Recombinant human interferon alpha increases oestrogen receptor expression in human breast cancer cells (ZR-75-1) and sensitizes them to the anti-proliferative effects of tamoxifen

Recombinant Interferon-β in the Treatment of Polycythemia Vera and Related Neoplasms: Rationales and Perspectives

About 30 years ago, the first clinical trials of the safety and efficacy of recombinant interferon-α2 (rIFN-α2) were performed. Since then, several single-arm studies have shown rIFN-α2 to be a highly potent anticancer agent against several cancer types. Unfortunately, however, a high toxicity profile in early studies with rIFN-α2 -among other reasons likely due to the high dosages being used-disqualified rIFN-α2, which was accordingly replaced with competitive drugs that might at first glance look more attractive to clinicians. Later, pegylated IFN-α2a (Pegasys) and pegylated IFN-α2b (PegIntron) were introduced, which have since been reported to be better tolerated due to reduced toxicity. Today, treatment with rIFN-α2 is virtually outdated in non-hematological cancers, where other immunotherapies-e.g., immune-checkpoint inhibitors-are routinely used in several cancer types and are being intensively investigated in others, either as monotherapy or in combination with immunomodulatory agents, although only rarely in combination with rIFN-α2. Within the hematological malignancies, rIFN-α2 has been used off-label for decades in patients with Philadelphia-negative chronic myeloproliferative neoplasms (MPNs)-i.e., essential thrombocythemia, polycythemia vera, and myelofibrosis-and in recent years rIFN-α2 has been revived with the marketing of ropeginterferon-α2b (Besremi) for the treatment of polycythemia vera patients. Additionally, rIFN-α2 has been revived for the treatment of chronic myelogenous leukemia in combination with tyrosine kinase inhibitors. Another rIFN formulation-recombinant interferon-β (rIFN-β)-has been used for decades in the treatment of multiple sclerosis but has never been studied as a potential agent to be used in patients with MPNs, although several studies and reviews have repeatedly described rIFN-β as an effective anticancer agent as well. In this paper, we describe the rationales and perspectives for launching studies on the safety and efficacy of rIFN-β in patients with MPNs.

Interferon Signaling in Estrogen Receptor-positive Breast Cancer: A Revitalized Topic

Cancer immunology is the most rapidly expanding field in cancer research, with the importance of immunity in cancer pathogenesis now well accepted including in the endocrine-related cancers. The immune system plays an essential role in the development of ductal and luminal epithelial differentiation in the mammary gland. Originally identified as evolutionarily conserved antipathogen cytokines, interferons (IFNs) have shown important immune-modulatory and antineoplastic properties when administered to patients with various types of cancer, including breast cancer. Recent studies have drawn attention to the role of tumor- and stromal-infiltrating lymphocytes in dictating therapy response and outcome of breast cancer patients, which, however, is highly dependent on the breast cancer subtype. The emerging role of tumor cell-inherent IFN signaling in the subtype-defined tumor microenvironment could influence therapy response with protumor activities in breast cancer. Here we review evidence with new insights into tumor cell-intrinsic and tumor microenvironment-derived IFN signaling, and the crosstalk of IFN signaling with key signaling pathways in estrogen receptor-positive (ER+) breast cancer. We also discuss clinical implications and opportunities exploiting IFN signaling to treat advanced ER+ breast cancer.

Enhanced IFNα Signaling Promotes Ligand-Independent Activation of ERα to Promote Aromatase Inhibitor Resistance in Breast Cancer

Aromatase inhibitors (AIs) reduce estrogen levels up to 98% as the standard practice to treat postmenopausal women with estrogen receptor-positive (ER+) breast cancer. However, approximately 30% of ER+ breast cancers develop resistance to treatment. Enhanced interferon-alpha (IFNα) signaling is upregulated in breast cancers resistant to AIs, which drives expression of a key regulator of survival, interferon-induced transmembrane protein 1 (IFITM1). However, how upregulated IFNα signaling mediates AI resistance is unknown. In this study, we utilized MCF-7:5C cells, a breast cancer cell model of AI resistance, and demonstrate that these cells exhibit enhanced IFNα signaling and ligand-independent activation of the estrogen receptor (ERα). Experiments demonstrated that STAT1, the mediator of intracellular signaling for IFNα, can interact directly with ERα. Notably, inhibition of IFNα signaling significantly reduced ERα protein expression and ER-regulated genes. In addition, loss of ERα suppressed IFITM1 expression, which was associated with cell death. Notably, chromatin immunoprecipitation experiments validated that both ERα and STAT1 associate with ERE sequences in the IFITM1 promoter. Overall, hyperactivation of IFNα signaling enhances ligand-independent activation of ERα, which promotes ER-regulated, and interferon stimulated gene expression to promote survival in AI-resistant breast cancer cells.

Interferons: role in cancer therapy

Interferons (IFNs) are a group of signaling cytokines, secreted by host cells to induce protection against various disorders. IFNs can directly impact on tumor cells or indirectly induce the immune system to protect host cells. The expression levels of IFNs and its functions of are excellently modulated in a way to protect host cells from probable toxicities caused by extreme responses. The efficacy of anticancer therapies is correlated to IFNs signaling. Although IFN signaling is involved in induction of antitumor responses, chronic stimulation of the IFN signaling pathway can induce resistance to various antineoplasm therapies. Hence, IFNs are expressed by both cancer and immune cells, and modulate their biological function. Understanding this mechanism of action might be a key target of combination therapies.

Follicular fluid vascular endothelial growth factor is associated with type of infertility and interferon alpha correlates with endometrial thickness in natural cycle in vitro fertilization

The aim of this study was to analyse the presence of vascular endothelial growth factor (VEGF) and interferon alpha (IFN-α) in the follicular fluid (FF) and their possible influence, as pro-angiogenic or anti-angiogenic factors, on in vitro fertilization outcome. The concentrations of VEGF and IFN-α were correlated with oocyte and embryo quality, concentrations of hormones in the serum, perifollicular blood flow and endometrial thickness. VEGF was detected in all FF samples (median 706.6 pg/ml, range 182.9-6638 pg/ml). IFN-α was detected in 60% of the samples (median 6.5 pg/ml, range 0-79.4 pg/ml), while in 40% of the samples its levels were below the test detection limit. VEGF and IFN-α concentrations did not correlate with the cause of infertility, concentrations of FSH, LH, E2 and prolactin, oocyte or embryo quality. Significantly higher concentrations of VEGF have been found in women with primary compared with secondary infertility (p = 0.011, Mann Whitney test). The concentrations of VEGF and IFN-α did not correlate with the resistance index (RI) on days of hCG administration, follicular aspiration and embryo transfer. However, the concentrations of IFN-α correlated with endometrial thickness on the day of embryo transfer (Spearman correlation coefficient ρ = 0.4107; P < 0.05) but not on days of hCG administration and follicular aspiration. The mechanism of VEGF association with the previous ability of having a child needs to be clarified in future studies. The results of this study indicate a possible role of IFN-α in pathways of endometrial remodelling.

Antitumour actions of interferons: implications for cancer therapy

The interferons (IFNs) are a family of cytokines that protect against disease by direct effects on target cells and by activating immune responses. The production and actions of IFNs are finely tuned to achieve maximal protection and avoid the potential toxicity associated with excessive responses. IFNs are back in the spotlight owing to mounting evidence that is reshaping how we can exploit this pathway therapeutically. As IFNs can be produced by, and act on, both tumour cells and immune cells, understanding this reciprocal interaction will enable the development of improved single-agent or combination therapies that exploit IFN pathways and new 'omics'-based biomarkers to indicate responsive patients.

The "Cross Talk" between the Receptors of Insulin, Estrogen and Progesterone in Neutrophils in the Synthesis of Maspin through Nitric Oxide in Breast Cancer

PURPOSE

The binding of either insulin, or estrogen or progesterone to their specific receptors on neutrophils has been reported to stimulate nitric oxide (NO) induced maspin synthesis in these cells. Experiments were carried out to determine the role of insulin receptor interaction in the nitric oxide induced maspin synthesis in neutrophils that was effected by estrogen or progesterone.

METHODS

Estrogen receptor positive (ER+) and progesterone receptor positive (PR+) neutrophils were isolated from the blood cancer subjects. Maspin was determined by enzyme linked immunosorbent assay after in vitro translation of maspin mRNA. NO was determined by methemoglobin method.

RESULTS

It was found that pre incubation of normal neutrophils with insulin to reach equilibrium binding decreased both ER and PR numbers by ≈50% without changing the dissociation constants of the steroids binding. The reduction of ER or PR numbers on neutrophils due to the pretreatment with insulin resulted in the decreased NO induced maspin synthesis from 2.383 ± 0.014 nM to 1.454 ± 0.004 nM in the case of estrogen and in the decrease of maspin synthesis from 2.329 ± 0.012 nM to 1.410 ± 0.002 nM in the case of progesterone. The incubation of ER+ neutrophils or PR+ neutrophils with insulin further decreased the maspin synthesis from 1.422 ± 0.029 nM to 0.790 ± 0.004 nM in the case of estrogen, and from 1.138 ± 0.024 nM to 0.555 ± 0.003 nM maspin in the case of progesterone respectively compared to normal control.

CONCLUSION

These results suggested that a "cross-talk" between the insulin receptors and the steroid receptors down regulated maspin synthesis in normal and in breast cancer neutrophils.

The Role of Interferon Regulatory Factor-1 (IRF1) in Overcoming Antiestrogen Resistance in the Treatment of Breast Cancer

Resistance to endocrine therapy is common among breast cancer patients with estrogen receptor alpha-positive (ER+) tumors and limits the success of this therapeutic strategy. While the mechanisms that regulate endocrine responsiveness and cell fate are not fully understood, interferon regulatory factor-1 (IRF1) is strongly implicated as a key regulatory node in the underlying signaling network. IRF1 is a tumor suppressor that mediates cell fate by facilitating apoptosis and can do so with or without functional p53. Expression of IRF1 is downregulated in endocrine-resistant breast cancer cells, protecting these cells from IRF1-induced inhibition of proliferation and/or induction of cell death. Nonetheless, when IRF1 expression is induced following IFNγ treatment, antiestrogen sensitivity is restored by a process that includes the inhibition of prosurvival BCL2 family members and caspase activation. These data suggest that a combination of endocrine therapy and compounds that effectively induce IRF1 expression may be useful for the treatment of many ER+ breast cancers. By understanding IRF1 signaling in the context of endocrine responsiveness, we may be able to develop novel therapeutic strategies and better predict how patients will respond to endocrine therapy.

IFNgamma restores breast cancer sensitivity to fulvestrant by regulating STAT1, IFN regulatory factor 1, NF-kappaB, BCL2 family members, and signaling to caspase-dependent apoptosis

Antiestrogens are effective therapies for the management of many estrogen receptor-alpha (ER)-positive breast cancers. Nonetheless, both de novo and acquired resistance occur and remain major problems in the clinical setting. IFNgamma is an inflammatory cytokine that induces the expression and function of IFN regulatory factor 1 (IRF1), a tumor suppressor gene that can increase antiestrogen responsiveness. We show that IFNgamma, but not IFNalpha, IFNbeta, or fulvestrant (ICI; ICI 182,780; Faslodex), induces IRF1 expression in antiestrogen-resistant MCF7/LCC9 and LY2 cells. Moreover, IFNgamma restores the responsiveness of these cells to fulvestrant. Increased IRF1 activation suppresses NF-kappaB p65 (RELA) activity, inhibits the expression of prosurvival (BCL2, BCL-W), and induces the expression of proapoptotic members (BAK, mitochondrial BAX) of the BCL2 family. This molecular signaling is associated with the activation of signal transducer and activator of transcription 1 and leads to increased mitochondrial membrane permeability; activation of caspase-7 (CASP7), CASP8, and CASP9; and induction of apoptosis but not autophagy. Whereas antiestrogen-resistant cells are capable of inducing autophagy through IFN-mediated signaling, their ability to do so through antiestrogen-regulated signaling is lost. The abilities of IFNgamma to activate CASP8, induce apoptosis, and restore antiestrogen sensitivity are prevented by siRNA targeting IRF1, whereas transient overexpression of IRF1 mimics the effects of IFNgamma treatment. These observations support the exploration of clinical trials combining antiestrogens and compounds that can induce IRF1, such as IFNgamma, for the treatment of some ER-positive breast cancers.

Gene network signaling in hormone responsiveness modifies apoptosis and autophagy in breast cancer cells

Resistance to endocrine therapies, whether de novo or acquired, remains a major limitation in the ability to cure many tumors that express detectable levels of the estrogen receptor alpha protein (ER). While several resistance phenotypes have been described, endocrine unresponsiveness in the context of therapy-induced tumor growth appears to be the most prevalent. The signaling that regulates endocrine resistant phenotypes is poorly understood but it involves a complex signaling network with a topology that includes redundant and degenerative features. To be relevant to clinical outcomes, the most pertinent features of this network are those that ultimately affect the endocrine-regulated components of the cell fate and cell proliferation machineries. We show that autophagy, as supported by the endocrine regulation of monodansylcadaverine staining, increased LC3 cleavage, and reduced expression of p62/SQSTM1, plays an important role in breast cancer cells responding to endocrine therapy. We further show that the cell fate machinery includes both apoptotic and autophagic functions that are potentially regulated through integrated signaling that flows through key members of the BCL2 gene family and beclin-1 (BECN1). This signaling links cellular functions in mitochondria and endoplasmic reticulum, the latter as a consequence of induction of the unfolded protein response. We have taken a seed-gene approach to begin extracting critical nodes and edges that represent central signaling events in the endocrine regulation of apoptosis and autophagy. Three seed nodes were identified from global gene or protein expression analyses and supported by subsequent functional studies that established their abilities to affect cell fate. The seed nodes of nuclear factor kappa B (NFkappaB), interferon regulatory factor-1 (IRF1), and X-box binding protein-1 (XBP1)are linked by directional edges that support signal flow through a preliminary network that is grown to include key regulators of their individual function: NEMO/IKKgamma, nucleophosmin and ER respectively. Signaling proceeds through BCL2 gene family members and BECN1 ultimately to regulate cell fate.

Subcutaneously administered recombinant human interleukin-2 and interferon alfa-2a for advanced breast cancer: a phase II study of the Cancer and Leukemia Group B (CALGB 9041)

New and more effective treatments are needed for metastatic breast cancer. This study aimed to determine the effectiveness of a combination of subcutaneously administered recombinant human interleukin-2 (rIL-2), 1.5 MU/m(2) for 5 consecutive days repeated for 3 weeks, and interferon alpha-2a (IFN), 7.5 MU/m(2), administered subcutaneously three times per week. Women who had previously received 1-2 prior chemotherapy regimens for measurable inoperable, recurrent, or metastatic breast cancer were eligible. Of 40 patients accrued to the study, 32 were evaluable for response assessment. Toxicities were frequent but manageable. The most common grade 3 and 4 toxicities were lymphopenia (17%) and malaise/fatigue (24%). There were no complete responses, one partial response (3%), and six patients with stable disease (19%). Of the seven patients with partial response or stable disease, all had tumors that expressed hormone receptors. The median survival was 8.9 months and all patients have died. Good performance status was the most important predictor of survival. In this group of women with metastatic breast cancer, the overall prognosis was poor. This combination of rIL-2 and IFN was ineffective.

A phase II trial of interferon-alpha and toremifene in advanced renal cell cancer patients

Treatment options for patients with metastatic renal cell carcinoma are limited. Interferon-alpha has an overall response rate of 10-15% in phase II and III clinical trials and is considered a standard option for patients. Though the anti-estrogen toremifene has shown only modest single agent activity in renal cell carcinoma, evidence for synergy of anti-estrogens with interferon-alpha exists in renal cell and other cancers. Therefore, a phase II trial was undertaken to test the combination of interferon-alpha and toremifene in advanced renal cell carcinoma. Thirteen patients with measurable metastatic or unresectable local disease were treated with interferon-alpha at a dose of 5 million units/m2 three times a week and daily oral toremifene at 300 mg daily in divided doses. Patients were treated for 12 weeks and then restaged. Clinical response was the primary endpoint of the trial. Four patients (31%) had evidence of stable disease at 12 weeks, while the remaining nine patients (69%) progressed on treatment. Toxicity was moderate, with grade 2 or 3 fatigue, nausea and anorexia each noted in 31% of patients. We conclude that the combination of interferon-alpha plus toremifene demonstrates no significant activity in advanced renal cell carcinoma.

Acute effects of interferon on estrogen receptor function do not involve the extracellular signal-regulated kinases p42mapk and p44mapk

Exposure to type I interferons (IFN) increased estrogen receptor (ER) ligand binding and induced protein kinase C (PKC) translocation within 30 min but had no effect on net incorporation of [32P] into ER in Madin Darby bovine kidney (MDBK) cells. Ligand binding was also increased within 30 min by phorbol ester and the protein phosphatase inhibitor okadaic acid. Mitogen-activated protein (MAP) kinase phosphorylation was initially inhibited between 2 and 30 min and subsequently activated between 30 and 60 min after treatment with IFN. The activatory response was blocked by the PKC inhibitor Ro 31-8220. Following transient transfection with an ERE-CAT reporter construct, IFN increased CAT expression after 6 h but decreased ER ligand binding, transcriptional activity and phosphorylation after 48 h, probably as a result of decreased ER concentrations. The results rule out rapid activation of ER ligand binding through phosphorylation at Ser118 by MAP kinase because (1) the increase in ligand binding preceded activation of MAP kinase, and (2) IFN had no short-term effect on [32P]incorporation or ER transcriptional activity. The rapid effect of IFN on ER ligand binding is postulated to reflect phosphorylation of the receptor at Tyr537 by p56lck, a member of the Src family of PKC-activated tyrosine kinases.

Effect of interferon alpha-2a on hormone receptor status in patients with advanced breast cancer

The aim of this study was to assess the effect of recombinant interferon alpha-2a (rh-IFN) on estrogen (ER) and progesterone (PR) receptor expression in patients with advanced breast cancer and the evaluation of the effect of rh-IFN pretreatment on response to endocrine therapy with tamoxifen (TAM). Between June 1990 and November 1992, 20 patients with disseminated breast cancer and with metastatic skin nodules suitable for biopsy were entered into this study. Eighteen assessable patients underwent biopsy before and 2 weeks after treatment with rh-INF. rh-INF 3 x 10(6) IU were administered subcutaneously per day. Patients with ER expression at second biopsy were subsequently treated with 20 mg TAM daily. One patient had rapid disease progression and died before rebiopsy could be performed, and an additional patient refused second biopsy. All other patients were considered assessable. Thirteen patients showed ER expression before rh-IFN treatment, and 5 PR presented with expression. Rh-IFN increased ER expression in three patients and PR in four patients. No change was observed in 8 patients for ER and in 12 patients for PR. ER expression decreased in seven patients and PR expression decreased in two patients, respectively. Two patients showed a partial remission after subsequent treatment with TAM. Adverse reactions caused by rh-IFN were mainly flu-like symptoms. In this trial we found no systematic impact of rh-IFN on hormone receptor expression and, subsequently, on the response rate in patients with advanced breast cancer.

Minimal residual disease in metastatic breast cancer: treatment with IFN-beta, retinoids, and tamoxifen

The aim of this pilot study was to investigate if chemotherapy (CT) followed by the combination of interferon-beta (IFN-beta), retinoids, and tamoxifen could be effective in the treatment of metastatic breast cancer (MBC). Thirty-six patients with stage IV carcinoma of the breast were treated with six courses of cyclophosphamide, 5-fluorouracil, 4-epidoxorubicin, vincristine, and prednisone every 3 weeks (FECPV), followed by two courses of non-cross-resistant drugs, methotrexate, mitomycin C, and mitoxantrone (MMM). Treatment was continued, in responders, with low dose IFN-beta, retinyl palmitate, and tamoxifen until relapse of the disease occurred. Among 36 evaluable patients, 23 achieved a clinical response (64 %) (95 % confidence interval [c.i.] 46 %-79 %), 7 had stable disease (19%), and 6 (17%) progressed. Leukopenia occurred in 15 patients, thrombocytopenia in 6, and anemia in 11. Sixteen patients had nausea/vomiting, stomatitis was observed in 9, and diarrhea occurred in 3. Toxicity from maintenance therapy was mild and mainly hepatic. Median response duration was 31 months (range 5-107). Median overall survival was 32 months (9-108). Our study shows that this combined approach for the treatment of MBC is feasible, with an acceptable toxicity.

Synergistic antitumor effects of a combination of interferon and tamoxifen on estrogen receptor-positive and receptor-negative human tumor cell lines in vivo and in vitro

Solid tumors are relatively resistant to growth inhibition by interferons (IFNs). To enhance sensitivity, we assessed combinations of IFNs with tamoxifen in estrogen receptor-positive (ER-positive) and ER-negative human tumor xenografts. In nude mice, the growth of MCF-7 human breast tumors (ER-positive) and NIH-OVCAR-3 ovarian tumors (functionally ER-negative) was suppressed completely when tamoxifen and IFN-alpha or IFN-beta was started 2 days after tumor inoculation. Established, 6-week-old MCF-7 and NIH-OVCAR-3 tumors regressed when treated with the combination of IFN-beta and tamoxifen but not with single-agent therapy. Treatment with the combination also resulted in an augmented antitumor response in vivo in an ER-negative breast tumor (MDA-MB-231), a colon carcinoma (HT-29), and a melanoma (SK-MEL-1). Antiproliferative studies in vitro suggested that growth of both MCF-7 and NIH-OVCAR-3 cells was inhibited to a greater degree by combination treatment with human IFN-alpha and tamoxifen or IFN-beta and tamoxifen compared with single agents. Median effect analysis defined synergy. Four ER-negative carcinomas (MDA-MB-231, MDA-MB-468, BT-20, and HT-29) also exhibited synergistic growth inhibition in response to the drug combination. The response of these four cell lines was particularly striking. Tamoxifen as a single agent had little effect (up to 2.0 microM) but caused enhanced antiproliferative activity when added to IFN-beta. Sequential treatment of MCF-7 cells in vitro with tamoxifen followed by IFN-beta was more effective at inhibiting growth than treatment with IFN-beta followed by tamoxifen, suggesting that tamoxifen modulated the anticellular response to IFN-beta rather than the converse. Similar results were obtained with IFN-alpha. Cell cycle analysis indicated that 7 days of exposure to the combination resulted in MCF-7 cell fragmentation and death. Together with our recent studies demonstrating enhancement of IFN-stimulated gene expression (ISG) by tamoxifen pretreatment in IFN-resistant cells, these data suggest that combination treatment with tamoxifen and IFNs may increase ISG expression in IFN-resistant tumors, leading to augmented antitumor effects. These effects appear to be independent of ER expression.

Tamoxifen enhances interferon-regulated gene expression in breast cancer cells

The molecular basis for the enhanced growth inhibition of MCF-7 human breast cancer xenografts by a combination of human interferon-beta (IFN-beta) and tamoxifen was investigated. Treatment of MCF-7, MDA-MB-231, and BT-20 cells with the combination of IFN-beta and tamoxifen resulted in enhanced antiproliferative effects in vitro. Treatment with the combination of IFN-beta and tamoxifen enhanced the expression of several IFN-beta-inducible genes in human breast carcinoma cell lines relative to levels induced by IFN-beta alone. Tamoxifen alone did not induce transcription of IFN-stimulated genes (ISGs). Augmentation of ISG expression by the combination of IFN-beta and tamoxifen was noted in breast tumor cell lines irrespective of their functional estrogen receptor (ER) status or their dependence on estradiol for growth, suggesting that upregulation of ISGs was independent of ER status. Enhancement of IFN-stimulated gene expression by tamoxifen occurred at the transcriptional level. Expression of transfected reporter genes under the control of IFN-alpha/beta regulated promoters was also enhanced in IFN-beta and tamoxifen-treated cells. Similarly, transcriptional induction of chimeric reporter plasmids driven by an IFN-gamma inducible promoter (GAS; IFN-gamma activated site) was also enhanced by the combination of IFN-gamma and tamoxifen. In tamoxifen treated cells, IFN-beta and IFN-gamma readily activated transcription factors ISGF-3 and GAF, respectively. Therefore, augmentation of ISG expression by tamoxifen is an early event in the antitumoral activity of this drug combination.

The treatment of disseminated malignant melanoma with special reference to the role of interferons, vinca alkaloids and tamoxifen

Malignant melanoma continues to increase in incidence. While early melanoma is highly curable by surgical means, the prognosis of patients with more advanced lesions and/or metastatic disease remains poor. Conventional chemotherapy with dacarbazine has a low frequency and short duration of response. Alternative drugs with single-agent activity include vinca alkaloids, nitrosoureas, procarbazine and platinum compounds. The addition of tamoxifen to chemotherapy, particularly cisplatin-based chemotherapy, appears to be beneficial. Recent studies suggest that combination chemotherapy may give better outcomes than single-agent treatment. Significant clinical activity has also been demonstrated with the use of interferons, particularly interferon alpha, and also with IL-2. Two recent studies suggest that the addition of interferon to chemotherapy may be beneficial. In addition, specific active immunotherapy with tumour vaccines has shown promise. The optimal methods of combining these treatment methods, such as chemotherapy and biological response modifiers/immunotherapy, however, remain to be defined.

Oncogene expression is modulated by recombinant human interferon-beta in human breast-cancer cells

The effect of recombinant human interferon-beta on growth and oncoprotein expression was investigated in several human breast-cancer cell lines with different characteristics. All cell lines tested were sensitive to the antiproliferative action of the drug, regardless of their estrogen sensitivity. The maximal inhibition of cell proliferation was seen after 6 days of treatment. In estrogen-sensitive CG-5 and ZR-75-1 cells, but not in MDA-MB-453 estrogen-insensitive cells, a reduction in c-myc and c-erbB2 oncoproteins occurred after 48-72 hr and became more pronounced after 120-168 hr of treatment, suggesting that this down-regulation is not direct but is mediated by undefined molecular mechanisms. The time-course of the IFN-mediated decrease in oncoproteins seems to indicate that this event is not strictly related to the IFN-regulation of cell proliferation. The expression of c-erbB2 and c-myc was also analyzed, after recombinant human interferon-beta treatment, at the mRNA level in CG-5 cells. Surprisingly, no statistically significant variation of c-erbB2 or of c-myc mRNA was found either before or after 120-168 hr. Thus, we surmise that the observed reduction of oncoproteins may be due to post-transcriptional mechanisms.

Specific binding of estradiol to rat coronary artery smooth muscle cells

We report the expression and characteristics of the estrogen receptor in rat coronary artery-derived smooth muscle cells. Polymerase chain reaction analyses of total and poly(A) + mRNA from rat coronary artery-derived smooth muscle cells indicate the presence of estrogen receptor mRNA. Binding analyses reveal the presence of high affinity binding sites for 17 beta-estradiol, with a Kd equivalent to that observed for authentic estrogen receptors in other estrogen responsive tissues. Scatchard and Hill plot analyses of the properties of receptor-ligand binding indicate the presence of a single site, and the absence of cooperative binding. Unlabeled E2 but not testosterone, dexamethasone or progesterone compete with [3H] 17 beta-estradiol for binding sites. The affinity, specificity and non-cooperative nature of the estrogen binding sites are identical to those observed in other estrogen-responsive tissues. These cells may provide a novel model in which to study the effects of estrogens on the proliferation, differentiation and function of vascular smooth muscle cells.

Interferon-beta, retinoids, and tamoxifen in the treatment of metastatic breast cancer: a phase II study

Based on the additive or synergistic antiproliferative effect of interferon and tamoxifen on breast cancer cell lines and on preclinical and clinical data on retinoids alone and in combination with antiestrogen or interferon, we designed a pilot phase II study to test the toxicity of simultaneous administration of interferon-beta (IFN-beta), retinoids (R), and tamoxifen (TAM) and the efficacy of this combination as salvage therapy in a group of patients with metastatic breast cancer (MBC). A total of 49 stage IV breast cancer patients, 11 pretreated with hormones, 26 with chemotherapy, and 12 with both, received 30 mg TAM and two dose levels of IFN-beta and retinyl palmitate. Among 49 evaluable patients, 27 achieved a clinical response (55%; 95% CI 41-69%), 10 had stable disease (20%), and in 12 (25%) the disease progressed. Toxicity with both dose levels was moderate and mainly hepatic. Median response duration, not statistically different in estrogen receptor-positive and negative patients, was 31.4 months (range 4.9-67). Median overall survival was 19.2 months (range 2-69). We have shown that long-term administration of TAM, IFN-beta, and retinyl palmitate is feasible with moderate toxicity. We have also demonstrated that this regimen is active in pretreated MBC patients and that responses are not influenced by receptor status.

Megestrol acetate plus alpha 2a interferon as second line therapy for postmenopausal patients with advanced breast cancer: results of a multicentric phase II trial

This phase II study was aimed to evaluate the activity of a combination of megestrol acetate (MA) and alpha 2a interferon (IFN) in a group of tamoxifen-responsive breast cancer patients. Thirty patients with metastatic breast cancer either previously treated with adjuvant tamoxifen for at least 24 months or treated with tamoxifen for metastatic disease and showing an objective response or stability of disease, were given MA (single daily dose of 160 mg per os) and alpha 2a IFN (3 million units-MU-three times per week intramuscularly-i.m.-). Of the 29 evaluable patients, 2 (6.8%) achieved a complete response and 4 (13.8%) a partial response for an overall response rate of 20.6% (95% confidence limits = 5.9%-35.4%). Treatment toxicity was mild and no patient had to discontinue or delay the treatment due to IFN side effects. Our results seem to rule out that alpha 2a IFN is able to improve the activity of MA as second-line therapy in tamoxifen-responsive patients.

Androgen receptors and hormone sensitivity of a human prostatic cancer cell line (PC-3) are modulated by natural beta-interferon

Androgen receptors are expressed at a low level in the cell line PC-3, which does not respond to either androgens or antiandrogens. If these cells are exposed to natural beta-interferon (beta-IFN) a reduction in cell growth and an increase in androgen receptors, evaluated by both biochemical and immunocytochemical techniques, occur. This increase seems not to be related to a selective block of PC-3 in any phase of the cell cycle. Pretreatment with beta-IFN determines in PC-3 cells a partial responsiveness to the androgen dihydrotestosterone as reflected by the increase in cell number. Moreover, the antiandrogen hydroxyflutamide shows agonistic properties by increasing the cell number of PC-3 cells pre-exposed to beta-IFN. When the antiandrogen is tested in combination with interferon, it produces a reduction in the beta-IFN-induced inhibition of cell growth. It is not known whether these unexpected effects are due to the increase in androgen receptors or to other mechanisms.

Phenotypic and functional profile of peripheral blood mononuclear cells isolated from melanoma patients undergoing combined immunotherapy and chemotherapy

In the present study we tested the phenotypic profile as well as several immunological responses of peripheral blood mononuclear cells (PBMC) isolated from melanoma patients. These patients underwent chemotherapy with dacarbazine and carboplatin from day 1 to day 22, followed by immunotherapy of low-dose recombinant interleukin-2 and recombinant interferon alpha administered subcutaneously from day 36 to day 75. The PBMC from 14 patients were isolated on day 0 before chemotherapy, on day 36 after chemotherapy and on day 76 after immunotherapy. After chemotherapy, a decrease in CD16+ cells and increase in CD3+ and CD4+ cells correlated with a significant decrease in the generation of lymphokine-activated killer (LAK) activity. After immunotherapy, an increase in CD16+ cells correlated with an increase in the induction of LAK activity. A comparison between responding and non-responding patients revealed statistically significant differences in LAK activity of PBMC and response to concanavalin A following chemotherapy, and in the percentage of CD8+ cells following immunotherapy. Our results point toward the value of continuing such a study on a larger population of cancer patients in order to select the appropriate bioassays for monitoring and predicting the clinical responsiveness to combined therapies.

Interferon plus tamoxifen treatment for advanced breast cancer: in vivo biologic effects of two growth modulators

The effects of interferon-alpha (IFN) plus tamoxifen (TMX) in the treatment of advanced breast cancer were assessed. Changes of in vivo biologic determinants including hormone receptors, P24 protein, Ki-67 and growth factor expression were evaluated. Seven patients with advanced, heavily pretreated, breast cancer with accessible disease, underwent biopsy prior to and after sequential treatment with IFN and IFN plus TMX. Clinically 4/7 patients responded to treatment with one complete and three partial remissions. Apart from the favourable response rate the sequential in vivo changes in expression of tumour variables were of considerable interest. IFN treatment consistently increased the expression of the estrogen receptor (ER) and of the estrogen regulated protein P24 while decreasing the expression of the proliferation associated antigen Ki-67. Addition of TMX on the other hand resulted in a reduction of ER expression to pre-IFN levels and a rise in progesterone receptor (PR) expression. When the effect of either IFN or IFN plus TMX on the expression of two growth factors was assessed they were found to be somewhat variable. While PDGF expression tended to be suppressed, there was no clinical correlation with response to therapy. TGF beta expression was found in all patients prior to treatment and while all non-responders showed reduction of TGF beta following treatment, the alterations were variable amongst responders (including two patients with increased expression, one with no change, and one with decreased expression). It is concluded that both IFN and TMX exert multiple effects on the expression of tumour biologic variables and that while the study confirmed some of the predictions from in vitro models, the in vivo effect are more complex than has been appreciated from the models. From the clinical point of view, it might be expected that treatment which enhances the expression of ER in tumours should have a positive effect on the response to TMX.

Enhancement of progesterone receptor levels by interferons in AE-7 endometrial cancer cells

BACKGROUND

Interferon (IFN) has been reported to increase hormone receptor expression in breast cancer cells and to sensitize them to antiproliferative hormones. Endometrial cancer cells with high progesterone receptor (PR) level respond better to progesterone therapy than cells with either low or absent PR level. The effect of four different interferons (alpha and beta, both natural [n] and recombinant [r]) on cell proliferation and steroid receptor levels was investigated in the PR positive AE-7 human endometrial cancer cell line over a period of 12 days.

METHODS

Cells were exposed to 10,100 and 1000 IU/ml of each IFN either for 3 days or continuously for 12 days. Hormone receptors were determined by the monoclonal enzyme immunoassay. Chemosensitivity was evaluated with the adenosine triphosphate-cell viability assay.

RESULTS

AE-7 has a low level of estrogen receptors, which was not significantly affected by IFN exposure. The four IFN showed significantly enhanced PR levels over 12 days in both the 3-day and continuous-exposure experiments. No significant difference of PR enhancement was observed between 3 days and continuous exposure to IFN. This increase of receptors did not appear to be dose related. IFN enhanced PR level to a maximum level of about two times control cells. IFN did not produce significant cytotoxicity. Antiproliferative activity was observed with nIFN beta and rIFN beta at 1000 IU/ml dose in continuous-exposure experiments, which showed survival values of 79% and 69% respectively, compared with control at day 12.

CONCLUSIONS

These preliminary data on PR expression modulation support other studies, which have shown that IFN modulate hormone receptor expression and, therefore, may play a role in the treatment of endometrial cancer.

Human recombinant interferon-beta SER and tamoxifen: growth suppressive effects for the human breast carcinoma MCF-7 grown in the athymic mouse

Tamoxifen is the endocrine treatment of choice for breast cancer. However, resistance to therapy and patient relapse inevitably occurs. In future treatment schedules, interferons could be administered with tamoxifen, in an attempt to prevent disease recurrence. Human recombinant interferon-beta SER (rIFN-beta SER) inhibited the growth in vitro of the estrogen receptor (ER) positive breast cancer cell line MCF-7 and the ER negative breast cancer cell line MDA-MB-231. This inhibitory effect was achieved at doses of 50 U/ml and above. The growth of MCF-7 tumors in estradiol-stimulated athymic mice was greatly inhibited by high dose rIFN-beta SER treatment (10(6)U/day). In spite of the impressive antitumor effects upon MCF-7 tumors, rIFN-beta SER had no effect upon ER levels within the tumors at either the RNA or protein level, as measured by Northern blotting and ER-EIA respectively. High dose rIFN-beta SER (10(6)U/day) did result in some inhibition in the growth in vivo of the tamoxifen-stimulated MCF-7 variant MCF-7 TAM, although not to the same extent as was observed with the estradiol-stimulated MCF-7 tumors. rIFN-beta SER was also administered to animals bearing MCF-7 tumors and treated with estradiol and tamoxifen. In the animals undergoing high dose therapy (10(6)U/day), tumor growth was completely suppressed. Furthermore, tumor growth continued to be suppressed in those animals in which the rIFN-beta SER therapy was halted and the tamoxifen capsule removed. No tumors were observed in spite of the environment of estradiol stimulation. Thus, the combination of interferon and tamoxifen was totally growth suppressive for MCF-7 xenografts in nude mice.

Lack of therapeutic efficacy of tamoxifen in advanced renal cell carcinoma

In the present study, we treated a total of 62 patients with advanced renal cell carcinoma with high-dose tamoxifen (100 mg/m2/day). Patients were treated in the outpatient setting, and were evaluated 8-12 weeks after initiation of therapy or sooner, when clinical disease progression was evident; a total of 15 patients were seen at short regular intervals for evaluation of clinical and laboratory parameters. Of these 62 patients, 59 were evaluable for treatment response, survival and systemic toxicity. One partial remission was achieved (1.7%; 95% confidence interval, 0.04-9.09%), response duration was 3 months. 10 patients presented with stable disease, for a median duration of 4.0 months, and 48 patients exhibited disease progression upon and after therapy. Systemic toxicity was significant; severe fatigue occurred in 5% of patients, and moderate anaemia, dyspnea, alopecia and malaise in almost 20% of patients. Antineoplastic efficacy of tamoxifen at this dosage in this cohort of patients was at best marginal and well in the range associated with the occurrence of spontaneous remissions. Toxicity was substantial, and it was not balanced by therapeutic benefit. This is consistent with the known lack of therapeutic efficacy of endocrine therapy in advanced renal cell carcinoma.

Steroid receptor enhancement by natural interferon-beta in advanced breast cancer

In the current study we investigated the effect of two different doses of natural interferon-beta (IFN-beta) on steroid hormone receptors in 45 patients with advanced breast cancer. IFN-beta seems to regulate the receptor mechanisms, inducing in cutaneous metastases an increase of oestrogen and progesterone receptors. Moreover, using IFN-beta and tamoxifen as a combined therapy in 23 receptor-positive patients, no negative interference of the two drugs was observed and no relevant side-effects due to the treatment were noticed. The modulation of steroid receptor content by IFN-beta in advanced breast cancer might represent an interesting way to ameliorate the clinical responsiveness to anti-oestrogens.

Recombinant human interferon-alpha 2a increases hormone receptor level of a human breast carcinoma xenograft in nude mice and enhances the anti-proliferative activity of tamoxifen

The effect of recombinant human interferon-alpha 2a (rhIFN-alpha 2a) on the hormone receptor level and antitumor activity of tamoxifen (TAM) was investigated in nude mice using ZR-75-1, an estrogen receptor (ER)-positive, and progesterone receptor (PgR)-negative human breast carcinoma xenograft. ER levels (maximum binding sites) of tumors treated with rhIFN-alpha 2a at a dose of 6 x 10(5) U/mouse/day for 1 or 3 wk were not significantly different from the control, whereas those with rhIFN-alpha 2a at a dose of 6 x 10(4) U/mouse/day for 1 or 3 wk were higher than the control (3.9- to 4.4-fold) with a significant difference at P < 0.01. The increase of ER by rhIFN-alpha 2a was investigated using a sucrose density gradient method. The peak was only seen at 8S in both rhIFN-alpha 2a-treated tumor and control ER, and the sedimentation patterns were almost the same, suggesting that both ERs were essentially equivalent. On the other hand, PgR of all the treated groups could be detected, while that of the control group was undetectable. The antitumor effect of the combination treatment of rhIFN-alpha 2a and TAM was compared with those of single treatments. While rhIFN-alpha 2a at a dose of 6 x 10(5) U/mouse/day and TAM did not show a combination effect, rhIFN-alpha 2a at a dose of 6 x 10(4) U/mouse/day and TAM showed a synergistic combination effect, and ER was decreased to the threshold of detection by the combination treatment. These findings indicated that a low dose of rhIFN-alpha 2a increased the ER levels of ER-positive human breast cancer in vivo as well as in vitro and enhanced the anti-proliferative effect of TAM, and the newly synthesized ER was essentially the same as the original ER.

Changes in epidermal growth factor receptor expression and response to ligand associated with acquired tamoxifen resistance or oestrogen independence in the ZR-75-1 human breast cancer cell line

We have examined the expression of receptors for epidermal growth factor (EGFR) by the ZR-75-1 human breast cancer cell line and tamoxifen resistant (ZR-75-9al 8 microM) and oestrogen independent/tamoxifen sensitive (ZR-PR-LT) variants. The parent line expressed a single class of high affinity binding sites (4,340 +/- 460 receptors/cell; Kd 0.23 +/- 0.04 nM). ZR-75-9al 8 microM cells, routinely maintained in medium containing 8 microM tamoxifen, were negative for oestrogen receptor (ER) and progesterone receptor (PGR) and expressed a markedly increased number of EGFR (14,723 +/- 2116 receptors/cell). Receptor affinity was unchanged. Time dependent reversal of the tamoxifen resistant phenotype was accompanied by a return to ER and PGR positivity and a fall in EGFR numbers to parent cell levels. In contrast ZR-PR-LT cells had a greatly reduced EGFR content (803 +/- 161 receptors/cell) accompanying elevated PGR numbers. Pre-treatment of these cells with suramin or mild acid stripping failed to expose receptors which may have been occupied by endogenously produced ligand. Increased proliferation of ZR-75-1 cells treated with EGFR (0.01-10 ng ml-1) was only observed in serum-free medium lacking insulin and oestradiol. Under these conditions untreated cells failed to proliferate. Both variant lines continued to proliferate in serum free medium in the absence or presence of insulin and oestradiol but failed to respond to exogenous EGF.

Tamoxifen and alpha interferon in advanced breast cancer

Thirteen pretreated advanced breast cancer patients received a combination of alpha interferon 5 million IU every 2 days, subcutaneously, plus tamoxifen 10 mg 3 times daily, until disease progression. The objective response rate was 15.4%: 1 patient achieved a complete response, 1 a partial response and 11 demonstrated stable disease; half of the patients were receptor negative and/or pretreated with hormonotherapy. Durations of response were 16 and 26 months for the CR and PR patients respectively; median progression-free survival was 4 months (range 0-26). Toxicities were registered according to WHO criteria: 4 patients stopped the treatment with interferon because of severe flu-like symptoms, while in the others the combination was generally accepted with good tolerance.

Phase II evaluation of interferon added to tamoxifen in the treatment of metastatic breast cancer

This phase II trial evaluated the clinical role of interferon (IFN) in overcoming tamoxifen (TAM) resistance in breast cancer. Twenty women and 1 man received recombinant alpha interferon (5 million units per meter squared intramuscularly, 5 times per week) plus TAM (10 mg orally, twice daily) for the treatment of metastatic breast cancer, either after failing tamoxifen therapy or as frontline hormonal therapy. Of the 9 evaluable patients with disease progression after an objective response to TAM, there were no partial or complete responses with the addition of IFN. Ten evaluable patients received TAM plus IFN as frontline hormonal therapy with 2 complete and 3 partial responses for an overall response rate (RR) of 50% (95% confidence interval = 19-81), a 71% RR for ER-positive patients (95% confidence interval = 29-96) and no responses in ER-unknown patients. Sixteen patients required dose reductions of IFN and 8 patients discontinued therapy due to toxicity. It is unlikely that the RR for TAM plus IFN is greater that than seen with TAM alone, or that the addition of IFN to TAM therapy can overcome clinical TAM resistance.

Increased epidermal growth factor receptor gene expression by gamma-interferon in a human breast carcinoma cell line

The interferons are a group of naturally occurring proteins that inhibit the growth of tumours in vivo and many transformed cell lines in vitro. The mechanisms of action of interferon, however, remain unclear. The IFN induced inhibition of growth of many epithelial cancer cell lines is associated with changes in Epidermal Growth Factor Receptor (EGFR) binding or expression. Therefore, we examined the effect of IFN treatment on the expression of EGFR in a human breast carcinoma cell line, MDA 468. We have found the IFN-gamma inhibited, in a dose dependent fashion, the growth of MDA 468 cells. IFN decreased cell surface binding of 125I-EGF to EGFR by changing receptor number rather than affinity. However, total cellular receptor protein, as measured by immunoprecipitation with monoclonal antibodies, was increased in IFN-treated cells. The half-life of the metabolically labelled receptor was unchanged by treatment with IFN. Increased amounts of EGFR mRNA were observed in MDA 468 cells treated with IFN-gamma for 3 days. The levels of mRNA increased with time in culture, reaching a peak of four times control values after 5 days of treatment. This effect was observable with as little as 10 U ml-1 of IFN-gamma. Treatment of the cells with Actinomycin D to inhibit new RNA synthesis suggested that the stability of EGFR mRNA was not enhanced in IFN-gamma treated cells. The increase in receptor mRNA induced by IFN was not inhibited by cycloheximide. These data suggest IFN-gamma can increase expression of EGFR mRNA and protein in MDA 468 cells. Increased expression of EGFR mRNA and protein by IFN-gamma is associated with inhibition of cell growth.

Interferon-induced modulation of epidermal growth factor-stimulated growth of a human breast tumor cell line

We have determined that interferon-alpha (IFN-alpha) and IFN-gamma inhibit the growth of a human breast tumor cell line, S4, in vitro. Cells were more sensitive to the antiproliferative effects of low-dose IFN-gamma than IFN-alpha. As the growth of the S4 cell line is enhanced by epidermal growth factor (EGF), we examined the effect of IFN on EGF-dependent growth of S4 cells. Cells plated in 2.5% serum alone failed to grow. EGF stimulated these cells to grow more than twofold. IFN substantially attenuated the EGF-stimulated growth of S4 cells. Binding of EGF to its receptor was unaffected by pretreatment of cells with IFN-alpha. However, a 24-h exposure of cells to IFN-gamma significantly increased the number of EGF receptors on S4 cells. Internalization of the EGF receptor was unaffected by IFN treatment. Binding remained elevated through 4 days of IFN-gamma exposure. Scatchard analysis of receptor binding data revealed that IFN-gamma increased the number of binding sites without changing the affinity of the receptor for its ligand. These results demonstrate that IFN inhibits EGF-stimulated growth of a breast tumor cell line and suggest that the antiproliferative effect of IFN may be due, in part, to its interaction with growth factor-initiated pathways.

Modulation by oestrogen and progestins/antiprogestins of alpha interferon receptor expression in human breast cancer cells

Human breast cancer ZR-75-1 cells expressed 1516 (105) (mean [S.D.]) interferon (IFN) receptors (IFNR) per cell with Kd of 0.61 (0.15) nmol/l. Oestrogen independent ZR-PR-LT and tamoxifen resistant ZR-75-9a1 8 mumol/l cells expressed similar numbers of IFNR. ZR-75-9a1 cells, which had been maintained in the absence of tamoxifen or known oestrogenic activity for 46 weeks, expressed a significantly higher number of IFNR (3170 [315]). Exposure of ZR-75-1 cells to 10(-9) mol/l 17 beta-oestradiol (E2) led to a consistent reduction in IFNR numbers whilst 10(-6) mol/l tamoxifen slightly increased IFNR expression. Since IFN increases oestrogen receptors in this cell line, IFN and E2 appear to have opposite effects on expression of each others' receptor. 10(-9) mol/l medroxy progesterone acetate and mifepristone significantly increased IFNR numbers whilst ORG 2058 decreased IFNR expression and ZK 98.299 had no effect. Progestin/antiprogestin induced IFNR increase in this cell line correlated with down-regulation of progesterone receptor (PR). Thus an IFN/ER/PR axis may exist in ZR-75-1 cells and variants.

Effect of recombinant human interferon-alpha 2b on receptors for steroid hormones and epidermal growth factor in patients with endometrial cancer

Interferons (IFNs) may modulate oestrogen (ER), progesterone (PR) and epidermal growth factor (EGFR) receptor expression in vitro. ER, PR and EGFR levels in tumour specimens taken from 13 patients with endometrial adenocarcinomas before and after 5 days' intramuscular treatment with 5 x 10(6) U per recombinant human leucocyte interferon-alpha 2b (rh IFN-alpha 2b). After treatment, ER (P less than 0.01) and PR (P less than 0.05) levels were significantly increased with a simultaneous reduction of EGFR content (P less than 0.05). Since the expression of ER and PR characterises more differentiated hormono-sensitive tumours, while EGFR are preferentially expressed in less differentiated tumours, the increase of steroid hormone receptor levels with the reduction of EGFR expression suggests that rh IFN-alpha 2b may induce endometrial cancer cell differentiation. Moreover, the decrease of EGFR levels may explain the antiproliferative effect of IFNs.

High progesterone receptor concentration in a variant of the ZR-75-1 human breast cancer cell line adapted to growth in oestrogen free conditions

Culture of ZR-75-1 human breast cancer cells for 5 days in the absence of oestrogens (phenol red-free medium supplemented with dextran coated charcoal stripped 5% fetal calf serum) resulted in a slowing of growth rate and loss of progesterone receptors. Oestradiol at 10(-9) M markedly stimulated growth and progesterone receptor synthesis over a 5-day period. While medroxyprogesterone acetate (10(-10) to 10(-6) M) inhibited growth of ZR-75-1 cells growing in complete medium, in the short-term absence of oestrogens low concentrations were growth stimulatory. Cells deprived of oestrogens for 5 days retained sensitivity to growth inhibition by 4-hydroxy tamoxifen. ZR-75-1 cells were also adapted to growth in the absence of oestrogens over a 5-month period. These cells (ZR-PR-LT) failed to express binding sites characteristic of the type 1 oestrogen receptor but progesterone receptor expression was at a level normally associated with oestrogen induction. Adapted cells were growth inhibited by oestradiol, 4-hydroxy tamoxifen and medroxyprogesterone acetate, but despite elevated progesterone receptor expression the progestin was only marginally more inhibitory than in the parent line. Our data indicate a poor quantitative relationship between response to progestins in vitro and progesterone receptor concentration and support previous findings that acquisition of an oestrogen independent phenotype does not necessarily result in resistance to anti-oestrogens.