CRP levels are strongly correlated with increased risk for cardiovascular disease and with endothelial dysfunction related to decreased NO bioavailability.1, 3, 20–22
We previously demonstrated that CRP potently antagonizes eNOS activation by diverse agonists at levels of CRP that have been associated with increased risk of cardiovascular disease,23
and that the inhibitory Fc receptor FcγRIIB is critically involved in this process.4
This represents a potentially major mechanism by which vascular NO bioavailability may be diminished under numerous disease conditions. Here we show that the activating Fc Receptor, FcγRI, and functional coupling of FcγRI to FcγRIIB are required for CRP inhibition of eNOS in endothelial cells, thereby identifying another key cell surface receptor that participates in the negative regulation of NO production by the endothelium. We further show that consistent with these observations in cell culture, both FcγRI and FcγRIIB are required for CRP to blunt endothelial repair in vivo, which is related to a diminution in bioavailable NO.5
Elucidation of these processes mediated by the FcγRI-FcγRIIB tandem provides further evidence that CRP is likely a causal factor in certain forms of cardiovascular disease, and not merely a marker of chronic, low-grade inflammation. In addition, since CRP impairs insulin signaling to eNOS in endothelial cells,10
which normally promotes blood flow that augments glucose disposal in skeletal muscle,24
these newly-identified mechanisms are also potentially relevant to the insulin resistance that accompanies chronic inflammatory conditions including obesity.
We further demonstrate that Src kinase is critically involved in the coupling of FcγRI to FcγRIIB in endothelial cells. In immune response cells in which Fc receptors serve their classical functions, Syk is the kinase that primarily links the activating phosphorylation of the ITAM within the γ subunit of FcγRI to the activating phosphorylation of the ITIM in FcγRIIB.25, 26
Interestingly, whereas we now show that in response to CRP, Src activation leads to eNOS antagonism, the stimulation of eNOS by numerous agonists including HDL, estrogen and shear stress also entails Src activation, which preceeds PI3 kinase and Akt activation that promotes eNOS Ser1177 phosphorylation.16, 27, 28
These disparate roles of Src may be explained by the participation of different Src family members. Alternatively, since Src activation entails poorly understood mechanisms that involve plasma membrane receptor clustering and Src interacts with a number of receptors,29
these disparate functions for Src may be explained by different responses by Src to FcγRI-FcγRIIB clustering versus eNOS activating receptor clustering, or differences in the mode of physical linkage between Src and Fc receptors versus eNOS activating receptors.30
Having demonstrated an inhibitory role for Src in the regulation of eNOS for the first time, additional studies are now warranted to distinguish between these possibilities.
Recognizing that the classical ligand for Fc receptors is IgG, we previously demonstrated that heat-aggregated IgG mimicking immune complex causes eNOS antagonism to a degree that is comparable to that attained by CRP.4
We now show that similar to CRP, the actions of aggregated IgG on eNOS are mediated by Src activation causing FcγRIIB ITIM phosphorylation and resulting activation of SHIP-1. Recognizing how eNOS contributes to insulin-mediated glucose disposal,24
these mechanisms may participate in the pathogenesis of not only the cardiovascular disease, but also the insulin resistance that frequently complicate immune complex-mediated diseases such as rheumatoid arthritis or systemic lupus erythematosis.31, 32
From a therapeutic perspective, the identification of FcγRI as the initiator of CRP action in endothelial cells provides a new cell surface target for potential interventions. Studies in humans and in mice indicate that attenuated function of FcγRIIB likely contributes to the pathogenesis of systemic lupus erythematosis.33
Therefore, seeking to normalize CRP-related endothelial dysfunction by decreasing FcγRIIB activity, which can be accomplished by selective antibody-mediated blockade,34
runs the risk of promoting autoimmune disease. Alternatively, strategies that selectively prevent FcγRI activation can now be considered to blunt CRP-induced endothelial dysfunction without shifting the balance of activating and inhibitory Fc receptor activity towards excessive immune function. Now having greater evidence of an important role for SHIP-1 in eNOS antagonism by both CRP and immune complex, the phosphatase may also warrant consideration as a therapeutic target in endothelial cells.
Although there is clear evidence of a causal role for CRP in hypertension and endothelial dysfunction in mice,6, 35, 36
CRP has not yet been implicated as a contributing factor in the pathogenesis of such disorders in humans. However, now knowing that FcγRI and FcγRIIB are both important participants in CRP signaling in endothelium, it becomes apparent that there are multiple modifiers of CRP actions of relevance to vascular health. There are numerous polymorphisms in Fc receptors and some have known impact on receptor function, and there is also great variability in Fc receptor gene copy number.37
Thus, as we continue to identify the participating signaling molecules, genetic and molecular modifiers of CRP action should be considered in our assessment of the role of CRP in cardiovascular disease pathogenesis and our use of CRP as a risk factor in patient populations.