Abstract
The heme oxygenase (HO) enzymes catalyze the rate-limiting step of heme breakdown, and may accelerate oxidative injury to neurons exposed to heme or hemoglobin. HO-1 and HO-2 are activated in vitro by the phos-phatidylinositol 3-kinase (PI3K)/Akt and protein kinase C (PKC)/CK2 pathways, respectively. The present study tested the hypotheses that CK2, PKC, and PI3K inhibitors would reduce both HO activity and neuronal vulnerability to hemoglobin in murine cortical cultures. Oxidative cell injury was quantified by LDH release and malondialdehyde assays. HO activity was assessed by carbon monoxide assay. Consistent with prior observations, treating primary cortical cultures with hemoglobin for 16h resulted in release of approximately half of neuronal LDH and a seven-fold increase in malondialdehyde. Both endpoints were significantly reduced by the CK2 inhibitors 4,5,6,7-tetrabromobenzotriazole (TBB) and 2-dimethyl-amino-4,5,6,7-tetrabromo-1H-benzimidazole (DMAT), and by the PKC inhibitor GF109203X; the PI3K inhibitors LY294002 and wortmannin had no effect. None of these inhibitors altered basal HO activity. The 1.9-fold activity increase observed after hemoglobin treatment was largely prevented by LY294002 and LY303511, a structural analog of LY294002 that does not inhibit PI3K activity. It was not reduced by wortmannin, TBB or GF109203X. These results suggest that the protective effect of CK2 and PKC inhibitors in this model is not dependent on reduction in HO activity. In this culture system that expresses both HO-1 and HO-2, HO activity does not appear to be primarily regulated by the PKC/CK2 or PI3K pathways.