Pancreatic beta cell-surface expression of glucose transporter 2 (Glut-2) is essential for glucose-stimulated insulin secretion, thereby controlling blood glucose homeostasis in response to dietary intake. We show that the murine GlcNAcT-IVa glycosyltransferase is required for Glut-2 residency on the beta cell surface by constructing a cell-type- and glycoprotein-specific N-glycan ligand for pancreatic lectin receptors. Loss of GlcNAcT-IVa, or the addition of glycan-ligand mimetics, attenuates Glut-2 cell-surface half-life, provoking endocytosis with redistribution into endosomes and lysosomes. The ensuing impairment of glucose-stimulated insulin secretion leads to metabolic dysfunction diagnostic of type 2 diabetes. Remarkably, the induction of diabetes by chronic ingestion of a high-fat diet is associated with reduced GlcNAcT-IV expression and attenuated Glut-2 glycosylation coincident with Glut-2 endocytosis. We infer that beta cell glucose-transporter glycosylation mediates a link between diet and insulin production that typically suppresses the pathogenesis of type 2 diabetes.
We previously demonstrated that Siglec-15, a member of the Siglec family of glycan-recognition proteins, is expressed on a subset of macrophages and preferentially recognizes the sialyl-Tn (sTn) antigen, a tumor-associated glycan structure. In this study, we report on the biological significance of the Siglec-15-mediated interaction between monocytes/macrophages and cancer cells. Siglec-15 is expressed on tumor-associated macrophages (TAMs) in various human tumor tissues. We further demonstrated that its expression is substantially elevated in macrophage colony-stimulating factor-induced M2-like macrophages, which produced more transforming growth factor-β (TGF-β) in response to sTn-positive cells than to negative cells. We designed a co-culture model of THP-1 (human monocytic leukemia) cells and H157 (human lung carcinoma) cells mimicking the interaction between monocytes/macrophages and cancer cells that recapitulated the enhanced TGF-β production in Siglec-15 expressing THP-1 cells by the cellular interaction with sTn expressing H157 cells. The enhanced TGF-β production required a direct interaction between the two cell lines through sialic acids. Siglec-15 associates with adaptor protein DNAX activation protein of 12 kDa (DAP12) at the binding determinant Lys(274) in the transmembrane domain and transduces a signal to spleen tyrosine kinase (Syk). The enhanced TGF-β secretion was significantly attenuated by Syk inhibitor treatment of THP-1 cells or by substitution of the Siglec-15 Lys(274) to Ala, which disrupts the molecular interaction between Siglec15 and DAP12. These findings indicate that Siglec-15 recognizes the tumoral sTn antigen and transduces a signal for enhanced TGF-β secretion in TAMs and further suggest that Siglec-15 on macrophages may contribute to tumor progression by the TGF-β-mediated modulation of intratumoral microenvironments.
Core fucosylation of the TCR is required for T-cell signaling and production of inflammatory cytokines and induction of colitis in mice. Levels of TCR core fucosylation are increased on T cells from intestinal tissues of patients with IBD; this process might be blocked as a therapeutic strategy.
Nonalcoholic fatty liver disease (NAFLD) is a growing medical problem; thus, discriminating nonalcoholic steatohepatitis (NASH) from NAFLD is of great clinical significance. For the diagnosis of NASH, liver biopsy-proven histological examination is the current gold standard, and noninvasive and reliable biomarkers are greatly needed. Recently, we found that two glycobiomarkers, fucosylated haptoglobin (Fuc-Hpt) and Mac-2 binding protein (Mac2bp), are useful independently for NASH diagnosis. In this study, we confirmed that serum Fuc-Hpt is suitable for the prediction of ballooning hepatocytes and that serum Mac2bp is suitable for the prediction of liver fibrosis severity in 124 biopsy-proven NAFLD patients (training cohort). In addition, we found that the combination of serum Fuc-Hpt and Mac2bp levels was an excellent tool for NASH diagnosis. Using receiver operating characteristic analyses, the area under the receiver operating characteristic curve, sensitivity, and specificity of the combination of these two glycobiomarkers were 0.854, 81.1%, and 79.3%, respectively. We established a prediction model for NASH diagnosis using logistic regression analysis: logit (p) 5 22.700 1 0.00242 3 Fuc-Hpt 1 1.225 3 Mac2bp. To validate the prediction model, another 382 biopsy-proven NAFLD patients were enrolled (validation cohort). In the validation cohort, the area under the receiver operating characteristic curve of this model for NASH diagnosis was 0.844, with 71.4% and 82.3% sensitivity and specificity, respectively. In addition, we investigated the significance of our developed NASH diagnosis model in ultrasound-diagnosed NAFLD subjects who received medical health checkups (n 5 803). Our model also could predict NAFLD disease severity in this larger population. Conclusion: The combination of serum Fuc-Hpt and Mac2bp can distinguish NASH from NAFLD patients. Our noninvasive model using two serum glycobiomarkers contributes to a novel NASH diagnostic methodology that could replace liver biopsy. (HEPATOLOGY 2015;62:1433-1443
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