Helicobacter pylori promotes apoptosis, activates cyclooxygenase (COX)-2 and inhibits heat shock protein HSP70 in gastric cancer epithelial cells

Objective Apoptosis plays an important role in the regulation of gastric epithelial cell number and gastrointestinal disorders induced by Helicobacter pylori (Hp). Heat shock proteins (HSPs) are involved in cell integrity, cell growth and in gastric mucosa colonized by Hp. COX-2 was implicated in Hp-induced carcinogenesis but the effects of this germ and CagA cytotoxin on HSP70, COX-2, Bax and Bcl-2 in gastric cancer epithelial cells have been little studied. Material and methods We determined the expression for HSP70, Bax and Bcl-2 in human gastric epithelial MKN7 cells incubated with live strain Hp (cagA + vacA+) with or without co-incubation with exogenous CagA and NS-398, the selective COX-2 inhibitor. After 3–48 h of incubation, the expression of HSP70, COX-2, Bax and Bcl-2 mRNA and proteins were determined by RT-PCR and immunoprecipitation. Results Hp inhibited expression for HSP70 and this was significantly potentiated by exogenous CagA. Co-incubation of epithelial cells with Hp, without or with CagA increased Bax expression and simultaneously decreased expression for Bcl-2. The increase in COX-2 mRNA and Bax expression were significantly inhibited by NS-398. We conclude that Hp promotes apoptosis in adenocarcinoma gastric epithelial cells in vitro and this is associated with activation of COX-2 and inhibition of HSP70.


Introduction
The gastric mucosa integrity depends on a balance between the renewal and the death of damaged or aged cells. Apoptosis, first proposed by Kerr et al. [1] in 1972, plays a complementary but opposite role to that of cell proliferation in regulation of normal growth and development of animal and human tissue. Gastric mucosa shows a high rate of renewal. The surface mucosal cells are constantly exfoliating into the gastric lumen, with a 3-5 day renewal rate under normal physiological conditions. Apoptosis has been reported to take place in all regions of stomach, occurring predominantly in the upper part of the gastric glands and involving 2-3 % of all epithelial cells [2,3]; however, this process in gastric cancer cells has been little investigated.
The regulation of apoptosis is a complex process that includes the activation of various apoptosis-related proteins such as the Bcl-2 family, p53, Fas and its ligand (FasL). The Bcl-2 family is one of the most thoroughly studied groups of proteins involved in the regulation of apoptosis under normal conditions. Some members of the Bcl-2 family, such as Bax, Bad or Bid promote apoptosis, whereas the other members, such as Bcl-2 and Bcl-x l were shown to inhibit this process [4][5][6]. Many bacterial pathogens have been identified as factors influencing the process of apoptosis in vivo and in vitro. Among the bacterial factors, the most important is Helicobacter pylori (Hp), which is a spiral, Gram-negative microaerophilic bacterial pathogen that has been categorized by WHO as a class I carcinogen, infecting the stomach of about 50 % of the world's population [7,8]. At present, the mechanisms by which Hp affects apoptosis in gastric epithelial cells still remains unclear because of the lack of availability of normal human epithelial cell line suitable to study these aspects in vitro conditions.
The major disease-associated genetic difference in Hp induced the upper gastrointestinal disorders is the presence (cag? and vacA?) or absence (cagA-and vacA-) of the pathogenicity island (cag-PAI), which is a locus of about 40-kilobase containing up to 27-31 genes [9,10]. One strain-specific Hp locus that has been associated with an increased risk of carcinogenesis is the vacA gene, which encodes a secreted bacterial cytotoxin identified as VacA. When added to mammalian cells in vitro, cytotoxin VacA induces multiple structural and functional alterations in the cell, the most prominent of which is the formation of large intracellular vacuoles [11][12][13]. Cytotoxin-associated gene A (cagA) is the only known Hp-protein translocated into host cells followed by tyrosine phosphorylation through host protein kinases. Hp strains possessing the cagA gene has been linked to an increased risk of the development of peptic ulceration and gastric cancer in infected individuals. However, this protein is not critical to the pathogenesis of the disease, due to the fact that many strains, which possess CagA do not cause disease and some strains that are associated with disease do not express cagA [14,15]. The cag-PAI encodes a type IV secretion system (TFSS) that forms a syringe-like structure, which is used to translocate the bacterial proteins into different types of eukaryotic cells. Once introduced into the host cells, bacterial toxins promote the synthesis and secretion of chemokines, such as interleukin-8 (IL-8) [16,17]. Moreover, TFSS activates intracellular signaling including NFjB and specific kinases such as mitogenactivated kinase (MAPK), that subsequently promote the transcription of genes responsible for inflammation, proliferation or apoptosis [18].
Heat shock proteins (HSPs) represent a nearly universal phylogenetically and highly conserved family of proteins that form the machinery able to prevent damage to the cell structure from a variety of environmental stresses and unfavorable conditions. HSPs act as molecular chaperons folding newly synthesized cell proteins and assisting in the refolding of damaged proteins. In particular, HSP70 is located in all cellular compartments including the mitochondria, endoplasmic reticulum, cytosol and nucleus [19][20][21]. HSP70 can be produced in response to environmental changes such as heat, ischemia, heavy metals, ethanol, toxin, oxidants, various viral and bacterial infections and it is believed that their primary function is to protect the body's cells against stress [22].
Cyclooxygenase (COX), the rate-limiting enzyme in the conversion of arachidonic acid to prostaglandin H 2 , is the main target of non-steroidal anti-inflammatory drugs (NSAIDs). Two isoforms of this enzyme have been identified: COX-1 and COX-2. The isoform COX-1 is constitutively expressed in most tissues and is involved in the production of prostaglandins (PG) to maintain normal physiological functions. COX-2 is involved in inflammation and triggered by mitogens, cytokines, hormones and growth factors. Several recent studies suggest that COX-2 might be an important factor in carcinogenesis, and COX-2 inhibitors were shown to possess anticancer effects [23]. Since Hp CagA and VacA positive and negative strains were reported to inhibit [24], and in some studies, to promote, the apoptosis in gastric mucosal cells in vivo and in vitro [25][26][27][28][29], we studied the effect of incubation with Hp of human MKN7 adenocarcinoma cell line with relation to expression HSP70, COX-2 and apoptosis. The purpose of our present study was several folds: (1) to determine the effect of live Hp strain expressing cagA and vacA on the expression of HSP70 in gastric epithelial MKN7 cells; (2) to examine the apoptosis rate in these cells by assessing the expression of mRNA for Bax and Bcl-2; (3) to compare the effect of cell incubation with Hp strain expressing cagA and vacA applied alone or in combination with exogenous CagA protein on the expression of HSP70, Bax and Bcl-2 in MKN7 cells; (4) to compare the effect of Hp strain expressing cagA and vacA and strains negative for cagA and vacA coincubated with or without the NS-398 on mRNA expression for COX-2 and apoptotic proteins Bax and Bcl-2 in MKN7 cells.

Materials and methods
All experimental procedures performed in this study were run in accordance to the Helsinki Declaration and approved by the Jagiellonian University Institutional Animal Care and Use Committee.

Bacterial strains and their characterization
Strains of Hp used in this study were isolated from gastric biopsy specimens of the patients with gastric ulcer who underwent upper endoscopy. The bacteria were grown on Columbia Agar supplemented with 5 % fresh horse blood (BioMerieux, Marcy l'Etoile, France). The plates were incubated under microaerophilic conditions at 37°C for 3-5 days.
Genomic DNA was isolated from Hp strains obtained from patients using DNAzol Reagent (Life Technologies, NY, USA) according to the manufacturer's protocol. For each single PCR reaction, 20 lg of DNA was used. Specific primers for the detection of cagA and vacA were synthesized by Sigma-Aldrich (St. Louis, USA). CagA and vacA positive and negative strains of Hp were used in experiments described in this study. Stock cultures were maintained at -70°C in Brucella Broth supplemented with 10 % fetal bovine serum and 10 % glycerol. Prior to the incubation with MKN7 cells, bacterial strains of Hp were suspended in sterile PBS.
Cell line and culture conditions MKN7 human gastric carcinoma cells were grown in RPMI 1640 medium (Sigma-Aldrich, USA) supplemented with 10 % fetal bovine serum at 37°C with 5 % CO 2 and humidified atmosphere in absence or in the presence of Hp alone or in combination with the recombinant CagA (Or-aVax Inc., Cambridge, USA) or NS-398, a selective COX-2 inhibitor. Before the experiments cells were seeded on 100 mm culture dish in RPMI 1640 with addition of 2 % fetal bovine serum without antibiotics. MKN7 cells were infected with 1 9 10 9 of live Hp per dish (calculated to approximately 300 Hp bacteria per cell) and co-incubated with 10 lg of CagA per 1 ml of RPMI medium or with 50 mM of NS-398.
Determination of Bax and COX-2 expression by RT-PCR After 3, 6, 24 and 48 h of incubation the cells were harvested and the total cellular RNA was isolated using Trizol Reagent (Invitrogen, Carlsbad, USA) according to the manufacturer's protocol. Following precipitation, RNA was resuspended in RNase-free water and its concentration was estimated by the absorbance at 260 nm wavelength. The RNA integrity in each sample was confirmed by 1 % agarose-formaldehyde gel electrophoresis and ethidium bromide staining. Aliquoted RNA samples were stored at -80°C until analysis. The synthesis of the first strand cDNA was performed with Reverse Transcription System (Promega, Madison, USA) using 2 lg of RNA. For the PCR, 2 ll of cDNA and oligo primers were used. All PCR reactions were carried out using a Promega PCR reagents. The DNA was amplified in the thermal cycler (Biometra T3, Berlin, Germany) with 20 cycles for b-actin (denaturation at 95°C for 1 min, annealing 54°C for 1 min and extension for 72°C for 2 min), 29 cycles for Bax (denaturation at 94°C for 45 s, annealing 55°C for 45 s and extension for 74°C for 2 min), 35 cycles for COX-2 (denaturation at 94°C for 1.5 min, annealing 58°C for 1.5 min and extension for 72°C for 1.5 min). Specific primers were synthesized by Sigma-Aldrich (St. Louis, USA). The following human primers were used: b-actin s-5 0 AGC GGG AAA TCG TCG GTG 3 0 , a-5 0 GGG TAC ATG GTG GTG CCG 3 0 , Bax s-5 0 TGG CAG CTG ACA TGT TTT CTG 3, a-5 0 CGT CCC AAC CAC CCT GGT CT 3 0 , Bcl-2 s-5 0 GAC AGC CAG GAG AAA TGA AA 3 0 , a-5 0 GAC TTC TTC CGC CGC TAC 3 0 , COX-2s-5 0 AGA TCA TCT CTG CCT GAG TA 3 0 , a-5 0 CCT TCT TAA CCT CTC CTA TTA TA 3 0 . PCR products were detected by electrophoresis on 2 % agarose gel containing ethidium bromide. Location of the predicted PCR products was confirmed using O'Gene Ruler 50 bp DNA Ladder (Fermentas Life Sciences, San Francisco, USA) as a standard-size marker.

Immunoprecipitation
Protein extracts from MKN7 cells were prepared as described by Cha et al. [30]. Samples containing 10 lg of proteins were incubated for 4 h at 4°C on a shaking platform with 5 ll of primary goat polyclonal anti HSP70, rabbit monoclonal anti Bax, and mouse polyclonal anti Bcl-2 antibodies. Five microliters of A-agarose were added to each sample and samples were incubated overnight at 4°C. Complexes were washed three times with RPB-buffer (150 mM NaCl, 1 % NP-40, 0.5 % deoxycholate, 0.1 % SDS, 50 mM Tris pH 8). Then 10 ll of western blot sample buffer (0.1 M Tris-HCl pH 6.8, 4 % SDS, 20 % glycerol, 2 % mercaptoethanol, 0.2 % bromophenol blue) was added to each pellet, boiled for 5 min at 95°C and loaded on the 12 % SDS-polyacrylamide gel. After electrophoresis and transfer of the samples, the PVDF membrane (BIORAD, Hercules, USA) was blocked with 5 % non-fat dried milk in PBS for 2 h at room temperature. Blocking procedure was followed with 1 h exposure to primary antibody diluted 1:1000 and secondary antibody anti goat, anti rabbit, and anti mouse diluted 1:1000 in blocking buffer. All antibodies were purchased from Santa Cruz Biotechnology (Santa Cruz, USA). After each antibody probing membrane was washed three times for 15 min in TBST buffer (0.1 M Tris Ph 8.0; 1.5 M NaCl; 0.5 % TritonX-100). Detection of membrane bound proteins was performed using West Pico Chemimiluminescent (Pierce, Rockford, USA).

Statistical analysis
Results are expressed as means ± SEM. Statistical analysis was done using analysis on variance and two ways ANOVA test when appropriate. Differences with p \ 0.05 were considered statistically significant. COX-2 and heat shock proteins 957

Results
The effect of live Hp (cagA and vacA positive) strain with or without coincubation with exogenous CagA on the HSP70, Bax and Bcl-2 expression in MKN7 gastric epithelial cells In this series of experiments, the MKN7 cells were exposed to live bacteria (1 9 10 9 per culture dish) over different periods of time starting from 3 h up to 48 h. As expected the basal expression of HSP70 and mRNA for COX-2 in MKN7 cells was relatively high due to the intensive growth rate of that kind of cell type. As shown in Fig. 1 (Fig. 1c). Ratio of Bax protein over GAPDH protein confirmed that the expression of Bax protein was significantly increased when MNK7 cells were incubated with or without coincubation with recombinant CagA ( As shown in Fig. 1 panel d, the expression for Bcl-2 was detected by immunoprecipitation in the MKN7 cells infected with Hp cagA and vacA positive strain, was significantly diminished during the time period of 3 h up to 48 h, with maximal inhibitory effect observed at 48 h of incubation. Densitometric analysis confirmed that the ratio of Bcl-2 protein to GAPDH protein was significantly reduced when MKN7 cells were incubated with or without recombinant CagA ( Fig. 1 panel g and Fig. 2 panel g). Figure 2 panel b shows that in the MKN7 cells which were incubated with a combination of Hp strain positive for cagA and vacA together with the addition of the recombinant CagA (10 lg per 1 ml of RPMI 1640 medium), the decrease in the signal intensity for HSP70 was not significantly different at 3 h and 24 h following the incubation as compared with that of Hp alone ( Fig. 1 panel b) without the addition of CagA. However, when the incubation of the MKN7 cells with the Hp strain positive for cagA and vacA combined with exogenous CagA was prolonged up to 48 h, a complete disappearance of the signal for HSP70 expression was observed. The ratio of HSP70 protein over GAPDH protein confirmed that expression of HSP70 was significantly reduced at 3 and 24 h when compared with the control group and that the expression was not detectable at all, when this Hp strain was coincubated with exogenous CagA for 48 h (Fig. 2, panel e).
The expression of Bax was significantly increased at 24 and 48 h after the addition of CagA to the cells infected with Hp cagA vacA positive strain. The addition of exogenous CagA resulted in a significant decrease in Bcl-2 expression with the most pronounced effect observed at 48 h (Fig. 2, panels c and d).
The effect of live Hp (cagA and vacA positive) and Hp (cagA and vacA negative) with or without coincubation with NS-398 on the COX-2 and Bax mRNA expression in MKN7 gastric epithelial cells In MKN7 cells infected with Hp positive for cagA and vacA, a significant increase in signal intensity for COX-2 mRNA at 3 h, 6 h and 24 h of incubation was recorded as compared to control cells (Fig. 3, panel b). The increase in COX-2 expression was first observed 3 h after Hp infection and the maximal rise of COX-2 mRNA expression was recorded at 24 h. The semi-quantitative ratio of COX-2 mRNA over b-actin mRNA confirmed that the expression of COX-2 reached the highest value in MKN7 cells incubated for 24 h with Hp strain positive for cagA and vacA (Fig. 3, panel c). Figure 4 shows that after 3 h, 6 h and 24 h of co-incubation MKN7 cells with Hp positive for cagA and vacA and NS-398, the signal for expression of mRNA for COX-2 was almost identical at each study period. Additionally, we found that incubation of MKN7 cells with NS-398 only, significantly reduced COX-2 mRNA in these cells at 24 h upon selective COX-2 inhibitor administration (Fig. 4,  panel b). Densitometry analysis of mRNA expression presented as ratio of mRNA for COX-2 over mRNA b-actin confirmed that the signal intensity for COX-2 mRNA was similar in control cells and those incubated with Hp cagA and vacA positive with the addition of NS-398 at 3, 6 and 24 h following the beginning of incubation with this COX-2 inhibitor. This ratio was significantly decreased when MKN7 cells were incubated only with NS-398 (Fig. 4, panel c).
The effects of co-incubation of Hp strain positive for cagA and vacA with NS-398 were then compared with those for Hp negative for cagA and vacA applied alone or co-incubated with NS-398. For this purpose, the MKN7 cells were incubated with live cagA and vacA negative Hp with or without NS-398 for 3, 6 and 24 h, exactly as in the case of experiments with Hp strain positive for cagA and vacA. In contrast to the Hp cagA and vacA positive strain which caused a time-dependent rise in COX-2 expression, the signal for COX-2 mRNA did not change during coinfection of the MNK7 cells with Hp cagA and vacA negative strain and this was confirmed by the ratio of mRNA for COX-2 over mRNA b-actin (Fig. 5, panels b and c).
As illustrated in Fig. 6, panel b, COX-2 inhibitor, NS-398 failed affect COX-2 expression at 3 h and 6 h but significantly decreased mRNA expression for COX-2 at 24 h of incubation. Similarly as presented in Fig. 4, NS-398 significantly decreased the expression of COX-2 in MKN-7 cells non infected with Hp.
The expression of Bax mRNA increased 24 h after Hp infection and this effect was significantly inhibited by NS-398 (Fig. 7, panel b). Ratio of Bax mRNA over b-actin mRNA confirmed that the expression of Bax mRNA was significantly increased in cells infected with Hp strain positive for cagA and vacA and this effect was significantly ameliorated by co-incubation with selective COX-2 inhibitor (Fig. 7, panel c).

Discussion
This study was designed to determine whether incubation of gastric mucosal cells with Hp influences the expression In vitro studies using gastric cancer cell lines have shown that incubation of isolated gastric cells with either live or dead Hp or sonicates of Hp cells leads to an induction of apoptosis [31,32] and results of our study with MKN7 cells are in agreement with these observations. At present, the mechanism by which Hp induces apoptosis in gastric cancer cells and interacts with HSP70 and COX-2 expression remains unknown.
There is increasing evidence that apoptosis plays an important role in the pathogenesis of a variety of infectious diseases. Although apoptosis as a process of cell death occurs in normal mucosal cells under physiological conditions, the excessive apoptosis results in tissue damage. We extended our and others previous observation in vivo [33,34] that cagA positive Hp strains induce apoptosis as evidenced by increasing expression of pro-apoptotic Bax and decreasing expression for anti-apoptotic Bcl-2. In addition, CagA negative Hp strain also enhanced apoptosis suggesting that CagA is not a prerequisite for apoptosis to occur as suggested previously [35]. This notion is to some extent in keeping with observation by Peek et al. [36] and Zhang et al. [37] who reported that in patients infected with cagA positive Hp strains the apoptotic index was lower than in those with cagA negative Hp strains. In clear contrast, the gastric infection with Hp cagA positive strains was shown to induce an overexpression of proapoptotic proteins in the gastric mucosa [38,39], similarly as in our present study with transformed cell line. Most in vitro and animal studies have shown that both Hp cagA positive and Hp cagA negative strains enhanced apoptosis sometimes with no relation to CagA expression [27,34,37,38] suggesting that our results with Hp infection of cancer cell line seems to mimic those obtained in normal gastric epithelial cells with respect to apoptosis at early time after infection. For example, Ashktorab et al. [38] reported that infection of gastric cells with a cagA positive Hp strain resulted in Bax translocation to mitochondria, caspase-3 activation, and ultimately cell death. Interestingly, Hp CagA positive strain induced apoptosis at 2 weeks after inoculation but this increased apoptosis almost declined to the baseline level after 12 months of inoculation in rats [40]. Therefore, it is not excluded that the early enhancement in apoptosis process in both, normal and cancer cells may involve the formation of monochloramine (NH 2 Cl) observed in normal rat gastric epithelial cell line RGM-1 [41] and in gastric cancer cells MKN45 [42].
In addition besides CagA another cytotoxin of Hp, VacA, has been reported to induce apoptosis [43]. Purified, recombinant VacA caused fragmentation of DNA in human gastric adenocarcinoma AGS cell line [13]. MKN7 cells were incubated with Hp expressing cagA and vacA and supplemented with exogenous CagA, the trend in protein expression of Bax and Bcl-2 was similar to that observed in cells infected with Hp without CagA supplementation suggesting that exogenous CagA itself exerts not a direct stimulatory influence on gastric epithelial cells apoptosis. However, the most pronounced effect on apoptosis was observed when exogenous CagA was added to Hp positive for cagA and vacA cytotoxins suggesting that action of CagA may involve indirect effect, e.g. inhibition of HSP70. Indeed, CagA injected by Hp into the host epithelial cells was shown to ''hijack'' physiological signal transduction and caused pathological cellular response such as increased cell apoptosis, proliferation and morphology changes [26]. Our study indicates that this promotion of cell apoptosis by Hp strain positive for cagA and vacA is also evident in epithelial adenocarcinoma cell line MKN7. In addition, the CagA by itself can induce ROS production causing DNA damage, thus contributing to enhanced apoptosis in early stages of gastric carcinogenesis. The major finding of our present study is that Hp can directly attenuate the expression of HSP70 in MKN7 cells which could by associated with the increase in apoptosis. Specific downregulation of HSP70 in MKN7 cells was observed, especially under condition where the live Hp expressing cagA and vacA was added to the cell culture and incubated for the period of 48 h. Moreover, when the exposure of the cells with Hp strain positive for cagA and vacA was combined with exogenous CagA administration, a complete disappearance of the signal of HSP70 mRNA was recorded, suggesting that the addition of CagA to the cells infected with Hp positive for cagA and vacA, accelerated the inhibitory effect of bacteria and its cytotoxin on HSP70 expression (see, our hypothesis presented in Fig. 8). It should be noted, however, that cell incubation with recombinant CagA alone failed to affect the expression of mRNA for HSP70 (not shown), suggesting that the CagA protein itself does not exhibit inhibitory influence on HSP70 expression but exerts this inhibition only in the presence of Hp. This is corroborative with the observation [43,44] that the genes of the cag PAI actually encode for a functional type IV secretion system, by which Hp delivers the CagA protein directly into gastric epithelial cells. The possible molecular mechanism of the attenuation of HSP70 expression by Hp involves TFSS (type IV secretion system) and the subsequent activation of STAT3 (signal transducers and activators of transcription 3) pathway and activation of caspase-3 dependent apoptosis pathway as has been recently proposed by our group, with regards to another human gastric epithelial cell line [45].
We conclude that Hp action involves the ''injection'' of CagA into the eukaryotic cells, or another not yet recognized mechanism related to the damage of cellular membrane and/or inhibitory effect of bacteria on tight by recruiting ZO-1 [46] that impaired cell junctions allowing for the transport of cytotoxin such as CagA by this bacteria and/or in the presence of this germ. Our working hypothesis is that Hp possesses an ability to deliver biologically active proteins such as CagA into eukaryotic cells, because the most pronounced inhibitory effect of Hp on HSP70 was observed in MKN7 cells additionally co-incubated with CagA. Our findings are, however, limited to the gastric adenocarcinoma cell line and further studies are needed to confirm this hypothesis in normal human gastric epithelial cells.
Our finding that Hp expressing cagA and vacA exerts direct effect on the structure of the epithelial cells causing its increased fragility and inhibits HSP70 expression suggests that downregulation of HSP70 and lack of its protective influence on cell defense system, might be one of the cause of many other Hp-associated alteration of the Bax/Bcl2 cellular equilibrium (Fig. 8) Fig. 8 Schematic hypothesis presenting model of Hp positive cagA and vacA strain interaction with human gastric epithelial MKN7 cells via enhancement of apoptosis, the inhibition of HSP70 and activation of COX-2 by this bacteria and its cytotoxin CagA exposure of the Hp-infected cells to short non-cytotoxic heat shock or geranylgeranylacetone (GGA) restored HSP70 expression in RGM-1 cells [47,48]. Hp upregulates COX-2 mRNA expression in gastric cancer cells, and both premalignant and malignant gastric lesions demonstrated high COX-2 expression levels in vivo, suggesting that, Hp-induced gastric carcinogenesis is associated with elevated expression of COX-2 in neoplastic gastric epithelium [49][50][51]. Herein, we have also demonstrated, that both Hp strains, namely Hp cagA and vacA positive and Hp cagA and vacA negative caused an up-regulation of COX-2 mRNA in MKN7 cells and this effect was inhibited by NS-398. Likewise, the NS-398 alone markedly inhibited the mRNA expression for COX-2 whether or not cells were infected with Hp cagA and vacA negative strain. Since COX-2 overexpression is involved in gastric carcinogenesis, the suppression of COX-2 expression and activity might be useful in a chemoprevention strategy [52]. In our present in vitro study, NS-398 not only inhibited the upregulation of mRNA expression for COX-2 in Hp-infected MKN7 cells, but also suppressed Bax expression. Li et al. [53] also showed that the selective COX-2 inhibitor attenuated the cell proliferation and apoptosis of the human gastric cancer cell line BGC-823, which may be attributed to the inhibition of cell cycle progress. On the other hand, Konturek et al. [54] reported that in gastric cancer patients 14-day treatment with selective COX-2 inhibitor a significant up-regulation of caspase-3 mediating apoptotic cell death was observed. This indicates that Hp apparently promotes an apoptosis in vitro at early stage of infection but could exert opposite effect in vivo in long term infected patients with chronic and atrophic gastritis. COX-2 derived products and COX-2 inhibitors may differ in apoptotic response as to whether apoptosis is determined in isolated cells or in vivo condition.
In summary, we found that co-incubation of MKN7 epithelial cells with Hp without or with addition of CagA changed the equilibrium between Bax and Bcl-2 expression towards proapoptotic state (Bax) while simultaneously decreasing expression for anti-apoptotic Bcl-2. Thus it is hypothesized (Fig. 8) that besides HSP, also the PG generated via COX-2 expression and activity could be considered as another important activator of apoptosis triggered by Hp in the presence of CagA cytotoxin.