Uncoupling Endothelial Nitric Oxide Synthase Is Ameliorated by Green Tea in Experimental Diabetes by Re-establishing Tetrahydrobiopterin Levels

Abstract

The current study investigated the potential of green tea (GT) to improve uncoupling of endothelial nitric oxide synthase (eNOS) in diabetic conditions. In rats with streptozotocin-induced diabetes, nitric oxide (NO) bioavailability was reduced by uncoupling eNOS, characterized by a reduction in tetrahydrobiopterin (BH4) levels and a decrease in the eNOS dimer-to-monomer ratio. GT treatment ameliorated these abnormalities. Moreover, immortalized human mesangial cells (ihMCs) exposed to high glucose (HG) levels exhibited a rise in reactive oxygen species (ROS) and a decline in NO levels, which were reversed with GT. BH4 and the activity of guanosine triphosphate cyclohydrolase I decreased in ihMCs exposed to HG and was normalized by GT. Exogenous administration of BH4 in ihMCs reversed the HG-induced rise in ROS and the decline in NO production. However, coadministration of GT with BH4 did not result in a further reduction in ROS production, suggesting that reduced ROS with GT was indeed secondary to uncoupled eNOS. In summary, GT reversed the diabetes-induced reduction of BH4 levels, ameliorating uncoupling eNOS, and thus increasing NO bioavailability and reducing oxidative stress, two abnormalities that are involved in the pathogenesis of diabetic nephropathy.

Nox4 NADPH Oxidase Mediates Peroxynitrite-dependent Uncoupling of Endothelial Nitric-oxide Synthase and Fibronectin Expression in Response to Angiotensin II

“…eNOS Is Uncoupled upon Exposure of MCs to Ang II and Is a Source of Intracellular ROS-As previously reported (35,36), we found that eNOS protein is present in MCs (Fig. 3A, left panel).…”

“…Peroxynitrite is known to alter eNOS function via oxidation of BH 4 , thereby reducing its availability to eNOS (27)(28)(29)(30). The possibility of eNOS dysfunction via alteration of BH 4 levels is supported by reports showing that BH 4 supplementation restores NO levels and eNOS function in MCs exposed to high glucose or in glomeruli of diabetic rats (35,48). Moreover, angiotensin receptor blockade reverses eNOS uncoupling by increasing BH 4 levels in glomeruli from diabetic rats (36).…”

Sestrin 2 and AMPK Connect Hyperglycemia to Nox4-Dependent Endothelial Nitric Oxide Synthase Uncoupling and Matrix Protein Expression

“…Although the bioavailability of NO varies with the stage of DN, it is now well recognized that progressive nephropathy is associated with a state of NO deficiency due to eNOS dysfunction, a phenomenon referred to as "uncoupling." Uncoupling of eNOS results not only in decreased bioavailability of NO but also in an increase in the generation of ROS (18)(19)(20)(21)(22)(23). The protective role of NO generation by eNOS in the kidney has been conclusively established by recent studies showing that eNOS knockout mice made diabetic (type 1 or type 2) develop advanced lesions and progressive nephropathy (24)(25)(26).…”

“…Under pathological conditions, eNOS dysfunction or uncoupling resulted in decreased bioavailability of NO and increased generation of superoxide anion by the enzyme (16,17,19,22,38,43). We assessed the temporal effects of HG on NO formation and intracellular superoxide generation.…”

“…The protective role of NO generation by eNOS in the kidney has been conclusively established by recent studies showing that eNOS knockout mice made diabetic (type 1 or type 2) develop advanced lesions and progressive nephropathy (24-26). In addition, eNOS is expressed in MCs and its uncoupling by HG has been linked to the decline in NO levels observed in the diabetic environment (22,27). However, the mechanisms by which eNOS is inactivated in DN are not clearly identified.…”