Activation and dysregulation of the unfolded protein response in nonalcoholic fatty liver disease

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.

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.

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.

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.