Expression and inducibility of antigens in severe combined immunodeficient mice recognized by human anti-P450 antibodies

Resistance to indomethacin-induced down-regulation of hepatic cytochrome P450 enzymes in the mice with non-functional Toll-like receptor 4

BACKGROUND/AIMS

Repetitive indomethacin administration induces down-regulation of hepatic cytochrome P450 (CYP) enzymes. We tested the hypothesis that an increase in intestinal permeability by indomethacin-induced intestinal injury leads to entry of bacterial endotoxin and reaching into liver via the portal vein, resulting in down-regulations of CYPs.

METHODS

C3H/HeJ mice, which are resistant to endotoxin, have a mutation in Toll-like receptor 4 gene. The sensitivity to indomethacin-induced impairment of hepatic CYPs in the lipopolysaccharide (LPS)-resistant mice was examined along with LPS-sensitive (C3H/He) mice.

RESULTS

Treatment of the LPS-sensitive mice with intraperitoneal indomethacin (5 mg/kg per day, 3 days) significantly decreased enzyme activities for CYP3A11, CYP2D9 and CYP1A2 but not CYP2E1. The LPS-resistant mice were resistant to the indomethacin-induced impairment of CYP2D9. The mice were also less sensitive to the effects on CYP3A11 and CYP1A2, but the activities for these isozymes in the indomethacin-treated mice were still lower than in untreated mice. Immunoblot analysis with anti-CYP3A2 and anti-CYP2D2 sera indicated that indomethacin-induced decreases in expression of the proteins recognized by the antibodies were attenuated in the LPS-resistant mice.

CONCLUSIONS

We conclude that Toll-like receptor 4 is involved in the indomethacin-induced down-regulation of hepatic CYP enzymes, indicating the pivotal role of gut-derived endotoxin in the hepatic effects.

Paraquat detoxicative system in the mouse liver postmitochondrial fraction

We examined the paraquat detoxicative system in mouse livers. The survival rate of mice receiving 50 mg/kg paraquat was 41% at 7 days and significantly rose to 88, 64, 69% with pretreatment with phenytoin, phenobarbital, and rifampicin, respectively. Phenytoin induced activity in NADPH-cytochrome P450 reductase, CYP3A, CYP2B, and CYP2C that was 3 to 4 times higher than that of the controls. Phenobarbital induced CYP2B and rifampicin induced CYP3A, respectively, in addition to NADPH-cytochrome P450 reductase. 3-Methylcholanthrene did not induce these enzymes and did not alter the survival rate. All the mice pretreated with CoCl(2) (a CYP synthesis inhibitor) or SKF 525-A (a CYP inhibitor) were dead after 5 days, and troleandomycin (a CYP3A-specific inhibitor) also reduced the survival rate. When cell homogenates were incubated with paraquat and NADPH, paraquat decreased and its metabolic intermediate paraquat-monopyridone was formed. Troleandomycin inhibited the decrease in paraquat and increased the monopyridone. After making a subfraction of the homogenate, monopyridone was produced in the postmicrosomal 105,000g supernatant, but not in the microsomes. The pretreatment of mice with phenytoin decreased the monopyridone in the postmitochondrial fraction, but did not affect the supernatant. These results indicated that paraquat was first metabolized in the postmicrosomal supernatant into monopyridone, and that may have been subsequently hydroxylated by the microsomes. Repeated intravenous injections of alpha-tocopherol to paraquat-loaded mice significantly reduced the paraquat mortality and when these mice were pretreated with rifampicin, 100% of them survived. These studies demonstrate that postmitochondrial fractions play an important role in paraquat detoxication metabolism, and that the combination of CYP induction and alpha-tocopherol administration is highly useful for the survival of paraquat-exposed mice.

In vitro analysis of the activity of the major human hepatic CYP enzyme (CYP3A4) using [N-methyl-14C]-erythromycin

Although the N-demethylation of erythromycin has found widespread use in a noninvasive assay with which to phenotype hepatic CYP3A function, currently, the routine in vitro analysis of erythromycin N-demethylase activity relies on the quantitation of liberated formaldehyde by relatively labor-intensive and insensitive colorimetric or fluorimetric detection. This report describes the development of a rapid, sensitive, and reproducible radioassay for human CYP3A4 using solid-phase extraction (SPE). The kinetics of erythromycin N-demethylation were best described by a one-site Michaelis-Menten model with autoinhibition and the apparent kinetic parameters for pooled human liver microsomes (HLM; Km=88 microM, Vmax=345 pmol/min/mg) and expressed CYP3A4 (Km=33 microM, Vmax=130 pmol/min/ mg) were in good agreement. Erythromycin N-demethylase activity was found to vary 14-fold in HLM and correlated with the rate of testosterone 6beta-hydroxylation (r2=0.92, p < 0.001; N=9). Ketoconazole was a potent inhibitor of the N-demethylation of erythromycin, and the estimated IC50 value (104+/-23 nM) agreed well with that obtained using testosterone as a probe for CYP3A (71+/-4 nM). The addition of this radioassay to those established for human CYP1A2, 2C9, 2D6, and 2E1 and its subsequent automation should enable the routine use of this methodology in the analysis of CYP-dependent reactions.