Eukaryotic cells deal with accumulation of unfolded proteins in the endoplasmic reticulum (ER) by the unfolded protein response, involving the induction of molecular chaperones, translational attenuation, and ER-associated degradation, to prevent cell death. Here, we found that the autophagy system is activated as a novel signaling pathway in response to ER stress. Treatment of SK-N-SH neuroblastoma cells with ER stressors markedly induced the formation of autophagosomes, which were recognized at the ultrastructural level. The formation of green fluorescent protein (GFP)-LC3-labeled structures (GFP-LC3 "dots"), representing autophagosomes, was extensively induced in cells exposed to ER stress with conversion from LC3-I to LC3-II. In IRE1-deficient cells or cells treated with c-Jun N-terminal kinase (JNK) inhibitor, the autophagy induced by ER stress was inhibited, indicating that the IRE1-JNK pathway is required for autophagy activation after ER stress. In contrast, PERK-deficient cells and ATF6 knockdown cells showed that autophagy was induced after ER stress in a manner similar to the wild-type cells. Disturbance of autophagy rendered cells vulnerable to ER stress, suggesting that autophagy plays important roles in cell survival after ER stress.

The NLRP3 (nucleotide-binding domain, leucine-rich-repeat-containing family, pyrin domain-containing 3) inflammasome mediates production of inflammatory mediators, such as IL-1β and IL-18, and as such is implicated in a variety of inflammatory processes, including infection, sepsis, autoinflammatory diseases, and metabolic diseases. The proximal steps in NLRP3 inflammasome activation are not well understood. Here we elucidate a critical role for Ca 2+ mobilization in activation of the NLRP3 inflammasome by multiple stimuli. We demonstrate that blocking Ca 2+ mobilization inhibits assembly and activation of the NLRP3 inflammasome complex, and that during ATP stimulation Ca 2+ signaling is pivotal in promoting mitochondrial damage. C/EPB homologous protein, a transcription factor that can modulate Ca 2+ release from the endoplasmic reticulum, amplifies NLRP3 inflammasome activation, thus linking endoplasmic reticulum stress to activation of the NLRP3 inflammasome. Our findings support a model for NLRP3 inflammasome activation by Ca 2+ -mediated mitochondrial damage.innate immunity | mitochondria

Inflammasomes are intracellular sensors that couple detection of pathogens and cellular stress to activation of Caspase-1, and consequent IL-1β and IL-18 maturation and pyroptotic cell death. Here, we show that the absent in melanoma 2 (AIM2) and nucleotidebinding oligomerization domain-like receptor pyrin domain-containing protein 3 (NLRP3) inflammasomes trigger Caspase-1-dependent mitochondrial damage. Caspase-1 activates multiple pathways to precipitate mitochondrial disassembly, resulting in mitochondrial reactive oxygen species (ROS) production, dissipation of mitochondrial membrane potential, mitochondrial permeabilization, and fragmentation of the mitochondrial network. Moreover, Caspase-1 inhibits mitophagy to amplify mitochondrial damage, mediated in part by cleavage of the key mitophagy regulator Parkin. In the absence of Parkin activity, increased mitochondrial damage augments pyroptosis, as indicated by enhanced plasma membrane permeabilization and release of danger-associated molecular patterns (DAMPs). Therefore, like other initiator caspases, Caspase-1 activation by inflammasomes results in mitochondrial damage.inflammasomes | mitochondrial damage | pyroptosis | mitophagy

Eukaryotic cells have signalling pathways from the endoplasmic reticulum (ER) to cytosol and nuclei, to avoid excess accumulation of unfolded proteins in the ER. We previously identified a new type of ER stress transducer, OASIS, a bZIP (basic leucine zipper) transcription factor, which is a member of the CREB/ATF family and has a transmembrane domain. OASIS is processed by regulated intramembrane proteolysis (RIP) in response to ER stress, and is highly expressed in osteoblasts. OASIS(-/-) mice exhibited severe osteopenia, involving a decrease in type I collagen in the bone matrix and a decline in the activity of osteoblasts, which showed abnormally expanded rough ER, containing of a large amount of bone matrix proteins. Here we identify the gene for type 1 collagen, Col1a1, as a target of OASIS, and demonstrate that OASIS activates the transcription of Col1a1 through an unfolded protein response element (UPRE)-like sequence in the osteoblast-specific Col1a1 promoter region. Moreover, expression of OASIS in osteoblasts is induced by BMP2 (bone morphogenetic protein 2), the signalling of which is required for bone formation. Additionally, RIP of OASIS is accelerated by BMP2 signalling, which causes mild ER stress. Our studies show that OASIS is critical for bone formation through the transcription of Col1a1 and the secretion of bone matrix proteins, and they reveal a new mechanism by which ER stress-induced signalling mediates bone formation.

Endoplasmic reticulum (ER) stress transducers IRE1, PERK and ATF6 are well known to transduce signals from the ER to the cytoplasm and nucleus when unfolded proteins are accumulated in the ER. Here, we identified OASIS as a novel ER stress transducer. OASIS is a basic leucine zipper (bZIP) transcription factor of the CREB/ATF family with a transmembrane domain that allows it to associate with the ER. The molecule is cleaved at the membrane in response to ER stress, and its cleaved amino-terminal cytoplasmic domain, which contains the bZIP domain, translocates into the nucleus where it activates the transcription of target genes that are mediated by ER stress-responsive and cyclic AMP-responsive elements. Intriguingly, OASIS was induced at the transcriptional level during ER stress in astrocytes of the central nervous system, but not in other cell types examined. Furthermore, overexpression of OASIS resulted in induction of BiP and suppression of ER-stress-induced cell death, whereas knockdown partially reduced BiP levels and led to ER stress in susceptible astrocytes. Our results reveal pivotal roles for OASIS in modulating the unfolded protein response in astrocytes, and the possibility that cell type-specific UPR signalling also exists in other cells.