Acetylation of Human Mitochondrial Citrate Carrier Modulates Citrate/Malate Exchange to Sustain NADPH Production During Macrophage Activation

Abstract

The mitochondrial citrate carrier (CIC), a known target of acetylation, is up-regulated in activated immune cells and has a role in the production of inflammatory mediators. However, the role of acetylation in CIC activity is elusive. We show that CIC is acetylated in activated primary human macrophages and U937 cells and the level of acetylation is higher in glucose-deprived compared to normal-glucose medium grown cells. Acetylation enhances CIC transport activity, leading to a higher citrate efflux from mitochondria in exchange with malate. Cytosolic citrate levels do not increase upon activation of cells in glucose-deprived medium, indicating that citrate, transported faster, is consumed at higher rates. Malate levels in the cytosol are very low in activated cells grown in glucose-deprived medium, indicating that this metabolite is used rapidly by the malic enzyme. Additionally, in activated cells CIC inhibition increases the NADP+/NADPH ratio in glucose-deprived cells; this ratio is unchanged in glucose-rich grown cells due to the activity of the pentose phosphate pathway. Consistently, we found the NADPH-producing isocitrate dehydrogenase level higher in glucose-deprived activated cells. These results demonstrate that, in the absence of glucose, activated macrophages increase CIC acetylation to enhance citrate efflux from mitochondria to meet the NADPH demand through the actions of isocitrate dehydrogenase and malic enzyme.