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Research in Toxicology 16:865-872.
signs have pointed to green tea's beneficial impact for health,
including multiple studies with animals and hints that green tea
drinkers and people living in green tea growing areas are less likely
to develop cancer.
this new study, Palermo and his colleagues probe deeply into one
of the mechanisms suggested by previous research for how green tea
might have beneficial effects. This suggestion is that plant compounds
in green tea interfere with a specific molecular action of dioxin,
preventing it from turning on genes that are involved in causing
in this publication in Chemical Research in Toxicology,
their results confirm this earlier work and identify the
specific plant compounds most likely to be responsible for green
tea's action at suppressing gene activation.
scientists remain uncertain about whether their findings reveal
the way green tea has beneficial health effects, however. Their
results indicate that the concentrations of the active compounds
required to work against dioxin via this mechanism are high compared
to the levels reached in people's blood after drinking green tea.
number of chemical contaminants cause their damaging effects through
the molecular pathway that green tea shuts down, including dioxins,
some PCBs, and some polycyclic aromatic hydrocarbons among others,
contaminants that are virtually ubiquitous. Hence this line of research
offers some promising possibilities for preventing diseases. Given
how much remains to be learned and confirmed about this, however,
long-term prospects for developing chemical treatments to prevent
diseases shouldn't diminish efforts to reduce and eliminate exposures
in the first place.
did they do? Palermo et al. experimented with
mouse cells that had been altered genetically to make it possible
to measure when and how much a specific gene was turned on via a
known to be activated by dioxin and related compounds, a pathway
that begins with dioxin binding with the Ah (aryl hydrocarbon) receptor.
The techniques they used are standard, widely employed for this
type of research.
first exposed the mouse cells to dioxin, and then added varying
amounts of green tea extract (made by brewing Lipton Green Tea for
20-30 minutes). This round of research confirmed the extract's ability
to suppress dioxin gene activation (see part
then used some tools out of analytical chemistry to separate the
crude extract into different components, as the crude is a complex
mixture of multiple plant compounds. That separation then allowed
them to confirm the identity of the components.
the next round of experiments, they bought commercial versions of
the identified components and performed a series of experiments
with these pure substances, and mixtures thereof. The experiments
again looked at whether or not different concentrations of the compounds
interfered with dioxin-mediated gene activation.
did they find?
Part one: Work with the Lipton green
tea extract showed that as the concentration of the extract was
increased, gene activation decreased.
graph to the right compares the percent of dioxin-induced
gene activation without green tea extract (GTE), the left-most
point on the curve, with what happens as GTE is increased
from 0 to 100 µg/ml. Between 10 and 100 µg/ml,
gene activation starts to drop sharply. GTE clearly suppresses
gene activation by dioxin at higher levels.
100 is a commercially-available mixture of green tea extracts.
two: The separation analysis revealed that the principal constituents
of their green tea extract were a series of plant compounds called
catechins, including EGCG (epigallocatechin-3-gallate), EGC (epigallocatechin)
and ECG (epicatechin-3-gallate). EGCG was the most abundant catechin
in the extract, amounting to 50%-80% of the total.
they tested effect of the different components of green tea
extract, 4 of 5 showed a substantial impact (graph to left).
those 4, however, only two reached high enough concentrations
in green tea extract to be likely to play a role in green
tea's overall effect, EGCG and EGC (The data on relative abundance
in extract are not shown).
three: The research team conducted a series of additional tests
in an effort to eliminate other interpretations of how plant compounds
from green tea extract reduced dioxin's impact on gene expression.
Each of these tests provided support for the theory that green tea
extracts, in the experimental system they used, interfered with
the chain of molecular events that allow dioxin to activate genes
by interacting with the Ah receptor.
does it mean? Most of these results provide support for
the idea that people drinking green tea may benefit from the plant
compounds' inteference with gene activation via the Ah receptor.
That pathway is one of the main molecular mechanisms for toxic effects
of dioxin and related compounds.
principal argument against this conclusion is that the concentrations
of the compounds necessary to cause a reduction in gene activation
are much higher than what can be measured in people's blood after
drinking green tea. Note in the graphs above that the percentage
of gene activation does not begin to fall until concentrations are
between 10 and 100 µg/ml. Direct measurements in people's
blood, in contrast, typically are less than one-tenth this amount.
evaluating that argument, Palermo et al. pointed to two
green tea is a complex mixture of many components, only a few
of which they tested in these experiments. Additive and synergistic
effects might dramatically alter the impact of low levels of the
plant compounds. Mixtures
are proving important for many toxicological analyses.
an impact through the Ah receptor is only one of many possible
biochemical routes by which green tea might have health benefits.
Their results provide biochemical support for one pathway, but
in no way exclude other possibilities.
sum, this study adds to a growing body of evidence indicating that
drinking green tea brings significant health benefits. It suggests
that additional work will further refine what we understand about
exactly how green tea has that effect, and this deeper knowledge
may point to treatments that prevent cancers even in the face of
continuing exposures to dioxins and related compounds.
how far we are from certainty about these findings, however, continued
investments in exposure reduction remain vitally important.