Strategy for studying the liver secretome on the organ level.

Secretome study presents new possibilities for understanding liver secretory function in a comprehensive and exploratory way. Perfusates from isolated perfused rat liver are good targets for liver secretome study on the organ level. There are two major concerns in this type of study, cytosolic and blood contaminations in the perfusates. Therefore, the perfusion conditions were carefully controlled and alanine aminotransferase levels in the perfusates were monitored as indicators of liver integrity and cytosolic contamination. The protein pattern of perfusate was significantly different from cell lysate, which showed low cytosolic contamination. The amount of immunoglobulins in the perfusates identified by both Western blot and MS/MS indicated low serum contamination. In total, 357 secretory protein candidates were identified by the Enrichment Index method or N-terminal signal peptide prediction. Secretory proteins annotated by Swiss-Prot were 5-fold enriched in the perfusates and around 10-fold enriched in the portion identified by the Enrichment Index method. Some cytokines, secretory proteins from liver interstitial cells, and components of the liver microenvironment were found in the perfusates, highlighting the advantages of studying the liver secretome on the organ level. The strategy can be used in physiology research and biomarker discovery for diseases in the liver as well as other organs.