Low-density lipoprotein activates the small GTPases Rap1 and Ral in human platelets.
Physiological concentrations of low-density lipoprotein (LDL) sensitize blood platelets to alpha-thrombin- and collagen-induced secretion, and after prolonged contact trigger secretion independent of other agonists. Here we report that LDL activates the small GTPases Rap1 and Ral but not Ras, as assessed by specific precipitation of the GTP-bound enzymes. In unstirred suspensions, the inhibitor SB203580 blocks Rap1 activation by 60-70%, suggesting activation via p38 mitogen-activated protein kinase and a second, unidentified route. Inhibitors of cyclooxygenase (indomethacin) and the thromboxane A(2) (TxA(2)) receptor (SQ30741) induce complete inhibition, indicating that Rap1 activation is the result of TxA(2) formation. Stirring reveals a second, TxA(2)-independent Rap1 activation, which correlates quantitatively with a slow induction of dense granule secretion. Both pathways are unaffected by inhibitors of ligand binding to integrin alpha(IIb)beta(3). The results suggest that Rap1 and Ral, but not Ras, may take part in signalling routes initiated by LDL that initially enhance the sensitivity of platelets to other agonists and later trigger LDL-dependent secretion.
Hackeng
C M
CM
Department of Haematology, Institute for Biomembranes, University Medical Center Utrecht, P.O. Box 85500, 3508 GA Utrecht, The Netherlands.
Franke
B
B
Relou
I A
IA
Gorter
G
G
Bos
J L
JL
van Rijn
H J
HJ
Akkerman
J W
JW
D016718:Arachidonic Acid / Q000378:metabolism; D001791:Blood Platelet Disorders / Q000378:metabolism; D001792:Blood Platelets / Q000201:enzymology* / Q000378:metabolism; D003094:Collagen / Q000378:metabolism; D016861:Cyclooxygenase Inhibitors / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005982:Glutathione Transferase / Q000378:metabolism; D006801:Humans; D007093:Imidazoles / Q000494:pharmacology; D007213:Indomethacin / Q000494:pharmacology; D008024:Ligands; D008077:Lipoproteins, LDL / Q000378:metabolism*; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D010741:Phospholipases A / Q000378:metabolism; D010766:Phosphorylation; D011725:Pyridines / Q000494:pharmacology; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D015398:Signal Transduction; D013917:Thrombin / Q000378:metabolism; D013928:Thromboxane A2 / Q000494:pharmacology; D013997:Time Factors; D048051:p38 Mitogen-Activated Protein Kinases; D020725:ral GTP-Binding Proteins / Q000378:metabolism*; D020827:rap1 GTP-Binding Proteins / Q000378:metabolism*
D016861:Cyclooxygenase Inhibitors; D004791:Enzyme Inhibitors; D007093:Imidazoles; D008024:Ligands; D008077:Lipoproteins, LDL; D011725:Pyridines; D011993:Recombinant Fusion Proteins; D016718:Arachidonic Acid; D013928:Thromboxane A2; D003094:Collagen; D005982:Glutathione Transferase; D019950:Mitogen-Activated Protein Kinase 1; D020928:Mitogen-Activated Protein Kinases; D048051:p38 Mitogen-Activated Protein Kinases; D010741:Phospholipases A; D013917:Thrombin; D020725:ral GTP-Binding Proteins; D020827:rap1 GTP-Binding Proteins; C093642:SB 203580; D007213:Indomethacin