Mitochondria and AMP-activated Protein Kinase-dependent Mechanism of Efferocytosis

Shaoning Jiang,Dae Won Park,William S Stigler,Judy Creighton,Saranya Ravi,Victor Darley-Usmar,Jaroslaw W Zmijewski

doi:10.1074/jbc.m113.489468

Abstract

Defective clearance of apoptotic cells is frequently associated with perpetuation of inflammatory conditions. Our results show a rapid activation of AMP-activated kinase (AMPK) in macrophages upon exposure to apoptotic cells or lysophosphatidylcholine, a specific phospholipid that is produced and released from dying cells. AMPK activation resulted from inhibition of mitochondrial oxygen consumption and ATP production and further depended on Ca(2+) mobilization and mitochondrial reactive oxygen species generation. Once activated, AMPK increased microtubule synthesis and chemokinesis and provided adaptation to energy demand during tracking and engulfment. Uptake of apoptotic cells was increased in lungs of mice that received lysophosphatidylcholine. Furthermore, inhibition of AMPK diminished clearance of apoptotic thymocytes in vitro and in dexamethasone-treated mice. Taken together, we conclude that the mitochondrial AMPK axis is a sensor and enhancer of tracking and removal of apoptotic cell, processes crucial to resolution of inflammatory conditions and a return to tissue homeostasis.

Highlights

Billions of cells undergo apoptosis in the human body every day, and the removal of dying cells is essential for tissue homeostasis
AMPK Inhibition Diminishes Migration of Macrophages toward a Lyso-PC Concentration Gradient—To further characterize the relationship between lyso-PC and AMPK, we examined the ability of lyso-PC to affect macrophage chemotaxis using a transmigration system in which peritoneal macrophages or RAW 264.7 cells were loaded into the upper chamber, whereas a lyso-PC attractant was placed in the lower chamber
Peritoneal macrophages or RAW 264.7 cells were treated with the AMPK inhibitor compound C (20 ␮M) for 30 min prior to inclusion of cells into the upper chamber

Summary

Background

Billions of cells undergo apoptosis in the human body every day, and the removal of dying cells (efferocytosis) is essential for tissue homeostasis. Our results show a rapid activation of AMP-activated kinase (AMPK) in macrophages upon exposure to apoptotic cells or lysophosphatidylcholine, a specific phospholipid that is produced and released from dying cells. The release of ATP and UTP or CX3CL1 from apoptotic cells were shown to activate macrophage chemotaxis in a P2Y2- or CX3R-dependent manner [10]. Another attractant is lysophosphatidylcholine (lyso-PC), a bioactive phospholipid that is produced and released from apoptotic cells that acts as a find me signal through activation of the macrophage G2A receptor [11]. The first step in AMPK activation is the binding of AMP to the ␥ regulatory subunit followed by ␣, ␤, and ␥ conforma-

The abbreviations used are

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION