Thioredoxin activation of iron regulatory proteins - Redox regulation of RNA binding after exposure to nitric oxide

Iron regulatory proteins (IRP1 and IRP2) are redox-sensitive RNA-binding pr oteins that modulate the expression of several genes encoding key proteins of iron metabolism. IRP1 can also exist as an aconitase containing a [4Fe-4 S] cluster bound to three cysteines at the active site. We previously showe d that biosynthesis of nitric oxide (NO) induces the transition of IRP1 fro m aconitase to apoprotein able to bind RNA. This switch is also observed wh en cytosolic extracts are exposed to NO donors. However, the activation of IRP1 under these conditions is far from maximal. In this study we examined the capacity of physiological reducing systems to cooperate with NO in the activation of IRP1. Cytosolic extracts from the macrophage cell line RAW 26 4.7 or purified IRP1 were incubated with NO donors and subsequently exposed to glutathione or to thioredoxin (Trx), a strong protein disulfide reducta se, Trx was the most effective, inducing a 2-6-fold enhancement of the RNA binding activity of NO-treated IRP1. Furthermore, the effect of NO on IRP1 from cytosolic extracts was abolished in the presence of anti-Trx antibodie s. We also studied the combined effect of NO and Trx on IRP2, which exhibit s constitutive RNA binding activity. We observed an inhibition of IRP2 acti vity following exposure to NO donors which was restored by Trx. Collectivel y, these results point to a crucial role of Trx as a modulator of IRP activ ity in situations of NO production.