Analysis of the nitric oxide-cyclic guanosine monophosphate pathway in experimental liver cirrhosis suggests phosphodiesterase-5 as potential target to treat portal hypertension

Abstract

Aim: To investigate the potential effect of inhibitors of phosphodiesterase-5 (PDE-5) for therapy of portal hypertension in liver cirrhosis.

Methods: In the rat model of thioacetamide-induced liver fibrosis/cirrhosis the nitric oxide-cyclic guanosine monophosphate (NO-cGMP) pathway was investigated. Expression and localization of PDE-5, the enzyme that converts vasodilating cGMP into inactive 5'-GMP, was in the focus of the study. Hepatic gene expression of key components of the NO-cGMP pathway was determined by qRT-PCR: Endothelial NO synthase (eNOS), inducible NO synthase (iNOS), soluble guanylate cyclase subunits α1 and β1 (sGCa1, sGCb1), and PDE-5. Hepatic PDE-5 protein expression and localization were detected by immunohistochemistry. Serum cGMP concentrations were measured using ELISA. Acute effects of the PDE-5 inhibitor Sildenafil (0.1 mg/kg or 1.0 mg/kg) on portal and systemic hemodynamics were investigated using pressure transducers.

Results: Hepatic gene expression of eNOS (2.2-fold; P = 0.003), sGCa1 (1.7-fold; P = 0.003), sGCb1 (3.0-fold; P = 0.003), and PDE-5 (11-fold; P = 0.003) was increased in cirrhotic livers compared to healthy livers. Overexpression of PDE-5 (7.7-fold; P = 0.006) was less pronounced in fibrotic livers. iNOS expression was only detected in fibrotic and cirrhotic livers. In healthy liver, PDE-5 protein was localized primarily in zone 3 hepatocytes and to a lesser extent in perisinusoidal cells. This zonation was disturbed in cirrhosis: PDE-5 protein expression in perisinusoidal cells was induced approximately 8-fold. In addition, PDE-5-expressing cells were also found in fibrous septa. Serum cGMP concentrations were reduced in rats with cirrhotic livers by approximately 40%. Inhibition of PDE-5 by Sildenafil caused a significant increase in serum cGMP concentrations [+ 64% in healthy rats (P = 0.024), + 85% in cirrhotic rats (P = 0.018)]. Concomitantly, the portal venous pressure was reduced by 19% in rats with liver cirrhosis.

Conclusion: Overexpression and abrogated zonation of PDE-5 likely contribute to the pathogenesis of cirrhotic portal hypertension. PDE-5 inhibition may therefore be a reasonable therapeutic approach for portal hypertension.

Keywords: Cyclic guanosine monophosphate; Hepatic stellate cells; Liver cirrhosis; Metabolic zonation; Nitric oxide; Phosphodiesterase-5; Portal hypertension; Sildenafil; Thioacetamide.

Figure 1

Dotplots showing hepatic gene expression of endothelial NO synthase (A), inducible NO synthase (B), phosphodiesterase-5 (C), soluble guanylate cyclase subunits α1 (D) and soluble guanylate cyclase subunits β1 (E), and serum cyclic guanosine monophosphate concentrations (pmol/mL) (F). Significant differences among groups are corrected for multiple comparisons and denoted by ^aP < 0.05. Gene expression levels are given as fold expression compared to CON. Since iNOS expression in CON was below the detection limit, it was arbitrarily set to “1.0”. iNOS: Inducible NO synthase.

Figure 2

Immunohistochemical localization of phosphodiesterase-5. A-C: Healthy liver; D-F: Cirrhotic liver. Red arrows: Localization in zone 3 hepatocytes; Blue arrows: Localization in perisinusoidal cells; Green Arrow: Fibrous septa.

Figure 3

Relative median (%) of mean arterial pressure and portal venous pressure over 60 min with error bars representing 95% confidence intervals. The time point “10 min” was set to 100%.