Glypican 3 (GPC3), CD81 and HCV E2 protein control Yap and Hippo Pathway
Authors: Xue Yuhua , Bown Bill , Orr Anne , Koral Kelly , Haynes Meagan , Mars Wendy , Michalopoulos George 
Published: November 2023

Liver failure is a common outcome of end‐stage liver diseases. Modern stem cell and tissue engineering techniques can't achieve functional unique liver with multi cell types. Transplantation can only save limited amount of patients, the realistic approach for the majority of patients is to use residual functional liver tissue to restore impaired liver function. Maintaining dynamic balance, mitogenic and mitoinhibitory signals coexist in the human organs. This balance determines the fate of single cell, and determines the size and morphologic characterization of tissue or organ finally. Therefore, theoretically, restoration of functional liver mass in liver failure setting depends on either negative modulation of mitoinhibitory effect or positive modulation of mitogenic effect. One group of Japanese liver failure patients showed aberrantly up regulation of mitogenic signals such as HGF, indicates this blocked regeneration might be dominantly mediated by mitoinhibitory signals. Thus, the investigation of the mechanism of involving mitoinhibitory signaling may provide novel therapeutic targets for liver failure treatment.

Previous studies from our laboratory have shown that GPC3 is associated with inhibition of hepatocyte growth and GPC3 transgenic mice with targeted expression in hepatocytes have decreased liver regeneration. Our previous investigation also showed that GPC3 overexpress intensively both in fulminant hepatic failure human liver tissues. We have also shown that GPC3 binds to the membrane tetraspanin known as CD81. Studies from other sources have shown that activation of CD81 is associated with highly targeted phosphorylation of the protein Ezrin. The latter has been shown to regulate the Hippo pathway, responsible for regulating nuclear levels of the protein known as Yes‐associated protein (Yap). Expression of Yap in the nucleus relates to enhanced liver size and activation of hepatocyte growth. Mice over‐expressing GPC3 with associated suppressed liver regeneration have decreased levels of nuclear Yap. We also found that treatment of primary rat hepatocytes with CD81 agonist antibody led to higher levels of phosphorylated Ezrin and lower Hippo activity, and this lower Hippo activity could be reversed by Spleen tyrosine kinase (Syk) inhibitor in highly differentiated hepatocytes. Partial hepatectomy (PHx) exhibited that loss of GPC3 antagonistic modulation during early liver regeneration, CD81 activated Ezrin and down regulated Hippo activity in vivo. Treatment with Hepatitis C virus (HCV) envelope protein E2 led to more Hippo activity and decrease in nuclear Yap in Hepa RG cells and primary human hepatocytes.

Conclusions

Dependent on Syk mediated CD81, GPC3 and HCV may function as growth inhibitory factor by modulating YAP expression pattern. The CD81‐Ezrin signaling enhances nuclear Yap levels. This pathway in normal hepatocytes is inhibited by GPC3. In human hepatocytes, GPC3 and the E2 protein of HCV also inhibit the CD81‐Ezrin effect on Hippo pathway and cause a decrease in nuclear Yap. Persistent HCV infection blocks the potential of liver regeneration by E2 protein interacting with CD81.

Support or Funding Information

NIH/NCI R01‐CA103958