Graduate Institute of Biomedical Sciences, College of Medicine, Chang Gung University, Taoyuan 333, Taiwan, Republic of China.
Graduate Institute of Biomedical Sciences, College of Medicine, Chang Gung University, Taoyuan 333, Taiwan, Republic of China; Department of Biochemistry and Molecular Biology, College of Medicine, Chang Gung University, Taoyuan 333, Taiwan, Republic of China.
Biochem Pharmacol. 2015 Jan 1;93(1):110-24. doi: 10.1016/j.bcp.2014.10.012. Epub 2014 Nov 3.
Prothymosin alpha (PTMA) is overexpressed in various human tumors, including hepatocellular carcinoma (HCC). The significance of PTMA overexpression and its underlying mechanism remain unclear. We show here that silencing PTMA sensitizes HCC cells to the kinase inhibitor sorafenib. In contrast, ectopic expression of PTMA induces cell resistance to the drug. While inhibitors targeting JNK, ERK or PI3K reduce PTMA expression, only ERK activation is suppressed by sorafenib. In addition, inhibition of ERK produces a dramatic decrease in both endogenous PTMA level and promoter activation. Ectopic expression of active MKK1/2 considerably induces PTMA expression. We also identify a sorafenib-responsive segment lying 1000-1500-bp upstream of the PTMA transcription start site and observe that it is controlled by c-Myc and ERK. Mutation in the PTMA promoter at the predicted c-Myc binding site and silencing of c-Myc both abrogate sorafenib's effect on PTMA transcription. We also find that silencing PTMA potentiates Bax translocation to mitochondria in response to sorafenib and this is associated with increased cytochrome c release from mitochondria and enhanced caspase-9 activation. These results indicate that PTMA is positively regulated by the oncoprotein c-Myc and protects HCC cells against sorafenib-induced cell death, thus identifying PTMA as a new target for chemotherapy against HCC.
胸腺素 α（PTMA）在多种人类肿瘤中过表达，包括肝细胞癌（HCC）。PTMA 过表达的意义及其潜在机制尚不清楚。我们在这里表明，沉默 PTMA 可使 HCC 细胞对激酶抑制剂索拉非尼敏感。相比之下，PTMA 的异位表达诱导细胞对药物产生耐药性。虽然针对 JNK、ERK 或 PI3K 的抑制剂可降低 PTMA 的表达，但只有索拉非尼抑制 ERK 的激活。此外，ERK 的抑制会导致内源性 PTMA 水平和启动子激活的急剧下降。活性 MKK1/2 的异位表达可显著诱导 PTMA 的表达。我们还鉴定了一个位于 PTMA 转录起始位点上游 1000-1500bp 的索拉非尼反应片段，并且观察到它受 c-Myc 和 ERK 控制。在预测的 c-Myc 结合位点处对 PTMA 启动子进行突变以及沉默 c-Myc 都可消除索拉非尼对 PTMA 转录的作用。我们还发现，沉默 PTMA 可增强 Bax 向线粒体的易位，以响应索拉非尼，这与线粒体中细胞色素 c 的释放增加和 caspase-9 的激活增强有关。这些结果表明，PTMA 受癌蛋白 c-Myc 的正向调节，可保护 HCC 细胞免受索拉非尼诱导的细胞死亡，从而将 PTMA 鉴定为 HCC 化疗的新靶点。
