The role of growth of normal and preneoplastic cell populations for tumor promotion in rat liver

Abstract

A number of different compounds, including phenobarbital, hypolipidemic drugs such as clofibrate and nafenopin, the sex steroids progesterone, cyproterone acetate, estradiol and mestranol, chlorinated hydrocarbons such as DDT, hexachlorocyclohexane, and TCDD and the antioxidant butylhydroxytoluene, appears to promote the development of liver tumors from previously induced initiated cells. The mechanisms of tumor promotion by several representative prototypes of these compounds were studied in rat liver in vivo.All liver tumor promoters mentioned above stimulate growth of normal liver. The growth response is due to cellular hypertrophy and/or increased rate of DNA (and cell) replication and/or decreased rate of cell death. Hepatocytes in foci or islands of altered cells (putatively preneoplastic) show higher rates of replication than normal liver cells; various different liver tumor promoters cause a further increase of proliferation of focal cells. The increased proliferative activity is found in different island phenotypes and thus seems to be a useful marker of the putative preneoplastic state. The focal cells respond to several factors limiting proliferation in normal liver, suggesting that they are not autonomous with respect to growth control.Early preneoplastic foci grow slowly without promotion, despite the relatively high rates of cell replication. Thus their cells seem to have a much shorter life-time than normal hepatocytes or to undergo reversion to the normal phenotype. Promoters seem to accelerate island enlargement by increasing cell replication and delaying cell death or remodeling. Thus, tumor promoters enhance the manifestation of the proliferation advantage of the putative initiated cell population.In addition, promoters cause increases in the number of detectable islands. This can partially be explained by enlargement of existing islands, but phenotypic changes that would enhance the probability of detection of remodelling islands and growth of dormant initiated cells, probably contribute to the apparent increase of island number.Putative preneoplastic foci of unknown origin are frequent in the liver of aged Wistar rats. They are morphologically and functionally very similar to those induced by carcinogens and are responsive to the mitogenic effect of tumor promoters. Promotion of these "spontaneous" foci may explain tumor appearance after long-term application of promoters.The findings may provide a basis for improved identification of initiated hepatocytes (and of initiating hepatocarcinogens) and for detection of tumor promoters. All suspected liver tumor promoters tested so far induced enhanced preneoplastic cell proliferation after single doses. The long-term carcinogenicity bioassay as currently performed does not discriminate between initiating and promoting properties of a test compound if the animals used develop spontaneous preneoplastic lesions in the organ affected.

Food restriction eliminates preneoplastic cells through apoptosis and antagonizes carcinogenesis in rat liver.

“…Previous (15,21) and the present studies suggest that the amount of consumed food, independent of its composition, largely determines which ofthe two processes is predominant. While fasting stops the generation of new cells and favors apoptosis in existing cells, feeding exerts the opposite effect.…”

“…From the present study, the maximum rate of apoptosis may be expected shortly before feeding, whereas the peak of cell replication is known to occur roughly 12 h later (15,16,21). Thus, the cyclic eating behavior of rats seems to result in a diurnal rhythm ofcell turnover in the liver.…”

“…PPF can be induced by treatment with genotoxic carcinogens or appear "spontaneously" during aging (20). We have found (21) that PPF are more sensitive to the antiproliferative effects offood restriction than normal liver cells.…”

Hepatocarcinogenesis by DDT in Rats

“…This cell proliferation pattern is consistent with that by nongenotoxic mitogenic hepatocarcinogens [16][17][18] . It is generally known that the hepatic cell proliferation response to nongenotoxic mitogenic hepatocarcinogens such as phenobarbital typically occurs through an initial burst of enhanced DNA synthesis followed by enhanced mitosis [17][18][19] . The enhanced cell proliferation, however, ceases after a few days even if treatment is continued.…”

In vivo cell lineage analysis in cyproterone acetate-treated rat liver using genetic labeling of hepatocytes

“…8 Initially, CPA was considered as a tumor promoter devoid of tumor-initiating activity. 9,10 The promoter activity of CPA is believed to result from its mitogenic activity. 7,11 Indeed, one of the earliest effects of CPA administration to rats is an increase in liver DNA synthesis leading to liver hyperplasia.…”