HEME-INDUCED MITOCHONDRIAL REMODELING CONNECTS ELECTRON TRANSPORT CHAIN ACTIVITY WITH INFLAMMATORY SIGNALING IN MACROPHAGES

Introduction. Heme, a cofactor of hemoglobin, is released during hemolysis and triggers reactive oxygen species (ROS) production, including mitochondrial ROS (mtROS), exerting pro-inflammatory effects on macrophages. Early activation of spleen tyrosine kinase (Syk) and MAPKs (ERK, JNK, p38) upon heme exposure is ROS-dependent and essential for cytokine release. Heme also disrupts mitochondrial respiration and induces mitochondrial fission within minutes, indicating a potential role for mitochondria in signaling. This study investigates the mechanisms of heme-induced mitochondrial fragmentation and its contribution to Syk and MAPK activation.

Methods and Results. In BMDMs, MAPK activation occurs in the cytosol, whereas Syk is phosphorylated in a mitochondria-enriched fraction. Syk and mtROS partially contribute to mitochondrial fission, but severe mitochondrial alterations induced by heme – such as disruption of mitochondrial cristae observed by electron microscopy – suggest that additional mechanisms are involved. Mitochondrial fission usually requires dynamin-related protein 1 (Drp1) and is a typical response to acute stress, including ROS production and membrane depolarization, both observed in heme-stimulated BMDMs. However, pharmacological inhibition of Drp1 did not prevent heme-induced mitochondrial fission. Notably, only inhibition of complex I or III of the electron transport chain (ETC) preserved the fused mitochondrial network after heme exposure. Inhibition of ETC complexes also impaired MAPK activation and TNF production, but accelerated cell death, suggesting a key role for mitochondrial ETC in heme-induced signaling.

Conclusion. Heme rapidly disrupts mitochondrial function and structure, initiating a complex signaling response in macrophages. Collectively, our results suggest that mitochondrial fission may represent an early adaptive response to heme-induced stress, limiting mitochondrial damage and reshaping downstream signaling.

Financial support: FAPERJ.