Effect of nodularin on the expression of glutathione S-transferase placental form and proliferating cell nuclear antigen in N-nitrosodiethylamine initiated hepatocarcinogenesis in the male Fischer 344 rat.

The tumor-promoting effect of nodularin during carcinogenesis was investigated. Male Fischer 344 rats were injected with nodularin for 10 weeks from week 3 after N-nitrosodiethylamine initiation without partial hepatectomy. Rats were further maintained for 10 weeks after the cessation of nodularin and were periodically killed. In contrast to the minimal foci in the DEN and nodularin alone groups, treatment with DEN and nodularin produced four kinds of nodules with eosinophilic, clear, mixed and basophilic cells. After the cessation of nodularin, the maximally increased number, but not the area, of glutathione S-transferase placental form-positive [GST-P(+)] nodules at week 12 decreased significantly and the appearance of two types of hyperplastic nodules was noted by GST-P immunostaining; homogeneously stained dense nodules (DN) and heterogeneously stained pale nodules (PN), which appeared only after the cessation of nodularin. DN were well circumscribed by enzyme-altered cells, as opposed to poorly in PN. Moreover, normal-appearing hepatocytes replaced the enzyme-altered cells in PN. In contrast to the higher PCNA index in GST-P(+) DN, the background level returned to that of the control at week 15. PCNA indices in DN were significantly higher than in PN, which were still higher than the control, indicating that nodularin affected the PCNA index differentially in the altered and unaltered hepatocytes. However, nodularin without DEN initiation significantly increased the PCNA index through initial cell death and subsequent hepatocyte proliferation. These results suggest that: (i) nodularin has a promoting effect by inducing hepatocyte proliferation in both enzyme-altered hyperplastic nodules and the surrounding parenchyma; (ii) proliferation is transient in background cells but not in enzyme-altered hepatocytes; (iii) GST-P(+) DN can be regarded as progressive and GST-P(+) PN as regressive, revealed by both immunohistochemistry and PCNA index.