Glycan-Mediated Regulation of Melanoma Growth and Survival

Abstract

Malignant transformation and tumor progression are often associated with aberrant glycosylation of cell surface proteins and lipids. Commonly observed changes in glycan composition during malignancy include truncation of serine/threonine O-linked glycans, altered expression of mucins, an increase in overall sialylation and enhanced synthesis of tri- and tetra-antennary complex N-glycans. Increasing evidence supports the idea that the presence of certain glycans correlates with cancer progression, affects tumor cell invasiveness, alters binding to pro-tumorigenic galectins, and promotes metastasis to distant organs. Yet the molecular details underlying how glycans functionally contribute to malignant transformation and progression are poorly understood. From expression-based analyses across multiple cancer lineages, we found that melanomas exhibit significant transcriptional changes in glycosylation-related genes. This glycosylation gene-signature revealed that compared to normal melanocytes, melanomas downregulate the β1,6 N-acetylglucosaminyltransferase, GCNT2, leading to a loss of asparagine(N)-linked I-branched glycans. Moreover, immunohistochemical analyses of clinical melanoma specimens revealed that GCNT2 expression is inversely correlated with progression. Functionally, we found that knockdown of GCNT2/loss of I-branched glycans significantly enhanced melanoma xenograft growth and promoted three-dimensional in vitro melanoma colony formation and survival. Whereas enforced expression of GCNT2/I-branched glycans decreased melanoma xenograft growth and inhibited in vitro colony formation and survival. Mechanistic studies further revealed that GCNT2/I-branched glycans negatively regulated growth factor receptor and adhesion-mediated cell growth and survival, specifically through insulin-like growth factor-1 receptor (IGF1R) and integrin-extracellular matrix mediated signaling pathways. In addition, we found that GCNT2/I-branched glycans reduced ligand binding to IGF1R and fibronectin-binding integrins. In all, our studies expand our current understanding of the role of aberrant glycans in melanoma and how subtle changes in glycan structure can regulate several malignancy-associated pathways to alter melanoma cell signaling, growth and survival.