Topological localization of monomeric C-reactive protein determines proinflammatory endothelial cell responses

The activation of endothelial cells (ECs) by monomeric C-reactive protein (mCRP) has been implicated in contributing to atherogenesis. However, the potent proinflammatory actions of mCRP on ECs in vitro appear to be incompatible with the atheroprotective effects of mCRP in a mouse model. Because mCRP is primarily generated within inflamed tissues and is rapidly cleared from the circulation, we tested whether these discrepancies can be explained by topological differences in response to mCRP within blood vessels. In a Transwell culture model, the addition of mCRP to apical (luminal), but not basolateral (abluminal), surfaces of intact human coronary artery EC monolayers evoked a significant up-regulation of MCP-1, IL-8, and IL-6. Such polarized stimulation of mCRP was observed consistently regardless of EC type or experimental conditions (e.g. culture of ECs on filters or extracellular matrix-coated surfaces). Accordingly, we detected enriched lipid raft microdomains, the major surface sensors for mCRP on ECs, in apical membranes, leading to the preferential apical binding of mCRP and activation of ECs through the polarized induction of the phospholipase C, p38 MAPK, and NF-κB signaling pathways. Furthermore, LPS and IL-1β induction of EC activation also exhibited topological dependence, whereas TNF-α did not. Together, these results indicate that tissue-associated mCRP likely contributes little to EC activation. Hence, topological localization is an important, but often overlooked, factor that determines the contribution of mCRP and other proinflammatory mediators to chronic vascular inflammation.