Mechanisms of protection against diabetes-induced impairment of endothelium-dependent vasorelaxation by Tanshinone IIA

Background: Impairment of endothelium-dependent vasorelaxation has been suggested to play a principle role of endothelial dysfunction in the development of cardiovascular complications of diabetes. Recent studies have demonstrated a protective effect of Tanshinone IIA (Tan) on endothelial nitric oxide synthase (eNOS)-NO pathway. However, its role in endothelium-dependent vasorelaxation in diabetes and precise mechanisms remain elusive. Methods: Sprague-Dawley rats were injected intraperitoneally with streptozotocin (STZ) to induce diabetes and then administered orally with Tan for 2 weeks. For the in vitro study, human umbilical vein endothelial cells (HUVECs) were co-incubated with Tan and high glucose for 48 h. Results: eNOS expression and NO generation were significantly decreased in diabetic rats. These decreases were accompanied by an impairment of endothelium-dependent relaxation. Administration of Tan ameliorated the aberrant changes in eNOS expression, NO generation and endothelium-dependent relaxation in diabetic rats. Expectedly, Tan also inhibited high glucose-induced decrease of eNOS expression and NO generation in a concentration-dependent manner in HUVECs. Mechanistically, high glucose attenuated eNOS transcriptional activity through inhibiting the binding activity and nuclear translocation of Sp1 and AP-1. However, Tan did not prevent these effects. At post-transcriptional level, Tan increased eNOS expression and activity through multiple mechanisms including regulation of mRNA and protein half-life, degradation, coupling and serine 1177 phosphorylation. Rather than affecting protein phosphatase 2A (PP2A) expression and activity, Tan markedly inhibited the translocation of PP2A-A from cytosol to membrane and subsequently impaired PP2A-A/eNOS interaction, leading to prevent eNOS dephosphorylation. All these alterations underlie the protective role of Tan on eNOS expression following high glucose stimulation. Conclusions: Our data demonstrate that high glucose decreases eNOS expression initiating at a transcriptional level, whereas Tan prevents such effect through multiple ways of post-transcriptional mechanism. General significance: Our work provided novel mechanisms for Tan in regulating vasorelaxation and may help to better understand the cardiovascular protective action of Tan. (C) 2015 Elsevier B.V. All rights reserved.