Hormonal regulation, tissue distribution, and content of aromatase cytochrome P450 messenger ribonucleic acid and enzyme in rat ovarian follicles and corpora lutea: relationship to estradiol biosynthesis.

The following study was undertaken to compare the content of aromatase cytochrome P450 (P450arom) mRNA with the content of the enzyme in rat ovarian tissues and to relate these changes with estradiol biosynthesis by follicles and corpora lutea isolated throughout pregnancy. A deoxyoligonucleotide (62 mer) probe derived from an amino acid sequence of purified human placental P450arom was used to screen a rat granulosa cell lambda gt11 cDNA expression library. Seven cDNA clones, ranging in size from 0.6-2.0 kilobases (kb), were identified and plaque purified. In vitro translation using mRNA that had been selected by hybridization to a 1.2-kb rat P450arom cDNA insert yielded an 35S-labeled translation product that bound antihuman aromatase immunoglobulin and comigrated with purified human placental aromatase on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, thus verifying that the clones do encode for P450arom. Using the 1.2-kb cDNA insert as a radiolabeled probe, the hormonal regulation, tissue distribution, content, and size of mRNA for P450arom were analyzed. Filter hybridization assays demonstrated that P450arom mRNA was low in small antral (SA) follicles, increased 16-fold in preovulatory (PO) follicles, and reached a peak in granulosa cells within 1 h after an ovulatory dose of hCG. In the corpus luteum of pregnancy, P450arom mRNA content was low on day 4, and increased 3-fold on days 7-11 and 10-fold on days 15-19 of gestation. P450arom mRNA then decreased on days 21 and 23, the day of parturition. Northern analyses of RNA from PO follicles and corpora lutea revealed three bands of P450arom mRNA that were 3.3, 2.6, and 1.9 kb in size. Immunoblots of soluble cell extracts of SA, PO, and luteinizing (PO plus hCG) follicles and corpora lutea of pregnancy demonstrated that aromatase enzyme was low in SA follicles, increased 1.5- to 3-fold in PO follicles, and decreased within 3-5 h after an ovulatory dose of hCG. Changes in the content of P450arom enzyme in luteal cells during pregnancy exhibited a pattern similar to that observed for P450arom mRNA. In contrast, changes in estradiol biosynthesis by follicles and corpora lutea were not directly related to the contents of P450arom mRNA and enzyme. For example, although corpora lutea isolated on days 15-21 of gestation contain the highest amount of P450arom mRNA and enzyme, these tissues did not produce the most estradiol when incubated for 5 h at 37 C in the presence of aromatizable androgen substrate.(ABSTRACT TRUNCATED AT 400 WORDS)