Structural and functional implications of plakoglobin compensation due to β‐catenin loss in the liver

Abstract

β‐Catenin is important in liver homeostasis as a part of Wnt signaling and cell‐cell adhesions. We previously reported that hepatocyte‐specific β‐catenin knockout (KO) mice show higher plakoglobin (PG; γ‐catenin) levels over wild‐type (WT) livers and also maintain intact adherens junctions (AJ) as a result of PG compensation. Since PG is a desmosomal protein we wanted to see if its increase in the KOs comes at the expense of desmosomes. Here we show no change in the desmosomes in the KO livers by protein analysis and TEM. Next we wanted to address functionality of PG's compensation for β‐catenin loss especially in Wnt signaling. We show that knocking down PG in Hep3B human hepatoma cells only slightly decreases TCF reporter (TOPflash) activity, indicating that PG is able bind TCF and participate in Wnt signaling. However, knock down of β‐catenin shows a dramatically greater drop in TOPflash activity. Next, we wanted to determine if changes in PG affect proliferation and migration. Changes in PG levels do not affect thymidine incorporation rates. However, scratch‐wound assays reveal that β‐catenin knock down (and subsequent PG increase) increases migration of Hep3Bs. Also, PG knock down decreases migration, indicating that PG may play a role in cell motility. Thus, β‐catenin loss is compensated by PG at the AJ without affecting desmosomes; however, the functions of β‐catenin as a part of the Wnt pathway remain unfulfilled by PG. Moreover, changes in PG levels also affect the migratory potential of human hepatoma cells. Research support: NIH grant 1R01DK62277.