Phosphorylation of eNOS initiates excessive NO production in early phases of portal hypertension

Abstract

Akt, also known as protein kinase B, is a serine/threonine kinase. Akt becomes active when phosphorylated by the activation of receptor tyrosine kinases, G protein-coupled receptors, and mechanical forces such as shear stress. Studies in vitro have shown that Akt can directly phosphorylate endothelial nitric oxide (NO) synthase (eNOS) and activate the enzyme, leading to NO production. The aim of this study was to test the hypothesis that the phosphorylation of eNOS plays a role in the enhanced NO production observed in early portal hypertension. Male Sprague-Dawley rats were subjected to either sham or portal vein ligation (PVL), and mesenteric arterial beds were used for ex vivo perfusion studies. Mesenteric arterial beds from PVL rats had an approximately 60-70% decrease in response to methoxamine (an alpha(1)-agonist and vasoconstrictor) compared with the sham group (P < 0.01). When N(G)-monomethyl-L-arginine (a NOS inhibitor) was added to the perfusion, the difference in perfusion pressure between the two groups was abolished, suggesting that enhanced NO production in the PVL group blunted the response to the vasoconstrictor. The reduced responsiveness in PVL was not due to changes in eNOS expression but was due to an increase in enzyme-specific activity, suggesting posttranslational modification of eNOS. The phosphorylation of eNOS at Ser(1176) was significantly increased by twofold (P < 0.05) in the PVL group. Furthermore, PVL significantly increased Akt phosphorylation (an active form of Akt) by threefold (P < 0.05). When vessels were treated with wortmannin (10 nM) to block the phosphatidylinositol-3-OH-kinase/Akt pathway, NO-induced vasodilatation was significantly reduced. These results suggest that the phosphorylation of eNOS by Akt activates the enzyme and may be the first step leading to an initial increase in NO production in portal hypertension.

Vascular biology and pathobiology of the liver: Report of a single-topic symposium

“…Degree of portal hypertension correlates with eNOS activation in the splanchnic circulation, in an animal model of portal hypertension 27 and is initiated by physical stimuli that induce Akt-dependent eNOS activation as well as through increased production of VEGF. [27][28][29] Thus, portal pressure itself, may be an important factor that regulates vasodilatation in the splanchnic arterial circulation by means of eNOS activation in the splanchnic circulation. 27 The formation of portosystemic collateral vessels and gastroesophageal varices are another important feature of portal hypertension that result in esophageal variceal hemorrhage.…”

Mild increases in portal pressure upregulate vascular endothelial growth factor and endothelial nitric oxide synthase in the intestinal microcirculatory bed, leading to a hyperdynamic state

“…A number of mechanisms, including shear stress (20), bacterial translocation (39, 40), or portosystemic shunting (PSS) (1, 2), have been proposed to explain the upregulation of endothelial NO synthase (eNOS), the isoform that accounts for the increased NO production in PHT. However, it has been demonstrated that eNOS activation occurs before any of these mechanisms is present (22,43), and the signals that initially trigger NO hyperproduction in PHT are still poorly understood.The partial portal vein ligation (PVL) model has been shown to reproduce the systemic and hemodynamic abnormalities in PHT (37). Furthermore, its highly predictable chronobiology has allowed the elucidation of the sequences of events leading to hyperdynamic circulation (8,33).…”

“…A number of mechanisms, including shear stress (20), bacterial translocation (39,40), or portosystemic shunting (PSS) (1,2), have been proposed to explain the upregulation of endothelial NO synthase (eNOS), the isoform that accounts for the increased NO production in PHT. However, it has been demonstrated that eNOS activation occurs before any of these mechanisms is present (22,43), and the signals that initially trigger NO hyperproduction in PHT are still poorly understood.…”

Systemic and pulmonary hemodynamics in patients with extrahepatic portal vein obstruction is similar to compensated cirrhotic patients

“…All the patients had HVPG level 6 mmHg or less, except one patient who had HVPG value of 10 mmHg. On the other hand, the median (range) HVPG levels in patients with compensated cirrhosis was Median Child-Pugh score (range) -6 (5-7) -significantly higher (14 [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] mmHg; P \ 0.01) than the EHPVO patients. None of the cirrhotic patients had HVPG less than 9 mmHg.…”

“…Furthermore, different degrees of portal pressure initiated physical stimuli to induce eNOS activation in the different parts of the splanchnic arterial circulation and with different molecular mechanisms [13]. For instance, an acute and higher portal pressure induced vasoconstriction first in large arteries of the splanchnic circulation owing to a myogenic reflex caused by a sudden increase in portal pressure, which then caused phosphorylation and activation of eNOS through Akt/PKB activation, ultimately leading to increased NO production and vasodilatation [14,15]. Activation of Akt might be due to an increase in shear stress induced by this myogenic reflex and vasoconstriction.…”