Emerging evidence indicates that in addition to native pentameric C-reactive protein (CRP), monomeric CRP (mCRP) also plays an active role in inflammation associated with cardiovascular diseases. mCRP activates endothelial cells, one of the critical events in cardiovascular diseases; however, the underlying molecular mechanisms are incompletely understood. Here we report that association of mCRP with human aortic and coronary artery endothelial cells is predominantly due to membrane insertion rather than binding to the surface proteins Fc gammaRs and proteoglycans. We identify lipid rafts as the preferential membrane microdomains for mCRP anchorage. mCRP binding depends on membrane cholesterol content and is synergistically mediated by the putative cholesterol binding consensus sequence of CRP (aa 35-47) and the C-terminal octapeptide (aa 199-206). Conversely, disrupting lipid rafts with methyl-beta cyclodextrin or nystatin abrogated mCRP-induced cytokine release, reactive oxygen species generation, and adhesion molecule expression in endothelial cells. Furthermore, ex vivo treatment of rabbit thoracic aorta and carotid artery segments with nystatin prevented mCRP-induced IL-8 release. Our data identify mCRP-lipid raft interaction as an important mechanism in mediating cellular responses to mCRP and lend further support to the notion of mCRP regulation of endothelial cell function during inflammation.

C-reactive protein (CRP) has been implicated in the regulation of inflammation underlying coronary artery disease; however, little is known about the molecular mechanisms responsible for the expression of its pro- or anti-inflammatory activities. Here, we have identified the intrasubunit disulfide bond as a conserved switch that controls the structure and functions of CRP. Conformational rearrangement in human pentameric CRP to monomeric CRP (mCRP) is the prerequisite for this switch to be activated by reducing agents, including thioredoxin. Immunohistochemical analysis revealed 36-79% colocalization of thioredoxin and mCRP in human advanced coronary atherosclerotic lesions. Nonreduced mCRP was largely inert in activating human coronary artery endothelial cells (HCAECs), whereas reduced or cysteine-mutated mCRP evoked marked release of IL-8 and monocyte chemoattractant protein-1 from HCAECs, with ~50% increase at a concentration of 1 μg/ml. Reduced mCRP was ~4 to 40-fold more potent than mCRP in up-regulating adhesion molecule expression, promoting U937 monocyte adhesion to HCAECs, and inducing cytokine release from rabbit arteries ex vivo and in mice. These actions were primarily due to unlocking the lipid raft interaction motif. Therefore, expression of proinflammatory properties of CRP on endothelial cells requires sequential conformational changes, i.e., loss of pentameric symmetry followed by reduction of the intrasubunit disulfide bond.

Background: It is currently unclear whether soluble oligomers of the amyloid-␤ peptide (A␤) can cause neurotoxicity by direct membrane incorporation. Results: A␤ monomers but not the soluble oligomers readily insert into the membrane followed by self-assembly into membrane-embedded oligomers. Conclusion: Solution-phase oligomerization and membrane insertion of A␤ are mutually exclusive processes that proceed through distinct pathways. Significance: The competing intra-and extra-membrane oligomerization of A␤ may determine distinct neurotoxic mechanisms.

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