Enhanced expression of endothelial nitric oxide synthase and caveolin-1 in human cirrhosis

Nitric oxide is synthesized in diverse mammalian tissues by a family of calmodulin-dependent nitric oxide synthases (NOS). Caveolin, the principal structural protein in caveolae, interacts with endothelial NOS (eNOS) leading to enzyme inhibition by a reversible process modulated by Ca++ -calmodulin. The aim of the present study was to examine the localizations of eNOS and caveolin-1 at protein level in normal human liver tissue, and how the expressions are altered in cirrhotic liver.

Fresh liver specimens were obtained from hepatic surgeries. Normal portions resected from cases of carcinoma metastasized to the liver were used as control specimens, and cirrhotic portions resected from cases of hepatocellular carcinoma with hepatitis C-related cirrhosis were used as cirrhotic specimens. Anti-eNOS and anticaveolin-1 antibodies were used for immunohistochemistry and Western blotting. Immunoelectron microscopy was conducted on ultra thin sections using immunoglobulin-gold combined with silver staining.

Immunohistochemistry revealed that both eNOS and caveolin-1 were sparsely expressed on hepatic sinusoidal lining in normal liver specimens, and these findings were confirmed by Western blot. Both immunohistochemistry and Western blotting demonstrated over-expression of eNOS and caveolin-1 in cirrhotic liver specimens. Morphometric analysis of immunogold particle labeling for eNOS and caveolin-1 was performed on immunoelectron micrographs. In normal liver tissue, hepatic stellate cells and sinusoidal endothelial cells (SEC) expressed low levels of caveolin-1, and SEC expressed a very low level of eNOS. In cirrhotic liver, both caveolin-1 and eNOS expressions were significantly increased by approximately four-fold on SEC compared to normal liver.

In cirrhotic human liver, marked increase of caveolin-1 in perisinusoidal cells may promote caveolin-eNOS binding and reduce the activity of eNOS despite an increased eNOS expression, leading to impaired NO production and increased hepatic microvascular tone.