EDITORIAL EXPERIMENTAL SKIN CANCER

Transcription

1 EDITORIAL EXPERIMENTAL SKIN CANCER THE experimental study of skin cancer is of historical interest in that it was the first type of malignant turnout to be induced by a pure synthetic chemical compound. The phases of the story are well known: the recognition by Percival Pott in 1775 of the high incidence of scrotal cancer in chimney-sweeps, followed by other occupational evidence of an association of skin cancer with tar, soot, and certain mineral oils; the interval of 14o years which elapsed before Yamagiwa and Itchikawa succeeded in demonstrating the carcinogenicity of tar for rabbits and, later, mice ; and the intensive study of tar itself and the turnouts which it produced which followed during the next fifteen years, before the team of research workers led by Kennaway at the Cancer Hospital, London, isolated the effective substance 3 : 4-benzpyrene (BP). Fortunately for medical history, Sir Ernest Kennaway (I955) put on record shortly before his death an account of the steps in reasoning and patient research by which the last phase was achieved. In the course of this work it was found that many, but not all, polycyclic hydrocarbons of a similar type of chemical structure are carcinogenic. Thus I : 2 : 5 : 6-dibenzanthracene (DBA), the first chemical carcinogen discovered, is not present in tar, nor are two other hydrocarbons--methylcholanthrene (MC) and 9 : Io-dimethyl-i : 2-benzanthracene (DMB)--which are more potent than BP or DBA, and have been more extensively used in recent research. Skin cancer can be induced by radiations of various kinds, e.g., ultraviolet light, X-rays, which are also known to constitute a hazard to man. There is also the virus-induced Shope papilloma of the rabbit, which frequently becomes malignant in the tame rabbit, but which does not have any demonstrable relevance to tumours of other species, including man. There are also a few chemical skin-carcinogens unrelated to the benzauthracene group. But the most reliable and controllable results are obtained by the use of the benzanthracene derivatives, and it is therefore not surprising that they have been used in the majority of investigations of the mechanism of carcinogenesis. Some of these hydrocarbons when applied to the skin can give rise to tumours in remote organs, e.g., MC gives a high yield of mammary cancer in mice, and DMB of mammary cancer in mice and rats and ovarian tumours in mice, but their carcinogenic effects as far as the skin is concerned are almost entirely local. Although a few tumours can be obtained by a single relatively large dose, it was at first thought that to obtain a good yield of tumours it was necessary to make successive applications over a period of some weeks. This statement has had to be modified since the discovery of the so-called co-carcinogens. The first discovered and most widely studied co-carcinogen is croton oil (Berenblum and Shubik, I947). After a single application of DMB, tumours can be elicited by continuing with weekly applications of croton oil. The interval between the DMB application and the beginning of the croton-oil regime does not affect the incidence of tumours. It has become customary to refer to the DMB as the initiating agent, and the croton oil as the promoting agent : this is the basis of the two-stage theory of the mechanism of carcinogenesis. 4 A z57

2 258 BRITISH JOURNAL OF PLASTIC SURGERY Various other promoting agents have been described since (see review by Salaman, i958), including a number of synthetic detergents, emulsifying, lipophilichydrophilic surface-tension lowering agents, of which the Tweens have attracted most interest (Setiil/i et al., 1957). Shubik (1958) has recently shown, however, that Tween 6o is itself a weak carcinogen, and this has raised the question as to whether the same applies to all co-carcinogens. All initiating agents are not overfly carcinogens. Salaman and Roe (1953) showed that urethane could act as an initiator or " incomplete carcinogen." Other such substances have since been found (Salaman, 1958). The situation is still further complicated by the demonstration that urethane is also an initiator for skin carcinogenesis when administered orally or parenterally (Berenblum and Haran-Ghera, 1957). This is a most surprising finding, having regard to the high solubility and diffusibihty of urethane, and its presumably rapid elimination. The two-stage theory of epidermal carcinogenesis originally put forward by Berenblum and Shubik, and developed by Berenblum (I954), postulated that the action of the initiator was to alter a minority of the epidermal cells to " latent " or " dormant" cancer cells, and that it was on these dormant cells that the promoting agent (co-carcinogen) subsequently acted to develop definitive tumour cells. The idea that promoting action is dependent simply on hyperplasia has to be rejected because most hyperplasia-producing irritants are not co-carcinogens. But there are also formidable objections to the dormant-cell idea, in the sense of cells which are immediately and irreversibly committed to this state after a single contact with the initiator. Not the least of these is the observation, made in the original experiments of Berenblum and Shubik, that if promoting treatment is delayed for several months the ultimate incidence of tumours is not reduced. This would imply that either the dormant cells do not take part in the normal process of surface desquamation, or that their rate of reparative proliferation is identical with that of the normal cells in spite of their altered constitution. Salaman (1958) has marshalled some of the other difficulties of the latent tumour-ceu theory in its original form. As a carcinoma consists essentially of neoplastic epithelial cells, it is natural that many investigators should have concentrated on finding a way in which the carcinogen might attack the epithelial cell directly and alter its growth qualities. But the epithelium is dependent for its metabolic requirements on the underlying stroma, and it is conceivable that modification of the latter might affect the epithelium which it maintains. It is therefore necessary to take into account the possibility of an indirect mechanism of this sort. A single application of a sufficiently concentrated solution of DMB results in complete necrosis of the epidermis and its epithelial appendages, the hair follicles and sebaceous glands (Orr, 1955). When regeneration occurs the hyperplasia is in no way different morphologically from that seen with smaller doses of DMB or with other carcinogens, although it depends on proliferation and ingrowth of cells outside the treated area of skin which are unlikely to have come into direct contact with DMB in significant amount. It is a reasonable inference from this that the primary action of all carcinogens on the epithelial cell may be one of damage, and the hyperplasia seen histologically the result of reparative regeneration rather than direct stimulation to growth. In mice which have received a dose of DMB sufficient to destroy the epithelium completely, the repair process takes a period of three to four weeks, by which time the epidermal hyperplasia has subsided,

3 EDITORIAL: EXPERIMENTAL SKIN CANCER 259 and hair follicles and sebaceous glands have been regenerated by differentiation de novo from the ingrowing superficial epidermis. If such mice are then treated with croton oil, tumours will develop in the usual way. In an experiment of this sort it seems almost certain that none of the epithelial cells can have come in contact with the amount of carcinogen necessary to obtain a reasonable yield of tumours by ordinary techniques. There is no doubt that changes occur in the dermis and subcutis during the induction period in experimental carcinogenesis (Orr, I938). There is a characteristic alteration in the fibrous texture of the collagen and elastic tissue, the progress of which varies in speed with the potency of the carcinogen used. This may not necessarily mean, of course, that these changes are material to the process of carcinogenesis as such, but they are different in quality to the stromal changes produced by non-carcinogenic skin irritants. There are indications, moreover, that the rate of carcinogenesis can be accelerated by various experimental procedures in which damage and repair of the subepidermal tissues is involved. Orr (I934, I935) found that tumours appeared more rapidly when linen threads were introduced subcutaneously for a short time beneath the surface which was subsequently to be treated with carcinogens; Linell (I947) showed that deep trauma accelerated carcinogenesis, while superficial trauma did not; the well-known experiments of Friedewald and Rous (I944), in which tumours appeared in relation to the healing of holes punched in carcinogen-treated rabbits' ears are relevant ; Salaman and Glendenning (I957) have found that intradermal injections of sclerosing agents have a promoting action. When thin split-skin grafts, or pure epidermis, are transplanted from a skin site which has received adequate carcinogenic treatment to an untreated site on the opposite side of the same mouse, tumours never develop in the grafts (Billingham, Orr, and Woodhouse, I95I). If such grafts are subsequently given co-carcinogenic treatment with croton oil, they are still resistant to carcinogenesis (Marchant and Orr, I953). On the other hand, tumours appear in the usual incidence at the original carcinogen-treated site, whether resurfaced with a graft of untreated skin or allowed to resurface itself by natural processes. Full-thickness grafts, or thick split-skin grafts, of carcinogen-treated skin proceed to tumour formation in their new untreated site. These experiments would seem to establish that the epidermis of a potential cancer site will not develop tumours if moved by itself to another body site. The fact that turnouts still occur at the site from which it has been removed is not so easily interpreted, in view of the fact that the roots of the hair follicles are left behind. As mentioned previously, there is no doubt that hair follicles can in certain circumstances re-differentiate in the mouse from superficial epidermis, but in the experiments just mentioned the proximal parts of the transected hair follicles would probably have been in contact with carcinogen and might have contributed to the resurfacing of the wound. King and Orr (I958) approached the problem in another way. Paraffin-wax pellets containing MC were inserted subcutaneously into the flanks of mice and removed a few weeks later. Split-skin grafts of ear or tail skin were inserted into the sacs which had been occupied by the pellets. Squamous carcinoma arose in many of these grafts rather more quickly than in an ordinary painting experiment. Spectrophotometric analysis of the tissue surrounding the pellets showed that it contained only io to 2o per cent. of the amount of MC necessary to obtain a

4 260 BRITISH JOURNAL OF PLASTIC SURGERY significant yield of tumours by painting the skin. This small amount of MC disappears from the pellet site in one to four days. It is difficult to avoid the interpretation that the pre-treatment of the recipient stroma was the major element in the effect of MC, and that the subsequent appearance of cancer in the graft was determined by the altered stromal environment. Gliicksmann (1958) is satisfied that a mechanism of this sort is operative in the case of irradiation carcinogenesis. With the use of/3-radiation it is possible to define very precisely the extent of the treated area, which undergoes necrosis followed by sloughing. The unstable repair tissue is again sloughed off several times, and when carcinoma eventuates, the cells it arises in are the descendants of cells which have never themselves been exposed to radiation. The induction of cancer by radiation takes three to four times as long as that by chemical carcinogens, and Gliicksmalm would seem to attribute this to the early hyperplasia induced by the latter. In the present writer's view, it may well be that this early hyperplasia is of a non-specific reparative nature, bearing little or no relation to the later neoplastic change which is determined in the epithelium by environmental conditions in both irradiation and chemical carcinogenesis. One of the classical properties of a malignant tumour is its increased rate of growth as compared with the corresponding normal tissue. The statement is true only in the limited sense that the comparison is made with resting normal tissues. Many normal tissues can grow much more rapidly than their tumours under conditions of functional activity or reparative prohferation, e.g., endometrium, mammary epithelium, hver. In the case of the skin, wounds heal and graft donor sites become resurfaced at a much greater rate of growth than that of the majority of epidermal neoplasms. When repair is complete, growth ceases. It is difficult to see how the mechanism for inhibiting growth could be indigenous to the prohferating cells themselves. It is equally difficult to attribute it to a generalised systemic effect, because in coincident unhealed lesions repair continues. One is almost forced to the conclusion that the control of epithelial growth is dependent on functional integrity of the stroma. If this is so, the cancer cell is a cell which has lost the capacity to respond to dermal control. Once the mahgnant change has occurred, the cancer cell does not recover this capacity, but it is possible that the reason the normal cell loses it is determined, at least to some extent, by the supporting tissues themselves. J.W. Oral Professor of Pathology and Director of Cancer Research, University of Birmingham. REFERENCES BERENBLUM, I. (x954). Cancer Res., x4, 47I. BERENBLUM, I., and HARAN-GHERA, N. (x957). Brit..7. Cancer, ix, 77. BERENBLUM, I., and SHtmm, P. (x947). Brit..7. Cancer, i, 383. BmLINGHAM, R. E., ORR, J. W., and WOODHOUSE, D. L. (I95I). Brit..7. Cancer, 5,417 FRIEDEWALD, W. F., and Rous, P. (1944)..7. exp. ivied., 80, xoi, I27. GL0CKSMANN, A. (I958). Brit. reed. Bull., I4, x78. KENNAWAY, Sm ERNST (I955). Brit. med..7., 2, 749. KING, V. ANN, and O~, J. W. (I958). Acta contra cancrum (in press), Proc. 7th Int. Cancer Cong., London.

5 EDITORIAL: EXPERIMENTAL SKIN CANCER LINELL, F. (1947). Acta path. microbiol, scand., Suppl. 71, I. MARCHANT, JUNE, and ORR, J. W. (1953). Brit. J. Cancer, 7, 329. ORR, J. W. (1934). Brit. ~. exp. Path., I5, 73. (1935). Brit. J. exp. Path., I6, 121. (1938). J. Path. Bact., 46, 495. (1955). Brit. ~. Cancer, 9, 623. SALAMAN, M. H. (1958). Brit. reed. Bull., I4, 116. SALAMAN, M. H., and GLENDENNING, O. M. (1957). Brit. J. Cancer, ii, 434. SALAMAN, M. H., and ROE, F. J. C. (1953). Brit. J. Cancer, 7, 472. SET~LA, K., HOLSTI, P., and LONDBOM, S. (1957). Acta contra cancrum, 13, 280. SHUBIK, P. (1958). Acta contra cancrum (in press), Proc. 7th Int. Cancer Cong., London. 261