Targeting of the mitochondrial membrane proteins to the cell surface for functional studies

Iron Promotes Cardiac Doxorubicin Retention and Toxicity Through Downregulation of the Mitochondrial Exporter ABCB8

In several cancers, the efflux and resistance against doxorubicin (DOX), an effective anticancer drug, are associated with cellular iron deficiency and overexpression of the mitochondrial exporter ABCB8. Conversely, decreased ABCB8 expression and disrupted iron homeostasis in the heart have been implicated in DOX-associated cardiotoxicity. While studies have demonstrated that altered iron status can modulate the susceptibility to DOX cardiotoxicity, the exact molecular mechanisms have not been clearly understood. Here, we hypothesized that iron stores influence cardiac ABCB8 expression and consequently cardiac retention and toxicity of DOX. First, we found that ABCB8 deficiency in cardiomyocytes decreased DOX efflux, increased DOX-induced toxicity, and decreased cell viability. Conversely, intracellular DOX retention and toxicity were ameliorated by ABCB8 overexpression. To determine if altered cardiac iron status modifies ABCB8 expression, we treated cardiomyocytes with high iron or iron chelators. Western blot and qPCR analyses revealed that ABCB8 levels were decreased in iron overload and increased in iron deficiency. Subsequently, DOX retention and toxicity were increased in cardiomyocytes with iron overload, whereas iron deficiency ameliorated these effects. Next, we validated our results using a mouse model of hereditary hemochromatosis (HH), a genetic iron overload disorder. HH mice exhibited decreased ABCB8 expression and increased DOX retention and toxicity. These changes were abolished by the treatment of HH mice with a low-iron diet. Finally, cardiac-specific overexpression of ABCB8 in HH mice prevented cardiac DOX accumulation and abrogated DOX-induced cardiotoxicity without altering iron overload in the heart. Together, our results demonstrate that ABCB8 mediates DOX efflux and that iron regulates DOX retention and toxicity by altering cardiac ABCB8 expression. Our study identifies a novel role of iron in DOX-induced cardiotoxicity and suggests potential therapeutic intervention for DOX and anthracycline-based cancer pharmacology.

Surface translocator protein 18 kDa (TSPO) localization on immune cells upon stimulation with LPS and in ART-treated HIV+ subjects

Translocator protein 18 kDa (TSPO) is a well-known outer mitochondrial membrane protein and it is widely used as a biomarker of neuroinflammation and brain injury. Although it is thought that TSPO plays key roles in a multitude of host cell functions, including steroid biosynthesis, apoptosis, generation of reactive oxygen species, and proliferation, some of these functions have recently been questioned. Here, we report the unexpected finding that circulating immune cells differentially express basal levels of TSPO on their cell surface, with a high percentage of monocytes and neutrophils expressing cell surface TSPO. In vitro stimulation of monocytes with LPS significantly increases the frequency of cells with surface TSPO expression in the absence of altered gene expression. Importantly, the LPS increase in TSPO cell surface expression in monocytes appears to be selective for LPS because two other distinct monocyte activators failed to increase the frequency of cells with surface TSPO. Finally, when we quantified immune cell TSPO surface expression in antiretroviral therapy-treated HIV⁺ donors, a chronic inflammatory disease, we found significant increases in the frequency of TSPO surface localization, which could be pharmacologically suppressed with ∆⁹ -tetrahydrocannabinol. These findings suggest that cell surface TSPO in circulating leukocytes could serve as a peripheral blood-based biomarker of inflammation.

20,000 picometers under the OMM: diving into the vastness of mitochondrial metabolite transport

The metabolic compartmentalization enabled by mitochondria is key feature of many cellular processes such as energy conversion to ATP production, redox balance, and the biosynthesis of heme, urea, nucleotides, lipids, and others. For a majority of these functions, metabolites need to be transported across the impermeable inner mitochondrial membrane by dedicated carrier proteins. Here, we examine the substrates, structural features, and human health implications of four mitochondrial metabolite carrier families: the SLC25A family, the mitochondrial ABCB transporters, the mitochondrial pyruvate carrier (MPC), and the sideroflexin proteins.

An uncharacterized region within the N-terminus of mouse TMC1 precludes trafficking to plasma membrane in a heterologous cell line

Mechanotransduction by hair cell stereocilia lies at the heart of sound detection in vertebrates. Considerable effort has been put forth to identify proteins that comprise the hair cell mechanotransduction apparatus. TMC1, a member of the transmembrane channel-like (TMC) family, was identified as a core protein of the mechanotransduction complex in hair cells. However, the inability of TMC1 to traffic through the endoplasmic reticulum in heterologous cellular systems has hindered efforts to characterize its function and fully identify its role in mechanotransduction. We developed a novel approach that allowed for the detection of uncharacterized protein regions, which preclude trafficking to the plasma membrane (PM) in heterologous cells. Tagging N-terminal fragments of TMC1 with Aquaporin 3 (AQP3) and GFP fusion reporter, which intrinsically label PM in HEK293 cells, indicated that residues at the edges of amino acid sequence 138–168 invoke intracellular localization and/or degradation. This signal is able to preclude surface localization of PM protein AQP3 in HEK293 cells. Substitutions of the residues by alanine or serine corroborated that the information determining the intracellular retention is present within amino acid sequence 138–168 of TMC1 N-terminus. This novel signal may preclude the proper trafficking of TMC1 to the PM in heterologous cells.

Neuropilin-1 Controls Endothelial Homeostasis by Regulating Mitochondrial Function and Iron-Dependent Oxidative Stress

The transmembrane protein neuropilin-1 (NRP1) promotes vascular endothelial growth factor (VEGF) and extracellular matrix signaling in endothelial cells (ECs). Although it is established that NRP1 is essential for angiogenesis, little is known about its role in EC homeostasis. Here, we report that NRP1 promotes mitochondrial function in ECs by preventing iron accumulation and iron-induced oxidative stress through a VEGF-independent mechanism in non-angiogenic ECs. Furthermore, NRP1-deficient ECs have reduced growth and show the hallmarks of cellular senescence. We show that a subcellular pool of NRP1 localizes in mitochondria and interacts with the mitochondrial transporter ATP-binding cassette B8 (ABCB8). NRP1 loss reduces ABCB8 levels, resulting in iron accumulation, iron-induced mitochondrial superoxide production, and iron-dependent EC senescence. Treatment of NRP1-deficient ECs with the mitochondria-targeted antioxidant compound mitoTEMPO or with the iron chelator deferoxamine restores mitochondrial activity, inhibits superoxide production, and protects from cellular senescence. This finding identifies an unexpected role of NRP1 in EC homeostasis.

•

A subcellular pool of NRP1 localizes in the mitochondria of endothelial cells (ECs)

•

NRP1 regulates mitochondrial function via ABCB8 transporter

•

NRP1 loss induces iron accumulation and iron-dependent oxidative stress in ECs

•

NRP1 protects ECs from iron-dependent premature cellular senescence

Structures and functions of mitochondrial ABC transporters

A small number of physiologically important ATP-binding cassette (ABC) transporters are found in mitochondria. Most are half transporters of the B group forming homodimers and their topology suggests they function as exporters. The results of mutant studies point towards involvement in iron cofactor biosynthesis. In particular, ABC subfamily B member 7 (ABCB7) and its homologues in yeast and plants are required for iron-sulfur (Fe-S) cluster biosynthesis outside of the mitochondria, whereas ABCB10 is involved in haem biosynthesis. They also play a role in preventing oxidative stress. Mutations in ABCB6 and ABCB7 have been linked to human disease. Recent crystal structures of yeast Atm1 and human ABCB10 have been key to identifying substrate-binding sites and transport mechanisms. Combined with in vitro and in vivo studies, progress is being made to find the physiological substrates of the different mitochondrial ABC transporters.

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.

Profiling of ABC transporters during active ulcerative colitis and in vitro effect of inflammatory modulators

BACKGROUND AND AIM

Inflammatory bowel disease is characterized by chronic inflammation of the gastro intestinal tract that manifests as ulcerative colitis and Crohn's disease. Comparative expression profiles of selected ABC transporter genes during active ulcerative colitis and intestinal tuberculosis were studied, and we also investigated the effect of inflammatory modulators on the expression of the transporters in HT-29 cells.

METHODS

Using the GEO database, we selected ABC transporter genes that are differentially regulated during active UC and validated the altered expression in biopsies samples by RT-PCR. We also analyzed the effect of inflammatory modulators like TNF-α, lipopolysaccharides (LPS) and drugs (5-ASA, prednisolone and hydrocortisone) on the expression of ABCA1, ABCB8, ABCF2 and ABCC4 using HT-29 cells.

RESULTS

We observed significant up-regulation of ABCA1 and ABCA3 while ABCF2, ABCC6, ABCB8 and ABCC4 were down-regulated during UC. ABCC4 was up-regulated in ITB but down-regulated in UC, whereas others showed similar patterns both in UC and ITB. Upon stimulation of HT29 cells by TNF-α, up-regulation of ABCA1, ABCB8, ABCF2 and ABCC4 was seen, and further using inhibitors we found that it was mediated through reactive oxygen species or NF-kB or both. LPS caused a dose dependent and significant down-regulation of ABCB8, ABCF2 and ABCC4 without any effect on ABCA1. The cells treated with drugs 5-ASA, prednisolone and hydrocortisone, exhibited up-regulation of transporters only at a higher dose.

CONCLUSION

Altered expression of the above transporters may be associated with the disease. The study also hints at possible mechanisms of differential expression.

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.

Molecular identification and functional characterization of a mitochondrial sulfonylurea receptor 2 splice variant generated by intraexonic splicing

RATIONALE

Cardioprotective pathways may involve a mitochondrial ATP-sensitive potassium (mitoK(ATP)) channel but its composition is not fully understood.

OBJECTIVE

We hypothesized that the mitoK(ATP) channel contains a sulfonylurea receptor (SUR)2 regulatory subunit and aimed to identify the molecular structure.

METHODS AND RESULTS

Western blot analysis in cardiac mitochondria detected a 55-kDa mitochondrial SUR2 (mitoSUR2) short form, 2 additional short forms (28 and 68 kDa), and a 130-kDa long form. RACE (Rapid Amplification of cDNA Ends) identified a 1.5-Kb transcript, which was generated by a nonconventional intraexonic splicing (IES) event within the 4th and 29th exons of the SUR2 mRNA. The translated product matched the predicted size of the 55-kDa short form. In a knockout mouse (SUR2KO), in which the SUR2 gene was disrupted, the 130-kDa mitoSUR2 was absent, but the short forms remained expressed. Diazoxide failed to induce increased fluorescence of flavoprotein oxidation in SUR2KO cells, indicating that the diazoxide-sensitive mitoK(ATP) channel activity was associated with 130-kDa-based channels. However, SUR2KO mice displayed similar infarct sizes to preconditioned wild type, suggesting a protective role for the remaining short form-based channels. Heterologous coexpression of the SUR2 IES variant and Kir6.2 in a K(+) transport mutant Escherichia coli strain permitted improved cell growth under acidic pH conditions. The SUR2 IES variant was localized to mitochondria, and removal of a predicted mitochondrial targeting sequence allowed surface expression and detection of an ATP-sensitive current when coexpressed with Kir6.2.

CONCLUSIONS

We identify a novel SUR2 IES variant in cardiac mitochondria and provide evidence that the variant-based channel can form an ATP-sensitive conductance and may contribute to cardioprotection.