c-Cbl negatively regulates platelet activation by glycoprotein VI.
BACKGROUND
The adapter protein c-Cbl has emerged as having a potential role in negative regulation of immune receptor signaling. The major platelet-signaling receptor for collagen, glycoprotein VI (GpVI), is associated with the Fc receptor (FcR) gamma-chain, and signals through a similar pathway to immune receptors. c-Cbl is tyrosine-phosphorylated in response to stimulation of GpVI, whereas phosphorylation of c-Cbl in thrombin-activated platelets is dependent on fibrinogen binding to the integrin GpIIb/IIIa.


OBJECTIVE
To investigate the role of c-Cbl in platelet signaling.


METHODS
Murine platelets lacking functional c-Cbl or Src family kinases were analyzed.


RESULTS
Phosphorylation of c-Cbl through GpVI is reduced in murine platelets deficient in the Src-family kinases Fyn and Lyn, demonstrating that they lie upstream of c-Cbl phosphorylation. Phosphorylation of several proteins of the GpVI-signaling pathway, including the FcR gamma-chain, Syk and phospholipase Cgamma2 (PLCgamma2), is increased in the absence of c-Cbl. In line with this, aggregation is potentiated in response to the GpVI-specific collagen-related peptide (CRP) after a slight delay. A delay in potentiation is also seen in response to stimulation by thrombin.


CONCLUSIONS
These observations demonstrate that c-Cbl negatively regulates platelet responses to GpVI agonists and to thrombin, with the latter effect possibly being mediated downstream of GpIIb/IIIa. c-Cbl may play a physiological role in helping to prevent unwanted platelet activation in vivo.
Auger
J M
JM
Department of Pharmacology, University of Oxford, Mansfield Road, Oxford, UK. jma299@bham.ac.uk
Best
D
D
Snell
D C
DC
Wilde
J I
JI
Watson
S P
SP
D033942:Adaptor Proteins, Vesicular Transport / Q000502:physiology; D000818:Animals; D001792:Blood Platelets; D015536:Down-Regulation; D006801:Humans; D051379:Mice; D008815:Mice, Inbred Strains; D010766:Phosphorylation; D015539:Platelet Activation / Q000187:drug effects; D010974:Platelet Aggregation / Q000187:drug effects; D010980:Platelet Membrane Glycoproteins / Q000819:agonists / Q000494:pharmacology*; D011518:Proto-Oncogene Proteins / Q000172:deficiency / Q000502:physiology*; D050721:Proto-Oncogene Proteins c-cbl; D015398:Signal Transduction; D013917:Thrombin / Q000494:pharmacology; D044767:Ubiquitin-Protein Ligases / Q000172:deficiency / Q000502:physiology*
D033942:Adaptor Proteins, Vesicular Transport; D010980:Platelet Membrane Glycoproteins; D011518:Proto-Oncogene Proteins; C098341:platelet membrane glycoprotein VI; D050721:Proto-Oncogene Proteins c-cbl; D044767:Ubiquitin-Protein Ligases; D013917:Thrombin; C067419:CBL protein, human; C493849:Cbl protein, mouse