Identification of a novel integrin signaling pathway involving the kinase Syk and the guanine nucleotide exchange factor Vav1.
BACKGROUND
. Integrins induce the formation of large complexes of cytoskeletal and signaling proteins, which regulate many intracellular processes. The activation and assembly of signaling complexes involving focal adhesion kinase (FAK) occurs late in integrin signaling, downstream from actin polymerization. Our previous studies indicated that integrin-mediated activation of the non-receptor tyrosine kinase Syk in hematopoietic cells is independent of FAK and actin polymerization, and suggested the existence of a distinct signaling pathway regulated by Syk.


RESULTS
. Multiple proteins were found to be activated by Syk, downstream of engagement of the platelet/megakaryocyte-specific integrin alphaIIbbeta3. The guanine nucleotide exchange factor Vav1 was inducibly phosphorylated in a Syk-dependent manner in cells following their attachment to fibrinogen. Together, Syk and Vav1 triggered lamellipodia formation in fibrinogen-adherent cells and both Syk and Vav1 colocalized with alphaIIbbeta3 in lamellipodia but not in focal adhesions. Additionally, Syk and Vav1 cooperatively induced activation of Jun N-terminal kinase (JNK), extracellular-signal-regulated kinase 2 (ERK2) and the kinase Akt, and phosphorylation of the oncoprotein Cbl in fibrinogen-adherent cells. Activation of all of these proteins by Syk and Vav1 was not dependent on actin polymerization.


CONCLUSIONS
. Syk and Vav1 regulate a unique integrin signaling pathway that differs from the FAK pathway in its proximity to the integrin itself, its localization to lamellipodia, and its activation, which is independent of actin polymerization. This pathway may regulate multiple downstream events in hematopoietic cells, including Rac-induced lamellipodia formation, tyrosine phosphorylation of Cbl, and activation of JNK, ERK2 and the phosphatidylinositol 3'-kinase-regulated kinase Akt.
Miranti
C K
CK
Department of Cell Biology Harvard Medical School Boston Massachusetts 02115 USA.
Leng
L
L
Maschberger
P
P
Brugge
J S
JS
Shattil
S J
SJ
D000199:Actins / Q000378:metabolism; D000818:Animals; D016466:CHO Cells; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D018797:Cell Cycle Proteins; D006224:Cricetinae; D004792:Enzyme Precursors / Q000235:genetics / Q000378:metabolism*; D020690:GTPase-Activating Proteins; D047908:Intracellular Signaling Peptides and Proteins; D019950:Mitogen-Activated Protein Kinase 1; D008954:Models, Biological; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000235:genetics / Q000378:metabolism*; D011505:Protein-Tyrosine Kinases / Q000235:genetics / Q000378:metabolism*; D011506:Proteins / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D050721:Proto-Oncogene Proteins c-cbl; D051567:Proto-Oncogene Proteins c-vav; D015398:Signal Transduction; D000072377:Syk Kinase; D014162:Transfection; D014443:Tyrosine / Q000378:metabolism; D044767:Ubiquitin-Protein Ligases
D000199:Actins; D018797:Cell Cycle Proteins; D004792:Enzyme Precursors; D020690:GTPase-Activating Proteins; D047908:Intracellular Signaling Peptides and Proteins; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; D011506:Proteins; D011518:Proto-Oncogene Proteins; D051567:Proto-Oncogene Proteins c-vav; D014443:Tyrosine; D050721:Proto-Oncogene Proteins c-cbl; D044767:Ubiquitin-Protein Ligases; D019869:Phosphatidylinositol 3-Kinases; D011505:Protein-Tyrosine Kinases; C000606896:SYK protein, human; D000072377:Syk Kinase; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D019950:Mitogen-Activated Protein Kinase 1; C067419:CBL protein, human