunoprecipitation showed GLI2 binding to the survivin promoter. The ectopic GLI2-evoked expression of endogenous survivin was observed in normal human fibroblasts. GANT61 decreased survivin level in nude mice tumors, mimicking the activity of GANT61 in cultured cells. The immunohistochemistry and double immunofluorescence of human tumors revealed a correlation between the tissue regions showing high GLI2 and survivin positivity. Thus, these results demonstrated that survivin is a classical transcriptional target of GLI2, a Hedgehog pathway signaling effector. This potentially reflects the high expression of survivin in human tumor cells. As the Hedgehog pathway is upregulated in virtually all types of cancer cells, these findings substantially contribute to the explanation of uniform survivin expression in tumors as a potential target for the development of a more effective treatment of cancers through the inhibition of GLI2 to restrain survivin activity.

Urogenital development in Pallister-Hall syndrome is disrupted in a cell-lineage-specific manner by constitutive expression of GLI3 repressor

Pallister-Hall syndrome (PHS) is a rare disorder caused by mutations in GLI3 that produce a transcriptional repressor (GLI3R). Individuals with PHS present with a variably penetrant variety of urogenital system malformations, including renal aplasia or hypoplasia, hydroureter, hydronephrosis or a common urogenital sinus. The embryologic mechanisms controlled by GLI3R that result in these pathologic phenotypes are undefined. We demonstrate that germline expression of GLI3R causes renal hypoplasia, associated with decreased nephron number, and hydroureter and hydronephrosis, caused by blind-ending ureters. Mice with obligate GLI3R expression also displayed duplication of the ureters that was caused by aberrant common nephric duct patterning and ureteric stalk outgrowth. These developmental abnormalities are associated with suppressed Hedgehog signaling activity in the cloaca and adjacent vesicular mesenchyme. Mice with conditional expression of GLI3R were utilized to identify lineage-specific effects of GLI3R. In the ureteric bud, GLI3R expression decreased branching morphogenesis. In Six2-positive nephrogenic progenitors, GLI3R decreased progenitor cell proliferation reducing the number of nephrogenic precursor structures. Using mutant mice with Gli3R and Gli3 null alleles, we demonstrate that urogenital system patterning and development is controlled by the levels of GLI3R and not by an absence of full-length GLI3. We conclude that the urogenital system phenotypes observed in PHS are caused by GLI3R-dependent perturbations in nephric duct patterning, renal branching morphogenesis and nephrogenic progenitor self-renewal.

The transcriptional repressor domain of Gli3 is intrinsically disordered

The transcription factor Gli3 is acting mainly as a transcriptional repressor in the Sonic hedgehog signal transduction pathway. Gli3 contains a repressor domain in its N-terminus from residue G106 to E236. In this study we have characterized the intracellular structure of the Gli3 repressor domain using a combined bioinformatics and experimental approach. According to our findings the Gli3 repressor domain while being intrinsically disordered contains predicted anchor sites for partner interactions. The obvious interaction partners to test were Ski and DNA; however, with both of these the structure of Gli3 repressor domain remained disordered. To locate residues important for the repressor function we mutated several residues within the Gli3 repressor domain. Two of these, H141A and H157N, targeting predicted helical regions, significantly decreased transcriptional repression and thus identify important functional parts of the domain.

Crosstalk between TGF-β and hedgehog signaling in cancer

Hedgehog (HH) and TGF-β signals control various aspects of embryonic development and cancer progression. While their canonical signal transduction cascades have been well characterized, there is increasing evidence that these pathways are able to exert overlapping activities that challenge efficient therapeutic targeting. We herein review the current knowledge on HH signaling and summarize the recent findings on the crosstalks between the HH and TGF-β pathways in cancer.

Reduction of human embryonal rhabdomyosarcoma tumor growth by inhibition of the hedgehog signaling pathway

Rhabdomyosarcoma (RMS) is the most frequent soft-tissue sarcoma in children. Embryonal rhabdomyosarcoma (E-RMS) represents the most common RMS subtype, but the molecular events driving this tumor are still largely unknown. The hedgehog (HH) pathway, a major signal transduction cascade, is linked with many cancers, including RMS. As we previously have detected loss of heterozygosity of PTCH1 in E-RMS, we now examined 8 E-RMS tumor samples and 5 E-RMS cell lines for the presence of PTCH1 mutations, but none was detected. However, in the E-RMS cell lines, a variable pattern of up-regulated expression of certain HH signaling target genes, including HHIP, PTCH1, SFRP1, and GLI1, was observed. Moreover, treatment with the small molecule HH signaling inhibitors cyclopamine and GANT61 inhibited cell proliferation in all E-RMS cell lines analyzed. Interestingly, GANT61 was more effective, and this was accompanied by increased apoptosis, while cyclopamine promoted necrotic events. Specific knockdown of SMO had no effect on the proliferation of E-RMS cells, indicating the presence of an SMO-independent HH signaling pathway in the E-RMS cell lines. Furthermore, in an in vivo xenograft model, tumor growth was significantly reduced by GANT61 treatment of E-RMS cells. Additionally, siRNA experiments provided evidence that inhibition of GLI1 or GLI3 but not GLI2 was sufficient to reduce proliferation of these cell lines. As GANT61 is known to block GLI1/GLI2 transcriptional activity, the inhibition of E-RMS growth by GANT61 is likely to be mediated through GLI1. In conclusion, our findings implicate that GLI1 could constitute an effective therapeutic target in pediatric E-RMS.

Holoprosencephaly in a family segregating novel variants in ZIC2 and GLI2

Holoprosencephaly (HPE) is the most common malformation of the human forebrain. Typical manifestations in affected patients include a characteristic pattern of structural brain and craniofacial anomalies. HPE may be caused by mutations in over 10 identified genes; the inheritance is traditionally viewed as autosomal dominant with highly variable expressivity and incomplete penetrance. We present the description of a family simultaneously segregating two novel variants in the HPE-associated genes, ZIC2 and GLI2, as well as the results of extensive population-based studies of the variant region in GLI2. This is the first time that multiple HPE-associated variants in these genes have been reported in one family, and raises important questions about how clinicians and researchers should view the inheritance of conditions such as HPE.

Genital abnormalities in Pallister-Hall syndrome: Report of two patients and review of the literature

We describe two patients with Pallister-Hall syndrome (PHS) with genital abnormalities: a female with hydrometrocolpos secondary to vaginal atresia and a male with micropenis, hypoplastic scrotum, and bilateral cryptorchidism. Nonsense mutations in GLI3 were identified in both patients. Clinical and molecular findings of 12 previously reported patients who had GLI3 mutations and genital abnormalities were reviewed. Genital features in the male patients included hypospadias, micropenis, and bifid or hypoplastic scrotum, whereas all the females had hydrometrocolpos and/or vaginal atresia. No hotspot for GLI3 mutations has been found. The urogenital and anorectal abnormalities associated with PHS might be related to dysregulation of SHH signaling caused by GLI3 mutations rather than hormonal aberrations. We recommend that clinical investigations of genital abnormalities are considered in patients with PHS, even those without hypopituitarism.

Sonic hedgehog signaling in the developing CNS where it has been and where it is going

Sonic Hedgehog (Shh) is one of three mammalian orthologs of the Hedgehog (Hh) family of secreted proteins first identified for their role in patterning the Drosophila embryo. In this review, we will highlight some of the outstanding questions regarding how Shh signaling controls embryonic development. We will mainly consider its role in the developing mammalian central nervous system (CNS) where the pathway plays a critical role in orchestrating the specification of distinct cell fates within ventral regions, a process of exquisite complexity that is necessary for the proper wiring and hence function of the mature system. Embryonic development is a process that plays out in both the spatial and the temporal dimensions, and it is becoming increasingly clear that our understanding of Shh signaling in the CNS is grounded in an appreciation for the dynamic nature of this process. In addition, any consideration of Hh signaling must by necessity include a consideration of data from many different model organisms and systems. In many cases, the extent to which insights gained from these studies are applicable to the CNS remains to be determined, yet they provide a strong framework in which to explore its role in CNS development. We will also discuss how Shh controls cell fate diversification through the regulation of patterned target gene expression in the spinal cord, a region where our understanding of the morphogenetic action of graded Shh signaling is perhaps the furthest advanced.

Frequent deregulations in the hedgehog signaling network and cross-talks with the epidermal growth factor receptor pathway involved in cancer progression and targeted therapies

The hedgehog (Hh)/glioma-associated oncogene (GLI) signaling network is among the most important and fascinating signal transduction systems that provide critical functions in the regulation of many developmental and physiological processes. The coordinated spatiotemporal interplay of the Hh ligands and other growth factors is necessary for the stringent control of the behavior of diverse types of tissue-resident stem/progenitor cells and their progenies. The activation of the Hh cascade might promote the tissue regeneration and repair after severe injury in numerous organs, insulin production in pancreatic beta-cells, and neovascularization. Consequently, the stimulation of the Hh pathway constitutes a potential therapeutic strategy to treat diverse human disorders, including severe tissue injuries; diabetes mellitus; and brain, skin, and cardiovascular disorders. In counterbalance, a deregulation of the Hh signaling network might lead to major tissular disorders and the development of a wide variety of aggressive and metastatic cancers. The target gene products induced through the persistent Hh activation can contribute to the self-renewal, survival, migration, and metastasis of cancer stem/progenitor cells and their progenies. Moreover, the pivotal role mediated through the Hh/GLI cascade during cancer progression also implicates the cooperation with other oncogenic products, such as mutated K-RAS and complex cross-talk with different growth factor pathways, including tyrosine kinase receptors, such as epidermal growth factor receptor (EGFR), Wnt/beta-catenin, and transforming growth factor-beta (TGF-beta)/TGF-beta receptors. Therefore, the molecular targeting of distinct deregulated gene products, including Hh and EGFR signaling components and other signaling elements that are frequently deregulated in highly tumorigenic cancer-initiating cells and their progenies, might constitute a potential therapeutic strategy to eradicate the total cancer cell mass. Of clinical interest is that these multitargeted approaches offer great promise as adjuvant treatments for improving the current antihormonal therapies, radiotherapies, and/or chemotherapies against locally advanced and metastatic cancers, thereby preventing disease relapse and the death of patients with cancer.

Dual function of UNC-51-like kinase 3 (Ulk3) in the Sonic hedgehog signaling pathway

The Sonic hedgehog (Shh) signaling pathway controls a variety of developmental processes and is implicated in tissue homeostasis maintenance and neurogenesis in adults. Recently, we identified Ulk3 as an active kinase able to positively regulate Gli proteins, mediators of the Shh signaling in mammals. Here, we provide several lines of evidence that Ulk3 participates in the transduction of the Shh signal also independently of its kinase activity. We demonstrate that Ulk3 through its kinase domain interacts with Suppressor of Fused (Sufu), a protein required for negative regulation of Gli proteins. Sufu blocks Ulk3 autophosphorylation and abolishes its ability to phosphorylate and positively regulate Gli proteins. We show that Shh signaling destabilizes the Sufu-Ulk3 complex and induces the release of Ulk3. We demonstrate that the Sufu-Ulk3 complex, when co-expressed with Gli2, promotes generation of the Gli2 repressor form, and that reduction of the Ulk3 mRNA level in Shh-responsive cells results in higher potency of the cells to transmit the Shh signal. Our data suggests a dual function of Ulk3 in the Shh signal transduction pathway and propose an additional way of regulating Gli proteins by Sufu, through binding to and suppression of Ulk3.

Birth defects caused by mutations in human GLI3 and mouse Gli3 genes

ABSTRACT GLI3 is the gene responsible for Greig cephalopolysyndactyly syndrome (GCPS), Pallister-Hall syndrome (PHS) and Postaxial polydactyly type-A (PAP-A). Genetic polydactyly mice such as Pdn/Pdn (Polydactyly Nagoya), Xt(H)/Xt(H) (Extra toes) and Xt(J)/Xt(J) (Extra toes Jackson) are the mouse homolog of GCPS, and Gli3(tmlUrtt)/Gli3(tmlUrt) is produced as the mouse homolog of PHS. In the present review, relationships between mutation points of GLI3 and Gli3, and resulting phenotypes in humans and mice are described. It has been confirmed that mutation in the upstream or within the zinc finger domain of the GLI3 gene induces GCPS; that in the post-zinc finger region including the protease cleavage site induces PHS; and that in the downstream of the GLI3 gene induces PAP-A. A mimicking phenomenon was observed in the mouse homolog. Therefore, human GLI3 and mouse Gli3 genes have a common structure, and it is suggested here that mutations in the same functional regions produce similar phenotypes in human and mice. The most important issue might be that GCPS and PHS exhibit an autosomal dominant trait, but mouse homologs, such as Pdn/Pdn, Xt(H)/Xt(H), Xt(J)/Xt(J) and Gli3(tmlUrt)/Gli3(tmlUrt), are autosomal recessive traits in the manifestation of similar phenotypes to human diseases. It is discussed here how the reduced amounts of the GLI3 protein, or truncated mutant GLI3 protein, disrupt development of the limbs, head and face.