K6-linked ubiquitylation marks formaldehyde-induced RNA-protein crosslinks for resolution

Reactive aldehydes are produced by normal cellular metabolism or after alcohol consumption, and they accumulate in human tissues if aldehyde clearance mechanisms are impaired. Their toxicity has been attributed to the damage they cause to genomic DNA and the subsequent inhibition of transcription and replication. However, whether interference with other cellular processes contributes to aldehyde toxicity has not been investigated. We demonstrate that formaldehyde induces RNA-protein crosslinks (RPCs) that stall the ribosome and inhibit translation in human cells. RPCs in the messenger RNA (mRNA) are recognized by the translating ribosomes, marked by atypical K6-linked ubiquitylation catalyzed by the RING-in-between-RING (RBR) E3 ligase RNF14, and subsequently resolved by the ubiquitin- and ATP-dependent unfoldase VCP. Our findings uncover an evolutionary conserved formaldehyde-induced stress response pathway that protects cells against RPC accumulation in the cytoplasm, and they suggest that RPCs contribute to the cellular and tissue toxicity of reactive aldehydes.

Dormant SOX9-Positive Cells Facilitate MYC-Driven Recurrence of Medulloblastoma

Relapse is the leading cause of death in patients with medulloblastoma, the most common malignant pediatric brain tumor. A better understanding of the mechanisms underlying recurrence could lead to more effective therapies for targeting tumor relapses. Here, we observed that SOX9, a transcription factor and stem cell/glial fate marker, is limited to rare, quiescent cells in high-risk medulloblastoma with MYC amplification. In paired primary-recurrent patient samples, SOX9-positive cells accumulated in medulloblastoma relapses. SOX9 expression anti-correlated with MYC expression in murine and human medulloblastoma cells. However, SOX9-positive cells were plastic and could give rise to a MYC high state. To follow relapse at the single-cell level, an inducible dual Tet model of medulloblastoma was developed, in which MYC expression was redirected in vivo from treatment-sensitive bulk cells to dormant SOX9-positive cells using doxycycline treatment. SOX9 was essential for relapse initiation and depended on suppression of MYC activity to promote therapy resistance, epithelial-mesenchymal transition, and immune escape. p53 and DNA repair pathways were downregulated in recurrent tumors, whereas MGMT was upregulated. Recurrent tumor cells were found to be sensitive to treatment with an MGMT inhibitor and doxorubicin. These findings suggest that recurrence-specific targeting coupled with DNA repair inhibition comprises a potential therapeutic strategy in patients affected by medulloblastoma relapse.

SIGNIFICANCE

SOX9 facilitates therapy escape and recurrence in medulloblastoma via temporal inhibition of MYC/MYCN genes, revealing a strategy to specifically target SOX9-positive cells to prevent tumor relapse.

TMOD-25. LATENT SOX9-POSITIVE CELLS BEHIND MYC-DRIVEN MEDULLOBLASTOMA RELAPSE

Tumor recurrence developing from therapy resistance, immune escape and metastasis is the leading cause of death in medulloblastoma, the most frequent malignant pediatric brain tumor. Amplification of MYC genes is the most common genetic alteration in Group 3 and Group 4 subgroups that constitute two thirds of medulloblastoma. SOX9 is a transcription factor present in stem cells in the normal brain but is limited to rare, quiescent cells in medulloblastoma patients with MYC gene amplifications. By studying paired primary-recurrent patient samples and patient-derived xenografts we here identified significant accumulation of SOX9-positive cells in Group 3 and Group 4 relapses. To follow relapse at the single cell level we developed an inducible dual Tet model of MYC-driven MB, where MYC was re-directed from the treatment-sensitive bulk cells to resistant, dormant SOX9-positive cells by doxycycline. In this model, distant recurrent tumors and spinal metastases developed. SOX9 promoted immune escape, DNA repair suppression and was essential for recurrence. Tumor cell dormancy was non-hierarchical, migratory and depended on MYC suppression by SOX9 to promote relapse. By using computational modeling and treatment we also showed how doxorubicin and MGMT inhibitors were specifically targeting recurrent cells that could be of potential use in future treatments for patients affected by these fatal relapses.

PTENP1-AS contributes to BRAF inhibitor resistance and is associated with adverse clinical outcome in stage III melanoma

BRAF inhibitors (BRAFi) selectively target oncogenic BRAFV600E/K and are effective in 80% of advanced cutaneous malignant melanoma cases carrying the V600 mutation. However, the development of drug resistance limits their clinical efficacy. Better characterization of the underlying molecular processes is needed to further improve treatments. We previously demonstrated that transcription of PTEN is negatively regulated by the PTEN pseudogene antisense RNA, PTENP1-AS, and here we investigated the impact of this transcript on clinical outcome and BRAFi resistance in melanoma. We observed that increased expression levels of PTENP1-AS in BRAFi resistant cells associated with enrichment of EZH2 and H3K27me3 at the PTEN promoter, consequently reducing the expression levels of PTEN. Further, we showed that targeting of the PTENP1-AS transcript sensitized resistant cells to BRAFi treatment and that high expression of PTENP1-AS in stage III melanoma correlated with poor survival. Collectively, the data presented here show that PTENP1-AS is a promising target for re-sensitizing cells to BRAFi and also a possible prognostic marker for clinical outcome in stage III melanoma.

K63-linked ubiquitylation induces global sequestration of mitochondria

Even though K63-linked polyubiquitin chains do not target proteins for proteasomal degradation, they play nevertheless a complementary protective role in maintaining protein homeostasis by directing malfunctioning proteins and organelles to inclusion bodies or autophagosomes. A paradigm for this process is the sequestration and autophagic degradation of dysfunctional mitochondria. Although studies have shown that K63-ubiquitylation of mitochondrial proteins by the ubiquitin ligase Parkin is important in this process, it is presently not clear if this modification also suffices to initiate this cascade of events. To address this question, we have engineered the ubiquitin ligase ProxE3, which in an inducible manner synthesizes K63-linked ubiquitin chains on the surface of mitochondria. We found that the presence of K63-linked ubiquitin chains on mitochondria resulted in the recruitment of the ubiquitin adaptor p62 and induced a dramatic redistribution of mitochondria, which was reminiscent to the Parkin-facilitated sequestration in response to mitochondrial uncoupler. However, ProxE3 did not induce autophagic degradation of mitochondria. Our data show that K63-linked ubiquitin chains at the mitochondrial membrane are sufficient for the induction of mitochondrial sequestration, but not mitophagy, without the need of extrinsically inflicting mitochondrial dysfunction.

MBRS-10. QUIESCENT SOX9-POSITIVE CELLS BEHIND MYC DRIVEN MEDULLOBLASTOMA RECURRENCE

Tumor recurrence is the leading cause of death in medulloblastoma, the most frequent malignant pediatric brain tumor. Recurrence occurs when subpopulations of cancer cells evade standard therapy by acquiring features of immune escape, metastatic spread, and treatment resistance. The transcription factor SOX9 correlated with treatment resistance and dissemination in aggressive Group 3 medulloblastoma. By studying paired primary-recurrent medulloblastoma samples and patient-derived xenograft models, we identified rare SOX9-positive slow-cycling, therapy-resistant tumor cells that accumulate in relapses and in metastases. In an inducible transgenic Group 3 tumor model, doxycycline treatment kills all tumor cells by turning MYC off. However, when MYC expression was redirected to the SOX9 promoter, recurrences from rare, dormant SOX9-positive cells developed with 100% penetrance. Expression profiling revealed that recurrences were more inflammatory, metastatic, and showed elevated MGMT methyltransferase levels which depleted recurrent cells when selectively inhibited. Our model explains how recurrences develop from SOX9-induced quiescence in MYC-driven brain cancer.

Ex vivo assessment of targeted therapies in a rare metastatic epithelial-myoepithelial carcinoma

Epithelial-myoepithelial carcinoma (EMC) is a rare subtype of salivary gland neoplasms. Since the initial description of the cancer, just over 300 cases have been reported. EMCs occupy a biphasic cellular differentiation-state defined by the constitution of two cell types representing epithelial and myoepithelial lineages, yet the functional consequence of the differentiation-state heterogeneity with respect to therapy resistance of the tumors remains unclear. The reported local recurrence rate of the cases is approximately 30%, and while distant metastases are rare, a significant fraction of these cases are reported to receive no survival benefit from radio- or chemotherapy given in addition to surgery. Moreover, no targeted therapies have been reported for these neoplasms. We report here the first use and application of ex vivo drug screening together with next generation sequencing to assess targeted treatment strategies for a rare metastatic epithelial-myoepithelial carcinoma. Results of the ex vivo drug screen demonstrate significant differential therapeutic sensitivity between the epithelial and myoepithelial intra-tumor cell lineages suggesting that differentiation-state heterogeneity within epithelial-myoepithelial carcinomas may present an outlet to partial therapeutic responses to targeted therapies including MEK and mTOR inhibitors. These results suggest that the intra-tumor lineage composition of EMC could be an important factor to be assessed when novel treatments are being evaluated for management of metastatic EMC.

PTEN and DNA-PK determine sensitivity and recovery in response to WEE1 inhibition in human breast cancer

Inhibition of WEE1 kinase by AZD1775 has shown promising results in clinical cancer trials, but markers predicting AZD1775 response are lacking. Here we analysed AZD1775 response in a panel of human breast cancer (BC) cell lines by global proteome/transcriptome profiling and identified two groups of basal-like BC (BLBCs): ‘PTEN low’ BLBCs were highly sensitive to AZD1775 and failed to recover following removal of AZD1775, while ‘PTEN high’ BLBCs recovered. AZD1775 induced phosphorylation of DNA-PK, protecting cells from replication-associated DNA damage and promoting cellular recovery. Deletion of DNA-PK or PTEN, or inhibition of DNA-PK sensitized recovering BLBCs to AZD1775 by abrogating replication arrest, allowing replication despite DNA damage. This was linked to reduced CHK1 activation, increased cyclin E levels and apoptosis. In conclusion, we identified PTEN and DNA-PK as essential regulators of replication checkpoint arrest in response to AZD1775 and defined PTEN as a promising biomarker for efficient WEE1 cancer therapy.

MEDU-26. LATENT SOX9-POSITIVE CELLS RESPONSIBLE FOR MYC-DRIVEN MEDULLOBLASTOMA RECURRENCE

Tumor recurrence is the leading cause of death among children with medulloblastoma, the most common type of malignant pediatric brain tumors. The mechanisms behind medulloblastoma recurrence are not fully understood. We previously showed that the transcription factor SOX9 promotes cisplatin treatment resistance in medulloblastoma. Here we show that SOX9 levels correlate with poor prognosis in Group 3 tumors. By studying paired primary-recurrent medulloblastoma samples and patient-derived xenograft (PDX) models we further identified rare SOX9-positive slow-cycling, therapy-resistant tumor cells that accumulate in relapses and in leptomenigeal metastases of Group 3 and Group 4 patients. By using an inducible Tet-OFF transgenic (GTML) mouse model for malignant MYCN-driven Group 3 tumors we identified rare SOX9-positive, quiescent brain tumor cells that are more resistant to cisplatin. Dox treatment normally cures GTML transgenic animals that developed aggressive medulloblastoma by turning MYCN off. However, when crossing the Tet-OFF GTML model with a Tet-ON rtTA-Sox9 model we can redirect MYCN expression to the Sox9 promoter ultimately driving brain tumor recurrence from rare SOX9-positive cells with 100% penetrance. In this novel animal model, recurrent tumors were actively disseminating from the hindbrain to the spinal cord and into the forebrain similar to distant relapses found in patients. By overexpressing SOX9 in human Group 3 tumor cells, MYC was directly inhibited and cell proliferation was decreased. PDX models of Group 3 tumors further showed increased levels of SOX9-positivity and less proliferative cells in metastatic compartments. Expression profiling revealed that recurrences were more inflammatory, metastatic, immune evasive and showed elevated MGMT methyltransferase levels which depleted recurrent cells and sensitized them for chemotherapy when using the MGMT inhibitor lomeguatrib. To summarize, our data clarify important and complex mechanisms by which latent medulloblastoma cells fail to respond to standard therapy and generate relapses.

Targeting SOX9 for degradation to inhibit chemoresistance, metastatic spread, and recurrence

Cancer cells with stem-like properties are believed to contribute to treatment resistance, dissemination, and recurrence. SOX9 controls stem cell plasticity and its deregulation may provide a basis for tumor progression. Here, we summarize our findings of targeted SOX9 destruction by SCF^FBW7 (Skp1/Cul1/F-box) in medulloblastoma and its potential for therapeutic intervention.

FBW7 suppression leads to SOX9 stabilization and increased malignancy in medulloblastoma

SOX9 is a master transcription factor that regulates development and stem cell programs. However, its potential oncogenic activity and regulatory mechanisms that control SOX9 protein stability are poorly understood. Here, we show that SOX9 is a substrate of FBW7, a tumor suppressor, and a SCF (SKP1/CUL1/F‐box)‐type ubiquitin ligase. FBW7 recognizes a conserved degron surrounding threonine 236 (T236) in SOX9 that is phosphorylated by GSK3 kinase and consequently degraded by SCF^FBW7α. Failure to degrade SOX9 promotes migration, metastasis, and treatment resistance in medulloblastoma, one of the most common childhood brain tumors. FBW7 is either mutated or downregulated in medulloblastoma, and in cases where FBW7 mRNA levels are low, SOX9 protein is significantly elevated and this phenotype is associated with metastasis at diagnosis and poor patient outcome. Transcriptional profiling of medulloblastoma cells expressing a degradation‐resistant SOX9 mutant reveals activation of pro‐metastatic genes and genes linked to cisplatin resistance. Finally, we show that pharmacological inhibition of PI3K/AKT/mTOR pathway activity destabilizes SOX9 in a GSK3/FBW7‐dependent manner, rendering medulloblastoma cells sensitive to cytostatic treatment.

TRIM16 overexpression induces apoptosis through activation of caspase-2 in cancer cells

TRIM16 exhibits tumour suppressor functions by interacting with cytoplasmic vimentin and nuclear E2F1 proteins in neuroblastoma and squamous cell carcinoma cells, reducing cell migration and replication. Reduced TRIM16 expression in a range of human primary malignant tissues correlates with increased malignant potential. TRIM16 also induces apoptosis in breast and lung cancer cells, by unknown mechanisms. Here we show that overexpression of TRIM16 induces apoptosis in human breast cancer (MCF7) and neuroblastoma (BE(2)-C) cells, but not in non-malignant HEK293 cells. TRIM16 increased procaspase-2 protein levels in MCF7 and induced caspase-2 activity in both MCF7 and BE(2)-C cells. We show that TRIM16 and caspase-2 proteins directly interact in both MCF7 and BE(2)-C cells and co-localise in MCF7 cells. Most importantly, the induction of caspase-2 activity is required for TRIM16 to initiate apoptosis. Our data suggest a novel mechanism by which TRIM16 can promote apoptosis by directly modulating caspase-2 activity.

CDK-mediated activation of the SCF(FBXO) (28) ubiquitin ligase promotes MYC-driven transcription and tumourigenesis and predicts poor survival in breast cancer

SCF (Skp1/Cul1/F-box) ubiquitin ligases act as master regulators of cellular homeostasis by targeting key proteins for ubiquitylation. Here, we identified a hitherto uncharacterized F-box protein, FBXO28 that controls MYC-dependent transcription by non-proteolytic ubiquitylation. SCF^FBXO28 activity and stability are regulated during the cell cycle by CDK1/2-mediated phosphorylation of FBXO28, which is required for its efficient ubiquitylation of MYC and downsteam enhancement of the MYC pathway. Depletion of FBXO28 or overexpression of an F-box mutant unable to support MYC ubiquitylation results in an impairment of MYC-driven transcription, transformation and tumourigenesis. Finally, in human breast cancer, high FBXO28 expression and phosphorylation are strong and independent predictors of poor outcome. In conclusion, our data suggest that SCF^FBXO28 plays an important role in transmitting CDK activity to MYC function during the cell cycle, emphasizing the CDK-FBXO28-MYC axis as a potential molecular drug target in MYC-driven cancers, including breast cancer.

Selenium-containing amino acids as direct and indirect antioxidants

Selenium is a trace element essential for normal physiological processes. Organic selenium-containing amino acids, such as selenocysteine (Sec) / selenocystine and selenomethionine (SeMet, the major dietary form), can provide antioxidant benefits by acting both as direct antioxidants as well as a source of selenium for synthesis of selenium-dependent antioxidant and repair proteins (e.g., glutathione peroxidases, thioredoxin reductases, methionine sulfoxide reductases). The direct antioxidant actions of these amino acids arise from the nucleophilic properties of the ionized selenol (RSe(-), which predominates over the neutral form at physiological pH values) and the ease of oxidation of Sec and SeMet. This results in higher rate constants for reaction with multiple oxidants, than for the corresponding thiols/thioethers. Furthermore, the resulting oxidation products are more readily and rapidly reversed by both enzyme and nonenzymatic reactions. The antioxidant effects of these seleno species can therefore be catalytic. Seleno amino acids may also chelate redox-active metal ions. The presence of Sec in the catalytic site of selenium-dependent antioxidant enzymes enhances the kinetic properties and broadens the catalytic activity of antioxidant enzymes against biological oxidants when compared with sulfur-containing species. However, while normal physiological selenium levels afford protection, when compared with deficiency, excessive selenium may induce damage and adverse effects, with this being manifest, for example, as an increased incidence of type 2 diabetes. Further studies examining the availability of redox-active selenium species and their mechanisms and kinetics of action are therefore of critical importance in the potential development of seleno species as a therapeutic strategy.

Photo-oxidation-induced inactivation of the selenium-containing protective enzymes thioredoxin reductase and glutathione peroxidase

Singlet oxygen ((1)O(2)) is a reactive oxygen species generated during photo-oxidation, inflammation, and via peroxidase-catalyzed reactions (e.g., myeloperoxidase and eosinophil peroxidase). (1)O(2) oxidizes the free amino acids Trp, Tyr, His, Cys, and Met, and those species present on peptides/proteins, with this resulting in modulation of protein structure and function. Impairment of the activity of antioxidant enzymes may be of relevance to the oxidative stress observed in a number of pathologies involving either light exposure or inflammation. In this study, the effects of (1)O(2) on glutathione peroxidase (GPx) and thioredoxin reductase (TrxR) activity, including the mechanisms of their inactivation, were investigated. Exposure of GPx or TrxR, either as purified proteins or in cell lysates, to Rose Bengal and visible light (an established source of (1)O(2)) resulted in significant, photolysis time-dependent reductions in enzyme activity (10-40%, P<0.05). More extensive inhibition (ca. 2-fold) was detected when the reactions were carried out in D(2)O, consistent with the intermediacy of (1)O(2). No additional inhibition was detected after the cessation of photolysis, eliminating a role for photo-products. Methionine, which reacts rapidly with (1)O(2) (k~10(7)M(-1) s(-1))(,) significantly reduced photo-inactivation at large molar excesses, presumably by acting as an alternative target. Reductants (NaBH(4), DTT, GSH, or NADPH) added after the cessation of (1)O(2) formation were unable to reverse enzyme inactivation, consistent with irreversible enzyme oxidation. Formation of nonreducible protein aggregates and/or fragments was detected for both photo-oxidized GPx and TrxR by SDS-PAGE. An oxidant concentration-dependent increase in protein carbonyls was detected with TrxR but not GPx. These studies thus demonstrate that the antioxidant enzymes GPx and TrxR can be irreversibly inactivated by (1)O(2).

Inhibition of lysosomal function in macrophages incubated with elevated glucose concentrations: a potential contributory factor in diabetes-associated atherosclerosis

OBJECTIVE

People with diabetes have an elevated risk of atherosclerosis. The accumulation of lipid within macrophage cells in the artery wall is believed to arise via the uptake and subsequent processing of modified low-density lipoproteins (LDL) via the endo-lysosomal system. In this study the effects of prolonged exposure to elevated glucose upon macrophage lysosomal function was examined to determine whether this contributes to modulated protein catabolism.

METHODS

Human monocytes were isolated from white-cell concentrates and differentiated, in vitro, into monocyte-derived macrophages over 11 days in medium containing 5-30 mmol/L glucose. Murine macrophage-like J774A.1 cells were incubated similarly. Lysosomal cathepsin (B, D, L and S) and acid lipase activities were assessed using fluorogenic substrates; cathepsin protein levels were examined by Western blotting. Lysosomal numbers were examined using the lysomotropic fluorescent dye LysoTracker DND-99, measurement of aryl sulfatase activity, and quantification of lysosome-associated membrane glycoprotein-1 (LAMP-1) by Western blotting.

RESULTS

Exposure to elevated glucose, but not mannitol, resulted in a concentration-dependent decrease in the activity, and to a lesser extent protein levels, of four lysosomal cathepsins. Acid lipase activity was also significantly reduced. Arysulfatase activity, LAMP-1 levels and lysosomal numbers were also decreased at the highest glucose concentrations, though to a lesser extent.

CONCLUSION

Long term exposure of human and murine macrophage cells to elevated glucose levels result in a depression of lysosomal proteolytic and lipase activities. This may result in decreased clearance and cellular accumulation of (lipo)proteins and contribute to the accumulation of modified proteins and lipids in diabetes-associated atherosclerosis.

Catalytic activity of selenomethionine in removing amino acid, peptide, and protein hydroperoxides

Selenium is a critical trace element, with deficiency associated with numerous diseases including cardiovascular disease, diabetes, and cancer. Selenomethionine (SeMet; a selenium analogue of the amino acid methionine, Met) is a major form of organic selenium and an important dietary source of selenium for selenoprotein synthesis in vivo. As selenium compounds can be readily oxidized and reduced, and selenocysteine residues play a critical role in the catalytic activity of the key protective enzymes glutathione peroxidase and thioredoxin reductase, we investigated the ability of SeMet (and its sulfur analogue, Met) to scavenge hydroperoxides present on amino acids, peptides, and proteins, which are key intermediates in protein oxidation. We show that SeMet, but not Met, can remove these species both stoichiometrically and catalytically in the presence of glutathione (GSH) or a thioredoxin reductase (TrxR)/thioredoxin (Trx)/NADPH system. Reaction of the hydroperoxide with SeMet results in selenoxide formation as detected by HPLC. Recycling of the selenoxide back to SeMet occurs rapidly with GSH, TrxR/NADPH, or a complete TrxR/Trx/NADPH reducing system, with this resulting in an enhanced rate of peroxide removal. In the complete TrxR/Trx/NADPH system loss of peroxide is essentially stoichiometric with NADPH consumption, indicative of a highly efficient system. Similar reactions do not occur with Met under these conditions. Studies using murine macrophage-like J774A.1 cells demonstrate a greater peroxide-removing capacity in cells supplemented with SeMet, compared to nonsupplemented controls. Overall, these findings demonstrate that SeMet may play an important role in the catalytic removal of damaging peptide and protein oxidation products.

Photo-oxidation of proteins

Photo-induced damage to proteins occurs via multiple pathways. Direct damage induced by UVB (λ 280-320 nm) and UVA radiation (λ 320-400 nm) is limited to a small number of amino acid residues, principally tryptophan (Trp), tyrosine (Tyr), histidine (His) and disulfide (cystine) residues, with this occurring via both excited state species and radicals. Indirect protein damage can occur via singlet oxygen ((1)O(2)(1)Δ(g)), with this resulting in damage to Trp, Tyr, His, cystine, cysteine (Cys) and methionine (Met) residues. Although initial damage is limited to these residues multiple secondary processes, that occur both during and after radiation exposure, can result in damage to other intra- and inter-molecular sites. Secondary damage can arise via radicals (e.g. Trp, Tyr and Cys radicals), from reactive intermediates generated by (1)O(2) (e.g. Trp, Tyr and His peroxides) and via molecular reactions of photo-products (e.g. reactive carbonyls). These processes can result in protein fragmentation, aggregation, altered physical and chemical properties (e.g. hydrophobicity and charge) and modulated biological turnover. Accumulating evidence implicates these events in cellular and tissue dysfunction (e.g. apoptosis, necrosis and altered cell signaling), and multiple human pathologies.

Cellular effects of photogenerated oxidants and long-lived, reactive, hydroperoxide photoproducts

Reaction of radicals and singlet oxygen ((1)O(2)) with proteins results in both direct damage and the formation of long-lived reactive hydroperoxides. Elevated levels of protein hydroperoxide-derived products have been detected in multiple human pathologies, suggesting that these secondary oxidants contribute to tissue damage. Previous studies have provided evidence for protein hydroperoxide-mediated inhibition of thiol-dependent enzymes and modulation of signaling processes in isolated systems. In this study (1)O(2) and hydroperoxides have been generated in J774A.1 macrophage-like cells using visible light and the photosensitizer rose bengal, with the consequences of oxidant formation examined both immediately and after subsequent (dark-phase) incubation. Significant losses of GSH (≤50%), total thiols (≤20%), and activity of thiol-dependent proteins (GAPDH, thioredoxin, protein tyrosine phosphatases, creatine kinase, and cathepsins B and L; 10-50% inhibition) were detected after 1 or 2 min photo-oxidation. Non-thiol-dependent enzymes were not affected. In contrast, NADPH levels increased, together with the activity of glutathione reductase, glutathione peroxidase, and thioredoxin reductase; these increases may be components of a rapid global cytoprotective cellular response to stress. Neither oxidized thioredoxin nor radical-mediated protein oxidation products were detected at significant levels. Further decreases in thiol levels and enzyme activity occurred during dark-phase incubation, with this accompanied by decreased cell viability. These secondary events are ascribed to the reactions of long-lived hydroperoxides, generated by (1)O(2)-mediated reactions. Overall, this study provides novel insights into early cellular responses to photo-oxidative damage and indicates that long-lived hydroperoxides can play a significant role in cellular damage.

Cellular effects of peptide and protein hydroperoxides

Hydroperoxides are major products of the reactions of radicals and singlet oxygen with amino acids, peptides, and proteins. These species can generate radicals in the presence of metal ions and oxidize thiols via nonradical reactions, but the effects of these materials on cells are poorly understood. In this study the exposure of murine macrophage-like cells to preformed peptide or protein hydroperoxides is shown to result in hydroperoxide consumption and cellular thiol oxidation; these effects precede loss of cell viability. N-acetyltryptophan methyl ester hydroperoxides, but not the decomposed species, decreased total cellular thiols and GSH, with the latter occurring more rapidly. Time-dependent inhibition of lysosomal cathepsins B and L was also observed, together with diminished caspase 3/7 activity. A number of other cytosolic thiol- and non-thiol-dependent enzymes were not affected significantly. Hydroperoxides formed on BSA did not deplete total thiols or GSH within cells, although such reactions are rapid in model systems. In contrast, selective inhibition of cathepsins B and L (but not cathepsin D or arylsulfatase) of the endosomal-lysosomal system was detected, consistent with localization within these compartments. Decomposed BSA hydroperoxides did not induce these effects, indicating a requirement for the hydroperoxide group. The differences between these hydroperoxides are ascribed to their mechanisms of penetration into cells. Overall these studies provide valuable data on the initial cellular events arising from exposure to exogenous protein and amino acid peroxides and indicate that cellular thiols are a major target. This selective oxidation may modulate cellular redox balance and subsequent cell behavior.

Role of ABCG1 and ABCA1 in regulation of neuronal cholesterol efflux to apolipoprotein E discs and suppression of amyloid-beta peptide generation

Maintenance of an adequate supply of cholesterol is important for neuronal function, whereas excess cholesterol promotes amyloid precursor protein (APP) cleavage generating toxic amyloid-beta (Abeta) peptides. To gain insights into the pathways that regulate neuronal cholesterol level, we investigated the potential for reconstituted apolipoprotein E (apoE) discs, resembling nascent lipoprotein complexes in the central nervous system, to stimulate neuronal [3H]cholesterol efflux. ApoE discs potently accelerated cholesterol efflux from primary human neurons and cell lines. The process was saturable (17.5 microg of apoE/ml) and was not influenced by APOE genotype. High performance liquid chromatography analysis of cholesterol and cholesterol metabolites effluxed from neurons indicated that <25% of the released cholesterol was modified to polar products (e.g. 24-hydroxycholesterol) that diffuse from neuronal membranes. Thus, most cholesterol (approximately 75%) appeared to be effluxed from neurons in a native state via a transporter pathway. ATP-binding cassette transporters ABCA1, ABCA2, and ABCG1 were detected in neurons and neuroblastoma cell lines and expression of these cDNAs revealed that ABCA1 and ABCG1 stimulated cholesterol efflux to apoE discs. In addition, ABCA1 and ABCG1 expression in Chinese hamster ovary cells that stably express human APP significantly reduced Abeta generation, whereas ABCA2 did not modulate either cholesterol efflux or Abeta generation. These data indicate that ABCA1 and ABCG1 play a significant role in the regulation of neuronal cholesterol efflux to apoE discs and in suppression of APP processing to generate Abeta peptides.