Putting the rap on integrin activation

Positional control of cell fate through joint integrin/receptor protein kinase signaling

Cells adhere to the extracellular matrix throughout most of their lifetime. This close, intimate contact with the matrix exerts an extraordinary control on the behavior of cells, determining whether they move or stay put, proliferate or remain quiescent, and even live or die. Attachment to the matrix not only enables cells to respond to soluble growth factors and cytokines but also determines the nature of the response. The integrins are a large family of receptors that attach cells to the matrix, organize their cytoskeleton, and cooperate with receptor protein tyrosine kinases to regulate cell fate. Research on integrin signaling is beginning to explain the complex and specific effects that the extracellular matrix exerts on cells.

Activation of the Rap GTPases in B lymphocytes modulates B cell antigen receptor-induced activation of Akt but has no effect on MAPK activation

Signaling by the B cell antigen receptor (BCR) activates the Rap1 and Rap2 GTPases, putative antagonists of Ras-mediated signaling. Because Ras can activate the Raf-1/ERK pathway and the phosphatidylinositol 3-kinase (PI3K)/Akt pathway, we asked whether Rap activation limits the ability of the BCR to signal via these pathways. To do this, we blocked the activation of endogenous Rap1 and Rap2 by expressing the Rap-specific GTPase-activating protein RapGAPII. Preventing Rap activation had no effect on BCR-induced activation of ERK. In contrast, BCR-induced phosphorylation of Akt on critical activating sites was increased 2- to 3-fold when Rap activation was blocked. Preventing Rap activation also increased the ability of the BCR to stimulate Akt-dependent phosphorylation of the FKHR transcription factor on negative regulatory sites and decreased the levels of p27Kip1, a pro-apoptotic factor whose transcription is enhanced by FKHR. Moreover, preventing Rap activation reduced BCR-induced cell death in the WEHI-231 B cell line. Thus activation of endogenous Rap by the BCR limits BCR-induced activation of the PI3K/Akt pathway, opposes the subsequent inhibition of the FKHR/p27Kip1 pro-apoptotic module, and enhances BCR-induced cell death. Consistent with the idea that Rap-GTP is a negative regulator of the PI3K/Akt pathway, expressing constitutively active Rap2 (Rap2V12) reduced BCR-induced phosphorylation of Akt and FKHR. Finally, our finding that Rap2V12 can bind PI3K and inhibit its activity in a manner that depends upon BCR engagement provides a potential mechanism by which Rap-GTP limits activation of the PI3K/Akt pathway, a central regulator of B cell growth and survival.

Neutrophil migration in the lung, general and bovine-specific aspects

Lung inflammation is often associated with sustained neutrophil migration into the lung tissue, causing undesired side effects, i.e. substantial damage of lung tissue and fibrin deposition, which hamper complete recovery. The need for additional anti-inflammatory treatment strategies focused attention on the function of cellular adhesion molecules (CAMs) on the leukocyte membrane, which guide migration of leukocytes across the endothelium to the site of inflammation. Recent data indicate that neutrophil migration in the lung is mediated by unique pathways, involving different CAM as compared to other organs. These pulmonary characteristics of neutrophil migration enable specific targeting of CAM for anti-inflammatory treatment of pneumonia. In addition, the recent interest in intracellular signaling revealed that most CAM not only function in adhesion, but also play an important role in initiation of intracellular signal transduction and vice versa may be modulated by intracellular signaling molecules (ISMs). In this review, the mechanisms of neutrophil migration in the lung and the role of ISM with respect to CAM function are described in the context of potential anti-inflammatory intervention strategies.

The immunological synapse: integrins take the stage

Adhesive interactions play important roles in coordinating T-cell migration and activation, specifically in the formation of the immunological synapse (IS), a specialized cell-cell junction. Recent demonstrations show several molecules implicated in T-cell signaling, including Vav, ADAP, and Rap-1, have major roles in integrin regulation and place adhesion molecules at center stage in addressing the question: what are the signals involved in the formation of the IS and full T-cell activation? This review focuses on the role of integrins as an essential system for both physical adhesion and signaling in T-cell activation. The role of integrins appears to be quite distinct from classical costimulation and has been largely overlooked due to the ubiquitous use of serum in lymphocyte functional assays. Each major signal transduction pathway has branches leading to the nucleus and others that feed back on cytoskeletal and membrane regulation at the IS.

Suppression of integrin activation by activated Ras or Raf does not correlate with bulk activation of ERK MAP kinase

The rapid modulation of ligand-binding affinity ("activation") is a central property of the integrin family of cell adhesion receptors. The Ras family of small GTP-binding proteins and their downstream effectors are key players in regulating integrin activation. H-Ras can suppress integrin activation in fibroblasts via its downstream effector kinase, Raf-1. In contrast, to H-Ras, a closely related small GTP-binding protein R-Ras has the opposite activity, and promotes integrin activation. To gain insight into the regulation of integrin activation by Ras GTPases, we created a series of H-Ras/R-Ras chimeras. We found that a 35-amino acid stretch of H-Ras was required for full suppressive activity. Furthermore, the suppressive chimeras were weak activators of the ERK1/2 MAP kinase pathway, suggesting that the suppression of integrin activation may be independent of the activation of the bulk of ERK MAP kinase. Additional data demonstrating that the ability of H-Ras or Raf-1 to suppress integrin activation was unaffected by inhibition of bulk ERK1/2 MAP kinase activation supported this hypothesis. Thus, the suppression of integrin activation is a Raf kinase induced regulatory event that can be mediated independently of bulk activation of the ERK MAP-kinase pathway.

H-Ras/mitogen-activated protein kinase pathway inhibits integrin-mediated adhesion and induces apoptosis in osteoblasts

We have studied the relevance of H-Ras and its downstream effectors to osteoblast functions. 1) Purified human osteoblasts highly expressed integrins beta1, alpha4, alpha5, alpha6 and the activation epitope of beta1. However, these molecules were markedly down-regulated on osteoblasts transfected with expression vector encoding fully activated H-Ras(V12), H-Ras(V12)T35S, activating Raf-1/mitogen-activated protein kinase (MAPK), or an active Raf-1 but not on cells having H-Ras(V12)Y40C, a phosphoinositide 3-kinase (PI3K)-binding mutant. 2) Although osteoblasts spontaneously adhered to fibronectin and laminin in beta1-dependent manner, the expression of H-Ras(V12) or H-Ras(V12)T35S, but not H-Ras(V12)Y40C, in osteoblasts reduced their adhesion. 3) Osteoblasts bearing H-Ras(V12), H-Ras(V12)T35S, or Raf-1 failed to proliferate, whereas those with H-Ras(V12)Y40C proliferated well. (4) The up-regulation of Fas and down-regulation of Bcl-2 were observed in osteoblasts expressing H-Ras(V12), H-Ras(V12)T35S, or Raf-1. (5) Most of the cells having H-Ras(V12), H-Ras(V12)T35S, or Raf-1 became annexin-V(high)/propidium iodide (PI)(high or low) and terminal deoxynucleotidyl-transferase-mediated dUTP nick-end labeling (TUNEL)(high)/PI(low) after 24 and 72 h incubation, respectively. Thus, we propose that H-Ras signals followed by Raf-1/MAPK pathway but not PI3K not only reduces beta(1)-mediated adhesion of osteoblasts to matrix proteins but induces apoptosis presumably via the Fas up-regulation and Bcl-2 down-regulation.

Vav1 controls integrin clustering and MHC/peptide-specific cell adhesion to antigen-presenting cells

Integrin-mediated adhesion is essential for the formation of stable contacts between T cells and antigen-presenting cells (APCs). We show that Vav1 controls integrin-mediated adhesion of thymocytes and T cells to ECM proteins and ICAM1 following TCR stimulation. In a peptide-specific system, Vav1 is required for T cell adhesion to peptide-loaded APCs. Intriguingly, TCR-induced cell adhesion and aggregation of integrins occurs independent of WASP. Whereas LFA-1 and actin caps colocalize in wasp(-/-) T cells in response to TCR stimulation, loss of WASP uncouples TCR caps from actin patches. Our data reveal a novel role for Vav1 and WASP in the regulation of TCR-induced integrin clustering and cell adhesion and show that integrin and TCR clustering are controlled by distinct pathways.