Rapid phosphorylation of heterogeneous nuclear ribonucleoprotein C1/C2 in response to physiologic levels of hydrogen peroxide in human endothelial cells

DUSP3 modulates IRES-dependent translation of mRNAs through dephosphorylation of the HNRNPC protein in cells under genotoxic stimulus

BACKGROUND INFORMATION

The dual-specificity phosphatase 3 (DUSP3) regulates cell cycle progression, proliferation, senescence, and DNA repair pathways under genotoxic stress. This phosphatase interacts with HNRNPC protein suggesting an involvement in the regulation of HNRNPC-ribonucleoprotein complex stability. In this work, we investigate the impact of DUSP3 depletion on functions of HNRNPC aiming to suggest new roles for this enzyme.

RESULTS

The DUSP3 knockdown results in the tyrosine hyperphosphorylation state of HNRNPC increasing its RNA binding ability. HNRNPC is present in the cytoplasm where it interacts with IRES trans-acting factors (ITAF) complex, which recruits the 40S ribosome on mRNA during protein synthesis, thus facilitating the translation of mRNAs containing IRES sequence in response to specific stimuli. In accordance with that, we found that DUSP3 is present in the 40S, monosomes and polysomes interacting with HNRNPC, just like other previously identified DUSP3 substrates/interacting partners such as PABP and NCL proteins. By downregulating DUSP3, Tyr-phosphorylated HNRNPC preferentially binds to IRES-containing mRNAs within ITAF complexes preferentially in synchronized or stressed cells, as evidenced by the higher levels of proteins such as c-MYC and XIAP, but not their mRNAs such as measured by qPCR. Under DUSP3 absence, this increased phosphorylated-HNRNPC/RNA interaction reduces HNRNPC-p53 binding in presence of RNAs releasing p53 for specialized cellular responses. Similarly, to HNRNPC, PABP physically interacts with DUSP3 in an RNA-dependent manner.

CONCLUSIONS AND SIGNIFICANCE

Overall, DUSP3 can modulate cellular responses to genotoxic stimuli at the translational level by maintaining the stability of HNRNPC-ITAF complexes and regulating the intensity and specificity of RNA interactions with RRM-domain proteins.

hnRNP C modulates MERS-CoV and SARS-CoV-2 replication by governing the expression of a subset of circRNAs and cognitive mRNAs

ABSTRACTHost circular RNAs (circRNAs) play critical roles in the pathogenesis of viral infections. However, how viruses modulate the biogenesis of host proviral circRNAs to facilitate their replication remains unclear. We have recently shown that Middle East respiratory syndrome coronavirus (MERS-CoV) infection increases co-expression of circRNAs and their cognate messenger RNAs (mRNAs), possibly by hijacking specific host RNA binding proteins (RBPs). In this study, we systemically analysed the interactions between the representative circRNA-mRNA pairs upregulated upon MERS-CoV infection and host RBPs. Our analysis identified heterogeneous nuclear ribonucleoprotein C (hnRNP C) as a key host factor that governed the expression of numerous MERS-CoV-perturbed circRNAs, including hsa_circ_0002846, hsa_circ_0002061, and hsa_circ_0004445. RNA immunoprecipitation assay showed that hnRNP C could bind physically to these circRNAs. Specific knockdown of hnRNP C by small interfering RNA significantly (P < 0.05 to P < 0.0001) suppressed MERS-CoV replication in human lung adenocarcinoma (Calu-3) and human small airway epithelial (HSAEC) cells. Both MERS-CoV and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection increased the total and phosphorylated forms of hnRNP C to activate the downstream CRK-mTOR pathway. Treatment of MERS-CoV- (IC₅₀: 0.618 µM) or SARS-CoV-2-infected (IC₅₀: 1.233 µM) Calu-3 cells with the mTOR inhibitor OSI-027 resulted in significantly reduced viral loads. Collectively, our study identified hnRNP C as a key regulator of MERS-CoV-perturbed circRNAs and their cognate mRNAs, and the potential of targeting hnRNP C-related signalling pathways as an anticoronaviral strategy.

Critical regulators of endothelial cell functions: for a change being alternative

SIGNIFICANCE

The endothelium regulates vessel dilation and constriction, balances hemostasis, and inhibits thrombosis. In addition, pro- and anti-angiogenic molecules orchestrate proliferation, survival, and migration of endothelial cells. Regulation of all these processes requires fine-tuning of signaling pathways, which can easily be tricked into running the opposite direction when exogenous or endogenous signals get out of hand. Surprisingly, some critical regulators of physiological endothelial functions can turn malicious by mere alternative splicing, leading to the expression of protein isoforms with opposite functions.

RECENT ADVANCES

While reviewing the evidence of alternative splicing on cellular physiology, it became evident that expression of splice factors and their activities are regulated by externally triggered signaling cascades. Furthermore, genome-wide identification of RNA-binding sites of splicing regulatory proteins now offer a glimpse into the splicing code responsible for alternative splicing of molecules regulating endothelial functions.

CRITICAL ISSUES

Due to the constantly growing number of transcript and protein isoforms, it will become more and more important to identify and characterize all transcripts and proteins regulating endothelial cell functions. One critical issue will be a non-ambiguous nomenclature to keep consistency throughout different laboratories.

FUTURE DIRECTIONS

RNA-deep sequencing focusing on exon-exon junction needs to more reliably identify alternative splicing events combined with functional analyses that will uncover more splice variants contributing to or inhibiting proper endothelial functions. In addition, understanding the signals mediating alternative splicing and its regulation might allow us to derive new strategies to preserve endothelial function by suppressing or upregulating specific protein isoforms. Antioxid. Redox Signal. 22, 1212-1229.

Phosphoproteomic profiling of selenate-treated Alzheimer's disease model cells

The reversible phosphorylation of proteins regulates most biological processes, while abnormal phosphorylation is a cause or consequence of many diseases including Alzheimer's disease (AD). One of the hallmarks of AD is the formation of neurofibrillary tangles (NFTs), which is composed of hyperphosphorylated tau proteins. Sodium selenate has been recently found to reduce tau hyperphosphorylation and NFTs formation, and to improve spatial learning and motor performance in AD mice. In the current study, the phosphoproteomics of N2aSW cells treated with selenate were investigated. To avoid missing low-abundance phosphoproteins, both the total proteins of cells and the phosphor-enriched proteins were extracted and subjected to the two-dimensional gel electrophoresis with Pro-Q diamond staining and then LC-MS/MS analysis. A total of 65 proteins were altered in phosphorylation level, of which 39 were up-regulated and 26 were down-regulated. All identified phosphoproteins were bioinformatically annotated according to their physiochemical features, subcellular location, and biological function. Most of these significantly changed phosphoproteins are involved in crucial neural processes such as protesome activity, oxidative stress, cysteine and methionine metabolism, and energy metabolism. Furthermore, decreases were found in homocysteine, phosphor-tau and amyloid β upon selenate treatment. Our results suggest that selenate may intervene in the pathological process of AD by altering the phosphorylation of some key proteins involved in oxidative stress, energy metabolism and protein degradation, thus play important roles in maintaining redox homeostasis, generating ATP, and clearing misfolded proteins and aggregates. The present paper provides some new clues to the mechanism of selenate in AD prevention.

Proteomic analysis of SP600125-controlled TrkA-dependent targets in SK-N-MC neuroblastoma cells: inhibition of TrkA activity by SP600125

The c-Jun N-terminal kinase (JNK) is well known to play an important role in cell death signaling of the p75 neurotrophin receptor. However, little has been studied about a role of JNK in the signaling pathways of the tropomyosin-related kinase A (TrkA) neurotrophin receptor. In this study, we investigated JNK inhibitor SP600125-controlled TrkA-dependent targets by proteomic analysis to better understand an involvement of JNK in TrkA-mediated signaling pathways. PDQuest image analysis and protein identification results showed that hnRNP C1/C2, α-tubulin, β-tubulin homolog, actin homolog, and eIF-5A-1 protein spots were upregulated by ectopic expression of TrkA, whereas α-enolase, peroxiredoxin-6, PROS-27, HSP70, PP1-gamma, and PDH E1-alpha were downregulated by TrkA, and these TrkA-dependent upregulation and downregulation were significantly suppressed by SP600125. Notably, TrkA largely affected certain PTM(s) but not total protein amounts of the SP600125-controlled TrkA-dependent targets. Moreover, SP600125 strongly suppressed TrkA-mediated tyrosine phosphorylation signaling pathways as well as JNK signaling, indicating that SP600125 could function as a TrkA inhibitor. Taken together, our results suggest that TrkA could play an important role in the cytoskeleton, cell death, cellular processing, and glucose metabolism through activation or inactivation of the SP600125-controlled TrkA-dependent targets.

Altered vascular activation due to deficiency of the NADPH oxidase component p22phox

BACKGROUND

Reactive oxygen species generated by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase play important roles in vascular activation. The p22(phox) subunit is necessary for the activity of NADPH oxidase complexes utilizing Nox1, Nox2, Nox3, and Nox4 catalytic subunits.

METHODS

We assessed p22(phox)-deficient mice and human tissue for altered vascular activation.

RESULTS

Mice deficient in p22(phox) were smaller than their wild-type littermates but showed no alteration in basal blood pressure. The wild-type littermates were relatively resistant to forming intimal hyperplasia following carotid ligation, and the intimal hyperplasia that developed was not altered by p22(phox) deficiency. However, at the site of carotid artery ligation, the p22(phox)-deficient mice showed significantly less vascular elastic fiber loss compared with their wild-type littermates. This preservation of elastic fibers was associated with a reduced matrix metallopeptidase (MMP) 12/tissue inhibitor of metalloproteinase (TIMP) 1 expression ratio. A similar decrease in the relative MMP12/TIMP1 expression ratio occurred in human coronary artery smooth muscle cells upon knockdown of the hydrogen peroxide responsive kinase CK1αLS. In the ligated carotid arteries, the p22(phox)-deficient mice showed reduced expression of heterogeneous nuclear ribonucleoprotein C (hnRNP-C), suggesting reduced activity of CK1αLS. In a lung biopsy from a human patient with p22(phox) deficiency, there was also reduced vascular hnRNP-C expression.

CONCLUSIONS

These findings indicate that NADPH oxidase complexes modulate aspects of vascular activation including vascular elastic fiber loss, the MMP12/TIMP1 expression ratio, and the expression of hnRNP-C. Furthermore, these findings suggest that the effects of NADPH oxidase on vascular activation are mediated in part by protein kinase CK1αLS.

Proteomic analysis of novel targets associated with TrkA-mediated tyrosine phosphorylation signaling pathways in SK-N-MC neuroblastoma cells

Tropomyosin-related kinase A (TrkA) is a receptor-type protein tyrosine kinase and exploits pleiotypic roles via nerve growth factor (NGF)-dependent or NGF-independent mechanisms in various cell types. Here, we showed that the inhibition of TrkA activity by GW441756 resulted in the suppression of tyrosine phosphorylation of cellular proteins including extracellular signal-regulated protein kinase (ERK) and c-Jun N-terminal kinase (JNK). To find novel targets associated with TrkA-mediated tyrosine phosphorylation signaling pathways, we investigated GW441756 effects on TrkA-dependent targets in SK-N-MC neuroblastoma cells by proteomic analysis. The major TrkA-dependent protein spots controlled by GW441756 were determined by PDQuest image analysis, identified by MALDI-TOF MS and MALDI-TOF/TOF MS/MS, and verified by 2DE/Western blot analysis. Thus, we found that most of the identified protein spots were modified forms in a normal condition, and their modifications were regulated by TrkA activity. Especially, our results demonstrated that the modifications of α-tubulin and heterogeneous nuclear ribonucleoproteins C1/C2 (hnRNP C1/C2) were significantly upregulated by TrkA, whereas α-enolase modification was downregulated by TrkA, and it was suppressed by GW441756, indicating that TrkA activity is required for their modifications. Taken together, we suggest here that the major novel TrkA-dependent targets such as α-tubulin, hnRNP C1/C2, and α-enolase could play an essential role in TrkA-mediated tyrosine phosphorylation signaling pathways via regulation of their posttranslational modifications.

Protein kinase CK1alphaLS promotes vascular cell proliferation and intimal hyperplasia

Protein kinase CK1alpha regulates several fundamental cellular processes including proliferation and differentiation. Up to four forms of this kinase are expressed in vertebrates resulting from alternative splicing of exons; these exons encode either the L-insert located within the catalytic domain or the S-insert located at the C terminus of the protein. Whereas the L-insert is known to target the kinase to the nucleus, the functional significance of nuclear CK1alphaLS has been unclear. Here we demonstrate that selective L-insert-targeted short hairpin small interfering RNA-mediated knockdown of CK1alphaLS in human vascular endothelial cells and vascular smooth muscle cells impairs proliferation and abolishes hydrogen peroxide-stimulated proliferation of vascular smooth muscle cells, with the cells accumulating in G(0)/G(1). In addition, selective knockdown of CK1alphaLS in cultured human arteries inhibits vascular activation, preventing smooth muscle cell proliferation, intimal hyperplasia, and proteoglycan deposition. Knockdown of CK1alphaLS results in the harmonious down-regulation of its target substrate heterogeneous nuclear ribonucleoprotein C and results in the altered expression or alternative splicing of key genes involved in cellular activation including CXCR4, MMP3, CSF2, and SMURF1. Our results indicate that the nuclear form of CK1alpha in humans, CK1alphaLS, plays a critical role in vascular cell proliferation, cellular activation, and hydrogen peroxide-mediated mitogenic signal transduction.

Analysis of PTEN complex assembly and identification of heterogeneous nuclear ribonucleoprotein C as a component of the PTEN-associated complex

PTEN (phosphatase and tensin homolog deleted on chromosome 10) is well characterized for its role in antagonizing the phosphoinositide 3-kinase pathway. Previous studies using size-exclusion chromatography demonstrated PTEN recruitment into high molecular mass complexes and hypothesized that PTEN phosphorylation status and PDZ binding domain may be required for such complex formation. In this study, we set out to test the structural requirements for PTEN complex assembly and identify the component(s) of the PTEN complex(es). Our results demonstrated that the PTEN catalytic function and PDZ binding domain are not absolutely required for its complex formation. On the other hand, PTEN phosphorylation status has a significant impact on its complex assembly. Our results further demonstrate enrichment of the PTEN complex in nuclear lysates, suggesting a mechanism through which PTEN phosphorylation may regulate its complex assembly. These results prompted further characterization of other protein components within the PTEN complex(es). Using size-exclusion chromatography and two-dimensional difference gel electrophoresis followed by mass spectrometry analysis, we identified heterogeneous nuclear ribonucleoprotein C (hnRNP C) as a novel protein recruited to higher molecular mass fractions in the presence of PTEN. Further analysis indicates that endogenous hnRNP C and PTEN interact and co-localize within the nucleus, suggesting a potential role for PTEN, alongside hnRNP C, in RNA regulation.

Role of hydrogen peroxide in NF-kappaB activation: from inducer to modulator

Hydrogen peroxide (H2O2) has been implicated in the regulation of the transcription factor NF-kappaB, a key regulator of the inflammatory process and adaptive immunity. However, no consensus exists regarding the regulatory role played by H2O2. We discuss how the experimental methodologies used to expose cells to H2O2 produce inconsistent results that are difficult to compare, and how the steady-state titration with H2O2 emerges as an adequate tool to overcome these problems. The redox targets of H2O2 in the NF-kappaB pathway--from the membrane to the post-translational modifications in both NF-kappaB and histones in the nucleus--are described. We also review how H2O2 acts as a specific regulator at the level of the single gene, and briefly discuss the implications of this regulation for human health in the context of kappaB polymorphisms. In conclusion, after near 30 years of research, H2O2 emerges not as an inducer of NF-kappaB, but as an agent able to modulate the activation of the NF-kappaB pathway by other agents. This modulation is generic at the level of the whole pathway but specific at the level of the single gene. Therefore, H2O2 is a fine-tuning regulator of NF-kappaB-dependent processes, as exemplified by its dual regulation of inflammation.

Modulation of Growth Factor Gene Expression in Vascular Cells by Oxidative Stress

Reactive oxygen species (ROS) generated in and around vascular endothelium may play a role in normal cellular signaling mechanisms but may also be an important causative factor in endothelial dysfunction underlying the development of atherosclerosis, diabetes complications, and ischemia-reperfusion injury. ROS influence a variety of molecular and cellular activities, including changes in the cellular localization of regulatory factors, protein modification, and altered gene expression, which in turn influence cellular phenotype. One mechanism by which ROS exert their cellular effects involves their ability to modulate the expression and function of vascular genes, such as vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), and platelet-derived growth factor (PDGF), which play key atherogenic roles by their regulation of cell growth, differentiation, and fibroproliferative responsiveness. In this review the authors describe the changes induced by oxidative stress on the profile of growth factor gene expression in endothelial cells, and the impact these modifications have on endothelial phenotype as well as on the behavior of neighboring vascular smooth muscle cells and fibroblasts. The authors also discuss the involvement of redox-sensitive transcription factors in these regulatory processes.

Up-regulation of a hydrogen peroxide-responsive pre-mRNA binding protein in atherosclerosis and intimal hyperplasia

BACKGROUND

Multiple lines of investigation have implicated hydrogen peroxide (H(2)O(2)) as an important endogenous mediator of cell proliferation in the vessel wall. Heterogeneous nuclear ribonucleoprotein C (hnRNP-C), a nuclear pre-mRNA binding protein that plays roles in vertebrate cell proliferation and differentiation, has been identified as a component of a vascular cell signaling pathway activated by low physiologic levels of H(2)O(2). The expression of hnRNP-C in human arteries has not previously been assessed.

METHODS

Segments of human proximal internal carotid arteries were evaluated for the expression of hnRNP-C by immunohistochemistry.

RESULTS

In normal proximal internal carotid arteries, hnRNP-C is expressed predominantly by the endothelium, with significantly lower expression by medial smooth muscle. In preatherosclerotic intimal hyperplasia, hnRNP-C is up-regulated in the artery wall, due to the robust expression by the intimal smooth muscle cells, without up-regulation in the medial smooth muscle cells. In arteries with atherosclerotic lesions, there is strong expression of hnRNP-C not only by intimal cells but also by medial smooth muscle cells.

CONCLUSIONS

The H(2)O(2) responsive pre-mRNA binding protein hnRNP-C is up-regulated in atherosclerosis and in preatherosclerotic intimal hyperplasia in humans, supporting the hypothesis that H(2)O(2) is a regulator of vascular cell proliferation in these conditions. These data also suggest that hnRNP-C may be useful as a marker of vascular cell activation.

Hydrogen peroxide inhibits cytochrome p450 epoxygenases: interaction between two endothelium-derived hyperpolarizing factors

The cytochrome P450 epoxygenase (CYP)-derived metabolites of arachidonic acid the epoxyeicosatrienoic acids (EETs) and hydrogen peroxide (H2O2) both function as endothelium-derived hyperpolarizing factors (EDHFs) in the human coronary microcirculation. However, the relative importance of and potential interactions between these 2 vasodilators remain unexplored. We identified a novel inhibitory interaction between CYPs and H2O2 in human coronary arterioles, where EDHF-mediated vasodilatory mechanisms are prominent. Bradykinin induced vascular superoxide and H2O2 production in an endothelium-dependent manner and elicited a concentration-dependent dilation that was reduced by catalase but not by 14,15-epoxyeicosa-5(Z)-enoic acid (EEZE), 6-(2-propargyloxyphenyl)hexanoic acid, sulfaphenazole, or iberiotoxin. However, in the presence of catalase, an inhibitory effect of these compounds was unmasked. In a tandem-bioassay preparation, application of bradykinin to endothelium-intact donor vessels elicited dilation of downstream endothelium-denuded detectors that was partially inhibited by donor-applied catalase but not by detector-applied EEZE; however, EEZE significantly inhibited dilation in the presence of catalase. EET production by human recombinant CYP 2C9 and 2J2, 2 major epoxygenase isozymes expressed in human coronary arterioles, was directly inhibited in a concentration-dependent fashion by H2O2 in vitro, as observed by high-performance liquid chromatography (HPLC); however, EETs were not directly sensitive to oxidative modification. H2O2 inhibited dilation to arachidonic acid but not to 11,12-EET. These findings suggest that an inhibitory interaction exists between 2 EDHFs in the human coronary microcirculation. CYP epoxygenases are directly inhibited by H2O2, and this interaction may modulate vascular EET bioavailability.

Regulation of protein kinase CK1alphaLS by dephosphorylation in response to hydrogen peroxide

Low levels of hydrogen peroxide (H(2)O(2)) are mitogenic to mammalian cells and stimulate the hyperphosphorylation of heterogeneous nuclear ribonucleoprotein C (hnRNP-C) by protein kinase CK1alpha. However, the mechanisms by which CK1alpha is regulated have been unclear. Here it is demonstrated that low levels of H(2)O(2) stimulate the rapid dephosphorylation of CK1alphaLS, a nuclear splice form of CK1alpha. Furthermore, it is demonstrated that either treatment of endothelial cells with H(2)O(2), or dephosphorylation of CK1alphaLS in vitro enhances the association of CK1alphaLS with hnRNP-C. In addition, dephosphorylation of CK1alphaLS in vitro enhances the kinase's ability to phosphorylate hnRNP-C. While CK1alpha appears to be present in all metazoans, analysis of CK1alpha genomic sequences from several species reveals that the alternatively spliced nuclear localizing L-insert is unique to vertebrates, as is the case for hnRNP-C. These observations indicate that CK1alphaLS and hnRNP-C represent conserved components of a vertebrate-specific H(2)O(2)-responsive nuclear signaling pathway.

Proteomic analysis of the response of the human neutrophil-like cell line NB-4 after exposure to anthrax lethal toxin

We used 2-D DIGE to analyze the early response of NB-4 cells, a human promyelotic leukemia cell line, exposed to lethal toxin from Bacillus anthracis at the proteome level. After a 2 h exposure, cells were still viable and 43% of spots (n = 1042) showed a significant change in protein level. We identified 59 spots whose expression had changed significantly, and these reflected cytoskeleton damage, mitochondrial lysis and endoplasmic reticulum stress. Actin filament assembly was disrupted as evidenced by an increase in both actin subunits and phosphorylated cofilin, whilst levels of tropomyosin, tropomodulin and actin related protein 2/3 complex subunit decreased. Lower levels of ATP synthase subunits and mitochondrial inner membrane protein were identified as markers of mitochondrial lysis. Levels of various stress response proteins rose and, uniquely, levels of Ca(2+) binding proteins such as translationally controlled tumor protein rose and hippocalcin-like protein 1 decreased. This response may have mitigated effects brought about by mitochondrial lysis and endoplasmic reticulum stress, and delayed or prevented apoptosis in NB-4 cells. These results resemble findings of similar proteomics studies in murine macrophages, although quantitative differences were observed.

Lipid Peroxidation During Human Cerebral Myelination

The critical period of human cerebral myelination is characterized by rapid production of cellular membranes. We hypothesize that this period is subject to the "physiological" generation of free radicals resulting in lipid peroxidation (LPO). In this study, oxidative markers were examined in developing human parietal white matter using 4-hydroxy-2-nonenal (HNE) protein adducts as an indicator of LPO. Immunocytochemistry showed an increase in HNE-positive glia from 40 gestational weeks to 1.5 postnatal years encompassing the peak period of myelin sheath synthesis at this site. Western blots showed a distinct pattern of HNE-modified proteins at fetal/term ages 26 to 42 gestational weeks and a second, different pattern at 45 gestational weeks to 2.5 postnatal years. Proteins modified by HNE in the latter period, corresponding to active myelination, were identified using mass spectrometry. The most prominent category of HNE modification included cytoskeletal proteins such as tubulins and neurofilaments. Other categories included cell type-specific proteins for mature oligodendrocytes and astrocytes and proteins involved in cell cycle and energy metabolism. We conclude that human brain development involves basal levels of oxidative stress and resulting LPO and that these processes target different proteins in an age-specific manner, thereby likely playing distinct roles during different periods of brain maturation.

Phosphoproteomic profiling of human SH-SY5Y neuroblastoma cells during response to 6-hydroxydopamine-induced oxidative stress

Dopaminergic neurons are known to be vulnerable to age-related neuronal disorders due to reactive oxygen species (ROS) generated during dopamine metabolism. However, it remains unclear what kinds of proteins are involved in the response to oxidative stress. We examined changes in whole proteins and phosphoproteins in the human dopaminergic neuroblastoma cell line SH-SY5Y under oxidative stress induced by the dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA). Proteins of SH-SY5Y cells at various stages of oxidative stress were separated by two-dimensional gel electrophoresis for comparative analysis. Increase in glutathione-S-transferase pi was detected on SYPRO Ruby-stained gels by computer-aided image analysis. Stress-induced alterations in phosphoproteins were detected by Pro-Q Diamond staining. Elongation factor 2, lamin A/C, T-complex protein 1, and heterogeneous nuclear ribonucleoprotein H3 were identified by MALDI-TOF mass spectrometry as stress-responsive elements.

Hydrogen peroxide: a signaling messenger

Hydrogen peroxide (H2O2) is a well-documented component of living cells. It plays important roles in host defense and oxidative biosynthetic reactions. In addition there is growing evidence that at low levels, H2O2 also functions as a signaling agent, particularly in higher organisms. This review evaluates the evidence that H2O2 functions as a signaling agent in higher organisms in light of the known biology and biochemistry of H2O2. All aerobic organisms studied to date from prokaryotes to humans appear to tightly regulate their intracellular H2O2 concentrations at relatively similar levels. Multiple biochemical strategies for rapidly reacting with these low endogenous levels of H2O2 have been elucidated from the study of peroxidases and catalases. Well-defined biochemical pathways involved in the response to exogenous H2O2 have been described in both prokaryotes and yeast. In animals and plants, regulated enzymatic systems for generating H2O2 have been described. In addition oxidation-dependent steps in signal transduction pathways are being uncovered, and evidence is accumulating regarding the nature of the specific reactive oxygen species involved in each of these pathways. Application of physiologic levels of H2O2 to mammalian cells has been shown to stimulate biological responses and to activate specific biochemical pathways in these cells.

Compartmentalization of hnRNP-K during cell cycle progression and its interaction with calponin in the cytoplasm

Coronary artery blockage, due to cardiovascular disease, is routinely treated by either balloon-angioplasty or bypass surgery. The limited success of these clinical interventions is due at least in part to smooth muscle cell (SMC) proliferation. Here we show that heterogeneous nuclear ribonucleoprotein complex K (hnRNP-K) protein levels increase in SMC with response to serum stimulation in vitro, in the aortas from an animal model of atherosclerosis, and in occluded human vein segments. hnRNP-K is a multi-functional protein that has been studied primarily in cancer cells and has been suggested to play a role in cell cycle progression. We show that in untransformed, cultured SMC, hnRNP-K protein sub-cellular localization modulates through the cell cycle in both the cytoplasm and nucleus. Using cycloheximide, we observed that cytoplasmic accumulation of hnRNP-K protein at later time points in the cell cycle occurred with a concomitant decrease in nuclear hnRNP-K protein, suggesting a translocation of nuclear hnRNP-K protein to the cytoplasm. Also, because we did not observe an increase in hnRNP-K protein at early time points in the cell cycle in the presence of cycloheximide, we propose that the early increase in cytoplasmic hnRNP-K protein following serum stimulation is due to new hnRNP-K protein synthesis. When present in the cytoplasm, hnRNP-K is part of a multi-protein complex that consists of at least two other proteins, calponin and ERK1/2. Our findings from this study are intriguing because they suggest that cytoplasmic hnRNP-K in SMC is part of a signaling complex that may be involved in growth-stimulated post-transcriptional regulation.

Protein kinase CK1alpha regulates mRNA binding by heterogeneous nuclear ribonucleoprotein C in response to physiologic levels of hydrogen peroxide

At low concentrations, hydrogen peroxide (H(2)O(2)) is a positive endogenous regulator of mammalian cell proliferation and survival; however, the signal transduction pathways involved in these processes are poorly understood. In primary human endothelial cells, low concentrations of H(2)O(2) stimulated the rapid phosphorylation of the acidic C-terminal domain (ACD) of heterogeneous nuclear ribonucleoprotein C (hnRNP-C), a nuclear restricted pre-mRNA-binding protein, at Ser(240) and at Ser(225)-Ser(228). A kinase activity was identified in mouse liver that phosphorylates the ACD of hnRNP-C at Ser(240) and at two sites at Ser(225)-Ser(228). The kinase was purified and identified by tandem mass spectrometry as protein kinase CK1alpha (formerly casein kinase 1alpha). Protein kinase CK1alpha immunoprecipitated from primary human endothelial cell nuclei also phosphorylated the ACD of hnRNP-C at these positions. Pretreatment of endothelial cells with the protein kinase CK1-specific inhibitor IC261 prevented the H(2)O(2)-stimulated phosphorylation of hnRNP-C. Utilizing phosphoserine-mimicking Ser-to-Glu point mutations, the effects of phosphorylation on hnRNP-C function were investigated by quantitative equilibrium fluorescence RNA binding analyses. Wild-type hnRNP-C1 and hnRNP-C1 modified at the basal sites of phosphorylation (S247E and S286E) both avidly bound RNA with similar binding constants. In contrast, hnRNP-C1 that was also modified at the CK1alpha phosphorylation sites exhibited a 14-500-fold decrease in binding affinity, demonstrating that CK1alpha-mediated phosphorylation modulates the mRNA binding ability of hnRNP-C.

Role of oxidative modifications in atherosclerosis

This review focuses on the role of oxidative processes in atherosclerosis and its resultant cardiovascular events. There is now a consensus that atherosclerosis represents a state of heightened oxidative stress characterized by lipid and protein oxidation in the vascular wall. The oxidative modification hypothesis of atherosclerosis predicts that low-density lipoprotein (LDL) oxidation is an early event in atherosclerosis and that oxidized LDL contributes to atherogenesis. In support of this hypothesis, oxidized LDL can support foam cell formation in vitro, the lipid in human lesions is substantially oxidized, there is evidence for the presence of oxidized LDL in vivo, oxidized LDL has a number of potentially proatherogenic activities, and several structurally unrelated antioxidants inhibit atherosclerosis in animals. An emerging consensus also underscores the importance in vascular disease of oxidative events in addition to LDL oxidation. These include the production of reactive oxygen and nitrogen species by vascular cells, as well as oxidative modifications contributing to important clinical manifestations of coronary artery disease such as endothelial dysfunction and plaque disruption. Despite these abundant data however, fundamental problems remain with implicating oxidative modification as a (requisite) pathophysiologically important cause for atherosclerosis. These include the poor performance of antioxidant strategies in limiting either atherosclerosis or cardiovascular events from atherosclerosis, and observations in animals that suggest dissociation between atherosclerosis and lipoprotein oxidation. Indeed, it remains to be established that oxidative events are a cause rather than an injurious response to atherogenesis. In this context, inflammation needs to be considered as a primary process of atherosclerosis, and oxidative stress as a secondary event. To address this issue, we have proposed an "oxidative response to inflammation" model as a means of reconciling the response-to-injury and oxidative modification hypotheses of atherosclerosis.

An assessment of proposed mechanisms for sensing hydrogen peroxide in mammalian systems

Despite much recent interest in the biochemistry of reactive oxygen species, the mechanisms by which hydrogen peroxide (H2O2) functions in mammalian cells remain poorly defined. Proposed mechanisms for sensing H2O2 in mammalian cells include inactivation of protein tyrosine phosphatases and dual specificity phosphatases as well as inactivation of peroxiredoxins. In this critical review, proteins proposed to serve as sensors for H2O2 in mammals will be compared to peroxidases, catalases, and the bacterial H2O2 sensor OxyR for their ability to react with H2O2, in the context of our current knowledge concerning the concentrations of H2O2 present in cells.

Decrease of H2O2 Plasma Membrane Permeability during Adaptation to H2O2 in Saccharomyces cerevisiae

Contrary to what is widely believed, recent published results show that H2O2 does not freely diffuse across biomembranes. The fast removal of H2O2 by antioxidant enzymes is able to generate a gradient if H2O2 is produced in a different compartment from that containing the enzymes (Antunes, F., and Cadenas, E. (2000) FEBS Lett. 475, 121-126). In this work, we extended these studies and tested whether an active regulation of biomembranes permeability characteristics is part of the cell response to oxidative stress. Using Saccharomyces cerevisiae as a model, we showed that: (a) H2O2 gradients across the plasma membrane are formed upon exposure to external H2O2; (b) there is a correlation between the magnitude of the gradients and the resistance to H2O2; (c) there is not a correlation between the intracellular capacity to remove H2O2 and the resistance to H2O2; (d) the plasma membrane permeability to H2O2 decreases by a factor of two upon acquisition of resistance to this agent by pre-exposing cells either to nonlethal doses of H2O2 or to cycloheximide, an inhibitor of protein synthesis; and (e) erg3Delta and erg6Delta mutants, which have impaired ergosterol biosynthesis pathways, show higher plasma membrane permeability to H2O2 and are more sensitive to H2O2. Altogether, the regulation of the plasma membrane permeability to H2O2 emerged as a new mechanism by which cells respond and adapt to H2O2. The consequences of the results to cellular redox compartmentalization and to the origin and evolution of the eukaryotic cell are discussed.

Monochloramine induces reorganization of nuclear speckles and phosphorylation of SRp30 in human colonic epithelial cells: role of protein kinase C

Intestinal epithelial cells are constantly stimulated by reactive oxidant metabolites (ROMs) in inflamed mucosa. Monochloramine (NH2Cl), a cell-permeant ROM, is particularly relevant to the pathogenesis of inflammation in the gastrointestinal tract. Nuclear speckles, a unique nuclear subcompartment, accumulate a family of proteins, namely, serine- and arginine-rich (SR) proteins. They play important roles in regulation of pre-mRNA splicing. Currently, little is known about the link between inflammatory stimulation and the pre-mRNA splicing process, although gene expression is changed in inflamed tissues. The present study was designed to investigate whether stimulation of human colonic epithelial cells (HT-29 and Caco-2 cell lines) with NH2Cl affects nuclear speckles and their components. By indirect immunofluorescence, nuclear speckles have been shown to undergo rapid aggregation after NH2Cl stimulation. By utilizing Western blotting, SRp30 (a subset of SR proteins) in intestinal epithelial cells was found to be phosphorylated after NH2Cl treatment, whereas other SR proteins were not responsive to NH2Cl stimulation. The cytotoxic effect of NH2Cl was excluded by both negative lactate dehydrogenase assay and propidium iodide staining. Therefore, NH2Cl-induced morphological changes on nuclear speckles and phosphorylated SRp30 do not result from intestinal epithelial injury. Furthermore, the effect of NH2Cl on nuclear speckles and SRp30 was blocked by bisindolylmaleimide I, a selective PKC inhibitor. Together, the available data suggest that stimulation of intestinal epithelial cells with NH2Cl results in a consequent change on pre-mRNA splicing machinery via a distinctive signal pathway involving activation of PKC. This effect may contribute to oxidant-induced pathophysiological changes in the gastrointestinal tract.

The role of hydrogen peroxide in endothelial proliferative responses

Hydrogen peroxide (H2O2) is a recently recognized second messenger regulating proliferation in mammalian cells. Endothelial cells possess NADPH oxidases, which produce the H202 precursor superoxide (.O2-) in response to receptor-mediated signaling. Multiple physiologic agents have been shown to stimulate endothelial cells to produce .O2-/H2O2, including growth factors, such as vascular endothelial growth factor and transforming growth factor-beta1, and alterations in biomechanical forces, such as shear stress and cyclic strain. Downstream effects of these stimuli can often be inhibited by scavenging H2O2. Low concentrations of H2O2 stimulate proliferation or enhanced survival in a wide variety of cell types. Also, low concentrations of H2O2 stimulate endothelial migration as well as tube formation in an in vitro model of angiogenesis. Although low concentrations of H2O2 have been shown to be involved in numerous signal transduction pathways and to independently stimulate mitogenesis, there has been little information presented on precisely how mammalian cells respond biochemically to these low concentrations of H2O2. Recently a functional proteomics approach has been utilized to identify proteins responsive to low concentrations of H2O2 in human endothelial cells.

Adenoviral-mediated overexpression of catalase inhibits endothelial cell proliferation

Although hydrogen peroxide (H(2)O(2)) induces proliferation of vascular smooth muscle cells, its role in endothelial cell proliferation is unclear. Our aim was to study the role of hydrogen peroxide in endothelial cell proliferation by overexpressing catalase. Human aortic endothelial cells were transduced with adenoviral vectors encoding beta-galactosidase (Adbetagal) or catalase (AdCat) or were exposed to diluent alone (control). Transgene expression was demonstrated by beta-galactosidase staining, Western analysis, and significantly increased enzyme activity in AdCat-transduced cells. Overexpression of catalase decreased DNA synthesis in AdCat compared with control and Adbetagal-transduced cells (536.8 +/- 31 vs. 1,875.1 +/- 132.9 vs. 1,347.5 +/- 93.7 dpm/well, respectively; P < 0.05 vs. control and Adbetagal). Six days after transduction with AdCat (multiplicity of infection = 50), cell numbers were significantly reduced (AdCat: 38 +/- 1.8% of cell counts in control, P < 0.05; and 45 +/- 2% of cell count in Adbetagal, P < 0.05). Incubation with aminotriazole 10 mmol/l, an inhibitor of catalase, prevented this effect. The number of apoptotic cells was increased one- and threefold 2 and 4 days, respectively, after transduction with AdCat. Exogenous administration of low concentrations of H(2)O(2) (50 microM) significantly increased cell proliferation, whereas it was inhibited by higher concentrations. These results suggest that H(2)O(2) is an important modulator of endothelial cell proliferation.

Changes in nuclear matrix proteins during the senescence-like phenomenon induced by 5-chlorodeoxyuridine in HeLa cells

5-Chlorodeoxyuridine (CldU) immediately induces a senescence-like phenomenon in any type of mammalian cells probably due to a change in nuclear matrix structure. We thus examined nuclear matrix proteins in HeLa cells cultured with CldU by high-resolution two-dimensional gel electrophoresis and peptide mass spectrometry. Three proteins were found to be down-regulated and one protein up-regulated by addition of CldU. In addition, one acidic protein accumulated in the nuclear matrix, although not quantitatively changed in the nuclei. Since these alterations were observed within 24h after addition of CldU, these proteins may be involved in an early step of the senescence-like phenomenon.