The biological importance of PTK2 protein tyrosine kinase 2, or Focal
adhesion kinase 1 (FAK1)-mediated signal transduction is
underscored by the fact that this tyrosine kinase plays a fundamental role in embryonic
development, in control of cell migration, cell cycle progression, and in apoptosis.
Binding of ECM proteins to Integrins leads
to activation of v-Src sarcoma viral oncogene homolog
(c-Src), thereby leading to direct phosphorylation and
activation of FAK1 by c-Src
[1].
FAK1 is one of the most important
components in Vascular endothelial growth factor (VEGF)-induced signaling in cardiac
myocytes. It plays a critical role in adhesive interactions between cardiac myocytes and
ECM. VEGF-A induces tyrosine phosphorylation and activation
of FAK1 via activation of VEGF-receptor 2 tyrosine kinase
(VEGFR-2) and c-Src.
G protein-coupled Bombesin receptor
activation by Bombesin induces rapid
FAK1 phosphorylation through transformation and activation
of G-protein alpha-q/11 that stimulates Phospholipase
C beta (PLC-beta)-dependent
increase of the intracellular Ca(2+) concentration.
Ca(2+) binding activates
Calmodulin and Calcium-calmodulin kinase II
(CaMKII). Activated CaMKII
directly phosphorylates the recombinant COOH-terminal region of
FAK1 [2].
FAK1 binds to the death domain kinase
receptor-interacting protein (RIPK1).
FAK1 provides a survival signal function by binding to
RIPK1. This binding leads to inhibition of the interaction
of RIPK1 with the adapter protein
TRAF3 [3].
FAK1 plays an anti-apoptotic role in
anchorage-dependent cells via activating the Phosphatidylinositol 3-kinase
(PI3K)/ v-AKT murine thymoma viral oncogene homolog
(AKT) survival pathway.
Phosphorylation by AKT inhibits Glycogen synthase kinase 3
beta (GSK3-beta) followed by activation of
Cyclin D3. This pathway ultimately leads to the inhibition
of the apoptosis and regulation of cell cycle [4].
FAK1 plays a major role in regulating Breast cancer anti-estrogen
resistance 1 (P130CAS) phosphorylation. It functions as a
docking or scaffolding protein that facilitates the recruitment of the
c-Src to phosphorylate P130CAS
[5].
FAK1 promotes reorganization of
cytoskeleton by activation of small GTPases Rac1 and
CDC42 through P130CAS/ v-CRK
sarcoma virus CT10 oncogene homolog (CRK)/ Dedicator of
cytokinesis 1 (DOCK1) and
p130CAS/ CRK/ Rap guanine
nucleotide exchange factor 1 (C3G)/ RAP1A member of RAS
oncogene family (RAP-1A)/ FERM RhoGEF and pleckstrin domain
protein 2 (FARP2) pathways, respectively.
Rac1 and CDC42 stimulate the formation of
protrusive structures such as membrane ruffles, lamellipodia and filopodia. Another small
GTPase, RhoA, regulates contractility and assembly of actin
stress fibers and focal adhesions. Integrin engagement
initially inactivates RhoA, in a
c-Src-dependent manner, but has no effect on the
activity of CDC42 or Rac1.
Additionally, early integrin signaling induces activation and tyrosine phosphorylation of
RhoA GTPase activating protein
p190RhoGAP via a mechanism that requires
c-Src [6].
FAK1 mediates attachment-induced
activation of Mitogen-activated protein kinase 8
(JNK1) in a Rac-dependent manner. Cell
attachment leads to the activation of the p21-Activated kinase 1
(PAK1)/ Mitogen-activated protein kinase kinase kinase 1
(MEKK1)/ Dual specificity mitogen-activated protein kinase
kinase 7 (MKK7)/ JNK1 pathway
[7].
JNK1 phosphorylates
Paxillin, a focal adhesion adaptor.
FAK1 directly interacts with Phospholipase
C gamma 1 (PLC-gamma 1). This leads to increase in the
Inositol phosphate (IP3) concentration and production of
Diacylglycerol (DAG) in
fibroblasts. FAK1 cannot phosphorylate
PLC-gamma 1 directly. It
recruits PLC-gamma 1 to the plasma membrane at sites of
cell-matrix adhesion, thereby promoting its enzymatic activity and inducing its
phosphorylation by the FAK1-associated
c-Src.
Increased FAK1 activity in the cells
contributes to the phosphorylation of Src homology 2 domain containing transforming
protein (Shc) by c-Src and
likely to the promotion of the Growth factor receptor bound 2
(GRB2)/ Son of sevenless proteins
(SOS)/ v-Ha-ras Harvey rat sarcoma viral oncogene homolog
(H-Ras)/ v-Raf-1 murine leukemia viral oncogene homolog 1
(c-Raf-1)/ Mitogen-activated protein kinase kinase 1 and
2 (MEK1 and
MEK2)/ Mitogen-activated protein kinases 1 and 3
(ERK1/2) signaling, which leads to cell proliferation [8]. ERK1/2 also directly phosphorylates and activates
proteinase Calpain 2 (m) [9].
Calpain 2 (m) is involved in cell
migration via its ability to regulate focal adhesion dynamics and rear retraction.
Calpain 2 (m), but not Calpain 1, is required for
proteolysis of the cytoskeletal and focal adhesion proteins
FAK1, Paxillin and
Talin. It limits membrane protrusions and regulates
lamellipodial dynamics at the leading edge of migrating cells [10].
Integrin-mediated cell spreading and the formation of focal adhesions
are down-regulated by PTEN phosphatase.
PTEN interacts with FAK1 and
reduces the level of its tyrosine phosphorylation [11].