Endoplasmic reticulum stress induces fibrogenic activity in hepatic stellate cells through autophagy

Abstract

Background & Aims Metabolic stress during liver injury enhances autophagy and provokes stellate cell activation, with secretion of scar matrix. Conditions that augment protein synthesis increase demands on the endoplasmic reticulum (ER) folding capacity and trigger the unfolded protein response (UPR) to cope with resulting ER stress. Generation of reactive oxygen species (ROS) is a common feature of hepatic fibrogenesis, and crosstalk between oxidative stress and ER stress has been proposed. The aim of our study was to determine the impact of oxidant and ER stress on stellate cell activation. Methods Oxidant stress was induced in hepatic stellate cells using H2O2 in culture or by ethanol feeding in vivo, and the UPR response was analyzed. Because the branch of the UPR mainly affected was IREα, we blocked this pathway in stellate cells and analyzed the fibrogenic response, together with autophagy and downstream MAPK signaling. The Nrf2 antioxidant response was also evaluated in stellate cells under oxidant stress conditions. Results H2O2 treatment in culture or ethanol feeding in vivo increased the UPR response based on splicing of XBP1 mRNA, which triggered autophagy. The Nrf2-mediated antioxidant response, as measured by qRT-PCR of its target genes was also induced under ER stress conditions. Conversely, blockade of the IRE1 pathway in stellate cells significantly decreased both their activation and autophagic activity in a p38 MAPK dependent manner, leading to a reduced fibrogenic response. Conclusions These data implicate mechanisms underlying protein folding quality control in regulating the fibrogenic response in hepatic stellate cells.

Reactive gamma-ketoaldehydes as novel activators of hepatic stellate cells in vitro

“…Considering the lack of IsoLG-mediated NF-kB activation in hHSC while there is evidence that 15-E 2 -IsoLG strongly activates NF-kB in macrophages, this confirms previous observations suggesting a strong celltype dependency in the response to this mediator [30]. Similarly, we did not detect any significant activation of p38 MAPK, a known target of ER stress activation [20,47]. Overall, the most profound effect of hHSC exposure to IsoLG is an effect on the synthesis and secretion of pro-inflammatory cytokines/chemokines.…”

“…This could contribute to the pro-fibrogenic role of HSC biology by multiple mechanisms. These include the activation of redox-dependent stress kinases such as JNK and ERK [55][56][57], and the stimulation of autophagy [47,58,59]. Along these lines, the results of this study provide evidence that IsoLG exposure in HSC activates autophagy.…”

“…This likely occurs as a compensatory anti-stress response, since inhibition of autophagy by bafilomycin increases the formation of proinflammatory cytokines. Indeed, autophagy can serve as a protective, pro-survival mechanism in cells exposed to oxidant stress, as reported in primary hepatocytes following ischemia/reperfusion [58], and HSC following H 2 O 2 treatment [47]. Interestingly, it has been suggested that enhanced autophagy is required to establish an activated phenotype in HSC, by providing energy for the process via lipid droplets mobilization [45,46].…”

“…Treatment of HSC with 10 nM bafilomycin induced a robust increase of LC3BII levels, consistent with its reduced turnover resulting from a block of autophagosome fusion with lysosome, thus failure to complete autophagy [45,46] In addition, bafilomycin treatment in control cells increased levels of IL1 Exposure to IsoLG promotes early activation of ER stress, which precedes autophagy in HSC. One of the triggers for autophagy is ER stress, and secondary activation of the unfolded protein response (UPR) [47,48]. Indeed activation of the unfolded protein response has previously been described in HUVEC cells following exposure to IsoLG [20].…”

Cellular and molecular mechanisms in liver fibrogenesis

“…In a more recent study, the same research group has provided further experimental evidence that correlates endoplasmic reticulum (ER) stress and generation of ROS to autophagy and HSCs activation [100]. The involvement of ER stress was hypothesized on the basis of the postulate that any condition that augments protein synthesis is likely to increase demands on the endoplasmic reticulum (ER) folding capacity, then potentially triggering the classic unfolded protein response (UPR) in order to cope with resulting ER stress.…”