ESTROGEN BIOSYNTHESIS IN THP1 CELLS IS REGULATED BY PROMOTER SWITCHING OF THE AROMATASE (CYP19) GENE

The expression of aromatase, the enzyme responsible for estrogen biosy nthesis, has been studied in THP-1 cells of human mononuclear leukemic origin, which exhibit high rates of aromatase activity. These cells h ave the capacity to differentiate in the presence of vitamin D into ce lls with osteoclast-like properties. Differentiated cells displayed hi gher rates of aromatase than undifferentiated cells, and, in both case s, activity was stimulated 10- to 20-fold by dexamethasone. Phorbol es ters also increased aromatase activity, but the effect was the same in differentiated as in undifferentiated cells. In a similar fashion to adipose stromal cells, serum potentiated the response to dexamethasone but had no effect on phorbol ester-stimulated activity. By contrast t o its action in adipose stromal cells, (Bu)(2)cAMP markedly inhibited aromatase activity of THP-1 cells, as did factors whose actions are me diated by cAMP, such as PTH and PTH-related peptide. This was true of control cells, as well as of dexamethasone-and phorbol ester-stimulate d cells. Previously we have shown that type 1 cytokines as well as tum or necrosis factor-alpha stimulate aromatase activity of adipose strom al cells in the presence of dexamethasone. By contrast, interleukin-6, interleukin-ll, and leukemia-inhibitory factor had no effect on aroma tase activity of THP-1 cells, whereas tumor were slightly inhibitory o f aromatase activity. Exon-specific Southern analysis of rapid amplifi cation of cDNA ends-amplified transcripts was employed to examine the distribution of the various 5'-termini of aromatase transcripts. In th e control group, most of the clones contained transcripts specific for the proximal promoter IS, whereas in dexamethasone-treated cells, mos t transcripts contained exon I.4. In the phorbol ester-treated cells, a broader spectrum of transcripts was present, with equal proportions of I.4, II, and I.3-containing clones. Additionally, one clone contain ing a new sequence, exon I.6, was found. This was shown to be located about 1 kb upstream of exon II. By contrast, all clones from cells hea ted with (Bu)(2)cAMP contained promoter II specific sequences. In addi tion to these transcripts, two clones in the library from the dexameth asone-treated cells contained the sequence previously defined as the b rain-specific sequence, If. In one of these, the If sequence was fused downstream of exon I.4, indicative that its expression likely employe d promoter I.4. These results point to similarities and important diff erences between aromatase expression in THP-1 cells and other cells su ch as adipose stromal cells, indicative of unique regulatory pathways governing aromatase expression in these cells.