The macrophage receptor mincle binds to trehalose dimycolate on the surface of Mycobacterium tuberculosis. Signaling initiated by this interaction leads to cytokine production, which underlies the ability of mycobacteria to evade the immune system and also to function as adjuvants. In previous work the mechanism for binding of the sugar headgroup of trehalose dimycolate to mincle has been elucidated, but the basis for enhanced binding to glycolipid ligands, in which hydrophobic substituents are attached to the 6-hydroxyl groups, has been the subject of speculation. In the work reported here, the interaction of trehalose derivatives with bovine mincle has been probed with a series of synthetic mimics of trehalose dimycolate in binding assays, in structural studies by x-ray crystallography, and by site-directed mutagenesis. Binding studies reveal that, rather than reflecting specific structural preference, the apparent affinity of mincle for ligands with hydrophobic substituents correlates with their overall size. Structural and mutagenesis analysis provides evidence for interaction of the hydrophobic substituents with multiple different portions of the surface of mincle and confirms the presence of three Ca2+-binding sites. The structure of an extended portion of the extracellular domain of mincle, beyond the minimal C-type carbohydrate recognition domain, also constrains the way the binding domains may interact on the surface of macrophages.

There has been considerable interest in understanding the epitopes that bind the lectin Helix pomatia agglutinin (HPA) in breast cancer as the lectin has been shown to identify glycosylation changes associated with the development of metastatic disease. HPA has previously been shown to recognize aberrant O-linked α-N-acetylgalactosamine (GalNAcα)/mucin glycosylation in cancer, including exposed Tn epitopes. However, recent glycan-array analysis reported that diverse epitopes are also recognized by the lectin, e.g. consortium for functional glycomics (CFG) data: GalNAcα1,3Gal; β-GalNAc; GlcNAcβ1,4Gal. The intriguing observations from the CFG array led to this study, in which HPA-binding epitopes were localized and characterized in an in vitro model of breast cancer metastasis. HMT3522 (benign disease), BT474 (primary cancer) and T47D/MCF7 (metastatic cancer) cells were assessed in confocal microscopy-based co-localization studies and a glycoproteomic analysis based on 2-dimensional electrophoresis (2DE), western blotting and mass spectrometry was adopted. HPA binding correlated with levels of integrin α6, transcription factors heterogeneous nuclear ribonuclear protein (HnRNP) H1, HnRNP D-like, HnRNP A2/B1 as well as heat shock protein 27 (Hsp27), glial fibrillary acidic protein and enolase 1 (ENO1). These glycoproteins were non-detectable in the non-metastatic breast cancer cell lines. The recognition of HnRNPs, Hsp27 and ENO1 by HPA correlated with O-GlcNAcylation of these proteins. Integrin α6 was the most abundant HPA glycoprotein in the breast cancer cells with a metastatic phenotype; this concurred with previous findings in colorectal cancer. This is the first report in which HPA has been shown to bind O-GlcNAcylated transcription factors. This class of proteins represents a new means by which HPA differentiates cancer cells with an aggressive metastatic phenotype.

We demonstrate that the natural product brartemicin, a newly discovered inhibitor of cancer cell invasion, is a high-affinity ligand of the carbohydrate-recognition domain (CRD) of the C-type lectin mincle.

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Mincle, the macrophage-inducible C-type lectin also known as CLEC-4E, binds to the mycobacterial glycolipid trehalose dimycolate and initiates a signaling cascade by serving as a receptor for Mycobacterium tuberculosis and other pathogenic mycobacterial species. Studies of the biological functions of human mincle often rely on mouse models, based on the assumption that the biological properties of the mouse receptor mimic those of the human protein. Experimental support for this assumption has been obtained by expression of the carbohydrate-recognition domain of mouse mincle and characterization of its interaction with small molecule analogs of trehalose dimycolate. The results confirm that the ligand-binding properties of mouse mincle closely parallel those of the human receptor. These findings are consistent with the conservation of key amino acid residues that have been shown to form the ligand-binding site in human and cow mincle. Sequence alignment reveals that these residues are conserved in a wide range of mammalian species, suggesting that mincle has a conserved function in binding ligands that may include endogenous mammalian glycans or pathogen glycans in addition to trehalose dimycolate.