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. 2012 Oct 5;12(21):3193–3205. doi: 10.1002/pmic.201200225

Correlation between TGF‐β1 expression and proteomic profiling induced by severe acute respiratory syndrome coronavirus papain‐like protease

Shih‐Wen Li 1,2, Tsuey‐Ching Yang 3, Lei Wan 4, Ying‐Ju Lin 4, Fuu‐Jen Tsai 4, Chien‐Chen Lai 2,4,✉, Cheng‐Wen Lin 1,5,6,✉
PMCID: PMC7168038  PMID: 22936401

Abstract

Severe acute respiratory syndrome (SARS) coronavirus (SARS‐CoV) papain‐like protease (PLpro), a deubiquitinating enzyme, demonstrates inactivation of interferon (IFN) regulatory factor 3 and NF‐κB, reduction of IFN induction, and suppression of type I IFN signaling pathway. This study investigates cytokine expression and proteomic change induced by SARS‐CoV PLpro in human promonocyte cells. PLpro significantly increased TGF‐β1 mRNA expression (greater than fourfold) and protein production (greater than threefold). Proteomic analysis, Western blot, and quantitative real‐time PCR assays indicated PLpro upregulating TGF‐β1‐associated genes: HSP27, protein disulfide isomerase A3 precursor, glial fibrillary acidic protein, vimentin, retinal dehydrogenase 2, and glutathione transferase omega‐1. PLpro‐activated ubiquitin proteasome pathway via upregulation of ubiquitin‐conjugating enzyme E2–25k and proteasome subunit alpha type 5. Proteasome inhibitor MG‐132 significantly reduced expression of TGF‐β1 and vimentin. PLpro upregulated HSP27, linking with activation of p38 MAPK and ERK1/2 signaling. Treatment with SB203580 and U0126 reduced PLpro‐induced expression of TGF‐β1, vimentin, and type I collagen. Results point to SARS‐CoV PLpro triggering TGF‐β1 production via ubiquitin proteasome, p38 MAPK, and ERK1/2‐mediated signaling.

Keywords: Microbiology, Papain‐like protease, SARS coronavirus, TGF‐β1, Ubiquitin proteasome, Vimentin

Abbreviations

CoV

Coronavirus

EMT

epithelial–mesenchymal transdifferentiation

ERK

extracellular signal‐regulated kinase

IFN

interferon

MAPK

mitogen‐activated protein kinase

NS

nonstructural

OAS

oligoadenylate synthetase

RANTES

regulated and normal T cell expressed and secreted

SARS

severe acute respiratory syndrome

TNFR

tumor necrosis factor receptor

TRAF

TNFR‐associated factor

1. Introduction

Severe acute respiratory syndrome (SARS) associated coronavirus (SARS‐CoV) is a causative agent of severe and atypical pneumonia 1, 2 that caused a global outbreak in 2003, resulting in over 8000 probable cases with a mortality of some 10%. Clinical pathology indicates bronchial epithelial denudation, multinucleated syncytial cells, loss of cilia, squamous metaplasia, inflammation infiltration of monocytes, macrophages, and neutrophils into lung tissue 2, 3. Clinical laboratory examination reveals SARS‐CoV triggering lymphopenia, thrombocytopenia, and leucopenia 4, 5 while rapidly elevating serum of inflammatory cytokines, e.g. IFN‐γ, IL‐18, TGF‐β1, TNF‐α, IL‐6, IP‐10, MCP‐1, MIG, and IL‐8 6, 7. SARS‐CoV‐induced proinflammatory cytokine storms are linked with recruitment of neutrophils, monocytes, and immune responder cells such as natural killer, T, and B cells into the lungs, resulting in acute lung injury, acute respiratory distress syndrome, even lung fibrosis in the late phase 7.

SARS‐CoV genome is an approximately 30 kbp positive‐stranded RNA with a 5′ cap and 3′ poly(A) tract, containing 14 ORFs 8, 9. Polyprotein replicases 1a and 1ab (∼450 and ∼750 kDa, respectively) encoded by 5′ proximal, producing nonstructural (NS) proteins primarily involved in RNA replication. Specifically, papain‐like protease (PLpro) and 3C‐like protease, two embedded proteases, mediate the processing of 1a and 1ab precursors into 16 NS proteins (termed NS 1 through NS16). PLpro recognizes a consensus motif LXGG as consensus cleavage sequence of cellular deubiquitinating enzymes, and cleaves polyprotein replicase 1a at NS1/2, NS2/3, and NS3/4 boundaries 10, 11. Modeling with crystal structures of herpes virus associated ubiquitin‐specific protease and in vitro cleavage assays reveal SARS‐CoV PLpro as a deubiquitinating enzyme 12, 13. These in vitro de‐ubiquination assays illustrate cleavage of interferon (IFN)‐induced 15‐kDa protein (ISG15)‐conjugated proteins by PLpro 12, 13. Deubiquitinating/de‐ISGylating activity of PLpro is believed to link with a mechanism that SARS‐CoV infection induces on type I IFNs in cell culture 14. PLpro is further identified by blocking type I IFN synthesis through inhibiting phosphorylation of IFN regulatory factor 3 15. PLpro antagonizes type I IFN signaling pathway by polyubiquitination and degradation of extracellular signal‐regulated kinase 1 (ERK1), resulting in the inhibition of STAT1 phosphorylation and decrease of IFN‐stimulated gene expression such as PKR, 2′‐5′‐oligoadenylate synthetase (OAS), IL‐6, and IL‐8 16. A deubiquitinating enzyme cylindromatosis that inhibits NF‐κB signaling via deubiquitination and inactivation of tumor necrosis factor receptor (TNFR)‐associated factor 2 (TRAF2) and TRAF6 17 has been reported to promote inflammatory responses via mitogen‐activated protein kinase (MAPK)‐mediated but NF‐κB independent pathways 18. Deubiquitinating protease A20 inhibits TLR4‐mediated NF‐κB activation, directly removing K63‐linked polyubiquitin chains of TRAF6 while reducing production of antiviral and proinflammatory cytokines by inactivation of RIG‐I dependent antiviral signaling 19. Yet the role of deubiquitinating enzyme PLpro in anti‐inflammation and proinflammatory response remains unclear. This study assesses possible effect of SARS‐CoV PLpro on cytokine induction: TGF‐β1, TNF‐α, IFN‐β, IL‐1α, regulated and normal T cell expressed and secreted protein (RANTES). Comparative proteomic analysis of PLpro expressing versus vector control cells yielded data on the relation between TGF‐β1 production and protein expression change induced by PLpro, affording insights into molecular mechanism(s) of SARS pathogenesis.

2. Materials and methods

2.1. Cell culture and transfection

SARS‐CoV PLpro gene amplified by RT‐PCR was cloned into pcDNA3.1/HisC vector (Invitrogen) as described in our previous report 16. This resulting construct pSARS‐PLpro (4.5 μg) or pcDNA3.1 empty vector was transfected into HL‐CZ cells, a human promonocyte cell line, using GenePorter reagent. Stable transfected cells were generated through long‐period incubation with RPMI‐1640 medium containing 10% FBS and 800 μg/mL of G418. To analyze expression, cells transfected with pSARS‐PLpro or empty vector were washed once in PBS, then fixed in 3.7% formaldehyde in PBS for 1 h. Cells subsequently underwent 1‐h incubation with anti‐PLpro sera from mice immunized with E. coli‐synthesized PLpro, followed by another 1‐h incubation with FITC‐conjugated antimouse IgG antibody (Abcam). Finally, cells were stained with 4′,6‐diamidino‐2‐phenylindole (DAPI, Sigma) for 10 min. After three PBS washes, imaging was analyzed by immunofluorescent microscopy (Olympus, BX50).

2.2. Western blot assay

To resolve protein expression, lysates of PLpro‐expressing and empty vector control cells were mixed in the ratio of 1:1 with 2× SDS‐PAGE sample buffer, then boiled for 10 min. Proteins in cell lysates were detected by SDS‐PAGE and transferred to nitrocellulose. After blocking with 5% skim milk, resulting blots were reacted with properly diluted antibodies, e.g. rabbit antivimentin (GeneTex), anti‐PLpro mouse sera, rabbit anti‐ERK1/2 (Cell signaling), rabbit antiphospho‐ERK1/2 (Thr202/Tyr204) (Cell signaling), anti‐HSP27 mAb (Chemicon), and anti‐β‐actin mAb (Abcam). Immune complexes were detected with HRP‐conjugated goat antimouse or antirabbit IgG antibodies, followed by enhanced chemiluminescence detection (Amersham Pharmacia Biotech).

2.3. Quantitative RT‐PCR

Total RNA was extracted from stable transfected cells with pSARS‐PLpro or empty vector incubated for 4 h in the presence or absence of 3000 U/mL IFN‐α, 20 μΜ MG‐132 (a proteasome inhibitor) or 15 μΜ U0126 (an ERK1/2 inhibitor), using PureLink Micro‐to‐Midi Total RNA Purification System kit (Invitrogen). Two‐step real‐time RT‐PCR using SYBR Green I quantified expression in response to SARS PLpro; cDNA was synthesized from 1 μg total RNA, using oligonucleotide d(T) primer and SuperScript III reverse transcriptase kit (Invitrogen), as previously described 16. Real‐time PCR mixture contained 5 μL of a cDNA mixture, 1 μL of primer pair (200 nM, Supporting Information Table S1), and 12.5 μL Smart Quant Green Master Mix with Dutp and ROX (Protech). PCR was performed with amplification protocol consisting of 1 cycle at 50°C for 2 min, 1 cycle at 95°C for 10 min, 40 cycles at 95°C for 15 s, and 60°C for 1 min. Amplification and detection of specific products were conducted in ABI Prism 7900 sequence detection system (PE Applied Biosystems). Relative changes in mRNA level of indicated genes were normalized relative to GAPDH mRNA.

2.4. ELISA analysis of TGF‐β1 in cultured medium and cell lysates

Those 2 × 106 vector control or PLpro‐expressing cells were treated with/without 20 μΜ MG‐132, 15 μΜ U0126, or 5 μΜ SB203580 (a p38 MAPK inhibitor) in 5 mL of serum‐free medium for 1 day, 400 μL of each cultured medium and cell lysates mixed with 40 μL of coating buffer; 100 μL of each mixture was immediately coated into a 96‐well plate at 4°C for 1 day. After tris‐buffered saline and Tween‐20 (TBST) washing, the 96‐well plate was blocked in 1% BSA in TBST for 1 h at room temperature, incubated with 1:2000 dilutions of rabbit anti‐TGF‐β1 antibodies (Cell Signaling) for 2 h, then reacted with HRP‐conjugated goat antirabbit IgG (Sigma) for another 1 h. Relative TGF‐β1 protein level was ascertained by chromogen solution of hydrogen peroxide and 2,2′‐azinodi‐3‐ethylbenzthiazoline‐6‐sulfonate, colored product at OD405nm detected by ELISA reader (BioTek, Elx808).

2.5. 2DE and protein spot analysis

For 2DE as described in our prior reports 16, total proteins from vector control and PLpro‐expressing cells were harvested, 100 μg of protein sample diluted with 350 μL of rehydration buffer, and applied to nonlinear Immobiline DryStrips (17 cm, pH 3–10; GE Healthcare), then applied for first‐dimensional isoelectric focusing, using Multiphor II system (GE Healthcare) and 2DE using 12% polyacrylamide gels (20 cm × 20 cm × 1.0 mm). Gels were fixed in 40% ethanol, 10% glacial acetic acid for 30 min, stained with silver nitrate solution for 20 min, then scanned by GS‐800 imaging densitometer with PDQuest software version 7.1.1 (Bio‐Rad). Three independent lysates under each condition served for the correction of spot intensity graphs and statistical analysis.

2.6. In‐gel digestion, nanoelectrospray MS, and database search

In‐gel digestion recovered peptides from gel spots for nanoelectrospray MS, as described in our prior reports 16, 20. Excised spots were soaked in 100% ACN for 5 min, dried in lyophilizer for 30 min, and rehydrated in 50 Mm ammonium bicarbonate buffer (pH 8.0) containing 10 μg/mL trypsin, followed by incubation at 30°C for 16 h. Digested peptides were extracted from supernatant of gel digestion solution (50% ACN in 5.0% trifluoroacetic acid) and dried in vacuum centrifuge. Peptides from digested protein spots were separated by RP C18 capillary column (flow rate 200 nL/min) and eluted with linear (10–50%) ACN gradient in 0.1% formic acid for 60 min. Eluted peptides from the capillary column were electrosprayed into mass spectrometer by PicoTip (FS360–20‐10‐D‐20; New Objective, Cambridge, MA, USA), data acquisition from Q‐TOF operated in data‐dependent mode to measure MS and MS2, using automatic Information Dependent Acquisition software (IDA; Applied Biosystem/MDS Sciex). The three most intense ions were sequentially isolated and fragmented in Q‐TOF by collision‐induced dissociation. Derived peak list generated by Mascot.dll v1.6b27 (Applied Biosystems) was searched as previously described 21 with a local version of the Mascot (2) program (v2.2.1; Matrix Science Ltd) and Mascot Daemon application (v2.2.0); parameters as follows: peptide and MS/MS tolerance, ±0.3 Da; trypsin missed cleavages, 1; variable modifications, carbamidomethylation and Met oxidation; and instrument type, ESI‐Q‐TOF. Protein identification was based on assignment of at least two peptides, protein function and subcellular location annotated by Swiss‐Prot (http://us.Expasy.org/sprot/). Proteins were also categorized according to biological process and pathway via PANTHER (Protein Analysis Through Evolutionary Relationships) classification (http://www.pantherdb.org/) described in prior studies 5, 22, 23, 24.

2.7. Statistical analysis

Each bar on the graph shows the mean of three independent experiments; error bars represent standard error of the mean. Chi‐square and student's t‐test analyzed all data, statistical significance between types of cells noted at p < 0.05.

3. Results

3.1. SARS‐CoV PLpro‐induced TGF‐β1 production in human promonocytes

PLpro was expressed in promonocyte cells transfected with recombinant plasmid containing SARS‐CoV PLpro gene. After two‐week selection with G418, quantitative real‐time PCR showed significant expression of PLpro mRNA in transfected versus control cells (Fig. 1A). Immunofluorescent staining with sera from hyper‐immunized mice with recombinant PLpro protein indicated immunoreactive fluorescence in transfected as opposed to empty vector cells (Fig. 1B). Western blot analysis with sera of mice hyper‐immunized with E.coli‐synthesized PLpro identified a 60‐kDa band recombinant PLpro protein in transfected cells but not controls (Fig. 1C), indicating SARS‐CoV PLpro stably expressed in human promonocyte cells.

Figure 1.

Figure 1

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Expression of SARS‐CoV PLpro in human promonocyte HL‐CZ cells. (A) Relative mRNA level of SARS‐CoV PLpro in transfected cells was measured by quantitative real‐time PCR and normalized by GAPDH mRNA. (B, C) Recombinant PLpro protein in transfected cells was detected by immunofluorescent staining, its molecular weight analyzed by Western blot. Lysates from cells transfected with pcDNA3.1 (lane 1) or pSARS‐PLpro (lane 2) were analyzed by 10% SDS‐PAGE prior to blotting; resulting blot was probed with mouse polyclonal sera against E . coli‐synthesized PLpro (top) and anti‐β actin monoclonal antibody as internal control.

Because PLpro suppressed IFN‐α signaling pathway via inactivation of ERK1 and STAT1 16, effect of SARS‐CoV PLpro expression on cytokine production (TGF‐β1, TNF‐α, IL‐1α, IFN‐β, and RANTES) was further characterized via SYBR Green real‐time PCR (Fig. 2). Cytokine mRNA levels in PLpro‐expressing and vector control cells were quantified and then normalized to housekeeping gene GAPDH expression. PLpro alone induced mRNA expression of TGF‐β1 (greater than fourfold), IL‐1α (approximately threefold), and RANTES (approximately twofold) (Fig. 2A, D, and E), yet no significant rise in TNF‐α and IFN‐β compared to vector controls (Fig. 2B and C). IFN‐α treatment stimulated higher transcriptional levels of TNF‐α and IFN‐β, but not TGF‐β1, IL‐1α, and RANTES in vector controls than in PLpro‐expressing cells (Fig. 2). To confirm PLpro‐induced TGF‐β1 expression, cultured media and lysates were harvested from identical amounts of vector control and PLpro‐expressing cells grown with FBS‐free medium for 24 h, coated into 96‐well plates for ELISA assay with rabbit anti‐TGF‐β1 Ab (Fig. 3). TGF‐β1 in cultured media and cell lysates of PLpro‐expressing cells were twice as high as in vector controls, indicating that PLpro plays a unique role in upregulating TGF‐β1 production.

Figure 2.

Figure 2

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Cytokine mRNA expression in vector control and PLpro‐expressing cells treated with and without IFN‐α. Vector control and PLpro‐expressing cells were treated with or without IFN‐α in 4 h, mRNA expression of cytokine gene in cells untreated or treated measured by quantitative PCR. Relative mRNA levels of (A) TGF‐β1, (B) TNF‐α, (C) IFN‐β, (D) IL‐1α, and (E) RANTES were normalized by GAPDH mRNA, presented as relative ratio.

Figure 3.

Figure 3

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Protein levels of TGF‐β1 in cultured media and lysates of PLpro‐expressing and vector control cells with or without IFN‐α. Vector control or PLpro‐expressing cells treated with or without 3000 U/mL IFN‐α were incubated in serum‐free medium for 1 day. Cultured media or cell lysate were coated into wells of a 96‐well plate at 4°C overnight, then reacted with rabbit anti‐TGF‐β1 antibodies and HRP‐conjugated goat antirabbit IgG. The colored product was detected at OD405 nm.

3.2. Proteomic analysis of expression profiles induced by SARS‐CoV PLpro

To differentiate specific protein profiling induced by PLpro, protein expression between vector control and PLpro‐expressing cells was analyzed by 2DE (Fig. 4): 34 protein spots (>1.5‐fold change in spot intensity) identified by trypsin digestion and nanoscale capillary LC/ESI‐Q‐TOF MS. Identified proteins matched with Mascot score above 75; MW and pI of indicated proteins in 2DE gel (Fig. 4, Tables 1, 2). Amino acid sequence coverage of identified proteins varied (4–71%). MS analysis of HSP27 (Spot ID 1) showed a Mascot score of 161, five matched peptides, and sequence coverage of 37%; vimentin (Spot ID 7) showed a Mascot score of 1420, sequence coverage of 71%, and 14 matched peptides. Peptide peaks from Q‐TOF MS analysis from three representative spots of interest, vimentin (Spot ID 7), glial fibrillary acidic protein (Spot ID 17), and HSP27 (Spot ID 1) appear in Supporting Information Fig. S1A–C. Analyzing cell lysates by Western blot confirms protein profiling induced by PLpro (Fig. 5), i.e. significant rise of vimentin and HSP27 coupled with substantial decline of ERK1 in PLpro‐expressing cells compared to vector controls, consistent with 2DE/Q‐TOF MS/MS analysis in Fig. 4, Tables 1, 2. Moreover, quantitative real‐time PCR indicated mRNA expression of ubiquitin‐conjugating enzyme E2–25 kDa (Spot ID 3) and protein disulfide isomerase A3 precursor (Spot ID 11) upregulated twofold in PLpro‐expressing cells, but no change in controls (Supporting Information Fig. S2), supporting 2D/MS data (Fig. 4 and Table 1). Thus, 21 up‐ and 13 downregulated proteins were identified in PLpro‐expressing cells.

Figure 4.

Figure 4

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Silver stain of 2D gel of PLpro‐expressing and vector control cells. Total protein of 100 μg from (A) vector control cells or (B) PLpro‐expressing cells was applied to the nonlinear Immobiline DryStrip (17 cm, pH 3–10) and transferred to the top of 12% polyacrylamide gels (20 × 20 cm × 1.0 mm). Protein size markers are shown at the left of each gel (in kDa).

Table 1.

Upregulated proteins in PLpro‐expressing cells compared to vector control cells

Spot ID PANTHER gene ID Protein identification Mascot score MW / pI Numbers of peptides identified Sequence coverage (%) Fold change Subcellular location
Mean SD
1 3315 Heat‐shock protein 27 (HSP27) 161 22.8/5.98 5 37 2.5 0.6 Cytoplasm, nucleus
2 23475 Nicotinate‐nucleotide pyrophosphorylase [carboxylating] (QPRT) 238 30.8/5.81 4 43 1.54 0.13 Cytosol
3 3093 Ubiquitin‐conjugating enzyme E2–25 kDa (UBE2K) 248 22.4/5.33 4 59 1.56 0.11 Cytoplasm
4 5686 Proteasome subunit alpha type 5 (PSA5) 157 26.4/4.74 4 30 1.68 0.15 Cytoplasm, nucleus
5 7178 Translationally controlled tumor protein (TCTP) 244 19.6/4.84 3 28 1.66 01656 Cytoplasm
6 5464 Inorganic pyrophosphatase (IPYR) 143 32.6/5.54 2 13 1.52 0.2 Cytoplasm
7 7431 Vimentin (VIM) 1420 53.6/5.06 14 71 1.63 0.1 Cytoplasm, cytoskeleton
8 9588 Peroxiredoxin‐6 (PRDX6) 97 25/6.0 4 24 2.675 1.015 Cytoplasm, lysosome, cytoplasmic vesicle
9 22948 T‐complex protein 1 subunit epsilon (CCT5) 702 59.6/5.45 8 32 1.838 0.045 cytoplasm, cytoskeleton, centrosome
10 6175 60S acidic ribosomal protein P0 (RPLP0) 517 34.3/5.71 5 41 1.69 0.165 Nucleus, cytoplasm
11 2923 Protein disulfide isomerase A3 precursor (PDIA3) 736 56.7/5.98 11 35 2.4 0.75 Endoplasmic reticulum lumen, melanosome
12 1832 Desmoplakin (DSP) 440 331.6/6.44 13 11 2.168 0.373 Cell junction, desmosome, cytoplasm, cytoskeleton
13 8854 Retinal dehydrogenase 2 (ALDH1A2) 630 56.7/5.79 10 29 2.561 0.181 Cytoplasm
14 3187 Heterogeneous nuclear ribonucleoprotein H (HNRNPH1) 233 49.2/5.89 6 21 9.469 1.627 Nucleus, nucleoplasm
15 3921 40S ribosomal protein SA (RPSA) 617 32.8/4.79 5 39 11.7 0.1 Cell membrane, cytoplasm, nucleus
16 3728 Junction plakoglobin (JUP) 174 81.6/5.95 7 9 1.77 0.35 Cell junction adherens junction, desmosome, cytoplasm, peripheral membrane protein
17 2670 Glial fibrillary acidic protein (GFAP), astrocyte 297 49.9/5.42 3 6 3.44 0.44 Cytoplasm, cytoskeleton
18 7170 Tropomyosin alpha‐3 chain (TPM3) 264 32.8/4.68 3 27 3.2 0.252 Cytoplasm, cytoskeleton
19 213 Serum albumin precursor (ALB) 124 69.3/5.92 4 10 3.6 1.9 Secreted
20 117159 Dermcidin precursor (DCD) 70 11.2/6.08 2 22 2.51 0.65 Secreted
21 9446 Glutathione transferase omega‐1 (GSTO1) 92 27.5/6.23 6 28 1.57 0.06 Cytoplasm
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Table 2.

Downregulated proteins in PLpro‐expressing cells compared to vector control cells

Spot ID PANTHER gene ID Protein identification Mascot score MW / pI Numbers of peptides identified Sequence coverage (%) Fold change Subcellular location
Mean SD
22 7453 Tryptophanyl‐tRNA synthetase (WARS2) 606 53.1 / 5.83 10 33 0.545 0.235 Mitochondrion matrix
23 10130 Protein disulfide‐isomerase A6 precursor (PDIA6) 227 48.1/4.95 9 25 0.44 0.14 Endoplasmic reticulum lumen, cell membrane, melanosome
24 3945 L‐lactate dehydrogenase B chain (LDHB) 498 36.6/5.71 7 35 0.455 0.095 Cytoplasm
25 5111 Proliferating cell nuclear antigen (PCNA) 385 28.8/4.57 6 50 0.562 0.108 Nucleus
26 506 ATP synthase subunit beta, mitochondrial precursor (ATP5E) 685 56.5 / 5.26 9 34 0.333 0.006 Mitochondrion, mitochondrion inner membrane
27 5595 Mitogen‐activated protein kinase 3 (ERK1) 109 43.1/6.28 2 14 0.195 0.077 Cytosol
28 4722 NADH dehydrogenase (NDUFC2) 182 30.2/6.99 2 10 0.257 0.043 Mitochondrion inner membrane
29 7001 Peroxiredoxin‐2 (PRDX2) 290 21.9/5.66 3 22 0.53 0.03 Cytoplasm
30 1933 Elongation factor 1‐beta (EEF1B2) 353 24.7/4.5 4 53 0.341 0.18 Cytosol
31 3312 Heat‐shock cognate 71 kDa protein (HSC70) 775 70.9/5.37 11 33 0.457 0.133 Cytoplasm, melanosome
32 7531 14–3‐3 protein epsilon (YWHAE) 681 29.1/4.63 6 67 0.251 0.014 Cytoplasm, melanosome
33 2324 Vascular endothelial growth factor receptor 3 (VEGFR3) precursor 146 145.5/5.89 3 4 0.507 0.264 Cell membrane, single‐pass type I membrane protein, cytoplasm, nucleus
34 10376 Tubulin alpha‐ubiquitous chain (Alpha‐tubulin ubiquitous) (TUBA1B) 75 50.1/4.94 1 3 0.335 0.058 Cytoplasm, cytoskeleton
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Figure 5.

Figure 5

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Western blot analysis of vimentin, HSP27, and ERK1/2 in vector control and PLpro‐expressing cells. Lysates from vector control and PLpro‐expressing cells were analyzed by 10% SDS‐PAGE prior to blot. Resulting blot was probed with antivimentin, anti‐ERK1/2, anti‐HSP27, and anti‐β actin antibodies, followed by enhanced chemiluminescence detection.

3.3. Inhibition of ubiquitin proteasome activity reduces PLpro‐induced expression of TGF‐β1 and vimentin

To examine the effect of PLpro‐induced activation of ubiquitin proteasome pathway via UBE2K upregulation of TGF‐β1 and vimentin expression, cells were analyzed for protein and mRNA of TGF‐β1 and vimentin with or without proteasome inhibitor MG‐132 (Fig. 6). Quantitative real‐time PCR assay indicated MG‐132 causing more than a twofold decrease

Figure 6.

Figure 6

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Effect of MG‐132 on TGF‐β1 and vimentin expression in PLpro‐expressing and vector control cells. (A) Vector control or PLpro‐expressing cells were treated with or without 20 μM MG‐132 for 4 h, mRNA expression of TGF‐β and vimentin in untreated or treated cells measured by quantitative PCR and normalized by GAPDH mRNA, presented as relative ratio. (B) Vector control and PLpro‐expressing cells treated with or without 20 μM MG‐132 were incubated in serum‐free media for 1 day. Cultured media or cell lysate were coated into wells of a 96‐well plate at 4°C overnight, then reacted with rabbit anti‐TGF‐β1 antibodies and HRP‐conjugated goat antirabbit IgG. The colored product was detected at OD 405 nm.

of TGF‐β1 and vimentin mRNA in PLpro‐expressing versus control cells (Fig. 6A). ELISA assays indicated MG‐132 reducing TGF‐β1 protein in lysates of PLpro‐expressing cells by twofold and no effect on controls (Fig. 6B), proving ubiquitin–proteasomal system involvement in PLpro‐induced expression of TGF‐β1 and vimentin.

3.4. SB203580 and U0126 reduce PLpro‐induced expression of TGF‐β1 and vimentin

To ascertain whether p38 MAPK and ERK1/2 signaling pathways linked with PLpro‐induced expression of TGF‐β1 and vimentin, treatment with(out) p38 MAPK inhibitor SB203580 and ERK1/2 inhibitor U0126 was further analyzed by ELISA and quantitative real‐time PCR assays (Fig. 7). ELISA assay showed less PLpro‐induced TGF‐β1 production by SB203580 (1.5‐fold) and U0126 (3.3‐fold) (Fig. 7A and B). Quantitative real‐time PCR assay indicated U0126 raising PLpro‐induced mRNA expression of TGF‐β1 (sixfold), vimentin (3.3‐fold), and type I collagen (8.4‐fold) (Fig. 7C). Results indicated the activation of p38 MAPK‐ and ERK1/2‐mediated signaling linking with induction of TGF‐β1 and upregulation of vimentin by PLpro.

Figure 7.

Figure 7

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Effect of SB203580 and U0126 on TGF‐β1, vimentin, and type I collagen expression in vector control and PLpro‐expressing cells. Vector control and PLpro‐expressing cells treated with and without (A) 5 μM SB203580 or (B) 15 μM U0126 were incubated in serum‐free media for 1 day. Cultured media or cell lysate were coated into wells of a 96‐well plate at 4°C overnight, then reacted with rabbit anti‐TGF‐β1 antibodies and HRP‐conjugated goat antirabbit IgG. Colored product was detected at OD 405 nm. (C) Vector control or PLpro‐expressing cells were treated with and without 15 μM U0126 for 1 day. The mRNA expression of TGF‐β1, vimentin, and type I collagen in both types of cells untreated or treated was measured by quantitative PCR, normalized by GAPDH mRNA, presented as relative ratio.

4. Discussion

Rapid elevation of serum cytokines, such as TGF‐β1, IL‐6, IL‐8, IFN‐γ, IL‐18, IP‐10, MCP‐1, and MIG, has been detected in SARS patients 6, 7. Of SARS‐CoV proteins, nucleocapsid increased expression of plasminogen activator inhibitor‐1 via the Smad3‐dependent TGF‐β1 signaling pathway 25. Baculovirus synthesized spike protein activated MAPKs and AP‐1, leading to IL‐8 release in lung cells 26. Nonstructure protein 1—but not nonstructured protein 5, envelope, and membrane—induced mRNA expression of CCL5, CXCL10, and CCL3 via activation of NF‐κB 27. This study demonstrated that PLpro‐potentiated induction of TGF‐β1, IL‐1α, and RANTES, particularly inducing more than a threefold increase of TGF‐β1 compared to vector controls (Figs. 2A,D, and E and 3). Of these three cytokines, TGF‐β1 has been reported to stimulate epithelial–mesenchymal transdifferentiation (EMT) and induced kidney and lung fibrosis 28, 29. PLpro can trigger the cytokine storm while playing a vital role in SARS‐CoV‐induced lung damage.

Schematic figure of proteomic profiling induced by SARS‐CoV PLpro (Fig. 8) was confirmed by Western blotting, quantitative real‐time PCR, and specific inhibitors (Figs 5–7), showing up‐ and downregulated proteins identified as linked with TGF‐β1 production in PLpro‐expressing cells. Proteomic analysis indicated PLpro upregulating expression of many TGF‐β1‐associated genes, e.g. HSP27 (Spot ID 1), vimentin (Spot ID 7), protein disulfide isomerase A3 precursor (Spot ID 11), retinal dehydrogenase 2 (Spot ID 13), glial fibrillary acidic protein (Spot ID 17), glutathione transferase omega‐1 (Spot ID 21) (Fig. 8, Table 1). Functional analysis using PANTHER classification system demonstrated PLpro upregulating cytoskeleton proteins such as vimentin (Spot ID 7), T‐complex protein 1 subunit epsilon (Spot ID 9), desmoplakin (Spot ID 12), junction plakoglobin (Spot ID 16), glial fibrillary acidic protein (Spot ID 17), and tropomyosin alpha‐3 chain (TPM3, Spot ID 18). These cytoskeleton proteins were involved in cell adhesion, cellular component morphogenesis, immune system process, signal transduction, and/or cell motion. Meanwhile, cytoskeleton protein changes and induction of such processes correlated with greater TGF‐β1 production by PLpro. Recent reports verified TGF‐β1‐mediated vimentin upregulation in alveolar epithelial cells as adequate for healing injured lung tissue 23, 24, 30. Vimentin upregulation was consistent with activation of profibrotic cytokines TGF‐β1 induced by SARS‐CoV PLpro. Vimentin, HSP27, glial fibrillary acidic protein, retinal dehydrogenase 2, and glutathione transferase omega‐1 linked with TGF‐β1‐induced EMT pathogenesis and fibrosis 23, 24, 31, 32. Protein disulfide isomerase A3 precursor influenced release and activation of latent TGF‐β1 from extracellular matrix 33, whereas glutathione transferase omega‐1 polymorphism correlated with expression of TGF‐β1 34.

Figure 8.

Figure 8

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Schematic chart of SARS‐CoV PLpro‐induced proteomic profile. Up‐ and downregulated proteins were identified, using 2DE and ESI‐Q‐TOF, Western blotting, and quantitative PCR, showing activation of p38 MAPK and ERK1/2 signaling pathways, as well as ubiquitin–proteasomal system, as correlated with PLpro‐induced TGF‐β1 production.

Biological pathways in Fig. 8 indicate PLpro activating ubiquitin proteasome via upregulation of UBE2K (Spot ID 3) and proteasome subunit alpha type 5 (Spot ID 4), and p38MAPK and ERK1/2 signaling via increase of HSP27 (Spot ID 1). Given proteomic profiling induced by PLpro, we hypothesized the activation of the ubiquitin–proteasomal system, p38 MAPK and ERK1/2 signaled as linked with PLpro‐induced upregulation of TGF‐β1 and vimentin. Notably, the inhibitor MG‐132 treatment confirmed the upregulation of ubiquitin–proteasomal system as involved in PLpro‐induced expression of TGF‐β1 and vimentin (Fig. 6). Ubiquitin‐proteasomal pathway modulated TGF‐β1 signaling by regulating expression and activity of effectors in a TGF‐β1 signaling cascade 35, 36. Also, proteasome inhibitor MG‐132 would definitely reduce TGF‐β1 signaling as well as inhibit activity of Smads and AKT 37, 38, e.g. E3 ubiquitin ligases of ubiquitin–proteasonal system increased ubiquitination and degradation of Smad7 and TGF‐β receptor I, which activate p38 MAPK‐ and JNK‐mediated TGF‐β1 signaling 39, 40, 41. Inhibiting PLpro‐induced expression of TGF‐β1 and vimentin by proteasome inhibitor MG‐132 support our hypothesis of ubiquitin–proteasonal system as involved in TGF‐β1 induction.

The p38 MAPK and ERK1/2 inhibitors (SB203580 and U0126) significantly decreased PLpro‐induced expression of TGF‐β1, vimentin, and type I collagen (Fig. 7, Table 1), which confirmed p38 MAPK and ERK1/2 modulating PLpro‐induced TGF‐β1 signaling cascades. Although SARS PLpro‐enhanced ERK1 degradation via activation of ubiquitin–proteasomal system and inhibited phosphorylation of STAT1 Ser727 in type I IFN signaling pathway, PLpro did not affect amount and activation of ERK2 in human promonocyte cells 16. ERK2, rather than ERK1, played a key positive role in TGF‐β1‐mediated EMT marker expression of type I collagen 42, 43. Other reports proved TGF‐β1 inducing EMT and increasing mesenchymal markers (cytokeratins 8 and 19, vimentin, and collagen type α1) via ERK2‐dependent signaling in pancreatic cancer cell lines 39. We suggest ERK2 activation as crucial in PLpro‐induced TGF‐β1‐mediated collagen synthesis, consistent with the previous report 43.

SARS‐CoV PLpro, a deubiquitinating enzyme, triggered TGF‐β1 production via ubiquitin‐proteasomal system, as well as p38 MAPK and ERK1/2 signaling pathways. Proteasome, p38 MAPK, and ERK1/2 inhibitors confirmed correlation between TGF‐β1 induction and proteome profiling induced by PLpro. The study highlights a pivotal role of PLpro in both TGF‐β1 production and SARS pathogenesis.

Supporting information

Disclaimer: Supplementary materials have been peer‐reviewed but not copyedited.

Table S1. Primers used in this study

Fig. S1. Tandem mass spectrometry spectra of up‐regulated proteins induced by PLpro. (A) Nanoelectrospray mass spectrum of doubly charged ion m/z 547.2859 for spot ID 7 is shown, amino acid sequence FADLSEAANR determined from mass differences in y‐ and b‐fragment ions series and matched residues 295–304 of vimentin. (B) Nanoelectrospray mass spectrum of the doubly charged ion m/z 639.4361 for spot ID 17 is shown, amino acid sequence LALDIEIATYR determined from mass differences in y‐ and b‐fragment ions series and matched residues 357–367 of glial fibrillary acidic protein. (C) Nanoelectrospray mass spectrum of doubly charged ion m/z 582.3241 for spot ID 1 is shown, amino acid sequence LFDQAFGLPR determined from mass differences in y‐ and b‐fragment ions series and matched residues 28–37 of heat shock protein 27. *Only y‐ and b‐fragment ions are labeled in the spectrum.

Fig. S2. Analysis of PDIA3 and UBE2K mRNA levels in vector control and PLpro‐expressing cells. Total RNA was extracted from vector control and PLpro‐expressing cells. The mRNA expression of PDIA3 and UBE2K was measured by quantitative real‐time PCR. The relative fold levels of PDIA3 and UBE2K mRNA levels are presented as the ratio with GAPDH mRNA. Each bar on the graph is the mean of 3 independent experiments and the error bars represent the standard error of the mean.

Click here for additional data file. (1.2MB, doc)

Acknowledgments

We would like to thank the National Science Council (Taiwan) and China Medical University for financial support (NSC101‐2320‐B‐039‐036‐MY3, CMU98‐P‐03‐M, and CMU99‐NSC‐08).

The authors have declared no conflict of interest.

Colour Online: See the article online to view Figures 1, 4, and 8 in colour.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Disclaimer: Supplementary materials have been peer‐reviewed but not copyedited.

Table S1. Primers used in this study

Fig. S1. Tandem mass spectrometry spectra of up‐regulated proteins induced by PLpro. (A) Nanoelectrospray mass spectrum of doubly charged ion m/z 547.2859 for spot ID 7 is shown, amino acid sequence FADLSEAANR determined from mass differences in y‐ and b‐fragment ions series and matched residues 295–304 of vimentin. (B) Nanoelectrospray mass spectrum of the doubly charged ion m/z 639.4361 for spot ID 17 is shown, amino acid sequence LALDIEIATYR determined from mass differences in y‐ and b‐fragment ions series and matched residues 357–367 of glial fibrillary acidic protein. (C) Nanoelectrospray mass spectrum of doubly charged ion m/z 582.3241 for spot ID 1 is shown, amino acid sequence LFDQAFGLPR determined from mass differences in y‐ and b‐fragment ions series and matched residues 28–37 of heat shock protein 27. *Only y‐ and b‐fragment ions are labeled in the spectrum.

Fig. S2. Analysis of PDIA3 and UBE2K mRNA levels in vector control and PLpro‐expressing cells. Total RNA was extracted from vector control and PLpro‐expressing cells. The mRNA expression of PDIA3 and UBE2K was measured by quantitative real‐time PCR. The relative fold levels of PDIA3 and UBE2K mRNA levels are presented as the ratio with GAPDH mRNA. Each bar on the graph is the mean of 3 independent experiments and the error bars represent the standard error of the mean.

Click here for additional data file. (1.2MB, doc)

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