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Family 19 of the P450 super family is responsible for the conversion of C19 androgenic steroids to the corresponding estrogens, a reaction known as aromatization because it involves conversion of the delta4-3-one A-ring of the androgens to the corresponding phenolic A-ring characteristic of estrogens. The gene encoding human aromatase has been cloned and characterized and shown to be unusual compared to genes encoding other P450 enzymes, because there are numerous untranslated first exons that occur in aromatase transcripts in a tissue-specific fashion due to differential splicing as a consequence of the use of tissue-specific promoters. Thus, expression in the ovary uses a proximal promoter that is regulated primarily by cAMP. On the other hand, expression in the placenta uses a distal promoter located at least 40 kb upstream of the start of transcription that is regulated by retinoids. Other promoters are used in brain and adipose tissue. In the latter case, class I cytokines such as IL-6 and IL-11, as well as TNF-alpha, are important regulatory factors. A common 3'-splice junction located upstream of the start of translation is used in all of the splicing events involved in the use of these various promoters. Thus, the coding region of the transcripts, and hence the protein, are identical regardless of the tissue site of expression; what differs in a tissue-specific fashion is the 5'-end of the transcripts. This pattern of expression has great significance both from a phylogenetic and ontogenetic standpoint, as well as for the physiology and pathophysiology of estrogen formation, as will be discussed in this review.

The expression of aromatase cytochrome P450 (P450arom) in the adrenal glands, testes, and placentas of fetal and newborn pigs was investigated. Western immunoblot analysis detected a single 48-50-kDa protein band in these tissues as well as in other porcine tissues known to express P450arom including Day 12 tubular conceptuses, theca interna, and granulosa. Slight differences in migration suggested that the P450arom protein expressed in the testis was larger than that in the adrenal gland, which was, in turn, larger than that in placenta, theca, and granulosa. Consistent with P450arom expression in these tissues, a cDNA encoding porcine P450arom hybridized to a 2.3-kb transcripts in Northern analyses of porcine blastocysts, placentas, and fetal and newborn adrenal glands and testes, as well as in theca and granulosa tissues from preovulatory follicles. No differences in transcript size were detectable among tissues. The identity of P450arom transcripts was confirmed by sequence analysis of partial cDNA clones amplified from porcine fetal adrenal glands, testes, and placentas according to the RACE procedure. The sequences of the adrenal and testis clones were identical but differed from the placental sequence, which represented the first 85 amino acids of porcine P450arom. Specifically, the adrenal and testis clones expressed transcripts that resembled the ovarian isoform of porcine P450arom, rather than the porcine placental isoform, predicting a two-amino acid deletion and 12 predicted amino acid substitutions. P450arom activity was examined to further define expression in these tissues. Activity in adrenal, testis, and placental homogenates was inhibited by 4-hydroxyandrostenedione (4OH-A4) whereas inhibition by etomidate was demonstrated in the adrenal and testis homogenates but not in the placental homogenates. The sensitivity of activity in the newborn porcine adrenal glands and testes to inhibition by etomidate was similar to that of ovarian P450arom activity. The level of P450arom activity was highest in the placenta and lowest in the adrenal gland, and no effect of fetal sex was noted in either tissue. Immunocytochemical studies localized the expression of P450arom in the adrenal gland of newborns to cells at the corticomedullary junction and, with greater intensity, to cells around the developing medullary lobules. The same cells expressed cytochrome P450 17 alpha-hydroxylase, the expression of which also extended throughout the zona fasciculata. The interstitial cells were the site of P450arom expression in the testis, but no expression could be detected in any cells with the spermatic tubules. These data demonstrate that fetal and newborn porcine adrenal glands and testes express an active P450arom that resembles the isoform expressed in the ovary. The localization of adrenal expression suggests a possible role in medullary maturation and function in the fetal and newborn pig.