THE CARCINOGENIC EFFECT OF A PAPILLOMA VIRUS ON THE TARRED SKIN OF RABBITS : II. MAJOR FACTORS DETERMINING THE PHENOMENON: THE MANIFOLD EFFECTS OF TARRING

The rabbit papillomavirus model: a valuable tool to study viral-host interactions

Cottontail rabbit papillomavirus (CRPV) was the first DNA virus shown to be tumorigenic. The virus has since been renamed and is officially known as Sylvilagus floridanus papillomavirus 1 (SfPV1). Since its inception as a surrogate preclinical model for high-risk human papillomavirus (HPV) infections, the SfPV1/rabbit model has been widely used to study viral-host interactions and has played a pivotal role in the successful development of three prophylactic virus-like particle vaccines. In this review, we will focus on the use of the model to gain a better understanding of viral pathogenesis, gene function and host immune responses to viral infections. We will discuss the application of the model in HPV-associated vaccine testing, in therapeutic vaccine development (using our novel HLA-A2.1 transgenic rabbits) and in the development and validation of novel anti-viral and anti-tumour compounds. Our goal is to demonstrate the role the SfPV1/rabbit model has played, and continues to play, in helping to unravel the intricacies of papillomavirus infections and to develop tools to thwart the disease. This article is part of the theme issue 'Silent cancer agents: multi-disciplinary modelling of human DNA oncoviruses'.

Free-radical polymer science structural cancer model: a review

Polymer free-radical lipid alkene chain-growth biological models particularly for hypoxic cellular mitochondrial metabolic waste can be used to better understand abnormal cancer cell morphology and invasive metastasis. Without oxygen as the final electron acceptor for mitochondrial energy synthesis, protons cannot combine to form water and instead mitochondria produce free radicals and acid during hypoxia. Nonuniform bond-length shrinkage of membranes related to erratic free-radical covalent crosslinking can explain cancer-cell pleomorphism with epithelial-mesenchymal transition for irregular membrane borders that "ruffle" and warp over stiff underlying actin fibers. Further, mitochondrial hypoxic conditions produce acid that can cause molecular degradation. Subsequent low pH-activated enzymes then provide paths for invasive cell movement through tissue and eventually blood-born metastasis. Although free-radical crosslinking creates irregularly shaped membranes with structural actin-polymerized fiber extensions as filopodia and lamellipodia, due to rapid cell division the overall cell modulus (approximately stiffness) is lower than normal cells. When combined with low pH-activated enzymes and lower modulus cells, smaller cancer stem cells subsequently have a large advantage to follow molecular destructive pathways and leave the central tumor. In addition, forward structural spike-like lamellipodia protrusions can leverage to force lower-modulus cancer cells through narrow openings. By squeezing and deforming even smaller to allow for easier movement through difficult passageways, cancer cells can travel into adjacent tissues or possibly metastasize through the blood to new tissue.

Personalizing lung cancer prevention through a reverse migration strategy

Lung cancer is the deadliest cancer in the United States and worldwide. Tobacco use is the one of the primary causes of lung cancer and smoking cessation is an important step towards prevention, but patients who have quit smoking remain at risk for lung cancer. Finding pharmacologic agents to prevent lung cancer could potentially save many lives. Unfortunately, despite extensive research, there are no known effective chemoprevention agents for lung cancer. Clinical trials in the past, using agents without a clear target in an unselected population, have shown pharmacologic interventions to be ineffective or even harmful. We propose a new approach to drug development in the chemoprevention setting: reverse migration, that is, drawing on our experience in the treatment of advanced cancer to bring agents, biomarkers, and study designs into the prevention setting. By identifying molecular drivers of lung neoplasia and using matched targeted agents, we hope to personalize therapy to each individual to develop more effective, tolerable chemoprevention. Also, advances in risk modeling, using not only clinical characteristics but also biomarkers, may help us to select patients better for chemoprevention efforts, thus sparing patients at low risk for cancer the potential toxicities of treatment. Our institution has experience with biomarker-driven clinical trials, as in the recently reported Biomarker-integrated Approaches of Targeted Therapy for Lung Cancer Elimination (BATTLE) trial, and we now propose to bring this trial design into the prevention setting.

THE EXPERIMENTAL DISCLOSURE OF LATENT NEOPLASTIC CHANGES IN TARRED SKIN

A carcinogenic tar applied to rabbit skin renders many more epidermal cells neoplastic than ever declare themselves by forming tumors. They may be present in large numbers and persist for a considerable time after brief tarring, yet give rise to no growths unless encouraged. The stimulus of wound healing will suffice to make some of them multiply and form tumors. No evidence has been obtained, in experiments specifically directed to the point, that the cells which tar renders neoplastic respond in this way because they are possessed of peculiarities not shared by the rest of the normal epithelium. The fact that non-specific stimulation (as e.g. wound healing) may act as the deciding influence in tumor formation brings out the need for a sharp distinction between the forces which induce neoplastic change and those which determine, or prevent, its realization in terms of a tumor. The distinction is vital to the appraisal of the many carcinogenic substances worked with nowadays. The ability of tumor cells to lie latent for long periods and respond to non-carcinogenic stimulation by multiplying into growths provides an explanation of those clinical instances in which cancer appears rapidly after acute injury to tissue that had seemed normal.

THE EFFECT OF CHEMICAL CARCINOGENS ON VIRUS-INDUCED RABBIT PAPILLOMAS

The application of methylcholanthrene and tar to virus-induced papillomas of the domestic rabbit caused them to become carcinomatous with great rapidity, and the malignant changes were frequently multiple. In bringing on the cancers the chemical agents acted in their specific capacity as carcinogens, not as ordinary stimulants of cell proliferation. The cancers derived from the virus-infected cells and were of the same types as arise from these elements spontaneously after a much longer time. The evidence would seem to indicate that the chemical carcinogens acted by way of the virus.

A COMPARISON OF VIRUS-INDUCED RABBIT TUMORS WITH THE TUMORS OF UNKNOWN CAUSE ELICITED BY TARRING

Tarring the ears of rabbits of one sort with a single kind of tar evoked epidermal tumors of a few sharply defined types, namely ordinary papillomas, carcinoids, carcinomas, and "frill horns." These last, relatively infrequent, are now recognized for the first time. The carcinoids have proved to be the expression of a spurious malignancy of papillomas, resulting from intercurrent influences, and they were wholly dependent upon these for their threatening aspect and behavior. Chief amongst such influences was continued tarring. It had the effect of establishing the papillomas, stimulated their proliferation, complicated their morphology, and rendered some of them disorderly, aggressive, and anaplastic. It brought all of the tissues of the ears into an excitable state, and often this state endured long after the skin had apparently returned to normal. The characters of the papilloma-carcinoids and of the frill horns were so different and distinctive as to imply the action of differing, specific causes. The papillomas were very like those induced with the Shope virus, and hence a point-to-point comparison was made of their manifestations, including the derivation of carcinomas from them. This comparison demonstrated that the unknown cause of the tar papillomas provoked neoplastic phenomena which were identical in all essential respects with those due to the virus. To suppose, for experimental purposes, that the papillomas which tarring elicits are caused by a virus rendered pathogenic by this procedure, is to demand least of the unknown. Yet it does not follow that they must be due to a virus.

CONDITIONAL NEOPLASMS AND SUBTHRESHOLD NEOPLASTIC STATES : A STUDY OF THE TAR TUMORS OF RABBITS

The "warts" which tar elicits on rabbit skin (papillomas, carcinomatoids, frill horns) are true tumors, benign growths expressive of slight yet irreversible deviations of epidermal cells from the normal. The neoplastic condition gives the cells a superiority over their neighbors when both are submitted to the same encouraging influences, and then they proliferate into tumors. Their state entails such disabilities, though, that they are unable to maintain themselves under ordinary circumstances, and consequently growths composed of them disappear when no longer aided. Often the neoplastic cells resume the normal aspect and habit of life long before the tumor mass is gone; and they may persist as part of an apparently normal epidermis, retaining their neoplastic potentialities for months after all signs of the growth have disappeared. In these instances it can be made to appear again, sometimes repeatedly, by non-carcinogenic stimulation of the skin (wound healing, turpentining). There is reason however to suppose that in the end the tumor cells, unless helped, die or are cast off. It is plain that the neoplastic state does not necessarily connote independence of behavior or success in tumor formation. On the contrary it may render cells unable to survive or endow them with powers which they can exert only under favoring conditions. This is the case with the cells composing the tar warts of rabbits. In the lack of such conditions the cells of these growths do not manifest themselves but remain in a subthreshold neoplastic state, whereas if aided they form neoplasms. The deviations from the normal represented by the benign tar tumors of rabbits are slight and limited in character, but further deviations in larger variety may be superimposed upon them, with result in malignant tumors, growths possessed of a greater, though not always absolute, independence. Tar cancers usually come about in this way, by successive, step-like deviations from the normal, and so also do the cancers which derive from virus-induced papillomas as well as many human carcinomas. After cells have become cancerous they frequently undergo further changes, some apparently step-like in character, and all taking the direction of greater malignancy. The hypotheses that tumors are due to somatic mutations and to viruses respectively are discussed in the light of these phenomena.

A TRANSPLANTABLE RABBIT CARCINOMA ORIGINATING IN A VIRUS-INDUCED PAPILLOMA AND CONTAINING THE VIRUS IN MASKED OR ALTERED FORM

A squamous cell carcinoma derived from a virus-induced rabbit papilloma has been propagated in fourteen successive groups of animals. It grows rapidly now in most individuals to which it is transplanted, killing early and metastasizing frequently. The original cancer was the outcome of alterations in epidermal cells already rendered neoplastic by the virus, and the latter, or an agent nearly related to it, has persisted and increased in the malignant tissue, as a study of the blood of the first ten groups of cancerous animals has shown. An antibody capable of specifically neutralizing the virus in vitro appeared in the blood of every new host in which the tumor enlarged progressively, and reached a titer comparable with that obtaining in animals which had long carried large papillomas. The antibody was absent from normal rabbits and those in which the cancer failed to grow. The implications of these facts are considered.

ORAL PAPILLOMATOSIS OF RABBITS: A VIRUS DISEASE

Papillomas occur frequently on the oral mucosa of domestic rabbits procured in the metropolitan area of New York. They are small and benign, and are situated mostly on the under side of the tongue. A filtrable virus can be extracted from them with which growths can be reproduced in the oral mucosa of several species of rabbits and hares but which fails to cause lesions when inoculated into other rabbit tissues and into the oral mucosa of other species. The virus differs notably from the Shope virus, which causes cutaneous papillomas in rabbits but proves innocuous to oral mucosa: rabbits solidly immune to the oral papilloma virus are fully susceptible to the Shope virus and vice versa. The oral papillomas are not highly contagious, for susceptible animals kept in individual cages in the same rooms with others carrying the growths, fed the same kind of food, and cared for by the same attendants, do not "catch" them. They are found much more frequently in the offspring of dams that carry the growths than in those of mothers free from them, and the causative virus can be recovered from the mouth washings of rabbits having no growths. The observations indicate that the virus may be spread by transfer from the mother to the young during the period of suckling, and that it may lie latent in the mouth, doing no harm unless the mucous membrane is injured. The slight trauma occurring now and then when coarse foods are chewed may furnish the required tissue nidus under natural conditions, for papillomas occasionally appear after virus has been dropped into the mouths of uninoculated rabbits; but the more extensive injury and healing resulting from experimental tattoo inoculations proves regularly effective in this respect. Tar can also act as an efficient adjuvant to the virus, the incidence of "spontaneous" oral papillomas being much higher in domestic rabbits that had had the opportunity to lick tar from their ears and paws during long periods than in normal control animals. The virus is recoverable in quantity from the oral papillomas of tarred domestic rabbits, and the findings indicate that it is their essential cause, the tar acting merely to prepare the tissue for the virus' action. For the same tar does not elicit oral papillomas in wild cottontail rabbits, which do not carry the causative virus though fully susceptible to it. The implications of the findings are discussed.

THE NEOPLASTIC POTENTIALITIES OF MOUSE EMBRYO TISSUES : II. CONTRIBUTORY EXPERIMENTS; RESULTS WITH THE SKIN OF C3H AND WEBSTER-SWISS EMBRYOS; GENERAL CONSIDERATIONS

Experiments were carried out to learn whether the widely differing liabilities to induced epidermal tumors of individual mice and rabbits are due to a previous localization out of the blood of an agent capable of undergoing change when the skin is exposed to carcinogenic influences, and of producing tumors in consequence. On the assumption that such an agent would localize in increased quantity where cutaneous inflammation exists, like various inert substances of large molecule and the epidermotrophic viruses when circulating, skin areas on adult and new-born animals were for some weeks kept inflamed, and months later, when the areas appeared normal, methylcholanthrene was applied to them and to control areas on the same or other individuals. No differences were observable in tumor incidence. These results led to attempts to test whether embryo epidermis is capable of undergoing neoplastic change, and the work of Paper I was done which showed that epidermal tumors arise with great rapidity and regularity from embryo skin transplanted to adults of homologous strain (C strain) together with methylcholanthrene. Webster-Swiss mice proved unsuited to experiments of the sort owing to heterogeneity of the breed, the transplanted embryo skin dying in most instances before the methylcholanthrene introduced with it could have been carcinogenic. The skin of C3H embryos also did badly, as if from incompatibility in some instances but mostly because its epidermal cells proliferated less vigorously than those of C embryos and did not tolerate methylcholanthrene nearly so well. Despite these difficulties, epidermal tumors were occasionally induced, as also in the transplanted skin of Webster-Swiss embryos, and the growths appeared quite soon, all things considered. The effect of methylcholanthrene on the skin of sucklings, their mothers, and young adult mice of the C strain was studied in order to find out whether the rapid rate of neoplastic change in the transplanted epidermis of embryos is indicative of some liability connected with its period of development. The skin of new-born animals proved very refractory to the carcinogen, hair coming in at the same rate as on control litters and no perceptible inflammation occurring for about 2 weeks, although within this period the mothers of the treated animals and the young adults became hairless where the methylcholanthrene had been put and their skin was much inflamed. Later on, as the applications were kept up, similar changes took place in the sucklings, but none of these developed tumors during some 6 weeks of observation whereas growths appeared within 3 weeks on more than half of the mother mice and on some of the young adults. The failure to produce tumors in the sucklings seems to have been due to cutaneous conditions preventing the necessary exposure of the deeper epidermal cells to methylcholanthrene. In any projected correlation of age differences with the response of cells to carcinogens allowance must be made for such factors. The present findings give no ground for the supposition that embryo skin has any special liability to neoplastic change. The results of transferring the tumors derived from embryo epidermis to new hosts have made plain that the neoplastic state not infrequently entails disabilities which are crucial, the tumor cells failing to succeed unless aided. This holds true of some carcinomas as well as of papillomas. By transplanting pieces of the organs of C embryos together with methylcholanthrene tumors of many sorts besides the epidermal have been obtained. As yet only those of the stomach have been worked with extensively. They can be elicited as quickly and regularly as those of the epidermis and can be as easily transplanted. The findings as a whole render it impossible to suppose that the neoplastic potentialities possessed by transplanted embryo tissues are due to the lodgement in them of tumor-producing viruses as specialized in their effects as those now known, or of precursor agents conferring neoplastic liabilities specialized to the same degree. Some other possibilities are mentioned. The rarity of neoplasms at birth is due to the circumstances of intrauterine life and to its brevity, not to any lack of capacity of the cells of the embryo to undergo neoplastic change.

CANCERS DERIVING FROM THE VIRUS PAPILLOMAS OF WILD RABBITS UNDER NATURAL CONDITIONS

The naturally occurring virus papillomas of western cottontail rabbits become malignant occasionally. The cancers derive from the papilloma cells, that is to say from elements already rendered neoplastic by the virus and still infected therewith. Papillomas produced with the virus in jack rabbits and snowshoe rabbits become cancerous in the same way but much more frequently, as is the case in domestic rabbits also. To all three species the virus is foreign. The character of the cancers of the wild rabbits is described and the relation of the virus to them discussed on the basis of experimental findings. The facts support the view that the cancers result from virus variation, this in many instances being but slight.

CELL STATE AS AFFECTING SUSCEPTIBILITY TO A VIRUS : ENHANCED EFFECTIVENESS OF THE RABBIT PAPILLOMA VIRUS ON HYPERPLASTIC EPIDERMIS

Rabbit skin can be rendered abnormally susceptible to papilloma virus infection by preliminary treatments with a variety of agents. The most effective agents thus far found are 0.3 per cent methylcholanthrene in benzene and a mixture in equal parts of turpentine and acetone, applied four or five times at 2 day intervals. When virus is inoculated into skin altered by these agents, either intradermally or by inunction after scarification, papillomas appear earlier and in greater number than on normal skin, and much higher dilutions give rise to growths. The method provides a means of detecting amounts of virus which cause no papillomas upon inoculation into normal skin. Papilloma virus material which is rubbed into scarified normal or hyperplastic skin is largely lost in the scabbing which ensues, and nearly all of it fails to reach susceptible cells. The preparatory agents which increase the effectiveness of the virus bring about marked epidermal hyperplasia, and the hyperplastic tissue regenerates with greater rapidity when scarified. The agents evidently act in large part by providing young epidermal cells in quantity to the virus, as also by inducing a richer vascularization than ordinary in support of the papillomatous proliferation. It is possible that they also act by providing especially susceptible cells. The implications of the findings are discussed.

Cancer prevention: from 1727 to milestones of the past 100 years

The rich, multidisciplinary history of cancer prevention recounted here begins with surgical and workplace recommendations of the 1700s and ends with 2009 results of the enormous (35,535 men) Selenium and Vitamin E [prostate] Cancer Prevention Trial (SELECT). This history comprises a fascinating array of chemopreventive, vaccine, surgical, and behavioral science research, both preclinical and clinical. Preclinical milestones of cancer prevention include the 1913 and 1916 mouse studies by Lathrop and Loeb of cancer development associated with pregnancy or cancer prevention through castration (oophorectomy), preventing chemically induced mouse carcinogenesis as early as 1929, energy restriction studies in the 1940s, the 1950s discoveries and later molecular characterizations of field cancerization and multistep carcinogenesis, and the effects of angiogenesis inhibition in genetically engineered mice reported in 2009. The extraordinary panoply of clinical research includes numerous large and smaller chemoprevention studies of nutritional supplements, other dietary approaches, a Bacillus Calmette-Guérin trial in 1976, molecular-targeted agents, and agents to prevent infection-related cancers such as hepatitis B virus vaccine to prevent liver cancer in 1984. Clinical surgical prevention includes removal of intraepithelial neoplasia detected by screening (including Pap testing developed in 1929 and culposcopy for cervical premalignancy and colonoscopy and polypectomy to prevent colorectal cancer begun in the 1960s) and prophylactic surgeries, such as in Lynch syndrome patients begun in 1977. Behavioral studies include smoking cessation and control beginning in the 1950s, obesity control rooted in studies of 1841, and genetic-counseling and cancer-survivorship studies. This history of pioneering events may help in better understanding who we are and what we want to achieve as cancer prevention researchers and practitioners.

Joint action of a chemical carcinogen and a neoplastic virus to induce cancer in rabbits; results of exposing epidermal cells to a carcinogenic hydrocarbon at time of infection with the Shope papilloma virus

Areas of rabbit skin previously rendered hyperplastic with turpentine were scarified, inoculated with the Shope papilloma virus, and covered with a dressing that contained 20-methylcholanthrene (MC) or 9:10-dimethyl-1:2-benzanthracene (9:10). The dressing was left on until healing had been well completed, a matter of 5 to 7 days. The papillomas which subsequently arose often appeared later, were fewer, and remained less vigorous than those due to the action of virus alone, but throughout several months they appeared to differ from these in no other ways. Then, more or less abruptly, the large majority became carcinomatous, frequently at several situations, whereas with few exceptions the control growths underwent no such alteration. The cancers were of the sorts ordinarily deriving, by secondary change, from epidermal cells infected with the virus. Collateral data have made plain that the hydrocarbons acted in their carcinogenic capacity to bring on the cancers. Indeed in control tests 9: 10 sometimes conferred latent neoplastic potentialities on uninoculated epidermis exposed to it while healing after scarification, a fact disclosed months later by painting these expanses with chloroform until hyperplasia occurred. Under the promoting influence of this agent papillomas formed which had the distinctive morphology of those induced by the chemical carcinogens. So strong and enduring were the effects of MC and 9:10 as to cause cancers to arise from many virus papillomas which were retrogressing after months of proliferation, that is to say under circumstances ordinarily unfavorable to malignant change. The facts justify the conclusion that the virus and the hydrocarbons acted jointly and in their carcinogenic capacities.

Experiments on the cause of the rabbit carcinomas derived from virus-induced papillomas. II. Loss by the Vx2 carcinoma of the power to immunize hosts against the papilloma virus

Tests were made to learn whether an anaplastic, epidermal carcinoma, the Vx2, which had originated more than 8 years previously from a virus papilloma in a domestic rabbit, still rendered its hosts immune to the virus. It had done so in the first 22 successive groups of animals to which it was transferred during a period of 3(1/2) years, its growth regularly eliciting a blood antibody that neutralized the Shope virus and fixed complement in mixture with it; and on the assumption that this would continue to be the case no further observations were made for nearly 4(1/2) years more. Then direct inoculation of animals carrying the tumor in its 46th Generation showed them to be as susceptible to the virus as normal rabbits; and sera procured from hosts of the 46th, 47th, 48th, and 50th Generations failed to neutralize the virus or fix complement with it. Tests of this last sort, repeated at intervals since,-most recently with sera from animals carrying the tumor in its 73rd Generation,-have yielded consistently negative findings. Loss of the power to immunize against the papilloma virus was not attended by any perceptible change in the Vx2 carcinoma. Manifestly the antigen responsible for the immunity cannot, as such, have been the actuating cause of the tumor. Attempts were made to infect the cells providing 48th Generation cancers, by mixing them with a suspension of the papilloma virus at time of implantation, or by injecting this agent into the blood stream of rabbits in which the tumog had already begun to proliferate. Its morphology and rate of growth remained unaltered; but tests of the animals to which transfers were next made yielded what appeared to be evidence of some slight immunity to the virus.

FURTHER EXPERIMENTS ON THE CAUSE OF SEQUENTIAL NEOPLASTIC CHANGES. THE EFFECTS OF 20-METHYLCHOLANTHRENE ON TRANSPLANTED EPIDERMAL MOUSE PAPILLOMAS AND THE DERIVATIVE CARCINOMAS

When crystalline 20-methylcholanthrene (MC) and the cells of tar-induced mouse papillomas (paps.) are injected together into the thigh muscles of mice the carcinogen exerts a marked promoting and chemotactic influence upon the cells while it is dissolving in the tissue fluid. Under such circumstances it strongly stimulates and attracts them, with result they surround and include the scattered crystals in small cysts that later coalesce to form a larger one from which the MC only very gradually escapes. Because of these findings intramuscular tests were made to learn whether MC would hasten the occurrence or increase the number of cancers that now and again derive from paps.; but the tests were repeatedly marred by the extraordinary behavior of such cancerous cells as happened to be already present in the implanted material. They responded far more actively to MC than did the pap. cells and soon took over the growths. Some carcinomas which failed to grow when transplanted alone, or only gradually formed small, regressing nodules, gave rise rapidly to huge growths of similar sort when exposed to MC. To exclude cancerous cells so far as possible from the later tests small grafts of pap. tissue with MC crystals adhering to them were implanted subcutaneously. The pap. cells promptly lined the graft pockets, encysting the crystals incidentally, and formed tumors that enlarged progressively by keratinizing inwards. While they did this their living layer of pap. tissue was continually bathed in dissolved MC throughout many weeks. Despite these apparently favorable conditions the carcinogen neither hastened the occurrence nor increased the number of visible epidermal cancers deriving from the paps. It also failed to bring about sequential malignant changes in the carcinomas. These negative results accord with those already obtained through long exposure of the benign pulmonary adenomas of mice to urethane or methylcholanthrene, agents which rapidly induce these benign tumors yet which were found to be incapable of furthering the cancerous changes to which such growths are prone. They accord also with another previous finding, namely that MC fails to bring on the malignant changes of discontinuous, sequential sort that mammary mouse carcinomas often undergo "spontaneously." Taken together these facts indicate that the change or changes whereby normal cells are converted into benign tumor cells differ in nature from those taking place when they become cancer cells, as also from those occurring when cancer cells undergo further, step-like, malignant changes. A study has been begun to learn whether the widely various carcinomas deriving from benign papillomas differ from these latter and from one and other in their chromosomal content.