Heme Oxygenase Activity and Heme Binding in a Neonatal Mouse Model

Abstract

Background: Severe hemolytic disease of the newborn leads to the release of pro-oxidative free heme (FH). Heme oxygenase (HO) is primarily responsible for detoxifying FH. Objective: To investigate the protective effects of HO in a model of heme overload. Methods: For in vitro studies, NIH3T3 HO-1-luc cells were incubated with 10, 30, or 60 µ<smlcap>M</smlcap> FH or methemalbumin (MHA). HO-1 promoter activity was assessed 3, 6, and 24 h after treatment. Cell survival was indexed by viability assays. For in vivo studies, 1- and 5-week-old wild-type (Wt) or HO-1-heterozygous (Het, HO-1^+/-) mice were given 60 µmol FH or MHA/kg intraperitoneally. After 24 h, plasma aspartate aminotransferease (AST)/alanine transaminase (ALT) and hemopexin, liver HO activity, and lipid peroxidation (LP) were determined. Results: In HO-1-luc cells, HO-1 promoter activity peaked 6 h after incubation with 30 µ<smlcap>M</smlcap> FH (1.6-fold) or 60 µ<smlcap>M</smlcap> MHA (2.1-fold) over baseline. Twenty-four hours after exposure to 60 µ<smlcap>M</smlcap> FH, a decrease in viability of 80% was found, compared with no decrease after exposure to 60 µ<smlcap>M</smlcap> MHA. In 1-week-old Wt and HO-1 Het pups given 60 µmol FH/kg, HO activity significantly increased 3.5- and 3.1-fold, respectively. No changes in LP or AST/ALT levels were observed. In adult Wt and HO-1 Het mice, HO activity increased (3.0- and 2.6-fold, respectively). LP and AST levels significantly increased 28.4- and 2.7-fold, respectively, in adult HO-1 Het mice. Hemopexin levels at baseline were higher in adults compared with newborns for both Wt and Het mice. In addition, FH induced hemopexin levels in both adults and newborns, but to a lesser degree in newborns. Conclusions: FH is highly toxic in vitro, but its toxicity is abolished when bound to albumin. Newborns appear to be protected from the pro-oxidative effects of FH, which may be mediated by heme binding and a higher absolute HO activity at baseline and after FH-mediated induction.