Abstract P6-04-22: Regulation of Notch localization by endocrine therapy in Estrogen Receptor positive breast cancer cells: Clinical implications for endocrine resistance

Abstract

Background: Estrogen receptors occur in about two-thirds of breast tumors and endocrine therapy is probably the most important systemic therapy for hormone receptor positive breast cancer. However, after an initial response to hormonal therapy, most tumors develop resistance leading to disease progression. Central mechanisms thought to be involved in this process include: 1) alterations in tumor cell physiology causing insensitivity to drug-induced apoptosis or induction of drug-detoxifying mechanisms; 2) expression of energy-dependent transporters that eject anti-cancer drugs from cells and 3) the emergence of cancer stem-like cell clones that are estrogen-independent and/or antiestrogen resistant. We and others have shown that Notch signaling mediates survival signals in ER+ breast cancer cells; induces expression of drug transporter ABCG2 (BCRP), activates ERα in the absence of estrogen and mediates survival of breast cancer stem-like cells. Previously, we demonstrated a crosstalk between ERα and Notch signaling whereby estrogen inhibits Notch activation and estrogen withdrawal or tamoxifen re-activate Notch, a possible mechanism of resistance. The mechanism of these effects has remained elusive. Methods: we used sub-cellular fractionation and immunofluorescence staining to investigate the total expression and cellular distribution of Notch signaling components in MCF-7 cells treated with 17-β-estradiol, or estrogen-free medium. Results: Our data shows that in cells treated with estrogen-free medium, Notch1 is located in the Lipid Rafts (LR) in the plasma membrane, along with Notch signaling components such as presenilin 1 (the catalytic subunit of γ-secretase), ADAM10 and Notch negative regulator NUMB. There is published evidence that γ-secretase is most active in lipid rafts.17-β-Estradiol modifies the membrane lipid composition and change the localization of Notch and Notch regulators (such as ADAM10, presenilin-1 and NUMB) at the plasma membrane, thereby altering the activation of Notch. These observations are consistent with a model in which the lipid composition of the plasma membrane is a critical factor in regulating the rate of Notch activation. FDA-approved cholesterol lowering drugs affect Notch signaling in ways consistent with this model. Conclusions: Our findings indicate that endocrine therapy affects Notch signaling at least in part through alterations in membrane composition and lipid raft localization of Notch signaling components. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P6-04-22.