Abstract 13202: Cellular Iron Regulates Mitochondrial Dynamics Through the RNA-Binding Protein Tristetraprolin (TTP)

Abstract

Background: Iron is a critical molecule for normal cellular and physiologic processes including mitochondrial energy production. Previous work in our lab has established the RNA-binding protein TTP as a protein whose expression is induced by iron deprivation. TTP functions by binding to AU-rich elements in the 3’UTR of mRNAs and promotes their degradation. Many of the targets of TTP encode metabolically active proteins that function in pathways requiring iron. Recently, we have identified Mitofusin 1 (MFN1), a critical component of the mitochondrial fusion machinery, as a novel target of TTP. Published reports have shown MFN1/2 double knockout mice develop spontaneous cardiomyopathy. Conversely, under conditions of ROS induced cellular stress, MFN1 knockout in cardiomyocytes has been associated with protection from cell death. We hypothesize that the level of MFN1 closely matches the metabolic demands of the cell and, therefore, induction of TTP by low iron reduces the level of MFN1 to fragment the mitochondria and limit the cell’s ability to proliferate during conditions of iron insufficiency. Results: Twenty-four hours of iron chelation using either desferoxamine (DFO) or 2-2, bipyridyl (BPD) induced TTP expression at both the mRNA and protein levels in a dose-dependent manner. In silico analysis of the MFN1 3’UTR revealed four conserved AU-rich elements. Basal levels of MFN1 were increased in TTP KO cells indicating TTP actively targets MFN1 mRNA. Additionally, iron chelation decreased MFN1 mRNA expression in WT but not TTP KO cells. Confocal imaging of mitochondria showed profound fragmentation in WT cells exposed DFO that was abrogated in TTP KO cells. Functionally, iron chelation induced complete growth arrest in WT cells that was partially escaped by TTP KO cells. The increase in proliferation was accompanied by higher rates of cell death in TTP KO cells. Conclusions: Our studies show that TTP expression is upregulated under conditions of iron deficiency and in turn represses the expression of the mitochondrial fusion protein MFN1 to alter mitochondrial dynamics. By remodeling the mitochondria under conditions of iron deprivation we propose a critical role for TTP in coordinating growth arrest to protect metabolically active cells from cell death.