Sanober Amin

Aromatase excess in cancers of breast, endometrium and ovary

Pathogenesis and growth of three common womens cancers (breast, endometrium and ovary) are linked to estrogen. A single gene encodes the key enzyme for estrogen biosynthesis named aromatase, inhibition of which effectively eliminates estrogen production in the entire body. Aromatase inhibitors successfully treat breast cancer, whereas their roles in endometrial and ovarian cancers are less clear. Ovary, testis, adipose tissue, skin, hypothalamus and placenta express aromatase normally, whereas breast, endometrial and ovarian cancers overexpress aromatase and produce local estrogen exerting paracrine and intracrine effects. Tissue-specific promoters distributed over a 93-kb regulatory region upstream of a common coding region alternatively control aromatase expression. A distinct set of transcription factors regulates each promoter in a signaling pathway- and tissue-specific manner. In cancers of breast, endometrium and ovary, aromatase expression is primarly regulated by increased activity of the proximally located promoter I.3/II region. Promoters I.3 and II lie 215 bp from each other and are coordinately stimulated by PGE(2) via a cAMP-PKA-dependent pathway. In breast adipose fibroblasts exposed to PGE(2) secreted by malignant epithelial cells, PKC is also activated, and this potentiates cAMP-PKA-dependent induction of aromatase. Thus, inflammatory substances such as PGE(2) may play important roles in inducing local production of estrogen that promotes tumor growth.

Regulation of Aromatase Expression in Estrogen-Responsive Breast and Uterine Disease: From Bench to Treatment

A single gene encodes the key enzyme for estrogen biosynthesis termed aromatase, inhibition of which effectively eliminates estrogen production. Aromatase inhibitors successfully treat breast cancer and endometriosis, whereas their roles in endometrial cancer, uterine fibroids, and aromatase excess syndrome are less clear. Ovary, testis, adipose tissue, skin, hypothalamus, and placenta express aromatase normally, whereas breast and endometrial cancers, endometriosis, and uterine fibroids overexpress aromatase and produce local estrogen that exerts paracrine and intracrine effects. Tissue-specific promoters distributed over a 93-kilobase regulatory region upstream of a common coding region alternatively control aromatase expression. A distinct set of transcription factors regulates each promoter in a signaling pathway- and tissue-specific manner. Three mechanisms are responsible for aromatase overexpression in a pathologic tissue versus its normal counterpart. First, cellular composition is altered to increase aromatase-expressing cell types that use distinct promoters (breast cancer). Second, molecular alterations in stromal cells favor binding of transcriptional enhancers versus inhibitors to a normally quiescent aromatase promoter and initiate transcription (breast/endometrial cancer, endometriosis, and uterine fibroids). Third, heterozygous mutations, which cause the aromatase coding region to lie adjacent to constitutively active cryptic promoters that normally transcribe other genes, result in excessive estrogen formation owing to the overexpression of aromatase in many tissues.

Mechanisms of excessive estrogen formation in endometriosis

Estrogen is produced in a number of human tissues including the ovary, placenta and extraglandular sites such as adipose tissue, skin and the brain. Aromatase is the key enzyme that regulates estrogen formation in these tissues. Aromatase activity is not detectable in normal endometrium. In contrast, aromatase is expressed aberrantly in endometriosis and is stimulated by PGE(2). This results in local production of estrogen, which induces PGE(2) formation and establishes a positive feedback cycle. Another abnormality in endometriosis, i.e. deficient 17beta-hydroxysteroid dehydrogenase (17beta-HSD) type 2 expression, impairs the inactivation of estradiol to estrone. These molecular aberrations collectively favor accumulation of increasing quantities of estradiol and PGE(2) in endometriosis. The clinical relevance of these findings was exemplified by the successful treatment of an unusually aggressive case of postmenopausal endometriosis using an aromatase inhibitor.

Regulation of Aromatase Expression in Breast Cancer Tissue

Epithelial−stromal interactions play key roles for aromatase expression and estrogen production in breast cancer tissue. Upregulated aromatase expression in breast fibroblasts increases the tissue concentration of estradiol (E2), which then activates a large number of carcinogenic genes via estrogen receptor‐α (ERα) in malignant epithelial cells. This clinically pertains, since aromatase inhibitors (AIs) are the most effective hormonal treatment of ERα‐positive breast tumors. A single gene encodes aromatase, the key enzyme in estrogen biosynthesis, the inhibition of which by an AI effectively eliminates E2 production. Since alternative promoters regulated by distinct signaling pathways control aromatase expression, it is possible to target these pathways and inhibit estrogen production in a tissue‐selective fashion. We and others previously found that the majority of estrogen production in breast cancer tissue was accounted for by the aberrant activation of the proximal promoter I.3/II region. PGE2 that is secreted in large amounts by malignant breast epithelial cells is the most potent known natural inducer of this promoter region in breast adipose fibroblasts. Signaling effectors/transcriptional regulators that mediate PGE2 action include the activator pathways p38/CREB‐ATF and JNK/jun and the inhibitory factor BRCA1 in breast adipose fibroblasts. Selective inhibition of this promoter region may treat breast cancer while permitting aromatase expression via alternative promoters in the brain and bone and thus obviate the key side effects of the current AIs. The signaling pathways that mediate the regulation of the promoter I.3/II region in undifferentiated fibroblasts in malignant breast tumors are reviewed.

A Novel Role of Sodium Butyrate in the Regulation of Cancer-associated Aromatase Promoters I.3 and II by Disrupting a Transcriptional Complex in Breast Adipose Fibroblasts

The aromatase gene encodes the key enzyme for estrogen formation. Aromatase enzyme inhibitors eliminate total body estrogen production and are highly effective therapeutics for postmenopausal breast cancer. A distal promoter (I.4) regulates low levels of aromatase expression in tumor-free breast adipose tissue. Two proximal promoters (I.3/II) strikingly induce in vivo aromatase expression in breast fibroblasts surrounding malignant cells. Treatment of breast fibroblasts with medium conditioned with malignant breast epithelial cells (MCM) or a surrogate hormonal mixture (dibutyryl (Bt2)cAMP plus phorbol diacetate (PDA)) induces promoters I.3/II. The mechanism of promoter-selective expression, however, is not clear. Here we reported that sodium butyrate profoundly decreased MCM- or Bt2cAMP + PDA-induced promoter I.3/II-specific aromatase mRNA. MCM, Bt2cAMP + PDA, or sodium butyrate regulated aromatase mRNA or activity only via promoters I.3/II but not promoters I.1 or I.4 in breast, ovarian, placental, and hepatic cells. Mechanistically, recruitment of phosphorylated ATF-2 by a CRE (–211/–199, promoter I.3/II) conferred inductions by MCM or Bt2cAMP + PDA. Chromatin immunoprecipitation-PCR and immunoprecipitation-immunoblotting assays indicated that MCM or Bt2cAMP + PDA stabilized a complex composed of phosphorylated ATF-2, C/EBPβ, and cAMP-response element-binding protein (CREB)-binding protein in the common regulatory region of promoters I.3/II. Overall, histone acetylation patterns of promoters I.3/II did not correlate with sodium butyrate-dependent silencing of promoters I.3/II. Sodium butyrate, however, consistently disrupted the activating complex composed of phosphorylated ATF-2, C/EBPβ, and CREB-binding protein. This was mediated, in part, by decreased ATF-2 phosphorylation. Together, these findings represent a novel mechanism of sodium butyrate action and provide evidence that aromatase activity can be ablated in a signaling pathway- and cell-specific fashion.

VCAM-1 blockade delays disease onset, reduces disease severity and inflammatory cells in an atopic dermatitis model.

We investigated the functions of critical adhesion molecules ICAM‐1 and VCAM‐1 in a keratin‐14 IL‐4‐transgenic (Tg) mouse model of atopic dermatitis, the skin lesions of which are characterized by prominent inflammatory cell infiltration, significantly increased mRNAs and proteins of ICAM‐1, VCAM‐1, E‐selectin, P‐selectin, L‐selectin, and PSGL‐1, and significantly increased numbers of dermal vessels expressing these adhesion molecules. We tested the hypotheses that deletion or blockade of these molecules may impede the inflammation by examining the disease progresses in the Tg mice crossed with ICAM‐1‐knockout mice and Tg mice received anti‐VCAM‐1‐neutralizing antibody. Although the findings of the ICAM‐1‐knockout Tg mice (Tg/ICAM‐1−/−) developed skin lesions similar to wide‐type ICAM‐1 Tg mice (Tg/ICAM‐1+/+) were surprising, a compensatory mechanism may account for it: the frequency of VCAM‐1 ligand, CD49d, on CD3+ T cells in the lesional skin significantly increased in the Tg/ICAM‐1−/− mouse, compared with the Tg/ICAM‐1+/+ mice. In contrast, anti‐VCAM‐1‐treated Tg/ICAM‐1−/− or Tg/ICAM‐1+/+ mice had significantly delayed onset of skin inflammation compared with isotype antibody‐treated groups. Moreover, anti‐VCAM‐1 significantly reduced the skin inflammation severity in Tg/ICAM‐1+/+ mice, accompanied with reduction of mast cell, eosinophil, and CD3+ T cell infiltration. VCAM‐1 is more critical in developing skin inflammation in this model.

Paracrine-stimulated gene expression profile favors estradiol production in breast tumors

Paracrine interactions between adipose fibroblasts and malignant epithelial cells are essential for structural and hormonal support of breast tumors. Factors derived from malignant epithelial cells inhibit adipogenic differentiation of fibroblasts and upregulate expression of aromatase, which stimulates estrogen synthesis and creates a localized, growth-stimulatory environment. Here, we characterized the gene expression profile of breast adipose fibroblasts in an in vitro model of malignancy to identify other paracrine interactions that support tumor growth. Primary breast adipose fibroblasts from cancer-free women were treated with conditioned media from malignant breast epithelial cells or normal breast epithelial cells, and differences in gene expression were identified by microarray. A total of 79 differentially regulated genes encoding cytokines, enzymes, angiogenic factors, cytoskeletal proteins, extra-cellular matrix remodeling proteins, signal transduction proteins and cell surface receptors were identified, and 6 of these were verified by real-time PCR. Among these, the expression of aldo-keto reductase family 1, member C3 (AKR1C3) was upregulated. AKR1C3 has multiple enzymatic properties, including conversion of estrone to estradiol and androstenedione to testosterone. Immunoreactive AKR1C3 was detected in epithelial and stromal components of benign lesions and ductal carcinomas in situ, and in 59.8% of epithelial and 69.6% of stromal cells in invasive breast carcinomas. AKR1C3 expression was significantly higher in myoepithelial cells surrounding the neoplastic epithelium of ductal carcinoma in situ compared with those surrounding benign epithelial lesions. Importantly, AKR1C3 and aromatase mRNA levels correlated positively in 61 malignant breast tumors (R=0.3967, p=0.00156). Malignant epithelial cell-conditioned medium significantly increased formation of testosterone and estradiol from androstenedione in breast adipose fibroblasts. In conclusion, malignant epithelial cell-derived factors significantly upregulate the enzymes AKR1C3 and aromatase that catalyze a series of complementary reactions to convert the circulating precursor androstenedione to biologically active estradiol in vitro in the stromal fibroblasts, and in vivo, in stromal component of breast tumors.