THE NEOPLASTIC POTENTIALITIES OF MOUSE EMBRYO TISSUES : I. THE FINDINGS WITH SKIN OF C STRAIN EMBRYOS TRANSPLANTED TO ADULT ANIMALS

Ricerche Sull'Azione Promovente della Follicolina nel Duplice Meccanismo della Carcinogenesi

Studies were undertaken on an eventual initiating or promoting action of follicular hormone in the twofold mechanism of carcinogenesis on the mouse skin. Animals were grouped into two series. The first series was divided into two groups: one was painted with a folliculin oleous solution (Estrene 5) every 3 days, the other with a 5 per cent dilution of croton oil in estrene. The mice of the second series, previously submitted to three paintings with 3:4-benzpyrene, were divided into four groups: 1) control group, 2) painted with Estrene every 3 days, 3) treated with 5 per cent croton oil solution in olive oil, and 4) treated with 5 per cent dilution of croton oil in oleous solution of follicular hormone. Results show that the follicular hormone, either alone or combined with croton oil, fails to produce any initiating and promoting action when painted on mouse skin previously treated with sub-maximal doses of 3:4-benzpyrene. Folliculin, on the contrary, enhances considerably the promoting action of croton oil when painted with it on the skin sensitized with benzpyrene: a rather early appearance of epithelial projections without basal membrane and composed of atypical elements is seen, whereas in animals of the same series, long treated with paintings of croton oil diluted with olive oil or with oleous folliculin this picture is not observed.

THE NEOPLASTIC POTENTIALITIES OF MOUSE EMBRYO TISSUES : III. THE TUMORS ELICITED FROM GASTRIC EPITHELIUM

Epithelial tumors have been readily obtained by the implantation of embryo stomach tissue together with olive oil containing methylcholanthrene (with or without Scharlach R) in adult mice of homologous strain. The implanted tissue from the squamous portion of the stomach rapidly encysted the oil, and benign and malignant papillomas and squamous cell carcinomas soon arose from the stratified squamous lining of the cysts. Bits of the glandular portion of the stomach also formed cysts, but the gland cells underwent metaplasia in response to the carcinogen, altering first to transitional epithelium and then to a stratified squamous layer. So swiftly did these changes take place that nearly all of the tumors took origin from epithelium that had already become stratified and squamous, and the growths themselves were of this type. A single transitional cell carcinoma and an adenoacanthoma were procured, but no adenocarcinomas; nor did any benign papillomas develop, though they often resulted from the action of methylcholanthrene on the squamous portion of the embryo stomach. Search failed to disclose any distinctive precancerous changes in the gastric tissue. Five of the cancers were transplanted and they grew in every host. No tumors arose from any of the numerous control implants. Those consisting of glandular tissue formed cysts lined partly with columnar epithelium secreting mucus and partly with tubular glands equipped with chief and parietal cells in good condition. Pepsin and rennin were found in the fluid contained in these cysts, but no free hydrochloric acid. The enzymes were present also when the cysts contained methylcholanthrene and the glands had not yet been wholly replaced by metaplastic epithelium. The tumors appeared months sooner than when methylcholanthrene is injected into the stomach of adult animals or given by mouth; some of them were well established after 5 or 6 weeks. They arose regularly when the requisite experimental conditions were provided. The utilization of transplanted embryo tissue provides a means whereby gastric tumors free from bacterial infection can be procured swiftly and easily.

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.

The neoplastic potentialities of mouse embryo tissues; lung adenomas in baby mice as result of prenatal exposure to urethane

The observation that adenomas develop very rapidly in the pulmonary tissue of mouse embryos implanted together with methylcholanthrene, in adult animals, has led to tests of the neoplastic potentialities of this tissue in utero. C strain females in the latter half of pregnancy were injected with urethane and the lungs of their young were searched for adenomas. None could be perceived with certainty in embryos at term or in mice just born, but they were several times found 3 days after birth and they were frequent and much larger in 10-day-old animals. The controls showed none. After 60 to 70 days they were often visible in the gross. Corroboratory findings were obtained in A mice. No parallelism could be perceived in the incidence of the tumors in mothers and offspring. The adenomas arose from tissue devoid of any sign of preliminary local disturbance. Mitoses were abundant in them and they grew rapidly for a while, but within 2 months cell division had almost ceased. By this time however many of the neoplasms were as big as any adenomas in the urethanized mother animals and in some instances twice as big. While growing fast they underwent little differentiation, but this took place when proliferation slowed and in consequence the tumors came to have the morphology of the spontaneous and induced adenomas of adults. The neoplastic cells were derived from alveolar elements, yet in proportion as differentiation of them occurred they came to resemble the epithelial cells lining the small bronchioles. Occasionally the resemblance to bronchial epithelium was complete, save that the cytoplasm of the tumor cells was slightly basophilic. The following conclusions seem justified:— 1. The injection of urethane into pregnant female mice of the C strain frequently initiates the development forthwith of pulmonary adenomas in the young she is carrying. 2. Some of the pulmonary cells of mouse embryos well along toward term possess the ability to be neoplastic. 3. The adenomatous change finds swift expression in young creatures because of conditions implicit in their youth. The rapid proliferation of the tumor cells is almost entirely due to these conditions, not to the neoplastic state as such. 4. Adenomatous change prior to birth is intrinsically the same process as that occurring in the adult creature. 5. The adenomatous state does not prevent the cells of young mice from undergoing the maturation that takes place in normal elements of the same sort as the organism grows older. Though the proliferative activity natural to youth and the unnatural activity consequent on neoplastic change are summated in the young organism, they still are separable.

The neoplastic potentialities of mouse embryo tissues; the tumors elicited with methylcholanthrene from pulmonary epithelium

The lung tissue of mouse embryos of the C strain proliferates to some extent after implantation in adult hosts of the same breed and rapidly differentiates, forming a parenchyma remarkably like the normal. The grafts persist long. When methylcholanthrene dissolved in olive oil has been introduced with them much more growth of them occurs. The carcinogen induces a pronounced metaplasia of the epithelium of the bronchial tree, and the altered cells are often aggressive, multiplying, invading, and largely replacing the parenchyma about them. So closely do they resemble malignant elements in aspect and behavior that it is frequently difficult to tell whether carcinomatous change is not actually present. Genuine tumors soon arise, multiple benign adenomas sometimes appearing within 3 weeks, and indubitable carcinomas a few weeks later. Not a few of the cancers are of transitional cell type, that is to say are expressive of an intermediate stage in the metaplasia. Under the influence of methylcholanthrene the cells lining the alveolar spaces of the graft sometimes undergo metaplasia also, with result in stratified squamous epithelium. It follows that there is reason to doubt the current assumption that all squamous cell carcinomas of the lung necessarily arise from the bronchial tree. The findings, taken with others previously reported, make it difficult to suppose, furthermore, that the generality of lung tumors can be due to neoplastic viruses entering the organism in postnatal life and having no broader scope than those thus far discovered.

Experiments on the cause of the rabbit carcinomas derived from virus-induced papillomas. I. Propagation of several of the cancers in sucklings, with etiological tests

Three out of four carcinomas arising from rims-induced, rabbit papillomas have grown well after transplantation to sucklings. Two were propagated serially, and it seems likely that all could have been maintained indefinitely had litters been available of newborn animals of the sort in which they arose. These successes are the more worthy of note because of the well-nigh uniform failure of similar growths on transfer to adults. The tumors enlarged with great rapidity in the sucklings, were extraordinarily destructive, and two of them metastaslzed within a few weeks. Many efforts were made to extract causative agents from the three carcinomas, on the assumption that these might be due to variants of the Shope virus. Highly favorable conditions for the demonstration of this latter were provided in the tests; yet their outcome was wholly negative although all of the cancers derived from papillomas caused by "recoverable" strains of virus, and although one of them appeared to be consequent upon only the slightest of alterations toward malignancy on the part of the papilloma from which it came. Extracts of another of the cancers, an anaplastic, squamous-cell carcinoma devoid of any morphological sign of the influence of the Shope virus, yielded typical virus papillomas on several occasions. The wholly negative results with the third cancer must be considered in the light of the fact that the "recoverable" strain of virus causing the papilloma from which it originated could no longer be recovered from such growths on collateral test.

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.

Studies of the mechanism of action of urethane in initiating pulmonary adenomas in mice. II. Its relation to nucleic acid synthesis

The process of carcinogenesis following exposure of mice to urethane is demonstrated in the present work to be intimately related to nucleic acid synthesis. Injection of animals with a DNA hydrolysate immediately prior to a single exposure of the animals to urethane markedly reduced the number of pulmonary adenomas initiated. Aminopterin, known to interfere in nucleic acid synthesis (46), potentiated the carcinogenic action of urethane and this potentiation was blocked by injection of a DNA hydrolysate. Of the components and precursors of nucleic acids the pyrimidine series seemed especially concerned. Alterations in the utilization of oxaloacetate, ureidosuccinic acid, dihydro-orotic acid, orotic acid, cytidylic acid, and thymine appeared to be critical steps in the oncogenic process, following upon the primary disorder of cellular metabolism initiated by the carcinogen. All these substances except oxaloacetate profoundly reduced the number of tumors initiated by urethane. Oxaloacetate potentiated the carcinogenic effect. When these results are viewed together and in relation to known facts concerning nucleic acid synthesis they provide evidence suggesting that the point of action of the carcinogen is in the pathway of nucleic acid synthesis below orotic acid and perhaps at the level of ureidosuccinic acid. The potentiating influence of adenine, 4-amino-5-imidazole carboxamide, and aminopterin, the lack of effect of uracil, and the inhibitory influence of thymine together suggest that DNA rather than RNA is the nucleic acid critical to the oncogenic response of mice to urethane.

Are carcinogens responsible for the superimposed neoplastic changes occurring in mouse tumor cells? The effect of methylcholanthrene and urethane on pulmonary adenomas and of methylcholanthrene on mammary carcinomas

Three spontaneous pulmonary adenomas of C mice, morphologically resembling those induced by methylcholanthrene or urethane, were propagated in host after host under conditions such that the neoplastic cells were directly exposed, while proliferating, to one or the other of these agents. The successive periods of test lasted for more than a year in some instances, the total exposure to the carcinogens far exceeding that required to change normal pulmonary cells into adenoma cells. One of the adenomas remained unaltered, and the others underwent cancerous changes; but these took place with equal frequency in the control growths, and their occurrence was neither hastened nor delayed by the carcinogens. Two polymorphous mammary carcinomas of "milk-factor" type, with the characteristic tendency to form acini and tubules, were exposed to methylcholanthrene in the same way as the pulmonary adenomas and for periods quite as long. Their cells continued to differentiate, and in other respects underwent no significant change. Urethane had no influence on the rate of growth of the adenomas exposed to it; methylcholanthrene, on the other hand, markedly retarded the enlargement both of them and of the mammary tumors. Its inhibitory influence was not passed on from cell to cell however; when freed of the carcinogen by further transplantation, the retarded tumors grew as fast as the controls. Furthermore the retardation caused no evident delay in the occurrence of cancerous changes in the adenomas. One of the adenomas was maintained in twelve parallel lines while under test and new tumors arose in nine of them, the earliest appearing more than fifteen months after initial transfer of the growth. Always it was an adenoma solidum, this appearing almost concurrently in eight of the nine lines. In six of them it was soon followed by carcinomas, the sequence of events and the morphological findings both indicating that they had derived from it. Individually the cancers were widely various, but they were similar on the whole from line to line. Carcinomas of a wholly different aspect arose from the other adenoma undergoing cancerous change, and they were not preceded by adenoma solidum. In both instances the character of the superimposed neoplastic alterations seemed to have been determined by some inherent trait of the adenoma concerned.

SUPPRESSION OF GROWTH OF BROWN-PEARCE TUMOR CELLS BY A SPECIFIC ANTIBODY : WITH A CONSIDERATION OF THE NATURE OF THE REACTING CELL CONSTITUENT

Experiments are reported in detail which show that an antibody which appears in the blood of certain rabbits implanted with the Brown-Pearce tumor or injected with cell-free extracts of it is capable of suppressing the growth of the tumor cells under a variety of experimental conditions, the effects of the antibody being wholly distinct from those of unknown factors that frequently bring about regression of the growth. The implications of the findings are discussed with particular reference to facts indicating that the distinctive cell constituent with which the antibody reacts may play a significant part in the proliferative activities of the Brown-Pearce tumor cell.