Interaction of carbamylated LDL with LOX-1 in the induction of endothelial dysfunction and atherosclerosis.

Urea is a normal component of human blood plasma, but it is not inert. Urea-derived cyanate modification of LDL-cholesterol, known as LDL carbamylation, has been discussed for more than a decade. Recent studies have linked LDL carbamylation to atherosclerosis in uraemic patients and animals with chronic kidney disease (CKD). – 4 More broadly, protein carbamylation has been linked with atherosclerotic cardiovascular disease (CVD) in CKD. Endothelial cell (EC) dysfunction induced by carbamylated LDL (cLDL) seems to be a plausible explanation for the high frequency of CVD in CKD. However, despite considerable efforts in determining the mechanistic basis of atherosclerosis induced by cLDL, the link remainsblurry since it is hard to relate the complexeventsbased on in vitro, in vivo, and clinical studies done independently. An important contribution by Speer and associates is that they were able to combine all these approaches into a unified concept. As opposed to many previous studies which used LDL artificially carbamylated by potassium cyanate, or measured protein carbamylation, or measured plasma cLDL by enzyme-linked immunosorbent assay (ELISA), these investigators isolated LDL from plasma samples and measured its carbamylation directly using HPLC. Surprisingly, this natural cLDL in CKD patients was found to contain more carbamylation sites than artificially ex vivo cLDL. This may be an indication of more than one mechanism of LDL carbamylation in the human body, or the presence of factors in blood that affect the efficiency of LDL carbamylation. The authors determined the link of LDL-carbamyl-lysine levels with cardiovascular (CV) outcomes in patients with CKD followed for a median duration of 4.7 years. Their remarkable finding was that LDL-carbamyl-lysine levels in these patients were significant predictors for CV events and all-cause mortality. Taken together with recent animal and clinical studies, these data link cLDL with vascular dysfunction and adverse outcome in patients with CKD. Findings by this team confirmed several previous observations and moved the discovery forward by providing evidence of an interesting new mechanism. The authors examined vascular reactivity in isolated aortic rings, and measured reactive oxygen species (ROS) and nitric oxide (NO) production by electron-spin resonance spectroscopy. While native LDL (nLDL) showed no effect, cLDL impaired EC-dependent relaxation, but not EC-independent relaxation. cLDL increased ROS production in aortic rings by activating NADPH oxidase, and stimulated endothelial nitric oxide synthase (eNOS) uncoupling apparently by promoting S-glutathionylation of eNOS. Importantly, by using aortic rings from lectin-like oxidized LDL (oxLDL) receptor (LOX-1) transgenic mice, Speer et al demonstrated that the impaired EC-dependent relaxation was more pronounced in the LOX-1 transgenic mice. These authors further confirmed that it is the activation of LOX-1 that mediates cLDL-induced EC dysfunction by examining NO production using small interfering RNA (siRNA) targeting LOX-1. They showed that silencing of LOX-1 abrogated the inhibitory effect of cLDL on EC NO release. Nonetheless, while the effect of cLDL being mediated through LOX-1 activation is clearly suggested, demonstration of LOX-1 gene up-regulation in ECs by cLDL and absence of cLDL’s effect on vasoreactivity in LOX-1 knockout mice might have provided additional evidence of a cLDL–LOX-1 pathway. These findings are an important addition to previous reports of cLDL-induceddysfunction of ECs shown mainly in in vitro studies. Previous studies showed that cLDL is capable of binding to ECs. cLDL may transmigrate through ECs, activate the mitogen-activated protein kinase (MAPK) pathway, and induce injury. It also induces monocyte adhesion and activation and their transformation into macrophages utilizing a unique spectrum of scavenger receptors. cLDL also activates smooth muscle cells (SMCs) and induces