Proteomics characterization of haptoglobin alpha-2 subunit in non-small cell lung cancer serum and various human materials

Abstract

Non-small cell lung cancer (NSCLC) is detected in all lung cancer patients and the serum samples as potential source are often used as a potential source for early diagnosis. In this study, we used the proteomic approaches combining 2-DE, 2-D DIGE and mass spectrometry to identify the potential serum biomarkers in NSCLC serum samples, and compare the result to normal serum samples. The haptoglobin alpha 2 subunit (HAP2) is one of the proteins we are interested in and showed the overexpression level in NSCLC serum amongst the NSCLC subtypes. The identity of HAP2 was confirmed by LC-MS/MS and NH2-terminal amino acid sequencing analyses, while the high expression level of HAP2 in NSCLC serum samples was validated using co-immunoprecipitation and western blotting. Interestingly, the high expression of HAP2 has specificity in human serum samples, but nonspecific expression in lung cancer tissues and is not observed in lung cancer cell lines except HepG2 hepatocellular carcinoma cell line. This indicates that HAP2 is not produced from lung tissues and/or cells, but may be secreted from the liver. In addition, HAP2 isoforms and its post-translational modifications (PTMs) could be detected by various staining methods such as phosphoprotein, glycoprotein stains and FITC-labeled lectins stain. The different carbohydrate specificities of HAP2 isoforms in NSCLC serum samples, especially sialyl Lewis x, was observed and may correlate to the development or metastasis of lung cancer. We suggest that HAP2 may become a potentially useful serum biomarker for early diagnostic and therapeutic applications. © 2013 Ma HT, et al.