Gordon W. McLean

The role of focal-adhesion kinase in cancer - a new therapeutic opportunity.

Focal-adhesion kinase (FAK) is an important mediator of growth-factor signalling, cell proliferation, cell survival and cell migration. Given that the development of malignancy is often associated with perturbations in these processes, it is not surprising that FAK activity is altered in cancer cells. Mouse models have shown that FAK is involved in tumour formation and progression, and other studies showing that FAK expression is increased in human tumours make FAK a potentially important new therapeutic target.

The role of focal-adhesion kinase in cancer |[mdash]| a new therapeutic opportunity

Focal-adhesion kinase (FAK) is an important mediator of growth-factor signalling, cell proliferation, cell survival and cell migration. Given that the development of malignancy is often associated with perturbations in these processes, it is not surprising that FAK activity is altered in cancer cells. Mouse models have shown that FAK is involved in tumour formation and progression, and other studies showing that FAK expression is increased in human tumours make FAK a potentially important new therapeutic target.

Src-induced de-regulation of E-cadherin in colon cancer cells requires integrin signalling

Although Src expression and activity are often elevated in colon cancer, the precise consequences of overexpression of the non-catalytic Src homology (SH) domains, or enhanced catalytic activity, are unknown. We show that, in KM12C colon cancer cells, elevated Src activity causes the components of adherens junctions, including vinculin, to be redistributed to Src-induced integrin–adhesion complexes. Specifically, elevated Src activity blocks proper assembly of cell–cell contacts after cells are switched from media containing a low level of calcium to media containing a high level of calcium, and E-cadherin remains internalized. In contrast, although elevated expression of the non-catalytic domains of Src is sufficient to induce assembly of integrin–adhesion complexes, it does not induce disorganization of E-cadherin-associated intercellular contacts. Surprisingly, Src-induced disruption of E-cadherin localization requires specific integrin signalling, because E-cadherin redistribution is blocked by loss of cell-matrix interaction, or by inhibitory antibodies to αv or β1 integrin subunits. Furthermore, phosphorylation of the integrin-regulated focal adhesion kinase (FAK) on Src-specific sites is required for Src-induced de-regulation of E-cadherin, demonstrating interdependence between integrin-induced signals and cadherin-associated adhesion changes induced by Src.

Focal adhesion kinase controls actin assembly via a FERM-mediated interaction with the Arp2/3 complex.

Networks of actin filaments, controlled by the Arp2/3 complex, drive membrane protrusion during cell migration. How integrins signal to the Arp2/3 complex is not well understood. Here, we show that focal adhesion kinase (FAK) and the Arp2/3 complex associate and colocalize at transient structures formed early after adhesion. Nascent lamellipodia, which originate at these structures, do not form in FAK-deficient cells, or in cells in which FAK mutants cannot be autophosphorylated after integrin engagement. The FERM domain of FAK binds directly to Arp3 and can enhance Arp2/3-dependent actin polymerization. Critically, Arp2/3 is not bound when FAK is phosphorylated on Tyr 397. Interfering peptides and FERM-domain point mutants show that FAK binding to Arp2/3 controls protrusive lamellipodia formation and cell spreading. This establishes a new function for the FAK FERM domain in forming a phosphorylation-regulated complex with Arp2/3, linking integrin signalling directly with the actin polymerization machinery.

Mammary epithelial-specific disruption of the focal adhesion kinase blocks mammary tumor progression

Elevated expression and activation of the focal adhesion kinase (FAK) occurs in a large proportion of human breast cancers. Although several studies have implicated FAK as an important signaling molecule in cell culture systems, evidence supporting a role for FAK in mammary tumor progression is lacking. To directly assess the role of FAK in this process, we have used the Cre/loxP recombination system to disrupt FAK function in the mammary epithelium of a transgenic model of breast cancer. Using this approach, we demonstrate that FAK expression is required for the transition of premalignant hyperplasias to carcinomas and their subsequent metastases. This dramatic block in tumor progression was further correlated with impaired mammary epithelial proliferation. These observations provide direct evidence that FAK plays a critical role in mammary tumor progression.

v-Src Induces Tyrosine Phosphorylation of Focal Adhesion Kinase Independently of Tyrosine 397 and Formation of a Complex with Src

The non-receptor tyrosine kinase FAK plays a key role at sites of cellular adhesion. It is subject to regulatory tyrosine phosphorylation in response to a variety of stimuli, including integrin engagement after attachment to extracellular matrix, oncogene activation, and growth factor stimulation. Here we use an antibody that specifically recognizes the phosphorylated form of the putative FAK autophosphorylation site, Tyr397. We demonstrate that FAK phosphorylation induced by integrins during focal adhesion assembly differs from that induced by activation of a temperature-sensitive v-Src, which is associated with focal adhesion turnover and transformation. Specifically, although v-Src induces tyrosine phosphorylation of FAK, there is no detectable phosphorylation of Tyr397. Moreover, activation of v-Src results in a net decrease in fibronectin-stimulated phosphorylation of Tyr397, suggesting possible antagonism between v-Src and integrin-induced phosphorylation. Our mutational analysis further indicates that the binding of v-Src to Tyr397 of FAK in its phosphorylated form, which is normally mediated, at least in part, by the SH2 domain of Src, is not essential for v-Src-induced cell transformation. We conclude that different stimuli can induce phosphorylation of FAK on distinct tyrosine residues, linking specific phosphorylation events to ensuing biological responses.

Focal adhesion kinase as a potential target in oncology.

Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that plays a pivotal role in signal transduction at integrin-linked cellular adhesions, which mediate cell contact with the extracellular matrix. It has been shown to play a role in the survival of anchorage-dependent cells and to be essential for integrin-linked cell migration – processes that are likely to play important roles in the development of malignancies. FAK is upregulated in a wide variety of human epithelial cancers, with expression being closely correlated to invasive potential. Recently, evidence has emerged directly linking FAK expression to tumour development in vivo, raising the possibility that intervention strategies to block FAK function may potentially provide an opportunity for the development of anticancer therapeutics.