Testosterone hydroxylation in bovine liver: enzyme kinetic and inhibition study

Generation and characterization of cytochrome P450 3A74 CRISPR/Cas9 knockout bovine foetal hepatocyte cell line (BFH12)

In human, the cytochrome P450 3A (CYP3A) subfamily of drug-metabolizing enzymes (DMEs) is responsible for a significant number of phase I reactions, with the CYP3A4 isoform superintending the hepatic and intestinal metabolism of diverse endobiotic and xenobiotic compounds. The CYP3A4-dependent bioactivation of chemicals may result in hepatotoxicity and trigger carcinogenesis. In cattle, four CYP3A genes (CYP3A74, CYP3A76, CYP3A28 and CYP3A24) have been identified. Despite cattle being daily exposed to xenobiotics (e.g., mycotoxins, food additives, drugs and pesticides), the existing knowledge about the contribution of CYP3A in bovine hepatic metabolism is still incomplete. Nowadays, CRISPR/Cas9 mediated knockout (KO) is a valuable method to generate in vivo and in vitro models for studying the metabolism of xenobiotics. In the present study, we successfully performed CRISPR/Cas9-mediated KO of bovine CYP3A74, human CYP3A4-like, in a bovine foetal hepatocyte cell line (BFH12). After clonal expansion and selection, CYP3A74 ablation was confirmed at the DNA, mRNA, and protein level. The subsequent characterization of the CYP3A74 KO clone highlighted significant transcriptomic changes (RNA-sequencing) associated with the regulation of cell cycle and proliferation, immune and inflammatory response, as well as metabolic processes. Overall, this study successfully developed a new CYP3A74 KO in vitro model by using CRISPR/Cas9 technology, which represents a novel resource for xenobiotic metabolism studies in cattle. Furthermore, the transcriptomic analysis suggests a key role of CYP3A74 in bovine hepatocyte cell cycle regulation and metabolic homeostasis.

Effects of acaricides on the activities of monooxygenases in bovine liver microsomes

Organophosphates (OPs), pyrethrins and fipronil, are acaricides commonly used in cattle, mainly as pour on formulations. Scant information is available on their potential interactions with hepatic xenobiotic metabolizing enzymes. This work aimed to evaluate in vitro the potential inhibitory effects of widely employed acaricides on catalytic activities mediated by hepatic cytochrome P450 (CYP) and flavin-monooxygenase (FMO) enzymes in cattle. Bovine (n = 4) liver microsomes were incubated in the absence (control assays) and in presence of different OPs (fenthion, chlorpyrifos, ethion, diazinon and dichlorvos), fipronil and cypermethrin at 0.1-100 μm. Five oxidative enzyme activities were assayed by spectrofluorimetric or HPLC methods: 7-ethoxyresorufin O-deethylase (for CYP1A1), methoxyresorufin O-demethylase (for CYP1A2), benzyloxyresorufin O-debenzylase (for CYP2B), testosterone 6-beta hydroxylase (for CYP3A) and benzydamine N-oxidase (for FMO). All acaricides, particularly phosphorothionate-containing OPs, inhibited to some extent more than one enzyme activity. The most frequent inhibitor was fenthion, which inhibited (p < .05) all enzyme activities tested (from 22% at 1 μm to 72% at 100 μm). However, low inhibitory potencies (IC50s higher than 7 μm) of all acaricides studied were observed against the catalytic activities assayed. Therefore, the risk of in vivo metabolic interactions due to inhibition of monooxygenases would be low under common husbandry conditions.

Induction by Phenobarbital of Phase I and II Xenobiotic-Metabolizing Enzymes in Bovine Liver: An Overall Catalytic and Immunochemical Characterization

In cattle, phenobarbital (PB) upregulates target drug-metabolizing enzyme (DME) mRNA levels. However, few data about PB's post-transcriptional effects are actually available. This work provides the first, and an almost complete, characterization of PB-dependent changes in DME catalytic activities in bovine liver using common probe substrates and confirmatory immunoblotting investigations. As expected, PB increased the total cytochrome P450 (CYP) content and the extent of metyrapone binding; moreover, an augmentation of protein amounts and related enzyme activities was observed for known PB targets such as CYP2B, 2C, and 3A, but also CYP2E1. However, contradictory results were obtained for CYP1A, while a decreased catalytic activity was observed for flavin-containing monooxygenases 1 and 3. The barbiturate had no effect on the chosen hydrolytic and conjugative DMEs. For the first time, we also measured the 26S proteasome activity, and the increase observed in PB-treated cattle would suggest this post-translational event might contribute to cattle DME regulation. Overall, this study increased the knowledge of cattle hepatic drug metabolism, and further confirmed the presence of species differences in DME expression and activity between cattle, humans, and rodents. This reinforced the need for an extensive characterization and understanding of comparative molecular mechanisms involved in expression, regulation, and function of DMEs.

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.

Effects in Rats of Maternal Exposure to Raspberry Leaf and Its Constituents on the Activity of Cytochrome P450 Enzymes in the Offspring

The goal of our study was to determine whether maternal exposure to red raspberry leaf (RRL) and its constituents can permanently alter biotransformation of fluorogenic substrates by cytochrome P450 (CYP) in the livers of male and female offspring. Nulliparous female rats received vehicle, raspberry leaf, kaempferol, quercetin, or ellagic acid orally once breeding had been confirmed until parturition. Hepatic microsomes were prepared from animals at birth (postnatal day 1 [PND1]), weaning (PND21), PND65, and PND120 to determine the biotransformation of 8 fluorogenic substrates. The pattern of biotransformation of all but 2 of the substrates was gender specific. Maternal consumption of RRL increased biotransformation of 3 substrates by female offspring at PND120 resulting in a more masculine profile. Kaempferol and quercetin had a similar effect to RRL. These results suggest that maternal consumption of either RRL or some of its constituents leads to long-term alterations of CYP activity in female offspring.