Shaping of Innate Immune Response by Fatty Acid Metabolite Palmitate

Abstract

Innate immune cells monitor invading pathogens and pose the first-line inflammatory response to coordinate with adaptive immunity for infection removal. Innate immunity also plays pivotal roles in injury-induced tissue remodeling and the maintenance of tissue homeostasis in physiological and pathological conditions. Lipid metabolites are emerging as the key players in the regulation of innate immune responses, and recent work has highlighted the importance of the lipid metabolite palmitate as an essential component in this regulation. Palmitate modulates innate immunity not only by regulating the activation of pattern recognition receptors in local innate immune cells, but also via coordinating immunological activity in inflammatory tissues. Moreover, protein palmitoylation controls various cellular physiological processes. Herein, we review the updated evidence that palmitate catabolism contributes to innate immune cell-mediated inflammatory processes that result in immunometabolic disorders.Fatty acid β-oxidation is a major process that provides energy by degrading fatty acids. The enzymes responsible for fatty acid β-oxidation are mainly located in the mitochondria and peroxisomes. Each β-oxidation cycle can generate one acetyl-CoA molecule, which serves as the source for tricarboxylic acid cycle progression and yields more ATP per carbon than that from sugars by oxidative phosphorylation [7]. In contrast to the removal of two carbons from long-chain fatty acids in each β-oxidation round, fatty acid synthesis is catalyzed by joining two carbon units to the growing acyl chain via the cytosolic fatty acid synthase complex [8]. Palmitate is a 16-carbon saturated fatty acid produced via the fatty acid synthesis pathway in a fatty acid synthase-dependent manner, and acts as the major lipid mediator in inflammatory response regulation. Palmitic Acid-Regulated Innate Immune ResponsesCell uptake of palmitate is mainly mediated by the membrane fatty acid transporter CD36, also known as scavenger receptor B2 [9], although palmitate can also enter cells by other mechanisms, including direct membrane interactions, albumin-mediated transfer, and lipoprotein lipase-mediated uptake [10]. Accumulating evidence suggests a critical role by palmitic acid in modulating the activation of pattern recognition receptors in innate immune cells (Figure 1). Indeed, accumulating evidence reveals that although not through direct engagement of Toll-like receptors (TLRs), palmitate can modulate TLR downstream signaling in response to their ligand stimulation [11,12]. Palmitate is believed to be a TLR4 agonist that promotes macrophage activation and lipid metabolism-associated inflammation. Recently, however, it has been demonstrated that palmitate-induced inflammatory effects are not triggered by the direct binding of palmitate to TLR4, but through a combination of palmitate-mediated and TLR4-dependent priming, which alters cellular lipid metabolic pathways, gene expression, and membrane lipid composition, the prerequisite changes that are ...

Role of TLR4 in the induction of inflammatory changes in adipocytes and macrophages

“…We hypothesized that the adipocyte micro-environment, might be important regulators in shaping macrophage functions and phenotype. Palmitate induces lipotoxicity with the overproduction of reactive oxygen species (ROS) and when converted to palmitoyl-CoA also serves as a precursor for ceramides generation, which further enhance ROS production [42]. Previous work from our lab showed that palmitic acid stimulated WT lineage negative bone marrow cells but not Tlr4 −/bone marrow cells when directly added to the cells [21].…”

“…Palmitate-induced metabolic reprogramming in innate immune cells modulates inflammatory responses and contributes to disease progression [42]. It is critical to investigate the cell type-specific responses of TLR4 to ligands such as palmitate to better understand TLR4's role in SFA-mediated inflammation during obesity.…”

The IgE production is initially induced in subcutaneous fat and depends on extrafollicular B cells

“…The transient nature of GCs formation in the regional lymph node requires particular explanations. We presume that certain adipocyte released mediators such as free fatty acids [73] may play a role by triggering unfolded protein response [74] in B cells resulting in enhanced plasma cells and plasmablasts development instead of stable GC persistence [75].…”

HIF2α is a direct regulator of neutrophil motility

“…It is well-accepted that the functionality of innate immune cells varies depending on the lipidtype composition of its cytoplasmic membrane. 45,46 Therefore, we evaluated if altered membrane lipid composition of the cKO P2 neutrophils could account for their different migratory ability, by performing high-throughput lipidomic analysis of freshly isolated BMDNs (see supplemental data). However, as there were no significant alterations between the cKO P2 and WT BMDNs (supplemental Figure 2D and Table 3), it appears unlikely that differences in the lipid composition are directly responsible for the dramatic difference in the migratory capacity of the cKOP2 neutrophils.…”