Gene expression and enzyme function of two cytochrome P450 3A isoenzymes in rat and cattle precision cut liver slices

In vitro metabolism of lidocaine in subcellular post-mitochondrial fractions and precision cut slices from cattle liver

In vitro models are widely used to study the biotransformation of xenobiotics and to provide input parameters to physiologically based kinetic models required to predict the kinetic behavior in vivo. For farm animals this is not common practice yet. The use of slaughterhouse-derived tissue material may provide opportunities to study biotransformation reactions in farm animals. The goal of the present study was to explore the potential of slaughterhouse-derived bovine liver S9 (S9) and precision cut liver slices (PCLSs) to capture observed biotransformation reactions of lidocaine in cows. The in vitro data obtained with both S9 and PCLSs confirm in vivo findings that 2,6-dimethylaniline (DMA) is an important metabolite of lidocaine in cows, being for both PCLSs and S9 the end-product. In case of S9, also conversion of lidocaine to lidocaine-N-oxide and monoethylglycinexylidine (MEXG) was observed. MEGX is considered as intermediate for DMA formation, given that this metabolite was metabolized to DMA by both PLCSs and S9. In contrast to in vivo, no in vitro conversion of DMA to 4-OH-DMA was observed. Further work is needed to explain this lack of conversion and to further evaluate the use of slaughterhouse-derived tissue materials to predict the biotransformation of xenobiotics in farm animals.

Midazolam oxidation in cattle liver microsomes: The role of cytochrome P450 3A

In humans, the cytochrome P450 3A (CYP3A) subfamily is involved in midazolam (MDZ) biotransformation into 1'- and 4-hydroxy metabolites, and the former serves as a probe for CYP3A catalytic activity. In veterinary species is still crucial to identify enzyme- and species-specific CYP substrates; thus, the aim of this study was to characterize MDZ oxidation in cattle liver. A HPLC-UV method was used to measure 1'- and 4-hydroxy MDZ (1'- and 4-OHMDZ, respectively) formation in cattle liver microsomes and assess the role of CYP3A by an immunoinhibition study. Moreover, MDZ hydroxylation was evaluated in 300 cattle liver samples and results were correlated with testosterone hydroxylation. Formation of both metabolites conformed to a single-enzyme Michaelis-Menten kinetics. Values of V_max and K_m were 0.67 nmol/min/mg protein and 6.16 μM for 4-OHMDZ, and 0.06 nmol/min/mg protein and 10.08 μM for 1'-OHMDZ. An anti-rat CYP3A1 polyclonal antibody inhibited up to 50% and 94% 1'- and 4-OHMDZ formation, respectively. MDZ oxidation in liver microsomes was poorly correlated with testosterone hydroxylation. In conclusion, cattle metabolized MDZ to 1'-OHMDZ and 4-OHMDZ. The immunoinhibition results indicated a major contribution of CYP3As to 4-OHMDZ formation and the involvement of other CYPs in 1'-OHMDZ production, paving the way for further investigations.

Functional impact of cytochrome P450 3A (CYP3A) missense variants in cattle

Cytochrome P450 3A is the most important CYP subfamily in humans, and CYP3A4/CYP3A5 genetic variants contribute to inter-individual variability in drug metabolism. However, no information is available for bovine CYP3A (bCYP3A). Here we described bCYP3A missense single nucleotide variants (SNVs) and evaluated their functional effects. CYP3A28, CYP3A38 and CYP3A48 missense SNVs were identified in 300 bulls of Piedmontese breed through targeted sequencing. Wild-type and mutant bCYP3A cDNAs were cloned and expressed in V79 cells. CYP3A-dependent oxidative metabolism of testosterone (TST) and nifedipine (NIF) was assessed by LC-MS/MS. Finally, SNVs functional impact on TST hydroxylation was measured ex vivo in liver microsomes from individually genotyped animals. Thirteen missense SNVs were identified and validated. Five variants showed differences in CYP3A catalytic activity: three CYP3A28 SNVs reduced TST 6β-hydroxylation; one CYP3A38 variant increased TST 16β-hydroxylation, while a CYP3A48 SNV showed enhanced NIF oxidation. Individuals homozygous for rs384467435 SNV showed a reduced TST 6β-hydroxylation. Molecular modelling showed that most of SNVs were distal to CYP3A active site, suggesting indirect effects on the catalytic activity. Collectively, these findings demonstrate the importance of pharmacogenetics studies in veterinary species and suggest bCYP3A genotype variation might affect the fate of xenobiotics in food-producing species such as cattle.

DNA elements for constitutive androstane receptor- and pregnane X receptor-mediated regulation of bovine CYP3A28 gene

The regulation of cytochrome P450 3A (CYP3A) enzymes is established in humans, but molecular mechanisms of its basal and xenobiotic-mediated regulation in cattle are still unknown. Here, ~10 kbp of the bovine CYP3A28 gene promoter were cloned and sequenced, and putative transcription factor binding sites were predicted. The CYP3A28 proximal promoter (PP; -284/+71 bp) contained DNA elements conserved among species. Co-transfection of bovine nuclear receptors (NRs) pregnane X and constitutive androstane receptor (bPXR and bCAR) with various CYP3A28 promoter constructs into hepatoma cell lines identified two main regions, the PP and the distal fragment F3 (-6899/-4937 bp), that were responsive to bPXR (both) and bCAR (F3 fragment only). Site-directed mutagenesis and deletion of NR motif ER6, hepatocyte nuclear factor 1 (HNF-1) and HNF-4 binding sites in the PP suggested either the involvement of ER6 element in bPXR-mediated activation or the cooperation between bPXR and liver-enriched transcription factors (LETFs) in PP transactivation. A putative DR5 element within the F3 fragment was involved in bCAR-mediated PP+F3 transactivation. Although DNA enrichment by anti-human NR antibodies was quite low, ChIP investigations in control and RU486-treated BFH12 cells, suggested that retinoid X receptor α (RXRα) bound to ER6 and DR5 motifs and its recruitment was enhanced by RU486 treatment. The DR5 element seemed to be recognized mainly by bCAR, while no clear-cut results were obtained for bPXR. Present results point to species-differences in CYP3A regulation and the complexity of bovine CYP3A28 regulatory elements, but further confirmatory studies are needed.

Gene expression and immunochemical localization of major cytochrome P450 drug-metabolizing enzymes in bovine nasal olfactory and respiratory mucosa

Despite tremendous advancement in the characterization of nasal enzyme expression, knowledge of the role of the nasal mucosa in the metabolism of xenobiotics is still inadequate, primarily due to the limited availability of in vitro models for nasal metabolism screening studies. An extensive knowledge of the oxidative and conjugative metabolizing capacity of the cattle (Bos taurus) olfactory and respiratory mucosa can aid in efficient use of these tissues for pre-clinical investigations of the biotransformation and toxicity of therapeutic agents following nasal administration or inhalation. Cows are also exposed to a variety of airborne pollutants and pesticides during their lifetime, the metabolism of which can have profound toxicological and ecological consequences. The aim of the present study was to characterize cytochrome P450 (CYP) enzyme expression in the bovine nasal mucosa. Amplification of the specific genes through RT-PCR confirmed expression of several CYP enzymes in bovine hepatic and nasal tissues. The results demonstrate that bovine nasal olfactory and respiratory mucosal and liver tissues express similar populations, families, and distributions of CYP enzymes, as has been previously reported with other species, including humans. Bovine ex vivo tissues can serve as a readily available reference tissue to elucidate preclinical toxico-kinetic effects resulting from exposure to substances in the environment or following drug administration.