Effects of hemoglobin on heme oxygenase gene expression and viability of cultured smooth muscle cells

Ferrous Hb contributes to cerebral vasospasm after subarachnoid hemorrhage, although the mechanisms involved are uncertain. The hypothesis that cytoto xic effects of ferrous Hb on smooth muscle cells contribute to vasospasm wa s assessed. Cultured rat basilar artery smooth muscle cells were exposed to pure Hb, dog erythrocyte hemolysate, or Hb breakdown products; and heme ox ygenase (HO-1 and HO-2) and ferritin mRNA and protein were measured. Cytoto xicity was assessed by lactate dehydrogenase release and fluorescence assay s. Pure Hb or hemolysate caused dose- and time-dependent increases in HO-1 mRNA and protein. Hemin was the component of Hb that increased HO-1 mRNA. C ycloheximide inhibited the increase in HO-1 mRNA in response to hemin. Ferr itin protein heavy chain but not mRNA increased upon exposure of cells to H b. Hemin and ferric but not ferrous Hb were toxic to smooth muscle cells. T oxicity was increased by exposure to Hb plus tin protoporphyrin IX. In conc lusion, exposure of smooth muscle cells to Hb induces HO-1 mRNA and protein through pathways that involve new protein synthesis. Hemin is the componen t of Hb that induces HO-1. Hemin and ferric but not ferrous Hb are toxic.