CAR and PXR expression and inducibility of CYP2B and CYP3A activities in rat and rabbit lungs.

Several CYP enzymes are expressed in the lung of mammals but studies on their regulation have been rather neglected. In this study, the CAR and PXR expression and the inducibility of CYP 2B and CYP 3A isoforms in the lung rats and rabbits were investigated. Rats were treated with phenobarbital, clotrimazole or a mixture of dexametasone plus pregnenolone 16alpha-carbonitrile, whereas rabbits were treated with phenobarbital or rifampicin. A low constitutive expression of CAR mRNA was demonstrated by RT-PCR analysis in the lung of rat but not in rabbit. Phenobarbital treatment did not change the CAR expression profiles and did not induce in either rats and rabbits the pulmonary CYP 2B isoforms, as judged by western blot analysis and the marker pentoxyresorufin O-dealkylase and 7-ethoxy-4-trifluoroethylcoumarin O-deethylase activities. On the contrary, these marker activities were strongly induced by phenobarbital in the liver of both species. A low constitutive level of PXR mRNA was also detected by RT-PCR in the lung of rabbit but not in rat. However, also in this case, their expressions were not altered by the administration of strong CYP 3A inducers such as clotrimazole or a mixture of dexametasone plus pregnenolone 16alpha-carbonitrile for the rat and rifampicin or phenobarbital for the rabbit. For the first time, it was demonstrated by RT-PCR that rat lung expresses CYP 3A2, 3A9, 3A18 and 3A23 whereas the rabbit lung expresses the CYP 3A6, the only CYP 3A isoform identified in the rabbit so far. However, notwithstanding the differences observed in the constitutive presence of PXR and CYP 3A transcripts in both species, the above mentioned treatments did not affect in their lungs, unlike their livers, neither the anti-rat 3A immunoreactive proteins nor the CYP 3A marker 7-benzyloxyquinoline O-debenzylase and the 6beta-testosterone hydroxylase activities. The results obtained indicate that the role of CAR and PXR in the lung of rat and rabbit is different from that observed in the liver or other extrahepatic tissues where the induction of the CYP 2B and CYP 3A isoforms is regulated by these receptors.