Deletion of hexose-6-phosphate dehydrogenase activates the unfolded protein response pathway and induces skeletal myopathy

A BioID-based approach uncovers the interactome of hexose-6-phosphate dehydrogenase in breast cancer cells and identifies anterior gradient protein 2 as an interacting partner

BACKGROUND

Hexose-6-phosphate dehydrogenase (H6PD) catalyzes the first two steps of the pentose-phosphate-pathway (PPP) within the endoplasmic reticulum, generating NADPH. H6PD modulates essential physiological processes, including energy and redox metabolism. Its sole reported interacting partner is 11β-hydroxysteroid dehydrogenase 1 (11β-HSD1), utilizing NADPH to reactivate glucocorticoids, linking energy status with hormonal response. Previous studies showed that loss of H6PD affects breast cancer cell properties, independent of 11β-HSD1. It remains unknown whether this is due to impaired concentrations of NADPH or PPP products downstream of H6PD. To gain insight into novel roles and pathways influenced by this enzyme, we aimed to assess the H6PD interactome.

RESULTS

We adapted the proximity-dependent Biotin Identification (BioID) method to identify novel H6PD interacting partners. First, we validated the method and confirmed the known interaction between H6PD and 11β-HSD1. Next, we constructed a triple-negative breast cancer MDA-MB-231 cell clone stably expressing a H6PD-biotin ligase fusion protein. Enriched biotinylated proteins were analyzed by mass-spectrometry and potential candidates assessed further by co-immunoprecipitation and functional assays. The resulting interactome revealed proteins of the calreticulin/calnexin cycle, unfolded-protein response (UPR) and chaperone activation pathways. Due to its known association with breast cancer, we examined the PDI Anterior gradient protein 2 (AGR2) as H6PD interacting partner. Gene set enrichment analysis revealed multiple overlapping pathways enriched in breast cancer tissues with relatively high H6PD and AGR2 expression. These included glycolysis, fatty acid metabolism, hypoxia, angiogenesis and epithelial to mesenchymal transition. Co-immunoprecipitation (Co-IP) from MCF7 cells confirmed a physical interaction between H6PD and AGR2. ARG2 knockdown in these cells increased H6PD protein levels but decreased activity. Coexpression with AGR2 in HEK-293 cells did not affect expression but enhanced H6PD activity.

CONCLUSION

BioID was successfully applied in the endoplasmic reticulum to identify AGR2 as H6PD interactor. This was confirmed using Co-IP from MCF7 cells endogenously expressing both proteins. The results indicate that AGR2 controls H6PD protein expression and enhances its activity. Whether higher H6PD activity due to increased AGR2 expression promotes a more aggressive cancer cell phenotype, for example by altering energy metabolism, Ca2+-related processes or UPR and chaperone activation pathways, warrants further investigations.

Common and divergent pathways in early stages of glutamate and tau-mediated toxicities in neurodegeneration

It has been shown that excitotoxicity and tau-mediated toxicities are major contributing factors to neuronal death in Alzheimer's disease (AD). The excitatory amino acid transporter 2 (EAAT2 or GLT-1), the major glutamate transporter in the brain that regulates glutamate levels synaptically and extrasynaptically, has been shown to be deficient in AD brains, leading to excitotoxicity and subsequent cell death. Similarly, buildup of neurofibrillary tangles, which consist of hyperphosphorylated tau protein, correlates with cognitive decline and neuronal atrophy in AD. However, common genes and pathways that are critical in the aforementioned toxicities have not been well elucidated. To investigate the impact of glutamate dyshomeostasis and tau accumulation on translational profiles of affected hippocampal neurons, we used mouse models of excitotoxicity and tau-mediated toxicities (GLT-1-/- and P301S, respectively) in conjunction with BAC-TRAP technology. Our data show that GLT-1 deficiency in CA3 pyramidal neurons leads to translational signatures characterized by dysregulation of pathways associated with synaptic plasticity and neuronal survival, while the P301S mutation induces changes in endocytic pathways and mitochondrial dysfunction. Finally, the commonly dysregulated pathways include impaired ion homeostasis and metabolic pathways. These common pathways may shed light on potential therapeutic targets for ameliorating glutamate and tau-mediated toxicities in AD.

Should the standard model of cellular energy metabolism be reconsidered? Possible coupling between the pentose phosphate pathway, glycolysis and extra-mitochondrial oxidative phosphorylation

The process of cellular respiration occurs for energy production through catabolic reactions, generally with glucose as the first process step. In the present work, we introduce a novel concept for understanding this process, based on our conclusion that glucose metabolism is coupled to the pentose phosphate pathway (PPP) and extra-mitochondrial oxidative phosphorylation in a closed-loop process. According to the current standard model of glycolysis, glucose is first converted to glucose 6-phosphate (glucose 6-P) and then to fructose 6-phosphate, glyceraldehyde 3-phosphate and pyruvate, which then enters the Krebs cycle in the mitochondria. However, it is more likely that the pyruvate will be converted to lactate. In the PPP, glucose 6-P is branched off from glycolysis and used to produce NADPH and ribulose 5-phosphate (ribulose 5-P). Ribulose 5-P can be converted to fructose 6-P and glyceraldehyde 3-P. In our view, a circular process can take place in which the ribulose 5-P produced by the PPP enters the glycolysis pathway and is then retrogradely converted to glucose 6-P. This process is repeated several times until the complete degradation of glucose 6-P. The role of mitochondria in this process is to degrade lipids by beta-oxidation and produce acetyl-CoA; the function of producing ATP appears to be only secondary. This proposed new concept of cellular bioenergetics allows the resolution of some previously unresolved controversies related to cellular respiration and provides a deeper understanding of metabolic processes in the cell, including new insights into the Warburg effect.

Contribution of local regeneration of glucocorticoids to tissue steroid pools

11β-Hydroxysteroid dehydrogenase 1 (11βHSD1) is a drug target to attenuate adverse effects of chronic glucocorticoid excess. It catalyses intracellular regeneration of active glucocorticoids in tissues including brain, liver and adipose tissue (coupled to hexose-6-phosphate dehydrogenase, H6PDH). 11βHSD1 activity in individual tissues is thought to contribute significantly to glucocorticoid levels at those sites, but its local contribution vs glucocorticoid delivery via the circulation is unknown. Here, we hypothesised that hepatic 11βHSD1 would contribute significantly to the circulating pool. This was studied in mice with Cre-mediated disruption of Hsd11b1 in liver (Alac-Cre) vs adipose tissue (aP2-Cre) or whole-body disruption of H6pdh. Regeneration of [9,12,12-2H3]-cortisol (d3F) from [9,12,12-2H3]-cortisone (d3E), measuring 11βHSD1 reductase activity was assessed at steady state following infusion of [9,11,12,12-2H4]-cortisol (d4F) in male mice. Concentrations of steroids in plasma and amounts in liver, adipose tissue and brain were measured using mass spectrometry interfaced with matrix-assisted laser desorption ionisation or liquid chromatography. Amounts of d3F were higher in liver, compared with brain and adipose tissue. Rates of appearance of d3F were ~6-fold slower in H6pdh-/- mice, showing the importance for whole-body 11βHSD1 reductase activity. Disruption of liver 11βHSD1 reduced the amounts of d3F in liver (by ~36%), without changes elsewhere. In contrast disruption of 11βHSD1 in adipose tissue reduced rates of appearance of circulating d3F (by ~67%) and also reduced regenerated of d3F in liver and brain (both by ~30%). Thus, the contribution of hepatic 11βHSD1 to circulating glucocorticoid levels and amounts in other tissues is less than that of adipose tissue.

Characteristics of glucose and lipid metabolism and the interaction between gut microbiota and colonic mucosal immunity in pigs during cold exposure

BACKGROUND

Cold regions have long autumn and winter seasons and low ambient temperatures. When pigs are unable to adjust to the cold, oxidative damage and inflammation may develop. However, the differences between cold and non-cold adaptation regarding glucose and lipid metabolism, gut microbiota and colonic mucosal immunological features in pigs are unknown. This study revealed the glucose and lipid metabolic responses and the dual role of gut microbiota in pigs during cold and non-cold adaptation. Moreover, the regulatory effects of dietary glucose supplements on glucose and lipid metabolism and the colonic mucosal barrier were evaluated in cold-exposed pigs.

RESULTS

Cold and non-cold-adapted models were established by Min and Yorkshire pigs. Our results exhibited that cold exposure induced glucose overconsumption in non-cold-adapted pig models (Yorkshire pigs), decreasing plasma glucose concentrations. In this case, cold exposure enhanced the ATGL and CPT-1α expression to promote liver lipolysis and fatty acid oxidation. Meanwhile, the two probiotics (Collinsella and Bifidobacterium) depletion and the enrichment of two pathogens (Sutterella and Escherichia-Shigella) in colonic microbiota are not conducive to colonic mucosal immunity. However, glucagon-mediated hepatic glycogenolysis in cold-adapted pig models (Min pigs) maintained the stability of glucose homeostasis during cold exposure. It contributed to the gut microbiota (including the enrichment of the Rikenellaceae RC9 gut group, [Eubacterium] coprostanoligenes group and WCHB1-41) that favored cold-adapted metabolism.

CONCLUSIONS

The results of both models indicate that the gut microbiota during cold adaptation contributes to the protection of the colonic mucosa. During non-cold adaptation, cold-induced glucose overconsumption promotes thermogenesis through lipolysis, but interferes with the gut microbiome and colonic mucosal immunity. Furthermore, glucagon-mediated hepatic glycogenolysis contributes to glucose homeostasis during cold exposure.

PRSS37 deficiency leads to impaired energy metabolism in testis and sperm revealed by DIA-based quantitative proteomic analysis

Our previous studies have reported that a putative trypsin-like serine protease, PRSS37, is exclusively expressed in testicular germ cells during late spermatogenesis and essential for sperm migration from the uterus into the oviduct and sperm-egg recognition via mediating the interaction between PDILT and ADAM3. In the present study, the global proteome profiles of wild-type (wt) and Prss37^-/- mice in testis and sperm were compared employing data independent acquisition (DIA) technology. Overall, 2506 and 459 differentially expressed proteins (DEPs) were identified in Prss37-null testis and sperm, respectively, when compared to control groups. Bioinformatic analyses revealed that most of DEPs were related to energy metabolism. Of note, the DEPs associated with pathways for the catabolism such as glucose via glycolysis, fatty acids via β-oxidation, and amino acids via oxidative deamination were significantly down-regulated. Meanwhile, the DEPs involved in the tricarboxylic acid cycle (TCA cycle) and oxidative phosphorylation (OXPHOS) were remarkably decreased. The DIA data were further confirmed by a markedly reduction of intermediate metabolites (citrate and fumarate) in TCA cycle and terminal metabolite (ATP) in OXPHOS system after disruption of PRSS37. These outcomes not only provide a more comprehensive understanding of the male fertility of energy metabolism modulated by PRSS37 but also furnish a dynamic proteomic resource for further reproductive biology studies.

Nicotinamide riboside kinases regulate skeletal muscle fiber-type specification and are rate-limiting for metabolic adaptations during regeneration

Nicotinamide riboside kinases (NRKs) control the conversion of dietary Nicotinamide Riboside (NR) to NAD+, but little is known about their contribution to endogenous NAD+ turnover and muscle plasticity during skeletal muscle growth and remodeling. Using NRK1/2 double KO (NRKdKO) mice, we investigated the influence of NRKs on NAD+ metabolism and muscle homeostasis, and on the response to neurogenic muscle atrophy and regeneration following muscle injury. Muscles from NRKdKO animals have altered nicotinamide (NAM) salvage and a decrease in mitochondrial content. In single myonuclei RNAseq of skeletal muscle, NRK2 mRNA expression is restricted to type IIx muscle fibers, and perturbed NAD+ turnover and mitochondrial metabolism shifts the fiber type composition of NRKdKO muscle to fast glycolytic IIB fibers. NRKdKO does not influence muscle atrophy during denervation but alters muscle repair after myofiber injury. During regeneration, muscle stem cells (MuSCs) from NRKdKO animals hyper-proliferate but fail to differentiate. NRKdKO also alters the recovery of NAD+ during muscle regeneration as well as mitochondrial adaptations and extracellular matrix remodeling required for tissue repair. These metabolic perturbations result in a transient delay of muscle regeneration which normalizes during myofiber maturation at late stages of regeneration via over-compensation of anabolic IGF1-Akt signaling. Altogether, we demonstrate that NAD+ synthesis controls mitochondrial metabolism and fiber type composition via NRK1/2 and is rate-limiting for myogenic commitment and mitochondrial maturation during skeletal muscle repair.

Balancing NAD+ deficits with nicotinamide riboside: therapeutic possibilities and limitations

Alterations in cellular nicotinamide adenine dinucleotide (NAD+) levels have been observed in multiple lifestyle and age-related medical conditions. This has led to the hypothesis that dietary supplementation with NAD+ precursors, or vitamin B3s, could exert health benefits. Among the different molecules that can act as NAD+ precursors, Nicotinamide Riboside (NR) has gained most attention due to its success in alleviating and treating disease conditions at the pre-clinical level. However, the clinical outcomes for NR supplementation strategies have not yet met the expectations generated in mouse models. In this review we aim to provide a comprehensive view on NAD+ biology, what causes NAD+ deficits and the journey of NR from its discovery to its clinical development. We also discuss what are the current limitations in NR-based therapies and potential ways to overcome them. Overall, this review will not only provide tools to understand NAD+ biology and assess its changes in disease situations, but also to decide which NAD+ precursor could have the best therapeutic potential.

The Two-Way Relationship Between Calcium and Metabolism in Cancer

Calcium ion (Ca²⁺) signaling is critical to many physiological processes, and its kinetics and subcellular localization are tightly regulated in all cell types. All Ca²⁺ flux perturbations impact cell function and may contribute to various diseases, including cancer. Several modulators of Ca²⁺ signaling are attractive pharmacological targets due to their accessibility at the plasma membrane. Despite this, the number of specific inhibitors is still limited, and to date there are no anticancer drugs in the clinic that target Ca²⁺ signaling. Ca²⁺ dynamics are impacted, in part, by modifications of cellular metabolic pathways. Conversely, it is well established that Ca²⁺ regulates cellular bioenergetics by allosterically activating key metabolic enzymes and metabolite shuttles or indirectly by modulating signaling cascades. A coordinated interplay between Ca²⁺ and metabolism is essential in maintaining cellular homeostasis. In this review, we provide a snapshot of the reciprocal interaction between Ca²⁺ and metabolism and discuss the potential consequences of this interplay in cancer cells. We highlight the contribution of Ca²⁺ to the metabolic reprogramming observed in cancer. We also describe how the metabolic adaptation of cancer cells influences this crosstalk to regulate protumorigenic signaling pathways. We suggest that the dual targeting of these processes might provide unprecedented opportunities for anticancer strategies. Interestingly, promising evidence for the synergistic effects of antimetabolites and Ca²⁺-modulating agents is emerging.

Lack of adipose-specific hexose-6-phosphate dehydrogenase causes inactivation of adipose glucocorticoids and improves metabolic phenotype in mice

Excessive glucocorticoid (GC) production in adipose tissue promotes the development of visceral obesity and metabolic syndrome (MS). 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) is critical for controlling intracellular GC production, and this process is tightly regulated by hexose-6-phosphate dehydrogenase (H6PDH). To better understand the integrated molecular physiological effects of adipose H6PDH, we created a tissue-specific knockout of the H6PDH gene mouse model in adipocytes (adipocyte-specific conditional knockout of H6PDH (H6PDHAcKO) mice). H6PDHAcKO mice exhibited almost complete absence of H6PDH expression and decreased intra-adipose corticosterone production with a reduction in 11β-HSD1 activity in adipose tissue. These mice also had decreased abdominal fat mass, which was paralleled by decreased adipose lipogenic acetyl-CoA carboxylase (ACC) and ATP-citrate lyase (ACL) gene expression and reduction in their transcription factor C/EBPα mRNA levels. Moreover, H6PDHAcKO mice also had reduced fasting blood glucose levels, increased glucose tolerance, and increased insulin sensitivity. In addition, plasma free fatty acid (FFA) levels were decreased with a concomitant decrease in the expression of lipase adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) in adipose tissue. These results indicate that inactivation of adipocyte H6PDH expression is sufficient to cause intra-adipose GC inactivation that leads to a favorable pattern of metabolic phenotypes. These data suggest that H6PDHAcKO mice may provide a good model for studying the potential contributions of fat-specific H6PDH inhibition to improve the metabolic phenotype in vivo. Our study suggests that suppression or inactivation of H6PDH expression in adipocytes could be an effective intervention for treating obesity and diabetes.

Induction of the nicotinamide riboside kinase NAD+ salvage pathway in a model of sarcoplasmic reticulum dysfunction

Background

Hexose-6-Phosphate Dehydrogenase (H6PD) is a generator of NADPH in the Endoplasmic/Sarcoplasmic Reticulum (ER/SR). Interaction of H6PD with 11β-hydroxysteroid dehydrogenase type 1 provides NADPH to support oxo-reduction of inactive to active glucocorticoids, but the wider understanding of H6PD in ER/SR NAD(P)(H) homeostasis is incomplete. Lack of H6PD results in a deteriorating skeletal myopathy, altered glucose homeostasis, ER stress and activation of the unfolded protein response. Here we further assess muscle responses to H6PD deficiency to delineate pathways that may underpin myopathy and link SR redox status to muscle wide metabolic adaptation.

Methods

We analysed skeletal muscle from H6PD knockout (H6PDKO), H6PD and NRK2 double knockout (DKO) and wild-type (WT) mice. H6PDKO mice were supplemented with the NAD⁺ precursor nicotinamide riboside. Skeletal muscle samples were subjected to biochemical analysis including NAD(H) measurement, LC-MS based metabolomics, Western blotting, and high resolution mitochondrial respirometry. Genetic and supplement models were assessed for degree of myopathy compared to H6PDKO.

Results

H6PDKO skeletal muscle showed adaptations in the routes regulating nicotinamide and NAD⁺ biosynthesis, with significant activation of the Nicotinamide Riboside Kinase 2 (NRK2) pathway. Associated with changes in NAD⁺ biosynthesis, H6PDKO muscle had impaired mitochondrial respiratory capacity with altered mitochondrial acylcarnitine and acetyl-CoA metabolism. Boosting NAD⁺ levels through the NRK2 pathway using the precursor nicotinamide riboside elevated NAD⁺/NADH but had no effect to mitigate ER stress and dysfunctional mitochondrial respiratory capacity or acetyl-CoA metabolism. Similarly, H6PDKO/NRK2 double KO mice did not display an exaggerated timing or severity of myopathy or overt change in mitochondrial metabolism despite depression of NAD⁺ availability.

Conclusions

These findings suggest a complex metabolic response to changes in muscle SR NADP(H) redox status that result in impaired mitochondrial energy metabolism and activation of cellular NAD⁺ salvage pathways. It is possible that SR can sense and signal perturbation in NAD(P)(H) that cannot be rectified in the absence of H6PD. Whether NRK2 pathway activation is a direct response to changes in SR NAD(P)(H) availability or adaptation to deficits in metabolic energy availability remains to be resolved.

TIGAR regulates mitochondrial functions through SIRT1-PGC1α pathway and translocation of TIGAR into mitochondria in skeletal muscle

TP53-induced glycolysis and apoptosis regulator (TIGAR), a glycolytic inhibitor, plays vital roles in regulating cellular metabolism and oxidative stress. However, the role of highly expressed TIGAR in skeletal muscle remains unexplored. In the present study, TIGAR levels varied in different skeletal muscles and fibers. An exhaustive swimming test with a load corresponding to 5% of body weight was utilized in mice to assess the effects of TIGAR on exercise-induced fatigue and muscle damage. The running time and metabolic indicators were significantly greater in wild-type (WT) mice compared with TIGAR knockout (KO) mice. Poor exercise capacity was accompanied by decreased type IIA fibers in TIGAR KO mice. Decreased mitochondrial number and mitochondrial oxidative phosphorylation were observed more in TIGAR KO mice than in WT mice, which were involved in sirtuin 1 (SIRT1)-mediated deacetylation of peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α), and resveratrol treatment in TIGAR KO mice can increase mitochondrial content and exercise time. Much more TIGAR was also detected in mitochondria during exhaustive exercise. In addition, TIGAR, rather than mitochondria-targeted TIGAR achieved by in vitro plasmid transfection, promoted SIRT1-PGC1α pathway. Glutathione S-transferase-TIGAR pull-down assay followed by liquid chromatography mass spectrometry found that TIGAR interacted with ATP synthase F1 subunit α (ATP5A1), and its binding to ATP5A1 increased during exhaustive exercise. Overexpression of mitochondrial-TIGAR enhanced ATP generation, maintained mitochondrial membrane potential and reduced mitochondrial oxidative stress under hypoxia condition. Taken together, our results uncovered a novel role for TIGAR in mitochondrial regulation in fast-twitch oxidative skeletal muscle through SIRT1-PGC1α and translocation into mitochondria, which contribute to the increase in exercise endurance of mice.-Geng, J., Wei, M., Yuan, X., Liu, Z., Wang, X., Zhang, D., Luo, L., Wu, J., Guo, W., Qin, Z.-H. TIGAR regulates mitochondrial functions through SIRT1-PGC1α pathway and translocation of TIGAR into mitochondria in skeletal muscle.

Dehydroepiandrosterone and Its CYP7B1 Metabolite 7α-Hydroxydehydroepiandrosterone Regulates 11β-Hydroxysteroid Dehydrogenase 1 Directions in Rat Leydig Cells

Background: The purpose of this study was to investigate cytochrome P450-7B1 (CYP7B1) in the human and rat testes to regulate 11β-hydroxysteroid dehydrogenase 1 (11β-HSD1) activity. We hypothesized that dehydroepiandrosterone (DHEA) and its product 7α-hydroxydehydroepiandrosterone (7αOHD) after catalysis of CYP7B1 played a critical role in driving the direction of 11β-HSD1, because 7αOHD is an alternative substrate for 11β-HSD1. Methods: We examined the influence of DHEA and 7αOHD on 11β-HSD1 activities in both intact Leydig cells and microsomes using radioactive substrates and identified the location of CYP7B1 in Leydig cells using immunohistochemical staining, Western blot, and qPCR. Results: We found that DHEA stimulated 11β-HSD1 oxidase activity in intact cells (EC₅₀ = 0.97 ± 0.11 μM) and inhibited its reductase activity (IC₅₀ = 1.04 ± 0.06 μM). In microsomes, DHEA was a competitive inhibitor of the reductase activity. The 11β-HSD1 oxidase activity in intact cells was inhibited by 7αOHD (IC₅₀ = 1.18 ± 0.12 μM), and the reductase activity was enhanced (EC₅₀ = 0.7 ± 0.04 μM). 7αOHD was a competitive inhibitor of 11β-HSD1 oxidase. CYP7B1 was present in rat Leydig cells, as shown by immunohistochemistry, Western blotting, and qPCR analysis. Conclusion: Our results are consistent with a conclusion that DHEA in the circulation driving 11β-HSD1 toward an oxidase in Leydig cells mainly through inhibiting the reductase of the enzyme, while 7αOHD (CYP7B1 catalytic product of DHEA) drives the enzyme toward the opposite direction.

Alterations of Cortisol Metabolism in Human Disorders

The interconversion of active and inactive corticosteroids - cortisol and cortisone, respectively, in humans - is modulated by isozymes of 11β-hydroxysteroid dehydrogenase (11-HSD). Studies of this process have provided crucial insights into glucocorticoid effects in a wide variety of tissues. The 11-HSD1 isozyme functions mainly as an oxoreductase (cortisone to cortisol) and is expressed at high levels in the liver and other glucocorticoid target tissues. Because it is required for full physiological effects of cortisol, it has emerged as a drug target for metabolic syndrome and type 2 diabetes. Mutations in the corresponding HSD11B1 gene, or in the H6PD gene encoding hexose-6-phosphate dehydrogenase (which supplies the NADPH required for the oxoreductase activity of 11-HSD1), cause apparent cortisone reductase deficiency, a rare syndrome of adrenocortical hyperactivity and hyperandrogenism. In contrast, the 11-HSD2 isozyme functions as a dehydrogenase (cortisol to cortisone) and is expressed mainly in mineralocorticoid target tissues, where it bars access of cortisol to the mineralocorticoid receptor. Mutations in the HSD11B2 gene encoding 11-HSD2 cause the syndrome of apparent mineralocorticoid excess, a severe form of familial hypertension. The role of this enzyme in the pathogenesis of common forms of low-renin hypertension remains uncertain.

The emergence of the nicotinamide riboside kinases in the regulation of NAD+ metabolism

The concept of replenishing or elevating NAD+ availability to combat metabolic disease and ageing is an area of intense research. This has led to a need to define the endogenous regulatory pathways and mechanisms cells and tissues utilise to maximise NAD+ availability such that strategies to intervene in the clinical setting are able to be fully realised. This review discusses the importance of different salvage pathways involved in metabolising the vitamin B3 class of NAD+ precursor molecules, with a particular focus on the recently identified nicotinamide riboside kinase pathway at both a tissue-specific and systemic level.

Oxidative stress-modulating drugs have preferential anticancer effects - involving the regulation of apoptosis, DNA damage, endoplasmic reticulum stress, autophagy, metabolism, and migration

To achieve preferential effects against cancer cells but less damage to normal cells is one of the main challenges of cancer research. In this review, we explore the roles and relationships of oxidative stress-mediated apoptosis, DNA damage, ER stress, autophagy, metabolism, and migration of ROS-modulating anticancer drugs. Understanding preferential anticancer effects in more detail will improve chemotherapeutic approaches that are based on ROS-modulating drugs in cancer treatments.

Hexose-6-phosphate dehydrogenase controls cancer cell proliferation and migration through pleiotropic effects on the unfolded-protein response, calcium homeostasis, and redox balance

Hexose-6-phosphate dehydrogenase (H6PD) produces reduced NADPH in the endoplasmic reticulum (ER) lumen. NADPH constitutes a cofactor for many reducing enzymes, and its inability to traverse biologic membranes makes in situ synthesis of NADPH in the ER lumen indispensable. The H6PD gene is amplified in several types of malignancies, and earlier work pointed toward a potential involvement of the enzyme in cancer cell growth. In the present study, we demonstrated a pivotal role of H6PD in proliferation and migratory potential of 3 human breast cancer cell lines. Knockdown of H6PD decreased proliferation and migration in SUM159, MCF7, and MDA-MB-453 cells. To understand the mechanism through which H6PD exerts its effects, we investigated the cellular changes after H6PD silencing in SUM159 cells. Knockdown of H6PD resulted in an increase in ER lumen oxidation, and down-regulation of many components of the unfolded protein response, including the transcription factors activating transcription factor-4, activating transcription factor-6, split X-box binding protein-1, and CCAAT/enhancer binding protein homologous protein. This effect was accompanied by an increase in sarco/endoplasmic reticulum Ca²⁺-ATPase-2 pump expression and an decrease in inositol trisphosphate receptor-III, which led to augmented levels of calcium in the ER. Further characterization of the molecular pathways involving H6PD could greatly broaden our understanding of how the ER microenvironment sustains malignant cell growth.-Tsachaki, M., Mladenovic, N., Štambergová, H., Birk, J., Odermatt, A. Hexose-6-phosphate dehydrogenase controls cancer cell proliferation and migration through pleiotropic effects on the unfolded protein response, calcium homeostasis, and redox balance.

Nicotinamide riboside kinases display redundancy in mediating nicotinamide mononucleotide and nicotinamide riboside metabolism in skeletal muscle cells

Objective

Augmenting nicotinamide adenine dinucleotide (NAD⁺) availability may protect skeletal muscle from age-related metabolic decline. Dietary supplementation of NAD⁺ precursors nicotinamide mononucleotide (NMN) and nicotinamide riboside (NR) appear efficacious in elevating muscle NAD⁺. Here we sought to identify the pathways skeletal muscle cells utilize to synthesize NAD⁺ from NMN and NR and provide insight into mechanisms of muscle metabolic homeostasis.

Methods

We exploited expression profiling of muscle NAD⁺ biosynthetic pathways, single and double nicotinamide riboside kinase 1/2 (NRK1/2) loss-of-function mice, and pharmacological inhibition of muscle NAD⁺ recycling to evaluate NMN and NR utilization.

Results

Skeletal muscle cells primarily rely on nicotinamide phosphoribosyltransferase (NAMPT), NRK1, and NRK2 for salvage biosynthesis of NAD⁺. NAMPT inhibition depletes muscle NAD⁺ availability and can be rescued by NR and NMN as the preferred precursors for elevating muscle cell NAD⁺ in a pathway that depends on NRK1 and NRK2. Nrk2 knockout mice develop normally and show subtle alterations to their NAD+ metabolome and expression of related genes. NRK1, NRK2, and double KO myotubes revealed redundancy in the NRK dependent metabolism of NR to NAD⁺. Significantly, these models revealed that NMN supplementation is also dependent upon NRK activity to enhance NAD⁺ availability.

Conclusions

These results identify skeletal muscle cells as requiring NAMPT to maintain NAD⁺ availability and reveal that NRK1 and 2 display overlapping function in salvage of exogenous NR and NMN to augment intracellular NAD⁺ availability.

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NRK1 and NRK2 are expressed in skeletal muscle and display redundancy in converting NR and NMN to NAD⁺.

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NRK1 and NRK2 are dispensable for maintaining basal skeletal muscle cell NAD⁺.

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Exogenous NMN salvage to NAD⁺ is NRK dependent.

Endoplasmic Reticulum Oxidative Stress Triggers Tgf-Beta-Dependent Muscle Dysfunction by Accelerating Ascorbic Acid Turnover

Endoplasmic reticulum (ER) and oxidative stress are two related phenomena that have important metabolic consequences. As many skeletal muscle diseases are triggered by oxidative stress, we explored the chain of events linking a hyperoxidized ER (which causes ER and oxidative stress) with skeletal muscle dysfunction. An unbiased exon expression array showed that the combined genetic modulation of the two master ER redox proteins, selenoprotein N (SEPN1) and endoplasmic oxidoreductin 1 (ERO1), led to an SEPN1-related myopathic phenotype due to excessive signalling of transforming growth factor (TGF)-beta. The increased TGF-beta activity in the genetic mutants was caused by accelerated turnover of the ER localized (anti-oxidant) ascorbic acid that affected collagen deposition in the extracellular matrix. In a mouse mutant of SEPN1, which is dependent on exogenous ascorbic acid, a limited intake of ascorbic acid revealed a myopathic phenotype as a consequence of an altered TGF-beta signalling. Indeed, systemic antagonism of TGF-beta re-established skeletal muscle function in SEPN1 mutant mice. In conclusion, this study sheds new light on the molecular mechanism of SEPN1-related myopathies and indicates that the TGF-beta/ERO1/ascorbic acid axis offers potential for their treatment.

Proteome-wide Profiling of Clinical PARP Inhibitors Reveals Compound-Specific Secondary Targets

Poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) are a promising class of targeted cancer drugs, but their individual target profiles beyond the PARP family, which could result in differential clinical use or toxicity, are unknown. Using an unbiased, mass spectrometry-based chemical proteomics approach, we generated a comparative proteome-wide target map of the four clinical PARPi, olaparib, veliparib, niraparib, and rucaparib. PARPi as a class displayed high target selectivity. However, in addition to the canonical targets PARP1, PARP2, and several of their binding partners, we also identified hexose-6-phosphate dehydrogenase (H6PD) and deoxycytidine kinase (DCK) as previously unrecognized targets of rucaparib and niraparib, respectively. Subsequent functional validation suggested that inhibition of DCK by niraparib could have detrimental effects when combined with nucleoside analog pro-drugs. H6PD silencing can cause apoptosis and further sensitize cells to PARPi, suggesting that H6PD may be, in addition to its established role in metabolic disorders, a new anticancer target.

Thrombospondin expression in myofibers stabilizes muscle membranes

Skeletal muscle is highly sensitive to mutations in genes that participate in membrane stability and cellular attachment, which often leads to muscular dystrophy. Here we show that Thrombospondin-4 (Thbs4) regulates skeletal muscle integrity and its susceptibility to muscular dystrophy through organization of membrane attachment complexes. Loss of the Thbs4 gene causes spontaneous dystrophic changes with aging and accelerates disease in 2 mouse models of muscular dystrophy, while overexpression of mouse Thbs4 is protective and mitigates dystrophic disease. In the myofiber, Thbs4 selectively enhances vesicular trafficking of dystrophin-glycoprotein and integrin attachment complexes to stabilize the sarcolemma. In agreement, muscle-specific overexpression of Drosophila Tsp or mouse Thbs4 rescues a Drosophila model of muscular dystrophy with augmented membrane residence of βPS integrin. This functional conservation emphasizes the fundamental importance of Thbs' as regulators of cellular attachment and membrane stability and identifies Thbs4 as a potential therapeutic target for muscular dystrophy.

Effects of Dietary Octacosanol on Growth Performance, Carcass Characteristics and Meat Quality of Broiler Chicks

Octacosanol, which has prominent physiological activities and functions, has been recognized as a potential growth promoter in animals. A total of 392 1-d-old male Arbor Acres broiler chicks with similar body weight were randomly distributed into four dietary groups of seven replicates with 14 birds each supplemented with 0, 12, 24, or 36 mg octacosanol (extracted from rice bran, purity >92%)/kg feed. The feeding trial lasted for six weeks and was divided into the starter (day 1 to 21) and the grower (day 22 to 42) phases. The results showed that the feed conversion ratio (FCR) was significantly improved in broilers fed a diet containing 24 mg/kg octacosanol compared with those fed the control diet in the overall phase (day 1 to 42, p = 0.042). The average daily gain and FCR both showed linear effects in response to dietary supplementation of octacosanol during the overall phase (p = 0.031 and 0.018, respectively). Broilers fed with 24 or 36 mg/kg octacosanol diet showed a higher eviscerated yield, which increased by 5.88% and 4.26% respectively, than those fed the control diet (p = 0.030). The breast muscle yield of broilers fed with 24 mg/kg octacosanol diet increased significantly by 12.15% compared with those fed the control diet (p = 0.047). Eviscerated and breast muscle yield increased linearly with the increase in dietary octacosanol supplementation (p = 0.013 and 0.021, respectively). Broilers fed with 24 or 36 mg/kg octacosanol diet had a greater (p = 0.021) pH_45min value in the breast muscle, which was maintained linearly in response to dietary octacosanol supplementation (p = 0.003). There was a significant decrease (p = 0.007) in drip loss value between the octacosanol-added and the control groups. The drip loss showed linear (p = 0.004) and quadratic (p = 0.041) responses with dietary supplementation of octacosanol. These studies indicate that octacosanol is a potentially effective and safe feed additive which may improve feed efficiency and meat quality, and increase eviscerated and breast muscle yield, in broiler chicks. Dietary supplementation of octacosanol at 24 mg/kg diet is regarded as the recommended dosage in the broilers’ diet.

Increased glycogen synthase kinase-3β and hexose-6-phosphate dehydrogenase expression in adipose tissue may contribute to glucocorticoid-induced mouse visceral adiposity

BACKGROUND

Increased adiposity in visceral depots is a crucial feature associated with glucocorticoid (GC) excess. The action of GCs in target tissue is regulated by GC receptor (GR) and 11ß-hydroxysteroid dehydrogenase type 1 (11ß-HSD1) coupled with hexose-6-phosphate dehydrogenase (H6pdh). Glycogen synthase kinase-3β (GSK3β) is known to be a crucial mediator of ligand-dependent gene transcription. We hypothesized that the major effects of corticosteroids on adipose fat accumulation are in part medicated by changes in GSK3β and H6pdh.

METHODS

We characterized the alterations of GSK3β and GC metabolic enzymes, and determined the impact of GR antagonist mifepristone on obesity-related genes and the expression of H6pdh and 11ß-HSD1 in adipose tissue of mice exposed to excess GC as well as in in vitro studies using 3T3-L1 adipocytes treated with GCs.

RESULTS

Corticosterone (CORT) exposure increased abdominal fat mass and induced expression of lipid synthase ACC and ACL with activation of GSK3β phosphorylation in abdominal adipose tissue of C57BL/6J mice. Increased pSer⁹ GSK3β was correlated with induction of H6pdh and 11ß-HSD1. Additionally, mifepristone treatment reversed the production of H6pdh and attenuated CORT-mediated production of 11ß-HSD1 and lipogenic gene expression with reduction of pSer⁹ GSK3β, thereby leading to improvement of phenotype of adiposity within adipose tissue in mice treated with excess GCs. Suppression of pSer⁹ GSK3β by mifepristone was accompanied by activation of pThr³⁰⁸ Akt and blockade of CORT-induced adipogenic transcriptor C/EBPα and PPARγ. In addition, mifepristone also attenuated CORT-mediated activation of IRE1α/XBP1. Additionally, reduction of H6pdh by shRNA showed comparable effects to mifepristone on attenuating CORT-induced expression of GC metabolic enzymes and improved lipid accumulation in vitro in 3T3-L1 adipocytes.

CONCLUSION

These findings suggest that elevated adipose GSK3β and H6pdh expression contribute to 11ß-HSD1 mediating hypercortisolism associated with visceral adiposity.

Metabolic Coupling Determines the Activity: Comparison of 11β-Hydroxysteroid Dehydrogenase 1 and Its Coupling between Liver Parenchymal Cells and Testicular Leydig Cells

Background

11β-hydroxysteroid dehydrogenase 1 (11β-HSD1) interconverts active 11β-hydroxyl glucocorticoids and inactive 11keto forms. However, its directionality is determined by availability of NADP+/NADPH. In liver cells, 11β-HSD1 behaves as a primary reductase, while in Leydig cells it acts as a primary oxidase. However, the exact mechanism is not clear. The direction of 11β-HSD1 has been proposed to be regulated by hexose-6-phosphate dehydrogenase (H6PDH), which catalyzes glucose-6-phosphate (G6P) to generate NADPH that drives 11β-HSD1 towards reduction.

Methodology

To examine the coupling between 11β-HSD1 and H6PDH, we added G6P to rat and human liver and testis or Leydig cell microsomes, and 11β-HSD1 activity was measured by radiometry.

Results and Conclusions

G6P stimulated 11β-HSD1 reductase activity in rat (3 fold) or human liver (1.5 fold), but not at all in testis. S3483, a G6P transporter inhibitor, reversed the G6P-mediated increases of 11β-HSD1 reductase activity. We compared the extent to which 11β-HSD1 in rat Leydig and liver cells might be coupled to H6PDH. In order to clarify the location of H6PDH within the testis, we used the Leydig cell toxicant ethane dimethanesulfonate (EDS) to selectively deplete Leydig cells. The depletion of Leydig cells eliminated Hsd11b1 (encoding 11β-HSD1) expression but did not affect the expression of H6pd (encoding H6PDH) and Slc37a4 (encoding G6P transporter). H6pd mRNA level and H6PDH activity were barely detectable in purified rat Leydig cells. In conclusion, the availability of H6PDH determines the different direction of 11β-HSD1 in liver and Leydig cells.

Glucose-6-phosphate dehydrogenase deficiency and Alzheimer's disease: Partners in crime? The hypothesis

Alzheimer's disease is a multifaceted brain disorder which involves various coupled irreversible, progressive biochemical reactions that significantly reduce quality of life as well as the actual life expectancy. Aging, genetic predispositions, head trauma, diabetes, cardiovascular disease, deficiencies in insulin signaling, dysfunction of mitochondria-associated membranes, cerebrovascular changes, high cholesterol level, increased oxidative stress and free radical formation, DNA damage, disturbed energy metabolism, and synaptic dysfunction, high blood pressure, obesity, dietary habits, exercise, social engagement, and mental stress are noted among the risk factors of this disease. In this hypothesis review I would like to draw the attention on glucose-6-phosphate dehydrogenase deficiency and its relationship with Alzheimer's disease. This enzymopathy is the most common human congenital defect of metabolism and defined by decrease in NADPH+H(+) and reduced form of glutathione concentration and that might in turn, amplify oxidative stress due to essentiality of the enzyme. This most common enzymopathy may manifest itself in severe forms, however most of the individuals with this deficiency are not essentially symptomatic. To understand the sporadic Alzheimer's disease, the writer of this paper thinks that, looking into a crystal ball might not yield much of a benefit but glucose-6-phosphate dehydrogenase deficiency could effortlessly give some clues.

ER stress and hexosamine pathway during tumourigenesis: A pas de deux?

Both the hexosamine biosynthetic pathway (HBP) and the endoplasmic reticulum (ER) are considered sensors for the nutritional state of the cell. The former is a branch of the glucose metabolic pathway that provides donor molecules for glycosylation processes, whereas the second requires co-translational N-glycosylation to ensure proper protein folding. It has become clear that the microenvironment of solid tumours, characterised by poor oxygen and nutrient supply, challenges optimal functions of the ER and the HBP. Here, we review recent advances demonstrating that the ER stress (ERS) response and HBP pathways are interconnected to promote cell viability. We then develop the idea that communication between ER and HBP is a survival feature of neoplastic cells that plays a prominent role during tumourigenesis.

Histone H3K4 methyltransferase Mll1 regulates protein glycosylation and tunicamycin-induced apoptosis through transcriptional regulation

Disrupting protein glycosylation induces ER (endoplasmic reticulum) stress, resulting in the activation of UPR (unfolded protein response) pathways. A key function of the UPR is to restore ER homeostasis, but prolonged or unsolved ER stress can lead to apoptosis. MLL1 (Mixed Lineage Leukemia 1, also named ALL-1 or HRX), a histone H3K4 methyltransferase in mammals, plays important roles in leukemogenesis, transcriptional regulation, cell cycle and development. Here, we find that Mll1 deficiency enhances UPR and apoptosis induced by the glycosylation inhibitor TM (tunicamycin). The abnormal regulation of the UPR appears to be caused by a defect in protein glycosylation. Furthermore, Mll1 directly binds to the promoters of H6pd, Galnt12 and Ugp2, which regulates H3K4 trimethylation and the subsequent expression of these genes. The knockdown of H6pd, Galnt12 or Ugp2 enhances TM-induced apoptosis in Mll1(+/+)MEF cells, whereas the ectopic expression of these proteins inhibits TM-induced apoptosis in Mll1(-/-) MEF cells. Together, our data suggest that the maturation of glycoproteins in the ER is subject to regulation at the epigenetic level by a histone methyltransferase whose abnormality can lead to cancer and developmental defects.

Differential proteomic profiling unveils new molecular mechanisms associated with mitochondrial complex III deficiency

We have analyzed the cellular pathways and metabolic adaptations that take place in primary skin fibroblasts from patients with mutations in BCS1L, a major genetic cause of mitochondrial complex III enzyme deficiency. Mutant fibroblasts exhibited low oxygen consumption rates and intracellular ATP levels, indicating that the main altered molecular event probably is a limited respiration-coupled ATP production through the OXPHOS system. Two-dimensional DIGE and MALDI-TOF/TOF mass spectrometry analyses unambiguously identified 39 proteins whose expression was significantly altered in complex III-deficient fibroblasts. Extensive statistical and cluster analyses revealed a protein profile characteristic for the BCS1L mutant fibroblasts that included alterations in energy metabolism, cell signaling and gene expression regulation, cytoskeleton formation and maintenance, and intracellular stress responses. The physiological validation of the predicted functional adaptations of human cultured fibroblasts to complex III deficiency confirmed the up-regulation of glycolytic enzyme activities and the accumulation of branched-chain among other amino acids, suggesting the activation of anaerobic glycolysis and cellular catabolic states, in particular protein catabolism, together with autophagy as adaptive responses to mitochondrial respiratory chain dysfunction and ATP deficiency. Our data point to an overall metabolic and genetic reprogramming that could contribute to explain the clinical manifestations of complex III deficiency in patients.

BIOLOGICAL SIGNIFICANCE

Despite considerable knowledge about their genetic origins, the pathophysiological mechanisms that contribute to the clinical manifestations of mitochondrial disorders remain poorly understood. We have investigated the molecular pathways and metabolic adaptations that take place in primary skin fibroblasts from patients with mutations in the BCS1L gene, a primary cause of mitochondrial complex III enzyme deficiency. Two-dimensional DIGE together with MALDI-TOF/TOF mass spectrometry and physiological validation analyses revealed a significant metabolic and genetic reprogramming as an adaptive response to mitochondrial respiratory chain dysfunction. Our data provide information about specific protein targets that regulate the transmitochondrial functional responses to complex III deficiency, thereby opening new doors for future research.

Caspase-12 ablation preserves muscle function in the mdx mouse

Duchenne muscular dystrophy (DMD) is a devastating muscle wasting disease caused by mutations in dystrophin. Several downstream consequences of dystrophin deficiency are triggers of endoplasmic reticulum (ER) stress, including loss of calcium homeostasis, hypoxia and oxidative stress. During ER stress, misfolded proteins accumulate in the ER lumen and the unfolded protein response (UPR) is triggered, leading to adaptation or apoptosis. We hypothesized that ER stress is heightened in dystrophic muscles and contributes to the pathology of DMD. We observed increases in the ER stress markers BiP and cleaved caspase-4 in DMD patient biopsies, compared with controls, and an increase in multiple UPR pathways in muscles of the dystrophin-deficient mdx mouse. We then crossed mdx mice with mice null for caspase-12, the murine equivalent of human caspase-4, which are resistant to ER stress. We found that deleting caspase-12 preserved mdx muscle function, resulting in a 75% recovery of both specific force generation and resistance to eccentric contractions. The compensatory hypertrophy normally found in mdx muscles was normalized in the absence of caspase-12; this was found to be due to decreased fibre sizes, and not to a fibre type shift or a decrease in fibrosis. Fibre central nucleation was not significantly altered in the absence of caspase-12, but muscle fibre degeneration found in the mdx mouse was reduced almost to wild-type levels. In conclusion, we have identified heightened ER stress and abnormal UPR signalling as novel contributors to the dystrophic phenotype. Caspase-4 is therefore a potential therapeutic target for DMD.

11β-hydroxysteroid dehydrogenases: intracellular gate-keepers of tissue glucocorticoid action

Glucocorticoid action on target tissues is determined by the density of "nuclear" receptors and intracellular metabolism by the two isozymes of 11β-hydroxysteroid dehydrogenase (11β-HSD) which catalyze interconversion of active cortisol and corticosterone with inert cortisone and 11-dehydrocorticosterone. 11β-HSD type 1, a predominant reductase in most intact cells, catalyzes the regeneration of active glucocorticoids, thus amplifying cellular action. 11β-HSD1 is widely expressed in liver, adipose tissue, muscle, pancreatic islets, adult brain, inflammatory cells, and gonads. 11β-HSD1 is selectively elevated in adipose tissue in obesity where it contributes to metabolic complications. Similarly, 11β-HSD1 is elevated in the ageing brain where it exacerbates glucocorticoid-associated cognitive decline. Deficiency or selective inhibition of 11β-HSD1 improves multiple metabolic syndrome parameters in rodent models and human clinical trials and similarly improves cognitive function with ageing. The efficacy of inhibitors in human therapy remains unclear. 11β-HSD2 is a high-affinity dehydrogenase that inactivates glucocorticoids. In the distal nephron, 11β-HSD2 ensures that only aldosterone is an agonist at mineralocorticoid receptors (MR). 11β-HSD2 inhibition or genetic deficiency causes apparent mineralocorticoid excess and hypertension due to inappropriate glucocorticoid activation of renal MR. The placenta and fetus also highly express 11β-HSD2 which, by inactivating glucocorticoids, prevents premature maturation of fetal tissues and consequent developmental "programming." The role of 11β-HSD2 as a marker of programming is being explored. The 11β-HSDs thus illuminate the emerging biology of intracrine control, afford important insights into human pathogenesis, and offer new tissue-restricted therapeutic avenues.

Green fluorescent protein-based monitoring of endoplasmic reticulum redox poise

Pathological endoplasmic reticulum (ER) stress is tightly linked to the accumulation of reactive oxidants, which can be both upstream and downstream of ER stress. Accordingly, detrimental intracellular stress signals are amplified through establishment of a vicious cycle. An increasing number of human diseases are characterized by tissue atrophy in response to ER stress and oxidative injury. Experimental monitoring of stress-induced, time-resolved changes in ER reduction-oxidation (redox) states is therefore important. Organelle-specific examination of redox changes has been facilitated by the advent of genetically encoded, fluorescent probes, which can be targeted to different subcellular locations by means of specific amino acid extensions. These probes include redox-sensitive green fluorescent proteins (roGFPs) and the yellow fluorescent protein-based redox biosensor HyPer. In the case of roGFPs, variants with known specificity toward defined redox couples are now available. Here, we review the experimental framework to measure ER redox changes using ER-targeted fluorescent biosensors. Advantages and drawbacks of plate-reader and microscopy-based measurements are discussed, and the power of these techniques demonstrated in the context of selected cell culture models for ER stress.

Using natural variation in Drosophila to discover previously unknown endoplasmic reticulum stress genes

Natural genetic variation is a rich resource for identifying novel elements of cellular pathways such as endoplasmic reticulum (ER) stress. ER stress occurs when misfolded proteins accumulate in the ER and cells respond with the conserved unfolded protein response (UPR), which includes large-scale gene expression changes. Although ER stress can be a cause or a modifying factor of human disease, little is known of the amount of variation in the response to ER stress and the genes contributing to such variation. To study natural variation in ER stress response in a model system, we measured the survival time in response to tunicamycin-induced ER stress in flies from 114 lines from the sequenced Drosophila Genetic Reference Panel of wild-derived inbred strains. These lines showed high heterogeneity in survival time under ER stress conditions. To identify the genes that may be driving this phenotypic variation, we profiled ER stress-induced gene expression and performed an association study. Microarray analysis identified variation in transcript levels of numerous known and previously unknown ER stress-responsive genes. Survival time was significantly associated with polymorphisms in candidate genes with known (i.e., Xbp1) and unknown roles in ER stress. Functional testing found that 17 of 25 tested candidate genes from the association study have putative roles in ER stress. In both approaches, one-third of ER stress genes had human orthologs that contribute to human disease. This study establishes Drosophila as a useful model for studying variation in ER stress and identifying ER stress genes that may contribute to human disease.

Baf60c drives glycolytic metabolism in the muscle and improves systemic glucose homeostasis through Deptor-mediated Akt activation

A shift from oxidative to glycolytic metabolism has been associated with skeletal muscle insulin resistance in type 2 diabetes^1–5. However, whether this metabolic switch is deleterious or adaptive remains controversial^6–8, in part due to limited understanding of the regulatory network that directs the metabolic and contractile specification of fast-twitch glycolytic muscle. Here we show that BAF60c, a transcriptional cofactor enriched in fast-twitch muscle, promotes a switch from oxidative to glycolytic myofiber type through Deptor-mediated AKT activation. Muscle-specific transgenic expression of BAF60c activates a program of molecular, metabolic, and contractile changes characteristic of glycolytic muscle. In addition, BAF60c is required for maintaining glycolytic capacity in adult skeletal muscle in vivo. BAF60c expression is significantly decreased in skeletal muscle from obese mice. Unexpectedly, transgenic activation of the glycolytic muscle program by BAF60c protects mice from diet-induced insulin resistance and glucose intolerance. Further mechanistic studies revealed that Deptor is induced by the BAF60c/Six4 transcriptional complex and mediates activation of AKT and glycolytic metabolism by BAF60c in a cell-autonomous manner. This work defines a fundamental mechanism underlying the specification of fast glycolytic muscle and illustrates that the oxidative to glycolytic metabolic shift in skeletal muscle is potentially adaptive and beneficial in the diabetic state.

The microsomal enzyme 17β-hydroxysteroid dehydrogenase 3 faces the cytoplasm and uses NADPH generated by glucose-6-phosphate dehydrogenase

Recent studies proposed a functional coupling between 17β-hydroxysteroid dehydrogenase 3 (17β-HSD3)-dependent testosterone formation and 11β-hydroxysteroid dehydrogenase 1 (11β-HSD1)-mediated interconversion of glucocorticoids through competition for the luminal pyridine nucleotide pool. To test this hypothesis, we used human embryonic kidney-293 cells transfected with 17β-HSD3 and/or 11β-HSD1, in the absence or presence of hexose-6-phosphate dehydrogenase that generates reduced nicotinamide adenine dinucleotide phosphate (NADPH) in the endoplasmic reticulum and determined enzyme activities. As an endogenous cell model, mouse MA-10 Leydig cells were used. 17β-HSD3-dependent reduction of Δ4-androstene-3,17-dione was affected by neither coexpression with 11β-HSD1 nor overexpression or knockdown of hexose-6-phosphate dehydrogenase. In contrast, knockdown of glucose-6-phosphate dehydrogenase decreased 17β-HSD3 activity, indicating dependence on cytoplasmic NADPH. Upon selective permeabilization of the plasma membrane by digitonin, 17β-HSD3 but not 11β-HSD1 was detected by antibodies against C-terminal epitope tags, suggesting a cytoplasmic orientation of 17β-HSD3. The cytoplasmic orientation was confirmed using proteinase K digestion of microsomal preparations and by analysis of glycosylation of wild-type 17β-HSD3 and chimera in which the N-terminal anchor sequences between 17β-HSD3 and 11β-HSD1 were exchanged. In conclusion, the results demonstrate a cytoplasmic orientation of 17β-HSD3 and dependence on glucose-6-phosphate dehydrogenase-generated NADPH, explaining the lack of a direct functional coupling with the luminal 11β-HSD1-mediated glucocorticoid metabolism.

Biomarkers of hypothalamic-pituitary-adrenal axis activity in mice lacking 11β-HSD1 and H6PDH

Glucocorticoid concentrations are a balance between production under the negative feedback control and diurnal rhythm of the hypothalamic-pituitary-adrenal (HPA) axis and peripheral metabolism, for example by the enzyme 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1), which catalyses the reduction of inactive cortisone (11-dehydrocorticosterone (11-DHC) in mice) to cortisol (corticosterone in mice). Reductase activity is conferred upon 11β-HSD1 by hexose-6-phosphate dehydrogenase (H6PDH). 11β-HSD1 is implicated in the development of obesity, and selective 11β-HSD1 inhibitors are currently under development. We sought to address the concern regarding potential up-regulation of the HPA axis associated with inhibition of 11β-HSD1. We assessed biomarkers for allele combinations of 11β-HSD1 and H6PDH derived from double heterozygous mouse crosses. H6PDH knock out (KO) adrenals were 69% larger than WT while 11β-HSD1 KO and double KO (DKO) adrenals were ~30% larger than WT - indicative of increased HPA axis drive in KO animals. ACTH-stimulated circulating corticosterone concentrations were 2.2-fold higher in H6PDH KO animals and ~1.5-fold higher in 11β-HSD1 KO and DKO animals compared with WT, proportional to the observed adrenal hypertrophy. KO of H6PDH resulted in a substantial increase in urinary DHC metabolites in males (65%) and females (61%). KO of 11β-HSD1 alone or in combination with H6PDH led to significant increases (36 and 42% respectively) in urinary DHC metabolites in females only. Intermediate 11β-HSD1/H6PDH heterozygotes maintained a normal HPA axis. Urinary steroid metabolite profile by gas chromatography/mass spectrometry as a biomarker assay may be beneficial in assaying HPA axis status clinically in cases of congenital and acquired 11β-HSD1/H6PDH deficiency.

G6PT-H6PDH-11βHSD1 triad in the liver and its implication in the pathomechanism of the metabolic syndrome

The metabolic syndrome, one of the most common clinical conditions in recent times, represents a combination of cardiometabolic risk determinants, including central obesity, glucose intolerance, insulin resistance, dyslipidemia, non-alcoholic fatty liver disease and hypertension. Prevalence of the metabolic syndrome is rapidly increasing worldwide as a consequence of common overnutrition and consequent obesity. Although a unifying picture of the pathomechanism is still missing, the key role of the pre-receptor glucocorticoid activation has emerged recently. Local glucocorticoid activation is catalyzed by a triad composed of glucose-6-phosphate-transporter, hexose-6-phosphate dehydrogenase and 11β-hydroxysteroid dehydrogenase type 1 in the endoplasmic reticulum. The elements of this system can be found in various cell types, including adipocytes and hepatocytes. While the contribution of glucocorticoid activation in adipose tissue to the pathomechanism of the metabolic syndrome has been well established, the relative importance of the hepatic process is less understood. This review summarizes the available data on the role of the hepatic triad and its role in the metabolic syndrome, by confronting experimental findings with clinical observations.

Crosstalk and barriers between the electron carriers of the endoplasmic reticulum

SIGNIFICANCE

The lumen of the endoplasmic reticulum (ER) constitutes a separate compartment with a special proteome and metabolome. The characteristic redox environment required for the optimal functioning of local pathways is defined by the redox couples of the main electron carriers. These molecules, glutathione, pyridine nucleotides, and ascorbic acid, are present within the ER, but their composition, concentration, and redox state are characteristically different from those observed in other subcellular compartments. Spatial and kinetic barriers contribute to the generation and maintenance of this special redox environment.

RECENT ADVANCES

The ER redox has usually been considered from the perspective of oxidative protein folding, one of the major functions of the ER. Thus, the lumen has been described as a relatively oxidizing subcellular compartment.

CRITICAL ISSUES

The ER redoxome has been scantily mapped. However, recent observations suggest that the redox systems in reduced and oxidized states are present simultaneously. The concerted actions of transmembrane uptake processes and local oxidoreductases as well as the absence of specific transport and enzyme activities maintain the oxidized state of the thiol-disulfide systems and the reduced state of the pyridine nucleotide redox systems. These states are prerequisites for the normal redox reactions localized in the ER.

FUTURE DIRECTIONS

An outline of the interactions between the major electron carriers of the ER will contribute to a better understanding of human diseases related to ER redox homeostasis.

Tissue-specific modulation of mineralocorticoid receptor function by 11β-hydroxysteroid dehydrogenases: an overview

In the last decade significant progress has been made in the understanding of mineralocorticoid receptor (MR) function and its implications for physiology and disease. The knowledge on the essential role of MR in the regulation of electrolyte concentrations and blood pressure has been significantly extended, and the relevance of excessive MR activation in promoting inflammation, fibrosis and heart disease as well as its role in modulating neuronal cell viability and brain function is now widely recognized. Despite considerable progress, the mechanisms of MR function in various cell-types are still poorly understood. Key modulators of MR function include the glucocorticoid receptor (GR), which may affect MR function by formation of heterodimers and by differential genomic and non-genomic responses on gene expression, and 11β-hydroxysteroid dehydrogenases (11β-HSDs), which determine the availability of intracellular concentrations of active glucocorticoids. In this review we attempted to provide an overview of the knowledge on MR expression with regard to the presence or absence of GR, 11β-HSD2 and 11β-HSD1/hexose-6-phosphate dehydrogenase (H6PDH) in various tissues and cell types. The consequences of cell-specific differences in the coexpression of MR with these proteins need to be further investigated in order to understand the role of this receptor in a given tissue as well as its systemic impact.

Inhibition of fatty acid oxidation enhances oxidative protein folding and protects hepatocytes from endoplasmic reticulum stress

The unfolded protein response regulates lipid metabolism, but the functional benefit of this regulation to ER function is not clear. This work shows that inhibition of fatty acid oxidation raises cellular oxidation potential, facilitates ER oxidative folding, and protects hepatocytes from ER stress.

The unfolded protein response (UPR) signals protein misfolding in the endoplasmic reticulum (ER) to effect gene expression changes and restore ER homeostasis. Although many UPR-regulated genes encode ER protein processing factors, others, such as those encoding lipid catabolism enzymes, seem unrelated to ER function. It is not known whether UPR-mediated inhibition of fatty acid oxidation influences ER function or, if so, by what mechanism. Here we demonstrate that pharmacological or genetic inhibition of fatty acid oxidation renders liver cells partially resistant to ER stress–induced UPR activation both in vitro and in vivo. Reduced stress sensitivity appeared to be a consequence of increased cellular redox potential as judged by an elevated ratio of oxidized to reduced glutathione and enhanced oxidative folding in the ER. Accordingly, the ER folding benefit of inhibiting fatty acid (FA) oxidation could be phenocopied by manipulating glutathione recycling during ER stress. Conversely, preventing cellular hyperoxidation with N-acetyl cysteine partially negated the stress resistance provided by blocking FA oxidation. Our results suggest that ER stress can be ameliorated through alteration of the oxidizing environment within the ER lumen, and they provide a potential logic for the transient regulation of metabolic pathways by the UPR during stress.

Calsequestrin (CASQ1) rescues function and structure of calcium release units in skeletal muscles of CASQ1-null mice

Amplitude of Ca(2+) transients, ultrastructure of Ca(2+) release units, and molecular composition of sarcoplasmic reticulum (SR) are altered in fast-twitch skeletal muscles of calsequestrin-1 (CASQ1)-null mice. To determine whether such changes are directly caused by CASQ1 ablation or are instead the result of adaptive mechanisms, here we assessed ability of CASQ1 in rescuing the null phenotype. In vivo reintroduction of CASQ1 was carried out by cDNA electro transfer in flexor digitorum brevis muscle of the mouse. Exogenous CASQ1 was found to be correctly targeted to the junctional SR (jSR), as judged by immunofluorescence and confocal microscopy; terminal cisternae (TC) lumen was filled with electron dense material and its width was significantly increased, as judged by electron microscopy; peak amplitude of Ca(2+) transients was significantly increased compared with null muscle fibers transfected only with green fluorescent protein (control); and finally, transfected fibers were able to sustain cytosolic Ca(2+) concentration during prolonged tetanic stimulation. Only the expression of TC proteins, such as calsequestrin 2, sarcalumenin, and triadin, was not rescued as judged by Western blot. Thus our results support the view that CASQ1 plays a key role in both Ca(2+) homeostasis and TC structure.

Expression of hexose-6-phosphate dehydrogenase in rat tissues

Hexose-6-phosphate dehydrogenase (H6PD) is the main NADPH generating enzyme in the lumen of the endoplasmic reticulum. H6PD is regarded as an ancillary enzyme in prereceptorial glucocorticoid activation and probably acts as a nutrient sensor and as a prosurvival factor. H6PD expression was determined in a variety of rat and human tissues by detecting mRNA and protein levels, and by measuring its dehydrogenase and lactonase activities. It was found that H6PD was present in all investigated tissues; both expression and activity remained within an order of magnitude. Correlation was found between the dehydrogenase activity and protein or mRNA levels. The results confirmed the supposed housekeeping feature of the enzyme.

A systems biology approach identifies molecular networks defining skeletal muscle abnormalities in chronic obstructive pulmonary disease

Chronic Obstructive Pulmonary Disease (COPD) is an inflammatory process of the lung inducing persistent airflow limitation. Extensive systemic effects, such as skeletal muscle dysfunction, often characterize these patients and severely limit life expectancy. Despite considerable research efforts, the molecular basis of muscle degeneration in COPD is still a matter of intense debate. In this study, we have applied a network biology approach to model the relationship between muscle molecular and physiological response to training and systemic inflammatory mediators. Our model shows that failure to co-ordinately activate expression of several tissue remodelling and bioenergetics pathways is a specific landmark of COPD diseased muscles. Our findings also suggest that this phenomenon may be linked to an abnormal expression of a number of histone modifiers, which we discovered correlate with oxygen utilization. These observations raised the interesting possibility that cell hypoxia may be a key factor driving skeletal muscle degeneration in COPD patients.

Chronic Obstructive Pulmonary Disease (COPD) is a major life threatening disease of the lungs, characterized by airflow limitation and chronic inflammation. Progressive reduction of the body muscle mass is a condition linked to COPD that significantly decreases quality of life and survival. Physical exercise has been proposed as a therapeutic option but its utility is still a matter of debate. The mechanisms underlying muscle wasting are also still largely unknown. The results presented in this paper show that diseased muscles are largely unable to coordinate the expression of muscle remodelling and bioenergetics pathways and that the cause of this phenomena may be tissue hypoxia. These findings contrast with current hypotheses based on the role of chronic inflammation and show that a mechanism based on an oxygen driven, epigenetic control of these two important functions may be an important disease mechanism.

Manifold effects of palmitoylcarnitine on endoplasmic reticulum metabolism: 11β-hydroxysteroid dehydrogenase 1, flux through hexose-6-phosphate dehydrogenase and NADPH concentration

With the exception of the oxidation of G6P (glucose 6-phosphate) by H6PDH (hexose-6-phosphate dehydrogenase), scant information is available about other endogenous substrates affecting the redox state or the regulation of key enzymes which govern the ratio of the pyridine nucleotide NADPH/NADP. In isolated rat liver microsomes, NADPH production was increased, as anticipated, by G6P; however, this was strikingly amplified by palmitoylcarnitine. Subsequent experiments revealed that the latter compound, well within its physiological concentration range, inhibited 11β-HSD1 (11β-hydroxysteroid dehydrogenase 1), the bidirectional enzyme which interconnects inactive 11-oxo steroids and their active 11-hydroxy derivatives. Notably, palmitoylcarnitine also stimulated the antithetical direction of 11β-HSD1 reductase, namely dehydrogenase. This stimulation of H6PDH may have likewise contributed to the NADPH accretion. All told, the result of these enzyme modifications is, in a conjoint fashion, a sharp amplification of microsomal NADPH production. Neither the purified 11β-HSD1 nor that obtained following microsomal sonification were sensitive to palmitoylcarnitine inhibition. This suggests that the long-chain amphipathic acylcarnitines, given their favourable partitioning into the membrane lipid bilayer, disrupt the proficient kinetic and physical interplay between 11β-HSD1 and H6PDH. Finally, although IDH (isocitrate dehydrogenase) and malic enzyme are present in microsomes and increase NADPH concentration akin to that of G6P, neither had an effect on 11β-HSD1 reductase, evidence that the NADPH pool in the endoplasmic reticulum shared by the H6PDH/11β-HSD1 alliance is uncoupled from that governed by IDH and malic enzyme.

Polymyositis and dermatomyositis: pathophysiology

Recent advances have increased the understanding of the pathogenesis of polymyositis and dermatomyositis. Clearly, the pathogenesis is complex, and adaptive (eg, autoimmune) and innate and nonimmune pathways play a role in the disease mechanisms, but the relative contribution may vary between patients and in different phases of the disease. Phenotyping patients using autoantibody profiling has resulted in information on molecular pathways that may be relevant in certain subsets of patients with polymyositis or dermatomyositis, but combining the autoantibody profiles with molecular signatures of innate and nonimmune mechanisms would enhance our ability to classify, diagnose, and treat these disorders more effectively.

Biochemistry and physiology of hexose-6-phosphate knockout mice

Hexose-6-phosphate dehydrogenase (H6PDH) has emerged as an important factor in setting the redox status of the endoplasmic reticulum (ER) lumen. An important role of H6PDH is to generate a high NADPH/NADP(+) ratio which permits 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) to act as an oxo-reductase, catalyzing the activation of glucocorticoids (GCs). In H6PDH knockout mice 11β-HSD1 assumes dehydrogenase activity and inactivates GCs, rendering the target cell relatively GC insensitive. Consequently, H6PDHKO mice have a phenotype consistent with defects in the permissive and adaptive actions of GCs upon physiology. H6PDHKO mice have also offered an insight into muscle physiology as they also present with a severe vacuolating myopathy, abnormalities of glucose homeostasis and activation of the unfolded protein response due to ER stress, and a number of mechanisms driving this phenotype are thought to be involved. This article will review what we understand of the redox control of GC hormone metabolism regulated by H6PDH, and how H6PDHKO mice have allowed an in-depth understanding of its potentially novel, GC-independent roles in muscle physiology.

Hexose-6-phosphate dehydrogenase contributes to skeletal muscle homeostasis independent of 11β-hydroxysteroid dehydrogenase type 1

Glucose-6-phosphate (G6P) metabolism by the enzyme hexose-6-phosphate dehydrogenase (H6PDH) within the sarcoplasmic reticulum lumen generates nicotinamide adenine dinucleotide phosphate (reduced) to provide the redox potential for the enzyme 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) to activate glucocorticoid (GC). H6PDH knockout (KO) mice have a switch in 11β-HSD1 activity, resulting in GC inactivation and hypothalamic-pituitary-adrenal axis activation. Importantly, H6PDHKO mice develop a type II fiber myopathy with abnormalities in glucose metabolism and activation of the unfolded protein response (UPR). GCs play important roles in muscle physiology, and therefore, we have examined the importance of 11β-HSD1 and GC metabolism in mediating aspects of the H6PDHKO myopathy. To achieve this, we examined 11β-HSD1/H6PDH double-KO (DKO) mice, in which 11β-HSD1 mediated GC inactivation is negated. In contrast to H6PDHKO mice, DKO mice GC metabolism and hypothalamic-pituitary-adrenal axis set point is similar to that observed in 11β-HSD1KO mice. Critically, in contrast to 11β-HSD1KO mice, DKO mice phenocopy the salient features of the H6PDHKO, displaying reduced body mass, muscle atrophy, and vacuolation of type II fiber-rich muscle, fasting hypoglycemia, increased muscle glycogen deposition, and elevated expression of UPR genes. We propose that muscle G6P metabolism through H6PDH may be as important as changes in the redox environment when considering the mechanism underlying the activation of the UPR and the ensuing myopathy in H6PDHKO and DKO mice. These data are consistent with an 11β-HSD1-independent function for H6PDH in which sarcoplasmic reticulum G6P metabolism and nicotinamide adenine dinucleotide phosphate-(oxidized)/nicotinamide adenine dinucleotide phosphate (reduced) redox status are important for maintaining muscle homeostasis.

Decreased prereceptorial glucocorticoid activating capacity in starvation due to an oxidative shift of pyridine nucleotides in the endoplasmic reticulum

Redox state of pyridine nucleotides of the endoplasmic reticulum (ER) lumen was determined in different nutritional conditions. NADPH-dependent cortisone reduction and NADP(+)-dependent cortisol oxidation were measured in rat liver microsomes, by utilizing the luminal 11β-hydroxysteroid dehydrogenase type 1 activity. Cortisone reduction decreased, while cortisol oxidation increased during onward starvation, showing that the luminal NADPH/NADP(+) ratio was substantially decreased. Cortisone or metyrapone addition caused a smaller decrease in NADPH fluorescence in microsomes from starved rats. The results demonstrate that nutrient supply is mirrored by the redox state of ER luminal pyridine nucleotides.

Contribution of hexose-6-phosphate dehydrogenase to NADPH content and redox environment in the endoplasmic reticulum

BACKGROUND

Hexose-6-phosphate dehydrogenase (H6PD) has been considered to be a main source of NADPH in the endoplasmic reticulum. It provides reducing equivalents to 11-hydroxysteroid dehydrogenase type 1 for in situ re-activation of glucocorticoids. H6PD null mice indeed show signs of glucocorticoid deficiency, but also suffer from a skeletal myopathy mainly affecting fast twitch muscles, in which the unfolded protein response (UPR) is activated. Thus, H6PD may have additional functions in muscle.

MATERIALS AND METHODS

To determine the contribution of H6PD to total microsomal NADPH content, we measured NADPH in microsomes from liver and quadriceps, gastrocnemius and soleus muscles. To evaluate the effect of H6PD deficiency on microsomal thiol-disulfide redox environment, we measured reduced and oxidized glutathione and free protein thiols.

RESULTS AND CONCLUSIONS

H6PD deficiency decreased but did not eliminate NADPH content in liver and soleus microsomes. Thus there must be other sources of NADPH within the endoplasmic/sarcoplasmic reticulum. Levels of reduced glutathione and free protein thiols were decreased in gastrocnemius muscle from null mice, indicating a more oxidative environment. Such alterations in redox environment may underlie the myopathy and UPR activation in H6PD null mice.

GENERAL SIGNIFICANCE

H6PD plays a role in maintaining normal NADPH levels and redox environment inside the endoplasmic reticulum. Intrinsic differences in ER metabolism may explain the differing effects of H6PD deficiency in different tissues.