The Ah locus: biochemical basis for genetic differences in brain tumor formation in mice

Tissue- and cell type-specific expression of cytochrome P450 1A1 and cytochrome P450 1A2 mRNA in the mouse localized in situ hybridization

We used in situ hybridization to examine organ- and cell type-specific constitutive and 3-methylcholanthrene (3MC)-inducible cytochrome P450 (CYP)1A1 and CYP1A2 mRNA expression in various tissues of the C57BL/6N mouse. In situ hybridization was carried out 10 hr after the mice had received intraperitoneal 3MC, or vehicle alone. We detected levels of 3MC-induced CYP1A1 mRNA in: liver (centrilobular, more so than periportal, regions); lung (Clara Type II cells much more than Type I epithelial cells); brain, especially endothelial cells lining the vascular surface of the choroid plexus; the digestive tract (duodenum > jejunum > ileum > colon > esophagus > stomach--in particular, the villous epithelium, plus cells surrounding glands in the lamina propria); renal corpuscles of the kidney; the ovary (medulla more so than cortex); and the endothelial cells of blood vessels throughout the animal. Constitutive CYP1A1 mRNA was not detectable by in situ hybridization in any of these tissues. In contrast, constitutive CYP1A2 mRNA was measurable in liver, and 3MC-inducible CYP1A2 mRNA was observed only in liver, lung, and duodenum (having cell-type locations similar to those of CYP1A1); the other above-mentioned tissues were negative for CYP1A2 mRNA. These data demonstrate the striking differences in tissue- and cell type-specific expression between the two members of the mouse Cypla subfamily. Because of the ubiquitous nature of 3MC-inducible CYP1A1 throughout the animal rather than just "portals of entry," these results support our hypothesis that CYP1A1, induced by particular endogenous signals in various tissues and cell types, might participate in one or more critical life processes--in addition to its well-established role of metabolism of polycyclic hydrocarbons, certain drugs, and other environmental pollutants.

Expression analysis of the mixed function oxidase system in rat brain by the polymerase chain reaction

Metabolism of therapeutic drugs in the body by the mixed function oxidase system is an important consideration in the analysis of a drug's effectiveness. P450-dependent metabolism within the brain of a neuro-specific drug may affect the drug's course of action. To determine whether cytochrome P450 was expressed in brain, RNA was isolated from the whole brains of rats treated with a variety of known hepatic P450 inducers, including amitriptyline, imipramine, isosafrole, phenobarbital, and beta-naphthoflavone. The RNA was analyzed for the presence of P450 isozymes by the PCR technique. Differential expression of P450IA1, P450IIB1, P450IIB2, P450IID, and P450IIE1 was detected in the brain samples, depending on the treatment. Cytochrome P450 reductase expression was also detected in the brain samples, giving strong evidence that the brain contains a competent mixed function oxidase system under all conditions studied.

Characterization of cerebral 7-ethoxycoumarin-O-deethylase: evidence for multiplicity of cytochrome P450 in brain

Characterization of 7-ECOD regulated by a number of cytochrome P450 species in liver has been undertaken in brain to understand whether it can metabolize a wide variety of substrates. Maximum activity of the enzyme was recorded when 0.8 to 2.0 mg of brain microsomal protein was added to the assay mixture at pH 7.4 and incubated for 30 min. The enzyme activity was found to be localized in the microsomal fraction with the maximum activity being in the olfactory lobes and the minimum in the pons medulla region. Kinetic studies revealed that rat brain microsomes exhibited a monophasic pattern of enzyme activity with a Km value of 2 mM. Rats pretreated with phenobarbital exhibited no change in enzyme activity whereas 3-methylcholanthrene preferentially stimulated enzyme activity (two- to threefold). The differential induction of enzyme activity after MC pretreatment and the differential response of alpha-naphthoflavone and metyrapone in vitro on 7-ECOD activity in control and MC-induced microsomes of rats, mice, guinea pigs, and rabbits do suggest the possibility of the presence of multiple forms of cytochrome P450 in the brain. Significant age, sex, and species-related differences observed in the activity of ECOD have also indicated that different forms of cytochrome P450 may be regulating the enzyme activity in the brain.

On the choice of subject populations for research in neurobehavioral toxicology

The choice of experimental subjects for research in neurobehavioral toxicology influences the questions that may be asked and the generality of findings obtained. In particular, the relative merits of "random bred" animals, heterogeneous stocks, selected lines, and inbred strains are discussed. Several aspects of genetic-environmental coaction that may influence data are described, and general issues for their analyses discussed.