Limited Role for the Bilirubin-Biliverdin Redox Amplification Cycle in the Cellular Antioxidant Protection by Biliverdin Reductase

Abstract

In mammalian cells, heme is degraded by heme oxygenase to biliverdin, which is then reduced to bilirubin by biliverdin reductase (BVR). Both bile pigments have reducing properties, and bilirubin is now generally considered to be a potent antioxidant, yet it remains unclear how it protects cells against oxidative damage. A presently popular explanation for the antioxidant function of bilirubin is a redox cycle in which bilirubin is oxidized to biliverdin and then recycled by BVR. Here, we reexamined this putative BVR-mediated redox cycle. We observed that lipid peroxidation-mediated oxidation of bilirubin in chloroform, a model of cell membrane-bound bilirubin, did not yield biliverdin, a prerequisite for the putative redox cycle. Similarly, H 2 O 2 did not oxidize albumin-bound bilirubin to biliverdin, and in vitro oxidation of albumin or ligandin-bound bilirubin by peroxyl radicals gave modest yields of biliverdin. In addition, decreasing cellular BVR protein and activity in HeLa cells using RNA interference did not alter H 2 O 2 -mediated cell death, just as BVR overexpression failed to enhance protection of these cells against H 2 O 2 -mediated damage, irrespective of whether bilirubin or biliverdin were added to the cells as substrate for the putative redox cycle. Similarly, transformation of human BVR into hmx1 (heme oxygenase) mutant yeast did not provide protection against H 2 O 2 toxicity above that seen in hmx1 mutant yeast expressing human heme oxygenase-1. Together, these results argue against the BVR-mediated redox cycle playing a general or important role as cellular antioxidant defense mechanism.

Human Biliverdin Reductase Suppresses Goodpasture Antigen-binding Protein (GPBP) Kinase Activity

“…We propose the idea that GPS pathophysiology may be in part related to a suppressed hBVR activity and/or expression in the affected tissues that would result both in an attenuated ability to temper GPBP kinase activity and to generate the quencher of oxygen radicals, bilirubin (30). ROS have been shown to expose cryptic epitopes associated with autoimmune GPS (35) Furthermore, GPBP has been identified as a product of a human-specific TNF-␣-responsive transcriptional unit (4) and has been implicated in apoptotic pro-autoimmune signaling (44).…”

Plasma Bilirubin and UGT1A1*28 Are Not Protective Factors Against First-Time Myocardial Infarction in a Prospective, Nested Case–Referent Setting

“…34 Oxidation of bilirubin partly yields biliverdin, which is rapidly reduced back to bilirubin, but irreversible oxidation to other products also occurs. 35 These compounds, which are thought to promote vasospasm, are rapidly excreted in the urine. 36 However, in the present study, there was no distinct correlation with the inflammation marker hsCRP (Tables 4 and 5).…”

Elucidating the Role of Biliverdin Reductase in the Expression of Heme Oxygenase-1 as a Cytoprotective Response to Stress

“…A multitude of studies have implicated Nrf2 in the induction of HO-1 during oxidative stress (Alam et al, 2003;Gong and Cederbaum, 2006a;Gong and Cederbaum, 2006b;Sun Jang et al, 2009). Finally, in two studies looking at the effects of oxidative stress (by hydrogen peroxide treatment), it was found that cell viability was only marginally affected (Baranano et al, 2002) or not affected at all (Maghzal et al, 2009) by silencing BVR (with interference RNA). The latter study also showed that BVR induction and over-expression also did not provide protection to the cells exposed to hydrogen peroxide.…”

“…Thus, BVR may have an important role in extending the antioxidant potency of bilirubin. It should be noted that a recent study concluded that the cytoprotective role of this redox cycle was limited as BVR overexpression and inhibition of BVR expression with antisense RNA did not seem to influence hydrogen peroxide-mediated cytotoxicity (Maghzal et al, 2009). It is now known that BVR also functions as a dual-specific kinase of serine/threonine and tyrosine residues in proteins, and in this capacity, BVR affects the signaling and cellular responses to a variety of stimuli (Reviewed in (Kapitulnik and Maines, 2009)).…”