HuR-Driven Reversible Mitochondrial Shuttling Buffers Cytosolic miRNA Levels in Hepatic Cells to Control Apoptosis

Mitochondria, the “powerhouse” of mammalian cells, also serve as key storage sites for ions, metabolites, and enzymes vital for metabolic regulation. Exploring the regulatory processes that control the activities of miRNAs, the key non-coding RNA in mammalian cells, we found a context-dependent reversible localization of specific miRNAs to the mitochondrial matrix. Our data suggests a de novo role of mitochondria as miRNA sinks in mammalian cells. miR-122 is a key hepatic miRNA regulating metabolic processes in the mammalian liver. In this study, we observed increased mitochondrial targeting of miR-122 in amino acid-starved hepatic cells. Interestingly, when cells were refed with amino acids, mitochondrial miR-122 gets relocalized and reused in the cytosol for the translational repression process. Moreover, this phenomenon is not limited to miR-122 as several mitochondrial miRNAs (mito-miRs) follow similar transient storage inside mitochondria in stressed cells. Remarkably, mitochondria-localized mito-miRs preferentially target mRNAs encoding crucial mitochondrial components related to apoptosis. Hence, hepatic cells regulate apoptosis pathways during the starvation-refeeding cycle by shuttling a specific set of miRNAs to and from mitochondria, thereby balancing cytosolic miRNA content and homeostasis. Stress response miRNA binder ELAVL1 or HuR protein was found to be both necessary and sufficient for transporting the mature mito-miRs to the mitochondrial matrix - a process also controlled by the interaction between mitochondria and the endoplasmic reticulum.