Fluctuations in cell proliferation, apoptosis, and cell cycle regulation at the early stage of tumor promotion in rat two-stage carcinogenesis models

We previously demonstrated that 28-day administration of carcinogens that evoked cell proliferation as determined by immunoreactivity for Ki-67 or minichromosome maintenance 3 (Mcm3), in many target organs, increased the numbers of topoisomerase (Topo) IIα+, ubiquitin D (Ubd)+, and TUNEL+ apoptotic cells. We also found increased co-expression of Topo IIα and Ubd, suggestive of increased spindle checkpoint disruption at the M phase. To investigate the roles of these markers in the early stages of carcinogenesis, we examined distribution changes in several carcinogenic target organs using rat initiation-promotion models. Promoting agents targeting the liver (piperonyl butoxide, methapyrilene hydrochloride), thyroid (sulfadimethoxine), urinary bladder (phenylethyl isothiocyanate), and forestomach and glandular stomach (catechol) were administered to rats after initiation treatment for each target organ. Numbers of Ki-67+, Mcm3+, Topo IIα+ and TUNEL+ cells increased within preneoplastic lesions as determined by glutathione S-transferase placental form in the liver or phospho-p44/42 mitogen-activated protein kinase in the thyroid, and hyperplastic lesions having no known preneoplastic markers in the urinary bladder, forestomach and glandular stomach. On the other hand, Ubd+ cells did not increase within these preneoplastic lesions, but increased within hyperplastic lesions. These results suggest that both cell proliferation and apoptosis may be involved in the formation of preneoplastic lesions in the liver and thyroid examined here; however, spindle checkpoint disruption may not be involved in this process. Changes in hyperplastic lesions of the urinary bladder, forestomach and glandular stomach are similar to the 28-day carcinogen-treated cases, suggestive of the hyperplastic cellular character before the preneoplastic state.