Regulated expression of platelet factor 4 in human monocytes—role of PARs as a quantitatively important monocyte activation pathway

Human mononuclear phagocytes have recently been shown to express constitutively and even more so, upon stimulation with bacteria, fungi, lipopolysaccharide (LPS), zymosan, or thrombin platelet basic protein (PBP). This CXC chemokine as well as platelet factor 4 (PF4), which is located genomically at a short distance from the PBP, were previously considered to be specific markers for the megakaryocyte cell lineage. Both chemokines have signaling and antimicrobial activity. In the present studies, transcriptional and expressional regulation of PF4 and related chemokines was studied in human monocytes. As shown by quantitative mRNA analysis, Western blots, radioimmunoprecipitation of cell extracts, and immunofluorescence and quantitatively with enzyme‐linked immunosorbent assay, human monocytes express PF4 in the same order of magnitude as the known, regulated CXC chemokine interleukin (IL)‐8. Expression of PF4 is up‐regulated at the mRNA and protein level by thrombin and mediated by proteinase‐activated receptors (PARs), resulting in a 32‐ to 128‐fold higher mRNA level and leading to an up‐to‐sixfold increase of the peptide concentration in monocyte culture supernatants. Thrombin and the synthetic ligand of PAR‐1 and PAR‐2, SFLLRN, also induced comparable increases in the levels of mRNA for PBP, IL‐8, regulated on activation, normal T expressed and secreted (RANTES), monocyte chemoattractant protein‐1, and macrophage‐inflammatory protein‐1α and increased synthesis of these chemokines as shown by immunofluorescence or a quantitative immunobead‐based method. The induction of increased mRNA levels for all chemokines by SFLLRN was unsurpassed by LPS, zymosan, interferon‐γ (IFN‐γ), tumor necrosis factor α (TNF‐α), and IL‐1. Activation of monocytes through PARs represents an alternate activation mechanism, independent from IFN‐γ, TNF‐α, or other signaling pathways.