A fundamental feature of cellular plasma membranes (PM) is the asymmetric distribution of lipids between the bilayer leaflets. This asymmetry is central to cellular physiology, regulating signaling, apoptosis, coagulation, and cell-cell fusion. While the broad transbilayer distributions of some lipid types are well established, the detailed lipidomic asymmetry of the PM has not been characterized and thus the compositions of the individual leaflets remain poorly understood. Further, how these compositional differences contribute to structural asymmetries between PM leaflets in living cells is not defined. Here, we report the distinct lipidomes and biophysical properties of both monolayers in living mammalian PMs. Using mass spectrometry coupled to enzymatic digestion, we report the detailed compositions of PM leaflets of human erythrocytes. We find a dramatic asymmetry in phospholipid unsaturation, with the cytoplasmic leaflet being ~2-fold more unsaturated than the exoplasmic. Atomistic simulations of lipid mixtures compiled from the lipidomic observations suggested significant asymmetries in lipid order, packing, and dynamics between the two PM leaflets. These were probed directly in the PM of living cells by Fluorescence Lifetime Imaging Microscopy (FLIM) of leaflet-selective, environment-sensitive probes. The outer PM leaflet is highly packed and ordered, resembling a liquid ordered phase, whereas the inner leaflet is significantly more disordered. This biophysical asymmetry is maintained in the endocytic system. These observations reveal the detailed compositional and biophysical asymmetry of mammalian plasma membranes, elucidating fundamental design principles of living membranes. SIGNIFICANCE STATEMENT:The asymmetric distribution of lipids between the two bilayer leaflets is a fundamental feature of mammalian plasma membranes. Here, we quantify the comprehensive lipidomes of the two PM leaflets in living mammalian cells and examine their consequences on leaflet physical properties. Using computer simulations, we predict distinct biophysical features for the two leaflets and confirm these with a variety of spectroscopic and microscopic techniques. We find that the outer leaflet of the membrane contains many more fully saturated lipids, which endows it with higher lipid packing and lower diffusivity.

Background: Effective treatment options for triple-negative inflammatory breast cancer (TN-IBC), the most aggressive form of breast cancer, are currently lacking. We previously reported that mediators of inflammation promote the growth of TN-IBC xenografts. Eicosapentaenoic acid (EPA), an omega-3 fatty acid (fish oil) with anti-inflammatory properties, is an emerging FDA-approved therapeutic with a favorable toxicology profile. Here we aimed to develop a novel approach to enhance EPA efficacy against TN-IBC by identifying a kinase inhibitor that synergizes with EPA's antitumor activity. Methods and Results: Using a high-throughput siRNA screen in the TN-IBC cell line SUM149PT, we identified inhibition of ephrin type-A receptor 2 (EPHA2), an oncogenic receptor tyrosine kinase, as a target that sensitizes TN-IBC cells to EPA therapy. To determine the clinical relevance of EPHA2, we investigated a meta-analysis of breast cancer mRNA expression data sets and found that high EPHA2 tumor expression, compared with low expressing, correlated significantly with poor overall survival in TN-IBC patients (P = 0.01), while not with other subtypes. Similar findings were observed in vitro, were EPHA2 protein and mRNA overexpression occurred predominantly in the TN subtypes among 49 and 51 breast cancer cell lines (63% and 47%, respectively), highlighting EPHA2 translational potential. Functional expression studies using proliferation and apoptosis assays in vitro, and xenografts in vivo, were performed in two EPHA2-expressing TN-IBC cell lines, SUM149PT and BCX010, to validate EPHA2 as a synergistic combinational target with EPA. EPHA2 gene silencing in combination with EPA significantly reduced cell growth, and enhanced apoptosis, compared with untreated and monotherapy in vitro (P < 0.05), and in vivo (P < 0.001). To translate our findings to the clinic, we validated dasatinib, an FDA-approved small molecule inhibitor of EPHA2, in combination to EPA to significantly enhance apoptosis of TN-IBC cells in vitro (P < 0.05) and in vivo (P < 0.05), compared with untreated and monotherapies. Using membrane fluidity assessment and cholesterol quantification we determined that apoptosis induction after combination therapy was due to increased membrane rigidity and cholesterol concentrations in the plasma membrane of TN-IBC cells (P < 0.05, compared with monotherapies). Finally, we discovered by western blot and gain/loss-of-expression studies that combination therapy inhibited the cholesterol efflux protein ATP-binding cassette sub-family A member 1 (ABCA1), which plays a significant role mediating increased cellular cholesterol (P < 0.05), cell membrane rigidity (P < 0.05), and induction of apoptosis (P < 0.05) in TN-IBC after EPA and EPHA2-targeting combination therapy. Conclusions: This is the first study demonstrating that EPA can enhance conventional targeted therapy against breast cancer. Our study provides molecular and preclinical evidence to support the development of an EPA/EPHA2-inhibition–based phase I clinical trial for patients with EPHA2-positive TN-IBC; our study further suggests the use of EPHA2 and ABCA1 protein expression as biomarkers for patient selection and therapeutic response. Citation Format: Torres-Adorno AM, Vitrac H, Qi Y, Tan L, Levental KR, Fan Y-Y, Yang P, Chapkin RS, Eckhardt BL, Ueno NT. EPHA2-targeting enhances eicosapentaenoic acid cytotoxicity against triple-negative inflammatory breast cancer via ABCA1 inhibition–mediated membrane rigidity [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P1-10-09.