Activation and dysregulation of the unfolded protein response in nonalcoholic fatty liver disease.
OBJECTIVE
Nonalcoholic fatty liver (NAFL) and nonalcoholic steatohepatitis (NASH) are associated with known triggers of the unfolded protein response (UPR). The aims were to (1) evaluate the activity of UPR in NAFL and NASH and (2) correlate expression of UPR pathways with liver histology.


METHODS
Messenger RNA (mRNA) and protein expression were measured by quantitative real-time PCR and Western blot, respectively. Apoptosis was assessed by TUNEL assay. Liver histology was scored using the NASH clinical research network criteria.


RESULTS
Compared with subjects with the metabolic syndrome and normal liver histology (n = 17), both NAFL (n = 21) and NASH (n = 21) were associated with increased eukaryotic initiation factor-2alpha (eIF-2alpha) phosphorylation. Activating transcription factor 4 (ATF4) mRNA and protein, C/EBP homologous protein (CHOP), and growth arrest, DNA damage-34 (GADD34) mRNA were not increased in NAFL or NASH. Whereas immunoglobulin heavy chain binding protein mRNA was significantly increased in NASH, unspliced X-box protein-1 (XBP-1) protein did not increase. Also, endoplasmic reticulum degradation-enhancing alpha-mannosidase-like protein mRNA levels were inversely related to spliced XBP-1 mRNA in NASH. NASH was specifically associated with low sXBP-1 protein and increased JNK phosphorylation. This correlated with increased TUNEL activity in NASH. The histologic severity correlated with sXBP-1 mRNA and JNK phosphorylation.


CONCLUSIONS
There is a variable degree of UPR activation in NAFL and NASH. Although both NAFL and NASH are associated with eIF-2alpha phosphorylation, there is a failure to activate downstream recovery pathways, ie, ATF4-CHOP-GADD34. NASH is specifically associated with (1) failure to generate sXBP-1 protein and (2) activation of JNK.
Puri
Puneet
P
Division of Gastroenterology, Hepatology, and Nutrition, Department of Internal Medicine, Virginia Commonwealth University Medical Center, Richmond, Virginia, USA.
Mirshahi
Faridoddin
F
Cheung
Onpan
O
Natarajan
Ramesh
R
Maher
James W
JW
Kellum
John M
JM
Sanyal
Arun J
AJ
D051701:Activating Transcription Factor 4 / Q000378:metabolism; D051702:Activating Transcription Factor 6 / Q000378:metabolism; D000328:Adult; D000943:Antigens, Differentiation / Q000378:metabolism; D018797:Cell Cycle Proteins / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000502:physiology*; D004722:Endoribonucleases / Q000378:metabolism; D015852:Eukaryotic Initiation Factor-2 / Q000378:metabolism; D005234:Fatty Liver / Q000378:metabolism / Q000503:physiopathology*; D005260:Female; D006801:Humans; D008099:Liver / Q000378:metabolism / Q000473:pathology / Q000503:physiopathology; D048670:MAP Kinase Kinase 4 / Q000378:metabolism; D008297:Male; D008565:Membrane Proteins / Q000378:metabolism; D024821:Metabolic Syndrome / Q000378:metabolism / Q000503:physiopathology; D008875:Middle Aged; D017510:Protein Folding; D054645:Protein Phosphatase 1; D011499:Protein Processing, Post-Translational / Q000502:physiology*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D000071701:Regulatory Factor X Transcription Factors; D051742:Transcription Factor CHOP / Q000378:metabolism; D014157:Transcription Factors / Q000378:metabolism; D000071717:X-Box Binding Protein 1; D019892:eIF-2 Kinase / Q000378:metabolism
D051701:Activating Transcription Factor 4; D051702:Activating Transcription Factor 6; D000943:Antigens, Differentiation; D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D004722:Endoribonucleases; D015852:Eukaryotic Initiation Factor-2; D048670:MAP Kinase Kinase 4; D008565:Membrane Proteins; D054645:Protein Phosphatase 1; D017346:Protein Serine-Threonine Kinases; D000071701:Regulatory Factor X Transcription Factors; D051742:Transcription Factor CHOP; D014157:Transcription Factors; D000071717:X-Box Binding Protein 1; D019892:eIF-2 Kinase; C497109:ATF4 protein, human; C497111:ATF6 protein, human; C497249:DDIT3 protein, human; C000605543:XBP1 protein, human; C403982:ERN2 protein, human; C087519:PPP1R15A protein, human