Cytochrome P450 in the rat brain : characterization and regulation

Abstract

Cytochrome P450s (CYPs) are key enzymes in the metabolism of cholesterol and fatty acids and their derivatives in the body. In the adrenals and gonads these enzymes are involved in steroid hormone biosynthesis, in the liver in bile acid synthesis, in the kidney in synthesis of the calcitropic hormone, calcitriol, and in the cardiovascular system in synthesis of thromboxane A2, a potent inducer of vasoconstriction and platelet aggregation. In addition, in the liver, lungs, intestines and kidneys, members of this superfamily of enzymes are involved in metabolism of dietary components, therapeutic drugs, environmental pollutants and solvents, a group of chemicals collectively referred to as xenobiotics. The level of CYPs in the brain is low, but the pattern of isozymes expressed is unique. This suggests specific functions for brain CYPs, but very few such functions have been defined.

The aims of this thesis were to characterize the isozymes in the brain, to identify cells which harbor these enzymes and to determine how these enzymes are regulated in the brain, in particular why many inducers of liver CYPs do not induce brain CYPs. We found that the hepatic CYP isozymes, CYP2B1/2 and CYP3A1/2 proteins, could not be detected in the brain, and concluded that published evidence from immunohistochemical studies indicating the abundant expression of these enzymes in the brain were methodological artifacts. With protein microsequencing we identified several CYP isozymes in the brain, including. CYP2C22, CYP2C24, CYP3A13, CYP4F1, CYP4F4 and CYP4F6. CYP2C22 was particularly interesting since it was highly expressed in the newborn and was sexually differentiated in the adult brain. Since one of its substrates was dihydroprogesterone the inference was made that this enzyme may be involved in regulating the level of anesthetic steroids (5[alpha]-reduced metabolites of progesterone) in the brain.

CNS-active drugs, such as clozapine, mianserin and sulpiride, as well as the solvent toluene increased the overall level of CYPs in the brain. Specific isozymes were induced but there were no measurable changes in the level of mRNA in any of the induced CYPs. From this we concluded that posttranscriptional mechanisms are important in regulating brain CYP levels. In order to understand why the transcriptional mechanisms which are so prominent in regulating liver CYPs do not apply to the brain, we investigated the pregnane X receptor (PXR) in brain. This receptor, which is involved in the induction of xenobiotic-metabolizing forms of CYP in liver, was expressed in the brain, but the predominant splice variant in brain was PXR.2. This receptor variant has a shortened ligand binding domain and is activated by fewer ligands than the liver predominant form, PXR.1. The presence of PXR.2 in brain may be one likely explanation for the observation that many drugs which induce CYPs in the liver are not inducers in the brain. Finally, with immunohistochemical methods we found that although the overall level of CYP in the brain is low, certain specific isozymes are highly expressed in selected neurons. These enzymes may therefore play important roles in neuronal function.