A circulating ligand for galectin-3 is a haptoglobin-related glycoprotein elevated in individuals with colon cancer.
OBJECTIVE
Galectin-3 is a beta-galactoside-binding protein implicated in tumor progression and metastasis of colorectal cancers. To determine whether circulating galectin-3 ligands are related to the presence of colon cancer, we sought to identify and quantify ligands in serum that bind to galectin-3.


METHODS
Sera from patients with colon cancer, adenomas, and normal individuals were desialylated, reduced, and separated by sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS-PAGE) and blots probed with biotinylated galectin-3.


RESULTS
In colon cancer sera, the major galectin-3 ligand was a 40-kilodalton band distinct from mucin, carcinoembryonic antigen, and Mac-2 binding protein. Serum 40-kilodalton ligand was 10- to 30-fold higher in patients with colon cancer than in healthy subjects. Ligand was purified by gel filtration, affinity precipitation on galectin-3/agarose, and SDS-PAGE. When tryptic peptides were analyzed by matrix-assisted laser-desorption ionization mass spectrometry and protein database searching, the 40-kilodalton ligand was identified as haptoglobin beta subunit. In confirmation of this finding, depletion of haptoglobin by immunoprecipitation also eliminated the 40-kilodalton ligand. Colon cancer sera had only a modest increase in total haptoglobin as compared with healthy subjects, suggesting that the structure rather than the amount of haptoglobin is altered in patients with colon cancer. Immunohistochemical staining confirmed the absence of haptoglobin in normal colon and the ectopic expression of haptoglobin in colon cancers and adenomatous polyps.


CONCLUSIONS
A major circulating ligand for galectin-3, which is elevated in the sera of patients with colon cancer, is a cancer-associated glycoform of haptoglobin.
Bresalier
Robert S
RS
Department of Gastrointestinal Medicine and Nutrition, University of Texas M.D. Anderson Cancer Center, 1515 Holcombe Boulevard, Unit 436, Houston, Texas 77030, USA. rbresali@mdanderson.org
Byrd
James C
JC
Tessler
David
D
Lebel
Joseph
J
Koomen
John
J
Hawke
David
D
Half
Elizabeth
E
Liu
Kai-Feng
KF
Mazurek
Nachman
N
D000236:Adenoma / Q000097:blood / Q000378:metabolism*; D014408:Biomarkers, Tumor / Q000096:biosynthesis* / Q000097:blood; D003110:Colonic Neoplasms / Q000097:blood / Q000378:metabolism*; D018450:Disease Progression; D037502:Galectin 3 / Q000096:biosynthesis* / Q000097:blood; D006242:Haptoglobins / Q000032:analysis / Q000096:biosynthesis; D006801:Humans; D008024:Ligands; D009362:Neoplasm Metastasis; D011446:Prospective Studies
D014408:Biomarkers, Tumor; D037502:Galectin 3; D006242:Haptoglobins; D008024:Ligands