Faustino Bisaccia

[25] Mitochondrial metabolite carrier proteins: Purification, reconstitution, and transport studies

Publisher Summary This chapter also discusses the methods for purifying metabolite carriers from mitochondria, reconstituting them in liposomes, and their transport activities in the reconstituted system. The general purification scheme applied for the isolation of mitochondrial metabolite carriers in functional state involves solubilization by nonionic detergents; chromatography on hydroxyapatite, recovering the respective carrier in the eluate; and final purification using various further chromatographic procedures. The detailed procedures for the purification of the mitochondrial carriers for phosphate, oxoglutarate/malate, dicarboxylates, tricarboxylates (citrate), carnitine/acylcarnitines, aspartate/glutamate, and ornithine/citrulline in functionally active state are described. For functional characterization in terms of transport activity, the purified carrier proteins have to be reconstituted into liposomal membranes. The selection of methods for the functional reconstitution of mitochondrial carrier proteins is restricted by the fact that these carriers are denatured by detergents with high critical micellar concentration; thus, dialysis and related methods cannot be applied.

Identification of elastin peptides with vasorelaxant activity on rat thoracic aorta

Elastin peptides obtained in vivo from the enzymatic degradation of elastic fibers are present in the circulating human blood. In order to verify the role that these peptides may have in the regulation of the vascular tone, the activity of several peptides identified in the elastolytic digest of human elastin and some of their structural homologues has been tested. Three of these peptides show a vasorelaxant activity in isolated rat aorta precontracted by phenylephrine. The activity observed is higher in the absence of the endothelium; in these conditions the IC50 for the peptides Val-Gly-Val-Ala-Pro-Gly, Val-Gly-Val-Pro-Gly and Val-Gly-Val-Hyp-Gly was 40 +/- 2, 73 +/- 2 and 10 +/- 1 ng/ml, respectively. They are active in the range of the pathological circulating concentration and their role could be important in the regulation of vascular tone during several elastin degradative diseases.

Role of FOXA in mitochondrial citrate carrier gene expression and insulin secretion

In this study, we have investigated the transcriptional role of the FOXA site present in the promoter of the mitochondrial citrate carrier (CIC) gene. We have shown that wild-type (but not mutated) CIC FOXA site cloned in front of the luciferase promoter confers transcriptional activation of the gene reporter, particularly in cells overexpressing FOXA1. We have also demonstrated that overexpression and silencing of FOXA increases and reduces CIC transcript and protein levels, respectively. In addition, FOXA1 silencing in INS-1 cells decreases not only CIC mRNA and protein but also the amount of citrate in the cytosol and glucose-stimulated insulin secretion. These results show that FOXA plays a role in the transcriptional regulation of CIC and in insulin secretion.

Novel properties of peptides derived from the sequence coded by exon 26A of human elastin

The exon 26A is a rarely expressed human elastin exon that codes for a hydrophilic and charged amino acid sequence. The functional role of elastin containing this additional sequence is unknown. The present investigation was aimed to determine the effect of synthetic peptides derived from this exon on the vascular tone of rat thoracic aorta. On phenilephrine-preconstricted rat thoracic aortic rings the peptides LSPELREGD and REGD cause dose-dependent relaxation in the concentration range from 10(-9) to 10(-5) M. omega-nitro-L-arginine methyl ester, a known inhibitor of the NO synthase, highly inhibits, although to a different extent, the relaxation induced by these peptides. Removal of endothelium and blocking of ATP-sensitive potassium channels by glibenclamide significantly inhibited the vasorelaxant activity of LSPELREGD but not that of REGD, suggesting a different mechanism of action and possibly a different receptor.

The mitochondrial oxoglutarate carrier: from identification to mechanism

The 2-oxoglutarate carrier (OGC) belongs to the mitochondrial carrier protein family whose members are responsible for the exchange of metabolites, cofactors and nucleotides between the cytoplasm and mitochondrial matrix. Initially, OGC was characterized by determining substrate specificity, kinetic parameters of transport, inhibitors and molecular probes that form covalent bonds with specific residues. It was shown that OGC specifically transports oxoglutarate and certain carboxylic acids. The substrate specificity combination of OGC is unique, although many of its substrates are also transported by other mitochondrial carriers. The abundant recombinant expression of bovine OGC in Escherichia coli and its ability to functionally reconstitute into proteoliposomes made it possible to deduce the individual contribution of each and every residue of OGC to the transport activity by a complete set of cys-scanning mutants. These studies give experimental support for a substrate binding site constituted by three major contact points on the even-numbered α-helices and identifies other residues as important for transport function through their crucial positions in the structure for conserved interactions and the conformational changes of the carrier during the transport cycle. The results of these investigations have led to utilize OGC as a model protein for understanding the transport mechanism of mitochondrial carriers.

Role of FOXA and Sp1 in mitochondrial acylcarnitine carrier gene expression in different cell lines.

This study investigates the transcriptional role of the human mitochondrial carnitine/acylcarnitine carrier (CAC) proximal promoter. Through deletion analysis, an activation domain (-334/-80 bp) was identified which contains FOXA and Sp1 active sites. The wild-type (but not mutated) -334/-80 bp region of the CAC gene conferred 74% LUC transgene activity in HepG2 cells, 17% in HEK293 cells and 14% in SK-N-SH cells as compared to that observed with the entire -1503/+3 bp proximal promoter. Overexpression and silencing of FOXA2 or Sp1 in HepG2 cells enhanced and diminished, respectively, LUC activity, CAC transcript and CAC protein. In HEK293 and SK-N-SH cells, which do not contain FOXA1-3, LUC activity was increased by FOXA2 overexpression to a greater extent than in HepG2 cells. Both FOXA2 and Sp1 in HepG2, and only Sp1 in HEK293 and SK-N-SH cells, were found to be bound to the CAC proximal promoter. These results show that FOXA and Sp1 sites in HepG2 cells and only the Sp1 site in HEK293 and SK-N-SH cells have a critical role in the transcriptional regulation of the CAC proximal promoter.

Antioxidant and Proapoptotic Activities of Sclerocarya birrea [(A. Rich.) Hochst.] Methanolic Root Extract on the Hepatocellular Carcinoma Cell Line HepG2

The main goal of this study was to characterize the in vitro antioxidant activity and the apoptotic potential of S. birrea methanolic root extract (MRE). Among four tested extracts, obtained with different solvents, MRE showed the highest content of polyphenols, flavonoids, and tannins together with antioxidant activities tested with superoxide, nitric oxide, ABTS, and beta-carotene bleaching assays. Moreover, the cytotoxic effect of MRE was evaluated on the hepatocarcinoma cell line HepG2. In these cells, MRE treatment induced apoptosis and generated reactive oxygen species (ROS) in dose-dependent manner. The cytotoxic effect promoted by MRE was prevented by pretreatment of HepG2 cells with N-acetyl-L-cysteine (NAC), suggesting that oxidative stress was pivotal in MRE-mediated cell death. Moreover, we showed that the MRE treatment induced the mitochondrial membrane depolarization and the cytochrome c release from mitochondria into the cytosol. It suggests that the apoptosis occurred in a mitochondrial-dependent pathway. Interestingly, MRE showed a sensibly lower cytotoxicity, associated with a low increase of ROS, in normal human dermal fibroblasts compared to HepG2 cells. It is suggested that the methanolic root extract of S. Birrea is able to selectively increase intracellular ROS levels in cancer cells, promoting cell death.

Dysregulation of gene expression in ABCC6 knockdown HepG2 cells.

ABCC6 protein is an ATP-dependent transporter that is mainly found in the basolateral plasma membrane of hepatocytes. ABCC6 deficiency is the primary cause of several forms of ectopic mineralization syndrome. Mutations in the human ABCC6 gene cause pseudoxanthoma elasticum (PXE), an autosomal recessive disease characterized by ectopic calcification of the elastic fibers in dermal, ocular and vascular tissues. Mutations in the mouse ABCC6 gene were also associated with dystrophic cardiac calcification. Reduced levels of ABCC6 protein were found in a β-thalassemic mouse model. Moreover, some cases of generalized arterial calcification in infancy are due to ABCC6 mutations. In order to study the role of ABCC6 in the pathogenesis of ectopic mineralization, the expressions of genes involved in this process were evaluated in HepG2 cells upon stable knockdown of ABCC6 by small hairpin RNA (shRNA) technology. ABCC6 knockdown in HepG2 cells causes a significant upregulation of the genes promoting mineralization, such as TNAP, and a parallel downregulation of genes with anti-mineralization activity, such as NT5E, Fetuin A and Osteopontin. Although the absence of ABCC6 has been already associated with ectopic mineralization syndromes, this study is the first to show a direct relationship between reduced ABCC6 levels and the expression of pro-mineralization genes in hepatocytes.

The nucleotide-binding domain 2 of the human transporter protein MRP6

Multidrug-resistance-associated protein 6 (MRP6/ABCC6) belongs to the ABC transporter family, whose members share many characteristic features including membrane domains and two nucleotide-binding domains (NBD1 and NBD2). These function cooperatively to bind and hydrolyze ATP for the transport of substrates across biological membranes. In this study, MRP6-NBD2 (residues 1252–1503) was expressed in Escherichia coli, purified and structurally and functionally characterized. CD spectra suggested that the protein is folded. Furthermore, NBD2 is shown to be biologically active as it binds ATP and presents ATPase activity although significantly lower compared with isolated NBD1. The mixture of NBD2 and NBD1 exhibited an activity similar to the NBD2 alone, indicating that NBD1 and NBD2 form a heterodimer with the latter limiting ATP hydrolysis. These findings suggest that NBD1 has a higher tendency to form an active homodimer, which is also supported by in silico analysis of energy-minimized dimers of the homology models of the two domains.

Solution structure of the first and second transmembrane segments of the mitochondrial oxoglutarate carrier

The structures of the first and the second transmembrane segment of the bovine mitochondrial oxoglutarate carrier (OGC) were studied by circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopies. Peptides 21–46 and 78–108 of its primary sequence were synthesized and structurally characterized in membrane-mimetic environments. CD data showed that at high concentrations of TFE (>50%) and SDS (>2%) both peptides assume α-helical structures, whereas in more hydrophilic environments only peptide 78–108 has a helical structure. 1H-NMR spectra of the two peptides in TFE/water and SDS were fully assigned, and the secondary structures of the peptides were obtained from nuclear Overhauser effects, 3JαH-NH coupling constants and αH chemical shifts. The three-dimensional solution structures of the peptides in TFE/water were generated by distance geometry calculations. A well-defined α-helix was found in the region K24-V39 of peptide 21–46 and in the region A86–F106 of peptide 78–108. We cannot exclude that in intact OGC the extension of these helices is longer. The helix of peptide 21–46 is essentially hydrophobic, whereas that of peptide 78–108 is predominantly hydrophilic.