Glypican-3 binds to Frizzled and plays a direct role in the stimulation of canonical Wnt signaling.
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, thus facilitating 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 and Frizzled interact directly through the glycosaminoglycan chains of GPC3, indicating that this glypican stimulates the formation of signaling complexes between Wnt and Frizzled. Consistent with this, we show that the binding of Wnt at the cell membrane triggers the endocytosis of a complex that includes Wnt, Frizzled and GPC3. Additional support for 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.
Biological Sciences, Sunnybrook Research Institute, and Department of Medical Biophysics, University of Toronto, 2075 Bayview Avenue, Toronto, ON M4N 3M5, Canada.
Mariana
M
Capurro
Tonya
T
Martin
Wen
W
Shi
Jorge
J
Filmus
D006528:Carcinoma, Hepatocellular / Q000378:metabolism*; D051157:Frizzled Receptors / Q000378:metabolism*; D053673:Glypicans / Q000378:metabolism*; D057809:HEK293 Cells; D006801:Humans; D008113:Liver Neoplasms / Q000378:metabolism*; D051153:Wnt Proteins / Q000378:metabolism*; D060449:Wnt Signaling Pathway / Q000502:physiology*
D051157:Frizzled Receptors; C105937:GPC3 protein, human; D053673:Glypicans; D051153:Wnt Proteins
Frizzled; Glypican-3; Heparan sulphate; Hepatocellular carcinoma; Proteoglycan; Wnt