Sequential alterations in the hepatic content and metabolism of cyclic AMP and cyclic GMP induced by DL-ethionine: Evidence for malignant transformation of liver with a sustained increase in cyclic AMP

Abstract

There is evidence that adenosine 3′,5′-monophosphate (cAMP) and guanosine 3′,5′-monophosphate (cGMP) may have antagonistic actions on cell growth, with cAMP inhibiting and cGMP stimulating this process. However, reductions in cAMP and increases in cGMP are not characteristic of all neoplastic tissues. Thus, benign and malignant tissues from hepatoma-bearing rats exposed to the hepatic carcinogen DL-ethionine have elevated rather than depressed cAMP, compared to control liver, and parenteral administration of this drug increases hepatic cAMP within hours. In the present study, the effects of ethionine ingestion on the hepatic content and metabolism of both cAMP and cGMP were examined sequentially in rats at 2 and then 6 wk intervals, from the initiation of drug administration until the development of hepatomas. After 2 wk, cAMP content of quick-frozen liver from rats receiving ethionine (E) was significantly increased (826 ± 91 pmole/g wet weight) above that of liver from pair-fed controls (C, 415 ± 44), whether calculated by tissue wet weight, protein, or DNA content. In benign tissue from E, higher cAMP was still evident after in vitro incubations of slices with 2 m M 1-methyl-3-iso-butylxanthine (MIX) and was associated with enhanced adenylate cyclase and unchanged high or low K m cAMP-phosphodiesterase activities. These findings are compatible with accelerated cAMP generation in liver from E. Protein kinase activity ratios were significantly increased in frozen liver from E (0.52 ± 0.04 versus 0.36 ± 0.03 in C), and the per cent glycogen synthetase in the I form was clearly reduced (19% ± 2% in E versus 47% ± 5% in C). Incubation of hepatic slices from E or C with MIX and/or 10 μ M glucagon further increased cAMP and protein kinase activity ratios, data which imply higher effective, as well as total, cellular cAMP in E. Changes in cAMP metabolism and action observed at 2 wk persisted throughout the 38-wk period of drug ingestion. Adenylate cyclase activity, cAMP content, and protein kinase activity ratios of ethionine-induced hepatomas exceeded those of both the surrounding liver from tumor-bearing rats and that of control liver, but alterations in these parameters were qualitatively similar in both tissues from E. By contrast, while cGMP in quick-frozen surrounding liver from tumor-bearing rats (36 ± 4 pmole/g wet weight) did not differ from that of control liver (30 ± 3), cGMP in the hepatomas was increased. This change was evident in both frozen tumor (89 ± 10) and in tumor slices incubated in vitro with MIX (C, 90 ± 11; surrounding liver, 85 ± 10; hepatoma 231 ± 29). These results indicate that malignant conversion can occur in liver with a sustained elevation of both total and effective cAMP during the premalignant phase. The increase in cGMP detected in ethionine-induced hepatomas could also be a key determinant of malignant transformation in the model, although premalignant changes in cGMP were not apparent.