[CANCER RESEARCH 42, 1716-1721,
0008-5472/82/0042-OOOOS02.00
May 1982]
Correlation between Amounts of Cellular Membrane Components and
Sensitivity to Hyperthermia in a Variety of Mammalian Cell Lines
in Culture1
Anne E. Cress,2 Patrick S. Culver, Thomas E. Moon, and Eugene W. Gerner
Department of Radiology, Division of Radiation Oncology ¡A.E. C., P. S. C., E. W. G.J, and the Cancer Center Division [T. E. M.J, Arizona Health Sciences Center,
Tucson, Arizona 85 724
ABSTRACT
The weight ratio of either cholesterol or phospholipid to
protein contents in 7 different cell lines, growing exponentially
at 37°, correlates positively with increasing resistance of the
cells to subsequent hyperthermic cell killing. The relative heat
resistance of each cell line is derived from survival curves
obtained when the different cell lines are exposed to 43°.
Cholesterol and phospholipid amounts in the particulate frac
tion correlate with survival sensitivity to 43°when the values
are expressed per mg protein but not when expressed per cell
number. Also, cholesterokphospholipid
molar ratios and the
amount of protein in the particulate fraction do not display
linear correlations with sensitivity of the respective cell lines to
43°-induced cell killing. The relative degree of fatty acid satu
ration at 37°also is independent of whether cells show a higher
degree of heat resistance. These data suggest that lipid (both
cholesterol and phospholipid):protein
weight ratios correlate
with increasing resistance of cells to an elevation in tempera
ture. The major implication of these data is that major mem
brane components can influence and perhaps predict cellular
survival to hyperthermia.
INTRODUCTION
Our laboratory is currently studying the efficacy of hyperthermic treatments for tumor eradication. The basis for this
treatment is the exploitation of the apparent increased cytotoxicity of cells to elevations in temperature above 37° (3, 13).
We and others find empirically that the sensitivity to hyperther
mia varies with the type of cell lines grown in vitro (3, 4, 24). In
this study, we use 7 different cell lines to investigate a probable
cause for the disparate survival responses of cells exposed to
hyperthermia.
Our working hypothesis is that the primary constituents of
the mammalian cell plasma membrane are major factors in
determining cellular heat resistance. Although it is likely that
heat, as a nonspecific agent, has multiple cellular targets, we
believe a major cellular defense against hyperthermia-induced
cytotoxicity may be the integrity of the plasma membrane.
Components which would "rigidize" the plasma membrane
and/or decrease permeability would be expected to protect
against the lethal effects of hyperthermia. Hahn ef al. (15) have
shown increased cellular uptake of Adriamycin at elevated
temperatures (15), and we have noted that cells appear to
"leak" polyamines as a consequence of exposure to hyper
' This research was supported by USPHS Grants CA-18273,
30052, andCA-23074.
2 To whom requests for reprints should be addressed.
Received April 17. 1980; accepted February 10, 1982.
1716
CA-17343,
thermia (9-11 ). In addition, the known acclimation mechanisms
of organisms to environmental temperature fluctuations sug
gest the membrane as a likely candidate for the modulation of
the heat survival response (19).
We are particularly concerned with cholesterol and phospho
lipid concentrations and the degree of fatty acid saturation
since these factors are primarily responsible for the physical
characteristics of the membrane. For example, elevations of
cholesterol are known to decrease membrane permeability (23)
and alter the permeation rate or transport of a solute across
lipid bilayers (5). CHO3 cell mutants deficient in cholesterol
have more "fluid" membranes as measured by spin labeled
probes (26, 27). Cholesterol thus appears to be one candidate
capable of increasing the rigidity of the plasma membrane and
decreasing membrane permeability.
Decreased permeability can also be achieved by the in
creased saturation of fatty acids. It is known that increasing
the amount of saturated fatty acid by dietary manipulation in
Escherichia coli modifies the thermal sensitivity of these cells
(28). Thus, we have an empirical link between increased satu
ration, decreased permeability, and an increased resistance to
hyperthermia-induced cytotoxicity.
These various studies, using a number of systems, indicate
a relationship between the amount of cholesterol, fatty acid
saturation, and phospholipid and the physical state of the
membrane. Specifically, modifications in these components
are known to alter permeability which in turn might explain the
intrinsic differences in the specific cell line survival responses
to hyperthermia. In this study, we have asked whether the
amount of cholesterol or phospholipid and the degrees of fatty
acid saturation, measured in cells growing normally at 37°,
correlates with the eventual survival potential of these cells
after exposure to 43°.This report extends our previous studies
which indicated a positive correlation between cholesterol con
centrations and cellular resistance to hyperthermia (4).
MATERIALS
AND METHODS
Cell Culture Techniques. All cell lines were maintained and treated
in log-phase growth in McCoy's Medium 5A supplemented with 20%
fetal bovine serum, 100 units penicillin per ml, and 100 jug streptomycin
per ml (all from Grand Island Biological Co., Grand Island, N. Y.). All
lines were grown in a 5% CO2:95% air atmosphere at 37°.
Survival Experiments. All cell lines were maintained in exponential
growth in monolayer cultures in T-25 flasks (Falcon Plastics, Oxnard,
Calif.). Survival was determined by the ability of single cells to form
colonies and was used to determine the thermal sensitivity of the
CA3 The abbreviations used are: CHO, Chinese hamster ovary; LDX, 50% lethal
dose.
CANCER
RESEARCH
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VOL.
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Cholesterol, Phospholipids,
various cell lines studied. The procedure as used by us and the
hyperthermic treatment conditions in our temperature-controlled
water
bath (temperature uniform and constant to ±0.1°)have been described
previously in detail (8, 12). Control plating efficiencies and doubling
times, respectively, were: CHO, 85 to 95%, 13 to 15 hr; EMT6/AZ
(mouse mammary sarcoma), 60 to 80%, 15 to 16 hr; RT-9 (rat astrocytoma), 60 to 80%, 14 to 15 hr; Rat-1 (tsLA24/RSV)
[rat fibroblast
infected with a temperature-sensitive
mutant of Rous sarcoma virus
(see 22)], grown at 37°, 37 to 40%, 11 to 13 hr; Rat-1 (normal rat
fibroblast),
50 to 70%, 15 to 17 hr; Rat-1 (wt/RSV)
[rat fibroblast
infected with Rous sarcoma virus (22)], 43 to 45% 13 to 17 hr; HeLa
(human cervical carcinoma), 65 to 80%, 20 to 22 hr.
Cholesterol, Protein, and Phospholipid Determinations. For each
sample, 2.5 x 107 cells were harvested by scraping the culture flasks
with a rubber policeman, washed twice with 10 ml of Puck's Saline A
(Grand Island Biological Co.), sonicated into 0.01 M Tris buffer, pH 7.2,
and centrifuged for 20 min at 31,300 x g. Sonication results in
disruption of 95% of the cells as revealed by microscopic examination.
Cell number was determined using an electronic particle counter (Coul
ter Electronics, Inc., Hialeah, Fla.). The particulate fraction was ex
tracted with chloroform:methanol:water
(2:2:1), and the amount of
cholesterol was determined spectrophotometrically
by the method of
Glick ef al. (14). Chromatographically
pure cholesterol (Sigma Chemi
cal Co., St. Louis, Mo.) is used as a standard. Our extraction procedure
was monitored by thin-layer chromatography
using a chloroform:
eters and cell membrane
attempted here.
components.
Nonlinear
and Hyperthermia
analyses were not
RESULTS
Cell Survival to 43°.Chart 1 describes the survival response
of the various cell lines exposed to 43°for the times indicated.
All cell lines are in exponential growth prior to treatment and
are grown in identical media, which are at pH 7.4 throughout
all procedures. The relative resistance of the cells to hyperthermia is estimated from these survival curves and is repre
sented as a DQ.The D0 is the reciprocal slope of the exponential
portion of the survival curve and has units of time for a specific
temperature. For example, in Chart 1, Rat-1 (wt/RSV) has the
greatest D0 and CHO has the smallest D0, corresponding to
high resistance to heat and low resistance to heat, respectively.
Chart 1 shows the disparate survival responses of different
cell lines exposed to identical elevations in temperature. Table
1 supplies further detail about the data given in Chart 1,
methanoLacetic acid (90:10:1) solvent.
Protein determinations were done according to the method of Brad
ford (2). The particulate fraction was hydrolyzed using a final concen
tration of 1 N NaOH which does not interfere with the protein determi
nation.
Phospholipid concentrations were measured from the extract of the
particulate fraction by the colorimetrie method of Raheja ef al. (25). A
Chromatographically
pure reference standard of phosphatidyl choline
(Sigma) was used.
Saturated and Unsaturated Fatty Acid Measurements. The weight
ratios of saturated to unsaturated fatty acids were estimated here by
the ratios of stearic saturated to stearic one-double bond fatty acids.
Approximately
1.0 x 106 cells were used for this assay. After the
particulate fraction was isolated (as described previously), 1 ml of 2.5
N KOH in methanol and 40 /¿gof heptadecanoic acid (an internal
standard) were added, and this mixture was heated for 30 min at 70°.
After cooling, 1.5 ml of 1 M phosphoric acid were added and mixed
well. Then, 1 ml of hexane was added to the hydrolysate, mixed well,
and centrifuged at 1200 x g for 3 min. Two additional 0.5-ml hexane
extractions were carried out, and all the hexane solutions were com
bined. To the hexane solution, 200 n\ of 0.5 M aqueous trimethyltrifluorotolylammonium hydroxide (21) were added. Fatty acid methylesters
were formed on the gas Chromatographie column (2 mm x 0.5 meter;
3% Silar 10C on Gas Chrom Q 100/120) which was interfaced with a
mass spectrometer (Finnigan 3300). The quantitation of fatty acid
methyl esters was performed by comparing the relative weight re
sponse of the cellular fatty acids to the internal standard using the total
ion current of the selected ions.
Statistical Methods. Linear regression analysis was carried out
using the observed data (log survival versus time at 43° and survival
parameters versus measured membrane component values) to obtain
estimates of slope and intercept (6). The estimated values for D0 (final
slope parameter), Dq (quasithreshold dose), and LD50 were then deter
mined from the slope and intercept parameters as described by Elkind
and Whitmore (7), discriminating all survival values above 20%. The
estimated standard errors of the estimates were obtained using the
method of propagation of error (18). The goodness of fit of the regres
sion equations was evaluated by the multiple correlation coefficient,
R2, that quantifies the variation explained by the combined influence of
the independent variables. The p value associated with each ff indi
cates the degree of significance that R2 differs from zero. Only linear
correlation coefficients
MAY
1982
were determined between survival curve param-
Chart 1. Survival of 7 different cell lines in exponential growth after exposure
to 43° for varying times shown. All lines were maintained at 37° in McCoy's
Medium 5A supplemented with 20% fetal bovine serum. Survival is based upon
colony-forming ability as described in the text.
Table 1
Multiple correlation coefficient and standard errors of the various Dt>s
lineCHORT-9EMT6/AZRat-1
Cell
(37°)Rat-1HeLaRat-1
(tsLA24/RSV)
(wt/RSV)Do10.5112.8427.0432.9635.3850.8167.54S.E.0.471.536.762.0114.119.736.08fl2"0.990.730.490.
a The multiple correlation coefficient (fl2) indicates the proportion of variation
about the mean representing
the regression line.
1717
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A. E. Cress et al.
indicating the standard error about the mean for the D0s and
the multiple correlation coefficient (R2). It should be noted here
that the D0s for the Rat-1 and EMT6/AZ cell lines (R2 = 0.41
and R2 = 0.49, respectively) have the smallest proportion of
variation which is represented by the survival curve in Chart 1.
In other words, the D0s representing the Rat-1 and the EMT6/
AZ cell lines are the least accurate of all the D0s determined.
We have used the fact that cells in cultures are differentially
sensitive to heat to ask whether the amount of any major
membrane component correlates with an increasing O0.
Phospholipid, Cholesterol, and Protein Measurements.
Since there are suggestive data (see "Introduction")
that the
plasma membrane is involved in hyperthermic cell killing, it is
important to determine which major membrane components
are probable contributors to the resistance of cells to hyperthermia. The major membrane components described in this
report are phospholipid,
cholesterol, and general protein
amounts present in the particulate fraction of the various cell
lines. Chart 2 graphically depicts the functional dependence of
cell survival sensitivity to 43° heat, indexed by D0 values, on
MQCholesterol/106
Cells
tig Phospholipid/106
4
02468
100
8
Cells
12
16
and 0.737, respectively). This finding suggests that cell lines
which are more resistant to hyperthermia-induced
cell killing
contain both more cholesterol and more phospholipid relative
to protein in the particulate fraction. Interestingly, the D0s do
not correlate well with the amounts of either cholesterol or
phospholipid per cell number (Table 3, R2 = 0.152 and 0.102,
respectively; Chart 2, A and ß),possibly because each cell line
differs significantly in size.
These data indicate that cells which have higher amounts of
phospholipid and cholesterol relative to protein when growing
at 37°are more resistant to the cytotoxic effects of subsequent
hyperthermic exposures. The O0s are not significantly corre
lated with the molar ratio of cholesterol to phospholipid (Table
3, R2 = 0.091; Chart 2O). In addition to cholesterol and
80
60
phospholipid concentrations, the protein amount of cellular
particulate fraction is determined. Table 3 and Chart 2C show
that the D0s do not correlate significantly with the protein
concentration in the particulate fraction (R2 = 0.246).
40
20
Since heat is a nonspecific agent, it is likely that multiple
parameters may be important to correlate significantly with the
D0s. Therefore, a multivariate analysis was performed to deter
mine which combination of factors correlated most highly with
the D0s.
Cellular Parameter Combinations and the O0. We have
analyzed 5 different groups of combinations in order to deter
mine statistically which combinations best correlate with the
D0. The degree of correlation is reported in Table 4 in terms of
2 values, R2 and p. R2 indicates the proportion of variation
0
80
60
40
20-
0
cholesterol (A), phospholipid (6), and protein (C) contents
expressed per unit cell number. Chart 2D shows this same
relationship for cholesterol:phospholipid
molar ratios. The ac
tual values for each parameter by cell line are shown in Table
2, and the statistical analyses of linear correlations are seen in
Table 3.
Chart 3 describes the manner in which heat sensitivity (D,,)
changes as a function of the lipid (cholesterol or phospholipid):protein ratio in the particulate fraction of each cell line. A
positive correlation exists between D0 and the amount of either
cholesterol or phospholipid per protein, as indicated by rela
tively high multiple correlation coefficients (Table 3, R2 = 0.770
SO
100
150
200 0
HflProtein/10«
Cells
Chart 2. Sensitivity (D0) to 43°-irtduced
content present in the particulate fraction
content present in the particulate fraction
content
in the particulate
fraction
0.2
0.4
0.6
about the mean representing the regression line. In other
words, a higher number for R2 represents a higher correlation
0.8
Cholesterol/Phospholipid
Molar Ratio
cell killing as a function of: cholesterol
of 1.0 x 106 cells M); phospholipid
of 1.0 x 106 cells (B); and protein
of 1.0
x
106 cells (C). D,
cholesterol:phospholipid
molar ratio. Bars. S.E. Each survival curve was char
acterized by measuring survival after at least 5 different exposure times with 9
individual dishes/time interval at 3 different cell dilutions. Each curve was then
done in triplicate.
with the regression line. In the analysis of the different combi
nations, the highest R2 value with the lowest p value represents
the best combination of cellular factors correlating with high
sensitivity. We have found that all the parameter combinations
which include /ig cholesterol per mg protein correlate well with
Do values (R2 > 0.75). The combination of jug protein per unit
cell number and cholesterol:phospholipid
molar ratio displays
Table 2
Summary of various components of the particulate fraction of 7 different cell lines
lineCHO
Cell
cells)1
10e
.70
RT-9
2.98
EMT6/AZ
7.50
Rat-1 (tsLA24/RSV)
3.60
Rat-1
2.90
4.25
HeLa
Rat-1 (wt/RSV)Cholesterol
4.97
8 Mean ±S.E.
1718
(fig/
cells)7.75
10«
±0.07"
13.05
±0.29
±0.93
17.46
±0.18
10.92
±0.08
15.19
±0.27
12.70
±0.17Phospholipid
13.85
(¿ig/
(¿ig/
10"
cells)80.51
phospholipid
ratio0.48
molar
±1.21
±18.66
±0.03
±0.04
129.51 ± 8.02
0.43 ±0.03
±5.56
0.70 ±0.01
202.10 ± 6.39
±0.89
96.03 ± 5.22
0.54 ±0.03
±3.51
96.66 ± 3.92
0.71 ±0.03
±0.79
85.78 ± 8.63
0.49 ±0.04
±2.04Protein 43.53 ± 3.14Cholesterol:
0.61 ±0.08
CANCER
RESEARCH
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VOL. 42
Cholesterol, Phospholipids,
the lowest level of correlation (fl2 = 0.522 and p = 0.228).
The combination of all these parameters gives a R2 = 0.878
but has a high p value (p = 0.504). It is interesting to note
that, although several combinations of cellular factors give
relatively high correlations with high R2 and low p values (Table
4), individual cellular parameters (jug cholesterol per mg protein
and fig phospholipid per mg protein) give comparable R2 and
p values (Table 3). These data indicate that a measurement of
either cholesterol/mg protein or phospholipid/mg protein is as
good a correlate to D0 as the combination of factors, including
protein content/cell and cholesterohphospholipid
molar ratios.
The last phase of our analysis of primary membrane parameters
was the fatty acid content of the particulate fraction.
Fatty Acid Content. In addition to cholesterol, fatty acid
saturation is known to modify membrane permeability and may
also be predictive of the hyperthermic survival response. How
ever, Table 5 shows that there is no obvious trend relating the
amount of stearic saturated fatty acids in each cell line with the
corresponding D0. Note that these values are in terms of weight
per 1.0 x 106 cells. If the data were expressed in terms of
protein content in particulate fraction, there is still no apparent
correlation between the fatty acid content in the particulate
fraction and the D0. A calculation of the weight ratio of stearic
saturated to stearic one-double bond fatty acids does not
correlate with the D0. For example, the ratio of stearic saturated
to stearic one-double bond fatty acids is 1.4 for both a D0 =
35.38 and Do = 10.51.
Table 3
Correlation of individual cellular parameters with the D0
Parameter/ig
cholesterol/mg
protein
/jg cholesterol /1 .0 x 108 cells
0.152
0.737
0.102
0.246
0.091P0.009
/ig phospholipid/mg
protein
/ig phospholipid/ 1.0 x 106 cells
/ig protein/1.0
x 106 cells
0.387
0.029
0.485
0.257
0.510
Cholesterohphospholipid
molar ratioR230.770
" The multiple correlation coefficient (R2) indicates the proportion of variation
about the mean representing the regression line.
80-
60-
E.
<3 40
20-
100
200
300
MÕ!
Lipid/mg Protein
Chart 3. The relationship of D0 (heat sensitivity) as a function of either cho
lesterol (•)
or phospholipid (O) content/mg protein for the cell lines characterized
In Charts 1 and 2. Bars, S.E.
MAY
1982
and Hyperthermia
Table 4
Correlation of various cellular parameter combinations
Parameter combinations
fl2
and the D0
p
/ig cholesterol/mg protein
Cholesterolrphospholipid
0.779
0.049
ng cholesterol/mg protein
ng phospholipid/mg
protein
0.864
0.050
/ig cholesterol/mg protein
/ig protein/106 cells
0.773
0.051
0.522
0.228
0.878
0.504
/ig protein/106
cells
Cholesterohphospholipid
/ig cholesterol/mg protein
/tg phospholipid/mg
protein
/¿g
protein/10e cells
Cholesterohphospholipid
Table 5
A comparison of fatty acid content, cholesterol content, and heat sensitivity in 5
different cell lines
Cholesterol
</ig/10e
cells)16:0"17.0
acid (/ig/106
at 43°
(min)67.54
cells)Rat-1
Cell line
(wt/RSV)
Rat-1
EMT6/AZ
RT-9
CHO4.97
2.90
7.50
2.98
1.50Fatty
64.0
49.0
34.0
12.0
13.0
15.0
8.2
5.9
5.8
35.018:015.029.018:112.121.0Do
35.38
27.04
12.84
10.51
16:0, palmitic saturated fatty acid; 18:0, stearic saturated fatty acid; 18:1,
stearic one-double bond fatty acid.
DISCUSSION
In previous studies, we have found that a positive correlation
exists linking elevated cholesterol levels to an increased re
sistance of cells to hyperthermia (4, 9). We have extended
those studies in this report to include examining whether other
membrane components correlate equally well with increased
thermal resistance. An increased thermal resistance is judged
by an increase in the D0, which was defined earlier. We have
also correlated the membrane parameters in this report to other
indicators of heat sensitivity, namely LD50 and Dq. LD50 is
defined as the dose required to result in 50% survival and the
Dq corresponds to the width of the shoulder portion of the
survival curve. In all cases, the D0 was the best correlate with
every membrane parameter (data not shown).
We have found that, of all the parameters examined, the
cholesterol content/mg protein and the phospholipid levels/
mg protein are the best positive correlates with heat sensitivity
(Table 3). Interestingly, if the cholesterol and phospholipid
contents are normalized to cell number, there is not a positive
linear correlation (Table 3). A possible explanation for this
finding is the fact that all the cell lines tested vary with respect
to cell size. The amount of protein in the particulate fraction
does not significantly correlate with an increasing D0 (Table 3).
To date, we have analyzed only linear correlations between
D0s and various membrane components. It is possible that
other functional relationships exist.
Fatty acid saturation is another important parameter since it
is a major factor in modifying membrane physical properties.
We have found that the amount of saturated stearic and palmitic
fatty acids show no trend either in correlating with the spectrum
of D0s or in reflecting the amounts of cholesterol (Table
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A. E. Cress et al.
5). The observations argue against a role of fatty acid saturation
in predicting survival to hyperthermia and against a compen
sation for the elevated cholesterol levels by an increased
desaturation of fatty acid. It should be noted here that the
particular fatty acids were examined and chosen because of
their predominance in the membrane.
Cholesterol:phospholipid
molar ratios often are used as a
convenient index of "packing."
According to our working
hypothesis, one would expect that the D0 should positively
correlate with the "packing ratio." However, we show that the
Do is apparently Independent of the cholesterol:phospholipid
molar ratio. This conflict may be resolved with the knowledge
that cholesterol can exist in clusters, possibly similar to the
arrangement found by Higgins et al. (17) in erythrocyte ghosts.
In addition, our computation of a phospholipid molar ratio
necessarily depends upon an average molecular weight for
phospholipid. Finally, it is possible that the degree of fatty acid
saturation in fatty acids not measured (24:4 and 18:2, for
example) determines the membrane fluidity. Our data suggest
that, of all the membrane components we have studied, lipid
(cholesterol or phospholipid):protein
ratios appear to be the
closest individual link to predicting subsequent sensitivity to
hyperthermia-induced
cell killing in a variety of cell lines of
different species.
We have also examined which combination of individual
parameters would best correlate with the D0. This is important
since the membrane is a complex structure and it may be that
multiple components are involved in thermal resistance. We
have found that, of all the combinations studied, again the lipid
(cholesterol or phospholipid):protein ratio correlated as well as
the best combinations when both R2 and p values were consid
ered.
The work presented with the Rat-1 cell line addresses the
influence of the transformed state on heat sensitivity. Compar
ing the survival data from the Rat-1 normal fibroblasts and the
Rat-1 (wt/RSV) and Rat-1 (tsLA24/RSV) transformed cells, it
is evident that the transformed state does not correlate with
either sensitivity or resistance to 43°-induced cytotoxicity.
Rather, it is the state of the membrane (e.g., cholesterol levels,
etc.) which predict the subsequent heat response.
Our results argue strongly for a role of membrane compo
nents and their ratios, specifically cholesterohprotein and phospholipid:protein, in the mechanism of hyperthermic cell inactivation. In support of this idea is an observation by Hahn ef al.
(16) indicating that the polyene antibiotics, such as amphotericin B and nystatin, greatly enhance cell killing induced by
hyperthermia. These antibiotics purportedly act by binding to
accessible sterols, such as cholesterol, in mammalian cells. In
addition, Anderson ef al. (1) have shown that increased cholesterol:phospholipid
molar ratios correlate with a decreased
membrane fluidity.
It is important to note that cells do not lyse immediately
during or after heat treatments but rather die a proliferative
death. We do not argue that cholesterol or other membrane
components are the only factors involved in heat cell killing.
Rather, we believe our results are consistent with the specu
lations of Landry and Marceau (20). These authors have pro
posed that cell inactivation by heat results from multiple (2 to
3) mechanisms in the 41-49° temperature range. Thus, a
membrane step may comprise one aspect of the mechanism of
thermal cell death, with other events, possibly involving nuclear
1720
damage, also contributing
to proliferative death.
to the overall mechanisms leading
ACKNOWLEDGMENTS
We appreciate the expert assistance of Sai Y. Chang for the saturated and
unsaturated fatty acid measurements and Hsiao-Sheng George Chen, and Judith
Parsells for the statistical analyses.
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Correlation between Amounts of Cellular Membrane
Components and Sensitivity to Hyperthermia in a Variety of
Mammalian Cell Lines in Culture
Anne E. Cress, Patrick S. Culver, Thomas E. Moon, et al.
Cancer Res 1982;42:1716-1721.
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