Regulation of transglutaminase-mediated hepatic cell death in alcoholic steatohepatitis and non-alcoholic steatohepatitis.
OBJECTIVE
Transglutaminase 2 (TG2), catalyzing crosslinking between lysine and glutamine residues, is involved in many liver diseases. We previously reported that TG2, induced in the nucleus of ethanol- or free fatty acids (FFAs)-treated hepatic cells, crosslinks and inactivates a transcription factor Sp1, leading to reduced expression of c-Met and thereby caspase independent hepatic apoptosis in culture systems, animal models, and both alcoholic steatohepatitis (ASH) and non-alcoholic steatohepatitis (NASH) patients. FFAs increase endoplasmic reticulum (ER) stress, NFkB activation and nuclear TG2 (nTG2) through pancreatic ER kinase (PERK)-dependent pathway, whereas ethanol induces nTG2 via retinoid signaling. However, the molecular mechanism by which ethanol/FFAs induce nuclear localization of TG2 has been unclear.


METHODS
A similar nTG2-mediated cell death is induced in acyclic retinoid (ACR)-treated hepatocellular carcinoma. Using cultured cells, we investigated how to control this novel apoptotic pathway by regulating nuclear localization of TG2.


RESULTS
TG2 is composed of N-terminal b-sandwich, catalytic core, b-barrel 1, and C-terminal b-barrel 2 domains. In a previous work, we identified a 14 amino acid nuclear localization signal (NLS) within the b-barrel 1 domain and a putative leucine-rich nuclear export signal (NES) at position 657 to 664 (LHMGLHKL) near the C-terminus in the b-barrel 2 domain, and found that ACR downregulated exportin-1 levels, thereby accumulation of TG2 in the nucleus. Here, we found that both ethanol and FFAs provoked generation of truncated short form of TG2 (TG2-S) defects in the putative NES at least in part through alternative splicing, thereby causing accumulation of TG2-S in the nucleus.


CONCLUSIONS
The generation of TG2-S in ethanol or FFAs-treated hepatic cells is a novel therapeutic target for prevention of hepatic cell death associated with ASH/NASH.
Molecular Ligand Biology Research Team, Chemical Genomics Research Group, Chemical Biology Department, RIKEN Advanced Science Institute, Saitama, Japan. skojima@postman.riken.go.jp
Soichi
S
Kojima
Ting-Fang
TF
Kuo
Hideki
H
Tatsukawa
D021581:Active Transport, Cell Nucleus; D017398:Alternative Splicing; D016923:Cell Death; D002467:Cell Nucleus / Q000187:drug effects / Q000201:enzymology* / Q000473:pathology; D000431:Ethanol / Q000633:toxicity*; D005230:Fatty Acids, Nonesterified / Q000633:toxicity*; D005234:Fatty Liver / Q000201:enzymology* / Q000473:pathology; D005235:Fatty Liver, Alcoholic / Q000201:enzymology* / Q000473:pathology; D019204:GTP-Binding Proteins; D056945:Hep G2 Cells; D006801:Humans; D028884:Karyopherins / Q000378:metabolism; D008099:Liver / Q000187:drug effects / Q000201:enzymology* / Q000473:pathology; D065626:Non-alcoholic Fatty Liver Disease; D000091345:Protein Glutamine gamma Glutamyltransferase 2; D021382:Protein Sorting Signals; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism; D015398:Signal Transduction; D011503:Transglutaminases / Q000737:chemistry / Q000378:metabolism*; D000097606:Exportin 1 Protein
D000431:Ethanol; D005230:Fatty Acids, Nonesterified; D019204:GTP-Binding Proteins; D028884:Karyopherins; D000091345:Protein Glutamine gamma Glutamyltransferase 2; D021382:Protein Sorting Signals; D018160:Receptors, Cytoplasmic and Nuclear; D011503:Transglutaminases; D000097606:Exportin 1 Protein