Mechanisms of intermittent hypoxia-mediated macrophage activation - potential therapeutic targets for obstructive sleep apnoea.

ABSTRACT

Intermittent hypoxia (IH) plays a key role in the pathogenesis of insulin resistance (IR) in obstructive sleep apnoea (OSA). IH induces a pro-inflammatory phenotype of the adipose tissue with M1 macrophage polarisation, subsequently impeding adipocyte insulin signalling, and these changes are in striking similarity to those seen in obesity. However, the detailed molecular mechanisms of IH-induced macrophage polarisation are unknown and identification of same should lead to the identification of novel therapeutic targets. In the present study, we tested the hypothesis that IH acts through similar mechanisms as obesity, activating Toll-like-receptor (TLR)4/nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) and nucleotide-binding domain (NOD)-like receptor protein 3 (NLRP3) signalling pathways leading to the upregulation and secretion of the key cytokines interleukin (IL)-1ß and IL-6. Bone-marrow derived macrophages (BMDMs) from lean and obese C57BL/6 male mice were exposed to a state-of-the-art in vitro model of IH. Independent of obesity, IH led to a pro-inflammatory M1 phenotype characterised by increased inducible nitric oxide synthase and IL-6 mRNA expression, robust increase in NF-κB DNA-binding activity and IL-6 secretion. Furthermore, IH significantly increased pro-IL-1ß mRNA and protein expression and mature IL-1ß secretion compared to control treatment. Providing mechanistic insight, pre-treatment with the TLR4 specific inhibitor, TAK-242, prevented IH-induced M1 polarisation and upregulation of IL-1ß mRNA and pro-IL-1ß protein expression. Moreover, IH-induced increase in IL-1ß secretion was prevented in BMDMs isolated from NLRP3 knockout mice. Thus, targeting TLR4/NF-κB and NLRP3 signalling pathways may provide novel therapeutic options for metabolic complications in OSA.