Malignant MCF10CA1 cell lines derived from premalignant human breast epithelial MCF10AT cells.

The MCF10 series of cell lines was derived from benign breast tissue from a woman with fibrocystic disease. The MCF10 human breast epithelial model system consists of mortal MCF10M and MCF10MS (mortal cells grown in serum-free and serum-containing media, respectively), immortalized but otherwise normal MCF10F and MCF10A lines (free-floating versus growth as attached cells), transformed MCF10AneoT cells transfected with T24 Ha-ras, and premalignant MCF10AT cells with potential for neoplastic progression. The MCF10AT, derived from xenograft-passaged MCF10-AneoT cells, generates carcinomas in approximately 25% of xenografts. We now report the derivation of fully malignant MCF10CA1 lines that complete the spectrum of progression from relatively normal breast epithelial cells to breast cancer cells capable of metastasis. MCF10CA1 lines display histologic variations ranging from undifferentiated carcinomas, sometimes with focal squamous differentiation, to well-differentiated adenocarcinomas. At least two metastasize to the lung following injection of cells into the tail vein; one line grows very rapidly in the lung, with animals moribund within 4 weeks, whereas the other requires 15 weeks to reach the same endpoint. In addition to variations in efficiency of tumor production, the MCF10CA1 lines show differences in morphology in culture, anchorage-independent growth, karyotype, and immunocytochemistry profiles. The MCF10 model provides a unique tool for the investigation of molecular changes during progression of human breast neoplasia and the generation of tumor heterogeneity on a common genetic background.

Breast stroma plays a dominant regulatory role in breast epithelial growth and differentiation: implications for tumor development and progression.

Although growth factors and extracellular matrix (ECM) are recognized as important contributors to breast epithelial growth, morphogenesis, hormone responsiveness, and neoplastic progression, the influence of functional interactions between breast stromal and epithelial cells on these processes has not been defined. Using a novel three-dimensional cell-cell interaction model, we have compared the abilities of different mesenchymal cell types, including breast fibroblasts derived from reduction mammoplasty and tumor tissues, and human umbilical endothelial cells (HUVECs) to induce three-dimensional morphogenesis and growth of normal MCF10A and preneoplastic MCF10AT1-EIII8 (referred as EIII8) human breast epithelial cells. Our data demonstrate a requirement for organspecific fibroblasts in the induction of epithelial morphogenesis. Whereas inclusion of normal reduction mammoplasty fibroblasts inhibit or retard morphological conversion and growth of MCF10A and EIII8 cells, respectively, tumor-derived breast fibroblasts evoke ductal-alveolar morphogenesis of both MCF10A and EIII8 cells. The growth and morphogenesis inhibitory effects of normal fibroblasts remain even in the presence of estrogen because they are able to suppress the estrogen-induced growth of EIII8 cells, whereas tumor fibroblasts support and maintain estrogen responsiveness of EIII8 cells. The inductive morphogenic effects of tumor fibroblasts on EIII8 cells is further augmented by the inclusion of HUVECs because these cocultures undergo a dramatic increase in proliferation and branching ductal-alveolar morphogenesis that is accompanied by an increase in invasion, degradation of coincident ECM, and expression of MMP-9. Therefore, tumor fibroblasts confer morphogenic and mitogenic induction of epithelial cells, and further enhancement of growth and progression requires active angiogenesis. These data illustrate the importance of structural and functional interactions between breast stromal and epithelial cells in the regulation of breast epithelial growth and progression.

Adaptive hypersensitivity to estradiol: potential mechanism for secondary hormonal responses in breast cancer patients.

Women with hormone dependent breast cancer initially respond to hormone deprivation therapy with tamoxifen or oophorectomy for 12-18 months but later relapse. Upon secondary therapy with aromatase inhibitors, patients often experience further tumor regression. The mechanisms responsible for secondary responses are unknown. We postulated that hormone deprivation induces hypersensitivity to estradiol. Evidence of this phenomenon was provided in a model system involving MCF-7 cells grown in vitro and in xenografts. To determine if the ER transcriptional process is involved in hypersensitivity, we examined the effect of estradiol on ER reporter activity, PgR, PS2, and c-myc as markers and found no alterations in hypersensitive cells. Next, we examined whether MAP kinase may be upregulated in the hypersensitive cells as a reflection of increased growth factor secretion or action. Basal MAP kinase activity was increased both in vitro and in vivo in hypersensitive cells. Proof of principle studies indicated that an increase in MAP kinase activity induced by TGFalpha administration caused a two- to three-fold shift to the left in estradiol dose response curves in wild type cells. Blockade of MAP kinase with PD98059 returned the shifted curve back to baseline. These data suggested that MAP kinase overexpression could induce hypersensitivity. To determine why MAP kinase was increased, we excluded constitutive receptor activity and growth factor secretion by the demonstration that the pure anti-estrogen, ICI 182780, could inhibit MAP kinase activation. We also excluded hypersensitivity to estradiol induced growth factor secretion, and thus MAP kinase activation, since estradiol stimulated MAP kinase at 24, 48, and 72 h at the same concentrations in hypersensitive as in wild type cells. Surprisingly, a series of experiments suggested that MAP kinase increased in hypersensitive cells as a result of estrogen activation via a non-genomic pathway. We examined the classical signal pathway in which SHC is phosphorylated and binds to SOS and GRB-2 to activate Ras, Raf, and MAP kinase. With 5-20 min of exposure, estradiol caused binding of SHC to the estrogen receptor, phosphorylation of SHC, binding of GRB-2 to SOS, and activation of MAP kinase. All of these affects could be blocked by ICI 182780. Taken together, these observations suggest that the cell membrane ER pathway may be responsible for upregulation of MAP kinase and hypersensitivity in cells adapted to estradiol deprivation.

Tissue-specific synthesis and oxidative metabolism of estrogens.

Estrogen exposure represents the major known risk factor for development of breast cancer in women and is implicated in the development of prostate cancer in men. Human breast tissue has been shown to be a site of oxidative metabolism of estrogen due to the presence of specific cytochrome P450 enzymes. The oxidative metabolism of 17beta-estradiol (E2) to E2-3,4-quinone metabolites by an E2-4-hydroxylase in breast tissue provides a rational hypothesis to explain the mammary carcinogenic effects of estrogen in women because this metabolite is directly genotoxic and can undergo redox cycling to form genotoxic reactive oxygen species. In this chapter, evidence in support of this hypothesis and of the role of P4501B1 as the 4-hydroxylase expressed in human breast tissue is reviewed. However, the plausibility of this hypothesis has been questioned on the grounds that insufficient E2 is present in breast tissue to be converted to biologically significant amounts of metabolite. This critique is based on the assumption that plasma and tissue E2 levels are concordant. However, breast cancer tissue E2 levels are 10-fold to 50-fold higher in postmenopausal women than predicted from plasma levels. Consequently, factors must be present to alter breast tissue E2 levels independently of plasma concentrations. One such factor may be the local production of E2 in breast tissue through the enzyme aromatase, and the evidence supporting the expression of aromatase in breast tissue is also reviewed in this chapter. If correct, mutations or environmental factors enhancing aromatase activity might result in high tissue concentrations of E2 that would likely be sufficient to serve as substrates for CYP1B1, given its high affinity for E2. This concept, if verified experimentally, would provide plausibility to the hypothesis that sufficient E2 may be present in tissue for formation of catechol metabolites that are estrogenic and which, upon further oxidative metabolism, form genotoxic species at levels that may contribute to estrogen carcinogenesis.

Regulated CYP19 aromatase transcription in breast stromal fibroblasts.

Extraglandular estrogen synthesis mediates the proliferation of estrogen-responsive breast cancer in postmenopausal women. Aromatase, the cytochrome P450 Cyp19 enzyme, catalyzes the rate-limiting step in estrogen biosynthesis. Activity is present in both normal and neoplastic breast tissue, and Cyp19 protein is localized by immunohistochemistry predominantly in breast stromal fibroblasts. In cultured breast stromal fibroblasts, both activity and Cyp19 messenger ribonucleic acid are increased to a substantial degree by hormonal and growth factor regulators of transcription. Transcriptional regulation of CYP19 is complex in breast tissues, in which exon switching in the usage of alternative first exons occurs from predominantly EI.4 in breast tissue from cancer-free women to predominantly EI.3 and PII in breast tumors and quadrants with or without tumor. The present study questioned whether the first exon switch occurs as a result of an inherent difference between fibroblasts in normal and tumor tissues or because of differences in local regulators between these tissues. To distinguish between these two possibilities, we examined fibroblasts cultured from breast tumor, benign breast, and reduction mammoplasty tissues for the ability of various CYP19 transcriptional regulators to modulate first exon EI.3, EI.4, and PII usage. A semiquantitative RT-PCR method was used to identify transcripts containing six of the nine known CYP19 first exons. Combinations of cAMP and Dex regulated transcription from first exons EI.3, EI.4, and PII in fibroblasts cultured from all tissues, but not in reduction mammoplasty epithelial cells. These results provide evidence that the fibroblasts from these breast tissues are not inherently different in transcriptional regulation of CYP19 first exon usage and that transcriptional regulatory molecules are likely to mediate the exon switch phenomenon.

Progression of premalignant MCF10AT generates heterogeneous malignant variants with characteristic histologic types and immunohistochemical markers.

The MCF10AT premalignant human breast epithelial cells form benign ductal structures in immunodeficient mice which sporadically progress to carcinoma in situ and invasive cancers of different histologic types. MCF10CA1 cell lines are malignant variants derived by serially passing small pieces of tumors in athymic mice before establishing cells in culture. As these MCF10CA1 variants gave rise to heterogeneous tumors, some cell lines were cloned. Inoculated into immunodeficient mice, these variants produce squamous carcinomas with an undifferentiated component or adenocarcinomas also with an undifferentiated component. Immunohistochemistry utilized antibodies against DF3, c-erbB-2, cyclin Dl, m keratin, p keratin, p53, B72.3 and estrogen receptor. We detected characteristic patterns for squamous carcinomas, for adenocarcinomas, and for each undifferentiated component, that is the undifferentiated components of the squamous and glandular carcinomas were distinct. Only adenocarcinomas were focally ER positive. One uncloned variant that produced cancers with a glandular component, MCF10CA1h, was cloned and cells were injected into mice. This clone produced only undifferentiated carcinomas that, compared to tumors formed by the parental uncloned variant, had lost ER, DF3 and c-erbB-2 expression, but more strongly expressed p53. Our data demonstrate the potential of the premalignant MCF10AT model to generate heterogeneity, including both estrogen receptor-positive as well as estrogen receptor-negative tumors, during progression.

Determinants of tissue estradiol levels and biologic responsiveness in breast tumors.

Estradiol stimulates the growth of breast tumor cells in both pre- and post menopausal women. Following the menopause, the levels of estradiol in breast tumor tissues are similar to those from tumors obtained prior to cessation of ovarian function, even though plasma estrogen levels are 10-50 fold lower in post- than in premenopausal women. These observations suggested the possibility of enhanced estradiol uptake from plasma or in situ synthesis in post-menopausal women. We systematically studied these possibilities in a series of model systems. Initially we demonstrated a very high affinity estradiol binding site in tissues from castrated rats. Enhanced uptake occurred under conditions of low plasma estrogen levels when compared to animals with higher estradiol levels. In situ synthesis also occurred both through the sulfatase and aromatase pathways. In further studies, we compared uptake from plasma with in situ synthesis via aromatase in a nude mouse model. Under the conditions utilized, in situ synthesis resulted in much higher tissue estradiol levels and tumor growth rates than did uptake from plasma. During these studies we demonstrated that tumors deprived of estradiol developed mechanisms rendering them more sensitive to estrogen. This involved the ability of cells to adapt to estradiol deprivation to allow them to be responsive to four log lower amounts of estrogen than when studied under wild type conditions. In addition, cells adapted by increasing their level of aromatase and thus developing the capability to become more sensitive to estrogen precursors. Taken together, these studies demonstrate that breast cancer tissue is highly plastic and can adapt to conditions of estrogen deprivation via a variety of mechanisms.

Demonstration of aromatase activity and its regulation in breast tumor and benign breast fibroblasts.

Breast tumors from post-menopausal women contain higher amounts of estradiol than would be predicted from levels circulating in plasma. This observation raised the hypothesis that tumors may synthesize estradiol in situ and increase their tissue estradiol levels via this mechanism. The key enzyme involved in tissue estrogen synthesis, aromatase, is present in breast tumors but, according to some investigators, not in sufficient concentration to be biologically meaningful. We postulated that foci of cells in breast tumors might contain high amounts of aromatase and this locally produced estrogen might act in a paracrine or autocrine fashion. To test this hypothesis, we utilized immunohistochemistry to localize the aromatase enzyme, an histological scoring system to quantitate it, and culture of isolated breast cells to demonstrate its potential regulation. In 26 archival breast tumors, 16 (62%) contained aromatase by radiometric assay. With the immunohistochemical method, we detected areas with staining in the stroma as well as tumor epithelial cells. Staining ranged from the intensity approaching that seen in placenta to levels just distinguishable from background. We adopted an histological scoring system (H-score) from that used to quantitate progesterone receptor levels in tissue and used it to quantitate aromatase activity. A higher histologic score was found in stromal spindle cells (13) than in tumor epithelial cells (4.8). The biochemical aromatase results correlated with the H-score of stromal but not epithelial cells. To further study stromal cells from tumors, we isolated stromal cells from breast tumors and the benign areas of breast distal to the tumor and grew them in culture. Addition of dexamethasone, phorbol esters, and cyclic AMP analogues stimulated aromatase enzyme and messenger RNA levels substantially. Use of aromatase enzyme inhibitors such as letrozole blocked estrogen production but did not alter aromatase message levels. Epithelial cells, whether nonmalignant or cancer derived, exhibited no regulation by dexamethasone, phorbol esters, or cAMP analogues. These data, taken together, suggest that stromal cells may be more important than epithelial cancer cells for estrogen production in breast tumors. The ability to stimulate aromatase activity substantially with various enhancers of aromatase provides further credence for an important biologic role of estrogen production in tumor tissue.

Comparative rates of androgen production and metabolism in Caucasian and Chinese subjects.

Clinically apparent prostate cancer occurs more commonly among Caucasians living in Western countries than in Chinese in the Far East. Prior studies demonstrated diminished facial and body hair and lower levels of plasma 3 alpha-androstanediol glucuronide and androsterone glucuronide in Chinese than in Caucasian men. Based upon these findings, investigators postulated that Chinese men could have diminished 5 alpha-reductase activity with a resultant decrease in prostate tissue dihydrotestosterone levels and clinically apparent prostate cancer. An alternative hypothesis suggests that decreased 3 alpha-androstanediol glucuronide and androsterone glucuronide levels might reflect reduced production of androgenic ketosteroid precursors as a result of genetic or environmental factors. The present study examined 5 alpha-reductase activity, androgenic ketosteroid precursors, and the influence of genetic and environmental/dietary factors in groups of Chinese and Caucasian men. We found no significant differences in the ratios of 5 beta-:5 alpha-reduced urinary steroids (a marker of 5 alpha-reductase activity) between Chinese subjects living in Beijing, China, and Caucasians living in Pennsylvania. To enhance the sensitivity of detection, we used an isotopic kinetic method to directly measure 5 alpha-reductase activity and found no difference in testosterone to dihydrotestosterone conversion ratios between groups. Then, addressing the alternative hypothesis, we found that the Caucasian subjects excreted significantly higher levels of individual and total androgenic ketosteroids than did their Chinese counterparts. To distinguish genetic from environmental/dietary factors as a cause of these differences, we compared Chinese men living in Pennsylvania and a similar group living in Beijing, China. We detected a reduction in testosterone production rates and total plasma testosterone and sex hormone-binding levels, but not in testosterone MCRs in Beijing Chinese as a opposed to those living in Pennsylvania. Comparing Pennsylvania Chinese with their Caucasian counterparts, we detected no significant differences in total testosterone, free and weakly bound testosterone, sex hormone-binding globulin levels, and testosterone production rates. Taken together, these studies suggest that environmental/dietary, but not genetic, factors influence androgen production and explain the differences between Caucasian and Chinese men.

Effect of CGS 20267 on ovarian aromatase and gonadotropin levels in the rat.

Aromatase catalyzes the rate limiting step that converts androgens to estrogens. Postmenopausal women with hormone dependent breast cancer respond to first generation aromatase inhibitors such as aminoglutethimide with a marked suppression of circulating estradiol levels. In contrast, premenopausal women appear to be resistant to first generation aromatase inhibitors. The inability to block ovarian aromatase results from the low affinity of first generation inhibitors for the active site of the enzyme. Under these circumstances, the high substrate levels in the premenopausal ovary compete effectively with these inhibitors and do not allow binding of inhibitor to the active site of the enzyme. Second and third generation aromatase inhibitors with higher affinity for aromatase have now been developed and potentially could block ovarian aromatase. To test this possibility, we administered CGS 20267 (letrozole), a highly potent aromatase inhibitor, to cycling female rats. A dose dependent inhibition of uterine weight occurred with maximum effects produced by the 5 mg/kg/day dosage. During a period of 4 weeks, uterine weight was reduced to levels induced by ovariectomy. Ovarian tissue estradiol levels were inhibited by approximately 80%. As a reflection of inhibition of ovarian aromatase activity, the levels of androstenedione in the ovary increased by an order of magnitude. Both LH and FSH plasma levels increased but not to those observed after ovariectomy. The rise in gonadotropin levels induced a statistically significant but relatively small increase in ovarian weights. These results demonstrate the ability to persistently block ovarian aromatase activity in cycling rats with a potent aromatase inhibitor. This study provides a rationale for clinical trials of potent aromatase inhibitors in pre-menopausal women with breast cancer.

Estrogen production via the aromatase enzyme in breast carcinoma: which cell type is responsible?

Studies of breast tumor homogenates from women with breast cancer have demonstrated the synthesis of estrogens in situ through the enzyme aromatase. The present series of investigations sought to determine which cell type within the tumor is responsible for local estrogen biosynthesis, and whether or not the amount produced is biologically important. Accordingly, we utilized an indirect immunohistochemical scoring method (H-score) to determine the relative amount of enzyme present in tumor epithelial and stromal cells. This revealed a value of 13 for tumor stromal cells and 4.8 for the epithelial component. Contributing to this difference is the fact that a greater percentage of cells in the tumor were stromal (45%) than epithelial (37%). To obtain direct evidence that tumor stromal cells could synthesize estrogens, we isolated and grew these cells in tissue culture. Stromal cells originating from within the tumor could be stimulated by known enhancers of transcription to produce nearly as much aromatase as is found in placental microsomes. Stromal cells isolated from benign tissue distal to the tumor exhibited properties similar to those of the tumor stroma. Epithelial cells, in contrast, did not respond to these enhancers and had low levels of aromatase basally. To obtain proof of the principle that local estrogen synthesis can be biologically meaningful, we measured tumor tissue estradiol levels and growth rates in aromatase-transfected MCF-7 cells implanted into nude mice. Local synthesis resulted in tumor levels ranging from 300 to 800 pg/g and growth rates substantially higher than in non-aromatase-containing tumors. These data suggest that tumor stromal cells contribute the major portion of estrogen synthesized in tumors, and that this local synthesis can increase tumor estradiol levels and growth rates.

Mechanism for maintenance of high breast tumor estradiol concentrations in the absence of ovarian function: role of very high affinity tissue uptake.

Breast tumors from postmenopausal women contain levels of estradiol similar to those in premenopausal patients even though serum estradiol levels fall by an order of magnitude upon cessation of ovarian function. The present study sought to examine enhanced uptake from plasma as one potential mechanism for maintenance of high tissue estradiol levels in postmenopausal patients. Accordingly, we used osmotic minipumps to continuously infuse estradiol (E2) at rates producing serum concentrations ranging from pre- to postmenopausal levels for two weeks to oophorectomized Sprague-Dawley rats bearing nitrosomethylurea-induced mammary tumors. We then measured E2 concentrations in various tissues and sera and reasoned that tissue affinities for estradiol could be directly calculated from in vivo measurements by adapting Scatchard analysis to steroid infusion data. Using this method, we demonstrated a very high affinity estradiol binding component with a Kd two orders of magnitude higher (i.e., 0.35 x 10(-12) M) than determined with standard in vitro techniques. A second estradiol binding component with the expected Kd of 1 x 10(-10) M was also present. Estradiol bound to both classes of binding sites could be 98% displaced with diethylstilbestrol within a 6-hr period. In vivo steroid binding off-times calculated from log-linear slopes averaged approximately 60 min. These data demonstrated that the actual E2 binding affinity in target tissues in vivo, especially at low estrogen concentrations, is much higher than usually estimated from standard, in vitro estrogen receptor assays. These observations provide one mechanism to explain why estradiol concentrations remain high in breast cancer tissue from postmenopausal women and consequently can stimulate tumor proliferation.

Aromatase activity and expression in breast cancer and benign breast tissue stromal cells.

In situ estrogen synthesis by hormone-dependent breast cancers could potentially regulate cellular proliferation through autocrine or paracrine mechanisms. Several biochemical studies have demonstrated activity of the enzyme aromatase, the rate-limiting step for estrogen synthesis, in breast tumor homogenates. Prior immunohistochemical studies in breast neoplasms demonstrated aromatase antibody binding to both stroma and parenchyma, but biochemically measured enzyme activity significantly correlated only with the level of staining in the stromal component. The present study sought to provide more direct evidence of the predominant role for stromal cell aromatase in breast tumor tissue. Accordingly, breast tumor stromal and epithelial cells were examined for aromatase enzyme activity and messenger ribonucleic acid (mRNA) expression. Stromal and epithelial cells from benign tissue served as a means of comparing activity and regulation in benign and tumor tissue. Enzyme activity in stromal cells from breast tumor tissue was low basally, but increased by 30- to 1200-fold when induced by dexamethasone. Combining dexamethasone with phorbol esters and cAMP produced an additional 1.2- to 4.1-fold stimulation. Analyses of exons III/V and exons IX/X demonstrated that aromatase mRNA expression was also substantially increased by these treatments. Increases in enzyme activity and mRNA expression in cells from benign breast stroma paralleled those observed in tumor stroma, although the increases in enzyme activity were generally lower. In contrast to the responses observed in stromal cells, epithelial cells from breast tumor or nonmalignant breast tissue were unresponsive to dexamethasone, either added alone or in combination with phorbol esters and cAMP. This study provides direct biochemical evidence that aromatase is present in stroma within breast tumors, as in surrounding tissues, and suggests that estrogen synthesis within the tumor may modulate tumor growth via a paracrine mechanism.

Evidence of in situ estrogen synthesis in nitrosomethylurea-induced rat mammary tumors via the enzyme estrone sulfatase.

A variety of indirect evidence suggests that mammary tumors can synthesize free estrogens in situ via the sulfatase enzyme. The present study utilized an isotopic kinetic technique to provide direct confirmation of local tumor synthesis. Animals bearing nitrosomethylurea (NMU)-induced rat mammary tumors were infused with 14C-estrone as well as 3H-estrone sulfate and plasma:tissue gradients for each steroid measured. Liver, serving as a control tissue, uniformly synthesized free estrone from estrone sulfate with local synthesis in this organ providing an average of 78 +/- 1.0% of the estrone in this tissue. In rat mammary tumors, five out of seven synthesized estrone locally with individual values ranging from 19 to 50% synthesized in tissue. These data indicate that liver uniformly converts estrone sulfate to free estrone, whereas the majority, but not all, breast tumors synthesize estrogen locally via this pathway.

Three-component model of oestrogen formation and regulation of intratumoural oestrogen pool in breast neoplasms.

Although a two-component model system (i.e. epithelial and stromal spindle-shaped cells) may explain the regulation of aromatase activity and the location of possible intratumoural sources of oestrogen production in mammary neoplasms, a third component, consisting of lymphocytes and macrophages, may also be considered. Examples are presented, suggesting the importance of a three-component model for a more complete understanding of the regulation of oestrogen production in breast-tumour tissue.

Estrogen deprivation causes estradiol hypersensitivity in human breast cancer cells.

Genetic and environmental factors can modulate the level of sensitivity to various hormones, including estrogens. Enhanced sensitivity to estradiol (E2) has been demonstrated in several biological conditions, such as in sheep during the nonbreeding season, in untreated patients with Turner's syndrome, and in the prepubertal state in normal girls. We postulated that secondary responses to hormonal therapy in patients with breast cancer could also result from enhanced E2 sensitivity, developing as an adaptive mechanism to E2 deprivation. The present study used the MCF-7 human breast cancer cell line as a model system to test the concept that enhanced sensitivity to E2 may occur as a result of adaptation to low E2 levels. After depriving MCF-7 cells of estrogens in tissue culture medium for periods of 1-6 months, we established conditions under which replication could be stimulated maximally by 10(-14)-10(-15) mol/L E2. In contrast, wild-type cells not exposed to estrogen deprivation required 10(-10) mol/L E2 to grow at the same rate. Further, the concentration of the antiestrogen, ICI 164384, needed to inhibit growth by 50% in estrogen-deprived cells was much lower than that required in wild-type cells (i.e. 10(-15) vs. 10(-9) mol/L). Nude mice implanted with these estrogen-deprived cells demonstrated an earlier appearance of palpable tumors in response to E2 than animals bearing wild-type cells. Reexposure to 10(-10)-10(-9) mol/L E2, either in vivo or in vitro, returned these cells to the level of estrogen sensitivity observed in wild-type cells. Taken together, these observations suggest that breast cancer cells can adapt to low levels of estrogens by enhancing their sensitivity to E2.

Use of ultrasensitive recombinant cell bioassay to measure estrogen levels in women with breast cancer receiving the aromatase inhibitor, letrozole.

The development of well tolerated, potent, specific, and nontoxic aromatase inhibitors for the treatment of postmenopausal women with estrogen-dependent breast cancer has been a major goal of recent studies. The third generation inhibitors now under investigation are nearly 10,000-fold more potent than first generation compounds. Currently available RIAs for plasma estradiol lack sufficient sensitivity to measure levels during aromatase inhibition and, thus, to assess drug potency precisely. The availability of an ultrasensitive bioassay for estradiol provided the opportunity to accurately assess the potency of a new third generation triazole aromatase inhibitor, letrozole (CGS 20267). We used this assay to measure estradiol levels in 14 women with metastatic breast cancer given letrozole at doses of 100 micrograms to 5.0 mg/day over a 12-week period. The lack of differences between doses and sampling times allowed pooling of data. Basal estradiol levels of 7.2 +/- 1.9 pmol/L (mean +/- SEM, 1.95 +/- 0.52 pg/mL) fell to 0.26 +/- 0.11 pmol/L (0.07 +/- 0.03 pg/mL) during the first 6 weeks of therapy and to 0.48 +/- 0.18 pmol/L (0.13 +/- 0.05 pg/mL) during the second 6 weeks of therapy. Although plasma estradiol levels measured by RIA were significantly correlated with levels measured by bioassay (r = 0.79; P < 0.01), the degree of suppression assessed by the bioassay (95 +/- 2% after 6 weeks) was greater than that determined by the RIA (81 +/- 4%), presumably due to improved ability to measure very low estradiol levels. We conclude that plasma estradiol is suppressed by letrozole to lower levels than previously observed, with equivalent suppression at all doses studied. A slight, although not statistically significant, rebound in estradiol levels occurs during the second 6 weeks of therapy compared to the first 6 weeks. Maximum inhibition of aromatase is achieved at letrozole doses as low as 100 micrograms.

Aromatase inhibitor development for treatment of breast cancer.

Inhibition of estrogen production provides effective therapy for patients with hormone-dependent breast cancer. The source of estrogens in premenopausal women is predominantly the ovary, but after the menopause, estradiol is synthesized in peripheral tissues through the aromatization of androgens to estrogens. Uptake from plasma is the primary mechanism for maintenance of estradiol concentrations in breast cancer tissue in premenopausal women, whereas several steps may be operant in postmenopausal women. These include enzymatic synthesis of estradiol via sulfatase, aromatase, and 17 beta-hydroxysteroid dehydrogenase in the tumor itself. Aromatization of androgens secreted by the adrenal to estrogens in peripheral tissues and transport to the tumor via circulation in the plasma provides another means of maintaining breast tumor estradiol levels in postmenopausal women. These various sources contribute to the high tissue estrogen levels measured in breast tumor tissue. To effectively suppress tissue concentrations of estrogens and circulating estradiol in postmenopausal patients, various aromatase inhibitors have been developed recently. These include steroidal inhibitors such as 4-hydroxy-androstenedione as well as non-steroidal compounds with imidazole and triazole structures. The most potent of these, CGS 20267, is reported to suppress levels of active estrogens (i.e., estrone, estrone sulfatase, and estradiol) by more than 95%. This compound can suppress both serum and 24-hr urine estrogens to a greater extent than produced by the second generation inhibitor, CGS 16949A. CGS 20267 is highly specific since it does not affect cortisol and aldosterone serum levels during ACTH stimulation tests nor sodium and potassium balance in 24-hr urine samples.(ABSTRACT TRUNCATED AT 250 WORDS)

Stromal spindle cells contain aromatase in human breast tumors.

In situ synthesis of estrogens by breast cancer tissue provides a potential explanation for the high concentrations of estradiol in mammary neoplasms in postmenopausal women. A major metabolic pathway for estrogen biosynthesis is the conversion of androstenedione to estrone via the enzyme aromatase. Biochemical studies have demonstrated aromatase in tumor tissue, but at relatively low and not clearly biologically significant levels. The present study tested the hypothesis that tumor levels of aromatase, albeit low, could be biologically important if present in high concentrations in focal clusters of specific cell types. A pilot study used an immunohistochemical method in frozen sections of fresh breast tumors as an optimal means to detect aromatase. Twelve of 18 tumors contained aromatase-positive cells, some with highly intense staining. A follow-up study then attempted to precisely define the types of cells containing aromatase and correlate the immunohistochemical findings with biochemical aromatase activity. A modified H-score (histological scoring system) was used to semiquantitate the amount of aromatase staining in tumor epithelial, stromal spindle, stromal inflammatory, and normal breast epithelial cells. We found that immunohistochemical staining for aromatase predominated in stromal spindle cells with a median H-score of 13, whereas tumor epithelial, stromal inflammatory, and normal breast elements contained lesser amounts (median H-scores of 4.8, 0.03, and 0.5, respectively). The H-score for stromal spindle cells, but not those for other cell types, correlated highly with the biochemical aromatase assay (P < 0.01). Using a cut-off parameter estimated by a sensitivity/specificity (receiver operating curve) analysis, 62% of tumors were classified as aromatase positive based on stromal spindle cell staining. A similar number were also positive by biochemical assay, with concordance between the two methods of 77%. These observations provide substantial evidence for the presence of aromatase in human breast tumors, particularly in stromal spindle cells, and support the biological importance of aromatase for in situ production of estradiol.