The growth of the progeny of initiated cells in the stage of promotion (preneoplastic cells) and their transition to the biologically malignant cell population constitute the major portion of the natural history of neoplastic development. We have already discussed the characteristics of the stages of initiation and promotion (Chapter 7). The transition of preneoplastic to neoplastic cells in the stage of progression may be considered the critical point in the development of neoplasia, although malignancy does not necessarily develop immediately after such transition. While the changes occurring during this transition may differ from tissue to tissue and proceed along a variety of pathways (see below), certain morphological and biological characteristics of the stage of progression can be delineated from studies of both experimental and human neoplasia. A listing of some of the major morphological and biological characteristics of the stage of pro- gression may be seen in Table 9.1. Irreversibility of this stage is emphasized by the demonstrable alterations in the cellular genome—e.g., chromosomal translocations (Wolman, 1983), chromo- somal deletions (Sato et al., 1991), gene amplification (Tlsty, 1996), and other structural DNA alterations (Malins et al., 1996) that are found within this stage. Such genomic changes clearly distinguish the stage of progression from the reversible preceding stage of promotion (Sargent et al., 1989; Aldaz et al., 1987; Balaban et al., 1986). Although the “reversibility” of the stage of
Table 9.1 Morphological and Biological Characteristics of the Stage of Progression During Carcinogenesis
Morphologically and molecularly discernible alterations in the cellular genome
Evolving karyotypic instability
Growth of cells in the stage of progression sensitive to environmental factors during early phase of this stage
Benign and malignant neoplasms observed in this stage
“Progressor” agents act to induce the transition of cells in the stage of promotion to that of progression
Spontaneous (fortuitous) induction of the stage of progression from the stage of promotion
promotion is accompanied by increased apoptotic rates (Chapter 7), the rate of apoptosis during the stage of progression varies dramatically with the histogenetic type of neoplasm (Staunton and Gaffney, 1995). Furthermore, as the stage of progression is extended into metastatic growth, cells may become increasingly resistant to apoptosis (Glinsky et al., 1997). As noted in Table 9.1, although cells in the stage of progression are not completely dependent for their existence on the continued presence of the promoting agent, promoting agents and other environmental factors may enhance the growth and genetic expression of cells in the stage of progression, espe- cially early during the development of this stage. As growth of the neoplasm continues and kary- otypic instability evolves, responses to environmental factors may be altered or lost (Noble,
1977; Welch and Tomasovic, 1985). Agents that act only to effect the transition of a cell from the stage of promotion to that of progression may properly be termed progressor agents. Some examples are seen in Table 9.2. Such agents would presumably have the characteristic of induc- ing chromosomal aberrations but may not necessarily be capable of initiation; in some cases, they may serve to enhance the clastogenesis associated with evolving karyotypic instability. Fur- thermore, the transition of the stage of progression from that of promotion frequently occurs as a spontaneous event even in studies involving chemical carcinogens that possess progressor agent activity (Luebeck and Moolgavkar, 1996; Cohen and Ellwein, 1990). In addition, there has been substantial evidence to demonstrate that the enhanced proliferation of cells in the stage of pro- motion does itself increase the likelihood of major genetic changes, resulting in demonstrable chromosomal alterations characteristic of the stage of progression (Ames and Gold, 1990b). Fa- cilitation of the spontaneous transition from the stage of promotion to that of progression may occur, at least in part, through the mediation of active oxygen radicals, which occur commonly in the living cell (Chapter 3) and have been shown to be clastogenic in cells in vitro (Ochi and Kaneko, 1989; Emerit et al., 1991), as well as in the evolution of the stage of progression to the metastatic state in vivo (Malins et al., 1996).
The experimental demonstration of the stage of progression is somewhat more complex than that of initiation and promotion. In Figure 9.3 may be seen a general experimental format designed to demonstrate the effect of the administration of a progressor agent (lll) after a course of initiation and promotion with most of the appropriate controls. However, in this instance the end point that is quantitated is the number of neoplastic lesions (NL). In experimental systems, the most effective development of neoplasia involves the continued administration of the pro- moting agent even after application of the progressor agent has ceased. This might be expected, because cells early in the stage of progression respond to promoting agents, and this continuing proliferative response yields an increase in neoplastic lesions in the experimental system (Table 9.1). As noted, a lower yield of lesions, but usually still significantly different from the controls,
Figure 9.3 General experimental format for demonstration of the stage of progression and the effect of progressor agents in experimental systems. NL, neoplastic lesions; ±, occasional or infrequent; 1+, few; 2+, some; 4+, many; ❙❙ ❙, administration of progressor agent as single or several multiple doses; V, initiation; , promoting agent dosing.
may be obtained without additional administration of the promoting agent. Because of the dura- tion of the experiment, preneoplastic lesions occur to varying degrees in each of the experimen- tal groups. The difficulty in such studies is the quantitation of the neoplastic lesions, which is usually carried out by determining the incidence and multiplicity of malignant tumors. However, premalignant lesions already in the stage of progression occur quite commonly (cf. Henson and Albores-Saavedra, 1986); thus the appropriate end point for the effect of progressor agents is the quantitation of such lesions. As yet, this has been extremely difficult to do, and thus quantitative analyses of the effects of progressor agents remain relatively crude.