Inhibition of c‐Jun NH2‐terminal kinase switches Smad3 signaling from oncogenesis to tumor‐ suppression in rat hepatocellular carcinoma

Abstract

Transforming growth factor beta (TGF-β) signaling involves both tumor-suppression and oncogenesis. TGF-β activates the TGF-β type I receptor (TβRI) and c-Jun N-terminal kinase (JNK), which differentially phosphorylate the mediator Smad3 to become COOH-terminally phosphorylated Smad3 (pSmad3C) and linker-phosphorylated Smad3 (pSmad3L). TβRI-dependent pSmad3C transmits a tumor-suppressive TGF-β signal, while JNK-dependent pSmad3L promotes carcinogenesis in human chronic liver disorders. The aim of this study is to elucidate how SP600125, a JNK inhibitor, affected rat hepatocellular carcinoma (HCC) development, while focusing on the domain-specific phosphorylation of Smad3. The rats received subcutaneous injections of either SP600125 or vehicle 11 times weekly together with 100 ppm N-diethylnitrosamine (DEN) administration for 56 days and were sacrificed in order to evaluate HCC development 28 days after the last DEN administration. The number of tumor nodules greater than 3 mm in diameter and the liver weight/body weight ratio were significantly lower in the SP600125-treated rats than those in the vehicle-treated rats (7.9 ± 0.8 versus 17.7 ± 0.9: P < 0.001; 6.3 ± 1.2 versus 7.1 ± 0.2%: P < 0.05). SP600125 significantly prolonged the median survival time in rats with DEN-induced HCC (113 versus 97 days: log-rank P = 0.0018). JNK/pSmad3L/c-Myc was enhanced in the rat hepatocytes exposed to DEN. However, TβRI/pSmad3C/p21WAF1 was impaired as DEN-induced HCC developed and progressed. The specific inhibition of JNK activity by SP600125 suppressed pSmad3L/c-Myc in the damaged hepatocytes and enhanced pSmad3C/p21WAF1, acting as a tumor suppressor in normal hepatocytes. Conclusion: Administration of SP600125 to DEN-treated rats shifted hepatocytic Smad3-mediated signal from oncogenesis to tumor suppression, thus suggesting that JNK could be a therapeutic target of human HCC development and progression. (HEPATOLOGY 2009.)