Regulation of cytochrome P450 2E1 by hyperoxia and inflammatory factors, with special reference to the central nervous system

Sammanfattning: Cytochrome P450 2E1(CYP2E1) is a mammalian member of the large P450 gene superfamily. CYP2E1 is active in the metabolism of many small hydrophobic substrates, including general anesthetics, muscle relaxants, precarcinogens, and organic solvents. CYP2E1 is, however, active also in acetone hydroxylation and the enzyme is specifically regulated during conditions of fasting and during diabetic states, as well as by insulin. The pathway initiated by CYP2E1 by acetone hydroxylation may contribute up to 10% of gluconeogenetic demands. CYP2E1 is further characterized by a pronounced oxidase activity, and can generate substantial amounts of oxygen free radicals. It was hypothesized that CYP2E1 could contribute to oxygen toxicity during hyperoxia. Exposure of rats to 95% oxygen, originally a model for Adult Respiratory Distress Syndrome, lead to a 4-fold induction of CYP2E1 in liver and lung. A concomitant increase in NADPH dependent superoxide anion and H202 generation was seen. In addition, pretreatment of rats with the CYP2E1 inducer acetone aggravated the peroxidation of lipids, and decreased the survival time during hyperoxia. Next, we investigated the potential expression of CYP2E1 in the rat central nervous system, and found the enzyme to be present in distinct groups of neurons and glial cells. CYP2E1 mRNA and catalytic activity was detected, and CYP2E1 could be induced 4-fold by a low concentration of ethanol (25 mM) in cultured glial cells. Interestingly, the centrally acting muscle relaxant chlorzoxazone was oxidized with a Km of ~25 myM, which is lower than plasma concentrations of the drug (~ 60 myM) in humans. It was concluded that CYP2E1 substrates may be metabolically activated in situ in the brain Following ischemic or mechanical brain injury, CYP2E1 was induced in astrocytes in vivo, in the inflammatory phase. Addition of lipopolysaccharide or interleukin-1beta induced CYP2E1 up to 7-fold in cultured astrocytes. The induction was accompanied by an increase of CYP2E1 mRNA, an unusual mode of induction for this enzyme. Because inflammatory factors are very potent down regulators of hepatic CYP2E1, and P450s in general, the induction in astrocytes is tissue specific. This is, to my knowledge, the first example of such differential regulation among the drug-metabolizing P450s. Chlormethiazoie, an inhibitor of hepatic CYP2E1 transcription and a potent neuroprotective agent, inhibited the CYP2E1 induction jn astrocytes with an IC50 of 5 myM. Phorbol ester was also found to be an inducer in astrocytes, but probably acted by stimulating differentiation. Since both phorbol ester and lipopolysaccharide stimulated tyrosine phosphorylation in astrocytes, and both agents induced CYP2E1 in a way that could be inhibited by genistein, it is suggested that tyrosine phosphorylation is a necessary step during induction. It is concluded that CYP2E1 is inducible and active during inflammatory states in the lung and the brain and that CYP2E1 is subject to pronounced tissue specificity in the regulation by interleukins. It is suggested that CYP2E1 may be of importance for local energy provision in astrocytes, especially during inflammation.