Differential regulation of c-myc by progestins and antiestrogens in T-47D human breast cancer cells

MYC amplification in subtypes of breast cancers in African American women

Background

MYC overexpression is associated with poor prognosis in breast tumors (BCa). The objective of this study was to determine the prevalence of MYC amplification and associated markers in BCa tumors from African American (AA) women and determine the associations between MYC amplification and clinico-pathological characteristics.

Methods

We analyzed 70 cases of well characterized archival breast ductal carcinoma specimens from AA women for MYC oncogene amplification. Utilizing immune histochemical analysis estrogen receptor (ER), progesterone receptor (PR), and (HER2/neu), were assessed. Cases were Luminal A (ER or PR+, Ki-67 < 14%), Luminal B (ER or PR+, Ki-67 = > 14% or ER or PR+ HER2+), HER2 (ER-, PR-, HER2+), and Triple Negative (ER-, PR-, HER2-) with basal-like phenotype. The relationship between MYC amplification and prognostic clinico-pathological characteristics was determined using chi square and logistic regression modeling.

Results

Sixty-five (97%) of the tumors showed MYC gene amplification (MYC: CEP8 > 1). Statistically significant associations were found between MYC amplification and HER2-amplified BCa, and Luminal B subtypes of BCa (p < 0.0001), stage (p < 0.001), metastasis (p < 0.001), and positive lymph node status (p = 0.039). MYC amplification was associated with HER2 status (p = 0.01) and tumor size (p = 0.01). High MYC amplification was seen in grade III carcinomas (MYC: CEP8 = 2.42), pre-menopausal women (MYC: CEP8 = 2.49), PR-negative status (MYC: CEP8 = 2.42), and ER-positive status (MYC: CEP8 = 2.4).

Conclusions

HER2 positive BCas in AA women are likely to exhibit MYC amplification. High amplification ratios suggest that MYC drives HER2 amplification, especially in HER2 positive, Luminal B, and subtypes of BCa.

Hormonal Modulation of Breast Cancer Gene Expression: Implications for Intrinsic Subtyping in Premenopausal Women

Clinics are increasingly adopting gene-expression profiling to diagnose breast cancer subtype, providing an intrinsic, molecular portrait of the tumor. For example, the PAM50-based Prosigna test quantifies expression of 50 key genes to classify breast cancer subtype, and this method of classification has been demonstrated to be superior over traditional immunohistochemical methods that detect proteins, to predict risk of disease recurrence. However, these tests were largely developed and validated using breast cancer samples from postmenopausal women. Thus, the accuracy of such tests has not been explored in the context of the hormonal fluctuations in estrogen and progesterone that occur during the menstrual cycle in premenopausal women. Concordance between traditional methods of subtyping and the new tests in premenopausal women is likely to depend on the stage of the menstrual cycle at which the tissue sample is taken and the relative effect of hormones on expression of genes versus proteins. The lack of knowledge around the effect of fluctuating estrogen and progesterone on gene expression in breast cancer patients raises serious concerns for intrinsic subtyping in premenopausal women, which comprise about 25% of breast cancer diagnoses. Further research on the impact of the menstrual cycle on intrinsic breast cancer profiling is required if premenopausal women are to benefit from the new technology of intrinsic subtyping.

C-myc as a predictive marker for chemotherapy in metastatic breast cancer

C-myc is considered to have an important role in cancerogenesis and tumor progression. The aim of this study was to evaluate a possible significance of c-myc amplification as a clinically useful prognostic/predictive parameter in metastatic breast cancer (MBC). Eighty-seven MBC patients with known clinicopathological parameters were included in the study, at the time of diagnosis of metastatic disease. In metastatic setting, 52% of patients received CMF, 34% received FAC, and 32% received hormonal therapy (tamoxifen). C-myc amplification was analyzed by chromogenic in situ hybridization, according to the manufacturer's instructions. C-myc amplification was detected in 26% cases and showed a strong correlation with ER status, stage of disease (initial) and existence of distance metastasis. There was no statistically significant difference in MBC (post-relapse) survival between c-myc-nonamplified and c-myc-amplified subgroups regardless of or regarding the treatment. However, correlation was found between c-myc status and individual patient's outcomes. Patients with c-myc amplification treated with chemotherapy (CMF and FAC) had clinical benefit (complete remission, partial remission or stable disease) in contrast to patients without amplification. Lack of significant difference in MBC (post-relapse) survival according to c-myc status could be due to a better response of patients to appropriate treatment (chemotherapy). It is possible that negative prognostic impact of c-myc amplification is masked with increased responsiveness to chemotherapy.

Progestins in the menopause in healthy women and breast cancer patients

At present, more than 200 progestin compounds are synthetized, but their biological effects are different: this is function of their structure, receptor affinity, metabolic transformations, the target tissues considered, dose. The action of progestins in breast cancer is controversial; some studies indicate an increase in breast cancer incidence, others show no differences, and yet others indicate a decrease. Many studies agree that treatment with progestins plus estrogens at a low dose and during a limited period (less than 5 years) can have beneficial effects in peri- and post-menopausal women. It was demonstrated that various progestins (e.g. nomegestrol acetate, medrogestone, promegestone), as well as tibolone and its metabolites, can block the enzymes involved in estradiol bioformation (sulfatase, 17beta-hydroxysteroid dehydrogenase) in breast cancer. Progesterone is converted into various metabolic products: in normal breast tissue the transformation is mainly to 4-ene derivatives, whereas in the tumor tissue 5alpha-pregane derivatives are predominant. Aromatase activity is the last step in the formation of estrogens by the conversion of androgens. In recent studies it was shown that 20alpha-dihydroprogesterone, a metabolite found mainly in normal breast tissue and having anti-proliferative properties, can act as an anti-aromatase agent. The data suggest the possible utilization of this compound in breast cancer prevention. In conclusion, in order to clarify and better understand the response of progestins in breast cancer (incidence and mortality), as well as in hormone replacement therapy or in endocrine dysfunction, new clinical trials are necessary using other progestins in function of the dose and period of treatment.

Tamoxifen increases apoptosis but does not influence markers of proliferation in an MCF-7 xenograft model of breast cancer

Twenty-four nude mice bearing MCF-7 breast cancer cells grown as xenografts and treated with tamoxifen (2.5 mg slow-release pellet) were studied for up to 35 days. Tumour size was measured in 2 dimensions at regular time-intervals and tumours were harvested on each of days 2, 4, 7, 14, 28 and 35 after the start of treatment. Control animals (8) received no treatment and the tumours were harvested after 0 or 35 days. Tumour sections were assessed for prevalence of apoptosis and mitosis and examined immunocytochemically for Ki(67)(MIB-1) and bcl-2 expression. Tumours increased in size during tamoxifen-treatment, but at a significantly slower rate (max. 2.6-fold) than in the untreated control animals; thus tumours not actually regressing may, nevertheless, be responding significantly to tamoxifen. MIB-1 and bcl-2 immunostaining and mitosis failed to show any consistent change over the period of study. Apoptosis, however, increased progressively and significantly to day-28 in tamoxifen-treated tumours, reaching approximately a 5-fold increase over day-0 values, then decreasing again to nearly 3-fold by day-35 (P= 0.0002). The apoptosis: mitosis ratio in treated tumours also increased to approximately 10-fold on day-28 over day-0 values, decreasing to nearly 4-fold by day-35 (P= 0.037). Within the treated group, apoptosis was significantly inversely correlated with both mitosis (R = -0.38, P= 0.03) and expression of bcl-2 (R = -0.48, P= 0.0056) and strongly positively correlated with both time on tamoxifen (R = +0.63, P= 0.0003) and the % inhibition of growth by tamoxifen (R = +0.58,P = 0.0012) in the 28 individual, treated tumours (estimated relative to the mean growth rate in the controls). The apoptosis: mitosis ratio was also inversely correlated with bcl-2 expression (R = -0.56, P= 0.0021) and positively correlated with both time on tamoxifen (R = +0.50, P= 0.0068) and % inhibition of growth (R = +0.56, P= 0.0019). In this hormone-sensitive tumour model for breast cancer, in which tamoxifen caused inhibition rather than regression, it was not possible to detect significant changes in the marker proteins Ki(67)and bcl-2, or in the prevalence of mitosis in relation to treatment; these factors may therefore not be accurate indices of response to tamoxifen in all situations. By contrast, however, tamoxifen induced a significant, early increase in the prevalence of apoptosis associated with inhibition of tumour growth and an inverse relationship in both mitosis and bcl-2 expression, suggesting that apoptosis may be an accurate and sensitive early marker of even a moderate response to tamoxifen.

MAP kinases couple multiple functions of human progesterone receptors: degradation, transcriptional synergy, and nuclear association

Breast cancers often have increased mitogen-activated protein kinase (MAPK) activity; this pathway influences breast cancer cell growth in part by targeting steroid hormone receptors. Bidirectional cross-talk between these two pathways is well documented; progestins increase the expression of type I growth factor receptors that couple to MAPK activation, and in turn, activation of p42 and p44 MAPKs increases ligand-dependent progesterone receptor (PR) transcriptional activity, and parodoxically, augments PR downregulation. Breast cancers that have become steroid hormone resistant often remain highly sensitive to growth factors. We believe that the mechanism of steroid hormone resistance is biochemically linked to the acquisition of growth factor responsiveness. Using in vitro models, we have established numerous regulatory links between signal transduction pathways elicited by peptide growth factors and PR. Of note is the role of phosphorylation of human PRs by MAPKs. Phosphorylation of PR on a key serine residue (Ser294) by MAPKs couples multiple receptor functions, including ligand-dependent PR downregulation by the ubiquitin-proteasome pathway, transcriptional synergy between progestins and growth factors, and nuclear localization of PR proteins. Linkage of these events suggests a mechanism for steroid hormone receptor "hypersensitivity" induced by growth factors. The uncoupling of these events during breast cancer progression is predicted to profoundly influence hormone responsiveness, as PR with altered stability may be driven primarily by upregulated growth factors.

Biological effects of progestins in breast cancer

The action of progestins is derived from many factors: structure, affinity for the progesterone receptor or for other steroid receptors, the target tissue considered, the biological response, the experimental conditions, the dose and metabolic transformation. The proliferative response to progestins in human breast cancer cells is contradictory: some progestins inhibit, others stimulate, have no effect at all, or have a dual action. For instance, medroxyprogesterone acetate has a stimulatory effect on breast cancer cells after a short period of treatment, but this effect becomes inhibitory when treatment is prolonged. It has been demonstrated that, in hormone-dependent breast cancer cells, various progestins (nomegestrol acetate, medrogestone, promegestone) are potent sulfatase inhibitory agents. The progestins can also involve the inhibition of the mRNA expression of this enzyme. In another series of studies it was also demonstrated that some progestins are very active in inhibiting 17beta-hydroxysteroid dehydrogenase for the conversion of estrone to estradiol. More recently it was observed that the progestins promegestone and medrogestone stimulate sulfotransferase for the formation of estrogen sulfates. Consequently, the action of progestins in blocking estradiol formation via sulfatase, or in stimulating the effect on sulfotransferase activity, can open interesting and new possibilities in clinical applications in breast cancer.

Amplification of c-myc by fluorescence in situ hybridization in a population-based breast cancer tissue array

A total of 261 primary breast carcinomas were analyzed for amplification of the c-myc oncogene by fluorescence in situ hybridization performed on tumor tissue array samples. Results were compared with individual clinicopathologic and follow-up data. Thirty-eight (14.6%) of the tumors showed c-myc gene amplification (defined as two or more additional copies of c-myc gene in relation to the number of chromosome 8 centromere). The reproducibility of fluorescence in situ hybridization assay (defined by hybridization with two different myc probes) was good (kappa coefficient 0.402). Statistically significant associations were found between c-myc amplification and DNA aneuploidy (P =.0011), and progesterone receptor negativity (P =.0071), and c-myc amplification also tended to be associated with high histologic grade (P =.064), positive axillary nodal status (P =.080), and a high S-phase fraction (P =.052). c-myc amplification was not significantly associated with overall survival of patients with invasive cancer (P =.32). These data from a population-based tumor material suggest that c-myc amplification is a feature of aggressive breast cancers, but that it is unlikely to be a clinically useful prognostic factor.

Progestins and breast cancer

In the last years there has been an extraordinary development in the synthesis of new progestins. These compounds are classified, in agreement with their structure, in various groups which include progesterone, retroprogesterones, 17alpha-hydroxyprogesterones, 19-norprogesterones, 17alpha-hydroxyprogesterone derivatives, androstane and estrane derivatives. The action of progestins is a function of many factors: its structure, affinity to the progesterone receptor or to other steroid receptors, the target tissue considered, the biological response, the experimental conditions, dose, and metabolic transformation. The information on the action of progestins in breast cancer patients is very limited. Positive response with the progestins: medroxyprogesterone acetate and megestrol acetate was obtained in post-menopausal patients with advanced breast cancer. However, extensive information on the effect of progestins was obtained in in vitro studies using hormone-dependent and hormone-independent human mammary cancer cell lines. It was demonstrated that in the hormone-dependent breast cancer cells, various progestins (nomegestrol acetate, tibolone, medrogestone, promegestone) are potent sulfatase inhibitory agents. The progestins can also involve the inhibition of mRNA of this enzyme. In another series of studies it was also demonstrated that various progestins are very active in inhibiting the 17beta-hydroxysteroid dehydrogenase for the conversion of estrone to estradiol. More recently it was observed that the progestins promegestone or medrogestone stimulate the sulfotransferase for the formation of estrogen sulfates. Consequently, the blockage in the formation of estradiol via sulfatase, or the stimulatory effect on sulfotransferase activity, by progestins can open interesting and new possibilities in clinical applications in breast cancer.

Mechanisms of cyclin-dependent kinase inactivation by progestins

The steroid hormone progesterone regulates proliferation and differentiation in the mammary gland and uterus by cell cycle phase-specific actions. In breast cancer cells the predominant effect of synthetic progestins is long-term growth inhibition and arrest in G1 phase. Progestin-mediated growth arrest of T-47D breast cancer cells was preceded by inhibition of cyclin D1-Cdk4, cyclin D3-Cdk4, and cyclin E-Cdk2 kinase activities in vitro and reduced phosphorylation of pRB and p107. This was accompanied by decreases in the expression of cyclins D1, D3, and E, decreased abundance of cyclin D1- and cyclin D3-Cdk4 complexes, increased association of the cyclin-dependent kinase (CDK) inhibitor p27 with the remaining Cdk4 complexes, and changes in the molecular masses and compositions of cyclin E complexes. In control cells cyclin E eluted from Superdex 200 as two peaks of approximately 120 and approximately 200 kDa, with the 120-kDa peak displaying greater cyclin E-associated kinase activity. Following progestin treatment, almost all of the cyclin E was in the 200-kDa, low-activity form, which was associated with the CDK inhibitors p21 and p27; this change preceded the inhibition of cell cycle progression. These data suggest preferential formation of this higher-molecular-weight, CDK inhibitor-bound form and a reduced number of cyclin E-Cdk2 complexes as mechanisms for the decreased cyclin E-associated kinase activity following progestin treatment. Ectopic expression of cyclin D1 in progestin-inhibited cells led to the reappearance of the 120-kDa active form of cyclin E-Cdk2 preceding the resumption of cell cycle progression. Thus, decreased cyclin expression and consequent increased CDK inhibitor association are likely to mediate the decreases in CDK activity accompanying progestin-mediated growth inhibition.

Estrogen and progestin regulation of cell cycle progression

Estrogens and progesterone, acting via their specific nuclear receptors, are essential for normal mammary gland development and differentiated function. The molecular mechanisms through which these effects are mediated are not well defined, although significant recent progress has been made in linking steroid hormone action to cell cycle progression. This review summarizes data identifying c-myc and cyclin D1 as major downstream targets of both estrogen- and progestin-stimulated cell cycle progression in human breast cancer cells. Additionally, estrogen induces the formation of high specific activity forms of the cyclin E-Cdk2 enzyme complex lacking the cyclin-dependent kinase (CDK)3 inhibitor, p21. The delayed growth inhibitory effects of progestins, which are likely to be prerequisites for manifestation of their function in differentiation, also involve decreases in cyclin D1 and E gene expression and recruitment of CDK inhibitors into cyclin D1-Cdk4 and cyclin E-Cdk2 complexes. Thus estrogens and progestins affect CDK function not only by effects on cyclin abundance but also by regulating the recruitment of CDK inhibitors and, as yet undefined, additional components which determine the activity of the CDK complexes. These effects of estrogens and progestins are likely to be major contributors to their regulation of mammary epithelial cell proliferation and differentiation.

A sequence in the 5' flanking region confers progestin responsiveness on the human c-myc gene

Previous reports have shown that progestins stimulate the proliferation of the human breast cancer cell line T47D in culture. Under different conditions other reports have shown progestin stimulation, inhibition or no effect on growth. It has also been shown that c-myc expression is stimulated at early times by progestins. We are currently testing the hypothesis that the mechanism of growth enhancement by progestins involves the stimulation of expression of c-myc. This hypothesis predicts a progesterone regulatory region in or near the c-myc gene. We have identified a region, from -2327 to -1833, which serves this function. This region includes a 15 bp sequence with homology to the PRE (progesterone response element) consensus sequence. Human progesterone receptor (PR) binds to this sequence in a specific, ligand-enhanced manner in electrophoretic mobility shift assays (EMSA). A 3507 bp HindIII-XbaI fragment of the 5' flanking region of the c-myc gene, -2327 to +1180, containing the progestin regulatory region and the c-myc promoter, confers progestin responsiveness to the CAT (chloramphenicol acetyl transferase) reporter gene in progesterone receptor (PR)-rich T47D human breast cancer cells, but not in PR-negative MDA-MB-231 cells. Removal of the progestin regulatory region abrogates progestin responsiveness. These data demonstrate that the sequence from -2327 to -1833 of the human c-myc gene includes a positive progestin regulatory region.

Defining a role for c-Myc in breast tumorigenesis

The proto-oncogene c-myc is commonly amplified and overexpressed in human breast tumors, and the tumorigenic potential of c-myc overexpression in mammary tissue has been confirmed by both in vitro and in vivo models of breast cancer. However, the mechanisms by which Myc promotes tumorigenesis are not well understood. Recent evidence indicates that Myc can promote cell proliferation as well as cell death via apoptosis. These studies provide new insight and impetus in defining a role for c-Myc in breast tumorigenesis and may point toward novel targets for breast cancer therapy.

Estrogen receptors, progesterone receptors, and cell proliferation in human breast cancer

The breast is a target organ for estrogens and progesterone. These hormones control several functions of the normal and abnormal mammary epithelium including cell proliferation. Most of the actions of estrogens and progesterone are mediated via specific steroid receptors, and one would expect that proliferating cells should contain estrogen receptors (ER) and/or progesterone receptors (PR). However, the correlation between receptor expression and cell proliferation is still controversial. In the present study we have examined 29 human breast cancer samples; in 17 of them we evaluated the simultaneous ER and PR localization with that of proliferating cell nuclear antigen (PCNA) and silver-stained nucleolar organizer regions (AgNORs) in a cell-by-cell study. We found that in almost 50% of the tumor biopsies examined, the cells expressing ER were significantly associated with elevated cell proliferation. In another group (38%) there were not significant differences between ER expression and cell proliferation. In only one of the samples (6%) the cells expressing ER showed lower cell proliferation. The study also revealed that in 44% of the tumors the PR expressing cells were associated with elevated cell proliferation. In a second group the PR expression was not significantly associated with cell proliferation (33% of the cases). Finally, in 22% of the samples the cells carrying PR showed lower cell proliferation. We also detected lower ER immunoreactivity in 30% of the breast cancer biopsies with one of the monoclonal antibodies against ER (antibody 1D5 directed against the A/B domain). This group of tumors was PR-negative (or very weakly positive) and had high proliferation. The presence of tumors with 'abnormal' ER proteins and displaying ER/PR significantly associated with elevated cell proliferation could have implications in human breast cancer treatment.

Estradiol stimulates c-myc proto-oncogene expression in normal human breast epithelial cells in culture

The proto-oncogene c-myc is involved in the stimulation of cell proliferation, and its expression is known to be stimulated by estradiol (E2) in human breast cancer cell lines and various non-cancerous E2-dependent tissues. However, little information is currently available concerning its expression and regulation in normal human breast tissue. We therefore studied c-myc expression and hormone modulation in normal human breast epithelial (HBE) cells in culture, routinely obtained in our laboratory and which remain hormone-dependent. On these normal HBE cells, E2 induced a biphasic increase in c-myc mRNA level, with a first peak as early as 30 min, and a secondary increase after 2 h of treatment; this stimulation was dose-dependent, with an optimal concentration of 10 nM E2. Its primary action is probably at the transcriptional level since the half-life of c-myc mRNA measured in the presence of actinomycin D (12 +/- 3 min) was not modified by E2 treatment. In addition, E2 stimulation of c-myc mRNA does not require protein synthesis since it was not suppressed by cycloheximide treatment. Western blot studies of c-myc protein in HBE cells revealed the same biphasic pattern of stimulation, with a first peak after 60 min and a second one after 2 h of E2 treatment. In conclusion, the c-myc proto-oncogene is expressed in normal HBE cells, as in breast cancer cells. Moreover, E2 stimulates c-myc expression which, therefore, may partly mediate the growth-promoting effect of E2.

Antiestrogen regulation of cell cycle progression and cyclin D1 gene expression in MCF-7 human breast cancer cells

The molecular mechanisms by which antiestrogens inhibit breast cancer cell proliferation are not well understood. Using cultured breast cancer cell lines, we studied the effects of antiestrogens on proliferation and cell cycle progression and used this information to select candidate cell cycle regulatory genes that are potential targets for antiestrogens. Under estrogen- and serum-free conditions antiestrogens inhibited proliferation of MCF-7 cells stimulated with insulin. Cells were blocked at a point in G1 phase. These effects are comparable with those in serum- and estrogen-containing medium and were also seen to a lesser degree in nude mice bearing MCF-7 tumors. Similar observations with other peptide mitogens suggest that the process inhibited by antiestrogens is common to estrogen and growth factor activated pathways. Other studies have identified G1 cyclins as potential targets for growth factor and steroid hormone/steroid antagonist regulation of breast epithelial cell proliferation. In MCF-7 cells growing in the presence of fetal calf serum, cyclin D1 mRNA was rapidly down-regulated by steroidal and nonsteroidal antiestrogens by an apparently estrogen receptor mediated mechanism. Cyclin D1 gene expression was maximally inhibited before effects on entry into S phase and inhibition was therefore not merely a consequence of changes in cell cycle progression. Together with data on the effects of antiestrogens in serum-free conditions [1], these results suggest down-regulation of cyclin D1 by antiestrogens may be a general phenomenon in estrogen receptor-positive breast cancer cells, independent of culture conditions and class of antiestrogen. These observations are compatible with the hypothesis that reductions in cyclin D1 levels may mediate in part the action of antiestrogens in blocking entry of cells into S phase.

Growth factor, steroid, and steroid antagonist regulation of cyclin gene expression associated with changes in T-47D human breast cancer cell cycle progression

Cyclins and proto-oncogenes including c-myc have been implicated in eukaryotic cell cycle control. The role of cyclins in steroidal regulation of cell proliferation is unknown, but a role for c-myc has been suggested. This study investigated the relationship between regulation of T-47D breast cancer cell cycle progression, particularly by steroids and their antagonists, and changes in the levels of expression of these genes. Sequential induction of cyclins D1 (early G1 phase), D3, E, A (late G1-early S phase), and B1 (G2 phase) was observed following insulin stimulation of cell cycle progression in serum-free medium. Transient acceleration of G1-phase cells by progestin was also accompanied by rapid induction of cyclin D1, apparent within 2 h. This early induction of cyclin D1 and the ability of delayed administration of antiprogestin to antagonize progestin-induced increases in both cyclin D1 mRNA and the proportion of cells in S phase support a central role for cyclin D1 in mediating the mitogenic response in T-47D cells. Compatible with this hypothesis, antiestrogen treatment reduced the expression of cyclin D1 approximately 8 h before changes in cell cycle phase distribution accompanying growth inhibition. In the absence of progestin, antiprogestin treatment inhibited T-47D cell cycle progression but in contrast did not decrease cyclin D1 expression. Thus, changes in cyclin D1 gene expression are often, but not invariably, associated with changes in the rate of T-47D breast cancer cell cycle progression. However, both antiestrogen and antiprogestin depleted c-myc mRNA by > 80% within 2 h. These data suggest the involvement of both cyclin D1 and c-myc in the steroidal control of breast cancer cell cycle progression.

Growth factor, steroid, and steroid antagonist regulation of cyclin gene expression associated with changes in T-47D human breast cancer cell cycle progression

Cyclins and proto-oncogenes including c-myc have been implicated in eukaryotic cell cycle control. The role of cyclins in steroidal regulation of cell proliferation is unknown, but a role for c-myc has been suggested. This study investigated the relationship between regulation of T-47D breast cancer cell cycle progression, particularly by steroids and their antagonists, and changes in the levels of expression of these genes. Sequential induction of cyclins D1 (early G1 phase), D3, E, A (late G1-early S phase), and B1 (G2 phase) was observed following insulin stimulation of cell cycle progression in serum-free medium. Transient acceleration of G1-phase cells by progestin was also accompanied by rapid induction of cyclin D1, apparent within 2 h. This early induction of cyclin D1 and the ability of delayed administration of antiprogestin to antagonize progestin-induced increases in both cyclin D1 mRNA and the proportion of cells in S phase support a central role for cyclin D1 in mediating the mitogenic response in T-47D cells. Compatible with this hypothesis, antiestrogen treatment reduced the expression of cyclin D1 approximately 8 h before changes in cell cycle phase distribution accompanying growth inhibition. In the absence of progestin, antiprogestin treatment inhibited T-47D cell cycle progression but in contrast did not decrease cyclin D1 expression. Thus, changes in cyclin D1 gene expression are often, but not invariably, associated with changes in the rate of T-47D breast cancer cell cycle progression. However, both antiestrogen and antiprogestin depleted c-myc mRNA by > 80% within 2 h. These data suggest the involvement of both cyclin D1 and c-myc in the steroidal control of breast cancer cell cycle progression.

Role of androgens in breast cancer

There is a vast amount of information on the concentration of different androgens in human breast cancer in pre- and post-menopausal women. Many years ago it was suggested that some androgens, in particular dehydroepiandrosterone (DHA), could be one of the parameters to establish whether these androgens can predict breast cancer. However, the enormous data available on the plasma and tissue concentrations, particularly DHA and DHA sulfate (DHA-S) allow confirmation that the quantitative values of these androgens are not significantly different in normal women from those with breast cancer. Another important aspect of androgens in breast cancer is their function as precursors of estrogens, hormones which play an important role in breast cancerization. However, it is not clear at present what the quantitative contribution of androgens "via aromatase" is to the formation of estrogens, because more recently it was found that estradiol in breast cancer tissues originates mainly "via sulfatase" using estrone sulfate as precursor. A point of further interest is that a series of authors have demonstrated that the administration of DHA to experimental animals with breast cancer significantly decreased the evolution of the disease. This part of the data is also contradictory because other experimental information has shown that administration of DHA can increase the incidence of granulosa cell tumors. Finally, it has been suggested that androgens, in their capacity as anti-estrogens, can be used to substitute anti-estrogens in cases where treatment with classical anti-estrogens has no response. In conclusion, more information concerning the plasma and tissular concentrations of androgens, their contribution as estrogen precursors and their biological response(s), is needed in order to have a clearer idea of the role of these steroids in breast cancer.

C-myc gene chromatin of estrogen receptor positive and negative breast cancer cells

Expression of the c-myc protooncogene is estrogen regulated in estrogen receptor (ER) positive, hormone-dependent human breast cancer cells, but it is constitutively active in ER negative, hormone-independent breast cancer cells. To determine whether these differences are reflected in c-myc chromatin, DNase I hypersensitive sites (DHS) were mapped. Six DHS were detected in all cell lines studied, with DHS 3(2) being more prominent than DHS 3(1). The accessibility of DHS 2 was markedly greater in ER negative cells than in ER positive cells, and this relative accessibility remained unchanged when cells were grown in estrogen free medium. DHS 2, 3(1) and 3(2) map near the P0, P1 and P2 promoters, respectively. An analysis of promoter usage demonstrated that P2 was the preferred promoter. Thus, the differences in the accessibility of DHS 2 in c-myc chromatin of ER positive and negative cells likely reflects alterations in DNA-protein interactions in this region.

Tyrosine kinase receptor--nuclear protooncogene interactions in breast cancer

In summary, evidence is beginning to accumulate in support of a major role for tyrosine kinase receptors (and their activating growth factors) and steroid hormones and their receptors in normal development and differentiation of the mammary gland. A point of intersection of their mechanisms of action in growth control appears to be the induction of nuclear protooncogenes such as c-myc. When c-myc is amplified, as it is in many breast cancers, EGF and FGF receptor tyrosine kinase action becomes transforming, not simply mitogenic. A source of the transforming factors could be either stromal or epithelial. This mechanism could function early in the progression of breast cancer. c-erbB-2 and EGF receptor overexpression and amplification, when they occur, appear to render tumors even more malignant and of especially poor prognosis. These mechanisms could function late in the progression of breast cancer. Transgenic mouse studies have begun to echo these themes. They have established that a growth factor (TGF-alpha) and its receptor (EGF receptor), which appear to be important in normal mouse and human proliferation and gland development, and a protooncogene (c-myc), commonly amplified and overexpressed in human and mouse breast cancer, can each contribute to mammary carcinogenesis. The mechanisms of the two are likely to be distinct. myc is likely to be acting as a tumor initiator in combination with normal proliferative factors, whereas TGF-alpha is likely to be acting as a hyperproliferative (promotional) factor in combination with a normal background of mutational events. The role of unmutated but amplified erbB-2 in the transgenic mouse is not yet known.

Activated oncogenes and putative tumor suppressor genes involved in human breast cancers

Cytogeneticists first proposed that the karyotypic abnormalities identified on chromosomes 1, 3, 6, 11, 13, 16, 17, and 18 supported a genetic basis for breast cancer. Such abnormal banding patterns, however, may represent either loss-of-function or gain-of-function molecular events. RFLP analyses have since confirmed that 20-60% of primary and spontaneous human breast tumors exhibit allelic losses on these same chromosomes, although the exact genes involved at these chromosomal sites remain largely unknown. Knowledge gained about the Rb-1 and p53 tumor suppressor genes at 13q14 and 17p13 in breast and other human tumors supports the paradigm that for any chromosomal locus, allelic loss associated with a mutation in the remaining tumor allele signifies an involved tumor suppressor gene. Given this paradigm, there are nearly a dozen putative breast tumor suppressor genes under active investigation, with most investigators now focusing on various chromosome 17 loci. Among the known proto-oncogenes found activated in breast cancer, amplification of c-erbB-2 at 17q21 is the most widely studied and clinically significant gain-of-function event uncovered to date, occurring in about 20% of all primary breast tumors. The involvement of this overexpressed membrane receptor has engendered interest in related tyrosine kinase receptors, such as EGFR, IR, and IGF-I-R, as well as their respective ligands, which may be overexpressed in a greater fraction of tumors, contributing to the autocrine and paracrine regulation of breast cancer growth and metastasis. New attention is being given to the potentially oncogenic function of structurally altered nuclear transactivating steroid hormone receptors, such as ER, whose overexpression has long been used to determine endocrine therapy and prognosis for individual breast cancer patients. While c-myc was one of the first known proto-oncogenes to be found amplified and overexpressed in human breast cancers, the actual incidence and clinical significance of its activation remain disputed and in need of further study. Lastly, we can expect greater clarification about the importance of various 11q13 genes found coamplified in nearly 20% of primary breast cancers, and pursuit into the intriguing possibility that a cyclin-encoding gene represents the overexpressed locus of real interest in this amplicon. Virtually all of these important genetic abnormalities identified thus far are associated with but not restricted to human breast cancers. The absence of identifiable molecular defects relating to the tissue specificity of this malignancy must be considered a substantial gap in our basic understanding of breast carcinogenesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanisms of growth inhibition by antiestrogens and progestins in human breast and endometrial cancer cells

Marked changes in both growth factor and proto-oncogene expression occur due to treatment of hormonally-responsive human cancers with progestins and antiestrogens. In human endometrial cancer cell lines the antiproliferative effects of progestins and antiestrogens in a particular cell line appear to be associated with similar effects on growth factor and/or proto-oncogene expression. This suggests that although these compounds initially interact with different steroid hormone receptors, the molecular mechanisms of their growth inhibition may be essentially similar. In the case of human breast cancer cell lines, however, the effects of progestins and antiestrogens on gene regulation are often different, suggesting that the molecular mechanisms of progestin and antiestrogen growth inhibition may be essentially dissimilar.