Abstract
Autophagy is a cell housekeeping mechanism that has recently received attention in relation to its effects on the immune response. Genetic studies have identified candidate loci for Crohn's disease susceptibility among autophagy genes, while experiments in murine macrophages from ATG16L1 deficient mice have shown that disruption of autophagy increases processing of IL-1β and IL-18 through an inflammasome-dependent manner. Using complementary approaches either inducing or inhibiting autophagy, we describe modulatory effects of autophagy on proinflammatory cytokine production in human cells. Inhibition of basal autophagy in human peripheral blood mononuclear cells (PBMCs) significantly enhances IL-1β after stimulation with TLR2 or TLR4 ligands, while at the same time reducing the production of TNFα. In line with this, induction of autophagy by starvation inhibited IL-1β production. These effects of autophagy were not exerted at the processing step, as inflammasome activation was not influenced. In contrast, the effect of autophagy on cytokine production was on transcription level, and possibly involving the inhibition of p38 mitogen activated protein kinase (MAPK) phosphorylation. In conclusion, autophagy modulates the secretion of proinflammatory cytokines in human cells through an inflammasome-independent pathway, and this is a novel mechanism that may be targeted in inflammatory diseases.
Highlights
Autophagy is a conserved mechanism for degradation of defective organelles and long-lived proteins, that plays an important role in the homeostasis of the cell by recycling cytoplasmic cargo for aminoacid and energy re-use [1]
In order to verify the modulation of autophagy using the typical induction method by starvation and the inhibition method using the pharmacological inhibitor 3-Methyl Adenine (3MA) in the human peripheral blood mononuclear cells (PBMCs) system used in this study, Western blotting of the two light chain 3 (LC3) fractions was performed
Isolated human PBMCs treated with the starvation medium EBSS (Earle’s Balanced Salt Solution) showed a markedly increased level of LC3-II compared to RPMI controls
Summary
Autophagy is a conserved mechanism for degradation of defective organelles and long-lived proteins, that plays an important role in the homeostasis of the cell by recycling cytoplasmic cargo for aminoacid and energy re-use [1]. Autophagy comprises three main processes – chaperone-mediated autophagy, microautophagy and macroautophagy The latter, referred to as autophagy, is characterized by the sequestration of cytosolic proteins and organelles into double-membrane vesicles called autophagosomes. These autophagosomes maturate through fusion with lysosomes, a process that will eventually lead to the breakdown of the protein content [2]. Through these effects, autophagy has been demonstrated to be a biological response of the cell in stressful situations, traditionally during starvation and growth factor deprivation, ensuring the degradation of old structures with the purpose of sustaining the essential anabolic processes of the cell [3]. Through its main roles in survival and housekeeping, the process of autophagy has gained relevance in the context of human pathologies like neurodegenerative diseases [4], cancer [5], lysosomal diseases [6], and ageing [1]
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