Abstract
The mobilization and extracellular release of nuclear high mobility group box-1 (HMGB1) by ischemic cells activates inflammatory pathways following liver ischemia/reperfusion (I/R) injury. In immune cells such as macrophages, post-translational modification by acetylation appears to be critical for active HMGB1 release. Hyperacetylation shifts its equilibrium from a predominant nuclear location toward cytosolic accumulation and subsequent release. However, mechanisms governing its release by parenchymal cells such as hepatocytes are unknown. In this study, we found that serum HMGB1 released following liver I/R in vivo is acetylated, and that hepatocytes exposed to oxidative stress in vitro also released acetylated HMGB1. Histone deacetylases (HDACs) are a family of enzymes that remove acetyl groups and control the acetylation status of histones and various intracellular proteins. Levels of acetylated HMGB1 increased with a concomitant decrease in total nuclear HDAC activity, suggesting that suppression in HDAC activity contributes to the increase in acetylated HMGB1 release after oxidative stress in hepatocytes. We identified the isoforms HDAC1 and HDAC4 as critical in regulating acetylated HMGB1 release. Activation of HDAC1 was decreased in the nucleus of hepatocytes undergoing oxidative stress. In addition, HDAC1 knockdown with siRNA promoted HMGB1 translocation and release. Furthermore, we demonstrate that HDAC4 is shuttled from the nucleus to cytoplasm in response to oxidative stress, resulting in decreased HDAC activity in the nucleus. Together, these findings suggest that decreased nuclear HDAC1 and HDAC4 activities in hepatocytes following liver I/R is a mechanism that promotes the hyperacetylation and subsequent release of HMGB1.
Highlights
high mobility group box-1 (HMGB1) functions as an inflammatory cytokine when released from necrotic cells or actively secreted from stressed cells
One hour of ischemia without reperfusion induced a detectable difference in the amount of acetylated HMGB1, and it was detectable at 1 h of reperfusion
The Nullscript-treated hepatocytes maintained a constant nuclear-to-cytosolic ratio of HMGB1. These results show that Histone deacetylases (HDACs) inhibition promotes nuclear-to-cytoplasmic translocation and release of acetylated HMGB1 from hepatocytes, and they support a role for HDAC regulation of HMGB1
Summary
HMGB1 functions as an inflammatory cytokine when released from necrotic cells or actively secreted from stressed cells. We have previously shown that hepatocytes actively release HMGB1 in response to oxidative stress, suggesting that these parenchymal cells can provide danger signals to neighboring immune cells in the liver to promote inflammation and organ damage [11]. HMGB1 can be passively released following necrosis, our findings demonstrate that oxidative stress in hepatocytes leads to early shuttling from the nucleus to cytoplasm, followed by its subsequent release in the absence of cell death. This suggests that HMGB1 mobilization in these cells is an active, regulated process. We further investigate the role of the HDAC isoforms, HDAC1 and HDAC4, in regulating post-translational modifications of HMGB1 to influence its translocation and release from oxidative stressed hepatocytes
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