Oxidized Low-Density Lipoproteins Impair Endothelial Function by Inhibiting Non-Genomic Action of Thyroid Hormone–Mediated Nitric Oxide Production in Human Endothelial Cells

Abstract

Background:

Thyroid hormone (TH) plays an important role in the modulation of cardiac function, including contractility and systemic vascular resistance (SVR). 3,5,3′-triiodothyronine (T₃), the active form of TH, induces the activation of endothelial nitric oxide synthase via PI3K/AKT non-genomic signaling. Hypothyroidism is associated with an increase in SVR and serum low-density lipoproteins (LDL) levels, and accumulation of oxidized LDL (oxLDL) may impair endothelial-dependent vascular relaxation. The aim of this study was to investigate the effects of both native LDL (nLDL) and oxLDL on T₃-mediated AKT phosphorylation, nitric oxide (NO), and cyclic guanosine monophosphate (cGMP) production in human endothelial cells.

Methods:

Human umbilical vein endothelial cells were exposed to either nLDL or oxLDL for 3 hours and then stimulated with T₃ (10⁻⁷ M) or pretreated with an antioxidant mixture of vitamins E and C for 12 hours before treatment with LDL. An analysis of AKT phosphorylation was performed by Western blot, and NO production was evaluated by using 4,5-diaminofluorescein diacetate. Intracellular production of cGMP was measured by enzymatic immunoassay. LDL oxidation was carried out by incubating LDL with CuSO₄, and α-tocopherol content of LDL was evaluated by high-performance liquid chromatography.

Results:

OxLDL impaired T₃-mediated AKT phosphorylation at serine 473 and significantly decreased the production of both NO (oxLDL+T₃ vs. T₃, 9.79±0.5 AU vs. 80.75±2.8 AU, mean±standard deviation, p<0.0001) and cGMP. Furthermore, pretreatment with the antioxidant mixture obviated the inhibitory effect of LDL on T₃ action.

Conclusions:

The results of this study demonstrate that oxLDL may contribute to a blunting of the non-genomic action of T₃ and impair the effect of T₃ on NO and cGMP production in endothelial cells. These data suggest that oxLDL, apart from inducing the atherosclerotic process, may also promote a mechanism of peripheral resistance to T_3, further amplifying the impact of hypothyroidism on endothelial function by increasing SVR.