Metabolism of Immune Cells in the Tumor Microenvironment.

The tumor microenvironment (TME) is a complex biological structure surrounding tumor cells and includes blood vessels, immune cells, fibroblasts, adipocytes, and extracellular matrix (ECM) [1, 2]. These heterogeneous surrounding structures provide nutrients, metabolites, and signaling molecules to provide a cancer-friendly environment. The metabolic interplay between immune cells and cancer cells in the TME is a key feature not only for understanding tumor biology but also for discovering cancer cells' vulnerability. As cancer immunotherapy to treat cancer patients and the use of metabolomics technologies become more and more common [3], the importance of the interplay between cancer cells and immune cells in the TME is emerging with respect to not only cell-to-cell interactions but also metabolic pathways. This interaction between immune cells and cancer cells is a complex and dynamic process in which immune cells act as a determinant factor of cancer cells' fate and vice versa. In this chapter, we provide an overview of the metabolic interplay between immune cells and cancer cells and discuss the therapeutic opportunities as a result of this interplay in order to define targets for cancer treatment. It is important to understand and identify therapeutic targets that interrupt this cancerpromoting relationship between cancer cells and the surrounding immune cells, allowing for maximum efficacy of immune checkpoint inhibitors as well as other genetic and cellular therapies.

Targeting Metabolic Cross Talk Between Cancer Cells and Cancer-Associated Fibroblasts.

Although cancer has classically been regarded as a genetic disease of uncontrolled cell growth, the importance of the tumor microenvironment (TME) [1, 2] is continuously emphasized by the accumulating evidence that cancer growth is not simply dependent on the cancer cells themselves [3, 4] but also dependent on angiogenesis [5-8], inflammation [9, 10], and the supporting roles of cancer-associated fibroblasts (CAFs) [11-13]. After the discovery that CAFs are able to remodel the tumor matrix within the TME and provide the nutrients and chemicals to promote cancer cell growth [14], many studies have aimed to uncover the cross talk between cancer cells and CAFs. Moreover, a new paradigm in cancer metabolism shows how cancer cells act like "metabolic parasites" to take up the high-energy metabolites, such as lactate, ketone bodies, free fatty acids, and glutamine from supporting cells, including CAFs and cancer-associated adipocytes (CAAs) [15, 16]. This chapter provides an overview of the metabolic coupling between CAFs and cancer cells to further define the therapeutic options to disrupt the CAF-cancer cell interactions.

Uncovering metabolic reservoir cycles in MYC-transformed lymphoma B cells using stable isotope resolved metabolomics.

The use of metabolomic technologies and stable isotope labeling recently enabled us to discover an unexpected role of N-acetyl-aspartyl-glutamate (NAAG): NAAG is a glutamate reservoir for cancer cells. In the current study, we first found that glucose carbon contributes to the formation of NAAG and its precursors via glycolysis, demonstrating the existence of a glucose-NAAG-glutamate cycle in cancer cells. Second, we found that glucose carbon and, unexpectedly, glutamine carbon contribute to the formation of lactate via glutaminolysis. Importantly, lactate carbon can be incorporated into glucose via gluconeogenesis, demonstrating the existence of a glutamine-lactate-glucose cycle. While a glucose-lactate-glucose cycle was expected, the finding of a glutamine-lactate-glucose cycle was unforeseen. And third, we discovered that glutamine carbon is incorporated into γ-aminobutyric acid (GABA), revealing a glutamate-GABA-succinate cycle. Thus, NAAG, lactate, and GABA can play important roles as storage molecules for glutamate, glucose, and succinate carbon in oncogenic MYC-transformed P493 lymphoma B cells (MYC-ON cells) but not in non-oncogenic MYC-OFF cells. Altogether, examining the isotopic labeling patterns of metabolites derived from labeled 13C6-glucose or 13C515N2-glutamine helped reveal the presence of what we have named "metabolic reservoir cycles" in oncogenic cells.

Upregulation of the Glutaminase II Pathway Contributes to Glutamate Production upon Glutaminase 1 Inhibition in Pancreatic Cancer.

The targeting of glutamine metabolism specifically via pharmacological inhibition of glutaminase 1 (GLS1) has been translated into clinical trials as a novel therapy for several cancers. The results, though encouraging, show room for improvement in terms of tumor reduction. In this study, the glutaminase II pathway is found to be upregulated for glutamate production upon GLS1 inhibition in pancreatic tumors. Moreover, genetic suppression of glutamine transaminase K (GTK), a key enzyme of the glutaminase II pathway, leads to the complete inhibition of pancreatic tumorigenesis in vivo unveiling GTK as a new metabolic target for cancer therapy. These results suggest that current trials using GLS1 inhibition as a therapeutic approach targeting glutamine metabolism in cancer should take into account the upregulation of other metabolic pathways that can lead to glutamate production; one such pathway is the glutaminase II pathway via GTK.

Uncovering the Role of N-Acetyl-Aspartyl-Glutamate as a Glutamate Reservoir in Cancer.

N-acetyl-aspartyl-glutamate (NAAG) is a peptide-based neurotransmitter that has been extensively studied in many neurological diseases. In this study, we show a specific role of NAAG in cancer. We found that NAAG is more abundant in higher grade cancers and is a source of glutamate in cancers expressing glutamate carboxypeptidase II (GCPII), the enzyme that hydrolyzes NAAG to glutamate and N-acetyl-aspartate (NAA). Knocking down GCPII expression through genetic alteration or pharmacological inhibition of GCPII results in a reduction of both glutamate concentrations and cancer growth. Moreover, targeting GCPII in combination with glutaminase inhibition accentuates these effects. These findings suggest that NAAG serves as an important reservoir to provide glutamate to cancer cells through GCPII when glutamate production from other sources is limited. Thus, GCPII is a viable target for cancer therapy, either alone or in combination with glutaminase inhibition.

Targeting Metabolic Cross Talk between Cancer Cells and Cancer-Associated Fibroblasts.

KEY POINTS: Although tumorigenesis has classically been regarded as a genetic disease of uncontrolled cell growth, the importance of the tumor microenvironment (TME) is continuously emphasized by the accumulating evidence that cancer growth is not simply dependent on the cancer cells themselves [1, 2] but also dependent on angiogenesis [3­6], inflammation [7, 8], and the supporting roles of cancer-associated fibroblasts (CAFs) [9, 10]. After the discovery that CAFs are able to remodel the tumor matrix within the TME and provide the nutrients and chemicals to promote cancer cell growth [11], many studies have aimed to uncover the cross talk between cancer and CAFs. Moreover, a new paradigm in cancer metabolism shows how cancer cells act like "metabolic parasites" to uptake the high-energy metabolites, such as lactate, ketone bodies, free fatty acid, and glutamine from supporting cells, including CAFs and cancer-associated adipocytes (CAAs) [12, 13]. This chapter provides an overview of the metabolic coupling between CAFs and cancer to further define the therapeutic options to disrupt the CAF-cancer cell interactions.