Study of the interaction of the C-reactive protein monomer with the U937 monocyte.

C-reactive protein (CRP) has two structurally distinct isoforms, the CRP pentamer and the CRP monomer. A role for the CRP monomer in atherosclerosis is emerging, but the underlying mechanisms are only beginning to be understood. Monocytes are an important contributor to atherosclerosis, and foam cell formation is the hallmark of atherogenesis. However, whether the CRP monomer can directly interact with the monocytes and modulate their responses remains unknown. Furthermore, although FcgammaRIII (CD16) has been identified as the receptor for the CRP monomer on neutrophils, its role in mediating the CRP monomer's biological effects in other cell types has been questioned. In this study, we investigated the interaction of the CRP monomer with the monocytes using the U937 monocytic cell line. The CRP monomer specifically binds to U937 cells. This binding is unique in that it is independent of FcgammaRs and insensitive to protease digestion of the cell surface proteins. Further assays revealed that the CRP monomer directly incorporates into the plasma membrane. Interestingly, the presence of the CRP monomer efficiently retards oxidized low-density lipoprotein-induced foam cell formation of PMA-differentiated U937 macrophages and peripheral blood monocytic cell-derived macrophages. These findings provide additional evidence for the notion that the CRP monomer is an active CRP isoform that plays a role in atherogenesis via the direct modulation of the behavior of the monocytes.