Cross-resistance of tamoxifen and raloxifene on the growth of raloxifene-resistant breast cancer cells in vivo and in vitro

Raloxifene (Ral) is a selective estrogen receptor modulator and is widely used for the prevention of osteoporosis in postmenopausal women. Since it inhibits breast cancer growth in cell culture, and unlike tamoxifen (TAM), has virtually no stimulatory effect on the uterus in mice, Ral is currently being compared with TAM for the prevention of breast cancer in high risk women in the STAR trial. The broad use of Ral will result in the development of Ral-stimulated tumors. We developed a new model and addressed two possible hypotheses for Ral-stimulated growth 1) an increase expression in ERβ; 2) an increase in ER coactivator proteins. Methods: MCF-7/Ral were developed in culture by exposure to 1 μM Ral for > 12 months. ERβ expression was determined by real-time PCR. GRIP 1, AIB1, p300 and ERa were measured by western blot analysis. MCF-7/Ral cells (10⁷) were inoculated into athymic mice and treated orally with TAM or Ral (1.5 mg daily) to determine tumor growth. Results: MCF-7/ Ral cells expressed ERa at a level similar to the parental MCF-7 cells. They still responded to E, treatment in transient transfection assay using a Vit-ERE3 luciferase reporter. Paradoxically, Ral and 4-hydroxytamoxifen (4-OHT) had no agonist activities and inhibited E₂-induced luciferase activity. However, neither Ral nor 4-OHT inhibited E,-induced MCF-7/Ral cell growth in culture. Instead, Ral and 4-OHT stimulated cell growth. The levels of ERβ and coactivator GRIP I, AIB1 and p300 were not significantly different from the parental MCF-7 cells. Ral and TAM also stimulated MCF-7/Ral growth in vivo. Conclusion: TAM is cross-resistant to MCF-7/Ral in vitro and in vivo. We found no evidence to support the view that an increase in ERβ or coactivators caused Ral-stimulated growth. Our results may have important clinical relevance since TAM may not be a suitable treatment for women following exposure to raloxifene.