Bax Inhibitor-1 Regulates Endoplasmic Reticulum Stress-associated Reactive Oxygen Species and Heme Oxygenase-1 Expression

Abstract

The Bax inhibitor-1 (BI-1) is an anti-apoptotic protein that is located in endoplasmic reticulum (ER) membranes and protects cells from ER stress-induced apoptosis. The ER is associated with generation of reactive oxygen species (ROS) through oxidative protein folding. This study examined the role of BI-1 in the regulation of ER stress-induced accumulation of ROS and expression of unfolded protein response-associated proteins. BI-1 reduced the expression levels of glucose response protein 78, C/EBP homologous protein, phospho-eukaryotic initiation factor 2␣, IRE1␣, XBP-1, and phospho-JNK and inhibited the cleavage of ATF-6␣ p-90, leading to the inhibition of ROS. Although ROS scavengers offer some protection against ER stress-induced apoptosis, the expression of pro-apoptotic ER stress proteins was not affected. This study shows that the response of unfolded proteins is followed by ROS accumulation under ER stress, which is regulated in BI-1 cells. The mechanism for these BI-1-associated functions involves the expression of heme oxygenase-1 (HO-1) through nuclear factor erythroid 2-related factor 2. In BI-1 cells, the transfection of HO-1 small interfering RNA completely abolished the BI-1-induced protection. The endogenous expression of HO-1 through ER stressinitiated ROS is believed to be as a protection signal. In conclusion, these observations suggest that BI-1 can inhibit the ER stress proteins as well as the accumulation of ROS, thereby protecting the cells. Moreover, HO-1 plays an important role in the BI-1-associated protection against ER stress.

Bax Inhibitor 1 in apoptosis and disease

“…Inversely, BI-1 knockout cells exhibited hypersensitivity to the same ER stresses, indicating BI-1 aids to inhibit ER stress signalling pathways ( Figure 2b, Table 1) (Chae et al, 2004;Bailly-Maitre et al, 2006). Lee et al (2007) demonstrated that BI-1 can interfere with all three ER signalling pathways ( Figure 2b, Table 1). During ER stress BI-1 overexpression decreased the level of IRE1 expression with downstream repercussions on the activation of XBP-1 and proapoptotic signalling through JNK phosphorylation (Lee et al, 2007).…”

“…Lee et al (2007) demonstrated that BI-1 can interfere with all three ER signalling pathways ( Figure 2b, Table 1). During ER stress BI-1 overexpression decreased the level of IRE1 expression with downstream repercussions on the activation of XBP-1 and proapoptotic signalling through JNK phosphorylation (Lee et al, 2007). Moreover BI-1 was shown to bind IRE1 through its C-terminus (Lisbona et al, 2009), which regulates its ability to oligomerize, thereby blocking its RNase activity.…”

“…Overexpression of BI-1 in interleukin-3-dependent pro-B lymphocytes (FL5.12) significantly inhibited apoptosis induced by the topoisomerase inhibitor, etoposide and global kinase inhibitor, staurosporine, both initiators of the intrinsic pathway (Xu and Reed, 1998). Whilst human diploid fibroblasts (GM701) overexpressing BI-1 grew during growth factor deprivation and inhibited reactive oxygen species (ROS) accumulation (Lee et al, 2007). BI-1 antisense transfected into human embryonic kidney 293 cells induced apoptosis, independent of cell stress inducers (Xu and Reed, 1998).…”

“…Indeed, the level of phosphorylation of eIF2a is decreased when BI-1 is overexpressed (Lee et al, 2007) suggesting that BI-1 inhibits PERK by an unknown mechanism. Moreover, BI-1 overexpression also decreased the cleavage of ATF-6 (Lee et al, 2007) and therefore blocks the subsequent signalling pathway.…”

“…Overexpression of BI-1 in cells increased the activation of Nrf-2, promoting its translocation to the nucleus resulting in an increase in anti-oxidant enzymes, such as, Heme-Oxygenase-1. Heme-Oxygenase-1 is an oxygenase enzyme, which blocks ROS activity and is expressed to counteract ROS accumulation, thereby promoting cell survival (Lee et al, 2007). Secondly, BI-1 can directly inhibit the formation of ROS by interacting through its C-terminus with NPR, and to a lesser extent with CYP2E1 ( Figure 3a).…”