The emergence of antiestrogen resistance in breast cancer is an important clinical phenomenon affecting long-term survival in this disease. Identifying factors that convey cell survival in this setting may guide improvements in treatment. Estrogen (E2) can induce apoptosis in breast cancer cells that have been selected for survival after E2 deprivation for long periods (MCF-7:5C cells), but the mechanisms underlying E2-induced stress in this setting have not been elucidated. Here, we report that the c-Src kinase functions as a key adapter protein for the estrogen receptor (ER, ESR1) in its activation of stress responses induced by E2 in MCF-7:5C cells. E2 elevated phosphorylation of c-Src which was blocked by 4-hydroxytamoxifen (4-OHT), suggesting that E2 activated c-Src through the ER. We found that E2 activated the sensors of the unfolded protein response (UPR), IRE1α (ERN1) and PERK kinase (EIF2AK3), the latter of which phosphorylates eukaryotic translation initiation factor-2α (eIF2α). E2 also dramatically increased reactive oxygen species (ROS) production and up-regulated expression of heme oxygenase HO-1 (HMOX1), an indicator of oxidative stress, along with the central energy sensor kinase AMPK (PRKAA2). Pharmacological or RNAi-mediated inhibition of c-Src abolished the phosphorylation of eIF2α and AMPK, blocked E2-induced ROS production, and inhibited E2-induced apoptosis. Together, our results establish that c-Src kinase mediates stresses generated by E2 in long-term E2-deprived cells that trigger apoptosis. This work offers a mechanistic rationale for a new approach in the treatment of endocrine-resistant breast cancer.

The prevalence and epidemiology of important viral (equine influenza virus [EIV], equine herpesvirus type 1 [EHV-1] and EHV-4) and bacterial (Streptococcus equi subspecies equi) respiratory pathogens shed by horses presented to equine veterinarians with upper respiratory tract signs and/or acute febrile neurological disease were studied. Veterinarians from throughout the USA were enrolled in a surveillance programme and were asked to collect blood and nasal secretions from equine cases with acute infectious upper respiratory tract disease and/or acute onset of neurological disease. A questionnaire was used to collect information pertaining to each case and its clinical signs. Samples were tested by real-time PCR for the presence of EHV-1, EHV-4, EIV and S equi subspecies equi. A total of 761 horses, mules and donkeys were enrolled in the surveillance programme over a 24-month study period. In total, 201 (26.4 per cent) index cases tested PCR-positive for one or more of the four pathogens. The highest detection rate was for EHV-4 (82 cases), followed by EIV (60 cases), S equi subspecies equi (49 cases) and EHV-1 (23 cases). There were 15 horses with double infections and one horse with a triple infection. The detection rate by PCR for the different pathogens varied with season and with the age, breed, sex and use of the animal.

Purpose Our publications demonstrate that physiological concentrations of estrogen (E2) induce endoplasmic reticulum and oxidative stress which finally result in apoptosis in E2-deprived breast cancer cells, MCF-7:5C. c-Src is involved in the process of E2-induced stress. To mimic the clinical administration of c-Src inhibitors, we treated cells with either E2, a c-Src inhibitor PP2, or the combination for 8 weeks to further explore the apoptotic potential of the c-Src inhibitor and E2 on MCF-7:5C cells. Methods Protein levels of receptors and signaling pathways were examined by immunoblotting. Expression of mRNA was detected through real-time PCR. Cell cycles were analyzed by flow cytometry. Results Long-term treatment with PP2 alone or E2 alone decreased cell growth. In contrast, a combination of PP2 and E2 blocked apoptosis and the resulting cell line (MCF-7:PF) was unique, as they grew vigorously in culture with physiological levels of E2, which could be blocked by the pure antiestrogen ICI182,780. One major change was that PP2 collaborated with E2 to increase the level of insulin-like growth factor-1 receptor beta (IGF-1Rβ). Blockade of IGF-1Rβ completely abolished E2-stimulated growth in MCF-7:PF cells. Furthermore, combination treatment up-regulated transcription factors, Twist1 and Snail, and repressed E-cadherin expression which made MCF-7:PF cells display a characteristic phenotype of epithelial-mesenchymal transition (EMT). Conclusions These data illustrate the role of the c-Src inhibitor to block E2-induced apoptosis and enhance E2-stimulated growth. Caution must be exercised when considering c-Src inhibitors in clinical trials following the development of acquired resistance to aromatase inhibitors, especially in the presence of the patient’s own estrogen.

Translational research for the treatment and prevention of breast cancer depends upon the four Ms: models, molecules, and mechanisms in order to create medicines. The process, to target the estrogen receptor (ER) in estrogen-dependent breast cancer, has yielded significant advances in patient survivorship and the first approved medicines (tamoxifen and raloxifene) to reduce the incidence of any cancer in high- or low-risk women. This review focuses on the critical role of the few ER-positive cell lines (MCF-7, T47D, BT474, ZR-75) that continue to advance our understanding of the estrogen-regulated biology of breast cancer. More importantly, the model cell lines have provided an opportunity to document the development and evolution of acquired antihormone resistance. The description of this evolutionary process that occurs in micrometastatic disease during up to a decade of adjuvant therapy would not be possible in the patient. The use of the MCF-7 breast cancer cell line in particular has been instrumental in discovering a vulnerability of ER-positive breast cancer exhaustively treated with antihormone therapy. Physiologic estradiol acts as an apoptotic trigger to cause tumor regression. These unanticipated findings in the laboratory have translated to clinical advances in our knowledge of the paradoxical role of estrogen in the life and death of breast cancer.

Estrogen (E2) exerts a dual function on E2-deprived breast cancer cells, with both initial proliferation and subsequent induction of stress responses to causes apoptosis. However, the mechanism by which E2 integrally regulates cell growth or apoptosis associated pathways remains to be elucidated. Here, E2 deprivation results in many alterations in stress-responsive pathways. For instance, E2-deprived breast cancer cells had higher basal levels of stress-activated protein kinase, c-Jun N-terminal kinase (JNK), compared with wild-type MCF-7 cells. E2 treatment further constitutively activated JNK after 24 hours. However, inhibition of JNK (SP600125) was unable to abolish E2-induced apoptosis, whereas SP600125 alone arrested cells at the G2-phase of the cell cycle and increased apoptosis. Further examination showed that inhibition of JNK increased gene expression of tumor necrosis alpha (TNFα) and did not effectively attenuate expression of apoptosis-related genes induced by E2. A notable finding was that E2 regulated both JNK and Akt as the downstream signals of insulin-like growth factor-1 receptor (IGF1R)/phosphoinositide 3-kinase (PI3K), but with distinctive modulation patterns: JNK was constitutively activated, whereas Akt and Akt-associated proteins, such as PTEN and mTOR, were selectively degraded. Endoplasmic reticulum-associated degradation (ERAD) was involved in the selective protein degradation. These findings highlight a novel IGF-1R/PI3K/JNK axis that plays a proliferative role during the prelude to E2-induced apoptosis and that the endoplasmic reticulum is a key regulatory site to decide cell fate after E2 treatment. IMPLICATIONS This study provides a new rationale for further exploration of E2-induced apoptosis to improve clinical benefit.