The Role And Mechanism Of Endoplasmic Reticulum Stress On Hepatic Fibrosis Induced By Tyrosine Metabolism Disorder

Background&AimsHepatic fibrosis（HF）is a common pathological process of multiple chronic liver diseases.Advanced fibrosis may lead to cirrhosis,liver failure and portal hypertension.When the liver is stimulated by various internal and external factors,inflammation and repair mechanisms will be triggered.Damaged hepatocytes can activate the hepatic stellate cells to transform into muscle fibroblasts and secrete fibrogenic factors which could promote the deposition of extracellular matrix（ECM）.Thus,HF is happened.HF is a dynamic process which characterized by more ECM synthesis than degradation.As the progression of HF,the hyperplasia of collagen fibers will damage the normal lobular architecture and form pseudolobules,eventually leading to irreversible cirrhosis.Nevertheless,the detailed regulatory mechanism of HF progression remains far from fully understood.It will be very helpful to develop novel targets and more effective therapeutic strategy if the underlying mechanism of HF occurrence and reversal is dissected.In recent years,the role of endoplasmic reticulum stress（ERS）in the development of liver diseases has attracted more and more attention.In response to stimulations,protein synthesis and secretion will be increased in hepatocytes.Meanwhile,various injury factors act on the endoplasmic reticulum（ER）,jointly aggravating the load of the ER,leading to the accumulation of unfolded proteins in the ER,stimulating the receptor proteins on the membrane of ER,finally causing ERS.ERS is a self-regulating mechanism of cells that leads to the unfolded protein response（UPR）.The main functions of the UPR are to decrease the load of ER via inhibition the protein synthesis and induction of some enzymes expression as well as to promote the protein folding through induction of the expression of chaperones.The UPR can lead to a variety of results,including alleviation of ERS,inflammation,autophagy and apoptosis.However,the role of ERS and its detailed regulatory mechanism in HF progression is still unclear.Fah gene knockout(Fah^-/-) rats are prone to develop HF due to tyrosine metabolism disorder.Loss of the Fah gene in humans leads to hereditary type I hypertyrosinemia（HT1）.In the present study,we intend to dissect the roles of ERS and it induced hepatocyte apoptosis in the progression of HF using Fah^-/- rats.The results will deepen the understanding of the pathogenesis of HF and provide new targets for the development of effective therapeutic strategies.Methods Fah gene knockout(Fah^-/-) rats and mice were constructed,respectively.The levels of HF and ERS were examined by immunohistochemistry and western blot assay after removing the NTBC drugs.4-Phenyl butyric acid（4-PBA） was used to alleviate ERS.Then,the levels of HF in the Fah^-/- rats treated with 4-PBA were further assessed.The liver samples from type I hereditary tyrosinemia（HT1） patients were employed to explore the correlation of the levels of Far and ERS.Results After stopping NTBC administration in Fah^-/- rats,toxic metabolites continued to induce hepatocytes injury,and HF was developed remarkably.A significant increase of ERS was observed in the livers of these Fah^-/- rats,which further activated the related apoptosis pathway and lead to hepatocytes apoptosis.Furthermore,we found that 4-PBA,a chemical chaperone,could alleviate ERS and HF.In contrary to the Fah^-/- rats,Fah^-/- mice did not develop HF after removing the NTBC drugs.Although the levels of ERS were increased at some extent in the livers of Fah^-/- mice,but the apoptosis pathway was not activated.In clinical investigation,ERS and induced hepatocytes apoptosis were observed in the liver samples from HT1 patients.Conclusion Fah^-/- rats showed HF clinicopathological features after liver injury,including elevated AST and ALT,activation of hepatic stellate cells,secretion of inflammatory cytokines,and recruitment and activation of immune cells.Mechanistically,ERS was significantly increased in the hepatocytes of the Fah^-/- rats.Persistent ERS activated the downstream apoptotic signaling pathway and increased the expression of CHOP,which induced the apoptosis of hepatocytes.With the increase of apoptotic cells,a large number of apoptosis bodies were produced to activate the inflammatory response and further activate the HSCs.Activated HSCs secreted more fibrogenic factors which could promote the deposition of ECM and finally resulted in HF.Nevertheless,for Fah^-/- mice,the elevated ERS caused by the injury of hepatocytes was insufficient to induce cell apoptosis and HF.We further found that 4-PBA could alleviate ERS and HF through inhibition of PERK/IRE1-CHOP pathway in Fah^-/- rats,indicating a new target and strategy for HF treatment.