Heme oxygenase isoform-specific expression and distribution in the rat kidney

Background. The heme oxygenase (HO) genes, HO-1 and HO-2, are the limiting steps in heme degradation and in the regulation of renal heme-dependent enz ymes. Previously we reported that selective overexpression of renal HO-1 re sulted in a decrease of microsomal heme and the cytochrome P450-dependent a rachidonic acid metabolite, 20 HETE, a vasoconstrictor. The present study w as undertaken to explore the relative expression and contribution of each o f the HO isoforms to HO activity in the rat kidney. Methods and Results. Renal HO activity increased above control levels after an injection of the inducers of HO activity, heme or SnCl2. Stannous Mesop orphyrin (SnMP), a nonselective inhibitor of HO, when used alone or in comb ination with heme or SnCl2, decreased HO activity. Heme alone and combined with SnCl2 decreased the levels of heme content by 13 and 35%, respectively . Western blot analysis showed that both SnCl2 and heme readily induced HO- 1 protein, whereas HO-2 was constitutively expressed. Immunohistochemistry showed the distribution of the HO-1 isoform primarily in proximaI convolute d tubules. Western blot analysis exhibited relatively higher levels of HO-1 in isolated proximal tubules and relatively higher HO-2 levels in the thic k ascending limbs of the loop of Henle and preglomerular arterioles. In viv o administration of HO-1 and HO-2 antisense oligodeoxynucleotides further c onfirmed that HO-2, but not HO-1, contributed to the basal HO activity; how ever, following induction of HO with heme, antisense to HO-1, but not to HO -2, inhibited the induced levels of HO activity. Conclusion. These results suggest that HO-2 is constitutively expressed in the rat kidney mainly within tubular and arteriolar structures, and its act ivity may modulate physiological function under basal conditions. On the ot her hand, the basal levels of expression of HO-1 in the rat kidney are rela tively low, and its contribution to HO activity and the regulation of hemop roteins such as cytochrome P450 become apparent only under pathophysiologic al conditions causing HO induction.