Upregulation of endothelin receptor B in human endothelial cells by low-density lipoproteins

Low-density lipoproteins (LDLs) represent the most important treatable risk factors for coronary artery disease. Although it has been previously shown that hypercholesterolemia stimulates the endothelin system, the effects of increased levels of LDL on endothelial endothelin receptors have not been previously studied. In particular, the influence of native and oxidatively modified LDLs (nLDLs and oxLDLs) and the regulatory mechanisms in endothelial cells are currently unknown. Human endothelial cells almost exclusively express the endothelin receptor type B (ETB). Therefore, the effect of nLDL and oxLDL on the expression of ETB was studied in primary cultures of human umbilical vein endothelial cells (HUVEC). HUVEC were stimulated by nLDL and oxLDL in a time-dependent (1-12 hrs) and dose-dependent (25-100 mu g/ml) manner. To analyze signal transduction pathways involved in the regulation of ETB, protein kinase C (PKC) was inhibited using 100 nM Ro-31-8220. The mRNA expression of ETB was determined by quantitative reverse transcription-polymerase chain reaction and ETB protein expression by Western blot. Native LDL induced ETB mRNA after 1 hr (100 mu g/ml, 199 +/- 35%, n = 15, P < 0.05 vs. control). Stimulation of HUVEC with oxLDL increased ETB mRNA expression (1 hr, 100 mu g/ml oxLDL: 308 +/- 48%, n = 15, P < 0.05 vs. control) as well. Induction of ETB Was also found on the protein level. nLDL was even more potent than oxLDL in inducing ETB protein expression. Induction of ETB expression by oxLDL is mediated by PKC. These data demonstrate that low-density lipoproteins even independent of oxidative modification are potent inducers of ETB receptors at the mRNA and protein level in HUVEC. Given the nitric oxide-releasing capacity of endothelial ETB receptors, this effect may represent a possible vasoprotective mechanism.