Virtual lack of annexins in human platelets argues against a role in phospholipase A2 regulation and platelet secretion.
Annexins are calcium-phospholipid binding proteins which share structural similarities and common biochemical properties. These proteins seem to be involved in different pathways of cell activation such as regulation of phospholipase A2 activity, membrane-cytoskeleton interaction, and exocytosis. The aim of this study was to attempt to characterize annexins in human platelets. Our results were based on specific EGTA extraction of proteins from platelet homogenates, immunodetection with specific antibodies raised against annexins I, II, V and VI, and measurement of phospholipase A2 inhibition. Antibodies raised against annexins I, V and VI revealed only trace amounts of these proteins in platelet EGTA extracts which did not promote phospholipase A2 inhibition in an in vitro assay. In addition, upon precipitation of membranes in the presence of calcium followed by EGTA extraction, the main substrate of protein kinase C (40-47-kDa protein) displayed a behaviour strictly different from that of annexins. Although one cannot exclude that a small amount of annexin(s) becomes phosphorylated in activated human platelets, these data argue against a role of annexins in the regulation of intracellular phospholipase A2 or in the processes of exocytosis in activated human platelets.
Ferrières
J
J
Institut National de la Santé et de la Recherche Médicale, Unité 326, Toulouse, France.
Simon
M F
MF
Fauvel
J
J
Chap
H
H
D017305:Annexin A1 / Q000097:blood; D017306:Annexin A2 / Q000097:blood; D017304:Annexin A5 / Q000097:blood; D017317:Annexin A6 / Q000097:blood; D017302:Annexins / Q000097:blood*; D001792:Blood Platelets / Q000502:physiology*; D002118:Calcium / Q000494:pharmacology; D004533:Egtazic Acid; D006801:Humans; D015151:Immunoblotting; D010741:Phospholipases A / Q000037:antagonists & inhibitors / Q000097:blood*; D054467:Phospholipases A2
D017305:Annexin A1; D017306:Annexin A2; D017304:Annexin A5; D017317:Annexin A6; D017302:Annexins; D004533:Egtazic Acid; D010741:Phospholipases A; D054467:Phospholipases A2; D002118:Calcium