Rehearsing the ABCs. Protein translocation

P-glycoprotein regulates Staphylococcus aureus enterotoxin B-stimulated interleukin-5 and thymic stromal lymphopoietin secretion in organotypic mucosal explants

BACKGROUND

T-helper 2 (Th2) inflammation is a hallmark of chronic rhinosinusitis with nasal polyps (CRSwNP) although the pathogenesis is poorly understood. P-glycoprotein (permeability glycoprotein, P-gp) is an efflux pump that is capable of regulating cytokine transport and is expressed within sinonasal mucosa. The purpose of this study was to examine if the oversecretion of interleukin 5 (IL-5) and thymic stromal lymphopoietin (TSLP) in CRSwNP could be explained through P-gp-mediated secretory pathways.

METHODS

Fifteen ethmoid mucosal explants were harvested from patients with CRS (n = 10) and CRSwNP (n = 10) and stimulated with Staphylococcus aureus enterotoxin B (SEB). P-gp was inhibited using zosuquidar trihydrochloride (herein Zosuquidar). P-gp expression was measured using real-time polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA). IL-5, IL-8, and TSLP secretion were quantified using ELISA.

RESULTS

P-gp protein was overexpressed in CRSwNP (28.32 ± 25.94 ng/mL per mg explant) as compared to CRS (10.74 ± 8.61; p = 0.01, 2-tailed Mann-Whitney U test). There was no difference in messenger RNA (mRNA) expression. SEB induced a significant increase in IL-5 and TSLP but not IL-8 secretion relative to control in the CRSwNP explants only. Subsequent P-gp inhibition significantly reduced IL-5 and TSLP secretion (p = 0.04 for both, 2-tailed Student t test) to control levels. The concentration of IL-5 and TSLP secretion were strongly and significantly correlated to the concentration of P-gp within the same explant (IL-5: r = 0.791, p = 0.001; TSLP: r = 0.687, p = 0.003; 2-tailed Spearman's rank-order correlation).

CONCLUSION

P-gp protein is expressed at higher concentrations in CRSwNP as compared to CRS. This overexpression directly contributes to the relative hypersecretion of IL-5 and TSLP. These findings suggest a novel mechanism for Th2 skewing in CRSwNP.

P-glycoprotein functions as an immunomodulator in healthy human primary nasal epithelial cells

BACKGROUND

P-glycoprotein (P-gp) is an adenosine triphosphate (ATP)-dependent efflux pump that confers chemotherapeutic resistance in cancer cells. Recent studies suggest that P-gp may also function as an immunomodulator through regulation of cytokine transport. Sinonasal epithelial cells have been recognized as drivers of local innate and adaptive immunity and are known to overexpress P-gp in the setting of inflammation. The objective of this study is to therefore determine whether P-gp participates in the regulation of cytokine secretion in sinonasal epithelial cells.

METHODS

Primary nasal epithelial cell cultures (PNECCs) were cultivated from 5 healthy patients. Membranous P-gp was quantified through quantitative fluorescent immunohistochemistry (Q-FIHC) and confirmed by enzyme-linked immunosorbent assay (ELISA). Sensitivity to inhibition was determined using a rhodamine 123 accumulation assay. Baseline and lipopolysaccharide (LPS)-stimulated cytokine secretion of interleukin 6 (IL-6), IL-8, granulocyte macrophage colony stimulating factor (GM-CSF), and thymic stromal lymphopoietin (TSLP) were quantified by ELISA and compared to LPS stimulated secretion in the setting of P-gp-specific inhibition. Differences in P-gp expression and cytokine secretion were compared using 2-tailed Student t tests with post hoc testing using the Bonferroni procedure.

RESULTS

Membranous P-gp is detectable in PNECCs and upregulated following LPS exposure. P-gp is sensitive to inhibition by both PSC 833 and verapamil in a dose-dependent fashion. LPS stimulated secretion of normalized IL-6 (mean, 95% confidence interval [CI]) (79.67, 42.26-117.07), GM-CSF (39.92, 7.90-71.94), and TSLP (6.65, 5.35-7.96) was significantly reduced following P-gp inhibition (37.60, 11.54-63.65, p = 0.023; 7.64, 2.25-13.03, p = 0.044; and 5.13, 4.44-5.82, p = 0.038; respectively).

CONCLUSION

P-gp is functionally active in PNECCs. P-gp participates in modulation of epithelial secretion of LPS stimulated IL-6, GM-CSF, and TSLP.

Statins promote the degradation of extracellular amyloid {beta}-peptide by microglia via stimulation of exosome-associated insulin-degrading enzyme (IDE) secretion

Epidemiological studies indicate that intake of statins decrease the risk of developing Alzheimer disease. Cellular and in vivo studies suggested that statins might decrease the generation of the amyloid β-peptide (Aβ) from the β-amyloid precursor protein. Here, we show that statins potently stimulate the degradation of extracellular Aβ by microglia. The statin-dependent clearance of extracellular Aβ is mainly exerted by insulin-degrading enzyme (IDE) that is secreted in a nonconventional pathway in association with exosomes. Stimulated IDE secretion and Aβ degradation were also observed in blood of mice upon peripheral treatment with lovastatin. Importantly, increased IDE secretion upon lovastatin treatment was dependent on protein isoprenylation and up-regulation of exosome secretion by fusion of multivesicular bodies with the plasma membrane. These data demonstrate a novel pathway for the nonconventional secretion of IDE via exosomes. The modulation of this pathway could provide a new strategy to enhance the extracellular clearance of Aβ.

Proteomic identification of multitasking proteins in unexpected locations complicates drug targeting

Proteomics has revealed that many proteins are present in unexpected cellular locations. Moreover, it is increasingly recognized that proteins can translocate between intracellular and extracellular compartments in non-conventional ways. This increases gene pleiotrophy as the diverse functions of the protein that the gene encodes are dependent on the cellular location. Given that trafficking drug targets may exist in various forms--often with completely different functions--in multiple cellular compartments, careful interpretation of proteomics data is needed for an accurate understanding of gene function. This Perspective is intended to inspire the investigation of unusual protein localizations, rather than assuming that they are due to mislocalization or artefacts. Given a fair chance, proteomics could reveal novel and unforeseen biology with important ramifications for target validation in drug discovery.

Mechanisms of interleukin-1beta release

Interleukin-1beta (IL-1beta) is a potent proinflammatory cytokine that initiates and amplifies a wide variety of effects associated with innate immunity and host responses to microbial invasion and tissue injury. Production and release of IL-1beta are stimulated by either pathogen-associated molecular pattern molecules (PAMPs) or damage-associated molecular pattern molecules (DAMPs) and involve several steps. IL-1beta is first synthesized as biologically inactive pro-IL-1beta, then processed into mature, biologically active IL-1beta by caspase-1, and subsequently released into the extracellular milieu. Whereas a large body of recent publications has greatly increased our knowledge of the mechanisms involved in production and processing of IL-1beta, we are only beginning to understand mechanisms of IL-1beta secretion. This review highlights the different models of a non-classical secretory pathway used by monocytes, macrophages and dendritic cells to export the leaderless cytokine IL-1beta. In particular, five different release mechanisms have been suggested, namely (i) exocytosis of IL-1beta-containing secretory lysosomes, (ii) release of IL-1beta from shed plasma membrane microvesicles, (iii) fusion of multivesicular bodies with the plasma membrane and subsequent release of IL-1beta-containing exosomes, (iv) export of IL-1beta through the plasma membrane using specific membrane transporters, and (v) release of IL-1beta upon cell lysis. Reasons for the diversity of IL-1beta secretory pathways remain to be elucidated. A better understanding of IL-1beta release mechanisms is of great therapeutic relevance and may help in the development of strategies aimed at reducing the severity of inflammatory and autoimmune diseases.

Antioxidant gene expression and function in in vitro-developing Schistosoma mansoni mother sporocysts: possible role in self-protection

The ability of the larval forms of Schistosoma mansoni to invade and parasitize their molluscan host, Biomphalaria glabrata, is determined by a multitude of factors. In this study we sought to elucidate the possible mechanisms by which the invading larvae are able to counteract the potentially harmful oxidative environment presented by the host upon initial miracidial infection. This was attempted by examining the gene expression profile of parasite antioxidant enzymes of the linked glutathione-(GSH) thioredoxin (Trx) redox pathway during early intramolluscan larval development. Three such enzymes, the peroxiredoxins (Prx1, Prx2 and Prx3) were examined as to their activity and sites of expression within S. mansoni miracidia and in vitro-cultured mother sporocysts. Results of these studies demonstrated that the H(2)O(2)-reducing enzymes Prx1 and 2 are upregulated during early mother sporocyst development compared to miracidia. Immunolocalization studies further indicated that Prx1 and Prx2 proteins are expressed within the apical papillae of miracidia and tegumental syncytium of sporocysts, and are released with parasite excretory-secretory proteins (ESP) during in vitro larval transformation. Removal of Prx1 and Prx2 from larval ESP by immunoabsorption significantly reduced the ability of ESP to breakdown exogenous H(2)O(2), thereby directly linking ESP Prx proteins with H(2)O(2)-scavenging activity. Moreover, exposure of live sporocysts to exogenous H(2)O(2)stimulated an upregulation of Prx1 and 2 gene expression suggesting the involvement of H(2)O(2)-responsive elements in regulating larval Prx gene expression. These data provide evidence that Prx1 and Prx2 may function in the protection of S. mansoni sporocysts during the early stages of infection.

Cellular Localization and Function of the Antiviral Protein, Ovine Mx1 (oMx1): II. The oMx1 Protein Is a Regulator of Secretion in an Ovine Glandular Epithelial Cell Line

PROBLEM

Embryonic loss is a major contributor to infertility. Understanding factors affecting embryonic loss will help increase fertility.

METHOD OF STUDY

We investigated if ovine Mx1 (oMx1) mediated secretion by ovine glandular epithelial (oGE) cells using small interfering RNA (siRNA). Effects on secretion were examined through the conventional endoplasmic reticulum-Golgi pathway using beta2- microglobulin (beta2MG) as a marker, and interferon-stimulated gene 15 (ISG15) as a marker for unconventional secretion.

RESULTS

Mx1 siRNA reduced oMx1 mRNA levels at 12 and 24 hr after IFN-tau treatment (P < 0.05), without affecting levels of oMx2, ISG15, 2',5'-oligoadenylate synthetas or beta2MG. Mx1 siRNA reduced Mx1 protein levels at 48 and 120 hr after treatment (P < 0.05) and protein levels remained low at 120 hr. Transient oMx1 knock-down reduced secretion of oMx1 (P < 0.01). ISG15 protein in secretions was reduced without affecting intracellular levels (P < 0.05). Levels of beta2MG in secretions were not affected by Mx1 siRNA.

CONCLUSION

We showed that oMx1 protein is secreted by oGE cells and that reduction in oMx1 protein levels by siRNA reduced secretion of ISG15, but not beta2MG. Results support the hypothesis that oMx1 is a regulator of secretion through unconventional secretory pathway(s).

Unconventional secretory routes: direct protein export across the plasma membrane of mammalian cells

The vast majority of extracellular proteins are exported from mammalian cells by the endoplasmic reticulum/Golgi-dependent secretory pathway. For poorly understood reasons, however, a heterogenous group of extracellular proteins has been discovered that does not make use of signal peptide-dependent secretory transport. Both the release mechanisms and the molecular identity of the secretory machines involved have remained elusive. Recent studies now have established a subgroup of unconventional secretory proteins capable of translocating from the cytoplasm directly across the plasma membrane to get access to the exterior of eukaryotic cells. This review aims to focus on a detailed comparison of the subcellular site of membrane translocation of various unconventional secretory proteins such as the proangiogenic molecule fibroblast growth factor-2 (FGF-2) and Leishmania hydrophilic acylated surface protein B (HASP B). A potential link between membrane translocation and quality control as an integral part of unconventional secretory processes is discussed.

Characterization of mouse tGolgin-1 (golgin-245/trans-golgi p230/256 kD golgin) and its upregulation during oligodendrocyte development

As part of an effort to identify gene products that are differentially regulated during oligodendrocyte development, we isolated a mouse cDNA that encodes tGolgin-1, a homolog of the human protein known as golgin-245, trans-golgi p230, or 256 kD golgin. Human tGolgin-1 is the target of autoantibodies in patients with Sjögren's syndrome, and is thought to be involved in vesicular transport processes at the trans-Golgi network. Sequencing of cDNAs and EST clones comprising the full-length tGolgin-1 transcript predict marked homology with the amino- and carboxy-terminal regions of the human protein, but more limited homology within the central predicted coiled-coil region. Epitope tagged, truncated forms of mouse tGolgin-1, like those of its human homolog, were localized at steady state to the Golgi/trans-Golgi network in transfected cells. The tGolgin-1 message was expressed in all tissues examined, but was highly upregulated in oligodendrocyte precursors at a stage just prior to myelination. This expression pattern suggests that tGolgin-1 may play a role in specialized transport processes associated with maturation and/or differentiation of oligodendrocyte precursors.

Protein transport in the host cell cytoplasm and ATP-binding cassette proteins in Plasmodium falciparum-infected erythrocytes

The main interest of our experiments is the study of ATP-binding cassette (ABC) proteins in Plasmodium parasites and their infected host cells. Here, we report on results obtained by studying the plasmodial PfGCN20 ABC protein. Employing immunomicroscopy and cell fractionation techniques, we found that PfGCN20 is localized to multiple regions of the infected erythrocyte, including membranous and non-membranous compartments inside and outside of the parasite cell. PfGCN20 was found to complement the function of its yeast homologue Gcn20p by acting as part of the yeast translation regulatory pathway. These results open up several hypotheses about a possible biological function of PfGCN20, such as being a component of plasmodial translation regulation, or functioning as an ATP-binding subunit of a multimeric ABC transporter, or acting as a molecular chaperone-like enzyme contributing to the protein translocation across multiple membranes in infected erythrocytes. More experiments are presently being performed to fully understand the biological function of this protein, abundant in multiple compartments of erythrocytes infected with the Plasmodium falciparum malaria parasite.

Four distinct secretory pathways serve protein secretion, cell surface growth, and peroxisome biogenesis in the yeast Yarrowia lipolytica

We have identified and characterized mutants of the yeast Yarrowia lipolytica that are deficient in protein secretion, in the ability to undergo dimorphic transition from the yeast to the mycelial form, and in peroxisome biogenesis. Mutations in the SEC238, SRP54, PEX1, PEX2, PEX6, and PEX9 genes affect protein secretion, prevent the exit of the precursor form of alkaline extracellular protease from the endoplasmic reticulum, and compromise peroxisome biogenesis. The mutants sec238A, srp54KO, pex2KO, pex6KO, and pex9KO are also deficient in the dimorphic transition from the yeast to the mycelial form and are affected in the export of only plasma membrane and cell wall-associated proteins specific for the mycelial form. Mutations in the SEC238, SRP54, PEX1, and PEX6 genes prevent or significantly delay the exit of two peroxisomal membrane proteins, Pex2p and Pex16p, from the endoplasmic reticulum en route to the peroxisomal membrane. Mutations in the PEX5, PEX16, and PEX17 genes, which have previously been shown to be essential for peroxisome biogenesis, affect the export of plasma membrane and cell wall-associated proteins specific for the mycelial form but do not impair exit from the endoplasmic reticulum of either Pex2p and Pex16p or of proteins destined for secretion. Biochemical analyses of these mutants provide evidence for the existence of four distinct secretory pathways that serve to deliver proteins for secretion, plasma membrane and cell wall synthesis during yeast and mycelial modes of growth, and peroxisome biogenesis. At least two of these secretory pathways, which are involved in the export of proteins to the external medium and in the delivery of proteins for assembly of the peroxisomal membrane, diverge at the level of the endoplasmic reticulum.