Direct Cell Cycle Regulation by the Fibroblast Growth Factor Receptor (FGFR) Kinase through Phosphorylation-dependent Release of Cks1 from FGFR Substrate 2

Abstract

Kip1 is a key regulatory step in activation of the cyclin E/A-Cdk complex during the G 1 /S transition of the cell cycle, the results suggest a novel mitogenic pathway whereby FGF and other growth factors that activate FRS2 directly activate cyclin-dependent kinases. The fibroblast growth factor (FGF)1 family is a ubiquitously expressed transmembrane signaling family that elicits receptor-mediated regulatory effects on cell growth, function, differentiation, and death. The FGF ligands are single polypeptides consisting of 22 genetically distinct homologues, and the FGF receptors (FGFRs) are transmembrane tyrosine kinases encoded by four homologous gene products, which complex with pericellular matrix heparan sulfates independent of the FGF ligand (1, 2). Binding of FGF ligands to FGFR-heparan sulfate complexes activates the kinase activity and transmits regulatory signals to downstream signaling mediators or targets. Accumulating reports demonstrate that FGFRs elicit diverse regulatory activities that are cell type-and FGFR-specific as well as redundant among the four FGFRs. The molecular mechanisms that would explain both specificity and redundant activities are not well understood. Two proximal substrates (FRS1 and FRS2) interact with the activated FGFR tyrosine kinases that are tyrosine-autophosphorylated. FRS1 is phospholipase C␥, which binds to phosphorylated tyrosine 766 of FGFR1, and is phosphorylated by the kinase. Activation of FRS1 is not obligatory for the mitogenic activity of FGFR1 (3, 4). However, it is important for prostate epithelial cells to acquire a mitogenic response to FGFR1 and for other cellular activities stimulated by phospholipase C␥ (5).FRS2 (also called SNT1, for suc1-associating nuclear target protein 1) is an adaptor protein that is anchored to the plasma membrane by virtue of myristoylation at the N terminus (6 -8). FRS2 interacts with the intracellular juxtamembrane sequence of the FGFR through a conserved phosphotyrosine binding (PTB) domain adjacent to the C terminus of the myristoylation consensus sequence (9, 10). The PTB domain of FRS2 is homologous to and exhibits a similar three-dimensional structure as PTB domains of several other signaling adaptor proteins, including insulin receptor substrate 1. Phosphorylation of FRS2 in response to FGF treatment is associated with the FGF mitogenic signaling (6). The C-terminal half of FRS2 has multiple candidate phosphorylation sites as well as numerous signaling molecule binding sites, which include Grb2 that recruits upstream activators of the extracellular signal-regulated kinase 1/2 in the MAP kinase pathway. Therefore, FRS2 is considered an adaptor that connects FGFR to the MAP kinase pathway (5,(11)(12)(13)(14)(15). The fact that not all FGFR signaling is tightly associated with activation of the MAP kinase pathway (16, 17) prompted us to investigate alternative and potentially more direct means by which the FGFR transmits mitogenic signals.The interaction of FRS2 with p13 suc1

Expression of the SNT-1/FRS2 phosphotyrosine binding domain inhibits activation of MAP kinase and PI3-kinase pathways and antiestrogen resistant growth induced by FGF-1 in human breast carcinoma cells

“…The FGFR-dependent phosphorylation of FRS2 causes release of Cks1 from FRS2 thus promoting degradation of p27 Kip1 followed by activation of the cyclin A/E-Cdk complex and leading to G1/S transit. Moreover, Cks1 associates with FRS2 through the latter's PTB and myristylation domains (Zhang et al, 2004). We have also demonstrated that full-length SNT1, as well as SNT-1 PTB, associates with Cks1 in MCF-7 breast adenocarcinoma cells (unpublished data).…”

“…Kip1 (Zhang et al, 2004). The FGFR-dependent phosphorylation of FRS2 causes release of Cks1 from FRS2 thus promoting degradation of p27 Kip1 followed by activation of the cyclin A/E-Cdk complex and leading to G1/S transit.…”

FGF-receptor substrate 2 functions as a molecular sensor integrating external regulatory signals into the FGF pathway

“…Recent reports described several atypical mechanisms underlying FRS2-mediated signal transduction. FGF-induced phosphorylation of FRS2 causes departure of Cks1 from FRS2, leading to the ubiquitination and degradation of p27(Kip1), which results in the activation of the cyclin E/A-Cdk complex [41]. Similarly, GTPase Rnd1 is released from the phosphorylated FRS2 after FGF treatment and suppresses RhoA activity [42].…”

CKS Proteins Protect Mitochondrial Genome Integrity by Interacting with Mitochondrial Single-stranded DNA-binding Protein

“…CKS proteins covalently linked to Sepharose beads have been used historically as a reagent to purify active CDK complexes and to help identify CDK-dependent substrates (1).…”