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The Golgi protein RCAS1 controls cell surface expression of tumor-associated O-linked glycan antigens
Arne Engelsberg1, Ricardo Hermosilla, Uwe Karsten
1Department of Hematology, Oncology and Tumorimmunology, Max-Delbrück Center for Molecular Medicine, Berlin, Germany.
Abstract:
Tumor immunology has received a large impetus from the identification of tumor-associated antigens. Among them, a monoclonal antibody, 22.1.1, was instrumental in defining a novel tumor-associated antigen that was termed "receptor binding cancer antigen expressed on SiSo cells" (RCAS1). RCAS1 was proposed to induce growth arrest and apoptosis on activated immune cells, mediated by a putative death receptor. Structurally, RCAS1 was predicted to exist as a type II transmembrane protein and in a soluble form. Here, we analyzed occurrence, membrane topology, and subcellular localization of the RCAS1-encoded gene product. RCAS1 was shown to be a ubiquitously expressed type III transmembrane protein with a Golgi-predominant localization. Monoclonal antibody 22.1.1 failed to recognize RCAS1, as demonstrated by confocal microscopy. Instead, we showed that the cognate 22.1.1 epitope is identical with the tumor-associated O-linked glycan Tn (N-acetyl-d-galactosamine, GalNAc). Overexpression of RCAS1 in cell lines that are negative for 22.1.1 surface staining led to the generation of Tn and the closely related TF (Thomsen-Friedenreich, Galbeta1-3GalNAc) antigen, thus providing a functional link to the generation of the 22.1.1 epitope. We suggest that RCAS1 modulates surface expression of tumor-associated, normally cryptic O-linked glycan structures and contributes indirectly to the antigenicity of tumor cells.
Insights
This study reveals that the tumor antigen RCAS1 does not directly bind the antibody 22.1.1. Instead, RCAS1 influences the expression of tumor-associated O-linked glycans, like Tn antigen, indirectly affecting cancer cell antigenicity.
Area of Science:
- Tumor immunology
- Cancer research
- Glycobiology
Background:
- Tumor-associated antigens (TAAs) are crucial in tumor immunology.
- The monoclonal antibody 22.1.1 was used to identify a TAA termed RCAS1.
- RCAS1 was hypothesized to be a type II transmembrane protein inducing immune cell apoptosis.
Purpose of the Study:
- To investigate the expression, membrane topology, and subcellular localization of RCAS1.
- To clarify the interaction between RCAS1 and the monoclonal antibody 22.1.1.
- To understand RCAS1's role in generating tumor-associated glycan antigens.
Main Methods:
- Confocal microscopy to analyze RCAS1 localization and antibody binding.
- Analysis of RCAS1's membrane topology and subcellular distribution.
- Investigating glycan expression in cell lines with and without RCAS1 overexpression.
Main Results:
- RCAS1 is a ubiquitously expressed type III transmembrane protein, localized predominantly in the Golgi.
- Monoclonal antibody 22.1.1 does not recognize RCAS1.
- The 22.1.1 epitope is the Tn antigen (GalNAc), and RCAS1 overexpression induces Tn and TF antigen generation.
Conclusions:
- RCAS1 is a type III transmembrane protein, not recognized by antibody 22.1.1.
- The 22.1.1 epitope is the Tn antigen, functionally linked to RCAS1.
- RCAS1 indirectly contributes to tumor antigenicity by modulating O-linked glycan expression.