Inactivation of the Hepatic Cytochrome P450 System by Conditional Deletion of Hepatic Cytochrome P450 Reductase

Abstract

Cytochrome P450 (CYP) monooxygenases catalyze the oxidation of a large number of endogenous compounds and the majority of ingested environmental chemicals, leading to their elimination and often to their metabolic activation to toxic products. This enzyme system therefore provides our primary defense against xenobiotics and is a major determinant in the therapeutic efficacy of pharmacological agents. To evaluate the importance of hepatic P450s in normal homeostasis, drug pharmacology, and chemical toxicity, we have conditionally deleted the essential electron transfer protein, NADH:ferrihemoprotein reductase (EC 1.6.2.4, cytochrome P450 reductase, CPR) in the liver, resulting in essentially complete ablation of hepatic microsomal P450 activity. Hepatic CPR-null mice could no longer break down cholesterol because of their inability to produce bile acids, and whereas hepatic lipid levels were significantly increased, circulating levels of cholesterol and triglycerides were severely reduced. Loss of hepatic P450 activity resulted in a 5-fold increase in P450 protein, indicating the existence of a negative feedback pathway regulating P450 expression. Profound changes in the in vivo metabolism of pentobarbital and acetaminophen indicated that extrahepatic metabolism does not play a major role in the disposition of these compounds. Hepatic CPR-null mice developed normally and were able to breed, indicating that hepatic microsomal P450-mediated steroid hormone metabolism is not essential for fertility, demonstrating that a major evolutionary role for hepatic P450s is to protect mammals from their environment. The hepatic cytochrome P450 (CYP)1 -dependent monoxygenase system plays a central role in mammalian defense against harmful environmental chemicals (1); it is also a major determinant of the half-life and pharmacological properties of therapeutic drugs and in certain cases, mediates the activation of drugs, toxins, and carcinogens to their ultimate toxic species (2, 3). Several other functions have been ascribed to hepatic P450s, including control of cholesterol and steroid hormone metabolism and bile acid biosynthesis (4). However, for certain of these pathways, the exact role of P450s in normal homeostasis is unknown.Over the last four decades, there have been significant advances in understanding the functions, genetics, and regulation of these enzymes and more recently their structure (5). However, a great deal remains to be learned about the expression and regulation of P450s, and their endogenous function(s), particularly in individual tissues. The size and diversity of the P450 multigene family results in great difficulties in dissecting out the function(s) of individual enzymes, particularly as many of those involved in foreign compound metabolism exhibit overlapping substrate specificities and may be expressed to a greater or lesser extent in almost every cell and tissue. The contribution that P450s in any particular tissue make to the overall pharmacokinetics of a drug is still, in the majority of cases,...

The relationship between DNA adduct formation by benzo[a]pyrene and expression of its activation enzyme cytochrome P450 1A1 in rat

“…Using the Hepatic P450 Reductase Null (HRN) and the Reductase Conditional Null (RCN) mouse model we also showed that hepatic CYP enzymes appear to be more important for detoxification of BaP in vivo (Arlt et al, 2008;. In HRN and RCN mice NADPH:CYP oxidoreductase, the essential electron donor to CYPs is deleted specifically in hepatocytes, resulting in a decrease in hepatic CYP function (Henderson et al, 2003;Finn et al, 2007). We found however that the levels of dG-N 2 -BPDE adducts in livers of these mice treated with BaP were higher than in WT mice (Arlt et al, 2008;.…”

Application of Osmotic Pumps for Sustained Release of 1-Aminobenzotriazole and Inhibition of Cytochrome P450 Enzymes in Mice: Model Comparison with the Hepatic P450 Reductase Null Mouse

“…The HRN mouse only has residual hepatic P450 activity as a result of a liver-specific knockout of the P450 reductase enzyme during the postnatal period (Henderson et al, 2003), and extrahepatic P450 is unaffected. Marked differences in drug disposition have been observed in HRN mice compared with wild-type mice (Pickup et al, 2012;Boggs et al, 2014;Grimsley et al, 2014).…”

Hepatic cytochrome P450 deficiency in mouse models for intrahepatic cholestasis predispose to bile salt-induced cholestasis

“…18 All cytochrome P450s receive an electron from a single donor, a reaction mediated by the cytochrome P450 oxidoreductase (Cpr). 19,20 Whole body deletion of cytochrome P450 oxidoreductase inactivates all cytochrome P450s and is embryonic lethal. 21 Henderson et al 20 have generated a hepatocyte-specific cytochrome P450 oxidoreductase knockout mouse, also termed hepatic reductase null (Hrn) mouse.…”

“…19,20 Whole body deletion of cytochrome P450 oxidoreductase inactivates all cytochrome P450s and is embryonic lethal. 21 Henderson et al 20 have generated a hepatocyte-specific cytochrome P450 oxidoreductase knockout mouse, also termed hepatic reductase null (Hrn) mouse. In this model, bile salt synthesis and (re)hydroxylation are reduced by 95%.…”