2.7. Cellular caspase activities (Caspase-3/7, -8 and -9)
HT-29 cells (25,000 cells/well), in RPMI-1640 and 10% FBS, were
seeded in the white 96-well plate (SPL, Korea) and incubated overnight
at 37 _C with 5% CO2 and 37 _C. The medium was then
replaced with fresh medium and the cells treated with the IC50
concentration of WTE for different time periods, i.e., 2, 8, 16, 24
and 48 h. Cells without any treatment and media alone without
cells in triplicate for the different times incubated were used as
the control and blank. Caspases-3/7, -8 and -9 activities were carried
out by using commercial kits purchased from Promega Company
(USA) according to the manufacturer’s protocol. In brief,
after the treatment time, the reagents of Caspase-Glo™ -3/7, -8
and -9 reagents were prepared and added directly to the cells in
96-well plates and incubated for 30 min before recording luminescence.
To reduce nonspecific background activity in cell-based
assays, MG-132 inhibitor was added to Caspase-Glo_ -8 and -9
reagent. The plates were read in a luminometer (GloMax microplate
luminescence reader, Promega Company, USA). The raw data
were collected from the luminometer and the average calculated
from the replicates. Background readings were determined from
wells containing culture medium without cells. The ‘‘no cell
media’’ blank control value was subtracted from each value.
2.8. Fluorescence microscopic examination
The morphological characterisation of HT-29 cells was performed
by using propidium iodide (PI) and acridine orange (AO)
double staining according to the method described by Ng et al.
(2013) and observed under a fluorescence microscope. Briefly,
HT-29 cells were plated at a density of 1 _ 106 cells/ml in a
25 ml culture flask and treated with the IC50 concentration of
WTE determined from MTT assay at varying time points of incubation.
The cells were incubated in an atmosphere of 5% CO2 at 37 _C
for 8, 12 and 24 h. Then the cells were harvested and washed twice
using PBS after centrifuging at 1800 rpm for 5 min to remove the
remaining media. An equal volume of fluorescent dye (AO/PI) containing
AO (10 lg/ml) and PI (10 lg/ml) were added to the cellular
pellet and freshly stained cells were observed under a UVfluorescence
microscope within 30 min before the fluorescence
colour started to fade.
2.9. Analysis of DNA damage
Analysis of DNA damage was carried out by using the comet
assay in the murine embryonic fibroblast line, 3T3-L1 (ATCC).
The 3T3-L1 cells were cultured in 12-well tissue culture plates
(1 _ 105 cells/well) for 24 h at 37 _C, in a humidified atmosphere
containing 5% CO2. Then the cells were pre-treated for 24 h with
WTE at concentrations of 5–25 lg/ml. After 24 h of incubation,
the cells were treated with 100 lM of H2O2 for 60 min in an ice
bath to prevent the action of DNA repair mechanisms (Miller,
Thomas, & Buschbom, 1995), and then harvested using trypsin–
EDTA, centrifuged for 5 min at 1500 rpm and resuspended in
1 ml of PBS. A volume of 25 ll of cell suspension was mixed with
75 ll of 0.6% low melting agarose. The suspension was spread on
a frosted microscopic slide pre-coated with 250 ll of 0.8% normal
melting agarose, covered with a cover slip, and then allowed to
solidify on ice for 10 min. The cover slips were then removed and
the slides were immersed in cold lysis solution containing 1%
sodium dodecyl sulphate, 2.5 M NaCl, 100 mM Na2EDTA, 1% Triton
X-100, and 10% DMSO, with the DMSO for 1 h at 4 _C in the dark.
The slides were arranged in an electrophoresis tank filled with
pre-chilled electrophoretic buffer (1 mM Na2EDTA and 300 mM
NaOH) and incubated for 20 min. Electrophoresis was conducted
at 25 V (300 mA) for 20 min using a power supply. The slides were
washed with 0.4 M Tris–HCl (pH 7.5) and stained with 20 lg/ml
ethidium bromide. An Olympus BX50 fluorescence microscope
was used for viewing the slides. A total of 50 cells in triplicate
per group were used to calculate the DNA damage induced by
hydrogen peroxide. The comet tail length was measured using an
ocular micrometer and the DNA damage was calculated by the
following formula:
Comet tail length maximum total length _ head diameter
2.10. Statistical analysis
Experimental results are presented as means } SD, and all measurements
and analyses were carried out in triplicate. Excel 2007
and SPSS V.18.0 statistical software were used for the statistical
and graphical evaluations in this study. Statistical analysis was
performed by one-way analysis of variance (ANOVA) with Tukey’s
multiple comparisons and the Student’s t-test. All p-values <0.05
were considered significant.
3. Results and discussion
3.1. Total phenolic and flavonoid content
The total phenolic content (TPC) was quantified as gallic acid
equivalents (GAE). The TPC of Silver needle white tea was
863 } 5.6 mg GAE/g dried weight of sample in hot water extraction
(Fig. 1). In our study, white tea extracted with hot water showed
high amounts of total phenols within the extraction time of 5 min.
The TPC of the hot water extract was higher than that of the pure flavonoid,
rutin. According to (Venditti et al., 2010), the levels of total
polyphenols were high in green and white teas under two different
extraction methods, hot water for 7 min and cold water for 2 h.
However, we decided to steep the tea for only 5 min as this would
reflect the actual time commonly used in steeping tea.
The content of total phenols and flavonoids in white tea
extract are presented in Fig. 1. The total flavonoid content (TFC)
of Silver needle white tea infused in hot water for 5 min was
530 } 10.6 mg quercetin/g dried weight of sample. The TFC of the
controls, catechin and rutin, were 999.2 } 9.9 and
569.7 } 12.026 mg quercetin/g dried weight, respectively. These
results indicate that Silver needle white tea, extracted for only
5 min, is a rich source of flavonoids, when extracted in hot water,
since the TFC is comparable to that of rutin. However, a study by
Rusak reported that the TFC of green and white tea increased with
extraction time (Rusak, Komes, Likic, Horzic, & Kovac, 2008).
3.2. Ferric reducing antioxidant power (FRAP)
In this assay sample readings were referenced to the standard
curve of ferrous sulphate, FeSO4_H2O. The FRAP value was
expressed as mmol Fe2+/g sample. Table 1 shows the reductive
capability of WTE. The FRAP value of the sample in hot water
extractions was 1.6 } 0.044 mmol Fe2+/g and the FRAP values for
the controls, catechin, quercetin and rutin, were 6.5 } 0.004,
7.5 } 0.035 and 3.03 } 0.028 mmol Fe2+/g, respectively. Since the
positive controls, catechin, quercetin and rutin, used were pure
compounds, it is expected that they would have higher FRAP values.
Pearson correlation analysis was performed to assess the relationship
between the phenolic content of the extract and ferric
reducing activity. There was a strong positive correlation between
the TPC of WTE and FRAP value (r = 0.902).
3.3. Assay of DPPH radical scavenging activity
The complete reduction of the DPPH radical is related to the
high scavenging activity shown by a particular sample. A high level
of antioxidants in the extract would indicate greater radical
scavenging activity.
Other studies on tea extracts showed that the levels of catechin
affect the antioxidant capacity. Green and black teas were efficient
in reducing DPPH due to their high catechin levels (Katalinic, Milos,
Kulisic, & Jukic, 2006). The results of our study showed that white
tea effectively reduces the oxidant free radical, DPPH. This could be
due to the presence of high levels of EGCG (epigallocatechin
gallate), a tea polyphenol in white tea (Almajano et al., 2008).
Hot water extract of tea showed IC50 values of 99.9 } 4.9 lg/ml
(Table 1). White tea extract was able to reduce the stable radical
DPPH to the yellow coloured diphenylpicrylhydrazine. The
scavenging effect increased with increasing concentrations of the
extract. The results showed that the scavenging activity of catechin,
a known antioxidant, was higher than extracts of C. sinensis.
The IC50 value of catechin, quercetin and rutin was 39.3 } 4.6,
35.5 } 3.6 and 44.4 } 5.8 lg/ml, respectively. Other studies showed
the relationship between tea TPC and scavenging of DPPH radical.
An increase in TPC resulted in an increase of DPPH radical
scavenging activity (Horzˇic´ et al., 2009). In the present study, the
tea with high TPC exhibited high potential DPPH radical scavenging
activity.
3.4. Assay of hydroxyl free radical scavenging activity
In a state of oxidative stress, reactive oxygen molecules (ROS)
such as superoxide, hydroxyl and peroxyl radicals are generated.
A number of studies have shown that ROS play an important role
in the pathogenesis of various chronic diseases, such as, neurodegenerative
disorders, cancer, cardiovascular diseases, atherosclerosis,
cataracts, and inflammation (Aruoma, 1998). As shown in
Table 1, the samples tested showed IC50 values of hydroxyl ion
scavenging activity of 353.4 } 31.9 lg/ml, in hot water extract.
The results indicated the scavenging capacity of the hot water
extract of white tea used against hydroxyl radicals.
3.5. Assay of nitric oxide scavenging activity
Nitric oxide is a reactive oxygen species involved in inflammation,
cancer and other pathological conditions (Moncada, Palmer, &
Higgs, 1991). One of the major causes of nicotine-induced cancer
metastasis and DNA damage is nitric oxide formation. Tea prepared
using hot water for 5 min showed a protective effect against nitric
oxide radicals by scavenging the nitric oxide radicals (Table 1). It
has been hypothesized that scavenging of nitric oxide is one of
the main mechanisms for the bioactivity of flavonoids.
3.6. Assay of superoxide radical scavenging activity
The superoxide radical is known to be very harmful to cellular
components as a precursor of more reactive oxygen species
(Halliwell & Gutteridge, 1985). Table 1 shows the superoxide
radical (O2
_) sc
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