Immunometabolism in Cancer and Inflammation

Division Of Basic Sciences - Nci

Investigators

Linked publications, trials & patents

Abstract

The tumor microenvironment represents a complex multicellular milieu where various cell types compete for the resources necessary for their function(s). Tumor cells promote the expansion of host vasculature to maintain sufficient levels of the nutrients required for their rapid proliferation. At the same time, tumor cells subjugate host defense mechanisms to thwart rejection. Many mechanisms of tumor-mediated immunosuppression have been described, and several target metabolic pathways. For example, highly glycolytic tumors deplete glucose in the microenvironment, suppressing T cell proliferation and activation. The resulting elevated lactate from the tumor can transcriptionally reprogram tumor-associated macrophages into immunosuppressive cells promoting tumor growth and progression. Taking these types of interactions into consideration, we view the tumor as a unique metabolic environment, or niche, within which the tumor cells and immune cells compete for resources and adapt to each other's presence. The aggressive nature of many cancers reflects the ability of tumors to exert a dominant role in this niche, subjugating the attempted immune response to facilitate tumor expansion, vascularization, and dissemination. This project is focused on understanding stimulation-induced alterations in cellular metabolic processes and the critical role that they play in enabling immune cells to meet the enhanced metabolic demands associated with activation. Thus, our work involves precise dissection of metabolic networks that govern immune cell function, as well as the investigation of the metabolic foundations of disease. Our work in this area has already provided us with exciting possibilities for unraveling, and possibly therapeutically exploiting, the substantial metabolic interactions between inflammatory cells and the tumor. One of the enzymes we have studied is pyruvate kinase muscle-2 (PKM2). As the penultimate enzyme of glycolysis, PKM converts phosphoenolpyruvate and ADP into ATP and pyruvate that is either fermented to lactate or imported into the mitochondria for oxidation. Based in part on its high expression in tumor cells, several non-canonical roles for PKM2 have been suggested. These include interactions with transcription factors and direct phosphorylation of target proteins, both of which are controversial. In contrast to these activities, our work has shown canonical regulation of metabolism by PKM2 in Natural Killer cells. Most recently, we find that stimulated T cells upregulate both PKM2, the highly regulated isoform of the enzyme, and PKM1 the constitutively active isoform of the enzyme. In T cells engineers to lack PKM2, the over representation of PKM1 leads to high flux of glucose-derived carbon to pyruvate that enters the mitochondria and is oxidized yielding increased levels of mitochondrial ROS. Although CD4+ and CD8+ T cells are equally susceptible to exogenous redox insult, the higher levels of mitochondria in CD4+ T cells leads to generation of higher mitochondrial ROS levels in these cells causing them to preferentially die when stimulated in the absence of PKM2. This phenotype is evident in vitro and is detected in vivo as a steady decline in CD4+ cells over time in mice where T cells lack PKM2. To further provoke the PKM2 phenotype, we transferred CD4+ T cells into RAG null mice and assayed their ability to compete with wildtype cells and expand to fill the T cell niche. These experiments confirmed our in vitro findings by highlighting the substantial competitive disadvantage of the PKM2 null T cells. Lastly, we isolated naive CD4+ T cells from PKM2 null donor animals and transferred these into RAG null mice. Animals receiving wildtype T cells showed dramatic expansion of pathogenic cells and the development of substantial colitis. In contrast, naïve CD4+ T cells lacking PKM2 failed to substantially expand in vivo, did not accumulate in the colon and were unable to cause disease. Together these studies show the canonical role of PKM2 in the maintenance and expansion of T cells and cast doubt on the involvement of non-canonical, "moonlighting" effects of PKM2. In addition, we are investigating additional metabolic genes. We have an ongoing interest in Immune-Regulated Gene-1 (Irg1, a.k.a. aconitate decarboxylase-1, Acod1). We recently demonstrated a role for Irg1 in lipid processing in the liver. In short, Irg1-derived itaconate interferes with substrate level phosphorylation in the mitochondria resulting in an upregulation of lipid utilization by hepatocytes. This activity reduces lipid accumulation in hepatocytes and helps to control metabolic disorder. Ongoing studies are focused on the metabolism of arginine into either creatine for polyamines. Together, our work defines leukocyte metabolic enzymes as bona fide therapeutic targets and demonstrate the importance of understanding immunometabolism in the context of unique physiological niches relevant to cancer.