Adhesion Receptors in Adhesion Receptors in transmembrane signalingtransmembrane signaling

Introduction.

Section 1.

Section 2.

Section 3.

IntroductionIntroduction

CCell adhesion is critical for the genesis and maintenance ell adhesion is critical for the genesis and maintenance of both three-dimensional structure and normal function of both three-dimensional structure and normal function in tissues. The biochemical entities mediating cell adhesion in tissues. The biochemical entities mediating cell adhesion are multiprotein complexes comprising three broad classes are multiprotein complexes comprising three broad classes of macromolecules: of macromolecules:

the adhesion receptors,the extracellular matrix molecules, the adhesion receptors,the extracellular matrix molecules, and the adhesion plaque proteins .and the adhesion plaque proteins .

Cell adhesion receptors are typically transmembrane Cell adhesion receptors are typically transmembrane glycoproteins that mediate binding to extracellular matrix glycoproteins that mediate binding to extracellular matrix (ECM) molecules or to counter-receptors on other cells; (ECM) molecules or to counter-receptors on other cells; these molecules determine the specificity of cell-cell or cell-these molecules determine the specificity of cell-cell or cell-ECM interaction.ECM interaction.

The ECM proteins are usually fibrillar in nature and The ECM proteins are usually fibrillar in nature and provide a complex structural and functional network that provide a complex structural and functional network that can interact simultaneously with multiple cell surface can interact simultaneously with multiple cell surface receptors.receptors.

cell adhesion complexes are not simply static architectural cell adhesion complexes are not simply static architectural entities. Rather, they are dynamic units that are capable of entities. Rather, they are dynamic units that are capable of capturing and integrating signals from the extracellular capturing and integrating signals from the extracellular environment . Moreover, the functions of cell adhesion environment . Moreover, the functions of cell adhesion complexes are regulated precisely by biochemical events complexes are regulated precisely by biochemical events within the cell. Thus cell adhesion receptors are at a nexus within the cell. Thus cell adhesion receptors are at a nexus of two-way signaling between the cell and its external of two-way signaling between the cell and its external environment. All four types’ cell-adhesion environment. All four types’ cell-adhesion (integrin, IgSF, cadherin, selectin) molecules have the potential to carry out this function.

Cells reside in a protein network, the Cells reside in a protein network, the extracellular matrix (ECM), which they extracellular matrix (ECM), which they secrete and mold into the intercellular secrete and mold into the intercellular space. The ECM exerts profound control space. The ECM exerts profound control over cells. The effects of the matrix are over cells. The effects of the matrix are primarily mediated by integrins, a family of primarily mediated by integrins, a family of cell surface receptors that attach cells to the cell surface receptors that attach cells to the matrix and mediate mechanical and matrix and mediate mechanical and chemical signals from it.chemical signals from it.

Direct Signal Transduction by IntegrinsDirect Signal Transduction by Integrins

Integrins comprise a large family of cell surface receptors Integrins comprise a large family of cell surface receptors that are found in many animal species, ranging from spongthat are found in many animal species, ranging from sponges to mammals. They are composed of two subunits, α and es to mammals. They are composed of two subunits, α and β, and each αβ combination has its own binding specificity β, and each αβ combination has its own binding specificity and signaling properties. Most integrins recognize several and signaling properties. Most integrins recognize several ECM proteins. Conversely, individual matrix proteins, sucECM proteins. Conversely, individual matrix proteins, such as fibronectin, laminins, collagens, and vitronectin, bind h as fibronectin, laminins, collagens, and vitronectin, bind to several integrins. Integrins can signal through the cell mto several integrins. Integrins can signal through the cell membrane in either direction: The extracellular binding actiembrane in either direction: The extracellular binding activity of integrins is regulated from the inside of the cell (insivity of integrins is regulated from the inside of the cell (inside-out signaling), while the binding of the ECM elicits signde-out signaling), while the binding of the ECM elicits signals that are transmitted into the cell (outside-in signaling).als that are transmitted into the cell (outside-in signaling).

Recent studies have provided a better understanding of the signaling pathways activated by integrins in adherent cells, such as fibroblasts and epithelial cells. Adherent cells must be anchored to an appropriate ECM to survive. Depending partly on signals from the matrix, they either proliferate or exit the cell cycle and differentiate. This anchorage requirement is lost in neoplastic cells( 癌细胞 ). In this review, we focus on the integrin signals that control these basic cellular behaviors.

Integrin clusteringIntegrin clustering

The cytoplasmic tails of integrins are The cytoplasmic tails of integrins are generally short and always devoid of generally short and always devoid of enzymatic features. Hence, integrins enzymatic features. Hence, integrins transduce signals by associating with transduce signals by associating with adapter proteins that connect the integrin to adapter proteins that connect the integrin to the cytoskeleton, cytoplasmic kinases, and the cytoskeleton, cytoplasmic kinases, and transmembrane growth factor receptors. transmembrane growth factor receptors.

Integrin signaling and assembly of the cytoskeleton arIntegrin signaling and assembly of the cytoskeleton are intimately linked. As integrins bind to ECM, they bee intimately linked. As integrins bind to ECM, they become clustered in the plane of the cell membrane and come clustered in the plane of the cell membrane and associate with a cytoskeletal and signaling complex thassociate with a cytoskeletal and signaling complex that promotes the assembly of actin filaments (the α6β4 iat promotes the assembly of actin filaments (the α6β4 integrin associates with keratin filaments through the untegrin associates with keratin filaments through the uniquely large β4 cytodomain). The reorganization of aniquely large β4 cytodomain). The reorganization of actin filaments into larger stress fibers, in turn, causes ctin filaments into larger stress fibers, in turn, causes more integrin clustering, thus enhancing the matrix bimore integrin clustering, thus enhancing the matrix binding and organization by integrins in a positive feedbnding and organization by integrins in a positive feedback system. As a result, ECM proteins, integrins, and ack system. As a result, ECM proteins, integrins, and cytoskeletal proteins assemble into aggregates on each cytoskeletal proteins assemble into aggregates on each side of the membrane. Well developed aggregates can side of the membrane. Well developed aggregates can be detected by immunofluorescence microscopy and abe detected by immunofluorescence microscopy and are known as focal adhesions and ECM contacts. In thire known as focal adhesions and ECM contacts. In this manner, integrins serve as integrators of the ECM as manner, integrins serve as integrators of the ECM and cytoskeleton, the property for which integrins are nnd cytoskeleton, the property for which integrins are named.amed.

Several integrins have been found to associate laterally Several integrins have been found to associate laterally with the oligomeric membrane protein caveolin-1with the oligomeric membrane protein caveolin-1 （（窖蛋窖蛋白白）） , at least in primary cells. Although the biochemical , at least in primary cells. Although the biochemical nature of this interaction is not known, inhibiting caveolinature of this interaction is not known, inhibiting caveolin expression suppresses the formation of focal adhesions n expression suppresses the formation of focal adhesions and integrin signaling. Because of its ability to associate iand integrin signaling. Because of its ability to associate into oligomers, caveolin-1 may help integrins to cluster onto oligomers, caveolin-1 may help integrins to cluster on the plasma membrane. Integrin associated structural an the plasma membrane. Integrin associated structural and signaling proteins also aggregate with the integrins, and signaling proteins also aggregate with the integrins, and signaling is facilitated by the resulting high local concnd signaling is facilitated by the resulting high local concentrations of these proteins.entrations of these proteins.

FA is a dynamic ligation-aFA is a dynamic ligation-anchored cell-link which is nchored cell-link which is anchored to the extracellulanchored to the extracellular matrix through integrin.ar matrix through integrin. The Cytoplasmic end of in The Cytoplasmic end of integrin link with actin filamtegrin link with actin filament through other proteins.ent through other proteins.

Focal Adhesion Kinase-Mediated EventsFocal Adhesion Kinase-Mediated Events

Focal adhesions are dynamic structures that Focal adhesions are dynamic structures that can be rapidly disassembled if the adhesion can be rapidly disassembled if the adhesion cell is stimulated to move or enter mitosis. Tcell is stimulated to move or enter mitosis. The plasma membrane in the region of a focahe plasma membrane in the region of a focal adhesion contains large clusters of integrinl adhesion contains large clusters of integrins. The cytoplasmic domains of the integrin as. The cytoplasmic domains of the integrin are connected by various adaptors to actin filre connected by various adaptors to actin filaments of the cytoskeleton.aments of the cytoskeleton.

The binding of an extracellular ligand, such The binding of an extracellular ligand, such as fibronection or laminin, can activate protas fibronection or laminin, can activate protein kinases, such as FAK, that can transmit ein kinases, such as FAK, that can transmit signals throughout the cell, including the celsignals throughout the cell, including the cell nucleus. l nucleus.

Focal adhesion kinase is a important signaling tranFocal adhesion kinase is a important signaling transduction molecule, which join many important sigsduction molecule, which join many important signaling pathway and a variety of behavior regulationaling pathway and a variety of behavior regulation of cell and molecular biology. Itn of cell and molecular biology. It is a unique protis a unique protein tyrosine kinase of approximately 125 kDa; it cein tyrosine kinase of approximately 125 kDa; it contains a central consensus kinase domain, a C-terontains a central consensus kinase domain, a C-terminal domain having two proline-rich sequence, aminal domain having two proline-rich sequence, and a region required for focal adhesion targeting tnd a region required for focal adhesion targeting termed the “FAT” sequence.ermed the “FAT” sequence.

A lot of evidence lends its weight to a critical role fA lot of evidence lends its weight to a critical role for FAK in integrin-mediated signaling. First, immor FAK in integrin-mediated signaling. First, immunofluorescent staining shows that FAK co localizunofluorescent staining shows that FAK co localizes with proteins such as talines with proteins such as talin （踝蛋白） （踝蛋白） and tensand tensinin （张力蛋白） （张力蛋白） in focal adhesion sites of fibroblain focal adhesion sites of fibroblasts. Immunoprecipitation studies demonstrate that sts. Immunoprecipitation studies demonstrate that FAK undergoes enhanced tyrosine phosphorylatioFAK undergoes enhanced tyrosine phosphorylation upon adhesion to fibronectin or antibody-mediatn upon adhesion to fibronectin or antibody-mediated integrin clustering.ed integrin clustering.

Besides integrin-mediated events, Besides integrin-mediated events, FAK tyrosine phosphorylation is FAK tyrosine phosphorylation is increased through a variety of non-increased through a variety of non-integrin cell surface receptors integrin cell surface receptors including growth factor receptor including growth factor receptor tyrosine kinases and G-protein-tyrosine kinases and G-protein-coupled receptors.coupled receptors.

Summary

Activation of tyrosine kinases is a key proximal event for integrin-mediated signal transduction.

Cell adhesion receptors and the Cell adhesion receptors and the control of cell cyclecontrol of cell cycle

1.1. Cell adhesion impact the regulatory Cell adhesion impact the regulatory elements of cell cycle .elements of cell cycle .

2.2. Cell adhesion regulate the transfer of cell Cell adhesion regulate the transfer of cell cycle information . cycle information .

3.3. Adhesion disorders result in cell cycle out Adhesion disorders result in cell cycle out of control .of control .

1.1 cell adhesion can induce the 1.1 cell adhesion can induce the expression of cyclin D1 mRNAexpression of cyclin D1 mRNA

Someone detect the levels of cyclin D1 MrnSomeone detect the levels of cyclin D1 Mrna in adhesion state and suspended state,and a in adhesion state and suspended state,and find that the adhesion state is three to five tfind that the adhesion state is three to five times higher than the suspended state. late imes higher than the suspended state. late G1 cell can’t express in the suspension state G1 cell can’t express in the suspension state but express positive in the adhesion state.but express positive in the adhesion state.

In order to study relation of cyclin D1 and Rb phospIn order to study relation of cyclin D1 and Rb phosphorylated from G1 to S phase, Biologists take the rethorylated from G1 to S phase, Biologists take the retrovirus transfected NIH - 3T3 cells with cyclin D1 gerovirus transfected NIH - 3T3 cells with cyclin D1 gene to express cyclin D1 sustained. Even leaving the tne to express cyclin D1 sustained. Even leaving the transfection of NIH - 3T3 cells under suspension statransfection of NIH - 3T3 cells under suspension state, forced expressive cyclin D1 can keep phosphorylatie, forced expressive cyclin D1 can keep phosphorylation of Rb and overcome the G1 to S phase retardarce on of Rb and overcome the G1 to S phase retardarce caused by the loss of adhesion. the expression of adhecaused by the loss of adhesion. the expression of adhesion dependence cyclin D1 can induce continued actision dependence cyclin D1 can induce continued activity of extracellar signal-regulated kinase, but growtvity of extracellar signal-regulated kinase, but growth factor only induce ERK activity moment. h factor only induce ERK activity moment.

1.2 cell adhesion can activate 1.2 cell adhesion can activate the activity of cyclin E-CDK2the activity of cyclin E-CDK2

Under the state of Suspended, the activity of Under the state of Suspended, the activity of cyclin E-CDK2 kinase is Significantly lower than cyclin E-CDK2 kinase is Significantly lower than the state of adhesion. The effects can not be the state of adhesion. The effects can not be interpreted by cyclin E or complex changes interpreted by cyclin E or complex changes because the expressions of cyclin E and CDK2 is because the expressions of cyclin E and CDK2 is independent adhesion, but combination with CKI: independent adhesion, but combination with CKI: P21 and P27 are affected by adhesion. Western P21 and P27 are affected by adhesion. Western blot shows that under the state of suspended ,P21, blot shows that under the state of suspended ,P21, P27 are three times and two times higher than P27 are three times and two times higher than adhesion. Therefore, the activity of cyclin-adhesion. Therefore, the activity of cyclin-dependent adhesion E-CDK2 is regulated through dependent adhesion E-CDK2 is regulated through the realization of P21 and P27 .the realization of P21 and P27 .

1.3 the expression of cyclin 1.3 the expression of cyclin mRNA depend cell adhesionmRNA depend cell adhesion

Under the state of suspended, NRK, NIH - 3T3 Under the state of suspended, NRK, NIH - 3T3 and embryonic fibroblast cells can’t express and embryonic fibroblast cells can’t express cyclin A, so the cyclin A of S phase also need cell cyclin A, so the cyclin A of S phase also need cell adhesion to express. In normal cells, the adhesion to express. In normal cells, the expression of cyclin A begins after the expression expression of cyclin A begins after the expression of cyclin D, E and CDK, mRNA and protein of of cyclin D, E and CDK, mRNA and protein of cyclin A begin to appear in late G1. Cyclin A cyclin A begin to appear in late G1. Cyclin A gene promoter contains the regulation of cell gene promoter contains the regulation of cell cycle (E2F binding sites) and its expression cycle (E2F binding sites) and its expression phosphorylate Rb, release E2F, and induct E2F phosphorylate Rb, release E2F, and induct E2F gene transcription . gene transcription .

2. cell adhesion regulate the transfer 2. cell adhesion regulate the transfer of cell cycle informationof cell cycle information

When the cell adhesion links with ECM, integrin When the cell adhesion links with ECM, integrin clustering gather to join the ECM with cell clustering gather to join the ECM with cell cytoskeletal proteins , phosphorylate FAK, and cytoskeletal proteins , phosphorylate FAK, and produce a series of regulatory protein about SH2 produce a series of regulatory protein about SH2 region, e.g. Paxillin, Crk, Grb-2 etc. Grb-2 and region, e.g. Paxillin, Crk, Grb-2 etc. Grb-2 and Crk can activate the pathway of mitogen-Crk can activate the pathway of mitogen-activated protein kinase, and then transcript and activated protein kinase, and then transcript and translate cyclin D1 mRNA .translate cyclin D1 mRNA .

3. adhesion disorders result in cell 3. adhesion disorders result in cell cycle out of controlcycle out of control

Epidermal cell needs to adhere to ECM to Epidermal cell needs to adhere to ECM to survive. Once break away the ECM, it will survive. Once break away the ECM, it will apoptosis. Frisch names this apoptosis apoptosis. Frisch names this apoptosis Anoikis (homeless). The cell which Anoikis (homeless). The cell which phosphorylated sustained by FAK is the phosphorylated sustained by FAK is the key of adhesion to inhibit apoptosis. Tumor-key of adhesion to inhibit apoptosis. Tumor-derived growth of cells perform Non-derived growth of cells perform Non-growth factor and adhesion dependence. growth factor and adhesion dependence.

The protein which Cancer gene encoded is onThe protein which Cancer gene encoded is one of normal cell growth control pathways, but e of normal cell growth control pathways, but its over-expression or mutation lead the pathits over-expression or mutation lead the pathway irreversible activation. These proteins wway irreversible activation. These proteins which tumor gene encoded activate the pathwahich tumor gene encoded activate the pathway of Integrin-mediated and transform the cell y of Integrin-mediated and transform the cell to adhesion-independent, such as Ibc, dbl. Thto adhesion-independent, such as Ibc, dbl. The down regulation of Drs (one of tumor suppe down regulation of Drs (one of tumor suppressor genes) connect with Tumor-derived inressor genes) connect with Tumor-derived independent adhesion, and result in the abnordependent adhesion, and result in the abnormal expression of cyclin A. mal expression of cyclin A.