Abstract A36: Protein expression analysis of intratumor heterogeneity in a luminal-like breast cancer xenograft
Authors: Skrbo Nirma , Kirik Ufuk , Kristian Alexandr , Cifani Paolo , Antberg Linn , Moestue Siver A. , Engebraaten Olav , Mælandsmo Gunhild M. , Andersen Kristin , James Peter , Sørlie Therese 
Published: May 2016

Estrogen receptor is a key driver in breast cancer and is expressed in about 75% of breast tumors. ER positive tumors are susceptible to endocrine therapies; however, the major obstacle for curative treatment is recurrence due to resistance to anti-estrogens. Endocrine therapies may induce a selective pressure promoting growth of estrogen independent cell subclones. Our aim was to reveal molecular changes occurring in tumors in response to anti-estrogen treatment, and to identify subpopulations of cells able to withstand anti-estrogen treatment.

A luminal-like estrogen-dependent orthotopically growing xenograft model was treated with fulvestrant, or exposed to estrogen deprivation. The effect of ER-signaling inhibition was analyzed using quantitative mass spectrometry (MS) -based proteomic analysis and high resolution magic angle spinning magnetic resonance spectroscopy (HR MAS MRS). Cell surface marker expression (CD24 and SSEA-4) was monitored by flow cytometry, allowing detailed comparison of protein expression between intratumor cell subpopulations.

We found that both modes of anti-estrogen therapy restrained tumor growth and induced expression of enzymes involved in TCA cycle, oxidative phosphorylation and fatty acid beta-oxidation. This was accompanied by changes in levels of specific metabolites indicative of a possible reprogramming of cell metabolism and utilization of oxidative phosphorylation in preference to aerobic glycolysis (decrease in Warburg effect). Furthermore, anti-estrogen treatment seemed to have selective effects on intratumor cell subpopulations, specified by expression of the markers CD24 and SSEA-4. More specifically, highly tumorigenic CD24low/SSEA-4low (dbl. low) cells were eliminated and the seemingly more benign CD24high/SSEA-4high (dbl. high) cells were enriched in the residual tumor. When comparing the proteome in dbl. low verus dbl. high cells sorted from untreated tumors, metabolism was one of the most differentially enriched processes. Enzymes involved in glycolysis, TCA cycle, respiratory electron transport chain and fatty acid were more abundant in the dbl. high subpopulation.

These results suggest that cancer cells may reprogram their metabolism in response to anti-estrogen therapy to support a less estrogen-dependent phenotype. Moreover, subpopulations of cells with different metabolism may exist within the growing tumor, and these may respond differently to anti-estrogen treatment.

Citation Format: Nirma Skrbo, Ufuk Kirik, Alexandr Kristian, Paolo Cifani, Linn Antberg, Siver A. Moestue, Olav Engebraaten, Gunhild M. Mælandsmo, Kristin Andersen, Peter James, Therese Sørlie. Protein expression analysis of intratumor heterogeneity in a luminal-like breast cancer xenograft. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr A36.