Expression of the tumor necrosis factor alpha gene and early response genes by nodularin, a liver tumor promoter, in primary cultured rat hepatocytes.

Nodularin is a new liver carcinogen possessing a potent tumor-promoting activity in rat liver, mediated through inhibition of protein phosphatases 1 and 2A, and a weak initiating activity. Since we previously reported evidence that nodularin up-regulated expression of the tumor necrosis factor α gene (TNFα) and early-response genes in rat liver after its i.p. administration, and since TNFα had tumor-promoting activity in vitro, it is possible that TNFα itself is involved in liver tumor promotion. We investigated whether hepatocytes themselves induce expression of theTNFα gene and early-response genes in primary cultured rat hepatocytes treated with nodularin. Like nodularin, microcystin-LR, which is another liver tumor promoter belonging to the okadaic acid class, strongly inducedTNFα gene expression in rat hepatocytes, as well as TNFα release from those cells into the medium. On the other hand, 12-O-tetradecanoylphorbol-13-acetate, which has been reported to induce no tumor promotion in rat liver, induced no apparent expression of theTNFα gene in primary cultured rat hepatocytes. As for the expression of early-response genes, 1 μM nodularin or microcystin-LR induced expression of the c-jun, jun B,jun D, c-fos, fos B andfra-1 genes in the hepatocytes, and the expression of these genes was prolonged up to 24 h, suggesting mRNA stabilization induced by inhibition of protein phosphatases 1 and 2A. This paper presents new evidence that theTNFα gene and early-response genes were expressed in hepatocytes treated with a liver tumor promoter.