Adhesion Receptors in
transmembrane signaling
Introduction.
Section 1.
Section 2.
Section 3.
Introduction
Cell adhesion is critical for the genesis and maintenance
of both three-dimensional structure and normal function
in tissues. The biochemical entities mediating cell adhesion
are multiprotein complexes comprising three broad classes
of macromolecules:
the adhesion receptors,the extracellular matrix molecules,
and the adhesion plaque proteins .
Cell adhesion receptors are typically transmembrane
glycoproteins that mediate binding to extracellular matrix
(ECM) molecules or to counter-receptors on other cells;
these molecules determine the specificity of cell-cell or cellECM interaction.
The ECM proteins are usually fibrillar in nature and
provide a complex structural and functional network that
can interact simultaneously with multiple cell surface
receptors.
cell adhesion complexes are not simply static architectural
entities. Rather, they are dynamic units that are capable of
capturing and integrating signals from the extracellular
environment . Moreover, the functions of cell adhesion
complexes are regulated precisely by biochemical events
within the cell. Thus cell adhesion receptors are at a nexus
of two-way signaling between the cell and its external
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
extracellular matrix (ECM), which they
secrete and mold into the intercellular space.
The ECM exerts profound control over cells.
The effects of the matrix are primarily
mediated by integrins, a family of cell
surface receptors that attach cells to the
matrix and mediate mechanical and
chemical signals from it.
Direct Signal Transduction by Integrins
Integrins comprise a large family of cell surface receptors
that are found in many animal species, ranging from
sponges to mammals. They are composed of two subunits,
α and β, and each αβ combination has its own binding
specificity and signaling properties. Most integrins
recognize several ECM proteins. Conversely, individual
matrix proteins, such as fibronectin, laminins, collagens,
and vitronectin, bind to several integrins. Integrins can
signal through the cell membrane in either direction: The
extracellular binding activity of integrins is regulated from
the inside of the cell (inside-out signaling), while the
binding of the ECM elicits signals 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 clustering
The cytoplasmic tails of integrins are
generally short and always devoid of
enzymatic features. Hence, integrins
transduce signals by associating with
adapter proteins that connect the integrin to
the cytoskeleton, cytoplasmic kinases, and
transmembrane growth factor receptors.
Integrin signaling and assembly of the cytoskeleton
are intimately linked. As integrins bind to ECM, they
become clustered in the plane of the cell membrane
and associate with a cytoskeletal and signaling
complex that promotes the assembly of actin filaments
(the α6β4 integrin associates with keratin filaments
through the uniquely large β4 cytodomain). The
reorganization of actin filaments into larger stress
fibers, in turn, causes more integrin clustering, thus
enhancing the matrix binding and organization by
integrins in a positive feedback system. As a result,
ECM proteins, integrins, and cytoskeletal proteins
assemble into aggregates on each side of the
membrane. Well developed aggregates can be detected
by immunofluorescence microscopy and are known as
focal adhesions and ECM contacts. In this manner,
integrins serve as integrators of the ECM and
cytoskeleton, the property for which integrins are
named.
Several integrins have been found to associate laterally
with the oligomeric membrane protein caveolin-1（窖蛋
白）, at least in primary cells. Although the biochemical
nature of this interaction is not known, inhibiting
caveolin expression suppresses the formation of focal
adhesions and integrin signaling. Because of its ability to
associate into oligomers, caveolin-1 may help integrins to
cluster on the plasma membrane. Integrin associated
structural and signaling proteins also aggregate with the
integrins, and signaling is facilitated by the resulting
high local concentrations of these proteins.
FA is a dynamic ligationanchored cell-link which is
anchored to the
extracellular matrix
through integrin. The
Cytoplasmic end of
integrin link with actin
filament through other
proteins.
Focal Adhesion Kinase-Mediated Events
Focal adhesions are dynamic structures that
can be rapidly disassembled if the adhesion
cell is stimulated to move or enter mitosis.
The plasma membrane in the region of a
focal adhesion contains large clusters of
integrins. The cytoplasmic domains of the
integrin are connected by various adaptors
to actin filaments of the cytoskeleton.
The binding of an extracellular ligand, such
as fibronection or laminin, can activate
protein kinases, such as FAK, that can
transmit signals throughout the cell,
including the cell nucleus.
Focal adhesion kinase is a important signaling
transduction molecule, which join many
important signaling pathway and a variety of
behavior regulation of cell and molecular biology.
It is a unique protein tyrosine kinase of
approximately 125 kDa; it contains a central
consensus kinase domain, a C-terminal domain
having two proline-rich sequence, and a region
required for focal adhesion targeting termed the
“FAT” sequence.
A lot of evidence lends its weight to a critical role
for FAK in integrin-mediated signaling. First,
immunofluorescent staining shows that FAK co
localizes with proteins such as talin（踝蛋白）
and tensin（张力蛋白） in focal adhesion sites of
fibroblasts. Immunoprecipitation studies
demonstrate that FAK undergoes enhanced
tyrosine phosphorylation upon adhesion to
fibronectin or antibody-mediated integrin
clustering.
Besides integrin-mediated events,
FAK tyrosine phosphorylation is
increased through a variety of nonintegrin cell surface receptors
including growth factor receptor
tyrosine kinases and G-proteincoupled receptors.
Summary
Activation of tyrosine kinases is a key
proximal event for integrin-mediated signal
transduction.
Cell adhesion receptors and the
control of cell cycle
1. Cell adhesion impact the regulatory
elements of cell cycle .
2. Cell adhesion regulate the transfer of cell
cycle information .
3. Adhesion disorders result in cell cycle out
of control .
1.1 cell adhesion can induce the
expression of cyclin D1 mRNA
Someone detect the levels of cyclin D1
Mrna in adhesion state and suspended
state,and find that the adhesion state is
three to five times higher than the
suspended state. late G1 cell can’t express
in the suspension state but express positive
in the adhesion state.
In order to study relation of cyclin D1 and Rb
phosphorylated from G1 to S phase, Biologists take
the retrovirus transfected NIH - 3T3 cells with cyclin
D1 gene to express cyclin D1 sustained. Even leaving
the transfection of NIH - 3T3 cells under suspension
state, forced expressive cyclin D1 can keep
phosphorylation of Rb and overcome the G1 to S
phase retardarce caused by the loss of adhesion. the
expression of adhesion dependence cyclin D1 can
induce continued activity of extracellar signalregulated kinase, but growth factor only induce ERK
activity moment.
1.2 cell adhesion can activate
the activity of cyclin E-CDK2
Under the state of Suspended, the activity of
cyclin E-CDK2 kinase is Significantly lower than
the state of adhesion. The effects can not be
interpreted by cyclin E or complex changes
because the expressions of cyclin E and CDK2 is
independent adhesion, but combination with CKI:
P21 and P27 are affected by adhesion. Western
blot shows that under the state of suspended ,P21,
P27 are three times and two times higher than
adhesion. Therefore, the activity of cyclindependent adhesion E-CDK2 is regulated through
the realization of P21 and P27 .
1.3 the expression of cyclin
mRNA depend cell adhesion
Under the state of suspended, NRK, NIH - 3T3
and embryonic fibroblast cells can’t express
cyclin A, so the cyclin A of S phase also need cell
adhesion to express. In normal cells, the
expression of cyclin A begins after the expression
of cyclin D, E and CDK, mRNA and protein of
cyclin A begin to appear in late G1. Cyclin A gene
promoter contains the regulation of cell cycle
(E2F binding sites) and its expression
phosphorylate Rb, release E2F, and induct E2F
gene transcription .
2. cell adhesion regulate the transfer
of cell cycle information
When the cell adhesion links with ECM, integrin
clustering gather to join the ECM with cell
cytoskeletal proteins , phosphorylate FAK, and
produce a series of regulatory protein about SH2
region, e.g. Paxillin, Crk, Grb-2 etc. Grb-2 and
Crk can activate the pathway of mitogenactivated protein kinase, and then transcript and
translate cyclin D1 mRNA .
3. adhesion disorders result in cell
cycle out of control
Epidermal cell needs to adhere to ECM to
survive. Once break away the ECM, it will
apoptosis. Frisch names this apoptosis
Anoikis (homeless). The cell which
phosphorylated sustained by FAK is the
key of adhesion to inhibit apoptosis. Tumorderived growth of cells perform Non-growth
factor and adhesion dependence.
The protein which Cancer gene encoded is
one of normal cell growth control pathways,
but its over-expression or mutation lead the
pathway irreversible activation. These
proteins which tumor gene encoded activate
the pathway of Integrin-mediated and
transform the cell to adhesion-independent,
such as Ibc, dbl. The down regulation of Drs
(one of tumor suppressor genes) connect with
Tumor-derived independent adhesion, and
result in the abnormal expression of cyclin A.