Targeting the Hedgehog pathway in cancer

A novel potent tumour promoter aberrantly overexpressed in most human cancers

The complexity and heterogeneity of tumours have hindered efforts to identify commonalities among different cancers. Furthermore, because we have limited information on the prevalence and nature of ubiquitous molecular events that occur in neoplasms, it is unfeasible to implement molecular-targeted cancer screening and prevention. Here, we found that the FEAT protein is overexpressed in most human cancers, but weakly expressed in normal tissues including the testis, brain, and liver. Transgenic mice that ectopically expressed FEAT in the thymus, spleen, liver, and lung spontaneously developed invasive malignant lymphoma (48%, 19/40) and lung-metastasizing liver cancer (hepatocellular carcinoma) (35%, 14/40) that models human hepatocarcinogenesis, indicating the FEAT protein potently drives tumorigenesis in vivo. Gene expression profiling suggested that FEAT drives receptor tyrosine kinase and hedgehog signalling pathways. These findings demonstrate that integrated efforts to identify FEAT-like ubiquitous oncoproteins are useful and may provide promising approaches for cost-effective cancer screening and prevention.

Stem cell biology and drug discovery

There are many reasons to be interested in stem cells, one of the most prominent being their potential use in finding better drugs to treat human disease. This article focuses on how this may be implemented. Recent advances in the production of reprogrammed adult cells and their regulated differentiation to disease-relevant cells are presented, and diseases that have been modeled using these methods are discussed. Remaining difficulties are highlighted, as are new therapeutic insights that have emerged.

The tissue-specific stem cell as a target for chemoprevention

While cancer treatment modalities are gradually improving due to increased knowledge about tumor heterogeneity and the cancer stem cell hypothesis, there remains a disconnect between tumor detection and mortality rates. The increasing knowledge of stem cell biology and its contribution to cancer progression illuminates the potential for chemopreventative regimens that effectively target the tissue-specific stem cell. Several signaling pathways have emerged that are critical for regulating stem cell self-renewal and multilineage differentiation over a range of tissue types, including Wnt, Hedgehog, and Notch signaling. Dysregulation of these genes can lead to cancer, which supports the cancer stem cell hypothesis. Several known chemopreventative agents have recently been shown to impact these and other pathways in the stem cell population, suggesting that their efficacies may be attributed in part to maintaining homeostasis of tissue-specific stem cells. Further understanding of the mechanisms of action of chemopreventative agents and of stem cell biology will generate better chemoprevention regimens that can be recommended especially to those in high-risk populations.

Exo-cyclopamine--a stable and potent inhibitor of hedgehog-signaling

The combination of theoretical and computational studies with organic synthesis and biological investigations has led to exo-cyclopamine. This stable and highly potent derivative of cyclopamine promises big potential as an experimental drug against several types of human cancer.

Targeting GLI1 expression in human inflammatory breast cancer cells enhances apoptosis and attenuates migration

BACKGROUND

Inflammatory breast cancer (IBC) is an aggressive subtype of breast cancer with distinct molecular profiles. Gene expression profiling previously identified sonic hedgehog (SHH) as part of a gene signature that is differentially regulated in IBC patients.

METHODS

The effects of reducing GLI1 levels on protein expression, cell proliferation, apoptosis and migration were determined by immunoblots, MTT assay, Annexin-V/PI assay and conventional and automated cell migration assays.

RESULTS

Evaluation of a panel of breast cancer cell lines revealed elevated GLI1 expression, typically a marker for hedgehog-pathway activation, in a triple-negative, highly invasive IBC cell line, SUM149 and its isogenic-derived counterpart rSUM149 that has acquired resistance to ErbB1/2 targeting strategies. Downregulation of GLI1 expression in SUM149 and rSUM149 by small interfering RNA or a small molecule GLI1 inhibitor resulted in decreased proliferation and increased apoptosis. Further, GLI1 suppression in these cell lines significantly inhibited cell migration as assessed by a wound-healing assay compared with MCF-7, a non-invasive cell line with low GLI1 expression. A novel high-content migration assay allowed us to quantify multiple effects of GLI1 silencing including significant decreases in cell distance travelled and linearity of movement.

CONCLUSION

Our data reveal a role for GLI1 in IBC cell proliferation, survival and migration, which supports the feasibility of targeting GLI1 as a novel therapeutic strategy for IBC patients.

Identification and characterization of KCASH2 and KCASH3, 2 novel Cullin3 adaptors suppressing histone deacetylase and Hedgehog activity in medulloblastoma

Medulloblastoma is the most common pediatric malignant brain tumor, arising from aberrant cerebellar precursors' development, a process mainly controlled by Hedgehog (Hh) signaling pathway. Histone deacetylase HDAC1 has been recently shown to modulate Hh signaling, deacetylating its effectors Gli1/2 and enhancing their transcriptional activity. Therefore, HDAC may represent a potential therapeutic target for Hh-dependent tumors, but still little information is available on the physiological mechanisms of HDAC regulation. The putative tumor suppressor REN(KCTD11) acts through ubiquitination-dependent degradation of HDAC1, thereby affecting Hh activity and medulloblastoma growth. We identify and characterize here two REN(KCTD11) homologues, defining a new family of proteins named KCASH, as "KCTD containing, Cullin3 adaptor, suppressor of Hedgehog." Indeed, the novel genes (KCASH2(KCTD21) and KCASH3(KCTD6)) share with REN(KCTD11) a number of features, such as a BTB domain required for the formation of a Cullin3 ubiquitin ligase complex and HDAC1 ubiquitination and degradation capability, suppressing the acetylation-dependent Hh/Gli signaling. Expression of KCASH2 and -3 is observed in cerebellum, whereas epigenetic silencing and allelic deletion are observed in human medulloblastoma. Rescuing KCASHs expression reduces the Hedgehog-dependent medulloblastoma growth, suggesting that loss of members of this novel family of native HDAC inhibitors is crucial in sustaining Hh pathway-mediated tumorigenesis. Accordingly, they might represent a promising class of endogenous "agents" through which this pathway may be targeted.

Hedgehog inhibition promotes a switch from Type II to Type I cell death receptor signaling in cancer cells

TRAIL is a promising therapeutic agent for human malignancies. TRAIL often requires mitochondrial dysfunction, referred to as the Type II death receptor pathway, to promote cytotoxicity. However, numerous malignant cells are TRAIL resistant due to inhibition of this mitochondrial pathway. Using cholangiocarcinoma cells as a model of TRAIL resistance, we found that Hedgehog signaling blockade sensitized these cancer cells to TRAIL cytotoxicity independent of mitochondrial dysfunction, referred to as Type I death receptor signaling. This switch in TRAIL requirement from Type II to Type I death receptor signaling was demonstrated by the lack of functional dependence on Bid/Bim and Bax/Bak, proapoptotic components of the mitochondrial pathway. Hedgehog signaling modulated expression of X-linked inhibitor of apoptosis (XIAP), which serves to repress the Type I death receptor pathway. siRNA targeted knockdown of XIAP mimics sensitization to mitochondria-independent TRAIL killing achieved by Hedgehog inhibition. Regulation of XIAP expression by Hedgehog signaling is mediated by the glioma-associated oncogene 2 (GLI2), a downstream transcription factor of Hedgehog. In conclusion, these data provide additional mechanisms modulating cell death by TRAIL and suggest Hedgehog inhibition as a therapeutic approach for TRAIL-resistant neoplasms.

Molecular characteristics of pancreatic ductal adenocarcinoma

Pancreatic cancer is an almost universally lethal disease and despite extensive research over the last decades, this has not changed significantly. Nevertheless, much progress has been made in understanding the pathogenesis of pancreatic ductal adenocarcinoma (PDAC) suggesting that different therapeutic strategies based on these new insights are forthcoming. Increasing focus exists on designing the so-called targeted treatment strategies in which the genetic characteristics of a tumor guide therapy. In the past, the focus of research was on identifying the most frequently affected genes in PDAC, but with the complete sequencing of the pancreatic cancer genome the focus has shifted to defining the biological function that the altered genes play. In this paper we aimed to put the genetic alterations present in pancreatic cancer in the context of their role in signaling pathways. In addition, this paper provides an update of the recent advances made in the development of the targeted treatment approach in PDAC.

Turning straw into gold: directing cell fate for regenerative medicine

Regenerative medicine offers the hope that cells for disease research and therapy might be created from readily available sources. To fulfil this promise, the cells available need to be converted into the desired cell types. We review two main approaches to accomplishing this goal: in vitro directed differentiation, which is used to push pluripotent stem cells, including embryonic stem cells or induced pluripotent stem cells, through steps similar to those that occur during embryonic development; and reprogramming (also known as transdifferentiation), in which a differentiated cell is converted directly into the cell of interest without proceeding through a pluripotent intermediate. We analyse the status of progress made using these strategies and highlight challenges that must be overcome to achieve the goal of cell-replacement therapy.

Clinicopathological significance of expression of Smo and Gli1 in esophageal squamous cell carcinoma

AIM: To investigate the expression of Smo and Gli1 proteins in esophageal squamous cell carcinoma (ESCC).

METHODS: The protein expression of Smo and Gli1 was detected by immunohistochemistry in 60 ESCC specimens, 45 tumor-adjacent atypical hyperplastic tissue specimens, and 60 normal esophageal mucosal specimens.

RESULTS: The expression of Smo and Gli1 protein was significantly higher in ESCC than in atypical hyperplastic tissue and normal esophageal mucosal tissue (53/60 vs 22/45, 19/60; 54/60 vs 18/45, 16/60; all P < 0.05). There was no significant difference in the expression of the two proteins between atypical hyperplasia tissue and normal esophageal mucosal tissue (both P > 0.05). Overexpression of Smo and Gli1 proteins was related with lymph node metastasis and the depth of tumor invasion in ESSC (both P < 0.05). The expression of Smo protein was significantly correlated with that of Gli1 protein in ESCC (r = 0.452, P < 0.01).

CONCLUSION: Abnormal sonic hedgehog signaling may play an important role in the development and evolution of ESCC, and this abnormality is associated with Smo protein overexpression, which can upregulate the expression of Gli1 protein, a downstream transcription factor in the sonic hedgehog signal transduction pathway. Combined detection of Smo and Gli1 proteins may be helpful to evaluate the prognosis of ESCC.

Phase I trial of hedgehog pathway inhibitor vismodegib (GDC-0449) in patients with refractory, locally advanced or metastatic solid tumors

PURPOSE

The hedgehog (Hh) signaling pathway, a key regulator of cell growth and differentiation during development is implicated in pathogenesis of certain cancers. Vismodegib (GDC-0449) is a small-molecule inhibitor of smoothened, a key component of Hh signaling. This phase I trial assessed GDC-0449 treatment in patients with solid tumors refractory to current therapies or for which no standard therapy existed.

EXPERIMENTAL DESIGN

Sixty-eight patients received GDC-0449 at 150 mg/d (n = 41), 270 mg/d (n = 23), or 540 mg/d (n = 4). Adverse events, tumor responses, pharmacokinetics, and pharmacodynamic down-modulation of GLI1 expression in noninvolved skin were assessed.

RESULTS

Thirty-three of 68 patients had advanced basal cell carcinoma (BCC), 8 had pancreatic cancer, 1 had medulloblastoma; 17 other types of cancer were also represented. GDC-0449 was generally well-tolerated. Six patients (8.8%) experienced 7 grade 4 events (hyponatremia, fatigue, pyelonephritis, presyncope, resectable pancreatic adenocarcinoma, and paranoia with hyperglycemia), and 27.9% of patients experienced a grade 3 event [most commonly hyponatremia (10.3%), abdominal pain (7.4%), and fatigue (5.9%)]. No maximum tolerated dose was reached. The recommended phase II dose was 150 mg/d, based on achievement of maximal plasma concentration and pharmacodynamic response at this dose. Tumor responses were observed in 20 patients (19 with BCC and 1 unconfirmed response in medulloblastoma), 14 patients had stable disease as best response, and 28 had progressive disease. Evidence of GLI1 down-modulation was observed in noninvolved skin.

CONCLUSIONS

GDC-0449 has an acceptable safety profile and encouraging anti-tumor activity in advanced BCC and medulloblastoma. Further study in these and other cancer types is warranted.

Genome-wide RNAi screen reveals disease-associated genes that are common to Hedgehog and Wnt signaling

The Hedgehog (Hh) and Wnt signal transduction pathways are master regulators of embryogenesis and tissue renewal and represent anticancer therapeutic targets. Using genome-wide RNA interference screening in murine cultured cells, we established previously unknown associations between these signaling pathways and genes linked to developmental malformations, diseases of premature tissue degeneration, and cancer. We identified functions in both pathways for the multitasking kinase Stk11 (also known as Lkb1), a tumor suppressor implicated in lung and cervical cancers. We found that Stk11 loss resulted in disassembly of the primary cilium, a cellular organizing center for Hh pathway components, thus dampening Hh signaling. Loss of Stk11 also induced aberrant signaling through the Wnt pathway. Chemicals that targeted the Wnt acyltransferase Porcupine or that restored primary cilia length by inhibiting the tubulin deacetylase HDAC6 (histone deacetylase 6) countered deviant pathway activities driven by Stk11 loss. Our study demonstrates that Stk11 is a critical mediator in both the Hh and the Wnt pathways, and our approach provides a platform to support the development of targeted therapeutic strategies.

Up-regulation of sonic hedgehog contributes to TGF-β1-induced epithelial to mesenchymal transition in NSCLC cells

Background

Lung cancer, especially non-small cell lung cancer (NSCLC) is the major cause of cancer-related deaths in the United States. The aggressiveness of NSCLC has been shown to be associated with the acquisition of epithelial-to-mesenchymal transition (EMT). The acquisition of EMT phenotype induced by TGF-β1in several cancer cells has been implicated in tumor aggressiveness and resistance to conventional therapeutics; however, the molecular mechanism of EMT and tumor aggressiveness in NSCLC remains unknown.

Methodology/Principal Findings

In this study we found for the first time that the induction of EMT by chronic exposure of A549 NSCLC cells to TGF-β1 (A549-M cells) led to the up-regulation of sonic hedgehog (Shh) both at the mRNA and protein levels causing activation of hedgehog signaling. These results were also reproduced in another NSCLC cell line (H2030). Induction of EMT was found to be consistent with aggressive characteristics such as increased clonogenic growth, cell motility and invasion. The aggressiveness of these cells was attenuated by the treatment of A549-M cells with pharmacological inhibitors of Hh signaling in addition to Shh knock-down by siRNA. The inhibition of Hh signaling by pharmacological inhibitors led to the reversal of EMT phenotype as confirmed by the reduction of mesenchymal markers such as ZEB1 and Fibronectin, and induction of epithelial marker E-cadherin. In addition, knock-down of Shh by siRNA significantly attenuated EMT induction by TGF-β1.

Conclusions/Significance

Our results show for the first time the transcriptional up-regulation of Shh by TGF-β1, which is mechanistically associated with TGF-β1 induced EMT phenotype and aggressive behavior of NSCLC cells. Thus the inhibitors of Shh signaling could be useful for the reversal of EMT phenotype, which would inhibit the metastatic potential of NSCLC cells and also make these tumors more sensitive to conventional therapeutics.

Systemic therapy for disseminated basal cell carcinoma: an uncommon manifestation of a common cancer

While basal cell carcinoma (BCC) is the most common human malignancy, distant metastases from this are rare. Current therapy for disseminated BCC is based on anecdotal reports in the absence of clinical trials to guide management. For many years, platinum based cytotoxic chemotherapy was the mainstay of treatment. Advances in the understanding of the biology of BCC have led to the development of targeted therapies (e.g. inhibitors of the hedgehog and the epidermal growth factor receptor pathways) that are currently being investigated in this disease. This review summarizes the available data on the epidemiology and management of disseminated BCC.

Opposite expression patterns of Sonic hedgehog and Indian hedgehog are associated with aberrant methylation status of their promoters in colorectal cancers

AIMS

Activation of the Hedgehog (Hh) signalling pathway in colorectal cancers (CRCs) is controversial, and its regulation mechanism remains to be elucidated. In the present study we attempted to clarify the regulatory mechanism of the expression of Hh ligands during colorectal carcinogenesis.

METHODS

Reverse transcriptase polymerase chain reaction and immunohistochemistry were used to characterise expressions of the SHH, IHH and GLI1 genes in 36 CRCs, and the findings compared to 21 hyperplastic polyps and 32 colorectal adenomas. In addition, the methylation status of the SHH and IHH promoters in these samples were investigated.

RESULTS

Expressions of SHH and GLI1 proteins were increased significantly in CRCs compared with those in hyperplastic polyps and colorectal adenomas (p<0.01 for both). In contrast, IHH was almost lost in both colorectal adenomas and CRCs. Furthermore, DNA methylation analysis revealed that the frequency of SHH methylation in CRCs (20.6%) was significantly lower than that in colorectal adenomas (72.4%, p<0.001) and hyperplastic polyps (64.7%, p = 0.002). IHH promoter methylation was frequently observed in colorectal adenomas (55.2%, p = 0.004) and CRCs (70.6%, p<0.001) compared with that in hyperplastic polyps (11.8%).

CONCLUSION

SHH hypomethylation could lead to the SHH dependent activation of Hh pathway in CRCs. On the other hand, down-regulation of IHH expression as a result of hypermethylation, may be an early event in colorectal carcinogenesis.

Clinical implications of hedgehog signaling pathway inhibitors

Hedgehog was first described in Drosophila melanogaster by the Nobel laureates Eric Wieschaus and Christiane Nüsslein-Volhard. The hedgehog (Hh) pathway is a major regulator of cell differentiation, proliferation, tissue polarity, stem cell maintenance, and carcinogenesis. The first link of Hh signaling to cancer was established through studies of a rare familial disease, Gorlin syndrome, in 1996. Follow-up studies revealed activation of this pathway in basal cell carcinoma, medulloblastoma and, leukemia as well as in gastrointestinal, lung, ovarian, breast, and prostate cancer. Targeted inhibition of Hh signaling is now believed to be effective in the treatment and prevention of human cancer. The discovery and synthesis of specific inhibitors for this pathway are even more exciting. In this review, we summarize major advances in the understanding of Hh signaling pathway activation in human cancer, mouse models for studying Hh-mediated carcinogenesis, the roles of Hh signaling in tumor development and metastasis, antagonists for Hh signaling and their clinical implications.

Targeting cancer stem cells by inhibiting Wnt, Notch, and Hedgehog pathways

Tumor relapse and metastasis remain major obstacles for improving overall cancer survival, which may be due at least in part to the existence of cancer stem cells (CSCs). CSCs are characterized by tumorigenic properties and the ability to self-renew, form differentiated progeny, and develop resistance to therapy. CSCs use many of the same signaling pathways that are found in normal stem cells, such as Wnt, Notch, and Hedgehog (Hh). The origin of CSCs is not fully understood, but data suggest that they originate from normal stem or progenitor cells, or possibly other cancer cells. Therapeutic targeting of both CSCs and bulk tumor populations may provide a strategy to suppress tumor regrowth. Development of agents that target critical steps in the Wnt, Notch, and Hh pathways will be complicated by signaling cross-talk. The role that embryonic signaling pathways play in the function of CSCs, the development of new anti-CSC therapeutic agents, and the complexity of potential CSC signaling cross-talk are described in this Review.

Cancer stem cells: repair gone awry?

Because cell turnover occurs in all adult organs, stem/progenitor cells within the stem-cell niche of each tissue must be appropriately mobilized and differentiated to maintain normal organ structure and function. Tissue injury increases the demands on this process, and thus may unmask defective regulation of pathways, such as Hedgehog (Hh), that modulate progenitor cell fate. Hh pathway dysregulation has been demonstrated in many types of cancer, including pancreatic and liver cancers, in which defective Hh signaling has been linked to outgrowth of Hh-responsive cancer stem-initiating cells and stromal elements. Hence, the Hh pathway might be a therapeutic target in such tumors.

Activation of the sonic hedgehog signaling controls human pulmonary arterial smooth muscle cell proliferation in response to hypoxia

The hedgehog signal pathway plays a crucial role in the angiogenesis and vascular remodeling. However, the function of this pathway in the pulmonary vascular smooth cell proliferation in response to hypoxia remains unknown. In this study, we have demonstrated that the main components of the hedgehog pathway, including sonic hedgehog (SHH), patched1 (PTCH1), smoothened (SMO), GLI and hypoxia-inducible factor 1 (HIF1) are expressed in the human pulmonary arterial smooth muscle cells (HPASMCs). Interestingly, hypoxia significantly enhanced the expression of SHH and HIF1, facilitated the translocation of GLI1 into the nuclei, and promoted the proliferation of HPASMCs. Furthermore, direct activation of the SHH pathway through incubation with the purified recombinant human SHH or with purmorphamine and SAG, two Smo agonists, also enhanced the proliferation of HPASMCs. Importantly, the treatment with anti-SHH and anti-HIF1 antibodies or cyclopamine, a specific SMO inhibitor, markedly inhibited the nuclear translocation of GLI1 and cell proliferation in the HPASMCs induced by hypoxia and activation of the SHH pathway. Moreover, the treatment with cyclopamine increased apoptosis in the hypoxic HPASMCs. These data strongly demonstrate for the first time that the SHH signaling plays a crucial role in the regulation of HPASMC growth in response to hypoxia.

Cross-signaling among phosphinositide-3 kinase, mitogen-activated protein kinase and sonic hedgehog pathways exists in esophageal cancer

The hedgehog (Hh) signaling pathway is essential for the development of tissues and organs. Hyperactive Hh signaling has been implicated in many gastric cancers, including esophageal cancer. However, the interaction between the Hh pathway and other potential signaling pathways in primary esophageal tumorigenesis has not been well investigated. In our study, we found that esophageal cancer cells expressed Hh signaling molecules and that the hyperexpression of Hh target genes was related to protein kinase B (AKT) activation but not extracellular signal-regulated kinase activation. We analyzed the relationship between Gli1 or p-AKT expression and clinicopathological features in esophageal carcinoma samples and found that Gli1 expression was associated with lymph vessel invasion (p = 0.016), blood vessel invasion (p = 0.006) and a poor prognosis (p = 0.003), and p-AKT expression was associated with blood vessel invasion (p = 0.031) and a poor prognosis (p = 0.031). We also studied the relationship between Hh and phosphinositide-3 kinase (PI3K)/AKT or mitogen-activated protein kinase (MAPK) signaling pathways in both TE-1 and TE-10 cell lines. We found that the PI3K/AKT pathway played a critical role in Hh signaling after stimulation with epidermal growth factor, Gβγ and N-Shh. Conversely, PI3K/AKT and MAPK signaling cooperated with the Shh pathway to promote esophageal cancer cell survival and proliferation. The results from esophageal cancer cells shed light on the significance of Hh signaling in esophageal tumor formation and the crosstalk of the Hh pathway with other basic signaling pathways, which is consistent with that observed in human tumor samples.

Gli-similar (Glis) Krüppel-like zinc finger proteins: insights into their physiological functions and critical roles in neonatal diabetes and cystic renal disease

GLI-similar (Glis) 1-3 proteins constitute a subfamily of the Krüppel-like zinc finger transcription factors that are closely related to the Gli family. Glis1-3 play critical roles in the regulation of a number of physiological processes and have been implicated in several pathologies. Mutations in GLIS2 have been linked to nephronophthisis, an autosomal recessive cystic kidney disease. Loss of Glis2 function leads to renal atrophy and fibrosis that involves epithelial-mesenchymal transition (EMT) of renal tubule epithelial cells. Mutations in human GLIS3 have been implicated in a syndrome characterized by neonatal diabetes and congenital hypothyroidism (NDH) and in some patients accompanied by polycystic kidney disease, glaucoma, and liver fibrosis. In addition, the GLIS3 gene has been identified as a susceptibility locus for the risk of type 1 and 2 diabetes. Glis3 plays a key role in pancreatic development, particularly in the generation of ß-cells and in the regulation of insulin gene expression. Glis2 and Glis3 proteins have been demonstrated to localize to the primary cilium, a signaling organelle that has been implicated in several pathologies, including cystic renal diseases. This association suggests that Glis2/3 are part of primary cilium-associated signaling pathways that control the activity of Glis proteins. Upon activation in the primary cilium, Glis proteins may translocate to the nucleus where they subsequently regulate gene transcription by interacting with Glis-binding sites in the promoter regulatory region of target genes. In this review, we discuss the current knowledge of the Glis signaling pathways, their physiological functions, and their involvement in several human pathologies.

KRAS, Hedgehog, Wnt and the twisted developmental biology of pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is characterized by near-universal mutations in KRAS and frequent deregulation of crucial embryonic signalling pathways, including the Hedgehog (Hh) and Wnt-β-catenin cascades. The creation of mouse models that closely resemble the human disease has provided a platform to better understand when and in which cell types these pathways are misregulated during PDAC development. Here we examine the central part that KRAS plays in the biology of PDAC, and how the timing and location of Hh and Wnt-β-catenin signalling dictate the specification and oncogenic properties of PDAC.

Requirement of TGFbeta signaling for SMO-mediated carcinogenesis

Hedgehog (Hh) signaling, via the key signal transducer Smoothened (SMO) and Gli transcription factors, is essential for embryonic development and carcinogenesis. At present, the molecular mechanism of Hh signaling-mediated carcinogenesis is not completely understood. Using a mouse model (K14cre/R26SmoM2) of SMO-mediated basal cell carcinoma development, we identified TGFβ2 as a major Hh-regulated gene. TGFβ2 expression was high in the keratinocytes, with activated TGFβ signaling (indicated by elevated expression of phosphorylated SMAD2/3) detected in both tumor and stroma. The significance of TGFβ signaling for SMO function was demonstrated in two assays. Down-regulation of TGFβ2 expression prevented Hh signaling-dependent osteoblast differentiation and motor neuron differentiation. Furthermore, inhibition of TGFβ signaling by TGFβ receptor I inhibitor SD208 significantly reduced tumor area in K14cre/R26SmoM2 mice. Tumor shrinkage in mice was associated with an increased number of lymphocytes, suggesting an immune suppression role of TGFβ signaling. The relevance of our results to human cancer is reflected by the fact that human basal cell carcinomas, which almost always harbor activated Hh signaling, have activated TGFβ signaling, as indicated by high levels of phosphorylated SMAD2 and SMAD3 in tumor and stroma. Together, our data indicate that TGFβ signaling is critical for Hh signaling-mediated carcinogenesis.

Genetic variations in the sonic hedgehog pathway affect clinical outcomes in non-muscle-invasive bladder cancer

Sonic hedgehog (Shh) pathway genetic variations may affect bladder cancer risk and clinical outcomes. Therefore, we genotyped 177 single-nucleotide polymorphisms (SNP) in 11 Shh pathway genes in a study including 803 bladder cancer cases and 803 controls. We assessed SNP associations with cancer risk and clinical outcomes in 419 cases of non-muscle-invasive bladder cancer (NMIBC) and 318 cases of muscle-invasive and metastatic bladder cancer (MiMBC). Only three SNPs (GLI3 rs3823720, rs3735361, and rs10951671) reached nominal significance in association with risk (P ≤ 0.05), which became nonsignificant after adjusting for multiple comparisons. Nine SNPs reached a nominally significant individual association with recurrence of NMIBC in patients who received transurethral resection (TUR) only (P ≤ 0.05), of which two (SHH rs1233560 and GLI2 rs11685068) were replicated independently in 356 TUR-only NMIBC patients, with P values of 1.0 × 10(-3) (SHH rs1233560) and 1.3 × 10(-3) (GLI2 rs11685068). Nine SNPs also reached a nominally significant individual association with clinical outcome of NMIBC patients who received Bacillus Calmette-Guérin (BCG; P ≤ 0.05), of which two, the independent GLI3 variants rs6463089 and rs3801192, remained significant after adjusting for multiple comparisons (P = 2 × 10(-4) and 9 × 10(-4), respectively). The wild-type genotype of either of these SNPs was associated with a lower recurrence rate and longer recurrence-free survival (versus the variants). Although three SNPs (GLI2 rs735557, GLI2 rs4848632, and SHH rs208684) showed nominal significance in association with overall survival in MiMBC patients (P ≤ 0.05), none remained significant after multiple-comparison adjustments. Germ-line genetic variations in the Shh pathway predicted clinical outcomes of TUR and BCG for NMIBC patients.

Activation of the hedgehog pathway confers a poor prognosis in embryonal and fusion gene-negative alveolar rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children and comprises two major histological subtypes: alveolar rhabdomyosarcoma (ARMS) and embryonal rhabdomyosarcoma (ERMS). Seventy-five percent of ARMS harbor reciprocal chromosomal translocations leading to fusion genes of the forkhead transcription factor FOXO1 and PAX3 or PAX7. The hedgehog (Hh) pathway has been implied in tumor formation and progression of various cancers including RMS. However, whether Hh pathway activation presents a general feature of RMS or whether it is restricted to specific subgroups has not yet been addressed. Here, we report that marker genes of active Hh signaling, that is, Patched1 (Ptch1), Gli1, Gli3 and Myf5, are expressed at significantly higher levels in ERMS and fusion gene-negative ARMS compared with fusion gene-positive ARMS in two distinct cohorts of RMS patients. Consistently, Gli1 expression correlates with Ptch1 expression in ERMS and fusion gene-negative ARMS, but not in fusion gene-positive ARMS. In addition, expression levels of MyoD1 are significantly lower in ERMS and fusion gene-negative ARMS, pointing to an inverse association of Hh activation and early muscle differentiation. Moreover, Myf5 is identified as a novel excellent class predictor for RMS by receiver operating characteristic analysis. Importantly, high expression of Ptch1 or low MyoD1 expression significantly correlate with reduced cumulative survival in fusion gene-negative RMS underscoring the clinical relevance of these findings. By showing that Hh signaling is preferentially activated in specific subgroups of RMS, our study has important implications for molecular targeted therapies, such as small molecule Hh inhibitors, in RMS.

Biology of colorectal cancer

Significant advances have been made in understanding the biology of colorectal cancer. In this chapter, we review the key signaling pathways that play key roles in maintaining the growth and proliferation of colorectal cancer. The pathways that will be discussed include Wnt/beta-catenin, TGF-beta/SMAD, Notch, Hedgehog, epidermal growth factor receptor, Ras, and PI3K/Akt. In addition, we review the growing role of colon cancer stem cells in mediating cellular drug resistance and cancer recurrence. This enhanced understanding of cancer biology provides important insights for developing novel therapies that target specific growth factor receptors and/or certain critical signal transduction pathways. These targeted therapies can then be used alone or in combination with standard cytotoxic chemotherapy regimens.

Novel agents in the treatment of metastatic colorectal cancer

Colorectal cancer (CRC) is one of the leading causes of cancer death in the United States. Research has led to an explosion of knowledge into the molecular basis of CRC in the past decades. Numerous receptors and intracellular proteins have been identified and implicated in the growth and progression of metastatic CRC, thus creating novel targets for drug development. Many agents are under development and have begun to enter early and even later-stage clinical trials. Results of these agents have demonstrated some encouraging activity but in a small number of patients. Research into predictive biomarkers aims to select the patients who may benefit from these novel agents. This review will address several of these promising new agents, their potential relevance to CRC, results from early clinical studies, and their incorporation into future and ongoing CRC clinical trials. Clearly, there is an urgent need for new agents in this disease, but as we learned from the experience with epidermal growth factor receptor-targeted antibodies, patient selection will be increasingly be required for individualized therapy to become a reality in CRC.

Antipsychotic drugs regulate hedgehog signaling by modulation of 7-dehydrocholesterol reductase levels

Recently we identified GANT61, a small-molecule antagonist of Gli transcription factors, which are the final effectors of the mammalian Hedgehog (HH) signaling pathway. Here we describe a diamine substructure of GANT61 that carries the biological activity and show that this part of the molecule is structurally related to trans-1,4-bis(2-chlorobenzaminomethyl)cyclohexane dihydrochloride (AY9944), an inhibitor of the enzymatic activity and transcriptional inducer of 7-dehydrocholesterol-reductase (Dhcr7, EC 1.3.1.21). Treatment of cells with the GANT61 diamine, AY9944, or overexpression of DHCR7 results in the attenuation of Smoothened-dependent and -independent HH signaling. Whereas GANT61 function is independent of Dhcr7, AY9944 does require up-regulation of endogenous Dhcr7. In line with these findings, Dhcr7-modulating antipsychotic (clozapine, chlorpromazine, haloperidol) and antidepressant (imipramine) drugs regulate HH signaling in vitro and in vivo. Modulation of HH signaling may represent a hitherto undiscovered biological (side) effect of therapeutics used to treat schizophrenia and depression.

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

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

GLI3-dependent repression of DR4 mediates hedgehog antagonism of TRAIL-induced apoptosis

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces apoptosis through its cognate receptors death receptor 4 (DR4) and death receptor 5 (DR5), preferentially in malignant cells. However, many malignant cells remain resistant to TRAIL cytotoxicity by poorly characterized mechanisms. Here, using cholangiocarcinoma cells, as a model for TRAIL resistance, we identified a role for the oncogenic Hedgehog (Hh)-GLI pathway in the regulation of TRAIL cytotoxicity. Blockade of Hh using pharmacological and genetic tools sensitizes the cells to TRAIL cytotoxicity. Restoration of apoptosis sensitivity coincided with upregulation of DR4 expression, while expression of other death effector proteins remained unaltered. Knockdown of DR4 mimics Hh-mediated resistance to TRAIL cytotoxicity. Hh regulates the expression of DR4 by modulating the activity of its promoter. Luciferase, chromatin immunoprecipitation and expression assays show that the transcription factor GLI3 binds to the DR4 promoter and Hh requires an intact GLI3-repression activity to silence DR4 expression. Finally, small interfering RNA (siRNA)-targeted knockdown of GLI3, but not GLI1 or GLI2, restores DR4 expression and TRAIL sensitivity, indicating that the Hh effect is exclusively mediated by this transcription factor. In conclusion, these data provide evidence of a regulatory mechanism, which modulates TRAIL signaling in cancer cells and suggest new therapeutic approaches for TRAIL-resistant neoplasms.

Cyclopamine and hedgehog signaling: chemistry, biology, medical perspectives

When Odysseus left the devastated city of Troy after ten years of siege he could not foresee the perils he still had to face. The encounter with the cyclops, a giant with only one eye placed in the middle of its forehead, was doubtlessly one of the creepiest and most dangerous of his adventures. In the end, Odysseus could only escape with the help of a sheep. Whether Homers cyclops was inspired by the observation of terribly malformed neonates remains speculative. However, when sheep herders in Idaho in the middle of the 20th century faced an increasing number of cyclops-like sheep in their herds, a unique cascade of chemical, biological, and medicinal discoveries was initiated. This Minireview tells this story and shows its impact on modern biomedical research.

Learning from Jekyll to control Hyde: Hedgehog signaling in development and cancer

The Hedgehog (Hh) cascade controls cell proliferation, differentiation and patterning of tissues during embryogenesis but is largely suppressed in the adult. The Hh pathway can become reactivated in cancer. Here, we assimilate data from recent studies to understand how and when the Hh pathway is turned on to aid the neoplastic process. Hh signaling is now known to have a role in established tumors, enabling categorization of tumors based on the role Hh signaling plays in their growth. This categorization has relevance for prognosis and targeted therapeutics. In the first category, abnormal Hh signaling initiates the tumor. In the second category, Hh signaling helps maintain the tumor. In the third category, Hh signaling is implicated but its role is not yet defined.

The animal within: carcinogenesis and the clonal evolution of cancer cells are speciation events sensu stricto

Heritable genomic variation and natural selection have long been acknowledged as striking parallels between evolution and cancer. The logical conclusion, that cancer really is a form of speciation, has seldom been expounded directly. My purpose is to reexamine the "cancer as species" thesis in the light of current attitudes to asexual speciation, and modern analyses of species definitions. The chief obstacles to accepting this thesis have been the asexual nature of cancer cell reproduction, the instability of the malignant genotype and phenotype, and our conditioning that speciation is an extremely rare and imperceptibly gradualistic process. However, these are not absolute barriers to the acceptance of cancers as bona fide species. Furthermore, although ongoing clonal evolution of extant cancers also results in a series of secondary speciation events, the initial emergence of a cancer requires a level of taxonomic reclassification even beyond the concept of speciation (i.e., phylogenation), and which is almost certain to provide a rich source of novel drug targets. The implications of the "cancer as species" idea may be as important for biology as for oncology, providing as it does an endless supply of observable if accelerated examples of a phenomenon once regarded as rare. From the perspective of cancer treatment, speciation guarantees the existence of causal molecular mechanisms which may have been neglected as exploitable targets for rational therapy; in particular, the mediators of metazoan life seem to have substantial overlap with components commonly deranged in cancer cells. However, the intractability of the drug resistance problem, residing as it does in the inherent plasticity of the genome, is traceable back to, and inseparable from, the very origins and nature of life.

Modulators of the hedgehog signaling pathway

Since its discovery by C. Nüsslein-Volhard and E. F. Wieschaus, hedgehog (hh) signaling has come a long way. Today it is regarded as a key regulator in embryogenesis where it governs processes like cell proliferation, differentiation, and tissue patterning. Furthermore, in adults it is involved in the maintenance of stem cells, and in tissue repair and regeneration. But hh signaling has a second-much darker-face: it plays an important role in several types of human cancers where it promotes growth and enables proliferation of tumor stem cells. The etiology of medulloblastoma and basal cell carcinoma is tightly linked to aberrant hh activity, but also cancers of the prostate, the pancreas, the colon, the breasts, rhabdomyosarcoma, and leukemia, are dependent on irregular hh activity. Recent clinical studies have shown that hh signaling can be the basis of an important new class of therapeutic agents with far-reaching implications in oncology. Thus, modulation of hh signaling by means of small molecules has emerged as a valuable tool in combating these hh-dependent cancers. Cyclopamine, a unique natural product with a fascinating history, was the first identified inhibitor of hh signaling and its story is closely linked to the progress in the whole field. In this review we will trace the story of cyclopamine, give an overview on the biological modes of hh signaling both in untransformed and malignant cells, and finally present potent modulators of the hh pathway-many of them already in clinical studies. For more than 30 years now the knowledge on hh signaling has grown steadily-an end to this development is far from being conceivable.

Small-molecule inhibitors of the hedgehog signaling pathway as cancer therapeutics

Inhibitors of the Hedgehog (Hh) molecular signaling pathway have emerged in recent years as a promising new class of potential therapeutics for cancer treatment. Numerous drug discovery efforts have resulted in the identification of a wide variety of small molecules that target different members of this pathway, including Smoothened (Smo), Sonic hedgehog protein (Shh), and Gli1. Several Smo inhibitors have now entered human clinical trials, and successful proof-of-concept studies have been carried out in patients with defined genetic mutations in the Hh pathway. This review provides a general overview of three main topics in this rapidly expanding area: 1) the various types of biological assays and in vivo models that have been employed for the identification and optimization of Hh pathway inhibitors; 2) Smo inhibitors reported to date, including recent clinical results where available; and 3) efforts toward the identification and characterization of inhibitors of other members of the Hh pathway.

Targeting breast cancer stem cells

The cancer stem cell (CSC) hypothesis postulates that tumors are maintained by a self-renewing CSC population that is also capable of differentiating into non-self-renewing cell populations that constitute the bulk of the tumor. Although, the CSC hypothesis does not directly address the cell of origin of cancer, it is postulated that tissue-resident stem or progenitor cells are the most common targets of transformation. Clinically, CSCs are predicted to mediate tumor recurrence after chemo- and radiation-therapy due to the relative inability of these modalities to effectively target CSCs. If this is the case, then CSC must be efficiently targeted to achieve a true cure. Similarities between normal and malignant stem cells, at the levels of cell-surface proteins, molecular pathways, cell cycle quiescence, and microRNA signaling present challenges in developing CSC-specific therapeutics. Approaches to targeting CSCs include the development of agents targeting known stem cell regulatory pathways as well as unbiased high-throughput siRNA or small molecule screening. Based on studies of pathways present in normal stem cells, recent work has identified potential "Achilles heals" of CSC, whereas unbiased screening provides opportunities to identify new pathways utilized by CSC as well as develop potential therapeutic agents. Here, we review both approaches and their potential to effectively target breast CSC.

Activation of Sonic hedgehog signaling pathway in S-type neuroblastoma cell lines

The effects of Sonic hedgehog (Shh) signaling pathway activation on S-type neuroblastoma (NB) cell lines and its role in NB tumorigenesis were investigated. Immunohistochemistry was used to detect the expression of Shh pathway components-Patched1 (PTCH1) and Gli1 in 40 human primary NB samples. Western blotting and RT-PCR were used to examine the protein expression and mRNA levels of PTCH1 and Gli1 in three kinds of S-type NB cell lines (SK-N-AS, SK-N-SH and SHEP1), respectively. Exogenous Shh was administrated to activate Shh signaling pathway while cyclopamine was used as a selective antagonist of Shh pathway. S-type NB cell lines were treated with different concentrations of Shh or/and cyclopamine for different durations. Cell viability was measured by using MTT method. Apoptosis rate and cell cycle were assayed by flow cytometry. The xenograft experiments were used to evaluate the role of Shh pathway in tumor growth in immunodeficient mice. High-level expression of PTCH1 and Gli1 was detected in both NB samples and S-type NB cell lines. Cyclopamine decreased the survival rate of the three cell lines while Shh increased it, and the inhibition effects of cyclopamine could be partially reversed by shh pre-treatment. Cyclopamine induced the cell apoptosis and the cell cycle arrest in G0/G1 phase, while Shh induced the reverse effects and could partially prevent effects of cyclopamine. Cyclopamine could also inhibit the growth of NB in vivo. Our studies revealed that activation of the Shh pathway is important for survival and proliferation of S-type NB cells in vivo and in vitro through affecting cell apoptosis and cell cycle, suggesting a new therapeutic approach to NB.

Discovery of NVP-LDE225, a Potent and Selective Smoothened Antagonist

The blockade of aberrant hedgehog (Hh) signaling has shown promise for therapeutic intervention in cancer. A cell-based phenotypic high-throughput screen was performed, and the lead structure (1) was identified as an inhibitor of the Hh pathway via antagonism of the Smoothened receptor (Smo). Structure-activity relationship studies led to the discovery of a potent and specific Smoothened antagonist N-(6-((2S,6R)-2,6-dimethylmorpholino)pyridin-3-yl)-2-methyl-4'-(trifluoromethoxy)biphenyl-3-carboxamide (5m, NVP-LDE225), which is currently in clinical development.

Addressing PXR liabilities of phthalazine-based hedgehog/smoothened antagonists using novel pyridopyridazines

Pyridopyridazine antagonists of the hedgehog signaling pathway are described. Designed to optimize our previously described phthalazine smoothened antagonists, a representative compound eliminates a PXR liability while retaining potency and in vitro metabolic stability. Moreover, the compound has improved efficacy in a hedgehog/smoothened signaling mouse pharmacodynamic model.