NADPH oxidase-dependent production of reactive oxygen species induces endoplasmatic reticulum stress in neutrophil-like HL60 cells.

Reactive oxygen species (ROS) primarily produced via NADPH oxidase play an important role for killing microorganisms in neutrophils. In this study we examined if ROS production in Human promyelocytic leukemia cells (HL60) differentiated into neutrophil-like cells (dHL60) induces ER stress and activates the unfolded protein response (UPR). To cause ROS production cells were treated with PMA or by chronic hyperglycemia. Chronic hyperglycemia failed to induce ROS production and did not cause activation of the UPR in dHL60 cells. PMA, a pharmacologic NADPH oxidase activator, induced ER stress in dHL60 cells as monitored by IRE-1 and PERK pathway activation, and this was independent of calcium signaling. The NADPH oxidase inhibitor, DPI, abolished both ROS production and UPR activation. These results show that ROS produced by NADPH oxidase induces ER stress and suggests a close association between the redox state of the cell and the activation of the UPR in neutrophil-like HL60 cells.

Transcriptional response to deletion of the phosphatidylserine decarboxylase Psd1p in the yeast Saccharomyces cerevisiae.

In the yeast, Saccharomyces cerevisiae, the synthesis of the essential phospholipid phosphatidylethanolamine (PE) is accomplished by a network of reactions which comprises four different pathways. The enzyme contributing most to PE formation is the mitochondrial phosphatidylserine decarboxylase 1 (Psd1p) which catalyzes conversion of phosphatidylserine (PS) to PE. To study the genome wide effect of an unbalanced cellular and mitochondrial PE level and in particular the contribution of Psd1p to this depletion we performed a DNA microarray analysis with a ∆psd1 deletion mutant. This approach revealed that 54 yeast genes were significantly up-regulated in the absence of PSD1 compared to wild type. Surprisingly, marked down-regulation of genes was not observed. A number of different cellular processes in different subcellular compartments were affected in a ∆psd1 mutant. Deletion mutants bearing defects in all 54 candidate genes, respectively, were analyzed for their growth phenotype and their phospholipid profile. Only three mutants, namely ∆gpm2, ∆gph1 and ∆rsb1, were affected in one of these parameters. The possible link of these mutations to PE deficiency and PSD1 deletion is discussed.

SDF2L1 interacts with the ER-associated degradation machinery and retards the degradation of mutant proinsulin in pancreatic ß-cells.

Stromal cell-derived factor 2-like 1 (SDF2L1) is an endoplasmic reticulum (ER)-localized protein whose function is undefined. Here we show that SDF2L1 protein levels are increased in response to ER stress-inducing compounds, but not other cell stressors that we tested in insulinoma cell lines. SDF2L1 protein levels were also induced by expression of misfolded proinsulin in insulinoma cells and in islets from diabetic mice. Immunoprecipitation and binding assays demonstrated that SDF2L1 interacts with the ER chaperone GRP78/BiP, the ER-associated degradation (ERAD) machinery and with misfolded proinsulin. Unexpectedly, knockdown of SDF2L1 in INS-1 (insulin 2 C96Y-GFP) cells increased the degradation kinetics of mutant proinsulin, suggesting that SDF2L1 regulates substrate availability for the ERAD system. We suggest that SDF2L1 increases the time that misfolded proteins have to achieve a correctly folded conformation and thus that SDF2L1 can act as a buffer for substrate availability for ERAD in pancreatic ß-cells.

A glucagon-like peptide-1 analog reverses the molecular pathology and cardiac dysfunction of a mouse model of obesity.

BACKGROUND: Cardiac consequences of obesity include inflammation, hypertrophy, and compromised energy metabolism. Glucagon-like peptide-1 is an incretin hormone capable of cytoprotective actions that reduces inflammation and endoplasmic reticulum stress in other tissues. Here we examine the cardiac effects of the glucagon-like peptide-1 analog liraglutide in a model of obesity, independent of changes in body weight. METHODS AND RESULTS: C57Bl6 mice were placed on a 45% high-fat diet (HFD) or a regular chow diet. Mice on HFD developed 46±2% and 60±2% greater body weight relative to regular chow diet-fed mice at 16 and 32 weeks, respectively (both P<0.0001), manifesting impaired glucose tolerance, insulin resistance, and cardiac ceramide accumulation by 16 weeks. One-week treatment with liraglutide (30 µg/kg twice daily) did not reduce body weight, but reversed insulin resistance, cardiac tumor necrosis factor-α expression, nuclear factor kappa B translocation, obesity-induced perturbations in cardiac endothelial nitric oxide synthase, connexin-43, and markers of hypertrophy and fibrosis, in comparison with placebo-treated HFD controls. Liraglutide improved the cardiac endoplasmic reticulum stress response and also improved cardiac function in animals on HFD by an AMP-activated protein kinase-dependent mechanism. Supporting a direct mechanism of action, liraglutide (100 nmol/L) prevented palmitate-induced lipotoxicity in isolated mouse cardiomyocytes and primary human coronary smooth muscle cells and prevented adhesion of human monocytes to tumor necrosis factor-α-activated human endothelial cells in vitro. CONCLUSIONS: Weight-neutral treatment with a glucagon-like peptide-1 analog activates several cardioprotective pathways, prevents HFD-induced insulin resistance and inflammation, reduces monocyte vascular adhesion, and improves cardiac function in vivo by activating AMP-activated protein kinase. These data support a role for glucagon-like peptide-1 analogs in limiting the cardiovascular risks of obesity.

Endoplasmic reticulum redox state is not perturbed by pharmacological or pathological endoplasmic reticulum stress in live pancreatic ß-cells.

Accumulation of unfolded, misfolded and aggregated proteins in the endoplasmic reticulum (ER) causes ER stress. ER stress can result from physiological situations such as acute increases in secretory protein biosynthesis or pathological conditions that perturb ER homeostasis such as alterations in the ER redox state. Here we monitored ER redox together with transcriptional output of the Unfolded Protein Response (UPR) in INS-1 insulinoma cells stably expressing eroGFP (ER-redox-sensor) and mCherry protein driven by a GRP78 promoter (UPR-sensor). Live cell imaging, flow cytometry and biochemical characterization were used to examine these parameters in response to various conditions known to induce ER stress. As expected, treatment of the cells with the reducing agent dithiothreitol caused a decrease in the oxidation state of the ER accompanied by an increase in XBP-1 splicing. Unexpectedly however, other treatments including tunicamycin, thapsigargin, DL-homocysteine, elevated free fatty acids or high glucose had essentially no influence on the ER redox state, despite inducing ER stress. Comparable results were obtained with dispersed rat islet cells expressing eroGFP. Thus, unlike in yeast cells, ER stress in pancreatic ß-cells is not associated with a more reducing ER environment.

Action of protein disulfide isomerase on proinsulin exit from endoplasmic reticulum of pancreatic ß-cells.

For insulin synthesis, the proinsulin precursor is translated at the endoplasmic reticulum (ER), folds to include its three native disulfide bonds, and is exported to secretory granules for processing and secretion. Protein disulfide isomerase (PDI) has long been assumed to assist proinsulin in this process. Herein we have examined the effect of PDI knockdown (PDI-KD) in ß-cells. The data establish that upon PDI-KD, oxidation of proinsulin to form native disulfide bonds is unimpaired and in fact enhanced. This is accompanied by improved proinsulin exit from the ER and increased total insulin secretion, with no evidence of ER stress. We provide evidence for direct physical interaction between PDI and proinsulin in the ER of pancreatic ß-cells, in a manner requiring the catalytic activity of PDI. In ß-cells after PDI-KD, enhanced export is selective for proinsulin over other secretory proteins, but the same effect is observed for recombinant proinsulin trafficking upon PDI-KD in heterologous cells. We hypothesize that PDI exhibits unfoldase activity for proinsulin, increasing retention of proinsulin within the ER of pancreatic ß-cells.

Diverse roles for the p24 family of proteins in eukaryotic cells.

Abstract Members of the p24 protein family form a highly conserved family of type I transmembrane proteins that are abundant components of the early secretory pathway. Topologically, the proteins have a large luminal domain and a short cytoplasmic domain that allows for targeting to both coat protein complex II and coat protein complex I vesicles, and thus these proteins cycle between the endoplasmic reticulum and Golgi compartments. Several functions have been proposed for these proteins including a role in coat protein complex I vesicle biogenesis, cargo protein selection, organization of intracellular membranes, and protein quality control. Recent studies have added to the list of potential cargo substrates for which p24 function is required for normal transport in the secretory pathway. This review focuses on recent developments in the study of p24 proteins and their requirement for secretory and membrane protein transport in eukaryotic cells.

Phosphatidylethanolamine synthesized by four different pathways is supplied to the plasma membrane of the yeast Saccharomyces cerevisiae.

In this study, we examined the contribution of the four different pathways of phosphatidylethanolamine (PE) synthesis in the yeast Saccharomyces cerevisiae to the supply of this phospholipid to the plasma membrane. These pathways of PE formation are decarboxylation of phosphatidylserine (PS) by (i) phosphatidylserine decarboxylase 1 (Psd1p) in mitochondria and (ii) phosphatidylserine decarboxylase 2 (Psd2p) in a Golgi/vacuolar compartment, (iii) incorporation of exogenous ethanolamine and ethanolamine phosphate derived from sphingolipid catabolism via the CDP-ethanolamine pathway in the endoplasmic reticulum (ER), and (iv) synthesis of PE through acylation of lyso-PE catalyzed by the acyl-CoA-dependent acyltransferase Ale1p in the mitochondria associated endoplasmic reticulum membrane (MAM). Deletion of PSD1 and/or PSD2 led to depletion of total cellular and plasma membrane PE level, whereas mutation in the other pathways had practically no effect. Analysis of wild type and mutants, however, revealed that all four routes of PE synthesis contributed not only to PE formation but also to the supply of PE to the plasma membrane. Pulse-chase labeling experiments with L[(3)H(G)]serine and [(14)C]ethanolamine confirmed the latter finding. Fatty acid profiling demonstrated a rather balanced incorporation of PE species into the plasma membrane irrespective of mutations suggesting that all four pathways of PE synthesis provide at least a basic portion of "correct" PE species required for plasma membrane biogenesis. In summary, the PE level in the plasma membrane is strongly influenced by total cellular PE synthesis, but fine tuned by selective assembly mechanisms.

Phosphatidylserine decarboxylases, key enzymes of lipid metabolism.

Phosphatidylserine decarboxylases (PSDs) (E.C. 4.1.1.65) are enzymes which catalyze the formation of phosphatidylethanolamine (PtdEtn) by decarboxylation of phosphatidylserine (PtdSer). This enzymatic activity has been identified in both prokaryotic and eukaryotic organisms. PSDs occur as two types of proteins depending on their localization and the sequence of a conserved motif. Type I PSDs include enzymes of eukaryotic mitochondria and bacterial origin which contain the amino acid sequence LGST as a characteristic motif. Type II PSDs are found in the endomembrane system of eukaryotes and contain a typical GGST motif. These characteristic motifs are considered as autocatalytic cleavage sites where proenzymes are split into alpha- and beta-subunits. The S-residue set free by this cleavage serves as an attachment site of a pyruvoyl group which is required for the activity of the enzymes. Moreover, PSDs harbor characteristic binding sites for the substrate PtdSer. Substrate supply to eukaryotic PSDs requires lipid transport because PtdSer synthesis and decarboxylation are spatially separated. Targeting of PSDs to their proper locations requires additional intramolecular domains. Mitochondrially localized type I PSDs are directed to the inner mitochondrial membrane by N-terminal targeting sequences. Type II PSDs also contain sequences in their N-terminal extensions which might be required for subcellular targeting. Lack of PSDs causes various defects in different cell types. The physiological relevance of these findings and the central role of PSDs in lipid metabolism will be discussed in this review.

The phosphatidylethanolamine level of yeast mitochondria is affected by the mitochondrial components Oxa1p and Yme1p.

The majority of phosphatidylethanolamine, an essential component of yeast mitochondria, is synthesized by phosphatidylserine decarboxylase 1 (Psd1p), a component of the inner mitochondrial membrane. Here, we report that deletion of OXA1 encoding an inner mitochondrial membrane protein translocase markedly affects the mitochondrial phosphatidylethanolamine level. In an oxa1Delta mutant, cellular and mitochondrial levels of phosphatidylethanolamine were lowered similar to a mutant with PSD1 deleted, and the rate of phosphatidylethanolamine synthesis by decarboxylation of phosphatidylserine in vivo and in vitro was decreased. This was due to a lower PSD1 transcription rate in the oxa1Delta mutant compared with wild-type and compromised assembly of Psd1p into the inner mitochondrial membrane. Lack of Mba1p, another component involved in the assembly of mitochondrial proteins into the inner mitochondrial membrane, did not affect the amount of phosphatidylethanolamine or the assembly of Psd1p. Deletion of the inner membrane protease Yme1p enhanced Psd1p stability suggesting that Yme1p contributed substantially to the proteolytic turnover of Psd1p in wild-type. In summary, our results demonstrate a link between the mitochondrial protein import machinery, assembly and stability of Psd1p, and phosphatidylethanolamine homeostasis in yeast mitochondria.