Serglycin as part of IL-1β induced inflammation in human chondrocytes

Highlights

• Proteoglycans are involved in inflammatory joint diseases.

• Chondrocytes guarantee the maintaining of cartilage functions and homeostasis.

• IL-1β induce serglycin expression and production in human chondrocytes.

• Serglycin-CD44 binding enhance NF-kB activity in IL-1β activated human chondrocytes.

• Serglycin plays a role in immune/inflammatory response.

Abstract

Serglycin (SRGN) is an intracellular proteoglycan produced and secreted by several cell types. The increased expression of SRGN was associated with greater aggressiveness in cancer and inflammation.

In this study, we demonstrated that SRGN is increased in human chondrocytes after IL-β stimulation. Furthermore, we found that secreted SRGN was able to bind the CD44 receptor thus participating in the extension of the inflammatory response. Using SRGN knockdown cells we observed a significantly decrease in specific inflammatory markers and NF-kB activation. Similar results were observed by blocking the CD44 receptor.

These data provide further evidences for a direct involvement of SRGN in the mechanisms regulating the non-infectious chondrocytes damage, and the consequent joint inflammation and cartilage destruction in arthritis.

Introduction

Proteoglycans (PGs) are complex macromolecules characterized by a central core protein covalently linked with glycosamminoglycan (GAGs) chains named chondroitin sulfate (CS), dermatan sulfate (DS), keratan sulphate (KS), heparin (HP), and heparan sulfate (HS) [1,2]. All cells synthetize PGs that, because of their ability to interact with a wide number of molecules, are involved in different physiological and pathological functions [[1], [2], [3]].

According to the classification proposed in 2015 by R V. Iozzo and L. Schaefer, PGs may be divided in four major groups: intracellular, cell surface, pericellular and extracellular [4]. Several reports have evidenced that the PGs expression is significantly altered during the onset and the progression of various diseases [2,[5], [6], [7], [8], [9], [10]]. Therefore, their overproduction has also been associated with the amplification and progression of the inflammatory mechanism. In the present study we focused our attention on SRGN the only intracellular proteoglycan characterized by its ability to interact with a number of biological molecules [2,5]. The human SRGN gene is located on chromosome 10q.22.1 and the translated protein has a molecular weight of 18kD characterized by a 9 serines in a serine-glycine repeat region [11,12]. An alternative splicing variant, lacking exon 2, was also described both in normal and cancer cells [13,14]. The Serine residues in these regions represent a potential GAGs attachment site [12].

In particular, it has been demonstrated that SRGN was involved in the storage and secretion of inflammatory mediators in hematopoietic lineages [15], but also in others non-hematopoietic cells such as smooth muscle cells [16], endothelial cells [16,17], fibroblasts [16,18] and chondrocytes [19].

In human hematopoietic and non-hematopoietic tumors SRGN enhances inflammation, promotes tumor growth and metastasis and its increase is associated with a poor prognosis [1,20].

SRGN is also released during inflammation induced following exogen treatment with liposaccharide (LPS) in macrophages [21], monocytes [22] and chondrocytes [23], as well as with tumor necrosis factor (TNF-α) in endothelial cells [17] and adipocytes [24], and treatment with interleukin 1β (IL-1β) of smooth muscle cells [25] and human umbilical cord vein endothelial cells (HUVECs) [26]. These data suggest that SRGN production represents a potent inflammatory mediator that takes part in different pathological conditions. CD44 is a transmembrane glycoprotein expressed on cell surface of several cell types [27] whose the principal ligand is recognized to be the GAG hyaluronic acid (HA) [28]. CD44 is also able to interact with other extracellular matrix components, including SRGN that is able to induce signaling activation [1,29,30].

On the basis of these previous evidences the aim of this study was to investigate whether SRGN play a role in the inflammatory response in human chondrocytes exposed to IL-1β and the involvement of the CD44 receptor.