Several studies (Grisham et al., 1980; Jones et al., 1976; McCormick and Bertram, 1982) have shown that the initiation of the neoplastic transformation in vitro is likely to be most effective at or near the interface of the G1 and S phases of the cell cycle. As with the natural history of neoplastic development in vivo (Chapter 8), the stages of initiation and promotion may be mod- ulated during carcinogenesis in vitro. As noted in Table 14.9, a variety of materials are effective in inhibiting transformation itself as well as altering the stages of initiation and promotion where such stages can be effectively identified. In primary cultures, most investigations have been car- ried out on the SHE cell system. Agents such as butylated hydroxytoluene and other antioxi- dants as well as cortisone act in a manner similar to that seen in vivo (Chapter 8). In contrast, fatty acids such as stearic acid and its derivatives, which have been found to enhance the stage of promotion in vivo, inhibit transformation of these cells in vitro (Embleton and Noy, 1991). Se- lenite ions added to mouse mammary gland organ culture were found to have opposite effects depending on their concentration in the culture (Chatterjee and Banerjee, 1982).
By use of cell lines, particularly the C3H10T1/2, a variety of inhibitors of transformation have been identified. A very interesting effect is the inhibition of transformation by various pro- tease inhibitors (Kennedy, 1985) that also alter the effectiveness of modulators of protein kinase C during the transformation process (Umans and Kennedy, 1992). As noted in the table, several other agents such as ascorbate, aspirin, and β-carotene, which do inhibit development of the stage of promotion in vivo (Chapter 8), are also effective in this cell line in inhibiting morpho- logical transformation. Unlike the effects in SHE cells, cortisone did not alter transformation induced by ionizing radiation in C3H10T1/2 cells (Kennedy and Weichselbaum, 1981). Arrest of proliferation of this cell line in isoleucine-free medium with subsequent replacement of this amino acid and exposure to MNNG causes an enhancement of transformation, probably by ef- fecting a more efficient rate of initiation because of increased cell cycling on release of the nutri- tional block (Grisham et al., 1979). Borek et al. (1983) noted the dependence of transformation
of C3H10T1/2 cells on the presence of triiodothyronine (T3). Complete removal of this hormone from the medium resulted in virtually complete loss of transformation effectiveness by ionizing radiation and chemical carcinogens. Addition of T3 back to the cultures restored transformation effectiveness.
Thus, while there are clear similarities between the modulation of transformation and the stages of initiation and promotion in vitro and in vivo, there are also significant differences. Un- til clearer definitions of the stages of initiation and promotion in vitro are obtained and the rela- tionship of morphological transformation to the neoplastic transformation is understood, it will be difficult to rectify these differences between the in vivo and in vitro systems.