Expression of the gene encoding aromatase cytochrome P450 (CYP19) in fetal tissues

The formation of estrogens from C₁₉ steroids is catalyzed by a specific form of cytochrome P450, aromatase cytochrome P450 (P450arom; the product of the CYP19 gene). In previous studies we have demonstrated that in adult human tissues and placenta, expression of the CYP19 gene is regulated in part by means of tissue-specific promoters through the use of alternative splicing mechanisms. In addition to placenta, a number of fetal tissues express aromatase, including liver, intestine, skin, and brain. To characterize the CYP19 transcripts present in these and other fetal tissues, we have used reverse transcription and polymerase chain reaction to amplify sequences corresponding to the various untranslated exons from RNA extracted from these tissues. In addition, we have prepared cDNA libraries using RNA from these tissues by the method of rapid amplification of cDNA ends. Sequencing of clones derived from these libraries has been employed to confirm the presence of sequence corresponding to untranslated exons at the 5'-ends of P450arom transcripts. Based on these findings, we conclude that in fetal tissues other than placenta, transcripts containing sequence found in the exon we have previously named 1.4 appear to be the most common. Such sequences have been found in cells in which P450arom expression is stimulated by glucocorticoids. Thus, the presence of such transcripts in fetal liver RNA is consistent with our previous observations that aromatase activity in fetal hepatocytes is stimulated by glucocorticoids. Secondly, transcripts are present in the fetal adrenal, although no aromatase activity has ever been detected in this tissue. Thirdly, the majority of clones in the brain terminate at a site between the 3'-acceptor splice junction in exon II and the start of translation. Thus, proximal genomic elements may serve to drive expression of the CYP19 gene in the brain as they do in the gonads, but employ a different transcriptional start site from that used in the gonads. These studies provide insight into the mechanisms regulating tissue-specific expression of aromatase and, hence, estrogen biosynthesis in the human and underscore further the multiplicity of splicing events that are involved in the regulation of expression of the CYP19 gene.