Efferocytosis potentiates the expression of arachidonate 15-lipoxygenase (ALOX15) in alternatively activated human macrophages through LXR activation

Abstract

Macrophages acquire anti-inflammatory and proresolving functions to facilitate resolution of inflammation and promote tissue repair. While alternatively activated macrophages (AAMs), also referred to as M2 macrophages, polarized by type 2 (Th2) cytokines IL-4 or IL-13 contribute to the suppression of inflammatory responses and play a pivotal role in wound healing, contemporaneous exposure to apoptotic cells (ACs) potentiates the expression of anti-inflammatory and tissue repair genes. Given that liver X receptors (LXRs), which coordinate sterol metabolism and immune cell function, play an essential role in the clearance of ACs, we investigated whether LXR activation following engulfment of ACs selectively potentiates the expression of Th2 cytokine-dependent genes in primary human AAMs. We show that AC uptake simultaneously upregulates LXR-dependent, but suppresses SREBP-2-dependent gene expression in macrophages, which are both prevented by inhibiting Niemann–Pick C1 (NPC1)-mediated sterol transport from lysosomes. Concurrently, macrophages accumulate sterol biosynthetic intermediates desmosterol, lathosterol, lanosterol, and dihydrolanosterol but not cholesterol-derived oxysterols. Using global transcriptome analysis, we identify anti-inflammatory and proresolving genes including interleukin-1 receptor antagonist (IL1RN) and arachidonate 15-lipoxygenase (ALOX15) whose expression are selectively potentiated in macrophages upon concomitant exposure to ACs or LXR agonist T0901317 (T09) and Th2 cytokines. We show priming macrophages via LXR activation enhances the cellular capacity to synthesize inflammation-suppressing specialized proresolving mediator (SPM) precursors 15-HETE and 17-HDHA as well as resolvin D5. Silencing LXRα and LXRβ in macrophages attenuates the potentiation of ALOX15 expression by concomitant stimulation of ACs or T09 and IL-13. Collectively, we identify a previously unrecognized mechanism of regulation whereby LXR integrates AC uptake to selectively shape Th2-dependent gene expression in AAMs.

Lysosome-Dependent LXR and PPARδ Activation Upon Efferocytosis in Human Macrophages

“…Our data also show that LXRs, PPARs and SREBPs are the main transcriptional regulators of lipid metabolism in efferocytotic human Mj. A strong downregulation of SREBP-2 targets was previously observed in efferocytotic LR73 hamster phagocytes (17), and in human efferocytotic Mj (27). In human Mj, accumulation of sterol biosynthetic intermediates upon efferocytosis likely contributes to LXR activation and SREBP-2 suppression (27).…”

“…A strong downregulation of SREBP-2 targets was previously observed in efferocytotic LR73 hamster phagocytes (17), and in human efferocytotic Mj (27). In human Mj, accumulation of sterol biosynthetic intermediates upon efferocytosis likely contributes to LXR activation and SREBP-2 suppression (27). In addition to sterol homeostasis targeted by LXRs and SREBP-2, fatty acid metabolism undergoes transcriptional regulation in efferocytotic Mj through PPARs.…”

microRNA-21-5p from M2 macrophage-derived extracellular vesicles promotes the differentiation and activity of pancreatic cancer stem cells by mediating KLF3

“…THP-1 cells of 90% confluence were detached by 0.25% trypsin (Solarbio Science & Technology Co., Ltd., Beijing, China) and passaged. Then, 100 ng/mL phorbol myristic acetate (Sigma-Aldrich, CA, USA) was utilized in 24-h culture of THP-1 cells and 20 ng/mL interleukin (IL)-4 and IL-13 (Peprotech, NJ, USA) in 48-h induction of M2 macrophage polarization (Chen et al 2019;Snodgrass et al 2020;Becerra-Diaz et al 2021). F4/80, CD86, and CD206 were determined by flow cytometry (Zhu et al 2020) whereas Arginase-1 (ARG-1) and IL-10 (M2 macrophage-related cytokines) by reverse transcription quantitative polymerase chain reaction (RT-qPCR) (Guan et al 2020).…”

Prostanoids and Resolution of Inflammation – Beyond the Lipid-Mediator Class Switch

“…On the one hand, PGE 2 appears to inhibit IL-2 production by T cells, thereby attenuating both T cell activation and activation-dependent apoptosis (104)(105)(106). On the other hand, while PGE 2 appears to contribute to sustained inflammation by differentiation and activation of Th1 and gd T cells, considered to support inflammation (107-109), other findings indicate that PGE 2 selectively inhibits Th1 cytokine production leaving Th2 cytokines, such as IL-4 and IL-5, unaffected (110,111), thus provoking a PGE 2 -dependent shift towards Th2 responses, which are associated with repair mechanisms instead (112)(113)(114). This notion is substantiated by the high levels of PGE 2 observed in Th2-driven diseases such as atopic allergy (115).…”

“…Moreover, while the pro-resolving activity of cyclooxygenase metabolites has long been attributed predominantly to PGD 2 and 15d-PGJ 2 ( 124 ), PGE 2 emerged as an important facilitator of the lipid-mediator class switch inducing not only the production of PGD 2 and its derivatives, but also of the so-called specialized pro-resolving mediators (SPM) ( 20 ). SPM, synthesized by 15-lipoxygenase (ALOX15) ( 31 , 125 – 127 ), are key players in the resolution of inflammation ( 114 , 128 ). PGE 2 induces the expression of the relevant lipoxygenases, thereby skewing the balance towards a pro-resolving lipid mediator profile ( 129 , 130 ).…”