Estradiol hypersensitivity and mitogen-activated protein kinase expression in long-term estrogen deprived human breast cancer cells in vivo

Women with breast cancer who have responded to initial hormonal therapy frequently experience additional remissions upon further endocrine manipulation. We postulate that hypersensitivity to estradiol (E-2) may serve as a mechanistic explanation for these secondary responses. We previously provided evidence of hypersensitivity using an in vitro breast cancer model system and demonstrated the role of mitogen-activated protein kinase (MAP kinase) in the process of adaptation to long-term estradiol deprivation. In the present study, we wished to demonstrate that hypersensitivity to E-2 could occur under more complex in vivo conditions and that MAP kinase activation is enhanced under these circumstances. We used an MCF-7 breast cancer model system involving long-term estradiol deprived (LTED) cells to produce xenografts in nude mice and an E-2 clamp method to precisely control sex steroid levels. The E-2 clamp was designed to maintain plasma E-2 at a series of doubling doses from 1.25 pg/ml to 20.0 pg/ml in oophorectomized nude mice. As evidence of the validity of the clamp method, a uterine weight bioassay revealed an excellent, linear dose-response relationship between the predicted level of plasma E-2 and stimulation of uterine weight. As evidence of hypersensitivity, we found that LTED xenograft tumors grew to a greater extent than wild-type in response to E, concentrations of 1.25pg/ml (P = 0.003) and 2.5 pg/ml (P = 0.0002). At the 10.0 and 20.0 pg/ml plasma concentrations, the LTED tumors were stimulated to a lesser extent than the wild-type. This pattern of increased growth at lower concentrations and reduced growth vs, the wild-type at higher concentrations mimics closely the pattern seen for LTED cells in vitro. All LTED cell tumors exhibited enhanced activation of MAP kinase ranging from 18 to 25%, and E, did not increase this further. In contrast, E-2 caused a linear increase in the percentage of activated MAP kinase positive cells (P < 0.0001) in wild-type tumors from basal levels of 2.66% to maximal levels of 6.40%. These observations suggest a dynamic interplay whereby activation of MAP kinase renders cells more sensitive to the proliferative effects of E-2. The precise mechanisms for this interplay are unknown but, when further understood, could potentially provide insight into approaches to prevent the evolution of tumors to a hormone insensitive state.