A central question in cell biology is how membrane-spanning receptors transmit extracellular signals inside cells to modulate cell adhesion and motility. Focal adhesion kinase (FAK) is a crucial signalling component that is activated by numerous stimuli and functions as a biosensor or integrator to control cell motility. Through multifaceted and diverse molecular connections, FAK can influence the cytoskeleton, structures of cell adhesion sites and membrane protrusions to regulate cell movement.
Many features of cell behavior are regulated by Rho family GTPases, but the most profound effects of these proteins are on the actin cytoskeleton and it was these that first drew attention to this family of signaling proteins. Focusing on Rho and Rac, we will discuss how their effectors regulate the actin cytoskeleton. We will describe how the activity of Rho proteins is regulated downstream from growth factor receptors and cell adhesion molecules by guanine nucleotide exchange factors and GTPase activating proteins. Additionally, we will discuss how there is signaling crosstalk between family members and how various bacterial pathogens have developed strategies to manipulate Rho protein activity so as to enhance their own survival.
The protein tyrosine kinase focal adhesion kinase (FAK) plays a prominent role in integrin signaling. FAK activation, demonstrated by an increase in phosphorylation of Tyr397 as well as other sites in the protein, is best understood in the context of the engagement of integrins at the cell surface. Activation of FAK results in recruitment of a number of SH2-domain- and SH3-domain-containing proteins, which mediate signaling to several downstream pathways. FAK-dependent activation of these pathways has been implicated in a diverse array of cellular processes, including cell migration, growth factor signaling, cell cycle progression and cell survival.
During progression from tumour growth to metastasis, specific integrin signals enable cancer cells to detach from neighbouring cells, re-orientate their polarity during migration, and survive and proliferate in foreign microenvironments. There is increasing evidence that certain integrins associate with receptor tyrosine kinases (RTKs) to activate signalling pathways that are necessary for tumour invasion and metastasis. The effect of these integrins might be especially important in cancer cells that have activating mutations, or amplifications, of the genes that encode these RTKs.
Protein kinase B or Akt (PKB/Akt) is a serine/threonine kinase, which in mammals comprises three highly homologous members known as PKBalpha (Akt1), PKBbeta (Akt2), and PKBgamma (Akt3). PKB/Akt is activated in cells exposed to diverse stimuli such as hormones, growth factors, and extracellular matrix components. The activation mechanism remains to be fully characterised but occurs downstream of phosphoinositide 3-kinase (PI-3K). PI-3K generates phosphatidylinositol-3,4,5-trisphosphate (PIP(3)), a lipid second messenger essential for the translocation of PKB/Akt to the plasma membrane where it is phosphorylated and activated by phosphoinositide-dependent kinase-1 (PDK-1) and possibly other kinases. PKB/Akt phosphorylates and regulates the function of many cellular proteins involved in processes that include metabolism, apoptosis, and proliferation. Recent evidence indicates that PKB/Akt is frequently constitutively active in many types of human cancer. Constitutive PKB/Akt activation can occur due to amplification of PKB/Akt genes or as a result of mutations in components of the signalling pathway that activates PKB/Akt. Although the mechanisms have not yet been fully characterised, constitutive PKB/Akt signalling is believed to promote proliferation and increased cell survival and thereby contributing to cancer progression. This review surveys recent developments in understanding the mechanisms and consequences of PKB/Akt activation in human malignancy.
Phosphoinositide 3-kinases (PI3Ks) generate specific inositol lipids that have been implicated in the regulation of cell growth, proliferation, survival, differentiation and cytoskeletal changes. One of the best characterized targets of PI3K lipid products is the protein kinase Akt or protein kinase B (PKB). In quiescent cells, PKB resides in the cytosol in a low-activity conformation. Upon cellular stimulation, PKB is activated through recruitment to cellular membranes by PI3K lipid products and phosphorylation by 3'-phosphoinositide-dependent kinase-1 (PDK1). Here we review the mechanism by which PKB is activated and the downstream actions of this multifunctional kinase. We also discuss the evidence that PDK1 may be involved in the activation of protein kinases other than PKB, the mechanisms by which this activity of PDK1 could be regulated and the possibility that some of the currently postulated PKB substrates targets might in fact be phosphorylated by PDK1-regulated kinases other than PKB.
To facilitate a rapid response to environmental change, cells use scaffolding - or adaptor - proteins to recruit key components of their signal-transduction machinery to specific subcellular locations. Paxillin is a multi-domain adaptor found at the interface between the plasma membrane and the actin cytoskeleton. Here it provides a platform for the integration and processing of adhesion- and growth factor-related signals.
Programmed cell death is crucial for the development and maintenance of multicellular organisms. The decision to live, or to die, depends, at the cellular level, upon the cell's interaction with extracellular cues that trigger cell signaling pathways promoting survival or death. The extracellular matrix (ECM) influences the execution of the apoptotic program through the actions of adhesion receptors. Among these, integrins initiate a variety of downstream signaling events in response to ECM ligation. Integrins directly activate survival pathways via the PI 3-kinase and MAPK pathways and act as essential cofactors for their stimulation by growth factors. Conversely, elevated integrin expression in the absence of appropriate ligands, or in the presence of natural or synthetic antagonists, can promote apoptosis under otherwise permissive growth conditions. Integrins thus act in a crucial biosensory role, coordinating survival or death responses as a function of ECM composition. This dual function provides an elegant mechanism through which tissue-remodeling events may regulate cell death or survival in a temporal, ECM-governed manner.
The ability of intracellular signaling networks to orchestrate a complex biological response such as cell motility requires that individual signaling proteins must act as integrators, responding to multiple extracellular inputs and regulating multiple signaling pathway outputs. In this review, we highlight recent findings that place focal adhesion kinase (FAK) in an important receptor-proximal position in the regulation of growth factor and integrin-stimulated cell motility. Emphasis is placed on the molecular mechanisms of FAK activation, connections of FAK to focal contact formation as well as turnover, and the potential that FAK function in promoting cell invasion may be distinct from its role in cell motility.
The acquisition of a motile and invasive phenotype is an important step in the development of tumors and ultimately metastasis. This step requires the abrogation of cell-cell contacts, the remodeling of the extracellular matrix and of cell-matrix interactions, and finally the movement of the cell mediated by the actin cytoskeleton. Evidence for participation of Rho GTPases in migration and invasion is addressed in this review with emphasis on epithelial cells and the contribution of Rho GTPases toward tumor invasion. The Rho GTPases, including Rac, Cdc42, and Rho, have been implicated in the establishment of cell-cell contacts and of cell-matrix interactions crucial to attaining a fully polarized epithelial state, and they are known for their regulation of the actin cytoskeleton and transcriptional activation. Under aberrant conditions, however, they have been implicated in motility, invasion, and some aspects of metastasis. It is well known that Rho GTPases are activated by different classes of transmembrane receptors and that they transmit these signals to their effector proteins. These downstream targets include not only adaptor proteins and kinases which affect the actin cytoskeleton, but also transcription factors leading to expression of genes necessary for the drastic morphological changes which accompany these processes.
Cell proliferation is controlled not only by soluble mitogens but also by components of the extracellular matrix (ECM) such as fibronectin, to which cells adhere via the integrin family of transmembrane receptors. Input from both growth factor receptors and integrins is required to stimulate progression through the G1 phase of the cell cycle, via induction of G1 cyclins and suppression of inhibitors of the G1 cyclin-dependent kinases. Extensive crosstalk takes place between integrin and growth factor receptor signaling pathways, and mitogenic signaling is weak and transient in the absence of integrin-mediated cell adhesion. In normal untransformed cells, all of the important mitogenic signal transduction cascades, namely those downstream of the Ras and Rho family small GTPases and the phosphoinositide 3-OH kinase-PKB/Akt pathway, are regulated by integrin-mediated cell adhesion. As a result, these cells are anchorage-dependent for growth. In contrast, constitutive activity of each of these pathways has been reported in cancer cells, which not only reduces their mitogen dependence but also allows these cells to grow in an anchorage-independent fashion.
Interactions of cells with the extracellular matrix are essential for the control of tissue remodelling, cell migration, and embryogenesis. At the cell-extracellular matrix contact points, specialized structures are formed and termed focal adhesions, where transmembrane adhesion receptors provide a structural link between the actin cytoskeleton and the extracellular matrix components. Numerous structural and regulatory proteins assemble at the cytoplasmic face of focal adhesions in a Rho-dependent fashion.
Adhesion molecules, although catalytically inactive, are able to translate environmental cues into complex intracellular signals. They can do this by associating with tyrosine kinase receptors for growth factors, which can prime, integrate or feedback adhesion-based signals. Recent results show that reciprocal crosstalk between the two systems is only one facet of such a collaboration, and that unconventional and alternative hierarchies can be established in which, on the one hand, cell adhesion can trigger ligand-independent activation of growth factor receptors, and, on the other, growth factors can induce adhesion molecules to propagate adhesion-independent signals.
Engagement of cells with the extracellular matrix (ECM) proteins is crucial for various biological processes, including cell adhesion, spreading, proliferation, differentiation, migration, apoptosis, and gene induction, contributing to maintenance of tissue integrity, embryogenesis, wound healing, and the metastasis of tumor cells (Hynes, 2002b; Juliano, 2002). The engagement involves cell adhesion mediated by integrins, a large family of cell adhesion receptors that are transmembrane glycoproteins which bind to ECM or to counter-receptors on neighbor cells. In this review, the molecular basis of signaling mediated by integrins and their collaboration with growth factor receptors will be discussed, based on recent observations. Although other cell adhesion receptors including cadherins, selectins, syndecans, and the immunoglobulin superfamily of cell adhesion molecules (IgCAMs) can play important roles or be involved in these processes, we suggest readers refer to recent outstanding reviews on them (Barclay, 2003; Brummendorf and Lemmon 2001; Panicker et al. 2003).
Integrin-linked kinase (ILK) is a serine/threonine protein kinase which interacts with the cytoplasmic domains of beta1 and beta3 integrins. ILK structure and its localization at the focal adhesion allows it not only to interact with different structural proteins, but also to mediate many different signalling pathways. Extracellular matrices (ECM) and growth factors each stimulate ILK signalling. Constitutive activation of ILK in epithelial cells results in oncogenic phenotypes such as disruption of cell extracellular matrix and cell to cell interactions, suppression of suspension-induced apoptosis, and induction of anchorage independent cell growth and cell cycle progression. More specifically, pathological overexpression of ILK results in down-regulation of E-cadherin expression, and nuclear accumulation of beta-catenin, leading to the subsequent activation of the beta-catenin/Tcf transcription complex, the downstream components of the Wnt signalling pathway. Here we review the data implicating ILK in the regulation of these two signalling pathways, and discuss recent novel insights into the molecular basis and requirement of ILK in the process of epithelial to mesenchymal transformation (EMT).
Intracellular levels of phosphorylation are regulated by the coordinated action of protein kinases and phosphatases. Disregulation of this balance can lead to cellular transformation. Here we review knowledge of the mechanisms of one protein phosphatase, the tumour suppressor PTEN/MMAC/TEP 1 apropos its role in tumorigenesis and signal transduction. PTEN plays an important role in the phosphatidyl-inositol-3-kinase (PI3-K) pathway by catalyzing degradation of phosphatidylinositol-(3,4,5)-triphosphate generated by PI3-K. This inhibits downstream targets mainly protein kinase B (PKB/Akt), cell survival and proliferation. PTEN contributes to cell cycle regulation by blockade of cells entering the S phase of the cell cycle, and by upregulation of p27(Kip1) which is recruited into the cyclin E/cdk2 complex. PTEN also modulates cell migration and motility by regulation of the extracellular signal-related kinase - mitogen activated protein kinase (ERK-MAPK) pathway and by dephosphorylation of focal adhesion kinase (FAK). We also emphasize the increasingly important role that PTEN has from an evolutionary point of view. A number of PTEN functions have been elucidated but more information is needed for utilization in clinical application and potential cancer therapy.
Integrins are cell surface transmembrane glycoproteins that function as adhesion receptors in cell-extracellular matrix interactions and link the matrix proteins to the cytoskeleton. The family of human integrins comprises 24 members, each of which is a heterodimer consisting of 1 of 18 alpha- and 1 of 8 beta-subunits. Integrins play an important role in the cytoskeleton organization and in transduction of intracellular signals, regulating various processes such as proliferation, differentiation, apoptosis, and cell migration. This review summarizes current views on the structure of integrins, integrin associated proteins, and biochemical mechanisms underlying their signaling functions.