Dissecting the Biological Functions of Various Isoforms of Ferredoxin Reductase for Cell Survival and DNA Damage Response

The ferredoxin reductase (FDXR) gene is expressed as seven isoforms: 1-6 by alternative splicing and 7 by an alternative promoter according to the Entrez Gene Database. Previous studies showed that FDXR, primarily the mitochondrial isoform 1, plays a role in biosynthesis of sterols, heme, and iron-sulfur clusters. However, the biological functions of FDXR isoforms 3-7 have not been characterized. Here, we first examined the expression profile of various FDXR isoforms. We found that isoform 1 is the most abundant one, accounting for ~70% of total FDXR, whereas isoforms 4 and 7 account for ~10% and ~7%, respectively. We found that isoforms 1 and 4 are mainly localized in the mitochondria, whereas isoform 7, which lacks a mitochondria localization signal (MLS), is expressed in the cytosol. We also found that like the promoter 1 for isoforms 1-6, the P2 promoter for isoform 7 can be induced by DNA damage in a p53-dependent manner. To determine isoform-specific activity, we generated multiple MCF7 cell lines in which one or more FDXR isoforms are knocked out. While total FDXR-KO MCF7 cells are non-viable, cells deficient in isoforms 1-6, isoform 4, or isoform 7 remain viable but are defective in cell proliferation, DNA damage response, and repair. These data suggest that each FDXR isoform contributes to cell survival and that isoform 7 has extra-mitochondrial activity that may be sufficient for cell survival.