Cytoprotective Mechanisms of the Lung in Sickle Cell Disease.

Abstract 1538

Poster Board I-561

Circulating plasma hemoglobin contributes to major vasculopathies including pulmonary hypertension in patients who have sickle cell disease (SCD). There is an emerging concept that such vasculopathies are relatively mild because of activation of several cytoprotective pathways in SCD. The biochemical profile of plasma and the transcriptome of peripheral blood cells in patients who have SCD offer indirect support for this concept. Indeed, heme oxygenase-1 (HO-1), an acute phase enzyme that degrades heme into intermediates and byproducts with vasculoprotective properties is markedly elevated in mononuclear leukocytes of patients who have SCD. Nonetheless, the scope of the cytoprotective mechanisms of the lung and other organs impacted directly by sickle vasculopathies remain poorly appreciated. We previously identified an array of cytoprotective enzymes in lung endothelial cells chronically exposed to non-toxic concentrations of hemin in vitro. In this study, we examined the expression of NAD(P)H oxidase and candidate cytoprotective enzymes in two models of transgenic mice with SCD, and examined HO-1 expression in sickle chronic lung disease. Although NAD(P)H oxidase catalyzes reactive oxygen species generation by heme and is responsible for increased adhesion of leukocytes to the endothelium in SCD mice, there was no elevation of any of its subunits (gp91^Phox and p22^Phox, p47^Phox, p67^Phox and p40 ^Phox) in sickle mice lungs compared to hemizygote control mice lungs. Quantitative RT-PCR analysis revealed unexpectedly no difference in HO-1 mRNA level in sickle and non-sickle control lungs. On the contrary, analysis of the same tissues showed significantly higher NAD(P)H quinone oxidoreductase-1 (NQO1) mRNA level in both Berkeley and Townes knock-in sickle mice compared to controls (p<0.001). Enhanced expression of NQO1 but not HO-1 in sickle mice lungs was confirmed at the protein level by western blot analysis and by immunohistochemistry. Finally, we studied cases of sickle chronic lung disease (n=17) and discovered that NQO1 is expressed in the pulmonary endothelium at significantly higher levels than in normal lung tissues (p<0.002). In agreement with the data in mice, we found no difference in HO-1 staining in sickle human lung and normal lung tissues. Our findings indicate that despite its established role in acute elimination of excess heme, HO-1 is not elevated in sickle lungs. This study highlights the importance of dissecting the cytoprotective phenotype of individual organs impacted by SCD towards a rationale approach to developing efficacious antioxidant therapy for this disorder.