Age alters expression and inducibility of heme oxygenase isozymes in mice

Heme oxygenase (HO) performs the rate limiting step in heme degradation and is induced by cell injury or stress. We wished to determine if dietary fat ty acid composition, increased age and/or an induced oxidative stress would alter the expression of HO-1 (constitutive and inducible isozyme) or of HO -2 (constitutive isozyme), in mouse liver, spleen and brain. Six- and 24-mo nth-old male B6C3F1 mice were fed AIN-76A diets containing either 5% corn o il (CO, moderately unsaturated, n=5 per age group) or 19% menhaden fish oil plus 1% corn oil (FO, highly polyunsaturated, n=20 per age group). After 2 weeks, 5 CO and 5 FO fed mice in each age group were sacrificed. The remai ning FO diet mice (n=15 per age group) were then challenged with a systemic oxidative stress by intraperitoneal injection of 125 mg iron/kg body weigh t as iron dextran. Five stressed mice from each age group were sacrificed 1 , 5, and 24 hours post injection; liver, spleen and brain were removed. Par t of each tissue was fixed in formalin, and microsomal protein isolated fro m the remaining tissue. HO-1 and HO-2 were detected by immunoblot of micros omal protein and by immunohistochemical staining of fixed tissue in the liv er and spleen, but only HO-2 was detected in the brain. There was no signif icant difference in HO-1 or HO-2 expression due to diet. The liver of old u nstressed mice had significantly more HO-1 than young mice. However, HO-1 w as significantly induced in the livers of young mice, but not of old mice, following oxidative stress. Spleen HO-1 expression was not significantly al tered by age or oxidative stress. HO-2 expression was not significantly alt ered by age or induced oxidative stress in any tissue examined. Age-related alterations in liver HO-1 isozyme expression and inducibility may contribu te to increased susceptibility to exogenous stress and disease.