The platelet proteasome and immunoproteasome are stable in buffy-coat derived platelet concentrates for up to 7 days.

OBJECTIVES: Characterization of the proteasome and its stability in buffy-coat derived platelet concentrates (PCs) during storage. BACKGROUND: The proteasome plays a key role in cell homeostasis by processing misfolded or abnormal proteins and regulating the levels and activities of a high number of proteins contributing to cell cycle, survival, and proliferation. Controversial data exist, whether inhibition of the proteasome affects platelet function. Little is known about function, expression, and stability of the proteasome in PCs during storage, and the potential role of the platelet proteasome in storage lesions. STUDY DESIGN AND METHODS: PCs were produced by the buffy-coat method in additive solution and stored at room temperature under agitation. Platelet aggregation was monitored by light transmission aggregometry. Proteasome complexes were assessed by immunoprecipitation and immunoblotting, and proteasome activity was measured using fluorogenic substrates specific for the three different proteolytic activities over 7 days of storage. RESULTS: Proteasome inhibition led to a decreased platelet aggregation response after activation with collagen, ADP, TRAP-6, and thrombin. There were no changes in the expression of the catalytic active subunits as well as the proteasome activity during storage of PCs, comparing baseline and day 7. DISCUSSION: Platelet proteasome function is relevant for platelet aggregation in response to various agonists. The constitutive and stable expression of the active standard- and immunoproteasome in platelets makes it unlikely that loss of proteasome function is a relevant cause of storage lesions.

Structure and function of the ubiquitin-proteasome system in platelets.

Platelets are small anucleate blood cells with a life span of 7 to 10 days. They are main regulators of hemostasis. Balanced platelet activity is crucial to prevent bleeding or occlusive thrombus formation. Growing evidence supports that platelets also participate in immune reactions, and interaction between platelets and leukocytes contributes to both thrombosis and inflammation. The ubiquitin-proteasome system (UPS) plays a key role in maintaining cellular protein homeostasis by its ability to degrade non-functional self-, foreign, or short-lived regulatory proteins. Platelets express standard and immunoproteasomes. Inhibition of the proteasome impairs platelet production and platelet function. Platelets also express major histocompatibility complex (MHC) class I molecules. Peptide fragments released by proteasomes can bind to MHC class I, which makes it also likely that platelets can activate epitope specific cytotoxic T lymphocytes (CTLs). In this review, we focus on current knowledge on the significance of the proteasome for the functions of platelets as critical regulators of hemostasis as well as modulators of the immune response.