Comprehensive and temporal analysis of secreted proteins in the medium from IL-6 exposed human hepatocyte

We have previously identified intracellular secretory acute phase response (sAPR) proteins in human hepatocytes following interleukin-6 (IL-6) induction by fluorogenic derivatization (FD)-liquid chromatography (LC)-tandem mass spectrometry (MS/MS). In this report, we utilized this method, which uses 7-chloro-N-[2-(dimethylamino)ethyl]-2,1,3-benzoxadiazole-4-sulfonamide (DAABD-Cl) as the FD reagent, to comprehensively and time-dependently analyze secreted proteins in the medium, including sAPR proteins. Since DAABD-Cl selectively reacts with thiol moieties of cysteinyl residues, direct derivatization, high-resolution LC separation and identification of the secreted proteins in the culture medium were successfully achieved without a pretreatment step. As a result, 14 sAPR proteins were identified simultaneously during a 72 h induction by IL-6. The secretion levels of 11 proteins increased, whereas the secretion levels of three important transport proteins decreased (albumin, retinol-binding protein 4 and transthyretin). In addition, the secretion level of a haptoglobin was found to increase significantly between 0 and 6 h by 1.88-fold compared with the control sample. The secretion levels of four cytoplasmic proteins increased: LDH, a known marker for cell damage, and GSTA1, FABP1 and ADH1B, which are marker proteins for hepatocellular damage. The secretion levels of the other two newly identified cytoplasmic proteins, profilin-1 and SOD2, were also found to increase, suggesting that these two proteins represent novel markers for cell damage. These results suggest that the FD-LC-MS/MS proteomics method can be used to analyze comprehensively and time-dependently the secreted proteins and thereby can offer information that aids our understanding of the dynamics of protein secretion affected by the exposure of cytokines such as IL-6.