Black tea prevents cigarette smoke-induced apoptosis and lung damage
© Banerjee et al; licensee BioMed Central Ltd. 2007
Received: 21 September 2006
Accepted: 14 February 2007
Published: 14 February 2007
Cigarette smoking is a major cause of lung damage. One prominent deleterious effect of cigarette smoke is oxidative stress. Oxidative stress may lead to apoptosis and lung injury. Since black tea has antioxidant property, we examined the preventive effect of black tea on cigarette smoke-induced oxidative damage, apoptosis and lung injury in a guinea pig model.
Guinea pigs were subjected to cigarette smoke exposure from five cigarettes (two puffs/cigarette) per guinea pig/day for seven days and given water or black tea to drink. Sham control guinea pigs were exposed to air instead of cigarette smoke. Lung damage, as evidenced by inflammation and increased air space, was assessed by histology and morphometric analysis. Protein oxidation was measured through oxyblot analysis of dinitrophenylhydrazone derivatives of the protein carbonyls of the oxidized proteins. Apoptosis was evidenced by the fragmentation of DNA using TUNEL assay, activation of caspase 3, phosphorylation of p53 as well as over-expression of Bax by immunoblot analyses.
Cigarette smoke exposure to a guinea pig model caused lung damage. It appeared that oxidative stress was the initial event, which was followed by inflammation, apoptosis and lung injury. All these pathophysiological events were prevented when the cigarette smoke-exposed guinea pigs were given black tea infusion as the drink instead of water.
Cigarette smoke exposure to a guinea pig model causes oxidative damage, inflammation, apoptosis and lung injury that are prevented by supplementation of black tea.
Cigarette smoking is a major cause for the increased incidence of Chronic Obstructive Pulmonary Diseases (COPD), worldwide. The pathogenesis of this disease is usually characterized by abnormal enlargement of airspaces of the lung accompanied by destruction of its walls . This is a major and increasing global health problem, which is currently the 4th leading cause of death, and is projected to become the 3rd commonest cause of death and the 5th commonest cause of disability in the world by the year 2020 . However, the cellular and molecular mechanism of COPD is not clear and there are no effective drug therapies for such lung damage that are able to significantly reduce disease progression.
Over the last few decades, inflammation and protease/antiprotease imbalance have been proposed to act as downstream effectors of the lung destruction following chronic cigarette smoking . It is now recognized that alveolar cell apoptosis is a major step in such damage process [1, 4–8]. It has been shown that chronic exposure of rats to mainstream cigarette smoke (CS) produces significant and time-dependent increase in the proportion of apoptotic cells in the bronchial and bronchiolar epithelium  and also of alveolar macrophages . However there is conflicting evidence for the induction of apoptosis. It is reported that exposure of airway epithelial cells to CS does not cause apoptosis but induces cell death by necrosis only . Notably, one prominent deleterious effect of CS is oxidative damage [12–15]. It is also reported that CS-induced oxidative stress is associated with apoptosis of human lung fibroblasts  as well as epithelial cells in vitro . In fact, it is now becoming progressively apparent that interactions among oxidative stress, apoptosis and excessive proteolytic damage of the alveolar cells may be responsible for the pathogenesis of cigarette smoke-induced lung damage . However, the question that remains to be addressed is whether oxidative damage precedes apoptosis or vice versa. Earlier we had shown that CS contains some stable water-soluble oxidant that causes significant oxidative damage to microsomal proteins and increased proteolysis [14, 15]. Altogether, these results would indicate that CS-induced oxidative damage and proteolysis may lead to apoptosis of alveolar cells and overall damage to the lung, which is likely to be prevented by antioxidants. In fact, oxidative stress has been implicated in the pathogenesis of lung damage as is seen in diseases like emphysema , and antioxidant therapy is considered to be a logical therapeutic approach in COPD . Black tea has strong antioxidant properties and this is well vindicated in our previous report which demonstrates that CS-induced oxidative damage of guinea pig lung microsomal proteins and increased proteolysis are markedly prevented by BT . In this paper we demonstrate that the initial event of exposure of guinea pigs to CS is oxidative damage, which is accompanied by inflammation, apoptosis and increased air space in the lung and that all these pathophysiological events are prevented when the CS-exposed guinea pigs are given black tea infusion as the drink instead of water.
Materials and methods
Chemicals and reagents
The source of black (CTC) tea was West Bengal Tea Development, Kolkata, India. Antibodies against p53, phosphorylated p53, Bax, Bcl-2, caspase 3 and anti-mouse-HRP, anti-rabbit HRP antibodies as well as the chemiluminescent kit for immunoblot analysis were obtained from Cell signaling Technology, Inc. USA. Anti-tubulin antibody was obtained from Santa Cruz Biotechnology, Inc. The in situ cell death detection kit was obtained from Roche. USA. Kit for protein estimation was obtained from Bio-Rad. BCIP/NBT (5-bromo-4-chloro-3-indolyl phosphate/nitro blue tetrazolium) was obtained from Bangalore Genei (India). All other chemicals were of analytical grade.
Preparation of tea infusion
Tea infusion was prepared as described as before . Two grams of black tea were added to 100 ml of boiling water, brewed for 5 min, cooled to room temperature and filtered. The filtrate has been designated as BT. The sample of black tea (CTC) used contained approximately 1% theaflavins (TF), 18% thearubigins (TR) and 6% catechins (CT) .
Exposure of guinea pigs to Cigarette Smoke (CS)
Male short hair guinea pigs weighing 350–450 g were used for all experiments. All animal treatment procedures met the NIH guidelines  and Institutional Animal Ethics committee guidelines. The guinea pig was used as a model animal, because, like humans, guinea pigs cannot synthesize ascorbic acid [21, 22]. The guinea pigs were fed an ascorbate-free diet for 7 days to minimize the ascorbate level of plasma and tissues . This is because ascorbate is a potential inhibitor of CS-induced oxidative damage of proteins [14, 15], which would otherwise counteract the damaging effect of CS. The diet given to the guinea pigs was similar to that described before , except that wheat flour was replaced by wheat bran. After 7 days of vitamin C deprivation, the guinea pigs were given oral supplementation of 1 mg vitamin C/day to prevent onset of scurvy. It is known that a dose of 0.8 mg vitamin C/day is adequate to maintain the guinea pigs .
After consuming the ascorbate-free diet for 7 days followed by supplementation of 1 mg vitamin C/animal/day, the guinea pigs were subjected to cigarette smoke exposure from 5 cigarettes/animal/day in a smoke chamber. An Indian commercial filter-tipped cigarette (74 mm) with a tar content of 15 mg and nicotine content of 1 mg was used. The smoke chamber was similar to that of a vacuum desiccator with an open tube at the top and a side tube fitted with a stop cock. The volume of the chamber was 5 litre. The cigarette placed at the top was lit and CS was introduced into the chamber containing the guinea pig by applying a mild suction of 4 cm water through the side tube for 10 sec. After then the vacuum was turned off and the guinea pig was further exposed to the smoke for another 30 sec. The total duration of exposure to smoke form one puff was thus 40 sec. The amount of suspended particle per puff was 2.3 mg. Altogether 2 puffs per cigarette was given, allowing the animal 1 min rest in smoke-free atmosphere to breathe air between each puff. The gap between one cigarette and the next was 1 hour. Pair-fed sham controls were subjected to air exposure instead of CS under similar conditions.
The guinea pigs were divided into the following experimental groups (n = 4/group). Air: exposed to air and given water to drink; CS: exposed to CS and given water to drink; CS + BT: exposed to CS and given the BT infusion (2 g/100 ml) to drink instead of water; BT: exposed to air and given the BT infusion to drink. The tea infusion was freshly prepared and replaced every morning and evening. The amount of tea infusion (2% solution) consumed per guinea pig per day was approximately 25 ml (≈0.5 g dry tea).
All guinea pigs were pair-fed individually with respect to a guinea pig in the CS group. The pairs were set up by their initial weights. The amounts of food consumed by the CS group (≈45 ± 5 g/guinea pig/day) was given to the guinea pigs of the other groups. After feeding ascorbate-free diet for 7 days following exposure to CS/air for further 7 days , both the sham controls and the CS-exposed guinea pigs were deprived of food overnight and sacrificed next day by diethyl ether inhalation. The lungs were then excised immediately and processed for analysis.
Histology and morphometric analysis for assessing pulmonary lung damage
The lungs were fixed in 10% formalin and embedded in paraffin. Sections (5 μm) were cut from the periphery of the middle lobes of lungs of each group more or less from similar positions. The paraffin embedded lung tissue sections (5 μm) were deparaffinized using xylene and ethanol (absolute, 95%, 90%, 80%, 70% diluted in water). The slides were washed with phosphate buffered saline (PBS) and permeabilised with permeabilisation solution (0.1 M citrate, 0.1% TritonX-100). The deparaffinized sections were stained with haematoxylin and eosin. Digital images were captured with Olympus CAMEDIA digital camera, Model C-7070 wide zoom (magnification, 10 ×). The individual area (A) and the perimeter (P, the contour length) of each alveolar air space were identified and measured using NIH image. Based on these measurements, a perimeter to area ratio (P/A) was calculated for each alveolar air space. The P/A value was transferred into surface density S/V, using the morphometric relationship S/V = π/4 × P/A . Two images were analyzed per lung section. Altogether 8 images were analyzed in 4 lung sections from each group.
Oxidative damage of proteins as evidenced by immunoblotting
Oxidative damage of lung proteins was evidenced by immunoblotting of the dinitrophenylhydrazone derivatives of protein carbonyls followed by densitometric scanning as described before , with the exception that whole lung lysates were used instead of microsomal membranes.
The paraffin embedded tissue sections (5 μm) were deparaffinized, washed and permeabilised as mentioned above under histology and morphometric analysis. The tunnel reaction was carried out using "In situ cell death detection kit, fluorescein" (Roche) according to manufacture's instruction. After reaction, the slides were washed with PBS and DNA fragmentation was detected by labeling with fluorescein labelled dUTP using terminal deoxynucleotidyl transferase. The cells were examined using a fluorescence microscope (Olympus Bx40) at excitation wavelength of 488 nm. Digital images were captured with cool CCD camera (Olympus; magnification, ×10). The nuclei were counted by counter staining with 4', 6'-diamidino-2-phenylindole (DAPI) at excitation wavelength, 350 nm. Two fields per section of four independent sections in each group were evaluated.
The tissue was homogenized in lysis buffer . Protein concentration was measured using Bio-Rad protein estimation kit. Thirty μg of tissue extract was resolved by SDS-PAGE, electro transferred to PVDF membrane, incubated with relevant primary antibodies of recommended dilution, washed and incubated further with HRP conjugated secondary antibodies of recommended dilution and detected using chemiluminiscent kit (Pierce). Caspase 3 and Bax were detected by chemiluminescence kit (Pierce) and Bcl-2 by color reaction using NBT-BCIP reaction. For primary antibodies, anti-rabbit antibodies were used against caspase 3, Bax, Bcl-2, p53, phosphorylated p53 (Ser 392), respectively. In case of tubulin, anti-mouse tubulin antibody was used.
All values are expressed as mean ± SD. Statistical significance was carried out using a two factor ANOVA, with factors being CS and BT. The P values were calculated using appropriate F-tests. Difference with P-values < 0.05 were considered significant.
Black tea prevents oxidation of lung proteins of guinea pigs exposed to CS
Lung cellular damage in guinea pigs exposed to CS and its prevention by black tea
Morphometric measurements of alveolar air space, perimeter and surface density.
No. of Air Spaces
Total Area (A)*
Total Perimeter (P)*
148 ± 10
25576 ± 1754
7646 ± 1282
0.299 ± 0.049
0.235 ± 0.038
84 ± 9
37879 ± 3931
7383 ± 919
0.197 ± 0.036
0.155 ± 0.028
176 ± 12
33978 ± 1704
10239 ± 758
0.302 ± 0.027
0.237 ± 0.021
184 ± 15
35389 ± 2733
10917 ± 1215
0.309 ± 0.037
0.243 ± 0.029
Black tea prevents CS- induced apoptosis in the guinea pig lung in vivo
Quantitative evaluation of the extent of DNA fragmentation indicates that compared to the air-exposed animals, there is 9 fold increase in the TUNEL positive cells in the lung of guinea pigs exposed to CS for 7 days (Figure 5B). However, when the guinea pigs are exposed to CS and given BT as the drink, the % of TUNEL positive cells (Figure 5D) is comparable with that of sham control and the BT group (Figures 5A,C).
Black tea inhibits CS- induced activation of caspase 3 in vivo
Phosphorylation of p53 in the lungs of guinea pigs and its prevention by black tea
Black tea inhibits over expression of Bax in the lungs of guinea pigs exposed to CS
We had previously demonstrated that cigarette smoke causes oxidation of guinea pig lung microsomal proteins [14, 15, 19]. Here we demonstrate that exposure of marginal vitamin C-deficient guinea pigs to CS causes oxidation of whole lung proteins. We have used vitamin C-depleted guinea pigs to minimize the ascorbate level in the tissues. This is because ascorbate is a potential inhibitor of CS-induced oxidative protein damage [14, 15]. If the guinea pigs were fed ascorbate-rich diet (15 mg vitamin C/animal/day), the animals failed to respond to CS . We further demonstrate that oxidative modification of proteins by cigarette smoke leads to inflammation, apoptosis and cellular damage of the lung (increased air space) and that black tea can prevent such cigarette smoke-induced lung damage. Others had also shown that one major deleterious effect of smoking is oxidative damage of proteins [13, 16]. Such oxidative modifications of structural proteins in the lung, including protein carbonylation, play a significant role in the etiology and progression of several human pulmonary diseases [13, 26, 29]. Oxidative stress plays an important role not only through direct injurious effects, but by involvement in the molecular mechanisms that control lung inflammation (13). One intriguing aspect of such oxidative protein damage is that the oxidized proteins become vulnerable to degradation by endogenous proteases present in the tissues [14, 29–32]. This may be a key cause of the degradation of lung structural proteins in smokers leading to degenerative diseases like emphysema, which is marked by the loss of structural matrix of the lung and its elasticity leading to impaired transfer of oxygen and carbon dioxide into and out of the blood. We have shown that protein oxidation is followed by inflammation, as evidenced by infiltration of inflammatory cells in the septal region and macrophages inside the alveoli. It is known that during phagocytosis macrophages undergo oxidative burst, accompanied by release of proteases . The proteases released from activated macrophages along with the endogenous proteases present in the tissue may be involved in degrading the cytoskeletal proteins leading to destruction of alveolar membranes and septal cells in emphysema. It is thus conceivable that if oxidation of lung proteins is prevented by antioxidants, subsequent proteolysis would be prevented, and this in turn would prevent lung damage like that observed in emphysema. Here we show that oxidation of proteins (Figure 1) and accompanied damage to the lung cells (Figure 2) are both inhibited by giving the CS-exposed guinea pigs BT as the drink. The extent of lung damage by CS exposure has been evidenced by the significant increase of the surface density (S/V) of the alveolar air space (Table 1). This represents the membrane interface of each alveolar air space per unit area. It is known that the efficiency of gas exchange (O2 and CO2) is greatly regulated by the surface density . The S/V is significantly increased by giving the CS-exposed guinea pigs BT as the drink. The possibility that the loss of alveoli accompanied by increased air space in the CS-exposed guinea pigs was due to inanition and comparatively less calorie intake  is not tenable. This is because the CS-exposed guinea pigs consumed ≈45 ± 5 g diet/day and the guinea pigs of all other groups, namely, sham control (air-exposed), BT, and CS + BT group were pair-fed with respect to the CS group. We had shown before that the inhibitory effect of BT is apparently a synergistic effect of the antioxidant flavonols present in BT, namely, theaflavins (TF), thearubigins (TR) and catechins (CT) . Based on the flavonol contents of BT, as determined before , the amount of flavonols consumed per guinea pig per day was approximately 5 mg TF, 90 mg TR and 30 mg CT. The BT flavonols probably act by quenching the stable oxidants, which might be long-lived radicals present in CS that are apparently responsible for oxidation of the lung proteins [14, 35, 36].
Our present data and other reports indicate that along with oxidative damage, apoptosis plays a crucial role in CS-induced lung damage [1, 4–8]. Although it is hypothesized that interaction of oxidative stress and apoptosis leads to pathophysiological conditions in emphysema, the question remains to be addressed: which one is the initial event, oxidative damage or apoptosis? It has been proposed that a vicious cycle may be established, because cells undergoing apoptosis display increased oxidative stress, which further contributes to the apoptosis . The role of apoptosis in such lung damage is not mere correlative, but potentially causative . Here we show that the oxidative damage is the initial event, which is followed by inflammation, apoptosis and increased air space indicating emphysematous change. The biochemical events that mark such apoptotic changes are DNA fragmentation, over-expression of Bax and activation of caspase 3. We have demonstrated that marked DNA fragmentation (increase in TUNEL positive cells) occurs in lungs of CS-exposed guinea pigs given water as the drink (Figures 4 and 5). When the CS-exposed guinea pigs are given BT instead of water, there is no observable increase in the DNA fragmentation. The percentage of TUNEL positive cells are comparable to that of sham controls (Figures 4 and 5). This indicates that BT prevents CS-induced DNA fragmentation.
Aoshiba et al.  reported that acute cigarette smoke exposure induces apoptosis of alveolar macrophages. However, Aoshiba et al. worked with rats and the present authors with partially vitamin C-deprived guinea pigs. Also, in the present study the authors used a relatively mild challenge while that used by Aoshiba et al.  was more severe, as evidenced by occurrence of some degree of alveolar bleeding. This is never observed in human smokers. Moreover, the incidence of alveolar macrophage (AM) apoptosis in CS-exposed rats obtained by Aoshiba et al.  was much lower (3.2 %) than observed by the present authors (≈16 %). This difference might be due to the fact that rats synthesize vitamin C  and vitamin C present in the respiratory tract of rats might have prevented the effect of CS inhalation on AM apoptosis.
Caspases contribute to apoptosis through disassembly of cell structures by disrupting the nuclear structure and also by cleaving several cytoskeletal proteins [30, 37]. Caspases are synthesized initially as inactive single polypeptide chains that undergo proteolytic cleavage to produce subunits having active protease activity. We have shown in this report that CS causes cleavage of procaspase 3 to active caspase 3 (17 KDa, Figure 6) in the guinea pig lung. When the CS-exposed guinea pigs were given BT as a drink, activation of caspase 3 was prevented (Figure 6).
It has already been demonstrated that phosphorylated form of p53 accumulates in the nucleus in response to DNA damage . We have shown here that although the level of p53 in the guinea pig lung remains unaltered after exposure to CS, the level of phosphorylated p53 is markedly increased (Figure 7). Phosphorylation of p53 and its trnslocation in the nucleus is accompanied by expression of Bax. Here we show that besides preventing CS-induced oxidation and fragmentation of DNA, BT also prevents CS-induced phosphorylation of p53 (Figure 7). In fact, we observed practically no accumulation of phosphorylated p53 in the lungs of guinea pigs exposed to CS and given BT as the drink (Figure 7).
Apoptosis is regulated by expression of a number of genes, including the Bcl-2 family [39, 40]. Out of these Bax is pro-apoptotic and Bcl-2 is anti-apoptotic. So, the ratio of Bax and Bcl-2 determines whether a cell will undergo apoptotic death or not. We have shown that CS exposure to guinea pigs given water as the drink has no effect on the level of Bcl-2, whereas the Bax protein is significantly increased, resulting in an overall increase of Bax/Bcl-2 ratio (Figure 8C, column 2). When the CS-exposed guinea pigs were given BT as the drink, there was no over expression of Bax (Figure 8A, column 4). This resulted in a reversal of the Bax/Bcl-2 ratio (Figure 8C, column 4). Although densitometric measurement shows that there is an increase of Bcl-2 proteins in the presence of BT (25% in lane 3 and 13% in lane B, over that of lanes 1 and 2, Fig 8), the significance of this increase is not clear.
In conclusion, we demonstrate that there is a close link between oxidative damage, apoptosis and lung cellular damage in our guinea pig model exposed to cigarette smoke. Apparently, the initial event in the pathophysiological condition is oxidative damage of proteins. This is followed by inflammation and apoptosis leading to destruction of alveolar membranes and septal cells, resulting in increased air space in the lung. When the CS-exposed guinea pigs are given BT as the drink, oxidative damage is prevented and this is accompanied by the prevention of apoptosis and lung damage.
The present study has some limitation for consideration of the smoke-induced guinea model as a model of COPD. In human smokers with COPD, marked inflammation associated with massive neutrophil influx is often seen. However, neutrophil accumulation is not a feature of the present model. Nevertheless, besides inflammation and neutrophil influx the CS-induced lung damage produced in guinea pigs may be comparable to that of human smokers. The structure of the guinea pig lung has similarity with that of the human lung with three major lobes on the right and two major lobes on the left as well as well-defined terminal bronchiole with subtending alveolar ducts (41). Also, the guinea pig develops morphologic and physiologic alterations after exposure to CS at the same pattern as humans . So the results obtained with guinea pigs in our present study would imply that regular intake of black tea may protect smokers from the risk of developing lung damage.
exposed to air and given water as the drink (also called Air group in Table 1)
- CS gr.:
exposed to CS and given water to drink
- CS + BT gr.:
exposed to CS and given BT infusion as the drink
- BT gr.:
exposed to air and given BT infusion as the drink.
We thank Dr. K. Ray for histopathological interpretation, and Kingshuk Roy Chowdhury for statistical analysis of the images. SB is Phulrenu Guha Research Fellow and IBC is INSA Honorary Scientist. 1Supported by National Tea Research Foundation, Tea Board, India.
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