Seminars in Cancer Biology 17 (2007) 370–376

Review

Oxidative breakage of cellular DNA by plant polyphenols: A putative

mechanism for anticancer properties

S.M. Hadi

∗

, Showket H. Bhat

1

, Asfar S. Azmi

1

,

2

, Sarmad Hanif, Uzma Shamim, M.F. Ullah

Department of Biochemistry, Faculty of Life Sciences, A.M.U., Aligarh 202002, UP, India

Abstract

Plant polyphenols are important components of human diet and a number of them are considered to possess chemopreventive and therapeutic

properties against cancer. They are recognized as naturally occurring antioxidants but also act as prooxidants catalyzing DNA degradation in the
presence of transition metal ions such as copper. We have shown that several of these compounds are able to bind both DNA and Cu(II) forming a
ternary complex. A redox reaction of the polyphenols and Cu(II) in the ternary complex may occur leading to the reduction of Cu(II) to Cu(I), whose
reoxidation generates a variety of reactive oxygen species (ROS). We have further confirmed that the polyphenol–Cu(II) system is indeed capable of
causing DNA degradation in cells such as lymphocytes. We have also shown that polyphenols alone (in the absence of added copper) are also capable
of causing DNA breakage in cells. Neocuproine (a Cu(I) sequestering agent) inhibits such DNA degradation. It also inhibits the oxidative stress
generated in lymphocytes indicating that the cellular DNA breakage involves the generation of Cu(I) and formation of ROS. It is well established
that tissue, cellular and serum copper levels are considerably elevated in various malignancies. Therefore, cancer cells may be more subject to
electron transfer between copper ions and polyphenols to generate ROS. Thus, our results are in support of our hypothesis that anticancer mechanism
of plant polyphenols involves mobilization of endogenous copper possibly chromatin bound copper and the consequent prooxidant action.
© 2007 Elsevier Ltd. All rights reserved.

Keywords: Plant polyphenols; Endogenous copper; Prooxidant; Anticancer; Apoptosis; Reactive oxygen species (ROS)

Contents

1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

370

2.

Oxidative DNA cleavage by plant polyphenols in vitro in the presence of copper ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

371

3.

Polyphenol–Cu(II) mediated DNA breakage in human peripheral lymphocytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

372

4.

Evidence for the prooxidant action of plant polyphenols as an important mechanism for their anticancer properties . . . . . . . . . . . . . . . .

372

5.

Oxidative DNA breakage by plant polyphenols in human peripheral lymphocytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

373

6.

Putative mobilization of endogenous copper by plant polyphenols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

373

7.

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

374

Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

375

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

375

1. Introduction

Plant-derived polyphenolic compounds that include flavo-

noids, tannins, curcuminoids, gallocatechins, stilbenes such as

∗

Corresponding author. Tel.: [phone]; fax: [phone].

E-mail address:

[e-mail]

(S.M. Hadi).

1

These authors contributed equally.

2

Present address: Department of Pathology, Karmanos Cancer Institute,

Wayne State University School of Medicine, Detroit, MI 48201, USA.

resveratrol, anthocyanidins such as delphinidin possess a wide
range of pharmacological properties the mechanisms of which
have been the subject of considerable interest (

Fig. 1

). They are

recognized as naturally occurring antioxidants and have been
implicated as antiviral and antitumor compounds

[1,2]

. In recent

years, a number of reports have appeared which have shown
that gallocatechins found in green tea and which include tan-
nic acid, gallic acid, epigallocatechin, epicatechin-3-gallate and
epigallocatechin-3-gallate (EGCG) induce apoptosis in various
cancer cell lines

[3,4]

. Similarly curcumin

[5]

from the spice

1044-579X/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:

10.1016/j.semcancer.2007.04.002

S.M. Hadi et al. / Seminars in Cancer Biology 17 (2007) 370–376

371

Fig. 1. Structures of (a) gallic acid, (b) syringic acid, (c) curcumin, (d) resveratrol, (e) delphinidin, (f) epigallocatechin-3-gallate, and (g) quercetin.

turmeric and resveratrol

[6]

which is found in grapes and red

wine have also been shown to be inducers of apoptosis in cancer
cells. The consumption of green tea is considered to reduce the
risk of various cancers such as that of bladder, prostate, esopha-
gus and stomach

[4]

. Of particular interest is the observation that

EGCG was found to induce internucleosomal DNA fragmenta-
tion in cancer cell lines such as human epidermoid carcinoma
cells, human carcinoma keratinocytes, human prostate carci-
noma cells, mouse lymphoma cells but not in normal human
epidermal keratinocytes

[4]

. Similarly gallic acid showed cyto-

toxicity for a number of tumor cell lines but primary cultured
rat hepatocytes and macrophages were found to be refractory to
the cytotoxic effect

[3]

. Resveratrol also was shown to induce

apoptotic cell death in HL60 human leukemia cell lines but not
in normal peripheral blood lymphocytes

[6]

. The hallmark of

apoptosis is internucleosomal DNA fragmentation, which dis-
tinguishes it from necrosis. Other changes such as shrinkage
of cells, membrane blebbing and the dissociation of the nucleus
into chromatoid bodies also occur. It is to be noted that most clin-
ically used anticancer drugs can activate late events of apoptosis
(DNA degradation and morphological changes) and there are
differences in essential signalling pathways between pharmaco-
logical cell death and physiological induction of programmed
cell death

[7]

. Based on our own observations and those of

others we propose a mechanism of DNA fragmentation in can-
cer cells by plant polyphenolics that involves mobilization of
intracellular copper. Studies on chemopreventive and therapeu-
tic plant-derived phytonutrients assume significance in view of
the fact that such compounds exhibit negligible or low toxicity

even at relatively high concentrations. Further they may also act
as lead compounds for the synthesis and development of novel
anticancer drugs.

2. Oxidative DNA cleavage by plant polyphenols in vitro
in the presence of copper ions

Studies in our laboratory have shown that a number of plant

polyphenols such as flavonoids

[8]

, tannic acid and its structural

constituent gallic acid

[9]

, curcumin

[10]

, gallocatechins

[11]

and resveratrol

[12]

cause oxidative strand breakage in DNA

either alone or in the presence of transition metal ions such
as copper. Recent studies by Liu and co-workers

[13]

demon-

strated that resveratrol as well as its certain synthetic analogs
namely 3,4,4



-trihydroxy-trans-stilbene, 3,4-dihydroxy-trans-

stilbene, 3,4,5-trihydroxy-trans-stilbene, which are generally
effective antioxidants, can switch to prooxidants in the presence
of Cu(II) to induce DNA damage. Copper is an important metal
ion present in chromatin and is closely associated with DNA
bases particularly guanine

[14,15]

. It is also one of the most

redox active of the various metal ions present in cells. We have
also shown that the flavonoid quercetin

[16]

and curcumin

[10]

are capable of binding to DNA and copper. Evidence deduced in
our laboratory has shown that polyphenols such as the flavonoid
quercetin and the stilbene resveratrol can not only bind copper
ions but also catalyze their redox cycling

[12]

. In the case of

quercetin a mechanism was proposed which involved the for-
mation of a ternary complex of DNA–quercetin–Cu(II)

[17,16]

.

A redox reaction of the compound and Cu(II) in the ternary com-

372

S.M. Hadi et al. / Seminars in Cancer Biology 17 (2007) 370–376

Fig. 2. Involvement of a ternary complex of quercetin, DNA and Cu(II)/Cu(I)
in the generation of active oxygen species. The oxidized forms of quercetin
(represented as quercetin*) are not necessarily identical.

plex may occur leading to the reduction of Cu(II) to Cu(I), whose
reoxidation generates a variety of ROS (

Fig. 2

). Most of the phar-

macological properties of plant polyphenols are considered to
reflect their ability to scavenge endogenously generated oxygen
radicals or those free radicals formed by various xenobiotics,
radiation etc. However, some data in the literature suggests that
the antioxidant properties of the polyphenolic compounds may
not fully account for their chemopreventive effects

[18,19]

. Most

plant polyphenols possess both antioxidant as well as prooxidant
properties

[3,8]

and we have proposed that the prooxidant action

of plant polyphenolics may be an important mechanism of their
anticancer and apoptosis inducing properties

[19]

. Such a mech-

anism for the cytotoxic action of these compounds against cancer
cells would involve mobilization of endogenous copper ions and
the consequent prooxidant action.

3. Polyphenol–Cu(II) mediated DNA breakage in
human peripheral lymphocytes

Using a cellular system of lymphocytes isolated from human

peripheral blood and alkaline single cell gel electrophoresis
(comet assay), we have confirmed that polyphenol–Cu(II) sys-
tem is indeed capable of causing DNA degradation in cells such
as lymphocytes

[20]

. Further, the DNA degradation of lympho-

cytes is inhibited by scavengers of ROS and neocuproine, a
Cu(I) specific sequestering agent. Also, similar to the in vitro
results, trans-stilbene which does not have any hydroxyl group
is inactive in the lymphocyte system. These findings demon-
strate that the polyphenol–Cu(II) system for DNA breakage is
physiologically feasible and could be of biological significance.

4. Evidence for the prooxidant action of plant
polyphenols as an important mechanism for their
anticancer properties

We give below several lines of indirect evidence in literature,

which strongly suggest that the prooxidant action of plant-

derived polyphenolics rather than their antioxidant activity may
be an important mechanism for their anticancer and apoptosis
inducing properties:

(1) Structure activity studies carried out in our laboratory with

gallic acid (a structural constituent of tannic acid) indicate
that if two of the three hydroxyl groups are methylated
(syringic acid), the DNA degrading capacity decreases
sharply

[9]

. The results correlate with those of Inoue et al.

[3]

who showed that modification of hydroxyl groups, such

as that resulting in the formation of syringic acid, abolishes
the apoptotic activity of gallic acid.

(2) Apoptotic DNA fragmentation properties of several anti-

cancer drugs

[21,22]

and

␥-radiation

[23]

are considered

to be mediated by ROS. It may also be mentioned that
doxorubicin induced apoptosis in human osteocarcinoma
Saos-2 cells is mediated by ROS and is independent of
p-53

[24]

. Interestingly, certain properties of polyphenolic

compounds, such as binding and cleavage of DNA and the
generation of ROS in the presence of transition metal ions

[16]

are similar to those of certain known anticancer drugs

[25]

.

(3) Fe

3+

and Cu

2+

are the most redox-active of the metal ions

in living cells. Wolfe et al.

[26]

have proposed that a copper

mediated Fenton reaction, generating site-specific hydroxyl
radicals, is capable of inducing apoptosis in thymocytes.
In a study with thiol-containing compounds, apoptosis was
induced in different cell lines when either free copper or
ceruloplasmin (a copper binding protein) was added; such
activity was not observed, however, when either free iron
or the iron-containing serum protein transferrin was added

[27]

. Most of the copper present in human plasma is associ-

ated with ceruloplasmin, which has six tightly held copper
atoms and a seventh, easily mobilized one

[28]

. In another

study supporting these observations, copper was found to
enhance the apoptosis-inducing activity of polyphenolic
antioxidants, whereas iron was inhibitory

[29]

. Although

iron is considerably more abundant in biological systems,
the major ions in the nucleus are copper and zinc

[15]

. Fur-

ther, although in general, tumors are considered to contain
less total iron and have less iron saturation in ferritin than
do normal cells that is not always the case

[30]

. As already

mentioned copper ions occur naturally in chromatin and
can be mobilized by metal chelating agents. Burkitt et al.

[31]

suggested that the internucleosomal DNA fragmenta-

tion might be caused not only by endonuclease but also by
metal-chelating agents such as 1,10-phenanthroline (OP),
which promotes the redox activity of endogenous copper
ions and the resulting production of hydroxyl radicals. Thus,
the internucleosomal DNA “laddering” often used as an
indicator of apoptosis may also reflect DNA fragmentation
by non-enzymatic processes. Several reports indicate that
serum

[32,33]

, tissue

[34]

and intracellular copper levels

in cancer cells

[35]

are significantly increased in various

malignancies. Indeed, such levels have been described as
a sensitive index of disease activity of several hematologic
and non-hematologic malignancies

[36]

.

S.M. Hadi et al. / Seminars in Cancer Biology 17 (2007) 370–376

373

(4) A comparison of the properties of complexes formed

between plant polyphenolics and Cu

2+

and Fe

3+

should

indicate which of these two metal ions could lead to DNA
fragmentation in the nucleus when complexed. Not much
is known about the properties of such complexes. However,
considerable information is available about OP chelation
of copper and iron ions. Burkitt et al.

[31]

cited several

reasons why Cu

2+

rather than Fe

3+

may be responsible for

OP-stimulated internucleosomal DNA fragmentation in iso-
lated nuclei. For example, the cumulative affinity constants
(

␤

3

in 0.1 M salt) for chelation of various metal ions by OP

are in the order Cu

2+

≈ Fe

2+

> Zn

2+

> Fe

3+

. The complex

formed between OP and Cu

2+

has a redox potential (E

◦

for Cu

2+

/Cu

+

= 0.17 V) that favors redox cycling, whereas

that for Fe

3+

/Fe

2+

is 1.1 V, presumably because of stabiliza-

tion in the ferrous state. Copper is also shown to be present
in chromosomes as Cu

+

ions because of stabilization in the

presence of DNA. This overcomes the need for the reduction
of Cu

2+

to Cu

+

and can directly generate the hydroxyl rad-

icals. Finally, copper and zinc are major metal ions present
naturally in chromosomes

[15]

. Because most polypheno-

lics are also polycyclic compounds similar in size to OP,
conceivably their metal binding properties are also similar.

(5) Evidence suggests that the antioxidant properties of

polyphenolics may not fully account for their chemopre-
ventive effects. For example, it was shown that although
ellagic acid is an antioxidant ten times more potent than
tannic acid, the latter was more effective in inhibiting skin
tumor promotion by 12-O-tetradecanoyl phorbol-13 acetate
than the former

[18]

. It was suggested that the antioxi-

dant effects of these polyphenols might be essential but not
sufficient for their antitumor promotion. In any case ROS
scavenging properties of plant polyphenols may account
for their chemopreventive effects but not for any thera-
peutic action against cancer cells

[22]

. Expression of the

bcl-2 proto-oncogene, which blocks apoptosis, decreases
cellular production of ROS

[37]

, whereas the coadministra-

tion of antioxidant enzymes such as superoxide dismutase
(SOD) and catalase prevents curcumin mediated apoptosis
in human leukemia cells

[5]

. Further it has been shown that

the programmed cell death induced by curcumin in human
leukemic T-lymphocytes is independent of the involvement
of mitochondria and caspases suggesting the existence of
pathways other than the ‘classical’ ones

[38]

. Caspases are

essential for both Fas and mitochondria mediated apop-
tosis. However, inhibition of caspases or the use of cells
with defective apoptosis machinery has demonstrated that
alternative types of programmed cell death could occur as
well and such alternative death mechanisms are divided into
“apoptosis-like” and “necrosis-like”

[39]

.

(6) Ascorbic acid is an essential micronutrient and is considered

to have an antioxidant function in living systems. However,
it may also act as a prooxidant and site specific DNA cleav-
age by ascorbic acid in the presence of Cu(II) has been
described

[40]

. Relatively high concentrations of ascor-

bic acid are able to induce apoptosis in various tumor cell
lines. Consequently, it has been shown to induce cell death,

nuclear fragmentation and internucleosomal DNA cleavage
in human myelogenous leukemia cell lines

[41]

. The apopto-

sis inducing activity of ascorbic acid has been ascribed to its
prooxidant action and is inhibited by catalase, antioxidants
like N-acetylcysteine and GSH, Ca

2+

and Fe

3+

depletion but

stimulated by H

2

O

2

, Cu

2+

, and iron chelators

[42]

.

(7) It has recently been shown that the polyphenol curcumin

mediated apoptosis of HL60 cells is closely related to an
increase in the concentrations of ROS possibly generated
through the reduction of transition metals in cells

[43]

.

5. Oxidative DNA breakage by plant polyphenols in
human peripheral lymphocytes

Using the comet assay our laboratory has shown that plant

polyphenols cause DNA breakage in isolated human peripheral
lymphocytes

[44]

. Photographs of comets seen on treatment with

different concentrations of resveratrol are shown in

Fig. 3

. At

50 and 100

␮M concentrations resveratrol did not damage the

lymphocyte DNA to any significant extent whereas at 200

␮M

concentration a comet with a tail indicative of DNA breakage
was observed. The results demonstrate that resveratrol alone
is capable of DNA breakage in lymphocytes. We have further
shown that several other polyphenols such as gallic acid, EGCG
and piceatannol are also able to catalyze DNA breakage in lym-
phocytes.

Table 1

gives the results of an experiment where three

scavengers of ROS have been tested namely, SOD and catalase,
which remove superoxide and H

2

O

2

, respectively and thiourea,

which is a scavenger of several ROS. All three cause signif-
icant inhibition of DNA breakage as evidenced by decreased
tail lengths. We concluded that superoxide anion and H

2

O

2

are

essential components in the pathway that leads to the forma-
tion of hydroxyl radical and other species which would be the
proximal DNA cleaving agents.

6. Putative mobilization of endogenous copper by plant
polyphenols

In a previous study

[20]

we have shown that the

resveratrol–Cu(II) mediated DNA degradation of lymphocyte
DNA is inhibited by neocuproine which is a Cu(I) specific
chelating agent and is membrane permeable

[45]

.

Fig. 4

gives

the results of an experiment where three progressively increas-
ing concentrations of neocuproine were tested on resveratrol
induced DNA breakage in lymphocytes. A progressive decrease
in the tail length as a function of increasing neocuproine con-

Table 1
Effect of scavengers of active oxygen species on resveratrol induced lymphocyte
DNA breakage

Dose

Tail length (

␮m)

Inhibition (%)

Untreated

1.22

± 0.08

a

–

Resveratrol (200

␮M)

20.84

± 1.28

–

Resveratrol + SOD (100

␮g/ml)

7.05

± 0.25

*

66

Resveratrol + catalase (100

␮g/ml)

8.86

± 0.29

*

57

Resveratrol + thiourea (1 mM)

11.93

± 1.01

*

45

a

All values represent S.E.M. of three independent experiments.

*

P <0.05 when compared to control.

374

S.M. Hadi et al. / Seminars in Cancer Biology 17 (2007) 370–376

Fig. 3. Alkaline single cell gel electrophoresis (comet assay) of human peripheral lymphocytes showing comets (100

×) after treatment with different concentrations

of resveratrol: (A) untreated, (B) 50

␮M, (C) 100 ␮M and (D) 200 ␮M.

centration was seen. From the results we concluded that the
DNA breakage by the polyphenol involves endogenous copper
ions and that Cu(I) is an essential intermediate that leads to
DNA breakage. We presume that the lymphocyte DNA break-
age is the result of the generation of hydroxyl radicals and
other ROS in situ. Oxygen radical damage to deoxyribose or
DNA is considered to give rise to thiobarbituric acid reactive
substance (TBARS)

[46,47]

. We therefore determined the for-

mation of TBARS in lymphocytes as a measure of oxidative
stress with increasing concentrations of resveratrol. The effect
of preincubating the cells with neocuproine and thiourea was
also studied. Results given in

Fig. 5

show that there is a dose

dependent increase in the formation of TBARS in lymphocytes.
However, when the cells were preincubated with neocuproine
and thiourea there was a considerable decrease in the rate of
formation of TBARS by resveratrol. These results indicate that
both DNA breakage and oxidative stress in cells is inhibited by
Cu(I) chelation and scavenging of ROS. Similar results were
obtained with caffeic acid

[48]

, a major polyphenolic compo-

nent of coffee. Thus, it can be concluded that the formation of
ROS by polyphenols in lymphocytes involves their interaction
with intracellular copper as well as its reduction to Cu(I).

It is well known that polyphenols autooxidize in cell cul-

ture media to generate H

2

O

2

and quinones that can enter cells

causing damage to various molecules

[49–51]

. This may lead to

Fig. 4. Effect of increasing concentrations of neocuproine on resveratrol induced
DNA breakage in human lymphocytes. Values reported are

±S.E.M. of three

independent experiments.

Fig. 5. Effect of preincubation of lymphocytes with neocuproine and thiourea on
TBARS generated by increasing concentrations of resveratrol. Resveratrol alone
(

䊉); resveratrol + neocuproine (1 mM) (); resveratrol + thiourea (1 mM) ().

The isolated cells (1

× 10

5

) suspended in RPMI 1640 were preincubated with the

indicated concentrations of neocuproine and thiourea for 30 min at 37

◦

C. After

pelleting the cells were washed twice with PBS (Ca

2+

and Mg

2+

free) before

resuspension in RPMI and further incubation for 1 h in the presence of increasing
resveratrol concentrations. Viability of lymphocytes after preincubation with
neocuproine and thiourea was more than 90%. Values reported are

±S.E.M. of

three independent experiments.

extracellular production of ROS that could account for lympho-
cyte DNA breakage. However, this does not appear to be the case
in our system since we have previously shown that no lympho-
cyte DNA breakage is observed on preincubating the cells with
resveratrol alone up to a concentration of 50

␮M. DNA breakage

could only be seen after incubating the pre-treated cells further
in the presence of Cu(II)

[20]

. Further, we could not detect any

H

2

O

2

formation on incubating resveratrol (up to a concentration

of 300

␮M) in the suspension medium (RPMI)

[44]

.

7. Conclusions

Most studies on anticancer mechanisms of plant polyphenols

invoke the induction of cell cycle arrest at S/G2 phase transition
brought about by an increase in cyclins A and E and inactivation
of cdc 2. Other mechanisms have also been proposed

[52]

. In

view of the above findings in our laboratory and those of oth-
ers in literature we suggest that the conclusion of our studies is

S.M. Hadi et al. / Seminars in Cancer Biology 17 (2007) 370–376

375

that the plant polyphenols possessing anticancer and apoptosis
inducing activities are able to mobilize endogenous copper ions
possibly the copper bound to chromatin. Essentially this would
be an alternative, non-enzymatic and copper-dependent path-
way for the cytotoxic action of certain anticancer agents that are
capable of mobilizing and reducing endogenous copper. As such
this would be independent of Fas and mitochondria mediated
programmed cell death. It is conceivable that such a mechanism
may also lead to internucleosomal DNA breakage (a hallmark of
apoptosis) as internucleosomal spacer DNA would be relatively
more susceptible to cleavage by ROS. Indeed such a common
mechanism better explains the anticancer effects of polyphenols
with diverse chemical structures as also the preferential cytotox-
icity towards cancer cells. The generation of hydroxyl radicals
in the proximity of DNA is well established as a cause of strand
scission. It is generally recognized that such reaction with DNA
is preceded by the association of a ligand with DNA followed
by the formation of hydroxyl radicals at that site. Among oxy-
gen radicals the hydroxyl radical is most electrophilic with high
reactivity and therefore possesses a small diffusion radius. Thus,
in order to cleave DNA it must be produced in the vicinity of
DNA

[53]

. The location of the redox-active metal is of utmost

importance because the hydroxyl radical, due to its extreme reac-
tivity, interacts exclusively in the vicinity of the bound metal

[54]

. As already mentioned cancer cells are known to contain

elevated levels of copper

[32–35]

and therefore may be more

subject to electron transfer with antioxidants to generate ROS

[13]

. Thus, because of higher intracellular copper levels in can-

cer cells it may be predicted that the cytotoxic concentrations of
polyphenols required would be lower in these cells as compared
to normal cells. Studies by Chen et al.

[55]

have earlier shown

that EGCG inhibited the growth of SV40 virally transformed
W138 cells but not their normal counterparts. The IC

50

value of

EGCG was estimated to be 120 and 10

␮M for normal versus

the transformed cells respectively. EGCG also showed a simi-
lar differential growth inhibitory effect with two other human
cancer cell lines, Caco-2 colorectal adenocarcinoma cells and
Hs578T breast ductal carcinoma cells. These authors have also
shown that the black tea polyphenol theaflavin-3



-monogallate

also exhibits a similar differential inhibitory effect for these can-
cer cell lines

[56]

. We have recently shown that ascorbate which

also acts as a prooxidant in the presence of copper ions is cyto-
toxic to normal lymphocytes

[57]

. Indeed it had been earlier

shown that ascorbate is cytotoxic to a leukemic cell line at a
lower concentration than normal lymphocytes

[58]

.

Acknowledgment

The authors acknowledge the financial assistance provided

by University Grants Commission, New Delhi, under the DRS
Programe.

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