Article Cite This: Chem. Res. Toxicol. 2018, 31, 666−679
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4‑Hydroxy-7-oxo-5-heptenoic Acid Lactone Is a Potent Inducer of the Complement Pathway in Human Retinal Pigmented Epithelial Cells Mikhail Linetsky,† Karina S. Bondelid,‡,# Sofiya Losovskiy,§,# Vadym Gabyak,∥ Mario J. Rullo,‡ Thomas I. Stiadle,† Vasu Munjapara,‡ Priyali Saxena,‡ Duoming Ma,† Yu-Shiuan Cheng,† Andrew M. Howes,‡ Emeka Udeigwe,† and Robert G. Salomon*,†,⊥ Downloaded via KAOHSIUNG MEDICAL UNIV on August 21, 2018 at 17:07:58 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, United States Department of Biochemistry, Case Western Reserve University, Cleveland, Ohio 44106, United States § Department of Chemistry, Cleveland State University, Cleveland, Ohio 44115, United States ∥ Department of Biological, Geological, and Environmental Sciences, Cleveland State University, Cleveland, Ohio 44115, United States ⊥ Department of Ophthalmology & Visual Sciences, Case Western Reserve University, Cleveland, Ohio 44106, United States ‡
ABSTRACT: We previously discovered that oxidative cleavage of docosahexaenoate (DHA), which is especially abundant in the retinal photoreceptor rod outer segments and retinal pigmented endothelial (RPE) cells, generates 4hydroxy-7-oxo-5-heptenoate (HOHA) lactone, and that HOHA lactone can enter RPE cells that metabolize it through conjugation with glutathione (GSH). The consequent depletion of GSH results in oxidative stress. We now find that HOHA lactone induces upregulation of the antioxidant transcription factor Nrf2 in ARPE-19 cells. This leads to expression of GCLM, HO1, and NQO1, three known Nrf2responsive antioxidant genes. Besides this protective response, HOHA lactone also triggers a countervailing inflammatory activation of innate immunity. Evidence for a contribution of the complement pathway to age-related macular degeneration (AMD) pathology includes the presence of complement proteins in drusen and Bruch’s membrane from AMD donor eyes, and the identification of genetic susceptibility loci for AMD in the complement pathway. In eye tissues from a mouse model of AMD, accumulation of complement protein in Bruch’s membrane below the RPE suggested that the complement pathway targets this interface, where lesions occur in the RPE and photoreceptor rod outer segments. In animal models of AMD, intravenous injection of NaIO3 to induce oxidative injury selectively destroys the RPE and causes secretion of factor C3 from the RPE into areas directly adjacent to sites of RPE damage. However, a molecular-level link between oxidative injury and complement activation remained elusive. We now find that sub-micromolar concentrations of HOHA lactone foster expression of C3, CFB, and C5 in ARPE-19 cells and induce a countervailing upregulation of CD55, an inhibitor of C3 convertase production and complement cascade amplification. Ultimately, HOHA lactone causes membrane attack complex formation on the plasma membrane. Thus, HOHA lactone provides a molecular-level connection between free-radical-induced oxidative cleavage of DHA and activation of the complement pathway in AMD pathology.
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INTRODUCTION
about 10% of patients. The susceptibility of RPE cells to oxidative stress and the prevalence of AMD progressively increase with age as antioxidant defenses deteriorate. Inappropriate Activation of the Complement Cascade in AMD. The involvement of the complement cascade in AMD pathology is suggested by the presence of complement pathway proteins in drusen and Bruch’s membrane from AMD donor eyes.2−5 Genetic AMD susceptibility loci for the
The pathogenesis of age-related macular degeneration (AMD) is a complex multifactorial process, and a molecular-level understanding of the etiology of AMD is incomplete. AMD is a progressive loss of central vision resulting from damage to the retinal pigmented epithelium (RPE) and neural retina that affects almost 50 million people worldwide, and its prevalence increases with age.1 For approximately 90% of individuals with AMD, there is atrophy and degeneration of the RPE (dry AMD), while choroidal neovascularization leads to leakage of blood into and detachment of the neural retina (wet AMD) in © 2018 American Chemical Society
Received: February 5, 2018 Published: June 8, 2018 666
DOI: 10.1021/acs.chemrestox.8b00028 Chem. Res. Toxicol. 2018, 31, 666−679
Article
Chemical Research in Toxicology complement pathway, including complement factor H (CFH),6,7 component 3 (C3),8,9 and factor B (CFB),10 have been identified that affect regulation of inflammatory responses, resulting in a hyperactive complement system. There are three complement pathwaysclassical, lectin, and alternativethat all lead to expression of C3. CFH regulates cleavage of C3, the common complement protein of all three pathways. CFB expression leads to the formation of a C3 convertase of the alternative pathway that amplifies a proteolytic cascade, resulting in C3 cleavage to form C3a and C3b that, in turn, form the C5 convertase. This promotes formation of C5a and C5b and ultimately formation of the C5b-9 membrane attack complex (MAC). The MAC is found in donor eyes with early (dry) AMD.4,11,12 In a mouse model of AMD, intravenous injection of NaIO3 selectively destroys the RPE, resulting in patchy RPE loss similar to that seen in dry AMD.13−16 In this model, secretion of C3 from the RPE was observed into areas directly adjacent to sites of RPE damage, and C3 mRNA expression in the RPE/choroid was maximally elevated 5-fold over that in control mice 3 days after injection of NaIO3.17 Immunolocalization of complement component C3d (Figure 1) in Bruch’s membrane below the
Scheme 1. Generation of HOHA Lactone and CEP from DHA-PL
Evidence is accumulating that supports the hypothesis that HOHA lactone and CEP are causally involved in initiating an inflammatory response in AMD.23,26−29 Previously we showed that exposure of ARPE-19 cells to >15 μM concentrations of HOHA lactone is cytotoxic inducing apoptosis, but submicromolar concentrations of HOHA lactone promote survival. We now find that sub-micromolar concentrations of HOHA lactone foster expression of C3, at both the mRNA and protein levels that are required for both classical and alternative complement activation pathways. This is accompanied by an increase in the level of CFB, a component of the alternative pathway of complement activation. CFB is the precursor of complement factor Bb that associates with C3b to form the alternative pathway C3 convertase that amplifies the proteolytic cascade resulting in C3 cleavage and ultimately MAC formation. We now find that sub-micromolar concentrations of HOHA lactone increase expression of CD59, also known as MAC-inhibitory protein (MAC-IP), that can prevent formation of the complement membrane attack complex (MAC). In contrast, micromolar concentrations of HOHA lactone seem to inhibit the expression of CD59, favoring MAC formation, which can lead to ARPE cell lysis and death. We previously showed that HOHA lactone causes depletion of glutathione (GSH) and oxidative stress in ARPE cells.30 We now find that HOHA lactone also triggers antioxidant protein expression that counteracts its contribution to oxidative stress. HOHA lactone elevates hemoxygenase-1 (HO-1) levels in ARPE cells consistent with the conclusion that oxidative stress, induced by HOHA lactone, contributes to activation of the complement pathway.31 Furthermore, ARPE cells respond to this HOHA lactone-induced GSH depletion and oxidative stress by upregulating expression of NAD(P)H dehydrogenase quinone 1 (NQO1) and glutamate−cysteine ligase modifier subunit (GCLM), the first rate-limiting enzyme of GSH synthesis. Finally, we discovered a dose-dependent increase in MAC formation on the ARPE-19 plasma membrane with increasing levels of HOHA lactone.
Figure 1. Immunocytochemistry of the mouse outer retina showing the presence of C3d (green) in Bruch’s membrane.18 Bar length represents 10 μm. Adapted with permission from Joe G. Hollyfield, Victor L. Perez, and Robert G. Salomon (2010), A Hapten Generated from an Oxidation Fragment of Docosahexaenoic Acid Is Sufficient to Initiate Age-Related Macular Degeneration, Mol. Neurobiol., Vol. 41, pp 290−298.
RPE in eye tissues from another mouse model of dry AMD18 suggested that the complement pathway targets this interface, where lesions occur in the RPE and photoreceptor rod outer segments. C3d is a degradation product of C3b, a key component in the generation of the C3 and C5 convertases required for complement activation through the classical, lectin, and alternate pathways.19 Lipid Oxidation in AMD. Docosahexaenoate (DHA) is the most oxidizable fatty acid in the body, and it is highly concentrated in the disc membranes of photoreceptor cells and in RPE cells that endocytose oxidatively damaged rod photoreceptor outer segments.20,21 The combination of high oxygen levels and the focus of environmental light on the retina fosters the generation of free radicals and lipid oxidation that can damage DHA. We discovered that oxidative cleavage of DHA phospholipids generates 4-hydroxy-7-oxo-5-heptenoate (HOHA) phospholipid esters (Scheme 1) that undergo rapid intramolecular transesterification to release HOHA lactone.22 The reaction of HOHA lactone with proteins generates carboxyethylpyrrole (CEP) derivatives of protein lysyl ε-amino groups.23 CEP accumulates in the rod outer segments and RPE in the retina and is present in AMD eye tissue and in the blood of AMD patients at higher levels than found in age-matched non-AMD tissues.24,25
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MATERIALS AND METHODS
Reagents. Ham’s F12 medium and Dulbecco’s modified Eagle medium (DMEM), phosphate-buffered saline (PBS), and fetal bovine serum (FBS) were purchased from Fisher Scientific (Pittsburgh, PA). The lactone of 4-hydroxy-7-oxohept-5-enoate (HOHA lactone) was synthesized as described elsewhere.23,32 Antibodies. Mouse monoclonal antihuman NRF2 (clone 1E9E3; 66504-1-Ig; observed molecular weight: 110 kDa), rabbit polyclonal complement factor B (10170-1-AP; observed molecular weight: 93− 100 kDa), rabbit polyclonal complement factor C3 antibodies (21337-1-AP; observed molecular weight: 115 kDa), rabbit polyclonal CD59 antibodies (10742-1-AP; observed molecular weight: 18−20 667
DOI: 10.1021/acs.chemrestox.8b00028 Chem. Res. Toxicol. 2018, 31, 666−679
Article
Chemical Research in Toxicology Table 1. PCR Primer Sequences and Product Sizes Used for Quantitative Real-Time RT-PCR target gene C335 C536 CFB37 CD4638 CD5538 CD5938 Nrf239 NQ0139 HO-139 GCLM40 β-actin
position
oligonucleotide sequence (5′→3′)
product size (bp)
Gene Bank code
313−330 373−356 3419−3438 3745−3726 1393−1411 1509−1487 830−851 1228−1209 1356−1377 1651−1630 226−245 435−416 819−838 917−893 280−300 502−483 622−641 771−752 608−631 803−782 432−455 586−566
ACTGTGCTGACCCCTGCC (f) TGTTGGCTGGGATCGTGA (r) AGGGTACTTTGCCTGCTGAA (f) TGTGAAGGTGCTCTTGGATG (r) TCCCTCCTGAAGGCTGGAA (f) TGTATAGCAAGTCCCGGATCTCA (r) GCTGCTCCAGAGTGTAAAGTGG (f) AACAATCACAGCAATGACCC (r) TACTACCCGTCTTCTATCTGGG (f) TTTTCAAGAGGTGTAGGTGTGC (r) ACTGCAAAACAGCCGTCAAT (f) AGGATGTCCCACCATTTTCA (r) AGCCCAGCACATCCAGTCAG (f) TGCATGCAGTCATCAAAGTACAAAG (r) TGAAGAAGAAAGGATGGGAGG (f) AGGGGGAACTGGAATATCAC (r) TTGCCAGTGCCACCAAGTTC (f) TCAGCAGCTCCTGCAACTCC (r) TCAACCCAGATTTGGTCAGGGAGT (f) TCCAGCTGTGCAACTCCAAGGA (r) CGAGAAGATGACCCAGATCATGTT (f) CCTCGTAGATGGGCACAGTGT (r)
61
NM_000064
173
NM_001735
117
NM_001710
399
NM_002389
296
NM_000574
210
NM_000611
99
NM_006164
223
NM_000903
150
NM_002133
196
NM_002061
155
NM_001101
a
f, forward; r, reverse.
kDa), and HRP-conjugated β-actin mouse monoclonal antibody (cytosolic proteins loading control, HRP-60008) were procured from Proteintech (Rosemont, IL). Mouse monoclonal antihuman C5 antibody (sc-52636), mouse monoclonal antihuman C5b-9 antibody (SC-58935), and HRP-conjugated laminin B1 (nuclear proteins loading control, sc-374015 HRP) were obtained from Santa Cruz Biotechnology (Dallas, TX). Goat anti-mouse IgG cross-adsorbed secondary antibody with Alexa Fluor 647 (A32728) was from ThermoFisher Scientific (Waltham, MA). Cell lysis buffer (TissuePELB) was from Gold Biotechnology (St. Louis, MO). Restore Plus Western blot stripping buffer as well as the enhanced chemiluminescence (ECL) Western blot detection system were from Pierce Biotechnology (Rockford, IL) via Fisher Scientific (Pittsburgh, PA). Reagents and precast gels (4−20% gradient) for SDS−PAGE were purchased from Invitrogen Life Technologies (Carlsbad, CA). The bicinchoninic acid (BCA) assay33 was used to determine the total amount of protein in the lysates. Cell Culture. The cell line ARPE-19 (ATCC CRL-2302), derived from spontaneously arising retinal pigment epithelia of a healthy person,34 was obtained from American Type Culture Collection (Manassas, VA). The stock cells were grown on 100 mm dishes in a humidified CO2 incubator at 37 °C and 5% CO2 in DMEM/F12 medium (50:50 ratio), containing L-glutamine and 10% heatinactivated FBS. Cells were trypsinized and passaged every 2−3 days. Dose-Dependent Effects of Exposing ARPE19 Cells to HOHA Lactone. Serum starved monolayers of ARPE19 cells (80− 90% confluence) on 60 mm plates in 5% CO2/95% air at 37 °C were challenged for 2 h with 0−30 μM HOHA lactone solutions (2 mL) in the respective basal medium. Cells were then scraped with a rubber policeman, transferred to 5 mL conical tubes, and centrifuged at 480g at 4 °C for 10 min. The medium was carefully aspirated, and the cells were washed three times with ice-cold PBS followed by centrifugation at 480g at 4 °C for 10 min. Cells were disrupted upon incubation with a RIPA lysis buffer, 50 mM HEPES (pH 7.4), 0.5% sodium deoxycholate, 0.1% sodium dodecyl sulfate, 1 mM phenylmethanesulfonyl fluoride (PMSF), and 1× HALT protease inhibitor cocktail for 15 min at 4 °C. Cell lysate was further placed on ice and sonicated at 40% power (5 cycles of 5 s on and 5 s off). The cell lysate was spun at 14000g at 4 °C for 20 min, and the supernatant was collected and
snap-frozen in liquid nitrogen. The BCA assay1 was used to determine the total amount of protein in the lysates. Time-Course of Effects Induced by Exposing ARPE19 Cells to HOHA Lactone. After 16 h of starvation in the basal medium, ARPE19 cells (80−90% confluence) in 100 mm plates were treated for 0, 15, 30, 60, 90, or 120 min with 20 μM HOHA lactone solution in a basal medium under 5% CO2/95% air at 37 °C. Handling cells, preparing the cell lysates, and measuring total protein concentration in the cell lysates were carried out in a manner similar to that described in the previous section. PAGE and Western Blot Analysis. ARPE19 cell extracts (25−30 μg protein/lane) were separated by SDS-PAGE (4−20% gels), and the proteins were electrotransferred onto a PVDF membrane (BioRad, Hercules, CA). The blot was blocked with 5% bovine serum albumin (BSA) in Tris buffered saline containing 0.1% Tween-20 (TBST) for an hour at room temperature. The blots were subsequently probed overnight with the indicated antibody at 4 °C in TBST buffer. After washing with TBST, the membrane was incubated with the appropriate secondary antibody-HRP conjugate at room temperature for 1 h. The membrane was again washed with TBST, and the immunoblot was developed with the SuperSignal West Pico Chemiluminescent Substrate or SuperSignal West Femto Chemiluminescent Substrate, both from Pierce Biotechnology (Rockford, IL) according to the manufacturer’s instructions. RNA Isolation and Real-Time Quantitative PCR. Total RNA was isolated from 60 mm Petri dishes using Rneasy Plus Mini Kit (Qaigen, Hilden, Germany). cDNA was synthesized from total cellular RNA using a reaction mix First Strand cDNA Synthesis Kit (11801-100) from Origene, Rockville, MD according to the manufacturer’s protocol. In brief, 750 ng of total RNA was reverse transcribed in a 20 μL volume containing 4 μL of cDNA Master Mix and 1 μL of Reverse Transcriptase for 30 min at 42 °C. cDNA was then incubated at 85 °C for 5 min. Quantitative real-time PCR was performed on a sequence-detection system (LightCycler 1.5; Roche, Basel, Switzerland) using heat-activated Taq DNA polymerase (FastStart Essential Green Master, 06402 702001; Roche), according to the manufacturer’s protocol. The cycling conditions were as follows: after 10 min of pre-incubation at 95 °C, the samples were cycled 45 times at 95 °C for 30 s, 55−60 °C depending on the Tm for the primers used for 60 s, and 72 °C for 30 s. The quantity of mRNA 668
DOI: 10.1021/acs.chemrestox.8b00028 Chem. Res. Toxicol. 2018, 31, 666−679
Article
Chemical Research in Toxicology
Figure 2. HOHA lactone induces upregulation of Nrf2 in RPE cells. Data in panels A and B are presented as fold change over control. (A) Representative Western blot that shows the time-course of Nrf2 accumulation in ARPE-19 cells as the result of HOHA lactone challenge (10 μM, 2 h). Below is the graph quantifying Nrf2 protein abundance. (B) Representative Western blot that shows a dose-dependent increase in the levels of Nrf2 in ARPE-19 cells challenged with HOHA lactone for 2 h under standard conditions. Below is the graph quantifying Nrf2 protein abundance. Panel C shows the time-course of intracellular glutathione-level changes in ARPE-19 cells after treatment with 10 μM HOHA lactone. All panels show a representative immunoblot of three independent experiments. Values represent means ± SD, n = 3. *p < 0.05, **p < 0.025, ****p < 0.005 compared to the respective controls. expression was analyzed by standard curve quantification for the target gene and the β-actin mRNA in the same sample using double delta CT analysis. All measurements were performed in triplicate. Controls consisting of bi-distilled H2O were negative in all runs. Experiments were repeated at least three times. The primer pairs used are listed in Table 1. Complement Attack and MAC Deposition Assay. ARPE19 cells (10 000 cells/per well) were plated on 8-chamber Nunc Lab-Tek II CC2 chamber slides in 500 μL of DMEM/F12 medium supplemented with 10% of heat-inactivated FBS and were grown to near confluence. The cells were starved in basal DMEM/F12 medium for 36 h and washed with basal DMEM/F12 medium. Solutions of 0, 0.1, 1.0, and 10.0 μM HOHA lactone were prepared in basal DMEM/ F12 medium and added to the respective wells, followed by 2 h incubation in a CO2 incubator at 37 °C and 5% CO2, followed by incubation in 10% normal human serum (NHS) or in 10% heatinduced NHS for 1 h at 37 °C and 5% CO2. At the end of the incubation, the chambers were aspirated and washed with PBS, and the cells were fixed with cold acetone (−25 °C) for 12 min. The slides were blocked with 3% BSA in PBST for 1 h at room temperature, aspirated, incubated with anti-human C5b-9 mouse monoclonal antibody (clone aE11; 1:100 dilution in 3% BSA in PBST) overnight at 4 °C, and washed with PBST the next day. The slides were treated with Alexa Fluor 647 goat anti-mouse (1:100 dilution; A32728, Invitrogen) antibody in PBST overnight at 4 °C, washed with PBST, and aspirated. The slides were incubated with 1:20 diluted in PBS stock solution of CF488A-Phalloidin (0.2 U/μL, Biotium, Fremont, CA) in the dark for 30 min at room temperature protected from light. After washing with PBST, slides were mounted in the DAPI Fluoromount-G (Southern Biotech, Birmingham, AL) sealed. All images were acquired with a Leica DMI 6000 B inverted fluorescent microscope using a Retiga EXI camera. Image analysis was performed using Metamorph imaging software (Molecular Devices, Downington, PA). The images were taken at 20× magnification. LDH Assay. Briefly, ARPE19 cells (20,000 cells/per well) were plated on 48-well plates in 500 μL of DMEM/F12 medium supplemented with 10% of heat-inactivated FBS and grown to near confluence. The cells were starved in basal DMEM/F12 medium for
48 h and washed with basal DMEM/F12 medium. Solutions of 0, 0.1, 1.0, 10.0 μM HOHA lactone and 1% Triton X-100 were prepared in basal DMEM/F12 medium and added to the respective wells (3 replicate wells were used for each concentration), followed by 2 h incubation in a CO2 incubator at 37 °C and 5% CO2. The cells were washed with PBS and incubated in 10%, 25%, and 50% NHS or in 10%, 25%, and 50% heat-induced NHS, respectively, for 2 h at 37 °C and 5% CO2. Cells were washed with DMEM/F12 basal cell culture medium and incubated for another 2 h in a CO2 incubator at 37 °C and 5% CO2. Aliquots of the conditioned extracellular cell medium were used to assess cell viability (cell’s plasma membrane integrity)41 by the LDH assay according to the manufacturer’s instructions. Quantification of Intracellular GSH in ARPE-19 Cells. Aliquots (10 μL) of ARPE-19 cell lysates from time-course and dose-dependence studies were assayed to determine intracellular GSH using a spectrofluorometric microplate method described earlier.42 In this experiment, all the reagents were prepared in 0.1 M potassium phosphate buffer with 5 mM EDTA disodium salt, pH 7.5 (KPE buffer). Briefly, 10 μL of KPE buffer, GSH standards, or samples was added to the corresponding microplate wells, followed by the addition of 120 μL of a freshly prepared mixture of DTNB (1 mg/3 mL) and glutathione reductase (1.7 U/mL). Then, 60.0 μL of β-NADPH (2 mg/3 mL) was added and mixed well after shaking a microplate for 10 min. The plate was immediately placed in a microplate reader (Molecular Devices) and absorbance was measured at λ = 412 nm. Statistical Analysis. Statistical analyses were performed by using Student’s t test. P value
