Mechanisms of Hedgehog signalling in cancer

Importance of targeting various cell signaling pathways in solid cancers

Most adult human cancers are solid tumors prevailing in vital organs and lead to mortality all over the globe. Genetic and epigenetic alterations in cancer genes or genes of associated signaling pathways impart the most common characteristic of malignancy, that is, uncontrolled proliferation. Unless the mechanism of action of these cells signaling pathways (involved in cell proliferation, apoptosis, metastasis, and the maintenance of the stemness of cancer stem cells and cancer microenvironment) and their physiologic alteration are extensively studied, it is challenging to understand tumorigenesis as well as develop new treatments and precision medicines. Targeted therapy is one of the most promising strategies for treating various cancers. However, cancer is an evolving disease, and most patients develop resistance to these drugs by acquired mutations or mediation of microenvironmental factors or due to tumor heterogeneity. Researchers are striving to develop novel therapeutic options like combinatorial approaches targeting multiple responsible pathways effectively. Thus, in-depth knowledge of cell signaling and its components remains a critical topic of cancer research. This chapter summarized various extensively studied pathways in solid cancer and how they are targeted for therapeutic strategies.

Cancer stem cells and angiogenesis

Cancer remains the primary cause of mortality in developed nations. Although localized tumors can be effectively addressed through surgery, radiotherapy, and other targeted methods, drug efficacy often wanes in the context of metastatic diseases. As a result, significant efforts are being made to develop drugs capable of not only inhibiting tumor growth but also impeding the metastasis of malignant tumors, with a focus on hindering their migration to adjacent organs. Cancer stem cells metastasize via blood and lymphatic vessels, exhibiting a high mutation rate, significant variability, and a predisposition to drug resistance. In contrast, endothelial cells, being less prone to mutation, are less likely to give rise to drug-resistant clones. Furthermore, the direct contact of circulating anti-angiogenic drugs with vascular endothelial cells expedites their therapeutic impact. Hence, anti-angiogenesis targeted therapy assumes a pivotal role in cancer treatment. This paper provides a succinct overview of the molecular mechanisms governing the interaction between cancer stem cells and angiogenesis.

Ligand-dependent hedgehog signaling maintains an undifferentiated, malignant osteosarcoma phenotype

TP53 and RB1 loss-of-function mutations are common in osteosarcoma. During development, combined loss of TP53 and RB1 function leads to downregulation of autophagy and the aberrant formation of primary cilia, cellular organelles essential for the transmission of canonical Hedgehog (Hh) signaling. Excess cilia formation then leads to hypersensitivity to Hedgehog (Hh) ligand signaling. In mouse and human models, we now show that osteosarcomas with mutations in TP53 and RB1 exhibit enhanced ligand-dependent Hh pathway activation through Smoothened (SMO), a transmembrane signaling molecule required for activation of the canonical Hh pathway. This dependence is mediated by hypersensitivity to Hh ligand and is accompanied by impaired autophagy and increased primary cilia formation and expression of Hh ligand in vivo. Using a conditional genetic mouse model of Trp53 and Rb1 inactivation in osteoblast progenitors, we further show that deletion of Smo converts the highly malignant osteosarcoma phenotype to benign, well differentiated bone tumors. Conversely, conditional overexpression of SHH ligand, or a gain-of-function SMO mutant in committed osteoblast progenitors during development blocks terminal bone differentiation. Finally, we demonstrate that the SMO antagonist sonidegib (LDE225) induces growth arrest and terminal differentiation in vivo in osteosarcomas that express primary cilia and Hh ligand combined with mutations in TP53. These results provide a mechanistic framework for aberrant Hh signaling in osteosarcoma based on defining mutations in the tumor suppressor, TP53.

RNA-seq and ChIP-seq Identification of Unique and Overlapping Targets of GLI Transcription Factors in Melanoma Cell Lines

BACKGROUND

Despite significant progress in therapy, melanoma still has a rising incidence worldwide, and novel treatment strategies are needed. Recently, researchers have recognized the involvement of the Hedgehog-GLI (HH-GLI) signaling pathway in melanoma and its consistent crosstalk with the MAPK pathway. In order to further investigate the link between the two pathways and to find new target genes that could be considered for combination therapy, we set out to find transcriptional targets of all three GLI proteins in melanoma.

METHODS

We performed RNA sequencing on three melanoma cell lines (CHL-1, A375, and MEL224) with overexpressed GLI1, GLI2, and GLI3 and combined them with the results of ChIP-sequencing on endogenous GLI1, GLI2, and GLI3 proteins. After combining these results, 21 targets were selected for validation by qPCR.

RESULTS

RNA-seq revealed a total of 808 differentially expressed genes (DEGs) for GLI1, 941 DEGs for GLI2, and 58 DEGs for GLI3. ChIP-seq identified 527 genes that contained GLI1 binding sites in their promoters, 1103 for GLI2 and 553 for GLI3. A total of 15 of these targets were validated in the tested cell lines, 6 of which were detected by both RNA-seq and ChIP-seq.

CONCLUSIONS

Our study provides insight into the unique and overlapping transcriptional output of the GLI proteins in melanoma. We suggest that our findings could provide new potential targets to consider while designing melanoma-targeted therapy.

Hedgehog Autoprocessing: From Structural Mechanisms to Drug Discovery

Hedgehog (Hh) signaling plays pivotal roles in embryonic development. In adults, Hh signaling is mostly turned off but its abnormal activation is involved in many types of cancer. Hh signaling is initiated by the Hh ligand, generated from the Hh precursor by a specialized autocatalytic process called Hh autoprocessing. The Hh precursor consists of an N-terminal signaling domain (HhN) and a C-terminal autoprocessing domain (HhC). During Hh autoprocessing, the precursor is cleaved between N- and C-terminal domain followed by the covalent ligation of cholesterol to the last residue of HhN, which subsequently leads to the generation of Hh ligand for Hh signaling. Hh autoprocessing is at the origin of canonical Hh signaling and precedes all downstream signaling events. Mutations in the catalytic residues in HhC can lead to congenital defects such as holoprosencephaly (HPE). The aim of this review is to provide an in-depth summary of the progresses and challenges towards an atomic level understanding of the structural mechanisms of Hh autoprocessing. We also discuss drug discovery efforts to inhibit Hh autoprocessing as a new direction in cancer therapy.

Trp53 and Rb1 regulate autophagy and ligand-dependent Hedgehog signaling

Ligand-dependent activation of Hedgehog (Hh) signaling in cancer occurs without mutations in canonical pathway genes. Consequently, the genetic basis of Hh pathway activation in adult solid tumors, such as small-cell lung cancer (SCLC), is unknown. Here we show that combined inactivation of Trp53 and Rb1, a defining genetic feature of SCLC, leads to hypersensitivity to Hh ligand in vitro, and during neural tube development in vivo. This response is associated with the aberrant formation of primary cilia, an organelle essential for canonical Hh signaling through smoothened, a transmembrane protein targeted by small-molecule Hh inhibitors. We further show that loss of both Trp53 and Rb1 disables transcription of genes in the autophagic machinery necessary for the degradation of primary cilia. In turn, we also demonstrate a requirement for Kif3a, a gene essential for the formation of primary cilia, in a mouse model of SCLC induced by conditional deletion of both Trp53 and Rb1 in the adult airway. Our results provide a mechanistic framework for therapeutic targeting of ligand-dependent Hh signaling in human cancers with somatic mutations in both TP53 and RB1.

The Role of Smoothened in Cancer

Smoothened (SMO) belongs to the Hedgehog (HH) signaling pathway, which regulates cell growth, migration, invasion and stem cells in cancer. The HH signaling pathway includes both canonical and noncanonical pathways. The canonical HH pathway functions through major HH molecules such as HH ligands, PTCH, SMO and GLI, whereas the noncanonical HH pathway involves the activation of SMO or GLI through other pathways. The role of SMO has been discussed in different types of cancer, including breast, liver, pancreatic and colon cancers. SMO expression correlates with tumor size, invasiveness, metastasis and recurrence. In addition, SMO inhibitors can suppress cancer formation, reduce the proliferation of cancer cells, trigger apoptosis and suppress cancer stem cell activity. A better understanding of the role of SMO in cancer could contribute to the development of novel therapeutic approaches.

p53 and RB1 regulate Hedgehog responsiveness via autophagy-mediated ciliogenesis

Loss of tumor protein p53 (p53) and RB transcriptional corepressor 1 (RB1) in developmental and small cell lung cancer models promotes primary cilia formation and hyper-responsiveness to Hedgehog ligand. This is mediated by impaired transcription of p53 and RB1 target genes involved in autophagic degradation of primary cilia.

Cancer Stem Cells: Acquisition, Characteristics, Therapeutic Implications, Targeting Strategies and Future Prospects

Since last two decades, the major cancer research has focused on understanding the characteristic properties and mechanism of formation of Cancer stem cells (CSCs), due to their ability to initiate tumor growth, self-renewal property and multi-drug resistance. The discovery of the mechanism of acquisition of stem-like properties by carcinoma cells via epithelial-mesenchymal transition (EMT) has paved a way towards a deeper understanding of CSCs and presented a possible avenue for the development of therapeutic strategies. In spite of years of research, various challenges, such as identification of CSC subpopulation, lack of appropriate experimental models, targeting cancer cells and CSCs specifically without harming normal cells, are being faced while dealing with CSCs. Here, we discuss the biology and characteristics of CSCs, mode of acquisition of stemness (via EMT) and development of multi-drug resistance, the role of tumor niche, the process of dissemination and metastasis, therapeutic implications of CSCs and necessity of targeting them. We emphasise various strategies being developed to specifically target CSCs, including those targeting biomarkers, key pathways and microenvironment. Finally, we focus on the challenges that need to be subdued and propose the aspects that need to be addressed in future studies in order to broaden the understanding of CSCs and develop novel strategies to eradicate them in clinical applications. Graphical Abstract Cancer Stem Cells(CSCs) have gained much attention in the last few decades due to their ability to initiate tumor growth and, self-renewal property and multi-drug resistance. Here, we represent the CSC model of cancer, Characteristics of CSCs, acquisition of stemness and metastatic dissemination of cancer, Therapeutic implications of CSCs and Various strategies being employed to target and eradicate CSCs.

Hepatoblastoma: current knowledge and promises from preclinical studies

The survival rate for patients with metastatic hepatoblastoma (HB) is steadily increased in the last thirty years from 27% to 79%. These achievements result from accurate risk stratification and effective chemotherapy and surgical care. However, patients with poor prognosis require more effective therapies. Recent years have witnessed new insights on the biology of HB, setting the stage for molecular classification and new targets of therapy. We review here the molecular pathology of HB, focusing on the driver genes involved in the process of oncogenesis and the identification of novel targets. We also address the role of in vivo models in elucidating the mechanisms of development of this disease and the pre-clinical phase of new treatment modalities.

Sonic Hedgehog Signaling in Organogenesis, Tumors, and Tumor Microenvironments

During mammalian embryonic development, primary cilia transduce and regulate several signaling pathways. Among the various pathways, Sonic hedgehog (SHH) is one of the most significant. SHH signaling remains quiescent in adult mammalian tissues. However, in multiple adult tissues, it becomes active during differentiation, proliferation, and maintenance. Moreover, aberrant activation of SHH signaling occurs in cancers of the skin, brain, liver, gallbladder, pancreas, stomach, colon, breast, lung, prostate, and hematological malignancies. Recent studies have shown that the tumor microenvironment or stroma could affect tumor development and metastasis. One hypothesis has been proposed, claiming that the pancreatic epithelia secretes SHH that is essential in establishing and regulating the pancreatic tumor microenvironment in promoting cancer progression. The SHH signaling pathway is also activated in the cancer stem cells (CSC) of several neoplasms. The self-renewal of CSC is regulated by the SHH/Smoothened receptor (SMO)/Glioma-associated oncogene homolog I (GLI) signaling pathway. Combined use of SHH signaling inhibitors and chemotherapy/radiation therapy/immunotherapy is therefore key in targeting CSCs.

Integrating lncRNAs and mRNAs expression profiles in terminal hindgut of fetal rats with anorectal malformations

BACKGROUND

The detailed embryonic etiology and pathogenesis of anorectal malformations (ARMs) remains unclear. Recent studies have shown that gene expression abnormalities were the key factors that result in ARMs. Long non-coding RNAs (lncRNAs) were reported as the 'transcriptional noise' within the genome. The expression profiles of lncRNA and mRNA remain less characterized in the pathogenesis of ARMs. Furthermore, the function of lncRNAs in the regulation of this process has not been investigated so far. Therefore, this current study was aimed to integrate lncRNA and mRNA expression profiles in terminal hindgut of ethylenethiourea (ETU)-induced ARM rats using Agilents lncRNA and mRNA co-expression microarrays.

METHODS

ARM model was induced with ethylenethiourea (ETU) on gestational day 10. Cesarean deliveries were conducted to collect the embryos on gestational day 20. For the extraction of total RNA, 1-cm terminal hindgut tissues were collected from three fetal rats with similair weights. The microarrays and quantitative RT-PCR analysis were conducted to evaluate the lncRNA and mRNA expression profiles in normal fetal rats and ARM fetal rats.

RESULTS

Compared with control group, 164 lncRNAs were observed to be aberrantly expressed (FC ≥ 2; P < 0.05) in ARM group, including 36 upregulated and 128 downregulated, while 772 mRNAs were observed to be aberrantly expressed (FC ≥ 2; P < 0.05) in the terminal hindgut, including 350 up-regulated and 422 down-regulated. The differential expression profiles between the ARM and the control group were used for gene ontology (GO) and pathway analysis. A subset of those RNAs was identified to be closely related to the development process of ARMs. The four RNAs that were differentially expressed between the two groups were selected for qPCR validation, and the results were in line with the microarray data. In addition, the lncRNAs and mRNA co-expression network was established according to the correlation analysis. We predicted the functions of transregulatory lncRNAs by the TFs (transcription factors) which might modulate their expression. In the core network of lncRNA-TF pairs, the lncRNAs can be classified into 5 categories of pathways governed by Jun, c-Myc, Usf1, Alf2, and Stat3.

CONCLUSION

From the above results, it can be suggested that these aberrant lncRNAs might participate in the pathogenesis of ARM, and our present work may provide new research directions for future studies of ARMs.

Novel smoothened antagonists as anti-neoplastic agents for the treatment of osteosarcoma

Osteosarcoma (OS) is an ultra-rare highly malignant tumor of the skeletal system affecting mainly children and young adults and it is characterized by an extremely aggressive clinical course. OS patients are currently treated with chemotherapy and complete surgical resection of cancer tissue. However, resistance to chemotherapy and the recurrence of disease, as pulmonary metastasis, remain the two greatest challenges in the management, and treatment of this tumor. For these reasons, it is of primary interest to find alternative therapeutic strategies for OS. Dysregulated Hedgehog signalling is involved in the development of various types of cancers including OS. It has also been implicated in tumor/stromal interaction and cancer stem cell biology, and therefore presents a novel therapeutic strategy for cancer treatment. In our work, we tested the activity of five potent Smoothened (SMO) inhibitors, four acylguanidine and one acylthiourea derivatives, against an OS cell line. We found that almost all our compounds were able to inhibit OS cells proliferation and to reduce Gli1 protein levels. Our results also indicated that SMO inhibition in OS cells by such compounds, induces apoptosis with a nanomolar potency. These findings suggest that inactivation of SMO may be a useful approach to the treatment of patients with OS.

The role of canonical and non-canonical Hedgehog signaling in tumor progression in a mouse model of small cell lung cancer

Hedgehog (Hh) signaling regulates cell fate and self-renewal in development and cancer. Canonical Hh signaling is mediated by Hh ligand binding to the receptor Patched (Ptch), which in turn activates Gli-mediated transcription through Smoothened (Smo), the molecular target of the Hh pathway inhibitors used as cancer therapeutics. Small cell lung cancer (SCLC) is a common, aggressive malignancy with universally poor prognosis. Although preclinical studies have shown that Hh inhibitors block the self-renewal capacity of SCLC cells, the lack of activating pathway mutations have cast doubt over the significance of these observations. In particular, the existence of autocrine, ligand-dependent Hh signaling in SCLC has been disputed. In a conditional Tp53;Rb1 mutant mouse model of SCLC, we now demonstrate a requirement for the Hh ligand Sonic Hedgehog (Shh) for the progression of SCLC. Conversely, we show that conditional Shh overexpression activates canonical Hh signaling in SCLC cells, and markedly accelerates tumor progression. When compared to mouse SCLC tumors expressing an activating, ligand-independent Smo mutant, tumors overexpressing Shh exhibited marked chromosomal instability and Smoothened-independent upregulation of Cyclin B1, a putative non-canonical arm of the Hh pathway. In turn, we show that overexpression of Cyclin B1 induces chromosomal instability in mouse embryonic fibroblasts lacking both Tp53 and Rb1. These results provide strong support for an autocrine, ligand-dependent model of Hh signaling in SCLC pathogenesis, and reveal a novel role for non-canonical Hh signaling through the induction of chromosomal instability.

Induced MiR-1249 expression by aberrant activation of Hedegehog signaling pathway in hepatocellular carcinoma

Aberrant activations of Hedegehog (Hh) signaling were found in hepatocellular carcinoma (HCC) and some other cancer types. However, the details have not been completely understood and the underlying mechanism remains unclear. Here we reported that miR-1249 transcription in HCC cells was regulated through direct binding to the conserved sequences in miR-1249 promoter region by Gli1, which functions as a transcription factor and is a component in the Hh signaling pathway. Interestingly, expression of tumor suppressor PTCH1, which is another component of the Hh signaling pathway, was inhibited by miR-1249 through targeting its 3'-untranslated region. Down-regulation of PTCH1 further enhanced the downstream effects mediated by Gli1. In consistent with these findings, miR-1249 expression level was correlated with degree of prognosis (p=0.005) in HCC patients. Taken together, our results suggested the existence of a positive feedback loop comprised of Gli1, miR-1249 and PTCH1. During the process of HCC progression, this positive feedback loop could be continuously activated to enhance tumor cell growth, migration and invasion.

Consumption of Red/Processed Meat and Colorectal Carcinoma: Possible Mechanisms Underlying the Significant Association

Epidemiology and experimental studies provide an overwhelming support of the notion that diets high in red or processed meat accompany an elevated risk of developing pre-neoplastic colorectal adenoma and frank colorectal carcinoma (CRC). The underlying mechanisms are disputed; thus several hypotheses have been proposed. A large body of reports converges, however, on haem and nitrosyl haem as major contributors to the CRC development, presumably acting through various mechanisms. Apart from a potentially higher intestinal mutagenic load among consumers on a diet rich in red/processed meat, other mechanisms involving subtle interference with colorectal stem/progenitor cell survival or maturation are likewise at play. From an overarching perspective, suggested candidate mechanisms for red/processed meat-induced CRC appear as three partly overlapping tenets: (i) increased N-nitrosation/oxidative load leading to DNA adducts and lipid peroxidation in the intestinal epithelium, (ii) proliferative stimulation of the epithelium through haem or food-derived metabolites that either act directly or subsequent to conversion, and (iii) higher inflammatory response, which may trigger a wide cascade of pro-malignant processes. In this review, we summarize and discuss major findings of the area in the context of potentially pertinent mechanisms underlying the above-mentioned association between consumption of red/processed meat and increased risk of developing CRC.

Hedgehog Signaling Non-Canonical Activated by Pro-Inflammatory Cytokines in Pancreatic Ductal Adenocarcinoma

Hedgehog(HH) pathway is found to be activated through a manner of canonical, or the non-canonical HH pathways. Distinct hyperplasia stroma around tumor cells is supposed to express pro-inflammatory cytokines abundantly, such as tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β), etc. in pancreatic ductal adenocarcinoma (PDAC) tissues. In this study we observed the effects of TNF-α and IL-1β on HH pathway activation in PDAC cells, and explored their activation manners. Our results showed that pro-inflammatory cytokines, TNF-α and IL-1β, could up-regulate the expression of GLI1 gene, increase its nuclear protein expression and promote malignant cell behaviors including migration, invasion, epithelial-mesenchymal transition (EMT) and drug resistance as well. Moreover, GLI1 promoter-reporter assay in combination with blocking either NF-κB or Smoothened (SMO) suggested that TNF-α and IL-1β could transcriptionally up-regulate expression of GLI1 completely via NF-κB, whereas ablation of SMO could not completely attenuate the regulation effects of TNF-α and IL-1β on GLI1 expression. Collectively, our results indicated that TNF-α and IL-1β in hyperplasia stroma can promote the PDAC cell development by activating HH pathway, through both the canonical and non-canonical HH activation ways.

GLI inhibitor GANT61 kills melanoma cells and acts in synergy with obatoclax

MEK kinase inhibitors (trametinib and selumetinib) or kinase inhibitors directed against mutated BRAF(V600E) (vemurafenib and dabrafenib) have initial encouraging effects in the treatment of melanoma but acquired resistance appears almost invariably after some months. Studies revealed mutually exclusive NRAS and BRAF activating mutations driving the MAPK/ERK pathway among human melanomas. Although combination therapy exerts significantly better antitumor cell efficacy, complete remission is rarely achieved. To employ an alternative approach, we have targeted the Hedgehog/GLI pathway, which is deregulated in melanomas, through the GLI1/2 inhibitor GANT61, alone or accompanied with the treatment by the BCL2 family inhibitor obatoclax in 9 melanoma cell lines. Thus, we targeted melanoma cells irrespective of their NRAS or BRAF mutational status. After GANT61 treatment, the cell viability was drastically diminished via apoptosis, as substantial nuclear DNA fragmentation was detected. In all tested melanoma cell lines, the combined treatment was more efficient than the application of each drug alone at the end of the cell growth with inhibitors. GANT61 was efficient also alone in most cell lines without the addition of obatoclax, which had only a limited effect when used as a single drug. In most cell lines, tumor cells were eradicated after 5-9 days of combined treatment in colony outgrowth assay. To conclude, GANT61 treatment might become a hopeful and effective anti-melanoma targeted therapy, especially when combined with the BCL2 family inhibitor obatoclax.

Differential role of Hedgehog signaling in human pancreatic (patho-) physiology: An up to date review

Since the discovery of the Hedgehog (Hh) pathway in drosophila melanogaster, our knowledge of the role of Hh in embryonic development, inflammation, and cancerogenesis in humans has dramatically increased over the last decades. This is the case especially concerning the pancreas, however, real therapeutic breakthroughs are missing until now. In general, Hh signaling is essential for pancreatic organogenesis, development, and tissue maturation. In the case of acute pancreatitis, Hh has a protective role, whereas in chronic pancreatitis, Hh interacts with pancreatic stellate cells, leading to destructive parenchym fibrosis and atrophy, as well as to irregular tissue remodeling with potency of initiating cancerogenesis. In vitro and in situ analysis of Hh in pancreatic cancer revealed that the Hh pathway participates in the development of pancreatic precursor lesions and ductal adenocarcinoma including critical interactions with the tumor microenvironment. The application of specific inhibitors of components of the Hh pathway is currently subject of ongoing clinical trials (phases 1 and 2). Furthermore, a combination of Hh pathway inhibitors and established chemotherapeutic drugs could also represent a promising therapeutic approach. In this review, we give a structured survey of the role of the Hh pathway in pancreatic development, pancreatitis, pancreatic carcinogenesis and pancreatic cancer as well as an overview of current clinical trials concerning Hh pathway inhibitors and pancreas cancer.

Neoexpression of a functional primary cilium in colorectal cancer cells

The Hedgehog (HH) signaling pathway is involved in the maintenance of numerous cell types both during development and in the adult. Often deregulated in cancers, its involvement in colorectal cancer has come into view during the last few years, although its role remains poorly defined. In most tissues, the HH pathway is highly connected to the primary cilium (PC), an organelle that recruits functional components and regulates the HH pathway. However, normal epithelial cells of the colon display an inactive HH pathway and lack a PC. In this study, we report the presence of the PC in adenocarcinoma cells of primary colorectal tumors at all stages. Using human colorectal cancer cell lines we found a clear correlation between the presence of the PC and the expression of the final HH effector, GLI1, and provide evidence of a functional link between the two by demonstrating the recruitment of the SMO receptor to the membrane of the primary cilium. We conclude that the primary cilium directly participates in the HH pathway in colorectal cancer cells.

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.

Relationship between Hedgehog signaling pathway and pancreatic cancer

Hedgehog (Hh) signaling pathway consists of ligands such as Hh, receptor (patched), transmembrane protein Smo, nuclear transcription factor Gli, and downstream target genes. This pathway plays an important role in cell differentiation, tissue development and organ formation in the embryonic stage. In recent years, the Hh signaling pathway has been reported to play an important role in the development of pancreatic cancer. It can induce differentiation, proliferation and invasion of pancreatic cancer cells. Blocking the Hh signaling pathway in pancreatic cancer cells will provide a new and effective method for the treatment of pancreatic cancer. In this review, we will summarize the composition of the Hh signaling pathway and its relationship with the development of pancreatic cancer.

Hedgehog Signaling in the Maintenance of Cancer Stem Cells

Cancer stem cells (CSCs) represent a rare population of cells with the capacity to self-renew and give rise to heterogeneous cell lineages within a tumour. Whilst the mechanisms underlying the regulation of CSCs are poorly defined, key developmental signaling pathways required for normal stem and progenitor functions have been strongly implicated. Hedgehog (Hh) signaling is an evolutionarily-conserved pathway essential for self-renewal and cell fate determination. Aberrant Hh signaling is associated with the development and progression of various types of cancer and is implicated in multiple aspects of tumourigenesis, including the maintenance of CSCs. Here, we discuss the mounting evidence suggestive of Hh-driven CSCs in the context of haematological malignancies and solid tumours and the novel strategies that hold the potential to block many aspects of the transformation attributed to the CSC phenotype, including chemotherapeutic resistance, relapse and metastasis.

Hedgehog signaling through GLI1 and GLI2 is required for epithelial-mesenchymal transition in human trophoblasts

BACKGROUND

Epithelial to mesenchymal transition (EMT) is critical for human placental development, trophoblastic differentiation, and pregnancy-associated diseases. Here, we investigated the effects of hedgehog (HH) signaling on EMT in human trophoblasts, and further explored the underlying mechanism.

METHODS

Human primary cytotrophoblasts and trophoblast-like JEG-3 cells were used as in vitro models. Quantitative real-time RT-PCR and Western blot analysis were performed to examine mRNA and protein levels, respectively. Lentiviruses expressing short hairpin RNA were used to knock down the target genes. Reporter assays and chromatin immunoprecipitation were performed to determine the transactivity. Cell migration, invasion and colony formation were accessed by wound healing, Matrigel-coated transwell, and colony formation assays, respectively.

RESULTS

Activation of HH signaling induced the transdifferentiation of cytotrophoblasts and trophoblast-like JEG-3 cells from epithelial to mesenchymal phenotypes, exhibiting the decreases in E-Cadherin expression as well as the increases in vimentin expression, invasion, migration and colony formation. Knockdown of GLI1 and GLI2 but not GLI3 attenuated HH-induced transdifferentiation, whereas GLI1 was responsible for the expression of HH-induced key EMT regulators including Snail1, Slug, and Twist, and both GLI1 and GLI2 acted directly as transcriptional repressor of CDH1 gene encoding E-Cadherin.

CONCLUSION

HH through GLI1 and GLI2 acts as critical signals in supporting the physiological function of mature placenta.

GENERAL SIGNIFICANCE

HH signaling through GLI1 and GLI2 could be required for the maintenance of human pregnancy.

Hedgehog signaling in the pathogenesis of neuro-oncology diseases

The review summarizes current knowledge on the Hedgehog signaling pathway, its role in normal embryogenesis and/or initiation and progression of neuro-oncological diseases, especially of high-grade gliomas, the most malignant neuroepithelial tumors. The main proteins forming the Hedgehog signaling pathway include Shh, PTCH1, SMO, HHIP, SUFU and GLI1 isoforms. Effects of other signaling pathways on the family of transcription factors GLI and other proteins are described. The review summarizes modern data about the impact of the Hedgehog signaling pathway on proliferation, migration activity and invasiveness, and also on tumor neoangiogenesis and tumor cell chemoresistance. The role of the Hedgehog signaling pathway in origin of cancer stem cells and epithelial-mesenchymal transition is also analyzed. Some prospects for new anticancer drugs acting on components of the Hedgehog signaling pathway inhibitors are demonstrated.

Recent advances in the design of Hedgehog pathway inhibitors for the treatment of malignancies

INTRODUCTION

The Hedgehog (Hh) signaling pathway is known to be dysregulated in several forms of cancer. Hence, specifically targeting this signaling cascade is a valid and promising strategy for successful therapeutic intervention. Several components within the Hh pathway have been proven to be druggable; however, challenges in the discovery and development process for small molecules targeting this pathway have been identified.

AREAS COVERED

This review details both the current state and future potential of Hh pathway inhibitors as anticancer chemotherapeutics that target a variety of human malignancies.

EXPERT OPINION

The initial development of Hh pathway inhibitors focused on small-molecule antagonists of Smoothened, a transmembrane protein that is a key regulator of pathway signaling. More recently, efforts to identify and develop inhibitors of pathway signaling that function through alternate mechanisms have been increasing. However, none of these have advanced into clinical trials. Further, early evidence suggesting the broad application of Hh pathway inhibitors as a monotherapy in a wide range of human cancers has not been validated. The potential for Hh pathway inhibitors as combination therapy has demonstrated promising preclinical results. However, more research to identify rational drug combinations to fully explore the potential of this anticancer drug class is warranted.

Identifying hedgehog signaling specific microRNAs in glioblastomas

Aberrant activation of hedgehog (Hh) signaling pathway plays an important role in the development and proliferation of glioblastoma (GBM) cells. However, its mechanism remains unknown. MicroRNAs (miRNAs) are short non-coding RNA molecules which are involved in the post-transcriptional regulation of genes, and enrolled in signaling transduction network in tumors. This study was designed to investigate the role of miRNAs targeting the Hh signaling pathway in GBMs. According to the expression level of Gli1 mRNA measured by real time PCR, GBM samples were assigned to Gli1 high or low expression group. MiRNA microarray was applied to screen the dysregulated miRNA. As a result, 17 miRNAs were differentially expressed between Gli1 high expression and low expression groups (p < 0.005). Thirteen miRNAs including miR-125b-1 were downregulated, while only 4 miRNAs including miR-144 were upregulated in Gli1 high expression group. In summary, our study presents a subset of miRNAs which target the Hh signaling pathway in GBMs, and throws some light on the aberrant activation mechanism.

Interaction of PACAP with Sonic hedgehog reveals complex regulation of the hedgehog pathway by PKA

Sonic hedgehog (Shh) signaling is essential for proliferation of cerebellar granule cell progenitors (cGCPs) and its aberrant activation causes a cerebellar cancer medulloblastoma. Pituitary adenylate cyclase activating polypeptide (PACAP) inhibits Shh-driven proliferation of cGCPs and acts as tumor suppressor in murine medulloblastoma. We show that PACAP blocks canonical Shh signaling by a mechanism that involves activation of protein kinase A (PKA) and inhibition of the translocation of the Shh-dependent transcription factor Gli2 into the primary cilium. PKA is shown to play an essential role in inhibiting gene transcription in the absence of Shh, but global PKA activity levels are found to be a poor predictor of the degree of Shh pathway activation. We propose that the core Shh pathway regulates a small compartmentalized pool of PKA in the vicinity of primary cilia. GPCRs that affect global PKA activity levels, such as the PACAP receptor, cooperate with the canonical Shh signal to regulate Gli protein phosphorylation by PKA. This interaction serves to fine-tune the transcriptional and physiological function of the Shh pathway.

Smoothened inhibition leads to decreased proliferation and induces apoptosis in esophageal adenocarcinoma cells

The Hedgehog (Hh) pathway is known to be active in Barrett's carcinogenesis. Therefore, we evaluated the efficacy and underlying mechanisms of inhibition of cancer cell growth by the smoothened (Smo) antagonist BMS-833923 in esophageal adenocarcinoma (EAC) cell lines. Cell proliferation and apoptosis were evaluated by flow cytometry, Western blotting, immunofluorescence, and quantitative reverse transcription polymerase chain reactions. Results showed that the Smo antagonist led to reduced Hh pathway activity, resulting in decreased cell proliferation and induction of apoptosis via the intrinsic pathway in the esophageal cancer cells. In conclusion, the Smo antagonist may have application as an EAC chemotherapeutic agent.

Inhibition of Hedgehog signaling in the gastrointestinal tract: targeting the cancer microenvironment

This review summarizes emerging information regarding the Hedgehog (Hh) signaling pathway during neoplastic transformation in the gastrointestinal tract. Although there is a role for the well-established canonical pathway in which Hedgehog ligands interact with their receptor Patched, there is sufficient evidence that downstream components of the Hh pathway, e.g., Gli1, are hijacked by non-Hh signaling pathways to promote the conversion of the epithelium to dysplasia and carcinoma. We review the canonical pathway and involvement of primary cilia, and then focus on current evidence for Hh signaling in luminal bowel cancers as well as accessory organs, i.e., liver, pancreas and biliary ducts. We conclude that targeting the Hh pathway with small molecules, nutriceuticals and other mechanisms will likely require a combination of inhibitors that target Gli transcription factors in addition to canonical modulators such as Smoothened.

The glioma-associated oncogene homolog 1 promotes epithelial--mesenchymal transition in human esophageal squamous cell cancer by inhibiting E-cadherin via Snail

The glioma-associated oncogene homolog 1 (GLI1) family of zinc finger transcription factors is the nuclear mediator of the Hedgehog pathway that regulates genes essential for various stages of tumor development and progression. However, the role and mechanism by which high expression of GLI1 contributes to the invasion and metastasis of human esophageal squamous cell cancer (ESCC) has not been fully elucidated. In the present study, we demonstrated that GLI1 was over-expressed in human ESCC tissues, especially in ESCC tissues with deep invasion and lymph-node metastasis. Moreover, GLI1 was also over-expressed in ESCC cell lines and correlated with the aggressiveness of ESCC cell lines. In addition, GLI signaling pathway agonist purmorphamine could increase the invasion and metastasis ability of ESCC cells in vitro. There is increasing evidence for the contribution of epithelial-mesenchymal transition (EMT) to ESCC invasion and metastasis, therefore we investigated GLI1's role in EMT. Our results showed that high expression of GLI1 dampened expression of E-cadherin and enhanced the expression of Vimentin, and it also improved the expression of Snail, indicative of its role in EMT occurrence. Mechanistic studies showed that down-expression of Snail reversed GLI1 activation-regulated expression of EMT markers, suggesting the role of Snail in GLI1-mediated EMT. Taken together, our results had revealed that GLI1 could participate in the invasion and metastasis of ESCC through EMT. These studies indicated that in ESCC, GLI1 could be a useful target for cancer prevention and therapy.

Hedgehog signaling inhibitor cyclopamine induces apoptosis by decreasing Gli2 and Bcl2 expression in human salivary pleomorphic adenoma cells

Pleomorphic adenoma is the most common benign neoplasm of the salivary gland. Few studies are currently available on pleomorphic adenoma cell apoptosis. The aim of this study was to investigate the effect of cyclopamine induction apoptosis in human salivary pleomorphic adenoma (HSPA) cells and the impact on Gli2 and Bcl2 mRNA levels. Cells were quantified and cell morphology was visualized under microscope. Flow cytometry was used to detect the apoptotic rate. Cyclopamine is considered an efficient blocker of the hedgehog (Hh) signaling pathway. Following treatment with 10 μmol/l cyclopamine for 48 h, the number of cells were reduced, and nuclear pycnosis or fragmentation, as well as chromatospherite disfiguration apoptotic morphology were observed under microscope. One-way ANOVA test results revealed a significantly greater decrease (P<0.01) of Gli2 and Bcl2 mRNA levels in the cyclopamine-treated group as compared to the blank control group and dimethyl sulfoxide (DMSO)-treated group. Following treatment with 10 μmol/l cyclopamine for 24 h, the apoptotic rate of the cyclopamine-treated group was significantly higher than that of the blank control and DMSO-treated group (P<0.01). Findings of this study showed that cyclopamine affected the mechanism of HSPA cell apoptosis, which may be associated with the downregulation of Gli2 and Bcl2 mRNA expression levels and the activation of the mitochondrial apoptotic pathways.

MEK1-RSK2 contributes to Hedgehog signaling by stabilizing GLI2 transcription factor and inhibiting ubiquitination

The transcription factor GLI2 has an important role in the transduction of Hedgehog signaling and thereby regulates tumorigenesis in a wide variety of human tumors. However, the mechanisms controlling GLI2 protein expression and stabilization are incompletely understood. In this study, we show that the mitogen-activated protein kinase MEK1 modulates GLI2 both at the mRNA and protein level. Constitutively activated MEK1 prolonged the half-life of GLI2 and increased its nuclear translocation, accompanied by attenuated ubiquitination of GLI2 protein. RSK2, a protein kinase lying downstream of MEK-ERK cascade, mimicked the effect of MEK on GLI2 stabilization. MEK1 and RSK2 failed to augment the half-life of GLI2 lacking GSK-3β phosphorylation sites, indicating that MEK-RSK stabilizes GLI2 by controlling targeting GSK-3β-mediated phosphorylation and ubiquitination of GLI2. The significance of MEK-RSK stabilization was demonstrated in experiments showing that activation of MEK-RSK paralleled higher protein level of GLI2 in several multiple myelomas (MM) cells relative to normal B cells. Moreover, combined treatment with RSK and GLI inhibitors led to an enhanced apoptosis of MM cells. Thus, our results indicate that MEK-RSK cascade positively regulates GLI2 stabilization and represses its degradation via inhibiting GSK-3β-dependent phosphorylation and ubiquitination of GLI2.

Overlapping activities of TGF-β and Hedgehog signaling in cancer: therapeutic targets for cancer treatment

Recent advances in the field of cancer therapeutics come from the development of drugs that specifically recognize validated oncogenic or pro-metastatic targets. The latter may be mutated proteins with altered function, such as kinases that become constitutively active, or critical components of growth factor signaling pathways, whose deregulation leads to aberrant malignant cell proliferation and dissemination to metastatic sites. We herein focus on the description of the overlapping activities of two important developmental pathways often exacerbated in cancer, namely Transforming Growth Factor-β (TGF-β) and Hedgehog (HH) signaling, with a special emphasis on the unifying oncogenic role played by GLI1/2 transcription factors. The latter are the main effectors of the canonical HH pathway, yet are direct target genes of TGF-β/SMAD signal transduction. While tumor-suppressor in healthy and pre-malignant tissues, TGF-β is often expressed at high levels in tumors and contributes to tumor growth, escape from immune surveillance, invasion and metastasis. HH signaling regulates cell proliferation, differentiation and apoptosis, and aberrant HH signaling is found in a variety of cancers. We discuss the current knowledge on HH and TGF-β implication in cancer including cancer stem cell biology, as well as the current state, both successes and failures, of targeted therapeutics aimed at blocking either of these pathways in the pre-clinical and clinical settings.

Cholesterol and its derivatives in Sonic Hedgehog signaling and cancer

The connection between the Hedgehog (HH) pathway and cholesterol has been recognized since the early days that shaped our current understanding of this unique pathway. Cholesterol and related lipids are intricately linked to HH signaling: from the role of cholesterol in HH biosynthesis to the modulation of HH signal reception and transduction by other sterols, passing by the phylogenetic relationships among many components of the HH pathway that resemble or contain lipid-binding domains. Here I review the connections between HH signaling, cholesterol and its derivatives and analyze the potential implications for HH-dependent cancers.

Metastatic gastric cancer - focus on targeted therapies

Gastric cancer (GC) is currently the second leading cause of cancer death worldwide; unfortunately, most patients will present with locally advanced or metastatic disease. Despite recent progress in diagnosis, surgery, chemotherapy, and radiotherapy, prognosis remains poor. A better understanding of GC biology and signaling pathways is expected to improve GC therapy, and the integration of targeted therapies has recently become possible and appears to be promising. This article focuses on anti-Her-2 therapy, specifically trastuzumab, as well as other epidermal growth factor receptor antagonists such as cetuximab, panitumub, matuzumab, nimotzumab, gefitinib, and erlotinib. Additionally, drugs that target angiogenesis pathways are also under investigation, particulary bevacizumab, ramucirumab, sorafenib, sunitinib, and cediranib. Other targeted agents in preclinical or early clinical development include mTOR inhibitors, anti c-MET, polo-like kinase 1 inhibitors, anti-insulin-like growth factor, anti-heat shock proteins, and small molecules targeting Hedgehog signaling.

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.

Cell death pathways as therapeutic targets in rhabdomyosarcoma

Resistance of rhabdomyosarcoma to current therapies remains one of the key issues in pediatric oncology. Since the success of most cytotoxic therapies in the treatment of cancer, for example, chemotherapy, depends on intact signaling pathways that mediate programmed cell death (apoptosis), defects in apoptosis programs in cancer cells may result in resistance. Evasion of apoptosis in rhabdomyosarcoma may be caused by defects in the expression or function of critical mediators of apoptosis or in aberrant expression of antiapoptotic proteins. Therefore, the identification of the molecular mechanisms that confer primary or acquired resistance to apoptosis in rhabdomyosarcoma presents a critical step for the rational development of molecular targeted drugs. This approach will likely open novel perspectives for the treatment of rhabdomyosarcoma.

Hedgehog signalling in gut development, physiology and cancer

The Hedgehog pathway is one of the most common signal transduction pathways used by mammalian cells. Most studies have focused on its role during development, primarily of the nervous system, skin, bone and pancreas. Due to the activation of this pathway during proliferation and neoplastic transformation, more recent studies have examined its role in adult tissues. Significant levels of sonic hedgehog are expressed in the gastric mucosa, which has served to direct analysis of its role during organogenesis, gastric acid secretion and neoplastic transformation. Therefore the goal of this review is to apply current knowledge of this pathway to further our understanding of gastrointestinal physiology and neoplasia, using the stomach as a prototype.