Endogenous HMGB1 contributes to ischemia-reperfusion-induced myocardial apoptosis by potentiating the effect of TNF-α/JNK

Xu H, Yao Y, Su Z, Yang Y, Kao R, Martin CM, Rui T. Endogenous HMGB1 contributes to ischemia-reperfusion-induced myocardial apoptosis by potentiating the effect of TNF-alpha/JNK. Am J Physiol Heart Circ Physiol 300: H913-H921, 2011. First published December 24, 2010; doi:10.1152/ajpheart.00703.2010.-High-mobility group box 1 (HMGB1) is a nuclear protein that has been implicated in the myocardial inflammation and injury induced by ischemia-reperfusion (I/R). The purpose of the present study was to assess the role of HMGB1 in myocardial apoptosis induced by I/R. In vivo, myocardial I/R induced an increase in myocardial HMGB1 expression and apoptosis. Inhibition of HMGB1 (A-box) ameliorated the I/R-induced myocardial apoptosis. In vitro, isolated cardiac myocytes were challenged with anoxia-reoxygenation (A/R; in vitro correlate to I/R). A/R-challenged myocytes also generated HMGB1 and underwent apoptosis. Inhibition of HMGB1 attenuated the A/R-induced myocyte apoptosis. Exogenous HMGB1 had no effect on myocyte apoptosis. However, inhibition of HMGB1 attenuated myocyte TNF-alpha production after the A/R was challenged; surprisingly, HMGB1 itself did not induce myocyte TNF-alpha production. Exogenous TNF-alpha induced a moderate proapoptotic effect on the myocytes, an effect substantially potentiated by coadministration of HMGB1. It is generally accepted that apoptosis induced by TNF-alpha is regulated by the balance of activation of c-Jun NH2-terminal kinase (JNK) and NF-kappa B. Indeed, in the present study, TNF-alpha increased the phosphorylation status of JNK and p65, a subunit of NF-kappa B; HMGB1 greatly potentiated TNF-alpha-induced JNK phosphorylation. Furthermore, inhibition of JNK (SP-600125) prevented the myocyte apoptosis induced by a TNF-alpha/HMGB1 cocktail. Finally, A/R increased HMGB1 production in both wild-type and toll-like receptor 4-deficient myocytes; however, deficiency in toll-like receptor 4 diminished A/R-induced myocyte apoptosis, TNF-alpha, and JNK activation. Our results indicate that myocyte-derived HMGB1 and TNF-alpha work in concert to promote I/R-induced myocardial apoptosis through JNK activation.