Glutamine deprivation and glutaminase 1 inhibitory effect in colorectal cancer cells: a metabolomic study

Cancer cells reprogram their metabolism to fulfil the rising bioenergetic demand due to the high proliferation rate. Some nutrients become essential for cancer growth and development and their lack determines an arrest of cell proliferation or a metabolic adaptation. Glutamine is considered a “conditionally essential” amino acid and plays a pivotal role in cancer cell metabolism. Indeed, glutamine addiction is an emergent hallmark of several types of cancer, including colorectal cancer. Identifying tumours sensitive to glutamine deprivation represents a promising approach movable to the clinic to improve targeted therapeutic strategies and personalised medicine. In this regard, metabolomics offers the opportunity to ameliorate the knowledge of pathophysiological aspects in cancer metabolism and to identify potential biomarkers. This study aimed to investigate glutamine addiction in colorectal cancer and to clarify the role of glutamine in cancer development by using an in vitro model consisting of four different cell lines: Caco-2, HCT116, HT29 and SW480. We investigated the effect of glutamine starvation or the pharmacological inhibition of its metabolism with CB839, a non-competitive allosteric inhibitor of glutaminase 1 (GLS1). The ultimate goal is to recognize and exploit tumoural metabolic dependencies to identify new therapeutic targets and to improve cancer therapy. Our experiments demonstrated that glutamine deprivation reduces the growth rate and proliferative capacity of all studied cancer cell lines. The absence of glutamine caused alterations in the levels of glutathione, one of the most important intracellular antioxidants, and its oxidized form, highlighting modifications of the redox homeostasis. Furthermore, glutamine-deprived cells exhibited greater glucose uptake and higher expression of the GLUT1 transporter probably to exploit the glucose carbons for biosynthetic purposes or to replenish the Pentose Phosphate Pathway and restore the redox balance. Glutamine starvation induced profound changes at the metabolic level: energetic processes, such as glycolysis and Krebs cycle, and amino acid pathways were deeply affected by the starvation. Additionally, glutamine deprivation sensitized cancer cells to chemotherapeutic drugs, such as camptothecin, while it is not a winning strategy with the antimetabolite 5-FU. Recently, studies on the glutamine dependency of cell lines in vitro motivated the testing of glutamine metabolism inhibitors as cancer therapeutics. CB839, a non competitive glutaminase 1 (GLS1) inhibitor, exerted antiproliferative effects and limited cell survival and proliferation of the colorectal tumour cell lines. Both glutamine deprivation and enzymatic inhibition of its metabolism similarly affected the energetic and amino acid pathways (alanine, aspartate and glutamate metabolism; glycine, serine and threonine metabolism; phenylalanine, tyrosine, and tryptophan metabolism; glutathione metabolism and glyoxylate and dicarboxylate metabolism). This work represents an intriguingly starting point to clarify the role of glutamine in colorectal cancer metabolism and to identify potential strategies useful to improve targeted therapies.