Regulation of GLI1 by cis DNA elements and epigenetic marks

Single-cell analysis of lizard blastema fibroblasts reveals phagocyte-dependent activation of Hedgehog-responsive chondrogenesis

Lizards cannot naturally regenerate limbs but are the closest known relatives of mammals capable of epimorphic tail regrowth. However, the mechanisms regulating lizard blastema formation and chondrogenesis remain unclear. Here, single-cell RNA sequencing analysis of regenerating lizard tails identifies fibroblast and phagocyte populations linked to cartilage formation. Pseudotime trajectory analyses suggest spp1+-activated fibroblasts as blastema cell sources, with subsets exhibiting sulf1 expression and chondrogenic potential. Tail blastema, but not limb, fibroblasts express sulf1 and form cartilage under Hedgehog signaling regulation. Depletion of phagocytes inhibits blastema formation, but treatment with pericytic phagocyte-conditioned media rescues blastema chondrogenesis and cartilage formation in amputated limbs. The results indicate a hierarchy of phagocyte-induced fibroblast gene activations during lizard blastema formation, culminating in sulf1+ pro-chondrogenic populations singularly responsive to Hedgehog signaling. These properties distinguish lizard blastema cells from homeostatic and injury-stimulated fibroblasts and indicate potential actionable targets for inducing regeneration in other species, including humans.

Noncanonical activation of GLI signaling in SOX2+ cells drives medulloblastoma relapse

SRY (sex determining region Y)-box 2 (SOX2)-labeled cells play key roles in chemoresistance and tumor relapse; thus, it is critical to elucidate the mechanisms propagating them. Single-cell transcriptomic analyses of the most common malignant pediatric brain tumor, medulloblastoma (MB), revealed the existence of astrocytic Sox2+ cells expressing sonic hedgehog (SHH) signaling biomarkers. Treatment with vismodegib, an SHH inhibitor that acts on Smoothened (Smo), led to increases in astrocyte-like Sox2+ cells. Using SOX2-enriched MB cultures, we observed that SOX2+ cells required SHH signaling to propagate, and unlike in the proliferative tumor bulk, the SHH pathway was activated in these cells downstream of Smo in an MYC-dependent manner. Functionally different GLI inhibitors depleted vismodegib-resistant SOX2+ cells from MB tissues, reduced their ability to further engraft in vivo, and increased symptom-free survival. Our results emphasize the promise of therapies targeting GLI to deplete SOX2+ cells and provide stable tumor remission.

Horizontal transfer of the stemness-related markers EZH2 and GLI1 by neuroblastoma-derived extracellular vesicles in stromal cells

Neuroblastoma (NB) is the most common extracranial pediatric solid cancer originating from undifferentiated neural crest cells. NB cells express EZH2 and GLI1 genes that are known to maintain the undifferentiated phenotype of cancer stem cells (CSC) in NB. Recent studies suggest that tumor-derived extracellular vesicles (EVs) can regulate the transformation of surrounding cells into CSC by transferring tumor-specific molecules they contain. However, the horizontal transfer of EVs molecules in NB remains largely unknown. We report the analysis of NB-derived EVs in bioengineered models of NB that are based on a collagen 1/hyaluronic acid scaffold designed to mimic the native tumor niche. Using these models, we observed an enrichment of GLI1 and EZH2 mRNAs in NB-derived EVs. As a consequence of the uptake of NB-derived EVs, the host cells increased the expression levels of GLI1 and EZH2. These results suggest the alteration of the expression profile of stromal cells through an EV-based mechanism, and point the GLI1 and EZH2 mRNAs in the EV cargo as diagnostic biomarkers in NB.

Arid1a-Plagl1-Hh signaling is indispensable for differentiation-associated cell cycle arrest of tooth root progenitors

Chromatin remodelers often show broad expression patterns in multiple cell types yet can elicit cell-specific effects in development and diseases. Arid1a binds DNA and regulates gene expression during tissue development and homeostasis. However, it is unclear how Arid1a achieves its functional specificity in regulating progenitor cells. Using the tooth root as a model, we show that loss of Arid1a impairs the differentiation-associated cell cycle arrest of tooth root progenitors through Hedgehog (Hh) signaling regulation, leading to shortened roots. Our data suggest that Plagl1, as a co-factor, endows Arid1a with its cell-type/spatial functional specificity. Furthermore, we show that loss of Arid1a leads to increased expression of Arid1b, which is also indispensable for odontoblast differentiation but is not involved in regulation of Hh signaling. This study expands our knowledge of the intricate interactions among chromatin remodelers, transcription factors, and signaling molecules during progenitor cell fate determination and lineage commitment.

CRISPR editing of the GLI1 first intron abrogates GLI1 expression and differentially alters lineage commitment

GLI1 is one of three GLI family transcription factors that mediate Sonic Hedgehog signaling, which plays a role in development and cell differentiation. GLI1 forms a positive feedback loop with GLI2 and likely with itself. To determine the impact of GLI1 and its intronic regulatory locus on this transcriptional loop and human stem cell differentiation, we deleted the region containing six GLI binding sites in the human GLI1 intron using CRISPR/Cas9 editing to produce H1 human embryonic stem cell (hESC) GLI1‐edited clones. Editing out this intronic region, without removing the entire GLI1 gene, allowed us to study the effects of this highly complex region, which binds transcription factors in a variety of cells. The roles of GLI1 in human ESC differentiation were investigated by comparing RNA sequencing, quantitative‐real time PCR (q‐rtPCR), and functional assays. Editing this region resulted in GLI1 transcriptional knockdown, delayed neural commitment, and inhibition of endodermal and mesodermal differentiation during spontaneous and directed differentiation experiments. We found a delay in the onset of early osteogenic markers, a reduction in the hematopoietic potential to form granulocyte units, and a decrease in cancer‐related gene expression. Furthermore, inhibition of GLI1 via antagonist GANT‐61 had similar in vitro effects. These results indicate that the GLI1 intronic region is critical for the feedback loop and that GLI1 has lineage‐specific effects on hESC differentiation. Our work is the first study to document the extent of GLI1 abrogation on early stages of human development and to show that GLI1 transcription can be altered in a therapeutically useful way.

GLI1/GLI2 functional interplay is required to control Hedgehog/GLI targets gene expression

The Hedgehog-regulated transcription factors GLI1 and GLI2 play overlapping roles in development and disease; however, the mechanisms underlying their interplay remain elusive. We report for the first time that GLI1 and GLI2 physically and functionally interact in cancer cells. GLI1 and GLI2 were shown to co-immunoprecipitate in PANC1 pancreatic cancer cells and RMS13 rhabdomyosarcoma cells. Mapping analysis demonstrated that the zinc finger domains of both proteins are required for their heteromerization. RNAi knockdown of either GLI1 or GLI2 inhibited expression of many well-characterized GLI target genes (BCL2, MYCN, PTCH2, IL7 and CCND1) in PANC1 cells, whereas PTCH1 expression was only inhibited by GLI1 depletion. qPCR screening of a large set of putative canonical and non-canonical Hedgehog/GLI targets identified further genes (e.g. E2F1, BMP1, CDK2) strongly down-regulated by GLI1 and/or GLI2 depletion in PANC1 cells, and demonstrated that ANO1, AQP1 and SOCS1 are up-regulated by knockdown of either GLI1 or GLI2. Chromatin immunoprecipitation showed that GLI1 and GLI2 occupied the same regions at the BCL2, MYCN and CCND1 promoters. Furthermore, depletion of GLI1 inhibited GLI2 occupancy at these promoters, suggesting that GLI1/GLI2 interaction is required for the recruitment of GLI2 to these sites. Together, these findings indicate that GLI1 and GLI2 co-ordinately regulate the transcription of some genes, and provide mechanistic insight into the roles of GLI proteins in carcinogenesis.

Cancer Stem Cells Equipped with Powerful Hedgehog Signaling and Better Epigenetic Memory: Avenues to Look for Cancer Therapeutics

Complex nature of the tumor is depicted at the cellular landscape by showing heterogeneity in the presence of cancer cells, cancer-associated stromal cells, mesenchymal stem cells and cancer stem cells (CSCs). One of the plausible views in cancer formation is suggested as the theory of cancer CSCs that is known as a source of initiation of tumorigenesis. In essence, these powerful CSCs are equipped with high Sonic Hedgehog (SHH) signaling and epigenetic memory power that support various tumor hallmarks. Truly, nature justifies its intent by limiting these stem cells with a potential to turn into CSCs and in turn suppressing the high risk of humans and other organisms. In short, this mini-review addresses the contribution of SHH signaling to allow reprogramming of epigenetic memory within CSCs that support tumor hallmarks. Besides, this paper explores therapeutic approaches to mitigate SHH signaling that may lead to a blockade of the pro-tumor potential of CSCs.

A Shh/Gli-driven three-node timer motif controls temporal identity and fate of neural stem cells

How time is measured by neural stem cells during temporal neurogenesis has remained unresolved. By combining experiments and computational modeling, we define a Shh/Gli-driven three-node timer underlying the sequential generation of motor neurons (MNs) and serotonergic neurons in the brainstem. The timer is founded on temporal decline of Gli-activator and Gli-repressor activities established through down-regulation of Gli transcription. The circuitry conforms an incoherent feed-forward loop, whereby Gli proteins not only promote expression of Phox2b and thereby MN-fate but also account for a delayed activation of a self-promoting transforming growth factor-β (Tgfβ) node triggering a fate switch by repressing Phox2b. Hysteresis and spatial averaging by diffusion of Tgfβ counteract noise and increase temporal accuracy at the population level, providing a functional rationale for the intrinsically programmed activation of extrinsic switch signals in temporal patterning. Our study defines how time is reliably encoded during the sequential specification of neurons.

The role of Shh signalling pathway in central nervous system development and related diseases

Sonic hedgehog (Shh) plays important roles in developmental of vertebrate animal central nervous system (CNS), and Gli is its downstream signal molecule. Shh signalling is essential for pattern formation, cell-fate specification, axon guidance, proliferation, survival and differentiation of neurons in CNS development. The abnormal signalling pathway of Shh leads to the occurrence of many nervous system diseases. The mechanism of Shh signalling is complex and remains incompletely understood. Nevertheless, studies have revealed that Shh signalling pathway is classified into canonical and non-canonical pathways. Here we review the role of the Shh signalling pathway and its impact in CNS development and related diseases. Specifically, we discuss the role of Shh in the spinal cord and brain development, cell differentiation and proliferation in CNS and related diseases such as brain tumour, Parkinson's diseases, epilepsy, autism, depression and traumatic brain injury. We also highlight future directions of research that could help to clarify the mechanisms and consequences of Shh signalling in the process of CNS development and related diseases. SIGNIFICANCE OF THE STUDY: This review summarized the role of Shh signalling pathway in CNS development and related diseases such as brain tumour, Parkinson's diseases, epilepsy, autism, depression and traumatic brain injury. It also presented the author's opinions on the future research direction of Shh signalling pathway.

The bromodomain inhibitor IBET-151 attenuates vismodegib-resistant esophageal adenocarcinoma growth through reduction of GLI signaling

The Hedgehog/GLI (HH/GLI) signaling pathway plays a critical role in human oncogenesis. Unfortunately, the clinical use of HH inhibitor(s) has been associated with serious adverse effects and mutation-related drug resistance. Since the efficacy of SMO (Smoothened) and GLI inhibitors is limited in clinical trials, there remains a critical need for the HH/GLI pathway inhibitors with different mechanisms of action. Here, we show that esophageal adenocarcinoma (EAC) cell lines are insensitive to vismodegib (SMO inhibitor) but respond to GANT61 (GLI1 inhibitor). Furthermore, we examine the role of GLI1 in tumorigenicity of EAC and how a selective bromodomain inhibitor IBET-151 downregulates transcriptional activity of the GLI1 transcription factor in EAC. Our study demonstrates that GLI1 plays an important role in tumorigenicity of EAC and that elevated GLI1 expression in patients’ ultrasound-assisted endoscopic biopsy may predict the response to neoadjuvant chemotherapy (NAC) FOLFOX. Importantly, IBET-151 abrogates the growth of vismodegib-resistant EAC cells and downregulates HH/GLI by reducing the occupancy of BRD4 at the GLI1 locus. IBET-151 also attenuates tumor growth of EAC-PDXs and does so in an on-target manner as it reduces the expression of GLI1. We identify HH/GLI signaling as a novel druggable pathway in EAC as well as validate an ability of clinically relevant GLI inhibitor to attenuate the viability of vismodegib-resistant EAC cells. Therefore, we propose that selective bromodomain inhibitors, such as IBET-151, could be used as novel therapeutic agents for EAC patients harboring GLI-dependent tumors.

Up-regulation of GLI1 in vincristine-resistant rhabdomyosarcoma and Ewing sarcoma

Background

The clinical significance of GLI1 expression either through canonical Hedgehog signal transduction or through non-canonical mechanisms in rhabdomyosarcoma (RMS) or Ewing sarcoma (EWS) is incompletely understood. We tested a role for Hedgehog (HH) signal transduction and GL11 expression in development of vincristine (VCR) resistance in RMS and EWS.

Methods

We characterized baseline expression and activity of HH pathway components in 5 RMS (RD, Rh18, Ruch-2, Rh30, and Rh41) and 5 EWS (CHLA9, CHLA10, TC32, CHLA258, and TC71) cell lines. We then established VCR-resistant RMS and EWS cell lines by exposing cells to serially increasing concentrations of VCR and determining the IC₅₀. We defined resistance as a ≥ 30-fold increase in IC₅₀ compared with parental cells. We determined changes in gene expression in the VCR-resistant cells compared with parental cells using an 86-gene cancer drug resistance array that included GLI1 and tested the effect of GLI1 inhibition with GANT61 or GLI1 siRNA on VCR resistance.

Results

We found evidence for HH pathway activity and GLI1 expression in RMS and EWS cell lines at baseline, and evidence that GLI1 contributes to survival and proliferation of these sarcoma cells. We were able to establish 4 VCR-resistant cell lines (Ruch-2VR, Rh30VR, Rh41VR, and TC71VR). GLI1 was significantly up-regulated in the Rh30VR, Rh41VR, and TC71VR cells. The only other gene in the drug resistance panel that was significantly up-regulated in each of these VCR-resistant cell lines compared with their corresponding parental cells was the GLI1 direct target and multidrug resistance gene, ATP-binding cassette sub-family B member 1 (MDR1). We established major vault protein (MVP), which was up-regulated in both vincristine-resistant alveolar RMS cell lines (Rh30VR and Rh41VR), as another direct target of GLI1 during development of drug resistance. Treatment of the VCR-resistant cell lines with the small molecule inhibitor GANT61 or GLI1 siRNA together with VCR significantly decreased cell viability at doses that did not reduce viability individually.

Conclusions

These experiments demonstrate that GLI1 up-regulation contributes to VCR resistance in RMS and EWS cell lines and suggest that targeting GLI1 may benefit patients with RMS or EWS by reducing multidrug resistance.

Cyclopamine functions as a suppressor of benign prostatic hyperplasia by inhibiting epithelial and stromal cell proliferation via suppression of the Hedgehog signaling pathway

Stromal-epithelial interaction serves a pivotal role in normal prostate growth, as well as the onset of benign prostatic hyperplasia (BPH). The present study aimed to explore the role of cyclopamine in the proliferation and apoptosis of epithelial and stromal cells in rats with BPH by blocking the Hedgehog signaling pathway. Cyclopamine (an inhibitor of the Hedgehog signaling pathway) was administered in a rat model of BPH, and the expression of Ki67 (proliferation factor) was determined by immunohistochemistry. In addition, epithelial and stromal cells were separated and cultured in order to investigate the role of cyclopamine in the progression of BPH. The expression of Hedgehog signaling pathway- and apoptosis-related genes, including basic fibroblastic growth factor (b-FGF) and transforming growth factor β (TGF-β), was evaluated using reverse transcription-quantitative polymerase chain reaction and western blot analysis. Cell proliferation, cell cycle and apoptosis were analyzed using an MTT assay and flow cytometry. We identified upregulated Ki67 expression and activated Hedgehog signaling pathway in rats with BPH. Cyclopamine inhibited the activation of the Hedgehog signaling pathway. In response to cyclopamine treatment, epithelial and stromal cell proliferation was inhibited; this was concomitant with decreased Ki67, TGF-β, and b-FGF expression. On the other hand, epithelial cell apoptosis was enhanced, which was associated with increased Bax and reduced Bcl-2 expression. Based on these findings, we proposed that cyclopamine may serve as a potential therapeutic agent in the treatment of BPH. Cyclopamine could inhibit epithelial and stromal cell proliferation, and induce epithelial cell apoptosis by suppressing the Hedgehog signaling pathway.