Tumor Viruses and the Cancer Problem: A Summation of the Conference - Cancer Research
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Tumor Viruses and the Cancer Problem: A Summation
of the Conference
ANDRÉ
LWOFF
(Institut Pasteur, Paris, France]
"The \iruses are actual workmen in the cellular world."
II. REGULATORY MECHANISMS
Peyton Rous, 1943
In order to understand what is going on in a
cell, the best procedure is certainly to study a
I. INTRODUCTION microorganism, i.e., a structure in which the
In 1943 a group of scientists of the Rockefeller problem of the cell and the problem of the or
Institute published a book entitled Virus Dis ganism are concentrated in one and the same
eases. Peyton Rous was responsible for the chap unit.
ter dealing with viruses and tumors. This paper, For the sake of simplicity, two main types of
certainly one of the classics in virology and can molecules will be considered: the enzymes which
cer, is perhaps too often ignored. Given here to catalyze simple reactions and the nucleic acid
day, it would appear almost entirely up to date. which perpetuates the genetic information for
I would like to quote one of Peyton Rous's per
the patternization of amino acids into specific
tinent statements, namely, that the host of the proteins. Nucleic acids are endowed with a dual
oncogenic virus is not the organism but the in function: (a) they reproduce their own struc
dividual cell. The problem of the tumor virus has ture, that is they replicate; (b) they are respon
to be considered at the cellular level, and the sible for enzyme synthesis.
study of the interrelations between virus and cell The studies performed in recent years have
is one of the essential tasks of virologists. Yet, if disclosed the essential laws of molecular balance
the host of the tumor virus is the cell, the host of and control. Schematically, it may be said that
the cell-virus system—the malignant cell—isthe the end-product of the activity of a chain of en
organism. zymes is built into a repressor as a result of the
activity of a "regulator" gene. The repressor it
An organism may be considered as an inde self acts on the "structural" genes, those genes
pendent unit of integrated and interdependent
which carry the information for the synthesis of
structures and functions. The cells, the constitu
enzymes. The repressors command the expres
tive parts of the organism, are the ultimate units sion of the structural genes—i.e., they decide
of integration and reproduction. The functioning whether and when the synthesis of the specific
of each cell, its activity, its movements, its enzymes should start or stop. The repressor is
growth and reproduction are controlled by the group-specific: it controls the synthesis of each
organism as a whole. Each cell knows what task member of a chain of enzymes, whether or not
it has to perform and when it has to perform its one enzyme is missing as a result of a mutation.
task. In a normal organism, each cell receives or Moreover, it seems as though the repressor syn
ders and obeys. thesized by the regulator gene would not act di
It happens that a cell may escape the regula rectly on the structural gene. Its action is prob
tion of control mechanism. It multiplies when it ably mediated by a third category of gene, the
should not multiply. It goes where it should not operator, one operator being common for a group
go. It invades and destroys the normal tissues, of structural genes corresponding to a chain of
and, as a result of such a pathological activity, reactions. The operator might or might not be
the organism is finally killed. This is cancer. And sensitive to the repressor. At any rate, it is the
we are assembled here because cancer may arise operator which seems to send the order "synthe
size" or "stop synthesizing" to the group of struc
as the consequence of a viral infection. Why and
how? tural genes.
820
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Research.LAYOFF—Tumor
Viruses and Cancer Problem 821
Another category of particles and mechanisms rial of a virus, but like a pathological cellular or-
certainly plays an important role in cell biology, ganelle. A defective prophage is a pathological
namely, the episomes. The episomes are en episome. Owing to the fact that a number of
dowed with genetic continuity. They can be ab viral functions can be lost, a number of interme
sent or present, i.e., they are dispensable parti diary phases or degrees exist between a virus and
cles. When present, they are either attached to an episome.
the chromosome or free in the cytoplasm. They Thus, the molecular balance of a microorgan
can be suppressed by certain chemicals. In bac ism is mainly controlled by a complex system of
teria, they may be responsible for differentiation. repressers involving three main types of genes:
For example, a male bacterium possesses the fer regulator, operator, and structural, which con
tility factor F+ which is missing in the female. trol the synthesis of enzymes by a feed-back
When the female bacterium receives the F+ fac mechanism. The final state of the cell can be
tor as a consequence of copulation, it is turned modified by episomal elements which are either
into a male. This is a sexual differentiation. The normal cellular organelles, viruses, or mixed, hy
problem of differentiation in animal cells has brid, intermediary structures. When one consid
been often discussed. Some biologists consider ers bacteria, and especially lysogenic bacteria, it
that differentiation has a chromosomal basis, becomes clear that viruses can play an impor
whereas others visualize it as a cytoplasmic tant role in the physiological balance of the or
event. Episomes establish the link between nu ganism, and this is also true for oncogenic vi
clear and cytoplasmic particles and reconcile two ruses.
apparently incompatible theories. Their role in III. TUMOR VIRUSES
differentiation in the animal cell is for the time
A. Viruses.—Inthe past the viral nature of tu
being hypothetical. If one considers, however,
that the activity of an episome is different when mor viruses has sometimes been disputed. How
attached to the chromosome or when free in the ever, since several years, our ideas concerning vi
cytoplasm, the episome should be seriously con ruses have been clarified, and before discussing
sidered as a new candidate for a theory of dif tumor viruses it seems useful to say a few words
about "true" classical viruses.
ferentiation.
Episomes are not only important for the prob Viruses represent a specific category of para
lem of cellular balance and cellular differentia sitic entities. In order that these entities can be
tion, but also as links or intermediaries between classified as viruses, they have to possess a few
normal cellular constituents and viruses. The ge characteristic features which are absent in bac
netic material of a temperate phage exhibits the teria, protozoa, and fungi (i.e., in microorgan
properties of an episome. It is endowed with ge isms). These features are the following:
netic continuity. It may be present or absent. It 1. A virus has to possess an infectious phase in
may be either attached to the bacterial chromo its life cycle: the viral particle. If not, it cannot
some or free in the cytoplasm. When attached to be recognized as a virus;
the chromosome, the genetic material of a bac- 2. a virus is devoid of the enzymes necessary
teriophage behaves as if it were a bacterial gene. for the synthesis of its building block and of the
It replicates in harmony with the bacterial chro enzymes necessary to manufacture the high-en
mosome. However, the viral functions of the ergy bonds necessary for biological synthesis;
prophage are not expressed—i.e., bacteriophage the corresponding information is absent in the
proteins are not synthesized, and no bacterio viral genetic material;
phage particles are produced. Sometimes, as a 3. viruses possess only one type of nucleic
result of an induction, the prophage ceases to be acid, either deoxyribonucleic acid or ribonucleic
a prophage, the vegetative phase is started, bac acid;
teriophage proteins are produced, and infectious 4. viruses carry a specific viral type of infor
particles are finally organized. mation for the synthesis of the subunits which
As a result of mutation, one or many of the are constitutive parts of the viral coat;
viral functions of the genetic material of the bac 5. the infectious particle of a vims is built of
teriophage can be lost. The prophage then be subunits, mainly proteinic, enclosing and protect
comes defective. Lethal syntheses can be started ing the genetic material. The viral infectious par
as a result of an induction, but the abortive vege ticle is metabolically inert. Its structure has no
tative phase does not culminate in infectious par equivalent among normal cellular organelles;
ticles. For all practical purposes, the prophage 6. viruses are reproduced from their sole ge
does not behave any more like the genetic mate netic material.
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All the features which have been considered mor virus depends on a variety of factors: spe
seem to be correlated, and any one of these prop cies and genetic constitution of the animal, its
erties will probably be enough to identify an in age, the nature of the cell and its physiological
fectious parasitic entity as a virus. Viruses can state as controlled by its environment.
accordingly be defined either as organized infec Thus, a cell infected by a tumor virus may die
tive particles reproduced from their sole genetic or survive. Nevertheless, in those cells which are
material or as pieces of genetic material carrying going to survive, the vegetative phase is some
the information for the production of organized times initiated, and infectious particles are pro
infectious particles. duced. An individual fibroblast infected by the
A viral infection may be defined as the intro Rous virus and isolated in a micro-drop has been
duction into a cell of the genetic material of a seen dividing after having liberated virus parti
virus. As a consequence of this infection, either cles. Why is the viral vegetative phase sometimes
the cell dies or it survives. Those viruses which lethal, sometimes not? Cellular death might be
always kill the cell they infect are called cellu- caused by a specific type of protein synthesized
licidal, and the infection is designated as non as a product of viral development. The rate of
integrative. Those viruses producing an infection viral development must also be considered. The
compatible with cell survival are said to be mod Rous fibroma virus and the Stewart Eddy poly-
erate (temperate in the case of bacteriophage). oma virus have a long eclipse phase of 40-24
However, the fate of a cell infected by a mod hours, and the release period lasts many days. If,
erate virus is variable. The cell may survive, but however, some cellulocidal viruses, such as, for
it may also die. When the infected cell survives, example, the poliovirus, exhibit much shorter
the infection is said to be integrative. Two eclipse phase, 2 hr. 30 min. to 3 hr., other cellulo
"parts," the cell and the virus, have been united cidal viruses such as adeno-viruses and salivary
into a whole: this is an integration. A new system gland viruses exhibit a very long eclipse phase.
has been formed. The new system, the cell/virus A long latent phase is, therefore, not a unique
system which behaves and reproduces as a characteristic of tumor viruses. What is perhaps
whole, is different from the original cell, for the more important is that the viral production per
"infected" cell carries and perpetuates the ge cell per unit time is generally small in tumor vi
netic material of the infecting virus. This genetic ruses as compared with cellulocidal viruses. One
material may interfere with normal cellular func of the differences between an integrative and
tions. nonintegrative vegetative phase could be a dif
When, as a consequence of a viral infection, a ference of rate of viral production. In any event,
cell becomes malignant, the virus is said to be the outcome of an infection is controlled not only
oncogenic. Tumor viruses may exhibit consider by the genetic constitution of the cell and the
able differences in size, morphology, morpho virus, but also by the environment. The influence
genesis, site of multiplication. Transcending of the environment is particularly striking in the
these nonessential differences, tumor viruses ex case of the fibroblast/Rous virus system. More
hibit all the common features recognized as over, one has to take into account not only the
characteristic of the viral category. It is known, influence of the culture medium as such, but also
for example, that tumor viruses contain only one the influence of the culture fluid as modified by
type of nucleic acid and are reproduced from the metabolism of the cellular population which
their sole genetic material. It is also known that has developed in this medium.
their coat is built of subunits which are organ It is of interest to remember that, under cer
ized just like the subunits of classical viruses. tain conditions, infected cells produce a sub
B. The integrative infection.—li the oncogenic stance, interferon, which inhibits or blocks non-
virus would kill all the cells it infects, there specifically the multiplication of many viruses.
would probably be no virus-induced malignancy. Whether the genetic information for the synthe
Viruses as causative agents or rumors may be sis of interferon, which is a protein, comes from
conceived only if they can produce integrative the cell or from the virus is not yet known. How
infections, that is, if they are moderate. This does ever, some indications have been gained recently
not mean that the infection has to be always in concerning the mode of action of this substance.
tegrative. As in the case of the temperate phage/ Cells treated with interferon show a threefold
bacterium system, the infection of a ceU by a increase of CO2 and lactic acid production. In
moderate tumor virus has only a certain proba view of the sensitivity of the development of
bility of being integrative. many viruses to a low pH, the action of inter
The outcome of the infection of a cell by a tu feron on viral reproduction could be due to a
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Research.LwoFF—Tumor Viruses and Cancer Problem 823
decrease of the cellular pH. Whatever the case creased. A hyperkeratinization takes place, and
may be, it is important to know that a cell may the production of infectious particles is multi
react to a viral infection by the production of a plied by a factor of 10,000. The same type of
substance which depresses viral multiplication. action has been observed after x-ray irradiation.
C. Loss of infectivity.—The original ability of If the papillomatous skin of a domestic rabbit
a malignant cell to produce infectious particles is is rubbed with tar, carcinomas are produced.
sometimes lost. This is the case in some of the When newborn mice of adequate genetic con
carcinoma developed from Shope's papilloma. stitution are given injections of the Gross virus,
The original malignant cell produces infectious malignant cells appear only after a few months.
particles and viral antigens. After a few transfers, In x-radiated animals, malignant cells may ap
the malignant cell produces viral antigens but no pear within a few weeks. Here, x-rays have ac
infectious particles. Finally, neither infectious celerated the transition of the normal cells to
particles nor antigens are formed. A similar situ ward malignancy or, let us say, have increased
ation has been described in hamster tumors in the probability of the malignant change in the
duced by the polyoma virus: the virus is lost virus-infected cell.
after a few transfers from hamster to hamster. As a result of the concerted action of tar and
It should be remembered that the viral vege of the fibroma virus, malignant tumors may ap
tative phase of ordinary viruses does not neces pear in the domestic rabbit.
sarily culminate in the production of infective What does all this mean? How do the carcino
particles. Sometimes, the development is abor gens act on the cell virus system?
tive. This is the case, for example, when myxo- In order to answer this question, lysogenic
viruses (influenza group of viruses) infect the bacteria have to be used once more as a model.
chorionic cells of the chick embryo. This also Two phases of the life cycle can be influenced
happens when the fowl plague virus infects the by carcinogenic agents: the response of the in
L strains of mouse fibroblast. The infection is fected bacterium and the transition from pro-
here abortive, apparently because one of the phage to vegetative phage.
proteins produced in the nucleus fails to be re The fate of a bacterium infected by a bac-
leased in the cytoplasm where the constituents teriophage is decided within a few minutes. The
of the infectious particles are "normally" assem response depends on whether a "vegetative" pro
bled at the end of a complete vegetative cycle. tein or a represser is synthesized first. If it is a
The loss of the capacity to produce viral anti protein, the vegetative phase is started and the
gens or viral particles could also be due to the bacterium is lysed. If it is a repressor, the viral
selection of a defective viral genetic material. functions are repressed, the infecting genetic ma
Lysogenic bacteria provide a model for this type terial cannot express its potentialities, and bac-
of event. Defective prophages are known which teriophage proteins are not synthesized. When
perpetuate one or a few mutated genes; as a con the repressed genetic material of the bacterio-
sequence, one or more viral functions are miss phage reaches the specific receptor site of the
ing, and the vegetative phases do not end with bacterial chromosome, the bacterium becomes
the production of infectious particles. lysogenic. From then on the genetic material of
Finally, it is theoretically conceivable that, as the bacteriophage multiplies in harmony with
a result of the presence of a tumor virus, the cell the bacterial chromosome and behaves as if it
has undergone an irreversible hereditary altera were a bacterial gene. Agents which block pro
tion responsible for malignancy. In this case, the tein synthesis considerably increase the proba
viral genetic material could disappear without bility of the lysogenic response, i.e., the chance
the virus-induced malignancy's being lost. of an integrative infection. Inducing agents, such
as, for example, ultraviolet light, shift the bac
IV. CARCINOGENS AS INDUCING terial response toward the vegetative phase and
AGENTS bacterial death, i.e., increase the probability of a
It has been seen that the response of a cell to nonintegrative infection.
an infection by a tumor virus is controlled by the In order to understand how inducing agents
environment. Once the integrative response has modify the response toward infection, it is best
taken place, the balance of the cell virus system to consider the factors which, in a lysogenic bac
can also be modified by extrinsic factors. terium, control the balance of the bacterium/
If the skin of the domestic rabbit infected with prophage system. In a lysogenic bacterium, bac
Shope's papilloma virus is painted with methyl-
teriophage proteins are not synthesized. Homol
cholanthrene, the cellular proliferation is in ogous superinfecting bacteriophages are unable
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to enter the vegetative phase. Elegant experi if an "initiating" agent has been allowed to act
ments have shown that the absence of the syn first. Whatever the part of speculation might here
thesis of bacteriophage proteins is due to the be, it should be remembered that the expression
presence of a specific represser produced by the of viral functions, like the expression of the syn
prophage. This repressor is absent in nonlysogenic thesis of enzymes, is controlled by specific re
bacteria. pressors.1
Some strains of lysogenic bacteria are induc- It is now interesting to speculate about the
ible: if irradiated with a sufficient dose of U.V. possible mode of action of carcinogens in ani
light, the vegetative phase is started after 20 mals. All carcinogens can induce malignancy in
minutes and infectious particles are formed. How the absence of detectable tumor viruses. The role
does the U.V. light exert its inducing activity? of repressors in the molecular balance of the cell
The most probable hypothesis is that inducing has already been discussed. It is clear that a
agents block the synthesis of the bacteriophage block in the synthesis of one or a few repressors
repressor. could unleash the expression of a gene, i.e., re
To account for the controlling action of re- lease the synthesis of a normally blocked enzy
pressors on the synthesis of enzymes, one has to matic system and thus be responsible for a new
assume that repressers have a short half-life, of type of steady state.
the order of a few minutes for a bacterium. If When tumor viruses are involved and when
repressers were stable, it would take a very long malignancy is due to the concerted action of a
time—many generations—for a bacterium to con virus and of a carcinogen, the role of carcinogens
trol its enzymatic balance. This being admitted, could be the same as when inducers act on lyso
the mechanism of the inducing effect of U.V. genic bacteria: they would upset the balance of
light appears to be simple: as a result of the the cell/virus system by interfering with the
inhibition of the synthesis of the new phage re synthesis of repressors.
pressers and of the decay of the preexisting ones, The part played by hypotheses in the logical
the repressor level decreases. When the level has picture of induction and of carcinogenesis which
reached a certain critical threshold value, repres has been presented is obvious. An attempt at a
sion ceases and the prophage can express its po unifying concept of two phenomena having so
tentialities: proteins are synthesized, and the much in common was, however, felt necessary.
vegetative phase is initiated. Moreover, the hypotheses might turn out to be
Some bacteriophages are known to produce a useful, since, it seems, they can be submitted to
large amount of repressor, others to be complete experimental tests.
ly insensitive to repressors, the two functions be Another hypothesis has to be discussed. Epi-
ing controlled by specific bacteriophage genes. somal elements, episomes, may play an impor
This explains why some lysogenic systems are tant role in cellular physiology and in differentia
inducible whereas others are not, and why in- tion. I would like to remind you that episomes
ducibility, like any other bacteriophage function, are either attached to the chromosome or free
can be modified by mutation. What is for us here in the cytoplasm and that their activity in one or
of utmost importance is that most of the inducing the other position is different. The action of car
agents tested so far have proved to be carcino cinogens could be to shift the position of an
gens. episome.
U.V. light and x-rays, nitrogen mustard, or Finally, it has been shown that the maintenance
ganic peroxides, epoxides, and ethyleneimines of a normal chromosomal outfit in the mammalian
have been found to act as inducers. All are carcin cell is controlled by environmental factors. A
ogenic agents. A few water-insoluble carcino phenotypic change of the cellular balance,
gens have failed to show an inducing action, but whether caused by a virus or by a physical or
this could of course be due to an absence of chemical agent, might be indirectly responsible
penetration into the bacterium. Finally, ethylu- for aneuploidy, that is, for a permanent genetic
rethan is devoid of inducing activity; this sub 1 Among the inducers are also azaserine and mitomycin
stance is, however, not a "complete carcinogen"
but only an "initiating" agent. It should be added C. Both are known to inhibit the synthesis of DNA, just
as do other carcinogenic agents. And it would be strange
that the inducing agents, when acting on lyso if both azaserine and mitomycin C did not possess carcino
genic bacteria, induce the vegetative phase only genic activity, just as the other inducers. Nevertheless, the
if the bacteria are in a given physiological state yet unsolved problem of the nature (nucleic or nucleic
called "aptitude." This recalls the fact that some acid-containing ) of the repressor is posed here once more.
"promoting" agents can induce malignancy only The solution is the key to a rational approach of antiviral
and antitumoral therapy.
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Research.LwoFF—Tumor Viruses and Cancer Problem 825
alteration which could play a role in the malig us today is that the viral expression is repressed.
nant alteration. In a lysogenic bacterium, induction necessarily
leads to bacterial death: the repression has to be
V. VIROGENY absolute, since the vegetative phase is always a
The question has often been raised concerning lethal process. The vegetative phase of a mod
the existence in animal cells of a situation homol erate virus is compatible with the survival and
ogous to lysogeny. All efforts to disclose "virog multiplication of the cells. Therefore, a highly
eny" with nontumor viruses have so far failed— repressed cell/virus system would be, from a
this despite the fact that, in some cases, such as, practical point of view, analogous to lysogeny.
for example, in the virus sensitizing Drosophila At any rate, in the two cases considered here:
to CO2, the infection is of the integrative type. (a) the genetic material of the tumor virus is
Nevertheless, many scientists have expressed the present in the cell which continues to divide;
view that virogeny should exist, but it was ob (fo) viral antigens are, as far as we know, not
vious that its existence could be conclusively produced, and, as a consequence, infectious par
proved only by the study of individual cells. ticles cannot be built; the viral functions of the
If one considers the data discussed during the viral genetic material are repressed; (c) as a
conference, it seems quite possible that a virog- consequence of a change in the physiology of
eny-like situation does exist in cell/tumor virus the host cell, the vegetative phase of the life
systems. Let us consider Shope's papilloma. In cycle is started, i.e., viral antigens are synthe
sized; the viral functions are now "derepressed";
the skin of the domestic rabbit, viral antigens
cannot be detected in the cells of the basal, mal- ( d ) whether infectious particles are produced or
pighian layer. Electron microscopy does not re not is irrelevant—their absence might be due to
veal any abnormal particle. The earliest signs of some genetic defects of the viral genetic material
the presence of a virus are to be seen in the or to an unsuitable physiological state of the host
keratohyalin layer, close to the germinal layer. cell; (e) it is of the utmost importance to re
Pathological granules first appear in the nucle- member that carcinogenic agents do modify the
olus, and viral antigens are synthesized. Here a cell/tumor virus system.
change in the physiology of the cell is responsible We are obviously dealing here with repression
and "derepression" of viral functions, and we
for the onset of the viral vegetative phase. When
the cell grows, multiplies, and does not form have to conclude that, from a practical point of
keratin, viral antigens are not produced. When view, a virogeny-like situation does exist.
cellular growth ceases and when keratin is syn From a theoretical point of view, virogeny, the
thesized, then viral proteins are synthesized. situation homologous to lysogeny, would imply
A similar type of situation has been disclosed in a complete repression and also the attachment of
the case of the virus of myeloblastosis. Electron the genetic material of the tumor virus to a specific
microscopy does not reveal the presence of viral receptor of the cell, whether chromosomal or not.
particles in the myeloblasts present in the blood; It will, of course, be essential to know whether
but, when grown in vitro, the malignant myelo or not the expression of viral functions is essential
blasts produce virus, around 70 particles/cell/ for the onset of virus-induced malignancy.
hour (normal myeloblasts do not multiply in
VI. TUMOR VIRUSES AS CARCINOGENIC
vitro ). Things happen as if a physiological altera
AGENTS
tion resulting from the transfer of myeloblasts
from the blood into a culture medium had "in As a result of a viral infection, cancer may
duced" the production of viral particles. One develop. Are tumor viruses able to produce ma
can of course speculate about the "absence" of lignant cells by themselves, or does their pres
virus in the blood myeloblasts. It is possible that ence merely increase the probability of the ma
lignant change? In order to answer this question,
the inability to detect infectious particles is a
it is necessary to learn first how a cell becomes
reflection of their rarity due, for example, to a malignant in the absence of a virus, under the
very slow vegetative phase. It is also possible influence of physical or chemical carcinogens.
that the vegetative phase is, in the blood, gen The change from normality to malignancy is a
erally abortive. long and complex process involving many steps.
Whether, in a malgnant myeloblast, the virus In this long "progression" two phases have been
is present in a proviral or in a "slow" undetectable
recognized, initiation and promotion. Initiation is
vegetative phase is, from a theoretical point of the process whereby normal cells are changed
view, an essential problem. What is important for into neoplastic cells, either benign or malignant.
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Research.826 Cancer Research Vol. 20, June, 1960
Promotion is a subsidiary process whereby neo- the cells—whether they are fibroblasts, epithelial
plastic cells are stimulated to multiply. cells of the iris, or epithelial cells of the kidney.
Urethan, for example, is an initiating agent, This change is hereditary.
devoid of promoting action. Croton oil is essen Thus, a cell has been infected by a tumor virus.
tially a promoting agent, although it might show The genetic material of the virus has penetrated
in some cases a slow initiating activity. Some into the cell. The infection has been integrative:
substances, such as the oncogenic hydrocarbons, the cell has survived, and it now perpetuates the
are "complete oncogens," i.e., have both an ini genetic material of the tumor virus. Sometimes
tiating and a promoting activity. What is the the viral genetic material expresses its potentiali
position of tumor viruses among carcinogens? ties, and viral antigens are produced. A yet hypo
Shope's fibroma virus can be considered as an thetical provirus could perhaps also be respon
initiating agent, the promotion being produced sible for new cellular functions by a mechanism
by tar (see Table 1). Shope's papilloma virus analogous to the "conversion" of bacteria by a
may behave in some cases like an initiating bacteriophage. As a result of the infection, a new
agent, sometimes like a promoting agent. Bittner's system has been produced, the cell/virus system :
virus, the milk agent, can be considered as an it perpetuates the viral information which is now
initiating agent, the promoting factor being the a part of its genetic make-up. As a consequence
of the new functions introduced by the virus,
TABLE1 moreover, the cell is now malignant.
VIRUSES
ASINITIATING
ANDPROMOTING
AGENTS
VII. THE MALIGNANT CELL
thevirus of
AND THE PROBLEM OF
by:ShopeInitiation Rous'sterminology
by:TarEstrogensSpontaneous P. CONTACT INHIBITION
1943ProvocativeDeterminingActuating
A. The malignant cell.—Malignancy can prob
virusBittner
fibroma ably not be ascribed to the alteration of one
(milk"agent")Shope
virus specific reaction or property. Moreover, the fac
virusTarPeyton
papilloma (un tors are still largely unknown which, in an or
known)Shope ganism, stop or start the multiplication and be
papillomavirusrus; havior of the specific cellular species. Hormones
are certainly operative. However, it is completely
Rous's sarcoma viPromotion
perma-nent direct unknown why a malignant cell does not respond
transformation. The virus is a com
plete oncogenAction to the unknown factors of cellular coordination.
Let us, however, discuss some data concerning
the difference between a normal and a malignant
estrogen. In all these cases, the concerted action cell. A phenomenon analogous to progression has
of a tumor virus and of a hormone, a carcinogen, been described in plants. Phytomonas tumefaciens
or an unknown factor are necessary in order that can induce different degrees of malignancy, the
a given cell becomes malignant. According to degree depending on the time during which the
Rous's terminology, their action is provocative in
bacterium has exerted its action.
one case, determining in the other. The virus and A normal plant cell needs to be provided with
the oncogenic agent both increase the probability a few essential metabolites in order to grow. The
of the change from normal to malignant which malignant cell can grow in the absence of these
might, in some cases, be considered as a somatic substances in the medium, and the number of
mutation. essential metabolites needed for rapid multipli
Some viruses, however, act as actuating agents, cation decreases with the degree of malignancy.
i.e., as a complete carcinogen. This is the case of The order in which the needs disappear is always
Rous virus and of the polyoma virus. When a the same in all plant species investigated so far.
normal fibroblast is infected with the Rous sar Whether the cancer has been induced by a bac
coma virus, its morphology is altered within 2 terium, by a virus such as the wound tumor virus,
days. The altered fibroblast, seeded on a layer of by a spontaneous mutation, or by radiation, the
normal cells, behaves like a malignant cell: it result is the same.
exhibits no more contact inhibition. Let us now consider two cases in which animal
The type of morphological change induced by viruses are involved. No argüÃ-ase is present in
the virus may be altered by a viral mutation. The the epidermal cells of the rabbit. Arginase is
infection with a certain mutant virus produces found when Shope papilloma virus is present. In
highly elongated cells regardless of the nature of the liver of the normal chick embryo, no cartilage
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Research.LwoFF—-Tumor Viruses and Cancer Problem 827
has ever been found. When the chick is infected embryonic tissue, such as, for example, the meso-
with De Ruyck's virus, cartilage appears: the nephros of a chick embryo, they invade the nor
same phenomenon has been observed in liver mal organ and destroy it. This type of behavior,
tissue grown in vitro. Whether or not De Ruyck's which is, up to now, specific for malignant cells,
virus is the etiologic agent of the human hydat- might be a useful tool especially to ascertain the
iform mole is, for our purpose, irrelevant. nature of the change produced in various cells
Finally, I would like to call attention to a veiy with material originating from human tumors.
important observation which remains alive thanks
to Peyton Rous. Cells which have absorbed fat- VIII. THE ANIMAL HOST
soluble dyes, Sudan 3, or scarlet red behave as The influence of environmental factors on the
if they were malignant. However, when as a balance and evolution of the cell virus system
consequence of growth and division, the dye has has been stressed again and again. We have to
been diluted out, the cells return to their normal remember that the host of the malignant cell is
state. The fat-soluble dyes have produced a the organism. It has been known for a long time
phenocopy of a malignant cell. that the physiological state of the organism can
It seems clear that an extensive search for the modify the outcome of an infection by a tumor
biochemical-physiological modifications induced virus. The role of estrogens in the mammary
carcinoma initiated by Bittner's agent has already
in a cell by a tumor virus is of great importance
for our understanding of the malignant change. been mentioned. In many cases, the age of the
B. Contact inhibition.—One of the main charac animal controls the outcome of a viral infection.
teristics of malignant cells is their invasive power. Shope fibroma virus produces malignant tumors
A normal cell stays where it should stay. A malig in young rabbits; in the adult, only benign tumors
nant cell migrates into forbidden areas. When a appear which regress spontaneously. Rous sar
normal fibroblast traveling in one direction makes coma virus produces a hemorrhagic disease in
contact with another normal fibroblast, it cannot young chickens, whereas a sarcoma is initiated
continue to move in that direction. This "contact in adult animals.
inhibition" takes place neither between two ma In some strains of mice, with a low incidence
lignant fibroblasts nor between a malignant fibro of leukemia, x-radiation considerably increases
blast and a normal one. After infection with the the incidence of the disease. In the nonirradiated
Rous virus, the infected fibroblasts no longer mice, virus cannot be detected, whereas virus is
respond to contact inhibition. present in those animals which develop leukemia
This contact inhibition probably plays an im after irradiation.
portant role in morphogenesis. Let me recall that If thymus is removed before or shortly after
the morphology of the fibroblast is rapidly mod x-radiation, leukemia does not develop. Finally,
ified by the Rous virus. Let me recall also that if newborn mice are given injections of leukemia
in some cases a compatibility has been disclosed virus and thymectomized 1 month later no leu-
between viral protein and some cell proteins: the kemias develop. Thymus is probably necessary
proteins of the viral coat can become part of the for the production of a lymphocytosis-stimulating
cell surface and, for example, confer hemag- factor (L.S.F.). The virus is not in the thymus.
glutinating power to the infected cell. How the virus acts is a mystery. It could act by
A mere change in the surface of the cell as the modifying either the synthesis of the L.S.F. or
direct effect of the virus could perhaps, in some the sensitivity of the cells to the L.S.F., or some
cases, be responsible for malignancy. The dem thing produced by the thymus could modify the
onstration of a difference between the membrane sensitivity of the cell to the virus.
of a normal and of a malignant cell would, there It is known also that the direct injection of
fore, be of utmost importance. carcinogens into the thymus increases the inci
Chimeras of different species can be produced. dence of leukemia. This is obviously a highly
The testis of duck and mouse can be associated, complex situation which is not yet understood.
and mixed tubuli are produced in which Sertoli
cells of both animals alternate. Mixed bronchiolar IX. CONCLUSIONS
tubules of chick and mouse can also be obtained. In a microorganism, in a bacterium, the molec
Thus normal cells of widely different animals ular balance is controlled by a system of repres-
recognize themselves. It is clear that any morpho- sors produced by regulator genes. In the absence
genetic process involves some sort of recognition of repressors, the synthesis of enzymes can only
and inhibition. be anarchic: one molecular species would out
When malignant cells are inoculated on an grow the others. This is what happens in effect
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Research.828 Cancer Research Vol. 20, June, 1960
when the represser system is disturbed by a mu tions are parts of molecular biology, as is also
tation of the regulator gene. One type of mole the pathological malignant function.
cule is synthesized in excess, and the result is a No single scientist can dominate the various
pathological condition: the diseased bacterium disciplines involved. That is why meetings such
is outgrown by the normal ones. as this, in which various viewpoints can be pre
The same type of regulatory mechanism is sented, are of utmost utility. Nobody, I hope,
known to exist in the cells of multicellular organ expected that the summing up would provide
isms. In the organism, the cell is, in addition, the solution of this highly complex problem.
submitted to the action of extrinsic substances
produced by other cells. And the problem of the X. APPENDIX
regulating mechanisms and of developmental This summing up is, naturally, based on the
functions in differentiation and morphogenesis reports, papers, and discussions given at the con
has to be reconsidered today in the light of the ference. The field covered has been wide, and
new ideas, brought about by the discovery of the the main speakers have presented valuable syn
represser system and of episomal elements. It is theses of a complex subject: this is exemplified
essential to remember that each molecule is a by a certain degree of overlapping, certainly a
dependent part of a cell, each cell a dependent healthy sign of a tendency toward unification
part of the organism, and that the tumor virus is and integration.
an anarchic infectious entity which disorganizes I arrived in Rye not completely unprepared,
an integrated system of structures and of func but, as far as I can judge, without preconceived
tions. We have to study the parts without ever views, except perhaps on one specific point. Can
forgetting that they belong to a whole. Whatever cer may develop either as a result of a viral
the carcinogenic factor might be, malignancy can infection or as the consequence of the action of
only be the consequence of the disturbance of physical or chemical agents. It seemed to me
the regulating system. that the problem of the mode of action of carcin
An infection by a tumor virus is the introduc ogens had perhaps been left somewhat too much
tion into a balanced dynamic system of a new in the background and that a constructive dis
type of genetic information. The expression of cussion on the subject could be useful. I have
viral functions, as controlled by "repression" and
"derepression," interacts with the normal cellular accordingly tried to do something in this direc
tion. For having introduced this personal view
functions. This is exemplified by the altered me point in the summing up, I have to apologize.
tabolism, the increased rate of multiplication, I also have to apologize for something much
the suppression of contact inhibition, and, finally, more serious. So many scientists have been re
invasiveness and invasion. The hereditary cellular sponsible for important contributions that it was
alterations caused by a tumor virus can only be felt impossible to give credit to anyone for his
a disturbance of the normal molecular order. The achievements and his theoretical views. A selec
most important problems are: (a) to identify tion would have been most arbitrary. Therefore,
those alterations which are the direct or indirect no name has been cited in the summing up. Only
consequence of the viral infection of the cell, one exception has been made—Dr. Peyton Rous
(&) to determine the sequence of these altera —andfor this no explanation is needed.
tions, (c) to find out which of these alterations While reading the preprints and hearing the
is or are responsible for malignancy. discussions, I was struck by the fact that a certain
Every cell/virus system amenable to a bio number of papers or reviews were not quoted
chemical/physiological analysis is suitable for and that a certain number of data had not been
the purpose. However, the data thus obtained taken into account. Of course, no one was ex
will reveal their full significance only when the pected to give an exhaustive bibliography or to
malignant cell can be compared with the normal discuss everything. And perhaps some of the pa
one. pers I have in mind have not been cited because
One of the great scientific achievements of our they were considered as too classical. Yet, some
time is the merging of a number of heretofore of them dealing with relatively new data could
separated disciplines. Cytology, biochemistry, have been overlooked. Because I found them
physiology, genetics are now integrated into a useful for myself, i.e., for an outsider in the field,
new discipline, molecular biology, which tran and because the proceedings of this conference
scends the various individual approaches and might be read by nonspecialists, these "missing"
confers a remarkable unity to modern biology. references are provided herewith. The others are
Developmental functions as well as viral func to be found in the reviews of my colleagues.
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Research.LwoFF—Tumor Viruses and Cancer Problem 829
XI. LITERATURE bactérieslysogènes defectives. I. Déterminisme
génétiquede la morphogenèse chez un bactério-
I. Introduction: phage tempéré.Ann. Inst. Pasteur, 90:282-302,
Rous, P. Viruses and Tumors. In: Virus Diseases, pp.
147-70. New York: Cornell University Press, 1956.
IV. Carcinogens as Inducing Agents:
1943.
. Concerning the Cancer Problem. Am. Scien FOULDS, L. Neoplastic Development. In: W. D.
tist, 34:329-58, 1946. MCELROY& B. GLASS( eds. ), The Chemical Basis
of Development, pp. 680-700. Baltimore: The
II. Regulatory Mechanisms and Episomes: Johns Hopkins Press, 1958.
JACOB, F. Genetic Control of Viral Functions, The JACOB,F., and WOLLMAN,E. L. Induction of Phage
Harvey Lectures, Series 54, pp. 1-39. New York:
Development in Lysogenic Bacteria. Cold Spring
Academic Press, 1958-1959. Harb. Symp. Quant. Biol., 18:101-21, 1953.
JACOB, F., and MONOD, J. Gènes de structure et LWOFF, A. L'induction. Ann. Inst. Pasteur, 84:225-
gènesde régulationdans la biosynthèse des pro 41, 1953.
téines.Compt. Rend. Acad. Se., 249:1282-84,
LWOFF, A., and JACOB,F. Induction de la produc
1949. tion de bactériophages et d'une colicine par les
JACOB, F.; SCHAEFFEB,P.; and WOLLMAN, E. L. peroxydes, les éthylèneimineset les halogénoal-
Episomic Elements in Bacteria. Xth Symp. Soc. coylamines. Compt. Rend. Acad. Se., 234:2308-
Gen. Microbiol., London, 1960. 10, 1952.
JACOB,F., and WOLLMAN,E. L. Les épisomes,élé OTSUJI, N.; SEKIGUCHI,M.; IIJIMA, T.; and TAKAGI,
ments génétiques ajoutés.Compt. Rend. Acad. Y. Induction of Phage Formation in thé Lysogenic
Se., 247:154-56, 1958.
Escherichia coli K12 by Mitomycin C. Nature,
MONOD,J. Biosynthese eines Enzyms. Information, 184:1079-80, 1959.
Induktion, Repression. Angew. Chemie, 71:685-
VII. The Malignant Cell and the Problem of Contact Inhi
91, 1959. bition
III. Tumor Viruses: ABERCROMBIE,M. Exchanges between Cells. In:
B. The integrative infection. W. D. MCELROY& B. GLASS(eds.), The Chemi
ISAACS,A. Metabolic Effects of Interferon on Chick cal Basis of Development, pp. 318-28. Baltimore:
Fibroblasts. Virology, 10:144-46, 1960. Johns Hopkins Press, 1958.
C. Loss of infectivity. SCHNEIDER,N. Sur les possibilitésde propagation
JACOB, F., and FUEHST, C. R. The Mechanism of d'un sarcome de souris sur des organes embryon
Lysis by Phage Studied with Defective Lysogenic naires de poulet à différentsstades du développe
Bacteria. J. Gen. Microbiol., 18:518-26, 1958. ment. Arch. Anat. Microsc. et morphol. expér.,
JACOB, F.; FUERST, C. R.; and WOLLMAN, E. L. 47:573-604, 1958.
Recherches sur les bactérieslysogènesdefectives. WOLFF, E., and WOLFF, E. Les résultatsd'une
II. Les types physiologiques liés
aux mutations du nouvelle méthodede culture de cellules cancé
prophage. Ann. Inst. Pasteur, 93:724-53, 1957. reuses "in vitro." Rev. franc, étudesclin, et biol.,
JACOB,F., and WOLLMAN,E. L. Recherches sur les 111:945-51, 1958.
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Research.Tumor Viruses and the Cancer Problem: A Summation of the
Conference
André Lwoff
Cancer Res 1960;20:820-829.
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