Cell-free transfer of the vesicular stomatitis virus G protein from an endoplasmic reticulum compartment of baby hamster kidney cells to a rat liver Golgi apparatus compartment for Man8-9 to Man5 processing.

We report the reconstitution of the transfer of a membrane glycoprotein (vesicular stomatitis virus glycoprotein, VSV-G protein) from endoplasmic reticulum to Golgi apparatus and its subsequent Man8-9GlcNAc2 to Man5GlcNAc2 processing in a completely cell-free system. The acceptor was Golgi apparatus from rat liver immobilized on nitrocellulose. The endoplasmic reticulum donor was from homogenates of VSV-G-infected BHK cells. Nucleoside triphosphate plus cytosol-dependent transfer and processing of radiolabeled VSV-G protein was observed with donor from BHK cells infected at 37 degrees C with wild-type VSV or at the permissive temperature of 34 degrees C with the ts045 mutant. With Golgi apparatus as acceptor, specific transfer at 37 degrees C in the presence of nucleoside triphosphate was eightfold that at 4 degrees C or in the absence of ATP. About 40% of the VSV-G protein transferred was processed to the Man5GlcNAc2 form. Processing was specific for cis Golgi apparatus fractions purified by preparative free-flow electrophoresis. Fractions derived from the trans Golgi apparatus were inactive in processing. With the ts045 temperature-sensitive mutant, transfer and processing were much reduced even in the complete system when microsomes were from cells infected with mutant virus and incubated at the restrictive temperature of 39.5 degrees C but were able to proceed at the permissive temperature of 34 degrees C. Thus, Man8-9GlcNAc2 to Man5GlcNAc2 processing of VSV-G protein occurs following transfer in a completely cell-free system using immobilized intact Golgi apparatus or cis Golgi apparatus cisternae as the acceptor and shows temperature sensitivity, donor specificity, requirement for ATP, and response to inhibitors similar to those exhibited by transfer and processing of VSV-G protein in vivo.     

Low temperature blocks exit of pro-opiomelanocortin from the endoplasmic reticulum but not subsequent delivery to the site of prohormone processing

The effect of reduced temperature on the delivery of the prohormone pro-opiomelanocortin (POMC) to the site of prohormone processing was investigated in the mouse anterior pituitary cell line AtT20. At 20 degrees C processing was substantially inhibited and was almost completely arrested at 18 degrees C. Earlier studies with membrane glycoproteins indicated that at these temperatures protein movement was blocked at the level of exit from the Golgi apparatus. In contrast it was found here that the inhibition of processing at reduced temperature was due to the retention of POMC in the endoplasmic reticulum. When POMC was allowed to progress to the Golgi before temperature was reduced, subsequent processing was only slightly retarded by incubation at 18 degrees C. This indicates either that Golgi exit is not inhibited at this temperature, or that the processing apparatus exists in the Golgi. A surprising incidental result was that when held in the endoplasmic reticulum at low temperature POMC is apparently subject to post-translational N-linked glycosylation.     

Effect of caffeine and reduced temperature (20 degrees C) on the organization of the pre-Golgi and the Golgi stack membranes

In the present study we have dissected the transport pathways between the ER and the Golgi complex using a recently introduced (Kuismanen, E., J. Jantti, V. Makiranta, and M. Sariola. 1992. J. Cell Sci. 102:505- 513) inhibition of transport by caffeine at 20 degrees C. Recovery of the Golgi complex from brefeldin A (BFA) treatment was inhibited by caffeine at reduced temperature (20 degrees C) suggesting that caffeine inhibits the membrane traffic between the ER and the Golgi complex. Caffeine at 20 degrees C did not inhibit the BFA-induced retrograde movement of the Golgi membranes. Further, incubation of the cells in 10 mM caffeine at 20 degrees C had profound effects on the distribution and the organization of the pre-Golgi and the Golgi stack membranes. Caffeine treatment at 20 degrees C resulted in a selective and reversible translocation of the pre- and cis-Golgi marker protein (p58) to the periphery of the cell. This caffeine-induced effect on the Golgi complex was different from that induced by BFA, since mannosidase II, a Golgi stack marker, remained perinuclearly located and the Golgi stack coat protein, beta-COP, was not detached from Golgi membranes in the presence of 10 mM caffeine at 20 degrees C. Electron microscopic analysis showed that, in the presence of caffeine at 20 degrees C, the morphology of the Golgi stack was altered and accumulation of numerous small vesicles in the Golgi region was observed. The results in the present study suggest that caffeine at reduced temperature (20 degrees C) reveals a functional interface between the pre-Golgi and the Golgi stack.     

The Hepatitis C Virus-induced NLRP3 Inflammasome Activates the Sterol Regulatory Element-binding Protein (SREBP) and Regulates Lipid Metabolism

Hepatitis C virus (HCV) relies on host lipids and lipid droplets for replication and morphogenesis. The accumulation of lipid droplets in infected hepatocytes manifests as hepatosteatosis, a common pathology observed in chronic hepatitis C patients. One way by which HCV promotes the accumulation of intracellular lipids is through enhancing de novo lipogenesis by activating the sterol regulatory element-binding proteins (SREBPs). In general, activation of SREBPs occurs during cholesterol depletion. Interestingly, during HCV infection, the activation of SREBPs occurs under normal cholesterol levels, but the underlying mechanisms are still elusive. Our previous study has demonstrated the activation of the inflammasome complex in HCV-infected human hepatoma cells. In this study, we elucidate the potential link between chronic hepatitis C-associated inflammation and alteration of lipid homeostasis in infected cells. Our results reveal that the HCV-activated NLRP3 inflammasome is required for the up-regulation of lipogenic genes such as 3-hydroxy-3-methylglutaryl-coenzyme A synthase, fatty acid synthase, and stearoyl-CoA desaturase. Using pharmacological inhibitors and siRNA against the inflammasome components (NLRP3, apoptosis-associated speck-like protein containing a CARD, and caspase-1), we further show that the activation of the NLRP3 inflammasome plays a critical role in lipid droplet formation. NLRP3 inflammasome activation in HCV-infected cells enables caspase-1-mediated degradation of insulin-induced gene proteins. This subsequently leads to the transport of the SREBP cleavage-activating protein·SREBP complex from the endoplasmic reticulum to the Golgi, followed by proteolytic activation of SREBPs by S1P and S2P in the Golgi. Typically, inflammasome activation leads to viral clearance. Paradoxically, here we demonstrate how HCV exploits the NLRP3 inflammasome to activate SREBPs and host lipid metabolism, leading to liver disease pathogenesis associated with chronic HCV.     

Multisite phosphorylation of glycogen synthase. Molecular basis for the substrate specificity of glycogen synthase kinase-3 and casein kinase-II (glycogen synthase kinase-5)

Glycogen synthase kinase-3 was isolated from rabbit skeletal muscle by an improved procedure. The purification was estimated to be 67000-fold and 0.2 mg of enzyme was isolated from 5000 g muscle, corresponding to an overall yield of 7%. The preparation was homogeneous by ultracentrifugal and electrophoretic criteria. The enzyme had a relative molecular mass of 47 kDa by sedimentation equilibrium centrifugation and 51 kDa by SDS-polyacrylamide gel electrophoresis. These values demonstrate that glycogen synthase kinase-3 is monomeric. The Stokes radius of 37 nm suggests the molecule to be asymmetric. The activating factor of the Mg-ATP dependent form of protein phosphatase-1 coeluted with glycogen synthase kinase-3 activity at the final step, establishing that these two activities reside in the same protein. Glycogen synthase kinase-3 phosphorylates glycogen synthase at sites-3, while casein kinase-II phosphorylates site-5, just C-terminal to sites-3 (Picton, C., Aitken, A., Bilham, T. and Cohen, P. (1982) Eur. J. Biochem. 124, 37-45). The basis for the substrate specificities of these protein kinases was investigated using chymotryptic peptides that contain the sites phosphorylated by each enzyme. These studies showed that efficient phosphorylation of sites-3, required the presence of phosphate in site-5 and a region of polypeptide more than 20 residues C-terminal to site-5. In contrast, efficient phosphorylation by casein kinase-II does not require this C-terminal region, and the results are consistent with the view that the enzyme recognises acidic residues immediately C-terminal to site-5.     

Piperine Induces Hepatic Low-Density Lipoprotein Receptor Expression through Proteolytic Activation of Sterol Regulatory Element-Binding Proteins

Elevated plasma low-density lipoprotein (LDL) cholesterol is considered as a risk factor for atherosclerosis. Because the hepatic LDL receptor (LDLR) uptakes plasma lipoproteins and lowers plasma LDL cholesterol, the activation of LDLR is a promising drug target for atherosclerosis. In the present study, we identified the naturally occurring alkaloid piperine, as an inducer of LDLR gene expression by screening the effectors of human LDLR promoter. The treatment of HepG2 cells with piperine increased LDLR expression at mRNA and protein levels and stimulated LDL uptake. Subsequent luciferase reporter gene assays revealed that the mutation of sterol regulatory element-binding protein (SREBP)-binding element abolished the piperine-mediated induction of LDLR promoter activity. Further, piperine treatments increased mRNA levels of several SREBP targets and mature forms of SREBPs. However, the piperine-mediated induction of the mature forms of SREBPs was not observed in SRD–15 cells, which lack insulin-induced gene–1 (Insig–1) and Insig–2. Finally, the knockdown of SREBPs completely abolished the piperine-meditated induction of LDLR gene expression in HepG2 cells, indicating that piperine stimulates the proteolytic activation of SREBP and subsequent induction of LDLR expression and activity.     

Translational regulation of the synthesis of a major heat shock protein in HeLa cells

The synthesis of a major heat shock protein (HSP 70) was measured in HeLa cells incubated at 42.5 degrees C and then transferred to 37 degrees C or 30 degrees C. After 90 min, synthesis of HSP 70 decreased by 54 and 85%, respectively, whereas HSP 70 mRNA was reduced at most by 20%. Therefore, the reduced synthesis of HSP 70 could not be accounted for by mRNA turnover. HSP 70 was associated with large polyribosomes (6-10 ribosomes) in cells kept at 42.5 degrees C, but with medium or small polyribosomes in cells transferred to 37 degrees C or 30 degrees C (5-6 or 2-3 ribosomes, respectively). Addition of puromycin to these cells resulted in the release of all ribosomes from HSP 70 mRNA, indicating that they were translationally active. The regulation of HSP 70 synthesis was investigated in cell-free systems prepared from heat-shocked or control cells and incubated at 30 degrees C and 42 degrees C. After 5 min at 42 degrees C, the cell-free system from heat-shocked cells synthesized protein at 3 times the rate of the control cell-free system. This difference was in large part due to synthesis of HSP 70. Addition of HSP mRNA to the control cell-free system stimulated protein synthesis at 42 degrees C, but not at 30 degrees C. These findings suggest that translation of HSP 70 mRNA is specifically promoted at high temperature and repressed during recovery from heat shock by regulatory mechanisms active at the level of initiation.