The strong interaction between NSP and CAS family proteins suggests that the 2 families are able to form stable complexes in which NSP proteins are critical modulators of CAS function. Studies with BCAR1 and NEDD9 have demonstrated the paramount importance of NSP-CAS downstream signaling pathways in normal physiology and cancer malignancy, whereas the mechanisms of upstream NSP-CAS regulation are less well understood.
CAS and NSP family proteins can both be phosphorylated on tyrosine and serine/threonine residues in response to cell substrate adhesion as well as cell stimulation with growth factors and cytokines. NSP proteins can potentiate CAS phosphorylation in normal and cancer cells through mechanisms that likely involve both anchoring CAS proteins to receptor tyrosine kinases and recruiting them near plasma membrane–localized SRC. This highlights their dual role as both adaptor and scaffolding proteins that can act in a positive feedback loop to further increase cell adhesion (). The fact that BCAR3, the long NSP3 isoform, and myristoylated NSP3 more readily induce CAS tyrosine and serine phosphorylation as compared to NSP1 or shorter NSP3 isoforms suggests that membrane targeting through the longer amino-terminal segment could act in concert with NSP SH2 domain interactions with cell surface receptors to more effectively promote CAS signaling. It will also be interesting to investigate if NSP proteins bound to the CAS carboxy-terminal FAT domain function together with proteins bound to the CAS SH3 domain as part of the mechanosensory machinery that stretches the central CAS substrate domain, leading to its phosphorylation by SRC.4,5
The mechanisms and kinases responsible for NSP-induced CAS family serine phosphorylation, and whether this phosphorylation may be a consequence of increased substrate adhesion, also remain to be elucidated.
The functional effects of NSP phosphorylation are also unknown, except for phosphorylation of a tyrosine in the NSP3 CDC25 homology domain, which has been reported to prevent interaction with BCAR1.90
A special connection between NSP3 and the ABL nonreceptor tyrosine kinase is suggested by the observed NSP3 phosphorylation downstream of the BCR-ABL oncogene88
) and by the predicted binding of the ABL SH2 and SH3 domains to NSP3 but also remains to be explored.
Further characterizing the NSP-CAS association will also help to understand the physiological and pathological activities of the various family members. For example, it will be important to know whether the binding affinities (which are below 30 nM for the BCAR3-BCAR1 and NSP3-BCAR1 complexes13
) differ depending on the NSP-CAS pair and if they can be physiologically regulated, for example, by posttranslational modifications.90
It will also be useful to determine the proportion of each protein found in NSP-CAS complexes in the cellular environment and clarify the subcellular localization of the complexes. Structure-based mutations designed to affect NSP-CAS interaction without causing overall conformational changes will be instrumental for these investigations.13
These types of studies have confirmed the importance of NSP-CAS complexes in processes such as the induction of CAS tyrosine phosphorylation and regulation of cell morphology and migration.13,90,98,99
Additionally, the effects of BCAR3 carboxy-terminal deletions support the importance of the CDC25 homology domain and the NSP-CAS association in antiestrogen resistance, SRC activation, CAS relocalization to membrane ruffles, and cell migration.89,101
According to some recent reports, however, substantial impairment of NSP-CAS association by single amino acid changes does not seem to impact NSP protein–dependent SRC and RAC1 activation and antiestrogen resistance induced by BCAR3 overexpression.89,98
Further work will be needed to resolve these discrepancies. Furthermore, deletion of the CDC25 homology domain has demonstrated that at least some BCAR1 serine phosphorylation events do not seem to require interaction with NSP proteins.94
On the other hand, gene transcription analyses and pathway activation mapping have revealed that overexpression of BCAR3 or BCAR1 activates signaling networks that have not only common elements but also distinctive features.109,120
Taken together, these findings highlight the complexities in the activities of NSP and CAS proteins and their modules, which remain to be further unraveled in future studies.
In summary, the role of NSP and CAS proteins in cancer appears to depend on their protein abundance and phosphorylation state and possibly on the particular family member or isoform expressed. Although BCAR1, BCAR3, and SRC mRNA levels may not show a positive correlation with aggressiveness or poor responsiveness to tamoxifen therapy,121
BCAR1 protein levels in breast cancer samples correlate with prognosis and could be useful to decide on treatment options.53,54
Screening for BCAR1 phosphorylation levels could be even more powerful.55,56
On the other hand, only a few mutations have been identified so far in each of the 3 NSP genes as well as in BCAR1 and NEDD9 in different types of cancers (sanger.ac.uk/cosmic), and no information is yet available on the functional effects of the mutations or their relevance to cancer development and malignancy. With regard to therapy, siRNA-based strategies will allow the reduction of NSP-CAS complexes and their malignant activities. Furthermore, better understanding of NSP-CAS signaling mechanisms will enable effective targeting of enzymatic activities associated with the complex or disruption of interactions with upstream regulators and downstream effectors. Developing agents that can disrupt NSP-CAS complexes will be more challenging due to the large interface and strong binding between the 2 classes of molecules.
An article published after submission of this review122
reports that BCAR3 recruits BCAR1 to integrin-mediated focal adhesions by binding through its SH2 domain to a tyrosine phosphorylated sequence of the receptor-like protein tyrosine phosphatase a (PTPa). This leads to BCAR1 phosphorylation by SRC and downstream signals that promote cell migration