Correction to: Buthionine sulfoximine sensitizes antihormone-resistant human breast cancer cells to estrogen-induced apoptosis.

After the publication of this work [1] an error was noticed in Fig. 3a and Fig. 5a.

The evolution of nonsteroidal antiestrogens to become selective estrogen receptor modulators.

The discovery of the first nonsteroidal antiestrogen ethamoxytriphetol (MER25) in 1958, opened the door to a wide range of clinical applications. However, the finding that ethamoxytriphetol was a "morning after" pill in laboratory animals, energized the pharmaceutical industry to discover more potent derivatives. In the wake of the enormous impact of the introduction of the oral contraceptive worldwide, contraceptive research was a central focus in the early 1960's. Numerous compounds were discovered e.g., clomiphene, nafoxidine, and tamoxifen, but the fact that clinical studies showed no contraceptive actions, but, in fact, induced ovulation, dampened enthusiasm for clinical development. Only clomiphene moved forward to pioneer an application to induce ovulation in subfertile women. The fact that all the compounds were antiestrogenic made an application in patients to treat estrogen responsive breast cancer, an obvious choice. However, toxicities and poor projected commercial returns severely retarded clinical development for two decades. In the 1970's a paradigm shift in the laboratory to advocate long term adjuvant tamoxifen treatment for early (non-metastatic) breast cancer changed medical care and dramatically increased survivorship. Tamoxifen pioneered that paradigm shift but it became the medicine of choice in a second paradigm shift for preventing breast cancer during the 1980's and 1990's. This was not surprising as it was the only medicine available and there was laboratory and clinical evidence for the eventual success of this application. Tamoxifen is the first medicine to be approved by the Food and Drug Administration (FDA) to reduce the risk of breast cancer in women at high risk. But it was the re-evaluation of the toxicology of tamoxifen in the 1980's and the finding that there was both carcinogenic potential and a significant, but small, risk of endometrial cancer in postmenopausal women that led to a third paradigm shift to identify applications for selective estrogen receptor (ER) modulation. This idea was to establish a new group of medicines now called selective ER modulators (SERMs). Today there are 5 SERMs FDA approved (one other in Europe) for applications ranging from the reduction of breast cancer risk and osteoporosis to the reduction of menopausal hot flashes and improvements in dyspareunia and vaginal lubrication. This article charts the origins of the current path for progress in women's health with SERMs.

Acquired resistance to selective estrogen receptor modulators (SERMs) in clinical practice (tamoxifen & raloxifene) by selection pressure in breast cancer cell populations.

Tamoxifen, a pioneering selective estrogen receptor modulator (SERM), has long been a therapeutic choice for all stages of estrogen receptor (ER)-positive breast cancer. The clinical application of long-term adjuvant antihormone therapy for the breast cancer has significantly improved breast cancer survival. However, acquired resistance to SERM remains a significant challenge in breast cancer treatment. The evolution of acquired resistance to SERMs treatment was primarily discovered using MCF-7 tumors transplanted in athymic mice to mimic years of adjuvant treatment in patients. Acquired resistance to tamoxifen is unique because the growth of resistant tumors is dependent on SERMs. It appears that acquired resistance to SERM is initially able to utilize either E2 or a SERM as the growth stimulus in the SERM-resistant breast tumors. Mechanistic studies reveal that SERMs continuously suppress nuclear ER-target genes even during resistance, whereas they function as agonists to activate multiple membrane-associated molecules to promote cell growth. Laboratory observations in vivo further show that three phases of acquired SERM-resistance exists, depending on the length of SERMs exposure. Tumors with Phase I resistance are stimulated by both SERMs and estrogen. Tumors with Phase II resistance are stimulated by SERMs, but are inhibited by estrogen due to apoptosis. The laboratory models suggest a new treatment strategy, in which limited-duration, low-dose estrogen can be used to purge Phase II-resistant breast cancer cells. This discovery provides an invaluable insight into the evolution of drug resistance to SERMs, and this knowledge is now being used to justify clinical trials of estrogen therapy following long-term antihormone therapy. All of these results suggest that cell populations that have acquired resistance are in constant evolution depending upon selection pressure. The limited availability of growth stimuli in any new environment enhances population plasticity in the trial and error search for survival.

Exploiting the apoptotic actions of oestrogen to reverse antihormonal drug resistance in oestrogen receptor positive breast cancer patients.

The ubiquitous application of selective oestrogen receptor modulators (SERMs) and aromatase inhibitors for the treatment and prevention of breast cancer has created a significant advance in patient care. However, the consequence of prolonged treatment with antihormonal therapy is the development of drug resistance. Nevertheless, the systematic description of models of drug resistance to SERMs and aromatase inhibitors has resulted in the discovery of a vulnerability in tumour homeostasis that can be exploited to improve patient care. Drug resistance to antihormones evolves, so that eventually the cells change to create novel signal transduction pathways for enhanced oestrogen (GPR30+OER) sensitivity, a reduction in progesterone receptor production and an increased metastatic potential. Most importantly, antihormone resistant breast cancer cells adapt with an ability to undergo apoptosis with low concentrations of oestrogen. The oestrogen destroys antihormone resistant cells and reactivates sensitivity to prolonged antihormonal therapy. We have initiated a major collaborative program of genomics and proteomics to use our laboratory models to map the mechanism of subcellular survival and apoptosis in breast cancer. The laboratory program is integrated with a clinical program that seeks to determine the minimum dose of oestrogen necessary to create objective responses in patients who have succeeded and failed two consecutive antihormonal therapies. Once our program is complete, the new knowledge will be available to translate to clinical care for the long-term maintenance of patients on antihormone therapy.

Activated estrogens and antiestrogens: a 30-year journey with David Kupfer.

David Kupfer had a passion for drug metabolism and used his talents to understand the putative metabolic activation of the insecticides o, p'DDT and methoxychlor to estrogens. His research helped to create a scientific foundation for the current interest in endocrine disruption. With the increasing clinical significance of tamoxifen in the late 1980s, and the proposal to test tamoxifen as a breast cancer chemopreventive in healthy women, David initiated laboratory studies on the mechanisms of tamoxifen metabolism. He was the first to note that tamoxifen is metabolically activated to alkylating species. Tamoxifen and insecticides covalently bind to microsomal proteins. His contribution presaged worldwide studies of the induction of rat liver carcinogenesis by tamoxifen.

Stable transfection of an estrogen receptor beta cDNA isoform into MDA-MB-231 breast cancer cells.

We previously reported stable transfection of estrogen receptor alpha (ERalpha) into the ER-negative MDA-MB-231 cells (S30) as a tool to examine the mechanism of action of estrogen and antiestrogens [J. Natl. Cancer Inst. 84 (1992) 580]. To examine the mechanism of ERbeta action directly, we have similarly created ERbeta stable transfectants in MDA-MB-231 cells. MDA-MB-231 cells were stably transfected with ERbeta cDNA and clones were screened by estrogen response element (ERE)-luciferase assay and ERbeta mRNA expression was quantified by real-time RT-PCR. Three stable MDA-MB-231/ERbeta clones were compared with S30 cells with respect to their growth properties, ability to activate ERE- and activating protein-1 (AP-1) luciferase reporter constructs, and the ability to activate the endogenous ER-regulated transforming growth factor alpha (TGFalpha) gene. ERbeta6 and ERbeta27 clones express 300-400-fold and the ERbeta41 clone express 1600-fold higher ERbeta mRNA levels compared with untransfected MDA-MB-231 cells. Unlike S30 cells, 17beta-estradiol (E2) does not inhibit ERbeta41 cell growth. ERE-luciferase activity is induced six-fold by E2 whereas neither 4-hydroxytamoxifen (4-OHT) nor ICI 182, 780 activated an AP-1-luciferase reporter. TGFalpha mRNA is induced in response to E2, but not in response to 4-OHT. MDA-MB-231/ERbeta clones exhibit distinct characteristics from S30 cells including growth properties and the ability to induce TGFalpha gene expression. Furthermore, ERbeta, at least in the context of the MDA-MB-231 cellular milieu, does not enhance AP-1 activity in the presence of antiestrogens. In summary, the availability of both ERalpha and ERbeta stable breast cancer cell lines now allows us to compare and contrast the long-term consequences of individual signal transduction pathways.

Tamoxifen, raloxifene and the prevention of breast cancer.

The recognition of a new group of drugs, now named selective estrogen receptor modulators (SERMs) has revolutionized prospects for the prevention of breast cancer. New agents will continue to be tested against tamoxifen, the first SERM and an established treatment of ER positive breast cancer. Raloxifene a related SERM is used to treat and prevent osteoporosis with the potential beneficial side effect of preventing breast cancer. The Study of Tamoxifen and Raloxifene (STAR) trial will establish whether raloxifene is an improvement over tamoxifen. Most importantly, emerging information about the molecular pharmacology of SERMs will be used to decipher the mechanism of action at specific target sites around a woman's body. This knowledge can be used to design new SERMs and advance the prospects for multifunctional medicine to prevent breast cancer, osteoporosis and coronary heart disease.