Proteomic analysis of macrophages: a potential way to identify novel proteins associated with activation of macrophages for tumor cell killing.

One major mechanism through which macrophages effectively kill tumor cells requires cell to cell contact, indicating that certain molecules expressed on cell surface of activated macrophages may mediate the tumoricidal capability. Tumor necrosis factor (TNF) and nitric oxide (NO) are the two classical mediators of tumor cell death. However, evidence of discrepancy is accumulating indicating these known mediators do not appear to account for the broad and potent tumoricidal activity of macrophages. To obtain a full repertoire of tumoricidal activation-associated membrane proteins, we combined one-dimensional SDS-PAGE with capillary liquid chromatography-tandem mass spectrometry (LC-MS/MS). Using this technique, we identified 454 activated macrophage specifically expressed proteins with extremely high confidence, including most known activation markers of macrophages, such as NO synthase (iNOS), Ym1, cyclooxygenase, etc. Membrane bound TNF-alpha was also identified on activated macrophages. However, it was also detected on thioglycolate elicited macrophages, indicating this molecule may not play a key role in conjugation-dependent tumor cell killing. In contrast, although NO has not been assigned as an effector molecule of conjugation-dependent tumoricidal pathway, iNOS was identified from membrane fraction of activated macrophages, suggesting NO may be involved in conjugation-dependent tumoricidal mechanism, because iNOS association with plasma membrane is ideally suited to deliver NO directly into the contacted tumor cells. This research provides not only new insights into macrophage conjugation-dependent tumoricidal mechanisms, but also a valuable data set of macrophage activation associated membrane proteins, thus providing better understanding of the functional mechanisms of macrophages in anti-tumor and other biological processes.