In the regulation of cytochrome P450 genes, phenobarbital targets LKB1 for necessary activation of AMP-activated protein kinase

Abstract

Transcriptional activation of cytochrome P450 (CYP) genes and various drug metabolizing enzymes by the prototypical inducer phenobarbital (PB) and many other drugs and chemicals is an adaptive response of the organism to exposure to xenobiotics. The response to PB is mediated by the nuclear receptor constitutive androstane receptor (CAR), whereas the chicken xenobiotic receptor (CXR) has been characterized as the PB mediator in chicken hepatocytes. Our previous results suggested an involvement of AMP-activated protein kinase (AMPK) in the molecular mechanism of PB induction. Here, we show that the mechanism of AMPK activation is related to an effect of PB-type inducers on mitochondrial function with consequent formation of reactive oxygen species (ROS) and phosphorylation of AMPK by the upstream kinase LKB1. Gain-and loss-of-function experiments demonstrate that LKB1-activated AMPK is necessary in the mechanism of drug induction and that this is an evolutionary conserved pathway for detoxification of exogenous and endogenous chemicals. The activation of LKB1 adds a proximal target to the so far elusive sequence of events by which PB and other drugs induce the transcription of multiple genes.drug metabolism ͉ induction ͉ mitochondria ͉ reactive oxygen species E volution has provided organisms with an elaborate defense system against foreign compounds (xenobiotics). The liver of vertebrates contains numerous enzymes that can transform potentially toxic xenobiotics (e.g., nutrients or drugs) or endobiotics (e.g., bile acids) to inactive and excretable metabolites. The expression of these enzymes can be adapted to the needs for detoxification by a process called induction. Phenobarbital (PB) is the prototype of a number of drugs that induce their own and the metabolism of other xenobiotics. Induction of drug metabolism is part of a pleiotropic response of the liver to xenobiotic exposure, which includes liver hypertrophy, tumor promotion, and induction of numerous genes in addition to those encoding for drug-metabolizing enzymes and drug transporters (1). PB also was shown to decrease the transcription of gluconeogenic enzymes such as phosphoenolpyruvate carboxykinase 1 (PEPCK1) and glucose-6-phosphatase (G6P), and of several hepatic genes responsible for fatty acid metabolism (2). Moreover, PB increases the transcription of ␦-aminolevulinic acid synthase 1 (ALAS1), the rate-limiting enzyme in the synthesis of heme, the prosthetic group of cytochromes P450 (CYPs) (3, 4). The molecular details of the mechanisms by which PB causes these effects are incompletely understood.The transcriptional activation by PB of genes encoding drugmetabolizing enzymes, such as Cyp2b10 in mouse and CYP2B6 in human, is mediated by the nuclear receptor constitutive androstane receptor (CAR) (5). The interaction of PB with CAR is complex. PB apparently does not bind directly to CAR, but rather triggers its translocation from cytoplasm to the nucleus by as yet unknown mechanisms. In addition, phosphorylation and dephosphorylation events str...

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