Phenyl acyl acids attenuate the unfolded protein response in tunicamycin-treated neuroblastoma cells

Screening of Cellular Stress Responses Induced by Ambient Aerosol Ultrafine Particle Fraction PM0.5 in A549 Cells

Effects of airborne particles on the expression status of markers of cellular toxic stress and on the release of eicosanoids, linked with inflammation and oxidative damage, remain poorly characterized. Therefore, we proposed a set of various methodological approaches in order to address complexity of PM0.5-induced toxicity. For this purpose, we used a well-characterized model of A549 pulmonary epithelial cells exposed to a non-cytotoxic concentration of ambient aerosol particle fraction PM0.5 for 24 h. Electron microscopy confirmed accumulation of PM0.5 within A549 cells, yet, autophagy was not induced. Expression profiles of various cellular stress response genes that have been previously shown to be involved in early stress responses, namely unfolded protein response, DNA damage response, and in aryl hydrocarbon receptor (AhR) and p53 signaling, were analyzed. This analysis revealed induction of GREM1, EGR1, CYP1A1, CDK1A, PUMA, NOXA and GDF15 and suppression of SOX9 in response to PM0.5 exposure. Analysis of eicosanoids showed no oxidative damage and only a weak anti-inflammatory response. In conclusion, this study helps to identify novel gene markers, GREM1, EGR1, GDF15 and SOX9, that may represent a valuable tool for routine testing of PM0.5-induced in vitro toxicity in lung epithelial cells.

24(S)-Hydroxycholesterol induces ER dysfunction-mediated unconventional cell death

Endoplasmic reticulum (ER) stress induced by disruption of protein folding activates the unfolded protein response (UPR), which while generally pro-survival in effect can also induce cell death under severe ER stress. 24(S)-hydroxycholesterol (24S-OHC), which is enzymatically produced in the ER of neurons, plays an important role in maintaining brain cholesterol homeostasis but also shows neurotoxicity when subjected to esterification by acyl-CoA:cholesterol acyltransferase 1 (ACAT1) in the ER. In this study, we demonstrated that the accumulation of 24S-OHC esters in human neuroblastoma SH-SY5Y cells evoked the UPR with substantially no pro-survival adaptive response but with significant activation of pro-death UPR signaling via regulated IRE1-dependent decay (RIDD). We further found that accumulation of 24S-OHC esters caused disruption of ER membrane integrity and release of ER luminal proteins into cytosol. We also found that de novo synthesis of global proteins was robustly suppressed in 24S-OHC-treated cells. Collectively, these results show that ER dysfunction and the accompanying RIDD-mediated pro-death UPR signaling and global protein synthesis inhibition are responsible for 24S-OHC ester-induced unconventional cell death.

Caspase-4 mediates cytoplasmic accumulation of TDP-43 in the primate brains

The cytoplasmic accumulation of the nuclear TAR DNA-binding protein 43 (TDP-43) is a pathologic hallmark in amyotrophic lateral sclerosis, frontotemporal lobar degeneration, and other neurological disorders. However, most transgenic TDP-43 rodent models show predominant nuclear distribution of TDP-43 in the brain. By expressing mutant TDP-43 (M337V) in the brains of rhesus monkeys and mice, we verified that mutant TDP-43 is distributed in the cytoplasm of the monkey brain and that the majority of mutant TDP-43 remains in the nuclei of the mouse brain. The primate-specific caspase-4, but not mouse homologue caspase-11, could remove the NLS-containing N-terminal domain and generate fragmented TDP-43 that accumulates in the cytoplasm. Moreover, increased expression of caspase-4 in the monkey brain promotes the cytoplasmic accumulation of endogenous TDP-43, and suppressing caspase-4 reduces the cytoplasmic distribution of endogenous TDP-43 in cultured human neural cells. Our findings suggest that primate-specific caspase-4-mediated cleavage of TDP-43 accounts for its cytoplasmic mislocalization in the primate brains and may serve as a potential therapeutic target.

Olanzapine-induced endoplasmic reticulum stress and inflammation in the hypothalamus were inhibited by an ER stress inhibitor 4-phenylbutyrate

Antipsychotics are the most important treatment for schizophrenia. However, antipsychotics, particularly olanzapine and clozapine, are associated with severe weight gain/obesity side-effects. Although numerous studies have been carried out to identify the exact mechanisms of antipsychotic-induced weight gain, it is still important to consider other pathways. Endoplasmic reticulum (ER) stress signaling and its associated inflammation pathway is one of the most important pathways involved in regulation of energy balance. In the present study, we examined the role of hypothalamic protein kinase R like endoplasmic reticulum kinase- eukaryotic initiation factor 2α (PERK-eIF2α) signaling and the inflammatory IkappaB kinase β- nuclear factor kappa B (IKKβ-NFκB) signaling pathway in olanzapine-induced weight gain in female rats. In this study, we found that olanzapine significantly activated PERK-eIF2α and IKKβ-NFκB signaling in SH-SY5Y cells in a dose-dependent manner. Olanzapine treatment for 8 days in rats was associated with activated PERK-eIF2α signaling and IKKβ-NFκB signaling in the hypothalamus, accompanied by increased food intake and weight gain. Co-treatment with an ER stress inhibitor, 4-phenylbutyrate (4-PBA), decreased olanzapine-induced food intake and weight gain in a dose- and time-dependent manner. Moreover, 4-PBA dose-dependently inhibited olanzapine-induced activated PERK-eIF2α and IKKβ-NFκB signaling in the hypothalamus. These results suggested that hypothalamic ER stress may play an important role in antipsychotic-induced weight gain.

Adaptive unfolded protein response promotes cell survival in rifampicin-treated L02 cells

An important concept in drug-induced liver injury (DILI) is adaptation, which means the injury reverses with the continuation of the drug. The mechanism of adaption of drugs remains enigmatic, adaptive unfolded protein response (UPR) is possibly involved. We once observed adaptation phenomenon of rifampicin (RFP) in animal models, in this study, we investigate the effects of RFP on adaptive UPR in L02 cells, and after inhibiting UPR by using 4-phenylbutyrate (4-PBA), the change of cell viability and cell apoptosis in RFP-treated cells. We found that with the concentration of RFP increased and the treatment time was prolonged, the glucose-regulated protein 78 (GRP78), a hallmark of the UPR, was upregulated, and was dose- and time-dependent. RFP also activates the p-eukaryotic initiation factor 2α (eIF2α) protein expression. 4-PBA decreased GRP78 and p-eIF2α protein expression levels. Moreover, FCA showed that cell apoptosis rate obviously increased, and MTT assay showed that cell survival rate obviously decreased, this indicates that after inhibiting the UPR, the cell damage increased, which shows that the UPR is an adaptation mechanism to protect cells against injury induced by RFP. This also proves that when the degree of UPR induced by RFP is relatively mild, adaptive UPR is helpful for cell survival.

Endoplasmic Reticulum Stress Facilitates the Survival and Proliferation of Nucleus Pulposus Cells in TNF-α Stimulus by Activating Unfolded Protein Response

Intervertebral disc (IVD) degeneration is closely related to inflammatory cytokines, such as tumor necrosis factor alpha (TNF-α). The endoplasmic reticulum (ER) serves several important cell functions, which are essential for normal cell metabolism and survival. This study aims to clarify the role of ER stress and unfolded protein response (UPR) in TNF-α-induced biological changes in rat nucleus pulposus cells (NPCs) and IVD degeneration. In our research, rat NPCs were cultured with different concentrations of TNF-α in the presence or absence of ER stress inhibitors. Related genes and proteins were measured by immunofluorescence staining, quantitative real-time PCR, and Western blot analyses to monitor ER stress. Cell proliferation was evaluated by CCK-8 assay and cyclin D1 expression. Apoptosis was detected by flow cytometry and Western blot analyses. Our results showed that TNF-α induced the apoptosis of some NPCs in the early stage and then accelerated the proliferation of surviving cells. In addition, TNF-α stimulus upregulated ER stress markers and initiated UPR. However, these effects could be reversed by inhibitors, thereby reducing cell proliferation and enhancing apoptosis. In conclusion, ER stress reinforces the survival and proliferation of NPCs in TNF-α stimulus by activating UPR signaling, which could be an important therapeutic target in the future.

Limited Unfolded Protein Response and Inflammation in Neuroserpinopathy

Familial encephalopathy with neuroserpin inclusion bodies (FENIB) is a rare disease characterized by the deposition of multiple intracytoplasmic neuronal inclusions that contain mutated neuroserpin. Tg-Syracuse (Tg-Syr) mice express Ser49Pro mutated neuroserpin and develop clinical and neuropathological features of human FENIB. We used 8-, 34-, 45- and 80-week-old Tg-Syr mice to characterize neuroinflammation and the unfolded protein response (UPR) in a neurodegenerative disease in which abnormal protein aggregates accumulate within the endoplasmic reticulum (ER). There were scattered neuroserpin inclusions in Tg-Syr mice at 8 weeks of age; the numbers of neurons involved and the amount of neuroserpin per neuron increased with age throughout the CNS to 80 weeks of age; no similar inclusions were found in wild type (Tg-WT) mice at any age. Increases in numbers of astrocytes and microglia occurred at advanced disease stages. Among 22 markers in 80-week-old Tg-Syr mice, only II1b and II10rb mRNAs in the somatosensory cortex and CxCl10 and Il10rb mRNAs in the olfactory bulb were upregulated when compared with Tg-WT mice indicating a limited relationship between neuroserpin inclusions and inflammatory responses. The changes were accompanied by a transient increase in expression of Xbp1 spliced at 45 weeks and increased ERdJ4 mRNAs at 80 weeks. The sequestration of UPR activators GRP78 and GRP94 in neuroserpin inclusions might explain the limited UPR responses despite the accumulation of neuroserpin in the ER in this FENIB mouse model.

The therapeutic effects of 4-phenylbutyric acid in maintaining proteostasis

Recently, there has been an increasing amount of literature published on the effects of 4-phenylbutyric acid (4-PBA) in various biological systems. 4-PBA is currently used clinically to treat urea cycle disorders under the trade name Buphenyl. Recent studies however have explored 4-PBA in the context of a low weight molecular weight chemical chaperone. Its properties as a chemical chaperone prevent misfolded protein aggregation and alleviate endoplasmic reticulum (ER) stress. As the ER is responsible for folding proteins targeted for use in membranes or secreted out of the cell, failure of maintaining adequate ER homeostasis may lead to protein misfolding and subsequent cell and organ pathology. Accumulation of misfolded proteins within the ER activates the unfolded protein response (UPR), a molecular repair response. The activation of the UPR aims to restore ER and cellular proteostasis by regulating the rate of synthesis of newly formed proteins as well as initiating molecular programs aimed to help fold or degrade misfolded proteins. If proteostasis is not restored, the UPR may initiate pro-apoptotic pathways. It is suggested that 4-PBA may help fold proteins in the ER, attenuating the activation of the UPR, and thus potentially alleviating various pathologies. This review discusses the biomedical research exploring the potential therapeutic effects of 4-PBA in various in vitro and in vivo model systems and clinical trials, while also commenting on the possible mechanisms of action.

ERp29 deficiency affects sensitivity to apoptosis via impairment of the ATF6-CHOP pathway of stress response

Endoplasmic reticulum protein 29 (ERp29) belongs to the redox-inactive PDI-Dβ-subfamily of PDI-proteins. ERp29 is expressed in all mammalian tissues examined. Especially high levels of expression were observed in secretory tissues and in some tumors. However, the biological role of ERp29 remains unclear. In the present study we show, by using thyrocytes and primary dermal fibroblasts from adult ERp29(-/-) mice, that ERp29 deficiency affects the activation of the ATF6-CHOP-branch of unfolded protein response (UPR) without influencing the function of other UPR branches, like the ATF4-eIF2α-XBP1 signaling pathway. As a result of impaired ATF6 activation, dermal fibroblasts and adult thyrocytes from ERp29(-/-) mice display significantly lower apoptosis sensitivities when treated with tunicamycin and hydrogen peroxide. However, in contrast to previous reports, we could demonstrate that ERp29 deficiency does not alter thyroglobulin expression levels. Therefore, our study suggests that ERp29 acts as an escort factor for ATF6 and promotes its transport from ER to Golgi apparatus under ER stress conditions.