A tripartite complex composed of ETV6-NTRK3, IRS1 and IGF1R is required for ETV6-NTRK3-mediated membrane localization and transformation.

ETV6-NTRK3 (EN), a chimeric tyrosine kinase generated by t(12;15) translocations, is a dominantly acting oncoprotein in diverse tumor types. We previously showed that insulin-like growth factor 1 receptor (IGF1R) is essential for EN-mediated oncogenesis and that insulin receptor substrate 1 (IRS1) is constitutively tyrosine phosphorylated and bound by EN in transformed cells. Given that IRS1 is also an adapter for IGF1R, we hypothesized that IRS1 might localize EN to IGF1R at the membrane to activate phosphatidylinositol 3-kinase (PI3K)-Akt, which is critical for EN oncogenesis. In this study, we examined EN/IRS1/IGF1R complexes in detail. We find that both IRS1 and kinase active IGF1R are required for EN transformation, that tyrosine phosphorylated IRS1 is present in high molecular weight complexes with EN and IGF1R, and that EN colocalizes with IGF1R at the plasma membrane. Both IGF1R kinase activity and an intact cytoplasmic Y950 residue, the IRS1-docking site of IGF1R, are required, confirming the importance of the IGF1R/IRS1 interaction for EN oncogenesis. The dual specificity IGF1R and insulin receptor (INSR) inhibitor, BMS-536924, blocks EN transformation activity, cell survival and its interaction with IRS proteins, and induces a striking shift of EN proteins to smaller sized molecular complexes. We conclude that a tripartite complex of EN, IRS1 and IGF1R localizes EN to the membrane and that this is essential for EN-mediated transformation. These findings provide an explanation for the observed IGF1R dependency of EN transformation. Blocking IGF1R kinase activity may, therefore, provide a tractable therapeutic strategy for the many tumor types driven by the EN oncoprotein.

Characterization of BMS-911543, a functionally selective small-molecule inhibitor of JAK2.

We report the characterization of BMS-911543, a potent and selective small-molecule inhibitor of the Janus kinase (JAK) family member, JAK2. Functionally, BMS-911543 displayed potent anti-proliferative and pharmacodynamic (PD) effects in cell lines dependent upon JAK2 signaling, and had little activity in cell types dependent upon other pathways, such as JAK1 and JAK3. BMS-911543 also displayed anti-proliferative responses in colony growth assays using primary progenitor cells isolated from patients with JAK2(V617F)-positive myeloproliferative neoplasms (MPNs). Similar to these in vitro observations, BMS-911543 was also highly active in in vivo models of JAK2 signaling, with sustained pathway suppression being observed after a single oral dose. At low dose levels active in JAK2-dependent PD models, no effects were observed in an in vivo model of immunosuppression monitoring antigen-induced IgG and IgM production. Expression profiling of JAK2(V617F)-expressing cells treated with diverse JAK2 inhibitors revealed a shared set of transcriptional changes underlying pharmacological effects of JAK2 inhibition, including many STAT1-regulated genes and STAT1 itself. Collectively, our results highlight BMS-911543 as a functionally selective JAK2 inhibitor and support the therapeutic rationale for its further characterization in patients with MPN or in other disorders characterized by constitutively active JAK2 signaling.

Synergistic cytotoxicity exhibited by combination treatment of selective retinoid ligands with taxol (Paclitaxel).

The focus of this study was to develop retinoic acid receptor (RAR) RAR alpha/beta selective agonists with anticancer efficacy and reduced toxicity associated with RAR gamma activity. In these studies, we report the identification and characterization of high-affinity RAR alpha/beta selective agonists with limited RAR gamma activity. These compounds inhibited human tumor cell line proliferation with similar efficacy to that observed for a pan-RAR agonist. However, for most tumor cell lines, the efficacy of these compounds was restricted to the micromolar range. To determine whether the RAR alpha/beta selective agonists could be additive or synergistic with existing agents, we investigated the effects of combining RAR alpha/beta selective agonists with various cytotoxic agents. Our results showed that the alpha/beta selective retinoids dramatically lowered the effective dose of Taxol needed to induce cytotoxicity of a wide range of tumor cell lines. This synergy was specific to tubulin-modifying agents and could not be observed with a variety of other cytotoxic agents of diverse function. Examination of pathways common to Taxol and retinoid signaling revealed that this synergy was related in part to effects on Bcl-2 expression/phosphorylation as well as the activity of the c-Jun NH(2)-terminal kinase and activator protein-1. In contrast, the tubulin polymerization induced by Taxol was not further affected by cotreatment with a variety of retinoid receptor ligands. These observations indicate that potent RAR alpha/beta selective agonists may be of therapeutic benefit in combination with Taxol therapy.

Circumventing tamoxifen resistance in breast cancers using antiestrogens that induce unique conformational changes in the estrogen receptor.

Tamoxifen inhibits estrogen receptor (ER) transcriptional activity by competitively inhibiting estradiol binding and inducing conformational changes in the receptor that may prevent its interaction with coactivators. In bone, the cardiovascular system, and some breast tumors, however, tamoxifen exhibits agonist activity, suggesting that the tamoxifen-ER complex is not recognized identically in all cells. We used phage display to demonstrate that the antiestrogen GW5638 induces a unique structural change in the ER. The biological significance of this conformational change was revealed in studies that demonstrated that tamoxifen-resistant breast tumor explants are not cross-resistant to GW5638. Because of these properties, this drug is currently being developed as a potential therapeutic for tamoxifen-resistant breast cancers.

Efficacy of LGD1069 (Targretin), a retinoid X receptor-selective ligand, for treatment of uterine leiomyoma.

The conventional treatment of uterine leiomyomas, or fibroids, with gonadotropin-releasing hormone (GnRH) agonists is often associated with serious side effects, necessitating short-term, palliative use of this therapy. Therefore, we examined a retinoid X receptor (RXR)-selective ligand, LGD1069, as a possible treatment for leiomyoma. LGD1069 has demonstrated efficacy as a chemopreventive agent in the N-nitroso-N-methylurea (NMU)-induced rat mammary carcinoma model and is a therapeutic agent in several epithelial tumor models. Previous studies have shown that it has both antitumor effects and antiestrogenic activity in the rat uterus, suggesting the potential utility of this agent for treatment of hormonally dependent uterine fibroids. The expression of retinoid receptors in tumors and cell lines derived from leiomyomas arising in the Eker rat was confirmed by Northern analysis. After treatment for 4 months with LGD1069, the number of grossly observable tumors was substantially reduced although the total incidence of tumors, including microscopic lesions, remained unaffected, suggesting an effect of the compound on tumor growth kinetics rather than on tumor initiation. Analysis of terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) staining and determination of 5-bromo-2-deoxyuridine (BrdU) incorporation indicated that the reduction in grossly observable tumors that occurred in treated animals was mediated by a significant increase in the level of apoptosis rather than a decrease in cell proliferation. These results suggest that LGD1069 may be an effective therapeutic agent for uterine leiomyoma that may inhibit tumor growth and, consequently, alleviate the symptoms associated with this disease.

9-cis-Retinoic acid suppresses mammary tumorigenesis in C3(1)-simian virus 40 T antigen-transgenic mice.

Retinoids have been investigated as potential agents for the prevention and treatment of human cancers. These compounds play an important role in regulating cell growth, differentiation, and apoptosis. 9-cis-Retinoic acid (9cRA) is a naturally occurring ligand with a high affinity for both the retinoic acid receptors and the retinoid X receptors. We hypothesized that treatment with 9cRA would prevent mammary tumorigenesis in transgenic mice that spontaneously develop mammary tumors. To test this hypothesis, C3(1)-SV40 T antigen (Tag) mice, which develop mammary tumors by the age of 6 months, were treated daily p.o. with vehicle or two different dose levels of 9cRA (10 or 50 mg/kg) from 5 weeks to 6 months of age. Tumor size and number were measured twice each week, and histological samples of normal and malignant tissue were obtained from each mouse at time of sacrifice. Our results demonstrate that 9cRA suppresses mammary tumorigenesis in C3(1)-SV40 Tag-transgenic mice. Time to tumor development was significantly delayed in treated mice; median time to tumor formation for vehicle-treated mice was 140 days versus 167 days for mice treated with 50 mg/kg 9cRA (P = 0.05). In addition, the number of tumors per mouse was reduced by >50% in mice treated with 9cRA (3.43 for vehicle, 2.33 for 10 mg/kg 9cRA, and 1.13 for 50 mg/kg 9cRA, P < or = 0.002). Histological analysis of the mammary glands from vehicle and treated mice demonstrated that 9cRA treatment also did not affect normal mammary gland development. Immunohistochemical staining of normal and malignant breast tissue and Western blot analysis demonstrated that SV40 Tag expression was not affected by treatment with retinoids. Single doses of 10 and 50 mg/kg resulted in peak plasma concentrations of 3.4 and 6.71 microM, respectively. Daily doses of 9cRA for 28 days resulted in plasma concentrations of 0.86 and 1.68 microM, respectively, concentrations consistent with that seen in humans treated with 9cRA in clinical trials. These results demonstrate that 9cRA suppresses mammary carcinogenesis in transgenic mice without any major toxicity and suggest that retinoids are promising agents for the prevention of human breast cancer.

Estrogen receptor activation via activation function 2 predicts agonism of xenoestrogens in normal and neoplastic cells of the uterine myometrium.

The possible contribution of endocrine disrupters to human disease, particularly those compounds that modulate the estrogen receptor (ER), has recently drawn considerable attention. The tissue specificity of effects mediated by the ER is well recognized, although the mechanism of this specificity is not understood sufficiently to predict the effects of a particular ligand in different target tissues. Although the divergence of ER-mediated effects in the breast, bone, and uterine endometrium has been described, a frequently overlooked site of estrogen action is the smooth muscle of the uterus. The uterine myometrium is the tissue of origin of an extremely common hormone-responsive tumor, uterine leiomyoma, a tumor with a significant impact on women's health and a possible environmental influence. This report describes an in vitro/in vivo system for identifying the effects of ER ligands in the myometrium and elucidating their mechanism of action. Several natural and synthetic xenoestrogens were evaluated at the cellular and molecular level for their ability to mimic estrogen action in uterine myometrial tissues. Diethylstilbestrol, coumestrol, genistein, naringenin, and endosulfan were able to activate the AF2 function of the ER in vitro and demonstrated agonist activity in estrogen-responsive myometrial cells, as determined by induction of proliferation and increased message levels of progesterone receptor. Compounds that could not activate AF2 function (4-hydroxy-tamoxifen, LY117018, and LY317783) did not act as estrogen agonists. For agonists, rank order of potency was predicted by receptor affinity; however, endosulfan displayed a surprising degree of activity, despite negligible receptor binding. Additionally, diethylstilbestrol and tamoxifen demonstrated prototypical agonist and antagonist effects, respectively, in the intact myometrium of sexually mature rats. The results presented here suggest that some exogenous ER ligands may mimic the effects of endogenous estrogens on uterine leiomyoma and may contribute to a complex hormonal milieu that impacts both normal and neoplastic myometrium.

5-Benzylidene 1,2-dihydrochromeno[3,4-f]quinolines, a novel class of nonsteroidal human progesterone receptor agonists.

A novel series of nonsteroidal progestins, 5-benzylidene-1, 2-dihydrochromeno[3,4-f]quinolines (2), was discovered, and a preliminary structure-activity relationship study around the 5-benzylidene ring generated several potent human progesterone receptor agonists (compounds 8, 16). These new progestins showed biological activities (EC50 = 5.7 and 7.6 nM) similar to progesterone (EC50 = 2.9 nM) in the cotransfection assay with high efficacy (132% and 166%) and binding affinity (Ki = 0.66 and 0.83 nM) similar to medroxyprogesterone acetate (MPA) (Ki = 0.34 nM). A representative analogue, 8, demonstrated similar oral potency to MPA in the uterine wet weight/mammary gland morphology assay in ovariectomized rats.

Preparation, resolution, and biological evaluation of 5-aryl-1, 2-dihydro-5H-chromeno[3,4-f]quinolines: potent, orally active, nonsteroidal progesterone receptor agonists.

Two potent nonsteroidal progestins from the 5-aryl-1, 2-dihydro-5H-chromeno[3,4-f]quinoline class (LG120746 and LG120747) were selected for scale-up, resolution, and biological evaluation of the purified enantiomers. For each quinoline, the levorotatory enantiomer was determined to be the more potent agonist of the human progesterone receptor isoform B (hPR-B) (EC50 < 3 nM), but the dextrorotatory enantiomers retained significant PR modulatory activity (EC50 < 200 nM). In two in vivo rodent models of progestational activity, a pregnancy maintenance assay and a uterine wet weight assay, the two eutomers displayed potent progesterone-like effects. In a third model for progestational activity, the mammary end bud assay, these compounds were significantly less active. These studies demonstrate that certain members of this class of selective progesterone receptor modulators display encouraging and potentially useful tissue-selective progestational effects.

5-Aryl-1,2-dihydrochromeno[3,4-f]quinolines: a novel class of nonsteroidal human progesterone receptor agonists.

The development of a novel class of nonsteroidal human progesterone receptor (hPR) agonists, 5-aryl-1,2-dihydro-5H-chromeno[3,4-f]quinolines 2, is described. The introduction of a 5-aryl group into the 1,2-dihydrocoumarino[3,4-f]quinoline core 1 is the key for progestational activities. The structure-activity relationship (SAR) studies of the 5-aryl substituents generated a series of potent hPR agonists, which exhibited similar biological activity (EC50 = 8-30 nM) to the natural hormone progesterone (EC50 = 2.9 nM) in cell-based assays with efficacies ranging from 28% to 96%. Most of the analogues displayed similar or greater binding affinity (Ki = 0.41-3.6 nM) than progesterone (Ki = 3.5 nM). Three representative analogues (13, 15, and 24) demonstrated in vivo activities in mammary gland morphology/uterine wet weight assay in ovariectomized rats.

5-Aryl-1,2-dihydro-5H-chromeno[3,4-f]quinolines as potent, orally active, nonsteroidal progesterone receptor agonists: the effect of D-ring substituents.

Several 5-(4-chlorophenyl)-1,2-dihydro-5H-chromeno[3,4-f]quinolines were prepared to determine the effects of substitution at C(8) and C(9) on the progestational activity of this pharmacophore. In combination with a halogen (F or Cl) at C(9), replacement of the C(5) aryl group with variously substituted aryl groups resulted in optimization of the progestational activity, affording compounds with in vitro activity greater than that of progesterone as measured by a cotransfection assay using human progesterone receptor subtype-B (hPR-B). Binding affinities (Ki) to hPR-A were subnanomolar in many cases. These in vitro effects were verified in vivo using a rodent model. Compound 10 (LG120794, 9-chloro-5-(4-chlorophenyl)-1,2-dihydro-2,2,4-trimethyl-5H-chromeno++ +[3,4-f] quinoline) was more potent than medroxyprogesterone acetate at counterpoising the effects of estradiol benzoate in the uterine wet weight assay using immature rats.

Beyond tamoxifen: the retinoid X receptor-selective ligand LGD1069 (TARGRETIN) causes complete regression of mammary carcinoma.

Recently, we reported that LGD1069, a high-affinity ligand for the retinoid X receptors (RXRs), was shown to have an efficacy equivalent to that of tamoxifen (TAM) as a chemopreventive agent in the N-nitroso-N-methylurea-induced rat mammary carcinoma model. Furthermore, LGD1069 was very well tolerated during 13 weeks of chronic therapy with no classic signs of "retinoid-associated" toxicities. Due to the high efficacy and benign profile of this RXR agonist as a suppressor of carcinogenesis, we examined its role as a therapeutic agent on established mammary carcinomas. In the rat mammary carcinoma model, N-nitroso-N-methylurea was used to induce tumors, and the tumors were allowed to grow to an established size prior to initiation of treatment. LGD1069-treated animals showed complete regression in 72% of treated tumors and had a reduced tumor load compared to control. In addition, the combination of LGD1069 and TAM showed increased efficacy over either agent alone. Histopathological analysis showed a reduction of LGD1069-treated tumor malignancy, an increase in differentiation, and a sharp decrease in cellular proliferation compared to vehicle-treated control tumors. These data demonstrate that the RXR-selective ligand LGD1069 is a highly efficacious therapeutic agent for mammary carcinoma and enhances the activity of TAM.

5-Alkyl 1,2-dihydrochromeno[3,4-f]quinolines: a novel class of nonsteroidal progesterone receptor modulators.

A series of nonsteroidal human progesterone receptor (hPR) agonists, 5-alkyl 1,2-dihydrochromeno[3,4-f]quinolines, was synthesized and evaluated in cotransfection and competitive receptor binding assays. The 5-alkyl substitution was shown to be responsible for the agonist activity and substitution at C9 dramatically enhanced the potency. A number of analogues in this series showed activities similar to or better than progesterone in the cotransfection and binding assays and analogue 15 exhibited similar in vivo activity as medroxyprogesterone acetate (MPA) in murine uterine wet weight/mammary gland morphology assays.

Decreased Egr-1 expression in human, mouse and rat mammary cells and tissues correlates with tumor formation.

We have examined several types of tumor cell lines and shown that they invariably expressed little or no Egr-1, in contrast to their normal counterparts. We have previously shown that the expression of exogenous Egr-1 in human breast and other tumor cells markedly reduces transformed growth and tumorigenicity. We therefore hypothesized that the loss of Egr-1 expression plays a role in transformation. All human and mouse breast cancer cell lines and tumors examined had reduced Egr-1 expression compared with their normal counterparts. Reduced Egr-1 expression was also observed in 7,12-dimethylbenz(a)anthracene (DMBA)-induced rat mammary tumors, and this level increased to normal levels in tumors that regressed after tamoxifen treatment. We concluded, therefore, that loss of Egr-1 expression may play a role in the deregulation of normal growth in the tumorigenic process and that Egr-1 acts as a tumor suppressor gene.

Chemoprevention of mammary carcinoma by LGD1069 (Targretin): an RXR-selective ligand.

Recently, 9-cis retinoic acid, a high affinity ligand for retinoic acid receptors and retinoid X-receptors (RXRs), was shown to have efficacy superior to all-trans retinoic acid as a chemopreventive agent in the N-nitroso-N-methylurea-induced rat mammary carcinoma model. To further explore the specific contribution RXR activation may play in suppression of carcinogenesis, the efficacy of LGD1069 (Targretin), an RXR-selective ligand, in the N-nitroso-N-methylurea-induced rat mammary tumor model was studied. LGD1069-treated animals showed a 90% reduction in tumor burden and tumor incidence compared with vehicle-treated rats with an efficacy similar to that achieved with tamoxifen. LGD1069 was very well tolerated during 13 weeks of chronic therapy with no classic signs of "retinoid-associated" toxicities. These data demonstrate that LGD1069, an RXR-selective ligand, can act as a highly effective and benign chemopreventive agent for mammary carcinoma.

Growth-inhibitory effects of vitamin D analogues and retinoids on human pancreatic cancer cells.

Retinoids and vitamin D are important factors that regulate cellular growth and differentiation. An additive growth-inhibitory effect of retinoids and vitamin D analogues has been demonstrated for human myeloma, leukaemic and breast cancer cells. We set out to study the effects of the vitamin D analogue EB1089 and the retinoids all-trans- and 9-cis-retinoic acid on the human pancreatic adenocarcinoma cell lines Capan 1 and Capan 2 and the undifferentiated pancreatic carcinoma cell line Hs766T. The cell lines investigated expressed vitamin D receptor, retinoic acid receptor (RAR)-alpha and gamma as determined by polymerase chain reaction after reverse transcription. RAR-beta was expressed only in Hs766T cells. Addition of all-trans-retinoic acid increased the amount of RAR-alpha mRNA in the three cell lines and induced RAR-beta mRNA in Capan 1 and Capan 2 cells. All-trans-retinoic acid at a concentration of 10 nM inhibited the growth of Capan 1 and Capan 2 cells by 40% relative to controls. 9-cis-Retinoic acid was less effective. Neither all-trans-retinoic acid nor 9-cis-retinoic acid affected the growth of Hs766T cells. EB1089, if added alone to the cells, did not significantly inhibit growth. However, the combination of 1 nM EB1089 with 10 nM all-trans-retinoic acid exerted a growth-inhibitory effect of 90% in Capan 1 cells and of 70% in Capan 2 cells. Our data suggest that vitamin D analogues together with retinoids inhibit the growth of human pancreatic cancer cells. However, in vivo studies are necessary to examine the potential use of retinoids and vitamin D analogues on pancreatic cancer.

Effects of AGM-1470 and pentosan polysulphate on tumorigenicity and metastasis of FGF-transfected MCF-7 cells.

Previously, we described FGF-1- or FGF-4-transfected MCF-7 breast carcinoma cells which are tumorigenic and metastatic in untreated or tamoxifen-treated ovariectomised nude mice. In this study, we have assessed the effects of AGM-1470, an antiangiogenic agent, and pentosan polysulphate (PPS), an agent that abrogates the effects of FGFs, on tumour growth and metastasis produced by these FGF-transfected MCF-7 cells. Untreated or tamoxifen-treated ovariectomised mice were injected with FGF-transfected cells, treated with AGM-1470 or PPS, and tumour growth and metastasis analysed. The sensitivity of FGF-transfected and parental MCF-7 cells to AGM-1470 or PPS was also determined in vitro. Both AGM-1470 and PPS inhibited tumour growth in otherwise untreated or tamoxifen-treated mice injected with either FGF- or FGF-4-transfected MCF-7 cells. This effect was more reliably seen in tamoxifen-treated animals. AGM-1470 was about 10(5) times less potent in inhibiting the anchorage-dependent growth of parental MCF-7 or FGF-transfected MCF-7 cells than in inhibiting the growth of human umbilical vein endothelial cells. PPS did not affect the in vitro growth of the transfectants or parental cells. Thus, the growth-inhibitory effect on tumours was in excess of the effect of either drug on the same cells in tissue culture, implying that stromal elements are important determinants of the effects of these drugs. There was a positive correlation between tumour size and the extent of proximal lymph node metastasis. However, neither drug had a significant effect on the extent of metastasis to proximal or distal lymph nodes or lungs. AGM-1470 or PPS may be helpful in cases of breast carcinoma in which angiogenesis is due to expression of FGFs by the tumour cells and may be more effective when combined with tamoxifen.

The efficacy of 9-cis retinoic acid in experimental models of cancer.

9-cis retinoic acid (9-cis RA) is a retinoid receptor pan-agonist that binds with high affinity to both retinoic acid receptors (RARs) and retinoid X receptors (RXRs). Using a variety of in vivo and in vitro cancer models, we present experimental data that 9-cis RA has activity as a potential chemotherapeutic agent. Treatment of the human promyelocytic leukemia cell line HL-60 with 9-cis RA decreases cell proliferation, increases cell differentiation, and increases apoptosis. Induction of apoptosis correlates with an increase in tissue transglutaminase (type II) activity. In vivo, 9-cis RA induces complete tumor regression of an early passage human lip squamous cell carcinoma xenograft. Finally, 9-cis RA inhibits the anchorage-independent growth of the human breast cancer cell lines MCF-7 and LY2 (an antiestrogen-resistant MCF-7 variant). Transient co-transfection assays indicate that 9-cis RA inhibits estrogen receptor transcription of an ERE-tk-LUC reporter through RAR or RXR receptors. These data suggest that retinoid receptors can antagonize estrogen-dependent transcription and provides one possible mechanism for the inhibition of cell growth by 9-cis RA in breast cancer cell lines. In summary, these findings present evidence that 9-cis RA has a wide range of activities in human cancer models.

Regulation of retinoblastoma gene expression in hormone-dependent breast cancer.

Studies have shown an increased risk for breast cancer in the mothers of children suffering from retinoblastoma and osteosarcoma, suggesting a role for the retinoblastoma susceptibility (Rb) gene product in breast cancer. We now show that estradiol decreases the expression of Rb at the level of protein and messenger RNA (mRNA) in estrogen-dependent breast cancer cell lines. Treatment of MCF-7 cells with 10(-9) M estradiol for 48 h resulted in a 70% decrease in the level of Rb protein. Ribonuclease protection assays showed a 50% decrease in the steady state levels of Rb mRNA by 12 h and a 70% decrease in Rb mRNA by 24 h. Treatment with estradiol had no effect on the rate of Rb gene transcription or on Rb mRNA stability, but resulted in an increase in the steady state level of Rb mRNA in the nucleus. The effect of estradiol was inhibited by 10(-7) M 4-hydroxytamoxifen. In the absence of estradiol, the antiestrogens 4-hydroxytamoxifen and ICI 164,384 increased Rb mRNA by 50% over that in estrogen-depleted conditions. Estradiol regulation of Rb mRNA also occurred in other estrogen-dependent breast cancer cell lines. Insulin-like growth factor I, insulin, progestins, and epidermal growth factor had no effect on Rb expression. In summary, these results show that estradiol specifically regulates the expression of the Rb susceptibility gene product in hormone-dependent breast cancer by a posttranscriptional mechanism that occurs in the nucleus. The results from this study suggest that the negative regulation of Rb expression by estradiol, rather than Rb loss or mutation, may play an important role in breast carcinogenesis.