Integrin-mediated activation of MAP kinase is independent of FAK: evidence for dual integrin signaling pathways in fibroblasts.
Integrin-mediated cell adhesion causes activation of MAP kinases and increased tyrosine phosphorylation of focal adhesion kinase (FAK). Autophosphorylation of FAK leads to the binding of SH2-domain proteins including Src-family kinases and the Grb2-Sos complex. Since Grb2-Sos is a key regulator of the Ras signal transduction pathway, one plausible hypothesis has been that integrin-mediated tyrosine phosphorylation of FAK leads to activation of the Ras cascade and ultimately to mitogen activated protein (MAP) kinase activation. Thus, in this scenario FAK would serve as an upstream regulator of MAP kinase activity. However, in this report we present several lines of evidence showing that integrin-mediated MAP kinase activity in fibroblasts is independent of FAK. First, a beta1 integrin subunit deletion mutant affecting the putative FAK binding site supports activation of MAP kinase in adhering fibroblasts but not tyrosine phosphorylation of FAK. Second, fibroblast adhesion to bacterially expressed fragments of fibronectin demonstrates that robust activation of MAP kinase can precede tyrosine phosphorylation of FAK. Finally, we have used FRNK, the noncatalytic COOH-terminal domain of FAK, as a dominant negative inhibitor of FAK autophosphorylation and of tyrosine phosphorylation of focal contacts. Using retroviral infection, we demonstrate that levels of FRNK expression sufficient to completely block FAK tyrosine phosphorylation were without effect on integrin-mediated activation of MAP kinase. These results strongly suggest that integrin-mediated activation of MAP kinase is independent of FAK and indicate the probable existence of at least two distinct integrin signaling pathways in fibroblasts.
Lin
T H
TH
Department of Pharmacology, University of North Carolina, Chapel Hill 27599, USA.
Aplin
A E
AE
Shen
Y
Y
Chen
Q
Q
Schaller
M
M
Romer
L
L
Aukhil
I
I
Juliano
R L
RL
D016475:3T3 Cells; D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism*; D002448:Cell Adhesion; D015815:Cell Adhesion Molecules / Q000502:physiology*; D002645:Chickens; D004789:Enzyme Activation / Q000187:drug effects; D005347:Fibroblasts / Q000201:enzymology / Q000502:physiology*; D051417:Focal Adhesion Kinase 1; D051416:Focal Adhesion Protein-Tyrosine Kinases; D051380:GRB2 Adaptor Protein; D019012:Integrin beta1 / Q000235:genetics / Q000502:physiology*; D051379:Mice; D010446:Peptide Fragments / Q000378:metabolism; D017354:Point Mutation; D011505:Protein-Tyrosine Kinases / Q000502:physiology*; D011506:Proteins / Q000502:physiology; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*; D014162:Transfection
D048868:Adaptor Proteins, Signal Transducing; D015815:Cell Adhesion Molecules; D051380:GRB2 Adaptor Protein; C495949:Grb2 protein, mouse; D019012:Integrin beta1; D010446:Peptide Fragments; D011506:Proteins; D011993:Recombinant Fusion Proteins; D011505:Protein-Tyrosine Kinases; D051417:Focal Adhesion Kinase 1; D051416:Focal Adhesion Protein-Tyrosine Kinases; C496051:Ptk2 protein, mouse; D017871:Calcium-Calmodulin-Dependent Protein Kinases