Cell migration in vivo, for instance during epithelial-to-mesenchymal transition (EMT) or tumor cell invasion/metastasis, requires the coordination of cell-ECM adhesion signaling and dynamics along with the remodeling of cell-cell adhesion. In the case of EMT, which is essential for tissue remodeling during embryonic development, cells lose their intercellular adhesion and remodel their cell-ECM adhesions and accompanying cytoskeleton resulting in a conversion from apicalbasal to a front-rear polarity ().48
The subsequent single cell migration is coordinately regulated by focal adhesion components such as paxillin and ILK as well as their dynamic interactions with various binding partners.48–50
Our lab previously demonstrated that TGFβ-induced EMT of breast and kidney epithelial cells was associated with the downregulation of a truncated form of paxillin, called paxillin δ49
and the coordinate upregulation of Hic-5,50
a close homologue of paxillin. It appears that paxillin and Hic-5 when coexpressed exhibit complimentary as well as antagonistic functions.14
Interestingly they can bind the PKL/GIT1-PIX-PAK complex.51
It will be important to determine if PKL tyrosine phosphorylation affects its interaction with Hic-5, as it does paxillin, as well as clarifying how PKL and GIT1 are involved in Hic-5 versus paxillin signaling in focal adhesions during EMT and tumor invasion.
Figure 2 Schematic representation of cell adhesion signaling during EMT or cell scattering. Epithelial cell stimulation by TGFβ or other scattering factors triggers a transition from apical-basal to front-rear polarity. The PKL-PIX-PAK or GIT1-PIX-PAK (more ...)
Neither paxillin nor Hic-5 is enriched in cell-cell adhesions.14
In contrast, recent studies suggest a direct functional role for the GIT1-PIX-PAK complex in modulating cell-cell junction homeostasis.52–54
The TGFβ receptor is associated with the GIT1-PIX-PAK complex and this interaction is differentially modulated during TGFβ-stimulated EMT,52
in agreement with the observation that PIX and PAK regulate contact inhibition between epithelial cells.55
Furthermore, GIT1 binds to EphA2 and EphrinB through the adaptor Nck/Grb4 in a tyrosine phosphorylation dependent manner56,57
and, although the involvement of PKL has not been determined, GIT family ArfGAP activity towards Arf6 GTPase is likely important for Eph-Ephrin dependent cell-cell contact homeostasis ().57
In view of the fact that Arf6 and Rac1 activities are important for adherens junction turnover and stability,39,58
it will be important to examine the precise role of PKL/GIT1 function as Arf6 GAPs as well as their phosphorylation dependent signaling to Rho family GTPase for adhesion receptor trafficking and cell-cell junction stabilization. In addition, the GIT1-PIX-PAK complex has also been reported to interact with the polarity component Scribble, revealing a further mechanism by which it may regulate EMT and invasion.41,59
In epithelial cells, Scribble is essential for apical-basal polarity while in migrating mesenchymal cells exhibiting front-rear polarity, Scribble changes its localization to the plasma membrane at the leading edge, where it facilitates the targeting of βPIX ().60
Interestingly, we have now demonstrated an essential role for PKL(GIT2) tyrosine phosphorylation in also regulating polarized βPIX redistribution, but in this case to paxillin-containing focal adhesions beneath the lamellipodia in front-rear polarized cells.6
Therefore, a GIT protein-PIX complex (or βPIX alone) might provide a mode of local crosstalk between the Scribble polarity complex and paxillin/Hic-5 dependent ECM-cell adhesion signaling. This represents a potentially central mechanism for coordinating the regulation of cytoskeleton organization and receptor trafficking at cell-cell junctions and the leading edge of migrating cells during EMT and invasion.
Finally, in animal models, genetic deletions of SFK, FAK, paxillin, GIT or PIX lead to pronounced developmental defects involved in the disruption of cell polarity and directed cell migration.16,35,37,61–66
Abnormal Rho GTPase and PAK signaling, as well as paxillin phosphorylation were frequently observed in embryonic-derived null cells.62
Additionally, point mutations in PKL/GIT1 and paxillin have been associated with human glioblastoma multiforme and lung cancers, respectively.67,68
Given the importance of cell adhesion signaling and cell migration in development and disease, it will be essential to precisely dissect the functional role of paxillin-PKL/GIT1 and their phosphorylation-dependent interactions in these processes.