Oxidative Inactivation of the Proteasome Augments Alveolar Macrophage Secretion of Vesicular SOCS3

Abstract

Extracellular vesicles (EVs) contain a diverse array of molecular cargoes that alter cellular phenotype and function following internalization by recipient cells. In the lung, alveolar macrophages (AMs) secrete EVs containing suppressor of cytokine signaling 3 (SOCS3), a cytosolic protein that promotes homeostasis via vesicular transfer to neighboring alveolar epithelial cells. Although changes in the secretion of EV molecules—including but not limited to SOCS3—have been described in response to microenvironmental stimuli, the cellular and molecular machinery that control alterations in vesicular cargo packaging remain poorly understood. Furthermore, the use of quantitative methods to assess the sorting of cytosolic cargo molecules into EVs is lacking. Here, we utilized cigarette smoke extract (CSE) exposure of AMs as an in vitro model of oxidative stress to address these gaps in knowledge. We demonstrate that the accumulation of reactive oxygen species (ROS) in AMs was sufficient to augment vesicular SOCS3 release in this model. Using nanoparticle tracking analysis (NTA) in tandem with a new carboxyfluorescein succinimidyl ester (CFSE)-based intracellular protein packaging assay, we show that the stimulatory effects of CSE were at least in part attributable to elevated amounts of SOCS3 packaged per EV secreted by AMs. Furthermore, the use of a 20S proteasome activity assay alongside treatment of AMs with conventional proteasome inhibitors strongly suggest that ROS stimulated SOCS3 release via inactivation of the proteasome. These data demonstrate that tuning of AM proteasome function by microenvironmental oxidants is a critical determinant of the packaging and secretion of cytosolic SOCS3 protein within EVs.