Impact of endoplasmic reticulum (ER) stress on cellular functions and role of natural products as modulators

Palmitic acid (PA) upregulates oxidized LDL receptor-1 (LOX-1), a scavenger receptor responsible for uptake of oxidized LDL (oxLDL), and enhances oxLDL uptake in macrophages. However, the precise underlying mechanism remains to be elucidated. PA is known to induce endoplasmic reticulum (ER) stress in various cell types. Therefore, we investigated whether ER stress is involved in PA-induced LOX-1 upregulation. PA induced ER stress, as determined by phosphorylation of PERK, eIF2α, and JNK, as well as induction of CHOP in macrophage-like THP-1 cells. Inhibitors [4-phenylbutyric acid (PBA), sodium tauroursodeoxycholate (TUDCA), and salubrinal] and small interfering RNA (siRNA) for the ER stress response decreased PA-induced LOX-1 upregulation. Thapsigargin, an ER stress inducer, upregulated LOX-1, which was decreased by PBA and TUDCA. We next examined whether unsaturated FAs could counteract the effect of PA. Both oleic acid (OA) and linoleic acid (LA) suppressed PA-induced LOX-1. Activation of the ER stress response observed in the PA-treated cells was markedly attenuated when the cells were cotreated with OA or LA. In addition, OA and LA suppressed thapsigargin-induced LOX-1 upregulation with reduced activation of ER stress markers. Our results indicate that activation of ER stress is involved in PA-induced LOX-1 upregulation in macrophages, and that OA and LA inhibit LOX-1 induction through suppression of ER stress.

A specific polymorphism in the hemochromatosis (HFE) gene, H63D, is over-represented in neurodegenerative disorders such as amyotrophic lateral sclerosis and Alzheimer disease. Mutations of HFE are best known as being associated with cellular iron overload, but the mechanism by which HFE H63D might increase the risk of neuron degeneration is unclear. Here, using an inducible expression cell model developed from a human neuronal cell line SH-SY5Y, we reported that the presence of the HFE H63D protein activated the unfolded protein response (UPR). This response was followed by a persistent endoplasmic reticulum (ER) stress, as the signals of UPR sensors attenuated and followed by up-regulation of caspase-3 cleavage and activity. Our in vitro findings were recapitulated in a transgenic mouse model carrying Hfe H67D, the mouse equivalent of the human H63D mutation. In this model, UPR activation was detected in the lumbar spinal cord at 6 months then declined at 12 months in association with increased caspase-3 cleavage. Moreover, upon the prolonged ER stress, the number of cells expressing HFE H63D in early apoptosis was increased moderately. Cell proliferation was decreased without increased cell death. Additionally, despite increased iron level in cells carrying HFE H63D, it appeared that ER stress was not responsive to the change of cellular iron status. Overall, our studies indicate that the HFE H63D mutant protein is associated with prolonged ER stress and chronically increased neuronal vulnerability.

ADAR1-Dependent RNA Editing Promotes MET and iPSC Reprogramming by Alleviating ER Stress.

Vitamin D receptor (VDR)-dependent mechanisms regulate human cathelicidin antimicrobial peptide (CAMP)/LL-37 in various cell types, but CAMP expression also increases after external perturbations (such as infection, injuries, UV irradiation, and permeability barrier disruption) in parallel with induction of endoplasmic reticulum (ER) stress. We demonstrate that CAMP mRNA and protein expression increase in epithelial cells (human primary keratinocytes, HaCaT keratinocytes, and HeLa cells), but not in myeloid (U937 and HL-60) cells, following ER stress generated by two mechanistically different, pharmacological stressors, thapsigargin or tunicamycin. The mechanism for increased CAMP following exposure to ER stress involves NF-κB activation leading to CCAAT/enhancer-binding protein α (C/EBPα) activation via MAP kinase-mediated phosphorylation. Furthermore, both increased CAMP secretion and its proteolytic processing to LL-37 are required for antimicrobial activities occur following ER stress. In addition, topical thapsigargin also increases production of the murine homologue of CAMP in mouse epidermis. Finally and paradoxically, ER stress instead suppresses the 1,25(OH)(2) vitamin D(3)-induced activation of VDR, but blockade of VDR activity does not alter ER stress-induced CAMP up-regulation. Hence, ER stress increases CAMP expression via NF-κB-C/EBPα activation, independent of VDR, illuminating a novel VDR-independent role for ER stress in stimulating innate immunity.

Details Unfold: The Endoplasmic Reticulum Stress Response in Intestinal Inflammation and Cancer

Endoplasmic reticulum (ER) stress is sensed by cells in different physiopathological conditions in which there is an accumulation of unfolded proteins in the ER. A coordinated adaptive program called the unfolded protein response is triggered and includes translation inhibition, transcriptional activation of a set of genes encoding mostly intracellular proteins, and ultimately apoptosis. Here we show that insulin-like growth factor (IGF)-binding protein-1 (IGFBP-1), a secreted protein that modulates IGF bioavailability and has other IGF-independent effects, is potently induced during ER stress in human hepatocytes. Various ER stress-inducing agents were able to increase IGFBP-1 mRNA levels, as well as cellular and secreted IGFBP-1 protein up to 20-fold. A distal regulatory region of the human IGFBP-1 gene (-6682/-6384) containing an activating transcription factor 4 (ATF4) composite site was required for promoter activation upon ER stress. Mutation of the ATF4 composite site led to the loss of IGFBP-1 regulation. Electrophoretic mobility shift assay revealed an ER stress-inducible complex that was displaced by an ATF4 antibody. Knockdown of ATF4 expression using two specific small interfering RNAs impaired up-regulation of IGFBP-1 mRNA, which highlights the relevance of ATF4 in endogenous IGFBP-1 gene induction. In addition to intracellular proteins involved in secretory and metabolic pathways, we conclude that ER stress induces the synthesis of secreted proteins. Increased secretion of IGFBP-1 during hepatic ER stress may thus constitute a signal to modulate cell growth and metabolism and induce a systemic adaptive response.

The efficient functioning of the endoplasmic reticulum (ER) is essential for most cellular activities and survival. Conditions that interfere with ER function lead to the accumulation and aggregation of unfolded proteins. ER transmembrane receptors detect the onset of ER stress and initiate the unfolded protein response (UPR) to restore normal ER function. If the stress is prolonged, or the adaptive response fails, apoptotic cell death ensues. Many studies have focused on how this failure initiates apoptosis, as ER stress-induced apoptosis is implicated in the pathophysiology of several neurodegenerative and cardiovascular diseases. In this review, we examine the role of the molecules that are activated during the UPR in order to identify the molecular switch from the adaptive phase to apoptosis. We discuss how the activation of these molecules leads to the commitment of death and the mechanisms that are responsible for the final demise of the cell.

ER Stress Triggers Apoptosis by Activating BH3-Only Protein Bim

415 Activated Protein C (APC) Alleviates Endoplasmic Reticulum (ER) Stress and ER Stress-Linked Apoptosis in Endothelial Cells

From adaption to death: endoplasmic reticulum stress as a novel target of selective neurodegeneration?

Stressing out the ER in aminoglycoside-induced hearing loss.

Best1 mitigates ER stress induced by the increased cellular microenvironment stiffness in epilepsy.

Gene Therapy Strategies to Restore ER Proteostasis in Disease.

Although short-term incubation of hepatocytes with oleic acid (OA) stimulates secretion of apolipoprotein B100 (apoB100), exposure to higher doses of OA for longer periods inhibits secretion in association with induction of endoplasmic reticulum (ER) stress. Palmitic acid (PA) induces ER stress, but its effects on apoB100 secretion are unclear. Docosahexaenoic acid (DHA) inhibits apoB100 secretion, but its effects on ER stress have not been studied. We compared the effects of each of these fatty acids on ER stress and apoB100 secretion in McArdle RH7777 (McA) cells: OA and PA induced ER stress and inhibited apoB100 secretion at higher doses; PA was more potent because it also increased the synthesis of ceramide. DHA did not induce ER stress but was the most potent inhibitor of apoB100 secretion, acting via stimulation of autophagy. These unique effects of each fatty acid were confirmed when they were infused into C57BL6J mice. Our results suggest that when both increased hepatic secretion of VLDL apoB100 and hepatic steatosis coexist, reducing ER stress might alleviate hepatic steatosis but at the expense of increased VLDL secretion. In contrast, increasing autophagy might reduce VLDL secretion without causing steatosis.

Upon detecting endoplasmic reticulum (ER) stress, the unfolded protein response (UPR) orchestrates adaptive cellular changes to reestablish homeostasis. If stress resolution fails, the UPR commits the cell to apoptotic death. Here we show that in hematopoietic cells, including multiple myeloma (MM), lymphoma, and leukemia cell lines, ER stress leads to caspase-mediated cleavage of the key UPR sensor IRE1 within its cytoplasmic linker region, generating a stable IRE1 fragment comprising the ER-lumenal domain and transmembrane segment (LDTM). This cleavage uncouples the stress-sensing and signaling domains of IRE1, attenuating its activation upon ER perturbation. Surprisingly, LDTM exerts negative feedback over apoptotic signaling by inhibiting recruitment of the key proapoptotic protein BAX to mitochondria. Furthermore, ectopic LDTM expression enhances xenograft growth of MM tumors in mice. These results uncover an unexpected mechanism of cross-regulation between the apoptotic caspase machinery and the UPR, which has biologically significant consequences for cell survival under ER stress.