Cloning of a cDNA coding for P-450 LM3c from rabbit liver microsomes and regulation of its expression.

Liver cytochromes P-450 LM3c in the rabbit and P-450p in the rat are two related forms, inducible by macrolide antibiotics such as triacetyloleandomycin (TAO) and glucocorticoids such as dexamethasone. We prepared a cDNA library from TAO induced rabbit liver mRNA and characterized a cDNA (pLM3c-4.1) that hybridized to pDex 3.22, a cDNA complementary to cytochrome P-450p mRNA. Northern blots of liver poly(A)RNA from untreated or TAO, erythromycin and rifampicin treated animals, revealed two mRNA species of approximately 1700 and 1850 nucleotides in length, that hybridized to LM3c cDNA and to pDEX 3.22. The level of both mRNAs was increased five fold over control by macrolide antibiotics but unaffected by both phenobarbital and B-naphthoflavone. After 5 days of TAO treatment LM3c mRNA had increased 5 fold while LM3c protein had increased 25 fold. However, the rate of P-450 LM3c gene transcription measured in isolated liver nuclei remained unchanged throughout five days of TAO treatment. We conclude that TAO may induce cytochrome P-450 LM3c by post-transcriptional effects.