Institute of Basic Medical Sciences, College of Medicine, Institute of Bioinformatics and Biosignal Transduction, National Cheng Kung University, Tainan, Taiwan.
Mol Pharmacol. 2012 Dec;82(6):1115-28. doi: 10.1124/mol.112.078485. Epub 2012 Sep 5.
Previous studies have shown that the inhibitory effect of betulinic acid (BA) on specificity protein 1 (Sp1) expression is involved in the prevention of cancer progression, but the mechanism of this effect remains to be delineated. In this study, we determined that BA treatment in HeLa cells increased the sumoylation of Sp1 by inhibiting sentrin-specific protease 1 expression. The subsequent recruitment of E3 ubiquitin-protein ligase RING finger protein 4 resulted in ubiquitin-mediated degradation in a 26S-proteosome-dependent pathway. In addition, both BA treatment and mithramycin A (MMA) treatment inhibited lung tumor growth and down-regulated Sp1 protein expression in Kras(G12D)-induced lung cancers of bitransgenic mice. In gene expression profiles of Kras(G12D)-induced lung cancers in bitransgenic mice with and without Sp1 inhibition, 542 genes were affected by MMA treatment. One of the gene products, cyclin A2, which was involved in the S and G(2)/M phase transition during cell cycle progression, was investigated in detail because its expression was regulated by Sp1. The down-regulation of cyclin A2 by BA treatment resulted in decreased retinoblastoma protein phosphorylation and cell cycle G(2)/M arrest. The BA-mediated cellular Sp1 degradation and antitumor effect were also confirmed in a xenograft mouse model by using H1299 cells. The knockdown of Sp1 in lung cancer cells attenuated the tumor-suppressive effect of BA. Taken together, the results of this study clarify the mechanism of BA-mediated Sp1 degradation and identify a pivotal role for Sp1 in the BA-induced repression of lung cancer growth.
先前的研究表明，白桦脂酸（BA）对特异性蛋白 1（Sp1）表达的抑制作用涉及癌症进展的预防，但这种作用的机制仍需阐明。在这项研究中，我们发现 BA 处理可通过抑制 sentrin 特异性蛋白酶 1 的表达来增加 Sp1 的 sumoylation。随后，E3 泛素蛋白连接酶 RING 指蛋白 4 的募集导致在 26S-蛋白酶体依赖性途径中发生泛素介导的降解。此外，BA 处理和米托蒽醌（MMA）处理均可抑制 Kras（G12D）诱导的双转基因小鼠肺癌肿瘤的生长，并下调 Sp1 蛋白表达。在具有和没有 Sp1 抑制的 Kras（G12D）诱导的双转基因小鼠肺癌的基因表达谱中，MMA 处理影响了 542 个基因。其中一个基因产物细胞周期蛋白 A2 参与细胞周期进展中的 S 和 G2/M 期过渡，因其表达受 Sp1 调节而受到详细研究。BA 处理导致细胞周期 G2/M 期阻滞和视网膜母细胞瘤蛋白磷酸化减少，从而下调 cyclin A2 的表达。BA 介导的细胞 Sp1 降解和抗肿瘤作用也通过使用 H1299 细胞的异种移植小鼠模型得到了证实。肺癌细胞中 Sp1 的敲低减弱了 BA 的肿瘤抑制作用。总之，本研究结果阐明了 BA 介导的 Sp1 降解机制，并确定了 Sp1 在 BA 诱导的肺癌生长抑制中的关键作用。
