Glypican-3 binds to frizzled and plays a direct role in the stimulation of canonical Wnt signaling

Abstract

Glypican-3 (GPC3) is a proteoglycan that is bound to the cell surface. It is expressed by most hepatocellular carcinomas (HCCs), but not by normal hepatocytes. GPC3 stimulates HCC growth by promoting canonical Wnt signaling. Because glypicans interact with Wnts, it has been proposed that these proteoglycans stimulate signaling by increasing the amount of Wnt at the cell membrane, facilitating in this way the interaction of this growth factor with its signaling receptor Frizzled. However, in this study we demonstrate that GPC3 plays a more direct role in the stimulation of Wnt signaling. Specifically, we show that, in addition to interacting with Wnt, GPC3 directly binds to Frizzled through its glycosaminoglycan chains, indicating that this glypican stimulates the formation of signaling complexes between these two proteins. Consistent with this, we show that Wnt binding at the cell membrane triggers the endocytosis of a complex that includes Wnt, Frizzled and GPC3. Additional support to our model is provided by the finding that Glypican-6 (GPC6) inhibits canonical Wnt signaling despite the fact that it binds to Wnt at the cell membrane.