GPC3 is a proteoglycan involved in the control of proliferation and survival, which has been linked to several tumor types. In this respect, we previously demonstrated that normal breast tissues exhibit high levels of GPC3, while its expression is diminished in tumors. However, the role of the GPC3 downregulation in breast cancer progression and its molecular and cellular operational machineries are not fully understood.

In this study we showed that GPC3 reverts the epithelial-to-mesenchymal transition (EMT) underwent by mammary tumor cells, blocks metastatic spread and induces dormancy at secondary site. Using genetically modified murine breast cancer cell sublines, we demonstrated that the phospho-Erk/phospho-p38 ratio is lower in GPC3 reexpressing cells, while p21, p27 and SOX2 levels are higher, suggesting a dormant phenotype. In vivo metastasis assays confirmed that GPC3 reexpressing cells reduce their metastatic ability. Interestingly, the presence of dormant cells was evidenced in the lungs of inoculated mice. Dormant cells could reactivate their proliferative capacity, remain viable as well as tumorigenic, but they reentered in dormancy upon reaching secondary site. We also proved that GPC3 inhibits metastasis through p38 pathway activation. The in vivo inhibition of p38 induced an increase in cell invasion of GPC3 reexpressing orthotropic tumors as well as in spontaneous and experimental metastatic dissemination.

In conclusion, our study shows that GPC3 returns mesenchymal-like breast cancer cells to an epithelial phenotype, impairs in vivo metastasis and induces tumor dormancy through p38 MAPK signaling activation. These results help to identify genetic determinants of dormancy and suggest the translational potential of research focusing in GPC3.

GPC3是一种蛋白聚糖，可参与增殖和存活的控制，与多种肿瘤类型有关。在这方面，我们先前证明了正常的乳腺组织表现出高水平的GPC3，而其在肿瘤中的表达却减少了。但是，GPC3下调在乳腺癌进展中的作用及其分子和细胞操作机制尚不完全清楚。

在这项研究中，我们表明GPC3可以逆转乳腺肿瘤细胞经历的上皮向间充质转化（EMT），阻断转移扩散并诱导次级部位的休眠。使用转基因的鼠类乳腺癌细胞亚系，我们证明了GPC3重表达细胞中的磷酸化Erk /磷酸化p38比率较低，而p21，p27和SOX2的水平较高，表明处于休眠表型。体内转移试验证实，GPC3重新表达的细胞可降低其转移能力。有趣的是，在接种小鼠的肺中已证明存在休眠细胞。休眠细胞可以重新激活其增殖能力，保持活力并具有致瘤性，但是到达次级位点后它们便重新进入休眠状态。我们还证明了GPC3通过p38途径激活抑制转移。在体内的p38的抑制诱导GPC3的细胞侵袭reexpressing各向异性肿瘤以及在自发和实验转移性传播的增加。

总之，我们的研究表明，GPC3使间充质样乳腺癌细胞恢复上皮表型，损害体内转移并通过p38 MAPK信号传导激活诱导肿瘤休眠。这些结果有助于确定休眠的遗传决定因素，并提示着眼于GPC3的研究的翻译潜力。