Proteoglycans (PGs) are pivotal components of extracellular matrices, involved in a variety of processes such as migration, invasion, morphogenesis, differentiation, drug resistance, and epithelial-to-mesenchymal transition (EMT). Cellular plasticity is a crucial intermediate phenotypic state acquired by cancer cells, which can modulate EMT and the generation of cancer stem cells (CSCs). PGs affect cell plasticity, stemness, and EMT, altering the cellular shape and functions. PGs control these functions, either by direct activation of signaling cascades, acting as co-receptors, or through regulation of the availability of biological compounds such as growth factors and cytokines. Differential expression of microRNAs is also associated with the expression of PGs and their interplay is implicated in the fine tuning of cancer cell phenotype and potential. This review summarizes the involvement of PGs in the regulation of EMT and stemness of cancer cells and highlights the molecular mechanisms.

Serglycin (SRGN) is a pro-tumorigenic proteoglycan expressed and secreted by various aggressive tumors including glioblastoma (GBM). In our study, we investigated the interplay and biological outcomes of SRGN with TGFβRI, CXCR-2 and inflammatory mediators in GBM cells and fibroblasts. SRGN overexpression is associated with poor survival in GBM patients. High SRGN levels also exhibit a positive correlation with increased levels of various inflammatory mediators including members of TGFβ signaling pathway, cytokines and receptors including CXCR-2 and proteolytic enzymes in GBM patients. SRGN-suppressed GBM cells show decreased expressions of TGFβRI associated with lower responsiveness to the manipulation of TGFβ/TGFβRI pathway and the regulation of pro-tumorigenic properties. Active TGFβRI signaling in control GBM cells promotes their proliferation, invasion, proteolytic and inflammatory potential. Fibroblasts cultured with culture media derived by control SRGN-expressing GBM cells exhibit increased proliferation, migration and overexpression of cytokines and proteolytic enzymes including CXCL-1, IL-8, IL-6, IL-1β, CCL-20, CCL-2, and MMP-9. Culture media derived by SRGN-suppressed GBM cells fail to induce the above properties to fibroblasts. Importantly, the activation of fibroblasts by GBM cells not only relies on the expression of SRGN in GBM cells but also on active CXCR-2 signaling both in GBM cells and fibroblasts.

Serglycin (SRGN) has been found overexpressed and secreted in glioblastoma (GBM), associated with tumorigenic signaling and poor prognosis. In this study, we aimed to elucidate the involvement of SRGN in the unfolded protein response (UPR), an oncogenic signaling pathway implicated in protein recycling and cell fate. Herein, we developed stably transduced LN-18^shSCR GBM cells, expressing high levels of SRGN, and SRGN-depleted LN-18^shSRGN cells. We observed significantly attenuated expression and activity of all UPR mediators upon SRGN suppression, in particular PERK, IRE1, ATF6 and downstream effectors. SRGN-expressing cells possessed a constitutively active UPR, as indicated by its active phosphorylation status and accumulated pool of nuclear ATF4 in LN-18^shSCR cells. Constitutive activation of the caspase-dependent apoptotic pathway was apparent in LN-18^shSRGN cells. Induction of endoplasmic reticulum (ER) stress pointed out that LN-18^shSRGN cells were predisposed to ER stress-associated cell death, whereas LN-18^shSCR cells activated adaptive UPR signaling and displayed resistance to apoptosis. The evaluation of TLRs, TNFRs, ILs and NF-kB also underscored that SRGN is essential for their expression and active inflammatory signaling. We concluded that SRGN-expressing cells acquire a pro-survival UPR mechanism, highlighting the novel regulatory role of SRGN in the adaptation and survival of GBM cells.