Cell adhesion Integrin inside-out signaling
Integrin inside-out signaling The integrin family of transmembrane adhesion receptors mediates both cell-cell andcell- extracellular matrix (ECM) adhesion. One important, rapid and reversible mechanismfor regulating adhesion is increasing the affinity of integrin receptors for theirextracellular ligands (integrin activation). This is controlled by intracellular signalsthat, through their action on integrin cytoplasmic domains, induce conformational changesin integrin extracellular domains that result in increased affinity for ligand(inside-out signaling) , . Several such inside-out signal pathways could be activated by a host ofG-protein-coupled receptors (GPCRs), including the Thromboxane A2 receptor (TBXA2R ), Thrombin receptors PAR1 and PAR4, AngiotensinII receptor type-1 ( AGTR1 ), and receptor for the C-X-C chemokineSDF-1 ( CXCR4 ). Ligand binding triggers conformational changes that promote receptor/G-proteincoupling and catalyzes the exchange of GTP for GDP on the G-alpha subunit of theheterotrimeric G protein, leading to dissociation of the GTP-bound G-alpha subunit fromthe G - beta/gamma subunit heterodimer . The G alpha-q family of G-proteins ( G-protein alpha-q/11 ) and G-proteinbeta/gamma subunits activate different phosphoinositide-specific phospholipase CPLC-beta isozymes , . These enzymes in turncatalyze the hydrolysis of phosphatidylinositol 4,5-bisphosphate ( PtdIns(4,5)P2 )to inositol 1,4,5-trisphosphate ( IP3 ) and diacylglycerol ( DAG ) . IP3 stimulates Ca(2+) release from endoplasmic reticulum storage sitesvia the inositol 1,4,5-trisphosphate receptor ( IP3 receptor ) .Ca(2+), in turn, activates diverse downstream targets, includingCalmodulin  and Calmyrin . DAG activates protein kinase C PKC-epsilon, that phosphorylates thecytoplasmatic tail of the beta-1 integrin subunit ( ITGB1 ) . PKC-epsilon is also activated in G-protein alpha-12 family signalingpathway . G-protein alpha-12 family subunits, in turn, arestimulated by the activity of TBXA2R  and AGTR1 . The Phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit, gamma isoform (PI3K cat class IB, gamma ) is activated by G-protein beta/gamma subunitsupon stimulation of GPCRs AGTR1 and CXCR4. PI3K cat class IB(p110-gamma) phosphorylates the membrane lipid PtdIns(4,5)P2 to generatephosphatidylinositol 3,4,5-trisphosphate PtdIns(3,4,5)P3 . Tyrosine-protein kinase c-Src is activated by G-protein alpha-q/11,G-protein alpha-12 family, and G-protein beta/gamma subunits. In most celltypes, c-Src stimulation is involved in GPCR-mediated activation of the Focaladhesion kinase FAK1 and themitogen-activated protein kinases ERK1 / 2 . Talin, a major cytoskeletal Actin -binding protein, plays a crucialrole in integrin activation. Talin binding to integrin beta-1 ( ITGB1 ),integrin beta-2 (ITGB2 ), integrin beta-3 ( ITGB3 ) cytoplasmic tailsinduces conformational changes in their extracellular domains, increasing integrinaffinity for ligands. Mechanisms that regulate Talin binding may therefore controlintegrin activation . The binding of PtdIns(4,5)P2 to Talin induces a conformational changethat enhances its association with integrin beta subunits. Talin binds to andactivates the PtdIns(4,5)P2 -producing enzyme: phosphatidylinositol phosphatekinase type I gamma ( PIPKI gamma ). Therefore, Talin can stimulatePtdIns(4,5)P2 production that enhances Talin - Integrininteractions, which suggests that PIPKI gamma may positively regulate integrinactivation. PIPKI gamma is also stimulated by c-Src  andFAK1 phosphorylation . However, PIPKI gamma and integrinbeta-1 tails compete for overlapping binding sites on the Talin and so, under someconditions, PIPKI gamma might inhibit integrin activation by displacingTalin from beta-1 tails . PtdIns(4,5)P2 also stimulates the transient, direct interactions of diversecytoskeleton actin-binding protein and couple adhesion to Actin assembly . The integrin beta-1 binding protein ICAP-1 inhibits Integrin -Talin association . Calcium/calmodulin-dependent protein kinase IICaMK II phosphorylates ICAP-1 and this phosphorylation negatively regulatesintegrin-mediated processes . Beta-3-endonexin ( NRIF3 ) binds specifically to ITGB3 and activatesalpha-IIb/beta-3 integrin . However, in the absence ofTalin, this activation is very weak. Therefore, NRIF3 may cooperate withTalin during alpha-IIb/beta-3 integrin activation in platelets. Calcium - and integrin-binding protein Calmyrin, which interactsdirectly with the alpha-IIb ( ITGA2B ) tail, inhibits alpha-IIb/beta-3integrin activation by competing with talin for binding to integrin . Guanine nucleotide exchange factors Cytohesin-1 and Cytohesin-3,activated by PI(3,4,5)P3, bind ITGB2 which leads to an increase celladhesion through an affinity-independent processes, such as integrin clustering, ratherthan integrin activation . Intracellular signals induce conformational changes in the integrin extracellulardomains that result in their increased affinity for ligands, focal adhesion formation andintegrin signal transduction (outside-in signaling) .