Regulation of EGFR signalling by palmitoylation and its role in tumorigenesis.

The epidermal growth factor receptor (EGFR) is an essential driver of oncogenic signalling, and EGFR inhibitors are some of the earliest examples of successful targeted therapies in multiple types of cancer. The tractability of EGFR as a therapeutic target is overshadowed by the inevitable drug resistance that develops. Overcoming resistance mechanisms requires a deeper understanding of EGFR regulation in cancer cells. In this review, we discuss our recent discovery that the palmitoyltransferase DHHC20 palmitoylates EGFR on the C-terminal domain and plays a critical role in signal regulation during oncogenesis. Inhibiting DHHC20 expression or mutating the palmitoylation site on EGFR alters the EGF-induced signalling kinetics from a transient signal to a sustained signal. The change in signalling is accompanied by a decrease in cell proliferation in multiple human cancer cell lines. Our in vivo studies demonstrate that ablating the gene Zdhhc20 by CRISPR/Cas9-mediated inhibition in a mouse model of oncogenic Kras-driven lung adenocarcinoma potently inhibits tumorigenesis. The negative effect on tumorigenesis is mediated by EGFR since the expression of a palmitoylation-resistant mutant form of EGFR also inhibits Kras-driven lung adenocarcinoma. Finally, reducing EGFR palmitoylation increases the sensitivity of multiple cancer cell lines to existing inhibitors of EGFR and downstream signalling effector pathways. We will discuss the implications of these effects and strategies for targeting these new vulnerabilities.

Differences in self-association between kindlin-2 and kindlin-3 are associated with differential integrin binding.

The integrin family of transmembrane adhesion receptors coordinates complex signaling networks that control the ability of cells to sense and communicate with the extracellular environment. Kindlin proteins are a central cytoplasmic component of these networks, directly binding integrin cytoplasmic domains and mediating interactions with cytoskeletal and signaling proteins. The physiological importance of kindlins is well established, but how the scaffolding functions of kindlins are regulated at the molecular level is still unclear. Here, using a combination of GFP nanotrap association assays, pulldown and integrin-binding assays, and live-cell imaging, we demonstrate that full-length kindlins can oligomerize (self-associate) in mammalian cells, and we propose that this self-association inhibits integrin binding and kindlin localization to focal adhesions. We show that both kindlin-2 and kindlin-3 can self-associate and that kindlin-3 self-association is more robust. Using chimeric mapping, we demonstrate that the F2PH and F3 subdomains are important for kindlin self-association. Through comparative sequence analysis of kindlin-2 and kindlin-3, we identify kindlin-3 point mutations that decrease self-association and enhance integrin binding, affording mutant kindlin-3 the ability to localize to focal adhesions. Our results support the notion that kindlin self-association negatively regulates integrin binding.

Chapter 22: Structural and signaling functions of integrins.

The integrin family of transmembrane adhesion receptors is essential for sensing and adhering to the extracellular environment. Integrins are heterodimers composed of non-covalently associated α and ß subunits that engage extracellular matrix proteins and couple to intracellular signaling and cytoskeletal complexes. Humans have 24 different integrin heterodimers with differing ligand binding specificities and non-redundant functions. Complex structural rearrangements control the ability of integrins to engage ligands and to activate diverse downstream signaling networks, modulating cell adhesion and dynamics, processes which are crucial for metazoan life and development. Here we review the structural and signaling functions of integrins focusing on recent advances which have enhanced our understanding of how integrins are activated and regulated, and the cytoplasmic signaling networks downstream of integrins.

Kindlin-2 interacts with a highly conserved surface of ILK to regulate focal adhesion localization and cell spreading.

The integrin-associated adaptor proteins integrin-linked kinase (ILK) and kindlin-2 play central roles in integrin signaling and control of cell morphology. A direct ILK-kindlin-2 interaction is conserved across species and involves the F2PH subdomain of kindlin-2 and the pseudokinase domain (pKD) of ILK. However, complete understanding of the ILK-kindlin-2 interaction and its role in integrin-mediated signaling has been impeded by difficulties identifying the binding site for kindlin-2 on ILK. We used conservation-guided mapping to dissect the interaction between ILK and kindlin-2 and identified a previously unknown binding site for kindlin-2 on the C-lobe of the pKD of ILK. Mutations at this site inhibit binding to kindlin-2 while maintaining structural integrity of the pKD. Importantly, kindlin-binding-defective ILK mutants exhibit impaired focal adhesion localization and fail to fully rescue the spreading defects seen in ILK knockdown cells. Furthermore, kindlin-2 mutants with impaired ILK binding are also unable to fully support cell spreading. Thus, the interaction between ILK and kindlin-2 is critical for cell spreading and focal adhesion localization, representing a key signaling axis downstream of integrins.This article has an associated First Person interview with the first author of the paper.