Mst1/2 signalling to Yap: gatekeeper for liver size and tumour development

Abstract

The mechanisms controlling mammalian organ size have long been a source of fascination for biologists. These controls are needed to both ensure the integrity of the body plan and to restrict inappropriate proliferation that could lead to cancer. Regulation of liver size is of particular interest inasmuch as this organ maintains the capacity for regeneration throughout life, and is able to regain precisely its original mass after partial surgical resection. Recent studies using genetically engineered mouse strains have shed new light on this problem; the Hippo signalling pathway, first elucidated as a regulator of organ size in Drosophila, has been identified as dominant determinant of liver growth. Defects in this pathway in mouse liver lead to sustained liver overgrowth and the eventual development of both major types of liver cancer, hepatocellular carcinoma and cholangiocarcinoma. In this review, we discuss the role of Hippo signalling in liver biology and the contribution of this pathway to liver cancer in humans.

Figure 1

Hippo pathway circuitry in Drosophila and in the mammalian liver. (A) Model of Drosophila Hippo signaling. Signaling may be initiated in response to the atypical cadherin Fat receptor activation through Ds binding. Signals are transduced through the FERM domain-containing cytoskeleton-associated protein, Merlin (Mer) and Expanded (Ex), and by Kibra, a protein that interacts with Mer and Ex. The Hippo kinase interacts with and phosphorylates the scaffold protein Sav promoting Hippo-mediated phosphorylation of the adaptor Mats and the Wts kinase. Wts is thereby activated and phosphorylates the transcriptional coactivator Yki. Phosphorylation of Yki induces its cytoplasmic retention through 14-3-3 binding. In the absence of Hippo pathway activation, Yki is mainly located in the nucleus, wherein it binds and activates various DNA biding transcription factors including Sd, Htx and Tsh to induce expression of genes implicated in cell growth and survival. B and C components of the Hippo pathway are highly conserved in mammals, wherein they have a critical role in proliferative control in the liver (mammalian orthologues are indicated with the same colour scheme as the corresponding Drosophila proteins). Although the circuitry is incompletely defined, it seems that two distinct models either in the oval cells or in hepatocytes can be proposed based on recent studies. In both cell types, inhibition of the Yki orthologue, Yap, is thought to be a critical output of the pathway. Defects upstream of Yap result in nuclear retention of Yap, which functions in association with DNA-binding transcription factors, for example, the TEAD domain transcription factors (orthologues of Sd) to regulate the expression of genes that control cell growth and survival. (B) In hepatocytes, Mst1/2 are required to phosphorylate Mob1. By analogy to Drosophila, phospho-Mob1 is likely to facilitate activation of an intermediary kinase, which phosphorylates Yap, resulting in both cytoplasmic retention by 14-3-3 binding, as well as cytoplasmic degradation after ubiquitinylation. Lats1/2 activity are unchanged by Mst1/2 inactivation suggesting the existence of a yet to be defined Mst1/2-regulated Yap kinase. The majority of the catalytically active Mst1/2 in the liver is in a truncated form that lacks the autoregulatory carboxy-terminus. The upstream activators of Mst1/2 are not defined, although Rassf family proteins, could link Mst1/2 to extracellular signals facilitating activation before the proteolytic cleavage. Sav1 does not seem to have a role in Yap regulation in hepatocytes. (C) In oval cells, Sav1 controls total Yap protein levels and levels in the nucleus through yet to be defined mechanisms. The relationship of Sav1 to the Mst1/2 activation state and the phosphorylation of Yap is not clear, although the Mst1/2-controlled phosphorylation of Yap-Ser127 is unaffected by Sav1 inactivation. Mst1/2 have not been studied specifically in oval cells, however, the increase in oval cells following Mst1/2 inactivation indicate an important regulatory role for these kinases in oval cells. The components upstream of these pathways are incompletely defined, although cell–cell contact is likely to be an important stimulus.

Figure 2

Model for the role of the Hippo pathway during liver regeneration and tumorigenesis. The normal adult liver is mainly composed of two parenchymal cell types, the hepatocytes and the cholangioctyes that surround the bile ducts. During embryogenesis (not shown), a common progenitor cell gives rise to these both of these cell types. In the adult, the liver is largely quiescent. The gradual replacement of cells during normal physiologic turnover is accomplished by the proliferation of the differentiated liver cells. Similarly, in response to various forms of liver injury or to partial hepatectomy, the liver mass is restored through cell cycle entry of remaining parenchymal cells. In contrast, when parenchymal cells are unable to proliferate (e.g., in response to hepatocyte toxins), rare cells associated with the bile ducts known as oval cells expand and then differentiate to restore liver mass. The Mst1/2 kinases seem to control hepatocyte quiescence by the inhibition of Yap activity. This inhibition may be periodically relieved during normal homostatic turnover, as well as in response partial hepatectomy. The quiescence of oval cells seem to be controlled both by Mst1/2 and Sav1, and again, Yap is a candidate downstream target of the pathway. Sustained defects in Mst1/2 result in hepatocyte and oval cell proliferation and the development of HCC and tumors of mixed HCC and CC histology. Sav1 defects produce comparable phenotypes except that no hepatocyte proliferation is observed.