Endogenous Control Mechanisms of FAK and PYK2 and Their Relevance to Cancer Development

Abstract

Focal adhesion kinase (FAK) and its close paralogue, proline-rich tyrosine kinase 2 (PYK2), are key regulators of aggressive spreading and metastasis of cancer cells. While targeted small-molecule inhibitors of FAK and PYK2 have been found to have promising antitumor activity, their clinical long-term efficacy may be undermined by the strong capacity of cancer cells to evade anti-kinase drugs. In healthy cells, the expression and/or function of FAK and PYK2 is tightly controlled via modulation of gene expression, competing alternatively spliced forms, non-coding RNAs, and proteins that directly or indirectly affect kinase activation or protein stability. The molecular factors involved in this control are frequently deregulated in cancer cells. Here, we review the endogenous mechanisms controlling FAK and PYK2, and with particular focus on how these mechanisms could inspire or improve anticancer therapies.

Keywords: FIP200; LKB1; PI3K; PTEN; anoikis; chaperon; dimerization; miRNA; motility; regulation.

Figure 1

Schematic domain structure of focal adhesion kinase (FAK) and PYK2. The three folded domains are shown in green, blue and magenta. Interaction motifs, sites of post-translational modification, and examples of binding sites of proteins discussed in the text are shown. The percentages show the sequence identity between corresponding regions of FAK and PYK2. The alternatively transcribed products FRNK and PRNK are schematically represented with respect to FAK and PYK2. Interacting proteins for FAK or PYK2 are shown boxed either above (FAK) or below (PYK2) the schematic structure with arrows pointing at the interacting domain or linker region. For a more complete list of interacting partners, please see [26].

Figure 2

Canonical FAK activation scheme. (A) In the absence of integrin activation, an interaction between the FERM and kinase domain (indicated by a lower-case ‘a’ in a red triangle) inhibits FAK kinase activity. (B) Ligand-mediated recruitment and clustering of FAK at focal adhesions promotes transient dimerization by stabilizing weak FERM:FERM interactions (lower-case ‘b’ in a red triangle) and promoting FERM:FAT binding in trans (lower-case ‘c’ in a red triangle). (C) FAK clustering and self-association allows trans-autophosphorylation of Y397 (red dots) in the FERM-kinase linker. When phosphorylated, Y397 and PR1 form a bidentate binding site for the SH2 and SH3 domains of Src (lower-case ‘d’ in a red triangle). Recruitment-activated Src phosphorylates the activation loop of the FAK kinase domain (lower-case ‘e’ in a red triangle) and other tyrosines on FAK, resulting in an open FAK conformation and full enzymatic activity. Triggered signaling may result in additional FAK modifications (e.g., serine phosphorylation; yellow dots) and may ultimately lead to dephosphorylation and/or displacement of FAK from focal adhesions (back to the closed monomeric inactive conformation of A), or to proteolytic cleavage and degradation (not shown in figure).

Figure 3

Schematic domain structures (left) and three-dimensional (3D) architectures (right) of protein regulators of FAK:LKB1, PTEN and FIP200 are negative regulators. LKB1 is shown as part of the activating complex formed with MO25α and STRADα (according to PDB accession 2wtk). The presented 3D structure of FIP200 (blue) is highly speculative and based on secondary structure predictions and homology modeling. The stabilizing function of HSP90 is upregulated in cancers. HSP90 is displayed with the kinase-specific adaptor CDC37, maintaining the separated N-terminal (NT) and C-terminal (CT) lobe of a kinase domain (taken from PDB 5fwl). Binding sites and post-transcriptional modifications relevant to their interaction with FAK and PYK2 are indicated. Abbreviations are: (A) LKB1:NTD, N-terminal domain; CTD, C-terminal domain. (B) PTEN:PD, phosphatase domain; PBM, PI(4,5)P2-binding module; PDZ-BD: PDZ binding domain. (C/D) FIP200/HSP90:NT, N-terminal domain; MD: middle domain; CT, C-terminal domain. FAK:KD, kinase domain; NT, N-terminal fragment.

Figure 4

Overview of selected endogenous mechanisms that influence functions of FAK or PYK2. The ‘Mechanism’ column shows examples of features that affect FAK and PYK2 function, i.e., their local protein concentration, the total amount of correctly folded protein, the degree of tyrosine phosphorylation, and the existence and stability of intramolecular associations, in particular the FERM:kinase autoinhibitory interaction. Selected examples of factors that decrease or increase these mechanisms are shown. For FIP200, the question mark indicates that its influence on intramolecular associations is highly speculative.