DIFFERENTIAL REGULATION OF IRP1 AND IRP2 BY NITRIC-OXIDE IN RAT HEPATOMA-CELLS

Iron-regulatory proteins (IRP1 and IRP2) are RNA-binding proteins that bind to stem-loop structures known as iron-responsive elements (IREs) . IREs are located in the 5'- or 3'-untranslated regions (UTRs) of spe cific mRNAs that encode proteins involved in iron homeostasis, The bin ding of IRPs to 5' IREs represses translation of the mRNA, whereas the binding of IRPs to 3' IREs stabilizes the mRNA. IRP1 and IRP2 IRE bin ding activities are regulated by intracellular iron levels, In additio n, nitric oxide (NO.) increases the affinity of IRP1 for IREs. The rol e of NO. in the regulation of IRP1 and IRP2 in rat hepatoma cells was investigated by using the NO.-generating compound S-nitroso-N-acetylpe nicillamine (SNAP), or by stimulating cells with multiple cytokines an d lipopolysaccharide (LPS) to induce NO. production, Mitochondrial and IRP1 aconitase activities were decreased in cells producing NO.. NO. increased IRE binding activity of IRP1, but had no effect on IRE bindi ng activity of IRP2, The increase in IRE binding activity of IRP1 was coincident with the translational repression of ferritin synthesis. Tr ansferrin receptor (TfR) mRNA levels were increased in cells treated w ith NO.-generating compounds, but not in cytokine-and LPS-treated cell s. Our data indicate that IRP1 and IRP2 are differentially regulated b y NO. in rat hepatoma cells, suggesting a role for IRP1 in the regulat ion of iron homeostasis in vivo during hepatic inflammation. (C) 1996 by The American Society of Hematology.