Reciprocal modulation of aconitase activity and RNA-binding activity of iron regulatory factor by nitric oxide

Inhibition of aconitase in citrus fruit callus results in a metabolic shift towards amino acid biosynthesis

Citrate, a major determinant of citrus fruit quality, accumulates early in fruit development and declines towards maturation. The isomerization of citrate to isocitrate, catalyzed by aconitase is a key step in acid metabolism. Inhibition of mitochondrial aconitase activity early in fruit development contributes to acid accumulation, whereas increased cytosolic activity of aconitase causes citrate decline. It was previously hypothesized that the block in mitochondrial aconitase activity, inducing acid accumulation, is caused by citramalate. Here, we investigated the effect of citramalate and of another aconitase inhibitor, oxalomalate, on aconitase activity and regulation in callus originated from juice sacs. These compounds significantly increased citrate content and reduced the enzyme's activity, while slightly inducing its protein level. Citramalate inhibited the mitochondrial, but not cytosolic form of the enzyme. Its external application to mandarin fruits resulted in inhibition of aconitase activity, with a transient increase in fruit acidity detected a few weeks later. The endogenous level of citramalate was analyzed in five citrus varieties: its pattern of accumulation challenged the notion of its action as an endogenous inhibitor of mitochondrial aconitase. Metabolite profiling of oxalomalate-treated cells showed significant increases in a few amino acids and organic acids. The activities of alanine transaminase, aspartate transaminase and aspartate kinase, as well as these of two γ-aminobutyrate (GABA)-shunt enzymes, succinic semialdehyde reductase (SSAR) and succinic semialdehyde dehydrogenase (SSAD) were significantly induced in oxalomalate-treated cells. It is suggested that the increase in citrate, caused by aconitase inhibition, induces amino acid synthesis and the GABA shunt, in accordance with the suggested fate of citrate during the acid decline stage in citrus fruit.

Alteration of iron regulatory proteins (IRP1 and IRP2) and ferritin in the brains of scrapie-infected mice

Considerable evidence suggests that oxidative stress may be involved in the pathogenesis of Transmissible Spongiform Encephalopathies (TSEs). To investigate the involvement of iron metabolism in TSEs, we examined the expression levels of iron regulatory proteins (IRPs), ferritins, and binding activities of IRPs to iron-responsive element (IRE) in scrapie-infected mice. We found that the IRPs-IRE-binding activities and ferritins were increased in the astrocytes of hippocampus and cerebral cortex in the brains of scrapie-infected mice. These results suggest that alteration of iron metabolism contributes to development of neurodegeneration and that some protective mechanisms against iron-induced oxidative damage may occur during the pathogenesis of TSEs.

Role of the ferroportin iron-responsive element in iron and nitric oxide dependent gene regulation

The newly described iron transporter, ferroportin (MTP1, IREG1), is expressed in a variety of tissues including the duodenum and cells of the mononuclear phagocyte system (MPS). In the MPS, ferroportin is hypothesized to be a major exporter of iron scavenged from senescent erythrocytes. Changes in iron metabolism, including the sequestration of iron in the MPS, are characteristic of both acute and chronic inflammation and these conditions induce changes in ferroportin expression. In a mouse model of acute inflammation, LPS administration is associated with reduced MPS ferroportin protein and mRNA expression. In addition, the ferroportin 5' UTR also has an iron-responsive element that binds to the iron-response proteins, but whether there is a role for this IRE in inflammation induced regulation of ferroportin has been unclear. A luciferase reporter gene under the control of the mouse ferroportin promoter and 5' UTR was used to determine if this 5' UTR conferred IRE-dependent regulation on this reporter gene. Stimulation of reporter gene transfected RAW 264.7 cells (a mouse macrophage cell line) with LPS resulted in IRE-dependent inhibition of luciferase production. Inhibitors of nitric oxide synthase abrogated the IRE-dependent effect of LPS. In addition, direct treatment of RAW 264.7 and with NO donor S-nitroso-N-acetylpenicillamine resulted in IRE-dependent down-regulation of luciferase expression. The effect of NO was consistent with IRP1/IRE mediated translation block. There are most likely both inflammation-mediated transcriptional and post-transcriptional (IRE-dependent) mechanisms for inhibiting ferroportin expression in MPS cells.

Characterization of the translation-dependent step during iron-regulated decay of transferrin receptor mRNA

Iron regulates the stability of the mRNA encoding the transferrin receptor (TfR). When iron is scarce, iron regulatory proteins (IRPs) stabilize TfR mRNA by binding to the 3'-untranslated region. High levels of iron induce degradation of TfR mRNA; the translation inhibitor cycloheximide prevents this. To distinguish between cotranslational mRNA decay and a trans effect of translation inhibitors, we designed a reporter system exploiting the properties of the selectable marker gene thymidine kinase (TK). The 3'-untranslated region of human transferrin receptor, which contains all elements necessary for iron-dependent regulation of mRNA stability, was fused to the TK cDNA. In stably transfected mouse fibroblasts, the expression of the reporter gene was perfectly regulated by iron. Introduction of stop codons in the TK coding sequence or insertion of stable stem-loop structures in the leader sequence did not affect on the iron-dependent regulation of the reporter mRNA. This implies that global translation inhibitors stabilize TfR mRNA in trans. Cycloheximide prevented the destabilization of TfR mRNA only in the presence of active IRPs. Inhibition of IRP inactivation by cycloheximide or by the specific proteasome inhibitor MG132 correlated with the stabilization of TfR mRNA. These observations suggest that inhibition of translation by cycloheximide interferes with the rate-limiting step of iron-induced TfR mRNA decay in a trans-acting mechanism by blocking IRP inactivation.