Murine cytochrome P450 2A5 (CYP2A5) is an interesting enzyme for its unique regulation and its involvement in liver injury caused by various well-known pathological conditions or hepatotoxins. It has been reported that CYP2A5 is upregulated following exposure to chemical hepatotoxins and during pathophysiological conditions in which the levels of most Cytochrome P450s are either unchanged or down-regulated. Recently bilirubin has been identified as the first endogenous substrate for CYP2A5 and it has been suggested that CYP2A5 plays a major role in bilirubin clearance as an alternative mechanism to BR conjugation by UGT1A1. This study investigated the mechanisms of gene regulation and cytoprotective role of CYP2A5 in response to bilirubin treatment in liver. Our results demonstrate that bilirubin induces CYP2A5 expression at the mRNA and protein levels by increasing CYP2A5 transcription via a mechanism that involves Nrf2 activation. Furthermore, our results suggest that induced CYP2A5 plays a cytoprotective role against bilirubin toxicity by directly lowering the cellular levels of bilirubin and by inhibiting caspase-3 activation.

Cadmium (Cd), is a non-essential metal with no known physiological function. It is known to alter redox state by disrupting the mitochondrial electron transport chain, as well as inactivating protein and non-protein thiols. It is thus believed that oxidative stress may comprise an important part of the mechanism of Cd toxicity. Accordingly, the initial cellular response to acute Cd exposure is defensive, where various anti-oxidant defence systems are triggered. One of the induced systems is the haem oxygenase-1 (HO-1). Its activation is mediated by the transcription factor Nrf2, which is the general regulator of cellular defence against oxidative stress. The protective effects of HO-1 are mediated, in part, through the generation of potent anti-oxidant bilirubin (BR) and its metabolites, which exploit the intrinsic antioxidant properties of these species at a cellular level. The oxidative metabolism of BR is an important route of detoxification in addition to glucuronidation. However, the major enzyme(s) involved in this oxidative degradation are not known. This thesis presents evidence for a major role of the hepatic cytochrome P450 2a5 (Cyp2a5) in BR degradation during Cd intoxication, where the BR levels are elevated following induction of HO-1. Treatment of DBA/2J male mice with CdCl2 induced both the Cyp2a5 and HO-1, and increased the microsomal BR degradation activity. By way of contrast, the total cytochrome P450 (CYP) content and the expression of Cyp1a2 were down-regulated by the treatment. The induction of the HO-1 and Cyp2a5 was significant at the mRNA, protein and enzyme activity levels. In each case, the up-regulation of the HO-1 preceded that of the Cyp2a5 with a 5-10 hr interval. In addition, BR totally inhibited the microsomal coumarin hydroxylase activity (a Cyp2a5-catalysed reaction) with an IC50 approximately equal to the substrate concentration. The MROD activity, catalysed mainly by the Cyp1a2, was inhibited up to 36% by BR. The microsomal BR degradation was inhibited by coumarin and by a monoclonal antibody against the Cyp2a5 by about 90%. In addition, 7-methoxyresorufin, a substrate for Cyp1a2, inhibited BR degradation activity by approximately 20%. A study using Nrf2 null mutant mice suggests that Cd-mediated induction of Cyp2a5 is dependent on the transcription factor Nrf2. Additionally, acute exposure to Cd activated localisation of Nrf2 from the cytoplasm to the nucleus. Furthermore, electrophoretic mobility shift assay (EMSA) analysis suggests that Cd induced sequence-specific binding of various species of the StRE-binding proteins on the 5’-flanking region of the Cyp2a5 gene. Collectively, these observations strongly suggest that BR may act as a substrate for the hepatic Cyp2a5, a major catalyst for BR degradation under conditions of substantial elevation of BR levels following induction of HO-1 by Cd. Secondly, the concurrent up-regulation of the HO-1 and Cyp2a5 during Cd-mediated injury implicates a coordinated regulation of two enzyme systems in the maintenance of balancing BR production and elimination. Finally, StRE-binding proteins, in particular Nrf2, may be involved in the regulation of the Cyp2a5 gene, which leads to the oxidation of BR. However, the respective roles of these factors in the regulation of the Cyp2a5 gene, as well as the coordinated regulation of ho-1 and Cyp2a5 genes remain an open question, requiring further investigations.