Carbamylated Low-density Lipoprotein Induces Death of Endothelial Cells: a Link to Atherosclerosis in Patients with Kidney Disease

Background: The presence of accelerated atherosclerosis in patients with kidney disease cannot be entirely explained by traditional cardiovascular risk factors. Exposure to urea, which is normally present in human blood plasma and elevated in patients with kidney disease, leads to the carbamylation of proteins. We postulated that low-density lipoprotein (LDL) carbamylated by urea has biologic effects relevant to atherosclerosis.

Methods: To produce carbamylated LDL (cLDL), human native LDL (nLDL) was chemically modified in vitro by exposure to potassium cyanate. Human coronary artery endothelial cells (HCAECs) and human coronary artery smooth muscle cells (CASMCs) were treated in vitro with cLDL or nLDL. Irreversible cell death was measured using the lactate dehydrogenase (LDH) assay, apoptosis was assessed by annexin V binding, and proliferation was determined using bromodeoxyuridine (BrdU) incorporation. Total plasma protein carbamylation and plasma cLDL were measured in hemodialysis patients using the homocitrulline assay and enzyme-linked immunosorbent assay (ELISA).

Results: Our studies demonstrated that cLDL but not nLDL induced dose-dependent vascular cell injuries relevant to atherosclerosis, which included the proliferation of vascular smooth muscle cells and endothelial cell death. Under light microscopy, endothelial cells treated with cLDL showed signs of morphologic alterations. The injury to endothelial cells measured by LDH release was time-dependent and correlated with the degree of LDL carbamylation. At least a part of the endothelial cell population treated with cLDL died by apoptosis. In patients with advanced renal disease on hemodialysis, total plasma protein carbamylation and plasma cLDL were several times higher than in control healthy individuals.

Conclusion: Collectively these data suggest the potential role of carbamylated LDL in accelerated atherosclerosis in patients with chronic renal disease and, possibly, in healthy individuals.