HIF 1 α and metabolic reprogramming in inflammation

Introduction ATP production is required to support physiological function in all cells. Methods of ATP production vary between cell types and cellular activation states. Glucose can be used to fuel ATP production through two linked metabolic pathways: glycolysis and the TCA cycle. In glycolysis, glucose is converted into pyruvate in the cytoplasm and phosphates are transferred to ADP to generate two molecules of ATP. Pyruvate can also be converted into acetyl-CoA, which enters the TCA cycle, linking the two processes. The TCA cycle produces NADH and flavin adenine dinucleotide (FADH2), which are used to fuel oxidative phosphorylation (OXPHOS) in the mitochondria to produce an additional 36 molecules of ATP. Cells can utilize other substrates aside from glucose depending on the context. Fatty acids and glutamine can both be used to fuel OXPHOS in some cells (1, 2). A key feature of metabolic pathways is their plasticity. Changes in nutrient availability or oxygen levels are the best-characterized drivers of metabolic reprogramming. For example, hypoxia is a well-known driver of glycolysis, as an oxygen deficit results in limited OXPHOS. Under these circumstances cells must rely on glycolysis to generate ATP. HIF1α is critical for this process, as it induces the expression of glycolytic enzymes such as hexokinase and phosphofructokinase, thereby allowing for sustained ATP production (3, 4). Hypoxia and inflammation are inherently linked. Decreasing oxygen levels induce metabolic changes to sustain ATP production. Similarly, quiescent immune cells can be viewed as metabolically inert and require significant metabolic reprogramming upon activation to provide sufficient ATP for effector functions. The HIF pathway provides a switch through which metabolic phenotypes can be amended in both of these scenarios and therefore is a critical transcriptional regulator of immunity and inflammation (2). HIF is a highly conserved member of the PER-ARNT-SIM (PAS) subfamily of the basic helix-loop-helix (bHLH) family of transcription factors (5). During active HIF signaling, HIF forms a heterodimeric complex that consists of an α and a β subunit. The α subunit can be of two primary forms, HIF1α or HIF2α. The β subunit is the constitutively expressed aryl hydrocarbon receptor nuclear translocator (ARNT). Upon dimerization, the HIFα/ ARNT complex translocates to the nucleus where it binds to the promoters of target genes containing hypoxia response elements (HREs). This binding initiates transcription of a battery of genes involved in cellular adaptation to hypoxia, metabolism, and cell function. HIF signaling is primarily regulated by the stability of its α subunit. In a resting cell, HIF1α is hydroxylated at conserved proline residues by the prolyl hydroxylases (PHDs). This hydroxylation allows for HIF1α ubiquitination by the von Hippel-Lindau (VHL) E3 ubiquitin ligase, marking it for rapid proteasomal degradation. The PHDs are oxygen dependent; thus, under normoxic conditions, HIF1α is continuously turned over by means of degradation, resulting in low basal HIF1α levels. In normoxia HIF1α has a remarkably short half-life of less than 5 minutes (6). Hypoxic conditions result in PHD inhibition and an attenuation of HIF1α hydroxylation. In the absence of proteasomal degradation, HIF1α accumulates, translocates to the nucleus, and increases transcription of HRE-containing genes. In addition to changing levels of oxygen, another well-characterized example of a HIF-mediated switch to glycolysis has been described in tumors, whereby under normoxia, glycolysis still predominates. This metabolic switch has been termed the Warburg effect (7) and can be driven by mutations in proto-oncogenes such as Myc or Ras, which ultimately result in HIF1α-mediated metabolic reprogramming towards a glycolytic phenotype. Hypoxia is a prominent component of solid tumors, primarily as a pathophysiological consequence of disturbed microcirculation due to insufficient vascularization following rapid tumor growth. HIF1α plays an important role in modifying tumor metabolism in response to an increasingly inhospitable hypoxic microenvironment. The changes in gene expression induced by HIF signaling contribute to many of the hallmarks of cancer that enable tumor growth, survival, and invasion. Hypoxia is seen as a poor prognostic marker in many cancer types. Importantly, HIF-inducible HIF1α is a common component of pathways involved in the control of cellular metabolism and has a role in regulating immune cell effector functions. Additionally, HIF1α is critical for the maturation of dendritic cells and for the activation of T cells. HIF1α is induced in LPS-activated macrophages, where it is critically involved in glycolysis and the induction of proinflammatory genes, notably Il1b. The mechanism of LPS-stimulated HIF1α induction involves succinate, which inhibits prolyl hydroxylases (PHDs). Pyruvate kinase M2 (PKM2) is also induced and interacts with and promotes the function of HIF1α. In another critical inflammatory cell type, Th17 cells, HIF1α acts via the retinoic acid–related orphan receptor-γt (RORγt) to drive Th17 differentiation. HIF1α is therefore a key reprogrammer of metabolism in inflammatory cells that promotes inflammatory gene expression. HIF1α and metabolic reprogramming in inflammation