TNF-α Inhibits Macrophage Clearance of Apoptotic Cells via Cytosolic Phospholipase A2 and Oxidant-Dependent Mechanisms

Abstract

Removal of apoptotic cells from inflammatory sites is an important step in the resolution of inflammation. Both murine and human macrophages stimulated with TNF-α or directly administered arachidonic acid showed an impaired ability to ingest apoptotic cells (efferocytosis). The inhibition was shown to be due to generation of reactive oxygen species, was blocked with a superoxide dismutase mimetic, MnTBAP, and was mimicked by direct addition of H2O2. To determine the mechanism of TNF-α-stimulated oxidant production, bone marrow-derived macrophages from gp91phox-deficient mice were examined but shown to still produce oxidants and exhibit defective apoptotic cell uptake. In contrast, a specific cytosolic phospholipase A2 inhibitor blocked the oxidant production and reversed the inhibited uptake. The suppressive effect of endogenous or exogenous oxidants on efferocytosis was mediated through activation of the GTPase, Rho. It was reversed in macrophages pretreated with C3 transferase to inactivate Rho or with an inhibitor of Rho kinase. During maturation of human monocyte-derived macrophages, only mature cells exhibited TNF-α-induced suppression of apoptotic cell clearance. The resistance of immature macrophages to such inhibition was shown to result not from defective generation of oxidants, but rather, from lack of response of these cells to the oxidants. Overall, the data suggest that macrophages in a TNF-α- and oxidant-rich inflammatory environment are less able to remove apoptotic cells and, thereby, may contribute to the local intensity of the inflammatory response.

Dysfunctional cystic fibrosis transmembrane conductance regulator inhibits phagocytosis of apoptotic cells with proinflammatory consequences

“…CFTR deficiency also increases oxidative stress in vitro and in vivo, likely through its role as a glutathione transporter and regulator of NADPH oxidase activity. This mechanism may be particularly important, because oxidative stress activates RhoA and inhibits efferocytosis (35), implying that a similar effect may occur in the CF airway.…”

Quantitative Proteomics Reveals the Induction of Mitophagy in Tumor Necrosis Factor-α-activated (TNFα) Macrophages

“…These analyses also confirmed the identification of several proteins known to be modulated by TNF-␣, such as cPLA 2 and TRAP1. Previous reports have described the activation of cPLA 2 upon TNF-␣ activation of macrophages (48,49). This protein is activated by calcium and comprises both lysophospholipase and transacylase activities.…”

From regulation of dying cell engulfment to development of anti-cancer therapy

“…The clearance of apoptotic cells is reduced by pretreating mature macrophages (but not immature macrophages) with tumor necrosis factor (TNF), an effect that is possibly related to the generation of ROS, which act through activation of the GTPase Rho. 41 Clearance of apoptotic cells apparently is inhibited by activated RhoA. 42 This implies that generation of ROS in phagocytes has a negative effect on clearance, in contrast to the positive role of oxidized lipids on recognition and uptake.…”

“…43 Although a molecular link between an altered redox state and activation of GTPase Rho is unknown, a recent study reported that redox agents, including the superoxide anion and nitrogen dioxide, can react with a conserved GXXXXGK(S/T)C motif in a number of Rho GTPases. 41 A soluble form of the myeloid epithelial reproductive (Mer) receptor tyrosine kinase, produced by cleavage of the membrane-bound Mer protein by a metalloproteinase, is released from macrophages in a constitutive manner. 44 As a decoy receptor for Gas6, soluble Mer prevented Gas6-mediated stimulation of membrane-bound Mer, resulting in defective apoptotic cell uptake by macrophages.…”