Identification of CYP2A3 as a major cytochrome P450 enzyme in the female peripubertal rat breast

On isolation of rat breast cytochrome P450, one of the proteins whose amino terminus was sequenced was CYP2A3. CYP2A3 was detected by Western blotting in cytochrome P450 fractions isolated from breast of 3-, 6-, and 9- week-old rats but was low during pregnancy and lactation. Reverse transcription-polymerase chain reaction analysis and sequencing of the PCR product confirmed the presence and identity of CYP2A3 in the rat breast. Breast microsomal coumarin-7-hydroxylase activity paralleled the developmental pattern observed for CYP2A3 on Western blots. In the lung, coumarin-7-hydroxylase activity was 10-fold higher than that in the breast, but the developmental pattern was similar to that in the breast. Lung microsomes from 9-week-old rats activated the heterocyclic amine 2-amino-3- methylimidazo[4,5-f]quinoline to mutagenic metabolites which could be detected with the Ames test. This activation could be inhibited by the CYP2A3 antiserum. With breast microsomes, which contain ≃10% of the cytochrome P450 in the lung, activation of 2-amino-3-methylimidazo[4,5-f]quinoline could not be reliably measured. Immunohistochemical localization revealed that CYP2A3 was expressed in a limited number of epithelial cells in the ducts of 6- week-old rat breast. Double staining with smooth muscle actin, a marker for myoepithelial cells, showed no staining of CYP2A3 immunoreactive cells, indicating that these cells were not myoepithelial. The data clearly show that a cytochrome P450 that can activate environmental procarcinogens is developmentally regulated and concentrated in specific cells in the breast. The peripubertal period seems to be a window in time when the breast may be more sensitive to procarcinogens that are substrates for CYP2A3.