Our present understanding of two-stage
carcinogenesis encompasses almost four decades of research. Evidence for chemical promotion or
cocarcinogenesis was first provided by Berenblum, who reported that a regimen of
croton oil (weak or noncarcinogenic) applied alternately with small doses of
benzo(a)pyrene (BP) to mouse skin induced a larger number of
tumors than BP alone. Subsequently, Moltram found that a single subcarcinogenic dose of BP followed by multiple applications of
croton oil could induce a large number of skin
tumors. These investigations as well as a number of others, such as Boutwell, Van Duuren and Hecker, were responsible in defining many important aspects of the initiation and promotion of two-stage
carcinogenesis. The initiation stage in mouse skin requires only a single application of either a direct-acting
carcinogen or a procarcinogen and is essentially an irreversible step which as data suggests probably involves a somatic cell mutation. The promotion stage in mouse skin can be accomplished by a wide variety of weak or noncarcinogenic agents and is initially reversible later becoming irreversible. Current information suggests that skin
tumor promoters are not mutagenic but bring about a number of important epigenetic changes, such as epidermal
hyperplasia, and an increase in
polyamines,
prostaglandins and dark basal keratinocytes as well as other embryonic conditions. Recently,
tumor promotion in mouse skin was shown to consist of at least two stages, in which each stage can be accomplished by either a known promoter or a weak or nonpromoting agent. Some of the important characteristics of the first stage of promotion are: (1) only one application of a first-stage promoter, such as
phorbol ester tumor promoters,
calcium ionophore A23187,
hydrogen peroxide and wounding is needed; (2) the action is partially irreversible; (3) an increase in dark basal keratinocytes and
prostaglandins is important; and (4) such an increase can be inhibited by antiinflammatory
steroids and
protease inhibitors. The second stage of promotion is initially reversible but later becomes irreversible.
Polyamines and epidermal cell proliferation are important events in the second stage of promotion. A number of weak or nonpromoting agents, such as
mezerein, are effective second-stage promoters which can be counteracted by
retinoic acid, antiinflammatory
steroids and
polyamine synthesis inhibitors. Although skin
tumor promotion has been extensively studied in mice, not all strains and stocks of mice are susceptible to
phorbol ester tumor promoters. In this regard, the C57BL/6 mice appear to be fairly resistant to
phorbol ester tumor promoters. In addition, not all species are equally susceptible to
phorbol ester tumor promotion. Recently the generality of the two-stage system of inducing
tumors has been shown to exist in a number of experimental
carcinogenesis systems, such as the liver, bladder, lung, colon, esophagus, stomach, mammary gland, pancreas and cells in culture. In these systems, a wide variety of promoting agents such as diet,
bile acids,
hormones,
saccharin,
tryptophan,
phenobarbital,
polychlorinated biphenyls,
polybrominated biphenyls and
butylated hydroxytoluene have been used to accomplish the
tumor promotion stage. It is not presently known if other experimental
carcinogenesis systems and the induction of human
cancer involves a series of stages similar to that in the mouse skin.