Developmental and endocrine regulation of P450 isoforms in rat breast.

Cytochrome P450 was partially purified from rat breast tissue from 1-, 2-, 3-, 6-, 9-, and 15-week-old pregnant, lactating, or 3-week postlactation rats. The detergent-solubilized P450 was spectrally quantified, and the P450 isozyme pattern in the different samples was characterized by Western blot analysis with antibodies against cytochromes P450 1A1, 1A2, 2A, 2B, 2D4, 3A, 4A, 2E, and 19. The yield of P450 was 5-60 pmol/g wet weight tissue, with the highest yields in 1- and 2-week-old pups and lactating rats. The cytochromes P450 expressed in the breast can be divided into six main groups on the basis of their pattern of regulation: (a) those present in all samples (4A, 2E1, and 2D4), (b) those highly expressed in 1- to 3-week-old rats (2D4 and 3A), (c) those expressed only after 9 weeks of age [P450 19 (aromatase)], (d) those induced in pregnancy and maintained during lactation (1A1), (e) those induced in pregnancy and not maintained during lactation (2A), and (f) those induced 3 weeks after lactation (2B, 2A, and 3A). Reverse transcription-polymerase reaction amplification was used to confirm the presence of P450 isoforms in the breast. The mRNAs of cytochromes P450 1A1, 2A1, 2B1-3, 2D1, 2D3, 2D4, 2E1, and 4A3 were detected on analysis of total breast RNA. The mRNA of CYP 3A1 was not convincingly detected in untreated rat breast but was inducible by treatment with pregnenolone-16-alpha-carbonitrile. The presence of these various forms of P450 in the breast and their regulation by age and endocrine status may have implications for in situ metabolism of steroids and steroid antagonists and for activation of procarcinogens.