Biliverdin inhibits activation of NF-κB: Reversal of inhibition by human biliverdin reductase

Abstract

hBVR functions in the cell as a reductase and as a kinase. In the first capacity, it reduces biliverdin, the product of HO activity, to the effective intracellular antioxidant, bilirubin; as a dual-specificity kinase (S/T/Y) it activates the MAPK and IGF/IRK receptor signal transduction pathways. NF-κB and the MAPK pathway are activated by ROS, which results in the activation of stress-inducible genes, including ho-1. Presently, we report on the negative effect of biliverdin on NF-κB activation and the converse effect of hBVR. Biliverdin, in a concentration- and time-dependent manner, inhibited transcriptional activity of NF-κB in HEK293A cells. Nuclear extracts from biliverdin-treated cells show reduced DNA binding of NF-κB in an electromobility shift assay, whereas extracts from cells treated with TNF-α showed enhanced binding. Coimmunoprecipitation data show hBVR binds to the 65 kDa subunit of NF-κB, and that this is dependent on activation by TNF-α. Overexpression of hBVR enhanced both the basal and TNF-α-mediated activation of NF-κB and also that of the NF-κB-activated iNOS gene. Also, overexpression of hBVR arrested the cell cycle in the G1/G0 phase and reduced the number of cells in S phase. Similar results were observed with MCF-7 cells. Because of the Janus nature of NF-κB activity in the cell and the inhibitory action of biliverdin, the present findings provide a foundation for therapeutic intervention in inflammatory diseases and cancer that may be attained by preventing reduction of biliverdin. On the other hand, by increasing BVR levels beneficial functions of NF-κB might be augmented. © 2007 Wiley-Liss, Inc.