Diabetic retinopathy is a leading cause of visual loss and blindness, characterized by microvascular dysfunction. Hyperglycemia is considered the major pathogenic factor for the development of diabetic retinopathy and is associated with increased oxidative/nitrosative stress in the retina. Since heme oxygenase-1 (HO-1) is an enzyme with antioxidant and protective properties, we investigated the potential protective role of HO-1 in retinal endothelial cells exposed to high glucose and oxidative/nitrosative stress conditions. Retinal endothelial cells were exposed to elevated glucose, nitric oxide (NO) and hydrogen peroxide (H2O2). Cell viability and apoptosis were assessed by MTT assay, Hoechst staining, TUNEL assay and Annexin V labeling. The production of reactive oxygen species (ROS) was detected by the oxidation of 2′,7′-dichlorodihydrofluorescein diacetate. The content of HO-1 was assessed by immunobloting and immunofluorescence. HO activity was determined by bilirubin production. Long-term exposure (7 days) of retinal endothelial cells to elevated glucose decreased cell viability and had no effect on HO-1 content. However, a short-time exposure (24 h) to elevated glucose did not alter cell viability, but increased both the levels of intracellular ROS and HO-1 content. Moreover, the inhibition of HO with SnPPIX unmasked the toxic effect of high glucose and revealed the protection conferred by HO-1. Oxidative/nitrosative stress conditions increased cell death and HO-1 protein levels. These effects of elevated glucose and HO inhibition on cell death were confirmed in primary endothelial cells (HUVECs). When cells were exposed to oxidative/nitrosative stress conditions there was also an increase in retinal endothelial cell death and HO-1 content. The inhibition of HO enhanced ROS production and the toxic effect induced by exposure to H2O2 and NOC-18 (NO donor). Overexpression of HO-1 prevented the toxic effect induced by H2O2 and NOC-18. In conclusion, HO-1 exerts a protective effect in retinal endothelial cells exposed to hyperglycemic and oxidative/nitrosative stress conditions.

This communication reports on the first electrochemical study of the human flavin-containing monooxygenase 3 (hFMO3) either absorbed or covalently linked to different electrode surfaces. Glassy carbon and gold electrodes gave reversible electrochemical signals of an active hFMO3. The midpoint potential measured for the immobilized enzyme on a glassy carbon electrode was -445 +/- 8 mV (versus Ag/AgCl). A monolayer coverage was obtained on gold functionalized with dithio-bismaleimidoethane that covalently linked surface accessible cysteines of hFMO3. A structural model of the enzyme was generated to rationalize electrochemistry results. The turnover of the active enzyme was measured with two specific drugs: tamoxifen and benzydamine. For tamoxifen, 1.7 and 8.0 microM of its N-oxide product were formed by the enzyme immobilized on glassy carbon and gold electrodes, respectively. In the case of benzydamine, a K(M) of 44 +/- 5 microM was measured upon application of a -600 mV bias to the enzyme immobilized on the glassy carbon electrode that is in good agreement with the values published for microsomal hFMO3 where NADPH is the electron donor.