Abstract
Limit of platelet life span (8-10 days) is determined by the activity of a putative "internal clock" composed of Bcl-2 family proteins, whereas the role of other molecular players in this process remains obscure. Here, we sought to establish a central role of proteasome in platelet life span regulation. Administration of mice with inhibitors of proteasome peptidase activity induced significant thrombocytopenia. This was associated with enhanced clearance of biotin-labeled platelets from circulation and reduction in average platelet half-life from 66 to 37 h. Cells pretreated in vitro with proteasome inhibitors exhibited augmented annexin V binding and a drop in mitochondrial transmembrane potential indicative of apoptotic cell death and decreased platelet life span. These cells were preferentially phagocytosed by monocyte-derived macrophages, thus linking proteasome activity with platelet survival. The decisive role of proteasome in this process was underscored from enhanced expression of conformationally active Bax in platelets with attenuated proteasome activity, which was consistent with pro-apoptotic phenotype of these cells. The present study establishes a critical role of proteasome in delimiting platelet life span ostensibly through constitutive elimination of the conformationally active Bax. These findings bear potential implications in clinical settings where proteasome peptidase activities are therapeutically targeted.
Highlights
The molecular players regulating platelet life span are largely unexplored
The results presented so far identify the proteasome peptidase complex as a novel regulator of platelet life span
Because Bax is a known substrate of proteasome [22], we evaluated the effect of proteasome inhibition on its levels
Summary
The molecular players regulating platelet life span are largely unexplored. Results: Proteasome inhibition induced apoptotic changes in platelets associated with a rise in active Bax and significant drop in platelet life span. Administration of mice with inhibitors of proteasome peptidase activity induced significant thrombocytopenia This was associated with enhanced clearance of biotin-labeled platelets from circulation and reduction in average platelet half-life from 66 to 37 h. Cells pretreated in vitro with proteasome inhibitors exhibited augmented annexin V binding and a drop in mitochondrial transmembrane potential indicative of apoptotic cell death and decreased platelet life span These cells were preferentially phagocytosed by monocyte-derived macrophages, linking proteasome activity with platelet survival. In a significant recent study platelet life span was demonstrated to be determined by putative internal clock based on opposing activities of anti- and pro-apoptotic Bcl-2 family proteins, Bcl-XL, Bak, and Bax [7, 8]. We show that attenuation of proteasome activity significantly shortens life span of platelets associated with accumulation of conformationally active Bax and phagocytic clearance of the cells by macrophages
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