Geetha Srikrishna

Proinflammatory S100 Proteins Regulate the Accumulation of Myeloid-Derived Suppressor Cells

Chronic inflammation is a complex process that promotes carcinogenesis and tumor progression; however, the mechanisms by which specific inflammatory mediators contribute to tumor growth remain unclear. We and others recently demonstrated that the inflammatory mediators IL-1β, IL-6, and PGE2 induce accumulation of myeloid-derived suppressor cells (MDSC) in tumor-bearing individuals. MDSC impair tumor immunity and thereby facilitate carcinogenesis and tumor progression by inhibiting T and NK cell activation, and by polarizing immunity toward a tumor-promoting type 2 phenotype. We now show that this population of immature myeloid cells induced by a given tumor share a common phenotype regardless of their in vivo location (bone marrow, spleen, blood, or tumor site), and that Gr1highCD11bhighF4/80−CD80+IL4Rα+/−Arginase+ MDSC are induced by the proinflammatory proteins S100A8/A9. S100A8/A9 proteins bind to carboxylated N-glycans expressed on the receptor for advanced glycation end-products and other cell surface glycoprotein receptors on MDSC, signal through the NF-κB pathway, and promote MDSC migration. MDSC also synthesize and secrete S100A8/A9 proteins that accumulate in the serum of tumor-bearing mice, and in vivo blocking of S100A8/A9 binding to MDSC using an anti-carboxylated glycan Ab reduces MDSC levels in blood and secondary lymphoid organs in mice with metastatic disease. Therefore, the S100 family of inflammatory mediators serves as an autocrine feedback loop that sustains accumulation of MDSC. Since S100A8/A9 activation of MDSC is through the NF-κB signaling pathway, drugs that target this pathway may reduce MDSC levels and be useful therapeutic agents in conjunction with active immunotherapy in cancer patients.

RAGE, carboxylated glycans and S100A8/A9 play essential roles in colitis-associated carcinogenesis.

Patients with inflammatory bowel diseases are at increased risk for colorectal cancer, but the molecular mechanisms linking inflammation and cancer are not well defined. We earlier showed that carboxylated N-glycans expressed on receptor for advanced glycation end products (RAGE) and other glycoproteins mediate colitis through activation of nuclear factor kappa B (NF-κB). Because NF-κB signaling plays a critical role in the molecular pathogenesis of colitis-associated cancer (CAC), we reasoned that carboxylated glycans, RAGE and its ligands might promote CAC. Carboxylated glycans are expressed on a subpopulation of RAGE on colon cancer cells and mediate S100A8/A9 binding to RAGE. Colon tumor cells express binding sites for S100A8/A9 and binding leads to activation of NF-κB and tumor cell proliferation. Binding, downstream signaling and tumor cell proliferation are blocked by mAbGB3.1, an anti-carboxylate glycan antibody, and by anti-RAGE. In human colon tumor tissues and in a mouse model of CAC, we found that myeloid progenitors expressing S100A8 and S100A9 infiltrate regions of dysplasia and adenoma. mAbGB3.1 administration markedly reduces chronic inflammation and tumorigenesis in the mouse model of CAC and RAGE-deficient mice are resistant to the onset of CAC. These findings show that RAGE, carboxylated glycans and S100A8/A9 play essential roles in tumor–stromal interactions, leading to inflammation-associated colon carcinogenesis.

S100A8/A9 activate key genes and pathways in colon tumor progression

The tumor microenvironment plays an important role in modulating tumor progression. Earlier, we showed that S100A8/A9 proteins secreted by myeloid-derived suppressor cells (MDSC) present within tumors and metastatic sites promote an autocrine pathway for accumulation of MDSC. In a mouse model of colitis-associated colon cancer, we also showed that S100A8/A9-positive cells accumulate in all regions of dysplasia and adenoma. Here we present evidence that S100A8/A9 interact with RAGE and carboxylated glycans on colon tumor cells and promote activation of MAPK and NF-κB signaling pathways. Comparison of gene expression profiles of S100A8/A9-activated colon tumor cells versus unactivated cells led us to identify a small cohort of genes upregulated in activated cells, including Cxcl1, Ccl5 and Ccl7, Slc39a10, Lcn2, Zc3h12a, Enpp2, and other genes, whose products promote leukocyte recruitment, angiogenesis, tumor migration, wound healing, and formation of premetastatic niches in distal metastatic organs. Consistent with this observation, in murine colon tumor models we found that chemokines were upregulated in tumors, and elevated in sera of tumor-bearing wild-type mice. Mice lacking S100A9 showed significantly reduced tumor incidence, growth and metastasis, reduced chemokine levels, and reduced infiltration of CD11b+Gr1+ cells within tumors and premetastatic organs. Studies using bone marrow chimeric mice revealed that S100A8/A9 expression on myeloid cells is essential for development of colon tumors. Our results thus reveal a novel role for myeloid-derived S100A8/A9 in activating specific downstream genes associated with tumorigenesis and in promoting tumor growth and metastasis. Mol Cancer Res; 9(2); 133–48. ©2011 AACR.

Two Proteins Modulating Transendothelial Migration of Leukocytes Recognize Novel Carboxylated Glycans on Endothelial Cells

We recently showed that a class of novel carboxylated N-glycans was constitutively expressed on endothelial cells. Activated, but not resting, neutrophils expressed binding sites for the novel glycans. We also showed that a mAb against these novel glycans (mAbGB3.1) inhibited leukocyte extravasation in a murine model of peritoneal inflammation. To identify molecules that mediated these interactions, we isolated binding proteins from bovine lung by their differential affinity for carboxylated or neutralized glycans. Two leukocyte calcium-binding proteins that bound in a carboxylate-dependent manner were identified as S100A8 and annexin I. An intact N terminus of annexin I and heteromeric assembly of S100A8 with S100A9 (another member of the S100 family) appeared necessary for this interaction. A mAb to S100A9 blocked neutrophil binding to immobilized carboxylated glycans. Purified human S100A8/A9 complex and recombinant human annexin I showed carboxylate-dependent binding to immobilized bovine lung carboxylated glycans and recognized a subset of mannose-labeled endothelial glycoproteins immunoprecipitated by mAbGB3.1. Saturable binding of S100A8/A9 complex to endothelial cells was also blocked by mAbGB3.1. These results suggest that the carboxylated glycans play important roles in leukocyte trafficking by interacting with proteins known to modulate extravasation.

N-Glycans on the receptor for advanced glycation end products influence amphoterin binding and neurite outgrowth

In this study we show that embryonic neurite growth‐promoting protein amphoterin binds to carboxylated N‐glycans previously identified on mammalian endothelial cells. Since amphoterin is a ligand for the receptor for advanced glycation end products (RAGE), and the ligand‐binding V‐domain of the receptor contains two potential N‐glycosylation sites, we hypothesized that N‐glycans on RAGE may mediate its interactions with amphoterin. In support of this, anti‐carboxylate antibody mAbGB3.1 immunoprecipitates bovine RAGE, and PNGase F treatment reduces its molecular mass by 4.5u2003kDa, suggesting that the native receptor is a glycoprotein. The binding potential of amphoterin to RAGE decreases significantly in presence of soluble carboxylated glycans or when the receptor is deglycosylated. Oligosaccharide analysis shows that RAGE contains complex type anionic N‐glycans with non‐sialic acid carboxylate groups, but not the HNK‐1 (3‐sulfoglucuronyl β1–3 galactoside) epitope. Consistent with the functional localization of RAGE and amphoterin at the leading edges of developing neurons, mAbGB3.1 stains axons and growth cones of mouse embryonic cortical neurons, and inhibits neurite outgrowth on amphoterin matrix. The carboxylated glycans themselves promote neurite outgrowth in embryonic neurons and RAGE‐transfected neuroblastoma cells. This outgrowth requires full‐length, signalling‐competent RAGE, as cells expressing cytoplasmic domain‐deleted RAGE are unresponsive. These results indicate that carboxylated N‐glycans on RAGE play an important functional role in amphoterin‐RAGE‐mediated signalling.

Reduced heparan sulfate accumulation in enterocytes contributes to protein-losing enteropathy in a congenital disorder of glycosylation.

Intestinal biopsy in a boy with gastroenteritis-induced protein-losing enteropathy (PLE) showed loss of heparan sulfate (HS) and syndecan-1 core protein from the basolateral surface of the enterocytes, which improved after PLE subsided. Isoelectric focusing analysis of serum transferrin indicated a congenital disorder of glycosylation (CDG) and subsequent analysis showed three point mutations in the ALG6 gene encoding an alpha1,3-glucosyltransferase needed for the addition of the first glucose to the dolichol-linked oligosaccharide. The maternal mutation, C998T, causing an A333V substitution, has been shown to cause CDG-Ic, whereas the two paternal mutations, T391C (Y131H) and C924A (S308R) have not previously been reported. The mutations were tested for their ability to rescue faulty N:-linked glycosylation of carboxypeptidase Y in an ALG6-deficient Saccharomyces cerevisiae strain. Normal human ALG6 rescues glycosylation and A333V partially rescues, whereas the combined paternal mutations (Y131H and S308R) are ineffective. Underglycosylation resulting from each of these mutations is much more severe in rapidly dividing yeast. Similarly, incomplete protein glycosylation in the patient is most severe in rapidly dividing enterocytes during gastroenteritis-induced stress. Incomplete N:-linked glycosylation of an HS core protein and/or other biosynthetic enzymes may explain the selective localized loss of HS and PLE.

Proinflammatory S100A12 Can Activate Human Monocytes via Toll-like Receptor 4

RATIONALEnS100A12 is overexpressed during inflammation and is a marker of inflammatory disease. Furthermore, it has been ascribed to the group of damage-associated molecular pattern molecules that promote inflammation. However, the exact role of human S100A12 during early steps of immune activation and sepsis is only partially described thus far.nnnOBJECTIVESnWe analyzed the activation of human monocytes by granulocyte-derived S100A12 as a key function of early inflammatory processes and the development of sepsis.nnnMETHODSnCirculating S100A12 was determined in patients with sepsis and in healthy subjects with experimental endotoxemia. The release of human S100A12 from granulocytes as well as the promotion of inflammation by activation of human monocytes after specific receptor interaction was investigated by a series of in vitro experiments.nnnMEASUREMENTS AND MAIN RESULTSnS100A12 rises during sepsis, and its expression and release from granulocytes is rapidly induced in vitro and in vivo by inflammatory challenge. A global gene expression analysis of S100A12-activated monocytes revealed that human S100A12 induces inflammatory gene expression. These effects are triggered by an interaction of S100A12 with Toll-like receptor 4 (TLR4). Blocking S100A12 binding to TLR4 on monocytes or TLR4 expressing cell lines (HEK-TCM) abrogates the respective inflammatory signal. On the contrary, blocking S100A12 binding to its second proposed receptor (receptor for advanced glycation end products [RAGE]) has no significant effect on inflammatory signaling in monocytes and RAGE-expressing HEK293 cells.nnnCONCLUSIONSnHuman S100A12 is an endogenous TLR4 ligand that induces monocyte activation, thereby acting as an amplifier of innate immunity during early inflammation and the development of sepsis.

Carboxylated N-glycans on RAGE promote S100A12 binding and signaling.

The receptor for advanced glycation end products (RAGE) is a signaling receptor protein of the immunoglobulin superfamily implicated in multiple pathologies. It binds a diverse repertoire of ligands, but the structural basis for the interaction of different ligands is not well understood. We earlier showed that carboxylated glycans on the V‐domain of RAGE promote the binding of HMGB1 and S100A8/A9. Here we study the role of these glycans on the binding and intracellular signaling mediated by another RAGE ligand, S100A12. S100A12 binds carboxylated glycans, and a subpopulation of RAGE enriched for carboxylated glycans shows more than 10‐fold higher binding potential for S100A12 than total RAGE. When expressed in mammalian cells, RAGE is modified by complex glycans predominantly at the first glycosylation site (N25IT) that retains S100A12 binding. Glycosylation of RAGE and maximum binding sites for S100A12 on RAGE are also cell type dependent. Carboxylated glycan‐enriched population of RAGE forms higher order multimeric complexes with S100A12, and this ability to multimerize is reduced upon deglycosylation or by using non‐glycosylated sRAGE expressed in E. coli. mAbGB3.1, an antibody against carboxylated glycans, blocks S100A12‐mediated NF‐κB signaling in HeLa cells expressing full‐length RAGE. These results demonstrate that carboxylated N‐glycans on RAGE enhance binding potential and promote receptor clustering and subsequent signaling events following oligomeric S100A12 binding. J. Cell. Biochem. 110: 645–659, 2010.

Carboxylated Glycans Mediate Colitis through Activation of NF-κB

The role of carbohydrate modifications of glycoproteins in leukocyte trafficking is well established, but less is known concerning how glycans influence pathogenesis of inflammation. We previously identified a carboxylate modification of N-linked glycans that is recognized by S100A8, S100A9, and S100A12. The glycans are expressed on macrophages and dendritic cells of normal colonic lamina propria, and in inflammatory infiltrates in colon tissues from Crohn’s disease patients. We assessed the contribution of these glycans to the development of colitis induced by CD4+CD45RBhigh T cell transfer to Rag1−/− mice. Administration of an anti-carboxylate glycan Ab markedly reduced clinical and histological disease in preventive and early therapeutic protocols. Ab treatment reduced accumulation of CD4+ T cells in colon. This was accompanied by reduction in inflammatory cells, reduced expression of proinflammatory cytokines and of S100A8, S100A9, and receptor for advanced glycation end products. In vitro, the Ab inhibited expression of LPS-elicited cytokines and induced apoptosis of activated macrophages. It specifically blocked activation of NF-κB p65 in lamina propria cells of colitic mice and in activated macrophages. These results indicate that carboxylate-glycan-dependent pathways contribute to the early onset of colitis.

A Novel Anionic Modification of N-Glycans on Mammalian Endothelial Cells Is Recognized by Activated Neutrophils and Modulates Acute Inflammatory Responses

We previously reported an unusual carboxylated modification on N-glycans isolated from whole bovine lung. We have now raised IgG mAbs against the modification by immunization with biotinylated aminopyridine-derivatized glycans enriched for the anionic species and screening for Abs whose reactivities were abrogated by carboxylate neutralization of bovine lung glycopeptides. One such Ab (mAb GB3.1) was inhibited by carboxylated bovine lung glycopeptides and other multicarboxylated molecules, but not by glycopeptides in which the carboxylate groups were modified. The Ab recognized an epitope constitutively expressed on bovine, human, and other mammalian endothelial cells. Stimulated, but not resting, neutrophils bound to immobilized bovine lung glycopeptides in a carboxylate-dependent manner. The binding of activated neutrophils to immobilized bovine lung glycopeptides was inhibited both by mAb GB3.1 and by soluble glycopeptides in a carboxylate-dependent manner. The Ab also inhibited extravasation of neutrophils and monocytes in a murine model of peritoneal inflammation. This inhibition of cell trafficking correlated with the increased sequestration but reduced transmigration of leukocytes that were found to be adherent to the endothelium of the mesenteric microvasculature. Taken together, these results indicate that these novel carboxylated N-glycans are constitutively expressed on vascular endothelium and participate in acute inflammatory responses by interaction with activated neutrophils.