Hedgehog inhibition as an anti-cancer strategy

Targeting the Hedgehog Pathway in Rhabdomyosarcoma

Aberrant activation of the Hedgehog (Hh) signalling pathway is known to play an oncogenic role in a wide range of cancers; in the particular case of rhabdomyosarcoma, this pathway has been demonstrated to be an important player for both oncogenesis and cancer progression. In this review, after a brief description of the pathway and the characteristics of its molecular components, we describe, in detail, the main activation mechanisms that have been found in cancer, including ligand-dependent, ligand-independent and non-canonical activation. In this context, the most studied inhibitors, i.e., SMO inhibitors, have shown encouraging results for the treatment of basal cell carcinoma and medulloblastoma, both tumour types often associated with mutations that lead to the activation of the pathway. Conversely, SMO inhibitors have not fulfilled expectations in tumours-among them sarcomas-mostly associated with ligand-dependent Hh pathway activation. Despite the controversy existing regarding the results obtained with SMO inhibitors in these types of tumours, several compounds have been (or are currently being) evaluated in sarcoma patients. Finally, we discuss some of the reasons that could explain why, in some cases, encouraging preclinical data turned into disappointing results in the clinical setting.

Targeting survival pathways in chronic myeloid leukaemia stem cells

Chronic myeloid leukaemia (CML) is a clonal myeloproliferative disorder characterized by the presence of a fusion oncogene BCR-ABL, which encodes a protein with constitutive TK activity. The implementation of tyrosine kinase inhibitors (TKIs) marked a major advance in CML therapy; however, there are problems with current treatment. For example, relapse occurs when these drugs are discontinued in the majority of patients who have achieved a complete molecular response on TKI and these agents are less effective in patients with mutations in the BCR-ABL kinase domain. Importantly, TKI can effectively target proliferating mature cells, but do not eradicate quiescent leukaemic stem cells (LSCs), therefore allowing disease persistence despite treatment. It is essential that alternative strategies are used to target the LSC population. BCR-ABL activation is responsible for the modulation of different signalling pathways, which allows the LSC fraction to evade cell death. Several pathways have been shown to be modulated by BCR-ABL, including PI3K/AKT/mTOR, JAK-STAT and autophagy signalling pathways. Targeting components of these survival pathways, alone or in combination with TKI, therefore represents an attractive potential therapeutic approach for targeting the LSC. However, many pathways are also active in normal stem cells. Therefore, potential targets must be validated to effectively eradicate CML stem cells while sparing normal counterparts. This review summarizes the main pathways modulated in CML stem cells, the recent developments and the use of novel drugs to target components in these pathways which may be used to target the LSC population.

LINKED ARTICLES

This article is part of a themed section on Emerging Therapeutic Aspects in Oncology. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.169.issue-8.

A computation using mutually exclusive processing is sufficient to identify specific Hedgehog signaling components

A system of more than one part can be deciphered by observing differences between the parts. A simple way to do this is by recording something absolute displaying a trait in one part and not in another: in other words, mutually exclusive computation. Conditional directed expression in vivo offers processing in more than one part of the system giving increased computation power for biological systems analysis. Here, I report the consideration of these aspects in the development of an in vivo screening assay that appears sufficient to identify components specific to a system.

G protein-coupled receptors revisited: therapeutic applications inspired by synthetic biology

G protein-coupled receptors (GPCRs) mediate the majority of cellular responses to hormones and neurotransmitters within the human body. They have much potential in the emerging field of synthetic biology, which is the rational, systematic design of biological systems with desired functionality. The responsiveness of GPCRs to a plethora of endogenous and exogenous ligands and stimuli make them ideal sensory receptor modules of synthetic gene networks. Such networks can activate target gene expression in response to a specific stimulus. Additionally, because GPCRs are important pharmacological targets of various human diseases, genes encoding their protein/peptide ligands can also be incorporated as target genes of the response output elements of synthetic gene networks. This review aims to critically examine the potential role of GPCRs in constructing therapeutic synthetic gene networks and to discuss various challenges in utilizing GPCRs for synthetic biology applications.

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

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

Heparanase-1 and components of the hedgehog signalling pathway are increased in untreated alveolar orbital rhabdomyosarcoma

BACKGROUND

To assess the activities of heparanase-1 and elements of the hedgehog signalling pathway in alveolar orbital rhabdomyosarcoma.

METHODS

Specimens (n = 23) were divided into two groups, those from patients with preoperative chemoradiotherapy or untreated patients; six samples of normal extraocular muscle were used as a normal muscle group. The presence of heparanase-1, patched, smoothened and glioma-associated oncogene homologue-1 protein expression was determined in 23 cases of archival paraffin-embedded alveolar orbital rhabdomyosarcoma after immunohistochemistry. RNA was extracted from three groups of paraffin-embedded specimens and messenger RNA expressions of heparanase-1, smoothened and glioma-associated oncogene homologue-1 compared using nested reverse transcriptase polymerase chain reaction and a limiting dilution analysis.

RESULTS

The heparanase-1, patched, smoothened and glioma-associated oncogene homologue-1 protein was expressed in 91.3%, 87.0%, 91.3% and 78.3%, respectively, of the alveolar orbital rhabdomyosarcoma specimens. Untreated rhabdomyosarcoma samples tended to stain intensely, but staining was relatively weak in tissue obtained from the chemoradiotherapy group. The expression levels of heparanase-1, smoothened and glioma-associated oncogene homologue-1 messenger RNA in untreated and chemoradiotherapy groups paralleled that seen with immunology, and there were no significant differences in heparanase-1, smoothened and glioma-associated oncogene homologue-1 messenger RNA levels between the chemoradiotherapy group and the normal muscle group (P > 0.05). However, the messenger RNA in the untreated group were all significantly higher than those in the chemoradiotherapy and normal muscle groups (P < 0.01).

CONCLUSIONS

Both heparanase-1 and hedgehog signalling pathway are involved in the pathogenesis of alveolar orbital rhabdomyosarcoma; however, chemotherapy and/or radiotherapy appears to significantly inhibit their upregulation.

Oxidative ipso substitution of 2,4-difluoro-benzylphthalazines: identification of a rare stable quinone methide and subsequent GSH conjugate

In vitro metabolite identification and GSH trapping studies in human liver microsomes were conducted to understand the bioactivation potential of compound 1 [2-(6-(4-(4-(2,4-difluorobenzyl)phthalazin-1-yl)piperazin-1-yl)pyridin-3-yl)propan-2-ol], an inhibitor of the Hedgehog pathway. The results revealed the formation of a unique, stable quinone methide metabolite (M1) via ipso substitution of a fluorine atom and subsequent formation of a GSH adduct (M2). The stability of this metabolite arises from extensive resonance-stabilized conjugation of the substituted benzylphthalazine moiety. Cytochrome P450 (P450) phenotyping studies revealed that the formation of M1 and M2 were NADPH-dependent and primarily catalyzed by CYP3A4 among the studied P450 isoforms. In summary, an unusual and stable quinone methide metabolite of compound 1 was identified, and a mechanism was proposed for its formation via an oxidative ipso substitution.