Activation of alphaIIbbeta3 is a sufficient but also an imperative prerequisite for activation of alpha2beta1 on platelets.
Platelet integrins alpha2beta1 and alphaIIbbeta3 play critical roles in platelet adhesion and thrombus formation after vascular injury. On resting platelets, both integrins are in a low-affinity state. However, agonist stimulation results in conformational changes that enable ligand binding that can be detected with conformation dependent monoclonal antibodies (mAbs). By using such conformation-dependent mAbs, we could demonstrate that activation of integrin alphaIIbbeta3 is not only sufficient, but also a prerequisite for alpha2beta1 activation. Compared with platelets in plasma, stimulation of washed platelets resulted in only a minor activation of alpha2beta1, as detected with the activation-sensitive mAb IAC-1. Addition of fibrinogen to stimulated washed platelets greatly potentiated activation of this integrin. Also, treatment of alphaIIbbeta3 with the ligand-mimetic peptide RGDS, resulting in outside-in signaling, led to a powerful alpha2beta1 activation, even in the absence of overall platelet activation, involving tyrosine kinase activity but no protein kinase C activation. The absolute necessity of alphaIIbbeta3 for proper alpha2beta1 activation on platelets was demonstrated by using the alphaIIbbeta3 antagonist aggrastat, which was able to completely abolish alpha2beta1 activation, both under static and flow conditions. In addition, analogous experiments with Glanzmann platelets lacking alphaIIbbeta3 confirmed the indispensability of alphaIIbbeta3 for alpha2beta1 activation.
Van de Walle
Gerlinde R
GR
Laboratory for Thrombosis Research, Katholieke Universiteit Leuven, KU Leuven Campus Kortrijk, E. Sabbelaan 53, B-8500 Kortrijk, Belgium, and Institut Fédératif No. 4, Hôpital Cardiologique, Pessac, France.
Schoolmeester
Anne
A
Iserbyt
Brecht F
BF
Cosemans
Judith M E M
JM
Heemskerk
Johan W M
JW
Hoylaerts
Marc F
MF
Nurden
Alan
A
Vanhoorelbeke
Karen
K
Deckmyn
Hans
H
D000818:Animals; D001792:Blood Platelets / Q000378:metabolism*; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D005340:Fibrinogen / Q000037:antagonists & inhibitors / Q000378:metabolism; D006801:Humans; D038982:Integrin alpha2beta1 / Q000378:metabolism*; D015539:Platelet Activation / Q000502:physiology*; D010975:Platelet Aggregation Inhibitors / Q000494:pharmacology; D019039:Platelet Glycoprotein GPIIb-IIIa Complex / Q000378:metabolism*; D011485:Protein Binding / Q000187:drug effects; D015398:Signal Transduction / Q000502:physiology
D038982:Integrin alpha2beta1; D010975:Platelet Aggregation Inhibitors; D019039:Platelet Glycoprotein GPIIb-IIIa Complex; D005340:Fibrinogen