The Soluble Form of CD74 Enhances MIF's Cardioprotective Properties

Objectives: The cardioprotective role of macrophage migration inhibitory factor (MIF) in myocardial ischemia and reperfusion (I/R) injury is well known whereas the involvement of the soluble form of the MIF receptor CD74 ectodomain (sCD74) is still elusive. There are first hints that complex formation of MIF with sCD74 might increase its cardioprotective effects, since patients with positive proof of MIF/sCD74 complexes showed a better post-operative outcome. Therefore, we aimed to assess the effect of MIF/sCD74 complex formation on cardioprotection by stimulating cardiac cells with sCD74 in complex with either recombinant MIF or redox inactive cysteine mutant MIFC60S.

Methods: Cardiac cells were isolated from neonatal rats. Cells were incubated with MIF, MIFC60S (100ng/ml) or sCD74 (5-fold molar excess) for 30 min followed by H₂O₂ treatment for 90 min. Cell survival was confirmed by a Trypan blue cell count. A hydroxyethyldisulfide transhydrogenase assay (HED) was performed to measure catalytic redox activity of MIF in presence and absence of sCD74. All experimental procedures were approved by the Animal Care and Use Committee of the local authorities.

Results: The cardioprotective effect of recombinant MIF in cardiac cells challenged with H₂O₂ could be further increased by sCD74/MIF complex formation, while the cell viability did not change in unchallenged cells (H₂O₂ versus MIF/H₂O₂: 58.2% versus 85.7% and H₂O₂ versus sCD74/MIF/H₂O₂: 58.2% versus 97.0%). The redox inactive mutant MIFC60S showed almost no cardioprotection as compared with the H₂O₂ value. To get a first idea how sCD74 enhances MIF's cardioprotective properties, the redox-activity of MIF in the presence and absence of sCD74 was measured by HED assay. MIF in complex with sCD74 showed an increased antioxidant activity compared with MIF alone.

Conclusion: Our results suggest that the antioxidant activity of MIF might be the most pivotal feature of MIF to rescue cardiac cells from oxidative stress-induced death. The cardioprotective effect of MIF can be increased by complex formation with sCD74 which seems to be facilitated by the increased redox-activity of MIF in the presence of sCD74. Therefore, sCD74 might be an attractive target to limit myocardial I/R injury. Nevertheless, further in vitro as well as in vivo studies are required to investigate the underlying molecular mechanism of sCD74/MIF complex formation in cardioprotection.