Self-renewal of acute lymphocytic leukemia cells is limited by the Hedgehog pathway inhibitors cyclopamine and IPI-926

Targeting cancer hallmark vulnerabilities in hematologic malignancies by interfering with Hedgehog/GLI signaling

Understanding the genetic alterations, disrupted signaling pathways, and hijacked mechanisms in oncogene-transformed hematologic cells is critical for the development of effective and durable treatment strategies against liquid tumors. In this review, we focus on the specific involvement of the Hedgehog (HH)/GLI pathway in the manifestation and initiation of various cancer features in hematologic malignancies, including multiple myeloma, T- and B-cell lymphomas, and lymphoid and myeloid leukemias. By reviewing canonical and noncanonical, Smoothened-independent HH/GLI signaling and summarizing preclinical in vitro and in vivo studies in hematologic malignancies, we elucidate common molecular mechanisms by which HH/GLI signaling controls key oncogenic processes and cancer hallmarks such as cell proliferation, cancer stem cell fate, genomic instability, microenvironment remodeling, and cell survival. We also summarize current clinical trials with HH inhibitors and discuss successes and challenges, as well as opportunities for future combined therapeutic approaches. By providing a bird's eye view of the role of HH/GLI signaling in liquid tumors, we suggest that a comprehensive understanding of the general oncogenic effects of HH/GLI signaling on the formation of cancer hallmarks is essential to identify critical vulnerabilities within tumor cells and their supporting remodeled microenvironment, paving the way for the development of novel and efficient personalized combination therapies for hematologic malignancies.

Hedgehog signaling in tissue homeostasis, cancers, and targeted therapies

The past decade has seen significant advances in our understanding of Hedgehog (HH) signaling pathway in various biological events. HH signaling pathway exerts its biological effects through a complex signaling cascade involved with primary cilium. HH signaling pathway has important functions in embryonic development and tissue homeostasis. It plays a central role in the regulation of the proliferation and differentiation of adult stem cells. Importantly, it has become increasingly clear that HH signaling pathway is associated with increased cancer prevalence, malignant progression, poor prognosis and even increased mortality. Understanding the integrative nature of HH signaling pathway has opened up the potential for new therapeutic targets for cancer. A variety of drugs have been developed, including small molecule inhibitors, natural compounds, and long non-coding RNA (LncRNA), some of which are approved for clinical use. This review outlines recent discoveries of HH signaling in tissue homeostasis and cancer and discusses how these advances are paving the way for the development of new biologically based therapies for cancer. Furthermore, we address status quo and limitations of targeted therapies of HH signaling pathway. Insights from this review will help readers understand the function of HH signaling in homeostasis and cancer, as well as opportunities and challenges of therapeutic targets for cancer.

The hedgehog pathway in hematopoiesis and hematological malignancy

The Hedgehog (HH) pathway is a promising therapeutic target in hematological malignancies. Activation of the pathway has been tied to greater chances of relapse and poorer outcomes in several hematological malignancies and inhibiting the pathway has improved outcomes in several clinical trials. One inhibitor targeting the pathway via the protein Smoothened (SMO), glasdegib, has been approved by the FDA for use with a low dose cytarabine regiment in some high-risk acute myeloid leukemia patients (AML). If further clinical trials in glasdegib produce positive results, there may soon be more general use of HH inhibitors in the treatment of hematological malignancies.While there is clinical evidence that HH inhibitors may improve outcomes and help prevent relapse, a full understanding of any mechanism of action remains elusive. The bulk of AML cells exhibit primary resistance to SMO inhibition (SMOi), leading some to hypothesize that that clinical activity of SMOi is mediated through modulation of self-renewal and chemoresistance in rare cancer stem cells (CSC). Direct evidence that CSC are being targeted in patients by SMOi has proven difficult to produce, and here we present data to support the alternative hypothesis that suggests the clinical benefit observed with SMOi is being mediated through stromal cells in the tumor microenvironment.This paper's aims are to review the history of the HH pathway in hematopoiesis and hematological malignancy, to highlight the pre-clinical and clinical evidence for its use a therapeutic target, and to explore the evidence for stromal activation of the pathway acting to protect CSCs and enable self-renewal of AML and other diseases. Finally, we highlight gaps in the current data and present hypotheses for new research directions.

Jerveratrum-Type Steroidal Alkaloids Inhibit β-1,6-Glucan Biosynthesis in Fungal Cell Walls

The limited number of available effective agents necessitates the development of new antifungals. We report that jervine, a jerveratrum-type steroidal alkaloid isolated from Veratrum californicum, has antifungal activity. Phenotypic comparisons of cell wall mutants, K1 killer toxin susceptibility testing, and quantification of cell wall components revealed that β-1,6-glucan biosynthesis was significantly inhibited by jervine. Temperature-sensitive mutants defective in essential genes involved in β-1,6-glucan biosynthesis, including BIG1, KEG1, KRE5, KRE9, and ROT1, were hypersensitive to jervine. In contrast, point mutations in KRE6 or its paralog SKN1 produced jervine resistance, suggesting that jervine targets Kre6 and Skn1. Jervine exhibited broad-spectrum antifungal activity and was effective against human-pathogenic fungi, including Candida parapsilosis and Candida krusei. It was also effective against phytopathogenic fungi, including Botrytis cinerea and Puccinia recondita. Jervine exerted a synergistic effect with fluconazole. Therefore, jervine, a jerveratrum-type steroidal alkaloid used in pharmaceutical products, represents a new class of antifungals active against mycoses and plant-pathogenic fungi. IMPORTANCE Non-Candida albicans Candida species (NCAC) are on the rise as a cause of mycosis. Many antifungal drugs are less effective against NCAC, limiting the available therapeutic agents. Here, we report that jervine, a jerveratrum-type steroidal alkaloid, is effective against NCAC and phytopathogenic fungi. Jervine acts on Kre6 and Skn1, which are involved in β-1,6-glucan biosynthesis. The skeleton of jerveratrum-type steroidal alkaloids has been well studied, and more recently, their anticancer properties have been investigated. Therefore, jerveratrum-type alkaloids could potentially be applied as treatments for fungal infections and cancer.

Myeloid cell-based delivery of IFN-γ reprograms the leukemia microenvironment and induces anti-tumoral immune responses

The immunosuppressive microenvironment surrounding tumor cells represents a key cause of treatment failure. Therefore, immunotherapies aimed at reprogramming the immune system have largely spread in the past years. We employed gene transfer into hematopoietic stem and progenitor cells to selectively express anti-tumoral cytokines in tumor-infiltrating monocytes/macrophages. We show that interferon-γ (IFN-γ) reduced tumor progression in mouse models of B-cell acute lymphoblastic leukemia (B-ALL) and colorectal carcinoma (MC38). Its activity depended on the immune system's capacity to respond to IFN-γ and drove the counter-selection of leukemia cells expressing surrogate antigens. Gene-based IFN-γ delivery induced antigen presentation in the myeloid compartment and on leukemia cells, leading to a wave of T cell recruitment and activation, with enhanced clonal expansion of cytotoxic CD8+ T lymphocytes. The activity of IFN-γ was further enhanced by either co-delivery of tumor necrosis factor-α (TNF-α) or by drugs blocking immunosuppressive escape pathways, with the potential to obtain durable responses.

Mechanisms involved in selecting and maintaining neuroblastoma cancer stem cell populations, and perspectives for therapeutic targeting

Pediatric neuroblastomas (NBs) are heterogeneous, aggressive, therapy-resistant embryonal tumours that originate from cells of neural crest (NC) origin and in particular neuroblasts committed to the sympathoadrenal progenitor cell lineage. Therapeutic resistance, post-therapeutic relapse and subsequent metastatic NB progression are driven primarily by cancer stem cell (CSC)-like subpopulations, which through their self-renewing capacity, intermittent and slow cell cycles, drug-resistant and reversibly adaptive plastic phenotypes, represent the most important obstacle to improving therapeutic outcomes in unfavourable NBs. In this review, dedicated to NB CSCs and the prospects for their therapeutic eradication, we initiate with brief descriptions of the unique transient vertebrate embryonic NC structure and salient molecular protagonists involved NC induction, specification, epithelial to mesenchymal transition and migratory behaviour, in order to familiarise the reader with the embryonic cellular and molecular origins and background to NB. We follow this by introducing NB and the potential NC-derived stem/progenitor cell origins of NBs, before providing a comprehensive review of the salient molecules, signalling pathways, mechanisms, tumour microenvironmental and therapeutic conditions involved in promoting, selecting and maintaining NB CSC subpopulations, and that underpin their therapy-resistant, self-renewing metastatic behaviour. Finally, we review potential therapeutic strategies and future prospects for targeting and eradication of these bastions of NB therapeutic resistance, post-therapeutic relapse and metastatic progression.

The Complexity of the Tumor Microenvironment and Its Role in Acute Lymphoblastic Leukemia: Implications for Therapies

The microenvironment that surrounds a tumor, in addition to the tumor itself, plays an important role in the onset of resistance to molecularly targeted therapies. Cancer cells and their microenvironment interact closely between them by means of a molecular communication that mutually influences their biological characteristics and behavior. Leukemia cells regulate the recruitment, activation and program of the cells of the surrounding microenvironment, including those of the immune system. Studies on the interactions between the bone marrow (BM) microenvironment and Acute Lymphoblastic Leukemia (ALL) cells have opened a scenario of potential therapeutic targets which include cytokines and their receptors, signal transduction networks, and hypoxia-related proteins. Hypoxia also enhances the formation of new blood vessels, and several studies show how angiogenesis could have a key role in the pathogenesis of ALL. Knowledge of the molecular mechanisms underlying tumor-microenvironment communication and angiogenesis could contribute to the early diagnosis of leukemia and to personalized molecular therapies. This article is part of a Special Issue entitled: Innovative Multi-Disciplinary Approaches for Precision Studies in Leukemia edited by Sandra Marmiroli (University of Modena and Reggio Emilia, Modena, Italy) and Xu Huang (University of Glasgow, Glasgow, United Kingdom).

Understanding the Hedgehog Signaling Pathway in Acute Myeloid Leukemia Stem Cells: A Necessary Step toward a Cure

Simple Summary

The Hedgehog signaling pathway is related to the cell cycle. In particular, it is considered to play a fundamental role in the quiescence of leukemic stem cell (i.e., a temporary resting state without cell replication). Leukemic stem cells are the cells supposed to give rise to the relapses of the leukemia. Therefore, the Hedgehog pathway must be understood to improve the current treatments against acute myeloid leukemia and avoid the relapse of the disease. In this review, we gather the present knowledge about the physiological Hedgehog pathway function, the aberrant activation of Hedgehog in leukemia, and highlight the lack of evidence regarding some aspects of this important pathway. Finally, we summarize the acute myeloid leukemia treatments targeting this signaling pathway.

Abstract

A better understanding of how signaling pathways govern cell fate is fundamental to advances in cancer development and treatment. The initialization of different tumors and their maintenance are caused by the deregulation of different signaling pathways and cancer stem cell maintenance. Quiescent stem cells are resistant to conventional chemotherapeutic treatments and, consequently, are responsible for disease relapse. In this review we focus on the conserved Hedgehog (Hh) signaling pathway which is involved in regulating the cell cycle of hematopoietic and leukemic stem cells. Thus, we examine the role of the Hh signaling pathway in normal and leukemic stem cells and dissect its role in acute myeloid leukemia. We explain not only the connection between illness and the signaling pathway but also evaluate innovative therapeutic approaches that could affect the outcome of patients with acute myeloid leukemia. We found that many aspects of the Hedgehog signaling pathway remain unknown. The role of Hh has only been proven in embryo and hematopoietic stem cell development. Further research is needed to elucidate the role of GLI transcription factors for therapeutic targeting. Glasdegib, an SMO inhibitor, has shown clinical activity in acute myeloid leukemia; however, its mechanism of action is not clear.

Agrobacterium-Mediated Genetic Transformation of the Medicinal Plant Veratrum dahuricum

Veratrum dahuricum L. (Liliaceae), a monocotyledonous species distributed throughout the Changbai mountains of Northeast China, is pharmaceutically important, due to the capacity to produce the anticancer drug cyclopamine. An efficient transformation system of Veratrum dahuricum mediated with Agrobacterium tumefaciens is presented. Murashige and Skoog (MS) medium containing 8 mg/L picloram was used to induce embryogenic calli from immature embryos with 56% efficiency. A. tumefaciens LBA4404 carrying the bar gene driven by the cauliflower mosaic virus 35S promoter was employed for embryogenic callus inoculation. A. tumefaciens cell density OD660 = 0.8 for inoculation, half an hour infection period, and three days of co-culture duration were found to be optimal for callus transformation. Phosphinothricin (PPT, 16 mg/L) was used as the selectable agent, and a transformation efficiency of 15% (transgenic plants/100 infected calli) was obtained. The transgenic nature of the regenerated plants was confirmed by PCR and Southern blot analysis, and expression of the bar gene was detected by RT-PCR and Quick PAT/bar strips. The steroid alkaloids cyclopamine, jervine, and veratramine were detected in transgenic plants, in non-transformed and control plants collected from natural sites. The transformation system constitutes a prerequisite for the production of the pharmaceutically important anticancer drug cyclopamine by metabolic engineering of Veratrum.

The Hedgehog signal transducer Smoothened and microRNA-326: pathogenesis and regulation of drug resistance in pediatric B-cell acute lymphoblastic leukemia

Purpose

Multidrug resistance (MDR) and the subsequent disease relapse are the major causes of childhood acute lymphoblastic leukemia (ALL) related death. The Hedgehog (Hh) signaling pathway can contribute to cancer MDR. In the current study, Smoothened (Smo) was selected as the experimental target due to its importance in the Hh pathway in order to evaluate its probable role in pediatric B-ALL drug resistance.

Patients and methods

The study included 27 pediatric B-ALL and 16 control bone marrow samples. Quantitative RT-PCR was used to investigate the expression levels of Smo and miR-326 as the key players of the Hh pathway. Western blot analysis was performed. The presence of minimal residual disease was studied using PCR-SSCP. The association between Smo expression and drug resistance was analyzed statistically.

Results

Results showed a significant increase in the Smo expression levels in drug-resistant patients in comparison with drug-sensitive children with B-ALL (P=0.0128, AUC=0.82). A considerable negative association between miR-326 and Smo expression levels was identified (r=-0.624, P=0.002). A binomial test confirmed the regulatory role of miR-326 on the translational repression of Smo (P=0.031). Statistics showed no association between Smo and ABCA2 expression levels. However, a significant positive correlation was observed between the Smo and ABCA3 transcripts in the resistant ALL children (r=0.607, P=0.016).

Conclusion

Data revealed the possible oncogenic impact of Smo on leukemogenesis and drug resistance in pediatric B-ALL. Upregulation of Smo was introduced, for the first time, as a prognostic factor for drug resistance in childhood B-ALL. To the best of our knowledge, this is the first study that shows a positive correlation between Smo and ABCA3 expression levels in pediatric B-ALL, explaining a possible mechanism for the development of drug resistance in this cancer. Moreover, the current project revealed a negative modulatory effect of miR-326 on the expression levels of Smo.

Notch/CXCR4 Partnership in Acute Lymphoblastic Leukemia Progression

Acute lymphoblastic leukemia (ALL) is the most common cancer among children. Recent advances in chemotherapy have made ALL a curable hematological malignancy. In children, there is 25% chance of disease relapse, typically in the central nervous system. While in adults, there is a higher chance of relapse. ALL may affect B-cell or T-cell lineages. Different genetic alterations characterize the two ALL forms. Deregulated Notch, either Notch1 or Notch3, and CXCR4 receptor signaling are involved in ALL disease development and progression. By analyzing their relevant roles in the pathogenesis of the two ALL forms, new molecular mechanisms able to modulate cancer cell invasion may be visualized. Notably, the partnership between Notch and CXCR4 may have considerable implications in understanding the complexity of T- and B-ALL. These two receptor pathways intersect other critical signals in the proliferative, differentiation, and metabolic programs of lymphocyte transformation. Also, the identification of the crosstalks in leukemia-stroma interaction within the tumor microenvironment may unveil new targetable mechanisms in disease relapse. Further studies are required to identify new challenges and opportunities to develop more selective and safer therapeutic strategies in ALL progression, possibly contributing to improve conventional hematological cancer therapy.

Hedgehog Signaling in Cancer: A Prospective Therapeutic Target for Eradicating Cancer Stem Cells

The Hedgehog (Hh) pathway is a signaling cascade that plays a crucial role in many fundamental processes, including embryonic development and tissue homeostasis. Moreover, emerging evidence has suggested that aberrant activation of Hh is associated with neoplastic transformations, malignant tumors, and drug resistance of a multitude of cancers. At the molecular level, it has been shown that Hh signaling drives the progression of cancers by regulating cancer cell proliferation, malignancy, metastasis, and the expansion of cancer stem cells (CSCs). Thus, a comprehensive understanding of Hh signaling during tumorigenesis and development of chemoresistance is necessary in order to identify potential therapeutic strategies to target various human cancers and their relapse. In this review, we discuss the molecular basis of the Hh signaling pathway and its abnormal activation in several types of human cancers. We also highlight the clinical development of Hh signaling inhibitors for cancer therapy as well as CSC-targeted therapy.

Vitamin D and its receptor polymorphisms: New possible prognostic biomarkers in leukemias

Several factors such as chromosomal translocations, gene mutations, and polymorphisms are involved in the pathogenesis of leukemia/lymphoma. Recently, the role of vitamin D (VD) and vitamin D receptor (VDR) polymorphisms in hematologic malignancies has been considered. In this review, we examine the possible role of VD levels, as well as VDR polymorphisms as prognostic biomarkers in leukemia/lymphoma. Relevant English language literature were searched and retrieved from Google Scholar search engine (1985-2017). The following keywords were used: vitamin D, vitamin D receptor, leukemia, lymphoma, and polymorphism. Increased serum levels of VD in patients with leukemia are associated with a better prognosis. However, low VD levels are associated with a poor prognosis, and VDR polymorphisms in various leukemias can have prognostic value. VD biomarker can be regarded as a potential prognostic factor for a number of leukemias, including acute myeloblastic leukemia (AML), chronic lymphoblastic leukemia (CLL), and diffuse large B-cell lymphoma (DLBCL). There is a significant relationship between different polymorphisms of VDR (including Taq I and Fok I) with several leukemia types such as ALL and AML, which may have prognostic value.

Targeting GLI Transcription Factors in Cancer

Aberrant activation of hedgehog (Hh) signaling has been observed in a wide variety of tumors and accounts for more than 25% of human cancer deaths. Inhibitors targeting the Hh signal transducer Smoothened (SMO) are widely used and display a good initial efficacy in patients suffering from basal cell carcinoma (BCC); however, a large number of patients relapse. Though SMO mutations may explain acquired therapy resistance, a growing body of evidence suggests that the non-canonical, SMO-independent activation of the Hh pathway in BCC patients can also account for this adverse effect. In this review, we highlight the importance of glioma-associated oncogene (GLI) transcription factors (the main downstream effectors of the canonical and the non-canonical Hh cascade) and their putative role in the regulation of multiple oncogenic signaling pathways. Moreover, we discuss the contribution of the Hh signaling to malignant transformation and propose GLIs as central hubs in tumor signaling networks and thus attractive molecular targets in anti-cancer therapies.

The transcription factor Gli3 promotes B cell development in fetal liver through repression of Shh

Solanki et al. show that stromal activity of the transcription factor Gli3 is required for B cell development in the fetal liver. Gli3 functions to repress Shh expression, and Shh signals to developing B cells to regulate their development at multiple developmental stages.

Before birth, B cells develop in the fetal liver (FL). In this study, we show that Gli3 activity in the FL stroma is required for B cell development. In the Gli3-deficient FL, B cell development was reduced at multiple stages, whereas the Sonic hedgehog (Hh [Shh])–deficient FL showed increased B cell development, and Gli3 functioned to repress Shh transcription. Use of a transgenic Hh-reporter mouse showed that Shh signals directly to developing B cells and that Hh pathway activation was increased in developing B cells from Gli3-deficient FLs. RNA sequencing confirmed that Hh-mediated transcription is increased in B-lineage cells from Gli3-deficient FL and showed that these cells expressed reduced levels of B-lineage transcription factors and B cell receptor (BCR)/pre-BCR–signaling genes. Expression of the master regulators of B cell development Ebf1 and Pax5 was reduced in developing B cells from Gli3-deficient FL but increased in Shh-deficient FL, and in vitro Shh treatment or neutralization reduced or increased their expression, respectively.

Anti-Cancer Phytometabolites Targeting Cancer Stem Cells

Medicinal plants are a plentiful source of bioactive molecules with much structural diversity. In cancer treatment, molecules obtained from plants represent an attractive alternative to other treatments because several plant-derived compounds have exhibited lower toxicity and higher selectivity against cancer cells. In this review, we focus on the possible application of bioactive molecules obtained from plants against more primitive cell populations in cancers, cancer stem cells. Cancer stem cells are present in several kinds of tumors and are responsible for recurrences and metastases. Common anti-cancer drugs exhibit lower effectiveness against cancer stem cells because of their biological features. However, recently discovered natural phytometabolites exert cytotoxic effects on this rare population of cells in cancers. Therefore, this review presents the latest research on promising compounds from plants that can act as antitumor drugs and that mainly affect stem cell populations in cancers.

Targeting the PI3K/AKT pathway via GLI1 inhibition enhanced the drug sensitivity of acute myeloid leukemia cells

Combination targeted therapy is commonly used to treat acute myeloid leukemia (AML) patients, particularly in refractory/relapse (RR) population. However, concerns have been raised regarding the safety and patient tolerance of combination chemotherapy. It is critical to choose the appropriate treatment for precision therapy. We performed genome-wide RNA profiling using RNA-Seq to compare the RR group and the complete remission (CR) group (a total of 42 adult AML patients). The Hedgehog (Hh) and PI3K/AKT pathways were upregulated in the RR population, which was further confirmed by western blot and/or qPCR. Overexpression of GLI1 in AML cells led to increased AKT phosphorylation and decreased drug sensitivity, which was attenuated by GLI1 inhibition. By contrast, neither the expression of GLI1 nor apoptosis in response to Ara-C treatment of AML cells was significantly affected by PI3K inhibition. Furthermore, co-inhibition of GLI1 and PI3K induced apoptosis of hematopoietic stem/progenitor cells (HSPCs), which raised serious concerns about the side effects of this treatment. These results indicated that GLI1 inhibition alone, but not combined inhibition, is sufficient to enhance AML drug sensitivity, which provides a novel therapeutic strategy for AML treatment.

Triterpene Structural Diversification by Plant Cytochrome P450 Enzymes

Cytochrome P450 monooxygenases (P450s) represent the largest enzyme family of the plant metabolism. Plants typically devote about 1% of the protein-coding genes for the P450s to execute primary metabolism and also to perform species-specific specialized functions including metabolism of the triterpenes, isoprene-derived 30-carbon compounds. Triterpenes constitute a large and structurally diverse class of natural products with various industrial and pharmaceutical applications. P450-catalyzed structural modification is crucial for the diversification and functionalization of the triterpene scaffolds. In recent times, a remarkable progress has been made in understanding the function of the P450s in plant triterpene metabolism. So far, ∼80 P450s are assigned biochemical functions related to the plant triterpene metabolism. The members of the subfamilies CYP51G, CYP85A, CYP90B-D, CYP710A, CYP724B, and CYP734A are generally conserved across the plant kingdom to take part in plant primary metabolism related to the biosynthesis of essential sterols and steroid hormones. However, the members of the subfamilies CYP51H, CYP71A,D, CYP72A, CYP81Q, CYP87D, CYP88D,L, CYP93E, CYP705A, CYP708A, and CYP716A,C,E,S,U,Y are required for the metabolism of the specialized triterpenes that might perform species-specific functions including chemical defense toward specialized pathogens. Moreover, a recent advancement in high-throughput sequencing of the transcriptomes and genomes has resulted in identification of a large number of candidate P450s from diverse plant species. Assigning biochemical functions to these P450s will be of interest to extend our knowledge on triterpene metabolism in diverse plant species and also for the sustainable production of valuable phytochemicals.

Therapeutic target discovery and drug development in cancer stem cells for leukemia and lymphoma: from bench to the clinic

INTRODUCTION

Cancer stem cells (CSCs), also known as tumor initialing cells, have self-renewal capacity and are believed to play an important role in residual disease or tumor relapse. CSCs exhibit characteristic slow growth rate and are resistant to conventional chemotherapy/radiotherapy in experimental models. The type of cells commonly employs aberrant activity of the embryonic signal transduction pathways - Notch, Hedgehog (Hh), and Wnt - for uncontrolled proliferation and survival. Areas covered: The following article discusses key genetic and molecular alterations in Notch, Hh and Wnt pathways and drugs targeting the alterations for the treatment of leukemia and lymphoma. Expert opinion: Early signs of signal agent activity have been observed in certain types of leukemia and lymphoma with experimental therapeutics targeting the embryonic pathways in the CSC signaling network. However, clinical development of agents that inhibit the Wnt/β-catenin, Notch and Hh signaling appear to be more complex in relapsed or refractory malignancies. A strategy to effectively target signaling may rely on early application of biomarkers representative of the active signaling nodes companion to the molecularly targeted agents. Biomarkers for efficacy could potentially guide selective treatment of hematological malignancies or cancer with drugs that target the embryonic pathways.

Deregulated hedgehog pathway signaling is inhibited by the smoothened antagonist LDE225 (Sonidegib) in chronic phase chronic myeloid leukaemia

Targeting the Hedgehog (Hh) pathway represents a potential leukaemia stem cell (LSC)-directed therapy which may compliment tyrosine kinase inhibitors (TKIs) to eradicate LSC in chronic phase (CP) chronic myeloid leukaemia (CML). We set out to elucidate the role of Hh signaling in CP-CML and determine if inhibition of Hh signaling, through inhibition of smoothened (SMO), was an effective strategy to target CP-CML LSC. Assessment of Hh pathway gene and protein expression demonstrated that the Hh pathway is activated in CD34⁺ CP-CML stem/progenitor cells. LDE225 (Sonidegib), a small molecule, clinically investigated SMO inhibitor, used alone and in combination with nilotinib, inhibited the Hh pathway in CD34⁺ CP-CML cells, reducing the number and self-renewal capacity of CML LSC in vitro. The combination had no effect on normal haemopoietic stem cells. When combined, LDE225 + nilotinib reduced CD34⁺ CP-CML cell engraftment in NSG mice and, upon administration to EGFP⁺ /SCLtTA/TRE-BCR-ABL mice, the combination enhanced survival with reduced leukaemia development in secondary transplant recipients. In conclusion, the Hh pathway is deregulated in CML stem and progenitor cells. We identify Hh pathway inhibition, in combination with nilotinib, as a potentially effective therapeutic strategy to improve responses in CP-CML by targeting both stem and progenitor cells.

Integration of Hedgehog and mutant FLT3 signaling in myeloid leukemia

FMS-like tyrosine kinase 3 (FLT3) internal tandem duplication (ITD) mutations resulting in constitutive kinase activity are common in acute myeloid leukemia (AML) and carry a poor prognosis. Several agents targeting FLT3 have been developed, but their limited clinical activity suggests that the inhibition of other factors contributing to the malignant phenotype is required. We examined gene expression data sets as well as primary specimens and found that the expression of GLI2, a major effector of the Hedgehog (Hh) signaling pathway, was increased in FLT3-ITD compared to wild-type FLT3 AML. To examine the functional role of the Hh pathway, we studied mice in which Flt3-ITD expression results in an indolent myeloproliferative state and found that constitutive Hh signaling accelerated the development of AML by enhancing signal transducer and activator of transcription 5 (STAT5) signaling and the proliferation of bone marrow myeloid progenitors. Furthermore, combined FLT3 and Hh pathway inhibition limited leukemic growth in vitro and in vivo, and this approach may serve as a therapeutic strategy for FLT3-ITD AML.

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.

Elucidating steroid alkaloid biosynthesis in Veratrum californicum: production of verazine in Sf9 cells

Steroid alkaloids have been shown to elicit a wide range of pharmacological effects that include anticancer and antifungal activities. Understanding the biosynthesis of these molecules is essential to bioengineering for sustainable production. Herein, we investigate the biosynthetic pathway to cyclopamine, a steroid alkaloid that shows promising antineoplastic activities. Supply of cyclopamine is limited, as the current source is solely derived from wild collection of the plant Veratrum californicum. To elucidate the early stages of the pathway to cyclopamine, we interrogated a V. californicum RNA-seq dataset using the cyclopamine accumulation profile as a predefined model for gene expression with the pattern-matching algorithm Haystack. Refactoring candidate genes in Sf9 insect cells led to discovery of four enzymes that catalyze the first six steps in steroid alkaloid biosynthesis to produce verazine, a predicted precursor to cyclopamine. Three of the enzymes are cytochromes P450 while the fourth is a γ-aminobutyrate transaminase; together they produce verazine from cholesterol.

Shaping of the tumor microenvironment: Stromal cells and vessels

Lymphomas develop and progress in a specialized tissue microenvironment such as bone marrow as well as secondary lymphoid organs such as lymph node and spleen. The lymphoma microenvironment is characterized by a heterogeneous population of stromal cells, including fibroblastic reticular cells, nurse-like cells, mesenchymal stem cells, follicular dendritic cells, and inflammatory cells such as macrophages, T- and B-cells. These cell populations interact with the lymphoma cells to promote lymphoma growth, survival and drug resistance through multiple mechanisms. Angiogenesis is also recognized as an important factor associated with lymphoma progression. In recent years, we have learned that the interaction between the malignant and non-malignant cells is bidirectional and resembles, at least in part, the pattern seen between non-neoplastic lymphoid cells and the normal microenvironment of lymphoid organs. A summary of the current knowledge of lymphoma microenvironment focusing on the cellular components will be reviewed here.

Cooperative integration between HEDGEHOG-GLI signalling and other oncogenic pathways: implications for cancer therapy

The HEDGEHOG-GLI (HH-GLI) signalling is a key pathway critical in embryonic development, stem cell biology and tissue homeostasis. In recent years, aberrant activation of HH-GLI signalling has been linked to several types of cancer, including those of the skin, brain, lungs, prostate, gastrointestinal tract and blood. HH-GLI signalling is initiated by binding of HH ligands to the transmembrane receptor PATCHED and is mediated by transcriptional effectors that belong to the GLI family, whose activity is finely tuned by a number of molecular interactions and post-translation modifications. Several reports suggest that the activity of the GLI proteins is regulated by several proliferative and oncogenic inputs, in addition or independent of upstream HH signalling. The identification of this complex crosstalk and the understanding of how the major oncogenic signalling pathways interact in cancer is a crucial step towards the establishment of efficient targeted combinatorial treatments. Here we review recent findings on the cooperative integration of HH-GLI signalling with the major oncogenic inputs and we discuss how these cues modulate the activity of the GLI proteins in cancer. We then summarise the latest advances on SMO and GLI inhibitors and alternative approaches to attenuate HH signalling through rational combinatorial therapies.

Hedgehog signaling in cancer stem cells: a focus on hematological cancers

The stem cell paradigm was first demonstrated in hematopoietic stem cells. Whilst classically it was cytokines and chemokines which were believed to control stem cell fate, more recently it has become apparent that the stem cell niche and highly conserved embryonic pathways play a key role in governing stem cell behavior. One of these pathways, the hedgehog signaling pathway, found in all organisms, is vitally important in embryogenesis, performing the function of patterning through early stages of development, and in adulthood, through the control of somatic stem cell numbers. In addition to these roles in health however, it has been found to be deregulated in a number of solid and hematological malignancies, components of the hedgehog pathway being associated with a poor prognosis. Further, these components represent viable therapeutic targets, with inhibition from a drug development perspective being readily achieved, making the hedgehog pathway an attractive potential therapeutic target. However, although the concept of cancer stem cells is well established, how these cells arise and the factors which influence their behavior are not yet fully understood. The role of the hedgehog signaling pathway and its potential as a therapeutic target in hematological malignancies is the focus of this review.

The Veratrum and Solanum alkaloids

This survey on steroidal alkaloids of the Veratrum and Solanum family isolated between 1974 and 2014 includes 187 compounds and 197 references. New developments in the chemistry and biology of this family of natural products with a special focus on the medicinal relevance of the jervanine alkaloid cyclopamine are discussed.

HIF factors cooperate with PML-RARα to promote acute promyelocytic leukemia progression and relapse

Acute promyelocytic leukemia (APL) is epitomized by the chromosomal translocation t(15;17) and the resulting oncogenic fusion protein PML-RARα. Although acting primarily as a transcriptional repressor, PML-RARα can also exert functions of transcriptional co-activation. Here, we find that PML-RARα stimulates transcription driven by HIF factors, which are critical regulators of adaptive responses to hypoxia and stem cell maintenance. Consistently, HIF-related gene signatures are upregulated in leukemic promyelocytes from APL patients compared to normal promyelocytes. Through in vitro and in vivo studies, we find that PML-RARα exploits a number of HIF-1α-regulated pro-leukemogenic functions that include cell migration, bone marrow (BM) neo-angiogenesis and self-renewal of APL blasts. Furthermore, HIF-1α levels increase upon treatment of APL cells with all-trans retinoic acid (ATRA). As a consequence, inhibiting HIF-1α in APL mouse models delays leukemia progression and exquisitely synergizes with ATRA to eliminate leukemia-initiating cells (LICs).

Salinomycin exerts anticancer effects on human breast carcinoma MCF-7 cancer stem cells via modulation of Hedgehog signaling

Breast cancer tissue contains a small population of cells that have the ability to self-renew, these cells are known as breast cancer stem cells (BCSCs). The Hedgehog signal transduction pathway plays a central role in stem cell development, its aberrant activation has been shown to contribute to the development of breast cancer, making this pathway an attractive therapeutic target. Salinomycin (Sal) is a novel identified cancer stem cells (CSCs) killer, however, the molecular basis for its anticancer effects is not yet clear. In the current study, Sal's ability to modulate the activity of key elements in the Hedgehog pathway was examined in the human breast cancer cell line MCF-7, as well as in a subpopulation of cancer stem cells identified within this cancer cell line. We show here that Sal inhibits proliferation, invasion, and migration while also inducing apoptosis in MCF-7 cells. Interestingly, in a subpopulation of MCF-7 cells with the CD44(+)/CD24(-) markers and high ALDH1 levels indicative of BCSCs, modulators of Hedgehog signaling Smo and Gli1 were significantly down-regulated upon treatment with Sal. These results demonstrate that Sal also inhibits proliferation and induces apoptosis of BCSCs, further establishing it as therapeutically relevant in the context of breast cancers and also indicating that modulation of Hedgehog signaling is one potential mechanism by which it exerts these anticancer effects.

Low serum miR-19a expression as a novel poor prognostic indicator in multiple myeloma

Multiple myeloma (MM) is the second most common hematologic malignancy characterized by the clonal expansion of plasma cells. Despite continuing advances, novel biomarkers are needed for diagnosis and prognosis of MM. In our study, we characterized the diagnostic and prognostic potential of circulating microRNAs (miRNAs) in MM. Serum miRNA levels were analyzed in 108 newly diagnosed symptomatic MM patients and 56 healthy donors (HDs). Our analysis identified 95 dysregulated miRNAs in newly diagnosed MM patients. Of the 95 dysregulated miRNAs, dysregulation of miR-19a, miR-92a, miR-214-3p, miR-135b-5p, miR-4254, miR-3658 and miR-33b was confirmed by quantitative reverse transcription PCR (RT-qPCR). Receiver operating characteristic analysis revealed that a combination of miR-19a and miR-4254 can distinguish MM from HD with a sensitivity of 91.7% and specificity of 90.5%. Decreased expression of miR-19a was positively correlated with international staging system advancement, del(13q14) and 1q21 amplification. Furthermore, downregulation of miR-19a resulted in significantly decreased progression-free survival (PFS) and overall survival (OS). Our analysis indicated that the poor prognostic correlation of miR-19a expression was independent of genetic abnormalities in MM. Multivariate analysis revealed that miR-19a was a significant predictor of shortened PFS and OS. Interestingly, although miR-19a levels portend a poor prognosis, patients with low miR-19a levels had an improved response to bortezomib compared to those with high miR-19a profile. Patients with downregulated miR-19a experienced a significantly extended survival upon bortezomib-based therapy. These data demonstrate that the expression patterns of serum microRNAs are altered in MM, and miR-19a levels are a valuable prognostic marker to identify high-risk MM.

New insight into hedgehog signaling in hematological malignancies

The hedgehog (Hh) signaling pathway is well established as being evolutionarily conserved across vertebrates, and is involved in organogenesis, hematopoiesis, embryogenesis and homeostasis of adult tissues. At a microscopic level, the Hh signaling pathway controls the proliferation, apoptosis, cell-cycle and differentiation programs of stem and progenitor cells. Increasing evidence suggests that aberrant activation of the Hh signaling pathway is related to neoplasm, including solid tumors and hematologic malignancies. Currently the Hh signaling pathway has become one of the most studied potential therapeutic targets in hematological malignancies. In this review, we focus on findings related to Hh signaling in the initiation, maintenance, progression and chemoresistance of hematological malignancies, looking forward to better targeted treatment strategies.

Therapy response correlates with ALDH activity in ALDH low-positive childhood acute lymphoblastic leukemias

The malignant cells of childhood acute lymphoblastic leukemia (ALL) do not form a homogenous entity but a collection of differently maturated blasts. The most immature leukemia cells may be more resistant to therapy than the bulk of more differentiated blasts. We studied 42 patients with childhood ALL treated according to the ALL-BFM 2000 protocol. At diagnosis, we determined the immunophenotype and the aldehyde dehydrogenase (ALDH) activity of the leukemic cells. Additionally, we investigated the expression of CD34, CD38 and CD45 to define a population of immunophenotypically immature cells (CD34(+)/CD38(-)/CD45(-/low)). We then studied levels of minimal residual disease (MRD) after induction therapy (day 33) to determine therapy response. Including all cases (n = 42), there was no correlation between ALDH positive cells, CD34(+)/CD38(-)/CD45(-/low) cells and MRD levels. A subset of 18 ALLs displayed a more mature phenotype with low-ALDH positivity (< 1%). Analyzing this cohort, ALDH positive blasts overlapped with the CD34(+)/CD38(-)/CD45(-/low) population. The initial rate of ALDH positivity correlated with MRD levels at day 33 of therapy (r = 0.61, P < .01). We conclude that in pediatric ALL, ALDH positivity as a marker of immaturity and stemness has prognostic significance only in phenotypically mature cases when the ALDH activity is not a property of the majority of the leukemic blasts. In case of an immature ALL phenotype, ALDH activity might be an inherent characteristic of the whole leukemia and is not limited to a more immature subpopulation that could confer to resistance and increased MRD-levels during therapy.

Sonic Hedgehog activation is implicated in diosgenin-induced megakaryocytic differentiation of human erythroleukemia cells

Differentiation therapy is a means to treat cancer and is induced by different agents with low toxicity and more specificity than traditional ones. Diosgenin, a plant steroid, is able to induce megakaryocytic differentiation or apoptosis in human HEL erythroleukemia cells in a dose-dependent manner. However, the exact mechanism by which diosgenin induces megakaryocytic differentiation has not been elucidated. In this study, we studied the involvement of Sonic Hedgehog in megakaryocytic differentiation induced by diosgenin in HEL cells. First, we showed that different elements of the Hedgehog pathway are expressed in our model by qRT-PCR. Then, we focused our interest on key elements in the Sonic Hedgehog pathway: Smoothened receptor, GLI transcription factor and the ligand Sonic Hedgehog. We showed that Smoothened and Sonic Hedgehog were overexpressed in disogenin-treated cells and that GLI transcription factors were activated. Then, we showed that SMO inhibition using siSMO or the GLI antagonist GANT-61, blocked megakaryocytic differentiation induced by diosgenin in HEL cells. Furthermore, we demonstrated that Sonic Hedgehog pathway inhibition led to inhibition of ERK1/2 activation, a major physiological pathway involved in megakaryocytic differentiation. In conclusion, our study reports, for the first time, a crucial role for the Sonic Hedgehog pathway in diosgenin-induced megakaryocytic differentiation in HEL cells.

Targeting hedgehog signaling in myelofibrosis and other hematologic malignancies

Treatment of myelofibrosis (MF), a BCR-ABL-negative myeloproliferative neoplasm, is challenging. The only current potentially curative option, allogeneic hematopoietic stem cell transplant, is recommended for few patients. The remaining patients are treated with palliative therapies to manage MF-related anemia and splenomegaly. Identification of a mutation in the Janus kinase 2 (JAK2) gene (JAK2 V617F) in more than half of all patients with MF has prompted the discovery and clinical development of inhibitors that target JAK2. Although treatment with JAK2 inhibitors has been shown to improve symptom response and quality of life in patients with MF, these drugs do not alter the underlying disease; therefore, novel therapies are needed. The hedgehog (Hh) signaling pathway has been shown to play a role in normal hematopoiesis and in the tumorigenesis of hematologic malignancies. Moreover, inhibitors of the Hh pathway have been shown to inhibit growth and self-renewal capacity in preclinical models of MF. In a mouse model of MF, combined inhibition of the Hh and JAK pathways reduced JAK2 mutant allele burden, reduced bone marrow fibrosis, and reduced white blood cell and platelet counts. Preliminary clinical data also suggest that inhibition of the Hh pathway, alone or in combination with JAK2 inhibition, may enable disease modification in patients with MF. Future studies, including one combining the Hh pathway inhibitor sonidegib and the JAK2 inhibitor ruxolitinib, are underway in patients with MF and will inform whether this combination approach can lead to true disease modification.

Involvement and targeted intervention of dysregulated Hedgehog signaling in osteosarcoma

BACKGROUND

During development, the Hedgehog pathway plays important roles regulating the proliferation and differentiation of chondrocytes, providing a template for growing bone. In this study, the authors investigated the components of dysregulated Hedgehog signaling as potential therapeutic targets for osteosarcoma.

METHODS

Small-molecule agonists and antagonists that modulate the Hedgehog pathway at different levels were used to investigate the mechanisms of dysregulation and the efficacy of Hedgehog blockade in osteosarcoma cell lines. The inhibitory effect of a small-molecule Smoothened (SMO) antagonist, IPI-926 (saridegib), also was examined in patient-derived xenograft models.

RESULTS

An inverse correlation was identified in osteosarcoma cell lines between endogenous glioma-associated oncogene 2 (GLI2) levels and Hedgehog pathway induction levels. Cells with high levels of GLI2 were sensitive to GLI inhibition, but not SMO inhibition, suggesting that GLI2 overexpression may be a mechanism of ligand-independent activation. In contrast, cells that expressed high levels of the Hedgehog ligand gene Indian hedgehog (IHH) and the target genes patched 1 (PTCH1) and GLI1 were sensitive to modulation of both SMO and GLI, suggesting ligand-dependent activation. In 2 xenograft models, active autocrine and paracrine, ligand-dependent Hedgehog signaling was identified. IPI-926 inhibited the Hedgehog signaling interactions between the tumor and the stroma and demonstrated antitumor efficacy in 1 of 2 ligand-dependent models.

CONCLUSIONS

The current results indicate that both ligand-dependent and ligand-independent Hedgehog dysregulation may be involved in osteosarcoma. It is the first report to demonstrate Hedgehog signaling crosstalk between the tumor and the stroma in osteosarcoma. The inhibitory effect of IPI-926 warrants additional research and raises the possibility of using Hedgehog pathway inhibitors as targeted therapeutics to improve treatment for osteosarcoma.

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.

Arsenic trioxide inhibits the Hedgehog pathway which is aberrantly activated in acute promyelocytic leukemia

BACKGROUND/AIM

Dysregulated Hedgehog (Hh) signaling has been implicated in several human malignancies. Hh signaling inhibitors are predicted to have a minimal effect when the Smoothened receptor is mutated. Implications that Gli proteins are molecular targets of arsenic trioxide (ATO) action prompted us to investigate the expression of Hh signaling in acute promyelocytic leukemia (APL) and the influence of ATO on the Hh signaling pathway in APL.

METHODS

Quantitative real-time reverse transcription polymerase chain reaction and Western blot were employed to analyze the expression of Hh pathway components and the influence of ATO on the Hh signaling pathway in APL.

RESULTS

The expression of Hh pathway components was significantly upregulated in APL. In newly diagnosed APL patients, Gli2 expression was significantly positively correlated with Gli1 (R = 0.57, p < 0.001) and Smo (R = 0.56, p < 0.001) and the expression of Hh pathway components was significantly higher in the high WBC group (p < 0.05). ATO can significantly downregulate the expression of Hh pathway components in vitro and in vivo (p < 0.05).

CONCLUSION

The Hh pathway is aberrantly activated in APL and associated with a bad prognostic factor. ATO can effectively inhibit the expression of the Hh pathway. The obtained data give the first clinical evidence for the application of ATO in tumors exhibiting an aberrantly activated Hh pathway.

New prospects for drug development: the hedgehog pathway revealed. Focus on hematologic malignancies

The hedgehog (Hh) pathway is a critical regulator of vertebrate embryonic development and is involved in the function of processes such as stem cell maintenance and differentiation, tissue polarity and cell proliferation. Given how critical these functions are, it is not surprising that mutations in Hh pathway components are often implicated in the tumorigenesis of a variety of human cancers. Promotion of tumor growth has recently been shown by activated Hh signaling in the tumor itself, as well as by pathway activation within surrounding cells comprising the tumor microenvironment. Targeted disruption of various Hh pathway proteins has been successfully employed as an anticancer strategy with several synthetic Hh antagonists now available. Here, the molecular basis of Hh signaling, the therapeutic rationales for targeting this pathway and the current status of Hh pathway inhibitors in the clinic are reviewed.

Identification of somatic and germline mutations using whole exome sequencing of congenital acute lymphoblastic leukemia

Background

Acute lymphoblastic leukemia (ALL) diagnosed within the first month of life is classified as congenital ALL and has a significantly worse outcome than ALL diagnosed in older children. This suggests that congenital ALL is a biologically different disease, and thus may be caused by a distinct set of mutations. To understand the somatic and germline mutations contributing to congenital ALL, the protein-coding regions in the genome were captured and whole-exome sequencing was employed for the identification of single-nucleotide variants and small insertion and deletions in the germlines as well as the primary tumors of four patients with congenital ALL.

Methods

Exome sequencing was performed on Illumina GAIIx or HiSeq 2000 (Illumina, San Diego, California). Reads were aligned to the human reference genome and the Genome Analysis Toolkit was used for variant calling. An in-house developed Ensembl-based variant annotator was used to richly annotate each variant.

Results

There were 1–3 somatic, protein-damaging mutations per ALL, including a novel mutation in Sonic Hedgehog. Additionally, there were many germline mutations in genes known to be associated with cancer predisposition, as well as genes involved in DNA repair.

Conclusion

This study is the first to comprehensively characterize the germline and somatic mutational profile of all protein-coding genes patients with congenital ALL. These findings identify potentially important therapeutic targets, as well as insight into possible cancer predisposition genes.

Stromal cells expressing hedgehog-interacting protein regulate the proliferation of myeloid neoplasms

Aberrant reactivation of hedgehog (Hh) signaling has been described in a wide variety of human cancers including cancer stem cells. However, involvement of the Hh-signaling system in the bone marrow (BM) microenvironment during the development of myeloid neoplasms is unknown. In this study, we assessed the expression of Hh-related genes in primary human CD34⁺ cells, CD34⁺ blastic cells and BM stromal cells. Both Indian Hh (Ihh) and its signal transducer, smoothened (SMO), were expressed in CD34⁺ acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS)-derived cells. However, Ihh expression was relatively low in BM stromal cells. Remarkably, expression of the intrinsic Hh-signaling inhibitor, human Hh-interacting protein (HHIP) in AML/MDS-derived stromal cells was markedly lower than in healthy donor-derived stromal cells. Moreover, HHIP expression levels in BM stromal cells highly correlated with their supporting activity for SMO⁺ leukemic cells. Knockdown of HHIP gene in stromal cells increased their supporting activity although control cells marginally supported SMO⁺ leukemic cell proliferation. The demethylating agent, 5-aza-2′-deoxycytidine rescued HHIP expression via demethylation of HHIP gene and reduced the leukemic cell-supporting activity of AML/MDS-derived stromal cells. This indicates that suppression of stromal HHIP could be associated with the proliferation of AML/MDS cells.

Targeting Hedgehog signaling and understanding refractory response to treatment with Hedgehog pathway inhibitors

Hedgehog (Hh) signaling is a principal component of the morphogenetic code best known to direct pattern formation during embryogenesis. The Hh pathway remains active in adulthood however where it guides tissue regeneration and remodeling and Hh production in the niche plays an important role in maintaining stem cell compartment size. Deregulated Hh signaling activity is associated, depending on the context, with both cancer initiation and progression. Interestingly, the Hh pathway is remarkably druggable, raising hopes that inhibition of the pathway could support anticancer therapy. Indeed, a large body of preclinical data supports such an action, but promising clinical data are still limited to basal cell carcinoma (BSC) and medulloblastoma. Nevertheless cancer resistance against Hh targeting has already emerged as a major problem. Here we shall review the current situation with respect to targeting the Hh pathway in cancer in general and in chemotolerance in particular with a focus on the problems associated with the emergence of tumors resistant to treatment with inhibitors targeting the Hh receptor Smoothened (SMO).

Prevailing importance of the hedgehog signaling pathway and the potential for treatment advancement in sarcoma

The hedgehog signaling pathway is important in embryogenesis and post natal development. Constitutive activation of the pathway due to mutation of pathway components occurs in ~25% of medulloblastomas and also in basal cell carcinomas. In many other malignancies the therapeutic role for hedgehog inhibition though intriguing, based on preclinical data, is far from assured. Hedgehog inhibition is not an established part of the treatment paradigm of sarcoma but the scientific rationale for a possible benefit is compelling. In chondrosarcoma there is evidence of hedgehog pathway activation and an ontologic comparison between growth plate chondrocyte differentiation and different chondrosarcoma subtypes. Immunostaining epiphyseal growth plate for Indian hedgehog is particularly positive in the zone of pre-hypertrophic chondrocytes which correlates ontologically with conventional chondrosarcoma. In Ewing sarcoma/PNET tumors the Gli1 transcription factor is a direct target of the EWS-FLI1 oncoprotein present in 85% of cases. In many cases of rhabdomyosarcomas there is increased expression of Gli1 (Ragazzini et al., 2004). Additionally, a third of embryonal rhabdomyosarcomas have loss of Chr.9q22 that encompasses the patched locus (Bridge et al., 2000). The potential to treat osteosarcoma by inhibition of Gli2 and the role of the pathway in ovarian fibromas and other connective tissue tumors is also discussed (Nagao et al., 2011; Hirotsu et al., 2010). Emergence of acquired secondary resistance to targeted therapeutics is an important issue that is also relevant to hedgehog inhibition. In this context secondary resistance of medulloblastomas to treatment with a smoothened antagonist in two tumor mouse models is examined.

The regulation of normal and leukemic hematopoietic stem cells by niches

The origin and propagation of normal and leukemic hematopoietic cells critically depend on their interplays with the hematopoietic microenvironment (or so-called niche), which represent important biological models for understanding organogenesis and tumorigenesis. Nevertheless, the anatomic and functional characterizations of the niche cells for normal hematopoietic stem cells (HSCs) have proved a formidable task. It is uncertain whether the combinational effects of a few sets of molecular niche elements, behind the long-sought cellular architectures with preferred anatomic locations, actually meets the functional definition of HSC niche. Moreover, even much less is known about the niche components for numerous types of leukemia-stem cells (LSCs) that originate via discrete cellular and molecular transforming mechanisms. However, one interesting scenario is emerging, i.e., the leukemia cells can positively remodel the hematopoietic microenvironment favorable for their competition over the normal hematopoiesis that co-exists within the same eco-system. This property probably represents a previously unappreciated essential trait of a functional LSC. Obviously, the further exploration into how the hematopoietic microenvironment interplay with normal or malignant hematopoiesis will shed light onto the designing of novel types of niche-targeting therapies for leukemia.

Enhancement of cyclopamine via conjugation with nonmetabolic sugars

The Veratrum alkaloid cyclopamine, an inhibitor of cancer stem cell growth, was used as a representative scaffold to evaluate the inhibitory impact of glycosylation with a group of nonmetabolic saccharides, such as d-threose. In a five-step divergent process, a 32-member glycoside library was created and assayed to determine that glycosides of such sugars notably improved the GI50 value of cyclopamine while metabolic sugars, such as d-glucose, did not.

Acute myeloid leukemia: focus on novel therapeutic strategies

Acute myeloid leukemia (AML) is a heterogeneous disease with variable clinical outcomes. Cytogenetic analysis reveals which patients may have favorable risk disease, but 5-year survival in this category is only approximately 60%, with intermediate and poor risk groups faring far worse. Advances in our understanding of the biology of leukemia pathogenesis and prognosis have not been matched with clinical improvements. Unsatisfactory outcomes persist for the majority of patients with AML, particularly the elderly. Novel agents and treatment approaches are needed in the induction, post-remission and relapsed settings. The additions of clofarabine for relapsed or refractory disease and the hypomethylating agents represent recent advances. Clinical trials of FLT3 inhibitors have yielded disappointing results to date, with ongoing collaborations attempting to identify the optimal role for these agents. Potential leukemia stem cell targeted therapies and treatments in the setting of minimal residual disease are also under investigation. In this review, we will discuss recent advances in AML treatment and novel therapeutic strategies.