Hippo signaling in oval cells and hepatocarcinogenesis

Hepatocellular carcinoma (HCC) is one of the most common malignancies and the third leading cause of cancer mortality world wide. Despite continuing development of new therapies, the prognosis for patients with HCC remains extremely poor. In part, this may relate to molecular abnormalities that stimulate HCC tumorigenesis and also contribute to reduced sensitivity to standard treatment. Increasing evidence has revealed the importance of liver cancer stem cells in hepatocarcinogenesis. Although widely investigated, the signaling pathways important for liver cancer stem cells in liver tumor initiation and progression are poorly understood. The Hippo signaling pathway was identified in Drosophila as an essential regulator of cell proliferation and apoptosis. Recently, Hippo pathway has been implicated in multiple events during development and it has also been proposed to play a vital role in several tumor types, especially in hepatocellular carcinoma. Strong evidences also proved the significant role of the Hippo signaling pathway in oval cell activation. As suggested, hippo signaling has a dual regulation of Hippo in liver tumor suppression as well as transition of oval cells to fully differentiated hepatocytes. Delineation of the malfunction of Hippo signaling pathway in HCC may lead to better understanding of hepatocarcinogenesis, rational medical therapy for HCC and possible therapy for other tumors. Here, we provide a historical review of this potent growth-regulatory pathway in HCC and highlight outstanding questions that will likely be the focus of future investigation.