MODIFICATION OF GROWTH AND APOPTOSIS BY UBIQUITINATION IN HEPATOMA CELL

我们研究了类维甲酸对肝细胞癌的化学预防作用，并证实了合成类维甲酸类似物无环类维甲酸在原发性肝癌发生中的临床作用。我们报道了维甲酸X受体(RXR) α在人正常肝脏和肝细胞癌（HCC）中泛素化的差异。我们发现，泛素/蛋白酶体依赖的RXR α降解的开关可能是导致HCC异常生长和类维生素a抑制HCC的原因。干扰素(IFNs)- α和- β也能诱导HCC细胞凋亡，并在临床上用于预防HCC。无环类维甲酸增强了IFN-β和IFN-α的抗肿瘤作用，而天然维甲酸（全反式和9-顺式维甲酸）则没有这种作用。这种联合效应可能是由于以DNA片段化和染色质凝聚为特征的细胞凋亡诱导增强。用无环类维甲酸预处理HCC细胞后再用IFN-β，而不是相反，诱导了这种细胞毒性作用。无环类维甲酸增加了1型IFN受体（IFNR）的表达，并与抗1型IFNR抗体包涵消除了联合作用对生长的协同抑制作用。此外，类维甲酸上调IFNR细胞内信号转导分子STAT1的表达和dna结合活性。因此，无环类维甲酸似乎增强了HCC细胞对IFN-β的敏感性，从而促进了癌细胞的死亡。这些结果提示无环类维甲酸和IFN-β联合应用在HCC的生化预防中的临床应用前景。我们认为这些结果对建立化学预防癌症和开发新的癌症化学预防药物具有重要意义。

We have conducted investigations of chemoprevention of hepatocellular carcinoma by retlonoids and confirmed the clinical effects of acyclic retionid, a synthetic retinoid analog, on second primary hepatocarcinogenesis. We reported the difference in ubiquitination of retinoid X receptor (RXR) α in human normal liver and hepatocellular carcinoma (HCC). We revealed that switching of the ubiquitin/proteasome-dependent degradation of RXR α may be responsible for the aberrant growth of HCC and its suppression by retinoids. Interferons (IFNs)- α and - β also induce apoptosts of HCC cells and are used clinically in the prevention of HCC. Acyclic retinoid enhanced anti-tumor effects of not only IFN-β but also IFN-α, however, natural retinoic acid (all-trans and 9-cis retinoic acid) failed to exert such effect. This combination effect was likely due to enhanced apoptosis induction as characterized by DNA fragmentation and chromatin condensation. Pretreatment of the HCC cells with acyclic retinoid followed by IFN-β, but not the converse, induced such cytotoxic effect. Acyclic retinoid increased the expression of type 1 IFN receptor (IFNR) and inclusion with anti-type 1 IFNR antibody abolished the synergistic growth suppression by the combination. Moreover, the retinoid up-regulated the expression and DNA-binding activity of STAT1, an intracelluiar signal transducing molecule of IFNR. Thus, acyclic retinoid seemed to enhance the sensitivity of HCC cells to IFN-β and thereby promoted the cancer cell death. These results suggest a future clinical use of the combination of acyclic retinoid and IFN-β in the biochemoprevention of HCC. We evaluate these results as very important to establish cancer chemoprevention with retionids and to develop novel cancer chemopreventive agents.