Increased Susceptibility of Aged Rats To Hepatocarcinogenesis By the Peroxisome Proliferator Nafenopin and the Possible Involvement of Altered Liver Foci Occurring Spontaneously

We investigated the mechanism of the hepatocarcinogenic action of nafenopin (NAF), a nongenotoxic peroxisome proliferator. Groups of male rats aged 13 wk (designated "young") or 57 wk (designated "old") were fed NAF for 13 mo; additional groups received a basal diet or a phenobarbital (PB)-containing diet as positive control. The following results were obtained. (a) NAF produced numerous hepatocellular adenomas and carcinomas in old animals but very few in young animals. A similar result, although less pronounced, was seen with PB. Adenomas of PB-treated groups mostly consisted of eosinophilic and glycogen-storing cells. However, adenomas and carcinomas of NAF-treated livers were composed of weakly basophilic cells. (b) Phenotypically altered foci, evaluated in hematoxylin:eosin-stained sections, appeared spontaneously in untreated livers. The majority of these foci was either of the eosinophilic-clear cell or the tigroid cell type. In addition, we identified foci which are characterized by weak, diffuse cytoplasmatic basophilia. Their phenotype was similar to that of adenomas and carcinomas in NAF-treated rats. The number and size of eosinophilic-clear cell and of tigroid cell foci increased considerably with the age of the animals. At the end of the experiment, approximately 2.4% of liver tissue was occupied by focal cells. NAF, but not PB, treatment led to a selective increase in number and size of weakly basophilic foci. This subtype has previously been described as a likely precursor lesion for liver tumors induced by an aflatoxin B1-NAF initiation-promotion regimen (B. Kraupp-Grasl et al., Cancer Res., 50:3701-3708, 1990). These findings suggest that the peroxisome proliferator NAF leads to tumor development in aging rat liver by promotion of spontaneously occurring preneoplastic lesions. The type of lesion appears to be different from that promotable by PB

Control of Cell-death (apoptosis) By Diethylstilbestrol in An Estrogen-dependent Kidney Tumor

The role of cell death as a determinant for tumor growth and regression was studied using an estrogen-dependent, transplantable kidney tumor designated H301. H301 cells were injected s.c. into diethylstilbestrol(DES)-treated male Syrian hamsters and developed solid tumors of 1-2 g within 2-3 weeks. Upon withdrawal of estrogen the tumors regressed by 80-90% within 4 days. Mitoses, necrotic areas and single-cell death indicated by small, condensed cell residues, were counted in hematoxylin and eosin stained histological sections of the tumors. Coincident with tumor regression after DES withdrawal, mitotic activity decreased by approximately 90%; the rate of single-cell death increased (by approximately 2-fold at its maximum). The incidence of necrotic areas was not affected by DES withdrawal. DES re-treatment resulted in reduction of single-cell death by 80% within 8 h. Mitotic activity increased within 24 h to the level observed before DES withdrawal. Again, the incidence of necrotic areas did not change. As a result, tumors re-grew to their previous size within 2 days after resumption of DES treatment. These results led to the following conclusions: (i) DES treatment inhibits and DES withdrawal enhances single-cell death of H301 tumor cells. (ii) Both this functional property and its morphology characterize single-cell death in the tumors as apoptosis. (iii) Estrogen-dependent cell death determines, in addition to mitosis and necrosis, the growth rate of H301 tumors. (iv) This experimental model may provide a useful tool to study the interaction of potential anti-tumor drugs with apoptosis in neoplasia

Inherent increase of apoptosis in liver tumors: Implications for carcinogenesis and tumor regression

We quantitatively assessed rates of cell replication and of apoptosis during the development and regression of liver cancer. In rats, apoptotic activity gradually increased from normal liver to putative preneoplastic foci (PPF), to hepatocellular adenoma (HCA), and to hepatocellular carcinoma (HCC). At all stages, rates of cell replication were higher than of apoptosis, allowing a preferential net gain of (pre)neoplastic cells. As in rats, in human HCC, birth and death rates were increased manifold, indicating a species independent phenomenon. Implications of the increasing cell turnover were studied in rats using the administration and withdrawal of nafenopin (NAF), a liver mitogen and nongenotoxic carcinogen. Prolonged NAF treatment enhanced cell number in normal liver by 25%, while PPF and liver tumors were amplified at least 100-fold. After stopping NAF treatment, cell replication ceased, while cell elimination by apoptosis was increased in normal and (pre)neoplastic liver. HCA and HCC showed the most pronounced shifts from replication toward apoptosis. As a result, 5 weeks after halting NAF, 20% of cells in normal liver, but about 85% of (pre)neoplastic lesions including HCC, were eliminated. The implications of these findings include that nongenotoxic carcinogens can act as survival factors even for malignant cells. Furthermore, tumor cells not only exhibit excessive proliferation, but also undergo apoptosis at rates that far exceed those in normal tissue. Therefore, inhibition of cell death by the survival activity of nongenotoxic carcinogens results in selective growth of (pre)neoplastic lesions. On the other hand, blockade of survival effects leads to excessive apoptosis in (pre)neoplasia and seems promising as a therapeutic concept for the selective elimination of (liver) cancer