A VIRUS-INDUCED MAMMALIAN GROWTH WITH THE CHARACTERS OF A TUMOR (THE SHOPE RABBIT PAPILLOMA) : III. FURTHER CHARACTERS OF THE GROWTH: GENERAL DISCUSSION

Cottontail Rabbit Papillomavirus (CRPV) Related Animal Models for Head and Neck Cancer Research: A Comprehensive Review of the Literature

Having suitable animal models is crucial to mimic human disease states and for the successful transfer of experimental data into clinical practice. In the field of papillomavirus research, the domestic rabbit (Oryctolagus cuniculus) has served as an indispensable model organism for almost 100 years. The identification and characterization of the first papillomaviruses in rabbits, their carcinogenic potential and their immunogenicity have contributed significantly to the state of knowledge on the genetics and life cycle of papillomaviruses in general, as well as the development of antiviral strategies such as vaccination procedures. Due to the high species specificity of papillomaviruses, only rabbit papillomaviruses (RPVs) can be used for animal studies on papilloma-based tumor diseases in the rabbit. The major focus of this article is on cottontail rabbit papillomavirus (CRPV)-related rabbit squamous cell carcinoma (RSCC). A brief history outlines the discovery and generation of experimentally used RSCC tumors. A comprehensive overview of the current CRPV-associated VX2 carcinoma-based tumor models with a major focus on human head and neck squamous cell carcinoma (HNSCC) tumor models is provided, and their strengths in terms of transferability to human HNSCC are discussed.

High-Risk Human Papillomavirus and Tobacco Smoke Interactions in Epithelial Carcinogenesis

Cervical, anogenital, and some head and neck cancers (HNC) are etiologically associated with high-risk human papillomavirus (HR-HPV) infection, even though additional cofactors are necessary. Epidemiological studies have established that tobacco smoke (TS) is a cofactor for cervical carcinogenesis because women who smoke are more susceptible to cervical cancer when compared to non-smokers. Even though such a relationship has not been established in HPV-related HNC, a group of HPV positive patients with this malignancy are smokers. TS is a complex mixture of more than 4500 chemical compounds and approximately 60 of them show oncogenic properties such as benzo[α]pyrene (BaP) and nitrosamines, among others. Some of these compounds have been evaluated for carcinogenesis through experimental settings in collaboration with HR-HPV. Here, we conducted a comprehensive review of the suggested molecular mechanisms involved in cooperation with both HR-HPV and TS for epithelial carcinogenesis. Furthermore, we propose interaction models in which TS collaborates with HR-HPV to promote epithelial cancer initiation, promotion, and progression. More studies are warranted to clarify interactions between oncogenic viruses and chemical or physical environmental factors for epithelial carcinogenesis.

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.

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

A considerable variety of tumors, both benign and malignant, result from the localization of the rabbit papilloma virus in skin which has been prepared by repeated tarrings. They appear only in individuals highly susceptible to the action of the virus, and are more likely to be engendered by highly pathogenic inocula. No evidence has been found that differences in the potentialities of the virus entities are responsible for the diversity of the growths. This is referable to changes in the epidermal cells; and much more preliminary tarring is required to produce these changes than suffices to cause localization of the virus out of the blood stream with a resulting papillomatosis of the ordinary sort. The character of the individual anomalous tumors depends in some degree upon the extent of the preparatory changes in the cells, malignant growths being more frequent when the epidermis has been tarred for a relatively long period. All are focal or punctate in origin, and they exhibit their peculiar characters from the first, none being due to secondary alterations in ordinary papillomas. Tarring after the virus has localized in the epidermis does not significantly increase their number. They are the outcome of the state of the cells at the time of virus infection. Tarring exerts important influences in addition to changing the cells in such a way that unusual tumors result from the action of the virus. The procedure is notably effective in determining localization of the virus out of the blood stream; enables it to produce growths when otherwise it would not do so though present in the tarred skin; stimulates the proliferation of the tumors engendered; makes them disorderly and aggressive; and hastens the anaplasia of such of them as are malignant. It has similar effects upon the tar tumors, as will be demonstrated in a subsequent paper.

THE PROGRESSION TO CARCINOMA OF VIRUS-INDUCED RABBIT PAPILLOMAS (SHOPE)

The papillomas induced in domestic rabbits with virus procured from cottontails undergo progressive changes in the direction of malignancy when they grow vigorously. From the beginning they exhibit the traits whereby tumors are characterized, and they have malignant potentialities. In seven animals of a group of ten carrying papillomas for more than 200 days, cancer has developed, and in an eighth a tumor of problematic malignancy has arisen. One of the remaining two rabbits died early in the cancer period, and the papillomas of the other eventually retrogressed. Ten cottontails with induced growths of much longer duration have not developed cancer. The malignant tumors have all been acanthomatous in type, and have arisen directly from the papillomas by graded, continuous alterations. These have often gone further after malignancy has been attained, and have eventuated in great anaplasia. Metastasis has been frequent, and transplantation to another host has proved successful. Individual growths have occurred expressive of each stage of the transformation to cancer, as if through a stabilization at this stage; yet despite the variety thus afforded, the tumors must all be looked upon as the consequence of alterations in cells of a single sort, namely epidermal cells affected by the virus, and the alterations themselves have taken a single direction. In the morphology of many of the cancers the influence of the virus is still manifest. The better the papilloma grew, the more likely was cancer to occur, and the greater was the tendency to multiple tumors. In the most favorable rabbits malignant changes took place at numerous locations in the papillomatous tissue, and were imminent at many others. Intercurrent factors had much to do with determining frank carcinosis; and when the tendency to it was not marked their influence sometimes seemed crucial. Analogous instances of a graded alteration from papilloma to cancer are frequent in human pathology. The virus that gives rise to the rabbit papillomas must be looked upon as the primary cause of the cancers developing therefrom. Whether it is their proximate cause has yet to be determined.

SEROLOGICAL REACTIONS WITH A VIRUS CAUSING RABBIT PAPILLOMAS WHICH BECOME CANCEROUS : II. TESTS OF THE BLOOD OF ANIMALS CARRYING VARIOUS EPITHELIAL TUMORS

The serum of a rabbit with large cancers resulting from the transplantation of a squamous cell carcinoma that had arisen from a virusinduced papilloma, possessed the power to neutralize the virus, and so too in less degree did that of an animal of the same transplantation series in which a small nodule had developed. The sera of rabbits carrying tar papillomas or the Brown-Pearce carcinoma proved wholly devoid of effect on the virus. The implantation of Brown-Pearce tumor material mixed with virus did not lead to an enduring establishment of the latter in the resulting growths, nor to any immediate changes in their morphological character. The significance of the facts is discussed.

Multifactorial etiology of cervical cancer: a hypothesis

Cancer of the cervix is the second most common life-threatening cancer among women worldwide, with incidence rates ranging from 4.8 per 100,000 women per year in the Middle East to 44.3 per 100,000 in East Africa. Epidemiologic and clinical data demonstrate that human papillomaviruses (HPV), especially HPV-16 and HPV-18, play at least a major if not a necessary role in the etiology of cervical cancer. However, many investigators acknowledge that HPV is not sufficient to induce cervical cancer and that a multifactorial etiology is likely. HPV can be found in a growing proportion of patients with cervical cancer, approaching 100%, but is not yet found in every patient with disease. Other factors, such as herpes simplex virus type 2 infections, cigarette smoking, vaginal douching, nutrition, and use of oral contraceptives, have been proposed as contributing factors. In the first half of the 20th century, Peyton Rous and colleagues demonstrated the joint action of tars and Shope papillomavirus to consistently induce squamous cell carcinomas in rabbits. Using the Rous model as a prototype, one might hypothesize that some cases of cervical cancer arise from an interaction between oncogenic viruses and cervical tar exposures. Cervical tar exposures include cigarette smoking, use of tar-based vaginal douches, and long years of inhaling smoke from wood- and coal-burning stoves in poorly ventilated kitchens.

Viruses, chemicals and co-carcinogenesis

The etiology of cancers appears to be complex and multifactorial. Peyton Rous and others demonstrated the process of co-carcinogenesis by exposing rabbits to a virus and tars. Epidemiologists have proposed virus-chemical interactions to cause several cancers. For example, one might propose that the etiology of cervical cancer results from a complex interplay between oncogenic viruses and cervical tar exposures through tar-based vaginal douching, cigarette smoking, and/or long-term cooking over wood-burning stoves in poorly ventilated kitchens. Hepatocellular carcinoma may result from the joint effects of viruses and hepatotoxic chemical carcinogens. Kaposi's sarcoma might happen following reciprocal actions of human herpes virus-8 infection, immunosuppression, and chemical exposures, such as nitrite radicals and alumino-silicates. Use of Koch's postulates will not help one prove or disprove a multifactorial causation of disease; new criteria are needed. Delineating the web of causation may lead to additional strategies for prevention and treatment of several cancers.

Fatal keratomas due to deep homografts of the benign papillomas of tarred mouse skin; normal proclivities and neoplastic disabilities as determinants of tumor course

Six out of eight epidermal papillomas, induced with tar in mice of homogeneous strain, have grown after transfer to the subcutaneous tissue of sucklings and weanlings. Five of them have been thus maintained for nearly or quite a year and a half, and in seven to nine successive groups of mice. The tumor studied longest has been kept going in five parallel lines since its primary implantation. The papillomas have all grown progressively in most instances, and proved fatal. None has altered except through the occurrence of derivative cancers, but these have arisen so often as only to be excluded on transfer by a rigorous selection of grafts. Histologically the papillomas have been of a single, completely unaggressive kind, yet transfer has disclosed great differences in their abilities. The tumors they form are of unique sorts. The cells of some are able-bodied (Type A), capable of spreading along bare connective tissue and keratinizing like normal, reparative epidermis. They line graft pockets, differentiate into the free space these provide, and form cysts densely packed with keratin. The papilloma is thus turned outside in. The cysts become huge as keratin accumulates in them, and eventually they rupture with result either in subcutaneous dissecting cysts or keratinizing surface growths that are often prodigious in size and fantastic in shape, but sometimes are completely like the cutaneous papillomas ordinarily induced by carcinogens, and tend, when small, to regress or come away as these frequently do. One growth of Type A was placed in the peritoneal cavity or in the liver, spleen or lung, and at all these situations it formed introverted cysts resembling the subcutaneous. The cells of other papillomas are more or less crippled (Type C). In extreme instances they are unable to spread laterally, and produce relatively little keratin. They fail to line graft pockets, but their keratin inflames the exposed connective tissue, extravasation ensues, and a continually enlarging, fluid-filled cyst forms, with walls that are bare except where a stalked or cauliflower papilloma exists, projecting inwards. At last the cyst ruptures and a second dissecting cyst forms, also devoid of papilloma tissue; or else the overlying skin undergoes pressure necrosis, the cyst fluid escapes through a rent, and fatal infection ensues. All gradations exist between Type A and Type C. The cancers derivative from both exhibit a marked disability,-though invasive they are almost or quite unable to extend along bare connective tissue. The papillomas that are possessed of this faculty spread beyond them along the cyst wall, and kill the host through their unceasing activity. In collateral work a papilloma was transplanted that was found protruding from the external auditory canal of a mouse which had received an intramuscular injection of methylcholanthrene many months previously. The tumor is now in its 5th generation, after 15 months. The growths it forms are of Type A. All of the papillomas are functioning tumors, with their own cells as the functioning product. Their papilliferous shape, when on the skin, is due solely to inability of their cells to gain space in other ways. Intrinsically they are keratomas. The papillomas do well after transfer to deep situations because the growth of their cells is indirectly promoted, through favoring local conditions. No direct promotion takes place like that when the cells of prostatic and mammary tumors are stimulated to multiply by hormones. Doubtless many agents act in both ways, that is to say by dual promotion.