Blocking signaling at the level of GLI regulates downstream gene expression and inhibits proliferation of canine osteosarcoma cells

Hedgehog pathway in sarcoma: from preclinical mechanism to clinical application

Sarcomas are a diverse group of malignant neoplasms of mesenchymal origin. They develop rarely, but due to poor prognosis, they are a challenging and significant clinical problem. Currently, available therapeutic options have very limited activity. A better understating of sarcomas' pathogenesis may help develop more effective therapies in the future. The Sonic hedgehog (Shh) signaling pathway is involved in both embryonic development and mature tissue repair and carcinogenesis. Shh pathway inhibitors are presently used in the treatment of basal cell carcinoma. Its increased activity has been demonstrated in many sarcomas, including osteosarcoma, Ewing sarcoma, chondrosarcoma, rhabdomyosarcoma, leiomyosarcoma, and malignant rhabdoid tumor. In vitro studies have demonstrated the effectiveness of inhibitors of the Hedgehog pathway in inhibiting proliferation in those sarcomas in which the components of the pathway are overexpressed. These results were confirmed by in vivo studies, which additionally proved the influence of Shh pathway inhibitors on limiting the metastatic potential of sarcoma cells. However, until now, the efficacy of sarcomas treatment with Shh pathway inhibitors has not been established in clinical trials. The reason for that may be the non-canonical activation of the pathway or interactions with other signaling pathways, such as Wnt or Notch. In this review, we present the Shh signaling pathway's role in the pathogenesis of sarcomas, including both canonical and non-canonical signaling. We also propose how this knowledge could be potentially translated into clinics.

Expression of the hedgehog signaling pathway and the effect of inhibition at the level of Smoothened in canine osteosarcoma cell lines

Osteosarcoma (OSA) is the most common malignant bone cancer in dogs. Canine and human OSA share several features, including tumor environments, response to traditional treatment, and several molecular pathways. Hedgehog (Hh) signaling is known to contribute to tumorigenesis and progression of various cancers, including human OSA. This study aimed to identify the role of the Hh signaling pathway in canine OSA cell lines, including Abrams, D17, and Moresco, focusing on the signal transducer Smoothened (SMO). mRNA and protein levels of Hh pathway components, including SHH, IHH, SMO, and PTCH1, were aberrant in all examined OSA cell lines compared with canine osteoblast cells. The SMO inhibitor cyclopamine significantly decreased cell viability and colony-forming ability in the canine OSA cell lines in a dose-dependent manner. Moresco cells, which expressed the highest level of SMO protein, were the most sensitive to the anticancer effect of cyclopamine among the three canine OSA cell lines tested. Hh downstream target gene and protein expression in canine OSA cell lines were downregulated after cyclopamine treatment. In addition, cyclopamine significantly increased apoptotic cell death in Abrams and Moresco cells. The findings that Hh/SMO is activated in canine OSA cell lines and cyclopamine suppresses OSA cell survival via inhibition of SMO suggest that the Hh/SMO signaling pathway might be a novel therapeutic target for canine OSA. This article is protected by copyright. All rights reserved.

Cross-Species Genomics Reveals Oncogenic Dependencies in ZFTA/C11orf95 Fusion-Positive Supratentorial Ependymomas

Molecular groups of supratentorial ependymomas comprise tumors with ZFTA-RELA or YAP1-involving fusions and fusion-negative subependymoma. However, occasionally supratentorial ependymomas cannot be readily assigned to any of these groups due to lack of detection of a typical fusion and/or ambiguous DNA methylation-based classification. An unbiased approach with a cohort of unprecedented size revealed distinct methylation clusters composed of tumors with ependymal but also various other histologic features containing alternative translocations that shared ZFTA as a partner gene. Somatic overexpression of ZFTA-associated fusion genes in the developing cerebral cortex is capable of inducing tumor formation in vivo, and cross-species comparative analyses identified GLI2 as a key downstream regulator of tumorigenesis in all tumors. Targeting GLI2 with arsenic trioxide caused extended survival of tumor-bearing animals, indicating a potential therapeutic vulnerability in ZFTA fusion-positive tumors. SIGNIFICANCE: ZFTA-RELA fusions are a hallmark feature of supratentorial ependymoma. We find that ZFTA acts as a partner for alternative transcriptional activators in oncogenic fusions of supratentorial tumors with various histologic characteristics. Establishing representative mouse models, we identify potential therapeutic targets shared by ZFTA fusion-positive tumors, such as GLI2.This article is highlighted in the In This Issue feature, p. 2113.

LINC01123 enhances osteosarcoma cell growth by activating the Hedgehog pathway via the miR-516b-5p/Gli1 axis

The lncRNA LINC01123 has been reported to act as an oncogene in many human cancers. Nevertheless, the function and underlying mechanism of LINC01123 in osteosarcoma (OS) remain unclear. This study aimed to explore the roles and mechanisms of LINC01123 in OS progression. In this study, the expression of LINC01123 was significantly upregulated in OS cell lines than in a human osteoblast cell line. Furthermore, in vitro and in vivo experiments confirmed that knockdown of LINC01123 suppressed cell progression. Mechanistically, LINC01123 acted as a competing endogenous RNA by sponging miR‐516b‐5p, thus, increasing Gli1 expression by directly targeting its 3ʹUTR. Taken together, LINC01123 enhances OS proliferation and metastasis via the miR‐516b‐5p/Gli1 axis. Therefore, LINC01123 may be a potential therapeutic target for OS treatment.

GANT61 plays antitumor effects by inducing oxidative stress through the miRNA-1286/RAB31 axis in osteosarcoma

Osteosarcoma (OS) is a rare malignancy of bone associated with poor clinical outcomes. The antitumor effects of GANT61 on OS is unclear. To investigate antitumor effects and mechanism of GANT61 in OS cells and xenograft model. Effects of GANT61 on cell viability, clone formation, cell cycle, apoptosis, migration, and invasion ability of OS cells were assessed. Reactive oxygen species (ROS) levels measured by dichlorofluorescein fluorescence were used to evaluate oxidative stress. The Xenograft model was constructed to investigate the antitumor effects of GANT61 in vivo. The microRNA (miRNA)-1286 was downregulated, while RAB31 upregulated in OS tissues and cells. GANT61 inhibited viability, migration, and invasion ability of OS cells (SaOS-2 and U2OS), and induced apoptosis and the ROS production, along with miRNA-1286 upregulation and RAB13 downregulation. After knockdown of miRNA-1286, GANT6-induced cell inhibition was attenuated, along with RAB31 upregulation. Inversely, miRNA-1286 overexpression downregulated RAB31. Dual-luciferase reporter assay verified that miR-1286 negatively targeted RAB13. Moreover, the knockdown of RAB31 stimulated apoptosis and ROS production while inhibited viability, migration, and invasion of GANT61-treated cells. In vivo experiments further confirmed that GANT61 inhibited tumor growth and RAB13 expression, but enhanced miRNA-1286. The study demonstrated that GANT61 inhibited cell aggressive phenotype and tumor growth by inducing oxidative stress through the miRNA-1286/RAB31 axis. Our findings provided a potential antitumor agent for the OS clinical treatment.

Evaluation of canonical Hedgehog signaling pathway inhibition in canine osteosarcoma

Canine osteosarcoma (OSA), the most common canine primary bone malignancy, has a highly aggressive biologic behavior. Despite current standard of care therapies, including amputation and adjuvant chemotherapy, most dogs still succumb to metastatic disease. Further investigations into molecular mechanisms and pathways driving OSA are needed to improve therapeutic options. The Hedgehog (HH) cell-signaling pathway has demonstrated involvement in human OSA. Several studies in canine OSA have found changes in expression of some HH pathway genes and demonstrated a role for HH transcription factors. However, the role of this pathway as well as the translational value of its targeting in canine OSA are still undefined. The objectives of this study were to determine the expression of HH components directly in canine OSA tissues and to evaluate the biologic impact of HH signaling inhibition in canine OSA cells. In situ hybridization was used to detect HH family mRNA expression in archived canine OSA tissues and revealed variable expression levels of these mRNAs in canine OSA tissues. The effect of a commercially available Smoothened inhibitor, vismodegib, was studied in established canine OSA cell lines. Alterations in cellular growth as well as assessment of downstream HH targets were evaluated. Although changes in cell growth were noted following Smoothened inhibition, inconsistent decreases in target gene expression were found. While treatment with vismodegib had a negative impact on canine OSA cell growth and viability, the mechanism remains unclear. Further studies are warranted to evaluate the clinical significance of canonical HH signaling in canine OSA.

Melarsomine suppresses canine osteosarcoma cell survival via inhibition of Hedgehog-GLI signaling

The Hedgehog-GLI signaling pathway is activated in human and canine osteosarcoma (OSA) and represents a potential therapeutic target for cancers, including OSA. Arsenic trioxide represses GLI expression. Melarsomine, an arsenic compound-containing drug, has been approved for the treatment of canine heartworm disease. Hence, we hypothesized that melarsomine inhibits GLI signaling in canine OSA cell lines. The present study aimed to assess this hypothesis. Cell viability and colony formation were decreased in the canine OSA cell lines Abrams and D17 after treatment with melarsomine. Melarsomine-induced apoptotic cell death was assessed via cell cycle analysis using propidium iodide staining. Quantitative real-time reverse transcription polymerase chain reaction and western blot analyses revealed a downregulation of genes downstream of the Hedgehog signaling pathway, including GLI1, GLI2, and PTCH, after melarsomine treatment. The present results suggest that melarsomine exerts antitumor effects and serves as a GLI inhibitor in canine OSA cells. Additional studies are required to evaluate and confirm the anticancer effect and relevant therapeutic dose of melarsomine in vivo.

Effect of RAB31 silencing on osteosarcoma cell proliferation and migration through the Hedgehog signaling pathway

Osteosarcoma (OS) is a prevalent cancer that plagues people worldwide. Identifying prognostic markers would be useful in treating human OS. In this study, we aimed to explore the functions of Ras-related protein Rab-31 (RAB31) in OS-cell proliferation, migration, and invasion as well as its roles in the Hedgehog signaling pathway for better understanding of the mechanism. To assess the detailed regulatory mechanism of RAB31 silencing on OS, both RT-qPCR and Western blot analysis were employed to evaluate the expressions of RAB31 as well as the Hedgehog signaling pathway-related genes. Besides, we also investigated the effects of silenced RAB31 both in vitro and in vivo. First, we found that in OS tissues, both mRNA and protein expressions of RAB31 and PCNA had a significant increase. Second, the Hedgehog signaling pathway was detected to play an integral role in OS progression. Finally, after transfection of RAB31-siRNA to reduce the expression of RAB31, the Hedgehog signaling pathway was suppressed, along with cell proliferation, invasion, and migration. Therefore, we conclude that RAB31 plays an important role in OS development and its silencing delays the OS progression via suppression of the Hedgehog signaling pathway.

Safety and Tolerability of Sonic Hedgehog Pathway Inhibitors in Cancer

The hedgehog pathway, for which sonic hedgehog (Shh) is the most prominent ligand, is highly conserved and is tightly associated with embryonic development in a number of species. This pathway is also tightly associated with the development of several types of cancer, including basal cell carcinoma (BCC) and acute promyelocytic leukemia, among many others. Inactivating mutations in Patched-1 (PTCH1), leading to ligand-independent pathway activation, are frequent in several cancer types, but most prominent in BCC. This has led to the development of several compounds targeting this pathway as a cancer therapeutic. These compounds target the inducers of this pathway in Smoothened (SMO) and the GLI transcription factors, although targeting SMO has had the most success. Despite the many attempts at targeting this pathway, only three US FDA-approved drugs for cancers affect the Shh pathway. Two of these compounds, vismodegib and sonidegib, target SMO to suppress signaling from either PTCH1 or SMO mutations that lead to upregulation of the pathway. The other approved compound is arsenic trioxide, which can suppress this pathway at the level of the GLI proteins, although current evidence suggests it also has other targets. This review focuses on the safety and tolerability of these clinically approved drugs targeting the Shh pathway, along with a discussion on other Shh pathway inhibitors being developed.

Canine sarcomas as a surrogate for the human disease

Pet dogs are becoming increasingly recognized as a population with the potential to inform medical research through their treatment for a variety of maladies by veterinary health professionals. This is the basis of the One Health initiative, supporting the idea of collaboration between human and animal health researchers and clinicians to study spontaneous disease processes and treatment in animals to inform human health. Cancer is a major health burden in pet dogs, accounting for approximately 30% of deaths across breeds. As such, pet dogs with cancer are becoming increasingly recognized as a resource for studying the pharmacology and therapeutic potential of anticancer drugs and therapies under development. This was recently highlighted by a National Academy of Medicine Workshop on Comparative Oncology that took place in mid-2015 (http://www.nap.edu/21830). One component of cancer burden in dogs is their significantly higher incidence of sarcomas as compared to humans. This increased incidence led to canine osteosarcoma being an important component in the development of surgical approaches for osteosarcoma in children. Included in this review of sarcomas in dogs is a description of the incidence, pathology, molecular characteristics and previous translational therapeutic studies associated with these tumors. An understanding of the patho-physiological and molecular characteristics of these naturally occurring canine sarcomas holds great promise for effective incorporation into drug development schemas, for evaluation of target modulation or other pharmacodynamic measures associated with therapeutic response. These data could serve to supplement other preclinical data and bolster clinical investigations in tumor types for which there is a paucity of human patients for clinical trials.

Targeting Transcription Factors for Cancer Treatment

Transcription factors are involved in a large number of human diseases such as cancers for which they account for about 20% of all oncogenes identified so far. For long time, with the exception of ligand-inducible nuclear receptors, transcription factors were considered as “undruggable” targets. Advances knowledge of these transcription factors, in terms of structure, function (expression, degradation, interaction with co-factors and other proteins) and the dynamics of their mode of binding to DNA has changed this postulate and paved the way for new therapies targeted against transcription factors. Here, we discuss various ways to target transcription factors in cancer models: by modulating their expression or degradation, by blocking protein/protein interactions, by targeting the transcription factor itself to prevent its DNA binding either through a binding pocket or at the DNA-interacting site, some of these inhibitors being currently used or evaluated for cancer treatment. Such different targeting of transcription factors by small molecules is facilitated by modern chemistry developing a wide variety of original molecules designed to specifically abort transcription factor and by an increased knowledge of their pathological implication through the use of new technologies in order to make it possible to improve therapeutic control of transcription factor oncogenic functions.

Glaucocalyxin A exerts anticancer effect on osteosarcoma by inhibiting GLI1 nuclear translocation via regulating PI3K/Akt pathway

Osteosarcoma, the most common malignant bone tumor with recurring disease or lung metastases, has become one of the leading causes of death in humans. In the current study, we made an investigation on the anticancer effect of glaucocalyxin A, a bioactive ent-kauranoid diterpenoid isolated from Rabdosia japonica var., and unraveled the underlying mechanisms. Here, we found that Glaucocalyxin A inhibited the cell viability of numerous osteosarcoma cells. Our results showed that Glaucocalyxin A exerted the pro-apoptotic effect on human osteosarcoma cells, MG-63 and HOS cells. Glaucocalyxin A induced apoptosis by mitochondrial apoptotic pathway through several steps including increasing the Bax/Bcl-2 ratio, triggering the intracellular reactive oxygen species (ROS) generation, reducing mitochondrial membrane potential (MMP), and inducing cleavage of caspase-9 and caspase-3. We demonstrated that Glaucocalyxin A induced apoptosis via inhibiting Five-zinc finger Glis 1 (GLI1) activation by overexpression and knockdown of GLI1 in vitro. We also found that Glaucocalyxin A inhibited GLI1 activation via regulating phosphatidylinositol 3 kinase/protein kinase B (PI3K/Akt) signaling pathway. We further confirmed our findings by using PI3K activator and inhibitor to verify the inhibitory effect of Glaucocalyxin A on PI3K/Akt/GLI1 pathway. Moreover, our in vivo study revealed that glaucocalyxin A possessed a remarkable antitumor effect with no toxicity in the xenograft model inoculated with HOS tumor through the same mechanisms as in vitro. In conclusion, our results suggested that Glaucocalyxin A induced apoptosis in osteosarcoma by inhibiting nuclear translocation of GLI1 via regulating PI3K/Akt signaling pathway. Thus, Glaucocalyxin A might be a potential candidate for human osteosarcoma in the future.

Hedgehog signalling in the tumourigenesis and metastasis of osteosarcoma, and its potential value in the clinical therapy of osteosarcoma

The Hedgehog (Hh) signalling pathway is involved in cell differentiation, growth and tissue polarity. This pathway is also involved in the progression and invasion of various human cancers. Osteosarcoma, a subtype of bone cancer, is commonly seen in children and adolescents. Typically, pulmonary osteosarcoma metastases are especially difficult to control. In the present paper, we summarise recent studies on the regulation of osteosarcoma progression and metastasis by downregulating Hh signalling. We also summarise the crosstalk between the Hh pathway and other cancer-related pathways in the tumourigenesis of various cancers. We further summarise and highlight the therapeutic value of potential inhibitors of Hh signalling in the clinical therapy of human cancers.

Development of anticancer agents targeting the Hedgehog signaling

Hedgehog signaling is an evolutionarily conserved pathway which is essential in embryonic and postnatal development as well as adult organ homeostasis. Abnormal regulation of Hedgehog signaling is implicated in many diseases including cancer. Consequently, substantial efforts have made in the past to develop potential therapeutic agents that specifically target the Hedgehog signaling for cancer treatment. Here, we review the therapeutic agents for inhibition of the Hedgehog signaling and their clinical advances in cancer treatment.

CNOT1 cooperates with LMNA to aggravate osteosarcoma tumorigenesis through the Hedgehog signaling pathway

While treatments for childhood osteosarcoma have improved, the overall survival for this common type of bone cancer has not changed for three decades, and thus, new targets for therapeutic development are needed. To identify tumor-related proteins in osteosarcoma, we used isobaric tags in a relative and absolute quantitation proteomic approach to analyze the differentially expressed proteins between osteosarcoma cells and human osteoblastic cells. Through clinical screening and functional evaluation, CCR4-NOT transcription complex subunit 1 (CNOT1) correlated with the growth of osteosarcoma cells. To date, the mechanisms and regulatory roles of CNOT1 in tumors, including osteosarcoma, remain largely elusive. Here, we present evidence that knockdown of CNOT1 inhibits the growth of osteosarcoma in vitro and in vivo. Mechanistically, we observed that CNOT1 interacted with LMNA (lamin A) and functioned as a positive regulator of this intermediate filament protein. The RNA-seq analysis revealed that CNOT1 depletion inhibited the Hedgehog signaling pathway in osteosarcoma cells. A rescue study showed that the decreased growth of osteosarcoma cells and inhibition of the Hedgehog signaling pathway by CNOT1 depletion were reversed by LMNA overexpression, indicating that the activity of CNOT1 was LMNA dependent. Notably, the CNOT1 expression was significantly associated with tumor recurrence, Enneking stage, and poor survival in patients with osteosarcoma. Examination of clinical samples confirmed that CNOT1 expression positively correlated with LMNA protein expression. Taken together, these results suggest that the CNOT1-LMNA-Hedgehog signaling pathway axis exerts an oncogenic role in osteosarcoma progression, which could be a potential target for gene therapy.

Targeting the Sonic Hedgehog Signaling Pathway: Review of Smoothened and GLI Inhibitors

The sonic hedgehog (Shh) signaling pathway is a major regulator of cell differentiation, cell proliferation, and tissue polarity. Aberrant activation of the Shh pathway has been shown in a variety of human cancers, including, basal cell carcinoma, malignant gliomas, medulloblastoma, leukemias, and cancers of the breast, lung, pancreas, and prostate. Tumorigenesis, tumor progression and therapeutic response have all been shown to be impacted by the Shh signaling pathway. Downstream effectors of the Shh pathway include smoothened (SMO) and glioma-associated oncogene homolog (GLI) family of zinc finger transcription factors. Both are regarded as important targets for cancer therapeutics. While most efforts have been devoted towards pharmacologically targeting SMO, developing GLI-targeted approach has its merit because of the fact that GLI proteins can be activated by both Shh ligand-dependent and -independent mechanisms. To date, two SMO inhibitors (LDE225/Sonidegib and GDC-0449/Vismodegib) have received FDA approval for treating basal cell carcinoma while many clinical trials are being conducted to evaluate the efficacy of this exciting class of targeted therapy in a variety of cancers. In this review, we provide an overview of the biology of the Shh pathway and then detail the current landscape of the Shh-SMO-GLI pathway inhibitors including those in preclinical studies and clinical trials.

GLI2 cell-specific activity is controlled at the level of transcription and RNA processing: Consequences to cancer metastasis

High activity of GLI family zinc finger protein 2 (GLI2) promotes tumor progression. Removal of the repressor domain at the N terminus (GLI2∆N) by recombinant methods converts GLI2 into a powerful transcriptional activator. However, molecular mechanisms leading to the formation of GLI2∆N activator proteins have not been established. Herein we report for the first time that the functional activities of GLI2 are parted into different protein isoforms by alternative promoter usage, selection of alternative splicing, transcription initiation and termination sites. Functional studies using melanoma cells revealed that transcriptional regulation of GLI2 is TGFbeta-dependent and supports the predominant production of GLI2∆N and C-terminally truncated GLI2 (GLI2∆C) isoforms in cells with high migratory and invasive phenotype. Taken together, these results highlight the role of transcription and RNA processing as major processes in the regulation of GLI2 activity with severe impacts in cancer development.

Sonic Hedgehog Signaling Affected by Promoter Hypermethylation Induces Aberrant Gli2 Expression in Spina Bifida

GLI2 is a key mediator of the sonic hedgehog (Shh) signaling pathway and plays an important role in neural tube development during vertebrate embryogenesis; however, the role of gli2 in human folate-related neural tube defects remains unclear. In this study, we compared methylation status and polymorphisms of gli2 between spina bifida patients and a control group to explore the underlying mechanisms related to folate deficiency in spina bifida. No single nucleotide polymorphism was found to be significantly different between the two groups, although gli2 methylation levels were significantly increased in spina bifida samples, accompanied by aberrant GLI2 expression. Moreover, a prominent negative correlation was found between the folate level in brain tissue and the gli2 methylation status (r = -0.41, P = 0.014), and gli2 hypermethylation increased the risk of spina bifida with an odds ratio of 12.45 (95 % confidence interval: 2.71-57.22, P = 0.001). In addition, we established a cell model to illustrate the effect of gli2 expression and the accessibility of chromatin affected by methylation. High gli2 and gli1 mRNA expression was detected in 5-Aza-treated cells, while gli2 hypermethylation resulted in chromatin inaccessibility and a reduced association with nuclear proteins containing transcriptional factors. More meaningful to the pathway, the effect gene of the Shh pathway, gli1, was found to have a reduced level of expression along with a decreased expression of gli2 in our cell model. Aberrant high methylation resulted in the low expression of gli2 in spina bifida, which was affected by the change in chromatin status and the capacity of transcription factor binding.

BMI1 is expressed in canine osteosarcoma and contributes to cell growth and chemotherapy resistance

BMI1, a stem cell factor and member of the polycomb group of genes, has been shown to contribute to growth and chemoresistance of several human malignancies including primary osteosarcoma (OSA). Naturally occurring OSA in the dog represents a large animal model of human OSA, however the potential role of BMI1 in canine primary and metastatic OSA has not been examined. Immunohistochemical staining of canine primary and metastatic OSA tumors revealed strong nuclear expression of BMI1. An identical staining pattern was found in both primary and metastatic human OSA tissues. Canine OSA cell lines (Abrams, Moresco, and D17) expressed high levels of BMI1 compared with canine osteoblasts and knockdown or inhibition of BMI1 by siRNA or by small molecule BMI1-inhibitor PTC-209 demonstrated a role for BMI1 in canine OSA cell growth and resistance to carboplatin and doxorubicin chemotherapy. These findings suggest that inhibition of BMI1 in primary or metastatic OSA may improve response to chemotherapy and that the dog may serve as a large animal model to evaluate such therapy.

Digging a hole under Hedgehog: downstream inhibition as an emerging anticancer strategy

Hedgehog signaling is a key regulator of development and stem cell fate and its aberrant activation is a leading cause of a number of tumors. Activating germline or somatic mutations of genes encoding Hh pathway components are found in Basal Cell Carcinoma (BCC) and Medulloblastoma (MB). Ligand-dependent Hedgehog hyperactivation, due to autocrine or paracrine mechanisms, is also observed in a large number of malignancies of the breast, colon, skin, bladder, pancreas and other tissues. The key tumorigenic role of Hedgehog has prompted effort aimed at identifying inhibitors of this signaling. To date, only the antagonists of the membrane transducer Smo have been approved for therapy or are under clinical trials in patients with BCC and MB linked to Ptch or Smo mutations. Despite the good initial response, patients treated with Smo antagonists have eventually developed resistance due to the occurrence of compensating mechanisms. Furthermore, Smo antagonists are not effective in tumors where the Hedgehog hyperactivation is due to mutations of pathway components downstream of Smo, or in case of non-canonical, Smo-independent activation of the Gli transcription factors. For all these reasons, the research of Hh inhibitors acting downstream of Smo is becoming an area of intensive investigation. In this review we illustrate the progresses made in the identification of effective Hedgehog inhibitors and their application in cancer, with a special emphasis on the newly identified downstream inhibitors. We describe in detail the Gli inhibitors and illustrate their mode of action and applications in experimental and/or clinical settings.

Hedgehog signaling inhibitors as anti-cancer agents in osteosarcoma

Osteosarcoma is a rare type of cancer associated with a poor clinical outcome. Even though the pathologic characteristics of OS are well established, much remains to be understood, particularly at the molecular signaling level. The molecular mechanisms of osteosarcoma progression and metastases have not yet been fully elucidated and several evolutionary signaling pathways have been found to be linked with osteosarcoma pathogenesis, especially the hedgehog signaling (Hh) pathway. The present review will outline the importance and targeting the hedgehog signaling (Hh) pathway in osteosarcoma tumor biology. Available data also suggest that aberrant Hh signaling has pro-migratory effects and leads to the development of osteoblastic osteosarcoma. Activation of Hh signaling has been observed in osteosarcoma cell lines and also in primary human osteosarcoma specimens. Emerging data suggests that interference with Hh signal transduction by inhibitors may reduce osteosarcoma cell proliferation and tumor growth thereby preventing osteosarcomagenesis. From this perspective, we outline the current state of Hh pathway inhibitors in osteosarcoma. In summary, targeting Hh signaling by inhibitors promise to increase the efficacy of osteosarcoma treatment and improve patient outcome.

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.