Carbon monoxide as a guardian against hepatobiliary dysfunction

Carbon monoxide (CO) generated through the reaction of heme oxygenase (HO) has attracted great interest in regulation of hepatobiliary homeostasis. The gas generated by HO-2 in the hepatic parenchyma can modestly activate soluble guanylate cyclase (sGC) expressed in hepatic stellate cells in a paracrine manner and thereby constitutively relax sinusoids. Kupffer cells express HO-1, the inducible isozyme, even under normal unstimulated conditions and constitutes approximately 30% of the total HO activity in this organ. Upon exposure to a variety of stressors such as cytokines, endotoxin, hypoxia and oxidative stress, the liver induces HO-1 and over-produces CO. The stress-inducible CO has been shown to guarantee ample blood supply during detoxification of heme and thus to play a protective role in the liver. However, molecular mechanisms by which CO serves as a protectant for hepatocytes, the cells expressing little sGC, remain to be solved. Previous observation suggested that CO modulates intracellular calcium mobilization through inhibiting cytochrome P-450 activities and thereby maintain stroke volume of bile canalicular contraction in cultured hepatocytes. CO also stimulates mrp2-dependent excretion of bilirubin-IXalpha and helps heme catabolism. Although a direct molecular target responsible for the latter event remains unknown, such properties of CO could support xenobiotic metabolism through its actions on sinusoidal hemodynamics and hepatobiliary systems.