Ancillary Activity: Beyond Core Metabolism in Immune Cells

Abstract

Immune cell function and fate are intimately linked to engagement of metabolic pathways. The contribution of core metabolic pathways to immune cell bioenergetics has been vigorously investigated in recent years. However, precisely how other peripheral metabolic pathways support immune cells beyond energy generation is less well understood. Here we survey the literature and highlight recent advances in our understanding of several ancillary metabolic pathways and how they support processes beyond ATP production and ultimately contribute to protective immunity.

Keywords: O-GlcNAcylation; cholesterol; hexosamine synthesis pathway; immune cell metabolism; immunity; metabolism; nucleotides; one-carbon metabolism; pentose phosphate pathway; polyamines.

Figure 1. Ancillary metabolic pathways are intimately intertwined with core metabolism

Core metabolic pathways (grey shaded) use most of the carbon equivalents derived from nutrients for the production of energy, to control redox balance and to generate biomass. The ‘peripheral’ pathways we describe in this review are also intertwined with core metabolism. In this figure we focus our attention on some of the documented interactions between ‘peripheral’ and core metabolic pathways. The pentose phosphate pathway (PPP, purple shaded) branches off glycolysis, feeds ribose-5-phosphate (ribose-5-P) to nucleotide synthesis and represents a source of reducing equivalents in the form of NADPH. NADPH is involved in fatty acid and cholesterol synthesis, and also enters the 1-carbon (1-C) metabolism to balance redox state. The 1-C metabolism (red shaded), together with amino acids and ribose-5-P, supports de novo nucleotide biosynthesis. Furthermore, the 1-C metabolism is the key source of S-adenosylmethionine required for spermidine and spermine synthesis (green shaded). The polyamine pathway also utilizes the amino acids arginine, glutamine and ornithine as precursors for polyamine synthesis. The production of hexosamines (blue shaded) integrates fructose-6-P, glutamine, nucleotides and acetyl-CoA to generate the amino sugar UDP-GlcNAc, involved in the post-transcriptional modification of proteins. Finally, acetyl-CoA is used to synthesize cholesterol (orange shaded) that together with its intermediates and derivatives coordinates intracellular signaling. ETC: electron transport chain; F-6-P: fructose-6-phosphate; G-3-P: glycerol-3-phosphate; G-6-P: glucose-6-phosphate; Met: methionine; Non-ox PPP: non-oxidative branch of PPP; Ox PPP: oxidative branch of PPP; TCA: tricarboxylic acid; THF: tetrahydrofolate; UDP-GlcNAc: uridine diphosphate N-actetylglucosamine.

Figure 2. The polyamine biosynthesis pathway

Polyamine synthesis proceeds with the decarboxylation of ornithine to form putrescine via the action of ornithine decarboxylase (ODC). Spermidine and spermine are then formed in step-wise fashion by spermidine synthase (SPDS) and spermine synthase (SPMS). Both SPDS and SPMS utilize decarboxylated S-adenosylmethionine, a product of S-adenosylmethionine decarboxylase (AdoMetDC), as a substrate for spermidine and spermine synthesis. Together with ODC, AdoMetDC comprise the rate-limiting steps of polyamine synthesis. During catabolism, spermine can be directly oxidized to spermidine by spermine oxidase (SMO). Induction of Spermidine/Spermine N1-acetyltransferase (SSAT), the rate-limiting step of polyamine catabolism, results in N¹-acetylated forms of spermine and spermidine. Through the subsequent activity of polyamine oxidase (PAO) spermine can be converted to spermidine and spermidine to putrescine. The precursor to polyamine synthesis, ornithine, is synthesised by multiple routes. As an intermediate of the urea cycle, ornithine is synthesised from arginine by arginase. The urea cycle may also act as a vehicle to link the TCA cycle to polyamine synthesis as aspartate is used to form argininosuccinate that is subsequently combined with fumarate to make arginine. Arginine may also be taken up as a substrate and synthesised directly to ornithine. Both proline and glutamate can feed into ornithine synthesis through Δ1-Pyrroline 5-carboxylate that once catalyzed to Glutamate-γ-semialdehyde can be converted to ornithine by ornithine acetyltransferase (OAT). Finally, glutamine can also feed into the urea cycle via carbamoyl phosphate that enters the cycle at citrulline (not shown).

Figure 3. The cholesterol biosynthesis pathway

HMG-CoA reductase combines 3 molecules of acetyl-CoA with NADPH to synthesize mevalonate, the precursor of cholesterol biosynthesis. Several steps of phosphorylation generate activated isoprenes, later condensed to form squalene. Cyclization of squalene occurs in several steps that finally generate the four fused rings characteristic of cholesterol and other sterols. Intermediates of cholesterol biosynthesis, such as farnesyl pyrophosphate are used to post-translational modify proteins (ie: protein prenylation). Similarly, many cholesterol derivatives such as cholesterol esters, hydroxycholesterols and cholesterol sulfates play important roles in modulating the immune function.

Figure 4. The hexosamines biosynthesis pathway

The hexosamine biosynthesis pathway is an off-shoot of glycolysis that starts with the conversion of fructose-6-phosphate and the amino group donor glutamine to glucosamine-6-phosphate. Glucosamine-6-phosphate is then linked to the nucleotide UTP and N-acetylated to form the bioactive molecule UDP-N-acetylglucosamine. The enzymes O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) catalyse addition or removal of the post-translational modification O-GlcNAcylation. O-GlcNAcylation and phosphorylation often compete for the same amino acid residues and their competition influence protein stability and function. On the other hand, the enzymes MGATs use UDP-N-acetylglucosamine to modify the branching of N-glycans, a post-translational modification of surface proteins that regulates cell membrane dynamics.

Figure 5. The pentose phosphate pathway, nucleotide biosynthesis and one-carbon metabolism

The pentose phosphate pathway (PPP) is divided into an oxidative and a non-oxidative arm. In the former, the glycolytic intermediate glucose-6-phosphate is oxidized to ribose-5-phosphate that can then feed the non-oxidative PPP. The non-oxidative PPP converts ribose-5-phosphate to glucose-6-phosphate that re-enters the oxidative PPP and sustains a cycle aimed at regenerating NADP⁺ to NADPH. Ribose-5-phosphate can otherwise fuel the de novo pathway of nucleotide biosynthesis to generate the building blocks for DNA replication and RNA transcription. Quiescent cells mostly use the salvage pathway of nucleotide regeneration that recycles senescent DNA and RNA to feed the nucleotide pool. The one-carbon metabolism is compartmentalized between cytosol and mitochondria and provided essential carbon equivalents for purine and pyrimidine biosynthesis. The THF cycle is the entry point of the essential amino acid serine to sustain proliferation of activate T cells.