Hedgehog signalling as a target in cancer stem cells

Hedgehog (Hh) is one of the most important signalling pathways. Together with the Wnt, TGF-β/BMP and Notch pathways, it is involved in both embryonic development and adult tissue homeostasis. This is because Hh plays a central role in the proliferative control and differentiation of both embryonic stem cells and adult stem cells. In this way, an alteration in the Hh pathway, either by misexpression of components of that pathway or by changes in the expression of other cellular components that interfere with the Hh signalling system, may trigger the development of several types of cancer. This occurs because normal stem cells or their intermediaries toward differentiated mature cells are not part of the normal proliferative/differentiation balance and begin to expand without control, triggering the generation of the so-called cancer stem cells. In this review, we will focus on the molecular aspects and the role of Hh signalling in normal tissues and in tumour development.     

MEK inhibitors as a chemotherapeutic intervention in multiple myeloma

The Ras/Raf/MEK/extracellular signal regulated kinase (ERK) (Ras/mitogen-activated protein kinases (MAPK)) signal transduction pathway is a crucial mediator of many fundamental biological processes, including cellular proliferation, survival, angiogenesis and migration. Aberrant signalling through the Ras/MAPK cascade is common in a wide array of malignancies, including multiple myeloma (MM), making it an appealing candidate for the development of novel targeted therapies. In this review, we explore our current understanding of the Ras/MAPK pathway and its role in MM. Additionally, we summarise the current status of small molecule inhibitors of MEK under clinical evaluation, and discuss future approaches required to optimise their use.     

A Golgi-based KDELR-dependent signalling pathway controls extracellular matrix degradation

We recently identified an endomembrane-based signalling cascade that is activated by the KDEL receptor (KDELR) on the Golgi complex. At the Golgi, the KDELR acts as a traffic sensor (presumably via binding to chaperones that leave the ER) and triggers signalling pathways that balance membrane fluxes between ER and Golgi. One such pathway relies on Gq and Src. Here, we examine if KDELR might control other cellular modules through this pathway. Given the central role of Src in extracellular matrix (ECM) degradation, we investigated the impact of the KDELR-Src pathway on the ability of cancer cells to degrade the ECM. We find that activation of the KDELR controls ECM degradation by increasing the number of the degradative structures known as invadopodia. The KDELR induces Src activation at the invadopodia and leads to phosphorylation of the Src substrates cortactin and ASAP1, which are required for basal and KDELR-stimulated ECM degradation. This study furthers our understanding of the regulatory circuitry underlying invadopodia-dependent ECM degradation, a key phase in metastases formation and invasive growth.     

Platelet-derived growth factor (PDGF) and glial tumorigenesis

Platelet-derived growth factor (PDGF) has long been implicated in cancer and is known to be involved in many biological processes. In Central Nervous System (CNS) neoplasms, particularly gliomas, PDGF is often over-expressed. However, what role PDGF plays in tumor progression remains to be fully described. A wide range of work from in vitro studies to mouse models have implicated the PDGF pathway in various processes including proliferation, cellular migration, development, and angiogenesis. Being a secreted factor, PDGF not only has autocrine effects on producing cells but also has potential for paracrine effects on other tumor cells and the tumor microenvironment. The development of small molecules that inhibit the PDGF receptor and various subsequent signaling components promises to introduce new approaches to the treatment of gliomas.     

The PTEN-PI3K pathway: of feedbacks and cross-talks

The tumor suppressor PTEN was originally identified as a negative regulator of the phosphoinositide 3-kinase (PI3K) signaling, a main regulator of cell growth, metabolism and survival. Yet this function of PTEN is extremely relevant for its tumor-suppressive ability, albeit the recent characterization of many PI3K-independent tumor-suppressive activities. PI3K-mediated PIP(3) production leads to the activation of the canonical AKT-mTORC1 pathway. The implications of this signaling cascade in health and disease have been underscored by the high number of regulators within the pathway whose alterations give rise to different malignancies, including familiar syndromes, metabolic dysfunctions and cancer. Moreover, PI3K is tightly buffered at multiple levels by downstream components, which have turned this signaling pathway literally upside down. PI3K and its downstream components in turn cross-talk with a number of other pathways, thereby leading to a complex network of signals that may have dramatic consequences when perturbed. Here, we review the current status of the PTEN-PI3K signaling pathway with special emphasis on the most recent data on targets and regulation of the PTEN-PI3K axis. This provides novel provocative therapeutic implications based on the targeted modulation of PI3K-cross-talking signals.     

Sonic Hedgehog signaling pathway: from embryology to molecular targeted therapies

It has long been known that the hedgehog signaling pathway is crucial for embryonic development, in both Drosophila and vertebrate systems. During the past few years its implication in carcinogenesis has become clear and today it is acknowledged that this pathway plays a role in the malignant transformation of multiple cell types, either owing to the mutation of some of its components or to its erratic activation. New molecular targeted therapies that inhibit the pathway have shown their unquestionable efficiency in several tumours -among which basal cell carcinoma, medulloblastoma, or pancreatic adenocarcinoma. The assessment of these inhibitors in other types of tumours is currently underway with promising results, suggesting that the Sonic hedgehog signalling pathway may become one of the therapeutic targets of the future.     

Inhibition of doxorubicin-induced HER3-PI3K-AKT signalling enhances apoptosis of ovarian cancer cells

Resistance to chemotherapy is a serious problem for the successful treatment of ovarian cancer patients but signalling pathways that contribute to this chemoinsensitivity are largely unknown. We demonstrate that the chemotherapeutic drug doxorubicin induces activation of the HER3-PI3K-AKT signalling cascade in ovarian cancer cells. We further show that the induction of this anti-apoptotic signalling pathway is based on upregulated expression of HER3 ligands, their shedding by the metalloprotease ADAM17, and is dependent on the HER2 receptor. The doxorubicin-mediated activation of this important survival cascade can be blocked by the kinase inhibitors lapatinib or erlotinib as well as by the therapeutic monoclonal antibody trastuzumab. Inhibition of the doxorubicin-induced activation of HER3-PI3K-AKT signalling significantly increased apoptosis of ovarian cancer cells. Besides doxorubicin, treatment of cells with cisplatin resulted in activation of the HER3 receptor whereas other chemotherapeutics did not show this effect. The increase in HER3 phosphorylation was detected in well-established ovarian cancer cell lines which originate from patients previously treated with these chemotherapeutic drugs. Based on these results, we postulate that activation of the HER3-PI3K-AKT cascade represents a major mechanism of chemoresistance in ovarian cancer.     

Status of PI3K inhibition and biomarker development in cancer therapeutics

The phosphatidylinositol 3-kinase (PI3K) signalling pathway is integral to diverse cellular functions, including cellular proliferation, differentiation and survival. The ‘phosphate and tensin homologue deleted from chromosome 10’ (PTEN) tumor suppressor gene plays a critical role as a negative regulator of this pathway. An array of genetic mutations and amplifications has been described affecting key components of this pathway, with implications not only for tumorigenesis but also for resistance to some classic cytotoxics and targeted agents. Emerging preclinical research has significantly advanced our understanding of the PI3K pathway and its complex machinations and interactions. This knowledge has enabled the evolution of rationally designed drugs targeting elements of this pathway. It is important that the development of suitable biomarkers continues in parallel to optimize use of these agents. A new generation of PI3K inhibitors is now entering early clinical trials, with much anticipation that they will add to the growing armamentarium of targeted cancer therapeutics.     

PI3K/Akt signalling pathway and cancer

Phosphatidylinositol-3 kinases, PI3Ks, constitute a lipid kinase family characterized by their ability to phosphorylate inositol ring 3'-OH group in inositol phospholipids to generate the second messenger phosphatidylinositol-3,4,5-trisphosphate (PI-3,4,5-P(3)). RPTK activation results in PI(3,4,5)P(3) and PI(3,4)P(2) production by PI3K at the inner side of the plasma membrane. Akt interacts with these phospholipids, causing its translocation to the inner membrane, where it is phosphorylated and activated by PDK1 and PDK2. Activated Akt modulates the function of numerous substrates involved in the regulation of cell survival, cell cycle progression and cellular growth. In recent years, it has been shown that PI3K/Akt signalling pathway components are frequently altered in human cancers. Cancer treatment by chemotherapy and gamma-irradiation kills target cells primarily by the induction of apoptosis. However, the development of resistance to therapy is an important clinical problem. Failure to activate the apoptotic programme represents an important mode of drug resistance in tumor cells. Survival signals induced by several receptors are mediated mainly by PI3K/Akt, hence this pathway may decisively contribute to the resistant phenotype. Many of the signalling pathways involved in cellular transformation have been elucidated and efforts are underway to develop treatment strategies that target these specific signalling molecules or their downstream effectors. The PI3K/Akt pathway is involved in many of the mechanisms targeted by these new drugs, thus a better understanding of this crossroad can help to fully exploit the potential benefits of these new agents.     

Phosphatidylinositol 3-kinase (PI3K) pathway activation in bladder cancer

The phosphatidylinositol 3-kinase (PI3K) pathway is a critical signal transduction pathway that regulates multiple cellular functions. Aberrant activation of this pathway has been identified in a wide range of cancers. Several pathway components including AKT, PI3K and mTOR represent potential therapeutic targets and many small molecule inhibitors are in development or early clinical trials. The complex regulation of the pathway, together with the multiple mechanisms by which it can be activated, make this a highly challenging pathway to target. For successful inhibition, detailed molecular information on individual tumours will be required and it is already clear that different tumour types show distinct combinations of alterations. Recent results have identified alterations in pathway components PIK3CA, PTEN, AKT1 and TSC1 in bladder cancer, some of which are significantly related to tumour phenotype and clinical behaviour. Co-existence of alterations to several PI3K pathway genes in some bladder tumours indicates that these proteins may have functions that are not related solely to the known canonical pathway.     

Bcl-2 regulates a caspase-3/caspase-2 apoptotic cascade in cytosolic extracts

Apoptosis is accompanied by the activation of a number of apoptotic proteases (caspases) which selectively cleave specific cellular substrates. Caspases themselves are zymogens which are activated by proteolysis. It is widely believed that 'initiator' caspases are recruited to and activated within apoptotic signalling complexes, and then cleave and activate downstream 'effector' caspases. While activation of the effector caspase, caspase-3, has indeed been observed as distal to activation of several different initiator caspases, evidence for a further downstream proteolytic cascade is limited. In particular, there is little evidence that cellular levels of caspase-3 that are activated via one pathway are sufficient to cleave and activate other initiator caspases. To address this issue, the ability of caspase-3, activated upon addition to cytosolic extracts of cytochrome c, to cause cleavage of caspase-2 was investigated. It was demonstrated that cleavage of caspase-2 follows, and is dependent upon, activation of caspase-3. Moreover, the activation of both caspases was inhibited by Bcl-2. Together, these data indicate that Bcl-2 can protect cells from apoptosis by acting at a point downstream from release of mitochondrial cytochrome c, thereby preventing a caspase-3 dependent proteolytic cascade.     

RAS oncogenes: weaving a tumorigenic web

RAS proteins are essential components of signalling pathways that emanate from cell surface receptors. Oncogenic activation of these proteins owing to missense mutations is frequently detected in several types of cancer. A wealth of biochemical and genetic studies indicates that RAS proteins control a complex molecular circuitry that consists of a wide array of interconnecting pathways. In this Review, we describe how RAS oncogenes exploit their extensive signalling reach to affect multiple cellular processes that drive tumorigenesis.     

The hedgehog signalling pathway in tumorigenesis and development

The hedgehog signalling pathway is responsible for the embryonic patterning of a range of tissues, and it is now known that dysregulation of this pathway can result in the formation of several tumour types. This cascade is regulated at the cell surface by the opposing actions of the patched and smoothened molecules which together form a receptor complex for hedgehog. The discovery that inactivation of the human patched gene is responsible for familial and sporadic forms of basal cell carcinoma firmly established a role for dysregulation of hedgehog signalling in tumorigenesis. Other key members of this pathway have also been shown to be involved in tumour formation, as have more distal downstream targets of hedgehog signalling. Since it appears that tumorigenesis results from constitutive activation of hedgehog responsive genes, the identification of novel downstream targets of hedgehog signalling in given cell types is likely to increase our understanding of the molecular processes underlying tumour formation.     

Wnt signalling and its impact on development and cancer

The Wnt signalling pathway is an ancient system that has been highly conserved during evolution. It has a crucial role in the embryonic development of all animal species, in the regeneration of tissues in adult organisms and in many other processes. Mutations or deregulated expression of components of the Wnt pathway can induce disease, most importantly cancer. The first gene to be identified that encodes a Wnt signalling component, Int1 (integration 1), was molecularly characterized from mouse tumour cells 25 years ago. In parallel, the homologous gene Wingless in Drosophila melanogaster, which produces developmental defects in embryos, was characterized. Since then, further components of the Wnt pathway have been identified and their epistatic relationships have been defined. This article is a Timeline of crucial discoveries about the components and functions of this essential pathway.