Finally, we observe increased UPR levels and low NMD gene expression in fibrotic mice following chronic CCl4 treatment. activation on its own, as chemical induction of endoplasmic reticulum stress with tunicamycin in 3D cultured, quiescent stellate cells is not able to induce stellate cell activation. Inhibition of Jnk is usually important for the transduction of the unfolded protein response. Stellate cells isolated from Jnk knockout mice do not activate as much as their wild-type counterparts and do not have an induced expression of unfolded protein response genes. A timely termination of the unfolded protein response is essential to prevent endoplasmic reticulum stress-related apoptosis. A pathway known to be involved in this termination is the non-sense-mediated decay pathway. Non-sense-mediated decay inhibitors influence the unfolded protein response at early time points during stellate cell activation. Our data suggest that UPR in HSCs is usually differentially regulated between acute and chronic stages of the activation process. In conclusion, our data demonstrates that this unfolded protein response is usually a JNK1-dependent early event during hepatic stellate cell TZ9 activation, which is usually counteracted by non-sense-mediated decay and is not sufficient to drive the stellate cell activation process. TZ9 Therapeutic strategies based on UPR or NMD modulation might interfere with fibrosis, but will remain challenging because of the feedback mechanisms between the stress pathways. Introduction Sustained chronic liver injury leading to fibrosis, cirrhosis and finally organ failure causes significant morbidity and mortality world-wide1. Liver fibrosis is usually defined by hardening and scarring of the liver due to an excessive deposition of extracellular matrix (ECM) components. The major cellular source for the ECM production are the hepatic stellate cells (HSCs). During chronic liver injury, HSCs undergo a transdifferentiation from quiescent, lipid droplet made up of cells towards activated myofibroblast-like HSCs with an increased proliferation rate and high production of ECM2. HSC activation is usually a critical step in the fibrotic TZ9 response to liver injury3. Novel insights into mechanisms regulating HSC activation are considered key in developing new treatments for hepatic fibrosis. Sensing and responding to stress is essential for maintaining cellular homeostasis. There are numerous triggers that can cause stress in a cell, e.g., viral infections, hypoxia, chemical insults and alterations in substrates and energy. The process of protein folding is particularly sensitive to such insults4. TZ9 An unfolded protein response (UPR) is initiated to restore cellular homeostasis upon acute stress exposure, while chronic activation of the UPR leads to endoplasmic reticulum (ER) stress and ultimately to apoptosis. UPR transmits survival signals through three sensory systems, the PERK (protein kinase R (PKR)-like endoplasmic reticulum kinase), IRE1 (inositol-requiring enzyme-1) and ATF6 (activating transcription factor 6) cascades, which aim at reducing ER stress by increasing the folding and export capacity and by lowering general translation5. The three UPR arms have been associated with chronic liver disease6C9. Numerous studies report on UPR induction in hepatocytes in, for example, nonalcoholic fatty liver disease, but more recent studies have also attributed a role for the UPR to HSC activation and fibrotic wound healing. In general, it is found that chronic injury or HSC activation correlates with high levels of ER stress related genes and that chemical induction of ER stress further increases HSC activation10C14. Non-sense-mediated mRNA decay (NMD) is usually a mechanism to remove aberrant messenger Rabbit Polyclonal to HSL (phospho-Ser855/554) RNA (mRNA) transcripts, but also to finetune the expression of certain normal mRNAs. As unfolded protein response will block translation, mRNA accumulation is usually expected, and this can be controlled by NMD. It was shown that NMD can buffer cells from an overactive UPR. In addition, there is evidence that NMD?directly targets the mRNAs encoding several?UPR components15C17. In this study, we confirm that there is an UPR in chronically in vivo activated HSCs by showing increased expression of BIP, Chop and XBP1 spliced and that these high UPR levels are paralleled by low NMD marker expression. However, more interestingly, we also observe a transient, endogenous induction of these genes very early during in vitro and in vivo HSC activation. We propose that this early UPR is usually a compensatory mechanism to cope with the increased needs for protein production and secretion of, for example, collagens,.