Plakoglobin at adherens junctions after β‐catenin loss: Mechanism & biological implications in hepatocellular carcinoma

Abstract

β‐Catenin (β‐cat) is important in liver homeostasis as a part of Wnt signaling & cell‐cell adhesions, while aberrant activation is observed in hepatocellular carcinoma (HCC). We previously reported that hepatocyte‐specific β‐cat knockout mice (KO) lack adhesive defects, since its loss is compensated by increased plakoglobin (PG; γ‐catenin) at adherens junctions (AJ) over wildtype (WT) livers. Since PG is a desmosomal protein we asked if its increase in KOs might affect the junction. We saw no change in desmosomal ultrastructure (via TEM), proteins, or their association with PG, besides an increase in Plakophilin‐3. We then utilized an in vitro model to further elucidate the role & regulation of PG after β‐cat knockdown (KD). PG is unable to rescue β‐cat‐ TCF reporter activity; however, it suitably compensates for β‐cat loss at AJs as assessed by scratch‐wound assay, centrifugal assay for cell adhesion & hanging‐drop assay. PG increase is observed in HCC cells only upon decrease in β‐cat protein levels, not after blockade of β‐cat's transactivational activity. There was enhanced serine & threonine phosphorylation of PG after β‐cat KD which was abrogated by inhibiting protein kinases A (PKA) & PKG. In conclusion, PG compensates for β‐cat loss at the AJ without affecting desmosomes, but is unable to fulfill functions in Wnt signaling. Catenin‐sensing mechanism depends on β‐cat protein levels & not activity. Anti‐β‐cat therapies for HCC that affect β‐cat levels may target aberrant Wnt signaling without negatively impacting intercellular adhesion, if mechanisms responsible for PG stabilization are spared.