EXPRESSION OF THE TUMOR-NECROSIS-FACTOR-ALPHA GENE AND EARLY RESPONSEGENES BY NODULARIN, A LIVER-TUMOR PROMOTER, IN PRIMARY CULTURED RAT HEPATOCYTES

Nodularin is a new liver carcinogen possessing a potent tumor-promotin g activity in rat liver, mediated through inhibition of protein phosph atases 1 and 2A, and a weak initiating activity. Since we previously r eported evidence that nodularin up-regulated expression of the tumor n ecrosis factor alpha gene (TNF alpha) and early-response genes in rat liver after its i.p. administration, and since TNF alpha had tumor-pro moting activity in vitro, it is possible that TNF alpha itself is invo lved in liver tumor promotion. We investigated whether hepatocytes the mselves induce expression of the TNF alpha gene and early-response gen es in primary cultured rat hepatocytes treated with nodularin. Like no dularin, microcystin-LR, which is another liver tumor promoter belongi ng to the okadaic acid class, strongly induced TNF alpha gene expressi on in rat hepatocytes, as well as TNF alpha release from those cells i nto the medium. On the other hand, 12-O-tetradecanoylphorbol-13-acetat e, which has been reported to induce no tumor promotion in rat liver, induced no apparent expression of the TNF alpha gene in primary cultur ed rat hepatocytes. As for the expression of early-response genes, 1 m u M nodularin or microcystin-LR induced expression of the c-jun, jun B , jun D, c-fos, fos B and fra-1 genes in the hepatocytes, and the expr ession of these genes was prolonged up to 24 h, suggesting mRNA stabil ization induced by inhibition of protein phosphatases 1 and 2A. This p aper presents new evidence that the TNF alpha gene and early-response genes were expressed in hepatocytes treated with a liver tumor promote r.