Polyribosomes
in Rat Tissues
II. The Polyribosome Distribution in the Minimal
Deviation Hepatomas1
THOMAS
E.
@1vfcArdleLaboratory
WEBB,2
GÃœNTER BLOBEL,
for Cancer
of Biochemistry,
Research,
Medical
National Cancer Institute,
VAN
School,
R.
POTTER,
University
AND
of Wisconsin,
HAROLD
Madison,
P.
Wisconsin,
MoRRIs
and the Laboratory
NIH, Department of Health, Education and Welfare, Bethesda, Maryland
SUMMARY
In agreement
with
the concept
of the “minimal deviation―
bound polyribosomes
in the postmitochondrial
and regenerating adult liver, 40 % in Hepatomas
eral other hepatomas,
and approximately
0 in
hepatoma.
In the hepatomas the proportion
to correlate with their degree of differentiation
All
hepatomas
is revealed
examined
appear
C-ribosome
The possible physiologic
significance
as “free―
is of considerable
factors
It was
shown
to have
in the abnormal
that
previously
control
(42)
interest
enzyme
that
synthesis
several
(S@) and ‘I'the extent
ment to the endoplasmic
tion
wasstudied
by
or strength
reticulum.
density
gradient
concentration
ribosome
rat hepatomas
of their
attach
and
the
Morris
5i23,
7793,
and
7794A
recovery was always less than .50%.
has been applied
to additional
mini
mal deviation hepatomas.
The results offer further sup
port for the concept of the minimal deviation tumor (31,
1 Supported
in
part
by
USPHS
research
grant
CA-07125
from
the National Cancer Institute and by training grant CRTY-5002.
2 Present
address
: Department
of
which
operationally
defined
responses
to
and
suggests
that
there
may
be structural
defects
in
the enzyme-forming
system, which is assumed to involve
the endoplasmic reticulum (28) in addition to messenger
RNA (rnRNA), ribosomes, and amino acid transfer RNA.
i\1ETHODS
(polyribosomes)
wererecoveredfrom thesetumorswith
out DOC treatment, in contrast to normal liver, where the
analysis
dimers,
to the aberrant
C-ribosomes
(18, i9) showed characteristically
higher monomer and
dimer peaks than normal liver ; the relative concentration
of the heavier polyribosomes varied with the tumor and
the subline.
Furthermore,
80—iOO
% of the C-ribosomes
A similar
of free
Their size distribu
analysisof
hepatomas
of
liver.
32)
of the
super
prepared
in the presence of deoxycholate
(DOC), ac
cording to Wettstein et al. (45). It was possible (42) to
estimate the degree of association between the polyribo
somes and the endoplasmic reticulum by comparing the
yield of C-ribosomes obtained from the S2 fraction in the
presence of DOC with that obtained from the same volume
of S2 in the absence of DOC.
The results demonstrated
that the C-ribosome patterns of the Novikoff hepatoma
(22)
be related
normal
the fraction
patterns.
of those polyribosomes
in
differ from normal rat liver in the size distribution
polyribosomes isolated from the postmitochondrial
natant
a high
and total
and may
tumor,
supernatant
was 60—70% in normal
7787 and 7800, 20 % or less in 5ev
immature liver and in the Novikoff
of the bound polyribosomes
appears
and inversely with their growth rate.
Biochemistry,
School, University of Manitoba, Winnipeg, Manitoba,
Received for publication February 10, 1965.
The
Medical
Canada.
Details concerning the housing and feeding of the rats
were similar to those given previously (42) . Normal rat
liver was usually obtained from Holtzman rats (Holtzman
Co., Madison,
Wis.).
The Morris hepatomas
5i23C,
7787, 7793, 7794A,
and 7800 (cf. Refs.
i8, i9) were carried
intramuscularly
in Buffalo strain rats at the National
Cancer Institute;
the Reuber hepatoma H-35 (37) was
similarly carried in AXC
performed
according
to
H-35 tumor was grown
Laboratory
by Dr. P.
rats.
Partial hepatectomies
were
Higgins and Anderson
(15).
The
in tissue culture at the i\IcArdle
A. Morse, the tissue culture
cells
being designated H-4; the Novikoff Ni-Si cells were also
carried in tissue culture by Dr. Morse.
C-ribosomes were prepared from normal liver or hepa
toma as previously described (42, 45). After the C-ribo
somes were resuspended in a volume of water (rather than
buffer)
equal
to that
of the
original
postmitochondrial
supernatant
(52) from which they were prepared, their
size distribution was analyzed by density gradient centrifu
tion in an SW25 rotor of a Spinco centrifuge.
After cen
trifuging for 2 hr at 63,000 X g (average),
the eluate was
scanned at 265 m@ in a flow cell as it flowed from the bot
tom of the gradient
tube.
The analysis was facilitated
1219
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by
1220
Cancer Research
Vol. 25, September
1965
a recorder and a drop counter (42) . The linear sucrose
(iO—40%) gradient used in these studies was prepared by
forcing a dense sucrose solution from one syringe (the
reservoir) into an equal initial volume of light sucrose
solution in another syringe (mixing chamber) containing a
stirring
bar ; the
sucrose
solution
forced from the mixing chamber
was
simultaneously
into the bottom
of the
centrifuge
tube.
To achieve
the linear gradient,
syringes
were of equal cross-sectional
area, and
both
both
plungers were mechanically advanced at an identical and
uniform rate of speed.
As noted previously (42), the C-ribosomes contain only
a fraction of the monomers and dimers originally present in
the S2. In the present work the total ribosomes (i.e.,
monomers,
dimers,
and heavier
polyribosomes)
present
in
the crude S2 were roughly
analyzed in a manner similar to
that used for the purified
stitution
of an exponential
C-ribosomes,
but with the sub
gradient
increasing
from iO %
to 30 % sucrose (39).
reservoir
for
40
cross-sectional
%
In this case the syringe serving as
sucrose
in
the
gradient
machine
had
gradient
with the overlying
3 hr, and the effluent was
sucrose solutions used in the
the polyribosomes
were pre
pared in 0.05 i@iTris(hydroxymethyl)aminomethane,
and
0.005
i@iMgCl2,
pH
0.025
7.5.
As before (42) the flO/flr and —DOC/ +DOC
ratios are
used to report significant
findings.
The no/nt ratios as
calculated
mate
from the C-ribosome
the deviation
from
normal
night fa@st)of the concentration
containing
n
ribosome
units.
patterns
are used to esti
rat liver
(after
an over
of any particular
Thus
n0
and
species
n,. are
the
relative heights of the experimental tissue and normal liver
respectively, after both patterns have been corrected to
equivalent
polyribosome
concentrations.
1@1ore specifi
cally, flo/flr
@O
(flN X A 0/A @),where nN is the height of
peak n in normal adult rat liver, and where A@and A0 are
the areas of the polyribosome region (n equal to, or greater
thati,
liver
5) in the optical density
and the experimental
— DOC/
+
DOC
ratio
likewise
(O.D.)
tissue,
tracings of normal
respectively.
The
provides
an
estimate
(cf.
Ref. 42) of the fraction of polyribosomes that are bound
to the endopla.smic reticulum and the fraction that is op
erationally
2.—C-ribosome
patterns
with
and without
(+DOC)
of the
Morris
hepatoma
(—DOC)
7800
deoxycholate.
The dotted line in the +DOC pattern represents the difference
between the 2 curves.
Linear sucrose gradients
were used.
with and without DOC treatment are centrifuged through
a discontinuous gradient of 0.5 M/2.0 M sucrose under con
ditions identical to those used in the preparation
of the
C-ribosomes.
The ratios are calculated from the O.D. or
ribonucleic acid analysis (4) of aliquots of the solution of
C-ribosomes after resuspension of the pellets.
area twice that of the syringe used as the
mixing chamber
(2).
The
sample was centrifuged
for
monitored
at 265 m@t. All
preparation
and analysis of
M KC1,
a
CHART
prepared
defined
as “free.―Aliquots
of the
S2 both
RESULTS
The C-ribosome patterns of the Morris hepatomas 7787
and 7800 prepared with (+DOC) and without (—DOC)
deoxycholate pretreatment
of the postmitochondrial
super
natant (S2) are shown in Charts 1 and 2. The —DOC
pattern represents the distribution of that fraction of the
C-ribosomes which are operationally defined as “free―
; the
dotted line in the +DOC pattern represents the approxi@
mate distribution
of the polyribosomes
that are firmly
bound to the endoplasmic reticulum, as determined by the
difference between the 2 curves.
It is apparent that a
significant
fraction
of the polyribosomes
these transplantable
hepatomas
studied previously (42).
The abnormally
is firmly
in contrast
high monomer
bound
in
to the tumors
and dimer
peaks
charac-@
teristic of the C-ribosome patterns
of all hepatomas
studied thus far are also seeii in these 2 minimal deviation
hepatomas.
Since these ribosomal
species are in relatively
low concentration
in the C-ribosomes of normal rat liver,
they were iliterpreted (42) to be the result of some modifi
cation in the tumor.
The total
toma.s
crude
ribosome
7787,
7793,
S2 +DOC
pattern
and
for normal
7794A,
of these tissues
obtained
liver
by
directly
and Hepa
layering
the
over an exponen
tial gradient, is shown in Charts 3 and 4. In contrast to
the purified C-ribosomes, all of the monomers are recovered
in this crude system.
The ultraviolet-absorbing
material
at the top of the gradient is made up of hemoglobin, fer
ritin, and soluble components of the cytoplasmic fraction
of the cell.
In spite of the relatively
poor resolution,
the polyribo
some complement
and monomer peak of normal liver can
be easily distinguished
+DOC
CHART
prepared
1.—C-ribosome
patterns
with
and without
(+DOC)
of the
Morris
hepatoma
7787
(—DOC) deoxycholate.
The dotted line in the +I)OC pattern represents the difference
between the 2 curves. The top and bottom of the linear gradients
are to the left and right, respectively, of this and the subsequent
charts.
is treated
in Chart
with ribonuclease,
3 (curve a) .
If the S2
the polyribosomes
broken down and most of the ultraviolet-absorbing
rial appears in the monomer peak (curve b).
The
(curve
are
mate
“total ribosome― patterns
for Hepatomas
7787
a), 7793 (curve b), and 7794A (curve c) are shown
in Chart 4.
rat liver there
When compared with the patterns
is again
a significant
difference
for normal
in the hepa
Downloaded from cancerres.aacrjournals.org on June 18, 2017. © 1965 American Association for Cancer Research.
WEBB
toma
patterns,
mainly
with
et al.—Polyribosomes
respect
to
the
high
dimer
peak.
The high dimer peak was also present in the total
ribosome pattern of Hepatoma 7787 when the DOC treat
ment
was
omitted
(not
shown)
;
the
distribution
of
the
heavy polyribosomes
was masked in such instances by
the sedimenting
endopla.smic reticulum.
This aberrant
pattern appears to persist in Hepatoma 7793 at high dilu
tions
(curve
b).
It
is of significance
in this
respect
that
the no/nr ratios where n equals i or n equals 2 for the
C-ribosome pattern of tumors remain relatively constant
over a 4-fold range of dilution of the 52. These results
suggest that the abnormally high dimer peak originally
detected in the C-ribosome patterns is not an artifact
originating
during the preparation
bly represents
the in vivo situation.
of the latter but proba
The concentration
of
monomers does not, however, appear to be exceptionally
high in the hepatomas.
The total ribosome pattern for
>. 15
@
@
@
@
@
@
@
I.0
—I0.4
0.3
0.2
02
0.I
0.I
ELUATE (ml)
CHART 3.—Total
@LUAT@
(ml)
ribosome
pattern
(cf.
“Methods―) for normal
rat liver obtained by layering (a) 0.3 ml of 52 + DOC and (b)
0.15 ml of S2 + DOC after treatment with ribonuclease over an
exponential gradient.
In(b) the 52 + DOC was incubated with
0.1 @g
of pancreatic ribonuclease/ml
for 5 mm at 35°C.
CHART
4.—Total
ribosome
patterns
for
(a)
7787, 0.6 ml 52 + DOC; (b) Morris hepatoma
Morris
hepatoma
7793, 0.15 ml S2 +
DOC; and (c) Morris hepatoma 7794A, 0.6 ml S, + DOC. An
exponential
gradient
was used in each case.
The
_1)OC/+DOCa
2O.D.RNA1.Normal
—DOC/+DOC
ratios
and no/n,. ratios
for normal
and regenerating liver and several hepatomas are tabu
lated in Table i. As found previously (42), the —DOC/
+DOC ratio for normal liver (after an overnight fast) is
always
less than
0.40.
Approximately
0.5 and is usually
regenerating
liver.
consistently
gave
the
same
within
ratio
the range 0.30—
is found
The Morris hepatomas
ratios
of 0.60
in
12-hr
7787 and 7800
(±0.02),
respectively.
Recent estimates of the —DOC/+DOC
ratio for Hepa
tomas 7793 and 5i23C, with a few exceptions, give a value
of approximately 0.8 (based on 8 animals), rather than i.0,
as reported previously (42). This difference is attributed
to (a) better resuspension of the C-ribosome pellet when
water is used as the medium and (b) omission of the low
speed
centrifugation
of the resuspended
pellet.
AND no/nr
RATIOS
suggest
that
all the hepatomas
examined
have
a
high —DOC/+DOC
ratio as compared with normal liver.
As indicated in Table i, the —DOC/+DOC
ratio for the
embryonic and the newborn rat liver is approximately
i .0;
thus all of the polyribosomes of the S2 of immature rat
liver are operationally
free. Protein of unknown origin
contaminates
the C-ribosome pellet obtained from the S@
—
DOG
of
tein/RNA
immature
rat
ratios
and
liver,
which
results
—DOC/+DOC
in
ratios
high
pro
based
on
O.D. that are significantly greater than i.0. The change
in the —DOC/+DOC
ratio during the maturation
liver
elsewhere
will be presented
(G. Blobel,
of rat
to be pub
lished).
TABLE
THE
1221
Tissues
Hepatoma 7794A (Chart 4c) confirms the earlier finding
(42) of the relative paucity of polyribosomes in this tumor.
The data
0.5
0.4
0.3
Rat
Hepatoma .5i23C, like subline A (cf. Ref. 42), appears
to have slightly less than 20 % of its polyribosomes strongly
bound ; the bound polyribosomes
in the Reuber (H-35)
hepatoma is also of this order.
In contrast, most of the
polyribosomes appear to be free in the Novikoff hepatoma.
@:
IC
in
1
FOR THE CRIBOSOMES
OF SEVERAL
RAT
TISSUES5
FOR fl
ISSUE—DOC/+DOC@O/@r
EQUALS
liver0.34(63)(0.15—0.45)0.39(12)(0.25—0.46)1.0Regeneratingliver
adult
(12hrpostop
0.38(16)(0.21—0.52)0.42(2)(0.41—0.43)—erative)2.Hepatoma77870.61(12)(0.50—0.70)0.60(6)(0.53—0.
78000.57(5)(0.55-0.59)0.55(5)(0.54—0.56)2.84.Hepatoma
77930.80(9)(0.73—0.85)0.82(4)(0.80-0.86)6.75.Hepatoma
5123C0.83(2)(0.82—0.84)0.83(2)(0.75—0.90)—6.Reuber
hepatoma0.89(4)(0.85—0.92)0.83(4)(0.80-0.90)13.8H-4
culture)——3.87.Novikoff
(tissue
hepatoma1
.00(8)(0.99—1.10)1
18)8.2Novikoff
culture)——8.18.Immature
Ni-Si (tissue
.00(5)(0.96—1.
liverEmbryonicrat
(l8days)1.40(3)(1.00—1.55)1.10(3)(0.98—i.15)—1
day old1.03(3)(0.98—1.06)0.91(3)(0.88—1.00)—
a The abbreviations used are: DOC, deoxycholate; O.D., optical density; RNA, ribonucleic acid;
and S,, postmitochondrial supernatant.
b Shown
for each
tissue
are:
(a) the
average
—DOC/-1-DOC
ratios
based
on O.D.
and
RNA
analysis
and qualified in parentheses by the number of rats and the range of values and (b) the no/ne ratios of
the dimer peak calculated from the size distribution of C-ribosomes purified from the S@+ DOC.
Downloaded from cancerres.aacrjournals.org on June 18, 2017. © 1965 American Association for Cancer Research.
1222
Cancer Research
It can be concluded from the no/nt ratios (Table i) that
the high dimer peak is a characteristic of all the hepatomas,
even
For example,
high
ratios are found in the C-ribosomes of the Reuber
toma (H-4 cells) and Novikoff hepatoma
(Ni-Si
after
hepa
cells)
grown
transfer
in tissue
to tissue
culture.
culture.
DISCUSSION
The present
suggest
52
of
that
data and those presented
the fraction
hepatomas
Novikoff
(42)
of “bound―polyribosomes
varies
hepatoma
previously
from
to about
approximately
in the
0
40 % in the minimal
in
the
deviation
Hepatomas 7787 and 7800; the other hepatomas studied
fall between these extremes.
Although normal adult liver
has
(cf.
free
ratio
60—70% of the polyribosomes
bound, the present data
Ref. 24) suggest that most of the polyribosomes
are
in immature
rat liver.
Thus the —DOC/+DOC
for the Novikoff hepatoma
resembles that of newborn
rat liver, whereas the ratios of Hepatomas 7787 and 7800
are closer to the ratio of adult liver. These findings are
reasonable
in terms
of the
“minimal deviation―
concept
(3i—34),which suggests that the biochemical and morpho
logic deviations of such tumors (e.g., hepatomas) from the
cell of origin (e.g., the differentiated parenchymal cell) are
fewer
than
in
biochemical
any
previous
and morphologic
demonstrated
instance.
parameters
The
are usually
based
on the status of the marker enzymes and organelles typical
of the cell of Origm.
The —DOC/+DOC
ratios for the Novikoff hepatoma
and Hepatoma 7787 correlate well with published data
concerning their biochemical (6, 30) and morphologic (6,
16) differentiation.
The storage of glycogen in Hepatoma
7787 is in itself strong evidence for its high degree of dii
ferentiation.
Two out of 3 sublines of Hepatoma 5123,
Hepatoma 7793, and Hepatoma H-35 have a measurable
fraction
of bound
polyribosomes;
these
tumors
are also
known to have several marker enzymes (7, i8, 29 34, 43)
and moderate amounts of endoplasmic reticulum (6, 7,
18) . Insufficient morphologic data are available concern
ing Hepatoma 7800, although the available biochemical
data (19, 34) suggest that it is quite highly differentiated.
Since a correlation
has been found between the degree of
ultrastructural
differentiation
and the growth rate of 5ev
era! hepatomas
+DOC
(6), it might be expected that the —DOC/
ratios will also bear a similar
relationship
to the
growth rate.
In general those hepatomas which have a
high ratio have a rapid growth rate, whereas those with a
low ratio grow more slowly. For example, Hepatomas
7787 and 5i23 have very slow and moderate
growth
rates,
respectively; the former has a transplant generation time
of approximately
10 months, whereas the latter has a
transplant
generation time of 2—3months (cf. Ref. 6).
Hepatoma 7800 appears (i9) to have a growth rate that is
approximately half that of Hepatoma 5i23A.
The Reuber
hepatoma appears (19) to have a growth rate only slightly
higher than Hepatoma
7800. However, a comparison
between the Morris and Reuber hepatomas may not be
feasible, since they are carried in different strains of rats;
the growth rates of Hepatomas 5123 and 7800 are signifi
cantly modified by the sex of the host (19). The rapidly
Vol. 25, September
growing
Novikoff
hepatoma
has
a transplant
1965
generation
time of less than 1 week (unpublished data).
Preliminary studies suggest that the Morris hepatoma
7794B, which has numerous profiles of tubular endoplasmic
reticulum, but only small amounts of organized ergasto
plasm, and which has a slow growth rate (6) appears to
have somewhat fewer bound polyribosomes than predicted
(unpublished
data).
Although preliminary,
the results
show a maximum
generation
that
of 28 % of the polyribosomes
of this hepatoma
the growth
by hormones
to be bound.
rate of this tumor
modified
There is now evidence that
tumor is growing considerably
than previous generations.
The results of earlier studies
20, 23) ; these suggest
less
is temporarily
or other factors and will increase during sub
sequent transplants.
generation
of this
contain
of the 4th
It is possible
have been reviewed
that malignant
endopla.smic
the 5th
faster
reticulum
and embryonic
than
normal
(13,
cells
adult
cells
of the same tissue and that variations in the amount of
endoplasmic reticulum may be linked to differentiation.
A
correlation between dedifferentiation
and growth rate has
been found previously (6, 12). The present data suggest
that the fraction of bound polyribosomes
correlates well
with the degree of differentiation of the tissue.
The fact
that the —DOC/+DOC
ratio for regenerating liver is in
the range of normal liver supports the view (20) that regen
erating liver represents increased growth rate with the
maintenance
of the differentiated
state.
It is probably
significant
in this respect
that
regenerating
liver,
but
not
several hepatomas, retains the cholesterol-negative
feed
back system (38). The possible relationship of the bound
polyribosomes to the tissue specific anigen of adult liver
should also be considered.
The latter is localized in the
microsome
fraction
(9)
and
disappears
in
liver
cancer
in
duced by feeding 4-dimethylaminoazobenzene
to rats (44).
The titer in regenerating liver is apparently half that of
adult liver (8) but is very low or nil in several minimal
deviation hepatomas (R. Suss, personal communication).
With the possible exception of a method based on the
differential sensitivity to ribonuclease (40), the —DOC/
+DOC ratio is at present the only suitable biochemical
method for detecting and estimating the fraction of “free―
and “bound―
polyribosomes in the S@fraction of the cell.
There are 2 possible limitations to the method, which
should be considered.
First, during the preparationof
the
82, which involves centrifuging at 17,300 X gmax, a signifi
cant but undetermined
fraction of the rough-surfaced
endoplasmic reticulum sediments with the nuclei and mito
chondria (25, 36). The existence of this 2nd possible
source
of
bound
polyribosomes
may
explain
why
the
—DOC/+DOC ratio for the Novikoff hepatoma is 1.0
while small remnants of the rough endoplasmic reticulum
are found in electron micrographs (16); rough endoplasmic
reticulum is also visible in the liver of the i-day-old rat
(22) .
The release of the ribosomes
from this heavy
micro
some fraction under certain physiologic conditions would
result in the modification of the normal C-ribosome and
total ribosome patterns.
A second limitation of the method is that it involves,
first, homogenization
and
and, then, separation
“bound―polyribosomes
by centrifugation
of the “free―
through
Downloaded from cancerres.aacrjournals.org on June 18, 2017. © 1965 American Association for Cancer Research.
a
WEBB
et al.—Polyribosomes
in Rat
1223
Tissues
exception since it does not have a nucleus to maintain an
endoplasmic reticulum.
The significance of the high dimer concentration
in the
hepatomas remains to be determined.
It is possible that
the loss of endoplasmic reticulum and the presence of this
peak are in some manner related.
Pitot and Peraino (28)
have suggested that the endoplasmic reticulum stabilizes
(a) TheC-ribosome
preparations
arefreeof significantthe mRNA of the polyribosome complex and that defects
in the membrane are responsible for the anomalies in en
contaminating
endoplasmic
reticulum
as judged by elec
zyme induction that were invariably found in neoplastic
tron microscopy (1) and tests for glucose-6-phosphatase
tissues.
Pitot (27) has further
shown that the template
(unpublished data) and catalase (14). (b) The Novikoff
hepatoma, which has few bound polyribosomes
@cf.“Re half life for many inducible enzymes differs in normal and
discontinuous
gradient,
thereby rendering difficult the
possibility of distinguishing
between “free―
and “loosely
bound― polyribosomes.
In addition to the data discussed
above, the following studies suggest that the free poly
ribosomes are unattached
to the endoplasmic reticulum
in vivo and also supj)ort the suggestion that the ratio so
obtained reflects the cellular differentiation
of the tissue.
sults―), has
a decreased
phospholipid
content
neoplastic
compared
liver.
The
dimer
peak
might
then
be a by
product of a destabilized template.
The present results
with liver, especially with respect to the microsome frac
offer further support to the view that the endoplasmic
tion (22). (c) Results of earlier studies suggest that most
reticulum is defective in the hepatomas; however, defective
of the ribosomes in the Jensen sarcoma (26) and the Novi
enzyme induction (32) and the loss of feed-back in choles
koff hepatoma (16, 17) and most or all of the polyribosomes
terol biosyntheis (38) may also be the result of more subtle
in reticulocytes (iO, 41) are unattached.
(d) About 90%
It is
of the polyribosomes of fertilized sea urchin eggs are “free―defects, not detectable by the present procedure.
unlikely that the high dimer peak is due to decreased pro
yet active in in vivo amino acid incorporation(40).
(e) In
duction of mRNA although C-ribosome patterns obtained
electron microscope studies of established mouse fibroblast
from
rats
treated
with
actinomycin
D
also
have
a high
lines (ii) micrographs
show structures
resembling free
polyribosomes
in situ. The last study is of particular
dimer peak (39), and it has been suggested (21) that mon
interest
since
the
fibroblasts
showed
little
endoplasmic
omers free of mRNA tend to dimerize.
However, this
reticulum and many structures resembling free polyribo
mechanism requires that the relatively high monomer peak
in normal rat liver contains mRNA; yet the monomers do
somes
during
log-phase
growth,
whereas
in
the
resting
phase, when they actively synthesized
collagen, there was
not incorporate amino acids (3, 2i).
good development
of the rough endoplasmic reticulum.
It can be concluded that gross dedifferentiation
need
It is probable that the “clusters―or “rosettes―of free
not be a permanent characteristic of neoplasia.
Indeed,
ribosomes
often seen in electron micrographs
of cells from
the Morris hepatoma 7787 stores glycogen (6), the Reuber
many tissues are free polyribosomes.
Additional
support
hepatoma H-35 produces bile (37), and a mouse mammary
for this theory is found in electron micrographs of healthy
tumor is known (cf. Ref. 23) to secrete milk. Further
and degenerating
Sarcoma I cells (4) . The former have
more, if the temporary loss of any or all of the cytoplasmic
numerous clusters of free ribosomes in their cytoplasm
information
resident in the endoplasmic
reticulum
or the
whereas in the latter the free ribosomes are distributed
deletion (32) of a small part of such information
is an
diffusely.
essential attribute of neoplasia, it is probably concerned
The presence of numerous free polyribosomes and little
with the promotion,
rather than the initiation,
phase of
rough endoplasmic reticulum in the cytoplasm of rapidly carcinogenesis (35). It is important to determine whether
growing tissues raises the question whether the former are
the increased proportion of free polyribosomes
in tumor
actively engaged in protein synthesis; recent in vivo (G.
cells is diverted from the production of proteins character
Blobel, to be published) and in vitro (1) studies suggest
istic of the cell type (i.e., proteins of differentiation)
to
that the free polyribosomes incorporate labeled leucine.
the production
of proteins
for cellular proliferation,
Thus unless other organelles synthesize most of the cellular
thereby contributing in part to the increased growth rate
proteins, the involvement of the free polyribosomes would
usually accompanying dedifferentiation.
seem obligatory.
Two theories have been advanced (cf.
Refs. 13, 20) concerning the role of the rough endoplasmic
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Ag
of
Polyribosomes in Rat Tissues: II. The Polyribosome Distribution
in the Minimal Deviation Hepatomas
Thomas E. Webb, Günter Blobel, Van R. Potter, et al.
Cancer Res 1965;25:1219-1224.
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