Activation of Human Platelets by Misfolded Proteins

Abstract

Objective-Protein misfolding diseases result from the deposition of insoluble protein aggregates that often contain fibrils called amyloid. Amyloids are found in Alzheimer disease, atherosclerosis, diabetes mellitus, and systemic amyloidosis, which are diseases where platelet activation might be implicated. Methods and Results-We induced amyloid properties in 6 unrelated proteins and found that all induced platelet aggregation in contrast to fresh controls. Amyloid-induced platelet aggregation was independent of thromboxane A 2 formation and ADP secretion but enhanced by feedback stimulation through these pathways. Treatments that raised cAMP (iloprost), sequestered Ca 2ϩ (BAPTA-AM) or prevented amyloid-platelet interaction (sRAGE, tissue-type plasminogen activator [tPA]) induced almost complete inhibition. Modulation of the function of CD36 (CD36 Ϫ/Ϫ mice), p38 MAPK (SB203580), COX-1 (indomethacin), and glycoprotein Ib␣ (Nk-protease, 6D1 antibody) induced Ϸ50% inhibition. Interference with fibrinogen binding (RGDS) revealed a major contribution of ␣ IIb ␤ 3 -independent aggregation (agglutination). Conclusions-Protein misfolding resulting in the appearance of amyloid induces platelet aggregation. Amyloid activates platelets through 2 pathways: one is through CD36, p38 MAPK , thromboxane A 2 -mediated induction of aggregation; the other is through glycoprotein Ib␣-mediated aggregation and agglutination. The platelet stimulating properties of amyloid might explain the enhanced platelet activation observed in many diseases accompanied by the appearance of misfolded proteins with amyloid. Key Words: amyloid Ⅲ platelet activation Ⅲ sRAGE Ⅲ tissue plasminogen activator Ⅲ CD36 Ⅲ glycoprotein Ib␣ P roteins typically adopt a well-defined 3-dimensional structure. There is now an increasing amount of evidence that abnormalities in this process have far reaching consequences for human health. Certain mutations and posttranslational modifications such as glycation and oxidation interfere with proper folding, resulting in protein misfolding, aggregation, and ultimately polymerization into insoluble fibrils called amyloid. 1,2 The term amyloidosis defines a group of systemic and localized diseases associated with the deposition of amyloid in different tissues. Alzheimer disease is caused by abnormal folding of amyloid-␤ and formation of amyloid-rich plaques that obstruct neurons and microvessels of the brain. 3 One has argued that these plaques cause the hyperreactivity of platelets observed in these patients as illustrated by P-selectin positive platelets and increased levels of urinary thromboxane A 2 metabolite. 4,5 An environmental risk factor for Alzheimer disease is Herpes Simplex virus. It contains glycoprotein B, a protein fragment that assembles into fibrils that are ultrastructurally indistinguishable from amyloid-␤. 6 Protein misfolding is not restricted to Alzheimer disease but is a common feature in the pathology of atherosclerosis, diabetes mellitus, and systemic amyloidosis. 7-9 Atherosclerotic plaques contain ...

Platelet Aggregation Increased by Advanced Glycated Hemoglobin

“…This assumption is consisted with the results from other groups who have already shown that AGE-BSA induces PLT reactivity [15,18] while some glycated structures like CML (Nε-(carboxymethyl)lysine) and pentosidine have no effect on these cells [16]. They have discussed their observation based on the presence of some essential epitopes in AGE-BSA.…”

“…This PLT malfunction has been attributed in part to hyperglycemia and glycation reactions that might aggravate with a higher rate in hyperglycemia conditions [24]. Aforementioned reactions affect PLT function by different mechanisms [15]. Several studies have revealed that an increase in PLT reactivity upon treatment with AGE proteins; though their influences vary remarkably [15][16][17].…”

“…Aforementioned reactions affect PLT function by different mechanisms [15]. Several studies have revealed that an increase in PLT reactivity upon treatment with AGE proteins; though their influences vary remarkably [15][16][17].…”

“…The general mechanism by which AGE proteins influence different cell types has been described to be involved particular types of receptors such as RAGE (receptor for AGE) and SR (scavenger receptor) [15,25].…”

“…glycation of PLT membrane proteins [13], glycation of PLT cytosolic proteins [14], and interaction between glycated proteins or AGEs and PLTs [15][16][17]. Reports dealing with the interaction between AGEs and PLTs are limited to few cases in leterature.…”