PCSK9 Inhibitor Attenuates Platelet TLR4 in ACS and TLR4 Blockers Prevent PCSK9-Mediated Platelet Activation.

Background: Proprotein convertase subtilisin/kexin type 9 (PCSK9) regulates platelet activation through multiple signaling pathways. This study aimed to determine whether PCSK9 modulates platelet activation via Toll-like receptor 4 (TLR4). Methods: A total of 120 patients with acute coronary syndrome (ACS) were divided into a control group and an experimental group receiving subcutaneous evolocumab. Platelet surface TLR4 was quantified by flow cytometry. Platelet-rich plasma from healthy volunteers (n = 5) was incubated with normal saline, PCSK9, PCSK9 + evolocumab, or PCSK9 + the TLR4 inhibitor resatorvid (TAK-242). Platelets were then stimulated with adenosine diphosphate, and aggregation was measured. Plasma thromboxane A2 (TXA2) was measured by ELISA. Megakaryoblastic leukemia cells (MEG-01) were co-incubated with PCSK9 and stained for TLR4 and PCSK9 by immunofluorescence. Results: PCSK9 significantly enhanced platelet aggregation and TXA2 release in vitro; both effects were abolished by evolocumab and TAK-242. In ACS patients, evolocumab treatment reduced platelet surface TLR4 expression and decreased plasma TXA2 levels. Dual immunofluorescence staining demonstrated co-localization of TLR4 and PCSK9 in MEG-01 cells. Conclusions: This study provides the first evidence that PCSK9 inhibitors downregulate platelet TLR4 expression in ACS patients. The pro-aggregatory effects of PCSK9 on platelets were counteracted by TLR4 inhibition, indicating a mechanistic link between PCSK9 and TLR4 signaling in platelet activation. Co-localization of TLR4 and PCSK9 in MEG-01 cells supports a potential interaction between the two proteins.