Role of heme in cytochrome P450 transcription and function in mice treated with lead acetate.

Genetic and acquired heme deficiencies are associated with impaired cytochrome P450 (P450) function in experimental animals and in humans. The hypothetical explanations have been either a decreased supply of heme for saturation of apo-P450 or a requirement of heme for P450 gene transcription. We investigated the effect of heme deficiency on P450 function, mRNA, and transcription in C57BL/6 mice treated with lead acetate (75 mg of Pb2+/kg intraperitoneally). Lead caused an increase in delta-aminolevulinic acid levels in plasma (> 30-fold) and a decrease in the heme saturation of hepatic tryptophan-2,3-dioxygenase (15 +/- 4% versus 33 +/- 6% of heme saturation in controls; p < 0.001), which is consistent with an effective inhibition of heme synthesis and depletion of the free heme pool. P450-dependent activities (7-ethoxycoumarin O-deethylation and O-dealkylation of alkoxyresorufins) decreased progressively after lead injection to 56-69% of control levels within 20 hr. This effect was partially counteracted by injection of hematin (4 mg/kg intraperitoneally) to 73-93% of control activities (p < 0.01 for 7-ethoxycoumarin O-deethylation and p < 0.05 for O-dealkylation of alkoxyresorufins). The mRNA levels of the P450 Cyp3a11, measured by semiquantitative reverse transcription-polymerase chain reaction under the same experimental conditions, also decreased after lead injection to 45% of control levels. This decrease was accounted for by inhibition of Cyp3a11 gene transcription, as demonstrated by run-off experiments in liver nuclei isolated 12 hr after lead injection. Hematin did not restore the mRNA levels or the transcriptional activity of Cyp3a11 in nuclei as well as in vivo. We conclude that the decrease of P450 in lead poisoning is a consequence of two different mechanisms: (a) a mechanism unrelated to heme, in which lead decreases P450 transcription; and (b) a mechanism dependent on heme, in which lead inhibits heme synthesis, and this results in a decreased heme saturation of P450 and/or apo-P450 content.