ER Stress-Induced Sphingosine-1-Phosphate Lyase Phosphorylation Potentiates the Mitochondrial Unfolded Protein Response.

The unfolded protein response (UPR) is an elaborate signaling network that evolved to maintain proteostasis in the endoplasmic reticulum (ER) and mitochondria (mt). These organelles are functionally and physically associated, and consequently, their stress responses are often intertwined. It is unclear how these two adaptive stress responses are coordinated during ER stress. The inositol-requiring enzyme-1 (IRE1), a central ER stress sensor and proximal regulator of the UPRER, harbors dual kinase and endoribonuclease (RNase) activities. IRE1 RNase activity initiates the transcriptional layer of the UPRER, but IRE1's kinase substrate(s) and their functions are largely unknown. Here, we discovered that sphingosine 1-phosphate (S1P) lyase (SPL), the enzyme that degrades S1P, is a substrate for the mammalian IRE1 kinase. Our data show that IRE1-dependent SPL phosphorylation inhibits SPL's enzymatic activity, resulting in increased intracellular S1P levels. S1P has previously been shown to induce the activation of mitochondrial UPR (UPRmt) in nematodes. We determined that IRE1 kinase-dependent S1P induction during ER stress potentiates UPRmt signaling in mammalian cells. Phosphorylation of eukaryotic translation initiation factor 2α (eif2α) is recognized as a critical molecular event for UPRmt activation in mammalian cells. Our data further demonstrate that inhibition of the IRE1-SPL axis abrogates the activation of two eif2α kinases, namely double-stranded RNA-activated protein kinase (PKR) and PKR-like ER kinase upon ER stress. These findings show that the IRE1-SPL axis plays a central role in coordinating the adaptive responses of ER and mitochondria to ER stress in mammalian cells.

PACT establishes a posttranscriptional brake on mitochondrial biogenesis by promoting the maturation of miR-181c.

The double-stranded RNA-dependent protein kinase activating protein (PACT), an RNA-binding protein that is part of the RNA-induced silencing complex, plays a key role in miR-mediated translational repression. Previous studies showed that PACT regulates the expression of various miRs, selects the miR strand to be loaded onto RNA-induced silencing complex, and determines proper miR length. Apart from PACT's role in mediating the antiviral response in immune cells, what PACT does in other cell types is unknown. Strikingly, it has also been shown that cold exposure leads to marked downregulation of PACT protein in mouse brown adipose tissue (BAT), where mitochondrial biogenesis and metabolism play a central role. Here, we show that PACT establishes a posttranscriptional brake on mitochondrial biogenesis (mitobiogenesis) by promoting the maturation of miR-181c, a key suppressor of mitobiogenesis that has been shown to target mitochondrial complex IV subunit I (Mtco1) and sirtuin 1 (Sirt1). Consistently, we found that a partial reduction in PACT expression is sufficient to enhance mitobiogenesis in brown adipocytes in culture as well as during BAT activation in mice. In conclusion, we demonstrate an unexpected role for PACT in the regulation of mitochondrial biogenesis and energetics in cells and BAT.

Targeting IRE1 endoribonuclease activity alleviates cardiovascular lesions in a murine model of Kawasaki disease vasculitis.

Kawasaki disease (KD) is the leading cause of noncongenital heart disease in children. Studies in mice and humans propound the NLRP3/IL-1ß pathway as the principal driver of KD pathophysiology. Endoplasmic reticulum (ER) stress can activate the NLRP3 inflammasome, but the potential implication of ER stress in KD pathophysiology has not been investigated to our knowledge. We used human patient data and the Lactobacillus casei cell wall extract (LCWE) murine model of KD vasculitis to characterize the impact of ER stress on the development of cardiovascular lesions. KD patient transcriptomics and single-cell RNA sequencing of the abdominal aorta from LCWE-injected mice revealed changes in the expression of ER stress genes. Alleviating ER stress genetically, by conditional deletion of inositol-requiring enzyme 1 (IRE1) in myeloid cells, or pharmacologically, by inhibition of IRE1 endoribonuclease (RNase) activity, led to significant reduction of LCWE-induced cardiovascular lesion formation as well as reduced caspase-1 activity and IL-1ß secretion. These results demonstrate the causal relationship of ER stress to KD pathogenesis and highlight IRE1 RNase activity as a potential new therapeutic target.

Intercepting the Lipid-Induced Integrated Stress Response Reduces Atherosclerosis.

BACKGROUND: Eukaryotic cells can respond to diverse stimuli by converging at serine-51 phosphorylation on eukaryotic initiation factor 2 alpha (eIF2α) and activate the integrated stress response (ISR). This is a key step in translational control and must be tightly regulated; however, persistent eIF2α phosphorylation is observed in mouse and human atheroma. OBJECTIVES: Potent ISR inhibitors that modulate neurodegenerative disorders have been identified. Here, the authors evaluated the potential benefits of intercepting ISR in a chronic metabolic and inflammatory disease, atherosclerosis. METHODS: The authors investigated ISR's role in lipid-induced inflammasome activation and atherogenesis by taking advantage of 3 different small molecules and the ATP-analog sensitive kinase allele technology to intercept ISR at multiple molecular nodes. RESULTS: The results show lipid-activated eIF2α signaling induces a mitochondrial protease, Lon protease 1 (LONP1), that degrades phosphatase and tensin-induced putative kinase 1 and blocks Parkin-mediated mitophagy, resulting in greater mitochondrial oxidative stress, inflammasome activation, and interleukin-1ß secretion in macrophages. Furthermore, ISR inhibitors suppress hyperlipidemia-induced inflammasome activation and inflammation, and reduce atherosclerosis. CONCLUSIONS: These results reveal endoplasmic reticulum controls mitochondrial clearance by activating eIF2α-LONP1 signaling, contributing to an amplified oxidative stress response that triggers robust inflammasome activation and interleukin-1ß secretion by dietary fats. These findings underscore the intricate exchange of information and coordination of both organelles' responses to lipids is important for metabolic health. Modulation of ISR to alleviate organelle stress can prevent inflammasome activation by dietary fats and may be a strategy to reduce lipid-induced inflammation and atherosclerosis.

Prevention of atherosclerosis by bioactive palmitoleate through suppression of organelle stress and inflammasome activation.

De novo lipogenesis (DNL), the conversion of glucose and other substrates to lipids, is often associated with ectopic lipid accumulation, metabolic stress, and insulin resistance, especially in the liver. However, organ-specific DNL can also generate distinct lipids with beneficial metabolic bioactivity, prompting a great interest in their use for the treatment of metabolic diseases. Palmitoleate (PAO), one such bioactive lipid, regulates lipid metabolism in liver and improves glucose utilization in skeletal muscle when it is generated de novo from the obese adipose tissue. We show that PAO treatment evokes an overall lipidomic remodeling of the endoplasmic reticulum (ER) membranes in macrophages and mouse tissues, which is associated with resistance of the ER to hyperlipidemic stress. By preventing ER stress, PAO blocks lipid-induced inflammasome activation in mouse and human macrophages. Chronic PAO supplementation also lowers systemic interleukin-1ß (IL-1ß) and IL-18 concentrations in vivo in hyperlipidemic mice. Moreover, PAO prevents macrophage ER stress and IL-1ß production in atherosclerotic plaques in vivo, resulting in a marked reduction in plaque macrophages and protection against atherosclerosis in mice. These findings demonstrate that oral supplementation with a product of DNL such as PAO can promote membrane remodeling associated with metabolic resilience of intracellular organelles to lipid stress and limit the progression of atherosclerosis. These findings support therapeutic PAO supplementation as a potential preventive approach against complex metabolic and inflammatory diseases such as atherosclerosis, which warrants further studies in humans.

Suppression of intragenic transcription requires the MOT1 and NC2 regulators of TATA-binding protein.

Chromatin structure in transcribed regions poses a barrier for intragenic transcription. In a comprehensive study of the yeast chromatin remodelers and the Mot1p-NC2 regulators of TATA-binding protein (TBP), we detected synthetic genetic interactions indicative of suppression of intragenic transcription. Conditional depletion of Mot1p or NC2 in absence of the ISW1 remodeler, but not in the absence of other chromatin remodelers, activated the cryptic FLO8 promoter. Likewise, conditional depletion of Mot1p or NC2 in deletion backgrounds of the H3K36 methyltransferase Set2p or the Asf1p-Rtt106p histone H3-H4 chaperones, important factors involved in maintaining a repressive chromatin environment, resulted in increased intragenic FLO8 transcripts. Activity of the cryptic FLO8 promoter is associated with reduced H3 levels, increased TBP binding and tri-methylation of H3K4 and is independent of Spt-Ada-Gcn5-acetyltransferase function. These data reveal cooperation of negative regulation of TBP with specific chromatin regulators to inhibit intragenic transcription.