Secretion of proteases in serglycin transfected Madin-Darby canine kidney cells

Ensemble learning model for identifying the hallmark genes of NFκB/TNF signaling pathway in cancers

BACKGROUND

The nuclear factor kappa B (NFκB) regulatory pathways downstream of tumor necrosis factor (TNF) play a critical role in carcinogenesis. However, the widespread influence of NFκB in cells can result in off-target effects, making it a challenging therapeutic target. Ensemble learning is a machine learning technique where multiple models are combined to improve the performance and robustness of the prediction. Accordingly, an ensemble learning model could uncover more precise targets within the NFκB/TNF signaling pathway for cancer therapy.

METHODS

In this study, we trained an ensemble learning model on the transcriptome profiles from 16 cancer types in the TCGA database to identify a robust set of genes that are consistently associated with the NFκB/TNF pathway in cancer. Our model uses cancer patients as features to predict the genes involved in the NFκB/TNF signaling pathway and can be adapted to predict the genes for different cancer types by switching the cancer type of patients. We also performed functional analysis, survival analysis, and a case study of triple-negative breast cancer to demonstrate our model's potential in translational cancer medicine.

RESULTS

Our model accurately identified genes regulated by NFκB in response to TNF in cancer patients. The downstream analysis showed that the identified genes are typically involved in the canonical NFκB-regulated pathways, particularly in adaptive immunity, anti-apoptosis, and cellular response to cytokine stimuli. These genes were found to have oncogenic properties and detrimental effects on patient survival. Our model also could distinguish patients with a specific cancer subtype, triple-negative breast cancer (TNBC), which is known to be influenced by NFκB-regulated pathways downstream of TNF. Furthermore, a functional module known as mononuclear cell differentiation was identified that accurately predicts TNBC patients and poor short-term survival in non-TNBC patients, providing a potential avenue for developing precision medicine for cancer subtypes.

CONCLUSIONS

In conclusion, our approach enables the discovery of genes in NFκB-regulated pathways in response to TNF and their relevance to carcinogenesis. We successfully categorized these genes into functional groups, providing valuable insights for discovering more precise and targeted cancer therapeutics.

Serglycin activates pro-tumorigenic signaling and controls glioblastoma cell stemness, differentiation and invasive potential

Despite the functional role of serglycin as an intracellular proteoglycan, a variety of malignant cells depends on its expression and constitutive secretion to advance their aggressive behavior. Serglycin arose to be a biomarker for glioblastoma, which is the deadliest and most treatment-resistant form of brain tumor, but its role in this disease is not fully elucidated. In our study we suppressed the endogenous levels of serglycin in LN-18 glioblastoma cells to decipher its involvement in their malignant phenotype. Serglycin suppressed LN-18 (LN-18shSRGN) glioblastoma cells underwent astrocytic differentiation characterized by induced expression of GFAP, SPARCL-1 and SNAIL, with simultaneous loss of their stemness capacity. In particular, LN-18shSRGN cells presented decreased expression of glioma stem cell-related genes and ALDH1 activity, accompanied by reduced colony formation ability. Moreover, the suppression of serglycin in LN-18shSRGN cells retarded the proliferative and migratory rate, the invasive potential in vitro and the tumor burden in vivo. The lack of serglycin in LN-18shSRGN cells was followed by G2 arrest, with subsequent reduction of the expression of cell-cycle regulators. LN-18shSRGN cells also exhibited impaired expression and activity of proteolytic enzymes such as MMPs, TIMPs and uPA, both in vitro and in vivo. Moreover, suppression of serglycin in LN-18shSRGN cells eliminated the activation of pro-tumorigenic signal transduction. Of note, LN-18shSRGN cells displayed lower expression and secretion levels of IL-6, IL-8 and CXCR-2. Concomitant, serglycin suppressed LN-18shSRGN cells demonstrated repressed phosphorylation of ERK1/2, p38, SRC and STAT-3, which together with PI3K/AKT and IL-8/CXCR-2 signaling control LN-18 glioblastoma cell aggressiveness. Collectively, the absence of serglycin favors an astrocytic fate switch and a less aggressive phenotype, characterized by loss of pluripotency, block of the cell cycle, reduced ability for ECM proteolysis and pro-tumorigenic signaling attenuation.

Serglycin promotes breast cancer cell aggressiveness: Induction of epithelial to mesenchymal transition, proteolytic activity and IL-8 signaling

Serglycin is an intracellular proteoglycan that is expressed and constitutively secreted by numerous malignant cells, especially prominent in the highly-invasive, triple-negative MDA-MB-231 breast carcinoma cells. Notably, de novo expression of serglycin in low aggressive estrogen receptor α (ERα)-positive MCF7 breast cancer cells promotes an aggressive phenotype. In this study, we discovered that serglycin promoted epithelial to mesenchymal transition (EMT) in MCF7 cells as shown by increased expression of mesenchymal markers vimentin, fibronectin and EMT-related transcription factor Snail2. These phenotypic traits were also associated with the development of drug resistance toward various chemotherapy agents and induction of their proteolytic potential as shown by the increased expression of matrix metalloproteinases, including MMP-1, MMP-2, MMP-9, MT1-MMP and up-regulation of urokinase-type plasminogen activator. Knockdown of serglycin markedly reduced the expression of these proteolytic enzymes in MDA-MB-231 cells. In addition, serglycin expression was closely linked to a pro-inflammatory gene signature including the chemokine IL-8 in ERα-negative breast cancer cells and tumors. Notably, serglycin regulated the secretion of IL-8 in breast cancer cells independently of their ERα status and promoted their proliferation, migration and invasion by triggering IL-8/CXCR2 downstream signaling cascades including PI3K, Src and Rac activation. Thus, serglycin promotes the establishment of a pro-inflammatory milieu in breast cancer cells that evokes an invasive mesenchymal phenotype via autocrine activation of IL-8/CXCR2 signaling axis.

Serglycin is part of the secretory repertoire of LPS-activated monocytes

Monocytes play multiple roles in the immune system, and are active in both acute and chronic diseases. Patients exposed to bacterial infections depend on monocytes in defense reactions, but excessive immune reactions may also cause morbidity through systemic inflammatory responses. Few studies have addressed the importance of proteoglycans, and in particular, the hematopoietic serglycin, in such monocyte immune reactions. Adherent primary monocytes were cultured in absence and presence of LPS. Media were analyzed by ELISA for detection of serglycin. Lysed cell fractions were used to determine the mRNA level of serglycin. Monocytes were also cultured on chamber slides to investigate if serglycin could be detected intracellularly by immunocytochemistry. Monocytes secreted serglycin, and LPS-stimulation increased the secretion. Secretion of inflammatory cytokines increased to a larger extent than serglycin. mRNA levels of serglycin were also increased, suggesting both increased expression and secretion. Immunocytochemistry revealed the presence of serglycin in intracellular vesicles, many destined for secretion. Serglycin containing vesicles increased in number and size when the cells were exposed to LPS. Intracellular vesicle localization and secretion of the proteoglycan serglycin is shown for the first time in primary human monocytes. Monocyte activation by LPS increased the expression and secretion of serglycin, suggesting roles for serglycin in inflammatory processes.

Cell-matrix interactions: focus on proteoglycan-proteinase interplay and pharmacological targeting in cancer

Proteoglycans are major constituents of extracellular matrices, as well as cell surfaces and basement membranes. They play key roles in supporting the dynamic extracellular matrix by generating complex structural networks with other macromolecules and by regulating cellular phenotypes and signaling. It is becoming evident, however, that proteolytic enzymes are required partners for matrix remodeling and for modulating cell signaling via matrix constituents. Proteinases contribute to all stages of diseases, particularly cancer development and progression, and contextually participate in either the removal of damaged products or in the processing of matrix molecules and signaling receptors. The dynamic interplay between proteoglycans and proteolytic enzymes is a crucial biological step that contributes to the pathophysiology of cancer and inflammation. Moreover, proteoglycans are implicated in the expression and secretion of proteolytic enzymes and often modulate their activities. In this review, we describe the emerging biological roles of proteoglycans and proteinases, with a special emphasis on their complex interplay. We critically evaluate this important proteoglycan-proteinase interactome and discuss future challenges with respect to targeting this axis in the treatment of cancer.

Serglycin: at the crossroad of inflammation and malignancy

Serglycin has been initially characterized as an intracellular proteoglycan expressed by hematopoietic cells. All inflammatory cells highly synthesize serglycin and store it in granules, where it interacts with numerous inflammatory mediators, such as proteases, chemokines, cytokines, and growth factors. Serglycin is implicated in their storage into the granules and their protection since they are secreted as complexes and delivered to their targets after secretion. During the last decade, numerous studies have demonstrated that serglycin is also synthesized by various non-hematopoietic cell types. It has been shown that serglycin is highly expressed by tumor cells and promotes their aggressive phenotype and confers resistance against drugs and complement system attack. Apart from its direct beneficial role to tumor cells, serglycin may promote the inflammatory process in the tumor cell microenvironment thus enhancing tumor development. In the present review, we discuss the role of serglycin in inflammation and tumor progression.

Serglycin is implicated in the promotion of aggressive phenotype of breast cancer cells

Serglycin is a proteoglycan expressed by some malignant cells. It promotes metastasis and protects some tumor cells from complement system attack. In the present study, we show for the first time the in situ expression of serglycin by breast cancer cells by immunohistochemistry in patients' material. Moreover, we demonstrate high expression and constitutive secretion of serglycin in the aggressive MDA-MB-231 breast cancer cell line. Serglycin exhibited a strong cytoplasmic staining in these cells, observable at the cell periphery in a thread of filaments near the cell membrane, but also in filopodia-like structures. Serglycin was purified from conditioned medium of MDA-MB-231 cells, and represented the major proteoglycan secreted by these cells, having a molecular size of ~ 250 kDa and carrying chondroitin sulfate side chains, mainly composed of 4-sulfated (~ 87%), 6-sulfated (~ 10%) and non-sulfated (~ 3%) disaccharides. Purified serglycin inhibited early steps of both the classical and the lectin pathways of complement by binding to C1q and mannose-binding lectin. Stable expression of serglycin in less aggressive MCF-7 breast cancer cells induced their proliferation, anchorage-independent growth, migration and invasion. Interestingly, over-expression of serglycin lacking the glycosaminoglycan attachment sites failed to promote these cellular functions, suggesting that glycanation of serglycin is a pre-requisite for its oncogenic properties. Our findings suggest that serglycin promotes a more aggressive cancer cell phenotype and may protect breast cancer cells from complement attack supporting their survival and expansion.

Cell-surface serglycin promotes adhesion of myeloma cells to collagen type I and affects the expression of matrix metalloproteinases

Serglycin (SG) is mainly expressed by hematopoetic cells as an intracellular proteoglycan. Multiple myeloma cells constitutively secrete SG, which is also localized on the cell surface in some cell lines. In this study, SG isolated from myeloma cells was found to interact with collagen type I (Col I), which is a major bone matrix component. Notably, myeloma cells positive for cell-surface SG (csSG) adhered significantly to Col I, compared to cells lacking csSG. Removal of csSG by treatment of the cells with chondroitinase ABC or blocking of csSG by an SG-specific polyclonal antibody significantly reduced the adhesion of myeloma cells to Col I. Significant up-regulation of expression of the matrix metalloproteinases MMP-2 and MMP-9 at both the mRNA and protein levels was observed when culturing csSG-positive myeloma cells on Col I-coated dishes or in the presence of soluble Col I. MMP-9 and MMP-2 were also expressed in increased amounts by myeloma cells in the bone marrow of patients with multiple myeloma. Our data indicate that csSG of myeloma cells affects key functional properties, such as adhesion to Col I and the expression of MMPs, and imply that csSG may serve as a potential prognostic factor and/or target for pharmacological interventions in multiple myeloma.

Serglycin in human cancers

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 proteoglycan promotes apoptotic versus necrotic cell death in mast cells

The mechanisms that govern whether a cell dies by apoptosis or necrosis are not fully understood. Here we show that serglycin, a secretory granule proteoglycan of hematopoietic cells, can have a major impact on this decision. Wild type and serglycin(-/-) mast cells were equally sensitive to a range of cell death-inducing regimens. However, whereas wild type mast cells underwent apoptotic cell death, serglycin(-/-) cells died predominantly by necrosis. Investigations of the underlying mechanism revealed that cell death was accompanied by leakage of secretory granule compounds into the cytosol and that the necrotic phenotype of serglycin(-/-) mast cells was linked to defective degradation of poly(ADP-ribose) polymerase-1. Cells lacking mouse mast cell protease 6, a major serglycin-associated protease, exhibited similar defects in apoptosis as observed in serglycin(-/-) cells, indicating that the pro-apoptotic function of serglycin is due to downstream effects of proteases that are complex-bound to serglycin. Together, these findings implicate serglycin in promoting apoptotic versus necrotic cell death.

Serglycin is a theranostic target in nasopharyngeal carcinoma that promotes metastasis

Nasopharyngeal carcinoma (NPC) is known for its high-metastatic potential. Here we report the identification of the proteoglycan serglycin as a functionally significant regulator of metastasis in this setting. Comparative genomic expression profiling of NPC cell line clones with high- and low-metastatic potential revealed the serglycin gene (SRGN) as one of the most upregulated genes in highly metastatic cells. RNAi-mediated inhibition of serglycin expression blocked serglycin secretion and the invasive motility of highly metastatic cells, reducing metastatic capacity in vivo. Conversely, serglycin overexpression in poorly metastatic cells increased their motile behavior and metastatic capacity in vivo. Growth rate was not influenced by serglycin in either highly or poorly metastatic cells. Secreted but not bacterial recombinant serglycin promoted motile behavior, suggesting a critical role for glycosylation in serglycin activity. Serglycin inhibition was associated with reduced expression of vimentin but not other epithelial-mesenchymal transition proteins. In clinical specimens, serglycin expression was elevated significantly in liver metastases from NPC relative to primary NPC tumors. We evaluated the prognostic value of serglycin by immunohistochemical staining of tissue microarrays from 263 NPC patients followed by multivariate analyses. High serglycin expression in primary NPC was found to be an unfavorable independent indicator of distant metastasis-free and disease-free survival. Our findings establish that glycosylated serglycin regulates NPC metastasis via autocrine and paracrine routes, and that it serves as an independent prognostic indicator of metastasis-free survival and disease-free survival in NPC patients.

Serglycin and secretion in human monocytes

The human monocytic cell line U-937 has been widely used as a model system for human monocytes. The subclone U-937-B has been adapted to serum-free conditions. This particular U-937 clone and its parent clone U-937-1 were used to investigate the role of the proteoglycan serglycin in human monocytes. For this purpose cells were treated with hexyl-beta-D-thioxyloside to abrogate proteoglycan expression. U-937-B cells expressed and secreted exclusively chondroitin sulphate proteoglycans, and after treatment with this xyloside they only expressed and released free chondroitin sulphate chains. Western blotting showed that serglycin core protein was present in conditioned medium of control cells, but absent in medium from xyloside-treated cells. Also, serglycin core protein could be detected in the cell fractions of control cells, but not in the cell fractions from xyloside-treated cells. Furthermore, less proteoglycan-associated proteins could be detected in medium from cells incubated with xyloside, suggesting that the absence of secreted sergycin affects the secretion of such proteins. Cells incubated in the presence of xyloside were analyzed by transmission electron microscopy and shown to contain numerous large empty vesicles. The lack of serglycin, the dominant proteoglycan in U-937 monocyte-like cells, consequently, leads to effects on vesicle formation and secretion of some low molecular weight proteins, suggesting that this particular proteoglycan is of importance for secretory processes in human monocytes.