Multiple connections link FAK to cell motility and invasion
Introduction
Focal adhesion kinase (FAK) was first identified in 1992 as a highly tyrosine-phosphorylated protein associated with the v-Src oncogene and localized within integrin-enriched focal adhesion contact sites in normal cells (see [1] for review). FAK activity is regulated by integrin-mediated cell adhesion as well as being downstream of growth factor and G-protein-linked receptor activation 2., 3.. In many cells, FAK activation leads to the Src homology 2 (SH2) domain-mediated binding to Src-family protein tyrosine kinases (PTKs) to the motif surrounding the FAK Tyr-397 phosphorylation site. It is this dual FAK–Src complex that promotes the tyrosine phosphorylation of substrates such as paxillin and p130Cas and FAK–Src-associated signaling can lead to the activation of multiple protein kinase cascades 2., 3., 4..
Whereas early studies linked FAK activation to the formation of cell–substratum contact sites, knockout studies showed that FAK-null fibroblasts exhibit an increased numbers of focal contact sites and cell-motility defects [5]. Reconstitution of FAK-null cells, FAK overexpression studies, and the inhibition of FAK activity have all supported a role for FAK–Src signaling in promoting cell migration [2]. In transformed cells and in clinical analyses of human tumors, elevated FAK expression and activity have been correlated with the progression to a highly malignant and metastatic phenotype 6.••, 7., 8.. To this end, the possibility that FAK-mediated signaling may promote increased tumorigenicity has generated much interest to determine the mechanisms of FAK activation.
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Multiple mechanisms of FAK activation
Even though the initial cloning of FAK showed that it was activated by integrin clustering, the precise mechanism of FAK activation has eluded investigators for over twelve years. This is likely due to the fact that FAK can be activated by multiple inputs and in different manners. One constant is that cloning efforts have revealed that the FAK central kinase domain is highly conserved from humans to Drosophila and Caenorhabditis elegans. The FAK N-terminal region harbors a FERM (band four point
New roles elucidated for FAK phosphorylation sites
Src SH2-mediated binding to FAK Tyr-397 leads to the generation of an activated Src–FAK signaling complex. Src facilitates maximal FAK activation through phosphorylation at Tyr-576 and Tyr-577 within the FAK kinase domain activation loop and Src can phosphorylate additional sites within the FAK C-terminal domain at Tyr-861 and Tyr-925, the latter serving as a high affinity Grb2 SH2 binding site. Efforts to make constitutively-active FAK mutants by plasma membrane targeting result in elevated
FAK FERM domain interacting proteins
As FAK connects to integrins through various C-terminal domain-mediated interactions, FAK is also functionally linked to various other proteins through N-terminal FERM domain-mediated interactions. Structural analyses and sequence comparisons reveal that FERM domains comprise three lobes containing multiple sites for both lipid and protein binding. Recent studies have begun to identify FAK FERM targets and how these interactions may affect FAK function. For instance, ezrin binding to the FAK
FAK connections to focal contact formation
Many research groups have observed correlations between FAK activation, paxillin tyrosine phosphorylation, and the subsequent formation of actin stress fibers and focal contact sites 1., 3.. These events are often connected to Rho-family GTPase activation that act as switches existing in either an inactive GDP-bound or an active GTP-bound form. Guanine nucleotide exchange factors (GEFs) stimulate the GDP to GTP exchange reaction to facilitate an active RhoGTPase conformation. FAK can promote
FAK-null fibroblasts as a model system to elucidate FAK function
Although FAK is linked to the formation of focal contacts as discussed above, FAK-null fibroblasts exhibit motility defects in part as a result of elevated Rho activity, increased focal contact formation, and the inability to remodel contact sites in response to various motility stimuli 2., 41., 42.•. This cellular response is unique to the loss of FAK as null mutations in other FAK- and focal contact associated proteins such as paxillin [9], p130Cas [43], or Src-family PTKs [44] do not yield a
A mechanism by which FAK promotes focal contact turnover
In addition to functioning as an integrator and amplifier of signaling to ERK2 and JNK, FAK needs to be localized appropriately to focal contact sites to reverse the motility defects of FAK-null cells [45]. This raises the possibility that there may be critical target(s) or substrate(s) of FAK localized within focal contact sites that are not being regulated appropriately in the absence of FAK expression. In addition to the contributions of FAK signaling to JNK with respect to gene-expression
Connections of FAK to cell invasion extend beyond cell motility
Tumor cell invasion through matrix and tissue barriers requires the combined effects of increased cell motility and regulated proteolytic degradation of the matrix. Though FAK expression is elevated in invasive humans cancers 6.••, 7., 8., assessments of FAK function within tumor cells have often relied upon measurements of cell motility in two dimensions rather than cell invasion in three dimensions [61]. Src-mediated cell-transformation studies revealed that naturally occurring v-Src SH3
Conclusions
FAK has been demonstrated to play important yet differing roles in signaling pathways associated with normal and tumor cell movement. As FAK functions as both a scaffold and as a kinase, the future development of pharmacological inhibitors to FAK may be used to discriminate whether normal cell motility or malignant cell invasion events differentially depend on FAK catalytic activation. The continued characterization of FAK-interacting proteins will provide a better understanding of the
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
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of special interest
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of outstanding interest
Acknowledgements
DDS is supported by grants from the National Cancer Institute (CA75240, CA87038, and CA102310) and SKM is supported by a fellowship (12FT-0122) from the California Tobacco-Related Disease Research Program. This is manuscript 16074-IMM from the Scripps Research Institute.
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