Kazuto Takayama

The human CYP19 (aromatase P450) gene: update on physiologic roles and genomic organization of promoters ☆

The human CYP19 (P450arom) gene is located in the chromosome 15q21.2 region and is comprised of a 30 kb coding region and a 93 kb regulatory region. The Internet-based Human Genome Project data enabled us to elucidate its complex organization. The unusually large regulatory region contains 10 tissue-specific promoters that are alternatively used in various cell types. Each promoter is regulated by a distinct set of regulatory sequences in DNA and transcription factors that bind to these specific sequences. In most mammals, P450arom expression is under the control of gonad- and brain-specific promoters. In the human, however, there are at least eight additional promoters that seemed to have been recruited throughout the evolution possibly via alterations in DNA. One of the key mechanisms that permit the recruitment of such a large number of promoters seems to be the extremely promiscuous nature of the common splice acceptor site, since activation of each promoter gives rise splicing of an untranslated first exon onto this common junction immediately upstream of the translation start site in the coding region. These partially tissue-specific promoters are used in the gonads, bone, brain, vascular tissue, adipose tissue, skin, fetal liver and placenta for physiologic estrogen biosynthesis. The most recently characterized promoter (I.7) was cloned by analyzing P450arom mRNA in breast cancer tissue. This TATA-less promoter accounts for the transcription of 29-54% of P450arom mRNAs in breast cancer tissues and contains endothelial-type cis-acting elements that interact with endothelial-type transcription factors, e.g. GATA-2. We hypothesize that this promoter may upregulate aromatase expression in vascular endothelial cells. The in vivo cellular distribution and physiologic roles of promoter I.7 in healthy tissues, however, are not known. The gonads use the proximally located promoter II. The normal breast adipose tissue, on the other hand, maintains low levels of aromatase expression primarily via promoter I.4 that lies 73 kb upstream of the common coding region. Promoters I.3 and II are used only minimally in normal breast adipose tissue. Promoters II and I.3 activities in the breast cancer, however, are strikingly increased. Additionally, the endothelial-type promoter I.7 is also upregulated in breast cancer. Thus, it appears that the prototype estrogen-dependent malignancy breast cancer takes advantage of four promoters (II, I.3, I.7 and I.4) for aromatase expression. The sum of P450arom mRNA species arising from these four promoters markedly increase total P450arom mRNA levels in breast cancer compared with the normal breast.

Transcription factor adrenal 4 binding protein as a marker of adrenocortical malignancy.

Adrenal 4 binding protein (Ad4BP) is a transcription factor that regulates the expression of the steroidogenic enzymes and is expressed primarily in steroidogenic cells. We immunolocalized Ad4BP in adrenocortical carcinoma (eight cases) and various malignancies that histologically simulate an adrenocortical carcinoma to evaluate the value of Ad4BP as an immunohistochemical marker of adrenocortical carcinoma. These malignancies examined were renal cell carcinoma (20 cases), hepatocellular carcinoma (10 cases), malignant melanoma (eight cases), ovarian (six cases) and uterine (three cases) clear cell carcinoma, large cell carcinoma of the lung (five cases), and pheochromocytoma (three cases). Nuclear Ad4BP immunoreactivity was observed only in adrenocortical carcinoma cases but not in other tumors examined. Almost all of the adrenocortical carcinoma cells were immunohistochemically positive for Ad4BP including cells associated with bizarre nuclei. These results show that application of Ad4BP immunostain can contribute greatly to the differential diagnosis of adrenocortical carcinoma.

Aromatase as a Therapeutic Target in Endometriosis

In contrast to normal endometrium, the expression of aromatase is aberrant in endometriosis and is stimulated by prostaglandin E2 (PGE2). This results in local production of estrogen, which induces PGE2 formation and establishes a positive feedback cycle. Another abnormality in endometriosis--deficient 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) type 2 expression--impairs the inactivation of estradiol (E2) to estrone (E1). These molecular aberrations collectively favor accumulation of increasing quantities of E2, and PGE2 in endometriosis. The clinical relevance of these findings was exemplified by the successful treatment of an unusually aggressive case of postmenopausal endometriosis with an aromatase inhibitor.