Targeting GPC3 Strategies for Therapeutic Intervention in Cancer

What is GPC3？

The full name of GPC3 is Glypican-3, which is a member of the heparan sulfate proteoglycan family and is connected to the cell surface through the glycosylphosphatidylinositol anchor on the cell membrane. The GPC3 gene was first detected in a rat epithelial cell line in 1988. In the human body, the GPC3 protein expressed by the GPC3 gene is significantly different in different developmental stages and different tissues, such as low expression or no expression in gastric cancer, breast cancer, ovarian cancer and other cancers, but often in hepatocellular carcinoma is overexpressed.

The Structure of GPC3

GPC3, or Glypican-3, is an oncofetal glycoprotein tethered to the cell membrane via a glycophosphatidylinositol (GPI) anchor[1-2]. Its core protein comprises 580 amino acids and weighs in at 70 kDa. Two heparan sulfate (HS) side chains are attached near the C-terminal section. A furin-mediated cleavage event at the Arg358-Cys359 bond processes the single-chain GPC3 into its mature form, composed of a 40-kDa N-terminal subunit and a 30-kDa C-terminal subunit connected by disulfide bonds[3]. Various GPC3 forms have been observed in the culture supernatant of GPC3-expressing cells and in serum, suggesting potential proteolytic cleavages of its extracellular segment[4].

The GPC3 gene is situated on the X chromosome (Xq26.2) and is recognized as a pivotal regulator of cellular proliferation in embryonic mesodermal tissues. Deletion of the GPC3 gene leads to the development of gigantism/overgrowth syndrome known as Simpson–Golabi–Behmel syndrome (SGBS)[5-6]. Mechanistically, GPC3 likely participates in the regulation of signaling pathways such as Wnt, hedgehog, bone morphogenic protein, and FGF. Through these pathways, it exerts control over cell growth and apoptosis in specific cell types during development[7-8]. GPC3 exhibits widespread expression in embryonic tissues like the placenta, as well as in the liver, lungs, and kidneys. However, in most adult organs, GPC3 is scarcely detectable. This biological downregulation in adult tissues may stem from DNA methylation within the GPC3 promoter region[9-10].

Function of GPC3

GPC3 exhibits a versatile nature in its ability to regulate cell growth, with its effects varying depending on the cell type involved.

In mesodermal embryonic tissues, GPC3 takes on a predominantly negative role. The deletion of the GPC3 gene has been linked to the pathogenesis of Simpson-Golabi-Behmel overgrowth syndrome[11]. Intriguingly, research has unveiled that GPC3 can interact with IGF2, effectively dampening IGF2-mediated growth in vivo. This evidence points toward GPC3's capacity to negatively regulate embryonic and fetal development. Additionally, GPC3 emerges as a negative transcriptional regulator and tumor suppressor, actively curbing the growth of breast, ovary, and lung cancer cells[12-13].

Conversely, in hepatocellular carcinoma (HCC), GPC3 assumes a pro-growth role. It is highly expressed in 70–100% of HCC cases[18]. GPC3 engages with Wnt to facilitate Wnt/Frizzled binding, ultimately promoting HCC growth. When the expression of GPC3 is downregulated in cell cultures, Yap signaling is reduced. Remarkably, soluble GPC3 proteins (GPC3DGPI) function dominantly in a negative manner, competing with endogenous GPC3 to hinder HCC cell growth, possibly by neutralizing GPC3 binding molecules[14]. These studies underscore the proliferative impact of GPC3 in HCC.

GPC3 and Tumor Progression

GPC3 expression has been detected in a range of tumor types, including hepatocellular carcinoma (HCC), lung squamous cell carcinoma (SqCC), gastric carcinoma, ovarian carcinoma, melanomas, and pediatric embryonal tumors. Notably, its expression is particularly elevated in HCC[16-17].

In HCC cells, GPC3 seems to play a significant role in the Wnt/β-catenin signaling pathway, bolstering cell proliferation. The core protein of GPC3 interacts with the Wnt receptor Frizzled (FZD). Recent research has unveiled that the GPC3 core protein acts as a co-receptor for Wnt, promoting Wnt/β-catenin signaling in HCC cells[18]. Upregulated GPC3 accelerates lung SqCC cell progression through a Wnt/β-catenin-dependent mechanism[19]. Furthermore, an enzyme called sulfatase 2 (SULF2) is upregulated in HCC cells, leading to increased release of heparin-binding growth factors, like FGF and HGF, which are attached to the HS sidechains of GPC3. This, in turn, activates signaling pathways mediated by their specific receptors[20-21]. Since Wnt can also attach to HS, SULF2 may enhance Wnt signaling as well. Conversely, soluble GPC3 has been found to suppress cancer cell proliferation[22]. The cleavage of the GPI anchor by sheddases like phospholipase D is thought to release cell surface GPC3. While notum was initially considered a sheddase of GPC3, it has been established that notum acts as a deacetylase, cleaving the palmitoleate moiety of Wnt attached to the HS chain, not the GPI anchor of GPC3, and inhibiting the binding of Wnt to FZD[23]. This suggests that GPC3 may serve as a crucial molecule in cancer cell biology, with cell surface GPI-anchored GPC3 potentially acting as a reservoir and cofactor for paracrine or autocrine growth factors to efficiently transmit their outside-in signaling. Conversely, shedding of GPC3 from the cell surface likely disrupts these signaling pathways. Hence, further studies are necessary to comprehend the molecular mechanisms and regulation of GPC3 shedding from cancer cell surfaces.

Recently, microRNAs (miRNAs) and long noncoding RNAs (lncRNA) have emerged as key regulators of gene expression in cancer cells[24]. Several miRNAs and lncRNAs have been identified as promoters or suppressors of the GPC3/Wnt/β-catenin axis in HCC[25]. Further investigations are warranted to gain deeper insights into the role of miRNAs, lncRNAs, and related axes that are critical in governing GPC3 expression.

Signaling Pathway of GPC3

In the progression of hepatocellular carcinoma (HCC), the activation of canonical Wnt signaling is a prevalent molecular event[26]. In fact, roughly 95% of HCC cases display some form of Wnt/β-catenin deregulation. This Wnt signaling pathway goes awry in various human diseases, including cancers and metabolic disorders.

Humans boast a total of 19 Wnt proteins, secreted through autocrine and paracrine systems. Canonical Wnt signaling, which relies on β-catenin, is initiated when Wnt molecules bind to two coreceptors: frizzled (FZD), a seven-pass transmembrane G protein-coupled receptor (GPCR), and low-density lipoprotein receptor-related protein 5/6 (LRP5/6), a single-pass transmembrane receptor. There are a total of 10 human FZDs[27]. Wnt ligands latch onto FZD's extracellular cysteine-rich domain, which houses the Wnt binding domain. This binding triggers the assembly of the FZD-LRP5/6 receptor complex[28]. Subsequent conformational changes in FZD and LRP5/6, along with the phosphorylation of glycogen synthase kinase 3 and casein kinase 1, facilitate the recruitment of Axin, a critical component of the destruction complex. This, in turn, leads to the recruitment of DVL, a cytoplasmic protein, binding to the C-terminal tail of FZD. Consequently, the destruction complex, which includes DVL, Axin, and other binding partners, becomes stabilized[29]. Axin prevents the degradation of β-catenin, allowing it to accumulate in the cytoplasm. From there, β-catenin translocates to the nucleus, where it drives the transcription of genes responsible for cell proliferation and survival[30]. It's worth noting that Wnt signaling can also function through a β-catenin-independent pathway, known as the non-canonical or alternative pathway[31].

Given the crucial role of Wnt signaling in functions like hepatobiliary processes, cell differentiation, and repair, any dysregulation in this pathway can lead to HCC, hepatoblastoma, cholangiocarcinoma, or other liver diseases.

GPC3 Protein

Recombinant Human GPC3 Protein (C-6His)

Click here for more GPC3

Synonym : Glypican-3; GTR2-2; Intestinal protein OCI-5; MXR7; GPC3; OCI5

References:

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