M2 macrophages exhibit higher sensitivity to oxLDL-induced lipotoxicity than other monocyte/macrophage subtypes

Suleiman A Isa,Richard Webb,José S Ruffino,Keith Morris,Maninder Ahluwalia,Andrew W Thomas

doi:10.1186/1476-511x-10-229

Suleiman A Isa, Richard Webb + Show 4 more

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https://doi.org/10.1186/1476-511x-10-229

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Abstract

BackgroundIn obesity, phenotypic switches occur in macrophage populations such that the predominantly M2-polarised anti-inflammatory state seen in lean individuals changes to a predominantly M1-polarised pro-inflammatory state in those who are obese. However, the mechanisms by which these phenotypic shifts occur have not yet been fully elucidated.ResultsThe effects of oxLDL (1-40 μg/ml; 24 h) on several parameters relevant to the Unfolded Protein Response (UPR)-mediated lipotoxic effects of oxLDL (disruption of ER Ca2+ handling; activation of the UPR transcription factor XBP-1; upregulation of the UPR target genes BiP and CHOP; apoptosis; cell viability) were investigated in human primary monocyte-derived macrophages, and also in monocyte-macrophages derived from the THP-1 monocytic cell line. A consistent pattern was observed: M2-polarised macrophages were more sensitive to the lipotoxic effects of oxLDL than either non-polarised macrophages or non-differentiated monocytic cells. Specifically, M2-polarised macrophages were the only cell type to undergo significantly increased apoptosis (Primary cells: 1.23 ± 0.01 basal; THP-1-derived: 1.97 ± 0.12 basal; P < 0.05 in both cases) and decreased cell viability (Primary cells: 0.79 ± 0.04 basal; THP-1-derived: 0.67 ± 0.02 basal; P < 0.05 in both cases) when exposed to oxLDL levels similar to those seen in overweight individuals (ie. 1 μg/ml).ConclusionsWe propose that the enhanced susceptibility of M2-polarised macrophages to lipotoxicity seen in the present in vitro study could, over time, contribute to the phenotypic shift seen in obese individuals in vivo. This is because a higher degree of oxLDL-induced lipotoxic cell death within M2 macrophages could contribute to a decrease in numbers of M2 cells, and thus a relative increase in proportion of non-M2 cells, within macrophage populations. Given the pro-inflammatory characteristics of a predominantly M1-polarised state, the data presented here may constitute a useful contribution to our understanding of the origin of the pro-inflammatory nature of obesity, and of the pathogenesis of obesity-associated inflammatory disorders such as Type 2 Diabetes and atherosclerosis.

Highlights

In obesity, phenotypic switches occur in macrophage populations such that the predominantly M2-polarised anti-inflammatory state seen in lean individuals changes to a predominantly M1-polarised pro-inflammatory state in those who are obese
The lipotoxic effects of oxLDL in M2-polarised primary macrophages Because endoplasmic reticulum (ER) stress-mediated apoptosis depends on activation of Caspases-3 and -7 [24,25], we sought to determine the effect of oxLDL on apoptosis using a luminometric Caspase 3/7 assay
We investigated the effects exerted by treatment with oxLDL (24 h) on expression of BiP and C/EBP homologous protein (CHOP); while no oxLDL-induced BiP mRNA induction could be observed in THP1-Mon, oxLDL upregulated BiP at high oxLDL doses in THP1-M2Mon (40 μg/ml: 1.81 ± 0.13 basal (P < 0.05)) and dTHP1-MF (40 μg/ml: 1.58 ± 1.20 basal (P < 0.05)), and at both oxLDL doses in dTHP1M2MF (1 μg/ml: 1.64 ± 0.40 basal; 40 μg/ml: 1.44 ± 0.28 basal (P < 0.05 in both cases))

Summary

Introduction

Phenotypic switches occur in macrophage populations such that the predominantly M2-polarised anti-inflammatory state seen in lean individuals changes to a predominantly M1-polarised pro-inflammatory state in those who are obese. Increased circulating oxidized low-density lipoprotein (oxLDL) [1,2]), and with chronic inflammation [3] This can lead to intracellular lipid accumulation in non-adipocyte cells, which can in turn lead to cell death, a phenomenon known as “lipotoxicity” [2]. Reductions in M2 differentiation correlate with disruptions in cholesterol homeostasis and reverse cholesterol transport [11], and to increases in total body fat mass, adiposity, glucose intolerance, and insulin resistance [10] These findings led Odegaard et al to suggest that “macrophage polarisation towards the alternative [M2] state may be a useful strategy for treating T2D“ [10]

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