Mice Lacking bi-1 Gene Show Accelerated Liver Regeneration

Abstract

The liver has enormous regenerative capacity such that, after partial hepatectomy, hepatocytes rapidly replicate to restore liver mass, thus providing a context for studying in vivo mechanisms of cell growth regulation. Bax inhibitor-1 (BI-1) is an evolutionarily conserved endoplasmic reticulum (ER) protein that suppresses cell death. Interestingly, the BI-1 protein has been shown to regulate Ca 2+ handling by the ER similar to antiapoptotic Bcl-2 family proteins. Effects on cell cycle entry by Bcl-2 family proteins have been described, prompting us to explore whether bi-1-deficient mice display alterations in the in vivo regulation of cell cycle entry using a model of liver regeneration. Accordingly, we compared bi-1 +/+ and bi-1 À/À mice subjected to partial hepatectomy with respect to the kinetics of liver regeneration and molecular events associated with hepatocyte proliferation. We found that bi-1 deficiency accelerates liver regeneration after partial hepatectomy. Regenerating hepatocytes in bi-1 À/À mice enter cell cycle faster, as documented by more rapid incorporation of deoxynucleotides, associated with earlier increases in cyclin D1, cyclin D3, cyclin-dependent kinase (Cdk) 2, and Cdk4 protein levels, more rapid hyperphosphorylation of retinoblastoma protein, and faster degradation of p27 Kip1. Dephosphorylation and nuclear translocation of nuclear factor of activated T cells 1 (NFAT1), a substrate of the Ca 2+ -sensitive phosphatase calcineurin, were also accelerated following partial hepatectomy in BI-1-deficient hepatocytes. These findings therefore reveal additional similarities between BI-1 and Bcl-2 family proteins, showing a role for BI-1 in regulating cell proliferation in vivo, in addition to its previously described actions as a regulator of cell survival.

BAX inhibitor-1 is a Ca2+ channel critically important for immune cell function and survival

“…21 We observed that NFATc1 was also already translocated to a much higher degree in BI-1-deficient splenocytes even without stimulation, similar to NF-κB ( Figure 3d) but unchanged at the expression level ( Figure 3e). These data corroborate results obtained in BI-1-deficient hepatocytes, which also showed an increased constitutive translocation of NFATc1, 12 and support the assumption that the changes in Ca 2+ signaling are responsible for the increased abundance and nuclear translocation (p65) shown by quantitative immunocytochemistry in BI-1 KO splenocytes. Splenocytes were stained with an α-RelA (p65) antibody and analyzed by confocal microscopy.…”

“…This condition is caused by retention of proteins within the ER lumen, and in contrast to the UPR does not induce Grp78/BiP 23 which is also unchanged in the absence of BI-1. 12,13 Notably, NF-κB induction by the ER overload reaction can be attenuated by intracellular Ca 2+ chelators. 24,25 Therefore, both major functions of BI-1, its role in ER stress and in the maintenance of the intracellular Ca 2+ homeostasis, might be implicated in the observed phenotype.…”

“…11 Despite its evolutionarily conserved role in important functions such as ER stress and Ca 2+ regulation, bi-1 − / − mice were reported to have no phenotypic abnormalities but increased infarct volumes in a stroke model, and increased sensitivity to tunicamycin-induced kidney toxicity. 6 Moreover, livers from BI-1-deficient mice regenerate faster than those from wild-type (WT) mice and this correlates with increased nuclear translocation of nuclear factor of activated T cells (NFATs) 12 , a Ca 2+ -dependent process. BI-1 knockout (KO) mice also express more of the spliced form of X-box-binding protein-1 (sXBP-1) in their liver and kidney, 13 which is generated by the endoribonuclease activity of inositol requiring enzyme 1 (IRE1), and is considered an indicator of increased UPR activity.…”

Hepatocyte Proliferation During Liver Regeneration Is Impaired in Mice with Methionine Diet-Induced Hyperhomocysteinemia

“…In contrast, bax inhibitor-1 deficiency accelerates liver regeneration after PH, which is associated with an increase in cyclin D1 protein levels. 51 Akt can enhance expression of cyclin D1 through enhancing translation and inhibiting protein degradation. 52 Thus, the reduced expression of cyclin D1 is probably due to a decrease in Akt phosphorylation in HHcy mice.…”

Proteomic Profiling of Differentially Expressed Proteins from Bax inhibitor-1 Knockout and Wild Type Mice

“…When mice were exposed to hypoxic stress, the infarction size was significantly larger in bi-1 -/-than it was in bi-1 +/+ mice (Chae et al, 2004). Although one study found highly efficient regeneration of damaged liver in bi-1 knock-out mice (Bailly-Maitre et al, 2007), other studies have mainly focused on the protective functions of BI-1 in in vitro and in vivo systems (Bailly-Maitre et al, 2006;Chae et al, 2004;Dohm et al, 2006).…”