Functional protein prenylation is required for the brefeldin A-dependent retrograde transport from the Golgi apparatus to the endoplasmic reticulum

Fluvastatin Converts Human Macrophages into Foam Cells with Increased Inflammatory Response to Inactivated Mycobacterium tuberculosis H37Ra

Cholesterol biosynthesis inhibitors (statins) protect hypercholesterolemic patients against developing active tuberculosis, suggesting that these drugs could help the host to control the pathogen at the initial stages of the disease. This work studies the effect of fluvastatin on the early response of healthy peripheral blood mononuclear cells (PBMCs) to inactivated Mycobacterium tuberculosis (Mtb) H37Ra. We found that in fluvastatin-treated PBMCs, most monocytes/macrophages became foamy cells that overproduced NLRP3 inflammasome components in the absence of immune stimulation, evidencing important cholesterol metabolism/immunity connections. When both fluvastatin-treated and untreated PBMCs were exposed to Mtb H37Ra, a small subset of macrophages captured large amounts of bacilli and died, concentrating the bacteria in necrotic areas. In fluvastatin-untreated cultures, most of the remaining macrophages became epithelioid cells that isolated these areas of cell death in granulomatous structures that barely produced IFNγ. By contrast, in fluvastatin-treated cultures, foamy macrophages surrounded the accumulated bacteria, degraded them, markedly activated caspase-1 and elicited a potent IFNγ/cytotoxic response. In rabbits immunized with the same bacteria, fluvastatin increased the tuberculin test response. We conclude that statins may enhance macrophage efficacy to control Mtb, with the help of adaptive immunity, offering a promising tool in the design of alternative therapies to fight tuberculosis.

The secretion of the angiogenic and neurotrophic factor angiogenin is COPII and microtubule dependent

The RNaseA superfamily member Angiogenin (ANG) is a secreted protein involved in neovascularization, cell proliferation and stress response. Dysregulation of ANG expression is found in many cancers with poor prognosis and mutations in ANG are associated with neurodegenerative diseases. While the uptake and nuclear translocation of ANG is relatively well characterised, little is known about how it reaches the plasma membrane and its mode of secretion. We generated SH-SY5Y neuroblastoma cell lines constitutively expressing wild type (WT) Hemagglutinin (HA) epitope tagged mouse Ang1 (mAng1), and two amyotrophic lateral sclerosis associated ANG variants (C39W and K40I). Herein, we show that these cell lines secrete mAng1 into the culture media. Using small molecule inhibitors we probed the route taken between the endoplasmic reticulum and trans-Golgi network during secretion and have characterised it as COPII and microtubule dependent. In addition, we show that disruption by the PI3-kinase inhibitor wortmannin of the later stages of transit to the plasma membrane leads to mAng1 trafficking to lysosomal compartments. This suggests an autophagy dependent regulation of secretion.

Inhibition of Rab prenylation by statins induces cellular glycosphingolipid remodeling

Statins, which specifically inhibit HMG Co-A reductase, the rate-limiting step of cholesterol biosynthesis, are widely prescribed to reduce serum cholesterol and cardiac risk, but many other effects are seen. We now show an effect of these drugs to induce profound changes in the step-wise synthesis of glycosphingolipids (GSLs) in the Golgi. Glucosylceramide (GlcCer) was increased several-fold in all cell lines tested, demonstrating a widespread effect. Additionally, de novo or elevated lactotriaosylceramide (Lc3Cer; GlcNAcβ1-3Galβ1-4GlcCer) synthesis was observed in 70%. Western blot showed that GlcCer synthase (GCS) was elevated by statins, and GCS and Lc3Cer synthase (Lc3S) activities were increased; however, transcript was elevated for Lc3S only. Supplementation with the isoprenoid precursor, geranylgeranyl pyrophosphate (GGPP), a downstream product of HMG Co-A reductase, reversed statin-induced glycosyltransferase and GSL elevation. The Rab geranylgeranyl transferase inhibitor 3-PEHPC, but not specific inhibitors of farnesyl transferase, or geranylgeranyl transferase I, was sufficient to replicate statin-induced GlcCer and Lc3Cer synthesis, supporting a Rab prenylation-dependent mechanism. While total cholesterol was unaffected, the trans-Golgi network (TGN) cholesterol pool was dissipated and medial Golgi GCS partially relocated by statins. GSL-dependent vesicular retrograde transport of Verotoxin and cholera toxin to the Golgi/endoplasmic reticulum were blocked after statin or 3-PEHPC treatment, suggesting aberrant, prenylation-dependent vesicular traffic as a basis of glycosyltransferase increase and GSL remodeling. These in vitro studies indicate a previously unreported link between Rab prenylation and regulation of GCS activity and GlcCer metabolism.

Fluvastatin modulates renal water reabsorption in vivo through increased AQP2 availability at the apical plasma membrane of collecting duct cells

X-linked nephrogenic diabetes insipidus (XNDI), a severe pathological condition characterized by greatly impaired urine-concentrating ability of the kidney, is caused by inactivating mutations in the V2 vasopressin receptor (V2R) gene. The lack of functional V2Rs prevents vasopressin-induced shuttling of aquaporin-2 (AQP2) water channels to the apical plasma membrane of kidney collecting duct principal cells, thus promoting water reabsorption from urine to the interstitium. At present, no specific pharmacological therapy exists for the treatment of XNDI. We have previously reported that the cholesterol-lowering drug lovastatin increases AQP2 membrane expression in renal cells in vitro. Here we report the novel finding that fluvastatin, another member of the statins family, greatly increases kidney water reabsorption in vivo in mice in a vasopressin-independent fashion. Consistent with this observation, fluvastatin is able to increase AQP2 membrane expression in the collecting duct of treated mice. Additional in vivo and in vitro experiments indicate that these effects of fluvastatin are most likely caused by fluvastatin-dependent changes in the prenylation status of key proteins regulating AQP2 trafficking in collecting duct cells. We identified members of the Rho and Rab families of proteins as possible candidates whose reduced prenylation might result in the accumulation of AQP2 at the plasma membrane. In conclusion, these results strongly suggest that fluvastatin, or other drugs of the statin family, may prove useful in the therapy of XNDI.

Rab GTPases as regulators of endocytosis, targets of disease and therapeutic opportunities

Rab GTPases are well-recognized targets in human disease, although are underexplored therapeutically. Elucidation of how mutant or dysregulated Rab GTPases and accessory proteins contribute to organ specific and systemic disease remains an area of intensive study and an essential foundation for effective drug targeting. Mutation of Rab GTPases or associated regulatory proteins causes numerous human genetic diseases. Cancer, neurodegeneration and diabetes represent examples of acquired human diseases resulting from the up- or downregulation or aberrant function of Rab GTPases. The broad range of physiologic processes and organ systems affected by altered Rab GTPase activity is based on pivotal roles in responding to cell signaling and metabolic demand through the coordinated regulation of membrane trafficking. The Rab-regulated processes of cargo sorting, cytoskeletal translocation of vesicles and appropriate fusion with the target membranes control cell metabolism, viability, growth and differentiation. In this review, we focus on Rab GTPase roles in endocytosis to illustrate normal function and the consequences of dysregulation resulting in human disease. Selected examples are designed to illustrate how defects in Rab GTPase cascades alter endocytic trafficking that underlie neurologic, lipid storage, and metabolic bone disorders as well as cancer. Perspectives on potential therapeutic modulation of GTPase activity through small molecule interventions are provided.

Golgi protein GOLM1 is a tissue and urine biomarker of prostate cancer

Prostate cancer is the most common type of tumor found in American men and is the second leading cause of cancer death in males. To identify biomarkers that distinguish prostate cancer from normal, we compared multiple gene expression profiling studies. Through meta-analysis of expression array data from multiple prostate cancer studies, we identified GOLM1 (Golgi membrane protein 1, Golm 1) as consistently up-regulated in clinically localized prostate cancer. This observation was confirmed by reverse transcription-polymerase chain reaction (RT-PCR) and validated at the protein level by immunoblot assay and immunohistochemistry. Prostate epithelial cells were identified as the cellular source of GOLM1 expression using laser capture microdissection. Immunohistochemical staining localized the GOLM1 signal to the subapical cytoplasmic region, typical of a Golgi distribution. Surprisingly, GOLM1 immunoreactivity was detected in the supernatants of prostate cell lines and in the urine of patients with prostate cancer. The mechanism by which intact GOLM1 might be released from cells has not yet been elucidated. GOLM1 transcript levels were measured in urine sediments using quantitative PCR on a cohort of patients presenting for biopsy or radical prostatectomy. We found that urinary GOLM1 mRNA levels were a significant predictor of prostate cancer. Further, GOLM1 outperformed serum prostate-specific antigen (PSA) in detecting prostate cancer. The area under the receiver-operating characteristic curve was 0.622 for GOLM1 (P = .0009) versus 0.495 for serum PSA (P = .902). Our data indicating the up-regulation of GOLM1 expression and its appearance in patients' urine suggest GOLM1 as a potential novel biomarker for clinically localized prostate cancer.

Statins Reduce Amyloid-β Production through Inhibition of Protein Isoprenylation

Epidemiological evidence suggests that long term treatment with hydroxymethylglutaryl-CoA reductase inhibitors, or statins, decreases the risk for developing Alzheimer disease (AD). However, statin-mediated AD protection cannot be fully explained by reduction of cholesterol levels. In addition to their cholesterol lowering effects, statins have pleiotropic actions and act to lower the concentrations of isoprenoid intermediates, such as geranylgeranyl pyrophosphate and farnesyl pyrophosphate. The Rho and Rab family small G-proteins require addition of these isoprenyl moieties at their C termini for normal GTPase function. In neuroblastoma cell lines, treatment with statins inhibits the membrane localization of Rho and Rab proteins at statin doses as low as 200 nm, without affecting cellular cholesterol levels. In addition, we show for the first time that at low, physiologically relevant, doses statins preferentially inhibit the isoprenylation of a subset of GTPases. The amyloid precursor protein (APP) is proteolytically cleaved to generate beta-amyloid (Abeta), which is the major component of senile plaques found in AD. We show that inhibition of protein isoprenylation by statins causes the accumulation of APP within the cell through inhibition of Rab family proteins involved in vesicular trafficking. Moreover, inhibition of Rho family protein function reduces levels of APP C-terminal fragments due to enhanced lysosomal dependent degradation. Statin inhibition of protein isoprenylation results in decreased Abeta secretion. In summary, we show that statins selectively inhibit GTPase isoprenylation at clinically relevant doses, leading to reduced Abeta production in an isoprenoid-dependent manner. These studies provide insight into the mechanisms by which statins may reduce AD pathogenesis.

Rab-small GTPases are involved in fluvastatin and pravastatin-induced vacuolation in rat skeletal myofibers

Three-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) reductase inhibitors, known as statins, induce skeletal muscle injury including myalgia, myositis, and rhabdomyolysis. The mechanism of this myotoxicity remains unknown. This study examined the effect of statins on single skeletal myofibers enzymatically isolated from the rat flexor digitorum brevis muscles. Fluvastatin and pravastatin induced the formation of numerous vacuoles in the myofibers after 72 h of treatment. This effect progressed in a time- and concentration-dependent manner and, consequently, cell death occurred after 120 h. Electron micrographs revealed craters along the sarcolemma and swelling of the sarcoplasmic reticula and mitochondria, in addition to intracellular vacuoles. When caffeine was added after 72 h of fluvastatin treatment, contractile shortening of statin-treated myofibers was significantly attenuated and blebs formed on the surface of the myofibers. The coapplication of geranylgeranylpyrophosphate (GGPP) with fluvastatin prevented the morphological changes, while that of farnesylpyrophosphate (FPP) was ineffective. Furthermore, perillyl alcohol, an inhibitor of Rab geranylgeranyl transferase and geranylgeranyl transferase-I (GGTase-I), mimicked the effect of statins, while a specific GGTase-I inhibitor (GGTI-298) or a farnesyl transferase inhibitor (FTI-277) failed to do so. These results suggest that the inactivation of Rab GTPase, which involved in intracellular membrane transport, is a crucial factor in statin-induced-morphological abnormality in skeletal muscle fibers.

Cisternal rab proteins regulate Golgi apparatus redistribution in response to hypotonic stress

We show that a physiological role of the extensively studied cisternal Golgi rab protein, rab6, is modulation of Golgi apparatus response to stress. Taking exposure of cells to hypotonic media as the best-known example of mammalian Golgi stress response, we found that hypotonic-induced tubule extension from the Golgi apparatus was sensitive to GDP-rab6a expression. Similarly, we found that Golgi tubulation induced by brefeldin A, a known microtubule-dependent process, was inhibited by GDP-restricted rab6a, rab6a', and rab33b, the most commonly studied cisternal rab proteins. These GDP-rab levels were sufficient to inhibit rab-induced redistribution of Golgi glycosyltransferases into the endoplasmic reticulum (ER), also a microtubule-dependent process, and to depress Golgi membrane association of the GTP-conformer of rab6. Nocodazole-induced Golgi scattering, a microtubule-independent process, also was inhibited by GDP-rab6a expression. In comparison, we found similar GDP-rab expression levels had little inhibitory effect on another microtubule-independent process, constitutive recycling of Golgi resident proteins to the ER. We conclude that Golgi cisternal rabs, and in particular rab6a, are regulators of the Golgi response to stress and presumably the molecular targets of stress-activated signaling pathway(s). Moreover, we conclude that rab6a can regulate select microtubule-independent processes as well as microtubule-dependent processes.

The HDV large-delta antigen fused with GFP remains functional and provides for studying its dynamic distribution

Hepatitis D virus (HDV) requires the isoprenylated large delta antigen (LDAg) for interaction with hepatitis B surface antigen (HBsAg) to allow packaging and secretion out of the host cell. Phosphorylated LDAg has been found but, as yet, neither localization of LDAg within the nucleus nor any other function has been correlated with modification. In this study, we transfected HuH-7 or HeLa cells with plasmids encoding various lengths of LDAg [designated GFP-LD and GFP-LD(31-214) for full length and a deletion, respectively] or non-isoprenylated mutants of these [designated GFP-LDM and GFP-LD(31-214)M] fused to the green fluorescent protein (GFP). These fusion proteins were then characterized and it was found that: (i) the addition of the GFP did not interfere with the functioning of the full-length or N-terminally deleted LDAgs when interacting with HBsAg for secretion; (ii) the HDV small antigen (SDAg) together with the GFP-LD, but not the GFP-LD(31-214), could be cosecreted by HBsAg; and (iii) the GFP-LD, but not the GFP-LD(31-214), exerted a dominant-negative role on HDV genome replication. Analyses of transiently transfected cells and postmitotic permanent cells revealed the sequential appearance of GFP-LD in the nucleoplasm, then in the nucleolus, and finally in nuclear speckles (NS). Isoprenylation of LDAg seems to be important for targeting to and accumulating in the NS, which was evident from the dynamic and static localization of the non-isoprenylation mutant (GFP-LDM) and the distribution of wild-type (GFP-LD) when treated with an isoprenylation inhibitor, lovastatin, for more than 48 h. Permanently expressing GFP-LD cells allowed us to show the dynamic redistribution of dephosphorylated GFP-LD from the nucleolus to the SC-35 containing NS in the presence of dichlororibofuranosyl benzimidazole (DRB) and then the translocation back of the GFP-LD to the nucleolus within 2 h after removal of DRB. Our studies thus suggest that the various versions of the GFP-LD fusion protein, having the same function as their nonfusion counterparts, can be a powerful tool for the study of the dynamic localization of LDAg when correlated with the functional modification of this protein.

Inhibitors of HMG-CoA reductase sensitize human smooth muscle cells to Fas-ligand and cytokine-induced cell death

Hydroxymethylglutaryl CoA (HMG CoA) reductase inhibitors, or statins, have been shown to reduce atherosclerotic cardiovascular morbidity and mortality. Atherosclerotic plaque lesions can be chronically inflamed and vulnerable to rupture or stable and less rupture-prone. Human smooth muscle cells (SMC) are critically important in maintaining the stability of atherosclerotic plaques. This stability may be greatly influenced by pro-inflammatory mediators such as IFN-gamma, TNF-alpha, and Il-1beta and Fas ligand (FasL) that are present in human atheroma. The purpose of the present study was to examine the effect of the statins on apoptosis of SMC. We have found that SMC are normally resistant to Fas or cytokine-induced apoptosis, but can be sensitized to these agents with pharmacological concentrations of some statins. Simvastatin and lovastatin strongly sensitized the cells to apoptotic agents while atorvastatin was less effective. In contrast to the lipophilic statins, the hydrophilic statin pravastatin did not induce this sensitization of SMC to apoptosis. Treatment of SMC with either mevalonate, the product of the HMG-CoA reductase, or geranylgeranylpyrophosphate, a down stream intermediate, prevented lipophilic statin-induced sensitization to apoptosis. These results suggest that prenylation of one or more proteins is critically involved in regulating the sensitivity of SMC to apoptotic stimuli. Our data support the emerging evidence that through this pathway the various statins may have effects which are beyond a simple lowering of the levels of circulating cholesterol.

Lovastatin is a potent inhibitor of cholecystokinin secretion in endocrine tumor cells in culture

Lovastatin prevents isoprene synthesis thereby affecting the structural organization of proteins involved in protein transport and secretion. Lovastatin at 1 microM decreases CCK 8 secretion by over 50% in WE cells and in CCK 8 expressing AtT20 cells. At 10 microM CCK 8 secretion was inhibited by two thirds and at 100 microM, cytotoxic effects were observed in both cell types. Addition of mevalonate does not restore CCK secretion and stimulation of secretion by forskolin is also partially inhibited. Cellular content of CCK 8 and pro-CCK were not altered in either of these cell lines except at 100 microM lovastatin. Our results clearly demonstrate that lovastatin at 1 microM strongly inhibits CCK 8 secretion at multiple levels while having little or no effect on its synthesis. This effect on secretion may be partly responsible for the adverse gastrointestinal side effects of lovastatin in patients.

NSF is required for the brefeldin A-promoted disassembly of the Golgi apparatus

N-Ethylmaleimide-sensitive factor (NSF) is required for multiple pathways of vesicle-mediated protein transport. Microinjection of a monoclonal anti-NSF antibody almost completely blocked brefeldin A-promoted Golgi disassembly without affecting the rapid release of beta-COP, a subunit of the Golgi coat proteins (COPI), from the Golgi apparatus. Similar results were obtained using a dominant-negative NSF which is known to compete with endogenous NSF. The present results suggest that an NSF-mediated step is present in the brefeldin A-promoted disassembly of the Golgi apparatus.

Rab6 regulation of rhodopsin transport in Drosophila

Rab6 is a GTP binding protein that regulates vesicular trafficking within the Golgi and post-Golgi compartments. We overexpressed wild-type, a GTPase defective (Q71L), and a guanine nucleotide binding defective (N125I) Rab6 protein in Drosophila photoreceptors to assess the in vivo role of Rab6 in the trafficking of rhodopsin and other proteins. Expression of Drab6(Q71L) greatly reduced the steady state levels of two rhodopsins, Rh1 and Rh3, whereas Drab6(wt) and Drab6(N125I) showed weaker effects. Analysis of a strain carrying Rh1 rhodopsin under a heat shock promoter showed that Drab6(Q71L), but not Drab6(wt) or Drab6(N125I), prevents the maturation of rhodopsin beyond an immature 40 kDa form. Drab6(Q71L) is a GTPase defective mutant, indicating that anterograde transport of rhodopsin requires Rab6 GTPase function. The three Drab6 strains had no effect on the expression of several other photoreceptor proteins. The Drab6(Q71L) photoreceptors show marked histological defects at young ages and degenerate over a two week time span. These results establish that rhodopsin is transported via a Rab6 regulated pathway and that defects in trafficking pathways lead to retinal degeneration.