Beijing Institute of Liver Disease, Beijing, China.
Hepatology. 2012 Oct;56(4):1380-90. doi: 10.1002/hep.25891.
Glypican-3 (GPC3) is a heparan sulfate proteoglycan that has an important role in cell growth and differentiation, and its function in tumorigenesis is tissue-dependent. In hepatocellular carcinoma (HCC), the overexpression of GPC3 has been demonstrated to be a reliable diagnostic indicator. However, the mechanisms that regulate the expression and function of GPC3 remain unclear. The oncoprotein c-Myc is a transcription factor that plays a significant role in more than 50% of human tumors. We report here that GPC3 is a transcriptional target of c-Myc and that the expression of c-Myc is also regulated by GPC3, thus forming a positive feedback signaling loop. We found that the overexpression of c-Myc could induce GPC3 promoter-dependent luciferase activity in luciferase reporter experiments. Furthermore, mutational analysis identified c-Myc-binding sites within the GPC3 promoter. The exogenous overexpression of c-Myc increased the endogenous messenger RNA (mRNA) and protein levels of GPC3. Chromatin immunoprecipitation experiments revealed the binding of c-Myc to the endogenous GPC3 promoter, indicating that c-Myc can directly transcriptionally activate GPC3. Interestingly, GPC3 can also elevate c-Myc expression. Overexpression of GPC3 increased c-Myc protein levels, whereas the knockdown of GPC3 reduced c-Myc expression levels. Lastly, the elevated levels of c-Myc correlate with the overexpression of GPC3 in human HCC samples.
These data provide new mechanistic insight into the roles of GPC3 and of c-Myc in the development of HCC.
Glypican-3（GPC3）是一种硫酸乙酰肝素蛋白聚糖，在细胞生长和分化中具有重要作用，其在肿瘤发生中的作用具有组织依赖性。在肝细胞癌（HCC）中，已经证明 GPC3 的过表达是一种可靠的诊断指标。然而，调节 GPC3 的表达和功能的机制尚不清楚。癌蛋白 c-Myc 是一种转录因子，在超过 50%的人类肿瘤中发挥重要作用。我们在这里报告 GPC3 是 c-Myc 的转录靶标，并且 c-Myc 的表达也受 GPC3 调节，从而形成正反馈信号环路。我们发现 c-Myc 的过表达可以在荧光素酶报告实验中诱导 GPC3 启动子依赖性荧光素酶活性。此外，突变分析确定了 GPC3 启动子内的 c-Myc 结合位点。外源性过表达 c-Myc 增加了内源性信使 RNA（mRNA）和 GPC3 蛋白水平。染色质免疫沉淀实验显示 c-Myc 与内源性 GPC3 启动子结合，表明 c-Myc 可以直接转录激活 GPC3。有趣的是，GPC3 也可以提高 c-Myc 的表达。GPC3 的过表达增加了 c-Myc 蛋白水平，而 GPC3 的敲低则降低了 c-Myc 表达水平。最后，c-Myc 的升高水平与人类 HCC 样本中 GPC3 的过表达相关。
这些数据为 GPC3 和 c-Myc 在 HCC 发展中的作用提供了新的机制见解。
