Glypicans as Cancer Therapeutic Targets

Abstract

Glypicans are a group of cell-surface glycoproteins in which heparan sulfate (HS) glycosaminoglycan chains are covalently linked to a protein core. The glypican gene family is broadly conserved across animal species and plays important roles in biological processes. Glypicans can function as coreceptors for multiple signaling molecules known for regulating cell growth, motility, and differentiation. Some members of the glypican family, including glypican 2 (GPC2) and glypican 3 (GPC3), are expressed in childhood cancers and liver cancers, respectively. Antibody-based therapies targeting glypicans are being investigated in preclinical and clinical studies, with the goal of treating solid tumors that do not respond to standard therapies. These studies may establish glypicans as a new class of therapeutic targets for treating cancer.

Keywords: Wnt signaling; antibody-based therapy; childhood cancer; glypican; liver cancer.

Figure 1.. GPC3 Promotes Wnt Signaling through the Core Protein and HS Chains.

GPC3 interacts with frizzled through HS chains. Meanwhile, the HS chains also contain Wnt specific binding motif which needs 6-O-sulfation with 4 disaccharides in length (the light blue region). When an additional 3-O-sulfation presents, the region that recognized by Wnt could as short as 3 disaccharides in length (the light apricot region). GPC3 rescues circulating Wnt and acts as a Wnt storage site on cell surface through the HS chains. When SULF2 reduces the modification of the 6-O-sulfation on HS chains, the stored Wnt is released and forms a tripartite complex with glypican-3 core protein and frizzled, therefore activates canonical Wnt signaling, and promotes HCC cell proliferation.

Figure 2.. GPC3 Inhibits Hedgehog Signaling by Competing with Patched for Hh Binding.

GPC3 binds to Shh and Ihh with high affinity. LRP1 promotes glypican-3/Hh complex internalization in a HS dependent way. The internalized Hh is transported to lysosome for degradation. Therefore, patched without Hh binding inhibits Smoothened activation and blocks Hedgehog signaling.

Figure 3.. Antibody-based Therapies Targeting Glypicans in Cancer.

ADC: antibody- drug-conjugate; CAR: chimeric antigen receptor; CTL: cytotoxic T lymphocyte; TCR: T- cell receptor; MHC: major histocompatibility complex; APC: antigen-presenting cell; GPC: glypican; ER: endoplasmic reticulum (adapted with permission from [96]).

Figure I, Text Box 1.. Glypicans Are Highly Conserved in Animals.

Phylogenetic comparison was carried out for glypicans protein sequences from Homo sapiens, Mus musculus, Xenopus tropicalis, Danio rerio and Drosophila melanogaster. The human genome has six glypican family members that can be grouped into two subfamilies: GPC1/2/4/6 and GPC3/5. Phylogenetic comparison reveals that all mouse, frog and fish proteins cluster with human orthologs. There are two glypicans–Dally and Dally-like protein (Dlp) in Drosophila melanogaster. Dally is an ortholog of the human GPC3/5 subfamily, and Dlp is an ortholog of the GPC1/2/4/6 subfamily. The evolutionary tree was constructed using Neighbor-Joining method in MEGA7 software using the Poisson model and partially deleted dataset. 1000 bootstrap replications were used as a test of phylogeny and the values are indicated next to the branch. Branch length corresponds to evolutionary distances that denote the number of amino acid substitutions per site. Scale bar: 0.1.

Figure II, Text Box 2.. Structure of Dlp in Drosophila melanogaster and GPC1 in Humans.

The termini are labeled, and the N, M, and C lobes are indicated. The N-, M-, and C-lobes are named according to their relative spatial position in the protein. The disulfide bonds are indicated (red). Both proteins show cylindrical-like structures that are conserved between Drosophila melanogaster and humans.