c-Jun N-terminal kinase activation by oxidative stress suppresses retinoid signaling through proteasomal degradation of retinoic acid receptor α protein in hepatic cells

We previously reported that impaired retinoid signaling causes hepatocellular carcinoma (HCC) through oxidative stress. However, the interaction between oxidative stress and retinoid signaling has not been fully understood. To address this issue, the effects of hydrogen peroxide on the transcriptional activity of RAR/RXR heterodimers, RAR alpha and RXR alpha proteins and intracellular signaling pathways were examined. The transcriptional activity of RAR/RXR examined by the DR5-tk-Luc reporter assay was significantly suppressed. The RAR alpha protein level began to decrease at 6 h after treatment and declined thereafter. However, RAR alpha mRNA were not changed. Activation of extracellular regulated kinases (ERK), p38, c-Jun N-terminal kinase (JNK) and Akt was observed after treatment of hydrogen peroxide. SP600125, an inhibitor of JNK, reversed the RAR alpha protein level reduced by hydrogen peroxide. Anisomycin, an activator of JNK, reduced RAR alpha protein. Transfection of wild-type JNK-constitutive actively expressing plasmid, but not kinase-negative JNK-expressing plasmid caused reduction of RAR alpha protein. Proteasomal degradation of RAR alpha was observed after anisomycin treatment; however, the mutant RAR alpha, of which phosphorylation sites are replaced with alanines, was not degradated. In hepatitis C virus (HCV)-related human liver tissues, phospho-JNK and RAR alpha reciprocally expressed with the progression of liver disease. Finally, the staining of 8-OHdG and thioredoxin was increased with the disease progression. These data indicate that JNK activation by oxidative stress suppresses retinoid signaling through proteasomal degradation of RAR alpha, suggesting that a vicious cycle between aberrant retinoid signaling and oxidative stress accelerates hepatocarcinogenesis. (Cancer Sci 2011; 102: 934-941).