Serglycin in human cancers

Abstract

Serglycin belongs to a family of small proteoglycans with Ser-Gly dipeptide repeats, and it is modified with different types of glycosaminoglycan side chains. Intracellular serglycin affects the retention and secretion of proteases, chemokines, or other cytokines by physically binding to these factors in secretory granules. Extracellular serglycin has been found to be released by several types of human cancer cells, and it is able to promote the metastasis of nasopharyngeal carcinoma cells. Serglycin can bind to CD44, which is another glycoprotein located in cellular membrane. Serglycin's function of promoting cancer cell metastasis depends on glycosylation of its core protein, which can be achieved by autocrine as well as paracrine secretion mechanisms. Further investigations are warranted to elucidate serglycin signaling mechanisms with the goal of targeting them to prevent cancer cell metastasis.

Serglycin is a proteoglycan that has its core peptide coded by the gene SRGN in humans. It belongs to a family of small proteoglycans with serine-glycine dipeptide repeats and is modified with various glycosaminoglycan side chains. Serglycin is also known as a secretory granule proteoglycan core protein or hematopoietic proteoglycan core protein[1]. Rat Srgn gene was originally cloned and sequenced from the rat yolk sac carcinoma cell line L2 in 1985; human serglycin was isolated from platelets in 1986[2]^,[3]. The amino acid sequence of human serglycin is closely homologous to those of the mouse and rat[4].

Human serglycin consists of a core protein decorated with glycosaminoglycan chains, for example, chondroitin-4-sulfate (CS-4), CS-6, CS-E, CS-B, or heparin[5]. The core protein of human serglycin has 158 amino acid residues that form three functional domains. A signal peptide domain, encoded by exon 1, is composed of amino acid residues 1–27; the N-terminal domain, encoded by exon 2, is composed of amino acid residues 28–76; and the glycosylation domain, encoded by exon 3, is composed of amino acid residues 77–158[6], as shown in Figure 1.

Figure 1. The protein structure of serglycin.

Previous studies have shown that there are two forms of serglycin in human cells, intracellular and extracellular[7]. Intracellular serglycin is a key mediator of granulopoiesis in mast cells, cytolytic T lymphocytes (CTLs), and neutrophils[8]^–[11], and it is involved in the retention or secretion of proteases, histamine, cytokines, and chemokines in the storage granules of mast cells or canine kidney cells[12]^–[15]. Extracellular serglycin released from the storage granules of mast cells and CTLs (or from monocytes, macrophages, and endothelial cells that constitutively secrete serglycin) has been shown to interact with CD44 in hematopoietic cells, suggesting that serglycin may take on an important role in cell-cell interactions[16]^–[18]. Secretion of serglycin is involved in the release of tissue-type plasminogen activator from endothelial cells, tumor necrosis factor-α from macrophages, and matrix metalloproteinase-9 (MMP-9) from monocytes[19]^–[21]. Although serglycin-knockout mice have been generated by Abrink et al.[22] the biological functions of serglycin have not been studied in detail.

In this article, we focus on the biological roles of serglycin in tumor metastasis as well as some related physiological characteristics of this interesting molecule.

Expression of Serglycin in Hematopoietic Cells

Both serglycin mRNA and protein have been detected in human hematopoietic cancer cell lines. Niemann et al.[23] found that serglycin is distinctively expressed in acute myeloid leukemia (AML) relative to lymphoblastic leukemia. Serglycin is also a selective marker distinguishing AML from Philadelphia chromosome-negative chronic myeloproliferative disorders[23]. In multiple myeloma cells, serglycin is a secreted protein decorated predominantly with CS-4, suggesting that serglycin has an effect on inhibition of bone mineralization[24]. Moreover, serglycin demonstrates a role in protecting myeloma cells from complement attacks induced by antibody immunotherapy, therefore promoting the survival of malignant myeloma cells[25]. Up-regulation of serglycin expression is found in drug-resistant tumor cell lines of hematopoietic origin, indicating that serglycin may be involved in the drug resistance of human cancer cells[26]. In a variety of hematopoietic cells, including myelomonocytes, macrophages, and lymphoma, myeloma, mastochytoma, or thymoma cells, serglycin has been shown to interact with cell surface protein CD44 if serglycin has attached CS-4 or CS-6 moieties, but not heparin or heparan sulfate[16].

Serglycin Promotes Metastasis of Nasopharyngeal Carcinoma Cells

Serglycin is highly expressed in high-metastasis nasopharyngeal carcinoma (NPC) cells[27]. In primary NPC tissues, a higher level of serglycin serves as an independent prognostic indicator for disease-free survival and distant metastasis-free survival of patients. In vitro and in vivo studies have proven that serglycin can promote motility, invasion, and metastasis of NPC cells via induction of a mesenchymal molecule vimentin [27]. This important function of serglycin depends on full glycosylation of the core protein[27]. In NPC cells, intracellular serglycin has a molecular weight of about 130 kDa, but the secreted form is about 300 kDa. Overexpression of serglycin by NPC cells dramatically increases only the amount of secreted serglycin but does not alter the intracellular serglycin level, suggesting that its metastasis-promoting function is mainly determined by the secreted form[27].

Serglycin Secretory Mechanism in Human Cells

Sequence analyses of the serglycin core protein revealed that amino acid residues 1–27 form the signal peptide[5]. In some hematopoietic cells, including monocytes or macrophages, myeloma cells, and human endothelial cells, serglycin is constitutively secreted [19]^,[20]^,[24]^,[25]^,[28]^,[29]. In other hematopoietic cells, including neutrophils and mast cells, serglycin is stored in secretory granules and secreted after cell activation[9]^,[13]. Cytokines, chemokines, and proteases interact with serglycin during secretion from secretory granules and can still be in complex with serglycin after its release from cells[12]^,[30]^–[34]. In the extracellular matrix (ECM), serglycin is involved in the activation of MMPs, which have important roles in inflammation, wound repair, cellular invasion, and other fundamental processes[21]^,[35]^,[36]. Secretion of tumor necrosis factor from macrophages has been reported to be regulated by serglycin[20]. In addition, serglycin from CTLs or natural killer (NK) cells can form macro-complexes with granzyme B and perforin to induce the apoptosis of target cells[37]^–[40]. The apoptosis-promoting effect of serglycin was found in mast cells, and this effect was associated with the release of serglycin and serglycin-dependent proteases into the cytosol[41]. Studies of serglycin released from human cancer cells should provide insight into new signaling pathways through which serglycin is involved in tumor progression.

Diversity of Glycan Chains on Serglycin

The glycosaminoglycan chains attached to serglycin vary among different cell types. In hematopoietic cells—including platelets, CTLs, NK cells, and mucosal mast cells (other than mast cells in connective tissues or the peritoneal cavity)—chondroitin sulfate is the major glycan, and CS-4 is the dominant form in most of these cell types[42]. Heparin is the glycan attachment to serglycin in peritoneal or connective tissue mast cells[15].

The localization of serglycin is determined by the types of glycan attached to the serglycin protein. In Madin-Darby canine kidney (MDCK) cells, chondroitin sulfate serglycin is generally secreted into the apical medium and heparin serglycin is mainly transported to basolateral membrane of the cells[43]^–[45]. In human endothelial cells, serglycin is a major proteoglycan that carries mainly chondroitin sulfate chains and some heparin chains. A major amount of serglycin is secreted into apical medium and a small amount co-localizes with growth-related Oncogene α (GROα /CXCL1) in vesicles[46]. In acinar pancreatic cells, serglycin without attached glycans is not able to be sorted into secretory granules[47]. Glycan modification of serglycin is a key factor determining its localization and biological roles in mammalian cells. Although serglycin's function in promoting metastasis depends on full glycosylation[27], glycan modification of serglycin is poorly understood in human cancer cells.

Serglycin Signaling for Clinical Applications

As a secreted glycoprotein, serglycin could be an ideal serum marker for predicting and monitoring cancer progression. Further, serglycin's role in promoting metastasis depends on interaction between the secreted protein and cancer cells, making that interaction an ideal drug target. Serglycin promotes NPC cell metastasis via autocrine and paracrine signaling[27], but how secreted serglycin triggers the cascade leading to cellular motility remains unknown. It is therefore crucial to identify the membrane-binding protein(s) of serglycin that help promote migration, invasion, and metastasis.

Serglycin binds to CD44 on hematopoietic cells[16]. CD44 itself is a glycosylated protein found at cell surface, and it is involved in cell-cell interactions, cell adhesion, and cellular migration[48]^,[49]. CD44 is a cancer stem cell marker for a variety of cancer types, including carcinomas of the pancreas, colon, ovary, breast, and liver[50]^–[54]. Some aggressive behaviors of cancer cells, e.g., apoptosis resistance, epithelial-mesenchymal transition, and metastasis, have been linked to CD44[49]. It is therefore important to clarify whether serglycin signaling depends on binding to CD44 at the cell surface.

Summary

Serglycin is widely expressed in various hematopoietic cells and highly metastatic NPC cells, and it is an important molecule in regulating NPC metastasis. Although great potential for clinical applications can be predicted, the exact molecular mechanism of serglycin signaling remains unclear. Further explorations in identifying serglycin-binding proteins and its signaling cascade in promoting cellular motility are needed.