Vanadate inhibits degradation of short-lived, but not of long-lived, proteins in L-132 human cells

Annexin A5 stimulates autophagy and inhibits endocytosis

Macroautophagy is a major lysosomal catabolic process activated particularly under starvation in eukaryotic cells. A new organelle, the autophagosome, engulfs cytoplasmic substrates, which are degraded after fusion with endosomes and/or lysosomes. During a shotgun proteome analysis of purified lysosomal membranes from mouse fibroblasts, a Ca(2+)-dependent phospholipid-binding protein, annexin A5, was found to increase on lysosomal membranes under starvation. This suggests a role for this protein, an abundant annexin with a still unknown intracellular function, in starvation-induced lysosomal degradation. Transient overexpression and silencing experiments showed that annexin A5 increased lysosomal protein degradation, and colocalisation experiments, based on GFP sensitivity to lysosomal acidic pH, indicated that this was mainly the result of inducing autophagosome-lysosome fusion. Annexin A5 also inhibited the endocytosis of a fluid-phase marker and cholera toxin, but not receptor-mediated endocytosis. Therefore, we propose a double and opposite role of annexin A5 in regulating the endocytic and autophagic pathways and the fusion of autophagosomes with lysosomes and endosomes.

Characterization of human GTPBP3, a GTP-binding protein involved in mitochondrial tRNA modification

Human GTPBP3 is an evolutionarily conserved, multidomain protein involved in mitochondrial tRNA modification. Characterization of its biochemical properties and the phenotype conferred by GTPBP3 inactivation is crucial to understanding the role of this protein in tRNA maturation and its effects on mitochondrial respiration. We show that the two most abundant GTPBP3 isoforms exhibit moderate affinity for guanine nucleotides like their bacterial homologue, MnmE, although they hydrolyze GTP at a 100-fold lower rate. This suggests that regulation of the GTPase activity, essential for the tRNA modification function of MnmE, is different in GTPBP3. In fact, potassium-induced dimerization of the G domain leads to stimulation of the GTPase activity in MnmE but not in GTPBP3. The GTPBP3 N-terminal domain mediates a potassium-independent dimerization, which appears as an evolutionarily conserved property of the protein family, probably related to the construction of the binding site for the one-carbon-unit donor in the modification reaction. Partial inactivation of GTPBP3 by small interfering RNA reduces oxygen consumption, ATP production, and mitochondrial protein synthesis, while the degradation of these proteins slightly increases. It also results in mitochondria with defective membrane potential and increased superoxide levels. These phenotypic traits suggest that GTPBP3 defects contribute to the pathogenesis of some oxidative phosphorylation diseases.

Role of proteasomes in the degradation of short-lived proteins in human fibroblasts under various growth conditions

Degradation of proteins in the cells occurs by proteasomes, lysosomes and other cytosolic and organellar proteases. It is believed that proteasomes constitute the major proteolytic pathway under most conditions, especially when degrading abnormal and other short-lived proteins. However, no systematic analysis of their role in the overall degradation of truly short-lived cell proteins has been carried out. Here, the degradation of short-labelled proteins was examined in human fibroblasts by release of trichloroacetic acid-soluble radioactivity. The kinetics of degradation was decomposed into two, corresponding to short- and long-lived proteins, and the effect of proteasomal and lysosomal inhibitors on their degradation, under various growth conditions, was separately investigated. From the degradation kinetics of proteins labelled for various pulse times it can be estimated that about 30% of newly synthesised proteins are degraded with a half-life of approximately 1h. These rapidly degraded proteins should mostly include defective ribosomal products. Deprivation of serum and confluent conditions increased the degradation of the pool of long-lived proteins in fibroblasts without affecting, or affecting to a lesser extent, the degradation of the pool of short-lived proteins. Inhibitors of proteasomes and of lysosomes prevented more than 80% of the degradation of short-lived proteins. It is concluded that, although proteasomes are responsible of about 40-60% of the degradation of short-lived proteins in normal human fibroblasts, lysosomes have also an important participation in the degradation of these proteins. Moreover, in confluent fibroblasts under serum deprivation, lysosomal pathways become even more important than proteasomes in the degradation of short-lived proteins.

Effects of polyamine levels on the degradation of short-lived and long-lived proteins in cultured L-132 human lung cells

Biogenic polyamines have important regulatory functions in various biological processes and it has also been suggested that they could modulate intracellular protein degradation. For an overall assessment of the role of polyamines in this process, we have investigated the effect that the decrease in intracellular polyamine levels caused by inhibitors of polyamine biosynthesis brings about on the degradation of the pools of short- and long-lived proteins in cultured L-132 human lung cells. Treatment of cells with 100 microM (2R,5R)-delta-methyl acetylenic putrescine (MAP), a potent enzyme-activated irreversible inhibitor of ornithine decarboxylase, or with 100 microM MAP plus 50 microM N-butyl 1,3-diaminopropane, a specific inhibitor of spermine synthase, caused a similar decrease (65-70% of control) in the total intracellular levels of polyamines, although they affected the concentrations of spermidine and spermine differently. The effect of the two treatments on protein degradation was essentially the same. In polyamine-depleted cells we observed an inhibition of degradation in long-lived proteins of 16% (P<0.05), with a significant increase in the half-life (t12) of this pool from 100.5 to 120.1 h. This was concomitant with an increase of 26% (P<0. 05) in degradation in short-lived proteins, with a significant decrease in the t12 of this pool from 0.85 to 0.67 h. Recovery of polyamine levels by the addition of 50 microM spermidine to polyamine-depleted cells resulted in a restoration of the degradation rates in both pools of proteins. The way(s) by which polyamines could modulate proteolysis are discussed.

Protein-tyrosine-kinase-dependent expression of cyclo-oxygenase-1 and -2 mRNAs in human endothelial cells

Endothelial cells possess constitutive or inducible cyclo-oxygenase (COX) isoenzymes for prostacyclin production, but the mechanisms for their expression are largely unknown. We found that vanadate, an inhibitor of protein-tyrosine phosphatases, induced the expression of two COX isoenzyme mRNAs in human umbilical vein endothelial cells (HUVEC) in a time- and dose-dependent manner. Vanadate also stimulated an increase in COX-2 protein levels, but did not affect significantly the levels of constitutively expressed COX-1 protein. Synergistic enhancement of expression of the two COX isoenzyme mRNAs was observed on stimulation of HUVEC with vanadate plus interleukin-1alpha. Tyrphostin-47, which as an inhibitor of protein-tyrosine kinases abolished vanadate-induced protein-tyrosine phosphorylation, inhibited expression of the two COX isoenzyme mRNAs in HUVEC stimulated with vanadate or interleukin-1alpha. These data provide conclusive evidence that activation of protein-tyrosine kinases is causally linked to expression of the mRNAs for the two COX isoenzymes in HUVEC.

A mitogen-responsive promoter region that is synergistically activated through multiple signalling pathways

A regulatory region of the human transferrin receptor gene promoter was found to be required for increased expression in response to serum or growth factors. This region contains two elements that appear to cooperate for full responsiveness. We found that sodium orthovanadate treatment of cells significantly activated expression of promoter constructs containing these elements. 12-O-Tetradecanoylphorbol-13-acetate alone induced a twofold increase in expression but acted synergistically with vanadate to generate a highly elevated level of expression. Dibutyryl cyclic AMP alone had no effect on expression, but when added together with vanadate and 12-O-tetradecanoylphorbol-13-acetate, led to superinduction of the promoter construct. Induction of expression by these reagents was delayed several hours, and the kinetics were identical to those observed for serum induction.

Immunotoxicity of in vitro vanadium exposures: effects on interleukin-1, tumor necrosis factor-alpha, and prostaglandin E2 production by WEHI-3 macrophages

Treatment of cultured mouse macrophages with either of two different vanadium compounds was shown to affect the production/release of two major immunoregulatory cytokines. The pentavalent vanadium compound ammonium metavanadate was shown previously to disrupt cell-mediated immunity at the earliest stages of an in vivo anti-Listerial response, in that mice treated with vanadium displayed decreased accessory cell recruitment and numbers of activated macrophages at infection sites. To determine whether these effects were due to vanadium-induced alterations in the production of biologically-active mediators, mouse macrophage-like WEHI-3 cells were treated in vitro with ammonium metavanadate or vanadium pentoxide prior to stimulation with lipopolysaccharide endotoxin (LPS). After stimulation, monokine (tumor necrosis factor-alpha and interleukin-1) and prostaglandin E2 (PGE2) activities were assessed. Both vanadium compounds decreased recovered monokine activities; measured TNF alpha concentrations were also reduced. Spontaneous release of the IL-1/TNF-regulating prostanoid PGE2 was significantly increased by the highest concentration of vanadate tested, although LPS-stimulated PGE2 production was unaffected by either compound. These results indicate that, in vitro, pentavalent vanadium can interfere with immunoregulatory mediators critical for maintaining host immunocompetence.

A mitogen-responsive promoter region that is synergistically activated through multiple signalling pathways

A regulatory region of the human transferrin receptor gene promoter was found to be required for increased expression in response to serum or growth factors. This region contains two elements that appear to cooperate for full responsiveness. We found that sodium orthovanadate treatment of cells significantly activated expression of promoter constructs containing these elements. 12-O-Tetradecanoylphorbol-13-acetate alone induced a twofold increase in expression but acted synergistically with vanadate to generate a highly elevated level of expression. Dibutyryl cyclic AMP alone had no effect on expression, but when added together with vanadate and 12-O-tetradecanoylphorbol-13-acetate, led to superinduction of the promoter construct. Induction of expression by these reagents was delayed several hours, and the kinetics were identical to those observed for serum induction.

Vanadate modulates the activity of a subpopulation of asialoglycoprotein receptors on isolated rat hepatocytes: active surface receptors are internalized and replaced by inactive receptors

In the absence of ligand, sodium vanadate causes a time- and dose-dependent loss of up to approximately 50% of the surface galactosyl receptor (GalR) activity in rat hepatocytes at 37 degrees C. The effect on total (surface plus intracellular) GalR activity is also dependent on exposure time and vanadate concentration. At less than 1 mM, vanadate induces a transient decrease and then partial recovery of cell surface GalR activity. At greater than 3 mM vanadate, surface GalR activity decreases rapidly (t1/2 approximately 2 min). Lost surface activity is initially recovered in digitonin-permeabilized cells, indicating that active surface GalRs redistribute to the cell interior. However, an antibody assay for GalR protein showed that although surface activity decreased, there was no decrease in surface receptor protein. The active intracellular GalRs then slowly inactivate over 30-60 min. With 8 mM vanadate, the loss of both surface and total cellular GalR activity is more rapid and coincident; no lag is observed. Maximal activity loss, however, was still only approximately 50%. Again, no net change was seen in the distribution of GalR protein between the cell surface and the interior. These results indicate that vanadate causes active GalRs to move from the surface to the inside and be replaced by inactive receptors moving from the inside to the cell surface. The Gal receptor system is comprised of two functionally different receptor subpopulations that operate via two distinct intracellular pathways. Only the State 2 GalRs, which recycle constitutively, are sensitive to modulation by vanadate. Consistent with this, vanadate inhibits the endocytosis of 125I-asialoorosomucoid (ASOR) only partially. The rate of uptake and the steady state level of ASOR intracellular accumulation were maximally inhibited by 50 and 70%, respectively, at 0.2 mM vanadate. The rate and extent of degradation of 125I-ASOR were also inhibited by 50-70%. Residual ASOR uptake and degradation is accounted for by the minor vanadate-resistant State 1 Gal receptor pathway.

Effects of centrifugation on the degradation of short-lived proteins in exponentially growing cultured cells

The degradation mechanisms of short-lived proteins in cultured cells are unknown, probably due to the lack of procedures which specifically affect the degradation of these proteins. We found that centrifugation of cultured cells, growing either in monolayer or in suspension, between 5000 and 25,000g for 30 min, inhibits (more than 50%) the degradation of short-lived proteins but not of long-lived proteins. Protein synthesis or cell viability is not affected. Centrifugation also disorganizes the Golgi apparatus, as checked by routine electron microscopy, and inhibits the degradation of endocytosed proteins (a lysosomal process which is controlled by the Golgi apparatus). Using different centrifugation speeds, a good correlation was found between alteration of the Golgi apparatus and inhibition of protein degradation.