IRE1 phosphatase PP2Ce regulates adaptive ER stress response in the postpartum mammary gland

Genome-wide association study for high-temperature tolerance in the Japanese flounder

This study addresses the critical issue of high-temperature stress in Japanese flounder (Paralichthys olivaceus), a factor threatening both their survival and the growth of the aquaculture industry. The research aims to identify genetic markers associated with high-temperature tolerance, unravel the genetic regulatory mechanisms, and lay the foundation for breeding Japanese flounder with increased resistance to high temperatures. In this study, using a genome-wide association study was performed to identify single nucleotide polymorphisms (SNPs) and genes associated with high-temperature tolerance for Japanese flounder using 280 individuals with 342 311 high-quality SNPs. The traits of high-temperature tolerance were defined as the survival time and survival status of Japanese flounder at high water temperature (31℃) for 15 days cultivate. A genome-wide association study identified six loci on six chromosomes significantly correlated with survival time under high-temperature stress. Six candidate genes were successfully annotated. Additionally, 34 loci associated with survival status were identified and mapped to 15 chromosomes, with 22 candidate genes annotated. Functional analysis highlighted the potential importance of genes like traf4 and ppm1l in regulating apoptosis, impacting high-temperature tolerance in Japanese flounder. These findings provide a valuable theoretical framework for integrating molecular markers into Japanese flounder breeding programmes, serving as a molecular tool to enhance genetic traits linked to high-temperature tolerance in cultured Japanese flounder.

The Experimental and In Silico-Based Evaluation of NRF2 Modulators, Sulforaphane and Brusatol, on the Transcriptome of Immortalized Bovine Mammary Alveolar Cells

Changes during the production cycle of dairy cattle can leave these animals susceptible to oxidative stress and reduced antioxidant health. In particular, the periparturient period, when dairy cows must rapidly adapt to the sudden metabolic demands of lactation, is a period when the production of damaging free radicals can overwhelm the natural antioxidant systems, potentially leading to tissue damage and reduced milk production. Central to the protection against free radical damage and antioxidant defense is the transcription factor NRF2, which activates an array of genes associated with antioxidant functions and cell survival. The objective of this study was to evaluate the effect that two natural NRF2 modulators, the NRF2 agonist sulforaphane (SFN) and the antagonist brusatol (BRU), have on the transcriptome of immortalized bovine mammary alveolar cells (MACT) using both the RT-qPCR of putative NRF2 target genes, as well as RNA sequencing approaches. The treatment of cells with SFN resulted in the activation of many putative NRF2 target genes and the upregulation of genes associated with pathways involved in cell survival, metabolism, and antioxidant function while suppressing the expression of genes related to cellular senescence and DNA repair. In contrast, the treatment of cells with BRU resulted in the upregulation of genes associated with inflammation, cellular stress, and apoptosis while suppressing the transcription of genes involved in various metabolic processes. The analysis also revealed several novel putative NRF2 target genes in bovine. In conclusion, these data indicate that the treatment of cells with SFN and BRU may be effective at modulating the NRF2 transcriptional network, but additional effects associated with cellular stress and metabolism may complicate the effectiveness of these compounds to improve antioxidant health in dairy cattle via nutrigenomic approaches.

TGFβ1 regulates HRas-mediated activation of IRE1α through the PERK-RPAP2 axis in keratinocytes

Transforming Growth Factor β1 (TGFβ1) is a critical regulator of tumor progression in response to HRas. Recently, TGFβ1 has been shown to trigger ER stress in many disease models; however, its role in oncogene-induced ER stress is unclear. Oncogenic HRas induces the unfolded protein response (UPR) predominantly via the Inositol-requiring enzyme 1α (IRE1α) pathway to initiate the adaptative responses to ER stress, with importance for both proliferation and senescence. Here, we show a role of the UPR sensor proteins IRE1α and (PKR)-like endoplasmic reticulum kinase (PERK) to mediate the tumor-suppressive roles of TGFβ1 in mouse keratinocytes expressing mutant forms of HRas. TGFβ1 suppressed IRE1α phosphorylation and activation by HRas both in in vitro and in vivo models while simultaneously activating the PERK pathway. However, the increase in ER stress indicated an uncoupling of ER stress and IRE1α activation by TGFβ1. Pharmacological and genetic approaches demonstrated that TGFβ1-dependent dephosphorylation of IRE1α was mediated by PERK through RNA Polymerase II Associated Protein 2 (RPAP2), a PERK-dependent IRE1α phosphatase. In addition, TGFβ1-mediated growth arrest in oncogenic HRas keratinocytes was partially dependent on PERK-induced IRE1α dephosphorylation and inactivation. Together, these results demonstrate a critical cross-talk between UPR proteins that is important for TGFβ1-mediated tumor suppressive responses.

IRE1/XBP1 and endoplasmic reticulum signaling - from basic to translational research for cardiovascular disease

Most cellular protein synthesis, including synthesis of membrane-targeted and secreted proteins, which are critical for cellular and organ crosstalk, takes place at the endoplasmic reticulum (ER), placing the ER at the nexus of cellular signaling, growth, metabolism, and stress sensing. Ample evidence has established the dysregulation of protein homeostasis and the ER unfolded protein response (UPR) in cardiovascular disease. However, the mechanisms of stress sensing and signaling in the ER are incompletely defined. Recent studies have defined notable functions for the inositol-requiring kinase 1 (IRE1)/X-box- binding protein-1 (XBP1) branch of the UPR in regulation of cardiac function. This review highlights the mechanisms underlying IRE1 activation and the IRE1 interactome, which reveals unexpected functions for the UPR and summarizes our current understanding of the functions of IRE1 in cardiovascular disease.

Metal dependent protein phosphatase PPM family in cardiac health and diseases

Protein phosphorylation and dephosphorylation is central to signal transduction in nearly every aspect of cellular function, including cardiovascular regulation and diseases. While protein kinases are often regarded as the molecular drivers in cellular signaling with high specificity and tight regulation, dephosphorylation mediated by protein phosphatases is also gaining increasing appreciation as an important part of the signal transduction network essential for the robustness, specificity and homeostasis of cell signaling. Metal dependent protein phosphatases (PPM, also known as protein phosphatases type 2C, PP2C) belong to a highly conserved family of protein phosphatases with unique biochemical and molecular features. Accumulating evidence also indicates important and specific functions of individual PPM isoform in signaling and cellular processes, including proliferation, senescence, apoptosis and metabolism. At the physiological level, abnormal PPM expression and activity have been implicated in major human diseases, including cancer, neurological and cardiovascular disorders. Finally, inhibitors for some of the PPM members have been developed as a potential therapeutic strategy for human diseases. In this review, we will focus on the background information about the biochemical and molecular features of major PPM family members, with emphasis on their demonstrated or potential roles in cardiac pathophysiology. The current challenge and potential directions for future investigations will also be highlighted.

Inhibition of the PI 3-kinase pathway disrupts the unfolded protein response and reduces sensitivity to ER stress-dependent apoptosis

Class Ia phosphoinositide 3-kinases (PI3K) are critical mediators of insulin and growth factor action. We have demonstrated that the p85α regulatory subunit of PI3K modulates the unfolded protein response (UPR) by interacting with and regulating the nuclear translocation of XBP-1s, a transcription factor essential for the UPR. We now show that PI3K activity is required for full activation of the UPR. Pharmacological inhibition of PI3K in cells blunts the ER stress-dependent phosphorylation of IRE1α and PERK, decreases induction of ATF4, CHOP, and XBP-1 and upregulates UPR target genes. Cells expressing a human p85α mutant (R649W) previously shown to inhibit PI3K, exhibit decreased activation of IRE1α and PERK and reduced induction of CHOP and ATF4. Pharmacological inhibition of PI3K, overexpression of a mutant of p85α that lacks the ability to interact with the p110α catalytic subunit (∆p85α) or expression of mutant p85α (R649W) in vivo, decreased UPR-dependent induction of ER stress response genes. Acute tunicamycin treatment of R649W^+/- mice revealed reduced induction of UPR target genes in adipose tissue, whereas chronic tunicamycin exposure caused sustained increases in UPR target genes in adipose tissue. Finally, R649W^+/- cells exhibited a dramatic resistance to ER stress-dependent apoptosis. These data suggest that PI3K pathway dysfunction causes ER stress that may drive the pathogenesis of several diseases including Type 2 diabetes and various cancers.

Occludin protects secretory cells from ER stress by facilitating SNARE-dependent apical protein exocytosis

Tight junctions (TJs) are fundamental features of both epithelium and endothelium and are indispensable for vertebrate organ formation and homeostasis. However, mice lacking Occludin (Ocln) develop relatively normally to term. Here we show that Ocln is essential for mammary gland physiology, as mutant mice fail to produce milk. Surprisingly, Ocln null mammary glands showed intact TJ function and normal epithelial morphogenesis, cell differentiation, and tissue polarity, suggesting that Ocln is not required for these processes. Using single-cell transcriptomics, we identified milk-producing cells (MPCs) and found they were progressively more prone to endoplasmic reticulum (ER) stress as protein production increased exponentially during late pregnancy and lactation. Importantly, Ocln loss in MPCs resulted in greatly heightened ER stress; this in turn led to increased apoptosis and acute shutdown of protein expression, ultimately leading to lactation failure in the mutant mice. We show that the increased ER stress was caused by a secretory failure of milk proteins in Ocln null cells. Consistent with an essential role in protein secretion, Occludin was seen to reside on secretory vesicles and to be bound to SNARE proteins. Taken together, our results demonstrate that Ocln protects MPCs from ER stress by facilitating SNARE-dependent protein secretion and raise the possibility that other TJ components may participate in functions similar to Ocln.

Seipin deficiency leads to increased endoplasmic reticulum stress and apoptosis in mammary gland alveolar epithelial cells during lactation

Seipin is an integral endoplasmic reticulum (ER) membrane protein encoded by Berardinelli-Seip congenital lipodystrophy type 2 (BSCL2/Bscl2) gene. Most litters (59%) from Bscl2-/- dams mated with wild type (WT) (Bscl2+/+) males did not survive postnatal day 5 (PND5) and pups (Bscl2+/-) lacked milk in their stomachs. The survived litters had reduced pup survival rate at PND21. It was hypothesized that seipin was critical for lactation. Bscl2 was upregulated and highly detected in the lactation day 1 (LD1) WT mammary gland alveolar epithelial cells. LD1 Bscl2-/- mammary glands lacked adipocytes and alveolar clusters and had varied alveolar morphology: from interconnected mammary gland alveoli with dilated lumen and sloughed epithelial cells to undifferentiated mammary gland alveoli with unexpanded lumen. Comparable levels of whey acidic protein (WAP, a major component in rodent milk) staining and Nile Red lipid droplet staining between WT and Bscl2-/- LD1 alveolar epithelial cells indicated normal milk protein synthesis and lipid syntheses in LD1 Bscl2-/- mammary glands. Significantly reduced percentage of larger lipid droplets was detected in LD1 Bscl2-/- alveoli with unexpanded lumen. There was no obviously impaired proliferation detected by PCNA staining but increased apoptosis detected by cleaved caspase-3 staining in LD1 Bscl2-/- alveolar epithelial cells. Increased expression of protein disulfide isomerase and binding immunoglobulin protein in the LD1 Bscl2-/- mammary gland alveolar epithelial cells indicated increased ER stress. This study demonstrates increased ER stress and apoptosis in LD1 Bscl2-/- mammary gland alveolar epithelial cells and reveals a novel in vivo function of seipin in lactation.

Regulation of IRE1 RNase activity by the Ribonuclease inhibitor 1 (RNH1)

Adaptation to endoplasmic reticulum (ER) stress depends on the activation of the sensor inositol-requiring enzyme 1α (IRE1), an endoribonuclease that splices the mRNA of the transcription factor XBP1 (X-box-binding protein 1). To better understand the protein network that regulates the activity of the IRE1 pathway, we systematically screened the proteins that interact with IRE1 and identified a ribonuclease inhibitor called ribonuclease/angiogenin inhibitor 1 (RNH1). RNH1 is a leucine-rich repeat domains-containing protein that binds to and inhibits ribonucleases. Immunoprecipitation experiments confirmed this interaction. Docking experiments indicated that RNH1 physically interacts with IRE1 through its cytosolic RNase domain. Upon ER stress, the interaction of RNH1 with IRE1 in the ER increased at the expense of the nuclear pool of RNH1. Inhibition of RNH1 expression using siRNA mediated RNA interference upon ER stress led to an increased splicing activity of XBP1. Modulation of IRE1 RNase activity by RNH1 was recapitulated in a cell-free system, suggesting direct regulation of IRE1 by RNH. We conclude that RNH1 attenuates the activity of IRE1 by interacting with its ribonuclease domain. These findings have implications for understanding the molecular mechanism by which IRE1 signaling is attenuated upon ER stress.

A genetic variant in SLC30A2 causes breast dysfunction during lactation by inducing ER stress, oxidative stress and epithelial barrier defects

SLC30A2 encodes a zinc (Zn) transporter (ZnT2) that imports Zn into vesicles in highly-specialized secretory cells. Numerous mutations and non-synonymous variants in ZnT2 have been reported in humans and in breastfeeding women; ZnT2 variants are associated with abnormally low milk Zn levels and can lead to severe infantile Zn deficiency. However, ZnT2-null mice have profound defects in mammary epithelial cell (MEC) polarity and vesicle secretion, indicating that normal ZnT2 function is critical for MEC function. Here we report that women who harbor a common ZnT2 variant (T²⁸⁸S) present with elevated levels of several oxidative and endoplasmic reticulum (ER) stress markers in their breast milk. Functional studies in vitro suggest that substitution of threonine for serine at amino acid 288 leads to hyperphosphorylation retaining ZnT2 in the ER and lysosomes, increasing ER and lysosomal Zn accumulation, ER stress, the generation of reactive oxygen species, and STAT3 activation. These changes were associated with decreased abundance of zona occludens-1 and increased tight junction permeability. This study confirms that ZnT2 is important for normal breast function in women during lactation, and suggests that women who harbor defective variants in ZnT2 may be at-risk for poor lactation performance.

Transient and permanent changes in DNA methylation patterns in inorganic arsenic-mediated epithelial-to-mesenchymal transition

Chronic low dose inorganic arsenic exposure causes cells to take on an epithelial-to-mesenchymal phenotype, which is a crucial process in carcinogenesis. Inorganic arsenic is not a mutagen and thus epigenetic alterations have been implicated in this process. Indeed, during the epithelial-to-mesenchymal transition, morphologic changes to cells correlate with changes in chromatin structure and gene expression, ultimately driving this process. However, studies on the effects of inorganic arsenic exposure/withdrawal on the epithelial-to-mesenchymal transition and the impact of epigenetic alterations in this process are limited. In this study we used high-resolution microarray analysis to measure the changes in DNA methylation in cells undergoing inorganic arsenic-induced epithelial-to-mesenchymal transition, and on the reversal of this process, after removal of the inorganic arsenic exposure. We found that cells exposed to chronic, low-dose inorganic arsenic exposure showed 30,530 sites were differentially methylated, and with inorganic arsenic withdrawal several differential methylated sites were reversed, albeit not completely. Furthermore, these changes in DNA methylation mainly correlated with changes in gene expression at most sites tested but not at all. This study suggests that DNA methylation changes on gene expression are not clear-cut and provide a platform to begin to uncover the relationship between DNA methylation and gene expression, specifically within the context of inorganic arsenic treatment.

A Sarcoplasmic Reticulum Localized Protein Phosphatase Regulates Phospholamban Phosphorylation and Promotes Ischemia Reperfusion Injury in the Heart

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PP2Ce is Ser-Thr phosphatase specifically localized on SR and expressed in cardiomyocytes.

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PP2Ce has specific phosphatase activity to dephosphorylate Thr-17 site of phospholamban.

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PP2Ce expression is induced upon pathological stress, including beta-AR stimulation and ROS.

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PP2Ce induction suppresses cardiomyocyte calcium cycling, reduces beta-AR-induced contractility, and promotes oxidative ischemia/reperfusion injury.

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PP2Ce is a new molecular component of stress-mediated cardiomyocyte calcium regulation.

Phospholamban (PLN) is a key regulator of sarcolemma calcium uptake in cardiomyocyte; its inhibitory activity to sarcolemma-endoplasmic reticulum calcium ATPase is regulated by phosphorylation. PLN hypophosphorylation is a common molecular feature in the failing heart. The current study provided evidence at the molecular, cellular, and whole-heart levels to implicate a sarcolemma membrane-targeted protein phosphatase, PP2Ce, as a specific and potent PLN phosphatase. PP2Ce expression was elevated in failing human heart and induced acutely at protein level by β-adrenergic stimulation or oxidative stress in cardiomyocytes. PP2Ce expression in mouse heart blunted β-adrenergic response and exacerbated ischemia/reperfusion injury. Therefore, PP2Ce is a new regulator for cardiac function and pathogenesis.

PERK Is a Haploinsufficient Tumor Suppressor: Gene Dose Determines Tumor-Suppressive Versus Tumor Promoting Properties of PERK in Melanoma

The unfolded protein response (UPR) regulates cell fate following exposure of cells to endoplasmic reticulum stresses. PERK, a UPR protein kinase, regulates protein synthesis and while linked with cell survival, exhibits activities associated with both tumor progression and tumor suppression. For example, while cells lacking PERK are sensitive to UPR-dependent cell death, acute activation of PERK triggers both apoptosis and cell cycle arrest, which would be expected to contribute tumor suppressive activity. We have evaluated these activities in the BRAF-dependent melanoma and provide evidence revealing a complex role for PERK in melanoma where a 50% reduction is permissive for Braf^V600E-dependent transformation, while complete inhibition is tumor suppressive. Consistently, PERK mutants identified in human melanoma are hypomorphic with dominant inhibitory function. Strikingly, we demonstrate that small molecule PERK inhibitors exhibit single agent efficacy against Braf^V600E-dependent tumors highlighting the clinical value of targeting PERK.

PERK is critical for progression of specific cancers and has provided stimulus for the generation of small molecule PERK inhibitors. Paradoxically, the anti-proliferative and pro-death functions of PERK have potential tumor suppressive qualities. We demonstrate that PERK can function as either a tumor suppressor or a pro-adaptive tumor promoter and the nature of its function is determined by gene dose. Preclinical studies suggest a therapeutic threshold exists for PERK inhibitors.

Proteostasis control by the unfolded protein response

Stress induced by accumulation of misfolded proteins in the endoplasmic reticulum is observed in many physiological and pathological conditions. To cope with endoplasmic reticulum stress, cells activate the unfolded protein response, a dynamic signalling network that orchestrates the recovery of homeostasis or triggers apoptosis, depending on the level of damage. Here we provide an overview of recent insights into the mechanisms that cells employ to maintain proteostasis and how the unfolded protein response determines cell fate under endoplasmic reticulum stress.