C-reactive protein mediated Fc-gamma-receptor signal transduction is enhanced by low density lipoprotein in human macrophages

Introduction: C-reactive protein (CRP) and low density lipoprotein (LDL) are risk markers for cardiovascular disease. Both deposit in the arterial wall during atherogenesis. CRP binds to LDL and has been implicated to play a pivotal role in cardiovascular disease. Previous reports proposed that Fcg receptor I (FcgRI) and Fcg receptor IIa (FcgRIIa) are the major receptors for CRP. Syk kinase is essential for FcgR signaling in phagocytosis. Objectives: Our objectives is to examine whether CRP can trigger Syk kinase phosphorylation in human monocytes and macrophages and whether this effect can be enhanced by the involvement of LDL. Methods: Peripheral blood monocytes were isolated from buffy coats obtained from the blood of healthy donors. Cells were cultured for 7 days to transform into macrophages. Fluorescence activated cell sorting analysis were performed respectively to peripheral human blood monocytes and mature macrophages. Peripheral human blood monocytes and mature macrophages were exposed to CRP and LDL. After cell lysis, equal amounts of protein were assayed by immunoblotting in order to investigate Syk kinase phosphorylation. Results: Fluorescence activated cell sorting (FACS) analysis of monocytes and macrophages revealed 70% FcgRII and 61% FcgRI, and 79% FcgRII and 62% FcgRI positivity, respectively. Induction of Syk kinase phosphorylation was not observed in monocytes. In macrophages, however, Syk kinase phosphorylation in response to CRP (100 µg/mL) is equal to that induced by human IgG. Significant, and CRP dose-dependent, induction of the signal was observed in response to the CRP-LDL complex (50 µg/mL LDL). LDL by itself (50 µg/mL) only induced trivial Syk kinase signaling. Conclusions: Our experiments suggest that human CRP-mediated tyrosine phosphorylation of the immunoreceptor-tyrosine based activation motif (ITAM) of Syk kinase in human macrophages is enhanced by LDL. CRP may recognize human tissue-deposited LDL as a foreign antigen