Abstract
We previously showed that macrophages from macrophage-specific ATP-binding cassette transporter A1 (ABCA1) knockout (Abca1(-M/-M)) mice had an enhanced proinflammatory response to the Toll-like receptor (TLR) 4 agonist, lipopolysaccharide (LPS), compared with wild-type (WT) mice. In the present study, we demonstrate a direct association between free cholesterol (FC), lipid raft content, and hyper-responsiveness of macrophages to LPS in WT mice. Abca1(-M/-M) macrophages were also hyper-responsive to specific agonists to TLR2, TLR7, and TLR9, but not TLR3, compared with WT macrophages. We hypothesized that ABCA1 regulates macrophage responsiveness to TLR agonists by modulation of lipid raft cholesterol and TLR mobilization to lipid rafts. We demonstrated that Abca1(-M/-M) vs. WT macrophages contained 23% more FC in isolated lipid rafts. Further, mass spectrometric analysis suggested raft phospholipid composition was unchanged. Although cell surface expression of TLR4 was similar between Abca1(-M/-M) and WT macrophages, significantly more TLR4 was distributed in membrane lipid rafts in Abca1(-M/-M) macrophages. Abca1(-M/-M) macrophages also exhibited increased trafficking of the predominantly intracellular TLR9 into lipid rafts in response to TLR9-specific agonist (CpG). Collectively, our data suggest that macrophage ABCA1 dampens inflammation by reducing MyD88-dependent TLRs trafficking to lipid rafts by selective reduction of FC content in lipid rafts.
Highlights
We previously showed that macrophages from macrophage-specific ATP-binding cassette transporter A1 (ABCA1) knockout (Abca1-M/-M) mice had an enhanced proinflammatory response to the Toll-like receptor (TLR) 4 agonist, lipopolysaccharide (LPS), compared with wild-type (WT) mice
To explore the specific role of ABCA1 in macrophages in vivo and in vitro, we generated macrophage-specific ABCA1 knockout (Abca1-M/-M) mice. Using this unique mouse model, we demonstrated that macrophages from Abca1-M/-M mice have a significant increase in free cholesterol (FC) and are more proinflammatory in vivo and in vitro in response to LPS via Toll-like receptor 4 (TLR4) compared with WT
Macrophages subjected to cholesterol depletion secreted less interleukin (IL)-6 and IL-12p40, whereas macrophages overloaded with cholesterol secreted more of these cytokines, suggesting that cytokine secretion in TLR4 activated macrophages has a significant positive association with cellular FC content (Fig. 1B)
Summary
We previously showed that macrophages from macrophage-specific ATP-binding cassette transporter A1 (ABCA1) knockout (Abca1-M/-M) mice had an enhanced proinflammatory response to the Toll-like receptor (TLR) 4 agonist, lipopolysaccharide (LPS), compared with wild-type (WT) mice. We demonstrate a direct association between free cholesterol (FC), lipid raft content, and hyper-responsiveness of macrophages to LPS in WT mice. We hypothesized that ABCA1 regulates macrophage responsiveness to TLR agonists by modulation of lipid raft cholesterol and TLR mobilization to lipid rafts. Abca1-M/-M macrophages exhibited increased trafficking of the predominantly intracellular TLR9 into lipid rafts in response to TLR9-specific agonist (CpG). Our data suggest that macrophage ABCA1 dampens inflammation by reducing MyD88-dependent TLRs trafficking to lipid rafts by selective reduction of FC content in lipid rafts.—Zhu, X., J. ATP-binding cassette transporter A1 (ABCA1) is a plasma membrane protein that functions to eliminate excess free cholesterol (FC) from tissues by effluxing cellular FC and phospholipid (PL) to lipid-free apolipoprotein AI, forming nascent HDL particles [1, 2].
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