BILIVERDIN REDUCTASE B/ FLAVIN REDUCTASE (BLVRB/FLR) EXHIBITS HEME-REGULATED, NADPH-DEPENDENT REDUCTASE ACTIVITY AND CONFERS CYTOPROTECTION IN DEVELOPING ERYTHROID CELLS

Abstract

Biliverdin reductase B (BLVRB), also known as flavin reductase (FLR), is especially expressed in cells having high levels of heme synthesis (e.g. developing erythroid cells, fetal and adult liver). BLVRB exhibits NADPH-dependent reductase activity toward non-alpha isomers of biliverdin, flavins and redox cycling agents, and also has pharmacological importance in the treatment of methemoglobinemia. Nonetheless, the physiological function of BLVRB is obscure. In this work, the functional role of BLVRB in developing erythroid cells in relation to its heme-associated properties is elucidated. Recombinant mouse BLVRB binds ferric heme with a submicromolar affinity in association with inhibition of the NADPH-dependent reductase activity while displaying heme peroxidase activity. The interaction of BLVRB with heme in developing erythroid cells (MEL) was demonstrated by proteomic analysis of heme peroxidase-associated proteins. Upon induction of erythroid differentiation of MEL cells, the BLVRB protein expression is markedly up-regulated. Knockdown of BLVRB in the induced MEL cells aggravates intracellular oxidation and cytotoxicity mediated by thiol depleting agent and heme synthesis inhibitor, suggesting that BLVRB may maintain the cell viability under oxidative stress or heme deficient conditions. Our study suggests that BLVRB/FLR, which is usually regarded as a biliverdin reducing enzyme in the heme degradation pathway, is an erythroid active, heme-regulated NADPH-dependent reductase. It confers cytoprotection in developing erythroid cells under oxidative stress conditions in connection with cellular heme levels. This work was supported by General Research Fund (17160916) and Health and Medical Research Fund (04151846).