Focal adhesion kinase (FAK) has long been confirmed to have a major role in cancer as it involved in virtually every aspect of tumorigenesis and tumor progression (1,2). Known for being significantly overexpressed in over 80% of solid tumors, and even hematological cancers (3,4), FAK drives tumorigenesis by regulating many of the traditional hallmarks of cancer (5,6). Research continues to demonstrate FAK’s involvement in multiple aspects of cancer, primarily due to the role of FAK as an oncogenic scaffold (7). FAK has docking sites at both the N- and C-termini of the protein, allowing these regions to interact with a multitude of binding partners driving proliferation, survival, invasion/migration, and angiogenesis. As FAK expression is minimal in surrounding normal tissue, these phenotypes are a defining characteristic of FAK overexpression in cancer. Additionally, FAK plays a role in the sequestration of pro-apoptotic proteins (8), the manipulation of the tumor microenvironment (TME) (9), the immune response (10), and anoikis (11), all of which demonstrate the importance of FAK as a continually evolving, multifaceted protein in tumor biology.
Despite FAK’s ability to drive malignant phenotypes, identifying FAK as a novel therapeutic target and developing effective therapeutics that target FAK have been difficult and elusive. Why has it been so hard to drug FAK? FAK does not have a specific mutation that can be targeted like BRAF (V600E) (12) nor is it a receptor like HER2 (13) or EGFR (14). Compared to oncogenes that require a mutation to trigger a malignant phenotype, the overexpression of wild-type FAK in tumors relative to the basal expression in surrounding normal tissue is sufficient to drive malignancy within the tumor and TME (15,16).
The primary driver of FAK function is through the phosphorylation of Y397 which allows for the subsequent activation of downstream tyrosines 861 and 925 by Src. This Y397 autophosphorylation site is critical for development of FAK-targeted therapeutics, by developing small molecule inhibitors specific for the ATP-binding site of the FAK-kinase domain to inhibit FAK-kinase function and autophosphorylation of Y397. To date, efforts to drug FAK based exclusively on its kinase function have not shown dramatic clinical responses and are made more challenging by the lack of an appropriate biomarker of response. For example, there are recent failed clinical trials of defactinib, including a MERLIN biomarker-based clinical trial that was terminated (NCT01870609) (17), raising the question of whether strict FAK kinase inhibitors are effective to target FAK in patients. Central dogma surrounding FAK inhibition has focused on inhibition of the FAK kinase enzyme only, which has led to a lag in the development of therapeutics targeting the non-kinase function of FAK (18,19).
It becomes a challenge to identify the best strategy to target a protein like FAK. It has been clearly established that Y397 phosphorylation is the critical mediator of FAK function, allowing for downstream activation of FAK’s own kinase domain and subsequent effector tyrosines 861 and 925 to be phosphorylated by Src. All three of these activated phosphotyrosines function as new scaffolding hubs that maintain proliferation, survival, migration/invasion, and angiogenesis. A critical factor in effectively targeting FAK is to shut off Y397 signaling but, unfortunately, FAK kinase inhibitors do not effectively do this. Creating kinase domain inhibitors is a simpler method than attempting to identify FAK-scaffold inhibitors. However, there is now enough evidence to support the cross-reactivity and limited therapeutic efficacy of these kinase inhibiting small molecules. Nonetheless, these tyrosines are additional scaffold sites that, in theory, can be targeted with the development of protein-protein interaction (PPIs) inhibitors designed to target the binding interfaces between two proteins (20).
To overcome this challenge of targeting FAK effectively, we must look to the creation of small molecule inhibitors that can block the critical signaling events on the scaffold. The creation of FAK-scaffold inhibitors may have seemed unachievable years ago, but now with advances in in silico rational based drug design we can accurately target the multiple and unique binding interfaces of FAK. Ideally, we want to prevent the multitude of functions that each domain of FAK demonstrates. By targeting the FERM domain we can inhibit interactions with receptor tyrosine kinases (RTKs), which we know signal indirectly and now, directly, with this domain (21). The FERM domain is not only a hub for RTKs but also intracellular survival signals, such as p53, MDM2, and RIP. Additionally, Y397 resides within a flexible linker region that joins the FERM domain to the central kinase domain that is also accessible by RTKs and quite possibly additional intracellular TKs, i.e., Src.
Recently our group has shown that RTKS protect Y397 by maintaining its phosphorylation. These RTKs can directly phosphorylate FAK at Y397 independent of FAK-kinase function (i.e., FAK-kinase inhibitor defactinib, stable overexpression of kinase-dead FAK in FAK-/- MEFs) through the direct interaction of FAK and RTKs (21). This novel observation challenges the idea that FAK activation of Y397 is a mechanism regulated by FAK autophosphorylation and not by outside kinases. This demonstrates the importance of Y397 activation, suggesting the need for novel drug development approaches such as PPIs designed to these newly discovered binding interfaces (22) as FAK remains a lynchpin and key signaling transducer.
The C-terminal region of FAK, which includes the FAT domain, is another appropriate for the development of targeted PPIs. The FAT domain is functionally relevant for proper localization of the FAK molecule to focal adhesion complexes, whose structural dynamics regulate focal adhesion turnover, and in turn, drive migration, invasion, and angiogenesis. Additionally, Y925, which resides on the FAT domain, is the key regulator of focal adhesion turnover and is a juncture for paxillin and GRB2 binding dynamics. Blocking Y925 phosphorylation opens up a new realm of FAK inhibition, not by blocking a catalytic domain but by allosterically inhibiting the binding interfaces and tyrosines responsible for regulating focal adhesion turnover and FAT domain behaviors (23).
As we gain more insight into why FAK is expressed at such high levels in malignant cells, we are learning how FAK manipulates the immune landscape of the tumor, i.e., TME, platelets, immune response, T cell receptor signaling, etc. This includes the induction of FAK signaling in the tumor stroma in BRAF-mutant melanoma (15), the regulation of Tregs and CD8+ T cells through nuclear FAK activation of chemo/cytokines (10), and, most recently, FAK expression in platelets inducing tumor growth after anti-angiogenic therapy withdrawal (9). This novel finding suggests that systemic FAK inhibition to the TME in combination with anti-angiogenic therapy, such as bevacizumab or pazopanib may inhibit tumor regrowth by reducing platelet migration to the tumor. Beautifully demonstrated through the use of the platelet-specific knock-out, FAK is once again shown to be a modulator of the pro-cancer phenotype this time through the microenvironment (9). The resurgence of angiogenesis after anti-angiogenic drugs have been stopped appears to be overcome by the inhibition of FAK. As one of the main functions of FAK is regulating cell adhesion, it is no surprise that inhibiting FAK in platelets (24) would inhibit tumor revascularization by preventing platelet attachment. Haemmerle reiterates FAK’s role in the tumor; demonstrating FAK as a manipulator of the TME by regulating platelet activity.
Haemmerle’s data aligns with the literature in that the complete knockdown of FAK was better to inhibit FAK-mediated phenotypes in the tumor, even more than FAK-kinase inhibitors alone. This is another example of how the total inhibition of FAK function is the best approach to targeting FAK in cancer. This allows for the creation of multifaceted FAK inhibitors, ranging from traditional FAK-kinase inhibitors to novel FAK-scaffold inhibitors. Due to the complexity of FAK signaling, there is a need to develop a more effective link between FAK’s complex role in cancer biology with the development of targeted therapeutics against FAK. As scaffold inhibitors are in their infancy (25) it becomes an underlying concern that we continue to rely on only kinase inhibitors that can lead to tumor resistance. The simultaneous targeting of both scaffolding domains of FAK not only has the potential to circumvent kinase domain related complications but also to disrupt scaffolding interactions and subsequent downstream signaling, lending additional efficacy to the overall approach. Additionally, inhibition of the FAK scaffold (non-kinase) domains may prove similarly effective to inhibit FAK function in the TME. With the rapidly increasing use of immune checkpoint inhibitors (CPI) and other immuno-oncology (I-O) therapeutics in tandem with our knowledge of FAK in manipulating the immune landscape and TME in cancer, moving in a direction of incorporating FAK-targeted inhibitors with I-O therapies seems plausible. Given the multitude of processes and signaling pathways that require FAK, it would be naïve to think that one single FAK inhibitor would be effective. In the era of precision targeted therapeutics, it is more likely that FAK inhibitors would need to be tailored to an individual patient’s tumor utilizing more effective therapeutics that extend beyond the FAK-kinase domain.
Funding: Dr. Cance’s work on focal adhesion kinase has been supported by NCI R01-CA65910.
Provenance and Peer Review: This article was commissioned and reviewed by the Section Editor Zheng Li (Department of Gynecologic Oncology, the Third Affiliated Hospital of Kunming Medical University, Yunnan Tumor Hospital, Kunming, China).
Conflicts of Interest: Dr. Cance is Chief Scientific Officer of FAKnostics, LLC. The other authors has no conflicts of interest to declare.
Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.
Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.
- McLean GW, Komiyama NH, Serrels B, et al. Specific deletion of focal adhesion kinase suppresses tumor formation and blocks malignant progression. Genes Dev 2004;18:2998-3003. [Crossref] [PubMed]
- Golubovskaya VM, Kweh FA, Cance WG. Focal adhesion kinase and cancer. Histol Histopathol 2009;24:503-10. [PubMed]
- Weiner TM, Liu ET, Craven RJ, et al. Expression of focal adhesion kinase gene and invasive cancer. Lancet 1993;342:1024-5. [Crossref] [PubMed]
- Lark AL, Livasy CA, Calvo B, et al. Overexpression of focal adhesion kinase in primary colorectal carcinomas and colorectal liver metastases: immunohistochemistry and real-time PCR analyses. Clin Cancer Res 2003;9:215-22. [PubMed]
- Hanahan D, Weinberg RA. Hallmarks of cancer: the next generation. Cell 2011;144:646-74. [Crossref] [PubMed]
- Cavallo F, De Giovanni C, Nanni P, et al. 2011: the immune hallmarks of cancer. Cancer Immunol Immunother 2011;60:319-26. [Crossref] [PubMed]
- Cance WG, Kurenova E, Marlowe T, et al. Disrupting the scaffold to improve focal adhesion kinase-targeted cancer therapeutics. Sci Signal 2013;6:pe10. [Crossref] [PubMed]
- Golubovskaya VM, Finch R, Cance WG. Direct interaction of the N-terminal domain of focal adhesion kinase with the N-terminal transactivation domain of p53. J Biol Chem 2005;280:25008-21. [Crossref] [PubMed]
- Haemmerle M, Bottsford-Miller J, Pradeep S, et al. FAK regulates platelet extravasation and tumor growth after antiangiogenic therapy withdrawal. J Clin Invest 2016;126:1885-96. [Crossref] [PubMed]
- Serrels A, Lund T, Serrels B, et al. Nuclear FAK controls chemokine transcription, Tregs, and evasion of anti-tumor immunity. Cell 2015;163:160-73. [Crossref] [PubMed]
- Frisch SM, Schaller M, Cieply B. Mechanisms that link the oncogenic epithelial-mesenchymal transition to suppression of anoikis. J Cell Sci 2013;126:21-9. [Crossref] [PubMed]
- Fisher R, Larkin J. Vemurafenib: a new treatment for BRAF-V600 mutated advanced melanoma. Cancer Manag Res 2012;4:243-52. [PubMed]
- Gordon MS, Matei D, Aghajanian C, et al. Clinical activity of pertuzumab (rhuMAb 2C4), a HER dimerization inhibitor, in advanced ovarian cancer: potential predictive relationship with tumor HER2 activation status. J Clin Oncol 2006;24:4324-32. [Crossref] [PubMed]
- Zhou C, Wu YL, Chen G, et al. Erlotinib versus chemotherapy as first-line treatment for patients with advanced EGFR mutation-positive non-small-cell lung cancer (OPTIMAL, CTONG-0802): a multicentre, open-label, randomised, phase 3 study. Lancet Oncol 2011;12:735-42. [Crossref] [PubMed]
- Hirata E, Girotti MR, Viros A, et al. Intravital imaging reveals how BRAF inhibition generates drug-tolerant microenvironments with high integrin β1/FAK signaling. Cancer Cell 2015;27:574-88. [Crossref] [PubMed]
- Hazlehurst LA, Landowski TH, Dalton WS. Role of the tumor microenvironment in mediating de novo resistance to drugs and physiological mediators of cell death. Oncogene 2003;22:7396-402. [Crossref] [PubMed]
- Shapiro IM, Kolev VN, Vidal CM, et al. Merlin deficiency predicts FAK inhibitor sensitivity: a synthetic lethal relationship. Sci Transl Med 2014;6:237ra68 [Crossref] [PubMed]
- McLean GW, Carragher NO, Avizienyte E, et al. The role of focal-adhesion kinase in cancer - a new therapeutic opportunity. Nat Rev Cancer 2005;5:505-15. [Crossref] [PubMed]
- Brunton VG, Frame MC. Src and focal adhesion kinase as therapeutic targets in cancer. Curr Opin Pharmacol 2008;8:427-32. [Crossref] [PubMed]
- Bourgeas R, Basse MJ, Morelli X, et al. Atomic analysis of protein-protein interfaces with known inhibitors: the 2P2I database. PLoS One 2010;5:e9598 [Crossref] [PubMed]
- Marlowe TA, Lenzo FL, Figel SA, et al. Oncogenic Receptor Tyrosine Kinases Directly Phosphorylate Focal Adhesion Kinase (FAK) as a Resistance Mechanism to FAK-Kinase Inhibitors. Mol Cancer Ther 2016;15:3028-39. [Crossref] [PubMed]
- Frame MC, Patel H, Serrels B, et al. The FERM domain: organizing the structure and function of FAK. Nat Rev Mol Cell Biol 2010;11:802-14. [Crossref] [PubMed]
- Mitra SK, Hanson DA, Schlaepfer DD. Focal adhesion kinase: in command and control of cell motility. Nat Rev Mol Cell Biol 2005;6:56-68. [Crossref] [PubMed]
- Lipfert L, Haimovich B, Schaller MD, et al. Integrin-dependent phosphorylation and activation of the protein tyrosine kinase pp125FAK in platelets. J Cell Biol 1992;119:905-12. [Crossref] [PubMed]
- Cance WG, Golubovskaya VM. Focal adhesion kinase versus p53: apoptosis or survival? Sci Signal 2008;1:pe22. [Crossref] [PubMed]