Time and Demand are Two Critical Dimensions of Immunometabolism: The Process of Macrophage Activation and the Pentose Phosphate Pathway

Abstract

A process is a function of time; in immunometabolism, this is reflected by the stepwise adaptation of metabolism to sustain the bio-energetic demand of an immune-response in its various states and shades. This perspective article starts by presenting an early attempt to investigate the physiology of inflammation, in order to illustrate one of the basic concepts of immunometabolism, wherein an adapted metabolism of infiltrating immune cells affects tissue function and inflammation. We then focus on the process of macrophage activation and aim to delineate the factor time within the current molecular context of metabolic-rewiring important for adapting primary carbohydrate metabolism. In the last section, we will provide information on how the pentose phosphate pathway may be of importance to provide both nucleotide precursors and redox-equivalents, and speculate how carbon-scrambling events in the non-oxidative pentose phosphate pathway might be regulated within cells by demand. We conclude that the adapted metabolism of inflammation is specific in respect to the effector-function and appears as a well-orchestrated event, dynamic by nature, and based on a functional interplay of signaling- and metabolic-pathways.

Keywords: immunometabolism; inflammation; macrophage activation; metabolic reprograming; pentose phosphate pathway; primary carbohydrate metabolism; sedoheptulose kinase; time and demand.

Figure 1

Time-resolved metabolic reprograming during pro-inflammatory macrophage polarization. This model illustrates the activation of a macrophage as a function of time and is based on the literature discussed in the main text. LPS-induced activation can be grouped into an initiation-, an early metabolic-reprograming,- and an amplification-phase. The initiation phase of the metabolic response is characterized by an increase in glucose consumption and in the extracellular acidification rate (ECAR). The early metabolic reprograming phase depicts the increase and rerouting of carbon flux through glycolysis and the PPP, events which also regulate the cellular redox-state. In this setting, the mitochondrial association of hexokinase-II (HKII) appears to provide sufficient levels of glucose 6-phosphate (G6P), while the downregulation of sedoheptulose kinase (Shpk, previously known as CARKL) appears to be necessary to maintain appropriate carbon flux at the interface of glycolysis and the PPP. During the amplification phase, this pro-glycolytic metabolic-phenotype is further strengthened. A switch toward the more active 6-phosphofructo-2-kinase (PFK2) enzyme PFKFB3 produces higher levels of fructose 2-bisphosphate [F2,6bP], thus allosterically activating PFK1 and enhancing glycolytic flux. Dimers of the pyruvate kinase M2 (PKM2), as well as accumulating succinate further augment metabolic reprograming by supporting HIF-1α dependent transcriptional induction of glycolytic genes. In the amplification phase, also the export of intracellular glycolysis-derived lactate through monocarboxylate transporter 4 (MCT4) becomes obligatory, which may otherwise inhibit PFK1. These initial events lead to more prominent metabolic changes observed 24 h after macrophages have encountered the pro-inflammatory stimuli. However, further time-resolved data is required to refine these processes and our current perspective, how cellular metabolism of macrophages adapts during activation.

Figure 2

The function and regulation of the non-oxidative PPP. (A) represents a simplified model, which illustrates how transketolase (TK) and transaldolase (TALDO) may interconvert carbohydrate-phosphates of three to seven carbon-atoms in length (C3P to C7P) without the need of energy (carbon-scrambling) to account for the cellular demand, which in part defines cell function (indicated by the symbol f (_x)). In (B), we theoretically evaluate the regulatory effect of Shpk-derived sedoheptulose 7-phosphate ([C7P]_Shpk) on non-oxPPP flux in the presence of TK and TALDO. Flux through the non-oxPPP, by its reversible reactions, is dependent on the stoichiometry of the reactants (indicated by green arrowheads). In contrast to TK- and TALDO-derived S7P, the phosphorylation of free sedoheptulose to S7P by Shpk requires energy in form of ATP. Assuming a constant contribution by Shpk, this additional source of S7P may therefore act as a thermodynamic buffer, which can be actively regulated to induce a non-equilibrium. In theory, perturbation of Shpk can either increase the resistance (increased [C7P]_Shpk) or lower it (decreased [C7P]_Shpk) to support shunting through the non-oxPPP. However, in the presence of TK or TALDO, an increased [C7P]_Shpk will promote the incorporation of glycolytic-G3P into the PPP. This model further illustrates that non-oxPPP flux-direction is also dependent on the demand of respective molecules (indicated by red arrowheads).