Structural requirements for high affinity ligand binding by estrogen receptors: a comparative analysis of truncated and full length estrogen receptors expressed in bacteria, yeast, and mammalian cells

In order to better understand the structural requirements for effective high affinity binding of estrogens and antiestrogens by the human estrogen receptor (ER), a comparative study was undertaken in which we examined: 1) native ER from the MCF-7 ER-positive human breast cancer cell line; 2) full length ER expressed in yeast; 3) the ER hormone binding domain (amino acid residues 302-595) expressed in yeast; 4) a bacterially expressed protein A fusion product encoding a truncated ER (amino acid residues 240-595); and 5) a synthetic peptide encompassing amino acids 510-551 of the ER. The binding parameters studied included affinity, kinetics, structural specificity for ligands, and stability. Full length ER expressed in yeast was very similar to the MCF-7 ER in its affinity [dissociation constant (Kd), 0.35 +/- 0.05 nM], dissociation rate (t1/2, 3-4 h at 25 C), and structural specificity for both reversible and covalently attaching affinity ligands. While the truncated ER expressed in yeast was similar to MCF-7 ER in its specificity of ligand binding, it showed a slightly reduced affinity for estradiol (Kd, 1.00 +/- 0.17 nM). The bacterially expressed ER also had a lower affinity for estradiol (Kd, 1.49 +/- 0.16 nM), which may be due in part to an increase in the dissociation rate (t1/2, 0.5 h at 25 C). The attachment of covalent affinity ligands and structural specificity for a variety of reversible ligands was comparable in the bacterially expressed ER to that observed for the receptors expressed in MCF-7 cells and yeast.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of a temperature-sensitive mutation in the hormone binding domain of the human estrogen receptor. Studies in cell extracts and intact cells and their implications for hormone-dependent transcriptional activation

A previous report from this laboratory (Reese, J.C., and Katzenellenbogen, B. S. (1991) J. Biol. Chem. 266, 10880-10887) identified an estrogen receptor (ER) mutant which had a similar binding affinity for estradiol as wild-type ER but displayed a dose-response shift for estradiol in transactivation studies. In this study, we have utilized hormone binding, DNA binding, and gene transfer experiments to further characterize this mutant, which contains an alanine substitution for a cysteine at amino acid 447 in the hormone binding domain of the receptor. Hormone binding studies indicate that the C447A receptor is a temperature-sensitive mutant, whose instability is only apparent at elevated temperatures, and that ligand can stabilize the mutant receptor. Western blot analysis reveals that the temperature-sensitive loss of hormone binding is not attributable to a degradation of receptor protein, but rather is an inactivation of the receptor's hormone binding ability. In addition to the loss in the hormone binding capacity of the C447A mutant, this mutant shows a temperature-sensitive loss in the DNA binding ability of the receptor. Transactivation profiles of the mutant and wild-type receptors demonstrate that incubation of transfected cells with increasing concentrations of estradiol at more ambient temperatures shifts the mutant receptor's dose-response curves to the left, converging on the wild-type curve. Hence, these transactivation studies reveal that the dose-response shift observed for this mutant in cells reflects the measured instability of the hormone binding and DNA interaction of the C447A mutant that can be demonstrated in vitro. In addition, this temperature-sensitive ER mutant is of interest in that its DNA binding is now ligand-dependent with the result that transcriptional activation now parallels receptor occupancy by ligand, which is similar to other steroid hormone receptors.

Human estrogen receptor mutants with altered estrogen and antiestrogen ligand discrimination

A structural characteristic of many antiestrogens (AEs) is a bulky side chain with basic or polar functional groups that may interact with charged and polar amino acids near the hormone-binding site in the estrogen receptor. Recently, we have identified Cys530 as the residue of the human estrogen receptor that is the site of covalent labeling by aziridine analogs of estrogens (Es) and AEs (Harlow, K. W., Smith, D. N., Katzenellenbogen, J. A., Greene, G. L., and Katzenellenbogen, B. S. (1989) J. Biol. Chem. 264, 17476-17485). Since the aziridine function is on the bulky side chain of these ligands, Cys530 must be at or near the site of those side chain interactions. To probe these interactions, we have, by site-directed mutagenesis, made mutant human estrogen receptors in which charged and polar amino acids near Cys530 are changed (Glu523 to Gln, Lys529 and Lys531 to Gln, Asn532 to Asp, and Asp538 to Asn) so as to alter charge with minimal steric alteration. These receptors were expressed in mammalian (Chinese hamster ovary or COS-1) cells and assayed for their binding affinity for Es and AEs, their interaction with estrogen-responsive element DNA, and their ability to activate or suppress transcription of estrogen-responsive reporter genes. Two of the estrogen receptor mutants, KKN-QQD (mutation of Lys529, Lys531, and Asn532 to Gln529, Gln531, and Asp532, respectively) and KK-QQ (mutation of Lys529 and Lys531 to Gln529 and Gln531, respectively), in which the local charge is changed from +2 to -1 or 0, respectively, display an affinity for estradiol (E2) 5-10 times lower than that of the wild-type receptor, which is attributable to an enhanced rate of E2 dissociation. Although these mutant receptors have reduced affinity for a variety of Es, they retain unaltered affinity for AE. The profiles of transcriptional activation of reporter genes by various concentrations of E2 show that these two mutants (KKN-QQD and KK-QQ) require 40- and 15-fold higher E2 concentrations, respectively, to achieve half-maximal activity. In contrast, mutants E523Q and D538N, with changes at amino acids further from Cys530, were unaltered in their hormone binding and transactivation activity by E or AE. Interestingly, the AEs 4-hydroxytamoxifen, LY 117018, U 23469M, and ICI 164384 were 15-30-fold more effective in inhibiting E2-stimulated transcription by mutants KKN-QQD and KK-QQ compared to the wild-type receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential DNA-binding abilities of estrogen receptor occupied with two classes of antiestrogens: studies using human estrogen receptor overexpressed in mammalian cells

We have developed a transient transfection system using the Cytomegalovirus (CMV) promoter to express the human estrogen receptor (ER) at very high levels in COS-1 cells and have used it to study the interaction of agonist and antagonist receptor complexes with estrogen response element (ERE) DNA. ER can be expressed to levels of 20-40 pmol/mg or 0.2-0.3% of total soluble protein and all of the soluble receptor is capable of binding hormone. The ER binds estradiol with high affinity (Kd 0.2 nM), and is indistinguishable from native ER in that the receptor is capable of recognizing its cognate DNA response element with high affinity, and of transactivating a transgene in an estradiol-dependent manner. Gel mobility shift assays reveal interesting ligand-dependent differences in the binding of receptor complexes to ERE DNA. Receptors occupied by estradiol or the type I antiestrogen transhydroxytamoxifen bind to DNA response elements when exposed to the ligand in vitro or in vivo. Likewise, receptors exposed to the type II antiestrogen ICI 164,384 in vitro bind to ERE DNA. However, when receptor exposure to ICI 164,384 is carried out in vivo, the ER-ICI 164,384 complexes do not bind to ERE DNA, or do so only weakly. This effect is not reversed by subsequent incubation with estradiol in vitro, but is rapidly reversible by in vivo estradiol exposure of intact COS-1 cells. This suggests there may be some cellular process involved in the mechanism of antagonism by the pure antiestrogen ICI 164,384, which is not observed in cell-free extracts.

Differential regulation of gene expression by estrogen in estrogen growth-independent and -dependent MCF-7 human breast cancer cell sublines

We have examined the ability of estradiol (E2) to regulate the expression of three mRNAs [for pS2, progesterone receptor (PR), and estrogen receptor (ER)], known to be under E2 regulation in the parental E2 growth-responsive MCF-7 cells, in an E2 growth-independent MCF-7 K3), previously isolated from the parental estrogen-dependent MCF-7 K1 human breast cancer cells after long term growth in vitro in the absence of estrogen, acquired estrogen-independent growth in vitro as well as the ability to form tumors in nude mice in vivo without estrogen. We find that the content of pS2 mRNA and the transcription rate of the pS2 gene, while being markedly increased by E2 in MCF-7 K1 cells, are no longer stimulated by E2 in this subline, although protein kinase activators tremendously increase (greater than 10-fold) pS2 mRNA in both K1 and K3 cells. In fact, basal pS2 mRNA levels are elevated 2.8 +/- 0.4-fold in MCF-7 K3 cells, and E2 evokes a concentration-dependent suppression of the pS2 mRNA level. In contrast, PR mRNA in the K3 subline, as in the parental K1 cells, is still up-regulated by E2, and ER mRAN content and the ER mRNA transcription rate are still down-regulated by E2 and show normal E2 dose-response relationships, implying that the ER in this subline is functional. These results demonstrate that the progression to estrogen-independent growth in K3 cells is accompanied by a change in the regulation of some estrogen-induced genes by estrogen. While PR and ER retain normal patterns of regulation by E2, the pS2 gene in the estrogen growth-independent K3 subline is differentially affected and is no longer stimulated by E2. Our data suggest that this altered regulation of the pS2 gene is probably not caused by a defect of the ER or ER regulation in this subline.

Mutagenesis of cysteines in the hormone binding domain of the human estrogen receptor. Alterations in binding and transcriptional activation by covalently and reversibly attaching ligands

We have carried out experiments to determine the role of the cysteines in the hormone-binding domain (HBD) of the human estrogen receptor (ER) in receptor function. In each mutant receptor, 1 of the 4 cysteines in the HBD (cysteines 381, 417, 447, and 530) was changed by in vitro mutagenesis of the ER cDNA (containing Gly400) from cysteine to alanine; Cys530 was also mutated to a serine. The mutant and wild-type receptor cDNAs were expressed in Chinese hamster ovary cells using an expression vector containing the Rous sarcoma virus promoter. The mutant and wild-type receptors were assayed for hormone binding and for their ability to activate estrogen-responsive reporter plasmids. All ER mutants bound estradiol (E2) with affinity similar to wild-type ER, displaying a Kd between 0.3 and 0.8 nM (wild-type ER Kd = 0.45 +/- 0.10 nM). All were capable of covalent labeling by the affinity ligands ketononestrol aziridine, an estrogen agonist, and tamoxifen aziridine, an antagonist. Since in previous work we identified Cys530 as the site of covalent attachment of these ligands (Harlow, K.W., Smith, D.N., Katzenellenbogen, J.A., Greene, G.L., and Katzenellenbogen, B.S. (1989) J. Biol. Chem. 264, 17476-17485) it appears that an alternate residue(s) can be labeled in the absence of a cysteine at position 530; studies with methyl methanethiosulfonate, a cysteine-specific reagent, suggest that this residue is probably another cysteine in the HBD. The C381A, C417A, C530A and C530S ERs showed E2-stimulated transcriptional activation profiles similar to wild-type ER whereas the dose response for E2 for the C447A mutant was shifted to the right, requiring 50 x higher E2 concentrations to achieve half-maximal response. Tamoxifen aziridine inhibited E2-stimulated transcription, and ketononestrol aziridine stimulated transcription by wild-type, C530A, and C530S ER, but the effectiveness of these covalently attaching ligands was altered in the C530A and C530S mutants. Thus, these two mutant receptors are altered in their transactivation response to agonist and antagonist affinity labeling ligands but are unaltered in their response to reversibly binding estrogens and antiestrogens. In addition, we show that a mutant ER (C447A) can have an affinity for E2 similar to that of wild-type ER but differs in its ability to activate transcription in response to E2, indicating a decoupling of the hormone binding and transcriptional activation functions in this receptor.

Progesterone receptor regulation in uterine cells: stimulation by estrogen, cyclic adenosine 3',5'-monophosphate, and insulin-like growth factor I and suppression by antiestrogens and protein kinase inhibitors

Primary uterine cell cultures were used to study multifactor regulation of progesterone receptor (PR) and the signal transduction pathways which may serve to mediate that regulation. Increases in intracellular cAMP, brought about by treatment with cholera toxin plus isobutyl methyl xanthine or by addition of 8-bromo-cAMP, result in 6- to 7-fold increases in the intracellular content of PR as monitored by [3H]R5020 binding and by Western immunoblot using anti-PR antibodies. In these primary cultures of uterine cells isolated from 19-day-old immature rats, 8-bromo-cAMP evokes significant increases in PR by 8 h with maximal increases by 24 h. This time course and magnitude of PR stimulation are similar to those evoked by maximally effective concentrations of estradiol (3 x 10(-9) M) or IGF-I (20 ng/ml). Dose-response studies reveal that 10(-6) to 10(-4) M concentrations of 8-bromo-cAMP (8-Br-cAMP) elicit a maximal response. In contrast, 8-bromo-cGMP over a wide concentration range was unable to elevate cellular PR levels. Under these culture conditions, cell proliferation was not altered by treatment with any of these agents. Although estrogen, cAMP, and insulin-like growth factor I (IGF-I) may act via different pathways to increase PR, the effects evoked by maximally effective concentrations of these agents are not additive implying involvement of a common component. The increases in PR evoked by estradiol, cAMP, or IGF-I are markedly suppressed by treatment with antiestrogen (ICI 164,384) or the cyclic nucleotide-dependent protein kinase inhibitor H8 or the protein kinase A inhibitor PKI, indicating the involvement of the estrogen receptor and phosphorylation pathways in PR regulation by these three agents. The present studies identify cAMP, as well as estrogen and IGF-I, as important regulators of the level of PR in uterine cells and suggest that multiple factors, including those affecting intracellular cAMP levels, might influence responsiveness to progestins via regulation of the intracellular PR content.

Efficient and selective photoaffinity labeling of the estrogen receptor using two nonsteroidal ligands that embody aryl azide or tetrafluoroaryl azide photoreactive functions

3-(4-Azido-2,3,5,6-tetrafluorobenzoyl)-6-hydroxy-2-(4- hydroxyphenyl)benzo[b]thiophene 1 (tetrafluoroaryl azide, TFAA) and its protio analogue 3-(4-azidobenzoyl)-6- hydroxy-2-(4-hydroxyphenyl)benzo[b]thiophene 2 (protioaryl azide, PAA), photoaffinity labeling (PAL) reagents for the estrogen receptor (ER), have been prepared in high specific activity tritium-labeled form (19 Ci/mmol) and shown to undergo selective and efficient photocovalent attachment to ER from rat uterus. Both azides 1 and 2 demonstrate high binding affinity for ER as determined by both a competitive binding assay (relative binding affinities: estradiol = 100; TFAA = 9.3; PAA = 66) and a direct binding assay (Kd: estradiol = 0.24 nM; TFAA = 2.64 nM; PAA = 0.37 nM). When unlabeled TFAA and PAA are irradiated at greater than 315 nm, they demonstrate site-specific photoinactivation of ER that reaches 43% and 55%, respectively, by 30 min. Specific photocovalent attachment to ER can be effected by irradiation of the tritium-labeled azides; the covalent attachment efficiency is good (1 = 20-30%, 2 = ca. 25%) and the selectivity of ER labeling is high. Characterization of the photolabeled proteins by SDS-polyacrylamide gel electrophoresis shows specific labeling of a major component at Mr 60,000 and a minor species at Mr 46,000, the same two species that are labeled by [3H]tamoxifen aziridine, a well-characterized affinity label for ER. The ER-specific antibodies H222Sp gamma and D547Sp gamma show a clean precipitation of only these two species. In the MCF-7 human breast cancer cell line, PAA is a full estrogen agonist in terms of stimulation of cell proliferation and induction of progesterone receptor. These two azides provide the first system in which the photocovalent attachment efficiency of an aryl azide can be compared to its tetrafluorosubstituted aryl azide analogue in a complex biological receptor system. Azides 1 and 2 are the most efficient and selective PAL reagents prepared to date for ER, and they should be useful in further studies of the hormone-binding domain of this protein.

Cross-linking of estrogen receptor to chromatin in intact MCF-7 human breast cancer cells: optimization and effect of ligand

To investigate the effect of ligand (be it hormone, antihormone, or no hormone) on the interaction between estrogen receptor (ER) and chromatin, we have used formaldehyde as a cross-linking agent in intact MCF-7 human breast cancer cells. After a 1- to 2-h hormone treatment, the cells are exposed for 8 min to formaldehyde, which is added directly to their culture medium to minimize environmental perturbation. Nuclei are prepared from formaldehyde-treated cells and their contents are fractionated on CsCl density gradients to separate DNA-protein complexes from free protein. Peak gradient fractions are assayed for the presence of specific proteins by immunoblot of sodium dodecyl sulfate-polyacrylamide gel patterns. Using this approach, we find that 0.15% formaldehyde is optimal for cross-linking ER to chromatin. We detect ER and the large subunit of RNA polymerase II with DNA from formaldehyde-treated, but not from untreated cells. On the other hand, actin (a cytoplasmic protein) and small nuclear ribonucleoprotein particle proteins (nuclear RNA binding proteins) are not cross-linked to DNA. Therefore, cross-linking appears to be selective and fractionation is efficient. Interestingly, we detect similar levels of ER (as well as RNA polymerase II) with DNA from formaldehyde-treated cells, regardless of whether the cells are preexposed to estrogen (17 beta-estradiol at 10(-8) M), antiestrogen (ICI 164,384 at 10(-7) or 10(-6) M), or no hormone. These results, using covalent cross-linking in intact cells, indicate that both ligand-occupied and unoccupied ER are associated with chromatin.

Hormonal modulation of HER-2/neu protooncogene messenger ribonucleic acid and p185 protein expression in human breast cancer cell lines

Recent work has suggested that overexpression of the HER-2/neu protooncogene may play a role in the aggressive clinical behavior of some breast tumors. Since hormones are also known to change the proliferation rate and invasiveness of these cells, we have studied the effect of sex steroid hormones and antihormones on levels of the HER-2/neu mRNA and protein in human breast cancer cell lines using complementary DNA and antibody probes. In MCF-7 cells, which contain high levels of estrogen receptor and an estradiol (E2)-inducible progesterone receptor (PR), 1 nM E2 caused a rapid drop in HER-2/neu mRNA (4.8 kilobases), to 40% of control values by 6 h, and a more gradual decrease in HER-2/neu protein, to 50% by 24 h. HER-2/neu protein and mRNA levels remained reduced throughout 1 week of E2 treatment. The effect of E2 was dose dependent, with the maximal effect seen with concentrations of 10(-10) M E2 and above, and antiestrogen partly reversed the E2-induced decrease in HER-2/neu expression. These characteristics suggest that the observed modulation of HER-2/neu is an estrogen receptor-mediated process. In contrast, progestins did not change HER-2/neu mRNA or protein levels in E2-primed MCF-7 cells that contain high levels of PR; in T47D cells, which contain low levels of ER and high levels of PR, addition of E2 or the progestin R5020 or the antiprogestin RU38,486 had no significant effect on HER-2/neu mRNA or protein levels over 6 days of treatment. These results indicate that estrogen but not progestin modulates HER-2/neu protooncogene expression in these breast cancer cell lines and suggest that aggressiveness associated with high levels of HER-2/neu mRNA and protein may be uncoupled from estrogen-stimulated proliferation in these cells.

Multihormonal regulation of the progesterone receptor in MCF-7 human breast cancer cells: interrelationships among insulin/insulin-like growth factor-I, serum, and estrogen

Estrogen (E) is well known to be an important stimulator of progesterone receptor (PR) synthesis in target cells. We have observed that E stimulation of PR in MCF-7 human breast cancer cells (as monitored by progestin binding or Western blotting with anti-PR antibodies) increases as a function of serum concentration in the cell culture medium; PR stimulation by E is greatest in high serum medium (5% or 10% charcoal dextran-treated calf serum) and is not observed when cells are in medium containing serum concentrations below 1%, although estrogen receptor levels are well maintained. This suggests that some serum factor(s) may be essential for E to be able to stimulate PR. To better understand such factors, we have grown cells in serum-free medium and in serum-free medium supplemented with insulin (6.25 micrograms/ml) [corrected], transferrin (6.25 micrograms/ml), selenium (6.25 ng/ml), albumin (1.25 mg/ml) [corrected], and linoleic acid (5.35 micrograms/ml; ITS+). Unexpectedly, we found that addition of ITS+ (without E) increases PR levels in these cells, especially in the absence of serum and under low serum conditions where E stimulation of PR is poor. Analyses of the individual components in ITS+ reveal that insulin is the major active component. Dose-response studies indicate that high superphysiological (greater than 1 microgram/ml) concentrations of insulin are required. In contrast, low physiological levels of insulin-like growth factor-I (IGF-I; 10 or 40 ng/ml) are active, suggesting mediation by the IGF type I receptor system. At all serum concentrations (0-10%), the effects of ITS+ and E in increasing PR are synergistic. The fact that anti-E are able to suppress the insulin/IGF-I stimulation as well as the E stimulation of PR suggests that the anti-E can actively interfere with the action of the growth factor as well as the action of E. These results indicate that regulation of PR is multifactor and raise the possibility that PR may be regulated in vivo by both E and growth factors such as IGF-I that are known to be increased in these breast cancer cells by E.

Identification of cysteine 530 as the covalent attachment site of an affinity-labeling estrogen (ketononestrol aziridine) and antiestrogen (tamoxifen aziridine) in the human estrogen receptor

Radiosequence analysis of peptide fragments of the estrogen receptor (ER) from MCF-7 human breast cancer cells has been used to identify cysteine 530 as the site of covalent attachment of an estrogenic affinity label, ketononestrol aziridine (KNA), and an antiestrogenic affinity label, tamoxifen aziridine (TAZ). ER from MCF-7 cells was covalently labeled with [3H]TAZ or [3H]KNA and purified to greater than 95% homogeneity by immunoadsorbent chromatography. Limit digest peptide fragments, generated by prolonged exposure of the labeled receptor to trypsin, cyanogen bromide, or Staphylococcus aureus V8 protease, were purified to homogeneity by high performance liquid chromatography (HPLC), and the position of the labeled residue was determined by sequential Edman degradation. With both aziridines, the labeled residue was at position 1 in the tryptic peptide, position 2 in the cyanogen bromide peptide, and position 7 in the V8 protease peptide. This localizes the site of labeling to a single cysteine at position 530 in the receptor sequence. The identity of cysteine as the site of labeling was confirmed by HPLC comparison of the TAZ-labeled amino acid (as the phenylthiohydantoin and phenylthiocarbamyl derivatives) and the KNA-labeled amino acid (as the phenylthiocarbamyl derivative) with authentic standards prepared by total synthesis. Cysteine 530 is located in the hormone binding domain of the receptor, near its carboxyl terminus. This location is consistent with earlier studies using sodium dodecyl sulfate-polyacrylamide gel electrophoresis to analyze the size of the proteolytic fragments containing the covalent labeling sites for TAZ and KNA and the antigen recognition sites for monoclonal antibodies. The fact that both the estrogenic and antiestrogenic affinity labeling agents react covalently with the same cysteine indicates that differences in receptor-agonist and receptor-antagonist complexes do not result in differential covalent labeling of amino acid residues in the hormone binding domain.

Progression of human breast cancer cells from hormone-dependent to hormone-independent growth both in vitro and in vivo

We have isolated a series of sublines of the hormone-dependent MCF-7 human breast cancer cell line after selection both in vivo and in vitro for growth in the presence of subphysiological concentrations of estrogens. These sublines represent a model system for study of the processes leading to hormonal autonomy. The cells form growing tumors in ovariectomized athymic nude mice in the absence of estrogen supplementation but retain some responsivity to estrogen as determined by stimulation of the rate of tumor growth in vivo and by induction of progesterone receptor. An ovarian-independent but hormone-responsive phenotype may occur early in the natural progression to hormone-independent and unresponsive growth in breast cancer. We observed no change in the affinity or decrease in the level of expression of estrogen receptors and progesterone receptors among the sublines and the parental cells. Epidermal growth factor receptors are not overexpressed in ovarian-independent cells. Thus, altered hormone receptor expression may be a late event in the acquisition of a hormone-independent and unresponsive phenotype. Sublines isolated by in vivo but not in vitro selection are more invasive than the parental cells both in vivo and across an artificial basement membrane in vitro. Thus, as yet unknown tumor-host interactions may be important in the development of an invasive phenotype. Furthermore, acquisition of the ovarian-independent and invasive phenotypes can occur independently.

Regulation of estrogen receptor messenger ribonucleic acid and protein levels in human breast cancer cell lines by sex steroid hormones, their antagonists, and growth factors

Since sex steroid hormones and growth factors are known to modulate the proliferation of breast tumors, we have studied the effects of estrogen and progestin, their antagonists, and growth factors on the regulation of estrogen receptor (ER) mRNA and protein levels in T47D breast cancer cells, which contain low levels of ER, and in two sublines of MCF-7 cells which contain high ER levels. The mRNA levels were measured by Northern blot analysis using lambda OR8, a cDNA probe for ER, and protein levels were measured by hormone binding or Western blot analysis. Treatment of T47D cells with estradiol (E2) caused a 2.5-fold increase in ER mRNA (6.6 kilobases) levels after 48 h. The progestin R5020 evoked a marked decrease in ER mRNA and protein levels to 20% of control values, while the antiprogestin RU38,486 caused no change in ER. In MCF-7 cells, the effect of E2 on ER levels was dependent on the prior growth history of the cells. In cells grown in low estrogen [5% charcoal-dextran-treated calf serum with phenol red for 8 yr (MCF-7-K2)], which are still E2 responsive, treatment with E2, the antiestrogen LY117018, or both produced little change in ER mRNA or protein; in contrast, ER mRNA and protein were reduced by E2 to 40% and 50% of control levels, respectively, in MCF-7 cells (denoted MCF-7-K1) which had been maintained routinely in medium containing 5% calf serum. This decrease in ER mRNA was dose dependent; 10(-11) E2 reduced levels to 60%, and 10(-10) M E2 evoked the maximal drop to 40% of the control level in 2 days. LY117018 alone did not alter ER mRNA levels in these cells, but it completely prevented the down-regulation of ER by E2. Administration of progestin, but not antiprogestin, along with E2 partially prevented the decrease in ER evoked by E2. Addition of epidermal growth factor or insulin-like growth factor-I to MCF-7-K1 cells, which increased cell proliferation, had no detectable effect on ER levels. Treatment with transforming growth factor-beta, which decreased cell proliferation, reduced ER by about 20%.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of pS2 gene expression by affinity labeling and reversibly binding estrogens and antiestrogens: comparison of effects on the native gene and on pS2-chloramphenicol acetyltransferase fusion genes transfected into MCF-7 human breast cancer cells

We have examined the effects of reversibly and irreversibly binding estrogenic and antiestrogenic ligands for the estrogen receptor on pS2 RNA accumulation in MCF-7 human breast cancer cells and on pS2-chloramphenicol acetyl transferase (CAT) fusion gene expression in transfected MCF-7 cells. In MCF-7 cells grown in the absence of estrogens, the reversibly binding estrogen, estradiol, and the affinity labeling estrogen, ketononestrol aziridine, KNA, evoked a 13-fold increase in pS2 RNA level. The reversibly binding antiestrogen trans-hydroxytamoxifen and the affinity labeling antiestrogens tamoxifen aziridine or desmethylnafoxidine aziridine behaved as partial agonists/antagonists. In thymidine kinase-chloramphenicol acetyltransferase (tk-CAT) fusion genes containing a 1000 base pair fragment of the pS2 5'-flanking region encompassing the estrogen responsive element of the gene [pS2 (-1100/-90) tk-CAT], estradiol and ketononestrol aziridine evoked a marked stimulation of CAT activity and, in transfected cells grown in both the presence or absence of the weak estrogen phenol red, the antiestrogens behaved as partial agonists/antagonists. This pS2 5'-flanking region displayed both estrogen-dependent and estrogen-independent enhancer activity as monitored by stimulation of CAT activity. Hormonal regulation of the transfected pS2 fusion gene was similar to that observed in the native pS2 gene of MCF-7 cells; however, antiestrogens, while still partial agonists-antagonists, were relatively more agonistic on the transfected fusion gene than on the native gene. One antiestrogen (ICI 164,384) that behaved as a pure estrogen antagonist on the native gene was a partial agonist-antagonist of pS2 gene expression in the plasmid. This study illustrates that the hormonal regulation of the pS2 gene, as characterized by the agonist-antagonist balance of estrogens and antiestrogens, is influenced by the DNA context of the pS2 estrogen responsive element. Also, the fact that estrogens and antiestrogens that form covalent bonds with the estrogen receptor modulate activity of the native pS2 gene and the pS2-tk-CAT fusion gene in a manner similar to that of their reversibly binding counterparts suggests that it may be possible to use these irreversibly binding ligands to follow the interaction of hormone-receptor complexes with regions regulating estrogenic stimulation of the pS2 gene.

Lipophilic impurities, not phenolsulfonphthalein, account for the estrogenic activity in commercial preparations of phenol red

Previously, we found that Phenol Red, a pH indicator dye commonly used in tissue culture media, had weak estrogenic activity, demonstrable by competitive binding to the estrogen receptor, stimulation of the growth rate of human breast cancer (MCF-7) cells, and elevation of progesterone receptor levels in these cells. We have now examined in more detail the source of this estrogenic activity, present in commercially available preparations of Phenol Red. By high performance liquid chromatography and solvent partitioning, we find that the receptor binding and growth promoting activity does not correspond to the indicator dye itself (phenolsulfonphthalein), but rather to more lipophilic impurities present in these preparations. There are numerous such impurities, many of which show some competitive binding activity, but the major receptor binding activity is accounted for by a single impurity component. Commercial preparations of Phenol Red can be purified by ether extraction of the sodium salt, whereby 95-99% of the lipophilic estrogenic impurities are removed, and the growth stimulating activity towards MCF-7 cells is reduced.

Comparative analysis of estrogen receptors covalently labeled with an estrogen and an antiestrogen in several estrogen target cells as studied by limited proteolysis

Estrogen receptors covalently labeled with the estrogen affinity label [3H]ketononestrol aziridine (KNA) or with the antiestrogen affinity label [3H]tamoxifen aziridine (TAZ) were subjected to limited proteolysis with trypsin, alpha-chymotrypsin, and Staphylococcus aureus V8 protease and then analyzed on 10-20% sodium dodecyl sulfate-polyacrylamide gradient gels followed by fluorography. The similar molecular weights of intact receptors (Mr 66,000 daltons) and the proteolytic digest patterns indicate extensive homology among estrogen receptors from MCF-7 human breast cancer cells, GH4 rat pituitary cells and rat uterus when liganded with estrogen or antiestrogen. Each protease generated a distinctive ladder of estrogen receptor fragments, and the fragmentation patterns were virtually identical for estrogen receptors labeled with estrogen (KNA) or antiestrogen (TAZ). Each protease yielded a relatively "resistant" receptor fragment of about 28,000-35,000 daltons. Trypsin and chymotrypsin at higher concentrations generated a much smaller 6,000-8,000 dalton digest product that still contained the [3H]KNA- or [3H]TAZ-labeled receptor binding site. Moreover, the receptor digest patterns were similar for estrogen receptors from the three different target cells. Our studies suggest considerable structural relatedness among these three estrogen receptors and also indicate that these two affinity labels bind to a similar, perhaps identical, region of the receptor molecule.

Progesterone receptor regulation in T47D human breast cancer cells: analysis by density labeling of progesterone receptor synthesis and degradation and their modulation by progestin

We have examined the effect of progestin on the regulation of cellular progesterone receptor (PR) levels and have used dense amino acid-density shift experiments to determine the mechanism by which progestin markedly decreases PR. We have utilized T47D human breast cancer cells which contain high levels of PR and are progestin responsive. When these cells are exposed to the progestin R5020, there is a time- and concentration-dependent decrease in PR levels. Experiments with different concentrations of R5020 reveal that the rate and extent of PR decrease reflect the time course of receptor occupancy and the fractional saturation of receptor. With a high concentration of ligand (20 nM) that labels all receptors rapidly, reductions in PR levels (processing) occur immediately and proceed rapidly to levels that are 15-20% of the initial; at lower concentrations (5 nM), where it takes several hours to achieve full saturation of receptors, there is a delay before the maximal rate of processing develops and then continues to achieve final receptor levels that are 15-20% of the initial; with a low concentration of ligand (0.5 nM), binding is even slower and never reaches full receptor saturation, with the consequence that processing is not only delayed but also less complete. Immunochemical detection of PR with a monoclonal antibody (B39) reveals a good correspondence between the loss of immunoreactive and hormone binding PR, and analysis of the A (Mr 85,000) and B (Mr 115,000) receptor forms on Western blots demonstrates that both A and B receptor forms are reduced after exposure to R5020. Density labeling of PR by biosynthetic incorporation of 2H, 13C, 15N (dense) amino acids reveals that PR turns over with a half-life of 21 h in control cells. In cells exposed to 20 nM R5020, PR levels decline and receptor half-life is reduced to 6 h. In addition, there is also a time-dependent decrease in the rate constant of PR synthesis, k8, which decreases to less than 10% of its initial value after 24 h of R5020 exposure. Thus, the R5020-evoked reduction in PR levels in this progestin-sensitive cell line is due both to a marked increase in the rate of receptor degradation as well as a dramatic decrease in the rate of receptor synthesis.

Regulation of progesterone receptor messenger ribonucleic acid and protein levels in MCF-7 cells by estradiol: analysis of estrogen's effect on progesterone receptor synthesis and degradation

The human breast cancer cell line MCF-7 responds to estrogens with increased progesterone receptor (PR) levels. In this study, we use dense amino acid density shift analyses to address directly the question of whether estrogen increases PR levels in MCF-7 cells by altering rates of receptor synthesis and/or degradation. Using different concentrations of estradiol (E2), which achieve PR levels that are half-maximal (3 X 10(-11) M F2) or maximal (6 X 10(-11) M E2), we have done sucrose gradient density shift analyses using dense (15N, 13C, 2H) amino acid incorporation to study rates of PR synthesis and degradation. These studies reveal a nonlinear loss of preexisting normal density receptor with time. From kinetic modeling analyses, equivalent rates of degradation are estimated for PR whether maximal or half-maximal levels are maintained, indicating that the major effect of E2 on PR content is to increase the rate of PR synthesis while leaving the degradation rate unaltered. The E2-stimulated increase in PR protein is also associated with increased levels of PR mRNA, as demonstrated by the use of a human PR cDNA probe. These density shift data provide evidence that the increased PR levels after estrogen exposure in MCF-7 cells are the result of an increased rate of receptor synthesis, rather than modulation of the rate of receptor degradation.

Ligand-modulated regulation of progesterone receptor messenger ribonucleic acid and protein in human breast cancer cell lines

We have examined the effects of estrogen and progestin agonist and antagonist ligands on regulation of progesterone receptor (PR) protein and mRNA levels in a variety of human breast cancer cell lines. By Northern blot analysis, using human PR cDNA probes, PR mRNA in T47D and MCF-7 cells appears as five species of approximately 11.4, 5.8, 5.3, 3.5, and 2.8 kilobases. PR mRNA species are not detected in the PR protein-negative breast cancer cell lines MDA-MB-231 and LY2. T47D cells contain high levels of PR mRNA and protein (detected by hormone binding assay or Western blot analysis), and the PR protein and mRNA content of T47D cells are reduced to about 10% of the control level within 48 h of treatment with 10 nM promegestone; 17, 21-dimethyl-19-nor-pregna-4,9-diene-3, 20-dione (R5020) or 16 alpha-ethyl-21-hydroxy-19-nor-pregn-4-ene-3,20-dione (ORG2058), both potent progestins. In contrast, treatment of T47D cells with the antiprogestin 17 beta-hydroxy-11 beta-[4-dimethylaminophenyl]-17 alpha-(1-propynyl)-estra- 4, 9-dien-3-one) (RU38486) reduces PR protein and mRNA levels only transiently. PR protein and mRNA are virtually undetectable in control MCF-7 cells grown in the absence of estrogens. When estradiol is administered to MCF-7 cells, the PR mRNA and protein levels increase gradually and proportionately (10- or 40-fold, respectively, in 3 days).(ABSTRACT TRUNCATED AT 250 WORDS)

Plasminogen activators in human breast cancer cell lines: hormonal regulation and properties

To understand the hormonal regulation of plasminogen activators (PAs) in human breast cancer, we have examined the hormonal regulation and properties of PAs in four human breast cancer cell lines that differ markedly in their estrogen receptor (ER) content: MCF-7 cells contain high levels of ER (approx 7 pmol/mg DNA) and their PA activity was increased 3-4-fold by physiological concentrations of estradiol; T47-D and ZR-75-1 cells contain lower levels of ER (0.9 and 2.1 pmol/mg DNA respectively) and their PA activity was also increased 3-4-fold by estradiol. In contrast, MDA-MB-231 cells, which do not contain ER, showed a high level of PA activity that was not modulated by estradiol. SDS-PAGE followed by zymography indicated that MCF-7 cells secreted tissue-type PA (t-PA), T47-D and ZR-75-1 cells secreted urokinase-type PA (u-PA), and MDA-MB-231 cells secreted both types of PAs. The types of PAs secreted by these cell lines did not change upon treatment with estradiol. Dose-response curves for the stimulation of MCF-7 PA activity by different estrogens showed an excellent correlation between affinities of the estrogens for ER and their potency in stimulating PA activity. With a clonal subline of MCF-7 cells, MCF-L, a soluble inhibitor of both t-PA and u-PA was secreted. Incubation of purified t-PA or u-PA with the serum-free conditioned medium from MCF-L cells resulted in a shift in the mobility of t-PA and u-PA in SDS-polyacrylamide gels to forms increased in molecular mass by about 50,000-70,000. The shifts in molecular mass could be prevented by the presence of the competitive inhibitor p-aminobenzamidine, indicating that the active sites of the PAs were involved in the formation of these complexes. Furthermore, co-cultivation, of RT4-D rat neuroblastoma cells, which exhibit high levels of t-PA activity, with MCF-L cells resulted in a marked decrease in the PA activity of the RT4-D cells. Our results were consistent with the following conclusions: t-PA, u-PA or both were secreted by human breast cancer cells. In the ER-containing cell lines, depending upon the specific cell line, t-PA or u-PA was stimulated by estrogens. The unstimulated levels of PA activity and the magnitude of PA stimulation by estrogens were not closely related to ER content.(ABSTRACT TRUNCATED AT 400 WORDS)

Proliferation, hormonal responsiveness, and estrogen receptor content of MCF-7 human breast cancer cells grown in the short-term and long-term absence of estrogens

We have examined the effect of short-term and long-term growth in the absence of estrogens on the proliferation rate and estrogen and antiestrogen responsiveness of MCF-7 human breast cancer cells. The removal of phenol red, the pH indicator in tissue culture medium that is weakly estrogenic (Y. Berthois et al., Proc. Natl. Acad. Sci. USA, 83:2496-2500, 1986), immediately slows the cell proliferation rate, and MCF-7 cells grown in phenol red-free medium with charcoal dextran-treated serum for periods up to 1 mo maintain this reduced rate of cell proliferation. In these short-term phenol red-withdrawn cells, estradiol stimulates proliferation markedly and reproducibly, and antiestrogens inhibit estrogen-stimulated proliferation. Antiestrogens by themselves appear as partial agonists/antagonists; at low concentrations they stimulate proliferation weakly, but they show no stimulation at the high concentrations where they fully inhibit estrogen-stimulated proliferation. In contrast to the short-term phenol red-withdrawn cells, cells maintained for several months (5 to 6 mo) in the apparently complete absence of estrogens (no phenol red, with charcoal dextran-treated calf serum) show a markedly increased basal rate of cell proliferation; estradiol is unable to increase this rate of proliferation further, but antiestrogens are able to decrease proliferation. This change in growth pattern is associated with a 3-fold increase in cellular estrogen receptor levels. Despite their differing basal growth rates, cells grown in either the short-term (less than 1 mo) or long-term (greater than 6 mo) absence of estrogens both have progesterone receptor levels that are very low and, in both cases, estradiol increases progesterone receptor levels markedly. Thus, under long-term estrogen-free conditions, there is a dissociation between the stimulation of cell proliferation and of specific protein synthesis (progesterone receptor) by estrogen. The increase in the cell proliferation rate observed in cells grown in the long-term absence of estrogen may reflect altered regulation of growth factor production or altered sensitivity to growth factors in the medium or produced by the cells themselves. Hence, these breast cancer cells adapt significantly to long-term growth in estrogen-free conditions, an observation that may be relevant to understanding the growth of hormone-responsive human breast cancers in vivo.

Ketononestrol aziridine, an agonistic estrogen receptor affinity label: study of its bioactivity and estrogen receptor covalent labeling

Ketononestrol aziridine [(6R,TS)1-(N-aziridinyl)6,7-bis-(4-hydroxyphenyl)5-nonamone (KNA)], an aziridine derivative of hexestrol, is an estrogenic affinity label for the estrogen receptor (ER). It has an apparent relative binding affinity 8% that of estradiol and shows time-dependent irreversible binding to the ER in uterine cytosol preparations and intact human breast cancer cells (MCF-7). The agonistic activity of KNA is evident in MCF-7 cells in culture, where it increases the cell growth rate and elevates the level of progesterone receptor. KNA was prepared in high specific activity tritium-labeled form by iodination of a methanesulfonate precursor, followed by catalytic tritium-iodine exchange and aziridinylation; the material prepared has high radiochemical purity and a specific activity of 67 Ci/mmol. The covalent attachment of [3H]KNA to the ER can be followed directly by a solvent precipitation assay. In cytosol preparations of uterine ER, labeling with [3H]KNA proceeds in a time-, concentration-, and temperature-dependent manner; labeling is efficient and selective and, by competition studies, was shown to be estrogen specific. ER in intact MCF-7 cells can also be covalently labeled by treatment with [3H]KNA. Receptor covalently labeled with [3H]KNA sediments as a 4S species on high salt sucrose gradients, and its sedimentation position is shifted by treatment with monoclonal antireceptor antibodies. On sodium dodecyl sulfate-polyacrylamide gels, the principal labeled species migrates with a mol wt of 66,000. KNA should prove to be a useful probe for studies on receptor structure, function, and chromatin interactions, particularly when the behavior of a receptor-agonist complex is being investigated.