Hepatic tristetraprolin promotes insulin resistance through RNA destabilization of FGF21.

The role of posttranscriptional metabolic gene regulatory programs in diabetes is not well understood. Here, we show that the RNA-binding protein tristetraprolin (TTP) is reduced in the livers of diabetic mice and humans and is transcriptionally induced in response to insulin treatment in murine livers in vitro and in vivo. Liver-specific Ttp-KO (lsTtp-KO) mice challenged with high-fat diet (HFD) have improved glucose tolerance and peripheral insulin sensitivity compared with littermate controls. Analysis of secreted hepatic factors demonstrated that fibroblast growth factor 21 (FGF21) is posttranscriptionally repressed by TTP. Consistent with increased FGF21, lsTtp-KO mice fed HFD have increased brown fat activation, peripheral tissue glucose uptake, and adiponectin production compared with littermate controls. Downregulation of hepatic Fgf21 via an adeno-associated virus-driven shRNA in mice fed HFD reverses the insulin-sensitizing effects of hepatic Ttp deletion. Thus, hepatic TTP posttranscriptionally regulates systemic insulin sensitivity in diabetes through liver-derived FGF21.

mRNA-binding protein tristetraprolin is essential for cardiac response to iron deficiency by regulating mitochondrial function.

Cells respond to iron deficiency by activating iron-regulatory proteins to increase cellular iron uptake and availability. However, it is not clear how cells adapt to conditions when cellular iron uptake does not fully match iron demand. Here, we show that the mRNA-binding protein tristetraprolin (TTP) is induced by iron deficiency and degrades mRNAs of mitochondrial Fe/S-cluster-containing proteins, specifically Ndufs1 in complex I and Uqcrfs1 in complex III, to match the decrease in Fe/S-cluster availability. In the absence of TTP, Uqcrfs1 levels are not decreased in iron deficiency, resulting in nonfunctional complex III, electron leakage, and oxidative damage. Mice with deletion of Ttp display cardiac dysfunction with iron deficiency, demonstrating that TTP is necessary for maintaining cardiac function in the setting of low cellular iron. Altogether, our results describe a pathway that is activated in iron deficiency to regulate mitochondrial function to match the availability of Fe/S clusters.

Short communication: high cellular iron levels are associated with increased HIV infection and replication.

HIV is a pandemic disease, and many cellular and systemic factors are known to alter its infectivity and replication. Earlier studies had suggested that anemia is common in HIV-infected patients; however, higher iron was also observed in AIDS patients prior to the introduction of antiretroviral therapy (ART). Therefore, the relationship between iron and viral infection is not well delineated. To address this issue, we altered the levels of cellular iron in primary CD4(+) T cells and showed that higher iron is associated with increased HIV infection and replication. In addition, HIV infection alone leads to increased cellular iron, and several ART drugs increase cellular iron independent of HIV infection. Finally, HIV infection is associated with increased serum iron in HIV-positive patients regardless of treatment with ART. These results establish a relationship between iron and HIV infection and suggest that iron homeostasis may be a viable therapeutic target for HIV.

Cardiotoxicity of doxorubicin is mediated through mitochondrial iron accumulation.

Doxorubicin is an effective anticancer drug with known cardiotoxic side effects. It has been hypothesized that doxorubicin-dependent cardiotoxicity occurs through ROS production and possibly cellular iron accumulation. Here, we found that cardiotoxicity develops through the preferential accumulation of iron inside the mitochondria following doxorubicin treatment. In isolated cardiomyocytes, doxorubicin became concentrated in the mitochondria and increased both mitochondrial iron and cellular ROS levels. Overexpression of ABCB8, a mitochondrial protein that facilitates iron export, in vitro and in the hearts of transgenic mice decreased mitochondrial iron and cellular ROS and protected against doxorubicin-induced cardiomyopathy. Dexrazoxane, a drug that attenuates doxorubicin-induced cardiotoxicity, decreased mitochondrial iron levels and reversed doxorubicin-induced cardiac damage. Finally, hearts from patients with doxorubicin-induced cardiomyopathy had markedly higher mitochondrial iron levels than hearts from patients with other types of cardiomyopathies or normal cardiac function. These results suggest that the cardiotoxic effects of doxorubicin develop from mitochondrial iron accumulation and that reducing mitochondrial iron levels protects against doxorubicin-induced cardiomyopathy.

When less is more: novel mechanisms of iron conservation.

Disorders of iron homeostasis are very common, yet the molecular mechanisms of iron regulation remain understudied. Over 20 years have passed since the first characterization of iron-regulatory proteins (IRP) as mediators of cellular iron-deficiency response in mammals through iron acquisition. However, little is known about other mechanisms necessary for adaptation to low-iron states. In this review, we present recent evidence that establishes the existence of a new iron-regulatory pathway aimed at iron conservation and optimization of iron use through suppression of nonessential iron-consuming processes. Moreover, we discuss the possible links between iron homeostasis and energy metabolism uncovered by studies of iron-deficiency response.

Heme levels are increased in human failing hearts.

OBJECTIVES: The goal of this study was to characterize the regulation of heme and non-heme iron in human failing hearts. BACKGROUND: Iron is an essential molecule for cellular physiology, but in excess it facilitates oxidative stress. Mitochondria are the key regulators of iron homeostasis through heme and iron-sulfur cluster synthesis. Because mitochondrial function is depressed in failing hearts and iron accumulation can lead to oxidative stress, we hypothesized that iron regulation may also be impaired in heart failure (HF). METHODS: We measured mitochondrial and cytosolic heme and non-heme iron levels in failing human hearts retrieved during cardiac transplantation surgery. In addition, we examined the expression of genes regulating cellular iron homeostasis, the heme biosynthetic pathway, and micro-RNAs that may potentially target iron regulatory networks. RESULTS: Although cytosolic non-heme iron levels were reduced in HF, mitochondrial iron content was maintained. Moreover, we observed a significant increase in heme levels in failing hearts, with corresponding feedback inhibition of the heme synthetic enzymes and no change in heme degradation. The rate-limiting enzyme in heme synthesis, delta-aminolevulinic acid synthase 2 (ALAS2), was significantly upregulated in HF. Overexpression of ALAS2 in H9c2 cardiac myoblasts resulted in increased heme levels, and hypoxia and erythropoietin treatment increased heme production through upregulation of ALAS2. Finally, increased heme levels in cardiac myoblasts were associated with excess production of reactive oxygen species and cell death, suggesting a maladaptive role for increased heme in HF. CONCLUSIONS: Despite global mitochondrial dysfunction, heme levels are maintained above baseline in human failing hearts.

mTOR regulates cellular iron homeostasis through tristetraprolin.

Iron is an essential cofactor with unique redox properties. Iron-regulatory proteins 1 and 2 (IRP1/2) have been established as important regulators of cellular iron homeostasis, but little is known about the role of other pathways in this process. Here we report that the mammalian target of rapamycin (mTOR) regulates iron homeostasis by modulating transferrin receptor 1 (TfR1) stability and altering cellular iron flux. Mechanistic studies identify tristetraprolin (TTP), a protein involved in anti-inflammatory response, as the downstream target of mTOR that binds to and enhances degradation of TfR1 mRNA. We also show that TTP is strongly induced by iron chelation, promotes downregulation of iron-requiring genes in both mammalian and yeast cells, and modulates survival in low-iron states. Taken together, our data uncover a link between metabolic, inflammatory, and iron-regulatory pathways, and point toward the existence of a yeast-like TTP-mediated iron conservation program in mammals.

Disruption of ATP-binding cassette B8 in mice leads to cardiomyopathy through a decrease in mitochondrial iron export.

Mitochondrial iron levels are tightly regulated, as iron is essential for the synthesis of Fe/S clusters and heme in the mitochondria, but high levels can cause oxidative stress. The ATP-binding cassette (ABC) transporter ABCB8 is a mitochondrial inner membrane protein with an unknown function. Here, we show that ABCB8 is involved in mitochondrial iron export and is essential for baseline cardiac function. Induced genetic deletion of ABCB8 in mouse hearts resulted in mitochondrial iron accumulation and cardiomyopathy, as assessed by echocardiography and invasive hemodynamics. Mice with ABCB8 deletion in the heart also displayed mitochondrial damage, and higher levels of reactive oxygen species and cell death. Down-regulation of ABCB8 in vitro resulted in decreased iron export from isolated mitochondria, whereas its overexpression had the opposite effect. Furthermore, ABCB8 is needed for the maturation of the cytosolic Fe/S proteins, as its deletion in vitro and in vivo led to decreased activity of cytosolic, but not mitochondrial, iron-sulfur-containing enzymes. These results indicate that ABCB8 is essential for normal cardiac function, maintenance of mitochondrial iron homeostasis and maturation of cytosolic Fe/S proteins. In summary, this report provides characterization of a protein involved in mitochondrial iron export.