Expression of Heart Development Protein With EGF‐Like Domains 1 (HEG1) Decorated With Low‐Sulfated Keratan Sulfate in Human Malignant Pleural Mesothelioma

The glycoform of heart development protein with EGF-like domains 1 (HEG1) recognized by the SKM9-2 monoclonal antibody is a useful diagnostic marker for malignant pleural mesothelioma (MPM). The putative glycoform includes core 2 O-glycans carrying sialyl poly-N-acetyllactosamine (LacNAc), but sulfation modifications are undetermined. Since sialyl 6-sulfo LacNAc-capped core 2 O-glycans are expressed in MPM and their structure overlaps with low-sulfated keratan sulfate (KS), we asked whether low-sulfated KS is expressed in MPM and whether HEG1 is decorated with low-sulfated KS. We performed immunohistochemical analysis of 41 MPM cases using anti-KS monoclonal antibodies and endoglycosidases, reversed-phase ion-pair high-performance liquid chromatography analysis of KS/sulfated LacNAc isolated from human pleural tissue, and western blot analysis of HEG1·IgG recombinant fusion proteins secreted from low-sulfated KS-expressing human embryonic kidney cells. Most MPM tissues were stained with anti-low-sulfated KS antibodies and staining was eliminated by endo-β-galactosidase and keratanase II but not by peptide-N-glycosidase F. Disaccharide composition analysis revealed that mono-sulfated LacNAc disaccharide and di-sulfated LacNAc disaccharide accounted for 83.1% and 16.9% of pleural KS/sulfated LacNAc, respectively. Western blot analysis of HEG1·IgG glycoforms indicated that HEG1 functions as a core protein for low-sulfated KS. Thus, HEG1 protein decorated with low-sulfated KS is expressed in MPM.