Abstract
Clusterin is an enigmatic protein altered in tumors of colorectal cancer patients. Because there is no information available about serum clusterin regarding this pathology, we applied proteomic techniques to analyze its isoforms in donors and patients. First we separated serum proteins through concanavalin A, obtaining a fraction with non- and O-glycosylated proteins (FI) and a second fraction enriched in N-glycoproteins (FII) wherein clusterin was supposed to elute on the basis of its glycosylation. Surprisingly analysis of the FI fraction revealed the existence of an unexpected and aberrantly N-glycosylated clusterin that was overexpressed in patients and comprised at least five isoforms with different isoelectric points. On the other hand, two-dimensional electrophoretic analysis of the clusterin eluted in FII detected one isoform that was increased and 15 isoforms that were decreased or absent in serum of patients. Finally immunoquantification by slot blot showed that in total serum and in FI the clusterin levels were significantly increased in patients, whereas in FII there was no significant variation. Therefore, serum clusterin and some of its isoforms could have a potential value as colorectal tumor markers and are interesting subjects for biomarker studies.
Highlights
Clusterin is an enigmatic protein altered in tumors of colorectal cancer patients
Concanavalin A Chromatography of Serum from Donors and Patients—We performed chromatography through the lectin concanavalin A (Con A) to separate human serum, both from donors and patients, into two fractions. This lectin binds proteins with complex- and high mannose-type oligosaccharide chains, obtaining a flow-through fraction including albumin and other serum proteins mainly non-glycosylated and O-glycosylated and a second eluate enriched in N-glycoproteins [46]
Many works have focused their attention on the relationship between CLU and cancer [7,8,9,10,11,12,13,14,15,16,17,18,19], and in particular, studies in colorectal cancer (CRC) tumors have demonstrated an oncogenic role of CLU in carcinogenesis and overexpression related to tumor progression [9, 18, 19]
Summary
Clusterin is an enigmatic protein altered in tumors of colorectal cancer patients. Because there is no information available about serum clusterin regarding this pathology, we applied proteomic techniques to analyze its isoforms in donors and patients. Twodimensional electrophoretic analysis of the clusterin eluted in FII detected one isoform that was increased and 15 isoforms that were decreased or absent in serum of patients. Serum clusterin and some of its isoforms could have a potential value as colorectal tumor markers and are interesting subjects for biomarker studies. Regarding CLU and human colorectal cancer (CRC), Chen et al [18] reported that this protein is elevated in early intestinal lesions, benign polyps, adenocarcinomas, and normal epithelia adjacent to tumors. An overexpression of the secreted CLU in highly aggressive colon tumors and metastatic nodes has been reported [19]
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