br Introduction Breast cancer is the most frequently

Introduction Breast cancer is the most frequently occurring cancer in women, and despite the development of new therapies there has been little decline in the mortality rate over the past decade (Siegel et al., 2015). This is partly due to the genetic diversity of breast cancers, such that there is no single therapy that is effective against all breast cancer subtypes. Breast cancers are commonly grouped by their hormone receptor status or specific mutations, in genes such as BRCA1/2 and PIK3CA, which can be used as biomarkers to predict the most efficacious targeted therapy for a breast cancer case (McCubrey et al., 2015, 2014). However, many breast cancers do not contain a single, targetable driver mutation, and targeted therapies are unlikely to eliminate all breast cancer cells due to tumor heterogeneity. Recently, new interest in studying the relationship between cancer and metabolism has arisen as a potential avenue to identify novel biomarkers and therapeutic targets effective across multiple breast cancer subtypes. Cancer metabolism is not a new field; Otto Warburg's pioneering research first described the increased levels of aerobic glycolysis followed by lactate fermentation for energy production in cancer cells, since termed the “Warburg Effect” (Warburg, 1956). Since this discovery, glycolysis has been heavily explored in the context of cancer, resulting in the development of drugs that target glycolysis and oxidative phosphorylation, some of which are in clinical trials (Granchi et al., 2014; Kim, 2015; Ngo et al., 2015; Talekar et al., 2014). Other metabolic pathways have also been implicated in cancer, particularly folate metabolism, evidenced by the use of the antifolates methotrexate and pemetrexed as a frontline chemotherapies in multiple cancers, including breast cancer (Amelio et al., 2014). A potentially more efficacious strategy is metabolic starvation therapy, by removing or limiting the availability of a specific metabolite, as this has the potential to be less toxic to patients than chemotherapy or radiation (Changou et al., 2014). Nonessential Elacridar are especially promising metabolites for starvation therapy. They can be synthesized by normal cells and thus an extracellular source is not required for their health, while many tumor cells require an external supply of nonessential amino acids (Morris, 2009; Souba, 1993). This difference can be utilized to selectively inhibit tumor cell growth by the removal of specific amino acids. Amino acids may also serve as useful biomarkers for diagnosis as well as screening during treatment, since they can be easily measured in blood, saliva, and urine (Budczies et al., 2013; Kim et al., 2015). Recently, amino acid profiling of saliva from breast cancer patients identified fifteen amino acids with significantly changed levels between breast cancer patients and healthy controls, and which could serve as biomarkers for early detection and breast cancer diagnosis (Cheng et al., 2015).
Glutamine and glutamate metabolism Of all the amino acids, glutamine is the most consumed by cancer cells (Jain et al., 2012). Glutamine is used for nucleotide and lipid biosynthesis, and also to synthesize glutamate, which can then be converted to alpha-ketoglutarate and feed the tricarboxylic acid (TCA) cycle (Fig. 1) (Budczies et al., 2013; Jeon et al., 2015). Glutaminolysis, the conversion of glutamine to glutamate for the production of energy via lactate, is increased in cancer cells (Erickson and Cerione, 2010). Specifically, glutamine can drive oxidative phosphorylation in cells transformed by Ras and Akt, mutations that occur in some breast cancers (Fan et al., 2013). Increased glutamate levels in breast tumor versus normal breast tissue samples have been found to correlate with estrogen receptor (ER) status and endocrine resistance, indicating a change in regulation at the first step of glutaminolysis (Budczies et al., 2015, 2013; Cao et al., 2014; Shajahan-Haq et al., 2014). The observed patterns suggest that ER-negative breast cancer patients may benefit most from glutaminase (GLS) inhibition (Budczies et al., 2013). Among breast cancer subtypes, HER2-amplified breast cancers have the highest levels of glutamine metabolism, with increased GLS and glutamate dehydrogenase (GLUD1) expression (Cao et al., 2014; Kim et al., 2013), and triple-negative breast cancer (TNBC) exhibited increased expression of GLS and GLUL (van Geldermalsen et al., 2015), indicating that this molecular subtype of breast cancer may respond well to glutamine metabolic therapy. An increase in glutamine and glutamate also correlates with the expression of epithelial–mesenchymal transition (EMT)-associated transcription factors (Bhowmik et al., 2015), and thus has potential not only as a biomarker for choosing a metabolic therapy, but also for monitoring cancer progression and prognosis.