Liver – master and servant of serum proteome Deniz Kuscuoglu, Sabina Janciauskiene, Karim Hamesch, Johannes Haybaeck, Christian Trautwein, Pavel Strnad  Journal of Hepatology  Volume 69, Issue 2, Pages 512-524 (August 2018) DOI: 10.1016/j.jhep.2018.04.018 Copyright © 2018 European Association for the Study of the Liver Terms and Conditions

Fig. 1 Handling of secretory proteins by hepatocytes. (1) Secretory proteins contain a hydrophobic signal sequence that is recognised by the signal recognition particle (SRP). SRP binding leads to the translational arrest and a transfer of the mRNA-ribosome complex to the endoplasmic reticulum (ER) membrane. The synthesised polypeptide then enters the ER lumen through a translocon channel (composed either by Sec61 or Derlins). In the ER, the peptide is supplemented with preassembled glycans and the signal sequence is removed. (2) The folding of glycoproteins is mediated by a range of glycosylation/deglycosylation steps that are facilitated by the chaperones calnexin (CNX) and calreticulin (CRT). Non-glycosylated peptides are folded in CNX-/CRT-independent manner including chaperones BiP or ERdj3/6. (3) The correctly folded proteins dissociate from the folding machineries and are transported to the ER-exit site and subsequently either by COPII-dependent manner or by unconventional secretion pathway (UPS) to the Golgi apparatus. A failure in the folding process may lead to accumulation of misfolded proteins that are chelated by the chaperone binding immunoglobulin protein (BiP). (4) At the same time, BiP dissociates from the ER stress sensors, thus triggering unfolded protein response (UPR). The protein accumulation activates (both in UPR-dependent and –independent manner) a transcriptional programme that tries to restore the regulated protein flow through the ER. As another line of defence, the misfolded proteins are degraded either via (5) ER-associated degradation pathway (ERAD) or (6) autophagy. The former process is initiated by ER mannosidase I (ERManI) mediated cleavage of a mannose residue, that removes the proteins from the CNX-CRT folding cycle. After association with lectins and chaperones, that prevent protein aggregation, the polypeptides are translocated via the Sec61/Derlin-containing channel into the cytoplasm, where they become ubiquitinated and degraded via proteasome. Journal of Hepatology 2018 69, 512-524DOI: (10.1016/j.jhep.2018.04.018) Copyright © 2018 European Association for the Study of the Liver Terms and Conditions

Fig. 2 Endoplasmic reticulum stress response. Endoplasmic reticulum (ER) stress response is regulated by the availability of the central chaperone BiP. Under basal conditions, BiP binds to the ER stress sensors ATF6α, IRE1α and PERK. Accumulation of misfolded proteins leads to its dissociation from these sensors and their activation. The activation of IRE1α unleashes both its kinase and endoribonuclease functions. The latter causes decay of mRNAs (RIDD; regulated IRE1-dependent decay of mRNA) and a cleavage of XBP1 mRNA into the transcriptionally active form. Spliced XBP1 (sXBP1) transits into the nucleus and enhances the expression of genes related to ER trafficking / ER-associated degradation (ERAD). In the case of ATF6α, the dissociation from BiP leads to translocation to Golgi complex, where it becomes proteolytically cleaved. Its cytosolic fragment then induces the expression of various chaperones. PERK phosphorylates eIF2alpha, thereby inhibiting translation of proteins. Although phosphorylation of eIF2α reduces the overall mRNA translation, it also enhances the translation of specific mRNAs bearing short upstream open reading frames (uORFs) that boost protein transport or antioxidant stress response. While all these steps promote the survival of the cells, the prolonged stress induces apoptotic cell death mainly through the PERK–eIF2α–ATF4-CHOP pathway. CHOP induces the synthesis of pro-apoptotic genes. IRE1α also promotes apoptosis by inducing ASK1 and JNK. Other strategies of the ER to cope with stress include the activation of the transcription factor NF-κB and the liver specific transcription factor CREBH. While the former is induced by the efflux of Ca2+ and reactive oxygen intermediates from the ER, CREBH is proteolytically cleaved and its N-terminal fragment stimulates the production of acute-phase response (APR) genes. Journal of Hepatology 2018 69, 512-524DOI: (10.1016/j.jhep.2018.04.018) Copyright © 2018 European Association for the Study of the Liver Terms and Conditions

Fig. 3 Characteristic hepatocellular endoplasmic reticulum-storage diseases. Ground glass hepatocytes (GGHs, depicted by arrows) are visualised via (A) haematoxylin and eosin (H&E) staining and (B) immunohistochemistry with an antibody against hepatitis B surface protein in a patient with chronic hepatitis B infection. (C) H&E and (D) periodic acid-Schiff after diastase digestion (PAS-D) stained liver sections of a patient with severe alpha1-antitrypsin deficiency (PiZZ genotype). The arrows indicate the alpha1-antitrypsin (AAT) aggregates. (E) H&E staining reveals hepatocytes with amorphous, fibrillar pale bodies (arrows) in a patient with hepatocellular carcinoma (HCC). (F) These inclusions are not labelled in an immunohistochemistry staining with an antibody against glutamine synthetase. Journal of Hepatology 2018 69, 512-524DOI: (10.1016/j.jhep.2018.04.018) Copyright © 2018 European Association for the Study of the Liver Terms and Conditions