Bilirubin and the risk of common non-hepatic diseases Igino Rigato 1,2 , J. Donald Ostrow 2 and Claudio Tiribelli 1 1 Centro Studi Fegato, Bldg. Q, AREA Science Park, Basovizza and Department of BBCM, University of Trieste, 34012 Trieste, Italy 2 Gastroenterology/Hepatology Division, University of Washington School of Medicine and VA Puget Sound Health Care System/ Seattle Division, Seattle, WA 98195-6424, USA Bilirubin is a potent antioxidant but can be toxic at high concentrations. This article critically reviews the reported relationships of plasma bilirubin levels to the severity and/or incidence of various common non- hepatic diseases. Plasma bilirubin levels are reportedly negatively related to the risk of atherosclerotic diseases, cancers, demyelinating neuropathies and seasonal affective disorder. By contrast, the incidence and severity of schizophrenia are increased by elevated bilirubin levels. The data strongly suggest that the level of plasma bilirubin should be considered as a risk factor for several common non-hepatic diseases. Additional studies are needed to clarify the mechanisms of this influence, which are thought to be related to unconjugated bilirubin counteracting the oxidative stress underlying these disorders. Introduction Unconjugated bilirubin (UCB), which is the major degra- dation product of heme, was long considered to be simply a toxic metabolite that must be cleared from the organism. During the past two decades, it has become appreciated [1] that UCB is a ‘boon’ at physiological and mildly elevated concentrations (Box 1), owing to its potent protective action against oxidant cell damage [2], but might be a ‘curse’ at more elevated, yet still clinically relevant, concentrations [3], as a result of its cytotoxic effects, especially in the central nervous system (CNS) [3,4]. These considerations suggest that the plasma (and therefore tissue) levels of UCB might affect the incidence and/or severity of various diseases. Here, we critically examine the existing experimental evidence in vitro and in animal models and epidemiological evidence from human subjects that suggest protective or harmful effects of UCB in non-hepatic diseases. As a background for this survey, we first briefly summarize the properties of UCB and the proposed molecular mechanisms of its cytotoxic and cytoprotective effects. Physicochemical properties of UCB The pK a value is an important medical issue. Solvent- partition studies suggested that the solubility of UCB was 70 nM at pH 7.4 and that the pK a values of the two carboxyl groups were both O8.0 [5]. The pK a values have also been reported to be below 5.0, although this was determined by a different method [6]. If the pK a values are above 8.0, over 80% of unbound UCB (Bf) is in the diacid form at physiological pH and there is almost no dianion; by contrast, if the pK a values are below 5.0, then the UCB dianion predominates. The diacid is the UCB species that preferentially binds to [7] and freely diffuses through [8] phospholipid membranes, and presumably has the pre- dominant role in the effects of the pigment on cellular function [3,4]. UCB in plasma is tightly bound to albumin, but recent studies have shown that the affinity decreases markedly with increasing albumin and chloride concentrations [9,10]. Using these corrected binding affinities of UCB for human albumin, it was realized that damage can be observed in neurons and astrocytes in vitro when Bf concentrations in the medium only modestly exceeds its aqueous solubility of 70 nM [3]. The neurotoxicity of UCB is dose dependent, causing apoptosis at moderately elevated Bf concentrations and cell necrosis at higher Bf levels [3,11]. Cytoprotection by UCB in vitro The cytotoxic effects of UCB, particularly in the nervous system, have been reviewed recently, and will not be discussed here [4]. At physiological and slightly elevated concentrations, when Bf is below aqueous saturation, UCB has a significant cytoprotective action. This is thought to be mediated by its antioxidant properties [2,12], which are shared by all the bile pigments found in plasma and tissues, including UCB, conjugated bilirubins and biliverdin, whether unbound or bound to albumin [13]. Box 1. UCB is not simply a waste product, but has intriguing effects on the health of subjects Unconjugated bilirubin (UCB) has been regarded until recently as a waste product of heme metabolism that can be toxic if accumulated at high concentrations, particularly in the brains of newborns. At low physiological levels, however, UCB is a potent antioxidant that apparently protects individuals against common diseases related to oxidant stress. This is supported by a strong negative correlation between serum bilirubin levels and several non-hepatic diseases, such as atherosclerosis (in particular coronary artery disease), cancers, demyelinating neuropathies and seasonal affective dis- order. An exception is schizophrenia, which is more frequent and severe when the plasma UCB is increased. Corresponding author: Tiribelli, C. (ctliver@csf.units.it). Available online 12 May 2005 Opinion TRENDS in Molecular Medicine Vol.11 No.6 June 2005 www.sciencedirect.com 1471-4914/$ - see front matter Q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.molmed.2005.04.008