Processing by Convertases Is Not Required for Glypican-3-induced Stimulation of Hepatocellular Carcinoma Growth

Jorge Filmus,Wen Shi,Shaifali Sandal,Mariana I. Capurro

doi:10.1074/jbc.m507004200

Abstract

Glypicans are a family of heparan sulfate proteoglycans that are bound to the cell surface by a lipid anchor. Six members of this family have been identified in mammals (GPC1-GPC6). Glypicans act as regulators of the activity of various cytokines, including Wnts, Hedgehogs, and bone morphogenetic proteins. It has been reported that processing by a convertase is required for GPC3 activity during convergent extension in zebrafish embryos, for GPC3-induced regulation of Wnt signaling, and for the binding of GPC3 to Wnt5a. In our laboratory, we have recently demonstrated that GPC3 promotes the growth of hepatocellular carcinomas (HCCs) by stimulating canonical Wnt signaling. Because there is increasing evidence indicating that the structural requirements for GPC3 activity are cell type specific, we decided to investigate whether GPC3 needs to be processed by convertases to stimulate cell proliferation and Wnt signaling in HCC cells. We report here that a mutant GPC3 that cannot be processed by convertases is still able to play its stimulatory role in Wnt activity and HCC growth.

Highlights

Genetic and functional studies performed in Drosophila, Xenopus, zebrafish, and mammals have demonstrated that glypicans can stimulate the signaling activity of Wnts, Hedgehogs, and bone morphogenetic proteins [7,8,9,10,11,12,13,14,15,16,17]
We report here that a mutant GPC3 that cannot be cleaved by convertases is still able to stimulate the growth of hepatocellular carcinomas (HCCs) cells
GPC3 Cleavage by Convertases Is Not Required for the Stimulation of HCC Cell Proliferation—We have previously shown that ectopic GPC3 stimulates the in vitro and in vivo growth of PLC-PRF-5 and HLF, two HCC cell lines that express very little and no endogenous GPC3, respectively [21]

Summary

Introduction

Genetic and functional studies performed in Drosophila, Xenopus, zebrafish, and mammals have demonstrated that glypicans can stimulate the signaling activity of Wnts, Hedgehogs, and bone morphogenetic proteins [7,8,9,10,11,12,13,14,15,16,17]. It should be noted that a portion of the smear corresponds to the glycanation products of the full-length GPC3, because analysis with the anti-HA antibody shows that not all the GPC3 that is on the cell surface is processed by convertases in transiently transfected 293T cells.

Results

Conclusion