Douglas M. Wolf

The estrogen receptor from a tamoxifen stimulated MCF-7 tumor variant contains a point mutation in the ligand binding domain.

The nonsteroidal antiestrogen tamoxifen (TAM) is the most commonly used endocrine treatment for all stages of breast cancer in both pre- and postmenopausal women. However, the development of resistance to the drug is common, as most patients treated with TAM eventually experience a recurrence of tumor growth. One of the potential mechanisms of treatment failure is the acquisition by the tumor of the ability to respond to TAM as a stimulatory rather than inhibitory ligand. We (Gottardis and Jordan, Cancer Res 48: 5183-5187, 1988; Wolfet al., J Natl Cancer Inst 85: 806-812, 1993) and others (Osborneet al., Eur J Cancer Clin Oncol 23: 1189-1196, 1987; Osborneet al., J Natl Cancer Inst 83: 1477-1482, 1991) have extensively described the reproducible development of TAM stimulated growth in a laboratory model system using MCF-7 human breast cancer cells grown as solid tumors in athymic mice. In this paper we report on the isolation of an estrogen receptor (ER) from a TAM stimulated tumor (MCF-7/MT2) which contains a point mutation that causes a tyrosine for aspartate substitution at amino acid 351 in the ligand binding domain. The mutant appears to the major form of ER expressed by this tumor. We also report that only wild type ER was detected in three other TAM stimulated MCF-7 tumor variants, suggesting that multiple mechanisms are possible for the development of TAM stimulated growth. The implications of these findings are discussed.

An estrogen receptor positive MCF-7 clone that is resistant to antiestrogens and estradiol

The antiestrogen tamoxifen has been successfully used to control estrogen receptor (ER) and progesterone receptor positive breast cancer. However, the development of antiestrogen resistance is frequently observed in patients following long term treatment. We have studied the development of antiestrogen resistance in vitro and established an antiestrogen resistant variant of MCF-7 cells (clone 5C) after long term culture in estrogen free medium. The growth of clone 5C cells was not altered by either estradiol-17 beta or the antiestrogens 4-hydroxytamoxifen and ICI 164,384. Estrogen-stimulated progesterone receptor and reporter gene expression were markedly reduced in 5C cells compared to wild type MCF-7 cells. Only minor alteration in the levels of ER and no alteration in the affinity of ER for ligand were found in 5C cells. No mutation of ER cDNA in 5C cells was detected by polymerase chain reaction and DNA sequencing. This study demonstrates that change(s) in ER-mediated gene expression rather than the amino acid sequence of the ER itself may be associated with the development of at least one form of antiestrogen resistance.

Gynecologic complications associated with long-term adjuvant tamoxifen therapy for breast cancer

The antiestrogen tamoxifen was originally introduced as a therapy for advanced breast cancer. Today, tamoxifen is used to treat selected patients with all stages of breast cancer, and trials are underway to evaluate its effectiveness as a potential breast cancer preventive. When tamoxifen is used as an adjuvant or preventive, extended patient survival times can be expected, and concerns about iatrogenic complications arising from long-term treatment become important. This review discusses currently available laboratory and clinical data regarding the toxicology of tamoxifen and focuses in particular on the gynecologic complications potentially associated with long-term tamoxifen administration.

Characterization of tamoxifen stimulated MCF-7 tumor variants grown in athymic mice

The non-steroidal antiestrogen tamoxifen (TAM) is successfully used to treat all stages of breast cancer in both pre- and postmenopausal women. Unfortunately, most women treated with TAM eventually develop resistant tumor recurrences which require intervention with a second-line endocrine therapy, or cytotoxic chemotherapy if the recurrence is completely endocrine insensitive. There is evidence that some recurrences may in fact be TAM stimulated. MCF-7 human breast cancer cells grown as solid tumors in athymic mice chronically treated with TAM reproducibly develop a TAM stimulated phenotype (Osborneet al., Eur J Cancer Clin Oncol 23: 1189-1196, 1987; Gottardis and Jordan, Cancer Res 48: 5183-5187, 1988; Osborneet al., J Natl Cancer Inst 83: 1477-1482, 1991; Wolfet al., J Natl Cancer Inst 85: 806-812, 1993). Tumors of this type may provide a useful model for a subset of therapeutic failures in the clinic. Therefore, we have extensively studied this model in an attempt to define the mechanism or mechanisms leading to TAM stimulated growth. In this paper we describe the characteristics of 4 TAM stimulated MCF-7 tumor variants. All of these tumors are growth stimulated by TAM, but vary in their response to estradiol (E2) treatment, and grow poorly in placebo treated hosts. All tumor variants express estrogen receptor (ER) RNA and protein, which at the RNA level appear to be down regulated by TAM, and to a greater extent by E2. All tumors also express epidermal growth factor receptor (EGFR) RNA, which is down regulated by TAM, and further down regulated by E2. However, among the tumor variants analyzed, ER and EGFR levels appear to be inversely related. Further, despite the expression of ER by all 4 TAM stimulated tumor variants, E2 induction of progesterone receptor expression is very weak or entirely absent.

Drug resistance to tamoxifen during breast cancer therapy

SummaryBreast cancer is the most common malignancy occurring in Western women, and is one of the leading causes of cancer mortality. The nonsteroidal antiestrogen tamoxifen has been shown to be an effective treatment for pre and postmenopausal women with all stages of the disease. Tamoxifen provides effective palliation when used to treat patients with advanced disease, and adjuvant tamoxifen therapy produces significant increases in both disease-free and overall survival (Early Breast Cancer Trialists Collaborative Group. Lancet 339:1-15, 71-85, 1992). Data from the laboratory have shown that the primary action of tamoxifen is tumoristatic rather than tumoricidal, and long-term therapy is therefore recommended. Unfortunately, many patients experience disease progression while taking tamoxifen. Some tamoxifen resistant tumors may remain sensitive to alternative endocrine therapies, while others may become refractory to any hormonal manipulation. Many models have been developedin vitro andin vivo to study the progression of breast cancer growth from tamoxifen sensitive to tamoxifen resistant. We and others have used long-term estrogen deprivation and long-term tamoxifen exposure to develop cell lines and tumors capable of growth in the presence of clinically relevant tamoxifen concentrations. Recently our laboratory has also shown that mutations in the estrogen receptor can cause an antiestrogen-occupied receptor to behave as though it were occupied by an estrogen. Breast cancer is a highly heterogeneous disease and it is likely that the mechanisms which cause tamoxifen resistant growth are equally heterogeneous. Several of the models from our laboratory and others which may contribute to an understanding of this complex phenomenon are discussed here.

Therapeutic Efficacy, Side Effects and the Potential for Tamoxifen to Prevent Breast Cancer

Orginally intended for use as a fertility agent and possibly a contraceptive, tamoxifen entered clinical trials as a therapy for breast cancer in 1971, and thereafter rapidly became the treatment of choice for advanced estrogen-receptor-positive breast cancer. However, until the 1980s treatment was restricted to patients with advanced disease, so the number of women being treated with tamoxifen represented only a small fraction of the total breast cancer population which might potentially have benefited from this agent

Understanding the antiestrogenic actions of raloxifene and a mechanism of drug resistance to tamoxifen

ConclusionThe recent crystallization of the estradiol and raloxifene ER complex19 has provided fascinating clues to the mechanism of action of antiestrogens and confirmed, for the first time, a mechanism of drug resistance for tamoxifen. The new strategy to prevent breast cancer as a beneficial side effect of the prevention of osteoporosis7.11 is poised to revolutionize health care for postmenopausal women. The convergence of molecular biology and medicine has the potential to open up new therapeutic opportunities for the discovery of novel targeted antiestrogens to either the ERα or the newly discovered ERβ. There is currently intense interest in describing the distribution of ERα and ERβ throughout the female and male organ systems. As a direct result of this research new targeted drugs hold the promise for the specific control of diseases not only in women but also in men.