Molecular cloning of cDNA for guinea pig CYP1A2 comparison with guinea pig CYP1A1

Alternative splicing within the human cytochrome P450 superfamily with an emphasis on the brain: the convolution continues

The human cytochrome P450 (CYP) superfamily of enzymes regulate hepatic phase 1 drug metabolism and subsequently play a significant role in pharmacokinetics, drug discovery and drug development. Alternative splicing of the cytochrome CYP gene transcripts enhances gene diversity and may play a role in transcriptional regulation of certain CYP proteins. Tissue-specific alternative splicing of CYPs is significant for its potential to add greater dimension to differential drug metabolism in hepatic and extrahepatic tissues, such as the brain, and to our understanding of the CYP family. This review provides an overview of tissue-specific splicing patterns, splicing types, regulation and the functional diversities between liver and splice variant CYP proteins and further explores the relevance of tissue-specific alternative splicing of CYPs in the nervous system.

Human and animal hepatocytes in vitro with extrapolation in vivo

Human and animal hepatocytes are now being used as an in vitro technique to aid drug discovery by predicting the in vivo metabolic pathways of drugs or new chemical entities (NCEs), identifying drug-metabolizing enzymes and predicting their in vivo induction. Because of the difficulty of establishing whether the cytotoxic susceptibility of human hepatocytes to xenobiotics/drugs in vitro could be used to predict in vivo human hepatotoxicity, a comparison of the susceptibility of the hepatocytes of human and animal models to six chemical classes of drugs/xenobiotics in vitro have been related to their in vivo hepatotoxicity and the corresponding activity of their metabolizing enzymes. This study showed that the cytotoxic effectiveness of 16 halobenzenes towards rat hepatocytes in vitro using higher doses and short incubation times correlated well with rat hepatotoxic effectiveness in vivo with lower doses/longer times. The hepatic/hepatocyte xenobiotic metabolizing enzyme activities of various animal species and human have been reviewed for use by veterinarians and research scientists. Where possible, recommendations have been made regarding which animal hepatocyte model is most applicable for modeling the susceptibility to xenobiotic induced hepatotoxicity of those humans with slow versus rapid metabolizing enzyme polymorphisms. These recommendations are based on the best human fit for animal drug/xenobiotic metabolizing enzymes in terms of activity, kinetics and substrate/inhibitor specificity. The use of human hepatocytes from slow versus rapid metabolizing individuals for drug metabolism/cytotoxicity studies; and the research use of freshly isolated rat hepatocytes and "Accelerated Cytotoxicity Mechanism Screening" (ACMS) techniques for identifying drug/xenobiotic reactive metabolites are also described. Using these techniques the molecular hepatocytotoxic mechanisms found in vitro for seven classes of xenobiotics/drugs were found to be similar to the rat hepatotoxic mechanisms reported in vivo.

Proteasomal Degradation of Human CYP1B1: Effect of the Asn453Ser Polymorphism on the Post-Translational Regulation of CYP1B1 Expression

Allelic variations in CYP1B1 are reported to modulate the incidence of several types of cancer. To provide a mechanistic basis for this association, we investigated the impact of nonsilent allelic changes on the intracellular levels and post-translational regulation of CYP1B1 protein. When transiently expressed in COS-1 cells, either in the presence or absence of recombinant cytochrome P450 reductase, the cellular level of the CYP1B1.4 allelic variant (containing a Ser at the amino acid position 453; Ser453) was 2-fold lower compared with the other four allelic CYP1B1 proteins (containing Asn453), as analyzed by both immunoblotting and ethoxyresorufin O-deethylase activity. This difference was caused by post-translational regulation; as in the presence of cycloheximide, the rate of degradation of immunodetectable and enzymatically active CYP1B1.4 was distinctly faster than that of CYP1B1.1. Pulse-chase analysis revealed that the half-life of CYP1B1.4 was a mere 1.6 h compared with 4.8 h for CYP1B1.1. The presence of the proteasome inhibitor MG132 [N-benzoyloxycarbonyl (Z)-Leu-Leuleucinal] increased the stability not only of immunodetectable CYP1B1, but also--unexpectedly given the size of the proteasome access channel--increased the stability of enzymatically active CYP1B1. The data presented herein also demonstrate that CYP1B1 is targeted for its polymorphism-dependent degradation by polyubiquitination but not phosphorylation. Our results importantly provide a mechanism to explain the recently reported lower incidence of endometrial cancer in individuals carrying the CYP1B1*4 compared with the CYP1B1*1 haplo-type. In addition, the mechanistic paradigms revealed herein may explain the strong overexpression of CYP1B1 in tumors compared with nondiseased tissues.

Molecular cloning of CYP1A from the estuarine fish Fundulus heteroclitus and phylogenetic analysis of CYP1 genes: update with new sequences

Since we published a phylogenetic analysis of the CYP1A subfamily in 1995, several additional full-length sequences have been reported, including three members of an entirely new subfamily, CYP1B. Two avian sequences were recently published, so that CYP1A sequence data are now available from three of the five major vertebrate lineages. The two new branches that have been added to the CYP1 family tree significantly add to our understanding of P450 evolution. The inclusion of the CYP1Bs to the phylogenetic analysis allows us to root inferred trees. Addition of the avian CYP1As indicates that the CYP1A1/CYP1A2 duplication present in the mammalian lineage may have occurred after the divergence of birds and mammals. The number of fish species from which full-length coding regions of CYP1A genes have been sequenced has increased from four (trout, plaice, toadfish, and scup) to nine. These include CYP1A sequences from tomcod, butterflyfish, sea bream, sea bass, and the full-length sequence of CYP1A from the killifish Fundulus heteroclitus that is reported here. Phylogenetic analyses incorporating the new fish CYP1A sequences support our original conclusion that the fish CYP1As are monophyletic and indicate that the genes are evolving at very different rates in different species.

Induction of CYP1A1, but not CYP1A2, in adrenals of 3, 3'-methylcholanthrene-treated guinea pigs

To test the inducibility of CYP1A homologs in guinea pig adrenal, the effects of 3,3'-methylcholanthrene, an archetypal inducer of CYP1A, were compared in guinea pig adrenal and liver. Western blot analysis showed that levels of both CYP1A1 (53 kDa) and CYP1A2 (56 kDa) increasedin liver microsomes of 3,3'-methylcholanthrene-treated guinea pigs. In adrenals, an immunoreactive protein comigrating with liver CYP1A1 was detected only after 3,3'-methylcholanthrene treatment. Protein comigrating with CYP1A2 was never detected in adrenal microsomes. A third inducible immunoreactive protein (57 kDa) was seen in liver, but not adrenal, after 3, 3'-methylcholanthrene treatment. Another immunoreactive protein (52 kDa), present constitutively in liver and adrenal microsomes, was not induced in either tissue by 3,3'-methylcholanthrene. The precise identities of the inducible 57-kDa and the noninducible 52-kDa proteins remain to be determined. However, the identity of the 53-kDa protein in the adrenal as CYP1A1 was confirmed by RT-PCR, Northern blot, and sequence analysis. Similar analyses demonstrated that, despite the fact that the 56-kDa protein was not detectable in adrenal microsomes, CYP1A2 mRNA was present in adrenals of control animals. Strikingly, CYP1A2 mRNA decreased in adrenal, but increased in liver, following 3,3'-methylcholanthrene treatment, underscoring differences in the regulation of CYP1A expression in the two tissues. Levels of ethoxyresorufin and methyoxyresorufin metabolism correlated with levels of CYP1A1 and CYP1A2 protein, respectively.