Immunohistochemical demonstration of the gap junctional protein connexin 32 and proliferating cell nuclear antigen in glutathione S-transferase placental form-negative lesions of rat liver induced by diethylnitrosamine and clofibrate.

The distributions of a gap junctional protein, connexin 32 (cx 32), and proliferating cell nuclear antigen (PCNA) were examined immunohistochemically in glutathione S-transferase placental form (GST-P)-negative foci, induced in rat liver by initiation with diethylnitrosamine (DEN, 200 mg/kg) followed by promotion with clofibrate (1% in diet) in an in vivo medium-term assay system for hepatocarcinogenesis. The results were compared to those in GST-P-positive foci induced by DEN alone. The treatment with clofibrate caused the appearance of GST-P-negative foci, increased in size as compared to GST-P-positive foci in the same liver or induced by the DEN alone. The proportion of PCNA-positive hepatocytes in GST-P-negative foci was significantly higher than in the surrounding parenchyma, indicating increased cell proliferation. The numbers of cx 32-positive spots per hepatocyte in GST-P-negative foci were clearly decreased, reaching 65.4% at week 20 and 51.8% at week 30 of values for surrounding normal hepatocytes. In GST-P-positive foci induced by DEN, only a slight decrease (80%) was observed at week 8. These findings show that a positive association between the sustained inhibition of gap junctional intercellular communication and increased cell proliferation of GST-P-negative foci in Fischer-344 male rats induced with DEN and promoted with clofibrate.