Arthur M. Mercurio

Activation of Phosphoinositide 3-OH Kinase by the α6β4 Integrin Promotes Carcinoma Invasion

We demonstrate that the alpha6beta4 integrin promotes carcinoma invasion through a preferential and localized targeting of phosphoinositide-3 OH kinase (PI3K) activity. Stable expression of alpha6beta4 increased carcinoma invasion in a PI3K-dependent manner, and transient expression of a constitutively active PI3K increased invasion in the absence of alpha6beta4. Ligation of alpha6beta4 stimulated significantly more PI3K activity than ligation of beta1 integrins, establishing specificity among integrins for PI3K activation. Alpha6beta4-regulated PI3K activity was required for the formation of lamellae, dynamic sites of motility, in carcinoma cells. The small G protein Rac is required downstream of PI3K for invasion. These studies define a mechanism by which the alpha6beta4 integrin promotes carcinoma invasion and invoke a novel function for PI3K signaling.

Mouse egg integrin α6β1functions as a sperm receptor

Summary Binding between sperm and egg plasma membranes is an essential step in fertilization. Whereas fertilin, a mammalian sperm surface protein, is involved in this crucial interaction, sperm receptors on the egg plasma membrane have not been identified. Because fertilin contains a predicted integrin ligand domain, we investigated the expression and function of integrin subunits in unfertilized mouse eggs. Polymerase chain reactions detected mRNAs for α5, α6, αv, β1, β3, and β5. Immunofluorescence revealed α6β1 and αvβ3 on the plasma membrane. GoH3, a function-blocking anti-α6 monoclonal antibody, abolished sperm binding, but a nonfunction-blocking anti-a6 monoclonal antibody, a function-blocking anti-αvβ3 polyclonal antibody, and an RGD peptide had no effect. Somatic cells bound sperm avidly, but only if they expressed α6β1. A peptide analog of the fertilin integrin ligand domain inhibited sperm binding to eggs and α6β1 + cells and diminished GoH3 staining of eggs. Our results indicate a novel role for the integrin α6β1 as a cell-cell adhesion receptor that mediates sperm-egg binding.

VEGF targets the tumour cell

The function of vascular endothelial growth factor (VEGF) in cancer is not limited to angiogenesis and vascular permeability. VEGF-mediated signalling occurs in tumour cells, and this signalling contributes to key aspects of tumorigenesis, including the function of cancer stem cells and tumour initiation. In addition to VEGF receptor tyrosine kinases, the neuropilins are crucial for mediating the effects of VEGF on tumour cells, primarily because of their ability to regulate the function and the trafficking of growth factor receptors and integrins. This has important implications for our understanding of tumour biology and for the development of more effective therapeutic approaches.

Glycogen synthase kinase-3 is an endogenous inhibitor of Snail transcription implications for the epithelial–mesenchymal transition

We report that the activity of glycogen synthase kinase-3 (GSK-3) is necessary for the maintenance of the epithelial architecture. Pharmacological inhibition of its activity or reducing its expression using small interfering RNAs in normal breast and skin epithelial cells results in a reduction of E-cadherin expression and a more mesenchymal morphology, both of which are features associated with an epithelial–mesenchymal transition (EMT). Importantly, GSK-3 inhibition also stimulates the transcription of Snail, a repressor of E-cadherin and an inducer of the EMT. We identify NFκB as a transcription factor inhibited by GSK-3 in epithelial cells that is relevant for Snail expression. These findings indicate that epithelial cells must sustain activation of a specific kinase to impede a mesenchymal transition.

ERβ Impedes Prostate Cancer EMT by Destabilizing HIF-1α and Inhibiting VEGF-Mediated Snail Nuclear Localization: Implications for Gleason Grading

High Gleason grade prostate carcinomas are aggressive, poorly differentiated tumors that exhibit diminished estrogen receptor beta (ERbeta) expression. We report that a key function of ERbeta and its specific ligand 5alpha-androstane-3beta,17beta-diol (3beta-adiol) is to maintain an epithelial phenotype and repress mesenchymal characteristics in prostate carcinoma. Stimuli (TGF-beta and hypoxia) that induce an epithelial-mesenchymal transition (EMT) diminish ERbeta expression, and loss of ERbeta is sufficient to promote an EMT. The mechanism involves ERbeta-mediated destabilization of HIF-1alpha and transcriptional repression of VEGF-A. The VEGF-A receptor neuropilin-1 drives the EMT by promoting Snail1 nuclear localization. Importantly, this mechanism is manifested in high Gleason grade cancers, which exhibit significantly more HIF-1alpha and VEGF expression, and Snail1 nuclear localization compared to low Gleason grade cancers.

The α6β4 integrin and epithelial cell migration

Abstract Although the involvement of α6β4, an integrin laminin receptor, in hemidesmosome organization has dominated the study of this integrin, recent studies are revealing novel functions for α6β4 in the migration of epithelial and carcinoma cells. The engagement of laminin by α6β4 can stabilize actin-rich protrusions and mediate traction forces necessary for cell movement. This integrin also has a significant impact on signaling molecules that stimulate migration and invasion, especially PI3-K and Rho GTPases. Activation of PI3-K by α6β4 enhances the formation of actin protrusions, and it may stimulate the function of other integrins, such as α3β1, that are also important for epithelial migration. Signaling through α6β4 may not always depend on the adhesive functions of this integrin, a possibility that has profound implications for migration and invasion because it implies that the ability of α6β4 to stimulate these processes is not limited to specific matrix environments.

Transcriptional activation of integrin β6 during the epithelial-mesenchymal transition defines a novel prognostic indicator of aggressive colon carcinoma

We used a spheroid model of colon carcinoma to analyze integrin dynamics as a function of the epithelial-mesenchymal transition (EMT), a process that provides a paradigm for understanding how carcinoma cells acquire a more aggressive phenotype. This EMT involves transcriptional activation of the beta6 integrin subunit and a consequent induction of alphavbeta6 expression. This integrin enhances the tumorigenic properties of colon carcinoma, including activation of autocrine TGF-beta and migration on interstitial fibronectin. Importantly, this study validates the clinical relevance of the EMT. Kaplan-Meier analysis of beta6 expression in 488 colorectal carcinomas revealed a striking reduction in median survival time of patients with high beta6 expression. Elevated receptor expression did not simply reflect increasing tumor stage, since log-rank analysis showed a more significant impact on the survival of patients with early-stage, as opposed to late-stage, disease. Cox regression analysis confirmed that this integrin is an independent variable for these tumors. These findings define the alphavbeta6 integrin as an important risk factor for early-stage disease and a novel therapeutic candidate for colorectal cancer.

The epithelial-mesenchymal transition (EMT) and colorectal cancer progression

During embryonic development, epithelial cells must escape the structural constraints imposed by tissue architecture and adopt a phenotype more amenable to cell movement, a process known as the epithelial-mesenchymal transition (EMT). The progression of carcinomas to invasive and metastatic disease may also involve localized occurrences of EMT. However, data that support the actual occurrence of EMT in specific carcinomas and the relevance of this process to the progression of these tumors had been scant. This review highlights recent studies that substantiate the importance of the EMT to colorectal carcinoma. Specifically, a novel model for studying the EMT of colorectal carcinoma has been used to gain insight into the nature of the EMT itself and to identify molecular events that contribute to disease progression. Although loss of E-cadherin function is a primal event for the EMT, the expression of specific integrins such as αvβ6 as a consequence of the EMT enables invasive cells to interact with interstitial matrices and to sustain activation of TGF-β. Of note, αvβ6 expression in tumors is a marker of cells that have undergone an EMT and it is prognostic for tumors that will progress more rapidly to terminal disease. The EMT also induces autocrine signaling involving VEGF and Flt-1 that enable invasive cells to become ‘self-sufficient’ for survival. Thus, the EMT appears to be an integral component of colorectal cancer progression and its analysis can yield novel targets for prognosis and therapy.

Laminin receptors: achieving specificity through cooperation

The laminins are a large family of extracellular matrix proteins that can profoundly influence development, differentiation and disease progression. The biological effects of the laminins are mediated by surface receptors that link laminin matrices to intracellular signalling pathways. Several classes of receptors, including integrins and other molecules, may cooperate to provide the specificity apparent in the diverse array of laminin-mediated phenomena. This review assesses our current understanding of laminin receptors and discusses how such receptors could recognize structural differences among the laminins and relay these differences to the cell.

IAP regulation of metastasis

Inhibitor-of-Apoptosis (IAP) proteins contribute to tumor progression, but the requirements of this pathway are not understood. Here, we show that intermolecular cooperation between XIAP and survivin stimulates tumor cell invasion and promotes metastasis. This pathway is independent of IAP inhibition of cell death. Instead, a survivin-XIAP complex activates NF-kappaB, which in turn leads to increased fibronectin gene expression, signaling by beta1 integrins, and activation of cell motility kinases FAK and Src. Therefore, IAPs are direct metastasis genes, and their antagonists could provide antimetastatic therapies in patients with cancer.