The concepts of initiation, promotion and progression were derived from animal experiments to help explain phenomenologically distinct processes occurring during carcinogenesis (Bout well, 1974). Operationally, on the whole animal level, initiation refers to the stable or apparent irreversible conversion of a normal stem or progenitor cell (see later discussion of the de-differentiation hypothesis) to a “premalignant” cell (Trosko et al., 1990b). This premalignant or initiated cell has, by definition, several features; namely, it has the inability to terminally differentiate (Trosko et al., 1988); it has the potential to proliferate and maintain its inability to terminally differentiate; and lastly, it has the potential to acquire all the other genetic/epigenetic changes needed to become a neoplastic, invasive and meta-static cell. Promotion, on the other hand, is that phase of the multi-stage process which enhances the frequency and earlier appearance of tumors in the initiated animal (Bout well, 1974). Progression refers to the conversion of one of the promoted initiated cells to the neoplastic cell.
Although the mechanisms underlying each of these three phases are not vet known, some hypotheses to explain the apparent irreversible, initiation phase, the potentially interruptible or reversible promotion phase and the irreversible progression phase have been proposed. Since mutagenesis is an irreversible event, and since known mutagens, such as ionizing and ultraviolet radiations, have been shown to be initiators in several initiation-promotion animal model systems (Trosko and Chang, 1985), it would be consistent to hypothesize that initiation is based on a mutagenic mechanism. Many chemicals, considered to be mutagens based on interpretations of a wide range of cell-free and in vitro/in vivo data, are also speculated to act as initiators because of their presumed mutagenic action. However, as will be discussed later, just because an animal can have some of its cells promoted after it has been exposed to a known or suspected mutagen, and just because either DNA lesions or DNA damage are found in tissues of the exposed animal, it does not prove that the mutation ultimately found in the tumor cell was induced by the agent that was used to induce the irreversible or initiation event. In addition, conceivably, an agent, which could induce a stable epigenetic change, could be thought to be an initiating agent. In short, at present we can only speculate that initiation, induced either by a mutation or stable epigenetic event, is that process which prevents a stem or progenitor cell from terminally differentiating but not from dividing. This must be the case, for if an affected cell has the ability to terminally differentiate under normal conditions; it could not give raise to a cancer (see later discussion).
Promotion has been postulated to be that process by which a single initiated cell is selectively amplified by one of several mitogenic stimuli.
Here again, the underlying mechanisms are not known. However, several lines of evidence have been used to hypothesize that promoting agents and conditions are mitogens, not mutagens (Ames and Gold, 1990b; Cohen and Ellwein, 1990; Trosko and Chang, 1988a). Many natural chemicals, such as phorbol esters and palyiotoxins, endogenous factor, such as hormones and growth factors, nutrients, such as unsaturated fatty acids, pollutants, such as polybrominate biphenyls, pesticides, such as DDT, dieldrin and aldrin, cigarette condensates, and drugs, such as Phenobarbital, have been demonstrated to act as promoters in in vivo model systems (Trosko and Chang, 1988a). The weight of the other experimental results with these chemicals is that they are not initiators in in vivo animal systems, and do not induce DNA damage or mutations in systems which simulate the in vivo conditions. Other promoting conditions include normal cell growth (as might be evidenced by initiating young animals prior to their normal growth), cell removal (surgery or wounding), cell killing, and solid state object-induced hyperplasia (Trosko et al., 1990d). In the animal systems, the mitogenic stimulation must be sustained (Sisskin et al., 1982). In other words, even if an animal is initiated and exposed to mitogenic stimuli for a short time or for irregular times or at concentrations not sufficient to cause proliferation of the initiated cell (Bout well, 1974), then tumors will not appear at earlier times or higher frequencies.
While the implied cellular basis of promotion is mitogenesis, it does not necessarily mean that all promoting agents will stimulate cell division in all cells exposed to the agent. By definition, if a tumor is clonally derived from a single cell during the carcinogenic process, then a promoter needs only to stimulate the initiated cell. This has some important implications to those who wish to identify promoters from non-promoters. If an agent can be shown to stimulate cell division in an animal tissue, it might have the potential of being a promoter in humans if it can be shown that it also can stimulate human cells. Also, while a chemical could stimulate normal cells to proliferate, there is no guarantee that an initiated cell would also be stimulated to divide. Conversely, if a chemical does not stimulate normal tissues to divide, it does not necessarily means it would not cause an initiated cell to proliferate. In the case where a promoting agent causes both the normal and initiated cell to divide or to stimulate only the initiated cell to divide, the net effect is that the initiated cells will selectively accumulate in the tissue, since the normal progenitor cells will terminally differentiate, whereas the initiated cell will not. These terminally differentiated cells would eventually be replaced, whereas the initiated cell will have the potential to further proliferate, thereby increasing their numbers. At this point, a single initiated cell has increased its numbers by sustained mitogenic stimuli, such that a large mass of these undifferentiated cells occupy a tissue.
During that clonal expansion period whereby cells containing a stable alteration in a gene which affects terminal differentiation (This could be a number of genes; possibly a different gene for each tissue of different embryonic origin.), additional mutations or stable epigenetic changes could occur, allowing the initiated cell to acquire all the phenotypic alterations needed to become promoter independent (Trosko et al., 1983a). Clearly, it at least two or more rare events are needed for complete neoplastic conversion, the probability that these two or more independent events occurring in one initiated cell of the body would be very small. On the other hand, if one increases that number by promoting the single initiated cell, the larger that mass of initiated cells become, the chances for one of those cells containing one genetic “hit” to acquire the second or additional “hit” increase (Potter, 1981a; Trosko and Chang, 1980; Trosko and Chang, 1985). Agents, which are thought to be effective at this stage, have been speculated to be able to induce stable genetic or epigenetic events (Hennings et al., 1983; jaffe et al., 1987; Reddy and Fialkow, 1987; Taguchi et al., 1984). Once this has occurred the cell no longer would need an exogenous stimulator of cell division or of a blocker of cell suppression.
As a summary of the multi-stage multi-mechanism concept of carcinogenesis, three distinct steps have been identified in experimental animal systems. In reality, for the human being, we are never exposed in unique sequence to those physical and chemical agents and conditions which are known to be initiators, promoters and Progressors. Humans are constantly exposed to a wide and changing mixture of initiators, promoters and Progressors. Rarely are we exposed to physical or chemical agents at cytotoxic levels, except possibly the ultraviolet rays of sunlight, alcohol and therapeutic radiation or chemical treatments. We are all exposed to our growth factors at discreet periods of our lives, to chemicals which could interact additively, synergistically or antagonistically with each other (Aylsworth et al., 1989; Hirose et al., 1991; Warngard et al., 1987), as well as to the different and changing physiological endogenous chemicals of our bodies, due to genetic, sex developmental, life style and dietary factors.
