Chemical analysis of C-reactive protein synthesized by human aortic endothelial cells under oxidative stress

C-reactive protein (CRP) is a clinical biomarker of inflammation, and high levels of CRP correlate with cardiovascular disease. The objectives of this study were to test our hypothesis that oxidized low-density lipoprotein (ox-LDL) induces the release of CRP from human aortic endothelial cells (HAECs) and to optimize several analytical methods to identify CRP released from cultured cells in a model of atherogenic stress. HAECs were incubated with copper-oxidized LDL, and the supernatant was subsequently purified by diethylaminoethyl chromatography and analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). We identified an optimal buffer for the elution of CRP, which contained 0.05 M sodium phosphate and 2.0 M NaCl (pH 4.5). Purified CRP was digested with trypsin and subjected to high-performance LC with an optimal mobile phase of acetonitrile-water containing 0.1% formic acid (50:50, v/v) and an optimal mobile phase flow rate of 0.2 mL/min. We identified optimal parameters for MS/MS analysis of CRP, including sheath gas pressure (80 psi), capillary temperature (275 °C), collision energy (25%), tube lens offset (-5 V), auxiliary gas pressure (0 psi), and isolation width of parent ion (m/z value = 3). Characterization of CRP was based on the extracted ion chromatograms and selected multiple-reaction monitoring spectra of three peptides (peptide-1, -2, and -3) derived from trypsin-digested intact CRP standard. CRP peptide-2 and peptide-3 were identified in the supernatant of ox-LDL-treated HAECs. Confirmation of CRP was based on LC-MS/MS and enzyme-linked immunosorbent assay analysis of CRP in purified HAEC supernatant, as well as real-time PCR analysis of CRP mRNA levels in HAECs.