Heme Oxygenase-1: A Potential Modulator of Inflammation and Vaso-Occlusion in Sickle Cell Disease.

Hemolysis in sickle cell patients leads to elevated plasma hemoglobin S and bilirubin, increased numbers of circulating reticulocytes, reduced plasma haptoglobin and hemopexin, increased kidney and endothelial cell HO-1 expression and carbon monoxide (CO) production. Plasma hemoglobin S can transfer heme to endothelium thereby enhancing the production of reactive oxygen species (ROS), activating vascular endothelium and inducing the cytoprotective enzyme heme oxygenase-1 (HO-1). We hypothesize that HO-1, an adaptive, anti-inflammatory gene, and its downstream products, play a vital role in the inhibition and resolution of vaso-occlusion in sickle cell disease. S+S Antilles transgenic sickle mice have an activated vascular endothelium, that includes enhanced nuclear factor kappa B (NF-kB) activation and endothelial cell adhesion molecule (ECAM) expression. These transgenic sickle mice also hemolyze in vivo as evidenced by increased reticulocyte counts (10.2%), plasma hemoglobin (2.3 mg/dl) and bilirubin (1.7 mg/dl) and reduced plasma haptoglobin compared to normal control mice (p<0.01). HO-1 expression was increased in the lungs (5.8 fold), kidneys (6.5 fold) and spleens (3.4 fold) of sickle mice compared to normal mice (p<0.05). Treatment of sickle mice with hemin (40 μmoles/kg i.p./d x 3) further increased HO-1 expression in the lung, liver, spleen and kidney. HO-1 upregulation by hemin treatment decreased lung, liver and kidney NF-kB over-expression in sickle mice by 56–80% (p<0.05). VCAM-1 and ICAM-1 over-expression in sickle mouse livers was reduced by 50% and 70% respectively, after hemin treatment (p<0.05). Treatment of sickle mice with an HO-1 inhibitor, tin protoporphyrin, had no effect on liver VCAM-1 expression, but increased liver ICAM-1 expression by 75% (p<0.05). Treatment of sickle mice with the HO-1 gaseous enzymatic product, carbon monoxide (CO, inhaled at 250 ppm for 1h for 3d) diminished NF-kB activation in the liver by 70% and the kidney by 40% (p<0.05). Leukocyte rolling and adhesion in the dorsal skin venules of sickle mice were inhibited by 70% (p<0.01) after global HO-1 upregulation by hemin. In response to hypoxia/rexoygenation, blood flow in venules of the dorsal skin became static. Upregulation of HO-1 by hemin, or treatment with HO-1 products biliverdin (50 μmoles/kg/d x 3) or CO prevented stasis (p<0.05), while HO-1 inhibition by Sn-protporphyrin significantly prolonged and worsened vaso-occlusion (p<0.05). We hypothesize that HO-1 modulates vaso-occlusion through multiple mechanisms including NF-kB inhibition, endothelial cell adhesion molecule down-regulation, decreased RBC hemolysis and altered vascular tone. In sickle cell disease patients, adaptation of HO-1 activity may be cytoprotective, suggesting that normal levels of HO-1 are inadequate to handle the excessive heme burden. We believe that further upregulation of HO-1 activity and/or its downstream products will be important strategies for preventing and treating vaso-occlusion in sickle cell disease.