Bilirubin Production and Hemeoxygenase-1 Expression in Ischemia-Reperfused Rat Liver in vivo

Kupffer cells constitute a major source of heme oxygenase (HO)-1, a heme-degrading enzyme. This study aimed to examine roles of Kupffer cells in the modulation of accelerated heme catabolism in ischemia-reperfused rat livers. Livers treated with or without liposome-encapsulated clodronate, a Kupffer cell-depleting reagent, underwent a 20-min ligation of the portal vein followed by reperfusion (I/R), and the time course of the biliary output of bilirubin, the terminal heme-degrading product, and the expression of HO-1 mRNA and protein were monitored. HO-1 mRNA levels were elevated at 3-12hrs after I/R in both control and Kupffer cell-depleted rats. Immunohistochemistry in the controls revealed that Kupffer cells constitute a major cellular component expressing HO-1, while hepatocytes exhibited little expression. On the other hand, in Kupffer cell-depleted livers, periportal hepatocytes displayed a marked HO-1 expression. Under these conditions the two groups exhibited distinct profiles of biliary bilirubin excretion: In the controls, total bilirubin excretion increased 8-fold and peaked at 10hrs after I/R. On the other hand, the Kupffer cell-depleting treatment significantly accelerated the initial rise in bilirubin which peaked at 4hrs, and the total amount of bilirubin excreted within the initial 10hrs after reperfusion was reduced by 50% as compared with the controls. These results suggest that Kupffer cells serve as an I/R sensor that upregulates heme-degrading capacity and ameliorates excessive excretion of bilirubin into bile, functioning as a sink-buffer of bilirubin metabolism.