The idea that development of cancer is a multistage process arose from early studies of virusinduced
tumors and from the discovery of the cocarcinogenic effects of croton oil. Rous and colleagues found that certain virus-induced skin papillomas in rabbits regressed after a period of time and that papillomas could be made to reappear if the skin was stressed by punching holes in it or by applying such irritant substances as turpentine or chloroform. These findings led Rous and his associates to conclude that tumor cells could exist in a latent or dormant state and that the tumor induction process and subsequent
growth of the tumor involved different mechanisms, which they called ‘‘initiation and promotion.’’27 Theterm cocarcinogen was coined by Shear, who discovered that a basic fraction of creosote oil enhanced the production of mouse skin tumors by benzo(a)pyrene.28 In 1941, Berenblum 29 reported that among mice receiving a single skin painting of a carcinogen, such as methylcholanthrene, only a small number of animals developed papillomas, but if the same area of skin was later painted repeatedly with
croton oil, which by itself is not carcinogenic, almost all the animals developed skin carcinomas.
Taken together, the data of these investigators suggested a multistage mechanism for carcinogenesis.
Studies of the events involved in the initiation and promotion phases of carcinogenesis were
greatly aided by the identification of agents that have primarily an initiating activity, such as urethane
or a low dose of a ‘‘complete’’ carcinogen (see below), and by the purification of the components
of croton oil that have only a promoting activity. Diesters of the diterpene alcohol phorbol
were isolated from croton oil and found to be the tumor-promoting substances.30,31 Of these,
12-O-tetradecanoylphorbol-13-acetate (TPA) is the most potent promoter.32 A scheme used to study the initiation–promotion phases of mouse skin carcinogenesis is depicted in Figure 2–8. Typically, tumor initiation is brought about by the single application of an initiator, such as urethane, or a subcarcinogenic
dose of an agent with both initiating and promoting activity, such as the polycyclic hydrocarbon
benzo(a)pyrene; promotion is carried out by repeated application of a phorbol ester, such as TPA (e.g., three times a week).31,33 Benign papillomas begin to appear at 12 to 20 weeks and by about 1 year, 40% to 60% of the animals develop squamous cell carcinomas. If the promoting agent is given alone, or before the initiating agent, usually no malignant tumors occur.
The progression stage of carcinogenesis is an extension of the tumor promotion stage and results from it in the sense that the cell proliferation caused by promoting agents allows the cellular damage inflicted by initiation to be propagated, and the initiated cells are clonally expanded. This propagation of damaged cells in which genetic alterations have been produced leads to the production of more genetic alterations.
This genetic instability is the hallmark of the progression phase of carcinogenesis and leads to the chromosomal translocations and aneuploidy that are frequently seen in cancer cells.34 Such alterations in the genome of the neoplastic cell during the progression phase lead to the increased growth rate, invasiveness, and metastatic capability of advanced neoplasms. Some of the gene expression alterations
that occur during tumor initiation and promotion are shown in Figure 2–9 (see color insert). Evidence for multistage induction of malignant tumors has also been observed for mammary gland, thyroid, lung, and urinary bladder and in cell culture systems (reviewed in Reference 9), thus it seems to be a general phenomenon. This experimental evidence is consistent with the observed clinical history of tumor development in humans after exposure to known carcinogens—that is, initial exposure to a known
chemical or physical carcinogen, a long lag period during which exposure to promoting agents
probably occurs, and finally the appearance ofa malignant tumor.
Several characteristics of tumor initiation, promotion, and progression provide some insight into the mechanisms involved in these processes. Initiation can occur after a single, brief exposure to a potent initiating agent. The actual initiation events leading to transformation into a dormant tumor cell appear to occur within one mitotic cycle, or about 1 day for the mouse skin system.32 Furthermore, initiation appears to be irreversible; the promoting agent can be given for up to a year later and a high percentage of
tumors will still be obtained. Thus, the initiation phase only requires a small amount of time, it is irreversible, and it must be heritable because the initiated cell conveys the malignant alteration to
its daughter cells. All these properties are consistent with the idea that the initiation event involves a genetic mutation, although other ‘‘epigenetic’’ explanations are possible. The promotion phase, by contrast, is a slow, gradual process and requires a more prolonged exposure to the promoting agent. Promotion occupies the greater part of the latent period of carcinogenesis, is at least partially reversible,
and can be arrested by certain anticarcinogenic agents .Tumor promotion is a cell proliferation phase that propagates the initiated damage and leads to the emergence of an altered clone of cells. Most promoting agents are mitogens for the tissue in which promotion occurs. Tumor progression requires continued
clonal proliferation of altered cells, during which a loss of growth control and an escape from host defense mechanisms become predominant phenotypic traits. This process allows growth to progress to a clinically detectable tumor. The later events in the tumor progression phase are also thought to be irreversible because of the pronounced changes in the genome that have occurred leading into this phase. Agents thatare ‘‘pure’’ progression-causing agents are hard to identify, but the free radical–generating agent
benzoylperoxide appears to be a progressioninducing agent during experimental epidermal carcinogenesis.35 It should be noted that some potent carcinogens are ‘‘complete carcinogens’’ in that at certain doses they can by themselves induce a cancer. Such agents include polycyclic aromatic
hydrocarbons, nitrosamines, certain aromatic amines, and aflatoxin B1. When these agents are given in sufficient dose to animals during cancercausing protocols, they can cause DNA damage and produce tissue necrosis, which is itself enough to stimulate several rounds of cell proliferation in response to the tissue damage. In this situation, the promotion–progression phases are often collapsed in time, resulting in the production of aneuploid malignant cells.