Glutamate enrichment as new diagnostic opportunity in breast cancer

Exogenous glutamine is an important source of energy and molecular building blocks for many tumors. There is a renewed interest in therapeutically targeting glutamine metabolism due to the recent discovery of two novel glutaminase inhibitors. To quantify the dysregulation of the glutamate-glutamine equilibrium in breast cancer, metabolomics analysis of 270 clinical breast cancer samples and 97 normal breast samples was carried out using gas chromatography combined with time-of-flight mass spectrometry. Positive correlation between glutamate and glutamine in normal breast tissues switched to negative correlation between glutamate and glutamine in breast cancer tissues. Compared with the ratio of glutamate to glutamine in normal tissues, we found 56% of the ER+ tumor tissues and 88% of the ER- tumor tissues glutamate-enriched. The glutamate-to-glutamine ratio (GGR) significantly correlated with ER status (p=8.0E-09) and with tumor grade (p=3.3E-05). Higher levels of GGR were associated with prolonged overall survival in univariate analysis (HR=0.77, p=0.027) and in multivariate analysis (HR=0.73, p=0.038). GGR levels were reflected in an unsupervised clustering of metabolomics profiles. In a supervised analysis of metabolomics data and of genome-wide expression data, replacement of GGR by metabolite surrogate markers was feasible, while replacement of GGR by RNA markers had a limited accuracy. Functional analysis of the gene expression data showed negative correlation between glutamate enrichment and activation of peroxisome proliferator-activated receptor (PPAR) pathway. Our findings may have important implications for patient stratification related to utilization of glutaminase inhibitors. What's New? In cancer cells, glutamine isn't processed the same way it is in normal cells. These cells often overexpress an enzyme, glutaminase, that converts glutamine to glutamate, and one strategy for attacking the tumor cells is to inhibit glutaminase. Thus, the ratio of glutamate to glutamine can indicate something about whether a particular cancer would be vulnerable to treatment with glutaminase inhibitors. In this study, the authors show that breast cancer cells, particularly ER- tumor cells, do have a higher glutamate-to-glutamine ratio than normal cells, suggesting that glutaminase inhibitors would be well worth developing as a potential therapy.