Increased mtDNA Polymorphism Level and Defective Mitophagy are Associated With Proinflammatory Activation in THP-1-Based Cybrids

Background: Mitochondria play a key role in the regulation of inflammatory processes. It is well established that mitochondria contain their own mitochondrial DNA (mtDNA), which displays a substantially higher frequency of polymorphisms compared to the nuclear genome. To maintain mitochondrial functionality, cells employ a quality control mechanism known as mitophagy, which ensures the selective removal of dysfunctional mitochondria. The aim of this study was to investigate the relationship between mitochondrial polymorphisms, the efficiency of dysfunctional mitochondria clearance, and proinflammatory activation of inflammatory cells. Methods: In this study, we used cybrids derived from the THP-1 (human monocytic leukemia cell line) that differed in their mitochondrial genome. Results: Based on the levels of secreted proinflammatory cytokines (tumor necrosis factor (TNF), interleukin-1β (IL-1β), IL-6, IL-8, and C-C motif chemokine ligand 2 (CCL2)), cybrids were divided into two groups: one exhibiting a low and the other a high proinflammatory response. mtDNA sequencing revealed a higher number of polymorphisms in the high proinflammatory response group. Transcriptome analysis showed that signaling pathways associated with mitophagy were downregulated in cells with a high proinflammatory response. Functional assays confirmed that mitophagy was impaired in this group. Conclusions: Increased proinflammatory activity in cybrids correlated with a greater number of mtDNA polymorphisms and with impaired mitophagy. These observations point to an association between mitochondrial genomic variation, reduced mitochondrial quality control, and heightened proinflammatory activation of inflammatory cells. However, the data are associative and do not establish causality. Further mechanistic studies are required to determine whether and how mtDNA polymorphisms and defective mitophagy directly contribute to proinflammatory activation.