MODIFICATION OF GROWTH AND APOPTOSIS BY UBIQUITINATION IN HEPATOMA CELL

我们已经进行了维甲酸类化合物化学预防肝细胞癌的研究，并证实了非环维A酸类类似物对第二原发癌发生的临床效果。我们报道了维甲酸X受体α在人正常肝和肝细胞癌中泛素化的差异。我们发现RXRα泛素/蛋白酶体依赖降解的转换可能与肝癌的异常生长和维甲酸的抑制有关。干扰素(IFN)-α和-β也可以诱导肝癌细胞的凋亡，并被用于临床预防肝癌。非环维A酸类化合物不仅能增强干扰素-β和干扰素-α的抗肿瘤作用，而且天然维甲酸(全反式和9-顺式维甲酸)不能发挥这种作用。这种联合作用可能是由于增强了以DNA片段化和染色质凝集为特征的细胞凋亡诱导。用去环维A酸预处理肝癌细胞后再用干扰素-β，而不是相反，诱导了这种细胞毒效应。非环类维甲酸可增加I型干扰素受体(IFNR)的表达，并与抗I型IFNR抗体包合，可消除联合作用对细胞生长的协同抑制作用。此外，维甲酸还上调了IFNR细胞内信号转导分子STAT1的表达和DNA结合活性。因此，去环维A酸似乎增强了肝癌细胞对干扰素-β的敏感性，从而促进了癌细胞的死亡。这些结果提示，去环维A酸类药物和干扰素-β联合应用于肝细胞癌的生化学预防中具有潜在的临床应用前景。我们评估这些结果对于建立用维拉西林进行癌症化学预防和开发新的癌症化学预防药物非常重要。

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.