CD63 is regulated by iron via the IRE-IRP system and is important for ferritin secretion by extracellular vesicles

Extracellular vesicles (EVs) transfer functional molecules between cells. CD63 is a widely recognized EV marker that contributes to EV secretion from cells. However, the regulation of its expression remains largely unknown. Ferritin is a cellular iron storage protein that can also be secreted by the exosome pathway, and serum ferritin levels classically reflect body iron stores. Iron metabolism-associated proteins such as ferritin are intricately regulated by cellular iron levels via the iron responsive element-iron regulatory protein (IRE-IRP) system. Herein, we present a novel mechanism demonstrating that the expression of the EV-associated protein CD63 is under the regulation of the IRE-IRP system. We discovered a canonical IRE in the 5' untranslated region of CD63 messenger RNA that is responsible for regulating its expression in response to increased iron. Cellular iron loading caused a marked increase in CD63 expression and the secretion of CD63(+) EVs from cells, which were shown to contain ferritin-H and ferritin-L. Our results demonstrate that under iron loading, intracellular ferritin is transferred via nuclear receptor coactivator 4 (NCOA4) to CD63(+) EVs that are then secreted. Such iron-regulated secretion of the major iron storage protein ferritin via CD63(+) EVs, is significant for understanding the local cell-to-cell exchange of ferritin and iron.

Automated summary

Extracellular vesicles (EVs) transfer functional molecules between cells, with CD63 expression regulated by the IRE-IRP system in response to iron levels. Under iron loading, cellular ferritin is transferred via NCOA4 to CD63(+) EVs for secretion, providing insight into the local exchange of ferritin and iron between cells.