Positive and negative regulation of insulin action by genistein in the endothelium

Genistein is an isoflavone phytoestrogen with biological activities in management of metabolic disorders. This study aims to evaluate the regulation of insulin action by genistein in the endothelium. Genistein inhibited insulin-stimulated tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and attenuated downstream Akt and endothelial nitric oxide synthase (eNOS) phosphorylation, leading to a decreased nitric oxide (NO) production in endothelial cells. These results demonstrated its negative regulation of insulin action in the endothelium. Palmitate (PA) stimulation evoked inflammation and induced insulin resistance in endothelial cells. Genistein inhibited IKKβ and nuclear factor-кB (NF-кB) activation with down-regulation of tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) production and expression. Genistein inhibited inflammation-stimulated IRS-1 serine phosphorylation and restored insulin-mediated tyrosine phosphorylation. Genistein restored insulin-mediated Akt and eNOS phosphorylation, and then led to an increased NO production from endothelial cells, well demonstrating its positive regulation of insulin action under insulin-resistant conditions. Meanwhile, genistein effectively inhibited inflammation-enhanced mitogenic actions of insulin by down-regulation of endothelin-1 and vascular cell adhesion protein-1 overexpression. PA stimulation impaired insulin-mediated vessel dilation in rat aorta, while genistein effectively restored the lost vasodilation in a concentration-dependent manner (0.1, 1 and 10 μM). These results suggested that genistein inhibited inflammation and ameliorated endothelial dysfunction implicated in insulin resistance. Better understanding of genistein action in regulation of insulin sensitivity in the endothelium could be beneficial for its possible applications in controlling endothelial dysfunction associated with diabetes and insulin resistance.