Cell Biology International 31 (2007) 1027e1035www.elsevier.com/locate/cellbi

Apoptosis induced by NO via phosphorylation of p38 MAPK thatstimulates NF-kB, p53 and caspase-3 activation in

rabbit articular chondrocytes

Honglin Wang, Zhilun Wang*, Jinghong Chen, Jin Wu

Institute of Endemic Diseases, Medical School of Xi’an Jiaotong University, Key Laboratory of Environment and Genes Related to Diseases,

Ministry of Education, Xi’an 710061, Shaanxi, China

Received 14 November 2006; revised 23 January 2007; accepted 12 March 2007

Abstract

Nitric oxide (NO), reported as an important inducer of apoptosis, plays a considerable role in the pathogenetic mechanisms of articular dis-eases. This research aimed at investigating the role of p38 MAPK signal transduction pathway on apoptosis induced by NO in rabbit articularchondrocytes. In the present study, NO was produced by a novel NO donor NOC-18. Rabbit articular chondrocytes were cultured as monolayer,and the first passage cells were used for the experiments. We detected apoptosis induced by NO using Annexin V-FITC/PI flow cytometry andTUNEL assay. Measurement of caspase-3 has reflected its activity level. Western blotting was performed to show the protein expressions of p38,NF-kB, p53 and caspase-3. Furthermore, we examined the inhibitory effects in the NO pathway with p38-specific inhibitor SB203580. Treat-ment with NOC-18 caused accelerated apoptosis in a concentration dependent manner. This acceleration was able to be reduced when added toSB203580. Besides, the inhibitor could significantly decrease NO-induced p38, NF-kB, p53 and caspase-3 protein expressions, as well ascaspase-3 intracellular activity (P < 0.05). These results suggest that p38 MAPK signal transduction pathway is critical to NO-inducedchondrocyte apoptosis, and p38 plays a role by way of stimulating NF-kB, p53 and caspase-3 activation.� 2007 International Federation for Cell Biology. Published by Elsevier Ltd. All rights reserved.

Keywords: Nitric oxide; Apoptosis; p38; NF-kB; p53; Caspase-3; Chondrocytes

1. Introduction

Nitric oxide (NO), a gaseous free radical, is synthesizedfrom L-arginine by inducible nitric oxide synthase (iNOS).NO acts as an inter-and intracellular messenger molecule inmany cell types, including articular chondrocytes, and inducesapoptosis in several cell systems (Fukuo et al., 1996; Bonfoco

Abbreviations: NO, nitric oxide; NOC-18, Nitrosocompound-18; DME/F-

12, Dulbecco’s Modified Eagle’s Medium/Nutrient Mixture F-12 Ham’s;

MAPK, mitogen-activated protein kinase; NF-kB, nuclear factor kappa B;

FCM, flow cytometry; SDS-PAGE, sodium dodecyl sulfate-polyacrylamide

gel electrophoresis; TBST, Tris buffered saline tween; BSA, bovine serum

albumin.

* Corresponding author. Tel.: þ86 029 8265 5193; fax: þ86 029 8265 5032.

E-mail address: whlxjtu@163.com (Z. Wang).

1065-6995/$ - see front matter � 2007 International Federation for Cell Biology

doi:10.1016/j.cellbi.2007.03.017

et al., 1995). Apoptosis has been characterized morphologi-cally by cell shrinkage and chromatin condensation, and bio-chemically by DNA laddering (Thompson, 1995; Que andGores, 1996). Over production of NO in articular chondrocytesplays a central role in cartilage diseases such as osteoarthritis(OA), osteoporosis, rheumatoid arthritis and inflammatory ar-thritis for that NO causes cartilage destruction by inducing ap-optosis, dedifferentiation, and inflammatory responses (Sandelland Aigner, 2001). Apoptosis, which is mediated by ERK andp38 kinase (Kim et al., 2002a,b), may contribute to the chon-drocyte loss and subsequent cartilage degeneration (Kimet al., 2000). OA, the most common joint disorder in the agingpopulation, is characterized by deteriorative structural changesin cartilage, leading to loss of joint function (Kraan and Berg,2000). The versatile nature of NO is related to its small size,its ability to diffuse freely across membranes, and its high

. Published by Elsevier Ltd. All rights reserved.

1028 H. Wang et al. / Cell Biology International 31 (2007) 1027e1035

reactivity with many different compounds, which is amplifiedby the complex interplay between the three NO redox speciesNOþ, NO� and NO� (Wendehenne et al., 2001).

NO was shown to stimulate MAP kinase cascades and beimplicated in regulation of AP-1 and NF-kB transcription fac-tors (Callsen and Brune, 1999). Mitogen-activated proteinkinases (MAPKs) belong to the serine/threonine kinase groupand are important mediators in transduction of intracellularsignals during various biological events that include develop-ment, proliferation, differentiation, and apoptosis (Chang andKarin, 2001). The MAPK pathway consists of three proteinkinases: MAPK, MAPK kinase (MAPKK), and MAPKK ki-nase (MAPKK-K); MAPKK-K phosphorylates and activatesMAPKK, which in turn phosphorylates and activates MAPK(Su and Karin, 1996). In mammalian cells, three importantgroups of kinase pathways compose the MAPK family in-cluding the extracellular signal-regulated kinases (ERK), thep38 MAPK, and the c-Jun NH2-terminal kinase (JNK). TheERK-cascade appears to mediate signals promoting cell prolif-eration, differentiation, or survival, whereas the p38 MAPKand JNK cascades appear to be mainly involved in cellularstress responses. In the signaling pathway, p38 is currently be-ing intensively studied as a key regulator (O’Neill and Greene,1998) which is activated by dual phosphorylation into tyrosineand threonine residues. The p38 kinase is phosphorylated andactivated by its upstream MAPKKs (MKK4, MKK3, andMKK6) (Raingeaud et al., 1996), whereas its downstream ef-fectors include MAPKAP kinase-2, Mnk1/2, and the transcrip-tion factors ATF2, Elk-1, CHOP, MEF2C, and CREB (Wangand Ron, 1996; Iordanov et al., 1997; Wang et al., 1998;Zhao et al., 1999). The p38 phosphorylation pattern is dose-dependent; and in some systems, p38 phosphorylation lastsfor longer periods (Peng et al., 2003).

NF-kB is an inducible transcription factor that mediates sig-nal transduction between cytoplasm and nucleus in many celltypes (Baeuerle and Henkel, 1994). NF-kB is a member ofthe Rel family including p50 (NF-kB1), p52 (NF-kB2), Rel A(p65), c-Rel, Rel B, and Drosophila morphogen dorsal geneproduct (Siebenlist et al., 1994). In the inactive state, the NF-kB subunits p50 and p65 are non-covalently bound to the inhib-itory protein I-kB within the cytoplasm. Upon stimulation bya variety of pathogenic inducers such as viruses, mitogens,bacteria, agents providing oxygen radicals, and inflammatorycytokines, the I-kB subunit is phosphorylated and degraded,allowing the p50/p65 dimer to migrate into the nucleus andbind DNA recognition sites in the regulatory regions of thetarget genes (Karin and Ben-Neriah, 2000).

The tumor suppressor protein p53 is a nuclear phosphopro-tein that can potently regulate the growth of mammalian cells(Vogelstein and Kinzler, 1992). The p53 protein was initiallyidentified as being a normal cellular protein bound to largeT antigen in a Simian virus 40 transformed cell line (Laneand Crawford, 1979). A total of 10 years later, it was shownthat wild type p53 protein suppresses transformation in mam-malian cells (Eliyahu et al., 1989). This tumour suppressingactivity is assumed to result from p53’s ability to both blockDNA replication (Cox et al., 1995) and regulate expression

of genes involved in cell-cycle control, apoptosis and DNA re-pair by activating either apoptotic or growth arrest pathways inproliferating cells (Levine, 1997). Although the mechanism ofp53-mediated apoptosis after cellular stress remains unclear,current evidence showed that p53 induces cell death by a mul-titude of molecular pathways involving transactivation oftarget genes and direct signaling events (Li et al., 1999; Leeet al., 2000). Genes transcriptionally up-regulated by p53that have been implicated in promoting apoptosis include theBcl-2 family members Bax, Bak, and Noxa. It is impliedthat Bax or Bak function is required for the release of cyto-chrome c from the mitochondria to the cytosol during apopto-sis (Tsujimoto, 1998; Miyashita and Reed, 1995). Cytochromec is released from the mitochondria in the apoptotic signalingpathways implemented by p53 and TNF-a. This event is piv-otal in the regulation of apoptosis because cytochrome c com-plexes with APAF-1 in the cytosol which, in turn, promotescaspase-9 and caspase-3 (CPP32) activation.

The caspase family of proteolytic enzymes has been a fre-quent target for apoptotic inhibition experiments because theseenzymes are critical to many aspects of the apoptotic pathway.Different caspases serve as triggers, regulatory elements, anddeath effectors (Slee et al., 1999). Caspases 1 family members(1, 4, 5, and 13) are believed to be involved in the initiation ofthe apoptotic cascade while caspases 3 family members (2, 3,and 7) take part in the execution phase of programmed celldeath (Nicholson, 1996). The activation of caspase cascadeis required for p53-dependent apoptosis, and results in cleav-age of cellular proteins, such as poly (ADP-ribose) polymerase(PARP), as well as DNA fragmentation and cell death.

There seems to implicate complicated mechanism in the p38MAPK signal transduction pathway of NO-induced articularchondrocyte apoptosis from above review. However, it hasnot been completely elucidated. To assess the apoptosis effectof NO on chondrocytes, cells were treated with a NO donorNOC-18. To investigate the p38 signal transduction pathwayin NO-induced apoptosis, we measured several proteins afterNOC-18 stimulation in the presence or absence of SB203580. It has been reported that NO released from 1 mMNOC-18 results in steady state levels of 1e3 mM NO in me-dium without any cofactors (Bal-Price and Brown, 2000).This level of NO is comparable to endogenous concentrationsgenerated by iNOS-mediated pathway in cultured cells and inplasma after cytokine stimulation or lung injury (O’Brienet al., 2001; Okamoto et al., 2000). SB203580 is a highly spe-cific, cell-permeable inhibitor of the stress- and inflammatorycytokine-activated MAP kinase homologues p38a, p38b1,p38b2, and it has no significant effect on the activities ofERKs, JNKs, p38g or p38d (Cuenda, 1995).

In the present study, we examined p38 MAPK signaltransduction pathway induced by NO in rabbit articularchondrocytes treated with NO donor NOC-18. We also inves-tigated whether several proteins are associated with this path-way. To determine the role of p38 MAPK signal transductionpathway on apoptosis induced by NO in rabbit articular chon-drocytes, we performed Annexin V-FITC/PI flow cytometry(FCM) and TUNEL assay. The effects of SB203580 on the

1029H. Wang et al. / Cell Biology International 31 (2007) 1027e1035

apoptosis-related key proteins were investigated by caspase-3activity assay. To identify inhibitory effects of SB203580,we examined the changes of proteins of p38, NF-kB, p53,caspase-3 using Western blotting.

We show here that NO induces apoptosis in chondrocytesthrough the phosphorylation of p38 MAPK, and it is alsoconfirmed that in this pathway p38 stimulates NF-kB, p53,caspase-3. Further, it has been shown that SB203580 can blockthis pathway and NO-induced apoptosis.

2. Materials and methods

2.1. Materials

Hyaluronidase, trypsin and collagenase were purchased from Sigma (St.

Louis, MO, USA). DME/F-12 (Dulbecco’s Modified Eagle’s Medium/Nutrient

Mixture F-12 Ham’s) was from Hyclone (Logan, Utah, USA). NOC-18{(z)-

1-[2-(2-aminoethyl)-N-(2-ammonioethyl) amino] diazen-l-ium-1, 2-diolate}

was obtained from Merck (Darmstadt, Germany). p38 MAPK inhibitor

SB203580[4-(40-Fluoophenyl)-2-(40-methylsulfinylphenyl)-5-(40-phridyl) im-

idazole] and caspase-3 assay system(colorimetric) were purchased from Prom-

ega (Madison, WI, USA). The Annexin V-FITC kit was from Jingmei Biotech

Co., Ltd (Shenzhen, Guangdong, China). In situ Cell Apoptosis Detection Kit

was obtained from Sino-American Biotechnology Company (Luoyang, Helan,

China). Trizol Reagent was from Invitrogen life technologies (Carlsbad, CA,

USA). The rabbit anti-p38 antibody was from Stressgen Biotech (Victoria,

BC, Canada). The anti-phosphorylation p38 antibody was purchased from

Cell Signaling Technology Inc. (Beverley, MA, USA). The rabbit anti-b-actin

antibody was from Lab Vision Corporation (Fremont, CA, USA). The rabbit

anti-NF-kB p65, caspase-3, p53 antibodies and horseradish peroxidase-conju-

gated anti-rabbit secondary antibodies were bought from Santa Cruz Biotech,

Inc (Santa Cruz, CA, USA). PVDF membranes were purchased from Millipore

(Billerica, MA, USA). SuperSignal West Pico Chemiluminescent (ECL) West-

ern blot detection system was obtained from Pierce Biotech Inc. (Pierce, Rock-

ford, USA). All other reagents were obtained from commercial sources.

2.2. Chondrocytes isolation and monolayer culture

Rabbit articular chondrocytes were released from cartilage slices of

3-week-old New Zealand white rabbit by enzymatic digestion. Briefly, after

aseptic dissection, chondrocytes were obtained by sequential digestions with

hyaluronidase, trypsin and collagenase. The cells were gained as precipitation

by abandoning the supernatant after a brief centrifugation of 2000 rpm for

10 min and cultured to confluence at 37 �C in a humidified atmosphere contain-

ing 5% CO2, in complete medium (DME/F12 with 30% fetal bovine serum,

100 U/ml penicillin and 100 mg/ml streptomycin). The medium was changed

on alternate days, and cells reached confluence by day 7e10. Confluent original

chondrocytes were passaged to the first generation. All experiments were per-

formed using the first generation confluent chondrocytes to avoid any problems

of dedifferentiation.

2.3. Culture of chondrocytes with NOC-18 and SB203580

All cells were kept in serum-free DME/F-12 for 24 h at 37 �C under 5%

CO2 and divided into two groups. Then cells in the first group were treated

with NOC-18 of various concentration [0 (control 1), 0.025, 0.1, 0.4,

0.8 mM] for 24 h. Cells in the second group were treated with NOC-18 and/

or SB203580 for 24 h as following design: control 2 (complete medium

containing 0.2% DMSO, which is the vehicle of SB203580. The purpose of

designing control 2 was to avoid any side effect created by DMSO),

SB203580 1 mM, NOC-18 0.4 mM þSB203580 1 mM, SB203580 10 mM,

NOC-18 0.4 mM þSB203580 10 mM, SB203580 20 mM, NOC-18 0.4 mM

þSB203580 20 mM. SB203580 was added 30 min before treatment with

NOC-18.

2.4. Measurement of caspase-3 activity

Caspase-3 activity was determined by measuring the absorbance at 405 nm

after cleavage of Caspase-3 Substrate Ac-DEVD-pNA using Caspase-3 Assay

System, Colorimetric kit. There were three flasks of cells included in each con-

centration. Briefly, the first passage chondrocytes which cell density was

adjusted to 106 cells/ml were added NOC-18, SB203580 and 50 mM Z-

VAD-FMK inhibitor as experimental design. The mixtures were incubated

for 24 h at 37 �C. The cells were harvested by centrifugation at 2000 rpm

for 10 min at 4 �C, washed once with ice-cold PBS and resuspended in Cell

Lysis Buffer at a concentration of 108 cells/ml. The cells were lysed by freez-

ing and thawing repeatedly, and then incubated on ice for 15 min. The cell ly-

sates were centrifuged at 15,000 � g for 20 min at 4 �C and collected the

supernatant fraction (cell extract). Caspase-3 activity of cell extracts was mea-

sured in a total volume of 100 ml in 96-well plates at least 1 � 106 cells/assay.

Duplicate wells were prepared containing blank, negative control, induced and

inhibited extracts according to the manufacturer’s instructions. Cell extracts

were used as an enzyme source. Ac-DEVD-pNA Substrate (2 ml) was added

to all wells. The plate was covered with Parafilm laboratory film and incubated

at 37 �C for 4 h. The absorbance was read in the wells at 405 nm in an

enzyme-linked immunosorbent assay reader (BMG, Germany). The enzyme

activity was calculated from a standard curve prepared using pNA Standard.

The relative levels of pNA were normalized against the protein concentration

of each extract. The specific activity (SA) of caspase-3 was calculated via mi-

crograms of protein in each 100 ml sample volume which were determined by

the method of Bradford using a BSA standard curve.

2.5. Analysis of apoptosis rate by Annexin V-FITC/PI FCM

Apoptosis rate was measured by FCM according to the instruction provided

by the Annexin V-FITC kit. In brief, after treatment with NOC-18 and/or

SB203580, cells were harvested by centrifugation, washed 1 time with

ice-cold PBS and resuspended in binding buffer at a concentration of

1 � 106 cells/ml, in which 100 ml of cell suspension was added into 5 ml

FCM tube. A total of 5 ml of Annexin V-FITC and 10 ml of 20 mg/ml PI were

added and incubated for 15 min in the dark before a further addition of 400 ml

of PBS. Quantitative analysis of apoptotic level was performed using a Flow Cy-

tometer (BD, CA, USA). The apoptotic percentage of 10,000 cells was deter-

mined and all the experiments reported in this study were performed 3 times.

2.6. Terminal deoxynucleotidyl transferase-mediateddUTP nick end labeling assay (TUNEL)

TUNEL assay was performed according to In situ Cell Apoptosis Detec-

tion Kit’s protocol. In brief, the first passage chondrocytes were replanted

on coverslips, after exposure to NOC-18 and/or SB203580 for 24 h, cells on

coverslips were fixed with 4% paraformaldehyde for 30 min and stick on slides

with balsam. The non-specific chromogen reaction, induced by endogenous

peroxidase was inhibited with 3% H2O2 for 10 min at room temperature.

The TdT and Biotin-11-dUTP reaction were performed for 1 h at 37 �C in a

humidified box, and Blocking Reagent was applied for 30 min at room

temperature, followed by Avidin-HRP for 1 h at 37 �C in a humidified box.

Biochemical controls were made with positive control slides treated with

DNase, and negative control slides were treated with PBS instead of TdT.

The DNA fragments were stained using DAB as a substrate for the peroxidase,

and hematoxylin was used as a counterstain.

2.7. Western blot analysis

The first generation chondrocytes were incubated at 37 �C for 24 h with

NOC-18 and/or SB203580 for various concentrations as indicated. Total pro-

tein was extracted from cultured cells using Trizol reagent and other organics.

The protein was adjusted to coincident concentration by the BSA standard

curve and diluted with 1% SDS. Each sample of 28 ml (containing 20 mg total

protein) was run on 10% SDS-PAGE then electrophoretically transferred to

PVDF membranes. The PVDF membranes were dried at 37 �C for 1 h,

1030 H. Wang et al. / Cell Biology International 31 (2007) 1027e1035

blocked in blocking buffer (TBST containing 5% non-fat milk) for 1 h, incu-

bated overnight with primary antibodies at 4 �C, washed, and incubated with

second antibodies at 37 �C for 1 h. The membranes were developed with ECL

substrate and exposed to X-ray film. Densitometry was performed using Work-

Lab software (UVP, USA). The data were recorded as the ratio of sample to

b-actin. All the experiments reported in this study were performed 3 times

and the results were reproducible.

2.8. Statistical analysis

All assays were repeated 3 times to ensure reproducibility. The data were

expressed as mean þ SD and analyzed by SPSS13.0 for windows. Significant

differences were established at P < 0.05.

3. Results

3.1. p38 MAPK inhibitor reduces NOC-18-inducedcaspase-3 activity

As an effecter caspase integrating all major apoptotic path-ways, we assessed the specific activity of caspase-3. The roleof p38 in NO donor NOC-18-induced caspase-3 activity wasinvestigated by treating chondrocytes with SB203580, whichis a specific inhibitor of p38 MAPK. The results showed thatNOC-18 dose-dependently increased caspase-3 activity, how-ever, this increase was inhibited by p38 MAPK inhibitor,SB203580 (Fig. 1).

3.2. p38 MAPK inhibitor reduces NOC-18-inducedchondrocytes apoptosis rate

To confirm that whether NO donor NOC-18 could induceapoptosis, we determined the incidence of apoptotic cells in-duced by NOC-18 of different concentration using AnnexinV-FITC/PI FCM (Fig. 2A, B). To further study the role ofp38 on chondrocyte apoptosis rate, cells were treated withp38 MAPK inhibitor, SB203580, and then analyzed by An-nexin V-FITC/PI FCM (Fig. 2A, C). The FCM figure wasshown in Fig. 2A. The FCM data of the positive percentagein Fig. 2B show that different concentration of NOC-18 couldincrease the apoptosis rate in chondrocytes in a dose-depen-dent manner, compared with untreated control (P < 0.05). Itwas also demonstrated in Fig. 2C that SB203580 was able

to decrease the apoptosis rate induced by NOC-18, and couldpartly block the apoptosis of chondrocytes induced by NOC-18, compared with untreated control (P < 0.05).

3.3. TUNEL technique detects DNA strand breaks

After treatment with NOC-18, chondrocytes DNA strandbroke, and nuclei became condensation and shrinkage, there-fore nuclei of apoptotic chondrocytes were stained to brown.However, the negative cells were blue. Examination of DABstained cells has shown that the TUNEL positive cells consis-tently exhibited nuclear condensation with a subset also exhib-iting nuclear ‘‘blebbing’’, both hallmarks of apoptosis. Asconcentration of NOC-18 increasing, there were more andmore positive cells. Following chondrocytes further additionof p38 MAPK inhibitor, SB203580, evidence of reduction ofpositive cells was found according to TUNEL consequence.It certificated again that p38 MAPK inhibitor SB203580 couldpartly block chondrocytes DNA strand fragmentation and fur-ther apoptosis induced by NO donor NOC-18 (Fig. 3).

3.4. NOC-18 stimulates phosphorylation of p38 MAPK

To investigate the earlier molecular mechanisms of NOC-18-induced DNA strand fragmentation and apoptosis, chon-drocytes in serum-free medium were stimulated with indicatedNOC-18 for 24 h and protein extracts were analyzed to detectthe activation of p38 MAPK by Western blotting with thephosphorylation state-specific and total antibodies. As theconcentration of NOC-18 increasing, the ratio of phosphoryla-tion p38/total also enlarged in a dose-dependent manner,compared with untreated control (P < 0.05), as shown inFig. 4A, B. The results demonstrated that NOC-18 stronglystimulated the phosphorylation of p38 MAPK, which wasthe early event in the transduction of this signal without affect-ing the total level.

3.5. SB203580 inhibits phosphorylation ofp38 MAPK induced by NOC-18

To confirm the effect of SB203580 on p38 MAPK, chon-drocytes in serum-free medium were stimulated with DMSO

Fig. 1. Down-regulation of NOC-18-induced caspase-3 activity by p38 MAPK inhibitor, SB203580. Rabbit cartilage chondrocytes in serum-free medium were

exposed to NO donor NOC-18 of various concentrations for 24 h (A). Chondrocytes in serum-free medium were exposed to DMSO (SB203580 vehicle, control)

or pretreated with SB203580 for 30 min and followed by stimulation with NOC-18 for 24 h (B). The activity of caspase-3 from lysates was measured using the

substrate Ac-DEVD-pNA.

1031H. Wang et al. / Cell Biology International 31 (2007) 1027e1035

Fig. 2. Reduction of NOC-18-induced chondrocytes apoptosis rate by p38 MAPK inhibitor, SB203580. Rabbit cartilage chondrocytes in serum-free medium were

exposed to NO donor NOC-18 of various concentrations (0, 0.025, 0.1, 0.4, 0.8 mM) for 24 h (001e005, respectively). Chondrocytes in serum-free medium were

exposed to DMSO (SB203580 vehicle, control, 006) or pretreated with SB203580 for 30 min and followed by stimulation with NOC-18 (SB203580 1 mM, NOC-

18 0.4 mM þSB203580 1 mM, SB203580 10 mM, NOC-18 0.4 mM þSB203580 10 mM, SB203580 20 mM, NOC-18 0.4 mM þSB203580 20 mM) for 24 h (007e

012, respectively). The apoptosis rate was measured by FCM assay, and the representative scattergram of three experiments was shown in A. A two-parameter

cytogram of log (FL1) (Annexin) vs log (FL3) (PI) was plotted. The apoptotic cells (Annexin Vþ/PI�) were detected in the lower right quadrant and data

were mean � SD of three experiments. *P < 0.05 vs control, # P > 0.05 vs control.

(SB203580 vehicle, control) or pretreated with SB203580for 30 min before treatment with NOC-18 for 24 h. The pro-tein extracts were analyzed to determine the activation ofp38 MAPK by Western blotting with the phosphorylationstate-specific and total antibodies. Different concentration ofSB203580 could reduce the ratio of phosphorylation p38/total

dose-dependently, compared with the control (P < 0.05).However, there was no significantly difference without stimu-lation with NOC-18, compared with the same control(P > 0.05). Thus NOC-18 could stimulate the activation ofp38 MAPK, and SB203580 was the specific inhibitor of p38MAPK (Fig. 4C, D).

1032 H. Wang et al. / Cell Biology International 31 (2007) 1027e1035

Fig. 3. DNA strand fragmentation demonstrated by TUNEL technique (DAB staining along with hematoxylin counterstaining of nuclei). Chondrocytes were

exposed to NOC-18 and/or SB203580 for 24 h as previously described, and the corresponding concentration was shown in the upper figure. The positive

(apoptosis) nuclei were stained brown by the TUNEL reaction, and the negative were blue. Magnification: �200.

3.6. NOC-18 induces NF-kB p65, p53 and caspase-3protein expression

To examine proteins involved in NOC-18-induced chondro-cytes apoptosis, cells in serum-free medium were stimulatedwith indicated NOC-18 for 24 h and protein extracts were an-alyzed to measure the levels of NF-kB p65, p53 and caspase-3by Western blotting with the respective antibody. As the con-centration of NOC-18 increasing, the evidence of up-regulationof NF-kB p65, p53 and caspase-3 protein expression was foundcompared with untreated control (P < 0.05) (Fig. 5). ThusNOC-18 activated NF-kB p65, p53 and caspase-3 proteins.

3.7. p38 MAPK inhibitor SB203580 suppressesNOC-18-induced NF-kB p65, p53 and caspase-3protein expression

To explore NOC-18 responsiveness of p38 MAPK signaltransduction pathway, chondrocytes in serum-free mediumwere stimulated with DMSO (SB203580 vehicle, control) orpretreated with SB203580 for 30 min before treatment withNOC-18 for 24 h. The protein extracts were analyzed to detectthe levels of NF-kB p65, p53 and caspase-3 by Western blot-ting with the respective antibody. The results demonstrated

that this inhibitor down-regulated NF-kB p65, p53 andcaspase-3 protein expressions, compared with untreated con-trol (P < 0.05). However, there was no significantly differencewithout stimulation with NOC-18, compared with the samecontrol (P > 0.05) (Fig. 5). Stimulation of NF-kB p65, p53and caspase-3 proteins was involved in p38 MAPK pathway.

4. Discussion

This research demonstrates the p38 MAPK signal transduc-tion pathway of NO-induced chondrocytes apoptosis. NOstimulates p38, which activates NF-kB, p53, caspase-3.Caspase-3 is critical to many aspects of the apoptotic pathway,which activation will ultimately lead to apoptosis.

Although multiple pathways are involved in both the initi-ation and execution of chondrocyte apoptosis, NO has beenreported as the primary inducer of apoptosis (Blanco et al.,1995). Also, it has previously been known that p38 MAPKpathway mediates NO-induced apoptosis. However, little isknown about the detailed mechanism. The present study isto investigate which factors are involved in NO-induced rabbitcartilage chondrocytes apoptosis in p38 MAPK pathway.

In this study, we reported morphologic and biochemical ev-idences of apoptosis (TUNEL and Annexin V-FITC/PI FCM)

1033H. Wang et al. / Cell Biology International 31 (2007) 1027e1035

Fig. 4. Inhibition of NOC-18-promoted p38 MAPK activity in chondrocytes in response to SB203580. Confluent cultures of rabbit cartilage chondrocytes in serum-

free medium for 24 h were exposed to various concentrations of NO donor NOC-18 (A, B), and DMSO (SB203580 vehicle, control 2) or pretreated with SB203580

for 30 min then followed by addition of NOC-18 (C, D) for 24 h. The total protein extracts were subjected to PAGE and sequential Western blot analysis with

phosphorylation state-specific antibody against phosphorylation p38 followed by incubation with anti-rabbit secondary antibody and revelation by enhanced chem-

iluminescence. After stripping, the same membranes were reacted with the total antibody. The positions of total p38, phosphorylation p38, and b-actin were shown

(A, C). The mean densitometric value of the phosphorylation protein divided by total p38 bands from three independent experiments was presented graphically in

the panel (B, D). *P < 0.05 vs control. #P > 0.05 vs control.

implicating that NO would induce chondrocyte apoptosis ina dose-dependent manner, and this effect could be inhibitedby SB203580. Our results show that p38 MAPK pathway isnecessary for NO-induced apoptosis in chondrocytes. One lim-itation of TUNEL analysis is the potential non-specific stain-ing of necrotic cells compared to truly apoptotic cells. Still,because of its relative simplicity, TUNEL is by far the mostwidely utilized technique for the analysis of chondrocyteapoptosis in the literature. Furthermore, the combination ofTUNEL and Annexin V-FITC/PI FCM could better detect ap-optosis. The exact mechanism of NO leading to DNA damagein the cells undergoing apoptosis has not been clarified. Onepossible explanation is that the reaction of NO with oxygenradicals results in production of highly toxic nitrous radicalperoxynitrite.

We studied the involvement of p38 in the NO-induced ap-optosis with p38 specific inhibitor SB203580 using Westernblotting. Fig. 4 shows that the ratio of phosphorylation p38/to-tal is up-regulated dose-dependently following treatment withNOC-18, and SB203580 blocks this up-regulation. It is thenhypothesized that NO phosphorylates p38 kinase. Proteinphosphorylation-dephosphorylation is one of the major signal-ing mechanisms for modulating the functional properties ofproteins involved in gene expression, cell adhesion, cell cycle,cell apoptosis, cell proliferation and differentiation.

Recent reports suggested a role of p38 MAPK on the activa-tion of NF-kB (Chen and Wang, 1999). There are some evi-dences suggesting that, in chondrocytes, NF-kB and p38MAPK lie on two distinct pathways that seem to be indepen-dently required for IL-1-induced iNOS expression (FerreiraMendes and Margarida Caramona, 2002). Similar findingshave been reported in mouse astrocytes stimulated with TNF-a/IL-1a (DaSilva et al., 1997). The precise role of NF-kB inthe regulation of NO-induced apoptosis remains to be deter-mined. In the current study, we found from Western blottingresults that SB203580 reduced NO-activated NF-kB p65 ex-pression. Therefore, it can be proposed that NF-kB is involvedin p38 pathway.

Previous studies have shown that p38 kinase activity inducedapoptosis via p53 accumulation through NF-kB-dependenttranscription and stabilization by serine-15 phosphorylation(Kim et al., 2002a,b). The phosphorylation of p53 is known tobe mediated by multitude of protein kinases, including p38MAPK (Huang et al., 1999). Accumulated evidences suggesta role of p38 kinase on p53 phosphorylation (Takekawa et al.,2000; She et al., 2001). The p38 kinase phosphorylates p53 atserine 15 in response to UV irradiation, osmotic shock, and re-sveratrol (She et al., 2000, 2001). However, it is controversialwhether p38 kinase either directly or indirectly phosphorylatesp53. The present study revealed that NO activated both NF-kB

1034 H. Wang et al. / Cell Biology International 31 (2007) 1027e1035

Fig. 5. Inhibition of NOC-18-increaced NF-kB p65, p53 and caspase-3 protein expressions by p38 inhibitor, SB203580. Confluent cultures of rabbit cartilage chon-

drocytes in serum-free medium for 24 h were exposed to indicated concentration of NO donor NOC-18 (A, B), and DMSO (SB203580 vehicle, control) or pre-

treated with SB203580 for 30 min then followed by treatment of NOC-18 (A, C) for 24 h. The total protein extracts were subjected to PAGE and sequential

Western blot analysis with respective antibody against p65, p53, caspase-3 followed by incubation with anti-rabbit secondary antibodies and revelation by en-

hanced chemiluminescence. The positions of p65, p53, caspase-3 proteins and b-actin were shown (A). The mean densitometric value of each sample protein

divided by b-actin from three independent experiments was depicted as bar graphs (B, C). *P < 0.05 vs control. #P > 0.05 vs control.

p65 and p53, and the activation was inhibited by SB203580. So,our results demonstrate that p38 is the upstream activator of NF-kB, which could lead to p53 stimulation. This proposal needs tobe certificated with NF-kB inhibitor in future research.

The requirement for NF-kB activity in apoptosis is con-sistent with p53 transcription and subsequent activation ofcaspase-3 (Kim et al., 2002a,b). The p38 MAPK activation iseither upstream or independent of caspases (Frasch et al.,1998). However, in apoptosis of T. vaginalis-infected macro-phage, p38 is located downstream of caspase activation (Changand Kim, 2006). Although apoptosis can occur independentlyof caspase involvement in some cell types (Lorenzo et al.,1999), almost all existing data indicate that caspase activationis a requirement for chondrocyte apoptosis. Therefore, in thisstudy, we attempted to make clear in detail the signal cascadesbetween p38 and caspase-3 in chondrocytes. In Fig. 1 (caspase-3 activity assay) and Fig. 5 (protein expression with Westernblotting), there are consistent results showing that NOC-18dose-dependently increases caspase-3, furthermore, this in-crease is inhibited by p38 MAPK inhibitor SB203580. So itcan be concluded that caspase-3 is requirement for p38 path-way, and p38 activation is upstream of caspase-3.

In conclusion, we have demonstrated the previously un-known p38 signal transduction pathway of NO-induced rabbit

articular chondrocyte apoptosis. There seems to exist a path-way as follows: NO / p38 / NF-kB / p53 / caspase-3 / chondrocyte apoptosis. p38, which is a key modulatorof cartilage degradation and inflammation in articular carti-lage, is a potential therapeutic target of articular diseases.

Acknowledgments

This research was supported by National Natural ScienceFoundation of China (Nos. 30371245 and 39770667) fromKey Laboratory of Environment and Genes related to Dis-eases, Ministry of Education, China.

References

Baeuerle PA, Henkel T. Function and activation of NF-kappa B in the immune

system. Annu Rev Immunol 1994;12:141e79.

Bal-Price A, Brown GC. Nitric oxide-induced necrosis and apoptosis in PC12

cells mediated by mitochondria. J Neurochem 2000;75:1455e64.

Blanco FJ, Ochs RL, Schwarz H, Lotz M. Chondrocyte apoptosis induced by

nitric oxide. Am J Pathol 1995;146:75e85.

Bonfoco E, Krainc D, Ankarcrona M, Nicotera P, Lipton SA. Apoptosis and

necrosis: two distinct events induced, respectively, by mild and intense

insults with N-methyl-D-aspartate or nitric oxide/superoxide in cortical

cell cultures. Proc Natl Acad Sci U S A 1995;92:7162e6.

1035H. Wang et al. / Cell Biology International 31 (2007) 1027e1035

Callsen D, Brune B. Role of mitogen-activated protein kinases in S-nitrosoglu-

tathione-induced macrophage apoptosis. Biochemistry 1999;38:2279e86.

Chang L, Karin M. Mammalian MAP kinase signalling cascades. Nature

2001;410:37e40.

Chang J-H, Kim S-K. Apoptosis of macrophages induced by Trichomonasvaginalis through the phosphorylation of p38 mitogen-activated protein

kinase that locates at downstream of mitochondria-dependent caspase

activation. Int J Biochem Cell Biol 2006;38:638e47.

Chen CC, Wang JK. p38 but not p44/42 mitogen-activated protein kinase is

required for nitric oxide synthase induction mediated by lipopolysaccha-

ride in RAW 264.7 macrophages. Mol Pharmacol 1999;55:481e8.

Cox LS, Hupp T, Midgley CA, Lane DP. A direct effect of activated human

p53 on nuclear DNA replication. EMBO J 1995;14:2099e105.

Cuenda A. SB203580 is a specific inhibitor of a MAPK homologue which is

stimulated by cellular stresses and interleukin 1. FEBS Lett 1995;364:

229e33.

DaSilva J, Pierrat B, Mary JL, Lesslauer W. Blockade of p38 mitogen-

activated protein kinase pathway inhibits inducible nitric-oxide synthase

expression in mouse astrocytes. J Biol Chem 1997;272:28373e80.

Eliyahu D, Michalovitz D, Eliyahu S, Pinhasi-Kimhi O, Oren M. Wild-type

p53 can inhibit oncogene-mediated focus formation. Proc Natl Acad Sci

1989;8:8763e7.

Ferreira Mendes A, Margarida Caramona M. Role of mitogen-activated pro-

tein kinases and tyrosine kinases on IL-1-induced NF-kB activation and

iNOS expression in bovine articular chondrocytes. NITRIC OXIDE: Biol

Chem 2002;6:35e44.

Frasch SC, Nick JA, Fadok VA, Bratton DL, Worthen GS, Henson PM. p38

Mitogen-activated protein kinase-dependent and -independent intracellular

signal transduction pathways leading to apoptosis in human neutrophils.

J Biol Chem 1998;273:8389e97.

Fukuo K, Hata S, Suhara T, Nakahashi T, Shinto Y, Tsujimoto Y, et al. Nitric

oxide induces up-regulation of Fas and apoptosis in vascular smooth mus-

cle. Hypertension 1996;27:823e6.

Huang C, Ma WY, Maxiner A, Sun Y, Nel A. p38 kinase mediates UV induced

phosphorylation of p53 protein at serine 389. J Biol Chem 1999;274:

12229e35.

Iordanov M, Bender K, Ade T, Schmid W, Sachsenmaier C, Engel K, et al.

CREB is activated by UVC through a p38/HOG-1-dependent protein

kinase. EMBO J 1997;16:1009e22.

Karin M, Ben-Neriah Y. Phosphorylation meets ubiquitination: the control of

NF-kappa B activity. Annu Rev Immunol 2000;18:621e30.

Kim HA, Lee YJ, Seong SC, Choe KW, Song YW. Apoptotic chondrocyte

death in human osteoarthritis. J Rheumatol 2000;27:455e62.

Kim SJ, Hwang SG, Shin DY, Kang SS, Chun JS. p38 kinase regulates nitric

oxide-induced apoptosis of articular chondrocytes by accumulating p53 via

NF-kB-dependent transcription and stabilization by serine 15 phosphoryla-

tion. J Biol Chem 2002a;277:33501e8.

Kim SJ, Ju JW, Oh CD, Yoon YM, Song WK, Kim JH, et al. ERK-1/-2 and

p38 kinase oppositely regulate nitric oxide-induced apoptosis of chondro-

cytes in association with p53, caspase-3, and differentiation status. J Biol

Chem 2002b;277:1332e9.

Kraan PM, Berg WB. Anabolic and destructive mediators in osteoarthritis.

Curr Opin Clin Nutr Metab Care 2000;3:205e11.

Lane DP, Crawford LV. Tantigen is bound to a host protein in SV40-transformed

cells. Nature 1979;278:261e3.

Lee SW, Fang L, Igarashi M, Ouchi T, Lu KP, Aaronson SA. Sustained acti-

vation of Ras/Raf/mitogen-activated protein kinase cascade by the tumor

suppressor p53. Proc Natl Acad Sci U S A 2000;97:8302e5.

Levine AJ. p53, the cellular gatekeeper for growth and division. Cell 1997;

88:323e31.

Li PF, Dietz R, von Harsdorf R. p53 regulates mitochondrial membrane poten-

tial through reactive oxygen species and induces cytochrome c independent

apoptosis blocked by Bcl-2. EMBO J 1999;18:6027e36.

Lorenzo H, Susin SA, Penninger J, Kroemer G. Apoptosis inducing factor

(AIF): a phylogenetically old, caspase-independent effector of cell death.

Cell Death Differ 1999;6:516e24.

Miyashita T, Reed JC. Tumor suppressor p53 is a direct transcriptional activa-

tor of the human bax gene. Cell 1995;80:293e9.

Nicholson DW. Caspase structure, proteolytic substrates, and function during

apoptotic cell death. Cell Death Differ 1996;6:1028e42.

O’Brien WJ, Heimann T, Tsao LS, Seet BT, McFadden G, Taylor JL. Regula-

tion of nitric oxide synthase 2 in rabbit corneal cells. Invest Ophthalmol

Vis Sci 2001;42:713e9.

Okamoto I, Abe M, Shibata K, Shimizu N, Sakata N, Katsuragi T, et al.

Evaluating the role of inducible nitric oxide synthase using a novel

and selective inducible nitric oxide synthase inhibitor in septic lung in-

jury produced by cecal ligation and puncture. Am J Respir Crit Care

Med 2000;162:716e22.

O’Neill LA, Greene C. Signal transduction pathways activated by the IL-1

receptor family: ancient signaling machinery in mammals, insects, and

plants. J Leukoc Biol 1998;63:650e7.

Peng T, Lu X, Lei M, Feng Q. Endothelial nitric-oxide synthetase enhances

lipopolysaccharide-stimulated tumor necrosis factor-alpha expression via

cAMP-mediated p38 MAPK pathway in cardiomyocytes. J Biol Chem

2003;278:8099e105.

Que FG, Gores GJ. Cell death by apoptosis: basic concepts and disease rele-

vance for the gastroenterologist. Gastroenterology 1996;110:1238e43.

Raingeaud J, Whitmarsh AJ, Barrett T, Derijard B, Davis RJ. MKK3- and

MKK6-regulated gene expression is mediated by the p38 mitogen-activated

protein kinase signal transduction pathway. Mol Cell Biol 1996;16:

1247e55.

Sandell LJ, Aigner T. Articular cartilage and changes in arthritis, an introduc-

tion: cell biology of osteoarthritis. Arthritis Res 2001;3:107e13.

She QB, Chen NY, Dong Z. ERKs and p38 kinase phosphorylate p53 protein at

serine 15 in response to UV radiation. J Biol Chem 2000;275:20444e9.

She QB, Bode AM, Ma WY, Chen NY, Dong Z. Resveratrol-induced activa-

tion of p53 and apoptosis is mediated by extracellular-signal-regulated pro-

tein kinases and p38 kinase. Cancer Res 2001;61:1604e10.

Siebenlist U, Franzoso G, Brown K. Structure, regulation and function of

NF-kB. Annu Rev Cell Biol 1994;10:405e55.

Slee EA, Adrain C, Martin SJ. Serial killers: ordering caspase activation events

in apoptosis. Cell Death Differ 1999;6:1067e74.

Su B, Karin M. Mitogen-activated protein kinase cascades and regulation of

gene expression. Curr Opin Immunol 1996;8:402e11.

Takekawa M, Adachi M, Nakahata A, Nakayama I, Itoh F, Tsukuda H, et al.

p53-inducible wip1 phosphatase mediates a negative feedback regulation

of p38 MAPK-p53 signaling in response to UV radiation. Embo J 2000;

19:6517e26.

Thompson CB. Apoptosis in the pathogenesis and treatment of disease.

Science 1995;267:1456e62.

Tsujimoto Y. Role of Bcl-2 family protein in apoptosis: apoptosomes or mito-

chondria? Genes Cells 1998;3:697e707.

Vogelstein B, Kinzler KW. p53 function and dysfunction. Cell 1992;70:

523e6.

Wang XZ, Ron D. Stress-induced phosphorylation and activation of the tran-

scription factor CHOP (GADD153) by p38 MAP kinase. Science 1996;

272:1347e9.

Wang X, Wong SC, Pan J, Tsao SW, Fung KH, Kwong DL, et al. Evidence of

cisplatin-induced senescent-like growth arrest in nasopharyngeal carcinoma

cells. Cancer Res 1998;58:5019e22.

Wendehenne D, Pugin A, Klessig D, Durner J. Nitric oxide: comparative

synthesis and signaling in animal and plant cells. Trends Plant Sci 2001;

6:177e83.

Zhao M, New L, Kravchenko VV, Kato Y, Gram H, diPadova F, et al. Regu-

lation of the MEF2 family of transcription factors by p38. Mol Cell Biol

1999;19:21e30.