Modulation of insulin degrading enzyme activity and liver cell proliferation

Diabetes mellitus type 2 (T2DM), insulin therapy, and hyperinsulinemia are independent risk factors of liver cancer. Recently, the use of a novel inhibitor of insulin degrading enzyme (IDE) was proposed as a new therapeutic strategy in T2DM. However, IDE inhibition might stimulate liver cell proliferation via increased intracellular insulin concentration. The aim of this study was to characterize effects of inhibition of IDE activity in HepG2 hepatoma cells and to analyze liver specific expression of IDE in subjects with T2DM. HepG2 cells were treated with 10 nM insulin for 24 h with or without inhibition of IDE activity using IDE RNAi, and cell transcriptome and proliferation rate were analyzed. Human liver samples (n = 22) were used for the gene expression profiling by microarrays. In HepG2 cells, IDE knockdown changed expression of genes involved in cell cycle and apoptosis pathways. Proliferation rate was lower in IDE knockdown cells than in controls. Microarray analysis revealed the decrease of hepatic IDE expression in subjects with T2DM accompanied by the downregulation of the p53-dependent genes FAS and CCNG2, but not by the upregulation of proliferation markers MKI67, MCM2 and PCNA. Similar results were found in the liver microarray dataset from GEO Profiles database. In conclusion, IDE expression is decreased in liver of subjects with T2DM which is accompanied by the dysregulation of p53 pathway. Prolonged use of IDE inhibitors for T2DM treatment should be carefully tested in animal studies regarding its potential effect on hepatic tumorigenesis.

FGB mutations leading to congenital quantitative fibrinogen deficiencies: an update and report of four novel mutations

INTRODUCTION

Causative mutations leading to congenital quantitative fibrinogen are frequently clustered in FGA encoding the fibrinogen Aα-chain. Mutations of FGB encoding the Bβ-chain are less common and of interest since the Bβ-chain is considered the rate-limiting factor in the hepatic production of the fibrinogen hexamer.

METHOD

Four novel FGB mutations were identified in two afibrinogenemic (one new-born and one 30 years old male) and hypofibrinogenemic (a 49 years old female) patient, with heterogeneous thrombotic and bleeding phenotype. The human fibrinogen beta chain precursor protein sequence (P02675) was obtained from the UniProt database. The resulting models were analysed in SwissPdbViewer 4.1 and POV-Ray 3.7.

RESULTS

The FGB c.895T>C p.Y299H (numbering from the initiator Met) and the FGB c.1415G>T p.G472V were predicted to be deleterious by SIFT analysis. The first replaces an uncharged aromatic amino acid side chain by a positively charged side chain modifying the balance in the distribution of hydrophobic and hydrophilic of the 10 Å neighbourhood residues. The second replaces one non-charged aliphatic side chain by another without any changes for the 10 Å surrounding region. The FGB c.352C>T p.Q118X leads to a severe premature termination codon and the FGB intron 4: IVS4-1G>C (c719-1G>C) leads to skipping of exon 5 or usage of a cryptic acceptor site located upstream or downstream of the normal site.

CONCLUSIONS

The continuous characterization of novel molecular defects responsible for fibrinogen deficiency combined with modelling of mutant proteins will continue to provide a better comprehension of the complexity of fibrinogen synthesis and physiology.

HIV-1 Nef perturbs artificial membranes: investigation of the contribution of the myristoyl anchor

Nef, an accessory protein from human immunodeficiency virus type 1, is critical for optimal viral replication and pathogenesis. Here, we analyzed the influence of full-length myristoylated and nonmyristoylated Nef on artificial lipid bilayers composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). By means of cosedimentation assays, we found that neither nonmyristoylated nor myristoylated Nef stably binds to POPC unilamellar vesicles. Time-resolved ellipsometry rather indicates that the proteins perturb the assembly of POPC planar bilayers. This observation was corroborated by fluorescence and scanning force microscopy, suggesting that membrane disordering occurs upon interaction of full-length myristoylated and nonmyristoylated Nef with planar POPC membranes immobilized on SiO(2) surfaces resulting in loss of material from the surface. The membrane perturbations were further investigated by vesicle release experiments, demonstrating that the disordering results in defects through which the fluorophor carboxyfluorescein can pass. From these results, we conclude that Nef is capable of disordering and perturbing lipid membranes and that the myristoyl group is not the decisive determinant for the action of the protein on lipid membranes.

Fodrin inhibits phospholipases A2, C, and D by decreasing polyphosphoinositide cell content

Brain fodrin inhibited in a dose dependent manner the GTPgammaS-stimulated cytosolic PLA2 (cPLA2), PLC, and PLD activities in differentiated HL-60 cells permeabilized with streptolysin O. cPLA2 and PLD were inhibited by the same concentrations of fodrin (IC50=1.5-2 nM) but PLC was inhibited by lower concentrations (IC50=0.3 nM). Moreover, the rates of inhibition were different between the phospholipases. Spectrin, which shares 50% homology with fodrin, had similar effects on the three phospholipases. However, using cytosol-depleted cells or recombinant PLD1, we showed that fodrin was not a direct inhibitor. Studying the potential mechanisms of these inhibitions, we demonstrated that a major decrease in membrane phosphatidylinositol 4-monophosphate (PtdIns(4)P) and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) amounts was induced by fodrin. Exogenous PtdIns(4,5)P2 partly reversed fodrin inhibition of GTPgammaS-stimulated phospholipase C activity. Hence, inhibition of PLC, cPLA2, and PLD activities observed with fodrin could be related to the decrease of PtdIns(4,5)P2, substrate of PLC, a cofactor of PLD and an enhancer of cPLA2 activity.

Inhibition of cytosolic phospholipase A2 by annexin V in differentiated permeabilized HL-60 cells. Evidence of crucial importance of domain I type II Ca2+-binding site in the mechanism of inhibition

Annexin V belongs to a family of proteins that interact with phospholipids in a Ca2+-dependent manner. This protein has been demonstrated to have anti-phospholipase A2 activity. However, this effect has never yet been reported with the 85-kDa cytosolic PLA2 (cPLA2). We studied, in a model of differentiated and streptolysin O-permeabilized HL-60 cells, the effect of annexin V on cPLA2 activity after stimulation by calcium, GTPgammaS (guanosine 5'-O-(3-thiotriphosphate)), formyl-Met-Leu-Phe, or phorbol 12-myristate 13-acetate. Both recombinant and human placental purified annexin V inhibit cPLA2 activity whatever the stimulus used. The decrease of arachidonic acid release is of 40 and 50%, respectively, at [Ca2+] of 3 and 10 microM. The mechanism of inhibition was also analyzed. cPLA2 requires calcium and protein kinase C (PKC) or mitogen-activated protein kinase phosphorylation for its activation. As annexin V was shown to be an endogenous inhibitor of PKC, PKC-stimulated cPLA2 activity was analyzed. Using GF109203x, a specific PKC inhibitor, we demonstrated that this pathway is of minor importance in our model. cPLA2 inhibition by annexin V is not linked to PKC inhibition. To test the hypothesis of phospholipid depletion, mutants of annexin V were constructed using mutagenesis directed to Ca2+ site. We demonstrate that the Ca2+ site located in domain I is necessary for the inhibitory effect of annexin V on cPLA2 activity. The site in domain IV is also involved but with less efficiency. In contrast, mutations in site II and III do not modify this effect. Moreover, annexin V mutated on all sites does not inhibit cPLA2. Thus, we propose a predominant role of module (I/IV) in the biological action of annexin V, which, in physiological conditions, may control cPLA2 activity by depletion of the phospholipid substrate.

Inhibition of phospholipase D activity by fodrin. An active role for the cytoskeleton

Phospholipase D (PLD) is a major enzyme implicated in important cellular processes such as secretion and proliferation. The knowledge of its regulation is essential to understand the control of these phenomena. Several proteins activating PLD have been described in the last years. In this report, we chromatographed bovine brain cytosolic proteins to identify fodrin, the non-erythroid spectrin, as the first described inhibitor of PLD. A cytosolic fraction with an inhibitory effect on PLD activity loses its capacity after immunoprecipitation of fodrin. Moreover, at 1 nM, purified fodrin blocks fully and quickly PLD activity, whatever the stimuli used. In contrast, fodrin has no effect on adenylate cyclase activity. Fodrin-analogous proteins like dimeric or tetrameric erythroid spectrin have the same inhibitory effect on PLD, at higher concentrations. Other cytoskeletal proteins, actin and vimentin, are inefficient on PLD inhibition. The mechanisms implicated in PLD modulation such as post-translational modifications of fodrin and the role of small G-proteins on the cytoskeleton regulation are discussed. In conclusion, this study reveals that fodrin is involved in the control of PLD activity, suggesting that the cytoskeleton could have an active role in control of secretion and proliferation.

A soluble protein negatively regulates phospholipase D activity. Partial purification and characterization

Phosphatidylcholine-specific phospholipase D (PLD) is an important signalling phospholipase in mammalian cells. Recently, PLD activity has been shown to be positively regulated by the GTP-binding protein ARF (ADP-ribosylating factor). In the present work, we document the presence of a factor negatively regulating PLD activity in bovine brain cytosol. The inhibitory factor is characterized as a large protein or a complex of proteins with a molecular mass higher than 300 kDa. Using permeabilized and pre-permeabilized HL-60 cells depleted of their cytosol, we demonstrate that the inhibitor acts on GTP[S]-stimulated PLD activity. This effect is immediate, persistent and dose dependent for GTP[S]-stimulated PLD. Different possibilities for a mechanism of action of the inhibitory factor on the regulation of GTP binding of ARF were investigated. This inhibitory factor is not the guanine-dissociating inhibitor (GDI) for the small G-binding proteins Rho (Rho-GDI), reported to be a PLD inhibitor, since specific antibodies against this protein did not recognize a protein in the peak containing the inhibitory factor for PLD activity. Furthermore, the inhibitory factor does not prevent the binding of GTP[S] to ARF in the presence of HL-60 membranes. This excludes its possible role as an inhibitor of an ARF/guanine exchange factor. The inhibitory factor not only inhibits a pathway of PLD through GTP[S] activation in particular of the small GTP-binding protein, ARF, but it also inhibits PLD activated via either protein kinase C (PKC) or tyrosine kinase activation. The inhibitory factor also decreases PLC activity and this effect seems to be secondary to the inhibition of PLD activity. We discuss a mechanism of action of the inhibitor on PLD and the importance of this enzyme activity for membrane traffic.