430
Antitumor Agents: Taxol and Taxanes – Production by Yew Cell Culture
(polyethyleneglycol – PEG 4,000) were ca-
pable
of
improving
Taxol
yield
in
T. chinensis
cell cultures. Cells cultivated
in PEG-supplemented medium yielded
a threefold increase of Taxol
over the
control. The authors have suggested that
osmotic stress may improve secondary
metabolism by directly modulating en-
zymes or by reducing growth rates and
making more carbon available for sec-
ondary metabolism.
6.2
Cell Line Selection
Screening different cell lines for high
yields of Taxol
and/or taxanes may lead to
the establishment and propagation of elite
cell lines that can be used for large-scale
production purposes. The technique has
been successfully applied, for instance, to
the production of ajmalicine by
C. roseus
,
shikonin by
Lithospermum erythrorhizon
,
and berberine by
Coptis japonica
.Although
laborious and time-consuming, the ap-
proach may turn out to be advantageous
for long-term potential industrial applica-
tions of yew cell cultures as a source of
Taxol
and taxanes.
The presence of a visual feature asso-
ciated with high productivity is highly
desirable for selecting high yielding cell
lines (e.g. colored product, such as an-
thocyanin). In the case of
T. cuspidata
,
additional studies are required to iden-
tify a reliable visual feature associated with
taxane productivity. When such a facili-
tating screening parameter is not present
at all, however, as it often happens, the
use of standard phytochemical extraction
and separation procedures is required or,
if the product of interest can be made anti-
genic, immunoassays can be used in the
screening procedure. Antitaxol and several
other antitaxane antibodies are commer-
cially available.
6.3
Light and Temperature
These physical parameters can be used to
manipulate taxane production. The vast
majority of protocols for yew cell cultures
uses dark incubation. In a study on the ef-
fects of light on heterotrophic
T. cuspidata
cell suspension cultures and Taxol
yield,
it was established that white light of low
intensity caused signiFcant reduction in
Taxol
yield and growth; moreover, light-
grown cells retained more intracellular
Taxol
and had much lower amounts of
the taxane in the medium fraction (only
25% of Taxol
was released to the medium
versus 65% in dark-grown suspensions),
presumably contributing to negative feed-
back of biosynthetic enzymes. Diversion
of the phenylalanine pool from taxane
biosynthesis toward synthesis of phenolic
compounds in light-grown cells is an-
other possible reason for light inhibition
of taxol production. The effects of light
have been shown to be the result of a di-
rect effect of light on cell physiology and
not photochemical degradation of medium
components or taxanes, as judged from
HPLC analysis of light-exposed Taxol
so-
lutions and growth performance in light
irradiated versus control medium.
Temperature is generally maintained at
approximately 25
Cfo
ryewce
l
lcu
l
tu
re
studies. A detailed investigation on sus-
pensions of
T. chinensis
established that
the optimum temperature for cell growth
was 24
C, whereas for Taxol
produc-
tion this parameter was 29
C. By applying
a
strategy
of
temperature
shift
(from
24 to 29
C) after a high cell biomass
was achieved, a sustained higher Taxol
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