Expression of cytochromes P450 3A in mouse lung: effects of dexamethasone and pregnenolone 16alpha-carbonitrile.

Expression of cytochromes P450 3A (CYP3A) has been reported in the lung, but its regulation has received little attention. In the present study, we assessed lung levels of Cyp3a mRNA, protein and activity in control mice and in mice treated with either dexamethasone (DEX), pregnenolone 16alpha-carbonitrile (PCN) or a mixture of DEX+PCN. Lung expression of the pregnane X receptor (PXR) was also investigated. Constitutive levels of Cyp3a mRNA were found in the lung from control mice by polymerase chain reaction after reverse transcription of total RNA (RT-PCR). These levels were significantly increased (2.0-fold, P<0.05) in mice treated with DEX and further enhanced (2.7-fold increase, P<0.01) in mice treated with DEX+PCN. In control mice, basal levels of Cyp3a protein and activity were also found, as assessed by western blot and measure of testosterone 6beta-hydroxylation, respectively. In mice treated with DEX or DEX+PCN, changes in Cyp3a protein and activity exhibited the same pattern as those in Cyp3a mRNA. In contrast, PCN alone failed to trigger consistent increases in lung Cyp3a mRNA, protein and activity. PXR mRNA was not detected in the lung from control or PCN-treated mice by RT-PCR, but was found at significant levels in the lungs from mice treated with DEX or DEX+PCN. Our results show that expression of Cyp3a is upregulated by glucocorticoids in mouse lung, and that this effect is potentiated by antiglucocorticoids. This potentiation may involve PXR, expression of which is induced in the lung of glucocorticoid-treated mice.

Increased expression and decreased activity of cytochrome P450 1A1 in a murine model of toluene diisocyanate-induced asthma.

In the lung, cytochromes P450 (CYP) may be affected by inhaled pollutants. In a previous study, we showed that acute inhalation of toluene diisocyanate (TDI), a low molecular weight chemical known to cause occupational asthma, decreased CYP2B1 expression in rat lung. In the present work, we investigated the effect of TDI in a murine model of TDI-induced asthma. Mice were skin-sensitized with TDI on 2 consecutive days and challenged intranasally 8 days later. Lung expression and activity of CYP were assessed 24 h after the challenge. A significant increase in Cyp1a1 protein expression was detected by western blotting in lung from mice sensitized and challenged with TDI, whereas no modification of expression of other CYP, namely Cyp2b, Cyp2e1 and Cyp3a was observed. Increase in Cyp1a1 protein was associated with an increase in Cyp1a1 mRNA, as assessed by polymerase chain reaction after reverse transcription of total lung RNA. However, a decreased Cyp1a1 activity, as measured by O-deethylation of ethoxyresorufin was observed in lung from TDI-sensitized and challenged mice, suggesting that TDI may inhibit Cyp1a1 function. In agreement with this hypothesis, in vitro experiments conducted on liver microsomes overexpressing Cyp1a1 after treatment of mice with 3-methylcholanthrene showed that TDI markedly inhibited in a concentration-dependent manner Cyp1a1 activity. In conclusion, expression of Cyp1a1, known to exhibit rather negative functions in the lung, is increased in mice sensitized and challenged with TDI. However, this effect is associated with a decreased enzyme activity, which might limit the toxicological consequences of increased Cyp1a1 expression.