Hypercholesterolemia is a risk factor for estrogen receptor (ER) positive breast cancers and is associated with a decreased response of tumors to endocrine therapies. Here we show that 27-Hydroxycholesterol (27HC), a primary metabolite of cholesterol and an ER and Liver X receptor (LXR) ligand, increases ER-dependent growth and LXR-dependent metastasis in mouse models of breast cancer. The effects of cholesterol on tumor pathology required its conversion to 27HC by the cytochrome P450 oxidase CYP27A1, and were attenuated by treatment with CYP27A1 inhibitors. In human breast cancer specimens, CYP27A1 expression levels correlated with tumor grade. In high-grade tumors, both tumor cells and tumor-associated macrophages exhibited high expression levels of the enzyme. Thus, lowering circulating cholesterol levels or interfering with its conversion to 27HC may be a useful strategy to prevent and/or treat breast cancer.

Cancer cells can activate diverse signaling pathways to evade the cytotoxic action of drugs. We created and screened a library of barcoded pathway-activating mutant cDNAs to identify those that enhanced the survival of cancer cells in the presence of 13 clinically relevant, targeted therapies. We found that activation of the RAS– MAPK (mitogen-activated protein kinase), Notch1, PI3K (phosphoinositide 3-kinase)–mTOR (mechanistic target of rapamycin), and ER (estrogen receptor) signaling pathways often conferred resistance to this selection of drugs. Activation of the Notch1 pathway promoted acquired resistance to tamoxifen (an ER-targeted therapy) in serially-passaged breast cancer xenografts in mice, and treating mice with a γ-secretase inhibitor to inhibit Notch signaling restored tamoxifen sensitivity. Markers of Notch1 activity in tumor tissue correlated with resistance to tamoxifen in breast cancer patients. Similarly, activation of Notch1 signaling promoted acquired resistance to MAPK inhibitors in BRAFV600E melanoma cells in culture, and the abundance of Notch1 pathway markers were increased in tumors from a subset of melanoma patients. Thus, Notch1 signaling may be a therapeutic target in some drug-resistant breast cancers and melanomas. Additionally, multiple resistance pathways were activated in melanoma cell lines with intrinsic resistance to MAPK inhibitors, and simultaneous inhibition of these pathways synergistically induced drug sensitivity. These data illustrate the potential for systematic identification of the signaling pathways controlling drug resistance that could inform clinical strategies and drug development for multiple types of cancer. This approach may also be used to advance clinical options in other disease contexts.

Summary A genomic signature designed to assess the activity of the estrogen-related receptor alpha (ERRα) was used to profile more than eight hundred breast tumors, revealing a shorter disease-free survival in patients with tumors exhibiting elevated receptor activity. Importantly, this signature also predicted the ability of an ERRα antagonist, XCT790, to inhibit proliferation in cellular models of breast cancer. Using a chemical genomic approach, it was determined that activation of the Her2/IGF-1 signaling pathways and subsequent C-MYC stabilization upregulate the expression of peroxisome proliferator-activated receptor gamma coactivator-1 beta (PGC-1β), an obligate cofactor for ERRα activity. PGC-1β knockdown in breast cancer cells impaired ERRα signaling and reduced cell proliferation, implicating a functional role for PGC1β/ERRα in the pathogenesis of breast cancers. Significance Overexpression of ERRα has been correlated with progression of breast and ovarian cancers in several small studies. Using a genomic approach, we defined specific aspects of the activity of this receptor that track with shorter disease-free survival in multiple cohorts of breast cancer patients. Importantly, cellular models of breast cancer exhibiting high ERRα activity are more sensitive to growth inhibition by an ERRα antagonist. This finding highlights a promising treatment strategy for those aggressive tumors that currently have limited therapeutic options.

Summary Imaging studies in animals and in humans have indicated that the oxygenation and nutritional status of solid tumors is dynamic. Further, the extremely low level of glucose within tumors, while reflecting its rapid uptake and metabolism, also suggests that cancer cells must rely on other energy sources in some circumstances. Here we find that some breast cancer cells can switch to utilizing lactate as a primary source of energy, allowing them to survive glucose deprivation for extended periods, and that this activity confers resistance to PI3K/mTOR inhibitors. The nuclear receptor, estrogen-related receptor alpha (ERRα), was shown to regulate the expression of genes required for lactate utilization and isotopomer analysis revealed that genetic or pharmacological inhibition of ERRα activity compromised lactate oxidation. Importantly, ERRα antagonists increased the in vitro and in vivo efficacy of PI3K/mTOR inhibitors, highlighting the potential clinical utility of this drug combination.

Lysine specific demethylase 1 (LSD1, also known as KDM1) is a histone modifying enzyme that regulates the expression of many genes important in cancer progression and proliferation. It is present in various transcriptional complexes including those containing the estrogen receptor (ER). Indeed, inhibition of LSD1 activity and or expression has been shown to attenuate estrogen signaling in breast cancer cells in vitro implicating this protein in the pathogenesis of cancer. Herein we describe experiments that utilize small molecule inhibitors, phenylcyclopropylamines, along with small interfering RNA to probe the role of LSD1 in breast cancer proliferation and in estrogen-dependent gene transcription. Surprisingly, whereas we have confirmed that inhibition of LSD1 strongly inhibits proliferation of breast cancer cells, we have determined that the cytostatic actions of LSD1 inhibition are not impacted by ER-status. These data suggest that LSD1 may be a useful therapeutic target in several types of breast cancer; most notably inhibitors of LSD1 may have utility in the treatment of ER-negative cancers for which there are minimal therapeutic options.