Platelet‐derived growth factor (PDGF) signalling in cancer: rapidly emerging signalling landscape

Platelet‐derived growth factor (PDGF)‐mediated signalling has emerged as one of the most extensively and deeply studied biological mechanism reported to be involved in regulation of growth and survival of different cell types. However, overwhelmingly increasing scientific evidence is also emphasizing on dysregulation of spatio‐temporally controlled PDGF‐induced signalling as a basis for cancer development. We partition this multi‐component review into recently developing understanding of dysregulation PDGF signalling in different cancers, how PDGF receptors are quantitatively controlled by microRNAs. Moreover, we also summarize most recent advancements in therapeutic targeting of PDGFR as evidenced by preclinical studies. Better understanding of the PDGF‐induced intracellular signalling in different cancers will be helpful in catalysing the transition from a segmented view of cancer biology to a conceptual continuum.     

IAPs: guardians of RIPK1

Deregulation of innate immune signalling and cell death form the basis of most human disease pathogenesis. Inhibitor of APoptosis (IAP) protein-family members are frequently overexpressed in cancer and contribute to tumour cell survival, chemo-resistance, disease progression and poor prognosis. Although best known for their ability to regulate caspases, IAPs also influence ubiquitin-dependent pathways that modulate innate immune signalling by activation of NF-κB. Recent advances in our understanding of the molecular mechanisms through which IAPs influence cell death and innate immune responses have provided new insights into novel strategies for treatment of cancer. In this review we discuss our current understanding of IAP-mediated NF-κB signalling, as well as elaborate on unexpected insights into the involvement of IAPs in regulating the 'Ripoptosome', a novel intrinsic cell death-inducing platform. We propose an evolutionarily conserved concept whereby IAPs function as guardians of killer platforms such as the apoptosome in Drosophila and the Ripoptosome in mammals.     

ERK5 and its role in tumour development

The MEK5 [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase 5]/ERK5 pathway is the least well studied MAPK signalling module. It has been proposed to play a role in the pathology of cancer. In the present paper, we review the role of the MEK5/ERK5 pathway using the 'hallmarks of cancer' as a framework and consider how this pathway is deregulated. As well as playing a key role in endothelial cell survival and tubular morphogenesis during tumour neovascularization, ERK5 is also emerging as a regulator of tumour cell invasion and migration. Several oncogenes can stimulate ERK5 activity, and protein levels are increased by a novel amplification at chromosome locus 17p11 and by down-regulation of the microRNAs miR-143 and miR-145. Together, these finding underscore the case for further investigation into understanding the role of ERK5 in cancer.     

Programmed cell death pathways in cancer: a review of apoptosis,autophagy and programmed necrosis

Programmed cell death (PCD), referring to apoptosis, autophagy and programmed necrosis, is proposed to be death of a cell in any pathological format, when mediated by an intracellular program. These three forms of PCD may jointly decide the fate of cells of malignant neoplasms; apoptosis and programmed necrosis invariably contribute to cell death, whereas autophagy can play either pro‐survival or pro‐death roles. Recent bulk of accumulating evidence has contributed to a wealth of knowledge facilitating better understanding of cancer initiation and progression with the three distinctive types of cell death. To be able to decipher PCD signalling pathways may aid development of new targeted anti‐cancer therapeutic strategies. Thus in this review, we present a brief outline of apoptosis, autophagy and programmed necrosis pathways and apoptosis‐related microRNA regulation, in cancer. Taken together, understanding PCD and the complex interplay between apoptosis, autophagy and programmed necrosis may ultimately allow scientists and clinicians to harness the three types of PCD for discovery of further novel drug targets, in the future cancer treatment.     

Structural and mechanical functions of integrins

Integrins are ubiquitously expressed cell surface receptors that play a critical role in regulating the interaction between a cell and its microenvironment to control cell fate. These molecules are regulated either via their expression on the cell surface or through a unique bidirectional signalling mechanism. However, integrins are just the tip of the adhesome iceberg, initiating the assembly of a large range of adaptor and signalling proteins that mediate the structural and signalling functions of integrin. In this review, we summarise the structure of integrins and mechanisms by which integrin activation is controlled. The different adhesion structures formed by integrins are discussed, as well as the mechanical and structural roles integrins play during cell migration. As the function of integrin signalling can be quite varied based on cell type and context, an in depth understanding of these processes will aid our understanding of aberrant adhesion and migration, which is often associated with human pathologies such as cancer.     

The extracellular topographical environment influences ovarian cancer cell behaviour

The importance of the biophysical cellular environment in cancer development has been increasingly recognised but so far has been only superficially studied. Here we investigated the effect of cell-like substrate topography on ovarian cancer cell behaviour and potential underlying signalling pathways. We observed changes in cell morphology in response to substrate topography, which implies modification of structure-function associations. Differences in focal adhesion signalling and Rho/ROCK activity suggested their involvement in the biomechanically-driven cellular responses. Cell-like topography was also shown to modulate the MAPK pathway and hence potentially regulate cell proliferation. The selective regulation of the cells by the mechanotransduction pathways that we noted has wide ranging implications for understanding cancer development. We established that the physical architecture of cell culture substrate is sufficient to influence cancer cell behaviour, independent of genetic composition or biochemical milieu.     

Surface plasmon resonance spectroscopy in the study of membrane-mediated cell signalling.

Peptide-membrane interactions contribute to many important biological processes such as cellular signaling, protein trafficking and ion-channel formation. During receptor-mediated signalling, activated intracellular signalling molecules are often recruited into receptor-induced signaling complexes at the cytoplasmic surface of the cell membrane. Such recruitment can depend upon protein-protein and protein-lipid interactions as well as protein acylation. A wide variety of biophysical techniques have been combined with the use of model membrane systems to study these interactions and have provided important information on the relationship between the structure of these proteins involved in cell signalling and their biological function. More recently, surface plasmon resonance (SPR) spectroscopy has also been applied to the study of biomembrane-based systems using both planar mono- or bilayers or liposomes. This article provides an overview of these recent applications, which demonstrate the potential of SPR to enhance our molecular understanding of membrane-mediated cellular signalling.     

Chemokines and cancer: migration, intracellular signalling and intercellular communication in the microenvironment

Inappropriate chemokine/receptor expression or regulation is linked to many diseases, especially those characterized by an excessive cellular infiltrate, such as rheumatoid arthritis and other inflammatory disorders. There is now overwhelming evidence that chemokines are also involved in the progression of cancer, where they function in several capacities. First, specific chemokine-receptor pairs are involved in tumour metastasis. This is not surprising, in view of their role as chemoattractants in cell migration. Secondly, chemokines help to shape the tumour microenvironment, often in favour of tumour growth and metastasis, by recruitment of leucocytes and activation of pro-inflammatory mediators. Emerging evidence suggests that chemokine receptor signalling also contributes to survival and proliferation, which may be particularly important for metastasized cells to adapt to foreign environments. However, there is considerable diversity and complexity in the chemokine network, both at the chemokine/receptor level and in the downstream signalling pathways they couple into, which may be key to a better understanding of how and why particular chemokines contribute to cancer growth and metastasis. Further investigation into these areas may identify targets that, if inhibited, could render cancer cells more susceptible to chemotherapy.     

Intracellular adaptor molecules and AR signalling in the tumour microenvironment

Androgen deprivation therapy is the mainstay for treating advanced prostate cancer. A better understanding in the complexity of the androgen receptor (AR) signalling pathway has highlighted that this form of treatment is not sufficient. Since Huggins and Hodges made their crucial observations on the benefits of castration for prostate cancer, significant progress has been achieved in understanding the importance of the cross-talk between the hormone signalling pathway and the kinase signalling network. We now know that preventing androgen production or ligand binding to the AR does not necessarily mark the end of the road for prostate tumour growth. Emerging evidence suggests that there exists a complex set of compensatory mechanisms which allows growth factors to push the transformed cells into a ‘survival adaptation mode’ within the tumour microenvironment. An increase in autocrine and paracrine cascades of growth factor are the most commonly reported events to correlate with progression of androgen-dependent disease to a disseminated androgen independent state. The mechanism of how growth factors can sustain AR activation when cells are deprived of androgens is unknown. This is due to the lack of information about the critical factors linking the intracellular signalling molecules associated with the downstream AR signalling events triggered by growth factors. The aim of this mini review is to highlight a potentially new insight into how intracellular adaptor molecules activated by growth factors may influence and act as a molecular switch to allow the continuation of AR activity in the presence of therapeutic anti-androgens following chemical or surgical castration.     

GHR is involved in gastric cell growth and apoptosis via PI3K/AKT signalling

Growth hormone receptor (GHR), the cognate receptor of growth hormone (GH), is a membrane bound receptor that belongs to the class I cytokine receptor superfamily. GH binding GHR induces cell differentiation and maturation, initiates the anabolism inside the cells and promotes cell proliferation. Recently, GHR has been reported to be associated with various types of cancer. However, the underlying mechanism of GHR in gastric cancer has not been defined. Our results showed that silence of GHR inhibited the growth of SGC-7901 and MGC-803 cells, and tumour development in mouse xenograft model. Flow cytometry showed that GHR knockout significantly stimulated gastric cancer cell apoptosis and caused G1 cell cycle arrest, which was also verified by Western blot that GHR deficiency induced the protein level of cleaved-PARP, a valuable marker of apoptosis. In addition, GHR deficiency inhibited the activation of PI3K/AKT signalling pathway. On the basis of the results, that GHR regulates gastric cancer cell growth and apoptosis through controlling G1 cell cycle progression via mediating PI3K/AKT signalling pathway. These findings provide a novel understanding for the role of GHR in gastric cancer.     

Integrated morphodynamic signalling of the mammary gland

The mammary gland undergoes a spectacular series of changes as it develops, and maintains a remarkable capacity to remodel and regenerate for several decades. Mammary morphogenesis has been investigated for over 100 years, motivated by the dairy industry and cancer biologists. Over the past decade, the gland has emerged as a major model system in its own right for understanding the cell biology of tissue morphogenesis. Multiple signalling pathways from several cell types are orchestrated together with mechanical cues and cell rearrangements to establish the pattern of the mammary gland. The integrated mechanical and molecular pathways that control mammary morphogenesis have implications for the developmental regulation of other epithelial organs.     

The tipping points in the initiation of B cell signalling: how small changes make big differences

B cells are selected by the binding of antigen to clonally distributed B cell receptors (BCRs), triggering signalling cascades that result in B cell activation. With the recent application of high-resolution live-cell imaging, we are gaining an understanding of the events that initiate BCR signalling within seconds of its engagement with antigen. These observations are providing a molecular explanation for fundamental aspects of B cell responses, including antigen affinity discrimination and the value of class switching, as well as insights into the underlying causes of B cell tumorigenesis. Advances in our understanding of the earliest molecular events that follow antigen binding to the BCR may provide a general framework for the initiation of signalling in the adaptive immune system.     

Phospholipase D as a key modulator of cancer progression

The phospholipase D (PLD) family has a ubiquitous expression in cells. PLD isoforms (PLDs) and their hydrolysate phosphatidic acid (PA) have been demonstrated to engage in multiple stages of cancer progression. Aberrant expression of PLDs, especially PLD1 and PLD2, has been detected in various cancers. Inhibition or elimination of PLDs activity has been shown to reduce tumour growth and metastasis. PLDs and PA also serve as downstream effectors of various cell‐surface receptors, to trigger and regulate propagation of intracellular signals in the process of tumourigenesis and metastasis. Here, we discuss recent advances in understanding the functions of PLDs and PA in discrete stages of cancer progression, including cancer cell growth, invasion and migration, and angiogenesis, with special emphasis on the tumour‐associated signalling pathways mediated by PLDs and PA and the functional importance of PLDs and PA in cancer therapy.     

Mechanotransduction at focal adhesions: from physiology to cancer development

Living cells are continuously exposed to mechanical cues, and can translate these signals into biochemical information (e.g. mechanotransduction). This process is crucial in many normal cellular functions, e.g. cell adhesion, migration, proliferation, and survival, as well as the progression of diseases such as cancer. Focal adhesions are the major sites of interactions between extracellular mechanical environments and intracellular biochemical signalling molecules/cytoskeleton, and hence focal adhesion proteins have been suggested to play important roles in mechanotransduction. Here, we overview the current molecular understanding in mechanotransduction occurring at focal adhesions. We also introduce recent studies on how extracellular matrix and mechanical microenvironments contribute to the development of cancer.     

On the road to leucine-rich repeat kinase 2 signalling: evidence from cellular and in vivo studies

Parkinson's disease (PD) is the most common neurodegenerative movement disorder. Although PD has long been considered a purely sporadic disorder, genetic research has revealed an underlying genetic cause in at least 10% of all PD cases. To date, mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the most common cause of familial PD. Moreover, given the strong clinical and neuropathological similarities between LRRK2 PD and the sporadic forms of the disease, the notion is supported that the unravelling of the molecular pathways underlying LRRK2 PD will greatly contribute to our general understanding of PD. Therefore, intense research efforts have been focused on the understanding of the physiological function of LRRK2 and its relation to PD. To date, progress has been made in these fields based on the study of LRRK2 cell culture models, the identification of LRRK2 interaction partners and kinase substrates and the generation of LRRK2 animal models. In this review, the current insights into the cellular role of LRRK2 are discussed. The overview reveals a potential involvement of LRRK2 in major cell signalling pathways including apoptosis, cytoskeleton dynamics, protein translation, mitogen-activated protein kinase signalling and specific dopaminergic functions, consistent with its proposed role as a signal transduction protein.